Oxygenase mimicking immobilised iron complex catalysts for alkane hydroxylation with H2O2

Immobilised iron complex catalysts with hydrophobic reaction fields that mimic the active sites of alkane hydroxylating enzymes were constructed into the mesopores of an SBA-15 type silicate support. The reaction of a chelating ligand (= L) anchored SBA-15 type support with (EtO)(3)SiC2H4CnF2n+1 and (Me3Si)(2)NH yielded the corresponding fluoroalkyl (= FC(n)) and trimethylsilyl group (= TMS)-modified supports L-SBA-FC(n)TMS with n = 4, 6, and 8. The ligand-anchored supports reacted with Fe(OTf)(2) or FeCl3 to yield the corresponding iron complex-immobilised catalysts. The structure, stability, and catalytic activity of the formed iron complexes depended on the anions of the used iron sources and the lengths of the fluoroalkyl chains. Examination of the cyclohexane oxidation with H2O2 revealed that the support decorated by longer fluoroalkyl chains and TMS was effective in improving the activity and alcohol selectivity of the iron complex immobilised catalysts. In a series of catalysts derived from Fe(OTf)(2), the longest fluoroalkyl chain (= FC(8)) modified catalyst was the most reactive and stable. In the FeCl3-derived double-hydrophobised catalysts, the FC(6) modified one exhibited higher activity compared to the FC(8) derivative. Propane oxidation catalysis of mononuclear iron complex-immobilised catalysts Fe(OTf)(2)/L-SBA-FC(n)TMS (where n = 6 or 8) demonstrated the substrate condensation effect of the hydrophobic pocket formed by the longer fluoroalkyl pillars. Formation of not only 2-propanol and acetone but also 1-propanol and propionaldehyde suggested the synergy of the strong radical characteristics of the generated active oxidant and the substrate concentration effect. The most active catalyst for the cyclohexane oxidation, FeCl3/L-SBA-FC(6)TMS, catalysed methane oxidation with H2O2: the products were methanol, formic acid, and methyl hydroperoxide, whereas no alkyl hydroperoxides formed in the oxidation of propane. Higher bond dissociation energy (= BDEC-H) of methane compared to propane resulted in decelerating the H atom abstraction (HAT) from methane by the oxidant formed on the iron complex while relatively accelerating the decomposition of H2O2.

Automated summary

Immobilised iron complex catalysts were constructed with hydrophobic reaction fields similar to the active sites of alkane hydroxylating enzymes. These catalysts were formed by anchoring chelating ligands to the mesopores of an SBA-15 silicate support and reacting them with fluoroalkyl and trimethylsilyl groups. The resulting catalysts showed improved activity and selectivity in cyclohexane and propane oxidation reactions, with the longest fluoroalkyl chain modification being the most reactive and stable.