Macrocyclic ligands molecular neutral complexes

Receptor and substrate are terms describing the species involved in complex formation. Throughout the chapter the receptor will refer tp the macrocyclic ligand, the substrate to other interacting species. Substrates may be metal or molecular catibns, neutral molecules, or atomic or molecular anions. The terms receptor and substrate imply that the complex formed has the well-defined structural and chemical properties of a supermolecule, as in biological receptor-substrate associations. They exclude species formed only in the solid state (clathrates). They are also easily converted and understood in many languages. 

The complexation of various molecular anions by other types of macrocyclic ligands has been reported [3.1-3.4] in particular with cyclophane-type compounds. Two such receptors are represented by the protonated forms of the macropolycycles 40 [3.29] and 41 [3.30]. Quaternary polybipyridinium compounds also bind anionic substrates [3.31]. Progress is also being made towards the developments of neutral anion receptor molecules [3.32]. The thermodynamic and kinetic data for anion complexation by macrocyclic receptors have been reviewed [2.18c]. 

Historically, the concept of coordination chemistry was associated with complexation of a metal cation (Lewis acid) by a ligand behaving as a Lewis base. Such was traditionally the case for macrocyclic molecules as ligands. In the early 1970s, however, the concept of coordination chemistry was extended in the area of macrocyclic chemistry to include molecular cations, neutral molecules and anions as substrates. Complexes of all of these species are to be included in the scope of this chapter section. Examples of the types of substrates are discussed below. 

It is hard to find a topic in supramolecular chemistry that has not been touched, even briefly, by calixarenes or related macrocycles. This justifies the fact that we were compelled to give only a short summary of the properties of classical calixarenes as molecular receptors for ions and neutral molecules or as scaffolds for the construction of multivalent ligands able to interact with biomacromolecules. We believe, indeed, that these two aspects are the most peculiar and remarkable features of calixarenes, on which a variety of other complex supramolecular functions can be built. The future will certainly be bright for this class of synthetic macrocycles as their molecular recognition and scaffolding properties are likely to be exploited in the classical ways and also in novel forms within the framework of the third developing phase of supramolecular chemistry, namely, constitutional dynamic chemistry and adaptive chemistry. ... 

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