Kinetics and Magnetism of Phosphane Diadducts of Diruthenium(II,III) Tetraacetate

The kinetic parameters for the axial‐ligand‐substitution reactions of [Ru 2 (μ‐O 2 CCH 3 ) 4 (MeCN) 2 ] + in acetonitrile (MeCN) with PCy 3 and PCy 2 Ph (Cy = cyclohexyl and Ph = phenyl) were investigated under pseudo first‐order conditions of excess phosphane. The rate‐determining formation of [Ru 2 (μ‐O 2 CCH 3 ) 4 (MeCN)(PCy 2 R] + appears independent of the nature of the incoming phosphane with an average k 1 (25.0 °C) = (1.25 ± 0.06) × 10 3 M –1  s –1 . A dissociatively activated mechanism is proposed for substitution of the axially coordinated acetonitrile, and the rate parameters are compared to earlier measurements carried out on [Rh 2 (μ‐O 2 CCH 3 ) 4 (MeCN) 2 ]. Variable‐temperature magnetic susceptibility measurements were carried out on the two phosphane diadducts, [Ru 2 (μ‐O 2 CCH 3 ) 4 (PCy 3 ) 2 ]PF 6 , ( 1 ) and [Ru 2 (μ‐O 2 CCH 3 ) 4 (PCy 2 Ph) 2 ]PF 6 ( 2 ), and showed behaviour unique to a Boltzmann distribution of states over the temperature range 2–300 K. To support the magnetic measurements, the single‐crystal X‐ray structure of 1 was redetermined at various temperatures down to –150 °C and showed a distinct decrease in the Ru–Ru bond length with decreasing temperature, consistent with a σ 2 π 4 δ 2 π* 3 configuration at low temperature (ground state) and a σ 2 π 4 δ 2 π* 2 δ* 1 configuration at room temp. In addition, the structure redeterminations of 1 were better resolved than the earlier reported structure and also corrected a minor error in that the molecule of solvation was found to be 1,2‐dichloroethane instead of dichloromethane. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)