A structural investigation of the coordination behaviour of 4,6-dimethylpyrimidine-2-thiol with Zinc (II) at pH 6 – A zwitterionic complex exhibiting unusual ZnS3N2 chromophore in solid state

Complexation between Zn2+ and 4,6-dimethylpyrimidine-2-thiol was attempted with ZnO, ZnSO4 and Zn(OAc)2. In all cases, the same compound, tris-4,6-dimethylpyrimidine-2-thiolato zinc (II) was generated without anion incorporation. It crystallized from methanol in the space group P21/a, Z = 4. The molecular structure exemplifies an uncommon coordination of 3 ligands to Zn(II) through their thiolate centres; one of the ligands is protonated to effectively form a zwitterionic complex, which exhibits capped rectangular pyramidal coordination geometry of an anionic ZnN2S3 chromophore. In crystalline state, the ligand moieties differ in respect of orientations and interactions while in solution they behave as identical components of a symmetric molecule. The crystal structure is stabilized by π stacking and periodic H-bonding interactions along a. The presence of a protonated azomethine N in one of the ligands is confirmed from the high conductance and acidic pH of an aqueous methanolic solution of the complex.