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

Abstract

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 1Al₄F₄(μ₄-O)(μ-OⁱPr)₅[H(OⁱPr)₂] contains a tetranuclear unit consisting of two different five fold coordinated AlFO₄-units, with F exclusively in the terminal position. Compound 2, Al₄F₄(μ₄-O)(μ-OⁱPr)₅[H(OⁱPr)₂]·Al₅F₅(μ₅-O)(μ-OⁱPr)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, Al₁₆F₂₀(μ₄-O)₄(μ-OⁱPr)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 ²⁷Altriple quantumMAS NMR and ¹H, ¹H→¹³C CP, ¹⁹F and ²⁷Al MAS NMR. On the basis of the collected data, a reliable decomposition of ²⁷Alsingle pulseMAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO₄-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol–gel mechanism previously discussed.