Toward the next generation of designer catalyst: Controlled heterogenization of selective homogeneous oxidation catalysts.

摘要

Four methods for immobilizing porphyrin- and salen-based metal complexes were investigated in an effort to synthesize the next generation of oxidation catalysts that possess the high stability and the separation advantages afforded by heterogeneous catalysts as well as the exquisite selectivity well-known to homogeneous systems. In the first method, the epoxidation catalyst [5,15-bis(4-phosphonic acid)porphyrinato]MnIII was immobilized on indium-tin oxide (ITO)-coated glass via layer-by-layer assembly, which allows for excellent control over the catalyst loading. Increased catalyst lifetimes and enhanced activity were obtained in the epoxidation of olefins using this film-based catalyst relative to its homogeneous analogue. Attempts to exploit the film-based catalysts electrochemically, using water as both the solvent and source of oxygen for epoxidation, were only partially successful.; In the second method, a mesoporous anodized aluminum oxide (AAO) membrane was used as a support for chiral (salen)MnIII complexes. The resulting membrane-supported catalyst allowed for easy catalyst separation and recycling while maintaining comparable selectivity to its homogeneous counterparts. Furthermore, when used in a forced-through-flow reactor, it offered great flexibility in the control of substrate feed and product separation, with an unusually high activity (TOF = 135 min-1, total TON = 1670) and a high enantioselectivity (84% ee) for the epoxidation of chromene.; The third method involved the formation of solid coordination polymers via a metal-mediated assembly of [bis(catechol)salen]MnIIICl. These heterogenized catalysts can be assembled by several different metal ions (CrII, CdII, ZnII, Mg II, etc.) and functioned as active recyclable asymmetric catalysts for olefin epoxidation.; Finally, homogeneous complexes containing 5,5'-bis(4-pyridine)-functional group was employed as a coordination motif to construct porous metal-organic frameworks (MOFs). The framework-immobilized [bis(pyridyl)salen]Mn III offered tremendous advantages over conventional (salen)Mn III-catalyzed epoxidation: enhanced catalyst stability, great ease in catalyst separation and recycling, and promising substrate-size selectivity. Several [bis(pyridyl)salen]ZnII complexes were also investigated as versatile building blocks for novel porous architectures that can be used for enantioselective catalysis and separation of racemic substrates.