Aromatic C-H hydroxylation reactions catalysed by nickel(ii) complexes of monoanionic ligands

The selective conversion of benzene to phenol in a single step under mild reaction conditions could serve as a viable alternative to the cumene process. Herein, we report the catalytic activity of nickel(ii) complexes of the type [Ni(L)(H₂O)]ClO₄ (1-3) containing monoamidate pentadentate N5 ligands (L1(H)-L3(H)) in the oxidation of aromatic C-H bonds using H₂O₂ as the oxidant. Catalytic conditions were optimised by studying the effect of catalyst loading, H₂O₂ amount, temperature and reaction time. Among the series, catalyst 1 showed the best activity under optimised reaction conditions with a phenol yield of 24.6%, selectivity of 98%, and a turnover number (TON) of 492. Ligand donor moiety influence on the catalytic performances was evident as the substitution of one pyridyl moiety in catalyst 1 with imidazolyl methyl (2) or 6-methyl pyridyl 1-methyl (3) groups resulted in diminished catalytic performance. Furthermore, kinetic isotopic effect (KIE = 1.04) and radical trapping studies ruled out discarded the involvement of hydroxyl or carbon-centred radicals in the oxidation reaction. Moreover, preferential phenol formation over side-chain oxidation, as well as the product distribution pattern obtained during the oxidation of other aromatic substrates, suggested the electrophilic aromatic substitution pathway mediated by a nickel-oxygen species.