Chemistry of mixed-ligand methoxy bonded oxidovanadium(V) complexes with a family of hydrazone ligands containing VO 3+ core and their substituent controlled methoxy-bridged dimeric forms

Abstract

[V IVO(acac) 2] reacts with an equimolar amount of benzoyl hydrazones of 2-hydroxyacetophenone (H 2L 1), 2-hydroxy-5-methylacetophenone (H 2L 2) and 5-chloro-2-hydroxyacetophenone (H 2L 4) in methanol to afford the penta-coordinated mixed-ligand methoxy bonded oxidovanadium(V) complexes [V VO(L 1)(OCH 3)] ( 1), [V VO(L 2)(OCH 3)] ( 2), and [V VO(L 4)(OCH 3)] ( 4), respectively, whereas, the similar reaction with the benzoyl hydrazone of 2-hydroxy-5-methoxyacetophenone (H 2L 3) producing only the hexa-coordinated dimethoxy-bridged dimeric complex [V VO(L 3)(OCH 3)] 2 ( 3A). Similar type of hexa-coordinated dimeric analogue of 1 i.e., [V VO(L 1)(OCH 3)] 2 ( 1A) was obtained from the reaction of [V IVO(acac) 2] with the equimolar amount of H 2L 1 in presence of half equivalent 4,4′-bipyridine in methanol while the decomposition of [V IVO(L 2)(bipy)] complex in methanol afforded the dimeric analogue of 2 i.e., [V VO(L 2)(OCH 3)] 2 ( 2A). All these dimeric complexes 1A– 3A react with an excess amount of imidazole in methanol producing the respective monomeric complex. The X-ray structural analysis of 1–3 and their dimeric analogues 1A– 3A indicates that the geometry around the vanadium center in the monomeric form is distorted square-pyramidal while that of their respective dimeric forms is distorted octahedral, where the ligands are bonded to vanadium meridionally in their fully deprotonated enol forms. Due to the formation of bridge, the V–O(methoxy) bond in the dimeric complexes is lengthened to such an extent that it becomes equal in length with the V–O(phenolate) bond in 3A and even longer in 1A and 2A, which is unprecedented. The 1H NMR spectra of the complexes 1A– 3A in CDCl 3 solution, indicates that these dimeric complexes are converted appreciably into their respective monomeric form. Complexes are electro-active displaying one quasi-reversible reduction peak near +0.25 V versus SCE in CH 2Cl 2 solution. The E ½ values of the complexes show linear relationship with the Hammett parameter ( σ) of the substituents. All these VO 3+-complexes are converted to the corresponding complexes with V 2 O 3 4 + motif simply on refluxing them in acetone and to the complexes with VO 2 + motif on reaction with KOH in methanol.