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

Abstract

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 °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]·4H₂O 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° 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 Å 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 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 (L²)^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.