Bis(imidazolium) galactarate dihydrate

In the structure of the title salt, 2C3H5N2 +·C6H8O8 2−·2H2O, the galactarate dianions have crystallographic inversion symmetry and together with the water molecules of solvation form hydrogen-bonded sheet substructures which extend along (110). The imidazolium cations link these sheets peripherally down c through carboxylate O—H—N and N′—H⋯Ohydroxy bridges, giving a three-dimensional framework structure.

In the structure of the title salt, 2C 3 H 5 N 2 + ÁC 6 H 8 O 8 2À Á2H 2 O, the galactarate dianions have crystallographic inversion symmetry and together with the water molecules of solvation form hydrogen-bonded sheet substructures which extend along (110). The imidazolium cations link these sheets peripherally down c through carboxylate O-H-N and N 0 -HÁ Á ÁO hydroxy bridges, giving a three-dimensional framework structure.

Structure Reports Online
. 2H 2 O (I), and the structure is reported here.
In the structure of (I) (Fig. 1), the galactarate anions lie across crystallographic inversion centres which is also the case in the structure of the parent acid (Jeffrey & Wood, 1982). Hydrogen-bonded anion-water sheets extending across the <100> planes in the unit cell ( Fig. 2) are formed through hydroxyl O31-H···O12 iii carboxyl and water-bridging O31···O11 iv carboxyl interactions (for symmetry codes, see Table 1). These include R 2 2 (12) and R 3 3 (12) cyclic motifs (Etter et al., 1990). The layered substructures are linked peripherally down the c cell direction by the imidazolium cations through carboxyl O···H-N,N'-H···O`h ydroxyl bridges giving a three-dimensional framework structure (Fig. 3). The structure of (I) differs from those of the anhydrous 1:1 salts of the hydrogen dicarboxylates (MacDonald et al., 2001) in which the bridging imidazolium cations are incorporated within two-dimensional layered structures.

Experimental
The title compound was synthesized by heating together under reflux for 10 minutes 1 mmol of galactaric acid (mucic acid) and 2 mmol of imidazole in 50 ml of 50% ethanol-water. After concentration to ca 30 ml, partial room temperature evaporation of the hot-filtered solution gave large colourless plates of (I) (m.p. 435 K) from which a suitable analytical specimen was cleaved.

Refinement
Hydrogen atoms potentially involved in hydrogen-bonding interactions were located by difference methods and their positional and isotropic displacement parameters were refined. Other H atoms were included in the refinement in calculated positions (C-H aromatic = 0.95 Å and others = 1.00 Å) and allowed to ride, with U iso (H) = 1.2U eq (C). Fig. 1. The molecular configuration and atom-numbering scheme for the cation, dianion and water species in (I). The galactarate dianion has inversion symmetry [symmetry code: (i) -x + 1, -y + 1, -z + 2]. Non-H atoms are shown as 50% probability ellipsoids and inter-species hydrogen bonds are shown as dashed lines. Fig. 2. Hydrogen-bonded anion-water sheet substructures in (I), extending across (110) (imidazolium cations are omitted). For symmetry codes, see Table 1. Hydrogen bonds are shown as dashed lines. as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O11 0.18384 (15