Poly[μ-(1,3-dihydroxypropan-2-olato)-potassium]

The asymmetric unit of the title compound, [K(C3H7O3)]n or K[H2gl]n, common name potassium glycerolate, contains half the K+ cation and half of the glycerolate anion. The other half of the anion is generated through a mirror plane passing through the K atom, and a C, an H and an O atom of the glycerolate ligand. The K+ ion is coordinated by the O atoms of the OH groups, leading to a six-membered chelate ring that adopts a very distorted boat conformation. The negatively charged O atom of the glycerolate anion, [H2gl−], is found in the flagpole position and forms an ionic bond with the K+ ion. The O atoms of the hydroxo groups are coordinated to two K+ ions, whereas the negatively charged O atom is bonded to one K+ ion. The K+ ion is coordinated by three other symmetry-related monodentate H2gl− ligands, so that each H2gl− ligand is bonded to two K+ ions, and the potassium has a seven-coordinate environment. The H2gl− ligands are connected via a strong O—H⋯O hydrogen bond and, together with the K⋯O interconnections, form polymeric sheets which propagate in the directions of the a and b axes.

We found that adding glycerol to a hot alkali hydroxide solution under agitation is the preferred synthesis for mono alkali glyceroxides (Schatte et al., 2010).
Recently, we have reported the crystal structure of poly[µ-2,3-dihydroxypropan-1-olato-sodium], the until now only known crystal structure of mono alkali glyceroxide. The crystal structure of [K(C 3 H 7 O 3 )] n (I) was determined as part of our continuing research on catalysts which can be used in the production of biodiesel.
The monodentate H 2 glligand coordinates to the potassium atom via the hydroxo groups leading to 6-membered chelating ring with a very distorted boat conformation, Fig. 1. The negatively charged O atom, which is attached to the secondary carbon atom of the H 2 glion, is found in the flagpole position and forms an ionic bond with the potassium cation. This structure is in contrast to the sodium analogue, where the H 2 glligand is coordinated to the sodium atom by one oxo-and one hydroxo group forming a non-planar 5-membered ring and the hydroxo group attached to primary carbon atom of the glycerol is deprotonated. In the related structure of the vanadium-H 2 gl complex, however, the hydroxo group attached to secondary carbon atom is deprotonated as well (Rath et al., 1998).
Each H 2 glligand is bonded to two potassium cations. Each potassium cation is connected via K···O bonds ranging from 2.690 (2) to 2.8576 (15) Å to three symmetry related H 2 glligands and is 7-coordinated. In addition, five longer and much weaker K···O interactions ranging from 3.211 (2) to 4.0451 (16) are observed (sum of the van der Waals radii, 4.3 Å; Table   1). The observed intra-and inter-molecular K···O bond distances are elongated in comparison to the related bond distances reported for potassium phenolate complexes (Brooker et al., 1991) and potassium alkoxides (Weiss et al., 1968;Kennedy, et al., 2001).
The H 2 glligands are connected via one strong intermolecular O-H···O hydrogen bond interaction (Table 2 and Fig. 2). Both, the K···O and O-H···O interconnections are responsible for the formation of polymeric sheets which extend indefinitely in the directions of the a and b axes (Fig. 2).

Experimental
A potassium hydroxide solution (336 g, 50%) was freshly prepared by dissolving potassium hydroxide pellets (168 g, 3 mol) in water (168 g). Glycerol (92 g, 1 mol) was slowly added into the hot potassium hydroxide solution under agitation. The mixture was allowed to stand and to cool down to room temperature. Colourless crystals of Poly[µ-2,3-dihydroxypropan-1-supplementary materials sup-2 olato-potassium] started to form after six days. The crystals are only stable in a very basic solution at ambient temperatures and are less stable than those of the sodium analogue. A suitable single-crystal was quickly coated with oil, collected onto the aperture of a mounted MiTeGen Micromount TM (diameter of the aperture: 100 microns) and as quickly as possible transferred to the cold stream of the X-ray diffractometer. The crystals tend to dissolve upon eposure to the oil at ambient temperatures for more than one minute.