ORIGINAL RESEARCH Synthesis and Crystal Structure of [Ni(H 2 O) 2 (pdc) (4-CNpy)]  2H 2 O (pdc = Pyridine-2,6-dicarboxylate, 4-CNpy = 4-cyanopyridine) Manjit K. Bhattacharyya Æ Sanchay J. Bora Æ Birinchi K. Das Received: 10 July 2007 / Accepted: 16 October 2007 / Published online: 6 November 2007 Ó Springer Science+Business Media, LLC 2007 Abstract A new complex containing two different substituted pyridine ligands, viz. 4-CNpy (4-cyanopyridine) and pdc (pyridine-2,6-dicarboxylate), [Ni(H 2 O) 2 (pdc)(4- CNpy)]  2H 2 O(1), has been prepared using [Ni(4-CNpy) 2 (H 2 O) 3 (SO 4 )]  H 2 O(2) as the starting compound. Crystal data on 1 are: monoclinic, P2 1 /c, a = 11.533(2) A ˚ , b = 20.114(4) A ˚ , c = 7.306(1) A ˚ , b = 93.777(3) ° , V = 1,691.1(5) A ˚ 3 , Z = 2, D calc. = 1.57 g cm -3 , R 1 = 0.0585, wR 2 = 0.1236 for reflections with I [ 4r(F 0 ) and 268 parameters (R 1 = 0.0859, wR 2 = 0.1336 for all 4,024 reflections). The discrete molecules of the title complex remain hydrogen- bonded to assemble into supramolecular layers interleaved by disordered water molecules. In an attempt to obtain the cationic complex species cis-[Ni(H 2 O) 4 (4-CNpy) 2 ] 2+ we have isolated the compound [Ni(H 2 O) 4 (4-CNpy) 2 ]  (BPh 4 ) 2  2(4-CNpy)  4H 2 O(3) by adding NaBPh 4 to an aqueous solution of 2. Crystal data on 3 are: monoclinic, P2 1 / n, a = 10.5002(1) A ˚ , b = 21.1692(3) A ˚ , c = 15.3249(2) A ˚ , b = 95.120(1)°, V = 3392.84(7) A ˚ 3 , Z = 2, D calc. = 1.231 g cm -3 , R 1 = 0.0453, wR 2 = 0.1217 for reflections with I [ 4r (F 0 ) and 556 parameters (R 1 = 0.0688, wR 2 = 0.1364 for all 9,257 reflections). Keywords Nickel(II) complex  Pyridine-2,6-dicarboxylate  4-Cyanopyridine  Supramolecular layers  p-p interaction Introduction Preparation and structural characterization of metal- organic compounds involving substituted pyridine ligands have received attention from both the crystal engineering community as well as the materials chemists [1–3]. While the pyridyl-N atom strongly coordinates to metal ions, presence of hydrogen bond acceptors such as –CO 2 - and CN as substituents on the pyridine ring helps in various types of covalent and non-covalent associations that may lead to interesting supramolecular architectures [4]. We had previously described an interesting 3D supramolecular structure of complexes based on 4-pyridinecarboxylate (4-CO 2 py - ) anions as ligands [5]. In these complexes of type M(H 2 O) 4 (4-CO 2 py) 2 , the pyridine monocarboxylate ion acts as a pyridyl-N bound monodentate ligand. While the dangling –CO 2 - groups act as proton acceptors, the coordinated water molecules serve as excellent H-donors to form a robust hydrogen-bonded network in the solid state. Among pyridine ligands containing two carboxyl groups, pyridine-2,6-dicarboxylic acid (H 2 pdc, I) has frequently been utilized to prepare various coordination complexes. This particular substituted pyridine dicarbox- ylic acid binds to metal ions as either a monoanion [6, 7] or a dianion [6, 8–11]. In most cases, it occurs as a tridentate (O, N, O) ligand in the complexes for which X-ray crystal structures have been reported. Electronic supplementary material The online version of this article (doi:10.1007/s10870-007-9289-3) contains supplementary material, which is available to authorized users. M. K. Bhattacharyya  S. J. Bora  B. K. Das (&) Department of Chemistry, Gauhati University, Guwahati, Assam 781014, India e-mail: das_bk@rediffmail.com M. K. Bhattacharyya Department of Chemistry, Cotton College, Guwahati, Assam 781001, India 123 J Chem Crystallogr (2008) 38:195–203 DOI 10.1007/s10870-007-9289-3