Two new supramolecular metal diphosphonates: Synthesis, characterization, crystal structure and inhibiting effects on metallic corrosion

Two new divalent metal(II) aminodiphosphonates with layered structure, namely, Cu(H₃L¹)₂·2H₂O (1), [H4L¹=methyl-N(CH₂PO₃H₂)₂] and Cd₂(H₂L²)₄(2), [H₄L²=n-propyl-N(CH₂PO₃H₂)₂] were synthesized and characterized. The Cu(II) ions in complex 1 are octahedrally coordinated by four oxygen atoms from two chelating ligands and two phosphonate oxygen atoms from two neighboring Cu(H₃L¹)₂ units. The Cu(H₃L¹)₂ units are interconnected by bridging phosphonate groups, forming a 2-D metal phosphonate layer. The structure of complex 2 contains two unique Cd(II) ions octahedrally-coordinated by six phosphonate oxygen atoms from four H₂L² diphosphonate anions. Corrosion inhibition performances of 1 and 2 were also compared with each other in order to study the effect of combinations of externally added Cd/H₄L² and Cu/H₄L¹ (1:1 ratio) on corrosion rates of carbon steel. It was found that at pH 3.0, Cd/H₄L² or Cu/H₄L¹ combinations do not have noticeable corrosion inhibition efficiency for carbon steel. In contrast, at pH 7.0, higher corrosion inhibition efficiency was achieved for Cd/H₄L². Physical characterizations such as scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were applied to study the corrosion specimens and film material. - Graphical abstract: Two new metal phosphonates have been synthesized and characterized by single-crystal X-ray diffraction and thermogravimetric analysis. corrosion inhibition performances 1 and 2 are also compared.