ORIGINAL PAPER Characterization, crystal structures and solution studies of Zn(II), Cd(II) and Mg(II) complexes obtained from a proton transfer compound including pyridine-2-carboxylic acid and piperazine M. Ghadermazi • J. Soleimannejad • S. Sheshmani • M. Shamsipur • M. Ghanbari • M. R. Eslami Received: 18 September 2011 / Accepted: 2 December 2011 / Published online: 25 February 2012 Ó Iranian Chemical Society 2012 Abstract The three complexes [Zn(pyc) 2 (H 2 O) 2 ]H 2 O (1), (pipH 2 )[Cd(pyc) 3 ]3H 2 O(2) and [Mg(pyc) 2 (H 2 O) 2 ] H 2 O(3) (pycH: pyridine-2-carboxylic acid, pip: pipera- zine) were prepared using a proton transfer compound (pipH 2 )(pyc) 2 and corresponding metallic salts. The char- acterization was carried out using IR and NMR spectros- copies and single crystal X-ray diffraction. These complexes crystallize in the space group P2 1 /n of the monoclinic system. Cell parameters of the complexes are a = 9.769(2)A ˚ , b = 5.157(1)A ˚ , c = 14.539(3)A ˚ and b = 90.205(3)° for (1); a = 8.436(2)A ˚ , b = 14.616(4)A ˚ , c = 19.050(6)A ˚ and b = 96.830(5)° for (2); a = 11.639(6)A ˚ , b = 8.796(5)A ˚ , c = 14.936(8)A ˚ and b = 107.221(1)° for (3). The crystal structures of (1) and (3) complexes illustrate that the metal ions are coordinated by two pyridine-2-car- boxylate but in crystal structure (2) the metal ion is coordi- nated by three pyridine-2-carboxylate. The protonation constants of the building blocks of the pyridine-2-carboxylic acid–piperazine adduct and the equilibrium constants for the reaction of pyridine-2-carboxylate with piperazine and the stoichiometry and stability of the Zn 2? , Cd 2? and Mg 2? complexes with pycH in aqueous solution were accom- plished by potentiometric pH titration. The corresponding stability constants, stoichiometry and distribution of the species were determined with program BEST. The solution studies strongly support the self-association and stoichiom- etry similar to that observed for the isolated crystalline complexes. Keywords Zn(II), Cd(II) and Mg(II) complexes  Pyridine-2-carboxylic acid  Piperazine  Crystal structures  Solution studies Introduction The potential for constructing novel metal-containing systems that bridge the gap between classical coordination and organic supramolecular chemistry seems virtually unlimited [1]. The formation of discrete supramolecular entities driven and held together through metal coordi- nation has remained an intense area of study for the past decade. Non-covalent interactions form the backbone of supramolecular chemistry and include hydrogen bonds, p–p stacking, electrostatic, hydrophobic and charge- transfer interactions, as well as metal ion coordination [2]. Among these interactions, hydrogen bonds play a central role in the structure, function and dynamics of chemical and biological systems [3]. Creating new supramolecular architectures, tests and refines our understanding of the fundamental principles of molecular self-organization. The process of self-assembly and self-organization involves the non-covalent interactions of two or more molecular subunits to form a supramolecular arrangement, M. Ghadermazi (&) Department of Chemistry, Faculty of Science, University of Kurdistan, Sanandaj, Iran e-mail: mghadermazi@yahoo.com J. Soleimannejad School of Chemistry, College of Science, University of Tehran, Tehran, Iran S. Sheshmani Department of Chemistry, Shahr-e Rey Branch, Islamic Azad University, Tehran, Iran M. Shamsipur  M. Ghanbari Faculty of Chemistry, Razi University, Kermanshah, Iran M. R. Eslami Faculty of Chemistry, Tehran University, Tehran, Iran 123 J IRAN CHEM SOC (2012) 9:579–589 DOI 10.1007/s13738-012-0071-x