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

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 (tau, 0.49-0.88) with Co-N-amine and Co-N-Py bond distances of 2.104-2.254 angstrom and 2.043-2.099 angstrom, 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 degrees C, and a yield of 19% and TON of 191 at 25 degrees 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 (H2O2)-O-18 showed an 89% incorporation of O-18, 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.