Anion Coordination Chemistry and the Recognition of Anionic Substrates

Anionic substrates have specific features. They are large, compared with cations they possess a range of geometries, spherical (halides), linear (N3-, OCN , etc.), planar (NO3-, R-C02, etc.), tetrahedral (SO42-, CIO4-, phosphates, etc.), octahedral (M(CN)6 -). 

Positively charged or neutral electron-deficient groups may serve as interaction sites for anion binding. Ammonium and guanidinium units, which form +N-H X bonds, have mainly been used, but neutral polar hydrogen bonds (e.g., with -NHCO- or -COOH functions), electron-deficient centers (boron, tin, mercury, [3.6, 3.7] as well as perfluoro crown ethers and cryptands [3.8], etc.), or metal-ion centres in complexes also interact with anions. 

Polyammonium macrocycles and macropolycycles have been studied most extensively as anion receptor molecules. They bind a variety of anionic species (inorganic anions, carboxylates, phosphates, etc.) with stabilities and selectivities resulting from both electrostatic and structural effects. 

Spherical recognition of halide ions is displayed by protonated macropolycyclic polyamines. Thus, macrobicyclic diamines yield katapinates [3.9]. Anion cryptates are formed by the protonated macrobicyclic 16-6H+ [2.52] and macrotricyclic 21-4H+ [2.97] polyamines, with preferential binding of F and Cl- respectively in an octahedral and in a tetrahedral array of hydrogen bonds. 

3 Anion Coordination Chemistry and the Recognition of Anionic Substrates 

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