Preparation of Structurally and Electronically Diverse N -> B-Ladder Boranes by [2+2+2] Cycloaddition

We report the synthesis of a series of eight N -> B-ladder boranes through cobalt-mediated cyclotrimerization of (2-cyanophenyl)-dimesitylborane with different dialkynes. The resulting tetracoordinate boranes show variable electrochemical and optical properties depending on the substitution pattern in the backbone of the coordinating pyridine-derivatives. While boranes containing alkyl-substituted pyridines show lower electron affinities than the known parent compound, boranes featuring pi-extended pyridine derivatives show higher electron affinities in the range of acceptor substituted triarylboranes. All derivatives show larger Stokes shifts (8790-6920 cm(-1)) compared to the N -> B-ladder borane coordinated by an unsubstituted pyridine.