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

Four new methyloxorhenium(V) complexes were synthesized:  MeReO(PA)₂ (1), MeReO(HQ)₂ (2), MeReO(MQ)₂ (3), and MeReO(diphenylphosphinobenzoate)₂ (4) (in which PAH = 2-picolinic acid, HQH = 8-hydroxyquinoline, and MQH = 8-mercaptoquinoline). Although only one geometric structure has been identified crystallographically for 1, 2, and 3, two isomers of 3 and 4 in solution were detected by NMR spectroscopy. These compounds catalyze the sulfoxidation of thioethers by pyridine N-oxides and sulfoxides. The rate law for the reaction between pyridine N-oxides and thioethers, catalyzed by 1, shows a first-order dependence on the concentrations of pyridine N-oxide and 1. The second-order rate constants of a series of para-substituted pyridine N-oxides fall in the range of 0.27−7.5 L mol^-1 s^-1. 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, MeSC₆H₄-p-Y. The value of each rate was expressed relative to the reference compound that has Y = H. A Hammett analysis of k_Y/k_H gave ρ = −1.9. Oxygen-18 labeled 1 was used in a single turnover experiment for 4-picoline N-oxide and dimethyl sulfide. No ¹⁸O-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 N-oxide to access the primary coordination sphere of rhenium. The second intermediate is a cis-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, SR₂. Water inhibits the reaction because it hydrolyzes the rhenium(VII) intermediate.