Interligand communication in a metal mediated LL′CT system – a case study

A series of oxo-Mo(iv) complexes, [MoO(Dt(2−))(Dt(0))] (where Dt(2−) = benzene-1,2-dithiol (bdt), toluene-3,4-dithiol (tdt), quinoxaline-2,3-dithiol (qdt), or 3,6-dichloro-benzene-1,2-dithiol (bdtCl(2)); Dt(0) = N,N′-dimethylpiperazine-2,3-dithione (Me(2)Dt(0)) or N,N′-diisopropylpiperazine-2,3-dithione ((i)Pr(2)Dt(0))), possessing a fully oxidized and a fully reduced dithiolene ligand have been synthesized and characterized. The assigned oxidation states of coordinated dithiolene ligands are supported with spectral and crystallographic data. The molecular structure of [MoO(tdt)((i)Pr(2)Dt(0))] (6) demonstrates a large ligand fold angle of 62.6° along the S⋯S vector of the Dt(0) ligand. The electronic structure of this system is probed by density functional theory (DFT) calculations. The HOMO is largely localized on the Dt(2−) ligand while virtual orbitals are mostly Mo and Dt(0) in character. Modeling the electronic spectrum of 6 with time dependent (TD) DFT calculations attributes the intense low energy transition at ∼18 000 cm(−1) to a ligand-to-ligand charge transfer (LL′CT). The electron density difference map (EDDM) for the low energy transition depicts the electron rich Dt(2−) ligand donating charge density to the redox-active orbitals of the electron deficient Dt(0) ligand. Electronic communication between dithiolene ligands is facilitated by a Mo-monooxo center and distortion about its primary coordination sphere.