Synthesis, characterization and catalytic activity of dioxidomolybdenum(VI) complexes of tribasic pentadentate ligands

Abstract

[MoVIO2(acac)2] (Hacac=acetylacetone) reacts with the tribasic pentadentate Schiff bases H3sal-dahp (I), H3clsal-dahp (II) and H3brsal-dahp (III) (sal=salicylaldehyde, clsal=5-chlorosalicylaldehyde, brsal=5-bromosalicylaldehyde, dahp=1,3-diamino-2-hydroxypropane) in methanol under refluxing conditions to yield the dioxidomolybdenum(VI) complexes [MoVIO2(Hsal-dahp)(H2O)] (1), [MoVIO2(Hclsal-dahp)(H2O)] (2) and [MoVIO2(Hbrsal-dahp)(H2O)] (3), respectively. In these complexes, only one set of phenolic oxygen and azomethine nitrogen atoms, along with the alcoholic oxygen atom of the ligands coordinate to the molybdenum. The reactions of these complexes with pyridine result in the formation of [MoVIO2(Hsal-dahp)(py)] (4), [MoVIO2(Hclsal-dahp)(py)] (5) and [MoVIO2(Hbrsal-dahp)(py)] (6). A single crystal X-ray study of [MoVIO2(Hbrsal-dahp)(DMSO)] (3a), grown by slow evaporation of a DMSO solution of 3, confirms the non-participation of one set of phenolic oxygen and azomethine nitrogen atoms of the ligand. The reaction of a methanolic solution of the in situ generated H3clsal-dahp with [MoVIO2(acac)2] in DMSO gave [MoVIO2(Hclsal-hdap)(DMSO)]4[Mo8O26]·6DMSO (7) (H2clsal-hdap=Schiff base obtained by the 1:1 condensation of 5-chlorosalicylaldehyde and 1,3-diamino-2-hydroxypropane), where the non-coordinated azomethine nitrogen hydrolyzes giving a free protonated amine group, as confirmed by a single crystal X-ray study. Complexes 1, 2 and 3 catalyze the oxidative bromination of styrene to yield 1,2-dibromo-1-phenylethane, 1-phenylethane-1,2-diol and 2-bromo-1-phenylethane-1-ol; therefore they act as functional models of vanadium dependent haloperoxidases. It has also been demonstrated that complexes 1, 2 and 3 are catalyst precursors for the oxidation of methyl phenyl sulfide and benzoin.