Formation of ionic liquids of divalent metal complexes comprising N‑alkylethylenediamines and the solvation of the nickel(II) complexes

Abstract

A series of divalent N‑alkylethylenediamine (alkyl‑en) metal(II) (alkyl = hexyl, 2‑ethylhexyl, octyl, dodecyl; metal = Ni, Cu, and Zn) complexes was prepared and their phase behavior was studied using differential scanning calorimetry. This kind of metal complexes is very useful for systematically investigating the relationship between phase behavior and molecular structures. It was found that several of the zinc(II) and nickel(II) complexes form room-temperature ionic liquids (RTILs), despite the divalent cation.Although the solid-to-liquid transition temperatures of metal(II) complex-based ILs are typically higher than those of the more common monovalent ionic liquids (ILs), they are dependent on the nature and combination of the metal(II) ions, alkyl chains, and counter anions. The zinc(II) complexes coupled with weakly coordinating bis(fluorosulfonyl)amide (FSA) or bis(trifluoromethanesulfonyl)amide (Tf2N) anions have significantly lower melting points, which is attributable to the longer distance between the zinc(II) ions and the counter anions upon the formation of tetrahedral bis(alkyl‑en)zinc(II) complexes as compared to the corresponding distances in NO3 and trifluoroacetate (TFA) complexes. The correlation of melting points with the molecular structures of the zinc(II) complexes is similar to that for the silver(I) alkyl‑en complexes. The tris(alkyl‑en)nickel(II) complexes coupled with Tf2N counter ions do not readily solidify and have glass transition temperatures below 0 °C, whereas the corresponding bis complexes have much higher melting points despite the counter-ions residing in the outer-spheres of the nickel(II) ions. The interactions of NO3, TFA, FSA, and Tf2N anions with the nickel(II) ions of the bis(alkyl‑en) complexes and their solvation behaviors were also studied in organic solvents using visible absorption spectroscopy based on the structures of the neat states. The characteristic solvation behaviors of the nickel(II) complexes were rationalized in terms of the counter anions and solvents.