Mechanistic Insights and Catalytic Efficiency of a 2,2'-Bipyridine-Coordinated Peroxidovanadium Complex as a Haloperoxidase Biomimetic.

Abstract

The peroxidovanadium(V) complex [V^VO(O₂)(HO₂)(bpy)]·3H₂O·0.5bpy (1), where bpy = 2,2'-bypiridine, featuring η²-coordinated peroxide and hydroperoxide ligands, is reported as an efficient functional model of vanadium haloperoxidases (VHPOs). Structural and spectroscopic analyses indicate similarities between 1 and VHPO active sites, including peroxide ligand protonation. Mechanistic studies employing ab initio computational methods are based on the presence of [V^VO(O₂)(HO₂)(bpy)] and its aqueous equilibrium species [V^VO(O₂)(HO₂)(H₂O)], in solutions of 1 (pH = 5.8). For each compound, two reaction pathways are explored for the oxidation of iodide and bromide: 1) direct HOX, where X = Br or I, formation through nucleophilic attack on the protonated η²-peroxide, affording ΔG^‡= 20.0-26.5 kcal mol^-1 and 2) V-OX intermediate formation after the nucleophilic attack on the η²-peroxide resulting in ΔG^‡= 15.6-17.6 kcal mol^-1. Catalyst regeneration via end-on H₂O₂ coordination is exergonic (ΔG = -15.2 and -21.6 kcal mol^-1), indicating sustainable turnover. Complex 1 catalyzes the oxidative bromination of phenol red with a rate constant of 990 ± 90 mol^-2L²min^-1 and achieves high-yield halogenation of 8-hydroxyquinoline (73 and 86% for 5,7-dibromoquinolin-8-ol and 5,7-diiodoquinolin-8-ol) in mild conditions (30 °C, pH 5.8). The results highlight 1 as an efficient catalyst, with potential applications in the pharmaceutical and agrochemical industries.