Switching of shifting and relaxational NMR-thermosensor properties of iron (II) tris-(pyrazol-1-yl) methane complexes due to spin-crossover

[Display omitted] The processes of spin-crossover (SCO) transition in the solid phase and in the solution for complexes of [Fe(HC(Pz)3)2](CF3SO3)2 and [Fe(HC(Pz)3)2]SiF6 were studied by UV–vis-spectrometry and NMR methods. According to NMR, the both complexes in solution showed the monotonic decrease in the fraction of the low-spin (LS) state and the increase in the fraction of the high-spin (HS) state when the temperature increases from 295 to 360 K. The observed chemical shifts and signal half-widths change significantly on temperature, despite the fact that the fraction of the HS form increases with rising temperature not so sharp as in the solid phase. The increased in temperature sensitivity of the observed chemical shifts in comparison with the temperature sensitivity of the “pure” HS and LS forms is caused by a growth in the relative population of the HS form with temperature rising. The significant temperature dependences of the observed chemical shifts and signal half-widths are suggested to use for temperature control in liquid media using NMR and MRI.