4-Hydroxybenzoato-rare earth(iii) complexes – syntheses, a structural goldmine from coordination diversity and improved corrosion inhibition behaviour

Abstract

A range of rare earth(III) 4-hydroxybenzoato (4hob) complexes has been prepared by salt metathesis reactions in water and structurally characterized by X-ray single crystal and powder diffraction. Complexes in which 2,2′-bipyridine (bpy) replaces coordinated water were also prepared to compare their anti-corrosion properties, but were largely structurally elusive. [La(4hob)₃(H₂O)₂]_n and {[Ce(4hob)₃(H₂O)₂]·3H₂O}_n were obtained as carboxylate-bridged polymeric sheets, which differ in lattice water, arene ring orientations and the H₂O–Ln–OH₂ angles, but the bulk powders have the same structure as the La single crystals. The yttrium analogue is a polymeric chain, and [Pr₄(4hob)₁₂(H₂O)₈]·4H₂O is a dimer of dinuclear units. Smaller rare earths mainly formed dimeric species, namely [Tb₂(4hob)₆(H₂O)₄]·4H₂O, [RE₂(4hob)₆(H₂O)₄]·2H₂O (RE = Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Lu), and [RE₂(4hob)₆(H₂O)₆]·10H₂O (RE = Ho, Y, Er). The first two classes differ in having anti–anti and syn–anti carboxylate bridging, and cis and transoid diaqua ligands respectively. The last class not only has an additional water coordinated to each RE atom, but also has a terminal unidentate ligand bonded to each RE atom in place of a chelating 4hob of the first two classes, thereby maintaining eight-coordination. Thus, considerable coordination diversity is observed particularly for RE = Tb, Ho, Y, Er. The bpy complexes, RE(4hob)₃(bpy), were largely isolated as amorphous powders and were characterized by elemental analysis and IR spectroscopy. However [RE₂(4hob)₆(bpy)₂]·2bpy·4H₂O (RE = Tb, Dy) crystallized as dimers with four bridging (syn–syn) 4hob ligands and one chelating 4hob and one chelating bpy bound to each RE atom. The La, Ce and Y aqua complexes are good corrosion inhibitors for mild steel in 0.01 M NaCl solution with Y > La > Ce in order of effectiveness. Moreover, all aqua 4hob complexes examined outperformed the established inhibitor, cerium(III) salicylate. For RE = La or Y, replacement of coordinated water by bpy improved the anti-corrosion performance.