Roles of phenol groups and auxiliary ligand of copper(II) complexes with tetradentate ligands in the aerobic oxidation of benzyl alcohol

Herein, six copper(II) complexes with multidentate ligands, [Cu(HL1)(OAc)(HOAc)] (1), [Cu(HL2)(OAc)] (2), [Cu(HL3)(OAc)] (3), [CuL4(OAc)] (4), [Cu(HL2) Cl] (5), and [Cu(HL3) Cl] (6) {H2L1 = [bis(3-tert-butyl-2-hydroxybenzyl)]( 2-pyridylmethyl) amine, H2L2 = [(3-tert-butyl-2-hydoxybenzyl)(3-trifluoromethyl-2-hydroxybenzyl) (2-pyridylmethyl)] amine, H2L3 = [bis(3-trifluoromethyl-2-hydroxybenzyl)] (2-pyridylmethyl) amine, and HL4 = [bis(2-pyridylmethyl)] (3-tert-butyl-2-hydroxybenzyl) amine}, are reported. The complexes were characterized by UV-vis spectroscopy, high-resolution mass spectrometry, X-ray single-crystal diffraction analysis and electrochemistry. These copper(II) complexes have been investigated as catalysts for the aerobic oxidation of benzyl alcohol to benzaldehyde mediated by TEMPO (2,2,6,6-tetramethyl-piperidinyl- 1-oxyl) radical in water at ambient temperature. Mechanistic investigations have revealed that the phenolate/phenol is involved in the intramolecular proton transfer with a bound substrate in catalysis. Hence, the presence of the trifluoromethyl group on the phenol ring significantly affects the catalysis process since the substituent affects the acidity of phenol, and subsequently, the intramolecular proton transfer from the bound substrate. During catalysis, the dissociation of the auxiliary ligand (Cl-or OAc-) occurred in the SN1 pathway, and it is necessary for the substrate to bind. To complete the catalytic cycle, the cleaved auxiliary ligand rebinds to the metal center to regenerate the catalyst.