Electronic effects of para substituents of 2-hydroxybenzaldehyde co-ligands in a family of hydrazonato-oxidovanadium(v) complexes

Abstract

Eight mixed-ligand hydrazonato-oxidovanadium(V) complexes of the type [V^VO(HL¹)(A^1–4)] (1–4) and [V^VO(HL²)(A^1–4)] (5–8) have been investigated, where (HL¹)²⁻ and (HL²)²⁻ represent the dianionic forms of the 2-hydroxybenzoylhydrazone of acetylacetone (H₃L¹) and benzoylacetone (H₃L²) respectively, and (A^1–4)⁻ represents the monoanionic forms of 2-hydroxybenzaldehyde (HA¹), 2-hydroxy-5-methylbenzaldehyde (HA²), 2-hydroxy-5-methoxybenzaldehyde (HA³) and 5-bromo-2-hydroxybenzaldehyde (HA⁴) respectively. The complexes were synthesized by refluxing [V^IVO(acac)₂] and [V^IVO(bzac)₂], where acac⁻ and bzac⁻ are the monoanionic forms of acetylacetone (Hacac) and benzoylacetone (Hbzac) respectively with an equimolar amount of 2-hydroxybenzoylhydrazide in methanol followed by the addition of either an equivalent amount or a slight excess of each HA. ¹H NMR spectra of complexes 1–3 and 5–7 in CDCl₃ indicated the existence of two isomeric forms arising from the interchange of the coordinating positions of the bidentate A⁻ ligand between axial and equatorial positions, a conclusion which is also supported by ⁵¹V NMR spectra. The vanadium atom in these complexes is in a six-coordinate distorted octahedral environment in which the dinegative hydrazone ligands in the enol form bind the vanadium in a tridentate meridional fashion utilizing their enolic-O, one imine-N and amide-O atoms, leaving the phenolic-O and other imine-N atoms unbonded although they do form intramolecular hydrogen bonds as is evident from IR, ¹H NMR and X-ray diffraction studies. Analysis of electronic spectra and redox potential (E_1/2) values of these complexes indicate that the λ_max values for the LMCT transition and the E_1/2 values are linearly related to the Hammett σ constants of the substituents in the monoanionic aryloxy ring of the A⁻ co-ligand. On keeping in methanol for ∼5 days, the monomers 1–8 are converted to dimethoxide bridged dimeric [V^VO(HL)(OCH₃)]₂ (9 and 10) complexes while oxido bridged dimeric [V^VO(HL)]₂O (11 and 12) complexes are obtained from dichloromethane or chloroform solution at room temperature after ∼5 days. On dissolution in CHCl₃ or CH₂Cl₂, the [V^VO(HL)(OCH₃)]₂ complexes converted to their respective monomeric [V^VO(HL)(OCH₃)] analogues and also to oxido bridged dimeric [V^VO(HL)]₂O species which is evident from their ¹H NMR, ⁵¹V NMR and mass spectra.