Rare Earth 3-Hydroxyphenylacetate Complexes: Synthesis, Structural Characterization, and Surprising Improved Anticorrosion Performance on Mild Steel

Abstract

Rare earth (RE) aqua 3-hydroxyphenylacetate complexes prepared from a RE salt and sodium 3-hydroxyphenylacetate by salt metathesis, have two different structural motifs namely, {[La(3hpa)₃(H₂O)₂]·2H₂O}_n (1), and [RE(3hpa)₃(H₂O)]_n (RE = Ce (2), Nd (3), Gd (4), Dy (5), Y (6), Er (7), Yb (8)). All are 1-D coordination polymers with a mononuclear repeating unit, the La complex having a ten-coordinate metal, whilst 2–8 are nine-coordinate. The carboxylate groups of the 3-hydroxyphenylacetate ligand bind to the metal center through chelating, chelating bridging, and bidentate bridging coordination modes. Only in complex 1 does the 3-hydroxy group form intermolecular H-bonds. The composition of the bulk samples was shown to correspond to that of the single crystals by elemental analysis, thermogravimetric analysis, and X-ray powder diffraction. The infrared spectra were consistent with the binding modes. In weight loss corrosion experiments, Nd, Gd, Dy, Y, Er, and Yb complexes outperformed RE complexes of unsubstituted, 2-hydroxy- and 4-hydroxy-phenylacetates and 4-nitrophenylacetates, whilst electrochemically determined inhibitor efficiencies for the Nd, Gd, and Y complexes confirmed their excellent performance. The results indicate that 3-hydroxy substituted aromatic carboxylates, normally overlooked as corrosion inhibitors, may have a major future role.