NMR CONFORMATIONAL STUDY OF DIAMAGNETIC COMPLEXES OF SOME TRIAZATRIACETATE MACROCYCLES

The conformational properties of the diamagnetic complexes formed by f ive triazatriacetic macrocyclic ligands with monovalent (alkaline) and divalent (alkaline-earth, Zn2+ and Cd2+) cations have been studied by proton nuclear magnetic resonance spectroscopy (H-1 NMR) in aqueous s olution as a function of pH and temperature. These structurally relate d triazatriacetic ligands, 1,4,7-triazacyclononane-N,N',N ''-triacetic acid (NOTA), 1,4,7-triazacyclodecane-N,N',N ''-triacetic acid (DETA), 1,4,8-triazacycloundecane-N,N',N ''-triacetic acid (UNTA), 1,5,9-tria zacyclododecane-N,N',N ''-triacetic acid (DOTRA), and 9,10-benzylidene -1,4,7-triazacyclotridecane-N,N',N ''-triacetic acid, (BUNTA) have tri azamacrocyclic rings with different sizes and flexibilities, bearing t hree pendant acetate arms. In the case of the alkaline-earth, Zn2+ and Cd2+ cations, only one complex form (1:1) has been detected for all t he ligands studied. In most cases the metal ion is coordinated to all the ligand ring nitrogens and to all or some of the carboxylate groups of the pendant arms, except when a poor fit of the cations into the m acrocyclic hole occurs, such as for the Ba2+ complexes of DETA, UNTA a nd DOTRA and the Ca2+ and Sr2+ complexes of BUNTA. The resonance patte rns of the ring proton resonances, determined by the long lifetime of the metal-nitrogen bonds, indicate that the six-membered rings formed upon chelation are conformationally rigid and the five-membered rings are conformationally flexible. The multiplicity of the acetate proton resonances shows that the metal-oxygen bonds are long-lived or short l ived depending whether the acetates are bound to nitrogens involved in two ring bridges of equal or unequal sizes. (C) 1998 Elsevier Science S.A. All rights reserved.