Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure

• Main Authors: Peter J. Byrne, Patricia J. Richardson, John Chang, Anna Kusmartseva, David R. Allan, Anita C. Jones, Konstantin V. Kamenev, Peter A. Tasker, Simon Parsons
• Format: Article
• Language: English
• Subjects: High-pressure chemistry; Nickel; Other physical sciences not elsewhere classified; Physical Sciences not elsewhere classified; Transition metals; X-ray diffraction

The crystal structures of bis(3-fluoro-salicylaldoximato)nickel( II) and bis(3-methoxy-salicylaldoximato) nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato) nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x 2-y 2) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.