Manganese Triazacyclononane Oxidation Catalysts Grafted under Reaction Conditions on Solid Cocatalytic Supports

Manganese complexes of 1,4,7-trimethy1-1,4,7-triazacydononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H2O2. Here, carboxylic acid-functionalized SiO2 simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H2O2 and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn-IV(mu-O)(3)Mn-IV(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn-III(mu-O)(mu-RCOO)(2)Mn-III(tmtacn)](2+). This transformation is assigned on the basis of comparison of diffuse reflectance UV-visible spectra to known soluble models, assignment of oxidation state by Mn K-edge X-ray absorption near-edge spectroscopy, the dependence of rates on the acid/Mn ratios, and comparison of the surface structures derived from density functional theory with extended X-ray absorption fine structure. Productivity in cis-cyclooctene oxidation to epoxide and cis-diol with 2-10 equiv of solid cocatalytic supports is superior to that obtained with analogous soluble valeric acid cocatalysts, which require 1000-fold excess to reach similar levels at comparable times. Cyclooctene oxidation rates are near first order in H2O2 and near zero order in all other species, including H2O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H2O2 activation on the hydrated, supported complex. This general mechanism and the observed alkene oxidation activation energy of 38 +/- 6 kJ/mol are comparable to H2O2 activation by related soluble catalysts. Undesired decomposition of H2O2 is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 degrees C and initial H2O2 concentration of 0.5 M, increasing reactor throughput. These results show that immobilized carboxylic acids can be utilized and understood like traditional carboxylic acids to activate non-heme oxidation catalysts while enabling higher throughput and providing the separation and handling benefits of a solid catalyst.