Observation of reversible three-step solid-to-solid phase transitions in a nickel-bis-1,2-dithiolene magnetic molecule solid

An ion-pair compound 1,6-bis(1-methylimidazolium)hexane bis(maleonitrio-dithiolato)nickelate was synthesized and structurally characterized. This compound crystallizes in the triclinic space group P-1, and the anions form the alternating stacks in the crystal. Reversible three-step solid-to-solid phase transitions occur at ~ 210, ~ 320 and ~ 432 K upon cooling. The entropy change of phase transition was estimated to be 25.4 J·K ^{- 1}· mol ^{- 1} and 9.97 J · K ^{- 1}· mol ^{- 1} for the transitions at ~ 320 and ~ 432 K, respectively. The hysteresis effect was observed in the χ _m-T plot for the transitions at ~ 210 and ~ 320 K. The mechanisms of transitions are discussed based on the crystal structure and thermal analyses.