1,2-Bis[N-(N′-alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals
作者:Minjae Lee、U Hyeok Choi、Sungsool Wi、Carla Slebodnick、Ralph H. Colby、Harry W. Gibson
DOI:10.1039/c1jm10995b
日期:——
A new class of organic ionic plastic crystals was discovered. A series of alkylene 1,2-bis[N-(Nâ²-alkylimidazolium)] salts with Brâ and PF6â anions was prepared and most of the ethylene (C2) linked compounds undergo multiple solidâsolid phase transitions as determined by differential scanning calorimetry (DSC). The salts with longer spacers (C3 or C4) between the imidazolium units do not display any solidâsolid transitions. The PF6â salts with terminal C10 and C12n-alkyl moieties are âclassicalâ organic ionic plastic crystals by Timmermans' definition; they have low ÎSf (11 J Kâ1 molâ1 for C10 and 12 J Kâ1 molâ1 for C12). Additionally, the C2 linked bis-imidazolium salts with n-butyl, n-hexyl, n-heptyl and n-octyl termini all undergo two, three or four solid state transitions and exhibit ÎSf values in the range of 36â49 J Kâ1 molâ1, which are similar to those of other known ionic plastic crystalline materials. These materials were additionally characterized via ionic conductivity and solid state NMR. These ionic plastic crystals are presumably single-ion conductors, but the ionic conductivities appear to be too low for practical applications. The activation energy for conduction decreases as these compounds are heated through each solidâsolid transition. The lack of any change in solid state 2H NMR spectra with temperature indicates that there is no change in phenyl ring flipping, suggesting no change in the imidazolium local environment. A resolution of this apparent dichotomy is perhaps that the counterions reside with the ethylene spacers between imidazolium moieties.
发现了一类新的有机离子塑料晶体。合成了一系列烷撑1,2-双[N-(N'-烷基咪唑鎓)]盐,其阴离子为溴和PF6。通过差示扫描量热法 (DSC) 测定,大多数乙烯 (C2) 连接的化合物经历多次固-固相变。具有较长间隔基 (C3或C4) 的咪唑鎓单元的盐不显示任何固-固相变。端基为C10和C12n-烷基的PF6盐是蒂默曼斯定义的“经典”有机离子塑料晶体;它们的熔化熵变(ΔSf)较低 (C10为11 J K-1 mol-1,C12为12 J K-1 mol-1)。此外,端基为正丁基、正己基、正庚基和正辛基的C2连接双咪唑鎓盐均经历两、三或四个固态相变,熔化熵变值在36-49 J K-1 mol-1范围内,与已知其他离子塑料晶体材料类似。这些材料还通过离子电导率和固态核磁共振进行了表征。这些离子塑料晶体可能是单离子导体,但离子电导率似乎太低,无法实际应用。传导活化能随着这些化合物经历每个固-固相变而降低。固态2H核磁共振光谱随温度变化而没有变化表明,苯环翻转没有变化,暗示咪唑鎓局部环境没有变化。这种明显悖论的解释可能是反离子存在于咪唑鎓单元之间的乙烯间隔基中。