"Masked" Lewis-acidity of an aluminum alpha-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 = 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 = 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