Observation of a re-entrant phase transition in the molecular complex tris(μ-3,5-diiso-propyl-1,2,4-triazolato-κ:)trigold(I) under high pressure.

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(μ-3,5-diiso-propyl-1,2,4-triazolato-κ:)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 3.5 GPa. The ambient 2/ mother cell transforms to a 2/ phase above 1 GPa, followed by a 2/ phase above 2 GPa and a large-volume 2/ supercell at 2.70 GPa, with the previously observed 2/ phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure 2/ 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 14040 cm at 2.40 GPa, decreasing steeply to 13550 cm at 3 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.