Electrochemical activity of a New Class of Luminescent Tricarbonyl Rhenium(I) Complexes Containing Bridging 1,2 Diazine Ligands

A novel class of luminescent tricarbonyl rhenium(I) dinuclear complexes of general formula [Re2(μ-X)2(CO)6(μ-diaz)] (X = halogen and diaz = 1,2-diazine), featuring a diazine ligand and two halide atoms bridging two rhenium centers, has been investigated by cyclic voltammetry in acetonitrile, together with the relevant free ligands. All the complexes but the phthalazine derivative feature a chemically and electrochemically reversible ligand-centered reduction process, as well as a reversible metal-centered bielectronic oxidation process. The localization of the active centers was supported by the analysis of the inductive effects in the investigated molecular structure series, in terms of Hammett parameters of the ligand substituents and of the electronegativity of the halide ligands, both resulting in very good linear correlations; theoretical HOMO and LUMO calculations confirmed the interpretation of the CV data. Moreover, the voltammetric investigation afforded neat monitoring of the increasing competition in metal coordination between polar solvent and diazine ligand, leading, in cases 4, 5 and 6, to the hydrolysis of the dinuclear complex with formation of two molecules of single metal complex including acetonitrile instead of the diazine ligand. The observed hydrolysis rate increased with increasing number of chloride substituents on the diazine ring, weakening the electron-donating ability of the nitrogen atoms in the pyridazine ring.