(Chemical Equation Presented) 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 BPh 4) 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[BA 2]2-, and [EMINN]+3[BA 3]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.
All Science Journal Classification (ASJC) codes
- Organic Chemistry