1,4,7-Triazacyclononane

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, C₂H₄NH. Other members of the series include diazacyclohexane, C₄H₈(NH)₂, and the cyclic tetramer 1,4,7,10-tetraazacyclododecane.

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Synthesis

The ligand is prepared from diethylene triamine 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.^[3]

is prepared as follows from TACN trihydrochloride:

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

• Mn-TACN complexes catalyze epoxidation of alkenes such as styrene using H₂O₂ as an oxidant in a carbonate buffered methanol solution at a pH of 8.0.  These reagents are considered environmentally benign,^[4]

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

• Chromium (III) sources, e.g. generated by heating CrCl₃^.6H₂O in DMSO react with TACN to form both 1:1 Cr:and 2:1 complexes,^[5] e.g. yellow [(TACN)₂Cr]³⁺.^[6]

References

1. Chaudhuri, P. and Wieghardt, K., "The Chemistry of 1,4,7-Triazacyclononane and Related Tridentate Macrocyclic Compounds", Progress in Inorganic Chemistry, 1987, volume 35, 329-436.

2. K. Wieghardt, W. Schmidt, B. Nuber, and J. Weiss, "Darstellung und Struktur des trans-Diaqua-di-μ-hydroxo-bis[(1,4,7-triazacyclononan)cobalt(III)]-Kations; Kinetik und Mechanismus seiner Bildung"Chem. Ber., 1979, 112, 2220-2230.

3. K. Sibbons, K Shastri, and M Watkinson, "The application of manganese complexes of ligands derived from 1,4,7-triazacyclononane in oxidative catalysis", Dalton Transactions, 2006, 645-661.

4. Deal, K. A.; Burstyn, J. N., "Mechanisitic Studies of Dichloro(1,4,7-triazacyclononane)copper(II)-Catalyzed Phosphate Diester Hydrolysis", Inorg. Chem. 1996, 35, 2792-2798.

5. K. Wieghardt, W. Schmidt, H. Endres and C.R. Wolfe, Chem. Ber., 1979, 112, 2837.

6. K. Wieghardt, W. Schmidt, W. Herrmann, and H.-J. Kuppers, Inorg. Chem., 1983, 23, 2953.