Lanthanide coordination with alpha-amino acids under near physiological pHconditions: Polymetallic complexes containing the cubane-like [Ln(4)(mu(3)-OH)(4)](8+) cluster core

Tetranuclear lanthanide-hydroxo complexes of the general formula [Ln(4 mu ( 3)-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 bee n prepared by a-amino acid controlled hydrolysis of lanthanide ions under n ear physiological pH conditions (pH 6-7). The core component of these compo unds is a cationic cluster [Ln(4)(mu (3)-OH)(4)](8+) whose constituent lant hanide ions and triply bridging hydroxo groups occupy the alternate vertexe s of a distorted cube. The amino acid ligands coordinate the lanthanide ion s via bridging carboxylate groups. Utilizing L-glutamic acid as the support ing ligand, a cationic cluster complex (4) formulated as [Er-4(mu (3)-OH)(4 )(Glu)(3)(H2O)(8)](5+) has been obtained. Its extended solid-state structur e is composed of the cubane-like [Er-4(mu (3)-OH)(4)](8+) cluster building units interlinked by the carboxylate groups of the glutamate ligands. All c ompounds are characterized by using a combination of spectroscopic techniqu es and microanalysis (CHN and metal). Infrared spectra of the complexes sug gest the coordinated amino acids to be zwitterionic. The presence of mass ( MALDI-TOF) envelopes corresponding to the [Ln(4)(mu (3)-OH)(4)](8+) (Ln = t rivalent Sm, Nd, or Er) core containing fragments manifests the integrity o f the cubane-like cluster unit. Magnetic studies using Evans' method sugges t that exchange interactions between the lanthanide ions are insignificant at ambient temperature. The structural identities of all four compounds hav e been established crystallographically. The tetranuclear cluster core has been demonstrated to be a common structural motif in these complexes. A mec hanism responsible for its self-assembly is postulated.