Design of selective macrocyclic ligands for the divalent first-row transition-metal ions

The protonation constants of H2L1, 3,11-bis(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene and H3L2, 3,7,11-tris(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),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 strength 0.10 mol dm(-3) in NMe4NO3. Both compounds are very selective for the divalent first-row transition-metal ions, exhibiting very high stability constants for Cu2+, fairly high values for Ni2+, but sharply decreasing 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 determined. The copper atom is encapsulated by the macrocycle in a distorted octahedral environment. The equatorial plane contains the four nitrogen atoms of the tetraaza ring and six-co-ordination is completed via two oxygen atoms of the appended carboxylate groups. The angles at the metal centre are close to the expected values of 90 and 180 degrees for 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 accommodated by the macrocycle in a cis-octahedral environment. However, these ions allow co-ordination numbers higher than six and so other structures ought to be also considered. The low stability constants for metal complexes of Co2+ and Zn2+ indicate that these complexes do not have a ti trans-octahedral structure, while the molecular mechanics calculations reveal that the cis isomer is not the most stable form. Therefore, other structures with co-ordination numbers lower than six should be considered, implying that one or more donor atoms are not co-ordinated. Stability constants of metal complexes of(L-2)(3-) and EPR studies suggest that not all the donor atoms in this macrocycle are co-ordinated when complexes are formed with first-row-transition divalent metal ions.