Abstract
In order to develop novel, more efficient, and/or selective contrast agents for magnetic resonance imaging (MRI), different modi operandi are explored as alternatives to water-relaxation enhancement. In this work, cobalt(II/III) complexes of bis(N-trifluoroethyl)cyclam derivatives with two acetate or two phosphonate pendant arms, H(2)te2f2a and H(4)te2f2p, were prepared and investigated.
X-ray diffraction structures confirmed octahedral coordination with a very stable trans-III 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 (13)C NMR shifts of the trans-[Co(II)(te2f2p)](2-) complex were successfully assigned based on spin density delocalization within the ligand molecule. The obtained spin density also helps to describe d-metal-induced NMR relaxation properties of (19)F nuclei, including the contribution of a Fermi contact relaxation mechanism.
The paramagnetic complexes show convenient relaxation properties to be used as (19)F MRI contrast agents.