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

摘要

The mono-and dinuclear oxidovanadium(V) complexes [(VO)-O-V(L-1)(Cl)] (1) and [(LVO)-V-1-O-V(mu(2)-O)VO(L-1)] (2) of ONNO donor amine-bis(phenolate) ligand (H2L1) were readily synthesized by the reaction between H2L1 and VCl3 center dot(THF)(3) or VO(acac)(2) in MeOH or MeCN, respectively, and then characterized through mass spectroscopy, H-1-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: [(LClVV)-Cl-1=OH](+) - [(LClVV)-Cl-1-OH](+) (in the case of [(LClVV)-Cl-1=OH](+) oxo-O is just protonated, whereas in [(LClVV)-Cl-1-OH](+) it is a hydroxo species), with the shift in lambda(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