Three Distinct Iron-Doped POM-Based Hybrid Composites for the Hydroxylation of Benzene to Phenol

Developing an efficient and stable heterogeneous catalyst for the one-pot hydroxylation of benzene to phenol under mild conditions is of critical importance. This study reports three Keggin-type POM-based organic-inorganic hybrid composites: (H₂pzpy)[Fe^II(pzpy)₂]₂[H₃PW^VI₉W^V₂Fe^IIO₄₀]·H₂O (<b>1</b>), [Fe^II(pzpy)₂(H₂O)₂]{[Fe^II(pzpy)]₂[HPMo^VI₈Mo^V₄O₄₀]}·3H₂O (<b>2</b>), and {[Fe^II(pzpy)₂(H₂O)]₂[PMo^VI₈Mo^V₃V^IVO₄₀(V^IVO)₂]}·2H₂O (<b>3</b>) (pzpy = 2-(1H-Pyrazol-3-yl)pyridine). Single-crystal analysis indicates that the structural commonality of compounds <b>1</b>-<b>3</b> is that the metal complexes formed by Fe(II) and pzpy ligands are grafted to different POM units via Fe-O bonds. Notably, compound <b>3</b> exhibits exceptional performance in benzene oxidation, achieving a phenol yield of up to 20.8% with a selectivity of 96%. Compared to compounds <b>1</b> and <b>2</b>, compound <b>3</b> shows significantly improved activity and stability. The outstanding performance of compound <b>3</b> can be ascribed to the fact that vanadium doping enhances the system's Fenton reaction, forming a strong synergistic effect among Fe, Mo, and V metals. Additionally, its stability is significantly enhanced by the highly centrosymmetric double-support double-vanadium-capped end-sealing configuration constructed by self-assembly.