Effect of protonated aminopyridines on the structural divergences of M(II)–malonate complexes (M = Cu, Ni, Co)

Three metal complexes containing malonate as primary ligand and 4-aminopyridine as secondary ligand (counter ion) namely, {(ampyH)2[Cu(mal)2]}n (1), (ampyH)2[Ni(mal)2(H2O)2] (2) and (ampyH)2[Co(mal)2(H2O)2] (3) [ampyH = protonated 4-aminopyridine, malH2 = malonic acid] have been synthesized from purely aqueous media and their crystal structures have been determined by single crystal X-ray diffraction. The copper complex is polymeric, whereas the nickel and cobalt complexes are monomeric in nature. The most striking feature of the present study is the observed ligand induced topological variations of the [Cu(mal)2]2− unit, which is the building block not only for 1, but for several other copper–malonate complexes previously synthesized by us. It is also observed that this influence of the ligand is not reflected in the primary coordination of the analogs Ni(II) and Co(II) complexes, as the same monomeric unit [M(II)(mal)2(H2O)2]2−, [M = Ni/Co] exists not only in 2 and 3, but also in other Ni and Co complexes prepared by us, although there are subtle differences in the overall supramolecular arrangements in the solid state of these complexes (Ni and Co). Starting with 2-amino-4-picoline molecule as an auxiliary ligand [S. Ray Choudhury, A.D. Jana, E. Colacio, H.M. Lee, G. Mostafa, S. Mukhopadhyay, Cryst. Growth Des. 7 (2007) 212], we have found that protonated heterocyclic bases (2-amino-4-picoline and 2-aminopyridine) have a preferred hydrogen bonded recognition motif R22(8) with metal carboxylate frameworks, which has an important role in guiding the crystal packing in the solid state. In the present study, we have extended our investigation by choosing 4-aminopyridine as an auxiliary ligand and discuss its effect in the overall self-assembly. A comprehensive comparison of the present complexes with our previously reported related complexes revealed the delicate nature of the self-assembly process.

M(II)-malonate (M = Cu, Ni, Co) frameworks adopt interesting topological diversity in the presence of various protonated aminopyridines.Figure optionsDownload as PowerPoint slide