"Masked" Lewis-acidity of an aluminum α-phosphinoamide complex.

The reaction of Ph2P(DIPP)NH with AlMe3 cleanly gives an aluminum amide complex that crystallizes as a centrosymmetric dimer with a six-membered Al-N-P-Al-N-P ring. In aromatic solvents the dimer remains intact but the Al-P bond is readily broken upon addition of THF to form Ph2P(DIPP)NAlMe2·THF. Efforts to use [Ph2P(DIPP)NAlMe2]2 as a "masked" Lewis acidic activator for olefin polymerization catalysts were unsuccessful but the complex showed a Frustrated Lewis pair reactivity instead. The P/Al complex reacts with isocyanates to give the C[double bond, length as m-dash]O inserted product that crystallizes as a five-membered ring system Al-O-C(NR)-P-N. The reaction of [Ph2P(DIPP)NAlMe2]2 with CO2, however, gave an insertion in the N-Al bond and the dimeric product [Ph2P(DIPP)NCO2AlMe2]2 was isolated. The dimer [Ph2P(DIPP)NAlMe2]2 is one of the few Al/P FLPs that can activate C[double bond, length as m-dash]C double bonds irreversibly. A reaction with allyl methyl sulfide and 1-hexene led to the clean formation of the structurally similar activated alkene products [(DIPP)N-Ph2P-CH(CH2SMe)CH2]AlMe2 and [(DIPP)N-Ph2P-CH(C4H9)CH2]AlMe2.