Ionic liquids with more than one metal: optical and electrochemical properties vs. d-metal combinations.

Thirteen N-butylpyridinium salts, including three monometallic [C4Py]2[MCl4], nine bimetallic [C4Py]2[M1-xaMxbCl4] and one trimetallic compound [C4Py]2[M1-y-zaMybMzcCl4] (M = Co, Cu, Mn; x = 0.25, 0.50 or 0.75 and y = z = 0.33), were synthesized and their structure and thermal and electrochemical properties were studied. All compounds are ionic liquids (ILs) with melting points between 69 and 93°C. X-ray diffraction proves that all ILs are isostructural. The conductivity at room temperature is between 10-4 and 10-8 S cm-1. Some Cu-based ILs reach conductivities of 10-2 S cm-1, which is, however, probably due to IL dec. This correlates with the optical band-gap measurements indicating the formation of large band gap semiconductors. At elevated temperatures approaching the melting points, the conductivities reach up to 1.47 ∙ 10-1 S cm-1 at 70 oC. The electrochemical stability windows of the ILs are between 2.5 and 3.0 V.