Neutral receptors

The tripodal, trialkylbenzene-based core has proved to be an extremely versatile scaffold in both cation and particularly anion supramolecular chemistry and we will return to it in more detail in Section 4.8.1. 

Antonisse, M. M. G. and Reinhoudt, D. N., Neutral anion receptors, design and application , Chem. Commun., 1998,443 48. 

All of the hosts examined in Sections 4.4 and 4.5 possess a formal positive charge, which assists in their anion complexing ability through the formation of non-directional electrostatic interactions. Despite the strong binding by charged hosts, there are two potential disadvantages to the use of cations as anion 

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The molecular recognition of anionic guest species by positively chaiged or neutral receptors is a relatively new area of research of growing interest in view of the key roles that these anions play in biochemical and chemical processes. For this reason, as part of the electrochemical studies, we decided to examine the use of the redox-active ferrocenyl dendrimers 3 and 4 that contain multiple N-H linkages capable of participating in H-bonding, as well as characteristic internal cavities,... 

In the case of cationic host molecules, the binding of a target anion usually must occur in the presence of other counterions and hence association constants often represent more a measure of the effectiveness with which the target anion is bound relative to the others in the system. Neutral receptor molecules do not suffer from this drawback and also have the potential for greater anion selectivity since they do not rely upon nondirectional electrostatic forces to achieve anion coordination. ... 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

Liu, S. Y., He, Y. B., Wu, J. L., et al., Calix[4]arenes containing thiourea and amide moieties neutral receptors towards alpha,omega-dicarboxylate anions. Org. Biomol. Chem. 2004, 2, 1582-1586. 

Receptor 60 possesses similar ion-pair sensing properties to its [Ru(bpy)3]2+ analogue 47 [38]. The neutral receptor naturally showed lower overall stability constants for halides, but the anion binding enhancement observed on the addition of Li+ or Na+ was increased, with a 60-fold enhancement in the stability constant for bromide in the presence of Li+. 

Therefore, thermodynamics plays a fundamental role in supramolecular chemistry. However, thermodynamics is rigorous and as such, a great deal of ancillary information is required prior to the formulation of an equation representative of the process taking place in solution, such as, the composition of the complex and the nature of the speciation in solution. For the latter and when electrolytes are involved, knowledge of the ion-pair formation of the free and complex salts in the appropriate solvent is required particularly in non-aqueous solvents. This information would allow the establishment of the concentrations at which particular ions are the predominant species in solution. Similar considerations must be taken into account when neutral receptors are involved, given that in dipolar aprotic or inert solvents, monomeric species are not always predominant in solution. In addition, awareness of the scope and limitations of the methodology used for the derivation of thermodynamic data for the complexation process is needed and this aspect has been addressed elsewhere [18]. 

As far as the determination of the composition of the complex is concerned, this can be obtained from the variation of electrical conductance of an ionic solution titrated with a solution of the neutral receptor as a result of the different mobilities of the species in solution. Plots of molar conductances, Am, against the ratio of the concentrations of the receptor and anion can provide useful information regarding the strength of anion-receptor interaction. In fact, several conclusions can be drawn from the shape of the conductometric titration curves. 

Pedersen, Cram and Lehn respectively gives us the other thread to the present story. The first example of fluorescence enhancement caused by a supramolecu-lar interaction between a metal ion and a neutral receptor (8) was reported by the Bordeaux-Strasbourg collaboration of Bouas-Laurent, Desvergne and Lehn [15]. An older example of luminescence arising from a strong coordinate interaction between a neutral ligand and a metal ion is (9) [16]. Anionic ligands,... 

Otto S, Kubik S (2003) Dynamic combinatorial optimization of a neutral receptor that binds inorganic anions in aqueous solution. J Am Chem Soc 125 7804—7805... 

Neutral receptors based on multiple Lewis acid-anion orbital overlap interactions. 

Lewis acidic centers are, due to their electron deficiency, capable of interacting with anions through an orbital overlap, causing a bonding interaction. Many novel, neutral receptors incorporating multiple numbers of this kind of anion-binding interaction have been developed. 

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