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

Abstract

Various methods for the synthesis of tri-N-azolylboranes, Az3B (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°, 29.3°, 29.3°), 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 1b are strongly twisted against the BN3 plane (36.7°, 48.5°, 50.8°), 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 13C CP/MAS NMR spectra. The molecular structures and the analysis of δ11B and δ13C data point towards rather weak BN(pp) π interactions in Az3B. Thus, the hindered rotation about the BN bonds in tri-1-indolylborane (2a) (ΔG≠ (248 K) = 52.0 ± 1 kJ mol−1) and in tris[1-(2-methyl)indolyl]borane (2b) (ΔG≠ (363 K) = 98.0 ± 1 kJ mol−1) must be ascribed mainly to steric hindrance rather than to a significant B double bond character.