H₂O₂ Activation and Alkane Oxidation by Copper Complexes With R-dpa N₃-Tridentate Ligands: The Complex-Based Dicopper...

Three Cu(II) complexes [Cu(Rdpa)(MeCN)](ClO₄)₂ (1^R, 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 1^R. The crystal structures of 1^R 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 1^R 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.