DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 183–190 183 5,5'-Dicyano-2,2'-bipyridine silver complexes: discrete units or co-ordination polymers through a chelating and/or bridging metal–ligand interaction He-Ping Wu, a Christoph Janiak,* a Gerd Rheinwald b and Heinrich Lang b a Institut für Anorganische und Analytische Chemie, Universität Freiburg, Albertstr. 21, D-79104 Freiburg, Germany. E-mail: janiak@uni-freiburg.de b Institut für Chemie, Zehrstuhl für Anorganische Chemie, Technische Universität Chemnitz, Strasse der Nationen 62, D-09111 Chemnitz, Germany Received 24th September 1998, Accepted 6th November 1998 The ambidentate ligand 5,5'-dicyano-2,2'-bipyridine (L) was found to function as a bi-, tri- or tetra-dentate chelate or chelate/bridging ligand in the co-ordination of silver ions. The mode of co-ordination depends on the anion and the crystallization conditions and was elucidated by single crystal X-ray diffractometry. With metal-co-ordinating anions such as NO 3 - and CF 3 SO 3 - a tridentate co-ordination mode of L is observed which involves the two bipyridine nitrogen donor atoms and one cyano group. The latter bridges to a neighboring silver center so that a one-dimensional co-ordination polymer results. For NO 3 - this co-ordination polymer forms a 2 1 helix. With less co-ordinating anions such as BF 4 - and PF 6 - monomeric bis-chelate complexes are obtained, where L assumes a bidentate co-ordination mode involving only the bipyridine nitrogen donor atoms. In the case of the PF 6 - anion a variation in the solvent of crystallization also produced a two-dimensional hexagonal co-ordination polymer where L functions as a tetradentate ligand using all four nitrogen donor atoms in chelation and bridging to the silver centers. Introduction Metal complexes of chelating 2,2'-bipyridine or bridging 4,4'-bipyridine or derivatives thereof are of constant and general interest in metal co-ordination chemistry. 1 4,4'-Bipyr- idine has recently gained considerable interest in the synthesis of (rectangular) two-dimensional network structures. 2 The generation of such frameworks is a promising path in the search for stable microporous metal–organic networks that exhibit reversible guest exchange and possibly selective catalytic activity. 3,4 Ligands containing CN-donors such as 4,4'-dicyanobi- phenyl, 1,3,5-tris(4-cyanophenylethynyl)benzene (TEB) and others are also excellent bridging ligands to synthesize porous N N N N CN NC NC CN CN N N CN NC co-ordination polymers. 5 A highlighted example is the co- ordination of TEB with AgSO 3 CF 3 in benzene that led to the isolation of the compound [Ag(CF 3 SO 3 )(TEB)]2C 6 H 6 , 6 which exhibits 15 Å channels and is porous to benzene exchange. We investigate here the co-ordination chemistry of 5,5'- dicyano-2,2'-bipyridine (L). This ambidentate ligand can be thought of as combining the ligating properties of the chelating 2,2'-bipyridine and the bridging 4,4'-dicyanobiphenyl ligand. The idea behind the use of such ambidentate ligands is to have cross-connecting blocks (tectons) for co-ordination polymers based on the endo chelation of two ligands with an appropriate metal center (1) or to supply functional donor atoms within the walls of the co-ordination polymer when the ligands are solely exo bridging (2). Our research has been concerned with the utilization of ambi- dentate endo-chelating/exo-bridging modified 2,2'-bipyridine ligands such as 5,5'-diamino-2,2'-bipyridine 7 or 2,2'-dimethyl- 4,4'-bipyrimidine 8 and ligands of the tris(pyrazolyl)borate type for the assembly of metal co-ordination polymers. 9,10 In this paper we describe the results of synthetic and structural studies of metal complexes with the ligand 5,5'-dicyano-2,2'- bipyridine (L) and try to elucidate the factors which lead to bridging, chelating or simultaneous bridging/chelating metal co-ordination. As a metal we have chosen silver() which together with copper() is a preferred ligand linker because of their favored tetrahedral co-ordination mode. 5,11–13 In addition, silver can also assume a linear co-ordination 13 and would thus be ideal for testing the formation of cross-linkers as depicted in 1. Results and discussion The synthesis of 5,5'-dicyano-2,2'-bipyridine as reported in the literature is the reaction of 2,2'-bipyridine-5,5'-dicarboxamide with POCl 3 in CHCl 3 under sonication (50 kHz) to give the product in 86% yield. 14 Another method is the sublimation of a mixture of 2,2'-bipyridine-5,5'-dicarboxamide and P 4 O 10