Crystal structures of low-melting ionic transition-metal complexes with N-alkylimidazole ligands

A series of ionic liquids with copper(ii), nickel(ii) or cobalt(ii)-containing cations have been synthesised and characterised, where possible with single crystal X-ray diffraction. A number of N-methylimidazole (MeIm), N-ethylimidazole (EtIm), N-butylimidazole (BuIm) or N-hexylimidazole (HeIm) ligands are coordinated to the metal centres and bis(trifluoromethyl) sulfonyl imide (Tf ₂N) anions provide the charge balance. Two types of metal complexes have been analysed. One has the general formula [Cu(AlkIm) ₄][Tf ₂N] ₂ (AlkIm = MeIm, EtIm, BuIm and HeIm) and contains square planar copper(ii) centres with longer interactions to oxygen atoms of bis(trifluoromethylsulfonyl)imide anions. The other type of metal complex has a metal:ligand ratio of 1:6, with composition [M(MeIm) ₆][Tf ₂N] ₂ (M = Cu, Ni, Co) and contains octahedral metal centres. The ionic liquids have low melting points and the change in melting points is discussed as a function of alkyl chain length on the imidazole ligand and as a function of the metal centre. The metal-ligand interactions and intermolecular interactions in the crystal structures are analysed and related to the properties of the metal complexes.