Cyclen or 1,4,7,10-tetraazacyclododecane is a macrocycle and the aza analogue of the crown ether 12-crown-4. Derivatives of cyclen are larger cyclic polyamines but the repeating unit (ethyleneimine, –CH2CH2NH–) is always the same. Like crown ethers, cyclen compounds are capable of selectively binding cations. They are used as a ligand in chemistry for instance with chemicals used in MRI contrast agents.