Complexation-decomplexation rates

The overall mass-transfer rate can be limited by any of the diffusion resistances in the three liquid phases (diffusion-limited transport) and/or chemical reaction (complexation/decomplexation) rate resistances on the membrane-solution interfaces (reaction rate-limited transport). 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

Table 3.9 Complexation and decomplexation rate constants of picrate salts for representative compounds under standard conditions.

Table 3.9 shows that all of the complexation rates are very fast on the human time scale and cannot even be followed by H NMR spectroscopy (effectively, reactions occur immediately upon mixing). Decomplexation rates for more flexible hosts are also fast on the human time scale, but may be accessible by H NMR, while for spheraplexes the reverse reaction is very slow even on the human time scale at 25 °C. 

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... 

Table 14.1 Maltodextrin complexes with the transport protein of maltose (from Quiocho 1989) (reproduced with kind permission from the International Union of Pure and Applied Chemistry). K

Until recently, the role of slow rates of cation release in LM transport was unclear. From U NMR studies, it is known that the complexes can be kinetically stable [65, 66] and, as a consequence, decomplexation rates can be very slow. Influence of slow rates of alkah metal cations release was raised in transport through a BLM [58,64,67-69]. Recently, at cation transport experiments with different calix crown ether derivatives [42,70], was proven that the rate of decomplexation can be rate controlling in the transport through SLMs. 

These values are also strongly temperature dependent, with activation energies in the range 50 to 100 kJ.mol. The high values of the activation energies both for the reaction rates and the equilibrium constants are characteristic of these chalcogenide precursors. This is due to reaction mechanisms involving the existence of activated intermediate species, contrary to standard complexation-decomplexation reactions which are very labile. 

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). 

Ion complexation-decomplexation reactions must proceed at a sufficiently rapid rate in order for the ionophore to function as an efficient carrier. This is only possible when the ionophore is flexible enough to allow a stepwise rather than a concerted substitution of liganding moieties for solvent molecules Thus,... 

A critical point in NMR determinations of thermodynamic data is to have a proton whose chemical environment changes sufficiently upon binding to lead to a change in chemical shift, that is, 5//free 7 SHbound- As has been discussed above, binding determinations by NMR can be divided into two cases those that are slow on the NMR timeframe and those that are fast. The observed com-plexation and decomplexation rates (25) are those of uni-molecular processes (units s ), and it is the slower of these two processes that we must compare with the rate of NMR data acquisition. Our previous discussion emphasized how the timeframe of the NMR experiment was dependent on the external field strength of the instrument, but it is also important to note that whether a complexation is observed to be fast or slow on the NMR timescale depends on the chemical shift difference between the free and the bound state. The key equation is (37), which defines the boundary between fast and slow timescales, that is, the observed rate constant for coalescence ( coai) of the signals for the proton in question in the free and the bound state ... 

In this chapter, we discuss aspects of the assembly of carcerands and hemicarcerands, which is a templated process, and discuss their molecular recognition properties. Hemicarcerands show high size and shape selectivity in equilibrium binding experiments as well as with respect to rates of complexation/decomplexation. Finally, we highlight some of the dynamic aspects of hemicarcerands and their guests and give examples of their use as molecular reactions flasks. 

Reaction-Rate Limited Transport of Monovalent Ions. From H NMR studies it is known that decomplexation rates can be very slow and, as a consequence, complexes can be kinetically stable (35,36). Until recently the role of slow rates of cation release in SLM transport was unclear. Lehn et al (24) and Fyles (37) theoretically raised the question of the influence of slow rates of alkali metal cation release on transport through a BLM. Experimentally, this phenomenon has only been observed by Yoshida et al (38). They showed that cation transport through a BLM mediated by polynactin was limited by the rate of cation release from the membrane. In 1994, Echegoyen stated that in SLM transport the rate of cation release from the membrane could never... 

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 complexation and corresponding decomplexation rates are the same at equilibrium, d[C]... 

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