A novel thermally stable hydroperoxo–copper(ii) complex in a Cu(N2O2) chromophore of a potential N4O2 donor Schiff baseligand: synthesis, structure and catalytic studies

Abstract

The generation and study of metal–hydroperoxo/metal–peroxo (LCu^II–OOH or LCu^II–OO˙) complexes is a fascinating area of research of many chemical and biochemical researchers, because of their involvement as active intermediates in many biological and industrial catalytic oxidation processes. For this purpose we have designed a bulky hexa-coordinating ligand with potential N₄O₂ donor atoms which could provide an opportunity to synthesize a mononuclear Cu(II) complex with an aim to utilize it in the catalytic oxidation of aromatic hydrocarbons by an environmentally benign oxidant, H₂O₂. The Cu(II) complex (1) was structurally characterized and found to have square-planar geometry with the two pyrazolyl groups remaining in dangling mode. A novel mononuclear complex [Et₃NH][LCu^II–OOH] (2) was found to form in the reaction between 1 and H₂O₂ in the presence of Et₃N. The presence of this dangling groups favours the stability of hydroperoxo species, [LCu–OOH]⁻ (2) through H-bonding with the coordinated phenoxo oxygen atom, which was confirmed by ESI-MS⁺ and MS⁻ (m/z) mass analysis and DFT calculations. This complex was found to be thermally stable at room temperature [k_d = (5.67 ± 0.03) × 10⁻⁵ s⁻¹ at 25 °C] and may be due to the formation of O–O–H⋯O(phenoxo) H-bonding as delineated by the DFT calculations. Complex 1 was found to be an efficient catalyst for the oxidation of aromatic hydrocarbons to the corresponding aldehyde and alcohol in 2 : 1 mole ratio with TON ∼300.