Anion binding through second sphere coordination

Anion recognition plays very crucial role in chemistry and many biological processes. Despite their importance, little has been done in the field of design and synthesis of supramolecular anion receptors as compared to that of cations. In most of the anion receptor molecules, the basic approach utilized is to develop an organic framework with the suitable functionalities to bind the anion with the help of hydrogen bonding interactions. An alternate approach which may be employed is to utilize the second sphere interactions around the metal centre for the binding of anions, mainly utilizing the directional nature of the hydrogen bonds and charge transfer electrostatic interactions. In this regard we have undertaken extensive research program to explore cationic cobalt(III) metal complexes as anion receptors; in particular, we have focused our attention on complex salts of [Co(phen)3]3+ (phen = 1,10-phenantroline) with a series of anions differing in size, charge and nature (organic or inorganic). In this communication we report the crystal structures of the above listed salts and a detailed analysis of their packing modes, with particular reference to those weak intermolecular interactions that, besides electrostatic forces, contribute to stabilize the crystalline architecture. Understanding of such network interactions would be rewarding as it can provide means of constructing intricate and novel molecular entities based on second sphere coordination.