Towards Ni(II) complexes with spin switches for 19F MR-based pH sensing

Objectives Our aim was to demonstrate the potential of exploiting simultaneous changes in coordination geometry and spin state in fluorinated Ni(II) complexes as an avenue for 19 F magnetic-resonance (MR)-based pH sensing. Materials and methods Crystal structures were studied using an Agilent Technologies SuperNova Dual Source diffractometer. Solution magnetic moment was determined using Evan’s method. MR images were collected on a 7.0-T MR scanner equipped with a quadrature 19 F volume coil. Results NiL 1 and NiL 2 were synthesized; crystallographic and spectroscopic data supported NiL 1 as being diamagnetic and NiL 2 as being paramagnetic. In aqueous solution, ligand dissociation from Ni(II) center was observed for both complexes at around pH 6, precluding their use as reversible pH sensors. The two complexes have distinct 19 F nuclear magnetic resonance (NMR) signals in terms of both chemical shift and relaxation times, and selective imaging of the two complexes was achieved with no signal interference using two 19 F MRI pulse sequences. Conclusion The significant difference in the chemical shift and relaxation times between NiL 1 and NiL 2 allowed selective imaging of these species using 19 F MRI. While NiL 1 and NiL 2 were not stable to acidic environments, this report lays the framework for development of improved ligand scaffolds that stably coordinate Ni(II) in acidic aqueous solution and act as agents for ratiometric pH mapping by 19 F MRI.