Unusually sharp dependence of water exchange rate versus lanthanide ionic radii for a series of tetraamide complexes

The tetraamide ligand, DOTA-tetra(glycine ethyl ester), forms complexes with the lanthanide(III) cations that exist in solution predominantly as the square antiprism structure with single, slowly exchanging inner-sphere water molecule. Variable-temperature ¹H and ¹⁷O NMR studies revealed that the bound water lifetimes in these complexes were sharply dependent upon the ionic radius of Ln³⁺ cation. A novel lanthanide-induced shift technique was used to unmask the bound water ¹⁷O resonance of SmL³⁺ and YL³⁺ complexes from the bulk water resonance. The bound water lifetime (τ_M²⁹⁸) was ∼800 μs in the EuL³⁺ complex but became much shorter (several microseconds) for Ln³⁺ cations with larger and smaller ionic radii. This demonstrates that water exchange is exquisitely fine-tuned in this macrocyclic tetraamide system and that a variety of Ln³⁺ complexes meet with the exchange requirement, Δω*τ_M ≥ 1, necessary for an efficient MT agent.