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.