Size-selective encapsulation

Iwamoto T, Watanabe Y, Sadahiro T, Haino T, Yamago S (2011) Size-selective encapsulation of C60 by [lOjcycloparaphenylene formation of the shortest fullerene-peapod. Angew Chem Int Ed 50(36) 8342-8344... 

Figure 7.33b shows the UV-Vis spectrum of the chloroform solution for the (RB + CR)/25b mixed system. The spectrum showed that only RB was encapsulated in 25b, indicating that 25b has a size-selective encapsulation ability, that is, 25b can preferentially encapsulate small molecules from the mixture. 

The encapsulation ability of 25 toward water-soluble dyes indicated that 25 was a unimolecular reversed micelle with an encapsulation abihty toward hydrophilic dye molecules. In addition, 25 showed a molecular size-selective encapsulation ability and a pH-sensitive controlled-release property. Hence, 25 was a good candidate for mixture-separation systems and pH-sensitive controlled-release systems. 

Sat(, T., Kitajyo, Y., Sakai, R. et al. (2009) Size-selective encapsulation property of unimolecular reverse micelle consisting of hyperbranched D-glucan core andL-leucine ethyl ether shelL Macromolecular Symposia, 279,145—150. 

Ronson TK, Giri C, Beyeh NK, Topic F, Holstein JJ, Rissanen K, Nitschke JR (2013) Size-selective encapsulation of hydrophobic guests by self-assembled M4L6 cobalt and nickel cages. Chem Eur J 19 3374—3382... 

The fractal character of the dendritic interior provides unique possibilities for size-selective encapsulation, manipulation and release of different substrates. Surprisingly, publications on dendrimer networks are rather scarce (72). In addition, in all known studies only small molecules have been used to form the links between the dendritic fragments (network junctions). 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

Indeed, recent results from our laboratory indicate that dendrimer-encapsulated CdS QDs can be prepared by either of two methods [192]. The first approach is analogous to the methodology described earlier for preparing dendrimer-encapsulated metal particles. First, Cd and S salts are added to an aqueous or methanolic PAMAM dendrimer solution. This yields a mixture of intradendrimer (templated) and interdendrimer particles. The smaller, dendrimer-encapsulated nanoparticles may then be separated via size-selective photo etching [193], dendrimer modification and extraction into a nonpolar phase [19], or by washing with solvent in which the dendrimer-encapsulated particles are preferentially soluble. An alternative, higher-yield method relies on sequential addition of very small aliquots of Cd + and S " to alcoholic dendrimer solutions. 

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

Nature often exploits large pJQ shifts in enzymes to effect chemical catalysis similarly, we hoped to apply the large shifts in the effective basicities of encapsulated guests to reaction chemistry. Initial studies focused on the hydrolysis of orthoformates, a class of molecules responsible for much ofthe formulation ofthe Bronsted theory of acids almost a century ago [98]. While orthoformates are readily hydrolyzed in acidic solution, they are exceedingly stable in neutral or basic solution [99]. However, in the presence of a catalytic amount of 1 in basic solution, small orthoformates are quickly hydrolyzed to the corresponding formate ester [38]. Addition of NEt4 to the reaction inhibited the catalysis but did not affect the hydrolysis rate measured in the absence of 1. With a limited volume in the cavity of 1, substantial size selectivity was observed in the orthoformate hydrolysis. Orthoformates smaller than tripentyl... 

Capsule shell sizes are generally standardized with Lnite volumes [12,13] available for dispensing the API or powder blend. The capsule size selection would depend on the dose and the bulk density of the NCE. Historically, if the doses were within a reasonable range, it was feasible to dispense API directly into the capsule body utilizing manual, semiautomatic, or automatic high-speed encapsulators available for manufacturing. 

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. 

S.K. Oh, Y. Niu, R.M. Crooks, Size-Selective Catalytic Activity of Pd Nanoparticles Encapsulated within End-Group EunctionaUzed Dendrimers, Langmuir 21, 10209, 2005. 

Transformation of a substrate depends on its access to the active site. In heterogeneous catalysts where the active site is grafted or encapsulated in the pores of a 3D framework (inorganic or hybrid), the factor limiting reactivity is the section of the windows, which creates a size selectivity. 

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