Cation effect on the inorganic–organic layered structure of pyrazole-4-sulfonate networks and inhibitory effects on copper corrosion

Abstract

The present study demonstrates that the pyrazole-4-sulfonate anion (4-SO₃-pzH = L⁻) is a versatile ligand for the preparation of layered, 3-D solid state lattices. The ligand itself can coordinate to metals with both its sulfonate group and aromatic N-atom, as well as participate in hydrogen bonding both as donor and acceptor, and form various extended π–π networks. Five new complexes with Rb⁺, Cs⁺, Mg²⁺, Sr²⁺ and Cu²⁺ have been prepared and characterized including single crystal X-ray diffraction: RbL, tetragonal I4₁/a, a = 9.7332(4) Å, b = 9.7332(4) Å, c = 29.941(1) Å, V = 2836.5(2) Å, Z = 16; CsL, monoclinic P2₁/c, a = 8.821(1) Å, b = 8.109(1) Å, c = 9.889(2) Å, β = 91.560(3)°, V = 707.1(2) Å, Z = 4; Mg(H₂O)₆L₂, triclinic P, a = 7.0543(7) Å, b = 7.7307(8) Å, c = 8.6389(9) Å, α = 72.572(2)°, β = 74.370(2)°, γ = 76.045(2)°, V = 426.22(8) Å, Z = 1; SrL₂, triclinic P, a = 6.027(2) Å, b = 7.243(3) Å, c = 14.186(5) Å, α = 92.491(8)°, β = 101.383(7)°, γ = 97.471(7)°, V = 426.22(8) Å, Z = 2; Cu(H₂O)₂L₂·4H₂O, monoclinic P2₁/c, a = 9.032(1) Å, b = 13.297(1) Å, c = 7.7103(9) Å, β = 109.747(2)°, V = 871.5(2) Å, Z = 2. The varying size and charge of the cations in these five complexes produce unique, alternating inorganic–organic layered materials. We find a correlation between the size and charge of cations and the coordination mode of the ligand as well as hydration of the metal, but no correlation between the nature of the cation and the thickness of the inorganic/organic layers. We conclude that the overall 3-D structure of these layered materials is determined by a subtle balance between the coordination preferences of the metal ion and an intricate lattice of hydrogen bonds and aromatic interactions. Ligand HL and its complexes with Na, K, Rb, Cs, Mg, Ca, Sr and Ba were tested as potential corrosion inhibitors of copper metal surfaces at three different pH values (2, 3, and 4). All the above mentioned compounds show significant corrosion inhibition at pH 4 and 3, while no activity is observed at pH 2. The correlation of inhibition activity with pH of ligand HL and its different metal complexes is discussed.