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Impact of a Mechanical Bond on the Activation of a Mechanophore Min Zhang, and Guillaume De Bo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08590 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018
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Journal of the American Chemical Society
Impact of a Mechanical Bond on the Activation of a Mechanophore Min Zhang and Guillaume De Bo* School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom Supporting Information Placeholder ABSTRACT: Mechanical bonds are known to efficiently
absorb mechanical energy at low forces but their behavior at high forces is unknown. Here we investigate the impact of a mechanical bond on the rate of activation of a Diels-Alder mechanophore. Using a combination of experimental and computational techniques, we found that the rate of a retro-Diels–Alder reaction under tension is decreased when the mechanophore is embedded in the axle of a rotaxane due to the presence of a competing high-stress region at the junction between the macrocycle and the axle.
Mechanical bonds, with their capacity for large amplitude movements, are ideal candidates for stress dampening materials.1 In slide-ring materials for example, rotaxane junctions act as pulleys to absorb mechanical energy.2 This architecture confers additional resilience to the material they are embedded in; nevertheless, an excess of mechanical energy will ultimately lead to the material failure. The impact of this mechanical bond on the resistance of the polymer backbone in the range of force leading to a mechanochemical bond rupture is still unknown. So far only a limited number of studies have been reported on the mechanical properties of the mechanical bond.3 The mechanical properties of catenanes,4a-b rotaxanes5a-e and, more recently, oligorotaxanes6a-b have been investigated at low strain rate by atomic force microscopy. Higher strain rates can be achieved in solution using high intensity ultrasounds,7 and the disassembly of rotaxane architectures has been reported using this technique (although the nature of the dissociation pathway remains unknown in these cases).8a-b Finally, Craig has investigated the mechanical resistance of catenanes and has shown that polymers containing catenane linkages display the same mechanical resistance than non-interlocked counterparts.9 Here we show that the mechanical activation of a Diels–Alder mechanophore can be slowed down when a rotaxane is used as a force actuator. This result suggests that the mechanical resistance of a polymer can be manipulated by the incorporation of a mechanical bond into the backbone.
Scheme 1. Rotaxane as a force-actuator in a mechanochemical retro-Diels–Alder reaction*
* Red arrows indicate the direction of the force. Conditions: (i) US (20 kHz, 13.0 W/cm2, 1s ON/2s OFF), CH3CN, 5-10˚C, 240 min.
We have recently shown that the rate of retro-Diels– Alder (rDA) reactions of furan/maleimide adducts under tension depends on the relative proximity of the polymer arms to the scissile bond (proximal vs distal) rather than the stereochemistry (endo vs exo) as observed in the thermal variant.10 In essence, the geometry of the adduct dictates how well the force is transduced from
the polymer to the scissile bond (mechanochemical coupling).
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Because of the particular way the force is transduced in a mechanical bond,3 we expect the interlocked architecture to impact the mechanochemical coupling. We have designed rotaxane 1 in which a benzo-21-crown-7 (B21C7) macrocycle11 is stoppered on one side by the proximal-exo DA adduct previously investigated,10 and on the other side by a large stopper (a tris(4chlorophenyl)propanoic acid derivative) that should prevent the deslipping of the macrocycle even under tension (Scheme 1). The desired chain-centered macromolecular [2]rotaxane12a-f was obtained by single electron