Computational Exploration of Intramolecular Sn/N Frustrated Lewis Pairs for Hydrogen Activation and Catalytic Hydrogenation

Stannylium-based cationic Lewis acids (LAs) are gaining attention as an alternative to neutral boranes for their use in frustrated Lewis pair (FLP)-mediated hydrogenation catalysis. Although intermolecular FLP combinations of stannylium LAs and nitrogen-Lewis bases are studied in detail, reports on their intramolecular analogs are lacking at present. In this work, through density functional theory calculations, we investigate a family of intramolecular stannylium/nitrogen FLPs (1-R-X) for their ability to activate dihydrogen and subsequently perform catalytic hydrogenation of organic compounds with unsaturated functional groups. These FLPs are designed by embedding stannylium motifs into a cyclic guanidinate backbone that provides a complementary Lewis basic nitrogen site. By varying the substituents and the counter anion on the Sn center, we show how to significantly tune the reactivity of this family of FLPs. The mechanism of H2 activation by 1-R-X is elucidated in detail, and we find that the heterolytic cleavage of the H–H bond proceeds through cooperative action of the Sn and N centers. The energetics of the H2 cleavage is correlated with molecular descriptors to understand trends in reactivity and to easily filter candidates suitable for the hydrogen release step necessary to complete the hydrogenation cycle. Finally, we find that the release of the proton and the hydride from the Lewis centers to multiple bonds occur in a concerted manner enabling catalytic hydrogenation of the corresponding organic substrates.