Ligand Control of (59)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 (59)Co T(1) (spin–lattice) and T(2) (spin–spin) relaxation times in a set of Co(III) complexes: K(3)[Co(CN)(6)] (1); [Co(NH(3))(6)]Cl(3) (2); [Co(en)(3)]Cl(3) (3), en = ethylenediamine); [Co(tn)(3)]Cl(3) (4), tn = trimethylenediamine); [Co(tame)(2)]Cl(3) (5), tame = triaminomethylethane); and [Co(dinosar)]Cl(3) (6), dinosar = dinitrosarcophagine). Measurements indicate that (59)Co T(1) and T(2) increase with temperature for 1–6 between 10 and 60 °C, with the greatest ΔT(1)/ΔT and ΔT(2)/ΔT temperature sensitivities found for 4 and 3, 5.3(3)%T(1)/°C and 6(1)%T(2)/°C, respectively. Temperature-dependent T(2)* (dephasing time) analyses were also made, revealing the highest ΔT(2)*/ΔT sensitivities in structures of greatest encapsulation, as high as 4.64%T(2)*/°C for 6. Calculations of the temperature-dependent quadrupolar coupling parameter, Δe(2)qQ/ΔT, enable insight into the origins of the relative ΔT(1)/ΔT values. These results suggest tunable quadrupolar coupling interactions as novel design principles for enhancing temperature sensitivity in nuclear spin-based probes.