Heterolytic Cleavage of H₂ by Frustrated B/N Lewis Pairs

Treatment of the Lewis acid B(C₆F₅)₃ with the Lewis base 2,6-dimethylpiperidine (DMP) resulted in the formation of the classical Lewis acid·base adduct DMP-B(C₆F₅)₃, 1a, which was anticipated to undergo thermal dissociation to the “unquenched” Lewis centers. The free Lewis pair was able to form a frustrated Lewis pair (FLP), which induced heterolytic splitting of H₂, affording the ionic product [DMPH][HB(C₆F₅)₃], 1b. FLPs, derived from B(C₆F₅)₃ and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP) and 1,2,2,6,6-pentamethylpiperidine (PMP), could also heterolytically activate H₂, affording the salts [TMPH][HB(C₆F₅)₃], 2, and [PMPH][HB(C₆F₅)₃], 3, respectively. In a VT NMR study the TMP/B(C₆F₅)₃ reaction was studied in greater detail, trying to trace intermediates. The supposed most prominent intermediate, the TMP/H₂/B(C₆F₅)₃ complex, could, however, not be detected. The combination of B(C₆F₅)₃ with the even more sterically demanding Lewis base 1-ethyl-2,2,6,6,-tetramethylpiperidine (Et-TMP) displayed FLP reactivity with H₂, but required the high temperature of 110 °C, forming [2,2,6,6-(CH₃)₄C₅H₆NH(CH₂CH₃)][HB(C₆F₅)₃], 4a. In the absence of H₂ the combination of B(C₆F₅)₃ and Et-TMP generated at room temperature a mixture of 4a and [2,2,6,6-(CH₃)₄C₅H₆NCHCH₂-B(C₆F₅)₃], 4b. 4b was formed via consecutive hydride and proton abstractions with Et-TMP as the base, generating 4a. 2,4,6-Tri-tert-butylpyridine (TTBP), exhibiting reduced Lewis basicity as compared to piperidine derivatives, showed FLP reactivity with B(C₆F₅)₃, which gave in the presence of H₂ the [TTBPH][HB(C₆F₅)₃], 5, salt as the only product after several hours. The steric demand of the Lewis bases was evaluated by aid of DFT calculations on borane adducts, which roughly correlated with the reaction temperature of H₂ splitting. 1a, 1b, 3, 4a, and 4b were studied by single-crystal X-ray diffraction analyses.