Neighboring Component Effect in a Tri-stable [2]Rotaxane

Article abstract of DOI:10.1021/jacs.8b08519

The redox properties of cyclobis(paraquat-p-phenylene)cyclophane (CBPQT⁴⁺) render it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT⁴⁺) and radical-pairing interactions in its partially reduced state (CBPQT^2(a€￠+)). Although it is expected that the fully reduced neutral state (CBPQT⁽⁰⁾) might behave as a ?€-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tri-stable [2]rotaxane in which a CBPQT⁴⁺ ring is mechanically interlocked with a dumbbell component containing five recognition sites - (i) a bipyridinium radical cation (BIPY^(a€￠+)) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT⁴⁺ ring and the triazole units, and (v) the CBPQT^2(a€￠+) ring and the reduced BIPY^(a€￠+) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional non-covalent bonding interactions between the CBPQT⁽⁰⁾ ring, acting as a donor in its neutral state, and the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, covalently linked, organic molecules of neighboring group participation (anchimeric assistance giving way to transannular interactions) in small-, medium-, and large-membered rings.

Full text of DOI:10.1021/jacs.8b08519

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Article
The Neighboring Component Effect in a Tristable [2]Rotaxane
Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu,
Marco Frasconi, William A. Goddard, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08519 • Publication Date (Web): 25 Sep 2018
Downloaded from http://pubs.acs.org on September 25, 2018
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The Neighboring Component Effect in a Tristable [2]Rotaxane
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∇
Yuping Wang, Tao Cheng, Junling Sun, Zhichang Liu, Marco Frasconi,
William A. Goddard III,‡ and J. Fraser Stoddart*,†, ,║
⊥
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA
‡Materials and Process Simulation Center, California Institute of Technology, 1200 East California Boulevard,
Pasadena, California 91125, USA
∇School of Science, Westlake University, 18 Shilongshan Road, Hangzhou 310024, China
§Department of Chemical Sciences, University of Padova, Via Marzolo 1, Padova 35131, Italy.
⊥Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, P.R. China
║School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
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*E-mail: stoddart@northwestern.edu
MAIN TEXT
*Correspondence Address
Professor J Fraser Stoddart
Department of Chemistry
Northwestern University
2145 Sheridan Road
Evanston, IL 60208-3113 (USA)
Tel: (+1)-847-491-3793
E-Mail: stoddart@northwestern.edu
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Abstract: The redox properties of cyclobis(paraquat-p-phenylene) cyclophane (CBPQT4+) renders
it a uniquely variable source of recognition in the context of mechanically interlocked molecules,
through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT4+) and radical-
pairing interactions in its partially reduced state (CBPQT2(•+)). Although it is expected that the fully
reduced neutral state (CBPQT(0)) might behave as a p-donating recognition unit, resulting in a
dramatic change in its binding properties when compared with the other two redox states, its role
in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis
of a tristable [2]rotaxane in which a CBPQT4+ ring is mechanically interlocked with a dumbbell
component containing five recognition sites¾(i) one, a bipyridinium radical cation (BIPY(•+))
located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units
linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT4+
ring and the triazole units, and (v) the CBPQT2(•+) ring and the reduced BIPY(•+) unit in the
dumbbell component, investigations in solution have now confirmed the presence of additional
noncovalent bonding interactions between the CBPQT(0) ring, acting as a donor in its neutral state
towards the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this
piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler,
covalently linked, organic molecules of neighboring group participation (anchimeric assistance
giving way to transannular interactions) in small-, medium-, and large-membered rings.
