Mono- and dinuclear oxidovanadium(v) complexes of an amine-bis(phenolate) ligand with bromo-peroxidase activities: synthesis, characterization, catalytic, kinetic and computational studies

The mono- and dinuclear oxidovanadium(v) complexes [V^VO(L¹)(Cl)] (1) and [L¹V^VO(μ₂-O)VO(L¹)] (2) of ONNO donor amine-bis(phenolate) ligand (H₂L¹) were readily synthesized by the reaction between H₂L¹ and VCl₃.(THF)₃ or VO(acac)₂ in MeOH or MeCN, respectively, and then characterized through mass spectroscopy, ¹H-NMR and FTIR techniques. Both the complexes possess distorted octahedral geometry around each V centre. Upon the addition of 1 equivalent or more acid to a MeCN solution of complex 1, it immediately turned into the protonated form, which might be in equilibrium as: [L¹ClV^V[double bond, length as m-dash]OH]⁺ ↔ [L¹ClV^V-OH]⁺ (in the case of [L¹ClV^V[double bond, length as m-dash]OH]⁺ oxo-O is just protonated, whereas in [L¹ClV^V-OH]⁺ it is a hydroxo species), with the shift in λ_max from 610 nm to 765 nm. Similar was the case for complex 2. The complexes 1 and 2 could efficiently catalyze the oxidative bromination of salicylaldehyde in the presence of H₂O₂ to produce 5-bromo salicylaldehyde as the major product with TONs of 405 and 450, respectively, in the mixed solvent system (H₂O : MeOH : THF = 4 : 3 : 2, v/v). The kinetic analysis of the bromide oxidation reaction indicated a first-order mechanism in the protonated peroxidovanadium complex and a bromide ion and limiting first-order mechanism on [H⁺]. The evaluated k^Br and k^H values were 5.78 ± 0.20 and 11.01 ± 0.50 M^-1 s^-1 for complex 1 and 6.21 ± 0.13 and 20.14 ± 0.72 M^-1 s^-1 for complex 2, respectively. The kinetic and thermodynamic acidities of the protonated oxido species of complexes 1 and 2 were pK_a = 2.55 (2.35) and 2.16 (2.19), respectively, which were far more acidic than those reported by Pecoraro et al. for peroxido-protonation instead of oxido protonation. On the basis of the chemistry observed for these model compounds, a mechanism of halide oxidation and a detailed catalytic cycle are proposed for the vanadium haloperoxidase enzyme and these were substantiated by detailed DFT calculations.