Synthesis and Properties of Oxo-carboxylato- and Dioxo-Bridged Diosmium Complexes of Tris(2-pyridylmethyl)amine

A series of oxo-bridged diosmium complexes with tpa ligand (tpa = tris(2-pyridylmethyl)amine) are synthesized. The hydrolytic reaction of the mononuclear osmium complex [OsIIICl2(tpa)]PF6 in aqueous solution containing a sodium carboxylate yields a μ-oxo-μ-carboxylato-diosmium(III) complex, [OsIII2(μ-O)(μ-RCOO)(tpa)2](PF6)3 (R = C3H7 (1), CH3 (2), or C6H5 (3)). One-electron oxidation of 1 with (NH4)2CeIV(NO3)6 gives a mixed-valent [OsIIIOsIV(μ-O)(μ-C3H7COO)(tpa)2](PF6)4 complex (4). A mixed-valent di-μ-oxo-diosmium complex, [OsIIIOsIV(μ-O)2(tpa)2](PF6)3 (5), is also synthesized from 1 in an aerobic alkaline solution (pH 13.5). All the complexes exhibit strong absorption bands in a visible–near-infrared region based on interactions of the osmium dπ and oxygen pπ orbitals of the Os–O–Os moiety. The X-ray crystallographic analysis of 1, 3, and 4 shows that the osmium centers take a pseudo-octahedral geometry in the μ-oxo-μ-carboxylato-diosmium core. The mixed-valent osmium(III)osmium(IV) complex 4 has a shorter osmium–oxo bond and a larger osmium–oxo–osmium angle as compared with those of the diosmium(III) complex 1 having the same bridging carboxylate. Crystal structure of 5 reveals that the two osmium ions are bridged by two oxo groups to give an Os2(μ-O)2 core with the significantly short osmium–osmium distance (2.51784(7) Å), which is indicative of a direct osmium–osmium bond formation with the bond order of 1.5 (σ2π2δ2δ*2π*1 configuration). In the electrochemical studies, the μ-oxo-μ-carboxylato-diosmium(III) complexes exhibit two reversible OsIIIOsIII/OsIIIOsIV and OsIIIOsIV/OsIVOsIV oxidation couples and one irreversible redox wave for the OsIIIOsIII/OsIIOsIII couple in CH3CN. The irreversible reductive process becomes reversible in CH3CN/H2O (1:1 Britton–Robinson buffer; pH 5–11), where the {1H+/2e–} transfer process is indicated by the plot of the redox potentials against the pH values of the solution of 1. Thus, the μ-oxo-μ-butyrato-diosmium(III) center undergoes proton-coupled electron transfer to yield a μ-hydroxo-μ-butyrato-diosmisum(II) species. The di(μ-oxo) complex 5 exhibits one reversible OsIIIOsIV/OsIVOsIV oxidation process and one reversible OsIIIOsIV/OsIIIOsIII reduction process in CH3CN. The comproportionation constants Kcom of the OsIIIOsIV states for the present diosmium complexes are on the order of 1019. The values are significantly larger when compared with those of similar oxo-bridged dimetal complexes of ruthenium and rhenium.