DESIGN OF SELECTIVE MACROCYCLIC LIGANDS FOR THE DIVALENT FIRST-ROW TRANSITION-METAL IONS

The protonation constants of H2L1, etraazabicyclo[11.3.1]heptadeca-1(1 7),13,15-triene and H3L2, 7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(1 7),13, 15-triene, and stability constants of complexes formed by these macrocycles with Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2, Ga3+, Fe3+ and In3+ were determined at 25 degrees C and ionic streng th 0.10 mol dm(-3) in NMe4NO3. Both compounds are very selective for t he divalent first-row transition-metal ions, exhibiting very high stab ility constants for Cu2+, fairly high values for Ni2+, but sharply dec reasing ones for the remaining metal ions of this row. Their complexes with the alkaline-earth or larger metal ions, such as Pb2+, have low stability constants. The crystal structure of[CuL1]. 4H(2)O was determ ined. The copper atom is encapsulated by the macrocycle in a distorted octahedral environment. The equatorial plane contains the four nitrog en atoms of the tetraaza ring and six-co-ordination is completed via t wo oxygen atoms of the appended carboxylate groups. The angles at the metal centre are close to the expected values of 90 and 180 degrees fo r octahedral geometry. Molecular mechanics studies carried out for the cis and the trans octahedral [ML1] complexes were consistent with the structure found in the solid state. For a mean Cu-II-N distance of 2. 01 Angstrom the experimentally observed trans isomer is 6.5 kcal mol(- 1) more stable than the cis one. On the other hand these calculations suggest that larger ions such as Pb2+, Ca2+ or Mn2+ can be accommodate d by the macrocycle in a cis-octahedral environment. However, these io ns allow co-ordination numbers higher than six and so other structures ought to be also considered. The low stability constants for metal co mplexes of Co2+ and Zn2+ indicate that these complexes do not have a t i trans-octahedral structure, while the molecular mechanics calculatio ns reveal that the cis isomer is not the most stable form. Therefore, other structures with co-ordination numbers lower than six should be c onsidered, implying that one or more donor atoms are not co-ordinated. Stability constants of metal complexes of(L-2)(3-) and EPR studies su ggest that not all the donor atoms in this macrocycle are co-ordinated when complexes are formed with first-row-transition divalent metal io ns.