Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine ligands: Synthesis, structural investigation and catalytic evaluation

Abstract

Seven new dioxidomolybdenum(VI) complexes [MoO2L1(X)].X (1) and [MoO2L2-7(X)] (2-7) [Where X = EtOH in case of 1 and 5 and X = DMSO in case of 2-4 and 6, 7] of aroylazines containing a bulky 3-hydroxy- 2-naphthoic substituent, were isolated and structurally characterized. The aroylazine ligands H2L1-7 were derived from the condensation of 3-hydroxy-2-naphthoic acid hydrazide with several substituted aromatic aldehydes/ketones. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV-Vis and NMR spectroscopy. X-ray structures of 1-6 revealed that the ligands coordinate to the metal center as a dibasic tridentate ligand. Cyclic voltammetry of the complexes shows two irreversible reductive responses within the potential window -0.50 to -1.36 V, due to Mo-VI/Mo-V and Mo-V/Mo-IV processes. The synthesized complexes 1-7 were used as catalysts for the oxidation of benzoin, and for the oxidative bromination of salicylaldehyde, as a functional mimic of haloperoxidase. It was found that the percentage of conversion increased significantly in the presence of catalysts 1-7 which contained bulky substituents, and showed high percentage of conversion (>90%) with high turnover frequency (>1100 h(-1)) than previously reported catalysts. Benzil, benzoic acid and benzaldehyde-dimethylacetal were formed selectively for the oxidation of benzoin. Formation of 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde took place during the oxidative bromination of salicylaldehyde in presence of H2O2 as an oxidant and therefore 1-7 act as functional models of vanadium dependent haloperoxidases. (C) 2017 Elsevier B.V. All rights reserved.