Stable Pentaammineruthenium(III) Complexes of Reductic Acids: Synthesis, Linkage Isomers, and Autoxidation Kinetics

Article abstract of DOI:10.1021/ja00213a023

Kinetically stable, monodentate, pentaammineruthenium(III) complexes of the reductate ligands ascorbate and tetramethylreductate have been synthesized and characterized spectroscopically and electrochemically, and the autoxidation kinetics have been studied.For all the complexes, axial or rhombic ESR spectra and magnetic susceptibilities in the range 1.8-1.9 μ_B indicate the Ru(III) oxidation state.Both TMRA and ascorbate yield two distinct complexes with closely parallel properties, which form in varying amounts and interconvert depending on the pH of the reaction with an apparent pK_a around 4.2. 1H NMR of the paramagnetic TMRA complexes suggest that the metal ion is closer to one pair of methyl groups in the lower pH form than that in the higher pH form, consistent with the coordination sites of O(2) and O(3) for the low- and high-pH forms, respectively.At low pH this isomerisation takes place with a specific rate of 8.0 1E-4 s^-1, and the resulting complex dissociates with a specific rate of 1.0 1E-4 s^-1.At neutral pH the O(2) to O(3) isomerisation occurs rapidly, and reductate ligand dissociation proceeds with a half-life of about 6 h.Cyclic voltammetric studies reveal reversible (on the CV time scale) couples (E⁰ values of 0.237 V for <(Asc)(NH3)5Ru>⁺ and 0.306 V for <(TMRA)(NH3)5Ru>⁺), which exhibit a pH dependency consistent with 1e, 1H⁺ redox processes, suggesting that induced electron transfer to the metal ion occurs upon single-electron oxidation of the ascorbate ligand.Both the reductate complexes and a related squarate complex exhibit an irreversible reduction, apparently due to ligand loss from the Ru(II) complex.The rate of autoxidation of the reductate complexes at pH 7.0 and 25 deg C is first order in the ruthenium complex and first order in oxygen concentration.Specific rate constants at 25 deg C, ΔH(excit), and ΔS(excit.) for the ascorbate and TMRA complexes, respectively, are the following: 5.42 s^-1 M^-1, 60.9 kJ/mol, and -26.6 J/mol K; 2.51 s^-1 M^-1, 69.6 kJ/mol, and -5.5 J/mol K.