Template synthesis and crystal structure of an asymmetric square-planar complex: Pyridine-2,6-dicarbaldehyde-bis(S-methylisothiosemicarbazonato)nickel(II) iodide

The asymmetric complex [Ni(HL)]I (where H2 L is pyridine-2,6-dicarbaldehyde-bis(S-methylisothiosemicarbazone)) is synthesized by the [2 + 1] template condensation of S-methylisothiosemicarbazide hydroiodide with pyridine-2,6-dicarbaldehyde in the presence of nickel(II) acetate. The crystal structure of the [Ni(HL)]I complex (where HL is C11H14N7S2) is determined using X-ray diffraction. The square-planar coordination of the nickel(II) central atom is provided by four N donor atoms of the chelating ligand, namely, one N atom of the pyridine residue and three N atoms of the isothiosemicarbazide fragments. The deprotonated isothiosemicarbazide fragment in the imino form and the neutral ammonium isothiosemicarbazide fragment differ in the degree of deprotonation, the mode of coordination to the central atom (N1N3 and N2, respectively), and the conformation (E and Z, respectively). The structural features of the ammonium isothiosemicarbazide fragment are associated with the formation of zwitterions. It is established that the crystal structure of the compound under investigation contains centrosymmetric dimers. These dimers participate in the formation of the second coordination sphere N4S of the central atom