Molecular structures of tri-N-pyrrolylboranes and the dynamic behaviour of tri-N-indolylboranes in solution

Various methods for the synthesis of tri-N-azolylboranes Az(3)B (Az = 1-pyrrolyl (1a), 1-(2,5-dimethyl)pyrrolyl (1b), 1-indolyl (2a), 1-(2-methyl)indolyl (2b), 9-carbazolyl (3)), are compared. The molecular structures of 1a (orthorhombic; space group Pbcn) and 1b (monoclinic; space group C2/c) were determined by X-ray analyses. There is a slight twist of the N-pyrrolyl groups in 1a against the BN3 plane (26.7 degrees, 29.3 degrees, 29.3 degrees), and the distinct tri-N-pyrrolylborane molecules are arranged in piles in the crystal lattice, with the boron atoms on top of each other and a staggered conformation of the pyrrolyl rings. The 2,5-dimethylpyrrolyl rings in Ib are strongly twisted against the BN3 plane (36.7 degrees, 48.5 degrees, 50.8 degrees), and in the lattice the molecules of 1b are arranged in a way to minimize intermolecular interactions. The structural features of 1b are also reflected by solid state C-13 CP/MAS NMR spectra. The molecular structures and the analysis of delta(11)B and delta(13)C data point towards rather weak BN(pp) pi interactions in Az(3)B. Thus, the hindered rotation about the B-N bonds in tri-1-indolylborane (2a) (Delta G* (248 K) = 52.0 +/- 1 kJ mol(-1)) and in tris[1-(2-methyl) indolyl] borane (2b) (Delta G* (363 K) = 98.0 +/- 1 kJ mol(-1)) must be ascribed mainly to steric hindrance rather than to a significant B-N double bond character.