1,4,7-Triazacyclononane trihydrochloride
[CAS 58966-93-1]

Identification

• Classification: Chemical reagent >> Organic reagent >> Crown ether
• Name: 1,4,7-Triazacyclononane trihydrochloride
• Synonyms: Octahydro-1H-1,4,7-triazonine hydrochloride
• Molecular Formula: C₆H₁₅N₃.3(HCl)
• Molecular Weight: 238.59
• CAS Registry Number: 58966-93-1
• EC Number: 625-619-9
• SMILES: C1CNCCNCCN1.Cl.Cl.Cl

Properties

• Melting point: 288 °C (dec.)

Safety Data

• Hazard Symbols: GHS05 Danger
• Risk Statements: H314-H318
• Safety Statements: P260-P264-P264+P265-P280-P301+P330+P331-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P321-P363-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Serious eye damage	Eye Dam.	1	H318
Skin corrosion	Skin Corr.	1B	H314
Skin corrosion	Skin Corr.	1C	H314

• Transport Information: UN 3261

chemBlink Chemical Story

A ring can organize several donor atoms before a metal ever arrives. 1,4,7-Triazacyclononane does so with three nitrogen atoms spaced around a nine-membered ring. The product registered as CAS 58966-93-1 is its trihydrochloride salt, a distinction that matters when describing both its composition and its role as a starting material in coordination chemistry.

The parent macrocycle is commonly abbreviated TACN. Three ethylene segments connect its three secondary amine nitrogens, providing a compact framework that can bind a metal through more than one donor atom. Connecting donor atoms within one ring changes the organizational problem compared with bringing together several independent amines. The final geometry and binding strength nevertheless depend on the metal and on the actual ligand structure.

In the trihydrochloride, protonation and chloride counterions are part of the isolated material. A protonated amine is not equivalent to an available neutral nitrogen donor. Coordination or further nitrogen functionalization must therefore be understood in the context of the acid-base conditions used. Calling the salt a ligand precursor is often more informative than drawing the free base and overlooking the additional acid equivalents.

The ring also provides three sites for installing pendant groups. These groups can introduce additional donor atoms or other functionality beyond the original nitrogen trio. As a result, a TACN-derived ligand may have a substantially different binding environment from unmodified TACN. Counting donor atoms in a derivative requires looking at the whole structure rather than carrying over the parent's description.

One documented example is the tripodal ligand H3tpatcn, which attaches three pyridinecarboxylate-containing arms to the TACN framework. Gateau and colleagues investigated its lanthanide complexes in a 2003 Dalton Transactions study. The work connected solid-state structure with solution behavior, showing why a metal-binding design should be examined in more than one physical setting. The extended ligand offers more potential donor sites than the original three-nitrogen ring.

This application illustrates an important distinction between an anchor and a finished coordination environment. TACN supplies the cyclic anchor; the pendant arms help determine the properties of the resulting complex. Neither the optical response nor the solution behavior of a particular lanthanide complex can be assigned directly to the bottle of TACN trihydrochloride from which a synthesis begins.

The distinction between three donor atoms and a nine-membered ring is worth retaining. The ring includes six carbon atoms as well as the three nitrogens; nine-membered does not mean that nine metal-binding nitrogens are present. Additional donor sites in an elaborated ligand come from the appended groups. Ring size, donor count, and coordination number are related structural descriptions, but they are not synonyms.

Metal binding also has a solution context. Protonation competes with the availability of nitrogen lone pairs, while counterions and other ligands can influence the species present. An isolated crystal provides a valuable structural snapshot, but it does not automatically establish every species or equilibrium in solution. Comparing structural and solution measurements is therefore part of understanding how a macrocyclic design actually behaves, rather than simply confirming that a metal was added to it.

The broader chemical story is one of preorganization followed by elaboration. A small macrocyclic salt gives researchers access to a defined arrangement of nitrogen atoms, which can then be developed into more specialized binding structures. Understanding protonation, ring organization, and the contribution of appended groups makes it possible to connect a simple starting material with the much richer chemistry of its metal-containing descendants.

References:
1. Sigma-Aldrich. 1,4,7-Triazacyclononane trihydrochloride, CAS 58966-93-1; product 329495. https://www.sigmaaldrich.com/US/en/product/aldrich/329495
2. TCI. 1,4,7-Triazacyclononane Trihydrochloride; product T1600. https://www.tcichemicals.com/IN/en/p/T1600
3. Gateau C et al. Solid-state and solution properties of the lanthanide complexes of a new nonadentate tripodal ligand derived from 1,4,7-triazacyclononane. Dalton Trans. 2003:2428. https://doi.org/10.1039/B303079B

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