VIOLOGEN-BASED ROTAXANES FROM DIBENZO-30-CROWN-10

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VIOLOGEN-BASED ROTAXANES FROM DIBENZO-30-CROWN-10 Hanlie R. Wessels, Carla Slebodnick, and Harry W. Gibson J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Journal of the American Chemical Society

VIOLOGEN-BASED ROTAXANES FROM DIBENZO-30-CROWN-10

Hanlie R. Wessels, Carla Slebodnick and Harry W. Gibson* Department of Chemistry, Virginia Tech Blacksburg, VA 24060 ABSTRACT Three [2]rotaxanes (4, 7 and 12) and one [3]rotaxane (8) were synthesized based on the dibenzo-30-crown-10/viologen binding motif. To the best of our knowledge, these are the first rotaxanes formed from dibenzo-30-crown-10 and viologens. The rotaxanes were all characterized by 1H NMR,

13

C NMR, and HRMS. An

X-ray crystal structure of one of the [2]rotaxanes (7) was obtained. This work demonstrates for the first time that dibenzo-30-crown-10 does form pseudorotaxane complexes with viologens in solution. INTRODUCTION Crown ethers were the original macrocyclic hosts. Their discovery by C. J. Pedersen, the subsequent publication of the syntheses of 60 of these compounds 1 and the study of their properties, especially their complexation behavior with alkali metals, led to his share in the 1987 Nobel Prize for Chemistry. Crown ethers played an important role in the study of alkali metal cations in biological membranes and supramolecular chemistry in general.2 Crown ethers have been widely used as hosts in supramolecular chemistry. They have been used as the wheel components in rotaxanes and other interlocked molecular structures with functionalities such as molecular shuttles,3 molecular switches,4-6 ionic sensors,7 switchable catalysts,8-10 molecular muscles

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11,12

and simple mimics of

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ribosomes.13,14 A rotaxane which selectively complexes Ca2+ was reported to have potent anti-cancer properties.15 The selective shuttling, or switching, behavior of rotaxanes has led to investigation of their use in drug delivery.16 These hosts have also been used to construct supramolecular polymers

17-22

by self-assembly, which makes

more complex topologies and structures accessible,23 and gives rise to interesting properties, self-healing

24,25

and optoelectronic properties,26 for example. There have

even been examples of polymers that completely change their topology (for example, linear to cyclic,27,28 and star to linear

29,30

), based on an external stimulus, a feat which

would be impossible with conventional polymers. In order for a host to be useful in the construction of supramolecular assemblies and polymers, it is imperative to determine the binding configuration with the intended guest molecule. The Gibson group has been especially interested in the construction of polymers that self-assemble through the formation of pseudorotaxane linkages.1720,22b,31-33

These types of materials are switchable, or “smart”, since the crown ether

host/guest pairs respond to stimuli like heat, solvent polarity, addition and removal of K+ ions, and pH.34 Materials of this sort have been constructed using dibenzo-24-crown-8 (DB24C8),18 bis(m-phenylene)-32-crown-10 (BMP32C10),17,19,20 and benzo-21-crown-7 (B21C7).35 Crown ether-based cryptands have emerged as more powerful hosts for N,N’-dialkyl-4,4’-bipyridinium salts (viologens).36 The cavity size of the crown ether limits the size of the guest molecule that can be accommodated; a guest molecule that is larger than the cavity of the crown ether simply will not fit. The association constant between the host and the guest also plays a crucial role in the degree of polymerization

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that can be reached by self-assembly.19,36i,37 The association constant is therefore a crucial factor that contributes to the supramolecular polymer’s properties. Our group developed optimized syntheses for functionalized (monofuctionalized

36c

) and unfunctionalized

38

38

and di-

dibenzo-30-crown-10 (DB30C10). We can now

make multiple grams of DB30C10 and derivatives from inexpensive and readily available starting materials in high overall yields; templation of the cyclization step with KPF6 results in yields of 80-93 %. This is enabling for production of useful quantities of supramolecular polymers. However, in order to use this crown ether for the synthesis of brush polymers, star-polymers,20a graft-polymers

20b

and other

supramolecular

materials, we need to establish that the binding conformation of DB30C10 with viologens is a mechanically linked, i. e., pseudorotaxane, structure. We discovered some time ago that BMP32C10 has two possible binding conformations with viologens. The guest molecule can either thread through the cavity of the host, or the host can fold around the guest molecule, forming a so-called “tacocomplex” 39a (Figure 1). In the case of bis(p-phenylene)-34-crown-10 (BPP34C10), only pseudorotaxanes are formed, due to the geometry of the crown ether that prevents such folding. However, examples of BMP32C10 in both conformations are known.19,39

Host

guest

pseudorotaxane

Taco complex

Figure 1. Cartoon representation of binding conformations of bis(m-phenylene) and dibenzo crown ethers with viologen.



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The crystal structure of the K+ complex of DB30C10 was published in the 1970s.40 Theoretical studies produced structures of complexes between DB30C10 and Li+, Na+, K+, Rb+ and Cs+ that closely resembled the crystal structures.41 Colquhoun and Stoddart et al. reported the taco-like crystal structure of DB30C10@[Pt(NH3)2Bipy]+2 (

PF6-)2, in which the bipyridinium unit π–stacked with the host’s aromatic rings,42a and

also the taco-like crystal structure of DB30C10@[Rh(cod)(NH3)2]+ PF6- in which the guest chiefly interacts with the ethyleneoxy units.42b Semnani et al. reported charge transfer

complexes

between

the

neutral

electron

deficient

compounds

tetracyanoethylene and 2,3-dichloro-5,6-dicyanobenzoquinone and dibenzo crown ethers with unknown conformations.43 DB30C10 was shown by X-ray crystallography to form a taco complex with N,N’ethylene-2,2’-bipyridinium (diquat) in the 1980s.44 Diformyl and dimethyl derivatives of DB30C10 also formed such taco complexes.45 Later cis- and trans-DB30C10 diols were shown to form taco complexes with diquat.46 Pyridyl esters of cis-DB30C10 diol with diquat also afforded taco compexes.47

