The isotopic signature of U(V) during bacterial reduction

A. Brown; M. Molinas; Y. Roebbert; R. Faizova; T. Vitova et al. 

The two-step electron transfer during bacterial reduction of UVI to UIV is typically accompanied by mass-independent fractionation of the 238U and 235U isotopes, whereby the heavy isotope accumulates in the reduced product. However, the role of the UV intermediate in the fractionation mechanism is unresolved due to the challenges associated with its chemical stability. Here, we employed the UV stabilizing ligand, dpaea2−, to trap aqueous UV during UVI reduction by Shewanella oneidensis. Whilst the first reduction step from UVI to UV displayed negligible fractionation, reduction of UV to UIV revealed mass-dependent isotope fractionation (preferential reduction of the 235U), contrary to most previous observations. This surprising behaviour highlights the control the U-coordinating ligand exerts over the balance between reactant U supply, electron transfer rate, and UIV product sequestration, suggesting that UV speciation should be considered when using U isotope ratios to reconstruct environmental redox conditions.

Iron promoted end-on dinitrogen-bridging in heterobimetallic complexes of uranium and lanthanides

N. Jori; J. J. Moreno; R. A. K. Shivaraam; T. Rajeshkumar; R. Scopelliti et al. 

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N2 bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)2(N2)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [MIII{N(SiMe3)2}3] (M = U, Ce, Sm, Dy, Tm) and [LnII{N(SiMe3)2}2], (Ln = Sm, Yb). Despite the lack of reactivity of the U(iii), Ln(iii) and Ln(ii) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)2}(mu-eta 1:eta 1-N2)(M{N(SiMe3)2}3)], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe)2}(mu-eta 1:eta 1-N2)(Ln{N(SiMe3)2}2)], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe)2(mu-eta 1:eta 1-N2)}2{SmII{N(SiMe3)2}2}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N2 complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(ii) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N2 with both Ln(iii) and Ln(ii) complexes with the degree of N2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)-N2Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N2 compared to free N2 due to the higher N2 polarization.|End-on bridging dinitrogen binding to U(iii), Ln(iii) and Ln(ii) is favoured for the iron-bound N2 compared to free N2, resulting in increased N2 activation with increasing Ln Lewis acidity with a back-bonding contribution only found for U(iii).

Siloxide tripodal ligands as a scaffold for stabilizing lanthanides in the +4 oxidation state

M. C. G. Tricoire; F-C. Hsueh; M. R. Keener; T. Rajeshkumar; R. Scopelliti et al. 

Synthetic strategies to isolate molecular complexes of lanthanides, other than cerium, in the +4 oxidation state remain elusive, with only four complexes of Tb(iv) isolated so far. Herein, we present a new approach for the stabilization of Tb(iv) using a siloxide tripodal trianionic ligand, which allows the control of unwanted ligand rearrangements, while tuning the Ln(iii)/Ln(iv) redox-couple. The Ln(iii) complexes, [LnIII((OSiPh2Ar)3-arene)(THF)3] (1-LnPh) and [K(toluene){LnIII((OSiPh2Ar)3-arene)(OSiPh3)}] (2-LnPh) (Ln = Ce, Tb, Pr), of the (HOSiPh2Ar)3-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(iii) analogue complexes, [LnIII((OSi(OtBu)2Ar)3-arene)(THF)] (1-LnOtBu) and [K(THF)6][LnIII((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (2-LnOtBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OtBu)2Ar)3-arene. The cyclic voltammetry studies showed a cathodic shift in the oxidation potential for the cerium and terbium complexes of the more electron-donating phenyl substituted scaffold (1-LnPh) compared to those of the tert-butoxy (1-LnOtBu) ligand. Furthermore, the addition of the -OSiPh3 ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [CeIV((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (3-CeOtBu) and [CeIV((OSiPh2Ar)3-arene)(OSiPh3)(THF)2] (3-CePh), were prepared by chemical oxidation of the Ce(iii) analogues. Chemical oxidation of the Tb(iii) and Pr(iii) complexes (2-LnPh) was also possible, in which the Tb(iv) complex, [TbIV((OSiPh2Ar)3-arene)(OSiPh3)(MeCN)2] (3-TbPh), was isolated and crystallographically characterized, yielding the first example of a Tb(iv) supported by a polydentate ligand. The versatility and robustness of these siloxide arene-anchored platforms will allow further development in the isolation of more oxidizing Ln(iv) ions, widening the breadth of high-valent Ln chemistry.|Robust arene-anchored polydentate siloxide ligands allow to control unwanted ligand rearrangements for the isolation of Tb(iv) complexes thus enabling the use of the Tb(iii)/Tb(iv) couple for the separation of Tb from the neighboring Dy ion.

Two-Electron Redox Reactivity of Thorium Supported by Redox-Active Tripodal Frameworks

F-C. Hsueh; D. W. Chen; T. Rajeshkumar; R. Scopelliti; L. Maron et al. 

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene-tethered tris(siloxide) tripodal ligands [(KOSiR2Ar)3-arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(OtBu)2Ar)3-arene)(THF)2] (2OtBu) and [K(2.2.2-cryptand)][Th((OSiPh2Ar)3-arene)(THF)2](2Ph-crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two-electron reduction of a broad range of substrates (N2O, COT, CHT, Ph2N2, Ph3PS and O2) while retaining the ligand framework. This work shows that arene-tethered tris(siloxide) tripodal ligands allow implementation of two-electron redox chemistry at the thorium center while retaining the ligand framework unchanged.|Two electrons can be stored in the arene anchor of robust tripodal siloxide frameworks by chemical reduction of their Th(IV) complexes. The two electrons become available at the metal center for the controlled two-electron reduction of a broad range of substrates (N2O, COT, CHT, Ph2N2, Ph3PS and O2) while the ligand framework is retained in its original form.image

Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation

D. K. Modder; R. Scopelliti; M. Mazzanti 

A trinuclear metallasilsesquioxane of uranium(III)

M. C. G. Tricoire; N. Jori; F. F. Tirani; R. Scopelliti; I. Zivkovic et al. 

The silsesquioxane ligand (Bu-i)(7)Si7O9(OH)(3) ((POSSH3)-P-iBu) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U-3((POSS)-P-iBu)(3)]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage

M. S. Batov; I. del Rosal; R. Scopelliti; F. Fadaei-Tirani; I. Zivkovic et al. 

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-Bu-3(t)-2,4,6)(3)}(2)(mu-eta(2):eta(2)-N-2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N-2 moiety ((mu-eta(2):eta(2)-N-2)(center dot 3-)). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N-2)(center dot 3-) can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N-2 molecules.

Dinitrogen cleavage by a dinuclear uranium(III) complex

N. Jori; M. R. Keener; T. Rajeshkumar; R. Scopelliti; L. Maron et al. 

Understanding the role of multimetallic cooperativity and of alkali ion-binding in the second coordination sphere is important for the design of complexes that can promote dinitrogen (N-2) cleavage and functionalization. Herein, we compare the reaction products and mechanism of N-2 reduction of the previously reported K-2-bound dinuclear uranium(III) complex, [K-2{[U-III(OSi((OBu)-Bu-t)(3))(3)](2)(mu-O)}], B, with those of the analogous dinuclear uranium(III) complexes, [K(2.2.2-cryptand)][K{U-III(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-O)], 1, and [K(2.2.2-cryptand)](2)[{U-III(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-O)], 2, where one or two K+ ions have been removed from the second coordination sphere by addition of 2.2.2-cryptand. In this study, we found that the complete removal of the K+ ions from the inner coordination sphere leads to an enhanced reducing ability, as confirmed by cyclic voltammetry studies, of the resulting complex 2, and yields two new species upon N-2 addition, namely the U(III)/U(IV) complex, [K(2.2.2-cryptand)][{U-III(OSi((OBu)-Bu-t)(3))(3)}(mu-O){U-IV(OSi((OBu)-Bu-t)(3))(3)}], 3, and the N-2 cleavage product, the bis-nitride, terminal-oxo complex, [K(2.2.2-cryptand)](2)[{U-V(OSi((OBu)-Bu-t)(3))(3)}(mu-N)(2){U-VI(OSi((OBu)-Bu-t)(3))(2)(kappa-O)}], 4. We propose that the formation of these two products involves a tetranuclear uranium-N-2 intermediate that can only form in the absence of coordinated alkali ions, resulting in a six-electron transfer and cleavage of N-2, demonstrating the possibility of a three-electron transfer from U(III) to N-2. These results give an insight into the relationship between alkali ion binding modes, multimetallic cooperativity and reactivity, and demonstrate how these parameters can be tuned to cleave and functionalize N-2.

Multielectron Redox Chemistry of Uranium by Accessing the plus II Oxidation State and Enabling Reduction to a U(I) Synthon

M. Keener; R. A. K. Shivaraam; T. Rajeshkumar; M. Tricoire; R. Scopelliti et al. 

Thesynthesis of molecular uranium complexes in oxidation stateslower than +3 remains a challenge despite the interest for their multielectrontransfer reactivity and electronic structures. Herein, we report theone- and two-electron reduction of a U(III) complex supported by anarene-tethered tris(siloxide) tripodal ligand leading to the mono-reducedcomplexes, [K(THF)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(THF)] (2) and [K(2.2.2-cryptand)][U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(THF)](2-crypt), and to the di-reduced U(I) synthons, [K-2(THF)(3)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)](& INFIN;) (3) and [(K(2.2.2-cryptand))](2)[U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)] (3-crypt). EPR and UV/vis/NIR spectroscopies, magnetic, cyclic voltammetry,and computational studies provide strong evidence that complex 2-crypt is best described as a U(II), where the U(II) is stabilizedby & delta;-bonding interactions between the arene anchor and the uraniumfrontier orbitals, whereas complexes 3 and 3-crypt are best described as having a U(III) ion supported by the di-reducedarene anchor. Three quasi-reversible redox waves at E (1/2) = -3.27, -2.45, and -1.71 V wereidentified by cyclic voltammetry studies and were assigned to theU(IV)/U(III), U(III)/U(II), and U(II)/U(III)-(arene)(2-) redox couples. The ability of complexes 2 and 3 in transferring two- and three-electrons, respectively,to oxidizing substrates was confirmed by the reaction of 2 with azobenzene (PhNNPh), leading to the U(IV) complex, [K(Et2O)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(PhNNPh)(THF)] (4), and of complex 3 with cycloheptatriene, yielding the U(IV) complex, [(K(Et2O)(2))U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(& eta;(7)-C7H7)](& INFIN;) (6). These results demonstratethat the arene-tethered tris(siloxide) tripodal ligand provides anexcellent platform for accessing low-valent uranium chemistry whileimplementing multielectron transfer pathways as shown by the reactivityof complex 3, which provides the third example of a U(I)synthon.

Isolation and redox reactivity of cerium complexes in four redox states

F-C. Hsueh; T. Rajeshkumar; L. Maron; R. Scopelliti; A. Sienkiewicz et al. 

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides with an arene ring in a tripodal ligand can stabilize cerium complexes in four different redox states and can promote multielectron redox reactivity in cerium complexes. Ce(iii) and Ce(iv) complexes [(LO3)Ce(THF)] (1) and [(LO3)CeCl] (2) (LO3 = 1,3,5-(2-OSi((OBu)-Bu-t)(2)C6H4)(3)C6H3) were synthesized and fully characterized. Remarkably the one-electron reduction and the unprecedented two-electron reduction of the tripodal Ce(iii) complex are easily achieved to yield reduced complexes [K(2.2.2-cryptand)][(LO3)Ce(THF)] (3) and [K-2{(LO3)Ce(Et2O)(3)}] (5) that are formally “Ce(ii)” and “Ce(i)” analogues. Structural analysis, UV and EPR spectroscopy and computational studies indicate that in 3 the cerium oxidation state is in between +II and +III with a partially reduced arene. In 5 the arene is doubly reduced, but the removal of potassium results in a redistribution of electrons on the metal. The electrons in both 3 and 5 are stored onto d-bonds allowing the reduced complexes to be described as masked “Ce(ii)” and “Ce(i)”. Preliminary reactivity studies show that these complexes act as masked Ce(ii) and Ce(i) in redox reactions with oxidizing substrates such as Ag+, CO2, I-2 and S-8 effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry.

A Route to Stabilize Uranium(II) and Uranium(I) Synthons in Multimetallic Complexes

R. A. Keerthi Shivaraam; M. Keener; D. K. Modder; T. Rajeshkumar; I. Živković et al. 

Herein, we report the redox reactivity of a multimetallic uranium complex supported by triphenylsiloxide (−OSiPh3) ligands, where we show that low valent synthons can be stabilized via an unprecedented mechanism involving intramolecular ligand migration. The two- and three-electron reduction of the oxo-bridged diuranium(IV) complex [{(Ph3SiO)3(DME)U}2(μ-O)], 4, yields the formal “UII/UIV”, 5, and “UI/UIV”, 6, complexes via ligand migration and formation of uranium-arene δ-bond interactions. Remarkably, complex 5 effects the two-electron reductive coupling of pyridine affording complex 7, which demonstrates that the electron-transfer is accompanied by ligand migration, restoring the original ligand arrangement found in 4. This work provides a new method for controlling the redox reactivity in molecular complexes of unstable, low-valent metal centers, and can lead to the further development of f-elements redox reactivity.

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as Th II Synthons

F-C. Hsueh; T. Rajeshkumar; B. Kooij; R. Scopelliti; K. Severin et al. 

Thorium redox chemistry is extremely scarce due to the high stability of ThIV. Here we report two unique examples of thorium arenide complexes prepared by reduction of a ThIV-siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(OtBu)3)3Th(η6-C10H8)] (1) and the inverse-sandwich complex [K(OSi(OtBu)3)3Th]2(μ-η6,η6-C10H8)] (2). The electrons stored in these complexes allow the reduction of a broad range of substrates (N2O, AdN3, CO2, HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN2 to yield the unexpected ThIV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.

Assembling diuranium complexes in different states of charge with a bridging redox-active ligand

D. K. Modder; M. S. Batov; T. Rajeshkumar; A. Sienkiewicz; I. Zivkovic et al. 

Radical-bridged diuranium complexes are desirable for their potential high exchange coupling and single molecule magnet (SMM) behavior, but remain rare. Here we report for the first time radical-bridged diuranium(IV) and diuranium(III) complexes. Reaction of [U{N(SiMe3)(2)}(3)] with 2,2′-bipyrimidine (bpym) resulted in the formation of the bpym-bridged diuranium(IV) complex [{((Me3Si)(2)N)(3)U-}2(IV)(mu-bpym(2-))], 1. Reduction with 1 equiv. KC8 reduces the complex, affording [K(2.2.2-cryptand)][{((Me3Si)(2)N)(3)U)(2)(mu-bpym)], 2, which is best described as a radical-bridged UIII-bpym center dot-U-III complex. Further reduction of 1 with 2 equiv. KC8, affords [K(2.2.2-cryptand)](2)[{((Me3Si)(2)N)(3)U-III}(2)(mu-bpym(2-))], 3. Addition of AgBPh4 to complex 1 resulted in the oxidation of the ligand, yielding the radical-bridged complex [{((Me3Si)(2)N)(3)U-IV}(2)(mu-bpym center dot-)][BPh4], 4. X-ray crystallography, electrochemistry, susceptibility data, EPR and DFT/CASSCF calculations are in line with their assignments. In complexes 2 and 4 the presence of the radical-bridge leads to slow magnetic relaxation.

The mechanism of Fe induced bond stability of uranyl(v)

T. Vitova; R. Faizova; J. Amaro-Estrada; L. Maron; T. Pruessmann et al. 

The stabilization of uranyl(v) (UO2¹⁺) by Fe(ii) in natural systems remains an open question in uranium chemistry. Stabilization of U^VO2¹⁺ by Fe(ii) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(ii) induced stability and the change of the bonding properties have not been elucidated up to date. We demonstrate that U(v) – oaxial bond covalency decreases upon binding to Fe(ii) inducing redirection of electron density from the U(v) – oaxial bond towards the U(v) – equatorial bonds thereby increasing bond covalency. Our results indicate that such increased covalent interaction of U(v) with the equatorial ligands resulting from iron binding lead to higher stability of uranyl(v). For the first time a combination of U M4,5 high energy resolution X-ray absorption near edge structure (HR-XANES) and valence band resonant inelastic X-ray scattering (VB-RIXS) and ab initio multireference CASSCF and DFT based computations were applied to establish the electronic structure of iron-bound uranyl(v).

