From Classical Adducts to Frustrated Lewis Pairs: Steric Effects in the Interactions of Pyridines and B(C6F5)3

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 Delta H of -42(1) kJ/mol and Delta S of -131(5) J/mol. K. Addition of H-2 shifts the equilibrium and yields [2,6-Me2C5H3NH][HB(C6F5)(3)] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-bulylpyridine with B(C6F5)(3) showed no evidence of adduct formation and upon exposure to H-2 afforded [(2,6-Ph-2)C5H3NH][HB(C6F5)(3)] 11 and [(2tBu)C5H4NH][HB(C6F5)(3)] 12, respectively. The energetics of adduct formation and the reactions with H-2 are probed computationally. Crystallographic data for compounds 1-10 are reported.