NMR conformational study of diamagnetic complexes of some triazatriacetate macrocycles

The conformational properties of the diamagnetic complexes formed by five triazatriacetic macrocyclic ligands with monovalent (alkaline) and divalent (alkaline-earth, Zn²⁺ and Cd²⁺) cations have been studied by proton nuclear magnetic resonance spectroscopy (¹H NMR) in aqueous solution as a function of pH and temperature. These structurally related triazatriacetic ligands, 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA), 1,4,7-triazacyclodecane-N,N′,A″-triacetic acid (DETA), 1,4,8-triazacycloundecane-N,N′,N″-triacetic acid (UNTA), 1,5,9-triazacyclododecane-N,N′,N″-triacetic acid (DOTRA), and 9,10-benzylidene-1,4,7-triazacyclotridecane-N,N′N″-triacetic acid, (BUNTA) have triazamacrocyclic rings with different sizes and flexibilities, bearing three pendant acetate arms. In the case of the alkaline-earth, Zn²⁺ and Cd²⁺ cations, oniy one complex form (1:1) has been detected for all the 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 macrocyclic hole occurs, such as for the Ba²⁺ complexes of DETA, UNTA and DOTRA and the Ca²⁺ and Sr²⁺ complexes of BUNTA. The resonance patterns 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 lived depending whether the acetates are bound to nitrogens involved in two ring bridges of equal or unequal sizes.