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–Npy bond [2.535(1) Å], and the water exchange rate (kex298=2.7×106 s-1) is sufficiently fast so as not to limit the pH independent relaxivity (R1p298=5.3 mM-1 s-1, 20 MHz).