High selective hydroxylation of phenol catalyzed by PNP ligand‐containing [FeFe]‐hydrogenase model complexes

Abstract

AbstractBACKGROUNDDihydroxybenzene (DHB) which is an important raw material and intermediate of the organic chemical industry can be prepared by hydroxylation of phenol, so it promotes the study of highly selective catalyst for hydroxylation of phenol.RESULTSTwo novel PNP ((Ph2P)2NPh) ligand‐substituted di‐iron complexes were synthesized under controllable reaction conditions via carbon monoxide (CO)/ligand substitution of the parent complex (μ‐budt)Fe2(CO)6 [budt = SCH(CH3)CH2CH2)S]. The substituted complexes 1 and 2 were fully characterized by solution infrared spectroscopy, nuclear magnetoresonance spectroscopy, single crystal X‐ray diffraction, cyclic voltammetry and density functional theory calculations. Due to its favorable redox properties, Complex 1 was adopted first as an efficient catalyst for the hydroxylation of phenol to DHB (dihydroxybenzene) with hydrogen peroxide (H2O2) as oxidant and H2O as solvent. Under the optimum reaction conditions, the conversion of phenol achieved 42.0%, the selectivity of DHB was ≤88.6% and the catechol:hydroquinone ratio (mol mol−1) was 2.2. Furthermore, only trace amounts of benzoquinone, as a deeply oxidized product, was detected (yield c. 0.1%).CONCLUSIONComplex 1 could be a new type of catalyst for the hydroxylation of phenol to hydroquinone owing to the mild reaction conditions and excellent catalytic performance. In addition, a possible catalytic reaction mechanism was related to the special structure of Complex 1 which features an internal base of amine in PNP ligand.