Entrapment salen ligands

Successful complexation of the exchanged transition metals by the tetradentate ligand salen was indicated by the typical yellow/brownish colour of the prepared inclusion compounds. X-ray powder diffraction and scanning electron microscopy reveal that the crystallinity of the zeolite host is virtually retained during the modification steps. Furthermore, no crystals of the salen ligand or of metal-salen complexes are visible on the external surface of the zeolite crystallites after careful piuification via soxhlet extraction with acetone. Analysis of the used solvent by UVATS-spectroscopy revealed that only salen but no metal-salen complexes were removed from the zeolite during extraction. This indicates that the complexes are truly entrapped in the intracrystalline voids of the zeolite. It has been demonstrated [7] that the presence of salen and PdSalen in zeolite Y can be probed by IR-spectroscopy in... 

This method relies on the size of the metal complex rather than on a specific adsorptive interaction. There are two different preparation strategies One, often called the ship-in-a-bottle approach, is based on building up catalysts in well defined cages of porous supports. Recently, enantioselective Mn epoxidation catalysts with different salen ligands have been assembled in zeolites. In zeoHte EMT [35] ees up to 88% and in zeolite Y [36] ees up to 58% were obtained with czs-P-methylstyrene. However, both entrapped catalysts were much less active than their homogeneous counterparts. Rh diphosphine complex were entrapped in the interlayers of Smectite [37]. The resulting catalyst was active for the enantioselective hydrogenation of N-acetamidoacrylic acid (ee 75%). 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

Despite the fact that more than a decade ago the formation of a square planar Co2+complex was already demonstrated to be very difficult in faujasite,[119] further claims of its entrapment continued to appear. The ESR cobalt signature does not correspond to that of solution or solid Co(salen). Addition of the hexadentate H2bbpen ligand (14) to CuY via the ligand method resulted in a degree of coordination with iron ions of less than 46 %.[112]... 

IR and UV-vis spectroscopy show that the majority of the encapsulated complexes still have structural integrity. As observed for many other oxidation catalysts, entrapment shows significantly enhanced activity. This effect is more pronounced when the degree of ligand substitution with Cl is higher. The simultaneous occurrence of two mechanisms is confirmed again, the selectivity of the encapsulated Mn(salen) complexes for the formation styrene epoxide is only 30%, the dominant product being benzaldehyde. 

The ship-in-the-bottle approach to heterogenization just exemplified has also been frequently investigated in oxidation. Two recent papers reported the use of complexes of Fe(III), Mn(III), V(VI), Cu(II), Ni(II) modified with salen-type ligands entrapped into the cages of zeolite Typically, the complexes were... 

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