A new series of polyoxa tetraaza macrocyclic tetraacetates have been prepared and characterized. These derivatives chelate lanthanides and transition-metal ions in the tetraaza tetraacetate portion of the macrocycle, leaving the polyoxa portion available for binding with alkali-metal cations. The protonation constants and metal-binding properties of these chelates have been investigated and compared to those of the parent molecule 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Molecular mechanics calculations were performed to estimate the polyoxa cavity sizes, and these results were compared to those of the crown ether series. The Dy3+, Ni2+, and Co2+complexes were examined for their ability to induce a paramagnetic shift in the 23Na+NMR resonance of aqueous NaCl at pH 7. The Dy3+complex formed with the tetraoxa tetraaza macrocycle (O4N4) produced the largest 23Na paramagnetic shift, opposite in direction to that produced by Dy(DOTA)−. This verified that Na+(and other alkali-metal cations) binds in the polyoxa cavity, orthorogonal to the presumed symmetry axis of the Dy3+cation bound within the tetraaza tetraacetate portion of the macrocycle. Competitive titrations with Ca2+indicate that these ligands are considerably more specific for Na+ions than are the more common ion-pairing 23Na shift agents.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry