Poly[μ-2,3-dihydroxypropan-1-olato-sodium]

The Na+ cation in the title compound, [Na(C3H7O3)]n or Na[H2gl], is coordinated by five O atoms leading to a distorted trigonal-bipyramidal geometry. The negatively charged O atom of the glycerolate anion is in an equatorial position, and the O atom of the hydroxo group, attached to the secondary C atom, occupies an axial position completing a five-membered non-planar chelate ring; this defines the asymmetric unit. The Na+ cation is coordinated by three other symmetry-related monodentate H2gl− ligands, so that each H2gl− ligand is bonded to four Na+ ions. The H2gl− ligands are connected via strong O—H⋯O hydrogen bonds and these, together with the Na⋯O interconnections, are responsible for the formation of polymeric sheets which propagate in the directions of the b and c axes.

The Na + cation in the title compound, [Na(C 3 H 7 O 3 )] n or Na[H 2 gl], is coordinated by five O atoms leading to a distorted trigonal-bipyramidal geometry. The negatively charged O atom of the glycerolate anion is in an equatorial position, and the O atom of the hydroxo group, attached to the secondary C atom, occupies an axial position completing a five-membered non-planar chelate ring; this defines the asymmetric unit. The Na + cation is coordinated by three other symmetry-related monodentate H 2 gl À ligands, so that each H 2 gl À ligand is bonded to four Na + ions. The H 2 gl À ligands are connected via strong O-HÁ Á ÁO hydrogen bonds and these, together with the NaÁ Á ÁO interconnections, are responsible for the formation of polymeric sheets which propagate in the directions of the b and c axes.
Funding for this research was contributed by The Agriculture Development Fund (ADF), administered by Saskatchewan Agriculture (SMA) and the National Sciences and Engineering Research Council (NSERC).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: TK2652).

Comment
We have shown that alkali metal glyceroxides can be used as efficient catalysts in trans-esterification reactions to produce biodiesel. Earlier syntheses of the mono sodium glyceroxide, Na[OCH 2 CH(OH)CH 2 (OH)] (referred to as Na[H 2 gl]), involved the reaction of excess sodium dissolved in ethanol with glycerol (Letts, 1872;Fairbourne & Toms, 1921). A more elegant and less expensive method for the the preparation of the title compound, (I), involved heating and stirring together equimolar quantities of powdered sodium hydroxide and glycerol (Gross & Jacobs, 1926).
Crystal structures of putative alkali metal glycerolates, M[H 2 gl], have not been reported to our knowledge. The crystal been until now the only reported structure containing coordinated H 2 glions (Rath et al., 1998). The crystal structure of (I) was determined as part of our research on catalysts which can be used in the production of biodiesel. The results of our crystal structure determination confirmed the earlier proposed structure based on derivative chemistry (Fairbourne & Toms, 1921).
The H 2 glanion behaves as a multifunctional ligand in the structure of (I), Fig. 1. In the first mode, the H 2 glligand is coordinating to the sodium atom by one oxo-(O1) and one hydroxo (O2) group forming a non-planar 5-membered ring.
Symmetry related H 2 glligands form essentially monodentate attachments. Pseudo-five-membered chelate rings are formed if rather longer Na···O interactions are taken into account [Na···O distances ranging from 3.35 to 3.83 Å (sum of the van der Waals radii, 3.8 Å)]; Table 1 and Fig. 2.
The observed intra-and inter-molecular Na···O bond distances are elongated in comparison to the related bond distances reported for sodium phenolate complexes (Hogerheide et al., 1996;Brooker et al., 1991) and sodium tert-butoxide (Davies et al., 1982). The oxygen atoms O1 and O2 act as bridging atoms between sodium atoms forming a planar O···Na···O···Na ring with alternation between O1 in one ring and O2 in the following ring. Each H 2 glligand is bonded to four Na ions. The H 2 glligands are connected via two strong intermolecular O-H···O hydrogen bond interactions (Table 2 and Fig. 2). Both the Na···O and O-H···O interconnections are responsible for the formation of polymeric sheets which extends indefinitely in the directions of the b and c axes (Fig. 2). Finally, it is noted that in (I), the hydroxo group attached to primary carbon atom of the glycerol is deprotonated. This is in contrast to the reported structure for the vanadium-H 2 gl complex, where the hydroxo group attached to secondary carbon atom is deprotonated (Rath et al., 1998).

Experimental
A sodium hydroxide solution (240 g, 50%) was freshly prepared by dissolving sodium hydroxide pellets (120 g, 3 mol) in water (120 g). Glycerol (92 g, 1 mol) was slowly added into the hot sodium hydroxide solution under agitation. The mixture was allowed to stand and to cool down to room temperature. Colourless crystals of mono sodium glyceroxide started to form. The crystals are only stable in a very basic solution at ambient temperatures. A suitable single crystal was quickly supplementary materials sup-2 coated with oil, collected onto the nylon fiber of a mounted CryoLoop TM and quickly transferred to the cold stream of the X-ray diffractometer. The data collection was performed at -90°C instead of -100°C to prevent cracking of the crystals at the lower temperature.

Refinement
The C-bound H atoms were geometrically placed (C-H = 0.98-1.00 Å) and refined as riding with U iso (H) = 1.2U eq (parent atom). The hydrogen atoms of the hydroxo groups were located in the difference Fourier map and were allowed to refine freely. Fig. 1. Molecular structure of the asymmetric unit in (I) showing the labelling scheme. Nonhydrogen atoms are represented by displacement ellipsoids at the 30% propability level.

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 > 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )