The Impact of Ligand Oxidation State and Fold Angle on the Charge Transfer Processes of Mo^IVO-Dithione Complexes

We report a series of mononuclear monooxo Mo(IV) complexes possessing either one or two fully oxidized dithiolene ligands; [MoOCl(R₂Dt⁰)₂][X], (1 and 2), and MoO(p-SC₆H₄Y)₂(R₂Dt⁰), (3 and 4), (R=Me, ⁱ Pr; X= PF₆, SbF₆, BF₄; Y= H, Cl, F, CF₃, Me, ^t Bu, OMe). Either four or two quasi-reversible ligand-based redox couples are detected depending upon the number of fully oxidized dithiolene ligands present. The molecular structure of 3 and 4 exhibit a large (47° to 70°) fold angle along the S•••S vector of the dithione ligand. The UV-Vis spectra of 3 and 4 exhibit a low energy charge transfer band at ~540 nm that are red-shifted ~200 nm compared to the spectra of 1 and 2. Density Functional Theory (DFT) calculations show that the low energy charge transfer band of 3 and 4 is heavily influenced by ligand fold angle. Reducing the fold angle decreases the charge transfer energy, and the transition becomes more ligand-based.