Alkali metal cation binding

Examples such as this show the importance of alkali metal cation binding and transport in biochemistry, and a great deal of effort has been expended in supramolecular chemistry in attempts to understand natural cation binding and transport of the ionophore and channel type and to develop artificial systems capable of similar selectivities and reactivities. We will take a close look at many of these compounds in Chapter 3. 

A related reduced series (Figure 5.5) was also reported J74 Selectivity of these crowned arborols (100-102) towards alkali metal cation binding was examined and allosteric as well as conformational binding effects were studied. Also they showed that the sterically less crowded arborols (e.g., 102) are better for the dissolution of myoglobin in organic solvents, such as DMF. This example affords an interesting entrance to internal metal ion complexation at a specific loci see Section 8.3. 

M. Albrecht, H. Rottele, P. Burger, Alkali-Metal Cation Binding by Self-Assembled Cryptand-Type Supermolecules , Chem. Em. J., 2, 1264 (1996)... 

Lowe, N.D. Garner, C.D. Transition-metal complexes of crown ether benzodithiolenes. Part 2. The effects of alkali-metal cation binding. J. Chem. Soc., Dalton Trans. 1993, 3333-3340 and references therein. 

In the early years of supramolecular chemistry, a number of studies showed that in comparison to the cyclic crown ethers and bicyclic cryptands, alkali metal cation binding by podands of the oligoethylene glycol type is relatively weak in polar solvents such as water and methanol. Thus [18]crown-6 binds K+ some four orders of magnitude more strongly in methanol solution compared to its open chain podand analog, 2. Representative binding constants for cyclic and acyclic species are shown... 

Originally, the alkali metal picrates were employed in ion-binding experiments because of the ease with which the cation concentration could be determined. In these cases, the picrate anion is simply considered to be the gegenion of the alkali metal cation, and as a spectrophotometrie label by which the cation concentration could be determined. Smid later found that some polymeric systems with pendent ionophores not only bind cations in the usual way, but that these systems also bind the picrate anion directly. In this instance we are dealing with alkali metal cations binding to the ionophore, using the picrate anion only as a spectropho tome trically detectable gegenion. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Scheme 6 Known binding modes for [(RC)xNy] (wherex+y=5) to alkali metal cations.

There are two general classes of naturally-occurring antibiotics which influence the transport of alkali metal cations through natural and artificial membranes. The first category contains neutral macrocyclic species which usually bind potassium selectively over sodium. The second (non-cyclic) group contains monobasic acid functions which help render the alkaline metal complexes insoluble in water but soluble in non-polar solvents (Lauger, 1972 Painter Pressman, 1982). The present discussion will be restricted to (cyclic) examples from the first class. 

The electrochemical properties of ferrocene have been utilized by many workers in the field of electrochemical molecular recognition. Saji (1986) showed that the previously synthesized (Biernat and Wilczewski, 1980) ferrocene crown ether molecule (Fig. 3 [1]), whose binding properties had previously been studied only by nmr and UV/Vis techniques (Akabori et al., 1983), could be used as an electrochemical sensor for alkali metal cations involving a combination of through-space and through-bond interactions. 

FAB mass spectrometry and UV/Vis spectroscopy have been used to demonstrate the binding of the cations to the complexes, and CV studies have provided quantitative measures of the perturbations which result on cation binding. This behaviour is quite complex and readers are encouraged to consult the original papers. However, it does indicate that receptors [42] and [44] show most promise as sensors for alkali metal cations. 

The 2-(AuC1)4 and 2-(PtCl2SMe2)4 complexes (see above), show extractability properties vs. alkali metal ions, with a greater affinity for than for other alkali metal ions [48]. No structural data were available and the nature of the binding in the formation of these complexes was not investigated. Similarly, the anionic complexes [2-Cu4(/t-Cl)4(/t3-Cl)] and [2-Ag4(/t-Cl)4(/t4-Cl)] have been shown to act as host for the selective binding of alkali metal cations and divalent metal ions like or Pb. Both complexes... 

Table 2 Binding constants (Xa, L mol ) and free energies of complexation (-AG°, kj mol ) for complexes of 12b with alkali metal cations ...

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

A parallel investigation on the binding of alkali metal cations to nucleobases, employing guided ion beam mass spectrometry, has recently been reported by Rodgers and Armentrout . ... 

Li+ interacts with both alkyl chains is more stable, compared to the structure where it sits at the antiperiplanar position, by 3.0 kcalmoG. The two binding trends of the alkali metal cations to alkenes were rationalized, taking into account the polarization of the double bond by the cation, polarization of the alkyl chains and steric effects as well. Recently, Gal and coworkers reported a similar alkyl chain-Li+ attraction in alkylbenzenes, namely the scorpion effect ... 

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