Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes

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Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion Christopher R. Benson, Andrew I. Share, Matthew G. Marzo, and Amar H. Flood* Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: The existence of two rings in [3]pseudorotaxanes presents opportunities for those rings to undergo double switching and cooperative mechanical coupling. To investigate this capability, we identified a new strategy for bringing two rings into contact with each other and conducted mechanistic studies to reveal their kinetic cooperativity. A redox-active tetrazine ligand bearing two binding sites was selected to allow for two mobile copper(I) macrocycle ring moieties to come together. To realize this switching modality, ligands were screened against their ability to serve as stations on which the rings are initially parked, ultimately identifying 5,5′-dimethyl-2,2′-bipyridine. The kinetics of switching a macrocycle in a single-site [2]pseudorotaxane between bipyridine and single-site tetrazine stations were examined using electrochemistry. The forward movement was rate-limited by the bimolecular reaction between reduced tetrazine and bipyridine [2]pseudorotaxane. Two bipyridines were then used with a double-site tetrazine to verify double switching of two rings. Our results indicated stepwise movements, with the first ring moving 4 times more frequently (faster) than the second. While this behavior is indicative of anticooperative kinetics, positive thermodynamic cooperativity sets the two rings in motion even though just one tetrazine is reduced with one electron. Double switching in this [3]pseudorotaxane uniquely demonstrates how a series of independent thermodynamic states and kinetic paths govern an apparently simple mechanical motion.

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INTRODUCTION Molecular switches bearing two mobile rings, like [3]pseudorotaxanes and [3]rotaxanes, can access properties not possible when there is just the one ring.1−6 Perhaps the most iconic use of the two rings is amplification of the fewnanometer, inter-ring contractile motions for the reversible upand-down bending of a 0.5 mm cantilever beam.7 These muscle-like behaviors8,9 rely upon a detailed understanding of how two rings come together by undergoing double switching. More generally, determining the characteristics of multi-ring systems helps lay a foundational understanding of why (driving forces) and how (pathways) matter reorganizes itself when it responds to changes in its recognition elements. Furthermore, with multiple components, interactions between them can lead to the emergence of complex behaviors most commonly observed in biology, like cooperativity. The examination of these characteristics is aided by the high designability10 (forward engineering) of multicomponent architectures,11−13 as exemplified by [3]pseudorotaxanes14 and [3]rotaxanes,7a,15 which bear two rings encircling a central rod component. Among the other multi-ring systems that have been studied, some can switch between mechanical states,5,16,17 and some show control over the motion of multiple components.15j,18,11,16d Motivated by the need to understand these types of movements in detail, we used reverse engineering of © XXXX American Chemical Society

[3]rotaxanes to identify multi-ring pseudorotaxanes (Figure 1a) that are capable of double switching and used mechanistic studies to reveal their cooperative motions. Herein, we investigate [3]pseudorotaxanes (Figure 1a) that allow for the characterization of motion in a double-ring system. We start by employing a reverse engineering approach that allows us to identify a novel mode of double switching. To investigate that switching modality, we take advantage of a double-site tetrazine ligand but need to first identify a complementary ligand site. To this end, we compare a series of ligands (Figure 2) using cyclic voltammetry (CV) and identify one that could serve as the ligand to complement tetrazine-based switchable [2]pseudorotaxanes, [2]PR. Using this method, an ideal candidate was identified as 5,5′-dimethyl2,2′-bipyridine (bipy), and the relative stabilities of the bipyridine- and tetrazine-derived [2]pseudorotaxanes were quantified. We were able to build a kinetic model for the switching cycle and uncover the activation parameters (ΔG⧧, ΔH⧧, and ΔS⧧) for the rate-limiting step of the forward motion. Finally, we verified that tetrazine could be used to create a palindromic [3]pseudorotaxane, [3]PR, that undergoes double switching. We also determined how fast the first and second Received: November 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02554 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

