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.