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Introduction
The neighboring group effect,1-2 alias anchimeric assistance, occupies a special place in the annuls
of physical organic chemistry and organic stereochemistry. The intramolecular nature of the effect
which, not only leads to the speeding up of reactions3-8 when they are potentially operative, but
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also often dictates9-11 regio- and stereospecifically the constitutions and configurations of the
products obtained under tight stereochemical control in what are geometrically constrained
environments. This intramolecular effect, which was explored in acyclic as well as cyclic
molecules, is one of the keystones of what became known as conformational analysis.12 Already
back in 2012, we argued in a review13 that the thermodynamic parameters associated with model
host-guest complexes provide a good starting point to rationalize the ratio of ground-state and
metastable co-conformations in bistable[2]rotaxanes at equilibrium. We demonstrated that these
[2]rotaxanes exhibit a strong correlation between their ground-state distribution constants and the
relative free energies of closely related model complexes. We pointed out in the review that the
semiquantitative treatment of the ground-state distribution of co-conformations (translational
isomers)14 is reminiscent of the type of conformational analysis12 that can be carried out on
multiply substituted cyclohexane rings once the conformational free energies¾also referred to as
A-values12¾between equatorial and axial substituents on monosubstituted cyclohexane rings are
known. Although caution has to be exercised in applying the principle12 of the additivity of A-
values, the approach is generally one that can be used in a predictive manner. Now, let us assume
that we introduce into a rotaxane, a recognition site between the ring and dumbbell components
that is known to be an extremely weak one, at least in the context of a host-guest complex.15 Will
we witness an intramolecular neighboring component effect in the rotaxane that is analogous to
the neighboring group effect in molecules that only contain covalent bonds?
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Recently, we reported16 that, although a 1:1 inclusion complex between the fully reduced p-
electron-rich cyclobis(paraquet-p-phenylene) (CBPQT(0)) ring and the p-electron-deficient guest
1,4-dicyanobenzene (DCB) can be isolated as a solid-state superstructure, the binding constant for
this host-guest complex in (toluene) solution is too small to be measured by the usual techniques.
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Now, what if this 1:1 host-guest complex was to be transplanted, so to speak, into a rotaxane,
would the DCB unit constitute a recognition site for the CBPQT(0) ring? In this paper, we set out
to answer this question by designing a tristable [2]rotaxane, which (i) allows us to explore the
interactions of the CBPQT(0) ring with recognition sites that reflects the electronic properties of
DCB and, at the same time, (ii) makes it possible for us to understand more about the CBPQT ring
in its different redox states, i.e., CBPQT(0), CBPQT2(•+) and CBPQT4+ interacting with three
different recognition sites on the dumbbell component¾namely, p-electron-deficient, radical and
p-electron-rich sites, respectively¾which have been chosen for incorporation into the dumbbell
component of a tristable [2]rotaxane. A bipyridinium (BIPY2+) unit was introduced into the
dumbbell as the radical recognition site thanks to the well-established17-20 recognition between the
CBPQT2(•+) ring and the bipyridinium radical cation (BIPY•+). By contrast, the choice of the p-
electron-rich recognition sites for CBPQT4+ is subtle, since the ideal candidate should be able to
interact with the CBPQT4+ ring, while not competing with the desire of the p-electron-rich
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CBPQT(0) to interact with the p-electron-deficient recognition sites on the dumbbell component.
In order to meet these two requirements, we have introduced triazole units which act (i) as the p-
electron-rich recognition sites for CBPQT4+ and, at the same time, (ii) as the means of templating
the synthesis of the tristable [2]rotaxane by the copper(I) catalyzed azide-alkyne cycloaddition21
(CuAAC). Finally, for the p-electron-deficient sites, tetrafluorinated phenylene (ArF) units are
introduced16 in the knowledge that the changes in the chemical environments of these ArF units
can be monitored by 19F NMR spectroscopy, an additional tool which would provide us with clear-
cut evidence for the location of the ring component in both the fully oxidized and neutral states.
Based on these considerations, we report (Scheme 1a) herein a tristable [2]rotaxane, where the
CBPQT4+ ring is mechanically interlocked with a dumbbell component containing BIPY2+,
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triazole and ArF units. By performing experiments at different redox potentials on this [2]rotaxane,
any interactions between the CBPQT(0) ring and the p-electron-deficient ArF units in solution will
be revealed. We show, by (i) 1H and 19F NMR spectroscopies, (ii) UV/Vis/NIR spectroscopy, (iii)
cyclic voltammetry (CV) studies and (iv) DFT-based calculation that the different redox states of
the ring interact with different recognition sites on the dumbbell component, demonstrating that
this tristable [2]rotaxane can serve as a multi-state molecular switch.22
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Results and Discussion
Synthetic Protocols. The para-substituted tetrafluorophenylene building block 3, which was
prepared according to literature procedures,23 was subjected (Scheme 1b) to bromination,
nucleophilic substitution with 4,4'-bipyridine and counterion exchange (NH4PF6 / H2O) in order
to afford the viologen-based precursor 5•2PF6 in an overall yield of 60 %. While the radical and
the p-electron-deficient sites are already incorporated into 52+, the formation of the triazole unit
was achieved along with the formation of the [2]rotaxane, 1•6PF6, in the presence of the CBPQT4+
ring and the alkyne-functionalized stopper precursor 6, by employing (Scheme 1b) a Cu-mediated
synthetic methodology in a threading-followed-by-stoppering protocol developed recently by us.21
Briefly, the azide-functionalized, dumbbell precursor 5•2PF6 and CBPQT•4PF6 were dissolved in
MeCN in a 1:1 molar ratio under an atmosphere of Ar, followed by the addition of an excess of
Cu0 dust (diam < 425 µm). Upon stirring the reaction mixture for 20 min, the BIPY2+ units in both
52+ and CBPQT4+ are reduced by Cu0 to their corresponding radical cationic states, and the
solution turns dark purple on account of the formation of the inclusion complex 5•+ÌCBPQT2(•+)
.