switches (Figure 3, top) as a starting point, disconnection strategies can reverse engineer the double switching. Such [3]rotaxanes have two central (pink) and two outer (green) stations to define contracted and extended mechanical states for the rings, respectively. Disconnection along pathway A (Figure 3) deconstructs contraction into a pair of [2]rotaxanes capable of two single movements. This approach has been applied7a and exploits knowledge of mechanical switches.19 A complementary approach (pathway B, Figure 3), tested for the first time here, disconnects the two outer stations while fusing together the two inner stations. The two-station core defines a [3]pseudorotaxane (Figure 3, middle). Even though [3]pseudorotaxanes14 and their congeners (2:1 receptor/guest complexes20) are prevalent, it is rare to find ones that can also undergo switching.16a,f,17 Of these, it is rarer still to find ones16a displaying the symmetric double switching of two rings wherein a single stimulus event moves two rings. This outcome is mapped onto functional target I (Figures 1 and 3). Given that this contractile functionality is key to exploiting pathway B, it is important to first establish the viability of controllable double switching by using model systems. The rational design of such a model switch21 in the form of a pseudorotaxane requires an understanding of how stimuli can alter the noncovalent bonding between components and the identification of primary and secondary binding sites for the mobile ring. For the first requirement, we make use of a redoxactive tetrazine ligand, 3,6-bis(5′-methyl-2-pyridyl)-1,2,4,5tetrazine (BPTz; Figure 1a), threaded inside ring components constituted by copper(I) macrocycles (Figures 1 and 2).9d Alternative approaches with metal−ligand systems could take advantage of metal-centered oxidations22 and pH-modulated changes to the ligands.23 Unique to the tetrazines, however, is that they can be easily toggled between weak and strong ligands. Following reduction, negatively charged tetrazines bind copper(I) ions more tightly than when the tetrazines are neutral. In addition, ligands like BPTz can house two copper(I) macrocycles at the same time.17 In prior studies, however, it was shown that just one ring switched positions between tetrazine-based pseudorotaxanes. Only when two rings can be made to move will the palindromic symmetry latent to the BPTz tetrazine enable double switching (Figure 1a). For the second requirement, a complementary ligand is needed on which to park the two copper(I) macrocycles prior to the tetrazine’s reduction. Instead of comparing possible ligands at the same time as assessing the viability of double switching, we lowered the complexity by invoking a further disconnection along pathway C. This strategy allows us to investigate just one mobile ring, which will generate simpler electrochemical signatures that are easier to interpret. Thus, functional target II at the end of pathway C lays the foundation for the double switching system, functional target I. Characteristics of a Secondary Switching Partner. The first goal was to identify a ligand capable of undergoing exchange with redox-active tetrazines. Within the format of a [2]pseudorotaxane, we compared a small selection of redoxinactive ligands (green, Figure 2a) with 3-(5′-chloro-2′-pyridyl)6-(p-tolyl)-1,2,4,5-tetrazine (PTTz, colored pink in Figures 1b and 4). The PTTz ligand has a single binding site for copper(I) in order to direct formation of the [2]pseudorotaxane using a phenanthroline-based macrocycle24 (M; Figure 2b). The desired switching mode (Figure 1b) requires the candidate ligand to be a stronger coordination partner than the neutral tetrazine and then a weaker partner compared to the reduced

Figure 1. (a) Scheme of double switching for moving two macrocycles from two [2]pseudorotaxanes to a single [3]pseudorotaxane upon reduction of the tetrazine ligand BPTz. (b) Redox-driven scheme of a bistable [2]pseudorotaxane that switches the copper(I) macrocycle between bipyridine (bipy, green) and reduced tetrazine (PTTz, pink) ligands.

rings moved and observed negative kinetic cooperativity and positive thermodynamic cooperativity, novel features that add complexity to the operation of molecular machines.17a The discovery of such double switching opens a new route toward contractile molecular machines.

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RESULTS AND DISCUSSION Retrofunctional Analysis of a Two-Ring Switching Core and Design Strategy. Using palindromic [3]rotaxane B


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Figure 2. (a) Candidate ligands used in screening for the switching partner and (b) the macrocycle used in the pseudorotaxanes.