Instead of forming a rotaxane with a viologen,

DB30C10 formed a taco complex, in contrast to di(1,5-naphtho)-38-crown-10 and BPP34C10, which did form rotaxanes.48 A [3]rotaxane with two DB30C10 units and an axle containing two dibenzylammonium centers, a p-xylene spacer and 3,5-(t-Bu)2C6H3CH2 stoppers was synthesized in 1997.49 In another true rotaxane with a bis(benzimidazolium) axle DB30C10 was threaded, but folded in the taco or “clam shell” conformation.50 A study of the complexation between a series of benzimidazolium-alkane guests and DB30C10 highlighted the tendency of the crown ether to form taco or pseudorotaxane complexes


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depending on the nature of the guest.51 A catenane was synthesized by Takata et al. from a pseudorotaxane based on DB30C10 and a dibenzyl ammonium salt functionalized with terminal alkyne groups.52 Later an elegant molecular rotor or pump used DB30C10 as the macrocycle in the formation of catenanes and rotaxanes based on the dibenzylammonium guest motif.53 To the best of our knowledge, these are the only examples of rotaxanes or catenanes with DB30C10 in the literature at present. From the above discussion we can safely conclude that the conformation of DB30C10 in complexes is highly dependent upon the nature of the guest. The cavity is large and the ethyleneoxy arms are quite flexible, affording the host the ability to adapt to the guest and change its binding conformation to maximize favorable interactions. To our knowledge, until now there has been no direct evidence that DB30C10 adopts the pseudorotaxane conformation in complexation with viologen derivatives. All reported X-ray crystal structures of the complexes between DB30C10 derivatives and viologens are in the taco conformation.46-48 The pseudorotaxane conformation may exist in solution, but this is difficult to observe directly by any means. One way to obtain conclusive proof of the threaded structure is by synthesizing rotaxanes from the putative pseudorotaxanes. The formation of rotaxanes means that the guest must have been threaded through the cavity of the host in solution, since the taco conformation is not a precursor to a rotaxane.



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We therefore set out to synthesize rotaxanes based on the viologen/DB30C10 binding motif. We successfully isolated and characterized three new [2]rotaxanes and one

new

[3]rotaxane,

demonstrating

conclusively

that

DB30C10

does

form

pseudorotaxanes with viologens in solution. RESULTS AND DISCUSSION

Scheme 1. Syntheses of dumbbell 2 and [2]rotaxane 4.

A. Syntheses of Rotaxanes [2]Rotaxane 4, the shortest and most crowded of our targets, was synthesized by adding a solution of 3.25 equivalents of DB30C10 to a solution of 4,4’-bipyridine in a minimum amount of MeCN followed by 2.38 equivalents of 3,5-diphenylbenzyl bromide (1) (Scheme 1). After four weeks of stirring at room temperature and an extensive workup and purification procedure, 4 was isolated in 2.7% yield. The low yield was anticipated due to the fact that DB30C10 would have to complex the half-quat (monobipyridinium salt 3, Scheme 2) formed in situ in the initial quaternization, before the second quaternization with 1 took place. The complexation between the half-quat and crown ethers is relatively weak, leading to a low percentage of threaded viologen


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derivative.54 Dumbbell 2 was prepared analogously without the crown ether in 93% yield. In another approach to [2]rotaxane 4, half-quat 3 was pre-formed and purified (43% yield) (Scheme 2). Once the crown ether (3.25 equivalents) was complexed with 3 in MeCN and dichloromethane (DCM), the addition of an excess (2.4 equivalents) of blocking group 1 yielded 4 in an increased yield of 13%.

Scheme 2. Two-step synthesis of [2]rotaxane 4.

Rotaxanes 7 and 8 based on a bisviologen were synthesized by combining 4.6 equivalents of DB30C10 and bis(half-quat) 5 in DCM/MeCN (Scheme 3). The blocking group, 3,5-diphenylbenzyl bromide (1, 2.1 equivalents), was added and the solution was allowed to stir at room temperature for four weeks. After washing, treatment with an ion exchange resin and chromatography, two orange bands were isolated. The first one contained the [3]rotaxane 8 in 4% yield and the second the [2]rotaxane 7 in 28% yield. The dumbbell 6 was made analogously without the crown ether in 77% yield.



Scheme 3. Syntheses of dumbbell 6 and rotaxanes 7 and 8.

The diazido viologen 10 was used to prepare [2]rotaxane 12 by copper catalysed “click coupling”.55 10 and three equivalents of DB30C10 were combined in dry MeCN, under argon. To the bright red solution was added a solution of two equivalents of 4trityl-(3’-propynyloxy)benzene (9) in dichloromethane (DCM) (Scheme 4). The catalyst, Cu(CH3CN)4PF6, was added and DCM and MeCN were added dropwise until all solids were dissolved. Cu(CH3CN)4PF6 and DCM/MeCN were chosen as the catalyst/solvent system for the “click reaction”, instead of CuSO4 in water or MeOH, because the crown ether and blocking group are not soluble in water or MeOH. Even though the crown ether and blocking group are perfectly soluble in MeCN, we opted for the MeCN/DCM mixture to decrease the solvent polarity as much as possible, because very polar solvents and solvents that form hydrogen bonds interfere with the complexation of the viologen and crown ether. After several washing steps and chromatography 12 was isolated as a bright red solid in 6% yield. Without the crown ether the dumbbell was 8 ACS Paragon Plus Environment

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produced in 26% yield; this result indicates that this procedure is not optimal and the yield could probably be improved.

