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.