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Issue 8, 2018
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Mono- and dinuclear oxidovanadium(V) complexes of an amine-bis(phenolate) ligand with bromo-peroxidase activities: synthesis, characterization, catalytic, kinetic and computational studies

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Abstract

The mono- and dinuclear oxidovanadium(V) complexes [VVO(L1)(Cl)] (1) and [L1VVO(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: [L1ClVV[double bond, length as m-dash]OH]+ ↔ [L1ClVV–OH]+ (in the case of [L1ClVV[double bond, length as m-dash]OH]+ oxo-O is just protonated, whereas in [L1ClVV–OH]+ it is a hydroxo species), with the shift in λmax 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 kBr and kH 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 pKa = 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.

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

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Publication details

The article was received on 14 Dec 2017, accepted on 22 Jan 2018 and first published on 22 Jan 2018


Article type: Paper
DOI: 10.1039/C7DT04718E
Citation: Dalton Trans., 2018,47, 2799-2809
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    Mono- and dinuclear oxidovanadium(V) complexes of an amine-bis(phenolate) ligand with bromo-peroxidase activities: synthesis, characterization, catalytic, kinetic and computational studies

    M. Debnath, M. Dolai, K. Pal, S. Bhunya, A. Paul, H. M. Lee and M. Ali, Dalton Trans., 2018, 47, 2799
    DOI: 10.1039/C7DT04718E

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