A novel double-chain silver(I) coordination polymer: catena-poly[[[μ-aqua-aquadisilver(I)]-bis(μ3-5-methylpyrazine-2-carboxylato)] dihydrate]

In the title silver(I) coordination polymer, {[Ag2(C6H5N2O2)2(H2O)2]·2H2O}n, the [Ag2(μ2-H2O)(H2O)] cores are extended by antiparallel 5-methylpyrazine-2-carboxylate (L) ligands, forming a novel double-chain structure. Both Ag+ cations show a distorted square-pyramidal coordination. Ag1 is bonded to two water molecules, one L N atom, one N atom and one carboxylate O atom from a neighbouring L, whereas Ag2 is surrounded by two L N atoms, two L carboxylate O atoms and one bridging water molecule. O—H⋯O hydrogen-bonding interactions involving water clusters and carboxylate O atoms link the molecules into a three-dimensional supramolecular architecture, which is further consolidated by weak C—H⋯O interactions and π–π stacking interactions [centroid–centroid distance 3.643 (5) Å].


Comment
During the past two decades poly-carboxylic acid ligands have aroused great interest for chemists because many coordination polymers with this series of ligands have shown intriguing structures and potential applications in the optical, electric and magnetic areas. 5-methylpyrazine-2-carboxylic acid contains N and O donor atoms, which makes it a good building block for constructing functional materials. For example, Dong et al. (2000) use Cu(L) 2 (H 2 O) as a building block for constructing novel one-dimensional hetero-bimetallic Cu(II)-Ag(I) frameworks. Tanase et al. (2006) investigate the magnetic properties of Co(II), Ni(II) and Fe(II) compounds with HL, in structures where L is also involved in intricate supramolecular interactions. In this work we describe how using HL and corresponding silver(I) salts under hydrothermal conditions, a novel one-dimensional double silver(I) framework with Ag 2 (µ 2 -H 2 O)(H 2 O) cores can be isolated.
As is shown in Fig. 1, the title compound comprises two crystallographically independent silver(I) atoms, two deprotonated ligands L, one bridged coordinated water molecule, one terminal coordinated water molecule and two lattice water molecules. Ag1 is five-coordinated in the square-pyramidal geometry by two coordinated water molecules, one L nitrogen atom, one nitrogen atom and one carboxylate oxygen atom from a neighboring L. The coordinated water molecule O5 occupies the apical site and the other four atoms occupy the plane with the mean deviation of 0.0463 (1) Å. Ag1 lies above the plane at a distance of 0.3523 (2) Å. Ag2 is also five-coordinated in the square-pyramidal geometry by two L nitrogen atoms, two L carboxylate oxygen atoms and one bridged water molecule.
There exist two kinds of crystallographically different L ligands which make a dihedral angle of 13.786 (2)°. These ligands, in anti-parallel pairs, alternatively link Ag 2 (µ 2 -H 2 O)(H 2 O) cores, forming a novel one-dimensional double chain structure along the crystallographic [101] direction. The distances of Ag2-O5 and Ag2-O4 are longer than other Ag-O distances (Table 1). However all the Ag-N and Ag-O bond distances fall in the normal range.
The formation of this novel framework also reveals great potential in constructing silver(I) frameworks with HL. Solvent water molecules are key because they greatly affect the coordination geometries. Interestingly, although several Ni(II), Co(II) and Cd(II) compounds with HL have been prepared from solutions in water (Garribba et al., 2006;Shang et al.,2007;Ciurtin et al., 2003;Ciurtin et al., 2001;Ptasiewicz-Bak & Leciejewicz, 2000), such arrangement of different metal(II) coordination geometries induced by coordinated water molecules are not observed. This may be ascribed to the flexible and varied coordination geometries of silver atoms, i.e., a metal-directing effect.
The one-dimensional double chains of the title compound are extended into a three-dimensional supramolecular architecture by nine O-H···O hydrogen bonds ( Table 2). The detailed environments of the O-H···O interactions are represented in Fig. 2. Lattice water molecule O7 acts as hydrogen bond donors to lattice water molecule O8 forming binuclear water clusters. As shown in Fig. 3, O-H···O hydrogen bonds from carboxylate oxygen atoms and lattice water molecules link the chains into a two-dimensional supramolecular sheet: O8 acts as hydrogen donor to two carboxylate oxygen atoms (O3 and O4) forming a C 2 2 (4) ring (Etter, 1990) and one carboxylate oxygen O1 of neighboring L ligands. O7 also acts as hydrogen supplementary materials sup-2 bond acceptor to O5, O6 and acts as hydrogen bond donor to atom O3. Additionally O5 is also hydrogen bonded to O2 forming a strong O-H···O hydrogen bond, further consolidating the supramolecular sheet. Neighboring sheets are assembled into a three-dimensional supramolecular architecture by O6-H6B···O1 and O7-H7A···O8 hydrogen bonds (Fig. 3).
Besides classical O-H···O hydrogen bonds, also weaker non-classical C-H···O hydrogen bonds are observed (geometric details in Table 2), further extending the title compound into a three-dimensional supramolecular architecture. Additionally π-π stacking interactions are also be observed between two pyrazine groups with a distance of 3.643 (5) Å, which also help to stabilize the supramolecular architecture. The detailed environment of C-H···O interactions are also represented in Fig. 3.

catena-poly[[[µ-aqua-aquadisilver(I)]-bis(µ 3 -5-methylpyrazine-2-carboxylato)] dihydrate]
Crystal data [Ag 2 (C 6   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.