A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine

The highly Lewis acidic, cationic aluminum species [DIPP-nacnacAlMe]⁺[B­(C₆F₅)₄]⁻ (1, DIPP-nacnac = [HC­{C­(Me)­N­(2,6-ⁱPr₂C₆H₃)}₂]⁻) has been shown to undergo reactions with a wide variety of small molecules, in both the presence and absence of an external weak phosphine base, PPh₃. Cycloaddition reactions of unsaturated C–C bonds across the aluminum diketiminate framework are reported, and the first structural confirmation of this type of cycloaddition product is presented. Addition of PPh₃ to 1 produces the cationic aluminum phosphine complex [DIPP-nacnacAl­(Me)­PPh₃]⁺[B­(C₆F₅)₄]⁻, which undergoes fluxional dissociation/coordination of the phosphine in solution. This weak Al–P interaction can be utilized in frustrated Lewis pair type reactions to activate alkenes, alkynes, CO₂, propylene oxide, and the C–Cl bonds of CH₂Cl₂. The CO₂ adduct [DIPP-nacnacAl­(Me)­OC­(PPh₃)­O]⁺[B­(C₆F₅)₄]⁻ undergoes further stoichiometric reduction with Et₃SiH to produce an aluminum formate species.