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.2016 Mar 1;55(10):3328-33.
doi: 10.1002/anie.201509702. Epub 2016 Feb 2.

Synthesis and Dynamics of Nanosized Phenylene-Ethynylene-Butadiynylene Rotaxanes and the Role of Shape Persistence

Affiliations

Synthesis and Dynamics of Nanosized Phenylene-Ethynylene-Butadiynylene Rotaxanes and the Role of Shape Persistence

Christopher Schweez et al. Angew Chem Int Ed Engl..

Abstract

Phenylacetylene-based [2]rotaxanes were synthesized by a covalent-template approach by aminolysis of the corresponding prerotaxanes. The wheel and the bulky stoppers are made of phenylene-ethynylene-butadiynylene macrocycles of the same size. The stoppers are large enough to enable the synthesis and purification of the rotaxane. However, the wheel unthreads from the axle at elevated temperatures. The deslipping kinetics and the activation parameters were determined. We described theoretically the unthreading by state-of-the-art DFT-based molecular-mechanics models and a string method for the simulation of rare events. This approach enabled us to characterize in detail the unthreading mechanism, which involves the folding of the stopper during its passage through the wheel opening, a process that defies intuitive geometrical considerations. The conformational and energetic features of the transition allowed us to infer the molecular residues controlling the disassembly timescale.

Keywords: density functional calculations; interlocked systems; molecular modeling; rotaxanes; template synthesis.

© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

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Figures

Figure 1
Figure 1
Top: Two novel [2]rotaxanes,1@2 a and1@2 b, based on shape‐persistent macrocycles. Bottom: Illustration of the short‐term dynamic behavior of the [2]rotaxane: the shuttling of the wheel along the axis.
Scheme 1
Scheme 1
a) [PdCl2(PPh3)2], CuI, PPh3, THF, NEt3, 60 °C, 18 h,5 a: quant.,5 b: 51 %; b) TBAF, H2O (4 vol %), THF, room temperature, 50 h,6 a: 61 %,6 b: 96 %; c) [PdCl2(PPh3)2], CuI, toluene, NEt3, 100 °C, 18 h,8 a: 38 %,8 b: 22 %; d) n‐propylamine, NH4Cl, THF, 50 °C, 16 h,1@2 a: 21 %,1@2 b: 13 %. CPDIPS=(3‐cyanopropyl)diisopropylsilyl.
Figure 2
Figure 2
Precursor isomers. Only isomer A features prerotaxane geometry. The aminolysis of isomer A leads to the desired [2]rotaxane, whereas isomer B generates the free wheel and axis.
Figure 3
Figure 3
Theoretical description of the unthreading. a) Gibbs free energy (black line), entropy times temperature (red line), solvation free energy (green line), and bonding part of the enthalpy (blue line) of the optimized finite‐temperature string. A: associated state, T1–T3: transition regions, D: dissociated state. b) Conformational changes during the unthreading. The axle is depicted in blue and the wheel in red. The left and right columns show a top and a side view of the rotaxane, and the rows correspond to the regions in (a) according to the same naming convention. In the left column, the green vectors describe the angle formed by the phenyl rings of the stoppertert‐butylphenyl residues and the phenyl group at the attachment point. This angle gradually decreases, which correlates with the stopper folding. During folding, strain builds up mainly in the bisacetylene and phenoxy groups, as shown in (c). In the right column, the orange vectors describe the angle between the axis and the bisector of the angle formed by the green vectors. This angle steadily increases and portrays the rotation of the stopper about the attachment point to the axis during the passage through the wheel opening. c) Van der Waals representation of the transition state T2, in which only the actively participating wheel (metallic color) and stopper (full color) are depicted. The entire opening of the wheel is occupied by the bulky substituents of the stopper. The atom color depicts the change in the energy per atom with respect to the associated state (see the Supporting Information for a precise definition). The color ranges from blue to red as the magnitude of energy deviations increases. The red regions therefore correspond to hot spots of energy accumulation.
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