Syntheses of Some Medium Sized Cyclic Triamines and Their Cobalt(III) Complexes

Abstract

Abstract Four cyclic triamines of a medium ring size (9–12-membered ring) which form three mutually-adjacent five- and/or six-membered chelate rings on complexation, such as 1,4,7-triazacyclononane (abbreviated TACN), 1,4,7-triazacyclodecane (TACD), 1,4,8-triazacycloundecane (TACUD), and 1,5,9-triazacyclododecane (TACDD) were synthesized and isolated as their trihydrobromides. The complexation of TACN and TACD with a cobalt-(III) salt yielded metal complexes of a sandwich-type structure, such as [Co(TACN)2]Br3 and [Co(TACD)2]Br3·H2O, which were then isolated. In the case of TACUD, a complex [Co(TACUD)BrCl2] containing only one molecule of the triamine was isolated. However, although the formation of a cobalt (III) complex with TACDD in solution was inferred spectroscopically, no expected complex was isolable. On the basis of the spectrophotometric data, the ligand-field strength was found to increase in the following order: [Co(NH3)6]3+<[Co(TACD)2]3+<[Co en3]3+<[Co(TACN)2]3+.