Iron(II) phosphonates: a new series of molecule-based weak ferromagnets

Abstract

The complexes Fe[CH₃PO₃]·H₂O, Fe₂[O₃P(CH₂)₂PO₃]·2H₂O and Fe₂[O₃P(CH₂)₂PO₃] were synthesized and characterized by thermogravimetric analysis and UV-visible and infrared spectroscopy and their magnetic properties studied using a superconducting quantum interference device magnetometer. Preliminary X-ray single-crystal data suggest for Fe[CH₃PO₃]·H₂O a lamellar structure similar to that observed in Cd[CH₃PO₃]·H₂O. The structure of cadmium phosphonate consists of alternating inorganic and organic layers, with the metal ion octahedrally co-ordinated by five oxygens from phosphonate and one from a water molecule. The inorganic layers are then separated by bilayers of methyl groups, and van der Waals contacts are established between them. In Fe₂[O₃P(CH₂)₂PO₃]·2H₂O a pillared structure similar to that found in the corresponding zinc(II) ethylenebis(phosphonates) is suggested. All the compounds reported contain Fe^II in the d⁶ (S = 2) high-spin electronic configuration, as determined from magnetic measurements in the high temperature region. Below 100 K the effective magnetic moment of Fe[CH₃PO₃]·H₂O decreases down to 30 K, then rises rapidly to a maximum at 24 K, and decreases again. The 3-D long-range antiferromagnetic order, defined as the temperature of the onset of the magnetization, has been located at T_N = 25 K. Similar behavior has been observed in Fe₂[O₃P(CH₂)₂PO₃]·2H₂O. Below the critical temperature, T_N, both compounds behave as a “weak ferromagnet”. After dehydration Fe₂[O₃P(CH₂)₂PO₃]·2H₂O loses the water molecules, and its magnetic behavior changes drastically in the low temperature region. The magnetic data at low temperatures of anhydrous Fe₂[O₃P(CH₂)₂PO₃] are consistent with a magnetically disordered ground state, possibly a spin-glass like state.