1H, 13C, 15N NMR, and DFT Studies on Complex Formation of Zinc(II) Ion with Ethylenediamine in Ionic Liquid [C2mIm][TFSA]

Abstract

In bis(trifluoromethylsulfonyl)amide (TFSA^–)-based ionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA^– ([C₂mIm][TFSA]), the complex formation equilibria of zinc(II) ion (Zn²⁺) with ethylenediamine (EN) have been investigated. An EN molecule may coordinate with Zn²⁺ as a bidentate ligand. First, the formation of Zn²⁺–EN complexes in [C₂mIm][TFSA] was confirmed from the difference of ¹H and ¹³C NMR chemical shift values of EN molecules between [C₂mIm][TFSA]–EN binary solvents and the 0.1 mol dm^–3 Zn(TFSA)₂/[C₂mIm][TFSA]–EN solutions as a function of EN mole fraction x_EN. Second, the stability constants of Zn²⁺–EN complexes formed in the IL were determined from the concentration ratio [EN]/[Zn²⁺] dependence of ¹⁵N NMR chemical shift values of the TFSA^– N atom in the Zn²⁺/IL–EN solutions. In the IL, mono-, bis-, and tris-EN complexes are successively formed by 1:1 replacement of TFSA^– anions coordinated with Zn²⁺ by EN molecules with increasing EN content. Third, ¹H and ¹³C NMR measurements with the help of density functional theory (DFT) calculations were made on [C₂mIm][TFSA]–EN binary solvents as a function of x_EN to clarify key interactions to the mechanism of the complex formation. Fourth, the stability constants of Zn²⁺–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 ¹³C NMR chemical shift values of EN molecules in the solutions were compared with the theoretical values calculated by DFT using the stability constants determined.