Molecular Structure and Magnetic Properties of 1-Ethyl-2-(1-oxy-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)-3-methylimidazolium Arylcarboxylates and Other Salts

1-Ethyl-2-(1-oxy-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)-3-methylimidazolium bromide, [EMINN]+[Br]−, carrying nitronylnitroxide (NN) in the cation unit, was prepared as a parent molecule and converted to seven salts, [EMINN]+[X]− (X = I, TFSI (bis(trifluoromethanesulfonyl)imide), BPh4 (tetraphenylborate), [EMINN]+1−3[BA1−3](1−3)−; BA1 (benzoic acid), BA2 (terephthalic acid), and BA3 (trimesic acid), and [EMINN]+[BANN]−; BANN (4-NN-benzoic acid)), by the ion-exchange reaction. The molecular structure of the cation units for all salts revealed by X-ray crystallography is similar, where the dihedral angles between the imidazolium ring and the NN planes are 51−58°. In the crystal structure, [EMINN]+[X]− (X = Br, I, TFSI, and BPh4) formed head-to-tail dimers, while the uniquely shaped dimers consisting of two [EMINN]+[carboxylate]− units were connected by the hydrogen bonding of water molecules to form a tape structure for [EMINN]+[BANN]− and 2D sheet structure for [EMINN]+2[BA2]2− and [EMINN]+3[BA3]3−. In the crystalline state, [EMINN]+[X]− showed behavior typical of a paramagnetic species with S = 1/2. The χmolT vs T plot for [EMINN]+[BANN]− was analyzed using a four-spin model to give J1/kB = −0.27 and J2/kB = −0.16 K. The plots for [EMINN]+2[BA2]2− and [EMINN]+3[BA3]3− were analyzed using an antiferromagnetic chain model to give J/kB = −62.1 and −86.5 K, respectively. In aqueous solution, on the other hand, the ESR spectra for all salts showed similar five-line signals due to the isolated NN moiety. The relaxivities (r1 and r2; 25 °C, 0.59 T, and 25 MHz) for [EMINN]+[Br]−, [EMINN]+[BA]−, [EMINN]+[BANN]−, [EMINN]+2[BA2]2−, and [EMINN]+3[BA3]3−, are r1 = 0.13, 0.14, 0.32, 0.26, and 0.40 and r2 = 0.17, 0.13, 0.31, 0.30, and 0.46 mM−1 s−1, respectively.

1-乙基-2-(1-氧基-3-氧代-4,4,5,5-四甲基咪唑啉-2-基)-3-甲基咪唑鎓溴化物，其阳离子为[EMINN]+[Br]−，阳离子单元带有硝酰基自由基（nitronylnitroxide, NN），该化合物作为母体分子被合成，并通过离子交换反应转化为七种盐：[EMINN]+[X]−（X=I、双(三氟甲磺酰)亚胺（bis(trifluoromethanesulfonyl)imide, TFSI）、四苯基硼酸盐（tetraphenylborate, BPh4）），以及[EMINN]₁₋₃⁺[BA₁₋₃](¹⁻³)⁻；其中BA₁为苯甲酸（benzoic acid）、BA₂为对苯二甲酸（terephthalic acid）、BA₃为均苯三甲酸（trimesic acid），另有[EMINN]+[BANN]−，其中BANN为4-硝酰基苯甲酸（4-NN-benzoic acid）。通过X射线晶体学表征可知，所有盐的阳离子单元分子结构均相似，咪唑鎓环与NN平面之间的二面角为51°~58°。在晶体结构中，[EMINN]+[X]−（X=Br、I、TFSI、BPh4）形成了头尾相接的二聚体；而由两个[EMINN]+[羧酸根]−单元构成的独特二聚体，通过水分子的氢键相互作用连接：对于[EMINN]+[BANN]−，其晶体形成带状结构；对于[EMINN]₂⁺[BA₂]²−和[EMINN]₃⁺[BA₃]³−，则形成二维片状结构。在晶态下，[EMINN]+[X]−表现出S=1/2顺磁性物种的典型行为。针对[EMINN]+[BANN]−的摩尔磁化率-温度积（χ_molT）随温度T变化的曲线，采用四自旋模型进行拟合，得到交换耦合常数J₁/k_B = −0.27 K、J₂/k_B = −0.16 K。针对[EMINN]₂⁺[BA₂]²−和[EMINN]₃⁺[BA₃]³−的曲线，采用反铁磁链模型拟合，分别得到J/k_B = −62.1 K和−86.5 K。另一方面，在水溶液中，所有盐的电子顺磁共振（Electron Spin Resonance, ESR）谱均呈现出相似的五重峰信号，源于孤立的NN基团。针对[EMINN]+[Br]−、[EMINN]+[BA₁]−、[EMINN]+[BANN]−、[EMINN]₂⁺[BA₂]²−和[EMINN]₃⁺[BA₃]³−的弛豫率（r1和r2；测试条件为25 ℃、0.59 T、25 MHz）数值如下：r1分别为0.13、0.14、0.32、0.26和0.40 mM⁻¹·s⁻¹，r2分别为0.17、0.13、0.31、0.30和0.46 mM⁻¹·s⁻¹。