DFT supported experimental evaluation of air oxidation converting a thiosemicarbazone oxidovanadium(IV) to a dioxidovanadium(V) complex

Two novel vanadium complexes, viz., an oxidovanadium(IV) complex [VO(HL)(acac)] (1) and a dioxidovanadium(V) complex [VO₂(HL)(H₂O)] (2), were synthesized using the tridentate ONS-donor Schiff base ligand H₂L (N-(dehydroacetic acid)-4-phenyl-3-thiosemicarbazone). The complexes were characterized by elemental analysis, FT-IR, UV-Vis, EPR spectroscopy, cyclic voltammetry, magnetic susceptibility, conductance measurements, NMR (⁵ 1 V NMR, 1 H NMR, and 1 ³C NMR), and TGA. The combined experimental data support distorted octahedral geometries around vanadium and quasi-reversible redox behavior. Density functional theory (DFT) calculations at the B3LYP/LANL2DZ level provide corroboration for the experimental findings and offer insights into optimized molecular structures, frontier molecular orbitals, and global reactivity descriptors. Time-dependent DFT (TD-DFT) calculations were performed to assign the electronic transitions observed in the UV-Vis spectra. Natural bond orbital (NBO) analysis reveals significant charge transfer and stabilizing hyperconjugative interactions, elucidating the covalent nature of the metal-ligand bonds. The NBO findings are systematically correlated with molecular electrostatic potential (MESP) surfaces to provide a comprehensive picture of charge distribution and reactive sites. Theoretical evaluation of nonlinear optical (NLO) properties predicts that complex 1 exhibits a higher first hyperpolarizability (β₀ = 23.49 × 10 -3 1 esu) than complex 2 (17.39 × 10 -3 1 esu), which is attributed to its smaller HOMO-LUMO gap and more efficient intramolecular charge transfer. The predicted β₀ value for complex 1 is approximately 7.8 times that of urea (β₀ = 3.0 × 10 -3 1 esu), indicating its potential as a promising NLO material. While single-crystal X-ray structures are not yet available, powder X-ray diffraction was carried out for both complexes, and supports the proposed stoichiometries.