Chemically Induced Mismatch of Rings and Stations in [3]Rotaxanes

Abstract

The mechanical interlocking of molecular components can lead to the appearance of novel and unconventional properties and processes, with potential relevance for applications in nanoscience, sensing, catalysis, and materials science. We describe a [3]rotaxane in which the number of recognition sites available on the axle component can be changed by acid-base inputs, encompassing cases in which this number is larger, equal to, or smaller than the number of interlocked macrocycles. These species exhibit very different properties and give rise to a unique network of acid-base reactions that leads to a fine pK_a tuning of chemically equivalent acidic sites. The rotaxane where only one station is available for two rings exhibits a rich coconformational dynamics, unveiled by an integrated experimental and computational approach. In this compound, the two crown ethers compete for the sole recognition site, but can also come together to share it, driven by the need to minimize free energy without evident inter-ring interactions.

Figure 1

Cartoon representation of [2]rotaxanes (left) and [3]rotaxanes (right) in which the number of recognition sites, or stations (n_S), is respectively (a, d) equal to, (b, e) higher than, and (c, f) lower than the number of macrocyclic rings (n_R). The blue arrows in b and c highlight the kind of translational movements that are typically observed in these rotaxanes.

Scheme 1. Structural Formulas and Acid–Base Triggered Interconversion of the [3]rotaxanes Studied in This Work

Scheme 2. Synthetic Route to the [3]rotaxane RotH₂³⁺: (i) BrCH₂CCH, K₂CO₃, MeCN; (ii) HPF₆, THF; (iii) BrC₂H₄Br, K₂CO₃, MeCN; (iv) NaN₃, DMF; (v) HPF₆, THF; (vi) DB24C8, [(MeCN)₄Cu][PF₆], CH₂Cl₂; (vii) MeI, KPF₆

Figure 2

¹H NMR spectra (500 MHz, CD₂Cl₂, 298 K) of (a) trication RotH₂³⁺ (red trace) and the species observed upon deprotonation with the heterogeneous phosphazene base B1: (b) mixture of dications RotH²⁺-I and RotH²⁺-II (blue trace), and (c) monocation Rot⁺ (green trace). The peak marked with an asterisk is due to the residual solvent.

Figure 3

¹H NMR chemical shift variations upon deprotonation of RotH₂³⁺ into Rot⁺ (500 MHz, CD₂Cl₂, 298 K). The Δδ values in red and blue correspond to shielding and deshielding effects, respectively. For clarity, the two DB24C8 rings and the anion have been omitted.

Figure 4

Schematic representation of the equilibria involved in the interconversion between RotH₂³⁺, RotH²⁺ (I and II), and Rot⁺.

Figure 5

(a) Portion of the variable-temperature ¹H NMR spectra of Rot⁺ (500 MHz, CD₃CN). (b) Magnification of the chemical shift variation of the triazolium peak H_Tz. For peak assignment refer to Figure 2.

Figure 6

Free energy profile (kcal mol^–1) for the shuttling of the two macrocycles along the axle in Rot⁺. The collective variable CV (in Å) represents the displacement of the ethereal oxygen atoms of the two rings with respect to the Tz nitrogen atoms (see the Supporting Information for details). The three different coconformations of Rot⁺ characterized by deep free energy minima are labeled A, B, and C.

Figure 7

Graphical representations of the three coconformations extracted from the combined Metadynamics-DFT simulation of Rot⁺ at room temperature. The reported coconformations are representative of the three free energy minima labeled A, B, and C in Figure 6. Color code: C, cyan; N, blue; O, red; H, white. The corresponding structural formulas are also shown for clarity.

Figure 8

Ball-and-stick representation of a triazolium unit encircled by a macrocycle (red oval). Color code: C, cyan; N, blue; H, white. The red arrows represent two nonequivalent shuttling directions for the macrocycle.

Figure 9

Snapshots taken from the DFT-metadynamics simulation of Rot⁺. Top panels: right-to-left transit of a DB24C8 macrocycle over the Tz station, describing the B → A path. Bottom panels: left-to-right transit of a DB24C8 macrocycle over the Tz station, describing the B → C path. Color code: H, white; N, blue; O, red. C atoms belonging to the axle in gray, C atoms belonging to the DB24C8 rings in yellow, C atoms of the triazolium group in orange. Atoms of the axle are represented as spheres, atoms of the macrocycles are represented as sticks.