Jason B. Love

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.

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.

A macrocyclic approach to transition metal and uranyl Pacman complexes

Multielectron redox chemistry involving small molecules such as O2, H2O, N2, CO2, and CH4 is intrinsic to the chemical challenges surrounding sustainable, low-carbon energy generation and exploitation. Compounds with more than one metal reaction site facilitate this chemistry by providing both unique binding environments and combined redox equivalents. However, controlling the aggregation of metal cations is problematic, as both the primary coordination spheres of the metals and the metal-metal separations have to be defined carefully. We described recently a series of pyrrole-based macrocyclic ligands designed to manage metal aggregation and form molecular multimetallic complexes. In particular, we have shown that these compartmentalised Schiff-base calixpyrroles generally form rigid Pacman complexes that prescribe well-defined, metallo microenvironments within the molecular cleft. This article will review the development of this chemistry and its context, and will highlight structural facets and reaction chemistry of metal complexes from across the periodic table.

Encapsulation of a magnesium hydroxide cubane by a bowl-shaped polypyrrolic Schiff base macrocycle.

Hydrolysis of a Pacman-shaped binuclear magnesium complex of a polypyrrolic Schiff base macrocycle results in the formation of a new magnesium hydroxide cubane that is encapsulated by the macrocyclic framework through both coordinative and hydrogen-bonding interactions.

Zirconium complexes incorporating the new tridentate diamide ligand [(Me3 Si)N{CH2CH2N(SiMe3)}2]2–(L); the crystal structures of [Zr(BH4)2L] and [ZrCl{CH(SiMe3)2}L]

The lithium complex of the sterically demanding, polyfunctional amide ligand [(Me3Si)N-{CH2CH2N(SiMe3)}2]2–,Li2L reacted with [ZrCl4(thf)2](thf = tetrahydrofuran) to form dimeric [(ZrCl2L)2]2, in which the ligand is facially co-ordinated in a tridentate manner. Treatment of 2 with LiBH4 or Li[CH(SiMe3)2] generated [Zr(H3BH)L]3 and [ZrCl{CH(SiMe3)2}L]4, respectively. Variable-temperature NMR studies on 4 give evidence for restricted rotation of both the alkyl and amino SiMe3 groups. The crystal structures of 3 and 4 have been determined.

Binuclear Cobalt Complexes of Schiff-Base Calixpyrroles and Their Roles in the Catalytic Reduction of Dioxygen

The syntheses and characterization of a series of binuclear cobalt complexes of the octadentate Schiff-base calixpyrrole ligand L are described. The cobalt(II) complex [Co(2)(L)] was prepared by a transamination method and was found to adopt a wedged, Pac-man geometry in the solid state and in solution. Exposure of this compound to dioxygen resulted in the formation of a 90:10 mixture of the peroxo [Co(2)(O(2))(L)] and superoxo [Co(2)(O(2))(L)](+) complexes in which the peroxo ligand was found to bind in a Pauling mode in the binuclear cleft in pyridine and acetonitrile adducts in the solid state. The dioxygen compounds can also be prepared directly from Co(OAc)(2) and H(4)L under aerobic conditions in the presence of a base. The reduction of dioxygen catalyzed by this mixture of compounds was investigated using cyclic voltammetry and rotating ring disk electrochemistry and, in acidified ferrocene solutions, using UV-vis spectrophotometry, and although no formation of peroxide was seen, reaction rates were slow and had limited turnover. The deactivation of the catalyst material is thought to be due to a combination of the formation of stable hydroxy-bridged binuclear complexes, for example, [Co(2)(OH)(L)](+), an example of which was characterized structurally, and the catalytic resting point, the superoxo cation, is formed by a pathway independent of the major peroxo product. Collision-induced dissociation mass spectrometry experiments showed that, while [Co(2)(O(2))(L)]H(+) ions readily lose a single O atom, the resulting Co-O(H)-Co core remains resistant to further fragmentation. Furthermore, DFT calculations show that the O-O bond distance in the dioxygen complexes is not a good indicator of the degree of reduction of the O(2) unit and provide a reduction potential of ca. +0.40 V versus the normal hydrogen electrode for the [Co(2)(O(2))(L)](+/0) couple in dichloromethane solution.

Synthesis of bimetallic uranium and neptunium complexes of a binucleating macrocycle and determination of the solid-state structure by magnetic analysis.

Syntheses of the bimetallic uranium(III) and neptunium(III) complexes [(UI)(2)(L)], [(NpI)(2)(L)], and [{U(BH(4))}(2)(L)] of the Schiff-base pyrrole macrocycles L are described. In the absence of single-crystal structural data, fitting of the variable-temperature solid-state magnetic data allows the prediction of polymeric structures for these compounds in the solid state.

Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes

New, conformationally restricted ThIV and UIV complexes, [ThCl2(L)] and [UI2(L)], of the small-cavity, dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide (L)2− are reported and are shown to have unusual κ5:κ5 binding in a bent metallocene-type structure. Single-electron reduction of [UI2(L)] affords [UI(THF)(L)] and results in a switch in ligand binding from κ5-pyrrolide to η6-arene sandwich coordination, demonstrating the preference for arene binding by the electron-rich UIII ion. Facile loss of THF from [UI(THF)(L)] further increases the amount of U–arene back donation. [UI(L)] can incorporate a further UIII equivalent, UI3, to form the very unusual dinuclear complex [U2I4(L)] in which the single macrocycle adopts both κ5:κ5 and η6:κ1:η6:κ1 binding modes in the same complex. Hybrid density functional theory calculations carried out to compare the electronic structures and bonding of [UIIII(L)] and [UIII2I4(L)] indicate increased contributions to the covalent bonding in [U2I4(L)] than in [UI(L)], and similar U–arene interactions in both. MO analysis and QTAIM calculations find minimal U–U interaction in [U2I4(L)]. In contrast to the reducible U complex, treatment of [ThCl2(L)] with either a reductant or non-nucleophilic base results in metallation of the aryl rings of the macrocycle to form the (L−2H)4− tetraanion and two new and robust Th–C bonds in the –ate complexes [K(THF)2ThIV(μ-Cl)(L−2H)]2 and K[ThIV{N(SiMe3)2}(L−2H)].

Exploiting outer-sphere interactions to enhance metal recovery by solvent extraction

Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.

Controlled Deprotection and Reorganization of Uranyl Oxo Groups in a Binuclear Macrocyclic Environment

Switching on uranium(V) reactivity: The silylated uranium(V) dioxo complex [(Me(3)SiOUO)(2)(L)(2)] (A) is inert to oxidation, but after two-electron reduction to [(Me(3)SiOUO)(2)(L)](2-) (1), it can be desilylated to form [OU(μ-O)(2)UO(L)(2)](2-) (2) with reinstated uranyl character. Removal of the silyl group uncovers new redox and oxo rearrangement chemistry for uranium, thus reforming the uranyl motif and involving the U(VI/V) couple in dioxygen reduction.