Synthesis and reactivity of N-heterocycle-B(C6F5)(3) complexes. 4. Competition between pyridine- and pyrrole-type substrates toward B(C6F5)(3): Structure and dynamics of 7-B(C6F5)(3)-7-azaindole and [7-azaindolium](+)[HOB(C6F5)(3)](-)

Reaction between 7-azaindole and B(C6F5)(3) quantitatively yields 7-(C6F5)(3)B-7-azaindole (4), in which B(C6F5)(3) coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a second equivalent of B(C6F5)(3). Reaction of 7-azaindole with H2O-B(C6F5)(3) initially produces [7-azaindolium](+)[HOB(C6F5)(3)]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known complexes (C6F5)(3)B-pyrrole (1) and (C6F5)(3)B-indole (2), with formation of free pyrrole or indole, giving the more stable adduct (C6F5)(3)B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid state and in solution and compared to the structures of other (C6F5)(3)B-N-heterocycle complexes. Two dynamic processes have been found in compound 4. Their activation parameters (Delta H-double dagger= 66 (3) kJ/mol, Delta S-double dagger= -18 (10) J/mol K and Delta H-double dagger= 76 (5) kJ/mol, Delta S-double dagger= -5 (18) J/mol K) are comparable with those of other (C6F5)(3)B-based adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed.