1,4,7-Triazacyclononane, known as "TACN" which is pronounced "tack-en," is an aza-crown ether with the formula (C2H4NH)3.

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

The ligand is prepared from diethylene triamine as follows by macrocyclization using ethyleneglycol ditosylate.

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.