Observation of intramolecular vibrations cooperating with the magnetic phase transition in a nickel-bis-dithiolene compound
作者:Shan-Shan Yu、Hai-Bao Duan、Xuan-Rong Chen、Zheng-Fang Tian、Xiao-Ming Ren
DOI:10.1039/c2dt31519j
日期:——
A new one-dimensional (1-D) ion-pair compound, [1,7-bis(1-methylimidazolium)heptane][Ni(mnt)2]2 (mnt2− = 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 P 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 = R ln 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 mnt2− ligands and the rocking vibration mode of the methylene group γr(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.
一种新的单维(1-D)离子对化合物[1,7-双(1-甲基咪唑)庚烷][Ni(mnt)2]2(mnt2-=马来酰亚胺二硫代甲酸盐)被合成,并对其结构和磁性进行了表征。该化合物在235 K左右表现出自旋-佩尔尔斯型转变。其晶体结构属于单斜晶系,空间群为C2/c,磁性[Ni(mnt)2]阴离子在高温(HT)相中形成均匀堆积。在低温(LT)相中,晶体发生转变,转变为三斜晶系空间群P,伴随着磁性转变,阴离子堆积形成二聚体。根据DSC数据,自旋-佩尔尔斯型转变的熵变化(ΔS)估计为0.772 J K-1 mol-1,远小于自旋熵变化(ΔS = R ln 2 ≈ 5.76 J K-1 mol-1),这表明在转变温度以上存在大量短程有序。可变温度红外光谱显示,在1160和725 cm-1附近的波段的峰值位置和强度分别在240 K左右发生突变,接近转变温度。这一观察结果表明,阴离子和反离子分子内的振动可能与自旋耦合,以配合该1-D自旋