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.