Versatile Reactivity and Theoretical Evaluation of Mono- and Dinuclear Oxidovanadium(V) Compounds of Aroylazines: Electrogeneration of Mixed-Valence Divanadium(IV,V) Complexes

The substituted hydrazones H2L1–4 (L1–4 = dibasic tridentate ONO2– donor ligands) obtained by the condensation of 2-hydroxy-1-naphthaldehyde and 2-aminobenzoylhydrazine (H2hnal-abhz) (H2L1) , 2-hydroxy-1-naphthaldehyde and 2-hydroxybenzoylhydrazine (H2hnal-hbhz) (H2L2), 2-hydroxy-1-acetonaphthone and benzoylhydrazine (H2han-bhz) (H2L3), or 2-hydroxy-1-acetonaphthone and 2-aminobenzoylhydrazine (H2han-abhz) (H2L4) are prepared and characterized. Reaction of ammonium vanadate with the appropriate H2L1–4 results in the formation of oxidoethoxidovanadium­(V) [VVO­(OEt)­(L1–4)] (1–4) complexes. All compounds are characterized in the solid state and in solution by spectroscopic techniques (IR, UV–vis, 1H, 13C, and 51V NMR, and electrospray ionization mass spectrometry). Single-crystal X-ray diffraction analysis of 1, 3, and 4 confirms the coordination of the corresponding ligands in the dianionic (ONO2–) enolate tautomeric form. In solution, the structurally characterized [VVO­(OEt)­(L)] compounds transform into the monooxido-bridged divanadium­(V,V) [(VVOL)2-μ-O] complexes, with the processes being studied by IR and 1H, 13C, and 51V NMR. The density functional theory (DFT) calculated Gibbs free energy of reaction 2­[VVO­(OEt)­(L4)] + H2O ⇆ [(VVOL4)2-μ-O] + 2EtOH is only 2–3 kcal mol–1, indicating that the dinuclear complexes may form in a significant amount. The electrochemical behavior of the complexes is investigated by cyclic voltammetry, with the VV–VIV E1/2red values being in the range 0.27–0.44 V (vs SCE). Upon controlled potential electrolysis, the corresponding (L)­(O)­VIV–O–VV(O)­(L) mixed-valence species are obtained upon partial reduction of the [(VVOL)2-μ-O] complexes formed in solution, and some spectroscopic characteristics of these dinuclear mixed-valence complexes are investigated using DFT calculations and by electron paramagnetic resonance (EPR), with the formation of VIV–O–VV species being confirmed by the observation of a 15-line pattern in the EPR spectra at room temperature.