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

Reactions of [Ru-2(RCOO)(4)Cl] (R=Me, Ph) with K[HBpz(3)] (HBpz(3)(-)=hydro tris(1-pyrazolyl)borate) afforded the oxo-bridged diruthenium(III) complexe s, [Ru-2(mu -O)(mu -RCOO)(2)(HBpz(3))(2)] (R=Me (1a), Ph (1b)) in moderate yields. Complexes 1 were converted by treatment with HPF6 into the hydroxo- bridged diruthenium(III) complexes, [Ru-2(mu -OH)(mu -RCOO)(2)(HBpz(3))(2)] (PF6) (R=Me (2a), Ph (2b)), and by reaction with (NH4)(2)Ce(NO3)(6), to the oxo-bridged mixed-valence diruthenium(III, IV) complexes, [Ru-2(mu -O)(mu -RCOO)(2)(HBpz(3))(2)](PF6) (R=Me (3a), Ph (3b)). Complexes 1-3 were charac terized by elemental analysis, IR, UV-Vis, and H-1 NMR spectroscopies, and X-ray absorption and crystallographic analyses (1a and 2a). Complex 1a cons ists of a {Ru-2(mu -oxo)(mu -carboxylato)(2)} core with two terminal face-c apping HBpz(3)(-) ligands (Ru . . . Ru=3.2544(7) Angstrom). Protonation of the mu -oxo group of 1a expanded the Ru . . . Ru distance to 3.4490(9) Angs trom, which was elucidated by X-ray crystallography, and one-electron oxida tion of 1a also increases this length to 3.38 Angstrom, determined by EXAFS analysis. These complexes interestingly showed, in cyclic voltammograms, a wide range of redox processes, [Ru-2(II)]<----> [(RuRuIII)-Ru-II]<----> [R u-2(III)]<----> [(RuRuIV)-Ru-III]<----> [Ru-2(IV)], which are coupled with protonation/deprotonation at the monoatom bridge. When complex 1b was reduc ed with sodium amalgam, the mononuclear ruthenium(II) complex [Ru(PhCOO)(HB pz(3))(CH3CN)(2)] (4b) was isolated and characterized by spectroscopic tech niques and EXAFS analysis. Upon exposure to air or dioxygen, complex 4b rea dily regenerated the oxo-bridged dimer 1b, and the oxo atom was confirmed t o be derived from dioxygen by an isotopically-labeled experiment. The prese nt results demonstrate the versatile redox states of diruthenium centres te rminally capped by HBpz(3)(-) ligands.