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

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Abstract
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 (Tf2N) anions provide the charge balance. Two types of metal complexes have been analysed. One has the general formula [Cu(AlkIm)4][Tf2N]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][Tf2N]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.

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