Multi-electron reduction of sulfur and carbon disulfide using binuclear uranium(iii) borohydride complexes

Abstract

The first use of a dinuclear U^III/U^III complex in the activation of small molecules is reported. The octadentate Schiff-base pyrrole, anthracene-hinged ‘Pacman’ ligand L^A combines two strongly reducing U^III centres and three borohydride ligands in [M(THF)₄][{U(BH₄)}₂(μ-BH₄)(L^A)(THF)₂] 1-M, (M = Li, Na, K). The two borohydride ligands bound to uranium outside the macrocyclic cleft are readily substituted by aryloxide ligands, resulting in a single, weakly-bound, encapsulated endo group 1 metal borohydride bridging the two U^III centres in [{U(OAr)}₂(μ-MBH₄)(L^A)(THF)₂] 2-M (OAr = OC₆H₂^tBu₃-2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for 2-K, in addition to the endo-BH₄ ligand the potassium counter-cation is also incorporated into the cleft through η⁵-interactions with the pyrrolides instead of extraneous donor solvent. As such, 2-K has a significantly higher solubility in non-polar solvents and a wider U–U separation compared to the ‘ate’ complex 1. The cooperative reducing capability of the two U^III centres now enforced by the large and relatively flexible macrocycle is compared for the two complexes, recognising that the borohydrides can provide additional reducing capability, and that the aryloxide-capped 2-K is constrained to reactions within the cleft. The reaction between 1-Na and S₈ affords an insoluble, presumably polymeric paramagnetic complex with bridging uranium sulfides, while that with CS₂ results in oxidation of each U^III to the notably high U^V oxidation state, forming the unusual trithiocarbonate (CS₃)²⁻ as a ligand in [{U(CS₃)}₂(μ-κ²:κ²-CS₃)(L^A)] (4). The reaction between 2-K and S₈ results in quantitative substitution of the endo-KBH₄ by a bridging persulfido (S₂)²⁻ group and oxidation of each U^III to U^IV, yielding [{U(OAr)}₂(μ-κ²:κ²-S₂)(L^A)] (5). The reaction of 2-K with CS₂ affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS₂)²⁻ ligand bridging the two U centres (6a), but only the mono-bridged sulfido (S)²⁻ complex [{U(OAr)}₂(μ-S)(L^A)] (6) is isolated. The persulfido complex (5) can also be synthesised from the mono-bridged sulfido complex (6) by the addition of another equivalent of sulfur.