From Classical Adducts to Frustrated Lewis Pairs: Steric Effects in the Interactions of Pyridines and B(C6F5)3
datasetposted on 02.11.2009, 00:00 by Stephen J. Geier, Austin L. Gille, Thomas M. Gilbert, Douglas W. Stephan
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The pyridine adducts of B(C6F5)3, (4-tBu)C5H4NB(C6F5)3 1, ((2-Me)C5H4N)B(C6F5)3 2, ((2-Et)C5H4N)B(C6F5)3 3, ((2-Ph)C5H4N)B(C6F5)3 4, ((2-C5H4N)C5H4N)B(C6F5)3 5, (C9H7N)B(C6F5)3 6, and ((2-C5H4N)NH(2-C5H4N))B(C6F5)3 7, were prepared and characterized. The B−N bond lengths in 2−7 reflect the impact of ortho-substitution, increasing significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with B(C6F5)3 affords the zwitterionic species (5-Me)C5H3NH(2-NH)B(C6F5)3 8. In contrast, lutidine/B(C6F5)3 yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me2C5H3N)B(C6F5)3 9. This equilibrium has a ΔH of −42(1) kJ/mol and ΔS of −131(5) J/mol·K. Addition of H2 shifts the equilibrium and yields [2,6-Me2C5H3NH][HB(C6F5)3] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C6F5)3 showed no evidence of adduct formation and upon exposure to H2 afforded [(2,6-Ph2)C5H3NH][HB(C6F5)3] 11 and [(2-tBu)C5H4NH][HB(C6F5)3] 12, respectively. The energetics of adduct formation and the reactions with H2 are probed computationally. Crystallographic data for compounds 1−10 are reported.