Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-bis(aminomethyl)benzene-N,N′-bis(methylenephosphonic acid), (H2O3PCH2)2–N–CH2C6H4CH2–N(CH2PO3H2)2 (designated herein as p-H8L and m-H8L) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M[(HO3PCH2)2N(H)CH2C6H4CH2N(H)(CH2PO3H)2(H2O)2]·2H2O (M = Mg, Co, and Zn), M–(p-H6L), and M[(HO3PCH2)2N(H)CH2C6H4CH2N(H)(CH2PO3H)2]·nH2O (M = Ca, Mg, Co, and Zn and n = 1–1.5), M–(m-H6L) , were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural M–(p-H6L) is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. M–(m-H6L) compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of M–(p-H6L) showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of M–(p-H6L) (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10–5 S·cm–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved.