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]<sup>+</sup>[Br]<sup>−</sup>, carrying nitronylnitroxide (NN) in the cation unit, was prepared as a parent molecule and converted to seven salts, [EMINN]<sup>+</sup>[X]<sup>−</sup> (X = I, TFSI (bis(trifluoromethanesulfonyl)imide), BPh<sub>4</sub> (tetraphenylborate), [EMINN]<sup>+</sup><sub>1−3</sub>[BA<sub>1−3</sub>]<sup>(1−3)−</sup>; BA<sub>1</sub> (benzoic acid), BA<sub>2</sub> (terephthalic acid), and BA<sub>3</sub> (trimesic acid), and [EMINN]<sup>+</sup>[BANN]<sup>−</sup>; 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]<sup>+</sup>[X]<sup>−</sup> (X = Br, I, TFSI, and BPh<sub>4</sub>) formed head-to-tail dimers, while the uniquely shaped dimers consisting of two [EMINN]<sup>+</sup>[carboxylate]<sup>−</sup> units were connected by the hydrogen bonding of water molecules to form a tape structure for [EMINN]<sup>+</sup>[BANN]<sup>−</sup> and 2D sheet structure for [EMINN]<sup>+</sup><sub>2</sub>[BA<sub>2</sub>]<sup>2−</sup> and [EMINN]<sup>+</sup><sub>3</sub>[BA<sub>3</sub>]<sup>3−</sup>. In the crystalline state, [EMINN]<sup>+</sup>[X]<sup>−</sup> showed behavior typical of a paramagnetic species with <i>S</i> = 1/2. The χ<sub>mol</sub><i>T</i> vs <i>T</i> plot for [EMINN]<sup>+</sup>[BANN]<sup>−</sup> was analyzed using a four-spin model to give <i>J</i><sub>1</sub>/k<sub>B</sub> = −0.27 and <i>J</i><sub><i>2</i></sub>/k<sub>B</sub> = −0.16 K. The plots for [EMINN]<sup>+</sup><sub>2</sub>[BA<sub>2</sub>]<sup>2−</sup> and [EMINN]<sup>+</sup><sub>3</sub>[BA<sub>3</sub>]<sup>3−</sup> were analyzed using an antiferromagnetic chain model to give <i>J</i>/<i>k</i><sub>B</sub> = −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 (<i>r</i><sub>1</sub> and <i>r</i><sub>2</sub>; 25 °C, 0.59 T, and 25 MHz) for [EMINN]<sup>+</sup>[Br]<sup>−</sup>, [EMINN]<sup>+</sup>[BA]<sup>−</sup>, [EMINN]<sup>+</sup>[BANN]<sup>−</sup>, [EMINN]<sup>+</sup><sub>2</sub>[BA<sub>2</sub>]<sup>2−</sup>, and [EMINN]<sup>+</sup><sub>3</sub>[BA<sub>3</sub>]<sup>3−</sup>, are <i>r</i><sub>1</sub> = 0.13, 0.14, 0.32, 0.26, and 0.40 and <i>r</i><sub>2</sub> = 0.17, 0.13, 0.31, 0.30, and 0.46 mM<sup>−1</sup> s<sup>−1</sup>, respectively.