Asymmetry-enhanced 59Co NMR thermometry in Co(<sc>iii</sc>) complexes

Design strategies for molecular thermometers by magnetic resonance are essential for enabling new noninvasive means of temperature mapping for disease diagnoses and treatments. Herein we demonstrate a new design strategy for thermometry based on chemical control of the vibrational partition function. To do so, we performed variable-temperature Co-59 NMR investigations of four air-stable Co(III) complexes: Co(accp)(3) (1), Co(bzac)(3) (2), Co(tBu(2)-acac)(3) (3), and Co(acac)(3) (4) (accp = 2-acetylcyclopentanonate; bzac = benzoylacetonate; tBu(2)-acac = 2,2,6,6-tetramethyl-3,5-heptanedionate and acac = acetylacetonate). We discovered Co-59 chemical shift temperature sensitivity (Delta delta/Delta T) values of 3.50(2), 3.39(3), 1.63(3), and 2.83(1) ppm degrees C-1 for 1-4, respectively, at 100 mM concentration. The values observed for 1 and 2 are new records for sensitivity for low-spin Co(III) complexes. We propose that the observed heightened sensitivities for 1 and 2 are intimately tied to the asymmetry of the accp and bzac ligands versus the acac and tBu(2)-acac ligands, which enables a larger number of low-energy Raman-active vibrational modes to contribute to the observed Delta delta/Delta T values.