"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.