+ N3 O 9

PF6

PF6 N

N

N N N

CuSO4 H2O N3

ascorbic acid DMF

O

PF6

PF6

N

N

10

O N N N

11

Cu(CH3CN)4PF6 MeCN/ CH2Cl2

N N N O

PF6

PF6

N

N

O N N N

12

Scheme 4. Syntheses of dumbbell 11 and [2]rotaxane 12.

B. Mass Spectrometric Characterization All four rotaxanes were characterized by high-resolution mass spectrometry (HR MS). For [2]rotaxane 4 a peak at m/z 1323.5295 corresponded to M+ and the peak at m/z 589.2831 was due to (M-2PF6)2+. [2]Rotaxane 7 displayed m/z 864.8138, corresponding to the loss of two PF6- counter ions and m/z 528.2216 corresponding to (M-3PF6)3+. [3]Rotaxane 8 yielded m/z 1132.9465, corresponding to (M-2PF6)2+, and m/z 706.9765, corresponding to(M-3PF6)3+. [2]Rotaxane 12 displayed m/z 1894.6626, 1725.8084, and 790.8737, corresponding to (M + Na)+, (M-PF6)+, and (M-2PF6)2+, respectively.



C. Evidence of Charge Transfer Interactions Aromatic crown ethers typically associate with viologens through hydrogenbonding, π-π stacking and charge transfer interactions.5,56 The charge transfer can be observed visually when aromatic crown ethers and viologens are combined in solution. For example, during the synthesis of [2]rotaxane 4, the solution changed from clear to bright yellow after a few minutes (presumably corresponding to the formation of the halfquat) and the solution gradually became dark orange. The solution of half-quat 5 and DB30C10 (which is a colourless solid) was light yellow and became dark orange over time. The viologen diazide 10 has a beige color and the crown ether is colorless; the solution of the two went bright red as soon as the solutions were combined. All the rotaxanes are bright orange or bright red. Equimolar solutions of the dumbbells ( i. e., 2, 6 and 11) and DB30C10 in DMSO or MeCN were only slightly yellow, presumably due to taco complexation. D. 1D NMR Chracterization From the first pseudorotaxane and rotaxane complexes of phenylene crown ethers with viologens, 1H NMR spectra have been known to reveal the π–stacking and hydrogen bonding interactions in terms of significant upfield shifts of the aromatic signals from both the viologen and the crown ether and ethyleneoxy protons of the latter.5,56 The formation of the present rotaxanes led to such peak shifts in the 1H NMR spectra. Because hydrogen bonding and dipole-dipole interactions contribute to the complexation, the solvent influences the extent of complexation between the host and the guest. In order to illustrate the effect of the solvent, we examined the spectra of 10 ACS Paragon Plus Environment

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rotaxane 12 separately in both DMSO-d6 and CD3CN. 1H NMR spectra of dumbbell 11, DB30C10, an equimolar solution of 11 and DB30C10, and 12 in DMSO-d6 (Figure 2) are compared to those same spectra in CD3CN (Figure 3). 1. Dimethyl Sulfoxide Solutions DMSO is a highly polar, strongly hydrogen bonding solvent and competes so much that it essentially eliminates complexation between the viologen and the crown ether.20c,57 Consequently, the equimolar solution of dumbbell 11 and DB30C10 (Figure 2b) exhibits no peak shifts; the spectrum is an almost perfect superposition of the spectra of 11 (Figure 2a) and DB30C10 (Figure 2c; Table S3), indicating that there is no complexation between the crown ether and the dumbbell in solution. However, there are clear peak shifts in the 1H NMR spectrum of [2]rotaxane 12 in DMSO (Figure 2a, Table S3) because the host and guest are mechanically linked. The resonances of the α- or 2-viologen protons j and ethyleneoxy protons k shifted upfield significantly as did those of the aromatic protons of the crown ether (a’ and b’). The ethyleneoxy proton signals c’, d’, e’ and f’ shift upfield slightly in 12. However, the triazole peak (g) shifts downfield in the rotaxane. Similar results were observed for rotaxanes 4, 7 and 8 vs. their respective dumbbells in DMSO (Tables 1 and 2); that is, the equimolar solutions of the dumbbells and DB30C10 displayed no significant shifts, while significant shifts were observed for the viologen and crown ether protons in all of the rotaxanes.



HW C059_dried_DM SO _PR O TO N _01

g

a)

b

a

c ed

h, i, f

c’

a’ b’

k

j

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d’ 4

b)

crown_dum bbell_dilute_PRO TO N _01

+

3

e’ f’

DB30C10_D M SO _PRO TO N _01

c)

c’

d’ 2

c HW B113_PF6_Carbon_PRO TO N _01

d)

j

k

g

f b a

e d 1

h i

9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2 7.0 6.8 6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 f1 (ppm )

Figure 2. 1H NMR spectra of a) [2]rotaxane 12, b) an equimolar solution of dumbbell 11 and DB30C10, c) DB30C10 and d) 11 in DMSO-d6 at 400 MHz.

Table 1. Chemical shifts (ppm) of [2]rotaxane 4 and its components in DMSO-d6. Proton

Dumbbell 2

DB30C10

α-PQ (j) β-PQ (k) Crown a’ Crown b’ OCH2 c’ OCH2 d’ OCH2 e’ OCH2 f’

9.62 8.70 -------------

----6.95 6.87 4.06 3.73 3.62 3.55

2+ DB30C10 9.63 8.70 6.94 6.86 4.06 3.73 3.62 3.55

a

Change from individual components to 2 + DB30C10.

b

Change from individual components to [2]rotaxane 4.

