Mononuclear zinc(II) and mercury(II) complexes of Schiff bases derived from pyrrolealdehyde and cysteamine containing intramolecular NH⋯S hydrogen bonds

Two neutral group 12 metal complexes, bis(pyrrol-2-ylmethyleneaminoethylthio)zinc(II) (1) and bis(pyrrol-2-ylmethyleneaminoethylthio)mercury(II) (2), with the (N_imine)₂S₂ coordination mode were synthesized by using metal-templated Schiff base condensation, and their molecular structures were determined by X-ray diffraction analysis. Complex 1 exhibits a distorted tetrahedral geometry around the metal, whereas the metal center has a bisphenoidal configuration in complex 2. Both mononuclear complexes possess intramolecular NH⋯S hydrogen bonds, as evidenced by IR, ¹H NMR and X-ray crystallography. The hydrogen-bond donor (H-N_pyrrole) and acceptor (S atom) are coming from different ligands within a single molecule. Complex 2 represents the first example of a mercury complex in the N₂S₂ coordination mode with intramolecular NH⋯S hydrogen-bond interactions. An investigation of the effects of the NH⋯S hydrogen bonding on the stability of 1 and 2, using an N-methyl pyrrolyl analogue, demonstrated that the N-H hydrogen-bond donor from the pyrrolyl moiety probably played a role in the stability of 1, but not 2.