Figure 4. Stability diagram showing thermodynamic design criteria for switching. The free energies of pseudorotaxane formation for the neutral (pink) and reduced (blue) tetrazine are separated by a gap of approximately 30 kJ mol−1. An optimal switching partner (green) will allow Boltzmann population swings of around 100:1, a requirement that narrows the ideal stability window.

with copper(I) ions. The ligands selected, listed in order of their coordination strength, are derivatives of phenanthroline, isoquinoline, bipyridine, and pyridyltriazole.26 Yet, it is not possible to know a priori which one would be ideal. Identifying a Secondary Ligand Using CV. Electrochemical characterization of PTTz and its [2]pseudorotaxane (Figure 5a) establish peak positions to aid in analysis of the switching mixture. A CV of the PTTz ligand (Figure 5a, pink trace) shows a reversible redox couple at E1/2 = −0.995 V (vs Ag/AgCl). When the tetrazine forms the associated [2]pseudorotaxane, [2]PRPTTz+ (Figure 5a, blue trace), its reduction is stabilized by 280 mV to E1/2 = −0.715 V with a residual 5% of the free ligand present as related by mass balance. The positions of these two reduction potentials are consistent with previous work on tetrazines27 and thus reflect the redox properties of PTTz and its copper(I)-coordinated [2]pseudorotaxane, respectively. This potential difference corresponds to 27 kJ mol−1 of stabilization. The ideal ligand must first be strong enough to outcompete neutral PTTz for formation of a [2]pseudorotaxane with the copper(I) macrocycle (CuM+) moiety. When equimolar ratios of PTTz, the copper(I) macrocycle, and the candidate ligand are mixed together, we would like to see the formation of a [2]pseudorotaxane composed of the candidate plus 1 equiv of uncoordinated PTTz. This form of PTTz should show up as a cathodic reduction peak (Epc) in the negative running sweep of the CV situated at the same position as a solution of PTTz alone, at ca. −1.05 V. A comparison of the candidate ligands

Figure 3. Retrofunctional analysis (retrosynthesis of a f unctional core) of a switchable palindromic [3]rotaxane. Herein we explore functional targets I and II.

tetrazine, PTTz−. The difference in the ligand strength between the two redox states of tetrazine is about 30 kJ mol−1. This difference is based on the ∼300 mV separation in half-wave potentials (E1/2) for the tetrazine’s reduction when it is uncoordinated relative to when it is incorporated into [2]pseudorotaxanes.17a The ideal candidate ligand must land in a ∼30 kJ mol−1 stability window (Figure 4) so that it forms a more stable [2]pseudorotaxane than neutral PTTz but will lose out to the reduced tetrazine PTTz−. However, optimal switches enjoy a large population swing between the mechanical states. Thus, this stability window symmetrically narrows (Figure 4) to a mere 8 kJ mol−1 when ∼11 kJ mol−1 is shaved off either end to afford a Boltzmann weight of at least 100:1 in favor of the two desired mechanical states. Several candidate ligands (Figure 2a) were examined based on their known25 and expected stabilities C


