Complexation kinetics calixarenes

The dynamics of a supramolecular system are defined by the association and dissociation rate constants of the various components of the system. The time-scale for the dynamic events is influenced by the size (length-scale) and by the complexity of the system. The fastest time for an event to occur in solution is limited by the diffusion of the various components to form encounter complexes. This diffusion limit provides an estimate for the shortest time scale required for kinetic measurements. The diffusion of a small molecule in water over a distance of 1 nm, which is the length-scale for the size of small host systems such as CDs or calixarenes, is 3 ns at room temperature. In general terms, one can define that mobility within host systems can occur on time scales shorter than nanoseconds, while the association/dissociation processes are expected to occur in nanoseconds or on longer time scales. The complexity of a system also influences its dynamics, since various kinetic events can occur over different time scales. An increase in complexity can be related to an increase in the number of building blocks within the system, or complexity can be related to the presence of more than one binding site. 

The ability of calixarenes to bind large metal ions with high kinetic stability is important in the search for complexants for radionuclides such as Cs (ti/2 = 30.2 yr) and Sr (ti/2 = 65 d) from the reprocessing of exhausted nuclear fuel. There has been considerable interest in caesium-complexed calix[4]-bis-crowns as selective Cs-carriers. Transport isotherms of trace level Cs through supported liquid membranes containing calix[4]-bis-crowns have been determined as a function of the ionic concentration of the aqueous feeder solutions, and l,3-calix[4]-bis-o-benzo-crown-6 appears to be much more efficient in decontamination than mixtures of crown ethers and acidic exchangers, especially in highly acidic media. " ... 

Analysis of CP kinetics and the parameters obtained are a source of important information about host-guest interactions. From the classic I-S model of cross polarization, it was found that the values of T pH relaxation times for guest and host molecules are considerably different. This suggests that DMF carbons cross-polarize from the DMF protons, not from the remote protons, of calixarene. Moreover, such a result proves that the DMF is located outside the calices and between the calixarene molecules. The simple I-S model is not sufficient to describe the cross-polarization of the complex of tetra (C-undecyl)calix[4]resorcinarene with DMA. The best fit of experimental data was seen when the more advanced I-F-S model of cross polarization was adopted [57]. Analysis of CP kinetic parameters provides important information on molecular mobility (Fig. 19, Table 5). 

As discussed above, calix[4]aienes reversibly interact with NO2/N2O4 and entrap reactive NO cations within their 7t-electron-rich interiors, one per cavity (72, 73). Very high K ssoc 10 Af values (AG 8 kcal mol ) for these processes were determined, and the complexes were also kinetically stable. These features were used in the design of calixarene-based nanotubes... 

Reinhoudt and co-workers hybridized calixarene and spherand structural features to make a new host 28 [63] (Structure 16) which forms kinetically stable complexes with NaBr and KBr. Because of this high stability, the decomposition... 

The p-sulfonatocalix[n]arenes 4, originally synthesized by Shinkai et al." as water soluble calixarenes for catalytic studies in water solution, were more recently used by Ueoka et alP as catalysts in the specific acid catalyzed methanolysis of A -Ac-L-amino acids (Phe, lyr, Trp, His, Lys, Arg). Rates of methanolysis in the presence of the calix[n] arene catalysts, normalized per sulfonic group, were compared with rates obtained in the presence of the noncyclic analogue p-hydroxybenzenesulfonic add. Rate enhancements - ranging from 12- to 86-fold - relative to control were recorded only in the methanolysis of basic amino acid derivatives (His, Lys, Arg), but neutral amino add derivatives (Phe, Tyr, Trp) responded virtually in the same way to the presence of cyclic and non-cyclic catalyst. Michaelis-Menten kinetics and H NMR spectral evidence pointed to the intermediacy of inclusion complexes of 4 with the protonated form of basic amino acid derivatives, as shown in 5 for the His-4 (n = 4) combination. 

Upon mixing CX and 1-2PF6 in 2 1 molar ratio, the best fits were obtained by assuming the reaction sequence illustrated by Eqs. 1 and 2 in Scheme 6.2. First, the 1 1 adduct is formed in two steps (Eq. 1), with the same kinetic rate constants determined in the previous experiment. Successively, this complex reacts with a second calixarene molecule, forming the 2 1 complex with a second-order reaetion (Eq. 2). The values of the kinetic rate constants are gathered in Table 6.2. 

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