Novel Al-Based FLP Systems

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Al/P based frustrated Lewis pairs (FLPs) having coordinatively unsaturated aluminium and phosphorus atoms in a single molecule were obtained on a multigram scale by hydroalumination of alkynylphosphines. Steric shielding prevented the formation of adducts and the quenching of the conflicting Lewis acidic and basic functionalities. These FLPs reacted with terminal alkynes by C–H bond cleavage with the protons bonded to phosphorus and the alkynido groups coordinated to aluminium. Alternatively, a five-membered heterocycle was formed by C≡C triple bond activation. Similar heterocycles resulted from the reversible coordination of carbon dioxide. Particularly interesting is their unique propensity to react as effective ion pair receptors for the complexation of alkali metal hydrides and their capability to activate such ionic hydrides as phase transfer catalysts. Sterically less shielded compounds gave dimers via Al–P interactions. These compounds are still active as masked FLPs and form complexes with carbon dioxide or phenyl isocyanate. Alternative routes gave methylene bridged Al–P compounds which were also shown to coordinate CO2. Bimolecular systems exhibit similar properties. They activate terminal alkynes and isobutene or reduce CO2 to methanol and carbon monoxide. Hydroalumination of ynamines proved to be an excellent method for the generation of Al/N based Lewis pairs. These compounds were shown to activate phenylethyne reversibly or to undergo insertion reactions with carbodiimide. The constitution of the latter products is determined by cooperative interactions between aluminium and nitrogen.

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