Coordination of Terpyridine to Li⁺ in Two Different Ionic Liquids

On the basis of ⁷Li NMR experiments, the complex-formation reaction between Li⁺ and the tridentate N-donor ligand terpyridine was studied in the ionic liquids [emim]­[NTf₂] and [emim]­[ClO₄] as solvents. For both ionic liquids, the NMR data implicate the formation of [Li(terpy)₂]⁺. Density functional theory calculations show that partial coordination of terpyridine involving the coordination of a solvent anion can be excluded. In contrast to the studies in solution, X-ray diffraction measurements led to completely different results. In the case of [emim]­[NTf₂], the polymeric lithium species [Li­(terpy)­(NTf₂)]_n was found to control the stacking of this complex, whereas crystals grown from [emim]­[ClO₄] exhibit the discrete dimeric species [Li­(terpy)­(ClO₄)]₂. However, both structures indicate that each lithium ion is formally coordinated by one terpy molecule and one solvent anion in the solid state, suggesting that charge neutralization and π stacking mainly control the crystallization process.