Acta Crystallographica Section B

Structural Science

Volume 62, Part 6 (December 2006)

research papers

Acta Cryst. (2006). B62, 1099-1111 [ doi:10.1107/S0108768106031752 ]

Effect of pressure on the crystal structure of salicylaldoxime-I, and the structure of salicylaldoxime-II at 5.93 GPa

P. A. Wood, R. S. Forgan, D. Henderson, S. Parsons, E. Pidcock, P. A. Tasker and J. E. Warren

Abstract: The effect of pressure on the crystal structure of salicylaldoxime has been investigated. The ambient-pressure phase (salicylaldoxime-I) consists of pairs of molecules interacting through oximic OHO hydrogen bonds; taken with phenolic OHN intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an $R_4^4 \left({10} \right)$ motif. The dimers interact principally via [pi] [pi] stacking contacts. Salicylaldoxime derivatives are used industrially as selective solvent extractants for copper; the selectivity reflects the compatibility of the metal ion with the pseudo-macrocycle cavity size. On increasing the pressure to 5.28 GPa the size of the cavity was found to decrease by an amount comparable to the difference in hole sizes in the structures of the Cu²⁺ salicylaldoximato complex and its Ni²⁺ equivalent. On increasing the pressure to 5.93 GPa a new polymorph, salicylaldoxime-II, was obtained in a single-crystal to single-crystal phase transition. PIXEL calculations show that the phase transition is driven in part by relief of intermolecular repulsions in the dimer-forming OHO-bonded ring motif, and the ten-centre hydrogen-bonding ring motif of the phase I structure is replaced in phase II by a six-centre ring formed by oximic OHN hydrogen bonds. The transition also relieves repulsions in the [pi] [pi] stacking contacts. The intramolecular OHN hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OHO hydrogen bond, but the total interaction energy of the pairs of molecules connected by this new contact is very slightly repulsive because the electrostatic hydrogen-bond energy is cancelled by the repulsion term. The intra- to intermolecular hydrogen-bond conversion simply promotes efficient packing rather than contributing to the overall lattice energy.

Keywords: salicylaldoxime; pressure dependence; phase transitions; hydrogen bonding.


	Structure factor file (CIF format) (99.4 kbytes) [ doi:10.1107/S0108768106031752/so5004273Ksup2.hkl ] Contains datablock 273K

	Structure factor file (CIF format) (26.3 kbytes) [ doi:10.1107/S0108768106031752/so50040_75GPasup3.hkl ] Contains datablock 0_75GPa

	Structure factor file (CIF format) (22.7 kbytes) [ doi:10.1107/S0108768106031752/so50042_37GPasup4.hkl ] Contains datablock 2_37GPa

	Structure factor file (CIF format) (21.2 kbytes) [ doi:10.1107/S0108768106031752/so50043_46GPasup5.hkl ] Contains datablock 3_46GPa

	Structure factor file (CIF format) (20.3 kbytes) [ doi:10.1107/S0108768106031752/so50044_55GPasup6.hkl ] Contains datablock 4_55GPa

	Structure factor file (CIF format) (19.9 kbytes) [ doi:10.1107/S0108768106031752/so50045_28GPasup7.hkl ] Contains datablock 5_28GPa

	Structure factor file (CIF format) (14.4 kbytes) [ doi:10.1107/S0108768106031752/so50045_93GPasup8.hkl ] Contains datablock 5_93GPa

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