∆ 0.01 -0.00 -0.01 -0.01 0.00 0.00 0.00 0.00


Rotaxane 4

∆b

9.31 8.29 6.45 6.13 3.76 3.38 3.26 3.26

-0.31 -0.41 -0.50 -0.74 -0.30 -0.35 -0.36 -0.29

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Table 2. Chemical shifts (ppm) of [2]rotaxane 7 and [3]rotaxane 8 and their components in DMSO-d6. D’bell 6+ Rotax Rotax ∆a ∆b ∆c DB30C10 Proton 6 DB30C10 7 8 +0.01 9.44 -0.20 -0.32 9.64 --9.65 9.33 α-PQ (g) α’-PQ (j) 9.44 --9.45 +0.01 9.24 -0.20 9.14 -0.31 0.00 -0.27 8.70 --8.70 8.43 8.30 -0.40 β-PQ (h) β’-PQ (i) 8.70 --8.70 0.00 8.43 -0.27 8.30 -0.40 Crown a’ --6.95 6.92 -0.03 6.34 -0.61 6.45 -0.47 Crown b’ --6.87 6.86 -0.01 5.90 -0.97 6.13 -0.73 OCH2 c’ --4.06 4.06 0.00 3.72 -0.34 3.77 -0.29 OCH2 d’ --3.73 3.73 0.00 3.41 -0.32 3.45 -0.28 OCH2 e’ --3.62 3.63 0.01 3.22 -0.40 3.27 -0.36 OCH2 f’ --3.55 3.55 0.00 3.14 -0.41 3.27 -0.28 a

Change from individual components to 6 + DB30C10.

b


c


2. Acetonitrile Solutions Figure 3 depicts the proton NMR spectra of dumbbell 11, DB30C10, an equimolar solution of 11 and DB30C10 and 12 in CD3CN. CD3CN is a less competitive solvent than DMSO, and consequently we see a little complexation between the dumbbell and DB30C10 in the equmolar solution, as reflected by small upfield shifts; this is presumably the taco complex. The upfield shifts are much more pronounced in the rotaxane spectrum (Figure 3a, Table S4) as expected for the mechanically bonded structure. In particular, large shifts occurred for the signals of the viologen β-protons (k), crown ether aromatic protons a’ and b’, as well as ethylene protons c’.



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&

a, b,c

a)&

g

e k

j

i, f, h

d

e’, f’

DCM&

a’ b’

c’

d’

b)&

e’, f’

c)& c’

d’

a’, b’

a, b,c

d)& j

k

g

e

d f i

h

& Figure 3. 1H NMR spectra of a) [2]rotaxane 12, b) an equimolar solution of DB30C10 and

dumbbell 11, c) DB30C10 and d) 11 in CD3CN at 400 MHz.

Figure 4 shows and Table S1 summarizes the chemical shift changes among the proton NMR spectra of a solution of dumbbell 2 and DB30C10 separately, their equimolar solution and a solution of [2]rotaxane 4. The resonances of viologen protons g and h of 2 move very slightly upfield in the equimolar solution, signifying taco complexation; however, the signal for proton h moves significantly in the rotaxane. The resonances of the aromatic protons of DB30C10 (a’ and b’) split into two signals in the equimolar solution, but move significantly upfield in 4. The signals for ethyleneoxy


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ptotons c’ – f’ of DB30C10 move upfield in the equimolar solution and even further upfield in the rotaxane solution.

c, e

a) g

b

dh

a

c’

f

a’ b’

d’ e’, f’

b)

c)

c'd' e' f' b' O O O O O a'

a’, b’

O O O O O

d)

b g

h

f

c, e a

Figure 4. 1H NMR spectra of a) [2]rotaxane 4, b) an equimolar solution of dumbbell 2 and DB30C10, c) DB30C10 and d) 2 in CD3CN at 400 MHz.

Figure 5 illustrates the peak shifts that were observed in CD3CN for [2]rotaxane 7 and summarized in Table S2. There are only slight peak shifts (presumably indicating taco complexation) in the equimolar solution of dumbbell 6 and crown ether. However, signals for viologen protons h and i (the β-protons) shifted upfield significantly more in 7, as did signals for crown ether aromatic protons a’ and b’ and all of the ethyleneoxy



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protons. The rotaxane solution is also bright red as opposed to the light yellow color of the equimolar solution of 6 and DB30C10.

c, e

a) g, j

h, i d

f k

b

l

a’

a

b’

c’

d’ e’ f’

b)

c) a’, b’

l

d) g j

h, i

d

c, e

b f

a

k

Figure 5. 1H NMR spectra of a) dumbbell 6, b) DB30C10, c) an equimolar solution of 6 and DB30C10 and d) [2]rotaxane 7 in CD3CN at 400 MHz.

The [3]rotaxane 8 also displayed changes in the chemical shifts in the 1H NMR spectrum that were distinct from the equimolar dumbbell + crown ether solution, but overall very similar to [2]rotaxane 7 (Figure 6, Table S2). However, notably the resonance for β-viologen protons h and i shifted even further upfield in 8 relative to their position in 7.


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From this large upfield shift and the fact that there are four viologen aromatic signals in both rotaxanes 7 and 8, we can make some inferences about the positions of the crown ether(s). If the crown ether were stationary on one of the viologen sites as drawn in Scheme 1, eight viologen signals would be expected for 7: four for the viologen complexed to the crown ether and four for the uncomplexed viologen, since the viologen units are unsymmetrical. Instead there are only four signals, indicating that the single crown ether in 7 rapidly shuttles between the two viologen sites. This is consistent with similar systems with two degenerate binding sites on an axle.57d,58 At room temperature, the shuttling is fast enough that the two viologen sites appear equivalent in the 1H NMR spectrum, i. e., the signals are time averaged. The fact that signals for protons h and I are 0.18 ppm upfield in 8 (relative to 7, Table S2) indicates that in the [3]rotaxane the crown ethers are forced to spend essentially all of their time on the viologen binding sites due to the spatial constraints imposed by the dumbbell.



