Macrocyclic host structures cavity

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

A particularly interesting family of host-guest systems [1] is that in which a linear guest is threaded through the cavity of a macrocyclic host (Figure 1). A compound which exhibits such a supramolecular structure is usually called pseudorotaxane [2], with reference to the name rotaxane (see Volume III, Part 2, Chapter 7) used to indicate the species in which the ring is prevented to dethread by the presence of bulky stoppers at the ends of the linear guest. 

The shape-persistent, structurally well-defined nature of PAMs and PDMs make them attractive models for binding guest molecules within their cavities. In 1995, Hoger and Enkelmann reported the construction of the first meta/para-PAM/PDM hybrid designed to possess hydrophobic and hydrophilic substituents for subsequent use in host guest chemistry [71]. Macrocyclic amphiphile 116 was assembled via the straightforward manner depicted in Scheme 26. 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

A molecule that contains one or more binding sites that can accommodate inorganic or organic ions referred to as guests. The binding site could even be a cavity within a crystal structure. Although enzymes clearly qualify as examples of host molecules, the term is usually restricted to structures such as crown ethers, macrocycles, and cyclodextrins. Nevertheless, these hosts do serve as models for molecular recognition. See also Crown Ethers Macrocycles Inclusion Complexes... 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

Work by K Travis Holman and Jerry Atwood at the University of Missouri, USA has resulted in a tricationic host 4.66 based on the macrocycle cyclotriveratrylele (CTV, Section 7.7), which exhibits a deep anion binding pocket surrounded by three metal centres. A guest PF6 anion fits neatly into the cavity, stabilised by C—H F interactions, which may be differentiated from symmetry-related, noninteracting protons on the other side of the metallated aryl ring by H NMR spectroscopy. The X-ray crystal structure of this material is shown in Figure 4.26. 

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