Ultrafast Ligand Self-Exchanging Gadolinium Complexes in Ionic Liquids for NMR Field Probes

Concurrent magnetic field monitoring in MRI with an array of NMR field probes allows for reducing image imperfections. High 19F concentrations together with short relaxation times are basic probe properties. We present the NMR properties of [Gd(NTf2)4]− dissolved in an ionic liquid consisting of an imidazolium cation and the [NTf2]− anion. These solutions achieve fluorine concentrations as high as 26 M and rapid relaxation times in the sub-ms time range. The second order self-exchange rate of coordinated [NTf2]− with [NTf2]− as determined with [Y(NTf2)4]− is 4.5 × 105·M–1·s–1. X-ray structure analyses confirm the coordination number of eight around Gd3+ and Y3+. These ionic-liquid solutions are thus excellent candidates for dynamically probing the NMR fields in MRI.