H W C043_last_PF6_5000_PR O TO N _01 c, e

a)

b

h, i d

g j

f

a’ b’

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c’

k

d’

e’, f’

a 4

D B30C10_x_M eCN_PRO TO N_01

b)

3

D B30C10_M eCN _PR O TO N _01

c)

a’, b’ 2

l x_M eCN _PR O TO N _01

d)

g j

c, e h, i

d

b f k a 1

9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0 f1 (ppm )

5.5

5.0

4.5

4.0

3.5

3.0

Figure 6. 1H NMR spectra of a) [3]rotaxane 8, b) an equimolar solution of dumbbell 6 and DB30C10, c) DB30C10 and d) 6 in CD3CN at 400 Mz.

In fact the upfield shifts of the resonances of the viologen β-protons in the rotaxanes relative to the respective dumbbells in CD3CN scale with the confinement lengths: the longer the confinement length, the smaller the shift as a result of shuttling of the crown ether. That is, in terms of shifts: 7 < 12 < 4 < 8 for confinement lengths of 24, 15, 10 and 12 atoms, respectively (Figure 7). In the case of 8 the presence of two threaded cyclics is deemed to decrease the confinement length by a factor of two, but steric hindrance may reduce it even more.


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-0.10

Change in Chemical Shift of beta-Protons (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


y = 0.0138x - 0.5553 R² = 0.9746

-0.20

-0.30

-0.40

-0.50 6

8

10

12

14

16

18

20

22

24

26

28

30

Length of Dumbbell (atoms) Figure 7. Change in chemical shift of viologen β-protons of rotaxanes 4, 7, 8 and 12 in CD3CN vs. length of the dumbbell.

E. 2D NMR Characterization 2D COSY and NOESY spectra were obtained for the rotaxanes. Figure 8, a 2D NOESY spectrum of [2]rotaxane 4, confirms through-space interactions of protons on the dumbbell with those of the threaded crown ether. Starting from the upper right on the diagonal, crown ether “source” ethyleneoxy protons c’ and d’ correlate with guest signals “of interest”, blocking group protons c / e and d, viologen proton h and Nbenzylic proton f. Moving down the diagonal, the “source” N-benzylic proton f correlates with ethyleneoxy protons “of interest” d’ and e’. And crown ether aromatic proton a’ stimulates blocking group protons d / h. And conversely protons d / h are correlated with crown protons a’ and d / e. Blocking group “source” protons c / e bring out “signals of interest” in crown ether protons d / e.



Figure 8. 2D NOESY spectrum of [2]rotaxane 4 in CDCl3 at 400 MHz.

2D NOESY spectra of rotaxanes 7, 8 and 12 (Figures S18, S24 and S36) also revealed through-space correlations between protons on the host and protons on the guest species, confirming their mechanically linked rotaxane structures. F. X-Ray Crystallography Several attempts were made to grow crystals of all of the rotaxanes for single crystal X-ray diffraction. Only from [2]rotaxane 7 was a suitable crystal obtained, by slow vapor diffusion of pentane into an MeCN/MeOH solution. The crystal structure (Figure


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9) clearly shows that the guest molecule is threaded through the host cavity. However, the host adopted a folded conformation rather than an S-shaped or stair step conformation that has been observed for some rotaxanes.59,60 That is, the guest did not thread in the manner depicted in Figure 1, wherein it is positioned “parallel” to the ethyleneoxy groups of the host as frequently observed in crown ether-based pseudorotaxanes

61,62

and those with cryptands.36a-36c Here, both aromatic rings of the

crown ether are positioned to π–stack with the viologen moiety, but the ethyleneoxy moieties engage only one of the pyridinium rings. A similar “clam shell” conformation of the crown ether has been reported for [2]pseudorotaxanes from DB24C8 and a few secondary ammonium guests,

57b,63a

but this differs from the “stair step” conformation

normally seen in DB24C8 pseudorotaxanes and rotaxanes.

57b,63b

The centroid-to-

centroid distance between the benzo groups of the host is 6.98 Å. Thus the centroid-tocentroid distance between the benzo groups of the host and the bipyridinium moiety on the guest is approximately 3.5 Å. The bipyridyl twist is 13.1 ⁰. The overall length of the rotaxane is 27 Å. The N-benzylic hydrogen atoms are engaged in bifurcated hydrogen bonds with the ethyleneoxy oxygen atoms: a / b and c / d (Figure 9D). The 2- or α-protons of one of the pyridinium units also interact with the ethyleneoxy oxygen atoms in a trifurcated manner: e / f / g (Figure 9E) and h / I / j (Figure 9F). The 3- or β-hydrogens of the bipyridyl unit are not engaged in H-bonding at all. This lack of H-bonding involvement of one pair of viologen protons (which is also observed by the lack of significant upfield shift of paraqat protons g and j in Figure 5) could result in a lower association constant for pseudorotaxane formation in solution and explain the low rotaxane yields. 21 ACS Paragon Plus Environment


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A

B

c

C

a d b

D


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e

h

f

i g

F

j

E

Figure 9. The X-ray crystal structure of [2]rotaxane 7; counterions are not shown; hydrogen atoms have been removed from the DB30C10 for clarity. A: side view in stick rendering; carbon atoms are grey, oxygen atoms are red, hydrogen atoms are white; nitrogen atoms are blue. B: side view in space fill rendering with the host in red and the guest in blue. C: top view in stick rendering. D: view of hydrogen bonding of benzylic hydrogens. E: view of interaction of one α−proton of the pyridinium ring with oxygen atoms. F: view of interaction of the other α–proton of the pyridinium ring with oxygen atoms. Centroid to centroid distance between aromatic rings of host: 6.98 Å; angle 4.8 o. Centroid to centroid distance between host and guest aromatic rings: 3.5 Å. Bipyridyl twist: 13.1 ⁰. CH---O H-bond distances in Å and bond angles in degrees: a: 2.76, 146; b: 2.55, 122; c: 2.81,147; d: 2.56, 128; e: 2.72, 122; f: 2.47, 150; g: 2.57, 118; h: 2.64, 122; i: 2.52, 147; j: 2.64, 113.

