Supercages

Turro N J, Buchachenko A L and Tarasov V F 1995 How spin stereochemistry severely complicates the formation of a carbon-carbon bond between two reactive radicals in a supercage Acc. Chem. Res. 28 69-80... 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

Deuterium NMR is used to study the molecular mobility of benzene-de in Na and Cs forms of zeolite X. The systems studied were prepared with loadings in the range 0.7 molecules/supercage to 5.6 and 5.0 mole-cules/supercage for (Na)X and (Cs,Na)X, respectively. 

Lowering the temperature has a similar effect on the deuterium spectra as does increased loadings. In Figure 3, spectra for benzene-d6/(Na)X at 0.7 molecules/supercage over the temperature range 298 to 133 K are shown. It is observed that both benzene species are detected simultaneously between 228 and 188 K. Below this temperature the oriented benzene species becomes the predominant form. A similar situation occurs for polycrystalline benzene-dg in which two quadrupole patterns, one static and the other motionally narrowed due to C rotation, are observed to coexist at temperatures between 110 and 130 K (7). This behavior has been attributed to sample imperfections (8) which give rise to a narrow distribution in correlation times for reorientation about the hexad axis. For benzene in (Na)X and (Cs,Na)X such imperfections may result from the ion/benzene interaction, and a nonuniform distribution of benzene molecules and ions within the zeolite. These factors may also be responsible for producing the individual species. However, from the NMR spectra it is not possible to... 

The temperature range in which the two observed benzene species coexist, is shifted to lower temperatures for Cgde/(Na)X samples with fewer benzene molecules per supercage (see Figure 3 and 5). This is... 

Figure 4. Dependence of the integrated intensity on the length between pulses, T, in the solid spin echo sequence. C6De/(Cs,Na)X at 2.4 molecules/supercage and 283 K. SW = 500 kHz.

Equations (1) and (2) hold for all points along the coexistence lines in Figure 6. At the lowest loading, 0.7 molecules/supercage, AE /AEjj = 1.2. Since the two lines are approximately parallel... 

Zeolite Y modified with 1,3-dithiane, 1 molecule per supercage (7.2 wt%)... 

Taking into consideration the preparation procedures, the Mo content of 2.1Mo/SC, and the Co/Mo atomic ratio of ca. unity at the maximum HDS activity, highly dispersed Co-Mo binary sulfide clusters, possibly COjMOjSx, in the supercage of the NaY zeolite are suggested for catalytically active species. The HDS activity of the CoSx-MoSx/NaY was not changed even after a 20-h treatment at 673 K in a stream of HjS/H (Fig.3), demonstrating a high thermal stability of the active species. 

When the metal nanoparticles are inserted into zeolite supercages, the size of the metal particles is confined according to the size of the supercage. However, after reduction of the precursor metal ions in a stream of hydrogen, the protons replacing the metal ions in the cation exchange position also interfere with the metal particles, influencing thereby their chemisorption and catalytic properties. 

Figure 4.3. Infrared spectra of the HY sample upon TMA adsorption and NH3 saturation evidencing OH in the supercages at 3637 cm1, OH in the sodalite units at 3548 cm-1 and OH in the hexagonal prism at 3501 cm-1 [54].

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

Figure 2 Model for the Bronsted sites in the supercage of a dealuminated HY, depending on the nature and location of the extraframework aluminic phase (the drawing of the extraframework phase is only schematic). In the center is the unperturbed Bronsted site.

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