Kinetics and mechanisms of the oxidation of rhenium(V) oxo and thiolato complexes by alkyl hydroperoxides and orthoquinones

摘要

Mononuclear rhenium(V) compounds MeReO(mtp)NC5H4X where mtpH2 is 2-(mercaptomethyl)thiophenol have been prepared from MeReO(mtp)2 by the addition of pyridines with electron-donating substituents in the para or meta position; X = 4-Me, 4-But, 3-Me, 4-Ph, and H. Analogous compounds MeReO(edt)N5H4X, edtH2 = 1,2-ethanedithiol, were prepared similarly. The equilibrium constants for the reaction, Dimer + 2 Py = 2 M-Py, are in the range 2.5--31.6 x 102. Both groups of monomeric compounds react with quinones (phenanthrenequinone, PQ, and 3,5-tert-butyl-1,2-benzoquinone, DBQ) displacing the pyridine ligand and forming Re(VII) catecholate complexes. With PQ, the reaction M-Py + PQ = MPCat + Py is an equilibrium. The equilibrium constants, KQ, have been measured at 298 K. Second-order rate constants were determined in both directions. The reactions of M-Py complexes with DBQ proceeded to completion allowing the forward rate constants to be determined;Aqueous methyldioxorhenium (MDO), prepared in situ from methyltrioxorhenium (MTO) and hypophosphorous acid, abstracts an oxygen atom from tertiary alkyl hydroperoxides. This regenerates MTO and forms the tertiary alcohol with rate constants 3.71 x 104 L mol-1 s -1 (t-BuOOH) and 3.47 x 104 L mol -1 s-1 (t-AmOOH) at 25.0°C in aqueous 1.0 M HOTf. In a separate study, the concomitant slow decomposition of alkyl hydroperoxides and MTO (to ReO4-) was investigated. The rate law is v = k[MTO][ROOH]/[H+], with k = 7.4 x 10-5 s-1 (R = tBu) and k = 8.4 x 10-5 s-1 (R = t-Am) at 25.0°C in aq. solution at mu 1.0 M. 1H NMR spectroscopy and GC revealed organic products suggestive of radical reactions. The products from tBuOOH are acetone, methanol, tertbutyl methyl ether, methane, ethane, and tertbutyl methyl peroxide. With CH2DReO3, it could be shown that both t-BuOOH and MTO were sources of the methane.