Proton-Transfer-Coupled Spin Transitions in Mononuclear Iron(II) Compounds: Interplay of Intra- and Intercenter Interactions

Recently, a new trend in the field of spin transitions has been reported that is represented by iron(II) molecular compounds exhibiting proton-transfer-coupled spin-transition behavior. The present paper focuses on the elucidation of the origin of this type of transitions, and explanation on this basis of the observed behavior of the [Fe(HL-Cl)2](AsF6)2 compound proved to be the first exhibiting such a type of behavior. Since the proton lies on the nitrogen of the hydrazone or pyridine moiety in the ligand in the 5T2 and 1A1 states of the Fe(II) ion, respectively, the developed model takes into account the proton transfer between its two positions. The cooperativity in the model is assured by electron-deformational and intermolecular dipole–dipole interactions because the spin transition is accompanied by an appreciable elongation of the iron–nitrogen bonds and redistribution of the proton density inside the complex. The parameters of interactions involved are estimated with the aid of structural data and through density functional theory (DFT) calculations. The problem of interacting molecules is solved within the mean field approximation. The suggested model captures the main features of the phenomenon and provides quite a good description of the temperature dependence of the χT product and Mössbauer spectra of the [Fe(HL-Cl)2](AsF6)2 compound.