Redox induced electron transfer in doublet azo-anion diradical Rhenium(II) complexes. Characterization of complete electron transfer series

Abstract

Reactions of dirhenium decacarbonyl with the two azoaromatic ligands, L^a=(2-phenylazo)pyridine and L^b=(4-chloro-2-phenylazo)pyridine (general abbreviation of the ligands is L) afford paramagnetic rhenium(II) complexes, [Re^II(L^-)₂(CO)₂] (1) (S=½ ground state) with two one-electron reduced azo-anion radical ligands in an octahedral geometrical arrangement. At room temperature (300 K) the complexes 1a-b, showed magnetic moments (μeff) close to 1.94 μ_B, which is suggestive of the existence of strong antiferromagnetic interactions in the complexes. The results of magnetic measurements on one of the complexes, 1b, in the temperature range 2-300 K are reported. The above complexes showed two cathodic and two anodic responses in cyclic voltammetry where one-electron oxidation leads to an unusual redox event involving simultaneous reduction of the rhenium(II) and oxidation of the second ligand via intramolecular electron transfer. The oxidized complexes 1a⁺ and 1b⁺ are air stable and were isolated as crystalline solids as their tri-iodide (I₃^-) salts. The structures of the two representative complexes, 1b and [1b]I₃, as determined by X-ray crystallography, are compared. The anionic complexes, [1]^- and [1]^2- were characterized in solution by their spectral properties.