The technology used in Magnetic Resonance Imaging (MRI) relies on the observation of nuclear magnetic resonance (NMR) during complex choreographies of time-varying magnetic fields. Hardware imperfections as well as other external perturbances influence these field evolutions, limiting image quality and the feasibility of high-performance methods. Continuous concurrent magnetic field monitoring¹ is a method for probing the spatiotemporal magnetic field evolution in an MR system by the usage of a set of NMR field sensors. For these sensors, highly homo-perfluorinated liquid compounds with short fluorine NMR relaxation times (T₁ and T₂ < 1 ms) are required. Effective relaxation enhancement requires a close contact between the ¹⁹F atoms and a paramagnetic centre – ideally in the first coordination sphere. Herein we present that a homo-perfluorinated ionic liquid in which the anions also act as ligands for the paramagnetic metal complex, offers such a possibility. The model Gd(III) complex [NⁿBu₄][Gd(Tf₂N)₄] (Tf₂N^-= bis(trifluoromethylsulfonyl)imide), obtained from the reaction of its precursor [Gd(NTf₂)₃] with [NⁿBu₄](NTf₂), shows the extension of the coordination sphere from six to eightfold with transoid configuration of the NTf₂ ligands. Hence, the solution of the [Gd(NTf₂)₃] in ionic liquids of the triflimide type [cat⁺][Tf₂N^-] (cat⁺=AMIm=1-allyl-3-methylimidazolium, EthylMIm or PR₃R’⁺) is shown (by evaluation of exchange rates of the non-paramagnetic parent Yttrium compound) to yield a single ¹⁹F signal, reflecting rapid ligand self-exchange, and relaxation times in the sub-millisecond range as targeted. 

Figure 1. ORTEP of [NⁿBu₄][Gd(Tf₂N)₄]. Cation omitted and fluorides shown as spheres for clarity. The transoid configuration is clearly visible for the [NTf₂^-] ligands. Ellipsoids except for fluorides are drawn on the 30% probability level.
Figure 2.  Comparison of observed (upper) with calculated (lower) ¹⁹F NMR (282.39 MHz) spectra of a mixture of [Y(NTf₂)₃]  with [EthylMIm][NTf₂] (1:1.5) in CD₂Cl₂ displaying line broadening, coalescence and splitting into two signals upon cooling. 

[1] C. Barmet, N. De Zanche, B. J. Wilm and K. P. Pruessmann, Magnetic Resonance in Medicine, 2009, 62, 269.