Solution ¹H n.m.r. of paramagnetic Co²⁺ complexes with dioxobis(carboxymethyl) tetraazamacrocycles

Paramagnetic shifts in the 1H n.m.r. spectra were observed for high-spin Co2+ complexes with 12-14-membered tetraazamacrocycles incorporating two amide groups and two pendant carboxymethyl groups. The pseudocontact term due to the dipolar interaction between the metal ion and the resonant protons was calculated on the basis of X-ray structures, and the Fermi contact term due to spin delocalization was determined. The ethylenediamine moiety of the ligand molecule is coordinated in an unsymmetric manner, even in solution, and internal motion (involving conformational change of the macrocyclic frame and exchange of coordinate bonds) is much slower than the n.m.r. observation frequency. In the 12-membered macrocyclic complex Co(L12), three of the eight CH2 groups fix their orientation in the n.m.r. time scale; in Co(L13) only one CH2 group fixes its orientation; in Co(L14) all CH2 groups undergo a rapid reorientation. The facility in internal motion increases with the ring size, and shows a correlation with the chemical stabilities of these Co2+ complexes against oxidation; to atmospheric oxygen, Co2+ (L12) is the most resistant and Co 2+ (L14) is the most susceptible.