cryptand

A cryptand is a class of synthetic ligands used in coordination and supramolecular chemistry. They are three‑dimensional, cage‑like macrocycles that encapsulate a metal ion inside a central cavity. Cryptands are typically built from a triamine core linked by polyether chains, providing multiple donor atoms (usually three nitrogens and several ether oxygens) that cooperate to bind a cation strongly and selectively.

The three linking chains create a hollow interior with restricted access to the outside. Cryptands are often described by a bracket notation [a.b.c], where the numbers indicate the lengths of the three bridges between the nitrogens. For example, [2.2.2] cryptand has three two‑atom bridges, forming a tight, highly preorganized cavity. This arrangement enables encapsulation of a single metal ion, producing a cryptate complex that is typically thermodynamically very stable and kinetically inert.

Synthesis of cryptands usually involves condensation reactions that assemble the three strands around a central core, such as triamines with dihalide or dialdehyde linkers, or sequential assembly of preformed fragments. A widely known member of the family is Kryptand 2.2.2, and related ligands are marketed as Kryptofix. These ligands are valued for their ability to sequester cations in solution, enhancing selectivity in separations, catalysis, and phase‑transfer processes.

In practice, complexes formed with cryptands are called cryptates. The strong, selective binding of cryptands to ions makes them useful tools in ion separation, radiochemistry, NMR studies, and various areas of inorganic and analytical chemistry.