ABSTRACT
We report a molecular crystal that exhibits four successive phase transitions under hydro-static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(µ2-3,5-diiso-propyl-1,2,4-triazolato-κ2N1:N2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5â GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1â GPa, followed by a P21/a phase above 2â GPa and a large-volume C2/c supercell at 2.70â GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040â cm-1 at 2.40â GPa, decreasing steeply to 13550â cm-1 at 3â GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.
ABSTRACT
A systematic investigation into the relationship between the solid-state luminescence and the intermolecular Auâ â â Au interactions in a series of pyrazolate-based gold(I) trimers; tris(µ2 -pyrazolato-N,N')-tri-gold(I) (1), tris(µ2 -3,4,5- trimethylpyrazolato-N,N')-tri-gold(I) (2), tris(µ2 -3-methyl-5-phenylpyrazolato-N,N')-tri-gold(I) (3) and tris(µ2 -3,5-diphenylpyrazolato-N,N')-tri-gold(I) (4) has been carried out using variable temperature and high pressure X-ray crystallography, solid-state emission spectroscopy, Raman spectroscopy and computational techniques. Single-crystal X-ray studies show that there is a significant reduction in the intertrimer Auâ â â Au distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100â K is accompanied by a reduction in the shortest intermolecular Auâ â â Au contacts of between 0.04 and 0.08â Å. The solid-state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red-shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular Auâ â â Au interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630â cm(-1) per GPa between ambient and 3.80â GPa. The shifts in all the complexes can be correlated with changes in Auâ â â Au distance observed by diffraction.