The Ru-IV=O-catalyzed sulfoxidation: a gated mechanism where O to S linkage isomerization switches between different efficiencies

Two Ru-IV=O catalysts with either a pentadentate bispidine ligand L-1 or a bidentate pyrazolate L-2/terpy L-3 combination of ligands have very different efficiencies as oxygen transfer catalysts for the selective oxidation of sulfides to sulfoxides: the [Ru-II(L-1)(solvent)](2+)/iodosyl benzene system has an initial TOF of approx. 40 h(-1) and quantitative yield, with [Ru-II(L-2)(L-3)(solvent)](+) the initial TOF is approx. 12 h(-1) with a maximum yield of approx. 60%. By experiment (cyclovoltametry) it is shown that there is S- to O-linkage isomerization of the RuII sulfoxide product complex, and this may partially switch off the catalytic cycle for the L-2/L-3-based catalyst. It emerges that the reasons for the reduced efficiency in the case of the L-2/L-3, in comparison with the L-1-based catalyst, are a more efficient linkage isomerization, a more stable S-bonded, in comparison with the O-bonded, Ru-II-based isomer, and inefficient ligand exchange in the product (hydrolysis produces the free sulfoxide and the Ru-II precatalyst). These interpretations are qualitatively in good agreement with preliminary DFT-based data.