One step phenol synthesis from benzene catalysed by nickel(ii) complexes

Abstract

Nickel(II)complexes of N₄-ligands have been synthesized and characterized as efficient catalysts for the hydroxylation of benzene using H₂O₂. All the complexes exhibited Ni²⁺ → Ni³⁺ oxidation potentials of around 0.966–1.051 V vs. Ag/Ag⁺ in acetonitrile. One of the complexes has been structurally characterized and adopted an octahedral coordination geometry around the nickel(II) center. The complexes catalysed direct benzene hydroxylation using H₂O₂ as an oxygen source and afforded phenol up to 41% with a turnover number (TON) of 820. This is unprecedentedly the highest catalytic efficiency achieved to date for benzene hydroxylation using 0.05 mol% catalyst loading and five equivalents of H₂O₂. The benzene hydroxylation reaction possibly proceeds via the key intermediate bis(μ-oxo)dinickel(III) species, which was characterized by HR-MS, vibrational and electronic spectral methods, for almost all complexes. The formation constant of the key intermediate was calculated to be 5.61–9.41 × 10⁻² s⁻¹ by following the appearance of an oxo-to-Ni(III) LMCT band at around 406–413 nm. The intermediates are found to be very short-lived (t_1/2, 73–123 s). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral properties were calculated by TD-DFT calculations, which are comparable to experimental spectral data. The kinetic isotope effect (KIE) values (0.98–1.05) support the involvement of nickel-bound oxygen species as an intermediate. The isotope-labeling experiments using H₂¹⁸O₂ showed 92.46% incorporation of ¹⁸O, revealing that H₂O₂ is the key oxygen supplier to form phenol. The catalytic efficiencies of complexes are strongly influenced by the geometrical configuration of intermediates, and stereoelectronic and steric properties, which are fine-tuned by the ligand architecture.