Binding and Recognition of Neutral Molecules

The binding and recognition of neutral molecules make use of electrostatic, donor-acceptor and especially of hydrogen bonding interactions [2.119-2.123]. Polar organic molecules such as malonodinitrile form weak complexes with crown ethers and related ligands [2.120]. 

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In this chapter the scope and limitation of crownophanes are reviewed, focusing on supramolecular functions as ionophores and receptors. Most are designed for the recognition of metal cations and some organic cations, and done partly for neutral organic materials. Their structures are classified into five groups from benzene nuclei to heteroaromatic nuclei, hi the individual section their supramolecular properties are summarized and the properties are mainly focused on metal ion-binding and chiral and/or achiral neutral molecule complexation. Rotaxanes and catenanes are also included in this chapter. Their preparation almost always accompanies some supramolecular complexation. Some typical and important crownophanes are summarized in Table 1. 

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). 

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