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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.

The reaction of nickel chloride with phenyl phosphonic acid under hydrothermal conditions resulted in the isolation of yellow-green single crystals of Ni[(C(6)H(5)PO(3))(H(2)O)]. The structure of the compound has been solved by X-ray single-crystal diffraction studies. Ni[(C(6)H(5)PO(3))(H(2)O)] crystallizes in the orthorhombic space group Pmn2(1) and is isostructural with the Mn(II), Fe(II), and Co(II) analogues. It presents the typical features of the hybrid 2D structures, consisting of alternating inorganic and organic layers. The former are formed by six-coordinated nickel(II) ions bridged by oxygen atoms into the layers. The inorganic layers are capped by the phenyl phosphonate groups, with phenyl groups of two adjacent ligands forming a hydrophobic bilayer region, and van der Waals contacts are established between them. The magnetic properties investigated by means of dc and ac susceptibility measurements point to an AF exchange coupling between nearest neighboring Ni(II) ions. Below 5 K, the compound orders magnetically showing the typical features of a canted antiferromagnet. The magnetic behavior and magnetic dimensionality of Ni[(C(6)H(5)PO(3))(H(2)O)] have been fully analyzed and compared to those of the Ni(II) parent compounds Ni[(RPO(3))(H(2)O)] (where R = CH(3), C(18)H(37)), which exhibit different symmetries of the inorganic layers and lengths of the R groups.

The isolation of complexes 2 and 3, unambiguously characterized by single-crystal X-ray diffraction, demonstrates that nucleophilic additions to the aluminum-coordinated imino bond of salophen complex 1 can be achieved under very mild conditions.

Cr[(H(3)N-(CH(2))(2)-PO(3))(Cl)(H(2)O)], a rare example of a polar organic-inorganic hybrid material containing Cr(2+), was prepared from CrCl(2), 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) A, b = 14.133(3) A, c = 5.275(1) A, 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 Cr(2+) 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[(H(3)N-(CH(2))(2)-PO(3))(Cl)(H(2)O)] 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(c), 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.

The crystal and molecular structure of the layered weak-ferromagnet Fe[CH(3)PO(3)] x H(2)O has been solved by X-ray single-crystal diffraction techniques. Crystal data for Fe[CH(3)PO(3)] x H(2)O are the following: orthorhombic space group Pna2(1); a =17.538(2), b = 4.814(1), c = 5.719(1) A. The structure is lamellar, and it consists of alternating organic and inorganic layers along the a direction of the unit cell. The inorganic layers are made of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. Each phosphonate group coordinates four metal ions, through chelation and bridging, making in this way a cross-linked Fe-O network. The resultant layers are then separated by bilayers of the methyl groups, with van der Waals contacts between them. The compound is air stable, and it dehydrates under inert atmosphere at temperatures above 120 degrees C. The oxidation state of the metal ion is +2, and the electronic configuration is d(6)( )()high spin (S = 2), as determined from dc magnetic susceptibility measurements from 150 K to ambient temperature. Below 100 K, the magnetic moment of Fe[CH(3)PO(3)] x H(2)O rises rapidly to a maximum at T(max) approximately equal to 24 K, and then it decreases again. The onset of peak at T = 25 K is associated with the 3D antiferromagnetic long-range ordering, T(N). The observed critical temperature, T(N), is like all the other previously reported Fe(II) phosphonates, and it appears to be nearly independent of the interlayer spacing in this family of hybrid organic-inorganic layered compounds. Below T(N), the compound behaves as a "weak ferromagnet", and represents the third kind of magnetic materials with a spontaneous magnetization below a finite critical temperature, ferromagnets and ferrimagnets being the other two types.