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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

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posted on 21.11.2008, 00:00 by Hiroyuki Hayashi, Satoru Karasawa, Noboru Koga
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.