Why Does the Intramolecular Trimethylene‐Bridged Frustrated Lewis Pair Mes 2 PCH 2 CH 2 CH 2 B(C 6 F 5 ) 2 Not Activate Dihydrogen?

The methyl labelled C 3 ‐bridged frustrated phosphane borane Lewis pair (P/B FLP) 2 b was prepared by treatment of Mes 2 PCl with a methallyl Grignard reagent followed by anti‐Markovnikov hydroboration with Piers’ borane [HB(C 6 F 5 ) 2 )]. The FLP 2 b is inactive toward dihydrogen under typical ambient conditions, in contrast to the C 2 ‐ and C 4 ‐bridged FLP analogues. Dynamic NMR spectroscopy showed that this was not due to kinetically hindered P⋅⋅⋅B dissociation of 2 b . DFT calculations showed that the hydrogen‐splitting reaction of the parent compound 2 a is markedly endergonic. The PH + /BH − H 2 ‐splitting product of 2 b was indirectly synthesized by a sequence of H + /H − addition. It lost H 2 at ambient conditions and confirmed the result of the DFT analysis.