(41d) 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.