[VIVO]/[VVO]/[VVO2] Complexes of Trinucleating Ligands: Synthesis, Characterization, Structural Evidence for a Hexanuclear Complex, and Their Catalytic Potential for the Synthesis of Pyrimido[1,2- a]benzimidazoles

Abstract

Ligands H₆btzh(fp)₃ (I), H₆btzh(mp)₃ (II), H₆btzh(pp)₃ (III), and H₆btzh(bp)₃ (IV) obtained by the condensation of benzene-1,3,5-tricarbohydrazide (H₃bthz) and 4-formyl-3-methyl-1-phenyl-5-pyrazolone (Hfp), 4-acetyl-3-methyl-1-phenyl-5-pyrazolone (Hmp), 4-butyryl-3-methyl-1-phenyl-5-pyrazolone (Hpp), and 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone (Hbp) have three tridentate dibasic ONO coordination functionalities. Three different types of trinuclear vanadium complexes have been isolated from these ligands. These are (i) tris{[V^IVO]²⁺} type [{V^IVO(H₂O)}₃{btzh(fp)₃}] (1), [{V^IVO(H₂O)}₃{btzh(mp)₃}] (2), [{V^IVO(H₂O)}₃{btzh(pp)₃}] (3), and [{V^IVO(H₂O)}₃{btzh(bp)₃}] (4); (ii) tris{[V^VO(OMe)]²⁺} type [{V^VO(OMe)}₃{btzh(fp)₃}] (5), [{V^VO(OMe)}₃{btzh(mp)₃}] (6), [{V^VO(OMe)}₃{btzh(pp)₃}] (7), and [{V^VO(OMe)}₃{(btzh(bp)₃)}] (8); and (iii) salts of tris{cis-[V^VO₂]⁺} type M₃[(V^VO₂)₃{btzh(fp)₃}]·6H₂O [M⁺ = K⁺ (9), Cs⁺ (10)], M₃[(V^VO₂)₃{btzh(mp)₃}]·6H₂O [M⁺ = K⁺ (11), Cs⁺ (12)], M₃[(V^VO₂)₃{btzh(pp)₃}]·6H₂O [M⁺ = K⁺ (13), Cs⁺ (14)], and M₃[(V^VO₂)₃{btzh(bp)₃}]·6H₂O [M⁺ = K⁺ (15), Cs⁺ (16)]. Different instrumental methods were used to characterize these complexes, including single-crystal X-ray for two of them. Dimerization through [V-(μ-O)-V] interactions of two independent units each of trinuclear 8 in the solid state gives a unique hexanuclear structure. These complexes catalyze the formation of biologically active pyrimido[1,2-a]benzimidazoles in excellent yields (up to 98%) within 5 min under mild conditions and are better than their mononuclear analogues. An intermediate of this reaction has also been isolated to propose a suitable reaction mechanism, which has further been supported by a DFT study.