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indicating that all redox processes take place on PTTz. Among the other ligands, dmp and dtp showed evidence of partial switching (Figure S1) after reduction to PTTz−. The behavior of these two phenanthroline-based ligands is attributed to the high stabilities of their [2]pseudorotaxanes instead of sluggish switching kinetics. Consistently, slower CV scans did not show increases in the degree of population transfer between mechanical states. The triazole ligand pytri shows a reduction peak at around Epc = −0.75 V assigned to a [2]pseudorotaxane composed of PTTz, which disqualified it from further consideration. Competitive [2]Pseudorotaxane Exchange between Tetrazine and Bipyridine. A titration experiment was conducted to confirm the competitive formation of [2]PRbipy+ from [2]PRPTTz+ by displacement of PTTz by bipy. First, the formation of [2]PRPTTz+ was examined by 1H NMR spectroscopy (Figure 6; see Figures S3−S5 for full titration data). The addition of an equimolar quantity of PTTz to a solution of the copper(I) macrocycle, CuM+, showed the complete disappearance of peaks associated with the parent copper macrocycle moiety and the appearance of signals for the formation of [2]PRPTTz+. Peak assignments of [2]PRPTTz+ were determined with the aid of two-dimensional 1H NMR spectroscopy (Figure S6). The resonances associated with the phenanthroline (Ha, Hb, and Hc) and resorcinol (Hg, Hh, and Hi) moieties show downfield shifts consistent with copper(I) coordination. Characteristics of threading are the upfield-shifted resonances in Hd (0.48 ppm) and He (0.95) on the phenylenes that π stack with the central PTTz ligand. Subsequent titration of [2]pseudorotaxane with bipy resulted in the evolution of peaks (Figures 6 and S5) that match the ejected PTTz ligand and the presence of [2]PRbipy+ with the retention of π-stacked Hd and He resonances. This competitive titration shows a mixture of fast- and slow-exchanging resonances in the NMR spectrum. The peak intensities associated with the loss of [2]PRPTTz+ and the production of [2]PRbipy+ change in a manner that shows slow chemical exchange. At the same time, the peaks associated with the ejected PTTz grow in while simultaneously shifting position (e.g., HA and HB), suggesting the presence of additional PTTz-containing intermediates at substoichiometric quantities of bipy. These observations are echoed in UV−vis titrations carried out at a similar concentration regime (vida infra). Nevertheless, the peaks saturate at 1 equiv of added ligand, consistent with the tight binding between bipy and CuM+ to form a [2]pseudorotaxane at 1.0 mM concentrations. Thermodynamics of Single-Ring Switching between Bipyridine and Tetrazine. The greater stability of the bipybased pseudorotaxane was quantified using UV−vis spectroscopy (Figure 7a). The addition of bipy to a solution of [2]PRPTTz+ caused the disappearance of the parent pseudorotaxane’s metal-to-ligand charge-transfer (MLCT) band at 780 nm17a,c and the formation of peaks characteristic of the bipyridine-based [2]pseudorotaxane, [2]PRbipy+, at 537, 464, and 426 nm. Additional UV−vis titrations were conducted under medium binding conditions29 (Figures S7 and S8) to determine the formation constants of the [2]pseudorotaxanes formed from CuM+ and their respective ligands. The titration data were subjected to an equilibrium-restricted factor analysis, as implemented using SIVVU,30 which provided formation constants: log β([2]PRPTTz+) = 7.0 ± 0.3 and log β([2]PRbipy+) = 9.0 ± 0.1. The bipy-based [2]pseudorotaxane is more stable by 12 kJ mol−1, which falls nicely at the edge of the stability

Figure 5. (a) Comparison of the redox properties of the control compounds: the PTTz ligand alone (pink trace) and its corresponding [2]pseudorotaxane, [2]PRbipy+ (blue trace). (b) CV showing switching between [2]pseudorotaxanes. CV of a solution of a 1:1 mixture of [2]PRPTTz+ and PTTz (1.0 mM each). Conditions: 0.1 M TBAPF6; CH2Cl2 degassed with N2; 200 mV s−1 scan rate; a 1-mm-diameter glassy carbon working electrode; a platinum wire counter electrode; an Ag/AgCl (0.1 M TBAPF6) quasi-reference electrode; 300 K.

using CV (Figure S1) shows that most of the ligands display the peak required at −1.05 V, indicating that they are stronger ligands than PTTz. The second requisite property of the ideal complementary ligand is that it can be replaced by the reduced tetrazine, PTTz−, to allow formation of a [2]pseudorotaxane, [2]PRPTTz0. For this purpose, CV studies again proved useful. After PTTz− is formed at −1.05 V, we expect it to exchange with the candidate ligand and produce [2]PRPTTz0. Formation of this [2]pseudorotaxane will be revealed by growth in its signature anodic peak at ca. Epa = −0.65 V.28 Only bipy was found to completely exchange with PTTz− (Figure 5b). Multiple cycles conducted with the bipyridine ligand generate identical CV traces, indicating robust and reversible switching (Figure S2). Consistent with the expected redox inactivity of [2]PRbipy+, no bipyridine- or macrocycle-based reduction processes were observed out to the solvent window of −2.3 V (Figure S1f), D


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Figure 6. 1H NMR spectra showing the formation of [2]PRPTTz+ (red trace) and then [2]PRbipy+ (green trace). (a) To a solution of CuM+ was added PTTz to form (b) [2]PRPTTz+. The addition of bipy to the solution of [2]PRPTTz+ shows displacement of the PTTz ligand and (c) formation of [2]PRbipy+, as evidenced by correspondence to peaks of (d) free PTTz (pink trace). Conditions: 1.0 mM for all components, CD2Cl2, 500 MHz, and 298 K.