Cation assisted binding and cleavage of dinitrogen by uranium complexes

N. Jori; T. Rajeshkumar; R. Scopelliti; I. Zivkovic; A. Sienkiewicz et al. 

The role of alkali promoters in N-2 cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N-2. Here we report a series of alkali bound-oxo-bridged diuranium(iii) complexes that provide a unique example of decreasing N-2 binding affinity with increasing cation size (from K to Cs). N-2 binding was found to be irreversible in the presence of K. A N-2 complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N-2 binding was found to be reversible under vacuum. In the presence of the Cs cation N-2 binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N-2 binding affinity is due to steric rather than electronic effects. We also find that weak N-2 binding in ambient conditions does not prevent alkali assisted N-2 cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N-2 cleavage leading to the isolation of a N-2 derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide.

Nitrogen activation and cleavage by a multimetallic uranium complex

M. Keener; F. Fadaei Tirani; R. Scopelliti; I. Zivkovic; M. Mazzanti 

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N-2 to nitrides (N3-). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N-2 cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N-2. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N-2. Two heteroleptic diuranium nitride complexes, [K{U-IV(OSi((OBu)-Bu-t)(3))(N(SiMe3)(2))(2)}(2)(mu-N)] (1) and [Cs{U-IV(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}(2)(mu-N)] (3-Cs), containing different combinations of OSi(CYBu)(3) and N(SiMe3)(2) ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(III)/U(IV) analogues, and the complex, [K-2{U-IV/III(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}(2)(mu-N)] (6-K), could be further reduced to a putative U(III)/U(III) species that is capable of promoting the 4e(-) reduction of N-2, yielding the N-2(4-) complex [K-3{U-V(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2)))(2)(mu-N)(mu-eta(2):eta(2)-N-2)], 7. Parallel N-2 reduction pathways were also identified, leading to the isolation of N-2 cleavage products, [K-3{U-VI(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))( N)}(mu-N)(2){U-V(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}](2), 8, and [K-4{(OSi((OBu)-Bu-t)(3))(2)U-V)( N))(mu-NH)(mu-kappa(2):C,N-CH2SiMe2NSiMe3){U-V(OSi((OBu)-Bu-t)(3))(2)][K(N(SiMe3)(2)](2), 9. These complexes provide the first example of N-2 cleavage to nitride by a uranium complex in the absence of reducing alkali metals.

Design Principles for the Development of Gd(III) Polarizing Agents for Magic Angle Spinning Dynamic Nuclear Polarization

Y. Rao; C. T. Palumbo; A. Venkatesh; M. Keener; G. Stevanato et al. 

Nuclear magnetic resonance suffers from an intrinsically low sensitivity, which can be overcome by dynamic nuclear polarization (DNP). Gd(III) complexes are attractive exogenous polarizing agents for magic angle spinning (MAS) DNP due to their high chemical stability in contrast to nitroxide-based radicals. However, even the state-of-the-art Gd(III) complexes have so far provided relatively low DNP signal enhancements of ca. 36 in comparison to standard DNP biradicals, which show enhancements of over 200. Here, we report a series of new Gd(III) complexes for DNP and show that the observed DNP enhancements of the new and existing Gd(III) complexes are inversely proportional to the square of the zero-field splitting (ZFS) parameter D, which is in turn determined by the ligand-type and the local coordination environment. The experimental DNP enhancements at 9.4 T and the ZFS parameters measured with pulsed electron paramagnetic resonance (EPR) spectroscopy agree with the above model, paving the way for the development of more efficient Gd(III) polarizing agents.

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes br

A. R. Willauer; F. Fadaei-Tirani; I. Zivkovic; A. Sienkiewicz; M. Mazzanti 

Trinuclear molecular complexes of europium (II)and ytterbium(II) [Ln3{(Ph2SiO)2O}3(THF)6],1-Ln3L3(Ln = Euand Yb), supported by the dianionic tetraphenyl disiloxanediolateligand, were synthesized via protonolysis of the [Ln{N-(SiMe3)2}2(THF)2] complexes. In contrast, the reaction of[Sm{N(SiMe3)2}2(THF)2] with the (Ph2SiOH)2O ligand led tothe isolation of the mixed-valent Sm(II)/Sm(III) complex[Sm3{(Ph2SiO)2O}3{N(SiMe3)2}(THF)4],2-Sm3L3, which wascrystallographically characterized. The Eu(II) complex1-Eu3L3displays weak ferromagnetic coupling between the Eu(II) metalcenters (J= 0.1035 cm-1). The addition of 3 equiv of (Ph2SiOK)2Oto1-Eu3L3resulted in the formation of the polynuclear Eu(II)dimer of dimers [K4Eu2{(Ph2SiO)2O}4(Et2O)2]2,3-Eu2L4. Com-plexes1-Ln3L3(Ln = Eu and Yb) are stable in solution at room temperature, while3-Eu2L4shows higher reactivity and rapidlydecomposes to give the mixed-valent Eu(II)/Eu(III) species [K3Eu2{(Ph2SiO)2O}4],4-Eu2L4. Complex1-Yb3L3affects the slowreductive disproportionation of carbon dioxide, but1-Eu3L3does not display any reactivity toward CO2. However, the presence ofone additional (Ph2SiO-)2O per Eu(II) metal center in3-Eu2L4increases dramatically the reductive ability of the Eu(II) metalcenters, affording thefirst example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO2by3-Eu2L4is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO2

Multi-electron Transfer by U(II) and Masked U(II) Complexes

D. K. Modder; M. Mazzanti 

Complexes of uranium in low oxidation state have shown the ability to activate non-reactive small molecules such as N-2. However, the multi-electron transfer required for such activation remains limited in uranium chemistry. Here, we review our recent research on the use of different strategies to overcome this issue, which has led to the isolation of a diuranium(III) bridging oxide complex that reacts as a U(II) synthon able to effect one-electron transfer per uranium center to N-heterocycles and multi-electron transfer to diphenylacetylene and azobenzene. We also showed that a closely related molecular U(II) complex effects the same reactions providing the first unambiguous example of a monouranium four-electron transfer.

Heterometallic uranium/molybdenum nitride synthesis via partial N-atom transfer

L. Barluzzi; N. Jori; L. Maron; R. Scopelliti; P. Oyala et al. 

The reaction of a terminal Mo(II) nitride with a U(III) complex yields an heterodimetallic U-Mo nitride which is the first example of a transition metal-capped uranium nitride. The nitride is triply bonded to U(V) and singly bonded to Mo(0) and supports a U-Mo interaction. This compound shows reactivity toward CO oxidation.

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

F-C. Hsueh; L. Barluzzi; M. Keener; T. Rajeshkumar; L. Maron et al. 

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M3Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K–nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN–.

Gd3+-Functionalized Lithium Niobate Nanoparticles for Dual Multiphoton and Magnetic Resonance Bioimaging

R. De Matos; A. Gheata; G. Campargue; J. Vuilleumier; L. Nicolle et al. 

Harmonic nanoparticles (HNPs) have emerged as appealing exogenous probes for optical bioimaging due to their distinctive features such as long-term photostability and spectral flexibility, allowing multiphoton excitation in the classical (NIR-I) and extended near-infrared spectral windows (NIR-II and -III). However, like all other optical labels, HNPs are not suitable for whole-body imaging applications. In this work, we developed a bimodal nonlinear optical/magnetic resonance imaging (MRI) contrast agent through the covalent conjugation of Gd(III) chelates to coated lithium niobate HNPs. We show that the resulting nanoconjugates exert strong contrast both in T1-weighted MRI of agarose gel-based phantoms and in cancer cells by harmonic generation upon excitation in the NIR region. Their capabilities for dual T1/T2 MRI were also emphasized by the quantitative mapping of the phantom in both modes. The functionalization protocol ensured high stability of the Gd-functionalized HNPs in a physiological environment and provided a high r1 relaxivity value per NP (5.20 × 105 mM–1 s–1) while preserving their efficient nonlinear optical response.

Structure, reactivity and luminescence studies of triphenylsiloxide complexes of tetravalent lanthanides

A. R. Willauer; I. Douair; A-S. Chauvin; F. Fadaei-Tirani; J-C. G. Bunzli et al. 

Among the 14 lanthanide elements (Ce-Lu), until recently, the tetravalent oxidation state was readily accessible in solution only for cerium while Pr(iv), Nd(iv), Dy(iv) and Tb(iv) had only been detected in the solid state. The triphenylsiloxide ligand recently allowed the isolation of molecular complexes of Tb(iv) and Pr(iv) providing an unique opportunity of investigating the luminescent properties of Ln(iv) ions. Here we have expanded the coordination studies of the triphenylsiloxide ligand with Ln(iii) and Ln(iv) ions and we report the first observed luminescence emission spectra of Pr(iv) complexes which are assigned to a ligand-based emission on the basis of the measured lifetime and computational studies. Binding of the ligand to the Pr(iv) ion leads to an unprecedented large shift of the ligand triplet state which is relevant for future applications in materials science.

Nitride protonation and NH3 binding versus N-H bond cleavage in uranium nitrides

M. Keener; R. Scopelliti; M. Mazzanti 

The conversion of metal nitrides to NH3 is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H+). Herein, we report comparative studies for the reactions of H+ and NH3 with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by -N(SiMe3)(2) ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic -N(SiMe3)(2) ligands promote the N-H heterolytic bond cleavage of NH3, yielding a “naked” diuranium complex containing three bridging ligands, a nitride (N3-) and two NH2 ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by -OSi((OBu)-Bu-t)(3) ligands, the nitride group is easily protonated to afford NH3, which binds the U(iv) ion strongly, resulting in a mononuclear U-NH3 complex, where NH3 can be displaced by addition of strong acids. Furthermore, the U-OSi((OBu)-Bu-t)(3) bonds were found to be stable, even in the presence of stronger acids, such as NH4BPh4, therefore indicating that -OSi((OBu)-Bu-t)(3) supporting ligands are well suited to be used when acidic conditions are required, such as in the H+/e(-) mediated catalytic conversion of N-2 to NH3.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

N. Jori; L. Barluzzi; I. Douair; L. Maron; F. Fadaei-Tirani et al. 

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K-2{[U-V(OSi((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-eta 2(:)eta(2)-N-2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K-6{(OSi((OBu)-Bu-t)(3))(2)UIV}(3){(OSi((OBu)-Bu-t)(3))(2)UVI}(mu(4)-N)(3)(mu(3)-N)(mu(3)-O)(2)], 1, a novel example of N2 cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N=U-VI=N core analogue of UO22+. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N-2(4-) ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium(V) bis-nitride [K-4{[UV(OSi-((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-N)(2)}], X. Additionally, cooperative potassium binding to the U-bound N24- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess (CO)-C-13 to yield (KCN)-C-13 and (KNCO)-C-13. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Synthesis, structure, and reactivity of uranium(VI) nitrides

L. Barluzzi; F-C. Hsueh; R. Scopelliti; B. E. Atkinson; N. Kaltsoyannis et al. 

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-N)}(2)], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi((OBu)-Bu-t)(3))(3)(mu-N)}(2)], 2 and [K-2{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)(2)(mu-I)], 3 while oxidation with a stronger oxidant (“magic blue”) yields the U(vi)/U(vi) complex [{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)(2)(mu-thf)], 4. The three complexes show very different stability and reactivity, with N-2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-NH)}(2)], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-NH2)(2)(mu-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H-2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N sigma orbital well localised to U=N and pi orbitals which partially delocalise to form the U-N single bond with the other uranium.

Single metal four-electron reduction by U(II) and masked “U(II)” compounds

D. K. Modder; C. T. Palumbo; I. Douair; R. Scopelliti; L. Maron et al. 

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)](2)[{((Me3Si)(2)N)(3)U}(2)(mu-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(eta(2)-C2Ph2){N(SiMe3)(2)}(3)], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)(2){N(SiMe3)(2)}(3)], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe3)(2)}(3)], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N2Ph2){N(SiMe3)(2)}(3)], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a “clear-cut” single metal four-electron transfer in f-element chemistry.

Biological reduction of a U(V)-organic ligand complex

M. Molinas; R. Faizova; A. Brown; J. Galanzew; B. Schacherl et al. 

Metal-reducing microorganisms such as Shewanella oneidensis MR-1 reduce highly soluble species of hexavalent uranyl (U(VI)) to less mobile tetravalent uranium (U(IV)) compounds. The biologically mediated immobilization of U(VI) is being considered for the remediation of U contamination. However, the mechanistic underpinnings of biological U(VI) reduction remain unresolved. It has become clear that a first electron transfer occurs to form pentavalent (U(V)) intermediates, but it has not been definitively established whether a second one-electron transfer can occur or if disproportionation of U(V) is required. Here, we utilize the unusual properties of dpaea2– ((dpaeaH2═bis(pyridyl-6-methyl-2-carboxylate)-ethylamine)), a ligand forming a stable soluble aqueous complex with U(V), and investigate the reduction of U(VI)–dpaea and U(V)–dpaea by S. oneidensis MR-1. We establish U speciation through time by separating U(VI) from U(IV) by ion exchange chromatography and characterize the reaction end-products using U M4-edge high resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. We document the reduction of solid phase U(VI)–dpaea to aqueous U(V)–dpaea but, most importantly, demonstrate that of U(V)–dpaea to U(IV). This work establishes the potential for biological reduction of U(V) bound to a stabilizing ligand. Thus, further work is warranted to investigate the possible persistence of U(V)–organic complexes followed by their bioreduction in environmental systems.

Synthesis and Characterization of Water Stable Uranyl(V) Complexes

R. Faizova; F. Fadaei‐Tirani; A. Chauvin; M. Mazzanti 

The importance of uranyl(V) (UO2+) species associated with environmental and geologic applications is becoming increasingly evident, but the tendency of the uranyl(V) cation to disproportionate in water has prevented the isolation of stable complexes. Here we demonstrate that in the presence of the tridentate complexing dipicolinate (dpa2-), a ligand highly abundant in soil, the uranyl(V) species can be stabilized and isolated in anoxic basic water. Stable uranyl(V) dipicolinate complexes are readily formed from the reduction of the uranyl(VI) analogue both in organic solution and in basic water, and their solution and solid-state structure were determined. A bisdpa UVO2+ complex was obtained from water at pH10, while at higher pH values, a trinuclear mono-dpa cation-cation complex was isolated. These results present the second ever isolated water stable uranyl(V) complex. Moreover, we demonstrate that dipicolinate complexes of UVIO2 2+, UVO2 + and U(IV) are strongly luminescent with a signature characteristic of each oxidation state. This provides unique examples of luminescent U(V) and U(IV) compounds.

Delivery of a Masked Uranium(II) by an Oxide-Bridged Diuranium(III) Complex

D. K. Modder; C. T. Palumbo; I. Douair; F. Fadaei-Tirani; L. Maron et al. 

Oxide is an attractive linker for building polymetallic complexes that provide molecular models for metal oxide activity, but studies of these systems are limited to metals in high oxidation states. Herein, we synthesized and characterized the molecular and electronic structure of diuranium bridged U-III/U-IV and U-III/U-III complexes. Reactivity studies of these complexes revealed that the U-O bond is easily broken upon addition of N-heterocycles resulting in the delivery of a formal equivalent of U-III and U-II, respectively, along with the uranium(IV) terminal-oxo coproduct. In particular, the U-III/U-III oxide complex effects the reductive coupling of pyridine and two-electron reduction of 4,4 ‘-bipyridine affording unique examples of diuranium(III) complexes bridged by N-heterocyclic redox-active ligands. These results provide insight into the chemistry of low oxidation state metal oxides and demonstrate the use of oxo-bridged U-III/U-III complexes as a strategy to explore U-II reactivity.

Photochemical Synthesis of a Stable Terminal Uranium(VI) Nitride

L. Barluzzi; R. Scopelliti; M. Mazzanti 

Terminal uranium nitrides have so far proven impossible to isolate by photolysis of azides. Here we report the second ever example of an isolated terminal uranium(VI) nitride. We show that the terminal nitride [NBu4] [U(OSi((OBu)-Bu-t)(3))(4)(N)], 3, can be prepared upon photolysis with UV light of the U(IV) azide analogue. This is achieved by careful tailoring of the azide precursor and of the reaction conditions. Complex 3 is stable under ambient conditions but reacts readily with electrophiles (H+ and CO).

Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

N. Jori; D. Toniolo; B. C. Huynh; R. Scopelliti; M. Mazzanti 

Here we have explored the ability of Schiff bases to act as electron reservoirs and to enable the multi-electron reduction of small molecules by lanthanide complexes. We report the reductive chemistry of the Ln(iii) complexes of the tripodal heptadentate Schiff base H(3)trensal (2,2 ‘,2 ”-tris(salicylideneimino)triethylamine), [Ln(III)(trensal)],1-Ln(Ln = Sm, Nd, Eu). We show that the reduction of the [Eu-III(trensal)] complex leads to the first example of a Eu(ii) Schiff base complex [{K(mu-THF)(THF)(2)}(2){Eu-II(trensal)}(2)],3-Eu. In contrast the one- and two-electron reduction of the [Nd-III(trensal)] and [Sm-III(trensal)] leads to the intermolecular reductive coupling of the imino groups of the trensal ligand and to the formation of one and two C-C bonds leaving the metal center in the +3 oxidation state. The resulting one- and two electron reduced complexes [{K(THF)(3)}(2)Ln(2)(bis-trensal)],2-Ln, and [{K(THF)(3)}(2){K(THF)}(2)Ln(2)(cyclo-trensal)],4-Ln(Ln = Sm, Nd) are able to effect the reductive disproportionation of carbon dioxide by transferring the electrons stored in the C-C bonds to CO(2)to selectively afford carbonate and CO. The selectivity of the reaction contrasts with the formation of multiple CO(2)reduction products previously reported for a U(iv)-trensal system.

Structure and small molecule activation reactivity of a metallasilsesquioxane of divalent ytterbium

A. R. Willauer; A. M. Dabrowska; R. Scopelliti; M. Mazzanti 

The first metallasilsesquioxane of a divalent lanthanide, [Yb{Cy7Si7O11(OSiMe3)}(THF)](2),1, was synthesized and structurally characterized. The Cy7Si7O11(OSiMe3)(2-)ligands in1bind two Yb(ii) ions in a bridging mode. The dinuclear complex effects the two-electron reduction of azobenzene yielding the Yb(iii) complex [{Yb(Cy7Si7O11(OSiMe3))(THF)(2)}(2)(PhNNPh)],2, and the CO(2)reduction to CO and carbonate.

Carbon Dioxide Reduction by Multimetallic Uranium(IV) Complexes Supported by Redox-Active Schiff Base Ligands

N. Jori; M. Falcone; R. Scopelliti; M. Mazzanti 

The synthesis, structure, and reactivity with CO2 and CS2 of new U(IV) complexes with a redox-active Schiff base are reported. The reaction of UI3 with the heptadentate Schiff base ligand 2,2′,2 ”-tris(salicylideneimino)-triethylamine (trensal) did not lead to the formation of a U(III) complex but to the reductive coupling and C-C bond formation between two imino groups of the Schiff base, yielding the U(IV) complex [U-2(bis-trensal)], 1. Further reduction of 1 led to the dinuclear macrocyclic complex [{K(THF)(3)}(2)U-2(cyclotrensal)], 3-THF, through a second C-C bond formation reaction between two additional imino groups. Complexes 1 and 3 are oxidized by AgOTf resulting in the cleavage of the C-C bonds and leading to the formation of the U(IV) complex [U(trensal)]OTf, 2. Complex 1 does not reduce CO2 or CS2 but undergoes insertion of CO2 into one of the U-N bonds. In contrast, the reaction of 3 with 2 equiv of CO2 leads to the reductive disproportionation of CO2 to afford carbonate in 80% yield. In the presence of a large excess of CO2 multiple reactions take place, as supported by the isolation of the crystals of [{K(THF)(3)}U-2(mu-O)(CO2-CO-cyclo-trensal)(U(trensal))], 4. The higher reductive activity toward CO2 of complex 3 compared to previously reported U(IV) complexes of reduced Schiff bases is interpreted in terms of its redox properties.

Assembly of High-Spin [Fe-3] Clusters by Ligand-Based Multielectron Reduction

D. Toniolo; R. Scopelliti; I. Zivkovic; M. Mazzanti 

The hexanuclear [Na12Fe6(tris-cyclo-salophen)(2)(THF)(14)], 1-THF, and the trinuclear [Na6Fe3(tris-cyclo-salophen)(py)(9)], 1-py, Fe(II) clusters can be easily assembled in one step from the ligand-based reduction of the [FeII(salophen)(THF)] complex. These complexes consist of triangular cores where three Fe(II) ions are held together, within range of bonding interaction, by the hexa-amide, hexaphenolate macrocyclic ligand tris-cyclo-salophen(12-). The tris-cyclo-salophen(12-) ligand is perfectly suited for binding three Fe(II) centers at short distances, allowing for strong magnetic coupling between the Fe(II) centers. The macrocyclic ligand is generated by the reductive coupling of the imino groups of three salophen ligands, resulting in three new C-C bonds. The six electrons stored in the ligand become available for the reduction of carbon dioxide with selective formation of carbonate.

Accessing the plus IV Oxidation State in Molecular Complexes of Praseodymium

A. R. Willauer; C. T. Palumbo; F. Fadaei-Tirani; I. Zivkovic; I. Douair et al. 

Out of the 14 lanthanide (Ln) ions, molecular complexes of Ln(IV) were known only for cerium and more recently terbium. Here we demonstrate that the +IV oxidation state is also accessible for the large praseodymium (Pr) cation. The oxidation of the tetrakis(triphenysiloxide) Pr(III) ate complex, [KPr(OSiPh3)(4)(THF)(3)], 1-Pr-Ph, with [N(C6H4Br)(3)][SbCl6], affords the Pr(IV) complex [Pr(OSiPh3)(4)(MeCN)(2)], 2-Pr-Ph, which is stable once isolated. The solid state structure, UV-visible spectroscopy, magnetometry, and cyclic voltammetry data along with the DFT computations of the 2-Pr-Ph complex unambiguously confirm the presence of Pr(IV).

Ligand-Supported Facile Conversion of Uranyl(VI) into Uranium(IV) in Organic and Aqueous Media

R. Faizova; F. Fadaei-Tirani; R. Bernier-Latmani; M. Mazzanti 

Reduction of uranyl(VI) to U-V and to U-IV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl U-VI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [(UO2)-O-VI(dpaea)] complex is readily converted into the cis-boroxide U-IV species via diborane-mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2S2O4, a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water-insoluble trinuclear U-IV oxo-hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well-defined molecular U-IV species in aqueous conditions.

C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride

C. T. Palumbo; R. Scopelliti; I. Zivkovic; M. Mazzanti 

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe3)(2) ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H-2. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)(3)][((Me3Si)(2)N)(2)U(mu-N)(mu-kappa(2):CN-CH2SiMe2NSiMe3)U(N(SiMe3)(2))(2)] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt3BPh3 yielded the cationic U(IV)/U(IV) nitride complex, [{((Me3Si)(2)N)(2)U(THF)}(2)(mu-N)][BPh4] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe3)(2) ligand and the uranium-methylene bond. Reduction of 3 with KC8 afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me3Si2N)(2)U(THF)}(2)(mu-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe3)(2) ligand across the uranium-nitride moiety to give the U(III)/U(IV) inside cyclometalate complex, [{((Me3Si)(2)N)(2)(THF)U(mu-NH)(mu-kappa(2):C,N – CH2SiMe2NSiMe3)U(N(SiMe3)(2)))(THF)], 5. Complex 4 also reacts with toluene at -80 degrees C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me3SO2N)(2)U(THF)}(2)(mu-N)][{((Me3SO2N)(3)U(mu-NH)U(N(SiMe3)(2))](2)(C7H8)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me3Si)(2)N)(2)U(THF)[(2)(mu-N)][((Me3Si)(2)N)(2)U(eta(1)-CCPh)(mu(2)-NH)(mu(2)-eta(2): eta(1)-CCPh)U(N(SiMe3)(2))(2)], 7. Complex 4 also reacts with H-2 to produce an imide hydride U(III)/U(IV) complex, [{((Me3SO2N)(2)U(THF)}(2)(mu-NH)(mu-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

Anhydrous Conditions Enable the Catalyst-Free Carboxylation of Aromatic Alkynes with CO2 under Mild Conditions

D. Toniolo; F. D. Bobbink; P. J. Dyson; M. Mazzanti 

The direct carboxylation of aromatic alkynes with CO2, a cheap and widely available C1 source, is the most attractive method for the synthesis of carboxylic acids. Here we show that direct carboxylation of terminal alkynes can be simply performed in near-quantitative yield in four hours with anhydrous Cs2CO3 under mild conditions without need of a metal catalyst.

Stabilization of the Oxidation State plus IV in Siloxide-Supported Terbium Compounds

A. R. Willauer; C. T. Palumbo; R. Scopelliti; I. Zivkovic; I. Douair et al. 

The synthesis of lanthanides other than cerium in the oxidation state +IV has remained a desirable but unmet target until recently, when two examples of Tb-IV with saturated coordination spheres were isolated. Here we report the third example of an isolated molecular complex of terbium(IV), where the supporting siloxide ligands do not saturate the coordination sphere. The fully characterized six-coordinate complex [Tb-IV(OSiPh3)(4)(MeCN)(2)], 2-Tb-Ph, shows high stability and the labile MeCN ligands can be replaced by phosphinoxide ligands. Computational studies suggest that the stability is due to a strong pi(O-Tb) interaction which is stronger than in the previously reported Tb-IV complexes. Cyclic-voltammetry experiments demonstrate that non-binding counterions contribute to the stability of Tb-IV in solution by destabilizing the +III oxidation state, while alkali ions promote Tb-IV/Tb-III electron transfer.

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

C. T. Palumbo; L. Barluzzi; R. Scopelliti; I. Zivkovic; A. Fabrizio et al. 

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U & xe001;N & xe001;U bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{(((BuO)-Bu-t)(3)SiO)(3)U}(2)(mu-N)], [NBu4]-1, and [NBu4][((Me3Si)(2)N)(3)U}(2)(mu-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, [U(OSi((OBu)-Bu-t)(3))(2)(mu-OSi((OBu)-Bu-t)(3))](2) and [U(N(SiMe3)(2))(3)], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)(2)N)(3)U}(2)(mu-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)(3)][((Me3Si)(2))(2)U(mu-N)(mu-kappa(2)-C,N-CH2SiMe2NSiMe3)U(N(SiMe3)(2))(2)] (dme = 1,2-dimethoxyethane), [Na(dme)(3)]-3, provided a versatile precursor for the synthesis of the mixed-ligand U4+-U4+ nitride complex, [Na(dme)(3)][((Me3Si)(2)N)(3)U(mu-N)U(N(SiMe3)(2))(OSi((OBu)-Bu-t)(3))], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H-2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(((BuO)-Bu-t)(3)SiO)(3)U}(2)(mu-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO2 but not H-2. The nitride complexes [NBu4]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu4]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the chi versus T plot of 90 K and 55 K). Computational analysis indicates that the U(mu-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

Theoretical Investigation of the Electronic Structure and Magnetic Properties of Oxo-Bridged Uranyl(V) Dinuclear and Trinuclear Complexes

B. Teyar; S. Boucenina; L. Belkhiri; B. Le Guennic; A. Boucekkine et al. 

The uranyl(V) complexes [UO2(dbm)(2)K(18C6)](2) (dbm = dibenzoylmethanate) and [UO2(L)](3)(L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate), exhibiting diamond-shaped U2O2 and triangular-shaped U3O3 cores respectively with 5f(1)-5f(1 )and 5f(1)-5f(1)-5f(1) configurations, have been investigated using relativistic density functional theory (DFT). The bond order and QTAIM analyses reveal that the covalent contribution to the bonding within the oxo cores is slightly more important for U3O3 than for U2O2, in line with the shorter U-O distances existing in the trinuclear complex in comparison to those in the binuclear complex. Using the broken symmetry (BS) approach combined with the B3LYP functional for the calculation of the magnetic exchange coupling constants (J) between the magnetic centers, the antiferromagnetic (AF) character of these complexes was confirmed, the estimated J values being respectively equal to -24.1 and -7.2 cm(-1) for the dioxo and trioxo species. It was found that the magnetic exchange is more sensitive to small variations of the core geometry of the dioxo species in comparison to the trioxo species. Although the robust AF exchange coupling within the UxOx cores is generally maintained when small variations of the UOU angle are applied, a weak ferromagnetic character appears in the dioxo species when this angle is higher than 114 degrees, its value for the actual structure being equal to 105.9 degrees. The electronic factors driving the magnetic coupling are discussed.

Molecular Complex of Tb in the +4 Oxidation State

C. T. Palumbo; I. Zivkovic; R. Scopelliti; M. Mazzanti 

Lanthanides (Ln) usually occur in the +3, or more recently the +2, oxidation states. The only example of an isolated molecular Ln(4+) so far remains Ce4+. Here we show that the +4 oxidation state is also accessible in a molecular compound of terbium as demonstrated by oxidation of the tetrakis(siloxide)terbium(III) ate complex, [KTb(OSi((OBu)-Bu-t)(3))(4)], 1-Tb, with the tris(4-bromophenyl) amminium oxidant, [N(C6H4Br)(3)] [SbCl6], to afford the Tb4+ complex [Tb(OSi((OBu)-Bu-t)(3))(4)], 2-Tb. The solid state structures of 1-Tb and 2-Tb were determined by X-ray crystallography, and the presence of Tb4+ was unambiguously confirmed by electron paramagnetic resonance and magnetometry. Complex 2-Tb displays a similar voltammogram to the Ce4+ analogue but with redox events that are about 1 V more positive.

CO2 and CO/H-2 Conversion to Methoxide by a Uranium(IV) Hydride

M. Falcone; R. Scopelliti; M. Mazzanti 

Here we show that a scaffold combining siloxide ligands and a bridging oxide allows the synthesis and characterization of the stable dinuclear uranium(IV) hydride complex [K-2{[U(OSi((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-H)(2)}], 2, which displays high reductive reactivity. The dinuclear bis-hydride 2 effects the reductive coupling of acetonitrile by hydride transfer to yield [K-2{[U(OSi((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-kappa(2)-NC(CH3)NCH2CH3)}], 3. Under ambient conditions, the reaction of 2 with CO affords the oxomethylene(2-) reduction product [K-2{[U(OSi((OBu)-Bu-t)(3))(3)](2)(mu-CH2O)(mu-O)}], 4, that can further add H 2 to afford the methoxide hydride complex [K-2{[U(OSi((OBu)-Bu-t)(3))(3)](2)(mu-OCH3)(mu-O)(mu-H)}], 5, from which methanol is released in water. Complex 2 also effects the direct reduction of CO2 to the methoxide complex 5, which is unprecedented in f element chemistry. From the reaction of 2 with excess CO2, crystals of the bis-formate carbonate complex [K-2{[U(OSi((OBu)-Bu-t)(3))(3)](2)(mu-CO3)(mu-HCOO)(2)}], 6, could also be isolated. All the reaction products were characterized by X-ray crystallography and NMR spectroscopy.

A Factor Two Improvement in High-Field Dynamic Nuclear Polarization from Gd(III) Complexes by Design

G. Stevanato; D. J. Kubicki; G. Menzildjian; A-S. Chauvin; K. Keller et al. 

Gadolinium(III) complexes have recently been demonstrated to have potential as polarizing agents for high-field dynamic nuclear polarization (DNP) NMR spectroscopy. By tailoring the ligand design to reduce the zero-field splitting (ZFS), we demonstrate a quadratic improvement in DNP through the investigation of a stable, water-soluble, narrow-line Gd(III) complex, [Gd-(tpatcn)], doubling the magic-angle-spinning DNP enhancement of the previous state-of-the-art [Gd(dota)-(H2O)](-) at 9.4 T and 100 K.

CS2 Reductive Coupling to Acetylenedithiolate by a Dinuclear Ytterbium(II) Complex

M. Mazzanti; D. Toniolo; A. R. Willauer; R. Scopelliti; J. Andrez et al. 

The activation of CS2 is of interest in a broad range of fields and more particularly in the context of creating new C‐C bonds. The reaction of the dinuclear ytterbium(II) complex [Yb2L4], 1, (L = (OtBu)3SiO‐) with carbon disulphide led to the isolation of unprecedented reduction products. In particular the crystallographic characterization of complex [Yb2L4(µ‐C2S2)], 2 provides the first example of an acetylenedithiolate ligand formed from metal reduction of CS2. Computational studies indicate that this unprecedented reactivity can be ascribed to the unusual binding mode of CS22‐ in the isolated “key intermediate” [Yb2L4(µ‐CS2)], 3 which is resulting from the dinuclear nature of 1.

Correction: Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex (vol 10, pg 3543, 2019)

L. Barluzzi; L. Chatelain; F. Fadaei-Tirani; I. Zivkovic; M. Mazzanti 

Correction for Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex’ by Luciano Barluzzi et al., Chem. Sci., 2019, DOI: ; 10.1039/c8sc05721d.

