High Lewis Acidity at Planar, Trivalent, and Neutral Bismuth Centers

Geometric perturbation
away from classical structures can engender
unusual frontier MO situations leading to high Lewis acidity. Recently
we reported a T-shaped bismuth triamide, which exhibited planar Lewis
acidity that was unprecedented for neutral group 15 compounds. We
now report a comprehensive computational assessment of the origins
of planar Lewis acidity in such compounds. We subsequently use several
metrics such as MO energies, the Global Electrophilicity Index, ligand
coordination strengths, and fluoride ion affinity to show that electronic
variation can be combined with the unusual geometry at bismuth to
achieve fine-tuning of Lewis acid strength. Our calculations reveal
for the first time a surprisingly high electrophilicity, comparable
to or exceeding that of polyfluorinated triarylboranes, as well as
high rigidity at neutral planar bismuth triamides. These results foreshadow
the potentially broad applications of a hitherto unexplored class
of compounds, planar bismuthanes, in contexts that are currently dominated
by Lewis acidic triarylboranes.