Frustrated Lewis pairs comprising nitrogen Lewis acids for Si-H bond activation

Frustrated Lewis Pairs (FLP) were discovered in 2007 by Stephan and Erker. They reported the activation of a hydrogen molecule by a non-metallic species.^1-2 Therefore, they used molecules containing both an electron-accepting boron atom, and a bulky electron-donating phosphine. Despite having a significant driving force for bond formation, these two moieties do not form a Lewis adduct because of steric congestion. The subsequent "frustration" renders them available for chemical bond activations.

Recently, we have demonstrated an unprecedented Lewis acidic behavior of nitrenium cations.³ It represents the first examples of nitrogen-based Lewis acids. In contrast to common Lewis acids used in FLPs, which utilize the empty orbital of a boron atom, nitrogen Lewis acids utilize an electrophilic nitrogen atom. Considering the superior electronegativity of nitrogen compared with boron, these molecules have the potential to activate a plethora of chemical bonds.

We have recently synthesized⁴ a new nitrenium-based Lewis acid which effectively cleaves Si-H bonds in a quantitative yield. The generated triazane was fully characterized in a solution by multinuclear NMR techniques and in the solid state by single crystal X-ray crystallography. To the best of our knowledge, this represents a first example of a FLP consisting of a nitrogen-based Lewis acid, capable of cleaving an electrophile-hydride (E-H) type bond. Attempts to utilize this FLP for catalytic hydrosilylation are underway.

Figure – Si-H bond activation by our nitrogen-based FLP

References:

1. G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan, Science 2006, 314, 1124-1126.
2. P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme, D. W. Stephan, Chem. Commun. 2007, 2, 5072-5072.
3. A. Pogoreltsev, Y. Tulchinsky, N. Fridman, M. Gandelman, J. Am. Chem. Soc. 2017, 139, 4062-4067.
4. I. Avigdori, A. Pogoreltsev, M. Gandelman, Manuscript in preparation.