Chemistry of ruthenium(II) complexes of N-substituted 1,2-benzoquinone diimines. Synthesis, structure and redox properties‡

Abstract

A series of ruthenium complexes of N-aryl-1,2-benzoquinone diimine chelates [Ru(acac)₂L] have been isolated via ruthenium promoted oxidative dimerization of arylamines. These compounds were obtained from the reaction of [Ru(acac)₃] and arylamines. The crystal structure of one representative case has been determined to authenticate the formation of the compound from a heretofore unknown chemical transformation. The structural data revealed a planar diimine L. The two C–N (imine) and two conjugate C–C double bonds in the quinonoid ring are localized and are indicative of the bivalent state of the metal ion. A plausible reaction pathway is discussed. Extended Hückel calculations on [Ru(acac)₂L] revealed that the metal–ligand overlap is high. The electronic transitions of the complexes are discussed based on the frontier MO diagram. There are multiple transitions in the range 1100 to 250 nm. The highly intense transition at ca. 520 nm has been assigned to a transition involving two heavily mixed metal–ligand orbitals. The diimine complexes undergo four successive one electron transfer processes. Two are metal centered, occurring at positive potentials, and two are ligand reductions at negative potentials. The E°₂₉₈ values of all the four redox processes are dependent on the nature of substitution of the quinone diimine ligand. The structure and physicochemical properties of these complexes are compared with those of [Ru(acac)₂(bqdi)] (bqdi = o-benzoquinone diimine) obtained from the reaction of [Ru(acac)₃] and o-phenylenediamine.