Vibrational and magnetic properties of [C2H5NH3][FeIIIMII(HCOO)6] (MÂ =Â Mn, Ni) and [C2H5NH3][CrIIIMnII(HCOO)6] framework compounds

We report synthesis, X-ray diffraction, magnetic and vibrational studies of three novel heterometallic MOFs, [C2H5NH3][FeIIIMII(HCOO)6] with M = Ni (EtFeNi) and Mn (EtFeMn) as well as [C2H5NH3][CrIIIMnII(HCOO)6] (EtCrMn) crystallizing in the niccolite type architecture (space group P3¯1c) with disordered ethylammonium (EtA+) cations located in the large cavities. Magnetic studies show that EtFeNi and EtFeMn exhibit ferromagnetic order below 43 and 38 K, respectively, whereas EtCrMn remains paramagnetic at least down to 2 K. Analysis of the Raman and IR data allowed us to propose assignment of the observed bands to the respective internal and lattice modes. These data give also evidence for weaker amine-cavity interactions compared to the perovskite analogues and show that this difference has significant effect on structure of the EtA+ cation. We also report that synthesis of Mg-analogues resulted in formation of ethylammonium magnesium formate with partially substituted Mg2+ (by Cr3+ or Fe3+) and EtA+ cations (by HCOOH molecules). These mixed-metal compounds have most likely the same R3¯symmetry as the high-temperature phase of undoped perovskite-type [C2H5NH3][Mg(HCOO)3].