Suppression of a Structural Phase Transition by an Orientational Disorder of Counteranions in an Organic Conductor, β″-β″-(BEDT-TTF) 2 ClC 2 H 4 SO 3 .

In 2022, we reported that the β″-(BEDT-TTF) ₂ ClC ₂ H ₄ SO ₃ salt ( 1 ) has a phase transition at 210 K on cooling and 260 K on heating, which was suggested as a nondoped-to-doped transition. During the synthesis of 1 , a second salt was coproduced, the structure and properties of which are reported here. The new salt β″-β″-(BEDT-TTF) ₂ ClC ₂ H ₄ SO ₃ ( 2 ) has the same composition and is isomorphous to 1L , the low-temperature phase of 1 . However, the salt shows no phase transition. 1H , the high-temperature phase of 1 , displays rotational disorders of the -SO ₃ ^- groups. In contrast, the anions in 2 have no rotational disorders but have orientational disorders. Approximately 13% of the anions in 2 have their orientation as opposed to the remaining anions. The geometrically calculated shortest O···O distance is 2.24 Å if neighbor -SO ₃ ^- groups are able to rotate freely. This is 1.1 Å shorter than the sum of the van der Waals radii, and therefore, this is too close to allow free rotation. Therefore, it is proposed that 2 has no rotational disorder, which leads to no phase transition as is observed in 1 .