Alkane Oxidation with H2O2 Catalyzed by Dicopper Complex with 6-hpa Ligand: Mechanistic Insights as Key Features for Methane Oxidation

Alkane oxidations with H2O2 catalyzed by copper complexes [Cu-2(mu-OH)(6-hpa)](3+) (1) and [Cu(MeCN)(tpa)](2+) (2) were examined. In the oxidation of cyclohexane (CyH), cyclohexyl hydroperoxide (CyO2H) was formed as the first product and converted to cyclohexanol (CyOH) with PPh3. The turnover frequency (TOF/h) and turnover number (TON) of 1 are 150 and 1030, respectively. The kinetic studies showed that the product formation rate, d[CyO2H]/dt, is proportional to [1] and [H2O2], and partly to [Et3N] and [H2O]. Solvent kinetic isotope effect k(H2O)/k(D2O) was 2.2, showing that a H2O molecule is involved in the rate-limiting step. tert-BuO2H disturbs the formation of a di(hydroperoxo) intermediate [Cu-2(O2H)(2)(6-hpa)](2+) to reduce the d[CyO2H]/dt. The active species [Cu-2(O center dot)(O-2 center dot)-(6-hpa)](2+) was detected by CSI MS. The inhibitory effects of a radical trap reagent DMPO and CO gas revealed that 1 suppresses the HO center dot formation. Methane oxidation with H2O2 catalyzed by 1, 2, and related complexes was conducted using a high-pressure reactor. Key features for the high catalytic activity of 1 in the methane oxidation are the complex-based active species [Cu-2(O center dot)(O-2 center dot)-(6-hpa)](2+) capable of cleaving the strong C-H bond of methane and the long catalyst life enabled by the suppression of the HO center dot formation.