Diruthenium complexes with hydrotris(1-pyrazolyl)borate face-capping ligands involving {Ru2(µ-O or µ-OH)(µ-carboxylato)2} cores†

Abstract

Reactions of [Ru2(RCOO)4Cl] (R = Me, Ph) with K[HBpz3] (HBpz3− = hydrotris(1-pyrazolyl)borate) afforded the oxo-bridged diruthenium(III) complexes, [Ru2(μ-O)(μ-RCOO)2(HBpz3)2] (R = Me (1a), Ph (1b)) in moderate yields. Complexes 1 were converted by treatment with HPF6 into the hydroxo-bridged diruthenium(III) complexes, [Ru2(μ-OH)(μ-RCOO)2(HBpz3)2](PF6) (R = Me (2a), Ph (2b)), and by reaction with (NH4)2Ce(NO3)6, to the oxo-bridged mixed-valence diruthenium(III, IV) complexes, [Ru2(μ-O)(μ-RCOO)2(HBpz3)2](PF6) (R = Me (3a), Ph (3b)). Complexes 1–3 were characterized by elemental analysis, IR, UV-Vis, and 1H NMR spectroscopies, and X-ray absorption and crystallographic analyses (1a and 2a). Complex 1a consists of a {Ru2(μ-oxo)(μ-carboxylato)2} core with two terminal face-capping HBpz3− ligands (Ru ⋯ Ru = 3.2544(7) A). Protonation of the μ-oxo group of 1a expanded the Ru ⋯ Ru distance to 3.4490(9) A, which was elucidated by X-ray crystallography, and one-electron oxidation of 1a also increases this length to 3.38 A, determined by EXAFS analysis. These complexes interestingly showed, in cyclic voltammograms, a wide range of redox processes, [RuII2] ↔ [RuIIRuIII] ↔ [RuIII2] ↔ [RuIIIRuIV] ↔ [RuIV2], which are coupled with protonation/deprotonation at the monoatom bridge. When complex 1b was reduced with sodium amalgam, the mononuclear ruthenium(II) complex [Ru(PhCOO)(HBpz3)(CH3CN)2] (4b) was isolated and characterized by spectroscopic techniques and EXAFS analysis. Upon exposure to air or dioxygen, complex 4b readily regenerated the oxo-bridged dimer 1b, and the oxo atom was confirmed to be derived from dioxygen by an isotopically-labeled experiment. The present results demonstrate the versatile redox states of diruthenium centres terminally capped by HBpz3− ligands.