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-tripty-cene) framework shows a trans 1,2-arrangement of the Mes(2)P- and (C6F5)(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 center dot nitroxide radical as well as CO reduction by Piers' borane [HB(C6F5)(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)PCH2CH(2)B(C6F5)(2) system revealed some decisive indications for very active FLPs.