Figure 7. (a) UV−vis spectroscopic changes following titration of PTTz [2]pseudorotaxane (red trace) with the stronger bipy ligand to form [2]PRbipy+ (green trace) along the equilibrium defined by ΔG(compete) outlined in (b) a square scheme showing how ΔG(switch) can be derived from independent determination of the other three thermodynamic quantities. Conditions: 2.0 mM [2]PRpttz+ in CH2Cl2, in a 1-mm-path-length quartz cuvette, titrated with 1 equiv of bipyridine (blue trace represents 0.5 equiv).

the solution returned the spectrum for [2]PRbipy+, confirming the reversibility of the PTTz/bipy switch. Mechanistic Studies of Single-Ring Switching. Characterization of the kinetics is essential for gaining an understanding of the pathways followed during switching. Electrochemical studies31 can be particularly useful for studying reactions that occur on the millisecond-to-second time scales. Previous work showed that reduced tetrazines exchange with neutral ligands in copper-based pseudorotaxanes (Figure S11) by bimolecular associative interchange.32,17a,c In contrast, the rate-limiting step for the reverse reaction was attributed17c to a unimolecular dissociation of the copper macrocycle from the neutral tetrazine. These pathways represent reasonable starting points for analysis of the present system. We found that forward switching had well-behaved experimental signatures. Overall, we see systematic changes produced in the CV profiles when we vary conditions that were reproduced in the

window for providing a 100:1 population ratio under standard conditions and 10:1 at the experimental conditions. On the basis of the reduction potentials for PTTz and [2]PRPTTz+/0 and the value for ΔG(compete) and by using the thermodynamic formalism of a square scheme (Figure 7b), the driving force for the reduction-driven switching is ΔG(switch) = −15 kJ mol−1. This driving force correlates to a 480:1 population ratio under standard conditions and 50:1 at the experimental conditions. Redox-driven conversion of [2]PRbipy+ to [2]PRPTTz0 was also observed by UV−vis spectroscopy. The application of slow-scan-rate CV (0.25 mV s−1) out to a potential of −1.2 V displayed bleaching of the characteristic MLCT band of [2]PRbipy+ and growth of a spectrum characteristic of [2]PRPTTz0.17a Consistently, these changes occurred at the voltage required to reduce PTTz (Figure S10). Reoxidation of E


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Figure 8. Cycle of double switching activated at −0.95 V and reset at −0.25 V. (b) The forward sweep of the CV (magenta trace) shows a reduction wave (−0.95 V) assigned from (c) the BPTz (lower gray trace) and (a) a prominent reoxidation wave (−0.25 V) assigned from (a) the CV (top gray trace) of the [3]pseudorotaxane, [3]PRBPTz2+, alone. Conditions: 1.0 mM analyte; 0.1 M TBAPF6; CH2Cl2 degassed with N2; 200 mV s−1 scan rate; a 1-mm-diameter glassy carbon working electrode; a platinum wire counter electrode; an Ag/AgCl (0.1 M TBAPF6) pseudoreference; 300 K.

