Internal Adduct Formation of Active Intramolecular C₄‑bridged Frustrated Phosphane/Borane Lewis Pairs

The tetramethylene-bridged PMes2/B­(C6F5)2 frustrated Lewis pair (FLP) 8 was prepared by hydroboration of Mes2P–(CH2)2CHCH2 with HB­(C6F5)2. It is an active FLP that splits dihydrogen under mild conditions and, consequently, serves as a metal-free hydrogenation catalyst for a variety of substrates. It also reacts typically with terminal acetylenes. The C4-bridged FLP 23 was prepared by HB­(C6F5)2 hydroboration of 1-dimesitylphosphino-2-vinylferrocene. It represents a rare example of a FLP where the equilibrium between the open form and the closed internal P/B adduct form is experimentally observable. It also shows a variety of typical FLP reactions, including dihydrogen splitting. The FLPs 8 and 23 (open form) and many precursors and products were characterized by X-ray diffraction.