transfer living radical polymerization (SET-LRP)13 of methyl acrylate (Table 1) initiated from both the macrocycle and the DA adduct (See Supporting Information for details). It was hypothesized that the application of an elongational force onto the polymer should initially pull the macrocycle away from its ammonium station and elongate the mechanical bond until the macrocycle reaches the terminal stopper (2, Scheme 1). From this point onwards, the DA adduct should be coupled to the mechanical force and further elongation of the rotaxane should eventually trigger the rDA reaction to release furan-terminated polymer 3 and rotaxane 4 (the maleimide is sufficiently large to act as a stopper for B21C7).14 We performed the mechanical activation of rotaxane 1 in acetonitrile at 5-10˚C, using high-intensity ultrasound (20 kHz, 13.0 W/cm2, 1s ON/2s OFF). The progress of the reaction was monitored by GPC and most of the starting polymer was cleaved after 240 min (see Supporting Information for details). 1H NMR analysis of the sonicated sample confirms that the dissociation proceeds via a rDA pathway (Figure 1). The emergence of characteristic signals associated with the aromatic protons of the furan unit (Ha, 7.4 ppm; Hb/Hc, 6.35 ppm) and the olefinic protons of the maleimide (He, 6.7 ppm) is diagnostic of the retrocycloaddition. Interestingly, rotaxane 1 is mechanically planar chiral and exists as a mixture of diastereoisomers (Smp epimer shown in Scheme 1),15 as indicated by the splitting of macrocycle protons H1 and methylene protons Hd. Upon dissociation, the stereochemical information of the DA adduct is lost and rotaxane 4 is recovered as a racemic mixture. This is reflected in the shift and coalescence of the methylene linker signals in furan 3 (∆δHd = 0.30 ppm) and the macrocycle proton H1 (see Supporting Information for details). To assess the effect of a rotaxane as a force actuator in the activation of a mechanophore, we compared the thermal and mechanical reactivity of rotaxane 1, uncoupled rotaxane 5 (in which the macrocycle is not covalently linked to the polymer backbone and hence not coupled to the force), and mechanophore 6 (Figure 2a). Thermally (95˚C in toluene), rotaxanes 1 and 5 undergo faster retrocycloaddition than the corresponding noninterlocked adduct 6 (Figure 2b and Table 1); probably due to steric hindrance induced by the macrocycle in the vicinity of the DA adduct. Mechanically, the rate of
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dissociation of the two controls (5 and 6) is virtually identical, meaning that the rotaxane architecture itself has little impact when uncoupled to the force. However, when a rotaxane architecture (1) is used as a force actuator, the cleavage of the DA adduct is slower (Figure 2c) with an apparent rate constant (kmech*) of 4.6×10-5 and 5.5×10-5 kDa-1.min-1 for rotaxanes 1 and 5 respectively (Table 1).16a-c
Figure 1. Partial 1H NMR (400/500 MHz, CDCl3) of rotaxane 1 before (a) and after (b) sonication (240 min), along with reference compounds 3ref (d) and 4ref (c). The lettering refers to assignment in Scheme 1. H O
a
O
∆ or
N
O
+
O
N
CO2Et
H O
O
RO
RO
O Br
R= O H O O O
N
O
H H N
CO2Me
PF 6-
H O
O 3
O H O
n
OR
O
N
PF 6-
O
O
H H N
H O
5
RO
RO
b
c
OR 3
6
Figure 2. a) Thermal and mechanical activation of rotaxane 1, uncoupled rotaxane 5, and mechanophore 6. b) Thermal activation. Conditions: Toluene, 95˚C. Conversion determined by 1H NMR. Solid lines correspond to a linear fit (R2 = 0.930, 0.979, and 0.921 for 1, 5, and 6 respectively). c) Mechanical activation. Changes in Mn determined by GPC. Conditions: US (20 kHz, 11.5 W/cm2, 1s ON/2s OFF),
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Journal of the American Chemical Society CH3CN, 5-10˚C. Solid lines correspond to a linear fit (R2 = 0.991, 0.994, and 0.991 for 1, 5, and 6 respectively). Each point corresponds to the average over 3 experiments. Error bars represent the standard deviation.
