A good catalyst for semihydrogenation of alkynes must preclude both over-hydrogenation of alkene to alkane and isomerization to the other alkene isomer. In addition, it should balance the trade-off between selectivity and activity. In 2013, the Repo and Pápai groups reported a frustrated Lewis pair (FLP) (1-NMe2-2-B(C6F5)2-C6H4), 1, which is a metal-free catalyst and for the first time shows excellent reactivity for the hydrogenation of internal alkynes.1 However, it is unreactive for terminal alkynes. In this work, we have designed thirteen FLPs, a−m, based on 1 by varying the Lewis base site with N and P, and the Lewis acid site with B, Al, Ga, and In, and replacing pentafluorophenyl with 1,3,5-trifluorophenyl, phenyl, or trifluoromethyl. We apply density functional theory to study the activity, selectivity, and deactivation of FLP 1-m for acetylene semihydrogenation. The catalytic cycle consists of three steps: (1) alkyne insertion, (2) H2 heterolysis, and (3) intramolecular protonation. We found the activity does not change much by the modification of bulky ligands, while it decreases with the direct replacement of LA and LB sites. The overall activity depends on steps 1 and 3, which are respectively positively and negatively linear correlated with the charge of Lewis acid site. Most of FLPs in this work show comparable or better selectivity for semihydrogenation of acetylene than of 1. FLPs deactivation is due to the strong binding of acetylene and the elimination of electron-withdrawing bulky ligands at the pre-activated catalyst rather than at activated catalysts. Taking the selectivity and the stability of FLPs into account, we predict d and k are potentially active for terminal alkynes.

Understanding the reactivity, selectivity and deactivation of frustrated Lewis pairs for alkyne semihydrogenation

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