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 19 F 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.