Observation of intramolecular vibrations cooperating with the magnetic phase transition in a nickel-bis-dithiolene compound

A new one-dimensional (1-D) ion-pair compound, [1,7-bis(1- methylimidazolium)heptane][Ni(mnt)₂]₂ (mnt^2- = maleonitriledithiolate), was synthesized and characterized structurally and magnetically. This compound shows a spin-Peierls-type transition at around 235 K. Its crystal structure belongs to the monoclinic system with space group C2/c and the magnetic [Ni(mnt)₂]^- anions form uniform stacks in the high-temperature (HT) phase. The crystal undergoes a transformation into the triclinic space group P1 accompanied by the magnetic transition and the anion stacks become dimerized in the low-temperature (LT) phase. The entropy changes (ΔS) are estimated to be 0.772 J K^-1 mol^-1 for the spin-Peierls-type transition, from DSC data, which is much less than the spin entropy change (ΔS = Rln 2 ≈ 5.76 J K^-1 mol ^-1), indicating that a substantial short-range order persists above the transition temperature. The variable temperature IR spectra showed that the peak positions and intensities for the bands near 1160 and 725 cm^-1, which correspond respectively to the ν(C-C) + ν(C-S) mode of the mnt ^2- ligands and the rocking vibration mode of the methylene group γ_r(CH₂) in the cation moiety, undergo an abrupt change at around 240 K, close to the transition temperature. This observation demonstrates that the intramolecular vibrations of both the anion and the counter-cation probably couple with the spins to cooperate with the spin-Peierls-type phase transition in this 1-D spin system.