Mesoporous encapsulated complex

M. Eswaramoorthy, S. Neeraj, and C. N. R. Rao, High Catalytic Efficiency of Transition Metal Complexes Encapsulated in a Mesoporous Phase, J. Chem. Soc. Chem. Commun. 5, 615-616... 

Fig. 8 Kubelka-Munk optical absorption spectra of as-prepared mesostructured (black) and mesoporous NU-Ge-1 (red) semiconductors and NU-Ge-1 incorporating into the pores TCNE (blue) and TTF (green line) organic molecules. The recovered optical adsorption spectra of NU-Ge-1 by encapsulation of TCNE-TTF complexes are also given (dashed lines). Inset optical absorption spectrum of NU-Ge-1 encapsulating anthracene...

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

In another study, [Re(bpy)(CO)3(py)]+ was encapsulated into microp-orous NaY and mesoporous A1MCM-41 molecular sieves. Irradiation of the molecular sieve encapsulated rhenium complex in the presence of C02 and H20 (either gaseous or adsorbed onto the matrix) led to formation of CO and carbonate species. The proposed photoinduced process is outlined in Scheme 5 [95,96]. 

From the seminal work of Lunsford et al. in the early 1980s (DeWilde et al., 1980 Quayle and Lunsford, 1982), ship-in-a-bottle synthesis of metal complexes in the zeolite supercages, encapsulation of catalytically, optically, and/or electrochemically active species within micro- and mesoporous aluminosilicates, has received considerable attention (Alvaro et al., 2003). Site isolation of individual guest molecules, combined with shape and size restrictions imposed by the supercage steric limitations. 

Other complexes including phthalocyanines and metal clusters have also been encapsulated in several zeolites such as Y, EMT, VPI-5 and mesoporous materials [103-110],... 

Studies of the catalytic activity of MOFs are in their infancy with some encouraging results emerging in enantioselective catalysis. By contrast, meso-porous solids have already been studied extensively as catalytic supports, particularly of complexes too large to be encapsulated in zeolites. One of the most significant developments in this area is the observation that the constrained encapsulation of chiral catalysts in mesopores can raise the enantioselectivities of reactions well above those observed when the reaction is performed homogeneously. 

Scheme 23.74. Anchoring/encapsulation of a chiral Mn(salen) complex on a mesoporous silica support.

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