A unique method has been developed to synthesize an active heterogeneous oxidation catalyst by the in situ grafting of a 1,4,7-trimethyl-1,4,7-triazacyclononane manganese complex on carboxylic acid-functionalized supports serving dual roles as surface tether and necessary co-catalyst, massively increasing total turnovers as compared to the homogeneous analog.

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H(2)O(2). Here, carboxylic acid-functionalized SiO(2) simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H(2)O(2) and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn(IV)(μ-O)(3)Mn(IV)(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn(III)(μ-O)(μ-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 H(2)O(2) and near zero order in all other species, including H(2)O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H(2)O(2) 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(2)O(2) activation by related soluble catalysts. Undesired decomposition of H(2)O(2) is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 °C and initial H(2)O(2) 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.

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Immobilizing a homogeneous catalyst provides obvious handling benefits, but ideally can also enhance catalyst productivity or selectivity because of beneficial interactions between the surface and the active site. Here, Mn 1,4,7-trimethyl-1,4,7-triazacyclononane dimers (1) are activated by carboxylate-containing solids for cyclooctene epoxidation/dihydroxylation with H2O2 at 0 °C. The productivity (TON, moles per mole 1) and selectivity (to cis-diol) of cyclooctene oxidation by 1 are known to be tuned by choice of soluble carboxylate cocatalyst, and this concept is extended here to solid carboxylate cocatalysts. These solid cocatalysts are synthesized by covalently grafting benzoate and propanoate silanes to SiO2 or allowing terephthalic acid or dihydroxyhydrocinnamic acid to chemisorb on oxides. Comparing analogous structures, SiO2-grafted carboxylates outperform soluble benzoic acid (275 vs 25 TON at 2 equiv), hydrocinnamic acid (150 vs 40 TON at 2 equiv), and valeric acid (675 vs 70 TON at 10 equiv). Of the oxides tested as modifiers for carboxylate cocatalysts, TiO2 leads to the largest improvements in oxidation productivity, boosting productivity to 300 TON when combined with 2 equiv of benzoic acid and to 425 TON when combined with ditopic terephthalic acid. The latter enhancement may be due to both a buffering effect and a high surface concentration of chemisorbed species that encourages formation of the presumed carboxylate-bridged active state of the catalyst. Dihydroxylation selectivities are a function of the carboxylate employed as well as the nature of the other groups on the solid cocatalyst surface. SiO2 grafted with propionate groups gives ∼50% cis-diol selectivity, but further modification with alkyl or perfluoroalkyl silanes increase cis-diol selectivity up to ∼60%. Dihydroxylation selectivity is also ∼60% for SiO2 grafted with benzoate or ∼70% for terephthalic acid chemisorbed on TiO2. These solid cocatalysts introduce a number of additional tunable parameters that lead to enhanced productivity or selectivity for molecular oxidation catalysts like 1 that are activated by carboxylates.