Asymmetry-enhanced ⁵⁹Co NMR thermometry in Co(iii) 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.