Editor's choice paperAlkane oxidation reactivity of homogeneous and heterogeneous metal complex catalysts with mesoporous silica-immobilized (2-pyridylmethyl)amine type ligands

- Immobilized complex catalysts were prepared with two ligands.
- Catalytic activity for cyclohexane oxidation with mCPBA was investigated.
- Activities of Mn, Fe, Co, Ni and Cu complex catalysts were compared.
- The site density of immobilized complex catalysts affected to the activity.
- Effect of the ligands (pyridine versus carboxylate) was compared.

Attachment of N,N-bis(2-pyridylmethyl)amine (L1) and N-(2-pyridylmethyl)glycine (L2) ligands on the azide-functionalized SBA-15 type mesoporous silica support and subsequent insertion of metal salts into the ligand-immobilized supports yielded heterogeneous metal complex catalysts, M(A)/SBA*-Ln-x, where M, A, *, n, and x indicate the metal ion (MnII, FeII, CoII, NiII, CuII), counter anion (Cl, OAc, OTf), trimethylsilyl end-capped silica, the numbering of the ligands, and the initial content of the azide tether group (tether/Si mol%) respectively. The cyclohexane oxidation activity of these immobilized catalysts and corresponding homogeneous complexes have been evaluated with the use of m-chloroperbenzoic acid as an oxidant. The activity of the immobilized catalysts is influenced by the ligand density on the support. Both the site-isolated immobilized catalysts and the related homogeneous catalysts of ligand L1 show similar trends of their final TONs on the type of metal ions (Co ≈ Ni > Fe > Mn > Cu). These results suggest that all of the metal complex sites are successfully immobilized without structural changes under the site-isolated condition. The reactivity trends of the complexes with the L2 ligand and their dependence on the ligand density were complicated. The observed results may be explained by the formation of cluster complexes via bridging carboxylate moieties of the ligand.