1,4,7-Triazacyclononane, known as "TACN" which is pronounced "tack-en," is an
aza-crown ether with the formula (C
2H
4NH)
3. TACN is derived, formally speaking, from cyclononane by replacing three equidistant CH
2 groups with NH groups. TACN is one of the oligomers derived from
aziridine
Aziridine is an organic compound consisting of the three-membered heterocycle . It is a colorless, toxic, volatile liquid that is of significant practical interest. Aziridine was discovered in 1888 by the chemist Siegmund Gabriel. Its deriva ...
, C
2H
4NH. Other members of the series include
piperazine
Piperazine () is an organic compound with the formula . In term of its structure, it can be described as cyclohexane with the 1- and 4-CH2 groups replaced by NH. Piperazine exists as deliquescent solid with a saline taste. Piperazine is freely sol ...
, C
4H
8(NH)
2, and the cyclic tetramer
1,4,7,10-tetraazacyclododecane.
Synthesis
The ligand is prepared from
diethylene triamine as follows by macrocyclization using ethyleneglycol ditosylate.
:H
2NCH
2CH
2NHCH
2CH
2NH
2 + 3 TsCl → Ts(H)NCH
2CH
2N(Ts)CH
2CHH
2N(H)Ts + 3 HCl
:Ts(H)NCH
2CH
2N(Ts)CH
2CH
2N(H)Ts + 2 NaOEt → Ts(Na)NCH
2CH
2N(Ts)CH
2CH
2N(Na)Ts
:Ts(Na)NCHH
2CH
2N(Ts)CH
2CH
2N(Na)Ts + TsOCH
2CH
2OTs + →
2CH2N(Ts)">CH2CH2N(Ts)sub>3 + 2 NaOTs
:
2CH2N(Ts)">CH2CH2N(Ts)sub>3 + 3 H
2O →
2CH2NH">H2CH2NHsub>3 + 3 HOTs
Coordination chemistry
TACN is a popular tridentate ligand. It is threefold symmetric and binds to one face of an octahedron of metalloids and transition metals. The (TACN)M unit is kinetically inert, allowing further synthetic transformations on the other coordination sites. A bulky analogue of TACN, is the N,N',N"-trimethylated analogue
trimethyltriazacyclononane.
Illustrative complexes
*Although TACN characteristically coordinates to metals in mid- and high oxidation states, e.g. Ni(III), Mn(IV), Mo(III), W(III), exceptions occur. To illustrate, 1,4,7-triazacyclononane reacts readily with Mo(CO)
6 and W(CO)
6 to produce the respective air-stable tricarbonyl compounds,
3 -TACN)Mo(CO)3">κ3 -TACN)Mo(CO)3and
3-TACN)W(CO)3">κ3-TACN)W(CO)3 Both have an oxidation state of zero. After further reacting with 30% H
2O
2, the products are
3-TACN)MoO3">κ3-TACN)MoO3and
3-TACN)WO3">κ3-TACN)WO3 Both of these oxo complexes have an oxidation state of 6. The macrocyclic ligand does dissociate in the course of this dramatic change in formal oxidation state of the metal.
*The complex,
3-TACN)Cu(II)Cl2">�3-TACN)Cu(II)Cl2 a catalyst for hydrolytic cleavage of phosphodiester bonds in DNA, is prepared as follows from TACN trihydrochloride:
:TACN·3HCl + CuCl
2·3H
2O + 3 NaOH →
3-TACN)CuCl2">κ3-TACN)CuCl2+ 6 H
2O + 3 NaCl
*Mn-TACN complexes catalyze
epoxidation
In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...
of
alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s such as
styrene
Styrene is an organic compound with the chemical formula C6H5CH=CH2. Its structure consists of a vinyl group as substituent on benzene. Styrene is a colorless, oily liquid, although aged samples can appear yellowish. The compound evaporates easi ...
using H
2O
2 as an oxidant in a carbonate buffered
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
solution at a pH of 8.0. These reagents are considered environmentally benign,
:
3-TACN)Mn">κ3-TACN)Mn+ H
2O
2 + NaHCO
3 + (C
6H
5)C
2H
3→
3-TACN)Mn">κ3-TACN)Mn + 2H
2O + CO
2 + (C
6H
5)C
2H
2O
*Chromium (II) sources, e.g. created by heating CrCl
3.6H
2O in DMSO react with TACN to form both 1:1 Cr:and 2:1 complexes, e.g. yellow
2Cr">TACN)2Crsup>3+.
References
{{DEFAULTSORT:Triazacyclononane, 1,4,7-
Ethyleneamines
Chelating agents
Macrocycles
Secondary amines
Tridentate ligands