Polly L. Arnold

Reduction and selective oxo group silylation of the uranyl dication

Uranium occurs in the environment predominantly as the uranyl dication [UO2]2+. Its solubility renders this species a problematic contaminant which is, moreover, chemically extraordinarily robust owing to strongly covalent U–O bonds. This feature manifests itself in the uranyl dication showing little propensity to partake in the many oxo group functionalizations and redox reactions typically seen with [CrO2]2+, [MoO2]2+ and other transition metal analogues. As a result, only a few examples of [UO2]2+ with functionalized oxo groups are known. Similarly, it is only very recently that the isolation and characterization of the singly reduced, pentavalent uranyl cation [UO2]+ has been reported. Here we show that placing the uranyl dication within a rigid and well-defined molecular framework while keeping the environment anaerobic allows simultaneous single-electron reduction and selective covalent bond formation at one of the two uranyl oxo groups. The product of this reaction is a pentavalent and monofunctionalized [O = U...OR]+ cation that can be isolated in the presence of transition metal cations. This finding demonstrates that under appropriate reaction conditions, the uranyl oxo group will readily undergo radical reactions commonly associated only with transition metal oxo groups. We expect that this work might also prove useful in probing the chemistry of the related but highly radioactive plutonyl and neptunyl analogues found in nuclear waste.

Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N-2, Coupling of CO, and Deoxygenation Insertion of CO2 under Ambient Conditions

Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris(aryloxide) U(ODtbp)(3), U(OC(6)H(3)-Bu(t)(2)-2,6)(3), and the tri-tert-butyl analogue U(OTtbp)(3), U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3), in the form of bridging, side-on dinitrogen complexes [U(OAr)(3)](2)(μ-η(2):η(2)-N(2)), for which the tri-tert-butyl N(2) complex is the most robust U(2)(N(2)) complex isolated to date. Attempted reduction of the tris(aryloxide) complex under N(2) gave only the potassium salt of the uranium(III) tetra(aryloxide) anion, K[U(OAr)(4)], as a result of ligand redistribution. The solid-state structure is a polymeric chain formed by each potassium cation bridging two arenes of adjacent anions in an η(6) fashion. The same uranium tris(aryloxides) were also found to couple carbon monoxide under ambient conditions to give exclusively the ynediolate [OCCO](2-) dianion in [U(OAr)(3)](2)(μ-η(1):η(1)-C(2)O(2)), in direct analogy with the reductive coupling recently shown to afford [U{N(SiMe(3))(2)}(3)](2)(μ-η(1):η(1)-C(2)O(2)). The related U(III) complexes U{N(SiPhMe(2))(2)}(3) and U{CH(SiMe(3))(2)}(3) however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3) reacts with CO(2) to give an insertion product containing bridging oxo and aryl carbonate moieties, U(2)(OTtbp)(4)(μ-O)(μ-η(1):η(1)-O(2)COC(6)H(2)-Bu(t)(3)-2,4,6)(2), which has been structurally characterized. The presence of coordinated N(2) in [U(OTtbp)(3)](2)(N(2)) prevents the occurrence of any reaction with CO(2), underscoring the remarkable stability of the N(2) complex. The di-tert-butyl aryloxide does not insert CO(2), and only U(ODtbp)(4) was isolated. The silylamide also reacts with carbon dioxide to afford U(OSiMe(3))(4) as the only uranium-containing material. GGA and hybrid DFT calculations, in conjunction with topological analysis of the electron density, suggest that the U-N(2) bond is strongly polar, and that the only covalent U→N(2) interaction is π backbonding, leading to a formal (U(IV))(2)(N(2))(2-) description of the electronic structure. The N-N stretching wavenumber is preferred as a metric of N(2) reduction to the N-N bond length, as there is excellent agreement between theory and experiment for the former but poorer agreement for the latter due to X-ray crystallographic underestimation of r(N-N). Possible intermediates on the CO coupling pathway to [U(OAr)(3)](2)(μ-C(2)O(2)) are identified, and potential energy surface scans indicate that the ynediolate fragment is more weakly bound than the ancillary ligands, which may have implications in the development of low-temperature and pressure catalytic CO chemistry.

C3‐Symmetric Lanthanide Tris(alkoxide) Complexes Formed by Preferential Complexation and Their Stereoselective Polymerization of rac‐Lactide

Restoring order: YIII, EuIII, and ErIII tris(ligand) complexes of a new chiral alkoxide ligand, tBu2P(O)CH2CH(tBu)OH (HL), preferentially form as C3‐symmetric diastereomers. Thus racemic HL affords (RRR)‐ and (SSS)‐[LnL3] complexes, which are active catalysts for the stereoselective polymerization of rac‐lactide to afford highly isotactic polylactic acid.

Uranium-mediated activation of small molecules

Molecular complexes of uranium are capable of activating a range of industrially and economically important small molecules such as CO, CO(2), and N(2); new and often unexpected reactions provide insight into an element that needs to be well-understood if future clean-energy solutions are to involve nuclear power.

Chelating N-heterocyclic carbene alkoxide as a supporting ligand for PdII/IV C-H bond functionalization catalysis

A Pd(IV) complex that represents a viable catalytic intermediate in Pd-catalyzed C-H bond halogenation reactions has been isolated and structurally characterized. It contains the first examples of both a Pd(IV) NHC bond and a Pd(IV) alkoxide bond and serves as a precatalyst for C-H bond halogenation. As such, this represents a new class of tunable supporting ligand systems in Pd(IV) catalysis.

