Reactions of thiosemicarbazones derived from β-keto amides and β-keto esters with Zn(ii) and Cd(ii) acetates: influence of metal, substitution, reagent ratio and temperature on metal-induced cyclization

Abstract

Zinc(II) and cadmium(II) acetates were reacted in methanol under various experimental conditions with thiosemicarbazones derived from β-keto amides or β-keto esters (HTSC). Some of these reactions afforded thiosemicarbazonate complexes [M(TSC)₂] with IR and NMR spectra compatible with N,S-coordination, but most gave complexes [ML₂], where HL is a substituted 2,5-dihydro-5-oxo-1H-pyrazole-1-carbothioamide resulting from cyclization of the HTSC. Some of these pyrazolonates and two of the HL ligands were studied by X-ray diffractometry, and their structures are discussed. Surprisingly, the reactions of zinc(II) acetate with HTSC in 1 ∶ 1 mol ratio usually gave a third, previously unreported type of complex with a dideprotonated ligand, [Zn(L − H)], which was also formed when [ZnL₂] and Zn(OAc)₂ interacted at room temperature in 1 ∶ 1 mol ratio. These L − H complexes are highly insoluble in all common solvents, which hinders their characterization but suggests that they are polymeric in nature.