Tuning the electronic structure of Mo-Mo quadruple bonds by N for O for S substitution

A series of quadruply bonded dimolybdenum compounds of form Mo(2)(EE'CC≡CPh)(4) (EE' = {NPh}(2), Mo(2)NN; {NPh}O, Mo(2)NO;{NPh}S, Mo(2)NS; OO, Mo(2)OO) have been synthesised by ligand exchange reactions of Mo(2)(O(2)CCH(3))(4) with the acid or alkali metal salt of {PhC≡CCEE'}(-). The compounds Mo(2)NO, Mo(2)NS and Mo(2)OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo(2)NO adopts a cis-2,2 arrangement of the ligands about the Mo(2)(4+) core, whereas Mo(2)NS adopts the trans-2,2 arrangement. The influence of heteroatom substitution on the electronic structure of the compounds was investigated using cyclic voltammetry and UV-Vis spectroscopy. Simple N for O for S substitution in the bridging ligands significantly alters the electronic structure, lowering the energy of the Mo(2)-δ HOMO and reducing the Mo(2)(4+/5+) oxidation potential by up to 0.9 V. A different trend is found in the optoelectronic properties, with the energy of the Mo(2)-δ-to-ligand-π* transition following the order Mo(2)OO > Mo(2)NO > Mo(2)NN > Mo(2)NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.