Effects of hydrogen-bonding properties of formamide on complex formation with zinc(ii) ions in ionic liquid [C2mIm][TFSA]

Abstract

In a bis(trifluoromethylsulfonyl)amide (TFSA⁻)−based ionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA⁻ ([C₂mIm][TFSA]), the complex formation of zinc(II) ions (Zn²⁺) with molecular liquid (ML) of formamide (FA) has been observed using ¹H, ¹³C, and ¹⁵N NMR and mass spectroscopy combined with DFT calculations. The effects of the strong hydrogen bonding donicity and acceptability of the two amino H atoms and carbonyl O atoms of the FA molecule on the complex formation were discussed in terms of the microscopic interactions of FA molecules with the imidazolium ring of C₂mIm⁺, TFSA⁻, and FA themselves evaluated by ¹H and ¹³C NMR measurements, DFT calculations, and MD simulations. Mass spectrometry on Zn²⁺/[C₂mIm][TFSA] solution provided an answer to the controversial structure of the solvation complex of Zn²⁺ with TFSA⁻. Three TFSA⁻ anions coordinate to Zn²⁺; the anions play a role as the bidentate ligand and make chelate rings form [Zn(tfsa)₃]⁻. In Zn²⁺/[C₂mIm][TFSA]–FA solutions, each of the three TFSA⁻ anions coordinated with Zn²⁺ is successively replaced by two FA molecules with increasing FA contents. Di-, tetra-, and hexa-FA–Zn²⁺ complexes are formed in solutions. The high hydrogen-bonding ability of FA molecules markedly reduces the stability of the Zn²⁺–FA complexes. The stabilities of Zn²⁺ complexes with monodentate FA were compared with those with bidentate ethylenediamine (EN), which also has a strong self-hydrogen bonding ability. The main factors for the reduction of the complex stabilities are different from each other; the self-aggregation of FA molecules by hydrogen bonding significantly contributes to the stabilities in the former, whereas the hydrogen bonding of EN molecules with the imidazolium ring is remarkable in the latter.