“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 CO 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 CC 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.