Optical basicity of ionic liquids.

The concept of optical basicities initially developed by Duffy for solid oxides was successfully transferred to low melting salts such as ionic liquids (ILs). Several ionic liquids (ILs) were doped with Mn(NTf(2))(2) (NTf(2) = bis(trifluoromethanesulfonyl)imide) as a probe to investigate the IL anion basicity by means of UV-Vis spectroscopy. The (6)A(1)-->(4)A, (4)E(G) transition band of Mn(2+) is influenced by the electron donor properties, i.e., Lewis basicity, of the coordinating ligands but remains-to the first order-independent from the ligand field. The change in the optical spectra for various Mn(NTf(2))(2) doped ILs is predominantly dependent on the anion. No significant influence of the IL cation could be detected for the ILs under investigation except for functionalized cations with electron donor functionalities. This allows ranking the ILs with respect to their (optical) anion basicities. For the most prominent IL anions an order of BF(4)(-) < ClO(4)(-) < NTf(2)(-) approximately MeSO(3)(-) < NO(3)(-) < DCA(-) < TFA(-) < EtSO(4)(-) approximately OTos(-) < Cl(-) < Br(-) with respect to rising optical basicity could be established.