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′CCCPh) ₄ (EE′ = {NPh} ₂, Mo ₂NN; {NPh}O, Mo ₂NO;{NPh}S, Mo ₂NS; OO, Mo ₂OO) have been synthesised by ligand exchange reactions of Mo ₂(O ₂CCH ₃) ₄ with the acid or alkali metal salt of {PhCCCEE′} ^-. The compounds Mo ₂NO, Mo ₂NS and Mo ₂OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo ₂NO adopts a cis-2,2 arrangement of the ligands about the Mo ₂ ⁴⁺ core, whereas Mo ₂NS 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 ₂OO > Mo ₂NO > Mo ₂NN > Mo ₂NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.