(478a) Calixarenes as Scaffolds for Catalytic Structures

Surface organometallic chemistry is an enabling technology for the synthesis of grafted transition metals on oxide supports. We rely on surface organometallic chemistry to achieve our research goal of atomic scale control of connectivity and composition within a heterogeneous catalyst active site. Our approach involves the careful grafting of organometallic macrocycles onto oxide supports, such that the coordination geometry in the grafted form resembles that in the molecular precursor, which is known from single crystal X-ray diffraction. This coordination consists of ligands from the original homogeneous precursor as well as the oxide surface, and enables incorporation of multi-dentate ligands for greater robustness and definition of the active site during ligand exchange processes inherent to catalysis. With this definition comes the ability to make discrete alterations of the electronic and steric environment surrounding a catalyst active site, thereby enabling study of the behavior of heterogeneous catalysts. This methodology also allows the creation of novel catalysts with activity and selectivity not possible with traditional homogeneous or heterogeneous catalysts.

The approach above is exemplified by the synthesis of surface organometallic catalysts using calixarenes as macrocyclic oxo-ligands for grafted transition metals, leading to a highly active alkene epoxidation catalyst based on titanium on silica. These catalysts display turnover rates in excess of 1000 h-1 at >95% selectivity to cyclohexene epoxide at >98% conversion of the oxidant (cumene hydroperoxide), are stable against leaching and ligand exchange during catalysis and display rigorous first order kinetics in hydroperoxide irrespective of co-product alcohol, often implicated as an inhibitor. These results bear important implications for rational catalyst design since intrinsic catalyst activity is not obscured by deactivation or inhibition phenomena. We have also proven this material to be a single-site catalyst, meaning that the titanium sites behavior is independent of the local Ti-active site density, in both spectroscopic and catalytic epoxidation characterization. As a result, rational design of the organometallic precursor is directly translated into the final catalyst. We have recently shown that decreasing the electron density on the calixarene pi-system results in an increase in epoxidation rate as predicted for a Lewis acid catalyst. We will also present the results of further efforts in modifying the calixarene ligand and the support chemistry in order to rationally design and elucidate activity- and selectivity-structure relationships in grafted calixarene catalysts.