Interligand communication in a metal mediated LL'CT system - a case study

A series of oxo-Mo(iv) complexes, [MoO(Dt^2-)(Dt⁰)] (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 (bdtCl₂); Dt⁰ = N,N'-dimethylpiperazine-2,3-dithione (Me₂Dt⁰) or N,N'-diisopropylpiperazine-2,3-dithione ( ⁱ Pr₂Dt⁰)), 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 Pr₂Dt⁰)] (6) demonstrates a large ligand fold angle of 62.6° along the S⋯S vector of the Dt⁰ 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 Dt⁰ 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 Dt⁰ ligand. Electronic communication between dithiolene ligands is facilitated by a Mo-monooxo center and distortion about its primary coordination sphere.