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 a 1:1 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 P21/c, with a = 9.1908(7) Å, b = 16.0054(12) Å, c = 9.6791(7) Å, β = 115.2890(10)°, V = 1287.37(17) Å3, 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.