Complexation properties of macrocyclic polyoxadiazadiphosphonates

Abstract

The macrocycle 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyldimethylenediphosphonic acid (H₄L¹), its bis(ethyl ester)(H₂L³) and 1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diyldimethylenediphosphonic acid (H₄L⁵) were prepared. The protonation constants of the macrocycles and the stability constants of their complexes with alkaline-earth-metal ions, lanthanide ions, Zn²⁺, Cd²⁺ and Pb²⁺ were determined by pH-potentiometric titration. The phosphonate (L³)^2– forms only unprotonated complexes ML, while (L¹)^4– and (L⁵)^4– form ML, monoprotonated complexes, M(HL), and diprotonated complexes, M(H₂L). The trends in the stability constants through the lanthanide series are similar for all complexes: a weak maximum in log K is observed at around Pr³⁺, Nd³⁺ and a minimum at about Tb³⁺, Ho³⁺. The chemical shifts of the non-labile protons of L¹ indicate that on protonation the first two protons are attached to the two nitrogens, the next two at one of the oxygen atoms of each phosphonate group. The equilibrium data and ¹H NMR spectra of the complexes [LaL¹]^– and [LuL¹]^– indicate the increasing role of the metal–phosphonate group interaction in the complexation through the lanthanide series from La³⁺ to Lu³⁺. Attachment of the strongly co-ordinating phosphonate groups to the 15- and 18-membered ring macrocycles results in cessation of the size–match selectivity.