Multi-electron reduction of sulfur and carbon disulfide using binuclear uranium(iii) borohydride complexesElectronic supplementary information (ESI) available: Full synthetic and structural characterisation data. CCDC 1480072-1480076. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc00382j

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(BH 4 )} 2 (μ-BH 4 )(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-MBH 4 )(L A )(THF) 2 ] 2-M (OAr = OC 6 H 2 t Bu 3 -2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for 2-K , in addition to the endo -BH 4 ligand the potassium counter-cation is also incorporated into the cleft through η 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 CS 2 results in oxidation of each U III to the notably high U V oxidation state, forming the unusual trithiocarbonate (CS 3 ) 2− as a ligand in [{U(CS 3 )} 2 (μ-κ 2 :κ 2 -CS 3 )(L A )] ( 4 ). The reaction between 2-K and S 8 results in quantitative substitution of the endo -KBH 4 by a bridging persulfido (S 2 ) 2− group and oxidation of each U III to U IV , yielding [{U(OAr)} 2 (μ-κ 2 :κ 2 -S 2 )(L A )] ( 5 ). The reaction of 2-K with CS 2 affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS 2 ) 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. The first use of a dinuclear U III /U III complex in the activation of small molecules is reported.