Ionic Liquids in Wonderland: from Electrostatics to Coordination Chemistry

This work represents the first systematic study comparing a homologous series of alkali-based ionic liquids (ILs) - potassium 1-alkyl-3-methylcyclopentadienyl, K[C _n C ₁ Cp] - with their charge-inverted counterparts of the 1-alkyl-3-methylimidazolium chloride series, [C _n C ₁ Im]Cl. Three new compounds of the K[C _n C ₁ Cp] (n = 2, 8, or 10) were synthesized, purified, and analyzed by nuclear magnetic resonance, completing the series of previously reported derivatives (n = 4, 6). Further characterization involved differential scanning calorimetry and powder X-ray diffraction determinations. The results show that most of the alkali-based salts exhibit melting temperature values in the range commonly observed for halide-based ILs. However, striking structural differences were revealed by both X-ray diffraction and molecular dynamics results. These findings are consistent with the fact that alkali metal cations are efficient electron acceptors that can participate in interactions with a significant covalent character, namely, with aromatic moieties present in the cyclopentadienyl anions. This new concept extends the boundaries of ILs from the realm of noncovalent electrostatic systems to that of coordination chemistry.