Synthesis and characterization of ruthenium terpyridine dioxolene complexes:: Resonance equilibrium between RuIII-catechol and RuII-semiquinone forms

A series of [RuX(dioxolene)(terpy)] (terpy = terpyridine; X = Cl, OAc) and one-electron oxidized complexes were prepared. The molecular structures of [RuCl(O2C6H2-3,5-Bu-2)(terpy)] (1) and [Ru(OAc)(O2C6H4) (terpy)] (3) were determined by X-ray crystallography. Crystal data for 1: monoclinic, space group P2(1)/c, Z = 8, a = 11.548(1), b = 18.224(5), c = 30.002(8) Angstrom, beta = 96.51(2)degrees, and R = 0.077 (R-w = 0.068). Crystal data for 3. monoclinic, space group C2/c, Z = 8, a = 13.355(5), b = 12.131(4), c = 26.645(4) Angstrom, beta = 92.46(2)degrees, and R = 0.041 (R-w = 0.041). Although the binding mode of O2C6H2-3,5-Bu-2 to Ru was not determined by the molecular structure of 1, the carbon-oxygen and carbon-carbon bond lengths of O2C6H4 in 3 were consistent with those of catecholato ligands. Electronic absorption spectra of [(RuX)-X-II(dioxolene)(terpy)] were explained by the electronic structure of [(RuX)-X-II(semiquinone)(terpy)] rather than [(RuX)-X-III(catecholato) (terpy)], while the reverse assignment was deduced from the IR spectra. Moreover, ESR spectra showed hyper-fine structures due to the contribution of semiquinone superimposed on an axial pattern of the Ru(III) center, indicating a resonance equilibrium between [(RuX)-X-II(semiquinone)(terpy)] and [RuX(dioxolene)(terpy)].