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

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 OH...O hydrogen bonds; taken with phenolic OH...N intramolecular hydrogen bonds, these dimers form a pseudo-macrocycle bounded by an R4 4(10) 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 OH...O-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 OH...N hydrogen bonds. The transition also relieves repulsions in the pi...pi stacking contacts. The intramolecular OH...N hydrogen bond of phase I is replaced in phase II by a intermolecular phenolic OH...O 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.