EXAFS investigations of temperature-dependent structure in cobalt-59 molecular NMR thermometers

Abstract

Cobalt-59 nuclei are known for extremely thermally sensitive chemical shifts (δ), which in the long term could yield novel magnetic resonance thermometers for bioimaging applications. In this manuscript, we apply extended X-ray absorption fine structure (EXAFS) spectroscopy for the first time to probe the exact variations in physical structure that produce the exceptional thermal sensitivity of the ⁵⁹Co NMR chemical shift. We apply this spectroscopic technique to five Co(III) complexes: [Co(NH₃)₆]Cl₃ (1), [Co(en)₃]Cl₃ (2) (en = ethylenediamine), [Co(tn)₃]Cl₃ (3) (tn = trimethylenediamine), [Co(tame)₂]Cl₃ (4) (tame = 1,1,1-tris(aminomethyl)ethane), and [Co(diNOsar)]Cl₃ (5) (diNOsar = dinitrosarcophagine). The solution-phase EXAFS data reveal increasing Co–N bond distances for these aqueous complexes over a ∼50 °C temperature window, expanding by Δr_(Co–N) = 0.0256(6) Å, 0.0020(5) Å, 0.0084(5) Å, 0.0006(5) Å, and 0.0075(6) Å for 1–5, respectively. Computational analyses of the structural changes reveal that increased connectivity between the donor atoms encourages complex structural variations. These results imply that rich temperature-dependent structural variations define ⁵⁹Co NMR thermometry in macrocyclic complexes.