Methyloxorhenium(V) Complexes with Two Bidentate Ligands:  Syntheses and Reactivity Studies

Four new methyloxorhenium(V) complexes were synthesized:  MeReO(PA)<sub>2</sub> (<b>1</b>), MeReO(HQ)<sub>2</sub> (<b>2</b>), MeReO(MQ)<sub>2</sub> (<b>3</b>), and MeReO(diphenylphosphinobenzoate)<sub>2</sub> (<b>4</b>) (in which PAH = 2-picolinic acid, HQH = 8-hydroxyquinoline, and MQH = 8-mercaptoquinoline). Although only one geometric structure has been identified crystallographically for <b>1</b>, <b>2</b>, and <b>3</b>, two isomers of <b>3</b> and <b>4</b> in solution were detected by NMR spectroscopy. These compounds catalyze the sulfoxidation of thioethers by pyridine <i>N</i>-oxides and sulfoxides. The rate law for the reaction between pyridine <i>N</i>-oxides and thioethers, catalyzed by <b>1</b>, shows a first-order dependence on the concentrations of pyridine <i>N</i>-oxide and <b>1</b>. The second-order rate constants of a series of para-substituted pyridine <i>N</i>-oxides fall in the range of 0.27−7.5 L mol<sup>-1</sup> s<sup>-1</sup>. Correlation of these rate constants by the Hammett LFER method gave a large negative reaction constant, ρ = −5.2. The next and rapid step does not influence the kinetics, but it could be explored with competition experiments carried out with a pair of methyl aryl sulfides, MeSC<sub>6</sub>H<sub>4</sub>-<i>p</i>-Y. The value of each rate was expressed relative to the reference compound that has Y = H. A Hammett analysis of <i>k</i><sub>Y</sub>/<i>k</i><sub>H</sub> gave ρ = −1.9. Oxygen-18 labeled <b>1 </b>was used in a single turnover experiment for 4-picoline <i>N</i>-oxide and dimethyl sulfide. No <sup>18</sup>O-labeled DMSO was found. We suggest that the reaction proceeds by way of two intermediates that were not observed during the reaction. The first intermediate contains an opened PA-chelate ring; this allows the pyridine <i>N</i>-oxide to access the primary coordination sphere of rhenium. The second intermediate is a <i>cis</i>-dioxorhenium(VII) species, which the thioether then attacks. Oxygen-18 experiments were used to show that the two oxygens of this intermediate are not equivalent; only the new oxygen is attacked by, and transferred to, SR<sub>2</sub>. Water inhibits the reaction because it hydrolyzes the rhenium(VII) intermediate.