Naked anion effect

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs+ cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs+ cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs ([18] crown-6) 2] + -e-. In these, species the alkali metal anions are situated in the same localised cavities as their electride analogues. 

Fluorescence Sensing of Anions, p. 566 Guanidium-Based Anion Receptors, p. (575 Halogen Bonding, p. 628 Macrocyclic Synthesis, p. 830 Molecular Squares, Boxes, and Cubes, p. 909 Naked Anion Effect, p. 939 Organometallic Anion Receptors, p. 1006 Rotaxanes and Pseudorotaxanes, p. 1194 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 The Template Effect, p. 1493... 

Cyclophanes Definition and Scope, p. 414 Inclusion Reactions and Polymerization, p. 705 Macrocycle Synthesis, p. 830 Naked Anion effect, p. 939... 

In 1987, Pedersen and Lehn shared the Nobel Prize for the development of Supramolecular Chemistry with a third recipient, Donald Cram, also very well-known for his work in carbanion chemistry. Cram realised that the chemical reactivity of a carbanion could be increased by the separation of a counter-cation from its anion. This separation was achieved by fully encapsulating the cation with cyclic polyethers and represents an example of the naked anion effect. This is where the anion has been stripped away from its counter-ion, such that there is little or no association between the anion and cation. Cram and his team went about synthesising cyclic polyethers in which the oxygen atoms comprise a small cavity that is rigidly preorganised in an octahedral array. These macrocycles are termed spherands, of which the Li -selective spherand-6 (2.15) is one of the best-known examples. 

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