[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.

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{[VIVO]2+} type [{VIVO(H2O)}3{btzh(fp)3}] (1), [{VIVO(H2O)}3{btzh(mp)3}] (2), [{VIVO(H2O)}3{btzh(pp)3}] (3), and [{VIVO(H2O)}3{btzh(bp)3}] (4); (ii) tris{[VVO(OMe)]2+} type [{VVO(OMe)}3{btzh(fp)3}] (5), [{VVO(OMe)}3{btzh(mp)3}] (6), [{VVO(OMe)}3{btzh(pp)3}] (7), and [{VVO(OMe)}3{(btzh(bp)3)}] (8); and (iii) salts of tris{cis-[VVO2]+} type M3[(VVO2)3{btzh(fp)3}]·6H2O [M+ = K+ (9), Cs+ (10)], M3[(VVO2)3{btzh(mp)3}]·6H2O [M+ = K+ (11), Cs+ (12)], M3[(VVO2)3{btzh(pp)3}]·6H2O [M+ = K+ (13), Cs+ (14)], and M3[(VVO2)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.