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

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) Angstrom], and the water exchange rate (k(ex)(298) = 2.7 x 10(6) s(-1)) is sufficiently fast so as not to limit the pH independent relaxivity (R-1p(298) = 5.3 mM(-1) s(-1), 20 MHz).