A Bioinspired Model for Copper Monooxygenase: A Direct Aromatic Hydroxylation Using O2

Abstract

A novel copper(I) complex, [CuI(L)(CH3CN)]CF3SO3 (1) (L = 1,1,2-tri(pyridin-2-yl)propan-1-ol), has been synthesized, characterized, and investigated as a bioinspired model for copper monooxygenases. Under aerobic conditions in CH3CN, complex 1 undergoes conversion to a dicopper complex, [(CuIIL)(CuIIL H)(SO3CF3)2]·CF3SO3·H2O (2), whose molecular structure reveals a Cu-Cu distance of 2.96 Å. A dicopper(II) complex, [(LCuII)2(SO3CF3)2] (3), has been synthesized for comparison, which exhibits a similar Cu-Cu distance of 2.97 Å. EPR spectroscopy has ascertained the solution state geometries of complexes 2 and 3, which displayed g׀׀ > g┴ values, indicative of distorted square pyramidal geometries consistent with their solid-state structures. Complex 1 selectively hydroxylates benzene in the presence of O₂ and Et₃N, affording 7% phenol based on the substrate, without any side products. However, the use of H2O2 as the oxygen source under identical conditions significantly increases the phenol yield to 19%. The catalytically active intermediates have been generated by the reaction of complex 1 with dioxygen showed an O (π*σ) → Cu ligand-to-metal charge transfer (LMCT) transition at 360 nm and a d-d transition at 650 nm. These spectral features are more pronounced with H2O2, showed a new LMCT transition at 360 nm and a very weak d-d transition at 689 nm. It is supported by solution FT-IR spectroscopy, which showed an O-O stretching frequency at 890 cm⁻¹ (DFT spectra at 829 cm⁻¹) corresponds to Cu-OOH intermediate. The structure of the [(L)CuII-OOH]+ species was optimized by DFT calculations. Kinetic isotope effect (KIE) studies using C6H6/C6D6 (1:1) (kH/kD = 1.03) and isotopic labeling experiments using H218O2 support our proposed mechanism of benzene hydroxylation. In contrast, dinuclear complexes 2 and 3 exhibited poor benzene hydroxylation activity even with H2O2 and yielded only 4% and 6% of phenol, respectively, along with by-products such as biphenyl and quinone under identical conditions.