Synthesis and Characterization of Ruthenium Terpyridine Dioxolene Complexes: Resonance Equilibrium between Ru^III-Catechol and Ru^II-Semiquinone Forms

A series of [RuX(dioxolene)(terpy)] (teipy = terpyridine; X = Cl, OAc) and one-electron oxidized complexes were prepared. The molecular structures of [RuCl(O₂C₆H₂-3,5-Bu₂)(terpy)] (1) and [Ru(OAc)(O₂C₆H₄)(terpy)] (3) were determined by X-ray crystallography. Crystal data for 1: monoclinic, space group P2₁/c, Z=8, a = 11.548(1), b-18.224(5), c = 30.002(8) Å, β = 96.51(2)°, 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) Å, β = 92.46(2)°, and R = 0.041 (R_w = 0.041). Although the binding mode of O₂C₆H₂-3,5-Bu₂ 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(dioxolene)-(terpy)] were explained by the electronic structure of [Ru^IIX(semiquinone)(terpy)] rather than [Ru^IIIX(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 [Ru^IIX(semiquinone)(terpy)] and [RuX(dioxolene)(terpy)].