Cooperative interactions between an organic ligand and an inorganic oxide support allow the construction and understanding of well-defined transition metal-based catalytic reaction sites. These inorganic-organic assemblies move us towards atomic-scale control of connectivity and composition within solid-supported active sites and allow us to break free of the existing paradigm in which metal surface density and support identity exert a controlling force over the structure formed and limit the diversity catalytic metal-oxide structures able to be synthesized. Our approach involves the careful grafting of macrocycle-metal complexes onto oxide supports such that the grafted form resembles the molecular precursor, whose structure is known from techniques not generally accessible to designers of supported metal-oxide catalysts: single crystal X-ray diffraction and high-resolution NMR spectroscopy. Multidentate ligands are available with the diversity characteristic of synthetic organic chemistry and are essential structural elements that create greater robustness and definition of active sites during ligand exchange processes inherent to catalysis.

To elaborate this design method, we have synthesized a novel family of ligand-modified, single-site heterogeneous catalysts using calixarenes as macrocyclic oxo-ligands for transition metals, leading, in particular, to highly active alkene epoxidation catalysts based on Ti-silica.[1] These catalysts can display turnover rates in excess of 1000/h at >95% selectivity to cyclohexene epoxide at >98% conversion of the oxidant (alkyl hydroperoxide), making them comparable to the current state-of the art. Grafted calixarene-Ti structures are also 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 precursors have been designed to create ‘single-site' materials possessing a uniform population of active site structures controlled not by ransom reactions on a surface, but rather by the calixarene-Ti complex that is assembled with atomic precision in solution. The grafted Ti atoms are unaffected by surface density and are nearly indistinguishable from soluble model compounds as verified by NMR, Ti-K edge XANES, and UV-visible spectroscopies. Critically, the rate of alkene epoxidation by Ti-complexed alkyl hydroperoxides is independent of the surface density of the grafted Ti sites, in contrast to some materials assembled by traditional grafting or sol-gel routes.

We have further demonstrated the creation of Ti active site structures not accessible from traditional syntheses and used them to understand the structural requirements of these catalysts with unprecedented precision. By introducing macrocycle ligands with additional oxygens, we developed a method to control the oxygen coordination number around grafted Ti atoms, as probed by Ti K-edge XANES, while maintaining the single-site, site-isolated structure.[2] We have used these materials to show a structure-activity correlation spanning more than a factor of 20 in epoxidation rate.

Finally, we use these design paradigms to demonstrate control over the surface environment surrounding the active site while leaving the immediate Ti coordination sphere untouched. Local Bronsted acidity was controlled using calixarenes synthesized to possess different phenol pKa's; SiO₂, TiO₂, and Al₂O₃ supports; and molecular support analogues such as triphenylsilanol. In all materials, a similar Ti atom electronic environment was maintained, as probed by Ti K-edge XANES. The activity displayed by these precisely-designed materials demonstrates that the requirement for extended SiO₂ supports is not purely electronic in origin and provides experimental support for mechanistic hypotheses involving H-bonding. These hypotheses have been explored primarily using theory due to the difficulty in synthesizing systematically altered materials that do not exhibit multiple confounding effects.

Using multidentate organic ligands and extended inorganic oxide surfaces cooperatively to define the environment around a catalytic metal, we have demonstrated our ability to translate over the entire synthetic phase space of supported TiOx Lewis acid catalysts while maintaining well-defined single-site structures. We have shown that this design method enables independent and systematic alterations of a catalyst active site and its surrounding surface. Full synthesis-structure-activity correlations are developed, which leads to enhanced mechanistic understanding of a reaction and allows rapid transiting of a catalyst development cycle because the kinetically-relevant material structures are predictable from the precursors employed. This may ultimately allow for the creation of novel catalysts with activity and selectivity not possible with traditional homogeneous or heterogeneous catalysts.[3]

[1] Notestein, J.M.,; Iglesia, E.; Katz, A. J. Am. Chem. Soc. 2004, 126, 16478-16486.

[2] Notestein, J.M.; Andrini, L.R.; Kalchenko, V.I.; Requejo, F.J.; Katz, A.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 1122-1131.

[3] Notestein, J. M.; Katz, A. Chem. Eur. J. 2006, 12, 3954-3965.

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