Transition metals salen ligands

The use of a Schiff s base complex as a chelating ligand has been extensively studied for transition metals (31). An analogous complex has been produced for sodium perchlorate with two molecules of Cu(salen) (where salen = N,N -ethylenebis(salicylidene-iminato)). Each sodium, Fig. 4, is surrounded octahedrally by two oxygens from a perchlorate ion and four from the two chelating Cu(salen) ligands (32). 

The zeolitic host was ion-exchanged to introduce the desired transition metal cation. Synthesis of the salen ligands in the mesopores was conducted at room temperature under an inert atmosphere. To the ion-exchanged zeolitic material, the optically pure diamine in dichloromethane was added, in slight excess (1.2 equiv.) relative to the metal content. The appropriate amount of aldehyde was added to the slurry. The final material was Soxhlet-extracted with dichloromethane and toluene, respectively, until the solvent remained colorless. This procedure is described elsewhere in detail [51]. 

Fig. 6 shows the diffuse reflectance UV-visible spectra of typical salen Co(IH) complexes immobilized on MCM-41, the homogeneous salen complex of the same structure, and a pure Si-MCM-41. The chiral salen ligands of Co(IU) form showed the bands at near 250 and 370 nm on the UV spectra. But the pure siliceous MCM-41 no absorption peak at all. This broad band is probably due to the charge-transfer transitions between metal and ligand. This result indicates that the successful immobilization of chiral salen ligands was achieved. 

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

Transition-metal complexes with salen ligands can serve as catalysts for a multitude of stereogenic reactions including Diels-Alder reactions. We recently studied the application of polyglycerol-supported salen systems 18, 19 and 20 as chiral catalysts in the hetero Diels-Alder reaction between... 

To introduce the desired transition metal cation the zeolitic host was ion-exchanged. The synthesis of the salen ligands in the mesopores was conducted at room temperature under an inert atmosphere. To the ion-exchanged zeolitic material was added an amount... 

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... 

The salen-based catalysts mentioned above are not soluble in water, which constitutes a limitation this is overcome by the preparation of new amphiphilic salen-type transition metal complexes. Therefore, several bulky salen-type SB ligands containing both tert-butyl and methyl(triphenylphosphonium) substituents have been prepared.122 The introduction of both lipophilic and ionic substituents in the ligands increases the solubility of the complexes of these ligands, which are found to be soluble both in water and in most common organic solvents and this may enhance the catalytic properties of the complexes. 

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