Assembling lanthanide clusters under physiological or higher pH-conditions

High-nuclearity lanthanide complexes, having unique structures and potentially important applications, have been prepared via limited hydrolysis of lanthanide ions controlled by α-amino acids as supporting ligands. These novel complexes, synthesized under physiological or higher-pH conditions, exist as lanthanide-hydroxo clusters, reflecting the subtle balance between the hydrolysis of the metal ions and the multidentate supporting coordination of the ligands. A cuboid [Ln₄(μ₃-OH)₄]⁸⁺ unit with four lanthanide ions and four triply bridging hydroxo ligands occupying the alternate vertices of a distorted cube has been identified as a prevalent structural motif in these complexes, wherein lanthanide complexation may be achieved via only the carboxylate group or simultaneous coordination of both the carboxylate and the amino moieties of an amino acid ligand. Bridging ligands with more than one carboxylate group may support open framework structures utilizing the [Ln₄(μ₃-OH)₄]⁸⁺ unit as building blocks. In addition, "wheel-like" structures composed of four or five vertex-sharing [Ln₄(μ₃-OH)₄]⁸⁺ units can be realized by the halide-templated synthesis. It has been further demonstrated that octahedral [Ln₆(μ₆-O)(μ₃-OH)₈] ⁸⁺ core-containing clusters with an interstitial μ₆-O ligand can be self-assembled via direct hydrolysis of lanthanide nitrates and perchlorates in the absence of any organic supporting ligands. Using a structurally unprecedented lanthanide-EDTA complex, the significant influence of pH conditions on the reaction outcome is illustrated.