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

Nickel( ii )complexes of N 4 -ligands have been synthesized and characterized as efficient catalysts for the hydroxylation of benzene using H 2 O 2 . All the complexes exhibited Ni 2+ → Ni 3+ 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 H 2 O 2 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 H 2 O 2 . 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 H 2 18 O 2 showed 92.46% incorporation of 18 O, revealing that H 2 O 2 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. Nickel( ii )complexes of N 4 -ligands are reported as efficient catalysts for direct benzene hydroxylation via bis(μ-oxo)dinickel( iii ) intermediate species. The exclusive phenol formation is achieved with a yield of 41%.