Selective Direct Oxidation of Benzene to Phenol Catalyzed by Dinuclear Nickel Carbene Complexes

Phenol is industrially synthesized on an annual multimillion-ton scale through the cumene process, which involves three synthetic steps consisting of Friedel–Crafts alkylation of benzene, oxidation, and acidolysis. Transition-metal-catalyzed direct oxidation of benzene offers a single-step access to phenol, though overoxidation and low turnover numbers have limited the catalytic efficiency. Based on the proposed relevance of dimetallic oxo-bridged species, we investigated a series of nickel complexes with a Ni2(OH)2 core as catalyst precursors for the direct oxidation of benzene to phenol. Here, we show that complexes supported by N-heterocyclic carbene (NHC) ligands of the type [(NHC)2Ni(μ–OH)2Ni(NHC)2]2+ afford highly efficient catalysts for the direct oxidation of benzene to produce phenol with exquisite (>98%) selectivity. Mesoionic triazole-based carbenes impart higher activity than imidazole-derived NHCs and reach up to 36% phenol yield with thousands of turnover numbers, a considerable improvement to the hundreds for the current state-of-the-art nickel systems. The catalytic efficiency is limited by product inhibition, which is mitigated, to some extent, by using trifluoroethanol as a solvent. Detailed mechanistic investigation suggests that the reaction proceeds via an electrophilic oxidation pathway involving a [NiIII(μ-O)2NiIII]2+ active species, which was generated in situ both through chemical and electrochemical oxidation of the hydroxy-bridged NiII–NiII precursor and spectroscopically identified at low temperatures. Kinetic and mechanistic data as well as control experiments strongly suggest the integrity of the bimetallic nickel core as a quintessential motive for catalytic activity in benzene oxidation.