Mapping the supramolecular chemistry of pyrazole-4-sulfonate: layered inorganic–organic networks with Zn2+, Cd2+, Ag+, Na+ and NH4+, and their use in copper anticorrosion protective films

Abstract

Five compounds based on the versatile pyrazole-4-sulfonate anion (4-SO₃-pzH = L⁻) were synthesized by the reaction of ligand HL with ZnO, CdCO₃, Ag₂O, NaOH and NH₃, respectively. Crystals of Zn(4-SO₃-pzH)₂(H₂O)₂, Cd(4-SO₃-pzH)₂(H₂O)₂, Ag(4-SO₃-pzH), Na(4-SO₃-pzH)(H₂O) and NH₄(4-SO₃-pzH) were obtained from aqueous solutions upon evaporation, and were characterized by single-crystal X-ray diffraction, IR and NMR spectroscopy, thermogravimetric analysis and copper corrosion inhibition experiments. We found that the non-isomorphous, 3-dimensional inorganic–organic layered solid state structure of these compounds is determined by an intricate interplay between the size, charge and coordination preference of the cation, and an extended lattice of hydrogen bonds and aromatic interactions. Ligand L⁻ incorporates a host of different binding capabilities (metal coordination through the pyrazole N-atom and/or the sulfonate O-atom, hydrogen-bonding both as donor and acceptor, π–π and C–H⋯π interactions). Thin films formed by these complexes on copper metal surfaces were studied by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. ZnL₂, CdL₂, AgL and NH₄L, in addition to the free ligand HL, were tested as corrosion inhibitors on copper metal surfaces at three different pH values (2, 3 and 4), and the corrosion rates were quantified. Significant corrosion protection was observed with all compounds at pH 4 and 3.