Phase behavior of decamethyl- and octamethylferrocenium salts containing bulky anions: Effects of packing and the anion structure on ionic plastic crystal formation

Ionic plastic crystals (IPCs), in which nearly spherical ions undergo rotational motion in the solid state, have recently attracted attention because of their unique properties. To investigate the ability of bulky anions to induce IPC formation in organometallic salts, we prepared decamethylferrocenium salts ([1]X) and octamethylferrocenium salts ([2]X) containing a CB₁₁H₁₂⁻ carborane anion or a hexafluoropropane-1,3-disulfonamide anion (CPFSA⁻), of which [2]CPFSA was previously reported. Although all the salts underwent a solid–solid phase transition, only [1]CB₁₁H₁₂ formed an IPC phase above 494 K. This phase behavior correlated with the low-temperature crystal structures; [1]CB₁₁H₁₂ and [2]CB₁₁H₁₂ both adopted alternating cation–anion packing arrangements favorable for IPC formation, but the local cation–anion configuration of the latter hindered molecular rotation. In contrast, [1]CPFSA and [2]CPFSA adopted non-alternating cation–anion arrangements because of ion pairing induced by charge localization within the anion structure and face-to-face interactions between cation ligands. Thus, molecular properties, local cation–anion configurations, and packing arrangements influence IPC formation.