Encapsulated complexes

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... 

There are a few examples of 02 oxidations catalyzed by zeolite-encapsulated complexes. Encapsulated CoPc was active in the oxidation of propene to aldehyde, whereas the free complex was inactive.76 A triple catalytic system, Pd(OAc)2, benzoquinone, and a metal macrocycle, was used to oxidize alk-enes with molecular oxygen at room temperature.77,78 Zeolite-encapsulated FePc79-81 and CoSalophen80,82 complexes were used as oxygen-activating catalysts. 

The encapsulated complex with bulky alkyl groups was more active than the complex without alkyl groups. The catalytic activity increases on the addition of axial ligands as pyridine N-oxide, and the highest enantiomeric excess, 88%, was also achieved in the presence of the pyridine N-oxide. 

J. Santamaria, T. Martin, G. Hilmersson, S. L Craig, J. Rebek, Jr, Guest exchange in an encapsulation complex A supramolecular substitution reaction , Proc Natl. Acad. Sci. USA 1999, 96, 8344-8347. 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

FIGURE 11.18. EPR spectra of (i) Cu(salen) and (ii) Cu(5-Cl-salen) at 77 K (a) neat polycrystalline sample, (b) frozen CHas/CHsCN (3 1) solution, (c) frozen DMF solution and (d) zeolite-Y-encapsulated complexes. Dotted line (e) is a blown-up trace of curve (d). 

With these caveats, then, the following is intended to illustrate the synthetic strategies to encapsulate complexes, rather than to comprehensively cover the field, which has been the subject of several reviews [140-142]. 

MPa O2). The role of the encapsulated [Co(salophen)] complexes is to catalyze the aerobic oxidation of hydroquinone to p-benzoquinone, which in turn oxidizes Pd(0). For the oxidation of 1,3-cyclohexadiene to l,4-diacetoxy-2-cyclohexene, the most active catalyst system involved the encapsulated complex [Co(tetra-tert-butyl-salophen)], which afforded product yields of 85-95% after 3 h at room temperature with greater than 90% trans-selectivity. This complex displayed significantly higher activity than the encapsulated [Co(salophen)] complex (72% yield in 3h) and the analogous homogeneous complex (86% yield in 5h). The increased activity of the t-butyl substituted catalyst was attributed to distortion of the bulky complex by the... 

FePc, it is less active, but more stable, than a FePc/NaY material. These differences between the encapsulated complexes were attributed to the possibilities that reaction could occur only at the pore mouths of VPI-5, where oxidatively degraded FePc catalyst molecules were replenished by the layer beneath and that the tighter fit of FePc in the NaY supercage required a saddle-type distortion of the molecule, which, while making it more reactive, made the catalyst more prone to oxidation. 

It can be seen from the above discussion that the varying structural modes present in Figure 14 are readily available. For the partially encapsulated complexes there is not only a dependence upon the cation cavity best fit but also on the relative strengths of the competing counter donors, both anions and/or solvent molecules. This is shown in the structures of (79)NaI H20, where the water is held by the metal rather than the I-,402 and (72)NaBr-H20, where the sodium is either symmetrically coordinated with two H20 in axial positions, or non-symmetrically coordinated and lying out of the ligand plane towards the Br not the H20 (Figure 19).421... 

Figure 7-9. The condensation of the cobalt(m) complex of the hexadentate ligand 7.4, which contains three primary amino groups, with formaldehyde and ammonia, gives the encapsulated complex 7.5. A view of the cation 7.5 as found in the solid state structure of its perchlorate salt is also presented.

Tei, L., Blake, A.J., Devillanova, F.A., Garau, A., Lippolis, V., Wilson, C., and Schroder, M. Anew cofacial binucleating macropolycycle segregated versus encapsulated complexation, Chem. Commun. (2001), 2582-2583. 

N. Sabbatini, M. Guardigli and I. Manet, Antenna effect in encapsulation complexes of lanthanide ions 69... 

XPS data of the encapsulated complex show the V 2p3/2 peak at 516 eV, typical for V comparable V contents obtained by ICP as well as by XPS, amounting to about 1 V per unit cell, are evidence for the homogeneous distribution of V across... 

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