Synthesis, structural determination and magnetic properties of layered hybrid organic–inorganic, iron (II) propylphosphonate, Fe[(CH 3(CH 2) 2PO 3)(H 2O)], and iron (II) octadecylphosphonate, Fe[(CH 3(CH 2) 17PO 3)(H 2O)

Abstract

Fe[(CH 3(CH 2) 2PO 3)(H 2O)] ( 1) and Fe[(CH 3(CH 2) 17PO 3)(H 2O)] ( 2) were synthesized by reaction of FeCl 2·6H 2O and the relevant phosphonic acid in water in presence of urea and under inert atmosphere. The compounds were characterized by elemental and thermogravimetric analyses, UV–visible and IR spectroscopy. The crystal structure of ( 1) was determined from X-ray single crystal diffraction studies at room temperature: monoclinic symmetry, space group P2 1, a = 5.7 0 7 ( 1 ) Å , b = 4.8 1 1 ( 1 ) Å , c = 1 1.8 1 8 ( 2 ) Å , and β = 9 8.6 2 ( 3 ) ° . The compound is lamellar and the structure is hybrid, made of alternating inorganic and organic layers along the c direction. The inorganic layers consist of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from the water molecule, separated by bi-layers of propyl groups. A preliminary structure characterization of compound ( 2) suggests a similar layered structure, but with an interlayer spacing of 40.3 Å. The magnetic properties of the compounds were both studied by a dc and ac SQUID magnetometer. Fe[(CH 3(CH 2) 2PO 3)(H 2O)] ( 1) obeys the Curie–Weiss law at temperatures above 50 K ( C = 3.8 1 cm 3 K mol - 1 , θ = - 6 2 K ), indicating a Fe +II oxidation state, a high-spin d 6 ( S = 2 ) electronic configuration and an antiferromagnetic exchange couplings between the near-neighbouring Fe(II) ions. Below T = 2 2 K , Fe[(CH 3(CH 2) 2PO 3)(H 2O)] exhibits a weak ferromagnetism. The critical temperature of T N = 2 2 K has been determined by ac magnetic susceptibility measurements. Compound ( 2) shows the same paramagnetic behaviour of the iron (II) propyl derivative. The values of C and θ were found to be 3.8 cm 3 K mol - 1 and −44 K, respectively, thus suggesting the presence of Fe +II ion in the S = 2 spin state and antiferromagnetic interactions between Fe(II) ions at low temperatures. Zero-field and field cooled magnetic susceptibility vs. T plots do not overlap below T = 3 0 K , suggesting the presence of an ordered magnetic state. The critical temperature, T N , has been located by the peaks at T N = 2 6 K from the ac susceptibility ( χ′and χ″) vs. T plots. Below T N hysteresis loops recorded in the temperature region 1 6 < T < 2 6 K show an S-shape, while below 15 K assume an ellipsoid form. They reveal that compound ( 2) is a weak ferromagnet. The critical temperature T N in these layered Fe(II) alkylphosphonates is independent of the distance between the inorganic layers.