Heterolytic Cleavage of H₂by Frustrated B/N Lewis Pairs

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Abstract

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.

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