Aluminum Species and the Synthesis Mechanism of AlCl3–CuCl–Arene Solutions

Aluminum species in AlCl3–arene and AlCl3–CuCl–arene solutions (including benzene, toluene, o-xylene, m-xylene, p-xylene, and mesitylene) were investigated using 27Al NMR and infrared (IR) spectroscopy. The AlCl3·arene and Al2Cl6·arene complexes were demonstrated experimentally. AlCl3 could not coordinate with benzene and toluene at a low temperature (40 °C) but could form complexes at a high temperature (80 °C). AlCl3 could coordinate with xylene and mesitylene to form Al2Cl6·arene complexes at both low and high temperatures (40 and 80 °C). As for AlCl3–CuCl–arene solutions with a molar ratio of 1:1:2, two signals were observed in 27Al NMR spectra. The signal at around 101 ppm was assigned to CuAlCl4·2arene complexes, and the other signal at around 96 ppm belonged to AlCl3·arene complexes. The characteristic peaks of AlCl3–CuCl–arene solutions in IR spectra moved toward the low field due to the strong π-back-bonding. The CuAlCl4·2arene complexes would convert to CuAl2Cl7·2arene complexes with a further increase in AlCl3 content when the molar ratio of AlCl3/CuCl exceeded 1. The synthesis mechanism of AlCl3–CuCl–arene solutions was analyzed by in situ Fourier transform IR (FTIR) spectroscopy. Three reaction pathways originating from different aluminum species were proposed.