Trisilver(I) citrate

Trisilver(I) citrate, 3Ag+·C6H5O7 3−, was obtained by evaporation of a saturated aqueous solution of the raw material that had been obtained from sodium dihydrogen citrate and silver nitrate. It features one formula unit in the asymmetric unit. There is an intramolecular O—H⋯O hydrogen bond between the OH group and one of the terminal carboxylate groups. Different citrate groups are linked via the three Ag+ ions, yielding a three-dimensional network with rather irregular [AgO4] polyhedra.

Trisilver(I) citrate, 3Ag + ÁC 6 H 5 O 7 3À , was obtained by evaporation of a saturated aqueous solution of the raw material that had been obtained from sodium dihydrogen citrate and silver nitrate. It features one formula unit in the asymmetric unit. There is an intramolecular O-HÁ Á ÁO hydrogen bond between the OH group and one of the terminal carboxylate groups. Different citrate groups are linked via the three Ag + ions, yielding a three-dimensional network with rather irregular [AgO 4 ] polyhedra.
The Swedish Research Council (VR) is acknowledged for providing funding for the single-crystal diffractometer.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: KP2301).

Comment
The structures of many citrates of common metal ions are surprisingly sparsely investigated. Of the citrates of coinage metal cations, only ammonium disilver citrate monohydrate (Sagatys et al., 1993) and tetraammonium copper(II) bis(citrate) (Bott et al. 1991) have been reported. Here, we report the crystal structure of trisilver citrate, which was obtained from mixing solutions of sodium dihydrogen citrate and silver nitrate.
The basic structural of the 3D-polymeric structure shows an intramolecular hydrogen bond O-H···O bond between O5 and O7 ( Fig. 1). As expected from the charges, all three carboxy groups are deprotonated. The coordination polyhedra about the Ag + cations are quite irregular, the Ag-O distances are in the range 2.275 (3) to 2.566 (3) Å (Table 1). The Ag2-Ag3 contact of 2.8801 (6) Å is the shortest one in the structure. It can be noted that significantly shorter distances Ag + -Ag + are observed in many silver coordination compounds.

Experimental
An aqueous solution (0.5 mol/L) of sodium dihydrogen citrate was prepared by dissolving the respecitve amounts of trisodium citrate (Merck, p.a.) and citric acid in demineralised water. 1 mL of this solution was added to 1 mL of a solution of silver nitrate (0.5 mol/L), yielding a white precipitate. The latter was washed with demineralised water. The precipitate was then heated to 323 K with 1 mL of demineralised water. Upon cooling to room remperature, the saturated solution was filtered off and put aside for evaporation. Within a couple of days, small, colourless, rod-like crystals formed, that were suitable for structure determination. It can be noted that crystals of the title compound turned brown during the structure determination; however, no significant decrease in diffraction intensity could be observed. The initial precipitate formed from sodium dihydrogen citrate and silver nitrate was investigated by powder diffraction and it could be confirmed that it consisted of pure trisilver citrate. In addition, this powder pattern is identical with that of commercial "silver citrate hydrate".

Refinement
Methylene-H atoms were placed at calculated positions (C-H=0.97 Å, U iso =1.2 U eq of the respective C atom). The hydroxy-H atom was located from the Fourier map and was refined with a restraint (O-H=0.82 (2) Å) and U iso =1.5 U eq (O)).