Three Cu(II) complexes Cu(Rdpa)(MeCN)(ClO4)2 (1R, R = Me, Phe, t-Bu) were prepared with N3-tridentate ligands, based on bis(2-pycolyl)alkylamine (R-dpa), to examine the mechanism of cyclohexane (CyH) oxidation with H2O2 catalyzed by 1R. The crystal structures of 1R revealed that the steric hindrance of the R group strongly affects the bridging structure. Reaction analysis using the HO• trapping reagent DMPO revealed that the CyH oxidation proceeds in two reaction pathways via the formation of HO• and complex-based active species. The H2O2 activation of 1R was monitored by low-temperature stopped-flow techniques, where hydroperoxo Cu(II) and bis-μ-oxo Cu(III)2 species were formed as key intermediates, depending on the ease of bridging structure formation. Based on these results, two H2O2 activation pathways (Paths 1 and 2) are proposed. In Path 1, the hydroperoxo Cu(II) intermediate forms HO• as the active species, which shortens the catalyst lifetime due to nonselective oxidation of the supporting ligand. In Path 2, the bis-μ-oxo Cu(III)2 intermediate forms a μ-oxyl radical (μ-O•) bridged Cu(II)2 complex as a relevant complex-based active species which prolongs the catalyst lifetime to achieve a large turnover number (TON) in CyH oxidation. These findings pave the way for developing efficient Cu complex catalysts for alkane oxidation.
Fujikawa et al. (Thu,) studied this question.