3-Fluorosalicylaldoxime at 6.5 GPa

3-Fluorosalicylaldoxime, C7H6FNO2, unlike many salicylaldoxime derivatives, forms a crystal structure containing hydrogen-bonded chains rather than centrosymmetric hydrogen-bonded ring motifs. Each chain interacts with two chains above and two chains below via π–π stacking contacts [shortest centroid–centroid distance = 3.295 (1) Å]. This structure at 6.5 GPa represents the final point in a single-crystal compression study.

3-Fluorosalicylaldoxime, C 7 H 6 FNO 2 , unlike many salicylaldoxime derivatives, forms a crystal structure containing hydrogen-bonded chains rather than centrosymmetric hydrogen-bonded ring motifs. Each chain interacts with two chains above and two chains below viastacking contacts [shortest centroid-centroid distance = 3.295 (1) Å ]. This structure at 6.5 GPa represents the final point in a singlecrystal compression study.

Experimental
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; method used to solve structure: model taken from ambient pressure structure (Wood et al., 2007b); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: CRYSTALS. taken together form a secondary level C(5) chain running parallel to the crystallographic c axis.
The compression of the 3-fluorosalicylaldoxime structure is anisotropic (Fig. 2). It can be seen that the c-axis is the least compressible (decreases by 4.9% up to 6.5 GPa) and this direction also corresponds to the direction of the hydrogen-bonded chains in the crystal structure. The next least compressible direction is that of the crystallographic b axis (decreases by 7.8%), where the main interactions are H···F and H···H contacts between the chains. Finally, the direction that compresses the most up to 6.5 GPa (14.4%) is along the a axis which corresponds to the π-π stacking direction; closest contact = Cg()···Cg() i = 3.295 (1) Å for i: 0.5+x, 0.5-y, z. Fig. 3 shows the compression of the π-π stacking contact geometry in comparison with equivalent interactions in other salicylaldoximes and with general stacking contacts found in the CSD. It can be seen that the compression follows the trend of the earlier oxime pressure studies (Wood et al., 2008) in that the interaction compresses to the edge of what is seen at ambient conditions in the CSD. In this experiment the crystal disintegrated when the pressure was increased further and this may be indicative that a destructive phase transition occurred.

Experimental
All solvents and reagents were used as received from Aldrich and Fisher. 1 H and 13 C NMR were obtained using a Bruker AC250 spectrometer at ambient temperature. Chemical shifts (δ) are reported in p.p.m. relative to internal standards. Fast atom bombardment mass spectrometry (FABMS) was carried out using a Kratos MS50TC spectrometer with a thioglycerol matrix. Analytical data was obtained from the University of St Andrews Microanalytical Service.
supplementary materials sup-2 KOH (0.674 g, 10.20 mmol) and NH 2 OH.HCl (0.709 g, 10.20 mmol) were dissolved in EtOH, mixed thoroughly and a white KCl precipitate removed by filtration. 3-Fluorosalicylaldehyde (1.000 g, 7.14 mmol) was added to the filtrate, and the mixture refluxed for 3 h. The solvent was removed in vacuo, the residue redissolved in CHCl 3 , washed with water three times, and dried over MgSO 4 . The solvent was removed in vacuo to yield the crude product as a white powder (0.980 g, 88.5%). A pale-yellow block suitable for X-ray diffraction was grown by slow evaporation of a hexane/chloroform solution. The high-pressure experiments were carried out using a Merrill-Bassett diamond anvil cell (half-opening angle 40°), equipped with brilliant-cut diamonds with 600 µm culets and a tungsten gasket (Merrill & Bassett, 1974). A 1:1 mixture of n-pentane and isopentane was used as a hydrostatic medium. Due to the high volatility of the n-pentane/isopentane solution, the cell was cooled in dry-ice prior to loading. A small ruby chip was also loaded into the cell as the pressure calibrant, with the ruby fluorescence method being used to measure the pressure (Piermarini et al., 1975).
Following data collection, an absorption correction was applied using the program SADABS (Sheldrick, (2006). The T max /T min ratio is larger than calculated on the basis of the crystal dimensions. However, multi-scan procedures (such as SADABS used in the present study) correct for all systematic errors that lead to disparities in the intensities of equivalent data. It is likely that the larger than expected range of transmission is accounted for by crystal decay or absorption by the high pressure cell.

Refinement
The hydrogen atoms were located in a Fourier difference map. The positional and isotropic displacement parameters were then refined subject to restraints [C-H = 0.93 (2) Å, O-H = 0.82 (2) Å and U iso (H) = 1.5 U eq (C or O)]. In subsequent cycles of least-squares refinement all the U iso (H) values were fixed and the H-atoms attached to C were constrained to ride on their parent atoms. H1 and H5 were refined subject to distance restraints equal to 0.84 (5) Å.
In the absence of significant anomalous scattering effects, 316 Friedel pairs were averaged in the final refinement.
The crystal quality was beginning to deteriorate by this pressure and the number of reflections collected also dropped.
In order to deal with this, global vibration and thermal similarity restraints were applied to the model. Fig. 1. Molecular structure of (I) with probability ellipsoids drawn at the 50% level, showing atom labelling. Fig. 2. Graph of the fractional variation of the lattice parameters (a, b and c) of (I) as a function of pressure (GPa). The cell lengths are displayed as red squares, green triangles and blue circles for a, b and c, respectively. Fig. 3. Graph of stacking distance (in Å) against stacking offset (in Å) for phenyl group stacking interactions in the CSD. The five previously published salicylaldoxime compression studies have been highlighted; salicylaldoxime (dark-blue), 3-chlorosalicylaldoxime (green), 3methylsalicylaldoxime (pink), 3-methoxysalicylaldoxime (light-blue) and 3-tert-butylsalicylaldoxime (orange). Data for the compression study on (I) (red) has now also been added. Method, part 1, Chebychev polynomial, (Watkin, 1994, Prince, 1982 [weight] = 1.0/[A 0 *T 0 (x) + A 1 *T 1 (x) ··· + A n-1 ]*T n-1 (x)] where A i are the Chebychev coefficients listed below and x = F /Fmax Method = Robust Weighting (Prince, 1982)