Organo-functionalized mesoporous supports for Jacobsen-type catalyst: Laponite versus MCM-41

In order to exploit the different textural properties of Laponite and MCM-41, specifically in terms of their external versus internal surface areas, in the covalent anchoring of a chiral Mn(III) salen complex, these materials were functionalized with 3-aminopropyltriethoxysilane (APTES), subsequently activated with sodium ethoxide, and finally used to anchor the Jacobsen catalyst derivative C1. All the materials were characterized by nitrogen elemental analysis, XPS, PXRD, nitrogen adsorption at −196 °C, FTIR and for those with the immobilized complex, they were additionally characterized by Mn AAS. The APTES anchored at the edges of the Laponite single crystals and inside the MCM-41 pores. Moreover, under the same preparative conditions, higher amount of APTES was anchored onto MCM-41 than onto Laponite, which is due to the higher surface area of MCM-41 compared to Laponite, as well as to its more exposed SiO₄ tetrahedra. Activation of the two organo-functionalized materials with sodium ethoxide originated anionic nitrogen groups as deduced by the increase of surface sodium content of these materials and N1s binding energy changes, but led to a small decrease in N bulk content as a result of some APTES leaching. Moreover, for MCM-41 some disruption of the silica framework occurred as a consequence of the basic treatment, as suggested by XPS, PXRD, and nitrogen adsorption study. The APTES functionalized Laponite and MCM-41 materials, as well as the activated analogs, were able to anchor C1 through axial coordination of the metal centre to the grafted surface nitrogen atoms. APTES functionalized MCM-41 presented similar complex content to Laponite analog, what points out for the fact that, at least for the bulky complex used in this work, there was no clear benefit in using a material of high internal area; for the ethoxide activated analogs, Laponite showed the highest complex content of all materials, but MCM-41 was able to anchor the lowest complex quantity, probably as a consequence of damaging effect caused by the basic treatment within its porous structure.