4,4,4-Trifluoro-3-hydroxy-3-(trifluoromethyl)butanoic acid

The asymmetric unit of the title compound, C5H4F6O3, a polyfluorinated derivative of β-hydroxybutyric acid, comprises two molecules. Intramolecular O—H⋯O hydrogen bonds occur. In the crystal, intermolecular O—H⋯O hydrogen bonds give rise to the formation of carboxylic acid dimers. Along with these hydrogen bonds, C—H⋯O contacts connect the molecules into infinite strands along the a axis.


Comment
Chelate ligands have found widespread use in coordination chemistry due to the increased stability of coordination compounds they can form in comparison to monodentate ligands. Most work in this field has been done with chelate ligands capable of forming five-, six-and seven-membered chelate rings. The coordination behaviour of such ligands with respect to reaction products formed (e.g. the coordination number of the central atom) is a function of electronic as well as steric factors. In a larger study aimed at elucidating the coordination chemistry of multiply-fluorinated carboxylic acid derivatives, the structure of the title compound was determined to enable comparisons with reaction products obtained.
The title compound is a symmetric, polyhalogenated derivative of β-hydroxypropanecarboxylic acid which bears two trifluoromethyl-groups at the alcoholic carbon atom. The asymmetric unit ( Fig. 1) comprises two molecules of the title compound.
In the crystal structure, intra-as well as intermolecular hydrogen bonds are present. While the intramolecular hydrogen bonds are formed between the alcoholic hydroxyl group and the carbonylic O-atom of the carboxylic group, intermolecular hydrogen bonds can be observed between the carboxylic acid groups' OH-groups and carbonylic O-atoms. The latter interaction connects both molecules of the asymmetric unit to dimers. In terms of graph-set analysis, the descriptor for the intramolecular hydrogen bonds is S(6)S(6) on the unitary level while the intermolecular hydrogen bonds necessitate a R 2 2 (8) descriptor on the binary level. For the intramolecular hydrogen bond, a bifurcation could be discussed applying the O-atom of another hydroxyl group as acceptor. This would render it a mixed intra-intermolecular hydrogen bond, however, the D-H···A angle of only around 120° for the intermolecular hydrogen bond is comparatively small.
Apart from these hydrogen bonds, C-H···O contacts are present in the crystal structure whose ranges fall more than 0.2 Å below the sum of van-der-Waals radii of the respective atoms. These contacts can be observed between one of the H-atoms of the methylene group and the O-atom of a neighbouring hydroxyl group. Like the possible, bifurcated hydrogen bond mentioned above, these C-H···O contacts connect the molecules to infinite strands along the crystallographic a axis (Fig.   3). The descriptor for the C-H···O contacts on the unitary level is C 1 1 (4)C 1 1 (4).

Experimental
The structural analysis was done on a single-crystal taken from a commercially obtained (Fluorochem) batch of the title compound.

Refinement
Carbon-bound H-atoms were placed in calculated positions (C-H 0.99 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C). The H-atoms of the carboxylic acid group as well as of the hydroxyl supplementary materials sup-2 groups were allowed to rotate with a fixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)), their U(H) invariably set to 1.5U eq (C) Figures   Fig. 1. The asymmetric unit of the title compound with anisotropic displacement ellipsoids drawn at the 50% probability level.   (7) 0.0536 (9) 0.0100 (6) 0.0162 (7) 0.0177 (7)