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), Mo2NN; {NPh}O, Mo2NO;{NPh}S, Mo2NS; OO, Mo2OO) have been synthesised by ligand exchange reactions of Mo-2(O2CCH3)(4) with the acid or alkali metal salt of {PhC CCEE'}(-). The compounds Mo2NO, Mo2NS and Mo2OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo2NO adopts a cis-2,2 arrangement of the ligands about the Mo-2(4+) core, whereas Mo2NS 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-delta 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-delta-to-ligand-pi* transition following the order Mo2OO > Mo2NO > Mo2NN > Mo2NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.