Cyclopalladation of thiophene-substituted thiosemicarbazones

The thiosemicarbazones obtained from the condensation of thiosemicarbazide or 4,4′-dimethylthiosemicarbazide with 2-acetylthiophene react with K2PdCl4 in the presence of a base to give dark, poorly soluble materials in high yields. Treatment of these oligomeric compounds with Ph3P, dppe or dppf gives monomeric palladium(II) phosphine complexes in which the deprotonated thiosemicarbazones coordinate to the metal in a tridentate fashion through the C3-cyclometallated thiophene ring, the imine nitrogen atom and the sulphur atom. This coordination mode was confirmed by X-ray structure analysis of several derivatives.