Synthesis and Properties of a Triruthenium Hydrido Complex Capped by a μ₃-Oxoboryl Ligand

A triruthenium hydrido complex in which one of the Ru3 planes is capped by a μ3-BO ligand, [{Cp*Ru(μ-H)}3(μ3-BO)(μ3-H)] (3; Cp* = η5-C5Me5), was synthesized by the reaction of the μ3-borylene complex [{Cp*Ru(μ-H)}3(μ3-BH)] (2a) with water in the presence of Et2NH. The face-capping coordination of the oxoboryl ligand was unambiguously established by X-ray diffraction and exhibited short B–O (1.231(6) Å) and long Ru–B bonds (average 2.31 Å). Density functional theory (DFT) calculations of 3 reproduced the observed structure well, and the multiplicity of the BO bond suggested by the value of the Wiberg bond index is 1.62. The four hydrido ligands in 3, three μ-hydrides and one μ3-hydride, underwent site exchange on the NMR time scale via the formation of a μ3-hydroxyborylene intermediate, [{Cp*Ru(μ-H)}3(μ3-BOH)] (2c). The DFT calculations showed that 2c lies above 3 by 10.5 kcal mol–1 at 25 °C. The basic oxygen atom in 3 allowed the formation of a B(C6F5)3 adduct, [{Cp*Ru(μ-H)}3{μ3-BO···B(C6F5)3}(μ3-H)] (5), in which the site exchange of hydrides was retarded considerably. Complex 3 reacted with PMe3 and CO to afford [(Cp*Ru)3(μ-BO)(μ-H)3(μ3-H)(PMe3)] (6) and [{Cp*Ru(CO)}3(μ-BO)(μ-H)2] (7), respectively, which demonstrated that the coordination mode of the BO ligand changed from a face-capping to an edge-bridging mode at the Ru3 site.