Frustrated Lewis Pair Behavior of an Open, Noninteracting Phosphane/Borane Pair at a Rigid Organic Framework: Exploring Decisive Factors for FLP Activity

The open, noninteracting vicinal P/B frustrated Lewis pair (FLP) at the very rigid, dibenzobicyclo[2.2.2]octadiene (“semi-triptycene”) framework shows a trans 1,2-arrangement of the Mes 2 P– and (C 6 F 5 ) 2 B– Lewis base/Lewis acid pair. It is an active dihydrogen splitting reagent and shows a great variety of typical frustrated Lewis pair reactions under mild reaction conditions. This includes the 1,1-addition reaction to nitric oxide (NO) to form a persistent FLPNO • nitroxide radical as well as CO reduction by Piers’ borane [HB(C 6 F 5 ) 2 ] at the reactive FLP template. The new FLP undergoes thermodynamically favorable 1,4-addition reactions to 1,2-diketones. Most products were characterized by X-ray diffraction, and the 1,4-diketone addition reaction was analyzed by DFT calculation. A comparison with the internally interacting “parent” vicinal Mes 2 PCH 2 CH 2 B(C 6 F 5 ) 2 system revealed some decisive indications for very active FLPs.