Carbon-based Lewis acids are limited merely to trityl cations and some electron-poor allenes.^[1] Recently, Bazan and coworkers reported that C₆₀ and C₇₀ can behave as all-carbon Lewis acids to form the corresponding Lewis acid–base pairs with a normal NHC structure.^[2] This work opens a new perspective on the research of carbon-based Lewis acids. Meanwhile, as a novel class of metal-carbon hybrid molecules, endohedral metallofullerenes (EMFs) feature electron transfer from the internal metallic species to the carbon cage, forming zwitterionic compounds.^[3] Accordingly, it is of special interest to study whether the highly charged carbon cages of EMFs are willing to accept additional electrons to act as Lewis acids or not.

In this representation, taking several kinds of EMFs as a representative, we show that EMFs also exhibit excellent Lewis acidity to form Lewis acid-base pairs with NHCs. Our unambiguous X-ray results reveal that the NHC moiety uses either its normal site or abnormal active center to react with the EMFs under study to form the corresponding adducts with different structures (Scheme 1). It is conformed theoretically the the inherent characteristic of the EMFs is responsible for the addition patterns.

References：

[1] F. E. Ahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122.

[2] H. P. Li, C. Risko, J. H. Seo, C. Campbell, G. Wu, J. Bredas and G. C. Bazan, J. Am. Chem. Soc., 2011, 133, 12410.

[3] X. Lu, L. Echegoyen, A. L. Balch, S. Nagase and T. Akasaka, Endohedral Metallofullerenes: Basics and Applications, CRCPress, New York, 2015.

[4] M. Q. Chen, L. P. Bao, M. Ai, W. Q. Shen and X. Lu. Chem. Sci., 2016, 7, 2331-2334.