CONCLUSIONS In conclusion we have prepared and isolated three new [2]rotaxanes and one new [3]rotaxane with DB30C10 as the host and viologens as the guest species. These are the first examples of viologen rotaxanes using DB30C10 as the cyclic species. The new rotaxanes were characterized by 1H NMR,

13

C NMR, and HRMS. An X-ray crystal

structure of [2]rotaxane 7 clearly shows that the guest is threaded through the cavity. These findings demonstrate that DB30C10 does indeed form pseudorotaxane complexes with viologens in solution.



EXPERIMENTAL General: 3,5-Diphenylbenzyl bromide (1),64 4-trityl-(3’-propynyloxy)benzene (9),65 halfquat 5

66

and 2-azidoethyl tosylate

67

were synthesized according to literature

procedures. Melting points were measured with a Mel-Temp II device in capillary tubes and are uncorrected. 1H and

13

C NMR spectra were obtained at ambient temperatures

on JEOL Eclipse Plus 500 MHz, Varian Unity Plus or Varian MR 400 MHz spectrometers. 1H NMR and

13

C spectra were corrected relative to residual solvent

peaks. High-resolution mass spectra (HRMS) were obtained with an Agilent 6220 LCMS electrospray solution ioniation time-of-flight (ESI TOF) spectrometer using acetonitrile as solvent. Ion exchange was performed with Amberlite IRA-400, strongly basic anion exchange resin 100-200 mesh. Column chromatography was performed with silica gel, 40-63 µm, 60 Ȧ from Sorbent Technologies, or Flash Alumina-N from Agela Technologies. [2]Rotaxane 4: Method 1: Half-quat 3 (0.22 g, 0.4 mmol), and DB30C10 (0.65 g, 1.2 mmol) were combined in acetone. The solvent was removed. 3,5-Diphenylbenzyl bromide (1, 0.14 g, 0.4 mmol) was added and the mixture was dissolved in DCM (9 mL) and MeCN (1 mL) and stirred at room temperature. The solution went from yellow to dark orange over the course of several days. After a month of stirring at room temperature, the solid precipitate was collected by vacuum filtration and washed with DCM. The filtrate was collected, concentrated to dryness and the resulting solid was washed with ethyl acetate. The filtrate was concentrated and washed with toluene. The toluene filtrate was collected. Some crystals formed (crown ether) and they were filtered. The resulting bright orange residue from the toluene filtrate was dissolved in 24 ACS Paragon Plus Environment

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DCM to load onto a column. However, some yellow solid would not dissolve. This solid was filtered and the resulting filtrate was concentrated by rotary evaporation. The residue was dissolved in MeOH and water and a saturated solution of KPF6 was added. The organic solvent was removed by rotary evaporation and the solid was collected, washed with water and allowed to dry on the frit.

The resulting orange solid was

dissolved in a minimum amount of DCM and purified by column chromatography (silica gel; DCM: MeOH 10:1). The orange fractions were contaminated with unreacted dumbbell and crown ether. The orange fractions were collected, concentrated and subjected to another round of chromatography (neutral alumina; EA -> 5% MeOH in EA). The orange fractions were combined and concentrated to give a red solid, 80 mg (13%). Method 2: 4,4’-Bipyridyl (0.13 g, 0.8 mmol) was dissolved in MeCN (10 mL) and filtered into a round bottomed flask. DB30C10 (1.42 g, 2.6 mmol) was added and enough DCM was added to dissolve the solids. 3,5-Diphenylbenzyl bromide (1, 0.60 g, 1.9 mmol) was added and the solution was allowed to stir at room temperature. The solution went from orange to yellow with precipitate. After 4 weeks, the solid was filtered and washed well with MeCN. The filtrate was concentrated and DCM was added. The solid material was filtered and the filtrate was concentrated to give an orange solid. The solid was purified by column chromatography (DCM –> 10% MeOH in DCM –> 12:2:1 MeOH:2M NH4Cl: MeNO2). The orange fractions were collected and purified twice more (silica gel; 20:1 MeOH:2M NH4Cl and silica 30:1 MeOH:2M NH4Cl). The orange solid was dissolved in MeOH and water. A saturated solution of KPF6 was added. The organic solvent was removed by rotary evaporation and the red solid was collected, washed with water and



allowed to dry on the frit and then under vacuum to yield a dark orange solid, 32.8 mg (2.7%), mp 80.5 – 105 ⁰C. 1H NMR (400 MHz, CDCl3) δ 8.99 (d, J = 7 Hz, 4H), 7.92 (d, J = 7 Hz, 6H), 7.72 (m, 12H), 7.46 (t, J = 7 Hz, 8H), 7.38 (t, J = 7 Hz, 4H), 6.49 (dd, J = 6, 4 Hz, 4H), 6.26 (dd, J = 6, 4 Hz, 4H), 5.88 (s, 4H), 3.78 (m, 8H), 3.48 (m, 16H), 3.42 (m, 8H).

13

C NMR (101 MHz, CDCl3) δ 147.8, 147.3, 145.6, 143.4, 139.7, 133.7, 129.2,

128.3, 127.4, 126.7, 126.0, 121.8, 114.3, 70.4, 70.2, 68.8, 64.8 (17 peaks, theory 18). 1

H NMR (400 MHz, CD3CN) δ 8.97 (d, J = 7 Hz, 4H), 8.01 (d, J = 2 Hz, 2H), 7.96 (d, J =

