F/Cl-Exchange on AlCl₃-Pyridine Adducts: Synthesis and Characterization of <i>trans-</i>Difluoro-tetrakis-pyridine-aluminum-chloride, [AlF₂(Py)₄]⁺Cl^-

Whereas liquid CCl3F reacts with solid AlCl3 exothermically under chlorine−fluorine-exchange already above −20 °C, no reaction takes place between CCl3F and the pyridine complexes of AlCl3 (AlCl3·Py, AlCl3·2Py, or AlCl3·3Py) up to 100 °C. The desired chlorine by fluorine substitution on the monomer AlCl3−pyridine adducts occurs, however, easily using Me3SiF as fluorinating agent. By reacting AlCl3·3Py with Me3SiF (even up to 10-fold stoichiometric excess) in pyridine as a solvent, only two of the three Cl atoms can be substituted by fluorine, leading in good yield to the new “mixed aluminum halide”, AlF2Cl·4Py. Actually, it represents the first example of a stable solid donor−acceptor adduct of an aluminum-III halide with two different halogens of defined stoichiometry. It was characterized by multinuclear solid-state NMR (27Al and 19F), IR spectroscopy, as well as single-crystal structure analysis. The new compound has an ionic solid-state structure with helical trans-octahedral [(Py)4AlF2]+ cations and isolated Cl- anions. The comparison of its 27Al MAS solid-state NMR spectra with those of a compound bearing the analogous [(Py)4AlCl2]+ cation reveals an extreme increase in the quadrupolar coupling constants, from 0.24 MHz in case of the chlorine cation to about 16 MHz in case of the new [(Py)4AlF2]+ cation.