Europium-based dinuclear triple-stranded helicate vs. tetranuclear quadruple-stranded helicate : effect of stoichiometric ratio on the supramolecular self-assembly

Two europium-based helicates, namely the dinuclear triple-stranded helicate [Eu2L3] (1) and tetranuclear quadruple-stranded helicate [Eu4L4(NO3)](3+) (2), were achieved by the self-assembly of the succinohydrazone derived bis-tridentate ligand (H2L) and Eu3+ ions in 1:1 and 1:2 ratios, respectively. Both the helicates were characterized by single-crystal X-ray analysis. Structural analysis of both the helicates revealed optically inactive 50:50 mixtures of left- (Lambda) and right-hand (Delta) helicates in one unit cell. In 1, both the europium centers lie along the three-fold axis whereas in 2, four europium centers are arranged in a triangular fashion around the four-fold axis, which passes through the center of the quadruple stranded helicate. In 2, the presence of a nitrate ion on the nucleation of the aesthetically pleasing tetranuclear quadruple stranded helicate is significant. Apart from the molar ratio, the role of the bridging modes of the ligand, anion and noncovalent interaction on the self-assembly of two helicates are also discussed