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

Abstract

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 AlX(3)·3Py (X = Cl, Br, I) with Me(3)SiF in pyridine, two of the three halogens X were substituted by fluorine atoms forming the "aluminum mixed halide" complexes AlF(2)X·4Py with the ionic solid-state structures [Al(Py)(4)F(2)]X. Whereas the (27)Al solid state NMR spectra of AlX(3)·3Py (X = Cl, Br) confirmed the existence of the expected singular σ(6)λ(3)-Al centers in their structures, the corresponding spectrum of AlI(3)·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 (AlI(3)·2Py), which in accord to the (27)Al MAS NMR spectra possessed only one σ(4)λ(3)-Al side as in the ionic structure [Al(Py)(2)I(2)]I. AlBr(3)·3Py was also transformed by pyridine into the ionic complex [Al(Py)(4)Br(2)]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)(n)X(2)](+) cations are most probably the dominating species in pyridine solutions of AlX(3). Thus, only two Al-X covalent bonds underwent X/F- exchange and the halogen exchange reactions were terminated at " [Al(Py)(4)F(2)](+) stage". The hydrolysis of [Al(Py)(4)F(2)]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(H(2)O)(4)F(2)](+) cation was confirmed by single-crystal XRD analysis. By reacting [Al(Py)(4)F(2)]Cl with the cyclo-n-propyl-phosphonic acid anhydride [CH(3)CH(2)CH(2)-PO(2)](3), an unexpected F-migration from Al- to P- atoms was observed.