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

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posted on 23.08.2016, 13:04 by Subhashree P. Dash, Satabdi Roy, Monalisa Mohanty, M. Fernanda N. N. Carvalho, Maxim L. Kuznetsov, João Costa Pessoa, Amit Kumar, Yogesh P. Patil, Aurélien Crochet, Rupam Dinda
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)] (14) 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.