ions.36 The value for the activation enthalpy ΔH⧧ = +21 ± 1 kJ mol−1 is consistent with steric strain and partial loss of Cu−N bonding during associative interchange (vide infra; see the transition state of step 1 in Figure 9). The activation enthalpy only accounts for about half of the thermal activation, with the remainder resulting from a sharp decrease in the entropy (ΔS⧧ = −90 ± 10 J K−1 mol−1). This observation is within the range observed for ligand exchange about tetrahedral centers36 and consistent with our expectation: formation of the associative interchange transition state reduces the number of species in solution by half and generates a congested, and thus highly ordered, transition state. Interestingly, a high-molecularity intermediate was observed during studies of the forward switching, as evidenced by a peak at −0.25 V in the reoxidation sweep (Figure S15). It only appeared with faster scan rates (>50 V s−1), which corresponds to early evolution on ∼30 ms time scales, and at higher concentrations (4.0 mM), where bimolecular reactions grow in importance. The peak position is consistent with tetrazines that are coordinated with two copper(I) nuclei.17a,37 Mechanistic models involving such species were created to fit the CV data (Figure S16). After refinement, the model indicated the presence of a short-lived [4]pseudorotaxane complex (Figure S17). Other possible alternatives, e.g., Cu(PTTz)2+ or Cu(PTTz)(bipy)+, were experimentally investigated and simulated but failed to reproduce the observed CV profiles (Figure S18). This [4]pseudorotaxane requires association between the switching product [2]PRPTTz0 and any unreacted starting material, [2]PRbipy+, that is still present inside the diffusion volume close to the electrode surface. The resulting “dimer of [2]pseudorotaxanes” is likely a collision complex, and it only has reasonable concentrations early on in the switching reaction. As such, it is a nonequilibrium species. It is compelling that the structural form of this species matches primordial

simulations. The changes were reproduced with a single mechanism and a single set of rate constants across a wide range of scan rates (50−0.1 V s−1), temperatures (298−271 K), and concentrations (4.0, 1.0, and 0.25 mM). Individually, the experimental CV profiles showed broad waves, low-intensity features on the baseline, and large capacitive currents. These features led to the CV profiles observed under any single condition having less ideal correspondence to the simulations. All of these types of features are common to copper-based switches.33 While we have seen closer agreement between individual CV profiles with other switching systems,31b,34 the minimum degree of correspondence needed for kinetics is with the totality of the time- and concentration-dependent data. When switching processes have bimolecular rate-determining steps, they display CV profiles that strongly depend on the concentration.17a,c For the forward-switching process, involving population transfer from [2]PRbipy+ to [2]PRPTTz0, we observed a 50% change in the peak intensities following dilution from 4.0 to 0.25 mM (Figure S12). Assuming a bimolecular rate-limiting step, digital simulation35 of the CV was used to generate a model that fits the experimental data. A bimolecular rate constant of kf = 5.5 × 104 M−1 s−1 (298 K) showed the best agreement across a variety of scan rates, and it approximates previous values involving reduced tetrazines (kf = 1.2 × 104 M−1 s−1, 298 K).17a This mechanistic model was used to determine the rate constants for forward switching of a single ring across a range of temperatures (271−298 K) to produce an Eyring plot (Figures S13 and S14). The data from these studies on the forward process allowed the determination of detailed kinetics parameters ΔG⧧, ΔH⧧, and ΔS⧧. A comparison with related tetrazine switches17 shows that the behavior of the PTTz switch is consistent with the previous rate data (ΔG⧧ range = +46−55 kJ mol−1) and with behaviors expected of ligand substitution reactions about metal F


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Figure 9. (a) Stepwise mechanism of double switching, with the model systems for (b) step 1 and (c) step 2 also examined herein.

∼−57 kJ mol−1 after conversion with Faraday’s constant. For comparison, the overall stability of two bipy-based pseudorotaxanes is ΔG is −100 kJ mol−1 (see the Supporting Information), which is between the stabilities for the neutral and reduced [3]pseudorotaxanes. Consequently, we hypothesized that we could drive switching from 2 equiv of [2]PRbipy+ into 1 equiv of BPTz-based [3]pseudorotaxane (functional target I) upon single-electron reduction. The half-wave potentials of the species we expect in solution for BPTz, [2]PRBPTz+, and [3]PRBPTz2+ are at −0.90, −0.65, and −0.30 V (vs Ag/AgCl quasi-reference),17a respectively (Figure S21). These potentials indicate that the first step has a lower driving force of 19 kJ mol−1 than the second, 25 kJ mol−1 (Figure S 24). When the reduced tetrazine is doubly threaded, it is more stable than that with a single ring and is thus consistent with positive thermodynamic cooperativity.17a The relative intensities of the three signature anodic peaks in the CV profiles will

