Nitrogen versus oxygen co-ordination of carboxamide-functionalized triazacyclononane ligands in transition metal ion complexes

Abstract

Mononuclear complexes of zinc(II), copper(II), nickel(II), cobalt(II), iron(II), manganese(II), chromium(III), vanadium(III) and vanadyl(IV) with the ligands 1,4,7-tris(carbamoylmethyl)-1,4,7-triazacyclonanone (L) and its N-methyl derivative C₆H₁₂N₃(CH₂CONHMe)₃ (MeL) have been prepared and structurally, magnetochemically and spectroscopically characterized. The ligands on complexation provide, in general, the MN₃O₃ moiety, i.e. they co-ordinate through the carboxamide oxygens with the exception of chromium(III). Chromium(III) forms complexes not only with the carboxamide oxygens, but also with the deprotonated amide nitrogen, thus yielding both CrN₃O₃ and CrN₄O₂ chromophores, respectively, which have been confirmed by crystal structures. The crystal structure of the VO²⁺ complex reveals a typical six-co-ordinate geometry with one amide dangling free, while that of the vanadium(III) compound shows seven-co-ordinate pentagonal bipyramidal geometry with the amide co-ordinated to the metal via its carbonyl oxygen atoms together with a chloride ion. The pendant carboxamide groups hydrolyse slowly to the corresponding carboxylic acid groups. The ligands L and MeL provide weak ligand fields and, relative to the trimethyl substituted cyclic amine [9]aneN₃, i.e. 1,4,7-trimethyl-1,4,7-triazacyclononane, are more effective in stabilizing the metal centers in the oxidation state II.