Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H 2O2. 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)MnIV (μ-O)3MnIV (tmtacn)]2+ into the active, dicarboxylate-bridged [(tmtacn)MnIII (μ-O)(μ-RCOO) 2MnIII (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 H 2O2 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 H 2O2 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 °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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry