Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging

Abstract

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) and trivalent (Cr³⁺, Fe³⁺, Co³⁺) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co²⁺ to Zn²⁺) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn²⁺ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni²⁺, Cu²⁺ and Zn²⁺ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co²⁺ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in ¹⁹F magnetic resonance imaging. A significant shortening of the ¹⁹F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr³⁺, Mn²⁺ and Fe³⁺ with symmetric electronic states (t_2g³ and HS-d⁵), the millisecond range for the Ni²⁺ and Cu²⁺ complexes and tens of milliseconds for the Co²⁺ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5–6.5 Å). Among the redox-active complexes (Mn³⁺/Mn²⁺, Fe³⁺/Fe²⁺, Co³⁺/Co²⁺, Cu²⁺/Cu⁺), the cobalt complexes show sufficient stability and a paramagnetic–diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co³⁺/Co²⁺ complex pair can be potentially used as a smart redox-responsive contrast agent for ¹⁹F MRI.