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

The three complexes Zn(pyc)₂(H₂O)₂]·H₂O (1), (pipH₂)Cd(pyc)₃]·3H₂O (2) and Mg(pyc)₂(H₂O)₂]·H₂O (3) (pycH: pyridine-2-carboxylic acid, pip: piperazine) were prepared using a proton transfer compound (pipH₂)(pyc)₂ and corresponding metallic salts. The characterization was carried out using IR and NMR spectroscopies and single crystal X-ray diffraction. These complexes crystallize in the space group P2₁/n of the monoclinic system. Cell parameters of the complexes are a?=?9.769(2)?, b?=?5.157(1)?, c?=?14.539(3)? and ???=?90.205(3)° for (1); a?=?8.436(2)?, b?=?14.616(4)?, c?=?19.050(6)? and ???=?96.830(5)° for (2); a?=?11.639(6)?, b?=?8.796(5)?, c?=?14.936(8)? and ???=?107.221(1)° for (3). The crystal structures of (1) and (3) complexes illustrate that the metal ions are coordinated by two pyridine-2-carboxylate but in crystal structure (2) the metal ion is coordinated by three pyridine-2-carboxylate. The protonation constants of the building blocks of the pyridine-2-carboxylic acid?Cpiperazine adduct and the equilibrium constants for the reaction of pyridine-2-carboxylate with piperazine and the stoichiometry and stability of the Zn²⁺, Cd²⁺ and Mg²⁺ complexes with pycH in aqueous solution were accomplished 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 stoichiometry similar to that observed for the isolated crystalline complexes.