7 Hz, 4H), 7.80 (m, 12H), 7.50 (m, 8H), 7.47 (m, 4H), 6.48 (m, 4H), 6.31 (m, 4H), 5.89 (s, 4H), 3.73 (m, 8H), 3.43 (m, 8H), 3.36 (m, 16H). 1H NMR (400 MHz, DMSO-d6) δ 9.31 (d, J = 8 Hz, 4H), 8.29 (d, J = 8 Hz, 4H), 7.95 (app. s, 2H), 7.86 (app. s, 4H), 7.85 (t, J = 7 Hz, 8H), 7.48 (m, 8H), 7.40 (m, 4H), 6.45 (m, 4H), 6.13 (m, 4H), 5.97 (s, 4H), 3.76 (t, J = 4 Hz, 8H), 3.38 (t, J = 4 Hz, 8H), 3.26 (s, 16H). 13C NMR (101 MHz, DMSO-d6) δ 152.7, 150.9, 145.3, 141.91, 140.95, 139.3, 135.4, 129.0, 128.0, 127.1, 126.7, 125.9, 122.0, 63.1 (14 peaks, theory 18). HRMS (ESI TOF) C76H78F6N2O10P (M-PF6)+, m/z 1323.5293 (calc.), 1323.5295 (found), error 0.1 ppm; C76H78N2O10 (M-2PF6)2+, m/z 589.2823 (calc.), 589.2837 (found), error 2.4 ppm. [2}Rotaxane 7 and [3]Rotaxane 8: DB30C10 (2.23 g, 4.2 mmol) and bis(half-quat) 5 (0.6145 g, 0.9 mmol) were dissolved in the minimal amount of MeCN/DCM. The blocking group 1 (0.6093 g, 1.9 mmol) was added and the solution was allowed to stir at room temperature. The solution went from clear yellow to orange with a precipitate to murky over the course of two days. After 4 weeks, the yellow solid was collected by vacuum filtration and washed with acetone. The red filtrate was concentrated to dryness and re-dissolved in DCM. Ion exchange resin (Amberlite IRA 400 Cl-) was added and


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the solution was stirred at room temperature for 30 min. The ion exchange resin was filtered and the solvent was removed. The resulting residue was purified by column chromatography (silica gel; DCM:MeOH 20:4 -> MeOH -> MeOH:NH4Cl 10:1 -> MeOH:NH4Cl:MeNO2 11:2:1). A red band started to move in MeOH and moved more in MeOH:NH4Cl. Two orange bands were collected and concentrated. The solids were treated with MeCN and filtered to remove chloride salts; KPF6 was added and the resulting solution was evaporated.

The solids were washed with water to remove

excess KPF6 and dried. The first band yielded an orange solid, [3]rotaxane 8, 90.7 mg (4 %), mp 110 – 115 ⁰C. 1H NMR (400 MHz, CD3CN) δ 9.02 (d, J = 7 Hz, 4H), 8.86 (d, J = 7 Hz, 4H), 8.02 (t, J = 2 Hz, 2H), 7.95 (m, 8H), 7.80 (m, 12H), 7.55 (s, 4H), 7.49 (m, 8H), 7.42 (t, J = 7 Hz, 4H), 6.51 (dd, J = 6, 4 Hz, 8H), 6.33 (dd, J = 6, 4 Hz, 8H), 5.93 (s, 4H), 5.80 (s, 4H), 3.76 (m, 16H), 3.48 (m, 16H), 3.38 (m, 32H). 1H NMR (400 MHz, DMSO-d6) δ 9.33 (d, J = 7 Hz, 4H), 9.14 (d, J = 7 Hz, 4H), 8.30 (m, 8H), 8.03 (t, J = 2 Hz, 2H), 7.98 (d, J = 2 Hz, 2H), 7.86 (d, J = 7 Hz, 4H), 7.60 (s, 4H), 7.49 (m, 8H), 7.41 (m, 4H), 6.45 (m, 8H), 6.13 (m, 8H), 5.98 (s, 4H), 5.87 (s, 4H), 3.77 (m, 16H), 3.45 (m, 16H), 3.27 (m, 32H).

13

C NMR (101 MHz, CD3CN) δ 149.8, 149.6, 148.7, 146.9, 146.4,

143.8, 140.6, 135.6, 135.2, 130.6, 130.1, 129.2, 128.2, 128.0, 127.7, 127.4, 122.2, 118.4, 114.4, 71.1, 70.9, 70.6, 69.5, 65.6, 64.7 (25 peaks, theory 25). HRMS (ESI TOF) C122H134F12N4O20P2 (M-2PF6)2+, m/z 1132.9449 (calc.), 1132.9465 (found), error 1.4 ppm; C122H134F6N4O20P (M-3PF6)3+, m/z 706.9750 (calc.), 706.9765 (found) error 2.1 ppm. The second band also provided an orange solid, [2]rotaxane 7, 0.4841 g (28 %), mp 137.5-144.8 ⁰C. 1H NMR (400 MHz, DMSO-d6) δ 9.44 (d, J = 7 Hz, 4H), 9.24 (d, J = 7 Hz, 4H), 8.43 (dd, J = 16, 7 Hz, 8H), 8.00 (m, 6H), 7.80 (m, 8H), 7.57 (s, 4H), 7.48 (t, J



Page 28 of 38

= 7 Hz, 8H), 7.40 (t, J = 7 Hz, 4H), 6.34 (dd, J = 6, 4 Hz, 4H), 5.95 (s, 4H), 5.90 (dd, J = 6, 4 Hz, 4H), 5.86 (s, 4H), 3.72 (m, 8H), 3.41 (m, 8H), 3.22 (m, 8H), 3.14 (m, 8H).

1

H

NMR (400 MHz, CD3CN) δ 9.02 (d, J = 7 Hz, 4H), 8.95 (d, J = 7 Hz, 4H), 8.14 (dd, J = 7, 2 Hz, 8H), 8.02 (t, J = 2 Hz, 2H), 7.80 (m, 12H), 7.53 (s, 4H), 7.45 (m, 8H), 7.42 (m, 4H), 6.45 (dd, J = 6, 4 Hz, 4H), 6.21 (dd, J = 6, 4 Hz, 4H), 5.92 (s, 4H), 5.86 (s, 4H), 3.76 (m, 8H), 3.49 (m, 8H), 3.40 (m, 8H), 3.33 (m 8H).

