1,4,7-Triazacyclononane, known as "TACN" which is pronounced "tack-en," is a cyclic organic compound with the formula C₆H₁₂(NH)₃. TACN is derived, formally speaking, from cyclononane by replacing three equidistant CH₂ groups with NH groups. TACN is one of the oligomers derived from aziridine

Aziridine

Aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine group and two methylene groups...

, C₂H₄NH. Other members of the series include piperazine

Piperazine

Piperazine is an organic compound that consists of a six-membered ring containing two opposing nitrogen atoms. Piperazine exists as small alkaline deliquescent crystals with a saline taste....

, C₄H₈(NH)₂, and the cyclic tetramer 1,4,7,10-tetraazacyclododecane

Cyclen

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 is always the same. Like crown ethers, cyclen compounds are capable of selectively binding cations...

.

Synthesis

The ligand is prepared from diethylene triamine

Diethylene triamine

Diethylenetriamine is an organic compound with the formula HN2. This colourless hygroscopic liquid is soluble in water and polar organic solvents, but not simple hydrocarbons. Diethylenetriamine is structural analogue of diethylene glycol. Its chemical properties resemble those for ethylene...

 as follows by macrocyclization using ethyleneglycol ditosylate.

H₂NCH₂CH₂NHCH₂CH₂NH₂ + 3 TsCl → Ts(H)NCH₂CH₂N(Ts)CH₂CHH₂N(H)Ts + 3 HCl

Ts(H)NCH₂CH₂N(Ts)CH₂CH₂N(H)Ts + 2 NaOEt → Ts(Na)NCH₂CH₂N(Ts)CH₂CH₂N(Na)Ts

Ts(Na)NCHH₂CH₂N(Ts)CH₂CH₂N(Na)Ts + TsOCH₂CH₂OTs + → [(CH₂CH₂N(Ts)]₃ + 2 NaOTs

[(CH₂CH₂N(Ts)]₃ + 3 H₂O → [CH₂CH₂NH]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.

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, [(κ³ -TACN)Mo(CO)₃] and [(κ³-TACN)W(CO)₃]. Both have an oxidation state of zero. After further reacting with 30% H₂O₂, the products are [(κ³-TACN)MoO₃] and [(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, [κ³-TACN)Cu(II)Cl₂], a catalyst for hydrolytic cleavage of phosphodiester bonds in DNA, is prepared as follows from TACN trihydrochloride:

TACN·3HCl + CuCl₂·3H₂O + 3 NaOH → [(κ³-TACN)CuCl₂] + 6 H₂O + 3 NaCl

• Mn-TACN complexes catalyze epoxidation of alkene
  Alkene
  In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
  
  s such as styrene using H₂O₂ as an oxidant in a carbonate buffered methanol
  Methanol
  Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
  
  solution at a pH of 8.0.  These reagents are considered environmentally benign,

[(κ³-TACN)Mn] + H₂O₂ + NaHCO₃ + (C₆H₅)C₂H₃→ [(κ³-TACN)Mn] + 2H₂O + CO₂ +  (C₆H₅)C₂H₂O

• Chromium (II) sources, e.g. created by heating CrCl₃^.6H₂O in DMSO react with TACN to form both 1:1 Cr:and 2:1 complexes, e.g. yellow [(TACN)₂Cr]³⁺