A complete series of uranium(iv) complexes with terminal hydrochalcogenido (EH) and chalcogenido (E) ligands E = O, S, Se, Te

M. W. Rosenzweig; J. Hümmer; A. Scheurer; C. A. Lamsfus; F. W. Heinemann et al. 

We here report the synthesis and characterization of a complete series of terminal hydrochalcogenido, U–EH, and chalcogenido uranium(IV) complexes, U≡E (with E = O, S, Se, Te), supported by the (Ad,MeArOH)3tacn (1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) ligand system. Reaction of H2E with the trivalent precursor [((Ad,MeArO)3tacn)U] (1) yields the corresponding uranium(IV) hydrochalcogenido complexes [((Ad,MeArO)3tacn)U(EH)] (2). Subsequent deprotonation of the terminal hydrochalcogenido species with KN(SiMe3)2, in the presence of 2.2.2-cryptand, gives access to the uranium(IV) complexes with terminal chalcogenido ligands [K(2.2.2-crypt)][((Ad,MeArO)3tacn)U≡E] (3). In order to study the influence of the varying terminal chalogenido ligands on the overall molecular and electronic structure, all complexes were studied by single-crystal X-ray diffractometry, UV/vis/NIR, electronic absorption, and IR vibrational spectroscopy as well as SQUID magnetometry and computational analyses (DFT, MO, NBO).

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

A. R. Willauer; D. Toniolo; F. Fadaei-Tirani; Y. Yang; M. Laurent et al. 

Two dinuclear homoleptic complexes of lanthanides(II) supported by the polydentate tris(tertbutoxy) siloxide ligand ([Yb2L4], 1-Yb and [Sm2L4], 1-Sm, (L = (OtBu)3SiO−)) were synthesized in 70–80% yield and 1-Sm was crystallographically characterized. 1-Yb and 1-Sm are stable in solution at −40 °C but cleave the DME C–O bond over time at room temperature affording the crystal of [Yb2L4(μ-OMe)2(DME)2], 2. The 1-Yb and 1-Sm complexes effect the reduction of CO2 under ambient conditions leading to carbonate and oxalate formation. The selectivity of the reduction towards oxalate or carbonate changes depend on the solvent polarity and on the nature of the ion. For both the lanthanides, carbonate formation is favoured but oxalate formation increases if a non-polar solvent is used. Computational studies suggest that the formation of oxalate is favoured with respect to carbonate formation in the reaction of the dimeric lanthanide complexes with CO2. Crystals of the tetranuclear mixed-valence oxalate intermediate [Yb4L8(C2O4)], 3 were isolated from hexane and the presence of a C2O42− ligand bridging two [YbIIL2YbIIIL2] dinuclear moieties was shown. Crystals of the tetranuclear di-carbonate product [Sm4L8(μ3-CO3-κ4-O,O′,O′′)2], 4 were isolated from hexane. The structures of 3 and 4 suggest that the CO2 activation in non-polar solvents involves the interaction of two dimers with CO2 molecules at least to some extent. Such a cooperative interaction results in both oxalate and carbonate formation.

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

L. Barluzzi; L. Chatelain; F. Fadaei-Tirani; I. Zivkovic; M. Mazzanti 

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(V) bis-nitride complex [K2{[U(OSi(OtBu)3)3]2(μ-N)2}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-N3)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS2 and 1 equiv. of CO2 resulting in N–C bond formation to afford the diuranium(V) complexes [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-S)(μ-NCS)}], 5 and [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H2 in mild conditions to afford the bis-imido bridged diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-NH)2}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber–Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(V) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

Structural Snapshots of Cluster Growth from {U6 } to {U38 } During the Hydrolysis of UCl4

L. Chatelain; R. Faizova; F. Fadaei-Tirani; J. Pécaut; M. Mazzanti 

Herein we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl4 in organic solution and in the presence of the benzoate ligands. {U6}, {U13}, {U16}, {U24}, {U38} oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U6} building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U6} (few hours), {U16} (3 days), {U24} (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes

M. Falcone; L. Barluzzi; J. Andrez; F. Fadaei Tirani; I. Zivkovic et al. 

Synthesis and Characterization of a Water Stable Uranyl(V) Complex

R. Faizova; R. Scopelliti; A-S. Chauvin; M. Mazzanti 

We have identified a polydentate aminocarboxylate ligand that stabilizes uranyl(V) in water. The mononuclear [UO2(dpaea)]X, (dpaeaH2 = Bis(pyridyl-6-methyl-2-carboxylate)-ethylamine; X = CoCp2*+ or X = K(2.2.2.cryptand) complexes have been isolated from anaerobic organic solution, crystallographically and spectroscopically characterized both in water and organic solution. These complexes disproportionate at pH ≤ 6, but are stable in anaerobic water at pH 7–10 for several days.

The effect of iron binding on uranyl(V) stability

R. Faizova; R. Scopelliti; M. Mazzanti; S. White 

Here we report the effect of UO2+⋯Fe2+ cation–cation interactions on the redox properties of uranyl(V) complexes and on their stability with respect to proton induced disproportionation. The tripodal heptadentate Schiff base trensal3− ligand allowed the synthesis and characterization of the uranyl(VI) complexes [UO2(trensal)K], 1 and [UO2(Htrensal)], 2 and of uranyl(V) complexes presenting UO2+⋯K+ or UO2+⋯Fe2+ cation–cation interactions ([UO2(trensal)K]K, 3, [UO2(trensal)] [K(2.2.2crypt)][K(2.2.2crypt)], 4, [UO2(trensal)Fe(py)3], 6). The uranyl(V) complexes show similar stability in pyridine solution, but the presence of Fe2+ bound to the uranyl(V) oxygen leads to increased stability with respect to proton induced disproportionation through the formation of a stable Fe2+–UO2+–U4+ intermediate ([UO2(trensal)Fe(py)3U(trensal)]I, 7) upon addition of 2 eq. of PyHCl to 6. The addition of 2 eq. of PyHCl to 3 results in the immediate formation of U(IV) and UO22+ compounds. The presence of an additional UO2+ bound Fe2+ in [(UO2(trensal)Fe(py)3)2Fe(py)3]I2, 8, does not lead to increased stability. Redox reactivity and cyclic voltammetry studies also show an increased range of stability of the uranyl(V) species in the presence of Fe2+ with respect both to oxidation and reduction reactions, while the presence of a proton in complex 2 results in a smaller stability range for the uranyl(V) species. Cyclic voltammetry studies also show that the presence of a Fe2+ cation bound through one trensal3− arm in the trinuclear complex [{UO2(trensal)}2Fe], 5 does not lead to increased redox stability of the uranyl(V) showing the important role of UO2+⋯Fe2+ cation–cation interactions in increasing the stability of uranyl(V). These results provide an important insight into the role that iron binding may play in stabilizing uranyl(V) compounds in the environmental mineral-mediated reduction of uranium(VI).

A tetranuclear samarium(II) inverse sandwich from direct reduction of toluene by a samarium(II) siloxide

R. P. Kelly; D. Toniolo; F. Fadaei Tirani; L. Maron; M. Mazzanti 

The dinuclear SmII complex, [Sm2L4(dme)] (L = OSi(OtBu)3), is easily obtained from the protonolysis reaction of [Sm{N(SiMe3)2}(thf)2] with HOSi(OtBu)3. This complex reacts slowly with toluene, resulting in the isolation of the triple-decker arene-bridged SmII complex, [{Sm2L3}2(μ-η6:η6-C7H8)], in 44% yield. This reactivity provides the first example of unambiguous arene reduction by an isolated SmII species. In contrast, reduction of [SmL3]2 afforded the inverse sandwich complex, [{KSmL3}2(μ-η6:η6-C7H8)].

Four-electron Reduction and Functionalization of N₂ by a Uranium(III) Bridging Nitride

M. Falcone; M. Mazzanti 

N2 is a cheap and widely available but very unreactive molecule. Notably, the only industrial process for the conversion of dinitrogen is the Haber-Bosch process to form ammonia, but under very harsh conditions (high temperature and pressure). Here, we review our recent research leading to the reduction of dinitrogen by four electrons, under ambient conditions by a uranium(III) bridging nitride, K3 U-N-U, where two uranium(III) cations are linked by a nitride group and a flexible metal-ligand scaffold. We also show that the bound dinitrogen can be further functionalized under mild conditions: the addition of acid, hydrogen and protons or carbon monoxide to the uranium hydrazido complex yields to the cleavage of the N-N single bond and to the formation of new N-H and N-C bonds.

Reversible Dihydrogen Activation and Hydride Transfer by a Uranium Nitride Complex

M. Falcone; L. N. Poon; F. Fadaei Tirani; M. Mazzanti 

Cleavage of dihydrogen is an important step in the industrial and enzymatic transformation of N2 into ammonia. The reversible cleavage of dihydrogen was achieved under mild conditions (room temperature and 1 atmosphere of H2) by the molecular uranium nitride complex, [Cs{U(OSi-(OtBu)3)3}2(m-N)] 1, leading to a rare hydride–imide bridged diuranium(IV) complex, [Cs{U(OSi(OtBu)3)3}2(m-H)(m- NH)], 2 that slowly releases H2 under vacuum. This complex is highly reactive and quickly transfers hydride to acetonitrile and carbon dioxide at room temperature, affording the ketimide- and formate-bridged UIV species [Cs{U(OSi-(OtBu)3)3}2(m-NH)(m-CH3CHN)], 3 and [Cs{U(OSi-(OtBu)3)3}2(m-HCOO)(m-NHCOO)], 4.

Nitrogen reduction and functionalization by a multimetallic uranium nitride complex

M. Falcone; L. Chatelain; R. Scopelliti; I. Živković; M. Mazzanti 

Molecular nitrogen (N-2) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N-2 fixation can do this, but the industrial Haber–Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites (1, 2). And although molecular complexes capable of binding and even reducing N-2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N-2 reduction step (1-14), the multimetallic species involved are usually not well defined, and further transformation of N-2-binding complexes to achieve N–H or N–C bond formation is rare (2, 6, 8, 10, 15, 16). Haber noted (17), before an iron-based catalyst was adopted for the industrial Haber–Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N-2. However, few examples of uranium complexes binding N-2 are known (18-22), and soluble uranium complexes capable of transforming N-2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N-2 under ambient conditions by a fully characterized complex with two U-iii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H-2 and/or protons, or CO to the resulting N-2(4-)complex results in the complete cleavage of N-2 with concomitant N-2 functionalization through N–H or N–C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N-2 into NH3 or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N-2 under mild conditions.

CO Cleavage and CO2 Functionalization under Mild Conditions by a Multimetallic CsU2 Nitride Complex

M. Falcone; L. Chatelain; R. Scopelliti; M. Mazzanti 

Novel efficient chemical processes involving cheap and widely accessible carbon dioxide or carbon monoxide under mild conditions for the production of valuable chemical products are highly desirable in the current energetic context. Uranium nitride materials act as high activity catalysts in the Haber-Bosch process but the reactivity of molecular nitride compounds remains unexplored. Here we review recent results obtained in our group showing that a multimetallic nitride complex [Cs{[U(OSi(OtBu)(3))(3)](2)(mu-N)}] (1) with a CsUIV-N-U-IV core, is able to promote N-C bond formation due to its strong nucleophile behaviour. In particular, complex 1, in the presence of excess CO2 leads to a remarkable dicarbamate product. The multimetallic CsUIV-N-U-IV nitride also readily cleaves the C O bond under mild conditions.

Synthesis and SMM behaviour of trinuclear versus dinuclear 3d-5f uranyl(v)-cobalt(II) cation-cation complexes

L. Chatelain; F. Tuna; J. Pecaut; M. Mazzanti 

Trinuclear versus dinuclear heterodimetallic (UO2+)-O-V center dot center dot center dot Co2+ complexes were selectively assembled via a cation-cation interaction by tuning the ligand. The trimeric complex 2, with a linear [Co-OvUvO-Co] core, exhibits magnetic exchange and slow relaxation with a reversal barrier of 30.5 +/- 0.9 K providing the first example of a U-Co exchange-coupled SMM.

Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple-Decker Arene-Bridged Cerium(II) Sandwich

R. P. Kelly; L. Maron; R. Scopelliti; M. Mazzanti 

Organometallic multi-decker sandwich complexes containing f-elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the Ce-III siloxide complex, [KCeL4] (1; L = OSi(OtBu)(3)), with excess potassium in a THF/toluene mixture afforded a quadruple-decker arene-bridged complex, [K(2.2.2-crypt)](2)[{(KL3Ce)(mu-eta(6):eta(6)-C7H8)}(2)Ce] (3). The structure of 3 features a [Ce(C7H8)(2)] sandwich capped by [KL3Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of Ce-II ions involved in delta bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi-decker complex and the first containing non-classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf = O3SCF3) yielded the Ce-IV complex, [CeL4] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.

A versatile route to homo- and hetero-bimetallic 5f–5f and 3d–5f complexes supported by a redox active ligand framework

C. Camp; D. Toniolo; J. Andrez; J. Pécaut; M. Mazzanti 

The salt-elimination reaction of the complex [Na2U(bis-salophen)] with metal halides provides an entry to the synthesis of well-defined homobimetallic uranium–uranium and rare heterobimetallic uranium–cobalt and uranium–nickel complexes supported by a redox-active dinucleating ligand.

Ligand and Metal Based Multielectron Redox Chemistry of Cobalt Supported by Tetradentate Schiff Bases

J. Andrez; V. Guidal; R. Scopelliti; J. Pécaut; S. Gambarelli et al. 

We have investigated the influence of bound cations on the reduction of cobalt complexes of redox active ligands and explored the reactivity of reduced species with CO2. The one electron reduction of [CoII(Rsalophen)] with alkali metals (M = Li, Na, K) leads to either ligand-centered or metal-centered reduction depending on the alkali ion. It affords either the [CoI(Rsalophen)K] complexes or the [CoII2(bis-salophen)M2] (M = Li, Na) dimers that are present in solution in equilibrium with the respective [CoI(salophen)M] complexes. The two electron reduction of [CoII(OMesalophen)] results in both ligand centered and metal centered reduction affording the Co(I)–Co(II)–Co(I) [Co3(tris-OMesalophen)Na6(THF)6], 6 complex supported by a bridging deca-anionic tris-OMesalophen10– ligand where three OMesalophen units are connected by two C–C bonds. Removal of the Na ion from 6 leads to a redistribution of the electrons affording the complex [(Co(OMesalophen))2Na][Na(cryptand)]3, 7. The EPR spectrum of 7 suggests the presence of a Co(I) bound to a radical anionic ligand. Dissolution of 7 in pyridine leads to the isolation of [CoI2(bis-OMesalophen)Na2Py4][Na(cryptand)]2, 8. Complex 6 reacts with ambient CO2 leading to multiple CO2 reduction products. The product of CO2 addition to the OMesalophen ligand, [Co(OMesalophen-CO2)Na]2[Na(cryptand)]2, 9, was isolated but CO32– formation in 53% yield was also detected. Thus, the electrons stored in the reversible C–C bonds may be used for the transformation of carbon dioxide.

Metathesis of a U(V) imido complex: a route to a terminal U(V) sulfide

R. P. Kelly; M. Falcone; C. A. Lamsfus; R. Scopelliti; L. Maron et al. 

Herein, we report the synthesis and characterisation of the first terminal uranium(V) sulfide and a related UV trithiocarbonate complex supported by sterically demanding tris(tert-butoxy)siloxide ligands. The reaction of the potassium-bound UV imido complex, [U(NAd){OSi(OtBu)3}4K] (4), with CS2 led to the isolation of perthiodicarbonate [K(18c6)]2[C2S6] (6), with concomitant formation of the UIV complex, [U{OSi(OtBu)3}4], and S[double bond, length as m-dash]C[double bond, length as m-dash]NAd. In contrast, the reaction of the UV imido complex, [K(2.2.2-cryptand)][U(NAd){OSi(OtBu)3}4] (5), with one or two equivalents of CS2 afforded the trithiocarbonate complex, [K(2.2.2-cryptand)][U(CS3){OSi(OtBu)3}4] (7), which was isolated in 57% yield, with concomitant elimination of the admantyl thiocyanate product, S[double bond, length as m-dash]C[double bond, length as m-dash]NAd. Complex 7 is likely formed by fast nucleophilic addition of a UV terminal sulfide intermediate, resulting from the slow metathesis reaction of the imido complex with CS2, to a second CS2 molecule. The addition of a solution of H2S in thf (1.3 eq.) to 4 afforded the first isolable UV terminal sulfide complex, [K(2.2.2-cryptand)][US{OSi(OtBu)3}4] (8), in 41% yield. Based on DFT calculations, triple-bond character with a strong covalent interaction is suggested for the U–S bond in complex 7.

