Receptor cation

Crown derivatives which are able to bind simultaneously to more than two metal ions have been synthesized such species have been termed multiloop crowns (Weber, 1982). Polynuclear cation receptors of this type provide yet another series of novel crown derivatives whose metal complexes tend to exhibit a range of unusual properties. Because of the diversity of the structures falling in this category, only a brief mention of this ligand type is presented here. 

Such a solvent relaxation explains the increase in the red-shift of the fluorescence spectrum as the polarity of the solvent increases. The effect of polarity on fluorescence emission will be further discussed in Chapter 7, together with polarity probes. Moreover, when a cation receptor is linked to an intramolecular charge transfer fluorophore so that the bound cation can interact with either the donor group or the acceptor group, the ICT is perturbed the consequent changes in photophysical properties of the fluorophore can be used for sensing cations (see Section 10.3.3). 

This type of probe, often called fluorescent photoinduced electron transfer (PET) sensors, has been extensively studied (for reviews, see Refs. 22 and 23). Figure 2.2 illustrates how a cation can control the photoinduced charge transfer in a fluoroiono-phore in which the cation receptor is an electron donor (e.g., amino group) and the fluorophore (e.g., anthracene) plays the role of an acceptor. On excitation of the fluorophore, an electron of the highest occupied molecular orbital (HOMO) is promoted to the lowest unoccupied molecular orbital (LUMO), which enables photoinduced electron transfer from the HOMO of the donor (belonging to the free cation receptor) to that of the fluorophore, causing fluorescence quenching of the latter. On... 

Catalysis by Reactive Macrocyclic Cation Receptor Molecules... 

Cryptands of type 7-9 and derivatives thereof carry alkali cations [6.4], even under conditions where natural or synthetic macrocycles are inefficient. The selec-tivities observed depend on the structure of the ligand, the nature of the cation and the type of cotransported counteranion. Designed structural changes allow the transformation of a cation receptor into a cation carrier [6.1, 6.4]. The results obtained with cryptands indicated that there was an optimal complex stability and phase-transfer equilibrium for highest transport rates. Combined with data for various other carriers and cations, they give a bell-shaped dependence of transport rates on extraction equilibrium (Fig. 11), with low rates for too small or too large... 

Quinone type carriers perform the cotransport of two protons and two electrons (2e, 2H+ symport) [6.48, 6.49] and take part in mitochondrial and photosynthetic electron transport. Cation receptor sites such as crown ethers or cryptands bearing a quinone [6.50a] or a ferrocene [6.50b] group (see also Section 8.3.1), bind and carry cations with redox coupling through switching between a low affinity state (quinone, ferricinium) and a high affinity state (reduced quinone, ferrocene). 

A case of special interest is that of the transport of divalent ions such as calcium versus monovalent ones. The lipophilic carrier 86, containing a single cation receptor site and two ionizable carboxylic acid groups, was found to transport selectively Ca2+ in the dicarboxylate form and K+ when monoionized, thus allowing pH control of the process. This striking change in transport features as a function of pH involves pH regulation of Ca2+-K+ selectivity in a competitive (Ca2+, K+) symport... 

Pazianas M, Adebanjo OA, Shankar VS, James SY, Colston KW, Maxwell JD, Zaidi M. 1995. Extracellular cation sensing by the enterocyte. Prediction of a novel divalent cation receptor. Biochem Biophys Res Comm210 948-53. 

Ferrocene-based bisphosphanes, in hydrogenations, 10, 10 Ferrocene-based receptors anion receptors, 12, 476 cation receptors, 12, 466 for neutral guest species, 12, 483 Ferrocene-based sensors, screen-printed, in biosensing,... 

Gold-acetylides, cation receptors based on, 12, 473 Gold atoms... 

Metal carbonyl monoanions, with trivalent Group 13 element halides, 3, 347, 3, 357 Metal carbynes, Fe-containing, 6, 325 Metal-catalyzed cycloadditions, alkenyl- and alkynylboron compounds, 9, 180 Metal cation receptors characteristics, 12, 466 ferrocene-based, 12, 466 Pt- and Au-based receptors, 12, 471 types, 12, 473 Metal chalcogenides... 

The interplay of complex stability and cation exchange kinetics is very important in the uses of supramolecular cation hosts. On the basis of their behaviour, we may distinguish between cation receptors (slow kinetics, large stability constants) and cation carriers (fast kinetics, lower stability). In the next section we will see how fast exchange kinetics make cation carriers highly useful in applications such as phase transfer catalysis. 

In 2001, the SRS announced its choice of CSSX as the baseline cesium-removal technology over small-tank precipitation (a small-scale version of the ITP process) and ion exchange with CST for its Salt Waste Processing Facility (SWPF) to go into operation in 2010 [22], An optimized solvent system, model, and flowsheet were developed and demonstrated in 2001 and 2002 [37,49], and a modular concept was developed by ORNL in 2003 [68], Thus, the past decade has seen the emergence and maturation of a powerful new technology based on a macrocyclic cation receptor designed to function in solvent extraction to meet the critical need of the USDOE for a means of cleanly separating Cs from alkaline tank waste. 

The Re(I) bipyridyl unit has been exploited in another series of ion-pair sensors. Molecules 61-64 incorporate crown ether components to act as cation receptors [43,44]. H NMR titrations revealed 61-64 to be selective for acetate over chloride. In all the receptors (except 62 and 64 with acetate) enhanced binding was observed in the presence of K+ cations. The degree of enhancement is lower in the xylyl-spaced molecules 62 and 64 (40-50%) than in 61 and 63 (80-110%). It is also small compared to that seen in the calix[4]arene receptor 60. 

In summary, the most stable cryptates release the cation very slowly and function as cation receptors, while less stable ones undergo rapid cation exchange and may be regarded as cation carriers. 

The corresponding dependence of cation complex stability on the anion differs profoundly from that of most other cation receptors such as cyclophanes or calix-arenes [16]. For these cation complex stability decreases on changing the anion from picrate through iodide to tosylate, a dependence that has been attributed to ion-pair aggregation in non-polar solvents. Because the interaction of quaternary ammonium ions with tosylate or iodide in chloroform is considerably stronger than with picrate, cation complexes in the presence of the latter anion are usually more stable. Only when iodide or tosylate cooperatively contributes to cation binding, as in 3 or in some recently described calixarene derivatives [17], is reversal of this order observed. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

In this section, various crowned spirobenzopyrans for use as alkali metal cation receptors have been presented. This new type of chromoionphore is conceptually different from the crown ether dyes because, in the case of the crown ether dyes, the absorption bands of the chromophones are shifted by complexation of cations. Crowned spirobenzopyrans might provide sophisticated ion-sensing systems such as ion sensors whose ion selectivity can be photochemically switched between cations and anions.104... 

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