Cr[(H3N-(CH2)2-PO3)(Cl)(H2O)]:: X-ray single-crystal structure and magnetism of a polar organic-inorganic hybrid chromium(II) organophosphonate

Cr[(H3N-(CH2)(2)-PO3)(Cl)(H2O)], a rare example of a polar organic-inorganic hybrid material containing Cr2+, was prepared from CrCl2, 2-aminoethylphosphonic acid, and urea in water and isolated as light-blue crystals. It crystallizes in the noncentrosymmetric monoclinic space group P2(1), with a = 5.249(1) Angstrom, b = 14.133(3) Angstrom, c = 5.275(1) Angstrom, and beta = 105.55(2)degrees. The inorganic layer of the hybrid network is formed by Cr(II) five-coordinated by three oxygen atoms from the phosphonates and one from the water molecule in a square pyramidal unit, whose apical position is occupied by the Cl- ion. Hydrogen bonds are established between the coordinating water molecule and the oxygen atoms of adjacent phosphonate ligands. The inorganic network is interspersed by ethylammonium groups, and the terminal ammonium moiety is linked to the apical Cl- ions through hydrogen bonds. Electrostatic interactions as well as hydrogen bonds and the coordinated chlorine atoms ensure the cohesion of the 3D structure. The lattice is polar (lack of inversion center), and this fact determines the magnetic behavior of the compound at low temperatures. The magnetic susceptibility data in the temperature range from 300 to 50 K show Curie-Weiss behavior, with C = 2.716 cm(3) K mol(-1) and the Weiss constant theta = -2.2 K. The corresponding effective magnetic moment of 4.7 mu(B) compares well with the expected value for Cr2+ in d(4) high-spin configuration. A slight decrease of the chiT product versus T observed at temperatures below 50 K indicates nearest-neighbor antiferromagnetic exchange interactions. On cooling below T = 6 K, the magnetic susceptibility increases sharply up to a maximum at ca. 5 K and then decreases again. Below T = 6 K, hysteresis loops taken at different temperatures show that Cr[(H3N-(CH2)(2)-PO3)(Cl)(H2O)] behaves as a weak ferromagnet with the critical temperature T-N at 5.5 K. The spin canting is responsible of the long-range magnetic ordering. The values of the coercive field, H, and of remnant magnetization, M-r, obtained from the hysteresis loop at T = 4.5 K (the lowest measured temperature) are 30 Oe and 0.08 mu(B), respectively.