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 Ni 2 (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) 2 Ni(μ–OH) 2 Ni(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 [Ni III (μ-O) 2 Ni III ] 2+ active species, which was generated in situ both through chemical and electrochemical oxidation of the hydroxy-bridged Ni II –Ni II 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.