Cyclen

 ( Ethyleneamines ) 

Cyclen ( 1,4,7,10-tetraazacyclododecane) is an aza-crown ether with the formula (CH₂CH₂NH)₄. It is a white solid. It forms coordination complexes with metal cations and is used to synthesize the chelating agent DOTA which has several medical applications. Being structurally simple, symmetrical, and polyfunctional, cyclen has been widely investigated.

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Synthesis Some syntheses exploit the Thorpe-Ingold effect to facilitate ring-formation. Illustrative is the reaction of the deprotonated tosylamides with di:

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 amidine – also a bis(imidazoline) – followed by reduction and ring expansion with DIBAL.

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Coordination complexes Coordination complexes of cyclen have been studied extensively. With a 12-membered ring, it tends to bind to four contiguous sites on octahedral metal centers, forming four 4-membered rings (see example figure). In contrast the larger cyclam ligand forms complexes consisting of 4 coplanar rings.

Cyclen also forms complexes of the type Ln(cyclen)₂³⁺ where Ln = lanthanide.

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Further reading

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