Synthesis, crystal structures, spectroscopic, thermal analysis, electrochemical and solution studies of two new mixed metal coordination polymers based on dipicolinic acid and 3,4-diaminopyridine Zohreh Derikvand a,⇑ , Giuseppe Bruno b , Hadi Amiri Rudbari c , Ardeshir Shokrollahi d , Fereshteh Zarghampour d , Azadeh Azadbakht a a Department of Chemistry, Faculty of Science, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran b Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy c Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran d Department of Chemistry, Yasouj University, Yasouj, Iran article info Article history: Received 22 August 2013 Received in revised form 5 November 2013 Accepted 8 November 2013 Available online 18 November 2013 Keywords: Heterometallic coordination polymer Electrochemical behaviors Solution studies Dipicolinic acid abstract Two new 2D heteronuclear coordination polymers with Ni(II), Zn(II) and Ca(II), namely {(dapH) 2 [CaM 2 (pydc) 4 (H 2 O) 2 ]2H 2 O} n (M = Ni (1) and M = Zn (2)) where [pydcH 2 = pyridine-2,6-dicarboxylic acid (dipicolinic acid), dap = 3,4-diaminopyridine] have been synthesized and characterized by elemental analysis, infrared spectroscopy, thermal (TG/DTG/DTA) analysis, electrochemical as well as X-ray single crystal diffraction. The compounds 1 and 2 display isostructural features and crystallized in the monoclinic P2 1 /n space group. Both of the compounds contain cationic and anionic fragments and two uncoordinated water molecules. In 1 and 2 (pydc) 2 acts as tridentate ligands and connected to calcium ions through carboxylate bridging. Then each dipicolinate ligand bridges one calcium to a M(II) atom. The equilibrium constants of the binary, ternary and quaternary complexes of ligands or proton trans- fer system with metal ions in an aqueous solution were investigated by a potentiometric pH titration method. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction There is currently considerable interest in the design and con- struction of coordination polymers due to their potential applica- tions in gas adsorption, magnetism, luminescence and catalysis technologies, unusual electrochemical, photophysical and porous properties [1–3]. As we know, construction of coordination polymers (CPs) is mainly dependent on the metal centers and organic ligands [4]. The coordination features of the organic ligand including the coor- dination mode and orientation of the donor groups also play an important role in controlling CPs’ structures. Many reports demon- strate that the aromatic dicarboxylic acids acting as organic ligands have been extensively utilized in the preparation of coordination polymers [5–7]. Moreover, many other factors can affect the struc- tures of the final products such as the type of the ligand, pH value [8], temperature [9], and the solvent system [10]. Herein we designed two new isostructural heterobimetallic coordination polymers (Ni cation, Zn cation and Ca cation) based on pydcH 2 in the presence of 3,4-diaminopyridine as counter cat- ions. They were prepared with a traditional synthetic method. A general used strategy in building such heterometallic is that one kind of metal cations are assembled with ligands firstly, leaving one or more ligand sites open for subsequent bonding to the other types of metal cations [11]. Multifunctional ligands with many li- gand sites are good for this process. Pyridine-2,6-dicarboxylic acid (pydcH 2 ) is such a ligands with variety of coordination fashions [12] through its one nitrogen atom and four carboxylate oxygen atoms, diverse subsistent groups on the pyridine ring are possible to form hydrogen bonds, which is helpful for self assembly. The pydcH 2 is therefore an excellent candidate for constructing hetero- metallic complex. However, the construction of designed and pre- dictable heterometallic coordination frameworks with potential properties is still a formidable task because of the coordinative complexity of the heterometallic ions involved in the self-assembly process [13,14]. In this study we describe the synthesis, spectroscopic, elemen- tal and thermal analysis, crystal structures, electrochemical and 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.11.004 ⇑ Corresponding author. Tel.: +98 6616200399. E-mail address: zderik@yahoo.com (Z. Derikvand). Inorganica Chimica Acta 410 (2014) 221–229 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica