Cryptates

Cryptates are coordination compounds in which a cryptand—a rigid, three-dimensional cage-like polyether bearing three tertiary amine nitrogens—encapsulates a single metal cation inside its hollow cavity. The cation is held so tightly that it is effectively shielded from external solvent and counterions, often resulting in very high binding constants and allowing characterization of the ion in solution.

Common cryptands include the 2.2.2-cryptand (cryptand-222) and the 4.4.4-cryptand; the numbers refer to the size of each arm’s chain. The cavity size governs selectivity: smaller cryptands preferentially bind small monovalent ions such as Li+ and Na+, whereas larger cryptands can accommodate bigger cations like K+, Rb+, and Cs+. Some cryptands can support certain divalent cations under suitable conditions.

Synthesis involves assembling the three-dimensional cage from polyamines and dihaloalkyl linkers, followed by metallation to furnish the cryptate salts. Counteranions such as PF6-, BF4-, or BArF- are commonly used to minimize external coordination.

Applications of cryptates lie in solution and separation chemistry and include stabilizing “naked” ions in nonpolar media, studying ion solvation and transport, phase-transfer catalysis, and facilitating spectroscopic investigations of cations unreliant on solvent interactions.

The concept was introduced by Jean-Marie Lehn and coworkers in the 1960s–1970s as part of supramolecular chemistry; the term cryptand reflects the way the cation is hidden inside the cage.