Influence of ligand encapsulation on cobalt-59 chemical-shift thermometry

Thermometry via magnetic resonance imaging (MRI) would provide a powerful noninvasive window into physiological temperature management. Cobalt-59 nuclear spins demonstrate exceptional temperature dependence of their NMR chemical shifts, yet the insight to control this dependence via molecular design is lacking. We present the first systematic evidence that encapsulation of this spin system amplifies the temperature sensitivity. We tested the temperature dependence of the ⁵⁹Co chemical shift (Δδ/ΔT) in a series of five low-spin cobalt(iii) complexes as a function of increasing encapsulation within the 1st coordination sphere. This study spans from [Co(NH₃)₆]Cl₃, with no interligand connectivity, to a fully encapsulated dinitrosarcophagine (diNOsar) complex, [Co(diNOsar)]Cl₃. We discovered Δδ/ΔT values that span from 1.44(2) ppm °C^-1 in [Co(NH₃)₆]Cl₃ to 2.04(2) ppm °C^-1 in [Co(diNOsar)]Cl₃, the latter among the highest for a molecular complex. The data herein suggest that designing ⁵⁹Co NMR thermometers toward high chemical stability can be coincident with high Δδ/ΔT. To better understand this phenomenon, variable-temperature UV-Vis, ⁵⁹Co NMR relaxation, Raman spectroscopic, and variable-solvent investigations were performed. Data from these measurements highlight an unexpected impact of encapsulation - an increasingly dynamic and flexible inner coordination sphere. These results comprise the first systematic studies to reveal insight into the molecular factors that govern Δδ/ΔT and provide the first evidence of ⁵⁹Co nuclear-spin control via vibrational means.