Planar, Three-Coordinate Boron Monohydrides Ligated by Bis(3-methylindolyl)methanes

Reactions of tris­(3-methylindolyl)­methane, bis­(3-methylindolyl)-2-methoxyphenylmethane, and bis­(3-methylindolyl)­phenylmethane with THF·BH3 result in hydrogen elimination and yield the corresponding boron monohydride complexes (3-CH3C8H4NH)­HC­(3-CH3C8H4N)2BH (6), (2-CH3OC6H4)­HC­(3-CH3C8H4N)2BH (7), and PhHC­(3-CH3C8H4N)2BH (8), respectively. Compositions and structures were confirmed by NMR (1H, 11B, 13C) and IR spectroscopy, elemental analysis, and X-ray crystallography. The molecular structures of 6–8 each possess a trigonal-planar boron atom which resides in the plane of a bidentate diindolylmethane ligand. This planarity, the lack of Lewis acid–base complexation in the presence of THF, and the short B–N distances suggest significant N→B π donation. Thermolysis of 6 did not produce the three-coordinate, pyramidal borane HC­(3-CH3C8H4N)3B or a tripodal, four-coordinate acid–base complex, presumably due to the significant B–N π bonding in 6. The planarity of 6–8 and the isoelectronic relationship of BH to C: suggest that diindolylmethanes may be useful platforms for the preparation of a new family of N-heterocyclic carbenes.