Ligand Control of Co Nuclear Spin Relaxation Thermometry.

Studying the correlation between temperature-driven molecular structure and nuclear spin dynamics is essential to understanding fundamental design principles for thermometric nuclear magnetic resonance spin-based probes. Herein, we study the impact of progressively encapsulating ligands on temperature-dependent Co (spin-lattice) and (spin-spin) relaxation times in a set of Co(III) complexes: K[Co(CN)] (); [Co(NH)]Cl (); [Co(en)]Cl (), en = ethylenediamine); [Co(tn)]Cl (), tn = trimethylenediamine); [Co(tame)]Cl (), tame = triaminomethylethane); and [Co(dinosar)]Cl (), dinosar = dinitrosarcophagine). Measurements indicate that Co and increase with temperature for - between 10 and 60 °C, with the greatest Δ /Δ and Δ /Δ temperature sensitivities found for and , 5.3(3)% /°C and 6(1)% /°C, respectively. Temperature-dependent * (dephasing time) analyses were also made, revealing the highest Δ */Δ sensitivities in structures of greatest encapsulation, as high as 4.64% */°C for . Calculations of the temperature-dependent quadrupolar coupling parameter, Δ /Δ enable insight into the origins of the relative Δ /Δ values. These results suggest tunable quadrupolar coupling interactions as novel design principles for enhancing temperature sensitivity in nuclear spin-based probes.