Salicylaldoximes are organic molecules which self-assemble into dimers creating rings: the cavity at the centre of the ring can accommodate metal cations. This makes salicylaldoximes very important in metal extraction processes and they used in the extraction of some 20% of the world’s copper.
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.
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 patterns of intermolecular interactions 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.
The aim of the programme was to investigate the extent to which the size of the ring could be tuned using high pressure, thereby affecting take up of different metals.
The hypothesis that pressure can be used to compress the metal-containing cavity in salicylaldoxime complexes has been confirmed in Ni-salicylaldoximato complexes using high-pressure crystallography up to 6 GPa. The extent to which different Ni-O or Ni-N distances shorten was found to be due to supramolecular or packing effects, correlating with the distribution of intermolecular voids. This result reveals the importance of supramolecular effects in determining the path of compression. The compression of the cavity also leads to piezochromism: crystals which are green at ambient pressure are red at 5 GPa. Combination of the crystallographic results with DFT simulations and optical spectroscopy enable the colour change to be traced to changes in the electronic structure of the metal complex which occur on compression. Compression of related copper complexes shows quite different behaviour, with the metal atom expanding its coordination number from four to six. Real-life extraction processes occur in solution rather than the solid state and we have explored the effect of pressure on Ni- and Cu complexes in solution using EXAFS. It was shown for the first time that the coordination environment of the metal is very much more compressible in solution than in the solid state with distance changes an order of magnitude higher.