Oxovanadium(V) and cobalt(III) complexes of dithiocarbazate-based Schiff base ligands: Formation of a thiadiazole ring by vanadium-induced cyclization of the coordinated ligand

S-Methyl 3-((2-hydroxyphenyl)methyl)dithiocarbazate (H2L1) and its bromo derivative (H2L2), which are traditionally biprotic tridentate (ONS) ligands, behave in an unprecedented manner when allowed to react with [VO(acac)(2)] under an oxidative environment in acetonitrile-water medium containing a catalytic amount of alkali metal ion. The products obtained are oxovanadium(V) compounds [VOL(L-cylic)] (L = L-1, 1a, and L-2, 1b) that contain one molecule of ligand which undergoes metal-induced cyclization to form a thiadiazole ring. Compound la crystallizes in the triclinic space group P (1) over bar with a = 9.1830(9) Angstrom, b = 9,4165(12) Angstrom, c = 12.700(2) Angstrom, alpha = 100.988(8)degrees, beta = 100.195(7)degrees, gamma = 78.774(8)degrees, V= 1046.3(2) Angstrom(3), and Z= 2. With cobalt(III), however, the products [CoL(HL)].H2O (L = L-1, 2a, and L-2, 2b) have hydrogen-bonded dimeric structures with each ligand virtually carrying 1.5 units of negative charge as confirmed by X-ray crystal structure analysis of 2a. It also crystallizes in triclinic space group P (1) over bar with a = 12.0842(8) Angstrom, b = 13.5251(9) Angstrom, c = 14.1960(10) Angstrom, alpha = 78.122(6)degrees, beta = 73.888(6)degrees, gamma = 78.255(6)degrees, V = 2154.7(3) Angstrom(3), and Z = 4. In solution, 2a is a symmetric molecule as indicated by H-1 NMR, involving a characteristic hydrogen-bonded O-H-O broad feature in the downfield (at 14.5 ppm) connecting both monoprotonated (LH-) and deprotonated (L2-) forms of the ligand-a situation somewhat analogous to the classic H-F-H case as observed in bifluoride ion.