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

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 (H 2 O) 2 ]·2H 2 O} n ( 1 ), and [RE(3hpa) 3 (H 2 O)] 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.