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)-((<i>S</i>)-(-)-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.