| Using the sterically bulky TptBu,Me ligand (Tp tBu,Me = hydrotris(3-tert-butyl-5methylpyrazolyl)borate), a series of superoxochromium(III) complexes was isolated from direct reactions of cis-divacant chromium (II) complexes with dioxygen. The crystal structures of [TptBu,MeCr(O2)(pz′H)]BARF (7), TptBu,MeCr(O2)Cl (10), and TptBu,MeCr(O2)pz′ (11) have been determined (pz′H = 3-tert-butyl-5-methylpyrazole; BARF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)). These superoxochromium(III) complexes adopted an unusual ‘side-on’ binding style, which is different from the traditionally described ‘end-on’ binding mode for superoxo complexes.; A mixture of one equivalent of the superoxochromium (III) compound ( 7), with one equivalent of Cr(II) precursor (1a) in ether solution generated [TptBu,MeCr(OH)(pz′H)]BARF (13). However, when organic compounds with weak C-H bonds, i.e. diethyl ether, were strictly excluded, the chromium(IV) mono-oxo compound, [TptBu,MeCr(O)(pz′H)]BARF (14), was isolated and structurally characterized.; from ordinary organic compounds, such as, 9,10-dihydroanthracene, THF, diethyl ether, and toluene. It is reduced to [TptBu,MeCr(OH)(pz organics. Kinetic studies of the reactions of 14 with DHA and toluene revealed that the hydrogen atom transfer from C-H to 14 was the rate determine step. The large values of kinetic isotope effect and the temperature-dependence of kinetic isotope effect in the reaction of DHA with 14 indicated that hydrogen atom transfer might involve a tunneling contribution. Hydrogen atom self-exchange between 14 and 13 is a slow process (kHSE = (5.6 ± 0.7) × 10−4 M−1 s−1 at 70°C) with an unusually large intrinsic barrier. |