CoGEF calculations17 (DFT B3LYP/6-31G*) performed on rotaxane 1’ (Figure 3) shed some light on the origin of the polymer resilience when a rotaxane is used as a force actuator. In a flexible chain under tension, the elongation process occurs in two distinct phases. At low force, elasticity is mainly governed by entropic factors (i.e. the reduction of the number of conformations energetically accessible), while at higher force the elasticity is the mainly the result of enthalpic factors (resistance to bond deformation). Similarly, we observe that in the early stages of the elongation (∆D, Figure 3), the mechanical bond in 1’ bends and stretches without inducing any significant change (∆d) in the length of the scissile bond (Figure 3b). The transition between the mainly entropic (with a small enthalpic contribution from the host-guest interaction)18 and enthalpic regime coincides with the α angle plateauing at ~178˚. This indicates that the mechanical bond is fully stretched and any further elongation of the rotaxane induces the deformation of its constitutive covalent bonds. This provokes a sharp increase in the rate of deformation of the scissile bond (i.e. the mechanochemical coupling), ultimately leading to its rupture after a total elongation (∆D) of ~18.5Å. Surprisingly, the calculated force at rupture (Fmax) is lower in rotaxane 1’ than in reference adducts 5’ and 6’ (Table 1). The lower reactivity of rotaxane 1 is ultimately due to a weaker mechanochemical coupling compared to uncoupled rotaxane 5 and non-interlocked mechanophore 6 (Table 1, Figures 4b and S8). Crucially, the distribution of tensile stress in the intermediate of highest energy (Emax) reveals that in force-coupled rotaxane 1’, two regions of high tensile stress develop concomitantly (Figure 4a). The first one is common to all three DA adducts and is centered around the scissile bond. The second region, specific to force-coupled rotaxane 1’, is located where the macrocycle and the axle are in contact. The constriction of the axle by the stretched macrocycle induces a significant amount of deformation in the chain and results in the accumulation of tensile, bending, and torsional stress in this region of the mechanical bond (Figure S8c,e).19a-b The presence of a competing highstress region effectively diminishes the mechanochemical coupling around the DA adduct and explains the slower cleavage of polymer 1 upon sonication. Table 1. Structural and activation parameters of polymers 1, 5, and 6. PolMn ymer (kDa)
Đ
ktherm* b
(%/min)
kmech* c -1 -1
(kDa .min )
Fmax Cou(nN)a pling (Å-1)d
1
56
1.19
0.33
4.6×10-5
3.22
0.39
5
56
1.20
0.30
5.5×10-5
3.77
1.17
6
60
1.15
0.26
5.5×10-5
3.48
1.13
a
Determined from CoGEF calculations (see Supporting Information for details). b Determined from the slope of the linear fit in Figure 2b. c Determined from the slope of the linear fit in Figure 2c. d Determined from the slope of the exponential fit in Figure S8b.
Figure 3. Evolution of energy (top), and structural parameters α and ∆d (bottom), of rotaxane 1’ upon simulated elongation (CoGEF, DFT B3LYP/6-31G*). a 1'
5'
Tensile stress / %
5' 0
2
4
6
8
10 12
b
Figure 4. a) Distribution of tensile stress in model rotaxanes 1’ and 5’ at Emax. b) Evolution of tensile stress in the scissile bond in the enthalpic regime for 1’, 5’, 6’ (R = Piv). Values obtained from CoGEF calculations (DFT B3LYP/631G*). See Supporting Information for details.
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In conclusion, we have shown that the introduction of a mechanical bond in the vicinity of a mechanophore can influence its rate of activation. The slower activation of a DA adduct embedded in the axis of a rotaxane is due to the presence of a competing high-stress region that decreases the mechanochemical coupling at the scissile bond. This opens the possibility of manipulating the mechanical resistance of a polymer by incorporating mechanical bonds at strategic positions within the backbone. Indeed, the high tensile stress region developing at the rotaxane junction has the potential to either decrease, as observed here, or increase the rate of scission of a polymer chain depending on the presence or not of a competing weak bond. This latter concept is currently under investigation in our lab. ASSOCIATED CONTENT Supporting Information Detailed descriptions of CoGEF calculations, synthetic procedures, characterization of new compounds, and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
*
[email protected] ACKNOWLEDGMENT We thank the Royal Society for a Newton International Fellowship to M.Z. and a University Research Fellowship to G.D.B.
