Enhanced Activity from Coordinatively Unsaturated and Dynamic Zeolite-Bound Organoscandium Species

Abstract

Scandium borohydride grafted into the micropores of the faujasite zeolite HY 30 catalyzes the C−H borylation of benzene, whereas silica-grafted species are inactive. This catalytic activity may originate from grafting at a Brønsted acid site leading to a more electron-deficient rare earth center. Herein, we apply multinuclear double-resonance nuclear magnetic resonance (NMR) experiments to probe the structure and dynamics of zeolite-and silica-bound scandium borohydride complexes. The experiments reveal that scandium centers located within the zeolite micropores, in proximity to Al-created Brønsted sites, are more dynamic than rigid scandium sites grafted on silanols. Through a combination of NMR and molecular dynamics simulations, we show that the coordination of the scandium in the zeolite is labile, with the metal exchanging between two binding sites. The weak electron donation from the support that enables the movement of the Sc center leads to the formation of an undercoordinated metal center that cannot exist on silica, ultimately leading to the new catalytic activity of the species.