In this study, the solution chemistry and solidstate structures of a novel proton transfer compound, (pipzH2)(pyc)2H2O 1, and three metal–organic complexes of [Mn(pyc)2(H2O)2]22H2O 2, [Ni(pyc)2(H2O)2]H2O 3 and [Cu(pyc)2H2O]n 4, (pyc = pyridine-2-carboxylate, pipz = piperazine) were investigated. On the basis of crystallographic data, each MII ion is coordinated to pyridine- 2-carboxylate as bidentate ligand. Furthermore, these complexes form a three-dimensional structure by a variety of noncovalent interactions such as extensive O–HO, O–HN, N–HO, N–HN, C–HO hydrogen bonds, C–Hp and C–Op interactions. The equilibrium constants for the binary pyc-pipz proton transfer system, the stoichiometry and stability of complexation of this system with Mn2?, Ni2? and Cu2? ions in aqueous solution were investigated by potentiometric pH titration method. In each case, the stoichiometry of the dominant metal complex species in solution was compared with that obtained from the crystal structure.
