Helicity Induction in Helicate Self‐Organisation from Chiral Tris(bipyridine) Ligand Strands

Abstract

Dedicated to Vludimir Prelog on the occasion of his 85th birthday (25. X. 91)The chiral tris-and pentakis(bipyridine) ligand strands 3a and 4 were synthesised, each in optically pure form with (S,S)-configuration. Ligand 3a yielded substituted double-helical metal complexes, derived from the parent trihelicate structure 2, with Cu' and Ag' ions. The spectral data, in particular the 'H-NMR spectra and the large positive Cotton effect, indicate that helicate formation occurs with very high induction of helicity. Together with consideration of the steric effects in the two possible helical diastereoisomers (P)-1 and (M)-1 that may be formed, the data favour the preferential generation of the right-handed double helicate (P)-1 from the tris(bipyridine) strand 3a of (S,S)-configuration,

Self-Assembly of Tricuprous Double Helicates: Thermodynamics, Kinetics, and Mechanism

“…These data show a decreasing entropic contribution along the process and a high activation energy for the rearrangement of the helicate L H 2 Cu 3 . The mechanistic scheme is markedly complicated by the fact that the binding of two oligo(bipy) strands with Cu I ions may yield both non-entangled side-by-side and helical structures [75]. One could expect the formation of the first type to be kinetically faster than that of the second, where the ligands have to wrap around each other.…”

Self-assembly in self-organized inorganic systems: a view of programmed metallosupramolecular architectures

“…One method involves the synthesis of enantiomerically pure ligands with chiral links between the bipyridine units. In this way, the optically pure chiral ligand 46 with (S,S)-configuration was synthesized 102 . Its complexation with Cu + and Ag + yielded the substituted trinuclear helicate 47 ( Figure 15), and experimental data indicated that the product is formed with a very high induction of helicity.…”

“…Its complexation with Cu + and Ag + yielded the substituted trinuclear helicate 47 ( Figure 15), and experimental data indicated that the product is formed with a very high induction of helicity. Space-filling models and molecularmechanics calculations suggested that steric effects may be responsible for the preferential formation of the righthanded (P) double helicate, from the two possible helical diastereomers for helicate 47 102 . The induced sense of helicity by the presence of chiral centers in the ligand strands has been previously reported 66,103 and this highlights the importance of the ligand design in the organization of the binding centers during the selfassembly process aimed towards the formation of supramolecular species.…”

“Induced Fit” in Chiral Recognition: Epimerization upon Dimerization in the Hierarchical Self‐Assembly of Helicate‐type Titanium(IV) Complexes

“…[7] The introduction of enantiomerically pure ligands can afford diastereomerically and enantiomerically pure helicates. [8] Recently, hierarchically formed [9] helicates with bridging lithium cations were described, [10] in which three Li + ions connect two mononuclear complex moieties. In this case, the interlocking of the two propeller-type complexes, which allows effective coordination of the lithium ions, is only possible if the two propellers possess the same twist.…”