Strongly coupled binuclear uranium–oxo complexes from uranyl oxo rearrangement and reductive silylation

The most common motif in uranium chemistry is the d(0)f(0) uranyl ion [UO(2)](2+) in which the oxo groups are rigorously linear and inert. Alternative geometries, such as the cis-uranyl, have been identified theoretically and implicated in oxo-atom transfer reactions that are relevant to environmental speciation and nuclear waste remediation. Single electron reduction is now known to impart greater oxo-group reactivity, but with retention of the linear OUO motif, and reactions of the oxo groups to form new covalent bonds remain rare. Here, we describe the synthesis, structure, reactivity and magnetic properties of a binuclear uranium-oxo complex. Formed through a combination of reduction and oxo-silylation and migration from a trans to a cis position, the new butterfly-shaped Si-OUO(2)UO-Si molecule shows remarkably strong U(V)-U(V) coupling and chemical inertness, suggesting that this rearranged uranium oxo motif might exist for other actinide species in the environment, and have relevance to the aggregation of actinide oxide clusters.

Carbon monoxide coupling and functionalisation at a simple uranium coordination complex

A simple coordination complex of uranium(III), a uranium tris(amide), can selectively couple gaseous CO to the linear ynediolate [OCCO]2− dianion, at room temperature and pressure, regardless of the reagent stoichiometry. This product exhibits further reactivity upon warming in the form of the addition of a C–H bond of a methyl group across the CC triple bond, this second carbon–carbon bond forming reaction generating a functionalised enediolate dianion.

Covalency in CeIV and UIV Halide and N‐Heterocyclic Carbene Bonds

Oxidative halogenation with trityl chloride provides convenient access to Ce(IV) and U(IV) chloroamides [M(N{SiMe(3)}(2))(3)Cl] and their N-heterocyclic carbene derivatives, [M(L)(N{SiMe(3)}(2))(2)Cl] (L = OCMe(2)CH(2)(CNCH(2)CH(2)NDipp) Dipp = 2,6-iPr(2)C(6)H(3)). Computational analysis of the bonding in these and a fluoro analogue, [U(L)(N{SiMe(3)}(2))(2)F], provides new information on the covalency in this relative rare oxidation state for molecular cerium complexes. Computational studies reveal increased Mayer bond orders in the actinide carbene bond compared with the lanthanide carbene bond, and natural and atoms-in-molecules analyses suggest greater overall ionicity in the cerium complexes than in the uranium analogues.

CH Bond Activation by f-Block Complexes

Most homogeneous catalysis relies on the design of metal complexes to trap and convert substrates or small molecules to value-added products. Organometallic lanthanide compounds first gave a tantalizing glimpse of their potential for catalytic C-H bond transformations with the selective cleavage of one C-H bond in methane by bis(permethylcyclopentadienyl)lanthanide methyl [(η(5) -C5 Me5 )2 Ln(CH3 )] complexes some 25 years ago. Since then, numerous metal complexes from across the periodic table have been shown to selectively activate hydrocarbon C-H bonds, but the challenges of closing catalytic cycles still remain; many f-block complexes show great potential in this important area of chemistry.

Uranium−Nitrogen Multiple Bonding: Isostructural Anionic, Neutral, and Cationic Uranium Nitride Complexes Featuring a Linear U═N═U Core

Reaction of the uranium(III) tris(anilide) complex (THF)U(N[t-Bu]Ar)(3) (1, THF = tetrahydrofuran; Ar = 3,5-Me(2)C(6)H(3)) with MN(3) (M = Na, [N(n-Bu)(4)]) results in the formation of the bimetallic diuranium(IV/IV) complexes M[(mu-N)(U(N[t-Bu]Ar)(3))(2)] (M[3]), which feature a single nitride ligand engaged as a linear, symmetric bridge between two uranium centers. The stability of the U=N=U core across multiple charge states is illustrated by stepwise chemical oxidation of Na[3] to the diuranium(IV/V) complex (mu-N)(U(N[t-Bu]Ar)(3))(2) (3) and the diuranium(V/V) complex [(mu-N)(U(N[t-Bu]Ar)(3))(2)][B(Ar(F))(4)] {[3][B(Ar(F))(4)]; Ar(F) = 3,5-(CF(3))(2)C(6)H(3)}. M[3], 3, and [3][B(Ar(F))(4)] were characterized by NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The cyclic voltammogram of 3 reveals two clean, reversible one-electron electrochemical events at E(1/2) = -1.69 and -0.67 V, assigned to the [3](-)/3 and 3/[3](+) redox couples, respectively. The X-ray crystal structures of [N(n-Bu)(4)][3], 3, and [3][B(Ar(F))(4)] reveal a linear U=N=U core that contracts by only approximately 0.03 A across the [3](n) (n = -1, 0, +1) series, an effect that is rationalized as being primarily electrostatic in origin. [3][B(Ar(F))(4)] reacts with NaCN, eliminating Na[B(Ar(F))(4)] and forming the known diuranium(IV/IV) cyanoimide complex (mu-NCN)(U(N[t-Bu]Ar)(3))(2), suggesting that the U=N=U core has metallonitrene-like character.