Liquid Coordination Complexes Formed by the Heterolytic Cleavage of Metal Halides

Abstract

Liquids containing very high metal concentrations are of value for a number of applications: from electrochemistry and Lewis acidic catalysis through to metals refining and inorganic materials syntheses. In the ideal case, such liquids should have well-controlled and tunable metal coordination, leading to adjustable physical properties (i.e. viscosity modifiable from mobile liquid to glass). Some of these requirements have been met by concentrated aqueous or acetonitrile solutions, [1] molten metals/alloys, molten salts, [2] ionic liquids, [3] or deep eutectic solvents.[4] However, each approach has some drawbacks, such as high cost, solubility limits, corrosiveness, or limited control over the metal coordination environment; consequently, the ideal system remains elusive.Herein, we describe a synthetic route to liquid coordination complexes (LCCs, shown in Figure 1), identify the metal coordination environment referring to in such systems, and present LCCs as a promising addition to the metals in the liquid state toolbox.A large number of ligands were reported to induce asymmetric splitting of the "M 2 Cl 6 " unit, where M = Al III or Ga III , leading to the formation of an ionic compound of formula [MCl 2 L 2 ][MCl 4 ].[5] These were investigated primarily within two distinct disciplines, coordination chemistry and electrochemistry. The traditional methodology of a coordination chemist involves allowing a metal halide and ligand to react in a solvent, using excess or equimolar amount of ligand, followed by the isolation of a crystalline product, and its analysis by single-crystal X-ray diffraction. Hence, inevitably, the majority of reported products of asymmetric cleavage are crystalline solids. For example, Richard and Beavers [6]

An AlCl3 based ionic liquid with a neutral substituted pyridine ligand for electrochemical deposition of aluminum

“…The peak at 103 ppm was attributed to the symmetrical tetrahedral anion AlCl 4 À , and the other one at 108 ppm was assigned to the cationic aluminum species, corresponding to [AlCl 2 (4-Pr-Py) 2 ] + , with similar tetrahedral structure. The downfield shift was caused by the coordination legend changes [25]. The two coordinated aluminum species observed by NMR spectra were consistent with those observed by mass spectroscopy.…”

High Coulombic efficiency aluminum-ion battery using an AlCl 3 -urea ionic liquid analog electrolyte

“…4D). However, the spectrum of AlCl 3 /urea = (22). The resonance at 52.7 ppm was broad and low intensity and is shown clearly in Fig.…”

Electrodeposition of Al from a 1-butylpyrrolidine-AlCl3 ionic liquid

“…At a composition of 1:1 two signals were observed, one is at 103.4 and the other one is at 108.3 ppm. The peak at 103.4 ppm is attributed to be AlCl 4 À , according to the literature, where it is found $ 103 ppm [31]. The peak at 108.3 ppm is due to cationic species like AlCl 3 À x L þ y (where x¼ 1À 2), possibly to [AlCl 2 (1-butylpyrrolidine) 2 ] þ .…”