Spin-crossover and high-spin iron(ii) complexes as chemical shift 19F magnetic resonance thermometers† †Electronic supplementary information (ESI) available: Experimental details, additional crystallographic, magnetic, and spectroscopic data, and crystallographic information files (CIFs) for 1a·0.5CH3CN, 1b, 2a, 2b, and 3a. CCDC 1505471–1505473, 1505534, and 1505863. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc04287b Click here for additional data file. Click here for additional data file. Click here for additional data file.

The potential utility of paramagnetic transition metal complexes as chemical shift F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover Fe complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of Fe complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (Lx). Variable-temperature magnetic susceptibility measurements, in conjunction with UV-vis and NMR data, show thermally-induced spin-crossover for [Fe(L1)] 2+ in H2O, with T1/2 1⁄4 52(1) C. Conversely, [Fe(L2)] remains high-spin in the temperature range 4– 61 C. Variable-temperature F NMR spectra reveal the chemical shifts of the complexes to exhibit a linear temperature dependence, with the two peaks of the spin-crossover complex providing temperature sensitivities of +0.52(1) and +0.45(1) ppm per C in H2O. These values represent more than two-fold higher sensitivity than that afforded by the high-spin analogue, and ca. 40-fold higher sensitivity than diamagnetic perfluorocarbon-based thermometers. Finally, these complexes exhibit excellent stability in a physiological environment, as evidenced by F NMR spectra collected in fetal bovine serum.