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 M2[(O3P–R′–PO3)(2H2O)] 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 TN 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)2PO3H2, or the carboxyethylphosphonic acid, HO2C(CH2)2PO3H2, is used as a ligand, then a novel Cr(II) compound of formula Cr[NH3(CH2)2PO3(Cl)(H2O)] and a microporous Fe(III) salt (NH4)[Fe2(OH){O3P(CH2)2CO2}2] are isolated. The latter are both polar and, more interesting, Cr(II) ammoniumethylphosphonate chloride results to be a weak ferromagnet below TN = 5.0 K.