13

C NMR (101 MHz, CD3CN) δ

148.8, 148.7, 148.2, 146.6, 146.5, 143.7, 140.6, 135.6, 135.5, 130.4, 130.0, 129.0, 128.2, 127.74, 127.69, 126.9, 126.8, 122.2, 114.6, 71.0, 70.9, 70.6, 69.5, 65.2, 64.4 (25 peaks, theory 25). HRMS (ESI TOF) C94H94F12N4O10P2 (M-2PF6)2+, m/z 864.8138 (calc.), 864.8138 (found), error 0 ppm; C95H95F6N4O10P (M-3PF6)3+, m/z 528.2210 (calc.), 528.2216 (found), error 1 ppm. [2]Rotaxane 12: Viologen diazide 10 (0.61 g, 1 mmol) and DB30C10 (1.65 g, 3 mmol) were combined in a minimum amount of dry MeCN. The solution turned bright red. The alkyne blocking group 9 (0.81 g, 2 mmol) and dry DCM were added until all the solid was dissolved. The solution was degassed by bubbling Ar (g) through it for several min. Cu(CH3CN)4PF6 (0.38 g, 1 mmol) was added and DCM and MeCN were added until all solids were just dissolved. The solution was degassed with Ar (g) and allowed to stir at room temperature under positive pressure of Ar (g). After 20 days, the solution was opened to atmosphere. The solution went from red to brown and a precipitate formed. The solution was filtered through pad of a Celite®. The filtrate was concentrated and the resultant orange solid was washed with ethyl acetate, leaving a white solid, which was filtered off and the solvent was removed from the filtrate by rotary evaporation. This process was repeated with DCM and acetone. The red tacky solid was washed with


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toluene. The red compound did not dissolve in toluene. Red and white solids were collected on the frit and dissolved in MeOH/acetone/DCM; Amberlite IRA-400 (Cl) ion exchange resin was added. The solution was allowed to stir for several h. The solution went from orange and clear to cloudy with a white precipitate. The solid was removed by filtration and the filtrate was concentrated and the residue was purified by column chromatography (silica gel; MeOH:2M NH4Cl 50:1 –> 20:1 –> 10:1). The orange band was collected, and the solvent removed. The solid was dissolved in water with a little bit of MeOH and a saturated aqueous solution of KPF6 was added. The red sticky solid was collected, washed with water and air dried, 0.1105 g (6%), mp 126.1-145.1 °C. 1H NMR (400 MHz, CD3CN) δ 8.78 (d, J = 7 Hz, 4H), 8.04 (s, 2H), 7.87 (d, J = 7 Hz, 4H), 7.24 (m, 30H), 7.16 (d, J = 9 Hz, 4H), 6.92 (d, J = 9 Hz, 4H), 6.53 (dd, J = 6, 4 Hz, 4H), 6.45 (dd, J = 6, 4 Hz, 4H), 5.13 (m, 12H), 3.81 (m, 8H), 3.64 (m, 8H), 3.58 (m, 16H). 13C NMR (101 MHz, CD3CN) δ 157.3, 148.9, 148.2, 146.8, 145.0, 140.6, 132.9, 131.6, 128.6, 126.9, 126.7, 125.9, 122.7, 115.0, 114.6, 71.1, 70.7, 69.7, 65.2, 62.2, 60.6, 49.8 (22 peaks, theory 24). 1H NMR (400 MHz, DMSO-d6) δ 9.05 (d, J = 7 Hz, 4H), 8.34 (m, 6H), 7.29 (t, J = 7 Hz, 12H), 7.20 (t, J = 7 Hz, 6H), 7.12 (m, 12H), 7.04 (d, J = 9 Hz, 4H), 6.96 (d, J = 9 Hz, 4H), 6.56 (dd, J = 6, 4 Hz, 4H), 6.40 (dd, J = 6, 4 Hz, 4H), 5.17 (m, 12H), 3.84 (m, 8H), 3.60 (m, 8H), 3.49 (m, 16H).

13

C NMR (101 MHz, DMSO-d6) δ

156.3, 149.3, 147.0, 146.7, 143.4, 139.2, 132.0, 130.8, 128.1, 126.8, 126.3, 125.8, 114.0,

64.2,

61.3,

60.3,

49.6

(17

peaks,

theory

24).

HRMS

(ESI

TOF)

C98H100F12N8NaO10P2 (M + Na)+, m/z 1894.6670 (calc.), 1894.6626 (found), error -2.3 ppm; C98H100F6N8O10P (M – PF6)+, m/z 1725.7096 (calc.), 1725.7084 (found), error -



0.70 ppm; C98H100N8O10 (M-2PF6)2+, m/z 790.3724 (calc.), 790.3721 (found), error -0.4 ppm. ASSOCIATED CONTENT: Supporting Information. Syntheses of 2, 3, 6, 10 and 11. 1H and 13C NMR spectra, high resolution mass spectra of all compounds, COSY and NOESY spectra of 4, 7, 8 and 12 and X-ray crystallographic data for [2]rotaxane 7. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION. Corresponding Author. [email protected] ORCID Harry W. Gibson: 0000-0001-9178-6691 ACKNOWLEDGEMENTS We are grateful to the National Science Foundation for supporting this research through grants CHE-1106899 and CHE-1507553. We also acknowledge the National Science Foundation for funds to purchase the Agilent 6220 Accurate Mass TOF LC/MS Spectrometer (CHE-0722638). REFERENCES 1.

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Ka for this system has not been determined. However, the cis-diol of DB30C10 complexes pyridinium TFSI with Ka = 746 M-1 in DCM at 23 oC (refrence 36h). For comparison the cis-diol of DB30C10 complexes N,N'-dimethyl-4,4'bipyridinium bis(hexafluorophosphate) with Ka = 1.1 x 103 M-1 in acetone.

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VIOLOGEN-BASED ROTAXANES FROM DIBENZO-30-CROWN-10

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