Crystal growth and characterization of zinc-(amino-tris-(methylenephosphonate)) organic-inorganic hybrid networks and their inhibiting effect on metallic corrosion

Amino-tris-(methylenephosphonate) (AMP, N(CH2PO3H2)(3)) forms various organic-inorganic hybrids with divalent metal cations. Zn2+ reacts with AMP at 1:1 A ratio by replacing two of the six phosphonic acid protons, while amino N maintains its proton, thus forming a zwitter ion.. The product is a polymeric material, Zn[HN(CH2PO3H)(3)(H2O)(3)]. Its crystal is monoclinic, space group P2(1)/c, with a = 9.1908(7) Angstrom, b = 16.0054(12) Angstrom, c = 9,6791(7) Angstrom, beta = 115.2890(10)degrees, V = 1287.37(17) Angstrom(3), and Z = 4. The Zn2+ ions form zig-zag chains bridged by two of the three phosphonate groups. The third phosphonate group is non-coordinating and involved in hydrogen bonding. The Zn2+ centers are located in a slightly distorted octahedral environment and are coordinated by three H2O molecules in a mer fashion, two phosphonate oxygens from the same AMP ligand forming an eight-member chelate and another phosphonate oxygen from a neighboring AMP. Adjacent chains are hydrogen-bonded to each other through P-O-H and H-N donors, and the additional participation of all H2O hydrogens in H-bonding results in a corrugated sheet-like structure. Zn2+ and AMP form a synergistic combination of additives that acts as corrosion inhibitor for carbon steel surfaces. The composition of this protective layer is a Zn-AMP material based on spectroscopic comparisons (FT-IR, XRF and EDS) with authentically prepared Zn-AMP. (C) 2004 Elsevier B.V. All rights reserved.