Layered Inorganic-Organic 3,5-Dimethylpyrazole-4-Sulfonate Films for Protection of Copper Surfaces against Corrosion

A series of 11 compounds, including alkali (Li, Na, K, Rb, Cs) and alkaline-earth (Mg, Ca, Sr, Ba) coordination polymers, transition-metal (Cu, Cd) complexes, and the ammonium salt of 3,5-dimethylpyrazole-4-sulfonic acid (HL), were synthesized. Single crystals of HL·H2O, LiL(H2O), NaL(H2O), RbL, CsL, CaL2(H2O), SrL2(H2O), BaL2(H2O), CuL2(H2O)3·2.3H2O·0.5(acetone), and CdL2(H2O)3·2H2O were obtained from aqueous solutions either by evaporation or acetone vapor diffusion. Characterization by single-crystal X-ray diffraction reveals that the coordination compounds of L- (except Cu) possess alternating inorganic-organic layered structures, in which L- engages in extensive charge-assisted networks of H-bonding and aromatic interactions as well as metal coordination through the pyrazole N atom and/or the sulfonate O atom. A topological analysis and classification of underlying metal-organic or hydrogen-bonded networks uncover a number of distinct topological nets (3,5L2, hcb, 6,6L1, 3,5C1, 3,8L28, hex, and pcu). A thermogravimetric analysis shows that HL and NH4L are stable up to 285 and 90 °C, respectively, whereas the anhydrous metal compounds decompose above 200-230 °C. The pKa values of 3,5-dimethylpyrazole-4-sulfonic acid (HL) and pyrazole-4-sulfonic acid were determined by 1H NMR titrations with H2SO4. Copper corrosion experiments indicate that 3,5-dimethylpyrazole-4-sulfonic acid (HL) is a better anticorrosion agent than the parent pyrazole-4-sulfonic acid at pH 4, whereas the coordination polymers of L- offer weaker corrosion protection in comparison to the corresponding pyrazole-4-sulfonate complexes. The latter result is corroborated by the less compact and less robust thin films formed by metal-L compounds, as indicated by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) measurements, and the weaker acidity of HL, which allows for easier protonation of the conjugate base L- in metal-L compounds. © 2021 American Chemical Society.