Nucleophilic Reactivity of a Nitride-Bridged Diuranium(IV) Complex: CO2 and CS2 Functionalization

M. Falcone; L. Chatelain; M. Mazzanti 

Thermolysis of the nitride-bridged diuranium(IV) complex Cs{(-N)[U(OSi((OBu)-Bu-t)(3))(3)](2)} (1) showed that the bridging nitride behaves as a strong nucleophile, promoting N-C bond formation by siloxide ligand fragmentation to yield an imido-bridged siloxide/silanediolate diuranium(IV) complex, Cs{(-(NBu)-Bu-t)(-O2Si((OBu)-Bu-t)(2))U-2(OSi((OBu)-Bu-t)(3))(5)}. Complex 1 displayed reactivity towards CS2 and CO2 at room temperature that is unprecedented in f-element chemistry, affording diverse N-functionalized products depending on the reaction stoichiometry. The reaction of 1 with two equivalents of CS2 yielded the thiocyanate/thiocarbonate complex Cs{(-NCS)(-CS3)[U(OSi((OBu)-Bu-t)(3))(3)](2)} via a putative NCS-/S2- intermediate. The reaction of 1 with one equivalent of CO2 resulted in deoxygenation and N-C bond formation, yielding the cyanate/oxo complex Cs{(-NCO)(-O)[U(OSi((OBu)-Bu-t)(3))(3)](2)}. Addition of excess CO2 to 1 led to the unprecedented dicarbamate product Cs{(-NC2O4)[U(OSi((OBu)-Bu-t)(3))(3)](2)}.

Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes

L. Chatelain; R. Scopelliti; M. Mazzanti 

The reduction of the nitride-bridged diuranium(IV) complex Cs[{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)] affords the first example of a uranium nitride complex containing uranium in the +III oxidation state. Two nitride-bridged complexes containing the heterometallic fragments Cs-2[U-III – N – U-IV] and Cs-3[U-III – N – U-III] have been crystallographically characterized. The presence of two or three Cs+ cations binding the nitride group is key for the isolation of these complexes. In spite of the fact that the nitride group is multiply bound to two uranium and two or three Cs+ cations, these complexes transfer the nitride group to CS2 to afford SCN- and uraniurn(IV) disulfide.

Sensitisation of visible and NIR lanthanide emission by InPZnS quantum dots in bi-luminescent hybrids

J. K. Molloy; C. Lincheneau; M. M. Karimdjy; F. Agnese; L. Mattera et al. 

The synthesis of stable hybrid nanoparticles combining InPZnS@ZnSe/ZnS quantum dots (QDs) and grafted lanthanide complexes has been performed using two different approaches in organic and aqueous media. The final bi-luminescent hybrids exhibit Ln(III) (Ln = Eu and Yb) centred luminescence upon QD excitation, suggesting that an energy transfer occurs from the QD to the lanthanide.

Versatile pyridine-2,6-bis-tetrazolate scaffolds for the formation of highly luminescent lanthanide complexes

S. Di Pietro; N. Gautier; D. Imbert; J. Pécaut; M. Mazzanti 

Here we report the straightforward synthesis of the ligands L-tz, L-tzC8, L-tzPEG, L-tzAnis and L-=Anis presenting the same pyridine-bistetrazolate (pytz, L) chelating scaffold but different substituents on the para position of the pyridine ring. Its substitution allows the tuning of the solubility of the Ln(III) complexes [Ln(L-i)(3)](3-). These new ligands form homoleptic nona-coordinated Ln(III) complexes of analogous structure and comparable stability in water and methanol. The derivatization of the para position with triazole and substituted triazole groups does not lead to a significant shift of the absorption window, but a shift of 40 nm is observed in ligand L-=Anis due to the presence of the p-ethynyl anisole fragment. The L-tzX series sensitize well, both the Eu and the Tb ions with quantum yields up to 98% for Tb and 43% for Eu, while the ligand L-=Anis sensitizes well Eu (QY of 48%) but cannot sensitize Tb due to the position of its triplet state.

Facile CO Cleavage by a Multimetallic CsU2 Nitride Complex

M. Falcone; C. E. Kefalidis; R. Scopelliti; L. Maron; M. Mazzanti 

Uranium nitrides are important materials with potential for application as fuels for nuclear power generation, and as highly active catalysts. Molecular nitride compounds could provide important insight into the nature of the uranium–nitride bond, but currently little is known about their reactivity. In this study, we found that a complex containing a nitride bridging two uranium centers and a cesium cation readily cleaved the C≡O bond (one of the strongest bonds in nature) under ambient conditions. The product formed has a [CsU2(μ-CN)(μ-O)] core, thus indicating that the three cations cooperate to cleave CO. Moreover, the addition of MeOTf to the nitride complex led to an exceptional valence disproportionation of the CsUIV–N–UIV core to yield CsUIII(OTf) and [MeN=UV] fragments. The important role of multimetallic cooperativity in both reactions is illustrated by the computed reaction mechanisms.

Isolation of a Star-Shaped Uranium(V/VI) Cluster from the Anaerobic Photochemical Reduction of Uranyl(VI)

L. Chatelain; S. White; R. Scopelliti; M. Mazzanti 

Actinide oxo clusters are an important class of compounds due to their impact on actinide migration in the environment. The photolytic reduction of uranyl(VI) has potential application in catalysis and spent nuclear fuel reprocessing, but the intermediate species involved in this reduction have not yet been elucidated. Here we show that the photolysis of partially hydrated uranyl(VI) in anaerobic conditions leads to the reduction of uranyl(VI), and to the incorporation of the resulting U-V species into the stable mixedvalent star-shaped (UI)-I-V/U-V oxo cluster [U(UO2)(5)(mu(3)-O)(5)-(PhCOO)(5)(Py)(7)] (1). This cluster is only the second example of a (UI)-I-V/U-V cluster and the first one associating uranyl groups to a non-uranyl(V) center. The U-V center in 1 is stable, while the reaction of uranyl(V) iodide with potassium benzoate leads to immediate disproportionation and formation of the (U12U4VO24)-U-IV cluster {[K(Py)(2)](2)[K(Py)](2)[U16O24(PhCOO)(24)(Py)(2)]} (5).

CS2 activation at uranium(III) siloxide ate complexes: the effect of a Lewis acidic site

C. Camp; O. Cooper; J. Andrez; J. Pecaut; M. Mazzanti 

Multimetallic cooperative binding of heteroallenes provides an attractive route to their activation, but the reduction of CS2 at heterobimetallic sites, associating an electron-rich metal with a main group Lewis acid has not been explored. Here we show that the presence of a heterometallic U, K site plays an important role in the CS2 reduction by uranium(III) complexes of the electron-rich and the sterically demanding tris(tert-butoxy)siloxide ligand. Specifically, the ion-pair complex [K(18c6)][U(OSi((OBu)-Bu-t)(3))(4)], 1, leads preferentially to the reductive disproportionation of CS2 to K2CS3 and CS. The crystal structure of the thiocarbonate intermediate complex [U(OSi((OBu)-Bu-t)(3))(4) (mu(3)_kappa(2):kappa(2):kappa 2-CS3)K-2(18c6)(2)], 3, isolated from the toluene reaction mixture has been determined. In contrast, the heterobimetallic complex [U(OSi((OBu)-Bu-t)(3))(4)K], 2, promotes preferentially the reductive dimerization of CS2 to K2C2S4 and K2C3S5. The [K2C2S4(DMSO)(3)](n), 5, and [U(OSi((OBu)-Bu-t)(3))(4)K-2(C3S5)](n), 6, polymeric compounds were isolated from this reaction and structurally characterized.

Heterometallic Fe-2(II)-U-V and Ni-2(II)-U-V Exchange-Coupled Single-Molecule Magnets: Effect of the 3d Ion on the Magnetic Properties

L. Chatelain; J. Pecaut; F. Tuna; M. Mazzanti 

Uranium-based compounds have been put forward as ideal candidates for the design of single-molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange-coupled 3d-5f SMM containing uranium have been reported and both are based on the Mn-II ion. Here we have synthesized the first examples of exchange-coupled uranium SMMs based on Fe-II and Ni-II. The SMM behavior of these complexes containing a quasi linear {M-O=U=O-M} core arises from intramolecular Fe-U and Ni-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9 +/- 0.9 K in the UFe2 complex and of 27.4 +/- 0.5 K in the UNi2 complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium-based SMMs.

CO2 conversion to isocyanate via multiple N–Si bond cleavage at a bulky uranium(III) complex

C. Camp; L. Chatelain; C. E. Kefalidis; J. Pécaut; L. Maron et al. 

The reaction of the sterically saturated uranium(III) tetrasilylamido complex [K(18c6)][U(N(SiMe3)(2))(4)] with CO2 leads to CO2 insertion into the U-N bond affording the stable U(IV) isocyanate complex [K(18c6)][U(N(SiMe3)(2))(3)(NCO)(2)](n) that was crystallographically characterized. DFT studies indicate that the reaction involves the [2+2] cyclo-addition of a double bond of O=CO to the U-N(SiMe3)(2) bond and proceeds to the final product through multiple silyl migration steps.

Confinement of a tris-aqua Gd(III) complex in silica nanoparticles leads to high stability and high relaxivity and supresses anion binding

M. M. Karimdjy; G. Tallec; P. H. Fries; D. Imbert; M. Mazzanti 

The water soluble tris-aqua complex [Gd(dhqN-SO3)(H2O)(3)](3-) based on a hexadentate hydroxyquinoline ligand shows high thermodynamic stability and high relaxivity (12.54 mM(-1) s(-1) at 1.2 T). Its non-covalent confinement in 25 nm silica nanoparticles prevents transmetallation and endogenous anion binding and leads to higher relaxivity over a wide range of magnetic fields.

A zig-zag uranyl(V)- Mn(II) single chain magnet with a high relaxation barrier

L. Chatelain; F. Tuna; J. Pécaut; M. Mazzanti 

The synthesis, structural characterization and magnetic properties of a 1D zig-zag coordination polymer based on a cation-cation [((UO2)-O-V)Mn-II] repeated unit are reported; it shows single chain magnet (SCM) behaviour with a high energy barrier of 122 K.

Ferrocene-Based Tetradentate Schiff Bases as Supporting Ligands in Uranium Chemistry

C. Camp; L. Chatelain; V. Mougel; J. Pécaut; M. Mazzanti 

Uranyl(VI), uranyl(V), and uranium(IV) complexes supported by ferrocene-based tetradentate Schiff-base ligands were synthesized, and their solid-state and solution structures were determined. The redox properties of all complexes were investigated by cyclic voltammetry. The bulky salfen-Bu-t(2) allows the preparation of a stable uranyl(V) complex, while a stable U(IV) bis-ligand complex is obtained from the salt metathesis reaction between [UI4(OEt2)(2)] and K(2)salfen. The reduction of the [U(salfen)(2)] complex leads to an unprecedented intramolecular reductive coupling of the Schiff-base ligand resulting in a C-C bond between the two ferrocene-bound imino groups.

Lanthanide(II) Complexes Supported by N,O-Donor Tripodal Ligands: Synthesis, Structure, and Ligand-Dependent Redox Behavior

J. Andrez; G. Bozoklu; G. Nocton; J. Pécaut; R. Scopelliti et al. 

The preparation and characterization of a series of complexes of the Yb and Eu cations in the oxidation state II and III with the tetradentate N,O-donor tripodal ligands (tris(2-pyridylmethyl)amine (TPA), BPA(-) (HBPA=bis(2-pyridylmethyl)(2-hydroxybenzyl)amine), BPPA(-) (HBPPA=bis(2-pyridylmethyl)(3.5-di-tert-butyl-2-hydroxybenzyl)amine), and MPA(2-) (H(2)MPA=(2-pyridylmethyl)bis(3.5-di-tert-butyl-2-hydroxybenzyl)amine) is reported. The X-ray crystal structures of the heteroleptic Ln(2+) complexes [Ln(TPA)I-2] (Ln=Eu, Yb) and [Yb(BPA)I(CH3CN)](2), of the Ln(2+) homoleptic [Ln(TPA)(2)]I-2 (Ln=Sm, Eu, Yb) and [Eu(BPA)(2)] complexes, and of the Ln(3+) [Eu(BPPA)(2)]OTf and [Yb(MPA)(2)K(dme)(2)] (dme=dimethoxyethane) complexes have been determined. Cyclic voltammetry studies carried out on the bis-ligand complexes of Eu3+ and Yb3+ show that the metal center reduction occurs at significantly lower potentials for the BPA(-) ligand as compared with the TPA ligand. This suggests that the more electron-rich character of the BPA(-) ligand results in a higher reducing character of the lanthanide complexes of BPA(-) compared with those of TPA. The important differences in the stability and reactivity of the investigated complexes are probably due to the observed difference in redox potential. Preliminary reactivity studies show that whereas the bis-TPA complexes of Eu2+ and Yb2+ do not show any reactivity with heteroallenes, the [Eu(BPA)(2)] complex reduces CS2 to afford the first example of a lanthanide trithiocarbonate complex.

Single-Molecule-Magnet Behavior in Mononuclear Homoleptic Tetrahedral Uranium(III) Complexes

L. C. J. Pereira; C. Camp; J. T. Coutinho; L. Chatelain; P. Maldivi et al. 

The magnetic properties of the two uranium coordination compounds, [K(18c6)][U(OSi-((OBu)-Bu-t)(3))(4)] and [K(18c6)][U(N(SiMe3)(2))(4)], both presenting the U-III ion in similar pseudotetrahedral coordination environments but with different O- or N-donor ligands, have been measured. The static magnetic susceptibility measurements and density functional theory studies suggest the presence of different ligand fields in the two compounds. Alternating-current susceptibility studies conducted at frequencies ranging from 95 to 9995 Hz and at temperatures in the 1.7-10 K range revealed for both compounds slow magnetic relaxation already at zero static magnetic field with similar energy barriers U similar to 24 K.

A Uranium-Based UO2+-Mn2+ Single-Chain Magnet Assembled trough Cation-Cation Interactions

V. Mougel; L. Chatelain; J. Hermle; R. Caciuffo; E. Colineau et al. 

Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [[UO2(salen)(py)][M(py) 4](NO3)]n, (M=Cd (1) and M=Mn (2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp*2Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm-1), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K. Uranium in chains: 5f-3d heterometallic 1D chains are assembled from the reaction of pentavalent uranyl and CdII or MnII. The Mn-UO2-Mn coordination polymer exhibits slow relaxation of magnetization with a high relaxation barrier, and shows an open hysteresis cycle, thus affording the first example of an actinide-based single-chain magnet. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Two-electron versus one-electron reduction of chalcogens by uranium(III): synthesis of a terminal U(V) persulfide complex

C. Camp; M. A. Antunes; G. Garcia; I. Ciofini; I. C. Santos et al. 

The reaction of the tripodal tris-amido U(iii) complex [U(SiMe 2NPh)3-tacn] (tacn = 1,4,7-triazacyclononane), 1, with 0.0625 and 0.25 equiv. of elemental sulfur affords the sulfide-bridged U(iv) complex [U((SiMe2NPh)3-tacn)2(μ-S)], 2, and the terminal persulfide U(v) complex [U(SiMe2NPh) 3-tacn(η2-S2)], 4, respectively, in good yield. Two different electronic structures, U(v) persulfide and U(iv) supersulfide, were computed for complex 4 at the DFT level. The results show that complex 4 is best described as a U(v) persulfide species with a significant sulfur contribution. X-ray, magnetism and electrochemistry data support this description. Complex 4 is the first example of a terminal U(v) persulfide and of a two-electron reduction of S8 by a U(iii) complex. Complex 4 behaves as a S-atom transfer agent when reacted with PPh3, affording the persulfide-bridged diuranium(iv) complex [U((SiMe2NPh) 3-tacn)2(μ-η2:η2-S 2)], 5, and SPPh3. © 2014 The Royal Society of Chemistry.

Tuning Lanthanide Reactivity Towards Small Molecules with Electron-Rich Siloxide Ligands

J. Andrez; J. Pecaut; P-A. Bayle; M. Mazzanti 

The synthesis, structure, and reactivity of stable homoleptic heterometallic LnL(4)K(2) complexes of divalent lanthanide ions with electron-rich tris(tert-butoxy)siloxide ligands are reported. The [Ln(OSi(OtBu)(3))(4)K-2] complexes (Ln=Eu, Yb) are stable at room temperature, but they promote the reduction of azobenzene to yield the KPhNNPh radical anion as well as the reductive cleavage of CS2 to yield CS32- as the major product. The Eu-III complex of the radical anion PhNNPh is structurally characterized. Moreover, [Yb(OSi(OtBu)(3))(4)K-2] can reduce CO2 at room temperature. Release of the reduction products in D2O shows the quantitative formation of both oxalate and carbonate in a 1:2.2 ratio. The bulky siloxide ligands enforce the labile binding of the reduction products providing the opportunity to establish a closed synthetic cycle for the Yb-II-mediated CO2 reduction. These studies show that the presence of four electron-rich siloxide ligands renders their Eu-II and Yb-II complexes highly reactive.

