Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

Abstract

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaldehyde (H 2L 1) and 2-hydroxyacetophenone (H 2L 2) (general abbreviation, H 2L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H 2L and a bidentate NN donor [e.g., 2,2′-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [V IVO(L)(B)]; complexes ( 1)–( 4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [V VO(L)(hq)]; complexes ( 5) and ( 6). Aerial oxygen is most likely the oxidant (for the oxidation of V IV → V V) in the synthesis of pentavalent complexes ( 5) and ( 6). [V IVO(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [V VO(L)(hq)] complexes are diamagnetic. The X-ray structure of [V VO(L 2)(hq)] ( 6) indicates that H 2L 2 ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V–O (enolato) bond is longer than V–O (phenolato) bond by ∼0.07 Å and is identical with V–O (carboxylate) bond. 1H NMR spectrum of ( 6) in CDCl 3 solution indicates that the binding nature in the solid state is also retained in solution. Complexes ( 1)–( 4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes ( 5) and ( 6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near −0.10 V versus SCE in CH 2Cl 2 solution. The VO 3+–VO 2+ E 1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO 3+ binding via phenolato oxygen. For a given bidentate ligand, E 1/2 increases in the order: (L 2) 2− < (L 1) 2−.