Formation of cobalt(II)–piperazine supramolecular systems under different organic acid mediums: synthesis, characterization and crystal structures

The reactions of CoCl2 salt and piperazine (L) in the presence of different organic acid mediums afford a series of cobalt(II) complexes, the solid structures (determined by X-ray diffraction techniques) of which are controlled by the nature of the specific acid. When terephthalic acid was used, a novel mononuclear molecule [Co(HL)2Cl3]·Cl·(H2O) (1) was obtained [orthorhombic, Pnma, a=24.604(16) Å, b=9.882(7) Å, c=6.609(4) Å, Z=4], in which the CoII center takes the unusual compressed trigonal-bipyramidal coordination geometry (CoN2Cl3) and multiple hydrogen-bonding interactions extend this structure into a three-dimensional supramolecular network. However, with the replacement of terephthalic acid with malonic acid in the above reaction, a one-dimensional coordination polymer [CoLCl2]n (2) was obtained [monoclinic, P21/n, a=6.363(3) Å, b=10.244(5) Å, c=12.124(5) Å, β=103.560(8)°, Z=4], in which the tetrahedral CoII centers (CoCl2N2) are linked by the bidentate-coordinated piperazine molecules to form a zigzag chain array and these coordination chains are further expanded to a three-dimensional hydrogen-bonding architecture. In addition, a mononuclear complex [Co(gly)3]·(H2O) (3) [monoclinic, P21/c, a=6.261(5) Å, b=14.280(9) Å, c=12.182(8) Å, β=101.450(14)°, Z=4] was yielded when glycine was used in the similar procedure. A three-dimensional framework is also observed through the hydrogen-bonding interactions between [Co(gly)3] moieties, in which the guest water molecules are included. These results unequivocally indicate that the nature of the organic acid templates play the key role in formation of these complexes.