Syntheses and Characterization of Salts with the [Al(D)₄F₂]⁺ Cation (D = Pyridine or Water)

Our approach for preparation of tetrakis-(pyridine)-difluoro-aluminum chloride was successfully deployed for the synthesis of corresponding Br and I compounds, respectively. By reacting AlX3·3Py (X = Cl, Br, I) with Me3SiF in pyridine, two of the three halogens X were substituted by fluorine atoms forming the “aluminum mixed halide” complexes AlF2X·4Py with the ionic solid-state structures [Al­(Py)4F2]­X. Whereas the 27Al solid state NMR spectra of AlX3·3Py (X = Cl, Br) confirmed the existence of the expected singular σ6λ3-Al centers in their structures, the corresponding spectrum of AlI3·3Py does not contain any signal that belongs to a 6-fold coordinated Al atom. The elemental analysis data strongly support the 1:2-stoichiometry of the complex (AlI3·2Py), which in accord to the 27Al MAS NMR spectra possessed only one σ4λ3-Al side as in the ionic structure [Al­(Py)2I2]­I. AlBr3·3Py was also transformed by pyridine into the ionic complex [Al­(Py)4Br2]­Br. The later was isolated from pyridine solutions, and its structure was determined by X-ray single crystal analysis. On the basis of our results, solvated [Al­(Py)nX2]+ cations are most probably the dominating species in pyridine solutions of AlX3. Thus, only two Al–X covalent bonds underwent X/F- exchange and the halogen exchange reactions were terminated at “ [Al­(Py)4F2]+ stage”. The hydrolysis of [Al­(Py)4F2]Cl by very diluted hydrochloric acid in methanol proceeded smoothly under preservation of the Al–F bonds and displacement of pyridine by water. The formation of the stable helical trans-octahedron [Al­(H2O)4F2]+ cation was confirmed by single-crystal XRD analysis. By reacting [Al­(Py)4F2]Cl with the cyclo-n-propyl-phosphonic acid anhydride [CH3CH2CH2–PO2]3, an unexpected F-migration from Al- to P- atoms was observed.