Synthesis, magnetic and vibrational properties of two novel mixed-valence iron(II)-iron(III) formate frameworks

Highlights: • Novel mixed-valence iron(II)-iron(III) MOF compounds have been synthesized. • These compounds crystallize in niccolite-type architecture. • IR and Raman spectra were measured and assignment of modes was proposed. • Magnetization measurement revealed that EtMeAFeFe is N-type ferrimagnet. - Abstract: We report synthesis of two novel mixed-valence iron(II)-iron(III) formate frameworks templated by N-ethylmethylammonium (EtMeA⁺) and 2-Hydroxyethylammonium (HEA⁺) cations. X-ray diffraction shows that these compounds crystallize in the P $\overline{3}$1c niccolite-type structures. Optical studies confirm mixed-valence character of these compounds. Magnetic investigation reveals that both N-ethylmethylammonium iron formate (EtMeAFeFe) and 2-Hydroxyethylammonium iron formate (HEAFeFe) exhibit magnetic order at 36.5 and 37.0 K, respectively. EtMeAFeFe also exhibits large negative magnetization below 31.0 K indicating that this compound is an N-type ferrimagnet. We also present Raman and IR data for both compounds. In order to help assignment of modes corresponding to the EtMeA⁺ cation, we also performed quantum chemical calculations to derive the harmonic and anharmonic vibrational wavenumbers. The performed calculations revealed that protonation should affect most strongly modes related to vibrations of the N–H bonds as well as the stretching modes of the CH₂ and CH₃ groups attached to the N atom.