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 π-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.