Spiroborate-Based Double-Stranded Helicates: Meso-to- Racemo Isomerization and Ion-Triggered Springlike Motion of the...

A one-handed double-stranded spiroborate helicate exhibits a unique reversible extension-contraction motion coupled with a twisting motion in one direction triggered by binding and release of a Na⁺ ion while retaining its handedness. Here we report that an extended meso-helicate was also produced together with the racemo-helicate, and the meso-helicate was readily converted to the racemo-helicate assisted by a Na⁺ ion as a template in the presence of water. The thermodynamic analyses of the ion-triggered springlike motion of the racemo-helicate using a series of monovalent cations with different sizes revealed that the association constants of the extended racemo-helicate decreased in the following order: Li⁺ > Na⁺ > NH₄⁺ > Ag⁺ ≥ K⁺ > Cs⁺ > Rb⁺, which roughly agrees with the reverse order of their ionic radii except for the NH₄⁺ ion due to the more elongated contracted helicates when complexed with larger cations as supported by the crystal and DFT calculated structures. The one-handed contracted helicates showed characteristic CD spectra depending on the cation species due to the differences in their contracted helical structures, and its absolute handedness of the spiroborate helicate was determined by X-ray crystallography. The kinetic studies of the springlike motions of the racemo-helicate showed that the exchange rate between the extended and contracted helicates tend to increase in the following order: Li⁺ < Na⁺ < K⁺ ≤ NH₄⁺ < Rb⁺ < Cs⁺ < Ag⁺ as anticipated from the association constants, being in good agreement with the order of the cation sizes except for Ag⁺.