From classical adducts to frustrated Lewis pairs: steric effects in the interactions of pyridines and B(C6F5)3.

The pyridine adducts of B(C(6)F(5))(3), (4-tBu)C(5)H(4)NB(C(6)F(5))(3) 1, ((2-Me)C(5)H(4)N)B(C(6)F(5))(3) 2, ((2-Et)C(5)H(4)N)B(C(6)F(5))(3) 3, ((2-Ph)C(5)H(4)N)B(C(6)F(5))(3) 4, ((2-C(5)H(4)N)C(5)H(4)N)B(C(6)F(5))(3) 5, (C(9)H(7)N)B(C(6)F(5))(3) 6, and ((2-C(5)H(4)N)NH(2-C(5)H(4)N))B(C(6)F(5))(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(C(6)F(5))(3) affords the zwitterionic species (5-Me)C(5)H(3)NH(2-NH)B(C(6)F(5))(3) 8. In contrast, lutidine/B(C(6)F(5))(3) yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me(2)C(5)H(3)N)B(C(6)F(5))(3) 9. This equilibrium has a DeltaH of -42(1) kJ/mol and DeltaS of -131(5) J/mol x K. Addition of H(2) shifts the equilibrium and yields [2,6-Me(2)C(5)H(3)NH][HB(C(6)F(5))(3)] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C(6)F(5))(3) showed no evidence of adduct formation and upon exposure to H(2) afforded [(2,6-Ph(2))C(5)H(3)NH][HB(C(6)F(5))(3)] 11 and [(2-tBu)C(5)H(4)NH][HB(C(6)F(5))(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.