H, C, N NMR, and DFT Studies on Complex Formation of Zinc(II) Ion with Ethylenediamine in Ionic Liquid [CmIm][TFSA].

In bis(trifluoromethylsulfonyl)amide (TFSA)-based ionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA ([CmIm][TFSA]), the complex formation equilibria of zinc(II) ion (Zn) with ethylenediamine (EN) have been investigated. An EN molecule may coordinate with Zn as a bidentate ligand. First, the formation of Zn-EN complexes in [CmIm][TFSA] was confirmed from the difference of H and C NMR chemical shift values of EN molecules between [CmIm][TFSA]-EN binary solvents and the 0.1 mol dm Zn(TFSA)/[CmIm][TFSA]-EN solutions as a function of EN mole fraction . Second, the stability constants of Zn-EN complexes formed in the IL were determined from the concentration ratio [EN]/[Zn] dependence of N NMR chemical shift values of the TFSA N atom in the Zn/IL-EN solutions. In the IL, mono-, bis-, and tris-EN complexes are successively formed by 1:1 replacement of TFSA anions coordinated with Zn by EN molecules with increasing EN content. Third, H and C NMR measurements with the help of density functional theory (DFT) calculations were made on [CmIm][TFSA]-EN binary solvents as a function of to clarify key interactions to the mechanism of the complex formation. Fourth, the stability constants of Zn-EN complexes in the IL were compared with those in aqueous solutions. It was suggested that the hydrogen bonding of the EN molecule with the imidazolium ring H atoms and the TFSA O atoms reduces the stability of the mono-EN complex in the IL. In contrast, the intracomplex hydrogen bonds between EN and TFSA in the first coordination shell contribute to the higher stability of the bis-EN complex in the IL than that in aqueous solutions. The difference in the stability constants between the tris-EN complexes and hexaacetonitrile complexes, where acetonitrile (AN) molecules act as monodentate ligands, was interpreted in terms of the higher electron donicity of EN. Finally, to verify the present evaluation, the experimental C NMR chemical shift values of EN molecules in the solutions were compared with the theoretical values calculated by DFT using the stability constants determined.