Nanostructured Materials and Catalysts via SOMC@PMS
Surface organometallic chemistry (SOMC) is known to produce highly reactive surface species. Such enhanced reactivity originates from sterically unsaturated metal centers featuring a highly distorted coordination environment and from a strongly electron-withdrawing effect of the support. SOMC in confined spaces gives access to a new class of nanostructured catalysts. We are examining periodic mesoporous silica (PMS) of the various MCM and SBA topologies which feature nanosized cavities ideal for an extensive intraporous chemistry (“molecular factories”). Advanced methods of surface grafting as evidenced by the versatility of a novel “heterogenized silylamide route” and advanced concepts of surface silylation have important implications for the design of the nanoenvironment (microenvironment and mesoenvironment) of surface-confined oxophilic metal species such as the lanthanides, aluminum, and titanium. For this, the catalytically active interface is fine-tuned via adjustment of both the dispersion and accessibility of the catalytically active sites by steric and hydrophobicity effects. Variation of the inner coordination sphere of the active metal site, i.e., change of the local ligand environment, is crucial for the design of homogeneous single site catalysts, and consequently, is examined for the development of more efficient and specialized catalysts of the category discussed here. Moreover, surface morphology, surface polarity, and pore confinement are important factors to define the nanoenvironment of PMS materials. Conclusive methods of characterization, elaborating the importance of spectroscopic probe ligands and nitrogen physisorption, reinforce the excellent capacity of PMS materials as a model support, e.g., of amorphous silica. The importance of combining novel molecular chemistry and surface chemistry is emphasized by the application of tailor-made grafting reagents and model oxo–surfaces such as calix[n]arenes and oligosilsesquioxanes. The application of SOMC@PMS in order to emulate the incredible cooperativity between metal site, protein tertiary structure, and substrate molecule in natural enzymes by “mesozymes” is certainly challenging.