Modification of the magnetic properties in molecular magnets based on Prussian blue analogues through adsorbed species

The interaction of guest molecules (H₂O, N₂, CO ₂, ethanol, methanol) with the metal ions at the pore surface in porous molecular magnets (M₃[Fe(CN)₆]₂xH ₂O, M ≤ Mn, Co, Ni, Cu) was studied by x-ray diffraction, together with Mössbauer and magnetic data. These compounds can be dehydrated at relatively low temperature, usually below 100 °C. On the removal of water a cell contraction of about 4% of the cell volume is observed. This corresponds to a shortening of the metal-metal distance and to a strengthening for the metal-metal interaction through the CN bridge groups. In these materials the outer metal (M) is always found at the pore surface and the guest-host interactions modify the electronic structure of the host solid. Such interactions and their effect on the material electronic structure were studied by Mössbauer spectroscopy for the guest molecules mentioned. The most pronounced metal-metal charge overlapping was observed for the host solid free of adsorbed species. When the guest molecules were absorbed the observed changes in the solid electronic structure followed the order N₂∼CO ₂<ethanol<methanol<water. The most pronounced effect on the solid properties was observed for water, the most polar molecule within these guest species. The magnetic properties were evaluated for anhydrous and hydrated samples. The highest Curie temperature (T_C) and Curie-Weiss ( _CW) constant values were found for the anhydrous solids, when the metal at the pore surface only interacts with the CN bridge groups. This corresponds to an increase in the charge delocalization among metal centres on the removal of water, an effect already detected by x-ray diffraction and Mössbauer data. Since, during the water adsorption and desorption processes, the cell symmetry is preserved, such changes for the T_C and _CW values cannot be ascribed to a variation in the linearity of the overlapping path. The observed effects are common to all the compounds studied and not to a particular metal ion. An explanation of such behaviour for the materials studied based on a tetrahedral coordination for the metal (M) linked at the N end of the CN groups in the anhydrous phase was discarded. In the cubic structure for Prussian blue analogues a true tetrahedral coordination cannot be present.