Why Does the Intramolecular Trimethylene-Bridged Frustrated Lewis Pair Mes2PCH2CH2CH2B(C6F5)2 Not Activate Dihydrogen?

The methyl labelled C-3-bridged frustrated phosphane borane Lewis pair (P/B FLP) 2b was prepared by treatment of Mes(2)PCl with a methallyl Grignard reagent followed by anti-Markovnikov hydroboration with Piers' borane [HB(C6F5)(2))]. The FLP 2b 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 PB dissociation of 2b. DFT calculations showed that the hydrogen-splitting reaction of the parent compound 2a is markedly endergonic. The PH+/BH- H-2-splitting product of 2b 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.