New Low-Melting Triply Charged Homoleptic Cr(III)-Based Ionic Liquids in Comparison to Their Singly Charged Heteroleptic Analogues.

Tim Peppel,Martin Köckerling

doi:10.3390/ma14102676

Abstract

A series of new low-melting triply charged homoleptic Cr(III)-based ionic liquids of the general formula (RMIm)3[Cr(NCS)6] (R = methyl, ethyl, n-butyl, benzyl) is reported. Their syntheses and properties are described in comparison to their singly charged heteroleptic analogues of the general formula (RMIm)[Cr(NCS)4L2] (R = methyl, ethyl, n-butyl, benzyl; L = pyridine, γ-picoline). In total, sixteen new Reineckate related salts with large imidazolium cations are described. Out of these, five compounds were crystallized, and their structures determined by single-crystal X-ray structure analyses. They all consisted of discrete anions and cations with octahedrally coordinated Cr(III) ions. In the structures, various hydrogen contacts interconnect the entities to build up hydrogen bonded networks. Thermal investigations showed relatively low melting points for the homoleptic complexes. The compounds with the [Cr(NCS)6]3− anion melt without decomposition and are stable up to 200 K above their melting points. The complex salts with the [Cr(NCS)4L2]− anion, in contrast, start to decompose and lose L molecules (Pyr or Pic) already at the melting point.

Highlights

For more than 30 years, ionic liquids (ILs), designated as organic salts with melting points below 100 ◦ C, have been a broad field of investigation due to their in part unique properties
ILs play an important role as starting materials for N-heterocyclic carbenes (NHCs), which can be applied in a variety of different catalytic reactions [6,7,8]
We extend the series of Reineckate-analogue compounds to triply charged homoleptic as well as singly charged heteroleptic complexes of the general formula (RMIm)x [Cr(NCS)4 L2 ] (R = methyl, ethyl, n-butyl, benzyl; MIm = N-methylimidazolium; x = 3: L = NCS; x = 1: L = Pyr, Pic; Pyr = pyridine; Pic = γ-picoline = 4-methylpyridine)

Summary

Introduction

For more than 30 years, ionic liquids (ILs), designated as organic salts with melting points below 100 ◦ C, have been a broad field of investigation due to their in part unique properties. They exhibit, for instance, large electrochemical windows, liquid ranges, hardly measurable vapor pressures, special solubility characteristics, and applications in catalysis [1,2,3,4,5]. ILs have been often called “designer solvents” and have been used in research fields fitting into the “Green Chemistry” approach Due to their low vapor pressures at room temperature, they are used as substitutes for common organic solvents. ILs play an important role as starting materials for N-heterocyclic carbenes (NHCs), which can be applied in a variety of different catalytic reactions [6,7,8]

Methods

Results

Conclusion