Complexation properties of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7-malonate, -7,16-bis(malonate) and -7,16-bis(α-methylacetate)

The macrocycles 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-bis(malonate) (oddm), -7-malonate (odmm) and -7,16-bis(α-methylacetate) (oddp) have been synthesized. Their protonation constants and the stability constants of their complexes ML with Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ have been determined by pH-potentiometry at a 1:1 metal-to ligand ratio. The macrocycle oddm possesses high selectivity for the large metal ions. The complex [Sr(oddm)]^2- is two orders of magnitude more stable than [Ca(oddm)]^2- and the stability constant of [Pb(oddm)]^2- is six orders of magnitude larger than that of the corresponding zinc complex. The complexes of oddm with large metal ions (Sr²⁺ and Ba²⁺) possess rigid structures, while the average lifetimes of the metal-donor bonds are relatively large, and this is indicated by the appearance of multiplet signals in the ¹H NMR spectra.