Cyclen

Cyclen (1,4,7,10-tetraazacyclododecane) is a aza-crown ether with the formula (CH2CH2NH)4. It is a white solid. Being structurally simple, symmetrical, and polyfunctional, cyclen has been widely investigated.

Synthesis
Some syntheses exploit the Thorpe-Ingold effect to facilitate ring-formation. Illustrative is the reaction of the deprotonated tosylamides with ditosylates:
 * TsN(CH2CH2NTsNa)2 +  TsN(CH2CH2OTs)2   →  (TsNCH2CH2)4

The resulting macrocycle can be deprotected with strong acid. Base gives the tetramine.

High dilution conditions result in a low reaction rate penalty and this disadvantage is removed in an alternative procedure starting from triethylenetetraamine and dithiooxamide to a bisamidine – also a bis(imidazoline) – followed by reduction and ring expansion with DIBAL.


 * [[Image:Cyclen synthesis dithiooxamide method.png|500px|Cyclen synthesis dithiooxamide method]]

Coordination complexes
Coordination complexes of cyclen have been studied extensively. With a 12-membered ring, it tends to bind to four contiguous sites on octaheral metal centers. In contrast the larger cyclam ligand tens to form attach to four coplanar sites. It also forms complexes of the type [Ln(cyclen)2]3+ where Ln = lanthanide.