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Ashcroft, B. A.; Spadola, Q.; Qamar, S.; Zhang, P.; Kada, G.; Bension, R.; Lindsay, S. Small 2008, 4, 1468–1475. (d) Lussis, P.; Svaldo-Lanero, T.; Bertocco, A.; Fustin, C.-A.; Leigh, D. A.; Duwez, A.S. Nat. Nanotech. 2011, 6, 553–557. (e) Bowman, K. A.; Aarstad, O. A.; Stokke, B. T.; Skjåk-Bræk, G.; Round, A. N. Langmuir 2016, 32, 12814–12822. (6) (a) Sluysmans, D.; Devaux, F.; Bruns, C. J.; Stoddart, J. F.; Duwez, A.-S. Proc. Natl. Acad. Sci. U.S.A. 2017, 10, 201712790. (b) Sluysmans, D.; Hubert, S.; Bruns, C. J.; Zhu, Z.; Stoddart, J. F.; Duwez, A.-S. Nat. Nanotech. 2018, 47, 209–213. (7) May, P. A.; Moore, J. S. Chem. Soc. Rev. 2013, 42, 7497–7506. (8) (a) Stoll, R. S.; Friedman, D. C.; Stoddart, J. F. Org. Lett. 2011, 13, 2706–2709. (b) Sagara, Y.; Karman, M.; Verde-Sesto, E.; Matsuo, K.; Kim, Y.; Tamaoki, N.; Weder, C. J. Am. Chem. Soc. 2018, 140, 1584–1587. (9) Lee, B.; Niu, Z.; Craig, S. L. Angew. Chem. Int. Ed. 2016, 55, 13086–13089. (10) Stevenson, R.; De Bo, G. J. Am. Chem. Soc. 2017, 139, 16768–16771. (11) Zhang, C.; Li, S.; Zhang, J.; Zhu, K.; Li, N.; Huang, F. Org. Lett. 2007, 9, 5553–5556. (12) For recent examples of mechanically linked polymers see: (a) De Bo, G.; De Winter, J.; Gerbaux, P.; Fustin, C.-A. Angew. Chem. Int. Ed. 2011, 50, 9093–9096. (b) Aoki, D.; Uchida, S.; Takata, T. Polym. J. 2014, 46, 546–552. (c) Aoki, D.; Uchida, S.; Takata, T. ACS Macro Lett. 2014, 3, 324–328. (d) Daisuke, A.; Satoshi, U.; Takata, T. Angew. Chem. Int. Ed. 2015, 54, 6770–6774. (e) Sato, H.; Aoki, D.; Takata, T. ACS Macro Lett. 2016, 699–703. (f) De Bo, G.; Gall, M. A. Y.; Kuschel, S.; De Winter, J.; Gerbaux, P.; Leigh, D. A. Nat. Nanotech. 2018, 13, 381–385. (13) Anastasaki, A.; Nikolaou, V.; Nurumbetov, G.; Wilson, P.; Kempe, K.; Quinn, J. F.; Davis, T. P.; Whittaker, M. R.; Haddleton, D. M. Chem. Rev. 2016, 116, 835–877. (14) No dethreading was observed after heating rotaxane S19 for 4 hours at 95˚C (see Supporting Information for details). (15) Jamieson, E. M. G.; Modicom, F.; Goldup, S. M. Chem. Soc. Rev. 2018, 47, 5266–5311. (16) (a) Sato, T.; Nalepa, D. E. J. Appl. Polym. Sci. 1978, 22, 865− 867. Although this method is known to underestimate the rate of degradation of mechanophore-centered polymers (see (b) Akbulatov, S.; Boulatov, R. ChemPhysChem 2017, 18, 1422−1450), it has proved effective to compare the rate of activation of structurally similar mechanophores (for recent examples, see ref 10 and (c) Kryger, M. J.; Munaretto, A. M.; Moore, J. S. J. Am. Chem. Soc. 2011, 133, 18992−18998. (17) Beyer, M. J. Chem. Phys. 2000, 112, 7307–7312. (18) The energy associated with the dissociation of the ammoniumcrown ether interaction is probably overestimated because GoGEF calculations were performed in vacuum in the absence of a counterion. (19) A similar weakening of a molecular chain at the exit of a loop is also predicted to occur in knots: (a) Stauch, T.; Dreuw, A. Angew. Chem. Int. Ed. 2016, 55, 811–814. (b) Klein, M. L.; Saitta, A. M.; Soper, P. D.; Wasserman, E. Nature 1999, 399, 46–48.
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