Supports As Co-Catalysts for Activating Molecular Oxidation Catalysts

There is a wide class of non-heme oxidase enzymes that consist of histidine-coordinated first-row metal oxides, further activated by carboxylates, and these structures have been extensively mimicked by soluble inorganic complexes. Many of these amine and pyridine-ligated complexes are activated towards productive oxidation using H₂O₂ or O₂ by coordination of carboxylates. As one example, dimeric 1,4,7-triazacyclononane manganese oxides are known to be efficient and highly active homogeneous oxidation catalysts whose selectivity toward a number of oxidation reactions (epoxidation / cis-dihydroxylation, alkane oxidation, alcohol oxidation) can be controlled by the addition of various acid co-catalysts.

Here we present a synthetic route to supported triazacyclononane manganese oxides through an in-situ assembly method whereby the surface active-site is synthesized under reaction conditions by simply combining a preformed Mn dimer and the carboxylate modified metal oxide co-catalytic supports.  Self-assembly from MeCN/H₂O₂ solutions onto organo-functionalized oxide supports provides an inherent advantage by supplying a co-catalyst which also acts as the support tether, thus eliminating synthetically cumbersome ligand alterations which can lead to differences in active-site structure and reactivity or selectivity from the homogeneous analogues. For example, we have shown sterically hindered surfaces to increase selectivity toward cis-diol. Additionally, the high local concentrations of surface carboxylates lead to increased reaction rates and shorter reaction times required for maximal productivity than is observed with analogous homogeneous carboxylate co-catalysts. Recent work using other metal complexes (e.g. V), but the same general principles, are also described. Surface structures are understood through X-ray absorption fine structure, diffuse reflectance UV-visible spectroscopy, and DFT modeling.