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

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)(mu-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)(mu-MBH4)(L-A)(THF)(2)] 2-M (OAr = (OC6H2Bu3)-Bu-t-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 eta(5)-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 S-8 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)(mu-kappa(2):kappa(2)-CS3)(L-A)](4). The reaction between 2-K and S8 results in quantitative substitution of the endo-KBH4 by a bridging persulfido (S-2)(2-) group and oxidation of each U-III to U-IV, yielding [{U(OAr)}(2)(mu-kappa(2):kappa(2)-S-2)(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)(mu-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.