Differences in the Reactivity of Geminal Si-O-P and Al-O-P Frustrated Lewis Pairs

The new oxygen-bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu₂ P-O-Si(C₂ F₅ )₃ (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, but-3-en-2-one, nitriles and phenylacetylene to that of the Al/P FLP tBu₂ P-O-AlBis₂ (Bis=-CH(SiMe₃ )₂ ) (1). When reacted with benzil, both, 1 and 2, form the 1,2-addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but-3-en-2-one 2 forms the 1,2-addition product, while 1 binds in 1,4-position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert-butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu₂ P(H)-O-AlBis₂ (CCPh) and the FLP -C≡C 1,2-addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.