Unexpected Behaviour of Pyridine‐2‐carbaldehyde Thiosemicarbazonatocopper( II ) Entities in Aqueous Basic Medium – Partial Transformation of Thioamide into Nitrile

Unusual species are obtained from the HL/CuII/NO3– system (HL = pyridine‐2‐carbaldehyde thiosemicarbazone) in basic aqueous solutions. A partial desulfurization process occurs for 1:1 Cu/HL molar ratio which gives rise to the precipitation of the mixed ligand [Cu(L)(LCN)] (1) complex [HLCN = C7H6N4, (pyridin‐2‐ylmethylene)hydrazinecarbonitrile]. This compound contains CuII ions in a square‐pyramidal environment. On the other hand, distorted octahedral [Cu(L)2] (2) entities are isolated in excess of the thiosemicarbazone ligand. Desulfurization of the thiosemicarbazone is detected even at physiological pH values. However, this reaction does not take place in absence of CuII ions, and HL·2.25H2O (3) or HL·H2O are obtained in these conditions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)