Effects of hydrogen-bonding properties of formamide on complex formation with zinc(II) ion 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 of zinc(II) ion (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 boding donicity and acceptability of the two amino H atoms and carbonyl O atoms of 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 calculation, and MD simulations. Mass spectrometry on Zn2+/[C2mIm][TFSA] solution gave the 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 bidentate ligand and make chelate rings to form [Zn(tfsa)3]−. In Zn2+/[C2mIm][TFSA]–FA solutions, each of 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 the 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 has also the strong self-hydrogen bonding ability. The main factors for the reduction of the complex stabilities are different 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.