The origins of the coordination chemistry of alkali metal ions

Abstract: Alkali metal ions, spherical, diamagnetic, colourless, with a single oxidation number and devoid of any (+1/+n) redox activity, have been ignored by inorganic chemists and coordination chemists until the 1960s, when Charles John Pedersen, an American chemist at Dupont, at the threshold of retirement, serendipitously synthesised a new class of cyclic polyethers, which he named crown ethers, capable of forming stable complexes with alkali metal ions. It was observed that the stability of these complexes could be modulated through the design of crowns whose size could fit the radius of the metal ion, thus introducing the novel concept of geometric selectivity. Then, Jean-Marie Lehn in Strasbourg developed three-dimensional macropolycyclic ligands, named cryptands, capable of including alkali metal ions, thus forming complexes of higher stability and increased geometric selectivity. Crowns and cryptands excited the chemical community for their applications in analytical chemistry (sensing and development of specific electrodes) and in organic chemistry (solubilisation of inorganic salts, e.g. KMnO4, in the apolar solvents of organic chemistry) and helped to define new concepts of coordination chemistry: the macrocyclic effect and the cryptate effect, and the importance of preorganisation in the design of effective ligands for a given metal ion. Graphical abstract: [Figure not available: see fulltext.]