double-switching products (cf. Figure S17 and the transition state for step 2 in Figure 9). Thermodynamics of Contractile Double Switching: Functional Target I. With bipy identified as an ideal switching partner for tetrazines (functional target II), we evaluated the ligand’s use in double switching with the doublesite tetrazine ligand BPTz. The relative stability of the two bipy-based [2]pseudorotaxanes (Figures 6 and S9) should ensure their dominance in solution. The overall stability of the BPTz-based [3]pseudorotaxane is dramatically enhanced by reduction showing ΔG = −1.5 × 102 kJ mol−1 compared to −90 kJ mol−1 for the neutral BPTz-based [3]pseudorotaxane. The stability of [3]PRBPTz2+ was previously determined.17a The stability of [3]PRBPTz+ is the combination of its formation constant (90 kJ mol−1) and the extra stabilization occurring upon reduction, as determined from the electrochemical square scheme ΔE1/2 = −0.30 − −0.90 = 0.60 V, which equates to G


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Figure 10. (a) Fast-scan-rate CV (20 V s−1) showing the reduction and oxidation of [2]PRBPTz+. (b) Slow-scan-rate CV (0.20 V s−1) showing the switching of CuM+ from bipy to form the reduced [3]pseudorotaxane [3]PRBPTz+. Conditions: [BPTz] = [bipy] = 1.0 mM; [CuM+] = 2.0 mM; 0.1 M TBAPF6; CH2Cl2 degassed with N2; a 1-mm-diameter glassy carbon working electrode; a platinum wire counter electrode; an Ag/AgCl (0.1 M TBAPF6) quasi-reference; 298 K.

When the conditions of CV are altered, stepwise switching (Figure 9a) of the first and then the second ring is also made evident. Our data suggest that the first step is about 4 times faster than the second. The first step is modeled on the PTTz switch (Figure 9b) with k = 5.5 × 104 M−1 s−1. Both the model and the first step involve movement of CuM+ from bipy to a reduced tetrazine, either PTTz− or BPTz−, respectively. For the second step, we modeled it (Figure 9c) on a solution containing an equal mixture of the bipy- and BPTz-based [2]pseudorotaxanes. Such a solution composition is actually the halfway point through the “double reset” titration described above. When the 1:1 solution of these [2]pseudorotaxanes is examined by fast-scan-rate CV profiles, [2]PRBPTz0 is reduced at −0.70 V (Figure 10a) and then quickly reoxidized at −0.60 V. The reoxidation is only possible using higher scan rates like 20 V s−1 (Figure S25) that traverse from the reduction peak at −0.70 V to the CV’s vertex at −0.80 V and back to the −0.60 V anodic peak, some 300 mV, in just 15 ms. In this case, the bipybased [2]pseudorotaxane is a mere spectator. However, when the CV is slowed down by 2 orders of magnitude to 0.20 V s−1 (Figure 10b), 1.5 s is available and the bipy-based [2]pseudorotaxane becomes an active participant in switching. Consequently, the reoxidation peak at −0.60 V has completely disappeared and been replaced by that of the switching product [3]PRBPTz− at −0.25 V. This single-ring switching event is exactly the microscopic process defined by step 2 in double switching (Figure 9). Simulations of the CV profiles of the model recorded as a function of the scan rate provide a bimolecular rate constant of k2 = 1.3 × 104 M−1 s−1. A mechanism for double switching can be based on a faster step 1 followed by a slower step 2. Simulation of the doubleswitching CV profiles using the rate constants for steps 1 and 2 derived from the single switches reproduces the observed features of redox-driven contraction (Figure S22). The rate constant for step 1 was approximated by the behavior of the PTTz-based [2]pseudorotaxane. In agreement with this mechanism, we show experimentally that the concentration of the intermediate [2]pseudorotaxane formed in the first step builds up when using faster scan rates (10 V s−1; Figure S26). The slower second step is consistent with a slight increase in

