Structural reinforcement of a large macrocyclic ligand. A structural, molecular mechanics, and thermodynamic study

Abstract

The complex of the macrocycles L⁵(1,10-dioxa-4,7,13,16-tetra-azacyclo-octadecane), L⁶(4,13-dioxa-1,7,10,16-tetra-azabicyclo[14.2.2] eicosane) and L⁷(4,13-dioxa-1,7,10,16-tetra-aza-tricyclo[14.2.2.2^7,10]docosane) have been synthesized and their complex formation constants determined with Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Pb²⁺; the ions Sr²⁺ and Ba²⁺ were found not to complex with the ligands. The successive insertion of ethylene bridges into the 1,4-diaminoethane units in L⁵ enhances the selectivity of the ligand for large metal ions, with L⁷ binding only to Pb²⁺. The selectivity of these ligands and the depression of the formation constants with all metal ions upon successive insertion of the ethylene bridges are rationalised with the aid of molecular mechanics calculations. The crystal structure of the free ligand of L⁷ has been determined: space group P2₁/n, monoclinic, with a= 6.053(2), b= 14.190(8), c= 10.346(6)Å and β= 106.82(4)°, and Z= 2. The final conventional R factor was 0.0627.