Pressure-sensitive complex formation based on self-assembling ligands

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Salicylaldoximes and related ligands are important in metal extraction processes, being used in the extraction of some 20% of the world's copper. They self-assemble into H-bonded dimers in non-polar solvents and in the solid state, creating pseudo-macrocyclic rings. The cavity at the centre of the pseudomacrocycle can accommodate divalent metal cations to give complexes which are stabilized by the inter-ligand hydrogen bonds. Substitution on different sites around the ligand has been shown to influence the cavity size and other properties, thereby altering the extractant strength. By addition of appropriate arms, it has also been possible to develop ligands which extract not only metal cations, but also their attendant anions (metal salt extractants). Salicylaldoximes are therefore extremely versatile ligands, but we have found that their geometric properties may be tuned even further by application of high pressure. Pressures of 1000 to 10 000 atm (0.1 - 1 GPa) are modest by the standards of modern high-pressure research, and are readily attainable in the laboratory using diamond anvil cells; pressures in a similar range are even used industrially for food treatment. However, pressure in this range has important structural consequences for the ways in which organic molecules interact and for the internal structures of metal complexes. Many organic molecules form crystalline structures with different hydrogen bonding patterns from those seen at atmospheric pressure; the magnetic properties of nanomagnetic complexes can be altered; and metals can be driven to increase their coordination numbers. We will design salicylaldoxime and related ligands which form pseudomacrocycles with cavities which are sensitive to pressure. We will then use pressure to tune the cavity-sizes, changing the selectivity of ligands for specific metal cations and metal salts. We plan to study systems where pressure effects are seen in the range 0.1 - 1 GPa, as this is the region where practical applications are most likely, though pressures up to 10 GPa will be readily accessible. Ligands and their complexes will be studied in the solid state using X-ray crystallography, which will provide precise geometric data on the effects of pressure. In real-life applications extraction occurs in solution, and so we will also investigate the effect of pressure on ligands and their complexes in solution using EXAFS and laser spectroscopy. This will enable solution state equilibrium constants to be determined, providing quantitative information on competitive binding as a function of pressure. We will also scale-up the high pressure processes discovered, establishing high pressure as a tool in supramolecular synthesis.We have assembled a multi-disciplinary team to carry out this work, which depends on innovations in synthesis, crystallography, spectroscopy and engineering. This project will form a pioneering step in the development of Gigapascal Chemistry.