be used to determine the relative concentrations of the three tetrazine-based species at the electrode surface. When studied as a function of time (by varying the scan rate), we observe conversion of one species to another (vide infra). We start by verifying that the two bipyridine [2]pseudorotaxanes, [2]PRbipy+, are more stable in solution than a single BPTz-based [3]pseudorotaxane involving the chargeneutral tetrazine. To verify this idea, we analyze the peak intensities in the reductive sweep from 0 to −1.3 V because they reflect the contents of the solution at the beginning of the experiment (see the magenta trace in Figure 8b). A solution of [3]PRBPTz2+ alone serves as a model. It shows a characteristic reduction peak at −0.35 V during the outward sweep (Figure 8a, gray trace). Titration of that solution with 1 equiv of bipy (Figure S22) showed the appearance of [2]PRBPTz+ with its reduction peak at −0.70 V, a solution given closer examination below (see Figure 10). Upon the addition of a second 1 equiv of bipy, the reduction peak for [2]PRBPTz+ at −0.70 V disappeared and the peak for the uncomplexed tetrazine BPTz appeared at −0.95 V (Figure 8b, magenta trace). This peak is assigned by comparison to the CV of BPTz (Figure 8c). These titration data confirm that the copper macrocycle moieties prefer the two bipy stations and that they moved from the BPTz ligand over to the first and second equivalent of the bipy ligand. Thus, the “double reset” (Figure 8) favors bipy-based [2]pseudorotaxanes, as predicted from the thermodynamics. Redox-Driven Double Switching. The return sweep of the CV profiles (Figure 8b, blue trace) confirms the full redoxdriven cycle of double switching. At the end of the forward sweep, the solution surrounding the electrode contains the reduced BPTz−. During the return sweep, the reoxidation peak for the free tetrazine (−0.85 V) has minimal intensity, and instead the peak for [3]pseudorotaxane, [3]PRBPTz+, at −0.25 V appeared. Consistent with the thermodynamic cooperativity, there is little evidence of the intermediate [2]pseudorotaxane [2]PRBPTz+/0 at these scan rates based on the negligible peak intensity at −0.60 V. Thus, [3]pseudorotaxane is generated after switching the first and second rings from the two different bipy-based [2]pseudorotaxanes that were originally present in solution. Collection of multiple CV cycles shows that the double-switching process is reversible (Figure S23). H


Article

Inorganic Chemistry

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steric bulk of the incoming reactants because the tetrazine core of BPTz− (first switching step) is less hindered than that of [2]PRBPTz0 (second switching step).

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CONCLUSIONS Reversible double switching of two rings from two bipyridine [2]pseudorotaxanes over to a reduced double-site tetrazine to produce a [3]pseudorotaxane has been demonstrated for the first time. A rational design approach was used to identify the critical design criteria. These criteria helped inform the comparison of the candidate ligands, leading to the identification of bipyridine for pairing with tetrazines. The activation parameters of the elementary step for switching a single ring from a bipyridine [2]pseudorotaxane onto a singlesite tetrazine were characterized for the first time and showed a large entropic factor consistent with associative interchange. This [2]pseudorotaxane served as a model for the kinetic properties of the first elementary step in the double switch. Additional experiments involving the movement of a single ring from a bipyridine [2]pseudorotaxane to a tetrazine [2]pseudorotaxane allowed estimation of the kinetics of the second elementary step. Kinetically, however, the slower movement of the second ring is consistent with negative cooperativity between the movements of the two rings. These switches show that each copper(I) macrocycle offers ∼30 kJ mol−1 of stabilization to the tetrazine-based pseudorotaxanes. As a result, these pseudorotaxanes show high driving forces for switching when the double-site tetrazine ligand is reduced such that [3]pseudorotaxane enjoys ∼60 kJ mol−1 of stability with two CuM+ moieties. Interestingly, positive thermodynamic cooperativity allows [3]pseudorotaxane to form with high fidelity. The design principles laid out by this work, along with the discovery of the uncommon double switching motif and its dependence upon cooperative behavior of the rings, should be useful for the future design of functional palindromic molecular muscles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02554. Syntheses, titration data and analyses, additional CV experiments, and digital CV simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: afl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF (Grant CHE-1412401) for support. REFERENCES

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Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes

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