Clément Camp

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

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

Multimetallic Cooperativity in Uranium-Mediated CO2 Activation

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(O(t)Bu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O(t)Bu)3)4(μ-κ(2):κ(1)-CO3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(O(t)Bu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(O(t)Bu)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.

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

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.

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

The reaction of the tripodal tris-amido U(III) complex [U{(SiMe2NPh)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-S2)], 5, and SPPh3.

Multielectron redox chemistry of lanthanide Schiff-base complexes

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[Nd2(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.

Activation of white phosphorus by low-valent group 5 complexes: formation and reactivity of cyclo-P4 inverted sandwich compounds.

We report the synthesis and comprehensive study of the electronic structure of a unique series of dinuclear group 5 cyclo-tetraphosphide inverted sandwich complexes. White phosphorus (P4) reacts with niobium(III) and tantalum(III) β-diketiminate (BDI) tert-butylimido complexes to produce the bridging cyclo-P4 phosphide species {[(BDI)(NtBu)M]2(μ-η3:η3P4)} (1, M = Nb; 2, M = Ta) in fair yields. 1 is alternatively synthesized upon hydrogenolysis of (BDI)Nb(NtBu)Me2 in the presence of P4. The trinuclear side product {[(BDI)NbNtBu]3(μ-P12)} (3) is also identified. Protonation of 1 with [HOEt2][B(C6F5)4] does not occur at the phosphide ring but rather involves the BDI ligand to yield {[(BDI#)Nb(NtBu)]2(μ-η3:η3P4)}[B(C6F5)4]2 (4). The monocation and dication analogues {[(BDI)(NtBu)Nb]2(μ-η3:η3P4)}{B(ArF)4}n (5, n = 1; 6, n = 2) are both synthesized by oxidation of 1 with AgBArF. DFT calculations were used in combination with EPR and UV–visible spectroscopies to probe the nature of the metal–phosphorus bonding.

Cation-Mediated Conversion of the State of Charge in Uranium Arene Inverted-Sandwich Complexes

Two new arene inverted-sandwich complexes of uranium supported by siloxide ancillary ligands [K{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (3) and [K2{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (4) were synthesized by the reduction of the parent arene-bridged complex [{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (2) with stoichiometric amounts of KC8 yielding a rare family of inverted-sandwich complexes in three states of charge. The structural data and computational studies of the electronic structure are in agreement with the presence of high-valent uranium centers bridged by a reduced tetra-anionic toluene with the best formulation being U(V)-(arene(4-))-U(V), KU(IV)-(arene(4-))-U(V), and K2U(IV)-(arene(4-))-U(IV) for complexes 2, 3, and 4 respectively. The potassium cations in complexes 3 and 4 are coordinated to the siloxide ligands both in the solid state and in solution. The addition of KOTf (OTf=triflate) to the neutral compound 2 promotes its disproportionation to yield complexes 3 and 4 (depending on the stoichiometry) and the U(IV) mononuclear complex [U(OSi(OtBu)3)3(OTf)(thf)2] (5). This unprecedented reactivity demonstrates the key role of potassium for the stability of these complexes.

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

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((Me)naphtquinolen)2] (X = Cl, (1-Cl) ; I (1-I)) with potassium metal affords the U(IV) complex of the new tetranionic hexadentate ligand μ-bis-(Me)naphtquinolen formed through the intramolecular reductive coupling of the imino groups of each (Me)naphtquinolen unit. The solid state structure of the [U(μ-bis-(Me)naphtquinolen)]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)((Me)naphtquinolen)2], 3, and [UO2((Me)naphtquinolen)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.

Single-molecule-magnet behavior in mononuclear homoleptic tetrahedral uranium(III) complexes.

The magnetic properties of the two uranium coordination compounds, [K(18c6)][U(OSi(O(t)Bu)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 ∼24 K.

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

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