Coordination networks from the self-assembly of silver salts and the linear chain dinitriles NC(CH2)nCN (n = 2 to 7): a systematic investigation of the role of counterions and of the increasing length of the spacers

Abstract

We have systematically investigated the self-assembly of polymeric networks from six different Ag(I) salts and the flexible linear chain dinitriles NC(CH₂)_nCN (with n ranging from 2 to 7) in order to obtain some basic information useful for the crystal engineering of coordination frames upon variation of the counterions and of the length of the chains in the bidentate ligands. We have reacted the silver salts AgBF₄, AgClO₄, AgPF₆, AgAsF₆, AgSbF₆ and AgSO₃CF₃ with all the dinitriles in ethanol, using metal-to-ligand molar ratios from 1∶1 to 1∶2. All of the products have been structurally characterized, showing, for any combination, the formation of polymeric frameworks. With the exception of two derivatives of AgSO₃CF₃, all of the other species contain cationic networks with [Ag(dinitrile)₂]⁺ stoichiometry that display two distinct topologies, i.e. 3D diamondoid frames or 2D layers of (4,4)-topology. The diamondoid networks exhibit different degrees of interpenetration (4-fold, 5-fold and 6-fold) which are discussed. The (4,4)-layers form different structures, that include single non-interpenetrated sheets, showing varied stacking modes and interdigitation, 3D arrays deriving from inclined interpenetration of the sheets, and 2D systems of 3-fold parallel entangled layers (a quite unusual degree of interpenetration). Some trends are apparent: for instance, the structure type changes when the anion dimensions exceed some limits, and, moreover, the diamondoid topology is found only with dinitriles having an even number of carbon atoms. Other features, such as the correlation between the anionic size and the dimensions of the cages and of the square meshes, as well as the role of the ligand conformations on the degree of interpenetration, are also discussed.