A Novel 1D-AF Hybrid Organic-Inorganic Chromium(II) Methyl Phosphonate Dihydrate: Synthesis, X-Ray Crystal and Molecular Structure, and Magnetic Properties

Light-blue crystals of chromium(II) methyl phosphonate dihydrate, [Cr(CH(3)PO(3))(H(2)O)].H(2)O, were obtained in water by mixing filtered solutions of methylphosphonic acid and chromium(II) chloride in the presence of urea in an inert atmosphere. The compound was characterized by elemental analysis, TGA-DSC, X-ray crystallography, magnetic measurements, and UV-visible and FT-IR spectroscopies. The crystal and molecular structures (orthorhombic Pnma (no. 62): a = 4.4714(5) A, b = 6.8762(7) A, c = 19.180(2) A, Z = 4) have been solved using single-crystal X-ray diffraction. The chromium(II) ion is six-coordinated by oxygens (4 + 2) to form an elongated octahedron, with the four equatorial oxygen atoms belonging to [-PO(3)](2-) phosphonate groups. This stereochemistry of the Cr(II) ion (high-spin d(4) electronic configuration) is ascribed to the Jahn-Teller effect. The [CrO(6)] chromophore, the [CH(3)PO(3)](2-) anions, and the water molecules build a novel one-dimensional (1D) metal(II) oxide chain, anchored to each other within the ab plane by two oxygens of the phosphonate ligand. Within the chain, each Cr(2+) ion is connected through double oxygen bridges to its two neighbors, forming edge-sharing octahedra running along the b axis. The chains are further connected with the adjacent chains by phosphonate [-PO(3)](2-) groups of the ligand, forming an inorganic layer that alternates along the c axis of the unit cell with bilayers, consisting of methyl groups and water of crystallization. The thermal variation of the magnetic susceptibility follows the Curie-Weiss law, with a large negative Weiss constant, theta = -60 K, indicating the presence of antiferromagnetic AF exchange interactions between neighboring Cr(II) ions. The magnetic behavior and the magnetic dimensionality have been analyzed in terms of Fisher's classical limiting form of the Heisenberg chain theory, and a value of J = -9.3 cm(-1) was found. The negative value of the intra-chain exchange constant coupling J confirms the presence of an AF coupling. No sign of long-range magnetic ordering down to 2 K (the lowest measured temperature) is observed, in agreement with the predominant one-dimensional character of the exchange interactions.