Electrochemistry of tris(quinoxaline-2,3-dithiolato)molybdate(IV) in acidic solution; multi-electron ligand-based redox activity

Abstract

The cyclic voltammetry of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)₃]^2–, NN-dimethylformamide solution, during titration with trifluoroacetic acid (Htfa), is reported. In aprotic solution two reversible, Nernstian, waves were detected, at E_½= 0.265 and –1.28 V, corresponding to the one-electron oxidation and reduction of the complex, respectively. On addition of Htfa, the oxidation process shifted to higher potential and became non-Nerstian, with E_½ dependent upon [Htfa], while the one-electron reduction of the complex was replaced by a three-electron, quasireversible wave at E_½=ca.–0.35 V. It is proposed that the latter process corresponds to the addition of one electron and one proton to each ligand, to form a complex containing three independent, co-ordinated quinoxalinium radicals. Therefore, [Mo(qdt)₃]^2– contains two distinct redox-active sites, the metal atom and the quinoxaline group of the qdt ligands.