New crystalline aluminum alkoxide oxide fluorides: Evidence of the mechanism of the fluorolytic sol-gel reaction

摘要

This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al_4F_4(μ_4-O)(μ- O~iPr)_5[H(O~iPr)_2] contains a tetranuclear unit consisting of two different five fold coordinated AlFO _4-units, with F exclusively in the terminal position. Compound 2, Al_4F_4(μ_4-O)(μ-O~iPr) _5[H(O~iPr)_2]·Al_5F _5(μ_5-O)(μ-O~iPr)_8, contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al_(16)F _(20)(μ_4-O_)4(μ-O~iPr) _(20)·2(~iPrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including ~(27)Al triple quantum MAS NMR and ~1H, ~1H→ ~(13)C CP, ~(19)F and ~(27)Al MAS NMR. On the basis of the collected data, a reliable decomposition of ~(27)Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO_4-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol-gel mechanism previously discussed.