Solvation Structure of Ag+ in 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid: Evidence for Linear N-Bound Coordination

The coordination environment of the Ag+ ion in the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) ionic liquid (IL) was investigated through a combined experimental and theoretical approach including Raman and Fourier transform-infrared (FT-IR) spectroscopy, density functional theory (DFT) calculations, and X-ray absorption spectroscopy. Raman spectra in the 720–780 cm–1 region show the progressive conversion of “free” to coordinated [Tf2N]− anions upon addition of the silver salt to [C4mim][Tf2N], and quantitative analysis indicates an average coordination number of approximately two anions per Ag+ ion. DFT calculations on the possible coordination isomers of the [Ag(Tf2N)2]− complex reveal that the most stable species consists of two monodentate [Tf2N]− ligands bound to the Ag+ cation through the central nitrogen atom, in agreement with the characteristic shifts observed in the experimental FT-IR spectra. Extended X-ray absorption fine structure analysis supports the presence of a well-defined first coordination shell featuring two Ag–N interactions at 2.23(2) Å arranged in an almost linear N–Ag–N geometry. Overall, the combined results establish a predominantly linear, N-bound solvation motif for Ag+ in [C4mim][Tf2N]. This behavior contrasts with the coordination observed in other ILs and highlights the strong influence of the anion on metal ion solvation in these media.