Synthesis of mixed-ligand complexes of VO²⁺ and VO³⁺ incorporating hydrazone, 1,10-phenanthroline and 8-hydroxyquinoline

<p>2-Aminobenzoylhydrazide (abh) reacts with equimolar amounts of either [V^IVO(acac)₂] or [V^IVO(bzac)₂] (where acac⁻ and bzac⁻ are the monoanionic forms of acetylacetone (Hacac) and benzoylacetone (Hbzac), respectively) in the presence of equimolar amounts of 1,10-phenanthroline (phen) to form the octahedral mixed-ligand complexes [V^IVO(L¹)(phen)] (<b>1</b>) and [V^IVO(L²)(phen)] (<b>2</b>), where (L¹)²⁻ and (L²)²⁻ are the dianionic forms of the 2-aminobenzoylhydrazone of acetylacetone (H₂L¹) and benzoylacetone (H₂L²). Upon substituting phen by 8-hydroxyquinoline (Hhq), pentavalent [V^VO(L¹)(hq)] (<b>3</b>) and [V^VO(L²)(hq)] (<b>4</b>) complexes were instead obtained. In the crystal structures of <b>3</b> and <b>4,</b> the hydrazone ligands coordinate to the vanadium center through the enolic-O, one imine-N and amide-O in a <i>mer</i> geometry. The amine and the second imine nitrogen form intramolecular hydrogen bonds. Complexes <b>1</b> and <b>2</b> display quasi-reversible one-electron oxidation peaks near +0.60 V, while the pentavalent <b>3</b> and <b>4</b> exhibit quasi-reversible one-electron reduction peaks near −0.18 V <i>versus</i> Ag/AgCl in CH₂Cl₂ solution. EPR spectroscopic studies on <b>1</b> and <b>2</b> suggest that the unpaired electron is present in the d_xy orbital. DFT studies for <b>3</b> indicate that the d_xy orbital of vanadium is the main contributor to the LUMO.