Effects of the long octyl chain on complex formation of nickel(ii) with dimethyl sulfoxide, methanol, and acetonitrile in ionic liquid of [C8mim][TFSA]

Abstract

In the room-temperature ionic liquid (IL) of 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide ([C₈mim][TFSA]), the complex formation of Ni²⁺ with molecular liquids (MLs), dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN), has been examined using ultraviolet (UV)-visible spectroscopy. The overall stability constants log β_n, enthalpies , and entropies of the equilibria have been determined to elucidate the mechanism of complex formation. From a comparison of such thermodynamic parameters of the present [C₈mim][TFSA] systems with those of the previous systems of 1-ethyl-3-methylimidazolium-based IL, [C₂mim][TFSA], the effects of the octyl chain of the imidazolium cation, [C₈mim]⁺, on the complex formation of Ni²⁺ with MLs have been demonstrated. In [C₈mim][TFSA]–ML systems, more stable complexes are formed with MLs in the sequence of AN > DMSO ≫ MeOH. This sequence differs from that of DMSO ≫ AN > MeOH in [C₂mim][TFSA]. For the AN systems, the stabilities of [Ni(an)_n] in [C₈mim][TFSA] are higher as compared to those in [C₂mim][TFSA]. In contrast, for the DMSO systems, [Ni(dmso)_n] is less stable in the IL with the longer alkyl chain than that in the IL with the shorter chain. The dependence of the alkyl chain length on the stabilities of [Ni(meoh)_n] is the least significant among the three MLs. These varieties of the stabilities of Ni²⁺ complexes with the MLs have been interpreted from the thermodynamic parameters, together with the static interactions in the [C₈mim][TFSA]–ML and [C₂mim][TFSA]–ML solvents observed by means of ¹H and ¹³C NMR, small-angle neutron scattering (SANS), and infrared (IR) with an ATR diamond prism.