<sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N NMR,\nand DFT Studies on Complex Formation of Zinc(II) Ion with Ethylenediamine\nin Ionic Liquid [C<sub>2</sub>mIm][TFSA]

In bis(trifluoromethylsulfonyl)amide (TFSA<sup>–</sup>)-based\nionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA<sup>–</sup> ([C<sub>2</sub>mIm][TFSA]), the complex formation equilibria of\nzinc(II) ion (Zn<sup>2+</sup>) with ethylenediamine (EN) have been\ninvestigated. An EN molecule may coordinate with Zn<sup>2+</sup> as\na bidentate ligand. First, the formation of Zn<sup>2+</sup>–EN\ncomplexes in [C<sub>2</sub>mIm][TFSA] was confirmed from the difference\nof <sup>1</sup>H and <sup>13</sup>C NMR chemical shift values of EN\nmolecules between [C<sub>2</sub>mIm][TFSA]–EN binary solvents\nand the 0.1 mol dm<sup>–3</sup> Zn(TFSA)<sub>2</sub>/[C<sub>2</sub>mIm][TFSA]–EN solutions as a function of EN mole fraction <i>x</i><sub>EN</sub>. Second, the stability constants of Zn<sup>2+</sup>–EN complexes formed in the IL were determined from\nthe concentration ratio [EN]/[Zn<sup>2+</sup>] dependence of <sup>15</sup>N NMR chemical shift values of the TFSA<sup>–</sup> N atom in the Zn<sup>2+</sup>/IL–EN solutions. In the IL,\nmono-, bis-, and tris-EN complexes are successively formed by 1:1\nreplacement of TFSA<sup>–</sup> anions coordinated with Zn<sup>2+</sup> by EN molecules with increasing EN content. Third, <sup>1</sup>H and <sup>13</sup>C NMR measurements with the help of density\nfunctional theory (DFT) calculations were made on [C<sub>2</sub>mIm][TFSA]–EN\nbinary solvents as a function of <i>x</i><sub>EN</sub> to\nclarify key interactions to the mechanism of the complex formation.\nFourth, the stability constants of Zn<sup>2+</sup>–EN complexes\nin the IL were compared with those in aqueous solutions. It was suggested\nthat the hydrogen bonding of the EN molecule with the imidazolium\nring H atoms and the TFSA<sup>–</sup> O atoms reduces the stability\nof the mono-EN complex in the IL. In contrast, the intracomplex hydrogen\nbonds between EN and TFSA<sup>–</sup> in the first coordination\nshell contribute to the higher stability of the bis-EN complex in\nthe IL than that in aqueous solutions. The difference in the stability\nconstants between the tris-EN complexes and hexaacetonitrile complexes,\nwhere acetonitrile (AN) molecules act as monodentate ligands, was\ninterpreted in terms of the higher electron donicity of EN. Finally,\nto verify the present evaluation, the experimental <sup>13</sup>C\nNMR chemical shift values of EN molecules in the solutions were compared\nwith the theoretical values calculated by DFT using the stability\nconstants determined.