Crystalline Aluminum Hydroxy Fluorides: Structural Insights Obtained by High Field Solid State NMR and Trend Analyses R. Ko¨nig, † G. Scholz, † A. Pawlik, ‡ C. Ja¨ger, ‡ B. van Rossum, § H. Oschkinat, § and E. Kemnitz* ,† Institut fu¨r Chemie, Humboldt UniVersita¨t zu Berlin, Brook Taylor-Strae 2, 12489 Berlin, Germany, Bundesanstalt fu¨r Materialpru¨fung and -forschung, Richard Willsta¨tter-Strae 11, 12489 Berlin, Germany, and Leibniz-Institut fu¨r Molekulare Pharmakologie, Robert Roessle-Strae 10, 13125 Berlin, Germany ReceiVed: May 27, 2008; ReVised Manuscript ReceiVed: July 15, 2008 A series of crystalline aluminum hydroxy fluorides in cubic pyrochlore structure AlF x (OH) 3-x · H 2 O with variable F-content x were investigated by solid-state NMR by applying different magnetic fields up to 21.1 T. Distinguishable octahedral species AlF x (OH) 6-x (x ) 1-6) were identified in the crystalline aluminum hydroxy fluorides. The subsequent analysis of the highfield 27 Al MAS NMR data allows the derivation of the trend analysis graphs giving correlations between the 27 Al chemical shifts and the quadrupolar frequencies and the F-content x in AlF x (OH) 6-x . Clear trends were obtained for both, which are, along with the 19 F MAS chemical-shift trend analysis presented earlier, valuable tools for the interpretation of MAS NMR spectra of amorphous AlF x (OX) 3-x compounds (X ) H, alkyl). Following the dehydration of the pyrochlores by solid-state NMR eventually reveals a remarkable influence of the incorporated solvent molecules (H 2 O) on the 19 F chemical shift. On that basis, a new chemical-shift trend analysis for 19 F chemical shifts in correlation with x in AlF x (OH) 6-x units for proton-poor substances (in the Al, F, O, H system) was determined. By using this correlation, high-surface AlF 3 has a mean bulk Al:F ratio similar to that found for ACF, namely, AlF 2.8 (O/OH) 0.2 . Introduction High-surface aluminum fluoride (HS-AlF 3 ), an important representative of functionalized fluorides prepared by the sol-gel route, 1-3 has a strong Lewis acidity comparable to that of aluminum chloro fluoride (ACF) 4,5 or the homogeneous catalyst SbF 5 6 (Swarts catalyst). In comparison to these two, it is easier to handle, chemically more stable, and therefore a promising candidate for catalytic applications. The synthesis of HS-AlF 3 involves two steps, the first step being the reaction of Al(O i Pr) 3 with nonaqueous HF (dissolved in i PrOH) to form a wet jelly-like gel. The evaporation of the solvent yields a xerogel with a residual content of i PrOH/O i Pr groups: AlF x (O i Pr) 3-x · i - PrOH (x ≈ 2.3-2.7). 7-9 A second postfluorination step with chlorofluorocarbons (for example CHClF 2 ) then leads to the formation of high-surface area aluminum fluoride. Both HS-AlF 3 and its precursor, the aluminum alkoxide fluoride, are of special interest, because their distinct local structural features may give insights into the origin of the extremely high Lewis acidity. However, no structural informa- tion can be obtained from X-ray diffraction because both solids are X-ray amorphous. On the other hand, solid-state NMR techniques have proven to be capable to study local structures in crystalline as well as amorphous solids, which may involve the chemical surroundings of all participating nuclei. The main structural features derived from first studies concerning solid-state NMR for the aluminum alkoxide fluorides are (a) 6-fold coordinated Al-sites in a mixed oxygen/fluorine environment and (b) the incorporation of solvent molecules into the structure with the inclusion of certain F-sites in a H-bond network. 7,8 The 27 Al MAS NMR and the 19 F MAS NMR spectra suggest structural models (for the xerogel) considering more than one 6-fold AlF x (O i Pr) 6-x species. Nonetheless, literature concerning solid-state NMR investiga- tions on related AlF x O y compounds is rare, and with only the literature data in mind, an unambiguous interpretation of spectra of related solids is only barely possible (e.g., refs 10-13) and requires model systems. Recently, we succeeded in the synthesis of crystalline aluminum hydroxy fluorides with pyrochlore structure AlF x (OH) 3-x · H 2 O and varying F contents. This involved a novel sol-gel route, and the pyrochlores were reinvestigated by solid-state NMR at 9.4 T. 14 Therein (ref 14), the structural differences and similarities of the crystalline pyrochlores on one hand and amorphous aluminum alkoxide fluorides on the other hand are shown and elaborated. A principle transfer of the pyrochlore data derived from 19 F MAS solid-state NMR for an easier interpretation of the 19 F MAS NMR spectra of related amorphous compounds was demon- strated. This is possible because the involved species are comparable: for both, octahedral AlF x (OX) 6-x units (X ) Alkyl or H) are present. Additionally, for both structures, the incorporation of solvent molecules or water molecules was found, which are in both cases involved in H-bond networks. The obtained chemical-shift trend analysis straight for 19 F MAS NMR data of AlF x (OX) 6-x -containing solids now allows a consistent interpretation of 19 F MAS NMR spectra and an assignment of octahedral AlF x (OX) 6-x species. Furthermore, a prediction of an average chemical composition for an amorphous solid as well as the prediction of certain chemical shifts for unknown compounds with a distinct AlF x (OX) 6-x (e.g., x ) 5) environment becomes possible. To reveal more precise answers from 27 Al MAS NMR concerning the assignment of different species AlF x (OX) 6-x in the pyrochlores, the 27 Al MAS NMR spectra obtained at 9.4 T (400 MHz spectrometer) alone are hardly interpretable. The superimposition of the several species involved, the central lines * Corresponding author. E-mail: kemnitz@chemie.hu-berlin.de. Fax: +49 (0) 30 2093 7277. † Humboldt Universita¨t zu Berlin. ‡ Bundesanstalt fu¨r Materialpru¨fung and -forschung. § Leibniz-Institut fu¨r Molekulare Pharmakologie. J. Phys. Chem. C 2008, 112, 15708–15720 15708 10.1021/jp804662f CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008