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

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)]center dot 3H(2)O}(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 [Pr-4(4hob)(12)(H2O)(8)]center dot 4H(2)O is a dimer of dinuclear units. Smaller rare earths mainly formed dimeric species, namely [Tb-2(4hob)(6)(H2O)(4)]center dot 4H(2)O, [RE2(4hob)(6)(H2O)(4)]center dot 2H(2)O (RE = Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Lu), and [RE2(4hob)(6)(H2O)(6)]center dot 10H(2)O (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)]center dot 2bpy center dot 4H(2)O (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 anticorrosion performance.

Automated summary

Rare earth(III) 4-hydroxybenzoato (4hob) complexes were prepared in water and characterized by X-ray diffraction. Different structures were observed for La and Ce complexes, while Y complex showed good anticorrosion properties. The La, Ce, and Y aqua complexes exhibited better corrosion inhibition than cerium(III) salicylate. Replacement of coordinated water by bpy improved the anticorrosion performance for La and Y complexes.