Lanthanide Coordination with α-Amino Acids under Near Physiological pH Conditions: Polymetallic Complexes Containing the Cubane-Like [Ln4(μ3-OH)4]8+ Cluster Core

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Abstract
Tetranuclear lanthanide−hydroxo complexes of the general formula [Ln4(μ3-OH)4(AA)x(H2O)y]8+ (1, Ln = Sm, AA = Gly, x = 5, y = 11; 2, Ln = Nd, AA = Ala, x = 6, y = 10; 3, Ln = Er, AA = Val, x = 5, y = 10) have been prepared by α-amino acid controlled hydrolysis of lanthanide ions under near physiological pH conditions (pH 6−7). The core component of these compounds is a cationic cluster [Ln4(μ3-OH)4]8+ whose constituent lanthanide ions and triply bridging hydroxo groups occupy the alternate vertexes of a distorted cube. The amino acid ligands coordinate the lanthanide ions via bridging carboxylate groups. Utilizing l-glutamic acid as the supporting ligand, a cationic cluster complex (4) formulated as [Er4(μ3-OH)4(Glu)3(H2O)8]5+ has been obtained. Its extended solid-state structure is composed of the cubane-like [Er4(μ3-OH)4]8+ cluster building units interlinked by the carboxylate groups of the glutamate ligands. All compounds are characterized by using a combination of spectroscopic techniques and microanalysis (CHN and metal). Infrared spectra of the complexes suggest the coordinated amino acids to be zwitterionic. The presence of mass (MALDI-TOF) envelopes corresponding to the [Ln4(μ3-OH)4]8+ (Ln = trivalent Sm, Nd, or Er) core containing fragments manifests the integrity of the cubane-like cluster unit. Magnetic studies using Evans' method suggest that exchange interactions between the lanthanide ions are insignificant at ambient temperature. The structural identities of all four compounds have been established crystallographically. The tetranuclear cluster core has been demonstrated to be a common structural motif in these complexes. A mechanism responsible for its self-assembly is postulated.

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