Supramolecular structures ionic complexes

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. 

A lower level of organization in the supramolecular structure is evident even if the layered structure of the core and the stacking of the cyclohexane residues are retained. In this context, ordered H-bonded ionic triple-stranded helicates, involving (R,R)-frdicarboxylic acid, (R,R)-44, in D2O afforded the complex (R.R)-29 (R,R)-44 as large, colourless crystals suitable for X-ray analysis (Scheme 20). The same reaction performed in the presence of the mismatched (5,5)-tra -cyclohexane-l,2-dicarboxylic acid (45) did not give any solid material. A similar result was obtained with (S)-methylsuccinic acid. 

Chemical diagrams for the molecular structures included in this review are shown in Schemes 3.9.2-3.9.4. Initially, structures in which Sn. .. 7r contacts are found in solvates are described. Following these is a discussion of Sn. .. itt interactions occurring in two ionic complexes. The remaining structures are discussed in terms of increasing complexity guided by tin atom nuclearity and the number of interactions found in their supramolecular structures. 

In chloroplasts of higher plants the ferredoxin-thioredoxin system links light-triggered events in thylakoid membranes with the regulation of enzymes in the stroma (1,2). If the conformation of enzymes changes because of modulators action then the surface exposed to the solvent will be different from the native state (3). As a consequence, interactions of modified enzymes with supramolecular structures (membranes, protein complexes) will differ respect to native forms. Since thylakoid membranes are complex structures they are not adequate for uncovering molecular mechanisms that participate in protein interactions(4). In thfe respect, the well-defined structure of micelles of non-ionic detergents constitute model compounds for the analysis of hydrophobic interactions in proteins (5,6). We report herein that chloroplast fructose-1,6-bisphosphatase interacts with micelles of Triton X-114 in a pH-dependent process. 

A phenomenon, which was studied in the literature, is the role of extra water in ionic liquids. This phenomenon is complex and depends on the supramolecular structure of the ionic liquid. It is assumed that its structure and chemical reactivity is far from that of bulk water, as it is tightly bound and activated in the H-bonding system of the IL. As a result, reactions with water take place quite rapidly in these systems. On the other hand, water cannot function as a solvating ligand here since it is too involved in IL binding. This was deduced, for instance, from the absence of so-called solvent pores and represents a quite singular situation for colloid chemistry and material synthesis. 

Detailed discussions on supramolecular structures of giant polyoxometalate clusters have been reported by Muller and co-workers." " The number of ionic lattices formed from anionic POM clusters and organic cations-coordination complex cations resulting in supramolecular structures is substantial and literally more than hundreds of new compounds in this class appear annuallyIn this overview, the supramolecular features of POM-based systems will be discussed mainly under three headings (a) supramolecular features of polyoxometalate supported transition metal complexes, (b) polyoxometalate-crown ether complexes with supramolecular cations, and (c) supramolecular water clusters associated with polyoxometalates. 

Fullerenes C60 and C70 form supramolecular adducts with a variety of molecules, such as crown ethers, ferrocene, calixarene, and hydroquinone. In the solid state, the intermolecular interactions may involve ionic interaction, hydrogen bonding, and van der Waals forces. Figure 14.2.9 shows a part of the structure of [K(18C6)]3-C6o-(C6H5CH3)3, in which Cgg is surrounded by a pair of [K+(18C6)] complexed cations. 

When triethanolamine H3L13 (35) was reacted with sodium hydride and iron(III) chloride, the hexanuclear centrosymmetric ferric wheel [Nac Fe6(L13)6 )Cl (36) was isolated. Amidst a set of possibilities in the template-mediated self-assembly of a supramolecular system, the one combination of building blocks is realized that leads to the best receptor for the substrate [112]. Therefore, the six-membered cyclic structure 36 is exclusively selected from all the possible iron triethoxyamine oligomers, when sodium ions are present. The iron(III) complex 36 is present as an Sg-symmetric wheel, with an encapsulated sodium ion in the center and a chloride counterion. Consequently, the trianion (L13)3- acts as a tripodal, tetradentate, tetratopic ligand, which each links three iron(III) ions and one sodium ion. In the presence of cations with different ionic radii, different structures are expected. Therefore, when triethanolamine H3L13 (35) was reacted with cesium carbonate and iron(III) chloride, the octanuclear centrosymmetric ferric wheel [Csc Fe8(L13)8 ]Cl (37) was isolated (Scheme 13) [113]. 

In principle, all the suprastructures shown in a still highly simplified manner in Fig. 11.13 are in equilibria that depend on many parameters such as pH (Section 11.3.1), concentration (Section 11.3.2), ionic strength (Section 11.3.1), nature of the bilayer (Sections 11.3.4 and 11.3.7), and so on. This dynamic supramolecular polymorphism restricts meaningful structural studies to conditions that are relevant for function but often incompatible with routine analytical methods (e.g. nanomolar to low micromolar concentrations in lipid bilayer membrane. The fact that the active conformers or supramolecules are often not the thermodynamically dominant ones [21] (Section 11.3.2) calls for additional caution as well as selective methods of detection). As a general rule, the complexity of the supramolecular polymorphism of synthetic ion channels and pores decreases with increasing complexity (size) of the monomer (in other words, synthetic efforts are often worthwhile [2] compare Fig. 11.2). 

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