Hydrolytic Synthesis and Structural Characterization of Lanthanide Hydroxide Clusters Supported by Nicotinic Acid

Polynuclear lanthanide hydroxide complexes featuring the cubane-like [Ln4(μ3-OH)4]8+ [Ln = Eu(III), Gd(III)] cluster core have been synthesized by controlled hydrolysis of the lanthanide ions using nicotinic acid as the ancillary ligand. The synthetic procedure has been found to significantly influence the nature of the resulting cluster species. In a one-pot synthesis, adjusting the pH of the reaction mixture containing Ln(ClO4)3 and nicotinic acid afforded tetranuclear complexes of the general formula [Ln4(μ3-OH)4(Hnic)5(H2O)12](ClO4)8 with the [Ln4(μ3-OH)4]8+ (Ln = Eu, Gd) cluster core encapsulated by zwitterionic nicotinate ligands. In stark contrast, mixing aqueous solutions of Ln(ClO4)3 and nicotinic acid whose pH had been preadjusted produced assemblies composed of two of the cubane-like cluster cores that are related by a crystallographic inversion center and are doubly bridged by nicotinate ligands using both the carboxylate group and pyridyl N atom for coordination. The influences of pH conditions and synthetic procedures on the identity of the resulting cluster species are discussed, so is the structural relevance of the low-pH complexes to their cluster analogues obtained under higher-pH conditions.