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

The first use of a dinuclear U/U complex in the activation of small molecules is reported. The octadentate Schiff-base pyrrole, anthracene-hinged 'Pacman' ligand L combines two strongly reducing U centres and three borohydride ligands in [M(THF)][{U(BH)}(μ-BH)(L)(THF)] , (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 group 1 metal borohydride bridging the two U centres in [{U(OAr)}(μ-MBH)(L)(THF)] (OAr = OCH Bu-2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for , in addition to the -BH ligand the potassium counter-cation is also incorporated into the cleft through η-interactions with the pyrrolides instead of extraneous donor solvent. As such, has a significantly higher solubility in non-polar solvents and a wider U-U separation compared to the 'ate' complex . The cooperative reducing capability of the two U 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 is constrained to reactions within the cleft. The reaction between and S affords an insoluble, presumably polymeric paramagnetic complex with bridging uranium sulfides, while that with CS results in oxidation of each U to the notably high U oxidation state, forming the unusual trithiocarbonate (CS) as a ligand in [{U(CS)}(μ-κ:κ-CS)(L)] (). The reaction between and S results in quantitative substitution of the -KBH by a bridging persulfido (S) group and oxidation of each U to U, yielding [{U(OAr)}(μ-κ:κ-S)(L)] (). The reaction of with CS affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS) ligand bridging the two U centres (), but only the mono-bridged sulfido (S) complex [{U(OAr)}(μ-S)(L)] () is isolated. The persulfido complex () can also be synthesised from the mono-bridged sulfido complex () by the addition of another equivalent of sulfur.