Synthesis and application of FeIII, NiII and MnII complexes anchored to HMS as efficient catalysts for cycloalkane oxyfunctionalization

• Synthesis of methoxysilane Schiff-base pentacoordinate metal complexes.
• Immobilized complexes into HMS as supported catalysts.
• Gas–liquid phase O2 oxidation of cyclohexane.
• High catalytic TONs and product selective.
• Proposed reaction mechanism.

Methoxysilane Schiff-base pentacoordinate metal complexes, i.e. Fe[Sal(PMeO-Si)DPTA], Ni[Sal(PMeO-Si)DPTA] and Mn[Sal(PMeO-Si)DPTA], were synthesized and single site covalently anchored into the uniform porous texture of HMS (2–10 nm size) via condensation process. The correspondent supported catalysts (4.a, 4.b and 4.c, respectively) were characterized by FT-IR, SEM/EDS, XRD, TG, EPR and AAS analysis. In the catalytic tests, they showed high efficiency in the selective oxidation of cyclohexane using molecular O2 (overall conversion 40.7% with Cat. 4.a, 29.5% with Cat. 4.b and 26.2% with Cat. 4.c) under relatively mild condition in batch reactor. The Cat. 4.a system (HMS/3.a) exhibits highest TONs ca. 4.2 × 103 with good selectivity ca. 70% (48% ketone selectivity). The reaction mechanism involves free radicals, as it was proved by the addition of PPh3. Finally, these supported catalysts could be reuse up to several catalytic cycles.

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