Fine‐Tuning the Spring‐Like Motion of an Anion‐Based Triple Helicate by Tetraalkylammonium Guests

Abstract

Supramolecular springs are a class of molecular devices that may provide implications to the macroscopic spring behavior from the molecular level. Helical structures are suitable molecular springs because the specific twisting of the helical strands can cause spring‐like (extension‐contraction) movement along the axis upon external stimuli. Herein we report an anion‐based triple‐helicate spring, which can undergo reversible contraction–extension motion through introduction and removal of tetraalkylammonium cations, including TMA+ and analogous irregular tetrahedral cations bearing different alkyl chains, while the relative orientation of the two phosphate ions changes to facilitate guest inclusion. Notably, the degree of contraction (shortening of the helical pitch) can be fine‐tuned by varying the shape and size of guest cation. However, with the larger cations (TEA+, TPA+ and TBA+), the meso‐helicate configuration is obtained, which interconverts with the helicate by addition/removal of TMA+ ion.

Fine‐Tuning the Spring‐Like Motion of an Anion‐Based Triple Helicate by Tetraalkylammonium Guests

“…Schematic representation of the crystal structures of triple helicate with different entrapped tetraalkylammonium guests, showing the influence of guest shape and orientation on the contraction motion (helical pitch decreases from left to right). [26] Scheme 1. Tetraalkylammonium cation guests studied in this work.…”

“…3À anion; c) TPA + ion is bound peripherally in the groove of the C-shaped ligand through cation•••p, CÀH•••p and CÀ H•••O interactions (blue-, green-and red-dashed lines, respectively). [26] Angewandte Chemie Zuschriften 9478 www.angewandte.de…”

“…3À ion and urea units, with converged (to O1 atom; b) and diverged (to the plane defined by O2, O3, O4 atoms; c) urea arms. [26] In solution, the transformation from the expanded version of triple helicate to the contracted one can be monitored by NMR studies. When adding 1 equiv of TMA + cations to H-[K(18-C-6)], the signal of methylene protons (H2) on the pxylylene spacer split into two peaks (d = 4.54, 2.95 ppm), and the encapsulation of TMA + cation was evident (d = 1.77 ppm versus 3.01 ppm of free TMA + ; Figure 5), indicating the replacement of one (peripheral) [K(18-C-6)] + in H-[K(18-C-6)] by TMA + ion to form the contracted (TMA)&H-[K(18-C-6)].…”

“…3À ion and six urea units; c) A TMA + ion is encapsulated in the central cavity through cation-p interactions. [26]…”

“…Figure 1. a) Crystal structure of triple helicate H-[K(18-C-6)] (only a P isomer of the racemic mixture is shown); b,c) Hydrogen bonds between each PO 43À ion and urea units, with converged (to O1 atom; b) and diverged (to the plane defined by O2, O3, O4 atoms; c) urea arms [26]. …”