Cavity of coordination capsule

Nanoreactors can also be used for protecting one reaction site or blocking a reaction through encapsulation of a reactant. As an example, the regioselectiv-ity of nucleophilic substitution of aryl-substituted allylic chlorides can be controlled through encapsulation and shielding of the internal reaction site of the substrate within the cavity of coordination capsule [51]. 

Sol-gel condensation of encapsulated tetrame-thoxysilane guest molecules within the spherical cavities of coordination capsules 529 and 530 has been used in [4] for preparation of monodisperse silica nanoparticles by Scheme 5.4. 

Suitable aromatic guests within the cavities of coordination capsules 483 and 487 undergo both [2+2] and [2+4] pericyclic additions by Scheme 5.6 depending on the reaction conditions [6]. A thermal initiation of these Diels-Alder... 

Two encapsulated molecules of the constitutional isomers4i4and4i5ofbis(trimethoxysilyl) pyrene undergo dimerization within the cavity of coordination capsule 552 by Scheme 5.7 giving 1 1 cage complex of only one conformational isomer of the resulting dimer 416 having the parallel 5y -conformation and that with... 

Unusual chemical and photochemical reactivity (or, in contrast, incredible stability) of encapsulated species and the unexpected products of such reactions within the confined cavities of molecular capsules have been observed in many cases. This can be explained by steric restrictions due to relative rigidity of covalent and coordination capsules, by isolation of these guests from external factors such as solvent effects, and by relatively strong supramolecular host-guest interactions. 

Recently, it has been found that the outcome of some cycloadditions can be altered remarkably when performed inside the cavity of cyclodextrins (288), self-assembled molecular capsules (289), or coordination cages (290). This fact intrigued us greatly and stimulated our interest in the Diels-Alder reactivity of the calixarene-like [M2(L19)(L )]+ complexes bearing unsaturated carboxylate coligands L (215). 

Capsule-shaped Ir(l) and Rh(l) cationic complexes with a triphosphinocaltx[6]arene as a multidentate ligand were recently synthesized (Figure 33). These organometallic bis-caltxarene complexes showed dynamic behavior with size-selective molecular encapsulation, which was confirmed by variable-temperature P H NMR measurements in the presence of various molecules. X-ray crystal analysis showed that the calix[6]arene moiety adopted the same pinched-cone conformation as the non-coordinated caltxarene. Molecules such as CFlj,Cly or CICH2CH2CI are too small to fit the cavity of the iridium and rhodium bis-calixarene complexes and cannot restrict the dynamic behavior at 25 °C. On the contrary, molecules such as X2CHCHX2 (X = C1 or Br), benzene, toluene, o- or w-xylene just fit in the cavity and show the dynamic behavior. Finally, large molecules (p-xylene, cumene, mesitylene, etc.) could not enter the cavity. 

Using encapsulation of substrate within the cavity of a coordination capsule, the internal reaction site of a substrate can be shielded, and nucleophilic attack will be directed to a less protected terminal site (Fig. 9.22). In this way, regioselectivity of the reaction can be controlled without changing steric and electronic effects of nucleophiles and substrates, or solvent polarity. 

Initially, counterions were distributed randomly around the nanocapsule, which was filled and surrounded by water. In the internal cavity of the POM 172 water molecules were placed 72 waters fulfilling the molybdenum coordination sphere of the pentagonal Mo(Mo)s units and a structureless 100 H2O cluster. Following a standard protocol [24], a large number of configurations were collected through the MD trajectory and analyzed. Then, the radial (RDF) and spatial (SDF) distribution functions of the centers of the capsule-oxygen water atoms were computed. 

Schemes 4.9 and 4.10. The encapsulated aromatic guest forms stacking interactions with facial azine fragments of 488, while that with bulky adaman-tyl substiment occupies the remaining free space within its cavity the photomediated 1,4-radical reaction of the caged molecules is described in Sect. 5.2. The coordination capsule 483 also gave the corresponding homoguest 1 1 and 1 2 cage complexes shown in Scheme 4.10 [11].

A bowl-shaped Pd Ls precursor 269 undergoes dimerization in the presence of a nine-residue peptide in NaNOs aqueous solution by Scheme 4.24 to give a 1 1 cage complex of the Pd Lg coordination capsule 504 with large hydrophobic cavity this capsule is in equilibrium with its cavitand analog [25]. According to the NMR data, the guest peptide molecule adopts a-helical conformation within the hydrophobic cavity of 504 [25]. 

Similar Pdi2L24 long-chain alkyl-functionalized caging ligands 513-515 with a diameter of approximately 50 A have been synthesized in [28] by Scheme 4.27. These spherical coordination capsules have hydrophobic inner cavities that are formed by alkyl chains and are able to encapsulate hydrophobic guests... 

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