Mixing States of Ionic Liquid-Molecular Liquid Mixed Solvents and Their Effects on Metal Complex Formation

Abstract

In this chapter, the complex formation of nickel(II) ion (Ni²⁺) in imidazolium-based ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C_NmimTFSA, N represents the alkyl chain length) with molecular liquids (MLs), such as dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN), observed by ultraviolet (UV)-visible spectroscopy is explained on the molecular level. In C_NmimTFSA, Ni²⁺ is coordinated with the six oxygen atoms of TFSA⁻ to form an octahedral complex. On adding ML into the Ni²⁺-IL solutions, the replacement of the TFSA⁻ anions by the ML molecules is progressed due to the higher electron donicity of the MLs than that of TFSA⁻. The stability constants of the complex formation at various temperatures were determined from the UV-visible spectra. It is expected that the mixing state of IL and ML, including the various microscopic interactions, such as IL-cation-anion, IL-cation-ML, and IL-anion-ML, and ML-ML, may influence the complex formation equilibria. Moreover, the polar domains consisting of the charged imidazolium ring and TFSA⁻ and the nonpolar domains arising from the non-charged alkyl chains may also affect the equilibria. To discuss the mechanism of the complex formation equilibria, thermodynamic parameters, enthalpies ΔH° and entropies ΔS°, were estimated using the van’t Hoff plots on the stability constants of the complex formation. Moreover, the microscopic interactions were also clarified using infrared (IR), ¹H and ¹³C NMR, small-angle neutron scattering (SANS) techniques.