CuI ions are generated in situ during this process, allowing the CuAAC reaction to take place and
generate the [2]rotaxane following the addition of the alkyne-functionalized stopper precursor 6
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to the reaction mixture. The dumbbell 2•2PF6 was also prepared (Scheme 1b) in order to serve as
a reference. Compounds 1•6PF6 and 2•2PF6 were purified by column chromatography and
counterion exchange (NH4PF6 / H2O). The characterization of the compounds was carried out by
high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) and by both 1H and 13C
NMR spectroscopies. See the Supporting Information for detailed synthetic procedures and
characterizations.
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Fully oxidized states. With both the dumbbell and the [2]rotaxane in hand, we investigated their
properties in their fully oxidized states by performing 1H and 1H–1H COSY NMR experiments. In
comparison (Figure 1b) with the 1H NMR spectrum of the structurally symmetric 2•2PF6, that of
1•6PF6 displays a more complicated spectrum (Figure 1b) as a consequence of the presence of the
ring component, shuttling along the dumbbell component slowly on the 1H NMR timescale. Since
the CBPQT4+ ring is located (Figure 1a) on one portion (vide infra) of the dumbbell component, it
has little effect on the chemical shift of one set of protons, e.g., protons a and b, etc. As a result,
these protons have similar chemical shifts to those observed in the case of the dumbbell 2•2PF6.
On the other hand, their counterparts, such as protons a', b' and c', experience significant changes
in chemical shift on account of their being much closer to the ring component. These chemical
shifts define the location of the CBPQT4+ ring. Indeed, amongst all these shifts, that of the triazole
proton (HTR') experience the most significant changes in chemical shift (Figure 1b) in an upfield
direction by almost 5 ppm, i.e., from 8.36 to 3.72 ppm, compared with its counterpart (HTR). This
observation suggests that the shielding region of the CBPQT4+ ring, located inside the ring cavity
as a result of aromatic ring currents,24 has the profound influence upon one of the two triazole
protons. By contrast, the signals for both proton a' and Hα' are shifted downfield. In addition, the
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19F NMR spectrum of 1•6PF6 reveals that the signals for Fa' and Fb' are also downfield-shifted,
compared with those of Fa and Fb, respectively. This observation confirms unambiguously the
hypothesis that, in the fully oxidized state, the CBPQT4+ ring encircles preferentially the relatively
p-electron-rich triazole unit rather than the p-electron-deficient ArF units. It is also noteworthy that,
while the shuttling motion of the CBPQT4+ ring along the dumbbell component is often observed24
in analogous viologen-based [2]rotaxanes, variable-temperature (VT) 1H and 19F NMR spectra of
1•6PF6 show that the two sets of the peaks for the protons and 19F nuclei on the dumbbell
component do not undergo coalescence within the temperature range from 233 to 343 K, an
observation which indicates that shuttling does not occur on the NMR timescale. See SI, Section
5. Indeed, the limiting chemical shifts for the slowly exchanging methylene protons a' (6.21 ppm)
and a (6.16 ppm) at 343 K allows an estimation25 of the energy barrier which is greater than 17.5
kcal mol–1. This observation is rationalized by the fact that in addition to the Coulombic repulsion
arising from the centrally located BIPY2+ unit on the dumbbell component, the introduction of the
p-electron-deficient ArF units into the dumbbell component presumably further increases the
energy barrier, preventing the CBPQT4+ ring from shuttling.
