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) Å). Protonation of the μ-oxo group of 1a expanded the Ru ⋯ Ru distance to 3.4490(9) Å, which was elucidated by X-ray crystallography, and one-electron oxidation of 1a also increases this length to 3.38 Å, determined by EXAFS analysis. These complexes interestingly showed, in cyclic voltammograms, a wide range of redox processes, [Ru^II₂] ↔ [Ru^IIRu^III] ↔ [Ru^III₂] ↔ [Ru^IIIRu^IV] ↔ [Ru^IV₂], 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.