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 χ_molT vs T plot for [EMINN]⁺[BANN]⁻ was analyzed using a four-spin model to give J₁/k_B = −0.27 and J₂/k_B = −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/k_B = −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.