Moon, Josh, Guo, Zhifang, Beaumont, Owen A., Hamilton, Sophie, Bousrez, Guillaume, Mottram, Eleanor, Wang, Jun, Somers, Anthony E., Deacon, Glen B., and Junk, Peter C. (2023) 4-Hydroxybenzoato-rare earth(iii) complexes - syntheses, a structural goldmine from coordination diversity and improved corrosion inhibition behaviour. New Journal of Chemistry, 47 (36). pp. 1666843-16854.

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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.

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