On the crystal structures and magnetism of some hybrid organic-inorganic metal organophosphonates

We review our recent work in the field of molecule-based magnets showing the structural and magnetic properties of a special class of hybrid organic-inorganic compounds, i.e. metal(II) organophosphonates. The synthesis, the crystal structures and, in particular, the magnetic studies of selected examples of compounds of formulas M(II)[(R-PO3)(H2O)], and M-2[(O3P-R'-PO3)(2H(2)O)] M = Cr, Fe, Co; R = CnH2n+1, n = 1, 2, 3... and C6H5, R' = (CH2)(2) prepared in our laboratory are presented and discussed. Metal alkylphosphonates, except those of Co(II), are weak ferromagnets at low temperatures. The observed magnetic ordering temperature T-N varies from 4.2 to 25 K, depending on the transition metal ions and on crystal and molecular structure. Moreover, in the case of a bifunctional molecule like aminoethylphosphonic acid, NH2(CH2)(2)PO3H2, or the carboxyethylphosphonic acid, HO2C(CH2)(2)PO3H2, is used as a ligand, then a novel Cr(II) compound of formula Cr[NH3(CH2)(2)PO3(Cl)(H2O)] and a microporous Fe(III) salt (NH4)[Fe-2(OH){O3P(CH2)(2)CO2}(2)] are isolated. The latter are both polar and, more interesting, Cr(II) ammoniumethylphosphonate chloride results to be a weak ferromagnet below T-N = 5.0 K. (C) 2008 Elsevier B.V. All rights reserved.