Symmetrical Bis-Hydrazone Ligand-Based Binuclear Oxido/Dioxido-Vanadium(IV/V) Complexes: Synthesis, Reactivity, and Catalytic Applications for the Synthesis of Biologically Potent 2‑Phenylquinazolin-4-(3H)‑ones

Symmetrical bis(hydrazone)-based ligands, H₄dar(bhz)₂ (I), H₄dar(fah)₂ (II), H₄dar(nah)₂ (III), and H₄dar(inh)₂ (IV) obtained from 4,6-diacetylresorcinol (H₂dar) and different hydrazides [benzoylhydrazide (Hbhz), isonicotinoylhydrazide (Hinh), nicotinoylhydrazide (Hnah), and 2-furoylhydrazide (Hfah)], were used to prepare potassium salts of binuclear cis-[V^VO₂]⁺ complexes, {K(H₂O)₂}₂[(V^VO₂)₂dar(bhz)₂] (1), {K(H₂O)₂}₂[(V^VO₂)₂dar(fah)₂] (2), {K(H₂O)₂}₂[(V^VO₂)₂dar(nah)₂] (3), and {K(H₂O)₂}₂[(V^VO₂)₂dar(inh)₂] (4), and binuclear [V^IVO]²⁺ complexes, [{V^IVO(MeOH)}₂dar(bhz)₂] (5), [{V^IVO(MeOH)}₂dar(fah)₂] (6), [{V^IVO(MeOH)}₂dar(nah)₂] (7), and [{V^IVO(MeOH)}₂dar(inh)₂] (8). In the presence of warm MeOH/DMSO (4:1), 3 changed to {K(H₂O)₂}[(V^VO₂)₂Hdar(nah)₂]·DMSO (3a·DMSO). Single crystal XRD studies of 1 and 3a confirm a binuclear structure along with a distorted square pyramidal geometry of each vanadium center where bis{ONO(2−)} ligands coordinate through phenolate-O, azomethine-N, and enolate-O atoms of each unit. While growing crystals of 6 in EtOH, part of it oxidizes and gives [{V^VO(OEt)}₂dar(fah)₂] (9) along with powdery 6. Complex 9 has a distorted octahedral structure. These complexes were used as catalysts for the synthesis of biologically important 2-phenylquinazolin-4-(3H)-ones having different aryl aldehydes, and they all show excellent catalytic performance (up to 97% yield) in less reaction time and low temperature, in the presence of 70% aqueous TBHP/30% aqueous H₂O₂ as a greener oxidant. Generally, these complexes perform better than their mononuclear analogues. Spectroscopy, DFT studies, and isolated intermediates have helped in proposing a suitable reaction mechanism for the catalytic reaction.