⁵⁹Co Thermal Sensitivity in Co(III) Trisdithiocarbamate Complexes

Understanding temperature sensitivity in magnetic resonance is key to novel molecular probes for noninvasive temperature mapping. Herein, we report an investigation of the effects of heavy-donor-atom dithiocarbamate ligands on the variable-temperature ⁵⁹Co nuclear magnetic resonance (NMR) properties of six Co(III) complexes: Co(et₂-dtc)₃ (<b>1</b>), Co(bu₂-dtc)₃ (<b>2</b>), Co(hex₂-dtc)₃ (<b>3</b>), Co(pyrr-dtc)₃ (<b>4</b>), Co(benzyl₂-dtc)₃ (<b>5</b>) and Co(2,6-dmpip-dtc)₃ (<b>6</b>) (et₂-dtc = diethyldithiocarbamate; bu₂-dtc = dibutyldithiocarbamate; hex₂-dtc = dihexyldithiocarbamate; pyrr-dtc = pyrrolidine-dithiocarbamate; benzyl₂-dtc = dibenzyldithiocarbamate; and 2,6-dmpip-dtc = 2,6-dimethylpiperidine-dithiocarbamate). This study reveals ⁵⁹Co chemical-shift temperature dependences of 1.17(3)-1.73(4) ppm/°C as a function of ligand substituents. Solid-state Raman spectroscopic analyses show that more Raman-active Co-S₆ vibrational modes correlate to higher thermal sensitivities for these compounds, in line with our current model for temperature sensitivity. Short spin-lattice relaxation <i>T</i>₁ times in solution (<i>ca</i>. 200 μs) were observed, and correlation with <i>T</i>₂* times and solid-state ⁵⁹Co NMR analyses reveal that the solution-phase line widths are attributable to quadrupolar relaxation processes, which ultimately lower temperature-sensing resolution.