Synthesis and properties of new chromium(II) organophosphonates

A new series of chromium(II)-alkylphosphonates, Cr[RPO3]. H2O (R = CH3, C2H5), and chromium(II) ethylenebis(phosphonate), Cr-2[O3P-(CH2)(2)-PO3]. 3H(2)O, have been synthesized by reaction of CrCl2 and the corresponding phosphonic acid in water in the presence of urea. The (2-aminoethyl)phosphonic acid reacts with CrCl2 to give an unexpected compound of formula Cr[H3N-(CH2)(2)-PO3(Cl)]. All the compounds are sensitive to the air, except for the latter one. They were characterized by X-ray powder diffraction, thermogravimetry, and UV-visible and infrared spectroscopy, and their magnetic properties were studied by a SQUID magnetometer. The metal ion is in the high-spin d(4) configuration, and it appears to have an octahedral coordination. Cr[CH3PO3]. H2O behaves as a "weak" ferromagnet, with a Neel temperature of T-N = 35 K, and it represents an example of an unusual kind of molecular-based magnetic material, having a finite zero-field magnetization. The thermal variation of the magnetic susceptibility of Cr-2[O3P-(CH2)(2)-PO3]. 3H(2)O follows the Curie-Weiss law, with a large negative Weiss constant of theta = -73 K. Below 100 K, the susceptibility increases until a peak at T-N = 15 K is observed, and this is associated to the three-dimensional antiferromagnetic order. Cr[H3N-(CH2)(2)PO3(Cl)] is paramagnetic down to 5 K.