Analysis of the Spin State of the Iron Complexes With Hydroxysubstituted 2,6-Bis(pyrazol-3-Yl)pyridines for Deprotonation in Solution

Simple mixing of iron(II) perchlorate hexahydrate with the corresponding 2,6-bis(pyrazol-3-yl) pyridine bearing either a phenyl (L) or ortho-difluorophenyl (L) group at the 1-position of the pyrazolyl moiety and a deprotonatable hydroxyl group at its 5-position in deuterated methanol within an NMR tube yielded the previously described iron(II) complexes Fe(L)(ClO) and Fe(L)(ClO). Analysis of the temperature dependence of their H NMR chemical shifts confirmed that both complexes remain in a high-spin state. Using the same approach, the possibility of in situ reversible deprotonation of the pH-sensitive hydroxyl groups in their ligands under the action of 1,8-diazabicyclo5.4.0undec-7-ene (DBU) was demonstrated, and the potential for shifting the spin equilibrium via deprotonation was investigated.