Preferred Orientation of a Physisorbed Molecular Catalyst and Implications for Selectivity

Confinement effects in stereoselective catalysis have
been the
topic of prolonged inquiry. Results have been largely mixed, with
confinement having been reported to both enhance and degrade selectivity.
Our ability to understand the surface’s impact on catalytic
mechanisms, and thus selectivity, has been severely hindered by the
low level of details commonly seen in the structures of supported
metal complexes. Recent developments in sensitivity-enhanced NMR are
revealing not only the molecular structure of surface sites but also
their configuration, orientation, and proximities to neighboring molecules.
In studying [Rh(cyclooctadiene)((S)-(−)-2-aminomethyl-1-ethyl-pyrrolidine)]CF3SO3 noncovalently immobilized to silica, an enantioselective
hydrogenation catalyst, we observed a strong preference for a particular
orientation of the complex relative to the support surface. We discuss
how preferential adsorption may help reduce the number of competing
reaction pathways and, in turn, have outsized effects on selectivity.