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.