Multimetallic Cooperativity in Uranium-Mediated CO2 Activation

O. Cooper; C. Camp; J. Pecaut; C. E. Kefalidis; L. Maron et al. 

The metal-mediated redox transformation of CO2 in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris(tert-butoxy)siloxide ligands induces a large cooperative effect in the reduction of CO2. Specifically, the ion pair complex [K(18c6)][U(OSi(OtBu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O tBu)3)4(u-K2:K1-CO 3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(OtBu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(OtBu)3) 4K], 3, thus providing a remarkable example of two-electron transfer in U(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO2. © 2014 American Chemical Society.

Self-Assembly of a 3d-5f Trinuclear Single-Molecule Magnet from a Pentavalent Uranyl Complex

L. Chatelain; J. P. S. Walsh; J. Pecaut; F. Tuna; M. Mazzanti 

Mixed-metal uranium compounds are very attractive candidates in the design of single-molecule magnets (SMMs), but only one 3d-5f hetero-polymetallic SMM containing a uranium center is known. Herein, we report two trimeric heterodimetallic 3d-5f complexes self-assembled by cation-cation interactions between a uranyl(V) complex and a TPA-capped M-II complex (M=Mn (1), Cd (2); TPA = tris(2-pyridylmethyl) amine). The metal centers were strategically chosen to promote the formation of discrete molecules rather than extended chains. Compound 1, which contains an almost linear {Mn-O=U=O-Mn} core, exhibits SMM behavior with a relaxation barrier of 81 +/- 0.5 K-the highest reported for a mono-uranium system-arising from intra-molecular Mn-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. Compound 1 also exhibits an open magnetic hysteresis loop at temperatures less than 3 K, with a significant coercive field of 1.9 T at 1.8 K.

Diamine Bis(phenolate) as Supporting Ligands in Organoactinide(IV) Chemistry. Synthesis, Structural Characterization, and Reactivity of Stable Dialkyl Derivatives

E. Mora; L. Maria; B. Biswas; C. Camp; I. C. Santos et al. 

The homoleptic compounds [U(salan-R2)2] (R = Me (1), tBu (2)) were prepared in high yield by salt-metathesis reactions between UI4(L)2 (L = Et2O, PhCN) and 2 equiv of [K2(salan-R2)] in THF. In contrast, the reaction of the tetradentate ligands salan-R2 with UI3(THF)4 leads to disproportionation of the metal and to mixtures of U(IV) [U(salan-R2)2] and [U(salan-R2)I2] complexes, depending on the ligand to M ratio. The reaction of K 2salan-Me2 ligand with U(IV) iodide and chloride salts always leads to mixtures of the homoleptic bis-ligand complex [U(salan-Me 2)2] and heteroleptic complexes [U(salan-Me 2)X2] in different organic solvents. The structure of the heteroleptic complex [U(salan-Me2)I2(CH3CN)] (4) was determined by X-ray studies. Heteroleptic U(IV) and Th(IV) chloride complexes were obtained in good yield using the bulky salan-tBu 2 ligand. The new complexes [U(salan-tBu 2)Cl2(bipy)] (5) and [Th(salan-tBu 2)Cl2(bipy)] (8) were crystallographically characterized. The salan-tBu2 halide complexes of U(IV) and Th(IV) revealed good precursors for the synthesis of stable dialkyl complexes. The six-coordinated alkyl complexes [Th(salan-tBu2)(CH 2SiMe3)2] (9) and [U(salan-tBu 2)(CH2SiMe3)2] (10) were prepared by addition of LiCH2SiMe3 to the chloride precursor in toluene, and their solution and solid-state structures (for 9) were determined by NMR and X-ray studies. These complexes are stable for days at room temperature. Preliminary reactivity studies show that CO2 inserts into the An-C bond to afford a mixture of carboxylate products. In the presence of traces of LiCl, crystals of the dimeric insertion product [Th 2Cl(salan-tBu2)2(μ- η1:η1-O2CCH2SiMe 3)2(μ-η1:η2-O 2CCH2SiMe3)] (11) were isolated. The structure shows that CO2 insertion occurs in both alkyl groups and that the resulting carboxylate is easily displaced by a chloride anion. © 2012 American Chemical Society.

Multicolour Optical Coding from a Series of Luminescent Lanthanide Complexes with a Unique Antenna

N. Wartenberg; O. Raccurt; E. Bourgeat-Lami; D. Imbert; M. Mazzanti 

The bis-tetrazolate-pyridine ligand H2pytz sensitises efficiently the visible and/or near-IR luminescence emission of ten lanthanide cations (Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb). The LnIII complexes present sizeable quantum yields in both domains with a single excitation source. The wide range of possible colour combinations in water, organic solvents and the solid state makes the complexes very attractive for labelling and encoding. Colour codes: The bis-tetrazolate-pyridine ligand sensitises the visible and/or near-IR luminescence emission of ten lanthanide cations (see figure). The LnIII complexes present sizeable quantum yields in both domains with a single excitation source. The range of possible colour combinations in water, organic solvents and the solid state makes the complexes attractive for labelling and encoding. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Radioactive Europium-Chelate-Based Silica Nanoparticles as a Probe for Stability, Incorporation Efficiency and Trace Analysis

N. Wartenberg; O. Raccurt; E. Bourgeat-Lami; D. Imbert; M. Mazzanti 

Two luminescent terbium and europium lanthanide chelates were efficiently embedded into silica nanoparticles by using a reverse microemulsion process. The incorporation was achieved without covalent bonding between the lanthanide chelates and the silica matrix. To investigate the efficiency of the incorporation process and the stability of the silica-encapsulated lanthanide complex, a method based on a radioactive probe was developed; γ-emitting europium (152) chelates were synthesized and incorporated into silica nanoparticles. Measurements of the γ activity through the entire synthesis allowed the accurate characterization of the incorporation efficiency of the used chelates. A clear correlation was established between the physicochemical properties of the different chelates and the measured incorporation efficiencies. A very efficient noncovalent incorporation of lanthanide chelates in highly stable nanoparticles was achieved by tuning the chelate properties, thus rendering the development of lanthanide-based fluorescent nanoparticles easier. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Controlled Thermolysis of Uranium (Alkoxy) siloxy Complexes: A Route to Polymetallic Complexes of Low-Valent Uranium

C. Camp; C. E. Kefalidis; J. Pecaut; L. Maron; M. Mazzanti 

Decomposition into higher species: Intramolecular UIII-mediated homolytic C-O bond cleavage in UIII (alkoxy)siloxy complexes at low temperature and subsequent reduction with KC8 led to unprecedented polymetallic complexes containing siloxy, silanediolate, and silanetriolate ligands (see example: U green, Si yellow, K blue, O red). Such compounds may be useful precursors to uranium ceramics relevant for catalysis and the storage of spent nuclear fuel. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Optimizing the relaxivity of Gd(III) complexes appended to InP/ZnS quantum dots by linker tuning

G. J. Stasiuk; S. Tamang; D. Imbert; C. Gateau; P. Reiss et al. 

Three bimodal MRI/optical nanosized contrast agents with high per-nanoparticle relaxivity (up to 2523 mM-1 s-1 at 35 MHz and 932 mM-1 s-1 at 200 MHz) have been prepared connecting up to 115 tris-aqua GdIII complexes to fluorescent non-toxic InP/ZnS quantum dots. The structure of the linker has an important effect on the relaxivity of the final multimeric contrast agent. © 2013 The Royal Society of Chemistry.

A Gadolinium Complex Confined in Silica Nanoparticles as a Highly Efficient T1/T2 MRI Contrast Agent

N. Wartenberg; P. Fries; O. Raccurt; A. Guillermo; D. Imbert et al. 

Gd in silica: Noncovalent confinement of a monoaqua Gd chelate into biocompatible silica nanoparticles (NPs; see figure) by a sol-gel method affords a new example of nanosized contrast agents with very high per-Gd relaxivities. Such NPs provide a route to highly efficient multimodal contrast agents. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lanthanide Complexes Based on beta-Diketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers

E. S. Andreiadis; N. Gauthier; D. Imbert; R. Demadrille; J. Pecaut et al. 

A new type of octacoordinated ternary β-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (L) as a sensitizer of lanthanide luminescence in combination with two β-diketonates ligands 2-thenoyltrifluoroacetyl- acetonate (tta-) for Eu3+ and trifluoroacetylacetonate (tfac-) for Tb3+. The solid state structures of the two complexes [Tb(L)(tfac)2] (1) and [Eu(L)(tta)2] (2) have been determined by X-ray crystallography. Photophysical and 1H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two β-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers. © 2013 American Chemical Society.

Crystal structure diversity in the bis hydrotris-(3,5-dimethylpyrazolyl)borate iodouranium(III) complex: from neutral to cationic forms

M. A. Antunes; I. C. Santos; H. Bolvin; L. C. J. Pereira; M. Mazzanti et al. 

The iodouranium(iii) complex with two hydrotris(3,5-dimethylpyrazolyl) borate ligands is shown to adopt three closely related forms in the solid state. In addition to the previously reported structure for [U(TpMe2) 2I], in which one of the pyrazolyl rings coordinates side-on to the U atom, another structure incorporating solvent molecules presents undistorted pyrazol rings, and a third one is the ionic compound [U(TpMe2) 2]I. The implications of this structural diversity for the recently reported single ion magnet behaviour in this complex are discussed, namely on the basis of quantum chemistry calculations. The main effect of the bonding of the iodine atom to uranium is the increase of the size of the first coordination sphere and lowering of the symmetry of the molecule, resulting in a smaller crystal field splitting. © 2013 The Royal Society of Chemistry.

Synthesis of Electron-Rich Uranium(IV) Complexes Supported by Tridentate Schiff Base Ligands and Their Multi-Electron Redox Chemistry

C. Camp; J. Andrez; J. Pecaut; M. Mazzanti 

The synthesis, structure, and reactivity of a new complex of U(IV) with the tridentate Schiff base ligand Menaphtquinolen are reported. The reduction of the bis-ligand complexes [UX2(Menaphtquinolen) 2] (X = Cl, (1-Cl); I (1-I)) with potassium metal affords the U(IV) complex of the new tetranionic hexadentate ligand μ-bis- Menaphtquinolen formed through the intramolecular reductive coupling of the imino groups of each Menaphtquinolen unit. The solid state structure of the [U(μ-bis-Menaphtquinolen)]2 dimer 2 isolated from toluene confirms the presence of a U(IV) complex of the reduced ligand. Reactivity studies with molecular oxygen and 9,10-phenanthrenequinone show that complex 2 can act as a multielectron reducing agent releasing two electrons through the cleavage of the C-C bond to restore the original imino function of the ligand. In the resulting U(IV) and U(VI) complexes [U(9,10-phenanthrenediol)(Menaphtquinolen)2], 3, and [UO2(Menaphtquinolen)2], 4, the restored tridentate Schiff base allows for the coordination of the reduced substrate to the metal. Electrochemical studies of complex 2 show the presence of irreversible ligand centered reduction processes and of a reversible U(IV)/U(III) couple. © 2013 American Chemical Society.

New Bisaqua Picolinate-Based Gadolinium Complexes as MRI Contrast Agents with Substantial High-Field Relaxivities

A. M. Nonat; C. Gateau; P. H. Fries; L. Helm; M. Mazzanti 

Two new tripodal heptadentate ligands, H4dpaba (N,N’-bis[(6-carboxypyridin-2-yl)methyl]aspartic acid) and H3mpatcn(1,4-bis(methoxycarbonyl)-7-[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane), which bear one or two picolinate pendant arms, have been synthesised. Their lanthanide complexes have been characterised by NMR and fluorescence spectroscopy and potentiometry. Both ligands gave rise to water soluble 1:1 complexes with an increased thermodynamic stability (pGddpaba = 13.3, pGdmpatcn = 11.8, with pGdL = log[Gd]free at pH 7.4, [Gd]total = 1 mu M and [L]total = 10 mu M) with respect to the analogous bisaqua complex [Gd(tpaa)(H2O)2] [H3tpaa = alpha,alpha’,alpha”itrilotri(6-methyl-2-pyridinecarboxylic acid), pGdtpaa = 11.2). The two inner-sphere water molecules confer sizeable relaxivities (r1) to the complexes at high field at physiological pH: r1 = 8.90 and r1 = 7.35 mM1s1 have been measured in HOD at 200 MHz for [Gd(dpaba)(HOD)2] and [Gd(mpatcn)(HOD)2], respectively. Their relaxometric properties have been investigated by NMRD (Nuclear Magnetic Relaxation Dispersion) and 17O NMR spectroscopy. The formation of ternary complexes with physiological anions, such as acetate, hydrogen carbonate, hydrogen phosphate and citrate, has been monitored by 1H NMR spectroscopy at 200 MHz and pH 7.4. The addition of a large excess (0.6 M) of acetate, hydrogen phosphate and citrate led to the formation monoaqua ternary complexes. Even under these conditions, the average relaxivity remains higher or similar than most currently used contrast agents. Only hydrogen carbonate interacts strongly with the complexes and coordinates in a bidentate mode by displacing both water molecules to induce a twofold decrease in the relaxivity. Both complexes interact with serum albumin to form a macromolecular adduct with increased relaxivity. In particular, a twofold increase of relaxivity has been measured for [Gd(dpaba)(H2O)2] in bovine serum, which suggests that anion binding does not significantly affect the relaxivity under these conditions.

Magnetic communication and reactivity of a stable homometallic cation-cation trimer of pentavalent uranyl

L. Chatelain; V. Mougel; J. Pecaut; M. Mazzanti 

The reaction of the UO2+ precursor [(UO 2Py5)(KI2Py2)]n, with the potassium salt of the tetradentate aza β-diketiminate ligand L (L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate) affords the first homometallic cation-cation complex of pentavalent uranyl. The complex [UO2L] 3 has a new triangular geometry of the cation-cation interaction in the solid state, which gives rise to a clear magnetic interaction with a maximum in the plot of χ versus T at 12 K. It retains its solid state trinuclear structure in solution and is fully stable in organic anaerobic solvents, but reacts rapidly with molecular oxygen to form a rare dinuclear oxo complex of uranyl(vi), ([UO2(L)]2[μ2-O]). © 2012 The Royal Society of Chemistry.

Metal-Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence of a Chiral Heptanuclear Europium Wheel

G. Bozoklu; C. Gateau; D. Imbert; J. Pecaut; K. Robeyns et al. 

The chiral dissymmetric tetradentate ligand (S)-6′-(4-phenyloxazolin- 2-yl)-2,2′-bipyridine-6-carboxylate (S-Phbipox) leads to the diastereoselective assembly of a homochiral Eu3+ triangle and a highly emissive (quantum yield = 27%) heptanuclear wheel that is the largest example of a chiral luminescent complex of Eu3+ reported to date. The nuclearity of the assembly is controlled by the solvent and the Eu3+ cation. All of the compounds show large circularly polarized luminescence with an activity that varies with the nature of the assembly (highest for the homochiral trimer). © 2012 American Chemical Society.

New polynuclear U(IV)-U(V) complexes from U(IV) mediated uranyl(V) disproportionation

V. Mougel; J. Pecaut; M. Mazzanti 

U(iv) promotes the disproportionation of otherwise stable uranyl(v) Schiff base complexes affording U(iv)-U(v) oxo clusters with new geometries and the first example of a U(iv)⋯UO 2 + cation-cation interaction.

Multielectron redox chemistry of lanthanide Schiff-base complexes

C. Camp; V. Guidal; B. Biswas; J. Pecaut; L. Dubois et al. 

Multielectron redox chemistry, which is essential in metal catalysed chemical transformations, is not easily accessible in lanthanide complexes. Here we explored the reductive chemistry of lanthanide complexes with tetradentate Schiff bases acting as redox-active ligands with the objective of identifying new pathways to lanthanide multielectron redox transfer. The chemical reduction with alkali metals of heteroleptic [Nd(salophen)X] (salophen = N,N′-bis(salicylidene)phenylenediamine, X = I, OTf) and of a series of homoleptic K[Ln(Rsalophen)2] complexes of trivalent lanthanides has resulted respectively in the synthesis of the new dinuclear Nd(iii) complex K2[Nd2(cyclo-salophen)(THF)2] and in the synthesis of a series of mononuclear lanthanide(iii) complexes of general formula K3[Ln(bis-Rsalophen)] (R = H, Me, tBu). Ligand reduction and C-C bond formation are supported by X-ray crystal structures. Proton NMR studies demonstrate that the K2[Nd 2(cyclo-salophen)(py)2] complex can transfer four electrons in the reaction with oxidizing agents such as AgOTf through the breaking of the two C-C bonds. Moreover the electrochemistry and reactivity of the mononuclear complexes K3[Ln(bis-Rsalophen)] show that they can act as formal two electron reductants and that their oxidation potential can be tuned by changing the substituents on the ligand. These results illustrate that Schiff bases provide a new way to introduce multielectron redox events at lanthanide centers and a new route to highly reactive mono- and polynuclear complexes of lanthanides. © The Royal Society of Chemistry 2012.