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Radical cationic states
In order to understand the molecular behavior of the [2]rotaxane in its radical state, cyclic
voltammetry (CV) was performed on 16+ and 22+. Upon scanning the potential in the negative
direction, the dumbbell 22+ undergoes (Figure 2a) a single-electron reduction process at –244 mV,
generating the radical state 2•+. This potential, which is significantly higher than that17 (–460 mV)
of the analogous viologen-based compounds having no ArF rings, is ascribed to the fact that the
BIPY2+ unit in 22+ raises its propensity to receive an electron because of the adjacent electron-
withdrawing ArF rings. In comparison, the highly positively charged [2]rotaxane 16+ is more prone
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to receive electrons: the reduction process occurs in a stepwise manner. In the first instance, it
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takes up (Figure 2b) two electrons at –143 mV¾with one going18, 24 to the BIPY2+ unit in the
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dumbbell component and the other going to one of the two BIPY2+ units in CBPQT4+¾to generate
a bisradical tetracationic species 12+2(•+), an intermediate which is stabilized by the pimerization
between the two reduced BIPY•+ radical cations. Subsequently, a single electron reduction of the
other BIPY2+ unit in CBPQT2+(•+) at –217 mV leads to formation of the trisradical tricationic state
13(•+). The anodically shifted reduction potential of this overall three-electron reduction process, in
comparison with those of other [2]rotaxanes17, 24 containing CBPQT4+ encircling a BIPY2+ based
dumbbell, can also be ascribed to the electron-withdrawing effect of the ArF rings. Following an
addition of an excess of Zn dust to the MeCN solution (100 µM) of 16+ and 22+, respectively, the
radical cationic states of the [2]rotaxane and the dumbbell were probed by UV/Vis/NIR
spectroscopy. See SI, Section 6. The spectrum of the dumbbell 2•+ displays absorption bands at
wavelengths around 400 and 605 nm, both peaks characteristic26 of the BIPY•+ radical species.
The lack of the NIR absorption indicates that the intermolecular radical-pairing interaction
between two dumbbell molecules is very weak at 100 µM. On the other hand, an absorption band
center on 1091 nm—evidence for the formation of the trisradical tricationic species—is observed
in the case of the [2]rotaxane 13(•+), which is further confirmed (Figure S9c) as an intramolecular
process with the assistance of concentration-dependent UV/Vis/NIR experiments. Thus, we
conclude that, upon the reduction to its trisradical tricationic state 13(•+), the CBPQT2(•+) ring
encircles the BIPY•+ station on the dumbbell.
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Neutral states
The recognition between CBPQT(0) and the ArF rings in the [2]rotaxane in its neutral form was
investigated in detail. UV/Vis/NIR Spectroscopic investigations were performed on the neutral
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[2]rotaxane and its dumbbell counterpart¾namely 1(0) and 2(0)¾which can be generated16 by
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titrating the strong chemical reductant cobaltocene (CoCp2) into both MeCN solutions (50 µM) of
16+ and 22+, respectively, under an Ar atmosphere. Specifically, upon the addition of 1.0 equiv of
CoCp2 to the solution of 22+, the initial colorless solution (Figure 3a, blue trace) turns dark blue,
displaying (Figure 3a, purple trace) similar absorption bands to those of the solution of 22+ treated
with Zn dust, indicating the rapid formation of the radical state 2•+. After further addition of 5
equiv of CoCp2 to this solution,27 the reduction from 2•+ to 2(0) is confirmed by the disappearance
of the radical absorption band (605 nm) and the concomitant increase of the absorption band at
371 nm, characteristic16, 28-29 of the neutral BIPY(0) units (Figure 3a, red trace). A closer
examination of the spectra of 2(0) revealed the presence of a weak absorption band at 520 nm—a
feature that has not been observed in the previously reported BIPY(0)-contained systems. The
reduced forms of the of the [2]rotaxane were generated by titrating a solution of 16+ with CoCp2
as well. The conversion of all three BIPY2+ units in 16+ to their radical cationic states was achieved
(Figure 3b, purple trace) after the addition of 3 equiv of CoCp2. The resulting spectrum is
characterized by an absorption band at 1091 nm which corresponds to the formation of the
trisradical tricationic species. This absorption band decreased in its intensity upon the further
addition of 8 equiv of CoCp2, resulting in a new set of peaks at 376 (Figure 3b, red trace) and 550
nm (Figure 3b-c, red trace) which indicate the generation of the neutral species 1(0). It should be
noted that the absorption band at 550 nm has a much higher extinction coefficient when compared
with that of the peak at 520 nm for 2(0).
