Supramolecular coordination chemistry in aqueous solution: Lanthanide ion-induced triple helix formation

The self-assembly of dinuclear triple helical lanthanide ion complexes (hel icates), in aqueous solution, is investigated utilizing laser-induced, lant hanide luminescence spectroscopy. A series of dinuclear lanthanide(III) hel icates (Ln(III)) based on 2,6-pyridinedicarboxylic acid (dipicolinic acid, dpa) coordinating units was synthesized by linking two dpa moieties using t he organic diamines (1R,2R)-diaminocyclohexane (chxn-R,R) and 4,4'-diaminod iphenylmethane (dpm). Luminescence excitation spectroscopy of the Eu3+ F-7( 0) --> D-5(0) transition shows the apparent cooperative formation of neutra l triple helical complexes in aqueous solution, with a [Eu2L3] stoichiometr y. Eu3+ excitation peak wavelengths and excited-state lifetimes correspond to those of the [Eu(dpa)(3)](3-) model complex. CD studies of the Nd(III) h elicate Nd-2(dpa-chxn-R,R)(3) reveal optical activity of the f-f transition s, indicating that the chiral linking group induces a stable chirality at t he metal ion center. Molecular mechanics calculations using CHARMm suggest that the Delta Delta configuration at the Nd3+ ion centers is induced by th e chxn-R,R linker. Stability constants were determined for both ligands wit h Eu3+, yielding identical results: log K = 31.6 +/- 0.2 (K in units of M-4 ). Metal-metal distances calculated from Eu3+ --> Nd3+ energy-transfer expe riments show that the complexes have metal-metal distances close to those c alculated by molecular modeling. The fine structure in the Tb3+ emission ba nds is consistent with the approximate D-3 symmetry as anticipated for heli cates.