Self-assembly of highly luminescent lanthanide complexes promoted by pyridine-tetrazolate ligands

E. S. Andreiadis; D. Imbert; J. Pecaut; R. Demadrille; M. Mazzanti 

Two tridentate pyridine-tetrazolate ligands (H 2pytz and H 2pytzc), analogues of the well-known dipicolinate (H 2dpa) ligand, have been synthesized in a straightforward manner and used for lanthanide(iii) coordination. The structures of the resulting tris-ligand complexes were determined in solution ( 1H-NMR), where they remain undissociated, as well as in the solid state (X-ray diffraction). The solubility of these anionic complexes can be easily tuned by changing the countercation. The bis-tetrazolate-pyridine ligand H 2pytz sensitizes very efficiently both the visible and near-IR emission of the lanthanides, with unusually high luminescence quantum yields in solid state (61% and 65% for Eu and Tb, respectively, and 0.21% for Nd) and in water (63% for Tb and 18% for Eu). Furthermore, the absorption window of the complexes is significantly extended towards the visible region up to 330 nm. The results show that the bis-tetrazolate-pyridine ligand provides a very attractive alternative to the classic dipicolinate ligand. © 2012 The Royal Society of Chemistry.

A tetrameric neptunyl(v) cluster supported by a Schiff base ligand

R. Copping; V. Mougel; C. Den Auwer; C. Berthon; P. Moisy et al. 

The first tetrameric cation-cation neptunyl(v) cluster, [NpO 2(salen) 4(μ 8-K) 2][K(18C6)Py] 2, has been synthesized in non-aqueous solution from the reaction of [(NpO 2Py 5)(KI 2Py 2)] n with K 2salen and its structure determined in the solid state and in solution where the complex retains its tetrameric form. © 2012 The Royal Society of Chemistry.

Siloxides as Supporting Ligands in Uranium(III)-Mediated Small-Molecule Activation

V. Mougel; C. Camp; J. Pecaut; C. Coperet; L. Maron et al. 

Siloxides can support U! in the reduction of small molecules with uranium complexes. The treatment of [UN(SiMe3)23] with HOSi(OtBu)3 (3 equiv) yielded a novel homoleptic uranium(III) siloxide complex 1, which acted as a two-electron reducing agent toward CS 2 and CO2 (see scheme). Complex 1 also reduced toluene to afford a diuranium inverted-sandwich complex. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Uranium and manganese assembled in a wheel-shaped nanoscale single-molecule magnet with high spin-reversal barrier

V. Mougel; L. Chatelain; J. Pecaut; R. Caciuffo; E. Colineau et al. 

Discrete molecular compounds that exhibit both magnetization hysteresis and slow magnetic relaxation below a characteristic ‘blocking’ temperature are known as single-molecule magnets. These are promising for applications including memory devices and quantum computing, but require higher spin-inversion barriers and hysteresis temperatures than currently achieved. After twenty years of research confined to the d- block transition metals, scientists are moving to the f-block to generate these properties. We have now prepared, by cation-promoted self-assembly, a large 5f-3d U 12 Mn 6 cluster that adopts a wheel topology and exhibits single-molecule magnet behaviour. This uranium-based molecular wheel shows an open magnetic hysteresis loop at low temperature, with a non-zero coercive field (below 4 K) and quantum tunnelling steps (below 2.5 K), which suggests that uranium might indeed provide a route to magnetic storage devices. This molecule also represents an interesting model for actinide nanoparticles occurring in the environment and in spent fuel separation cycles. © 2012 Macmillan Publishers Limited.

Single-ion magnet behaviour in U(Tp(Me2))(2)I

J. T. Coutinho; M. A. Antunes; L. C. J. Pereira; H. Bolvin; J. Marcalo et al. 

[U(TpMe2)2I] exhibits at low temperatures single molecule magnet (SMM) behaviour comparable to its bipyridine derivative and related single ion U(iii) complexes recently reported as SMMs. The trend of variation of the energy barrier for the magnetic relaxation in these compounds is well reproduced by quantum chemistry calculations. © The Royal Society of Chemistry 2012.

Highly relaxing gadolinium based MRI contrast agents responsive to Mg2+ sensing

S. Abada; A. Lecointre; M. Elhabiri; D. Esteban-Gomez; C. Platas-Iglesias et al. 

A Gd complex based on a polyphosphonated pyridyl ligand shows a very high stability in aqueous solution (logK EuL = 25.7), a high relaxivity (8.5 mM -1 s -1 at 25 °C and 20 MHz) and a marked and selective relaxivity enhancement (37%) in the presence of Mg 2+, opening interesting perspectives for the design of cation responsive contrast agents. © 2012 The Royal Society of Chemistry.

U(Tp(Me2))(2)(bipy) (+): A Cationic Uranium(III) Complex with Single-Molecule-Magnet Behavior

M. A. Antunes; L. C. J. Pereira; I. C. Santos; M. Mazzanti; J. Marcalo et al. 

A versatile precursor for non-aqueous neptunyl(V) chemistry

R. Copping; V. Mougel; S. Petit; C. Den Auwer; P. Moisy et al. 

The polymeric complex [(NpO2Py5)(KI 2Py2)]n is prepared from dry “NpO 2Cl” by anion exchange with potassium iodide in pyridine affording the first convenient starting material for the development of NpO 2+ coordination chemistry in anhydrous organic media. © 2011 The Royal Society of Chemistry.

High Relaxivity and Stability of a Hydroxyquinolinate-Based Tripodal Monoaquagadolinium Complex for Use as a Bimodal MRI/Optical Imaging Agent

G. Tallec; P. H. Fries; D. Imbert; M. Mazzanti 

An octadentate ligand based on triazacyclononane and 8-hydroxyquinolinate/ phenolate binding units leads to very soluble, highly stable lanthanide complexes. The monoaquagadolinium complex shows a high relaxivity as a result of the unusually long rotational correlation time, fast water exchange rate, and slow electronic relaxation. The ligand also acts as sensitizer of the near-IR luminescence emission of the Yb and Nd ions. It appears as an excellent candidate for use as a bimodal imaging agent. © 2011 American Chemical Society.

MOLECULAR MAGNETISM: Uranium memory

M. Mazzanti 

Base-Driven Assembly of Large Uranium Oxo/Hydroxo Clusters

B. Biswas; V. Mougel; J. Pecaut; M. Mazzanti 

Uranium building blocks: Two new uranium cluster topologies are obtained from the stoichiometric hydrolysis of low-valent uranium in non-aqueous media (see picture). The organic base TMEDA directs the self-assembly process towards the formation of a nanosized oxo-hydroxo U16 cluster. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Cell-Permeable Ln(III) Chelate-Functionalized InP Quantum Dots As Multimodal Imaging Agents

G. J. Stasiuk; S. Tamang; D. Imbert; C. Poillot; M. Giardiello et al. 

Quantum dots (QDs) are ideal scaffolds for the development of multimodal imaging agents, but their application in clinical diagnostics is limited by the toxicity of classical CdSe QDs. A new bimodal MRI/optical nanosized contrast agent with high gadolinium payload has been prepared through direct covalent attachment of up to 80 Gd(III) chelates on fluorescent nontoxic InP/ZnS QDs. It shows a high relaxivity of 900 mM-1 s-1 (13 mM -1s-1 per Gd ion) at 35 MHz (0.81 T) and 298 K, while the bright luminescence of the QDs is preserved. Eu(III) and Tb(III) chelates were also successfully grafted to the InP/ZnS QDs. The absence of energy transfer between the QD and lanthanide emitting centers results in a multicolor system. Using this convenient direct grafting strategy additional targeting ligands can be included on the QD. Here a cell-penetrating peptide has been co-grafted in a one-pot reaction to afford a cell-permeable multimodal multimeric MRI contrast agent that reports cellular localization by fluorescence and provides high relaxivity and increased tissue retention with respect to commercial contrast agents. © 2011 American Chemical Society.

Phosphorescent Binuclear Iridium Complexes Based on Terpyridine-Carboxylate: An Experimental and Theoretical Study

E. S. Andreiadis; D. Imbert; J. Pecaut; A. Calborean; I. Ciofini et al. 

The phosphorescent binuclear iridium(III) complexes tetrakis(2- phenylpyridine)μ-(2,2′:6′,2″-terpyridine-6, 6″-dicarboxylic acid)diiridium (Ir1) and tetrakis(2-(2,4-difluorophenyl) pyridine))μ-(2,2′:6′,2″-terpyridine-6,6″- dicarboxylic acid)diiridium (Ir2) were synthesized in a straightforward manner and characterized using X-ray diffraction, NMR, UV-vis absorption, and emission spectroscopy. The complexes have similar solution structures in which the two iridium centers are equivalent. This is further confirmed by the solid state structure of Ir2. The newly reported complexes display intense luminescence in dichloromethane solutions with maxima at 538 (Ir1) and 477 nm (Ir2) at 298 K (496 and 468 nm at 77 K, respectively) and emission quantum yields reaching ∼18% for Ir1. The emission quantum yield for Ir1 is among the highest values reported for dinuclear iridium complexes. It shows only a 11% decrease with respect to the emission quantum yield reported for its mononuclear analogue, while the molar extinction coefficient is roughly doubled. This suggests that such architectures are of potential interest for the development of polymetallic assemblies showing improved optical properties. DFT and time-dependent-DFT calculations were performed on the ground and excited states of the complexes to provide insights into their structural, electronic, and photophysical properties. © 2011 American Chemical Society.

Diastereoselective Self-Assembly of a Homochiral Europium Triangle from a Bipyoxazoline-Carboxylate Ligand

G. Bozoklu; C. Marchal; C. Gateau; J. Pecaut; D. Imbert et al. 

(Figure Presented) A homochiral triangle: An enantiopure europium triangle selectively selfassembles from a new bipyoxazolinecarboxylate ligand in a concentrationdependent process (see scheme). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ligand assisted cleavage of uranium oxo-clusters

G. Nocton; J. Pecaut; Y. Filinchuk; M. Mazzanti 

Dibenzoylmethanate replaces the bridging triflate ligands in uranium triflate polyoxo-clusters and cleaves the U12O20 core yielding the new [U6O4(OH)4(η-dbm) 12] dibenzoylmethanate (dbm-) cluster which slowly dissociates into a monomeric complex. This reactivity demonstrates the importance of bridging ligands in stabilizing uranium polyoxo-clusters. © 2010 The Royal Society of Chemistry.

Structural and photophysical properties of trianionic nine-coordinated near-IR emitting 8-hydroxyquinoline-based complexes

G. Bozoklu; C. Marchal; J. Pecaut; D. Imbert; M. Mazzanti 

Multielectron Redox Reactions Involving C-C Coupling and Cleavage in Uranium Schiff Base Complexes

C. Camp; V. Mougel; P. Horeglad; J. Pecaut; M. Mazzanti 

The reaction of U(III) with Schiff base ligands and the reduction of U(IV) Schiff base complexes both promote C-C bond formation to afford dinuclear or mononuclear U(IV) amido complexes, which can release up to four electrons to substrates through the oxidative cleavage of the C-C bond. © 2010 American Chemical Society.

New insights into the acid mediated disproportionation of pentavalent uranyl

V. Mougel; B. Biswas; J. Pecaut; M. Mazzanti 

The reaction of benzoic acid with the uranyl(v) complex [(UO 2Py5)(KI2Py2)] in pyridine leads to immediate disproportionation with formation of a hexanuclear U(iv) benzoate cluster, a bis-benzoate complex of uranyl(vi) and water. © 2010 The Royal Society of Chemistry.

Cation-Cation Complexes of Pentavalent Uranyl: From Disproportionation Intermediates to Stable Clusters

V. Mougel; P. Horeglad; G. Nocton; J. Pecaut; M. Mazzanti 

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO 2py 5)(KI 2py 2)] n (1) with the Schiff base ligands salen 2-, acacen 2-, and salophen 2- (H 2salen=N,N′- ethylene-bis(salicylideneimine), H 2acacen=N,N′- ethylenebis(acetylacetoneimine), H 2salophen=N,N′-phenylene- bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen 2- in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, [UO 2(acacen)] 4[μ 8-] 2[K([18]C-6)(py)] 2 (3) and [UO 2(acacen)] 4[μ 8-]·2[K([222])(py) ] (4), [UO 2(salophen)] 4[μ 8-K] 2[μ 5-KI] 2[(K([18]C-6)]·2[K([18]C-6) (thf) 2]·2I (5), and [UO 2(salen) 4][μ 8-Rb] 2[Rb([18]C-6)] 2 (9) ([222]=[222]cryptand, py=pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U VO 2(salen)(py)][Cp 2Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K + and Rb +, whereas Li + leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U VO 2(salen)(py)][Cp 2Co] (Cp=pentamethylcyclopentadienyl) with its U VI analogue yields the oxo-functionalized dimer [UO 2(salen)(py)] 2[Cp 2Co] (8). The reaction of the [UO 2(salen) 4][μ 8-K] 2[K([18]C-6)] 2 tetramer with protons leads to disproportionation to U IV and U VI species and H 2O confirming the crucial role of the proton in the U V disproportionation. Size matters: Large alkali ions promote the assembly of Schiff base complexes of pentavalent uranyl into three new stable polynuclear cation-cation clusters, while small cations assemble unstable intermediates, resulting in disproportionation (see figure). Protons also promote disproportionation. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Highly stable and soluble bis-aqua Gd, Nd, Yb complexes as potential bimodal MRI/NIR imaging agents

G. Tallec; D. Imbert; P. H. Fries; M. Mazzanti 

A tripodal ligand based on the 8-hydroxyquinolinate binding unit yields a soluble and highly stable bis-hydrated Gd3+ complex in water (pGd = 19.2(3)) with relaxivity change in the pH range 4.5-7.4 and Nd3+, Yb3+ analogues with sizeable NIR emission upon excitation at 370 nm providing a new architecture for the development of bimodal agents. © 2010 The Royal Society of Chemistry.

Synthesis, Structure, and Bonding of Stable Complexes of Pentavalent Uranyl

G. Nocton; P. Horeglad; V. Vetere; J. Pecaut; L. Dubois et al. 

Lanthanide-chelate silica nanospheres as robust multicolor Vis-NIR tags

J. Samuel; G. Tallec; P. Cherns; W. L. Ling; O. Raccurt et al. 

Different lanthanide chelates have been simultaneously embedded in a silica matrix yielding bright dual-mode lanthanide doped nanospheres which are uniform in size distribution, tunable, photostable, and leakage free. Depending on the chelate combination, two color emission with a single light source or tunable emission with multiple sources is obtained. © 2010 The Royal Society of Chemistry.

