Complexation-decomplexation kinetics

Factors which influence the effectiveness of membrane separation systems are summarized. These factors include the complexation/decomplexation kinetics, membrane thickness, complex diffusivity, anion type, solvent type, and the use of ionic additives. 

In the presence of a-CD or y-CD the observed peak-to-peak splittings for the oxidation of these compounds do not change appreciably from the reversible 57-mV value. This observation combined with NMR spectroscopic results reveals that a-CD interacts with the aliphatic region of the ferrocene derivatives. This conclusion is consistent with the observed increase in the association constant with a-CD in going from to 2 (see Table 1). In contrast, y-CD interacts with the ferrocene moiety, as does p-CD, but the larger cavity size of the former probably gives rise to faster complexation/decomplexation kinetics, which would explain the absence of y-CD effects on the voltammetric peak-to-peak splittings (at moderate scan rates). 

Liquid membrane separation systems possess great potential for performing cation separations. Many factors influence the effectiveness of a membrane separation system including complexation/ decomplexation kinetics, membrane thickness, complex diffusivity, anion type, solvent type, and the use of ionic additives. The role that each of these factors plays in determining cation selectivity and flux is discussed. In an effort to arrive at a more rational approach to liquid membrane design, the effect of varying each of these parameters is established both empirically and with theoretical models. Finally, several general liquid membrane types are reviewed, and a novel membrane type, the polymeric inclusion membrane, is discussed. 

Complexation/Decomplexation Kinetics. Though most researchers report that diffusion predominates over complexation/decomplexation kinetics in determining traasport rates, reaction kinetics also play a role (37). The rate at which macrocycles bind to cations is determined principally by two factors-- the charge density of the cation and the degree of preorganization inherent in the macrocycle. 

The macrocydic polyether dibenzo[24]crown-8 (DB24C8, Scheme 3) binds124,251 dialkylammonium-containing guests in solution, as well as in the solid state, to generate complexes with pseudorotaxane co-conformations that are stabilized primarily by [0—H-N+] and [O —H-C] hydrogen bonds. The com-plexation and decomplexation kinetics of these recognition events are influenced by the size of the... 

The evolution of a new set of electrochemical waves (as opposed to the gradual shifting of the redox couple) on addition of guest species may be due to a number of factors. If the complex formed has a particularly high stability constant and has a redox potential which is markedly different from that of the free ligand, a new set of waves may be observed. However, if the decomplexation kinetics of the complex formed is particularly slow on the electrochemical time scale then, as the potential is scanned between the vertex points during a cyclic voltammetric experiment, the solution complexed species will be stable over this time period and the two sets of waves will correspond to free ligand and complex. Therefore care should be taken to determine the cause of the evolution of a new set of electrochemical waves and... 

This two-wave behaviour [which has been described previously (Chariot et al., 1962 Saji, 1986)] has been rationalized in terms of the high stability constant of [25] Na+ complex. Kinetic effects (i.e. slow decomplexation kinetics) were discounted on the basis that no change in the CV of a solution of [25] in the presence of 0.5 equiv Na+ cations was observed when the scan rate was varied between 0.02 and 5 V s 1. 

Cyclodextrins have a large range of industrial applications. The market for them is growing as a consequence of their unique inclusion properties and decomplexation kinetics in conjunction with their stability, non-toxicity and relative cheapness. Cyclodextrins are the main active ingredient in Procter and Gamble s deodorising product Febreze, for example, where their complexation ability binds molecules responsible for household odours. The principal areas of interest are summarised in Figure 6.26. Some 1,649 research papers were published with the word cyclodextrin in the title in 2006 alone. 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

Conformational flexibility is important in particular for transport processes where fast kinetics of complexation/decomplexation is desired. However, the rigidity of the all-homocalixarenes may be tailored by capping the cavity in order to fine tune them for specific guests. 

The interfacial transfer kinetics were then investigated by perturbing the equilibrium, through the depletion of Cu + in the aqueous phase, by reduction to Cu at an UME located in close proximity to the aqueous-organic interface. This process promoted the transfer of Cu into the aqueous phase, via the transport and decomplexation of the cupric ion-oxime complex, resulting in an enhanced steady-state current at the UME. Approach curve measurements of i/i oo) vs. d allowed the kinetics of the transfer process to be determined unambiguously [9,18]. 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

In 1991, Kessar and coworkers demonstrated that the kinetic barrier could be lowered by complexing the tertiary amine with BF3, snch that i-BuLi is able to deprotonate the ammoninm compound, which can be added to aldehydes and ketones as shown by the example in Scheme 4a. Note the selectivity of deprotonation over vinyl and allyl sites. A limitation of this methodology is that the ylide intermediate does not react well with alkyl hahde electrophiles. To get aronnd this, a seqnence that begins with the stannylation and decomplexation shown in Scheme 4b was developed. The stannane can be isolated in 94% yield (Scheme 4b) and snbseqnently snbjected to tin-lithium exchange to afford an unstabilized lithiomethylpiperidine that is a very good nucleophile. However, isolation of the stannane is not necessary and a procedure was devised in which the amine is activated with BF3, deprotonated, stannylated, decomplexed from BF3 with CsF, transmetalated back to lithium and alkylated, all in one pot (Scheme 4c). ... 

I would like to extend Prof. Simon s characterizations of these beautiful new molecules to include a description of the effects on lipid bilayers of his Na+ selective compound number 11, which my post-doctoral student, Kun-Hung Kuo, and I have found to induce an Na+ selective permeation across lipid bilayer membranes [K.-H. Kuo and G. Eisenman, Naf Selective Permeation of Lipid Bilayers, mediated by a Neutral Ionophore, Abstracts 21st Nat. Biophysical Society meeting (Biophys. J., 17, 212a (1977))]. This is the first example, to my knowledge, of the successful reconstitution of an Na+ selective permeation in an artificial bilayer system. (Presumably the previous failure of such well known lipophilic, Na+ complexing molecules as antamanide, perhydroan-tamanide, or Lehn s cryptates to render bilayers selectively permeable to Na+ is due to kinetic limitations on their rate of complexation and decomplexation). 

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