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-(Pr2C6H3)-Pr-i)}(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(C6F3)(4)](-) undergoes further stoichiometric reduction with Et3SiH to produce an aluminum formate species.