Effect of Ligand Deprotonation on the Spin Transition in Fe(II) Complexes

The effect of deprotonation of the ligands forming the second coordination sphere of the iron­(II) ion on its spin crossover behavior is examined. Due to the pronounced changes in the observed magnetic characteristics of the iron­(II) ion, the proton removal from the ligand in the second coordination sphere is modeled by the appearance of a negative charge on one of the neutral nitrogens in the nearest surrounding of the metal ion, thus significantly changing the crystal field acting on this ion and, respectively, the conditions for observation of the spin transition. Different symmetries of the nearest nitrogen surrounding of the iron­(II) ion and different locations of the negative charge in this surrounding are examined in the frames of the crystal field model. It is demonstrated that in the model accepted, depending on the location of the negative charge on the nitrogens of the first coordination sphere, the spin conversion can be either suppressed or facilitated. Within the framework of the suggested model, a qualitative explanation of the effect of deprotonation on the observed magnetic properties in [Fe4(H n L4)](n)+ (n = 8, 6, 4) compounds is given.