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

Abstract

Whereas liquid CCl3F reacts with solid AlCl3 exothermically under chlorine-fluorine-exchange already above -20 degrees C, no reaction takes place between CCl3F and the pyridine complexes of AlCl3 (AlCl3.Py, AlCl3.2Py, or AlCl3.3Py) up to 100 degrees 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.

Lewis Superacidic Ionic Liquids with Tricoordinate Borenium Cations

“…45.5 ppm for P888-BCl3-2MCl3 systems is consistent with [BCl2{P888}] + [23] and BCl3. Partial ligand transfer onto a metal centre was confirmed with 27 Al NMR spectroscopy (Figure 3-SI, [24][25][26] with a shoulder at 75±1 ppm, originating from a pentacoordinate complex, [AlCl3L2], and/or cationic complex, [AlCl2L2] + . [25,27] 27 Al NMR spectrum of P888-BCl3-2AlCl3 gave a broad feature with two maxima at 66 and 114 ppm, consistent with species analogous to those identified for Ndonors.…”

Spectroscopic characterization of crystalline AlF3 phases

“…More local structural peculiarities can be discovered applying 27 Al and 19 F MAS NMR spectroscopy. Although a rotation frequency of 25 kHz was applied the presence of possibly different fluorine sites, as shown by XRD, cannot be resolved by 19 F MAS NMR (cf.…”

“…Nonetheless we used in this work the more common C 1s binding energy reference (284.8 eV) to be compatible to NIST Xray Photoelectron Spectroscopy Database [17]. 19 F and 27 Al NMR parameters (chemical shifts and quadrupolar frequency) were exactly determined for a-AlF 3 [20][21][22]. NMR parameters for b-AlF 3 were only given by Chupas et al [20] with one 27 Al parameter set (n Q , h Q ) for the two crystallographically determined aluminium sites.…”

“…19 F and 27 Al NMR parameters (chemical shifts and quadrupolar frequency) were exactly determined for a-AlF 3 [20][21][22]. NMR parameters for b-AlF 3 were only given by Chupas et al [20] with one 27 Al parameter set (n Q , h Q ) for the two crystallographically determined aluminium sites. Herron et al [23][24][25] reported the preparation and structures of h-, k-and u-AlF 3 , estimated their BET surface areas and characterized the catalytic activities for halogen exchange reactions of all aluminium fluoride phases known so far.…”

“…For this purpose FT IR, XPS and particularly 27 Al and 19 F MAS NMR spectroscopic techniques were applied.…”