Catalytic promiscuity of mononuclear copper(II) complexes in mild conditions: Catechol and cyclohexane oxidations

Abstract

Two copper(II) mononuclear complexes were synthesized and investigated as promiscuous oxidative catalysts: [Cu(pymimi)Cl2]: [(2-(pyridyl-2-yl)ethyl)((1-methylimidazol-2-yl)methyl)imine](bischlorido)copper(II), C1, and [Cu(pymima)Cl2]: [(2-(pyridyl-2-yl)ethyl)((1-methylimidazol-2-yl)methyl)amine] (bischlorido)copper(II), C2. Both complexes were characterized by the following techniques: X-ray crystallography, decomposition point, elemental analysis, IR, UV–Vis and EPR spectroscopies, conductimetric analysis and electrochemistry. DFT calculations at the B3LYP level were carried out to corroborate structural, vibrational and spin Hamiltonian parameters (gx, gy and gz). TDDFT electronic spectra were calculated at CAMB3LYP level. Cyclohexane peroxidation promoted by C1 and C2 yielded the following overall conversions/turnover numbers: 48%/531s−1 and 46%/508s−1, respectively. Both complexes also catalyze the conversion of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) with turnover numbers of 6.0×10−2s−1 for C1 and 11.0×10−2s−1 for C2. Complexes C1 and C2 can be classified as substrate promiscuous catalysts due to their ability to promote the oxidation of two different substrates, in the presence of distinct oxidants (O2, for catechol and H2O2 for cyclohexane).