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

Abstract

The interaction of guest molecules(H2O,N2,CO2, ethanol, methanol) with the metal ions at the pore surface in porous molecular magnets(M3[Fe(CN)6]2·xH2O,M = Mn, Co, Ni, Cu) was studied by x-ray diffraction, together with Mössbauer and magneticdata. These compounds can be dehydrated at relatively low temperature, usually below100 °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 forthe metal–metal interaction through the CN bridge groups. In these materials the outermetal (M) is always found at the pore surface and the guest–host interactions modify theelectronic structure of the host solid. Such interactions and their effect on thematerial electronic structure were studied by Mössbauer spectroscopy for the guestmolecules mentioned. The most pronounced metal–metal charge overlapping wasobserved for the host solid free of adsorbed species. When the guest molecules wereabsorbed the observed changes in the solid electronic structure followed the orderN2∼CO2<ethanol<methanol<water. The most pronounced effect on the solid properties was observed for water, themost polar molecule within these guest species. The magnetic properties wereevaluated for anhydrous and hydrated samples. The highest Curie temperature(TC) andCurie–Weiss (θCW) constant values were found for the anhydrous solids, when the metal at the pore surfaceonly interacts with the CN bridge groups. This corresponds to an increase in the chargedelocalization among metal centres on the removal of water, an effect alreadydetected by x-ray diffraction and Mössbauer data. Since, during the water adsorptionand desorption processes, the cell symmetry is preserved, such changes for theTC and θCW 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 aparticular metal ion. An explanation of such behaviour for the materials studiedbased on a tetrahedral coordination for the metal (M) linked at the N end ofthe CN groups in the anhydrous phase was discarded. In the cubic structurefor Prussian blue analogues a true tetrahedral coordination cannot be present.