Formation and characterization of a reactive chromium(v)-oxo complex: mechanistic insight into hydrogen-atom transfer reactions.

A mononuclear Cr(v)-oxo complex, Cr(O)(6-COO-tpa) (; 6-COO-tpa = ,-bis(2-pyridylmethyl)--(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(iii) precursor complex with iodosylbenzene as an oxidant. Characterization of was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential () of was determined to be 1.23 V SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between and a one-electron donor, [Ru(bpy)] (bpy = 2,2'-bipyridine). The reorganization energy () of was also determined to be 1.03 eV in ET reactions from phenol derivatives to on the basis of the Marcus theory of ET. The smaller value in comparison with that of an Fe(iv)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of . When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e-oxidized products in moderate yields as determined from H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to . In sharp contrast, in the oxidation of trimethoxy-BA ( = 1.22 V) by , trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.