Disclosing a novel structure of dimethylaluminum fluoride and its reactivity with metallocenes: An old molecule can be a new instrument in the toolbox of olefin polymerization catalysts

The true structure of methylalumoxane (MAO), the key component for the success of homogeous metallocene and post-metallocene catalysts for olefin polymerization, is still rather obscure, although theoretical and experimental evidence suggests a mixture of different oligomeric clusters (MeAl-O-)n. While other more common aluminum alkyls are not effective cocatalyst, we reported that AlMe2F is an efficient activator for metallocene catalysts, resulting in propene polymerization activity comparable to MAO in polar solvents. Although AlMe2F was synthesized by K. Ziegler in the 50's, the literature concerning its chemistry is surprisingly scarce: early studies based on crioscopy and IR and Raman spectra suggested a tetrameric structure with 8-membered planar ring while gas phase electron diffraction studies proposed an Al4F4 puckered ring. With the aim to understand why AlMe2F behaves in catalysis more similarly to MAO than to other aluminum alkyls, we have re-examined its solution structure by high resolution multidimensional NMR analysis, revealing the formation, inter alia, of an unprecedented heterocubane [(Me2Al)4F4] tetramer (see Figure 1), as well as its reactivity with a prototypical ansa-metallocene, resulting in the formation of ion pairs containing monomeric, dimeric or oligomeric [Me(AlMe2F)n]- anions (see Figure 2). Our results suggest that use of AlMe2F can add a new instrument to the toolbox of olefin polymerization catalysts. Figure 1 Figure 2