Understanding the Chemical Reactivity of Phosphonium-Based Ionic Liquids with Tellurium

The chemical reactivity of phosphonium based ionic liquids (ILs) towards tellurium at temperatures above 220 °C was systematically investigated by a series of dissolution experiments, tracking the solute tellurium species by nuclear magnetic resonance, and characterizing the reaction products by X-ray diffraction and scanning electron microscopy. The initial step is the thermal elimination of an alkyl group of the phosphonium cation of the ILs, most probably via an S_N 2 mechanism. The addition of tellurium follows to form trialkylphosphane tellurides as evidenced by ³¹ P and ¹²⁵ Te NMR spectroscopic experiments. The trialkylphosphane tellurides can serve as a tellurium reservoir for the formation of metal tellurides, like Bi₂ Te₃ and Ag₂ Te. It was observed that trihexyltetradecylphosphonium chloride ([P_{6 6 6 14} ]Cl) shows a very weak reactivity that is reflected by a low solubility of tellurium, while trihexyltetradecylphosphonium dicyanamide/decanoate ([P_{6 6 6 14} ][N(CN)₂ ]/[P_{6 6 6 14} ][decanoate]) and tetrabutylphosphonium decanoate ([P_{4 4 4 4} ][decanoate]) dissolve tellurium to a much higher extent. We attribute these observations to the different Lewis basicity of the anions of the ILs as main influencing factor.