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)[BA(1-3)]((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 degrees. 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 chi(mol)T 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 degrees 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.