Novel bioinspired diiron core complexes with rigid chelating diphosphine ligands for highly efficient catalytic phenol hydroxylation

Abstract

Hydroquinone (HQ) and catechol (CAT) are the important raw materials in the chemical industry, which can be produced through a direct catalytic hydroxylation process using hydrogen peroxide as an oxidant. Herein, we report two novel diiron core complexes (μ-budt)Fe2(CO)5(DPEphos) (1) and (μ-budt)Fe2(CO)5(Xantphos) (2) supported by rigid heterocyclic xanthene-like diphosphine ligands which exhibit high selectivity and activity in phenol hydroxylation. The structures of complexes 1 and 2 were characterized by FT-IR, UV–vis, 1H NMR, 31P-NMR, X-ray crystallography, while the catalytic products were tested by using HPLC. The FT-IR and UV–vis spectra reveal that two diphosphine ligands exhibit similar impacts on the structure of parent complex (μ-budt)Fe2(CO)6 [budt = SCH(CH3)CH2CH2)S]. Despite the similarity of ligands’ impact on the structure of the parent complex, the catalytic behavior was strongly influenced by the electronic and steric effects. The factors influencing the catalytic activity were optimized, including catalyst dosage, phenol/ H2O2 molar ratio, reaction temperature, and time. Under the optimal conditions, 1 exhibits great catalytic performance with 57.7% phenol conversion and 49.6% DHB yield, both of which are remarkable. The mechanism of phenol hydroxylation using 1 as a catalyst was also proposed. Hydroxyl radicals (•OH) were generated in the catalytic reaction, as demonstrated by electron spin resonance (ESR).