Synthesis and Properties of New Chromium(II) Organophosphonates

A new series of chromium(II)-alkylphosphonates, Cr[RPO 3 ]·H 2 O (R = CH 3, C 2 H 5 ), and chromium(II) ethylenebis(phosphonate), Cr 2 [O 3 P−(CH 2 ) 2 −PO 3 ]·3H 2 O, have been synthesized by reaction of CrCl 2 and the corresponding phosphonic acid in water in the presence of urea. The (2-aminoethyl)phosphonic acid reacts with CrCl 2 to give an unexpected compound of formula Cr[H 3 N−(CH 2 ) 2 −PO 3 (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[CH 3 PO 3 ]·H 2 O behaves as a “weak” ferromagnet, with a Nèel 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 [O 3 P−(CH 2 ) 2 −PO 3 ]·3H 2 O follows the Curie−Weiss law, with a large negative Weiss constant of θ = −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[H 3 N−(CH 2 ) 2 PO 3 (Cl)] is paramagnetic down to 5 K.