Paramagnetic Cobalt(II) Complexes with Cyclam Derivatives: Toward F MRI Contrast Agents.

In order to develop novel, more efficient, and/or selective contrast agents for magnetic resonance imaging (MRI), different are explored as alternatives to water-relaxation enhancement. In this work, cobalt(II/III) complexes of bis(-trifluoroethyl)cyclam derivatives with two acetate or two phosphonate pendant arms, H and H, were prepared and investigated. X-ray diffraction structures confirmed octahedral coordination with a very stable - cyclam conformation and with fluorine atoms located about 5.3 Å from the metal center. The Co(II) complexes are kinetically inert, decomposing slowly even in 1 M aqueous HCl at 80 °C. The Co(II) complexes exhibited well-resolved paramagnetically shifted NMR spectra. These were interpreted with the help of quantum chemistry calculations. The C NMR shifts of the -[Co()] complex were successfully assigned based on spin density delocalization within the ligand molecule. The obtained spin density also helps to describe -metal-induced NMR relaxation properties of F nuclei, including the contribution of a Fermi contact relaxation mechanism. The paramagnetic complexes show convenient relaxation properties to be used as F MRI contrast agents.