Partial encapsulation

Figure 3.48. An artist impression of possible shapes of catalyst particles present on a support a. spherical particle with only one point contact to support, b. hemispherical particle, strongly bonded to support and partially poisoned, c. metal crystallite, strongly bonded to and partially encapsulated in support, d. complete wetting of the support by the active phase. After Scholten et al, 1985 and Ba.stein cr a/., 1987.

Figure 33. The partial encapsulation of a pair of tt-stacked C60 units in Ni[pz(Ar-Me2)g]2 C60 (lOOd).

Partially encapsulated Tl+ ions have been observed for the related doubly bridged compound 107.103 The transition metal fragment [ Me2Pt 2(/r-Ph2PC5H3NPPh2)2] is first assembled and then treated with TICIO4 to give the product in 60% yield. The Tl+ ion was displaced by Cu+ upon addition of [Cu(MeCN)4]C104 almost quantitatively. 

The cavity of diphenylglycoluril derivative 3 is well suited to partially encapsulate a [4Fe-4S] cluster. Compound 29 which contains four arms terminating with thiol groups was synthesized and treated with (n-Bu)4N 2 Fe4S4Cl4) in dimethylformamide to give cluster complex 30 [31]. The product was characterized by a number of techniques, including cyclic voltammetry and differential pulse polarography. The current response of 30 was very small, but improved upon addition of a modulator, e.g. Ba or Na" ions. This behavior is similar to that observed for certain redox active enzymes [32]. As in the natural systems, a maximum response is observed when the Ba concentration is... 

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 1.5 Schematic illustration of the synthesis principle for crystallization of mesoporous zeolite single crystals. The individual zeolite crystals partially encapsulate the nanotubes during growth. Selective removal of the nanotubes by combustion leads to formation of intracrystalline mesopores. Reproduced from Schmidt et alP51 by permission of American Chemical Society...

In this case, the mechanism of the porous structure formation can be subdivided into several stages that differ from one another by the gel particle size, composition of gel, and particle arrangement. Undoubtedly, these parameters will change with composition of the solution. In particular, at a high concentration of the first component (with a lower pH of precipitation) and insignificant content of the second, completely adsorbed on gel particles, precipitated by the first component, the structure of the adsorbent obtained will be represented by gel partially encapsulated by the other component. As the concentration of the second component increases, the number of encapsulated particles will... 

The tumors are either partially encapsulated or nonencapsulated. On low-power magnification, EMC tends to grow in a diffuse or multinodular pattern. The tumors are composed of small ducts composed of cuboi-dal cells with pink cytoplasm and centrally or basally located round nuclei. This epithelial cell component... 

The first bisphosphine calixarenes that have been used in catalysis are di(amide)-phosphine hybrids calix[4]ar-ene. Reaction of [RhCl(norbornadiene)]2 with these calixarene derivatives gave an organometallic complex whose norbornadiene-rhodium moiety lies above the cavity defined by the four substituents of the calixarene and between the two amide functionalities. This complex was applied in the hydroformylation reaction of styrene. The rather low reaction rate observed (7.5 turnovers per Rh per hour) has been attributed to a partial encapsulation of the metal center preventing the approach of the substrate. Indeed, the metal center may be viewed as located in a hemispherical ligand environment. 

There are two basic structural motifs in which a tetranuclear cluster could accommodate an interstitial atom - fully encapsulated within a tetrahedral cavity or partially encapsulated between the wings of a butterfly framework. 

As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The c/o5o-pentanuclear cluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore open-up if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters (E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as /i/Jo-octahedral species (Scheme 2). 

It is difficult to rationalize all the information relating to SMSI in terms of a single effect. Present evidence suggests that there are three main ways in which a support may modify the catalytic and chemisorptive properties of a metal particle. First, the metal and the support may interact to form a compound, or there may be a partial encapsulation of the metal by the support. Second, there may be a transfer of electrons between the metal and the support. Third, the particle size distribution and/or the particle morphology may be altered. 

Although as yet there is no direct evidence to support the contention that partial encapsulation of the metal may occur after high temperature reduction, this offers a possible explanation of the observed chemisorption effects. It is perhaps significant that even in the case of Si02-supported Ni there is evidence that OH ions can migrate from the support to the metal and inhibit the adsorption of H2... 

The efficient production of fully formed double-walled microspheres depended on the mass ratio of both polymers, and it occurred only above a critical polymer mass ratio. The core concentration, on the other hand, does not seem to have a significant effect on the critical polymer mass ratio needed for forming double-walled microspheres. Partial encapsulation of core by shell polymer was found in microspheres when a shell flow rate below the critical polymer mass ratio was selected. Above the critical polymer mass ratio, fully formed double-walled microspheres were observed with no sign of partial encapsulation confignration. The molecular weight of the shell layer did not influence the subsequent drug release from the microspheres as evidenced by the release profiles obtained experimentally. 

This structure also revealed that the fullerene is only partially encapsulated, with an average intermolecular distance of 3.4 A, similar to the interlayer distance in multi-walled carbon nanotubes and multi-layer graphene. From this discovery, Kawase was able to investigate further the dynamics of the [6]CPPA-fullerene interaction [21] and also to synthesize naphthyl- derivatives of CPPAs with deeper cavities and stronger interactions such as 16-18 (Figs. 11 and 12) [22,23] (Table 1). 

Reaction of an iron(lll) preassembled complex [Fe(L1020)3] with tris(2-ami-noethyl)amine proceeds to form the partially encapsulated product [Fe(L1021)] in 90 /o yield (Scheme 4-17). 

This coordination capsule 575 is reported in [147] to only partially encapsulate ruthenocenyl-based zwitterionic guests by Scheme 4.144. Whereas their cationic heads are situated within its cavity, the spacer to the anionic terminal group passes through one of the small openings at the centers of the triangular faces of its dodecaanionic cage framework. 

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