Characterization of a Structural Phase Transition in δ-(ET)2AuBr2 at 420K by ESR and Crystal Packing Studies

Abstract The ESR lineshapes of δ- and s′ - (ET)2AuBr2 (ET denotes bis(ethylenedithio)tetrathiafulvalene) were examined over a wide temperature range. The lineshape studies indicated that α′ −(ET)2AuBr2 was stable from 300 to 460 K, where decomposition occurred. In contrast, δ-(ET)2AuBr2 was converted to α' -(ET)2AuBr2 in a few minutes at 420 K. Due to the heavily twinned nature of the crystals obtained, the α' product could not be unamiguously identified by X-ray crystallography and was characterized by ESR spectroscopy. The ESR lineshape behavior (100 to 300 K) of the thermally-derived α' modification was identical within experimental error to that of independently synthesized α' - (ET)2AuBr2. Crystal packing analysis showed that α' - (ET)2AuBr2 has more short C-H … anion contacts per donor molecule than does δ - (ET)2AuBr2, thereby suggesting that α′ −(ET)2AuBr2 is thermodynamically more stable than δ- (ET)2AuBr2.