Modulation of the unpaired spin localization in Pentavalent Uranyl Complexes

V. Vetere; P. Maldivi; M. Mazzanti 

The electronic structure of various complexes of pentavalent uranyl species, namely UO2 +, is described, using DFT methods, with the aim of understanding how the structure of the ligands may influence the localisation of the unpaired 5f electron of uranium (V) and, finally, the stability of such complexes towards oxidation. Six complexes have been inspected: [UO2py5]+ (1), [(UO 2py5)KI2] (2), [UO2(salan- tBu2)(py)K] (3), [UO2(salophen-t- Bu2)(thf)K] (4), [UO2(salen-tBu 2)(py)K] (5), [and UO2-cyclo[6]pyrrole]1- (6), chosen to explore various ligands. In the five first complexes, the UO 2 + species is well identified with the unpaired electron localized on the 5f uranium orbital. Additionally, for the salan, salen and salophen ligands, some covalent interactions have been observed, resulting from the presence of both donor and acceptor binding sites. In contrast, the last complex is best described by a UO2 2+ uranyl (VI) coordinated by the anionic radical cyclopyrrole, the highly delocalized φ orbitals set stabilizing the radical behaviour of this ligand. © 2010 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Gadolinium(III) complexes of 1,4,7-triazacyclononane based picolinate ligands: simultaneous optimization of water exchange kinetics and electronic relaxation

A. Nonat; M. Giraud; C. Gateau; P. H. Fries; L. Helm et al. 

The two new tripodal picolinate H3ebpatcn (1-carboxyethyl-4,7-bis((6-carboxypyridin-2-yl)-methyl)-1,4,7-triazacyclononane) and H4pbpatcn (1-methylphosphonic-acid-4,7-bis((6-carboxypyridin-2-yl)methyl)-1,4,7-triazacyclononane) ligands based on the 1,4,7-triazacyclononane anchor were prepared and their lanthanide complexes were characterized by NMR, fluorescence and potentiometric studies. The [Gd(ebpatcn)(H2O)] complex displays a relaxivity of r1 = 4.68 mM-1 s-1 at 45 MHz and 298 K, whereas r1 = 4.55 mM-1 s-1 was measured for [Gd(Hpbpatcn)(H2O)] under the same conditions. The modified scaffold of the ligands with respect to the previously reported H3bpatcn (1-(carboxymethyl)-4,7-bis[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane) leads to an optimization of the properties of these gadolinium complexes. The replacement of an acetate binding group of the H3bpatcn ligand with a propionate group (H3ebpatcn) or a phosphonate group (H4pbpatcn) leads to a faster exchange rate of the coordinated water molecule in both mono-aquo gadolinium complexes. The resulting water exchange rate is optimized for the future design of high relaxivity macromolecular gadolinium based contrast agents with a value measured by O17 NMRD of kex = 34 ¥ 106 s-1 for [Gd(Hpbpatcn)(H2O)] falling in the range of optimum values of (30 to 50) ¥ 106 s-1 predicted by the SBM theory. The water exchange rate kex 298 = 86 ¥ 106 s-1 of the complex [Gd(ebpatcn)(H2O)] is the fastest reported in the literature for a neutral complex with only one inner-sphere water molecule. The relatively high stability of these modified gadolinium complexes (pGd = 14.1 for Gd(pbpatcn) and 13.1 for Gd(ebpatcn)) is similar to that of the [Gd(bpatcn)(H2O)] complex (pGd = 13.6). The high luminescence efficiency is also retained for the terbium complex. However, whereas the longitudinal electronic spin relaxation time keeps a value for [Gd(ebpatcn)(H2O)], which is long enough not to affect the relaxivity in macromolecular complexes (transient ZFS amplitude D2 [1020 rad2 s-2] = 0.39), the O17 relaxation and the 1H NMRD indicate a rather fast electron spin relaxation for the phosphonate containing complex (D2 [1020 rad2 s-2]= 1.3).

Stable Pentavalent Uranyl Species and Selective Assembly of a Polymetallic Mixed-Valent Uranyl Complex by Cation-Cation Interactions

V. Mougel; P. Horeglad; G. Nocton; J. Pecaut; M. Mazzanti 

Lanthanide-Based Coordination Polymers Assembled by a Flexible Multidentate Linker: Design, Structure, Photophysical Properties, and Dynamic Solid-State Behavior

C. Marchal; Y. Filinchuk; X-Y. Chen; D. Imbert; M. Mazzanti 

Remarkable Tuning of the Coordination and Photophysical Properties of Lanthanide Ions in a Series of Tetrazole-Based Complexes

E. S. Andreiadis; R. Demadrille; D. Imbert; J. Pecaut; M. Mazzanti 

Pentavalent uranyl stabilized by a dianionic bulky tetradentate ligand

P. Horeglad; G. Nocton; Y. Filinchuk; J. Pecaut; M. Mazzanti 

Water Stability and Luminescence of Lanthanide Complexes of Tripodal Ligands Derived from 1,4,7-Triazacyclononane: Pyridinecarboxamide versus Pyridinecarboxylate Donors

G. Nocton; A. Nonat; C. Gateau; M. Mazzanti 

A series of europium(III) and terbium(III) complexes of three 1,4,7-triazacyclononane-based pyridine containing ligands were synthesized. The three ligands differ from each other in the substitution of the pyridine pendant arm, namely they have a carboxylic acid, an ethylamide, or an ethyl ester substituent, i.e., these ligands are 6,6′,6″-[1,4,7-triazacyclononane-1,4,7-triyltris(methylene)]tris[pyridine-2-carboxylic acid] (H3tpatcn), -tris[pyridine-2-carboxamide] (tpatcnam), and -tris[pyridine-2-carboxylic acid] triethyl ester (tpatcnes) respectively. The quantum yields of both the europium(III) and terbium(III) emission, upon ligand excitation, were highly dependent upon ligand substitution, with a ca. 50-fold decrease for the carboxamide derivative in comparison to the picolinic acid (=pyridine-2-carboxylic acid) based ligand. Detailed analysis of the radiative rate constants and the energy of the triplet states for the three ligand systems revealed a less efficient energy transfer for the carboxamide-based systems. The stability of the three ligand systems in H2O was investigated. Although hydrolysis of the ethyl ester occurred in H2O for the [Ln(tpatcnes)](OTf)3 complexes, the tripositive [Ln(tpatcnam)](OTf)3 complexes and the neutral [Ln(tpatcn)] complexes showed high stability in H2O which makes them suitable for application in biological media. The [Tb(tpatcn)] complex formed easily in H2O and was thermodynamically stable at physiological pH (pTb 14.9), whereas the [Ln(tpatcnam)](OTf)3 complexes showed a very high kinetic stability in H2O, and once prepared in organic solvents, remained undissociated in H2O.

Structural and Photophysical Studies of Highly Stable Lanthanide Complexes of Tripodal 8-Hydroxyquinolinate Ligands Based on 1,4,7-Triazacyclononane

A. Nonat; D. Imbert; J. Pecaut; M. Giraud; M. Mazzanti 

A nitrido-centered uranium azido cluster obtained from a uranium azide

G. Nocton; J. Pecaut; M. Mazzanti 

A flexible tripodal ligand linking octametallic terbium rings into luminescent polymeric chains

X-Y. Chen; C. Marchal; Y. Filinchuk; D. Imbert; M. Mazzanti 

Polynuclear Cation-Cation Complexes of Pentavalent Uranyl: Relating Stability and Magnetic Properties to Structure

G. Nocton; P. Horeglad; J. Pecaut; M. Mazzanti 

Efficient sensitization of lanthanide luminescence by tetrazole-based polydentate ligands

M. Giraud; E. S. Andreiadis; A. S. Fisyuk; R. Demadrille; J. Pecaut et al. 

A comparative spectroscopic study of U(III)/Am(III) and Ln(III) complexed with N-donor ligands

M. A. Denecke; P. J. Panak; F. Burdet; M. Weigl; A. Geist et al. 

Self-assembly of polyoxo clusters and extended frameworks by controlled hydrolysis of low-valent uranium

G. Nocton; F. Burdet; J. Pecaut; M. Mazzanti 

Relating structural and thermodynamic effects of the Pb(II) lone pair: A new picolinate ligand designed to accommodate the Pb(II) lone pair leads to high stability and selectivity

A. Pellissier; Y. Bretonniere; N. Chatterton; J. Pecaut; P. Delangle et al. 

Structure, stability, dynamics, high-field relaxivity and ternary-complex formation of a new tris(aquo) gadolinium complex

A. Nonat; P. H. Fries; J. Pecaut; M. Mazzanti 

Selective Self-Assembly of Hexameric Homo- and Heteropolymetallic Lanthanide Wheels: Synthesis, Structure, and Photophysical Studies

X-Y. Chen; Y. Bretonnière; J. Pecaut; D. Imbert; J-C. Bunzli et al. 

A rational approach to the formation of pure heteropolymetallic lanthanide complexes that uses a two-step assembly strategy and exploits the different size requirements of the two metals included in the final structure is described. The investigation of the assembly of [LnL2](Otf) (L = 2,2′:6′,2′ ‘-terpyridine-6-carboxylate) complexes into hexametallic rings hosting an additional hexacoordinated lanthanide cation was crucial for the development of this strategy. The formation and size of the cyclic assembly are controlled by the ionic radius and by the coordination number of the lanthanides. The rather high luminescence quantum yield of the heptaeuropium complex (25%) indicates that the ring structure is well adapted to include highly luminescent lanthanide complexes in nanosized architecture. The use of a stepwise synthetic strategy leads to the selective assembly of large heteropolymetallic rings. The addition of a smaller lanthanide ion to the [EuL2](Otf) complex in anhydrous acetonitrile leads selectively to heterometallic species with the Eu ions located on the peripheral sites and the smaller ion occupying only the central site. The high selectivity is the result of the different size requirements of the two metal sites present in the cyclic structure. The heterometallic structure of the isolated [Lu C (EuL2)6](Otf)9 complex was confirmed by X-ray diffraction and by high resolution solid-state photophysical studies. The described synthetic approach allowed us to obtain the first example of selective assembly of two different lanthanide ions in a large polymetallic structure characterized in solution and in the solid state and will make the isolation of planned dimetallic combinations presenting different lanthanide emitters in the peripheral sites possible.

Toward the Rational Design of Lanthanide Coordination Polymers: a New Topological Approach

C. Marchal; Y. Filinchuk; D. Imbert; J-C. G. Bünzli; M. Mazzanti 

The implementation of four bidentate building blocks into a highdenticity linker with a flexible spacer leads to a predisposed ligand that allows one to direct the self- assembly of 1D functional coordination polymers. This is illustrated by the assembly under mild conditions of the luminescent metal-organic framework [Tb(Htpabn)]â14H2O¥ (1; H4tpabn ) N,N,N¢,N¢-tetrakis[(6-carboxypyridin-2-yl) methyl]butylenediamine). The X-ray crystal structure shows that the monoprotonated Htpabn binds two equivalent lanthanide ions to form a one-directional staircase chain. The high ligand denticity prevents solvent coordination and leads to a high luminescence quantum yield (Q ) 39%), which is maintained after solvent removal.

Multiple-frequency EPR spectra of two aqueous Gd3+ polyaminopolypyridinecarboxylate complexes: a study of high field effects

A. Borel; S. Laus; A. Ozarowski; C. Gateau; A. Nonat et al. 

In the search for highly efficient magnetic resonance imaging contrast agents, polyaminopolypyridinecarboxylate complexes of Gd3+ have shown unusual properties with both very rapid and very slow electron spin relaxation in solution observed by electron paramagnetic resonance. Since the relationship between the molecular structure and the electron spin properties remains quite obscure at this point, detailed studies of such complexes may offer useful clues for the design of Gd3+ compounds with tailored electronic features. Furthermore, the availability of very high frequency EPR spectrometers based on quasi-optical components provides us with an opportunity to test the existing relaxation theories at increasingly high magnetic fields and observation frequencies. We present a detailed EPR study of two gadolinium polyaminopolypyridinecarboxylate complexes, [Gd(tpaen)]- and [Gd(bpatcn)(H2O)], in liquid aqueous solutions at multiple temperatures and frequencies between 9.5 and 325 GHz. We analyze the results using the model of random zero-field splitting modulations through Brownian rotation and molecular deformations. We consider the effect of concentration on the line width, as well as the possible existence of an additional g-tensor modulation relaxation mechanism and its possible impact on future experiments. We use 17O-NMR to characterize the water exchange rate on [Gd(bpatcn)(H2O)], and find it to be slow (~ 0.6×10^6 s-1).

Variable Temperature and EPR Frequency Study of Two Aqueous Gd(III) Complexes with Unprecedented Sharp Lines

A. Borel; H. Kang; C. Gateau; M. Mazzanti; R. B. Clarkson et al. 

We present an EPR study of two Gd(III) complexes in aqueous solution at multiple temperatures and EPR frequencies. These two complexes, [Gd(TPATCN)] and [Gd(DOTAM)(H2O)]3+, display remarkably sharp lines (i.e. slow transverse electron spin relaxation) in comparison with all complexes studied in the past, especially at X-band (~9.08 GHz). These unprecedented spectra even show, for the first time in solution, a distinct influence of hyperfine coupling to two magnetically active Gd isotopes (155Gd 14.8%, I = 3/2, gamma = -0.8273 × 10^7 s-1 T-1 and 157Gd, 15.65%, I = 3/2, -1.0792 × 10^7 s-1 T-1). The hyperfine coupling splitting in [Gd(TPATCN)] was determined accurately for a 157Gd-enriched complex, and the value A(157Gd)/gB = 5.67 G seems to be a good estimation for most chelates of interest. Consequently, we can safely assert that neglecting the Gd isotopes in line shape studies is not a significant source of error as long as the apparent peak-to-peak width is greater than 10-20 G. This is generally the case, except at very high EPR frequencies (>150 GHz). Analyzing the spectra within the physical model of Rast et al. we find that the slow electron spin relaxation is due to a nearly zero static ZFS. We discuss some structural features that might explain this interesting electron structure.

Isolation of a tetrameric cation-cation complex of pentavalent uranyl

F. Burdet; J. Pecaut; M. Mazzanti 

Lanthanide complexes of a picolinate ligand derived from 1,4,7-triazacyclononane with potential application in magnetic resonance imaging and time-resolved luminescence imaging

A. Nonat; C. Gateau; P. H. Fries; M. Mazzanti 

The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes

N. Chatterton; C. Gateau; M. Mazzanti; J. Pécaut; A. Borel et al. 

The ligand N,N’-bis[6-carboxy-2-pyridylmethyl]ethylenediamine-N,N’-diacetic acid (H4bpeda) was synthesized using an improved procedure which requires a reduced no. of steps and leads to a higher yield with respect to the published procedure. It was obtained in three steps from diethylpyridine-2,6-dicarboxylate and com. available ethylenediamine-N,N’-diacetic acid with a total yield of .apprx.20%. The crystal structure of the hexa-protonated form of the ligand which was detd. by x-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of polynuclear [Gd(bpeda)(H2O)2]3[Gd(H2O)6]2Cl3 (2), isolated by slow evapn. of a 1:1 mixt. of GdCl3 and H4bpeda at pH .apprx. 1, was detd. by x-ray diffraction. In complex 2 three [Gd(bpeda)(H2O)2] units, contg. a Gd(III) ion ten-coordinated by the octadentate bpeda and two water mols., are connected in a pentametallic structure by two hexa-aqua Gd3+ cations through four carboxylato bridges. The protonation consts. (pKa1 = 2.9(1), pKa2 = 3.5(1), pKa3 = 5.2(2), and pKa4 = 8.5(1)) and the stability consts. of the complexes formed between Gd(III) and Ca(II) ions and H4bpeda (log bGdL = 15.1(3); log bCaL = 9.4(1)) were detd. by potentiometric titrn. The unexpected decrease in the stability of the gadolinium complex and of the calcium complex of the octadentate ligand bpeda4- with respect to the hexadentate ligand EDTA4- was interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent DHpp .apprx.800-1200 G at X-band and 90-110 G at Q-band depending on the temp.), indicating a rapid transverse electron spin relaxation. At X-band, Gd(bpeda) is among the fastest relaxing Gd3+ complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation.

Solid-state and solution properties of the lanthanide complexes of a new nonadentate tripodal ligand derived from 1,4,7-triazacyclononane

C. Gateau; M. Mazzanti; J. Pecaut; F. A. Dunand; L. Helm 

The synthesis of the potentially nonadentate ligand 1,4,7-tris[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H3tpatcn), a new derivative of 1,4,7-triazacyclononane N-functionalised with three pyridinecarboxylate arms, is described. The complexes of four lanthanide ions (Nd, Eu, Gd, Lu) of this ligand have been prepared and structurally characterised. These complexes, which have high water solubility, show highly rigid C3 symmetric solution structures. All the complexes present mononuclear nine-coordinated solid-state structures. The coordination polyhedron is a slightly distorted tricapped trigonal prism. The NMRD (Nuclear Magnetic Relaxation Dispersion) profiles measured for the [Gd(tpatcn)] complex indicate that the second-sphere contribution arising from the presence of water molecules tightly hydrogen-bonded to the carboxylate moieties on the surface of the complex are not large enough to explain the very high relaxivity of the previously reported [Gd(tpaa)(H2O)2] complex (H3tpaa = α,α′,α″-nitrilotri(6-methyl-2-pyridinecarboxylic acid)). In fact the low-field relaxivity of [Gd(tpatcn)] more likely points to a favorable electronic relaxation rate.

A new heptadentate tripodal ligand leading to a gadolinium complex with an improved relaxation efficiency

Y. Bretonniere; M. Mazzanti; J. Pecaut; F. A. Dunand; A. E. Merbach 

Solid-State and Solution Properties of the Lanthanide Complexes of a New Heptadentate Tripodal Ligand: A Route to Gadolinium Complexes with an Improved Relaxation Efficiency

Y. Bretonniere; M. Mazzanti; J. Pecaut; F. A. Dunand; A. E. Merbach 

Bis(diphenylphosphino)methanetetradecacarbonylhexairidium dichloromethane solvate

M. Mazzanti; R. Roulet; K. J. Schenk