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

The complexes Fe[CH3PO3].H2O, Fe-2[O3P(CH2)(2)PO3]. 2H(2)O and Fe-2[O3P(CH2)(2)PO3] 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 Fe-2[O3P(CH2)(2)PO3]. 2H(2)O 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 d(6) (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 Fe-2[O3P(CH2)(2)PO3]. 2H(2)O. Below the critical temperature, T-N, both compounds behave as a "weak ferromagnet". After dehydration Fe-2[O3P(CH2)(2)PO3]. 2H(2)O loses the water molecules, and its magnetic behavior changes drastically in the low temperature region. The magnetic data at low temperatures of anhydrous Fe-2[O3P(CH2)(2)PO3] are consistent with a magnetically disordered ground state, possibly a spin-glass like state.