(7g) Mechanistic Analysis of CO Oxidation over Mixed-Valence Oxo-Bridged Trimers

Immobilization of active site moieties on heterogeneous catalyst surfaces, although enabling their use in large-scale industrial applications, almost invariably results in a distribution of active site speciation. Metal-organic framework materials (MOFs) - crystalline porous networks formed through the interconnection of inorganic nodes and organic linkers - enable, in principle, the replication of organometallic catalyst coordination environment over the entirety of the active site pool of a heterogeneous catalyst surface. MIL-100 is a MOF material endowed with oxo-bridged trimeric nodes that each carry one M²⁺ and two M³⁺ sites that effectuate a variety of acid-catalyzed and redox turnovers. In this study, we elucidate (at the level of elementary steps) the mechanism of CO oxidation over mixed-valence oxo-bridged trimers.

Steady state and transient kinetic experiments combined with isotopic tracer studies suggest steps constituting the oxidation half cycle as being kinetically relevant over both Fe and Cr trimers, with activation energies reflecting standard state free energy differences between Fe(IV)=O intermediates and gas phase nitrogen, and the bare Fe²⁺ surface and gas phase N₂O. Cr trimers enable access to much greater coverages of metal-oxo intermediates, reflected in lower sensitivities to CO pressure relative to their Fe trimer counterparts (Figure 1). Overall, these results provide unprecedented levels of clarity as to the identity and kinetic relevance of steps mediating oxidative turnovers over multinuclear metal-oxo clusters, as well as the utility of metal organic framework materials in addressing questions hitherto rendered intractable by heterogeneity in active site speciation.