Crystal structure of catena-poly[silver(I)-μ-l-valinato-κ2 N:O]

The reaction of Ag2O with l-valine in a 1:2 molar ratio in water, followed by vapour diffusion, afforded polymeric N—Ag—O repeated units of the title silver(I) complex. It shows a weak Ag⋯Ag interaction and hydrogen bonds between amino groups and carboxylates.


Chemical context
Silver(I) complexes with amino acid ligands have been of interest not only due to their numerous medicinal applications but also as model protein-silver(I) interaction compounds (Banti & Hadjikakou, 2013;Eckhardt et al., 2013). Aside from S-containing amino acids, such as cysteine which forms an insoluble S-bridging silver(I) complex (Leung et al., 2013), we have focused on ligand-exchangeable silver(I) complexes with N and O donor atoms. Although many of them are difficult to crystallize and light-sensitive, several crystals of silver(I) complexes have been prepared (Nomiya et al., 2014). In comparison to gold(I) ions, silver(I) ions show various coordination numbers and modes with N and O atoms and tend to form polymeric structures. The polymeric structures of silver(I) complexes with non-S amino acid ligands are classified into four types based on the bonding modes of the silver(I) atom: type I contains only Ag-O bonds, e.g., silver(I) with aspartic acid (Hasp), {[Ag 2 (d-asp)(l-asp)]1.5H 2 O} n ; type II contains O-Ag-O and N-Ag-N bonds, e.g., silver(I) with glycine (Hgly), [Ag(gly)] n ; type III contains N-Ag-O units, e.g., silver(I) complexes with glycine, [Ag(gly)] n , and l-asparagine (l-Hasn), [Ag(l-asn)] n ; type IV contains only Ag-N bonds, e.g., silver(I) with l-histidine (l-H 2 his), [Ag(l-Hhis)] n (Nomiya et al., 2000;Nomiya & Yokoyama, 2002). Two types of complexes (types II and III) have been reported for [Ag(gly)] n . Here, we report the preparation and crystal structure of silver(I) with l-valine (l-Hval).

Structural commentary
The local coordination around the silver(I) atom of the title compound is shown in Fig. 1 [Ag(l-val)] n is classified as being a type III linear N-Ag-O polymer, as found in the silver(I) complexes with glycine (Acland & Freeman, 1971), with -alanine (Dé maret & Abraham, 1987) and with asparagine (Nomiya & Yokoyama, 2002).
Although the polymeric structures of N-Ag-O repeated units of [Ag(l-val)] n and [Ag(l-asn)] n are similar to each other, the AgÁ Á ÁAg distance [3.3182 (6) Å ] between the neighbouring chains in [Ag(l-val)] n is slightly shorter than that [3.4371 (9) Å ] in [Ag(l-asn)] n . This indicates the presence of a weak AgÁ Á ÁAg interaction between the two independent N-Ag-O chains in the title complex, considering the metallic and van der Waals radii of 1.44 and 1.72 Å , respectively, for Ag (Wells, 1975;Bondi, 1964 Symmetry code: (i) x À 1; y; z À 1.

Figure 1
Part of the polymeric structure of the title compound showing the local coordination around the silver(I) atoms. Displacement ellipsoids are drawn at the 50% probability level. The weak AgÁ Á ÁAg interaction is displayed as a grey line and the N-HÁ Á ÁO hydrogen bonds are drawn as blue dotted lines. [Symmetry code: (i) x À 1, y, z À 1.]

Supramolecular features
The two independent polymeric chains containing Ag1 and Ag2, respectively, are represented as green and blue in Fig. 2. The chains of Ag1 are connected to each other by N-HÁ Á ÁO hydrogen bonds [N1-H1AÁ Á ÁO2 ii ; symmetry code: (ii) x À 1, y, z] into a sheet structure. The chains of Ag2 are also linked into a sheet structure by N-HÁ Á ÁO hydrogen bonds [N2-H2AÁ Á ÁO3 iii ; symmetry code: (iii) x, y, z + 1]. Both sheets are parallel to the ac plane and the two sheets are stacked alternately along the b axis through the weak AgÁ Á ÁAg interactions and N-HÁ Á ÁO hydrogen bonds (N1-H1BÁ Á ÁO4 and N2-H22BÁ Á ÁO2; Table 2).

Synthesis and crystallization
To a suspension of 232 mg (1.0 mmol) of Ag 2 O in 20 ml of water was added 234 mg of l-valine (2.0 mmol), followed by stirring for 2 h at room temperature. The resulting grey suspension was filtered. Vapour diffusion was performed at room temperature by using the colourless filtrate as the inner solution and ethanol as the external solvent. The platelet crystals formed were collected and washed with acetone (30 ml) and ether (30 ml

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned geometrically and refined using a riding model with U iso (H) = 1.2 or 1.5U eq (C). H atoms of the amino groups were found in a difference Fourier map and their positions were refined with restraints of N-H = 0.86 (2) Å and HÁ Á ÁH = 1.40 (4) Å , and with U iso (H) = 1.2U eq (N).

catena-Poly[silver(I)-µ-L-valinato-κ 2 N:O]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.