Oxygenase mimicking immobilised iron complex catalysts for alkane hydroxylation with H2O2

Abstract

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)₃SiC₂H₄C_nF_2n+1 and (Me₃Si)₂NH 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 (= BDE_C–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.