Structure and relaxivity of macrocyclic gadolinium complexes incorporating pyridyl and 4-morpholinopyridyl substituents

Abstract

Octadentate N₅O₃ ligands containing a coordinating pyridyl group form kinetically stable complexes with Eu and Gd that resist protonation; the structure of a representative Gd complex reveals a short Gd–N_py bond [2.535(1) Å], and the water exchange rate (k_ex²⁹⁸=2.7×10⁶ s^-1) is sufficiently fast so as not to limit the pH independent relaxivity (R_1p²⁹⁸=5.3 mM^-1 s^-1, 20 MHz).