Kinetic Probes of the Origin of Activity in MOF-Based C–H Oxidation Catalysis
Aishanee Sur, Nicholas B. Jernigan, David C. Powers
Abstract
The development of homogeneous catalysis is enabled by the availability of a rich toolkit of kinetics experiments, such as the Hg-drop test, that differentiate catalytic activity at ligand-supported metal complexes from potential heterogeneous catalysts derived from the decomposition of molecular species. Metal–organic frameworks (MOFs) have garnered significant attention as platforms for catalysis at site-isolated, interstitial catalyst sites. Unlike homogeneous catalysis, a relatively few strategies have been advanced to evaluate the origin of catalytic activity in MOF-promoted reactions. Many of the MOFs that have been examined as potential catalysts are composed of molecular constituents that represent viable catalysts in the absence of the extended MOF lattice, and thus interfacial sites and leached homogeneous species represent potential sources of catalyst activity. Here, we demonstrate that the analysis of deuterium kinetic isotope effects (KIEs) and olefin epoxidation diastereoselectivity provides probes of the origin of catalytic activity in MOF-promoted oxidation reactions. These analyses support the involvement of lattice-based Fe sites in the turnover-limiting step of C–H activation with Fe-MOF-74-based materials (i.e., the MOF functions as a bona fide catalyst) and indicate that Cu2-based MOF MIL-125-Cu2O2 functions as a solid-state initiator for solution-phase oxidation chemistry and is not involved in the turnover-limiting step (i.e., the MOF does not function as a catalyst for substrate functionalization). We anticipate that the simple experiments described here will provide a valuable tool for clarifying the role of MOFs in C–H oxidation reactions.