Single-step benzene hydroxylation by cobalt(ii) catalysts via a cobalt(iii)-hydroperoxo intermediate

Abstract

The cobalt(II) complexes of 4N tetradentate ligands have been synthesized and characterized as the catalysts for phenol synthesis in a single step. The molecular structure of the complexes showed a geometry in between square pyramidal and trigonal bipyramidal (τ, 0.49–0.88) with Co–N_amine and Co–N_Py bond distances of 2.104–2.254 Å and 2.043–2.099 Å, respectively. The complexes exhibited a Co²⁺/Co³⁺ redox potential around 0.489–0.500 V vs. Ag/Ag⁺ in acetonitrile. The complexes catalyzed hydroxylation of benzene using H₂O₂ (30%) and afforded phenol selectively as the major product. A maximum yield of phenol up to 29% and turnover number (TON) of 286 at 60 °C, and a yield of 19% and TON of 191 at 25 °C are achieved. This is the highest catalytic performance reported using cobalt(II) complexes as catalysts to date. This aromatic hydroxylation presumably proceeded via a cobalt(III)-hydroperoxo species, which was characterized by ESI-MS, and vibrational and electronic spectral methods. The formation of key intermediate [(L)Co^III(OOH)]²⁺ was accompanied by the appearance of the characteristic O → Co(III) ligand to metal charge transfer (LMCT) transition around 488–686 nm and vibration modes at 832 cm⁻¹ (O–OH) and 564 cm⁻¹ (Co–O). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral properties were calculated by TD-DFT calculations. These data are comparable to the experimental observations. The kinetic isotope effect (KIE) values (0.98–1.07) support the involvement of cobalt-bound oxygen species as a key intermediate. Isotope-labeling experiments using H₂¹⁸O₂ showed an 89% incorporation of ¹⁸O, revealing that H₂O₂ is the main oxygen supplier for phenol formation from benzene. The catalytic efficiencies of cobalt complexes are tuned by ligand architectures via their geometrical configurations and steric properties.