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Taking all these observations into account, we have hypothesized that both the absorption bands
at 520 nm for 2(0) and 550 nm for 1(0) result from charge transfer interactions. In the case of 2(0),
charge transfer process takes place (Figure 3d) intramolecularly between the neutral BIPY(0) and
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the ArF units. See SI, Section 9. In contrast, rather than the ArF units interacting with the BIPY(0)
unit on the dumbbell, the mechanically interlocked structure of 1(0) allows the ArF units to interact
(Figure 3d) with the neutral CBPQT(0) as a result of its being encircled by the ring. In this manner,
charge transfer between the ArF unit and the two BIPY(0) units in the ring component can take
place in a face-to-face manner, while the shuttling motion of the ring component along the
dumbbell makes it possible for the ring to adopt the most optimized co-conformation so as to
maximize these interactions. In other words, charge-transfer is facilitated in the case of 1(0), giving
rise to the significantly higher molar extinction coefficient as well as the 30 nm red-shifted
absorption band on going from the neutral dumbbell to the neutral [2]rotaxane. By employing a
different reduction protocol (see SI, Section 9), the absorption spectra of these neutral species were
also measured in toluene, and shown to be consistent with the observations in MeCN.
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The results from CV also help us gain an understanding of the interactions between the ArF rings
and CBPQT(0) in the case of 1(0). In analogous viologen-based [2]rotaxanes24 without ArF rings,
the reduction from the trisradical tricationic state to the neutral state occurs in a stepwise manner,
wherein the formation of the bisradical dicationic intermediate¾stabilized by radical-pairing
interactions¾can be observed. By contrast, the [2]rotaxane 13(•+) undergoes (Figure 2b) a
simultaneous three-electron reduction at –686 mV to generate 1(0). This observation can be
rationalized by the fact that, once one of the BIPY•+ units in CBPQT2(•+) is reduced to the neutral
state, the ring will move away from the radical recognition site as a result of the interactions
between the ArF rings and the BIPY(0) unit, leading to the collapse of the bisradical dicationic
intermediate so that all the BIPY•+ units are reduced simultaneously.
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More evidence for the recognition between CBPQT(0) and the p-electron-deficient ArF rings were
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obtained from H and 19F NMR spectroscopies. Samples suitable for NMR experiments were
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prepared16 (see SI, Section 7) by extracting the neutral species 1(0) and 2(0) from a biphasic system
using CD3C6D5 as the organic layer under an Ar atmosphere. By comparing the chemical shifts of
the protons and fluorine nuclei in the dumbbell and [2]rotaxane, we can characterize the fully
reduced [2]rotaxane and identify the recognition sites encircled by the CBPQT(0) ring in the neutral
state. In the case of the dumbbell compound, the reduction from the fully oxidized state 22+ to the
neutral state 2(0) leads to (Figure 4b) significant upfield chemical shifts of (i) the protons Ha and
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Hb on the BIPY2+ unit, which resonate at 5.47 and 5.60 ppm, respectively, as well as (ii) a signal
for the methylene protons a at around 3.55 ppm, revealing the loss of the aromaticity upon
reduction of the BIPY2+ unit. In the case of the neutral [2]rotaxane 1(0), the presence of a single
species was confirmed (Figure S11) by diffusion-ordered 1H NMR spectroscopy (DOSY), which
afforded a diffusion coefficient of 8.2 ´ 10–10 m2 s–1, demonstrating that the mechanically
interlocked structure is retained during reduction. All the proton resonances of 1(0) can be assigned
with the assistance of 1H–1H NOESY spectroscopy. In particular, the peaks for Ha1 and Hb1 on the
BIPY(0) units in the CBPQT(0) ring appear, respectively, at 5.41 and 5.35 ppm (Figure 4c), values
which are close to those reported16 for the free neutral CBPQT(0). On the other hand, the 19F NMR
spectra help us pinpoint the location of the CBPQT(0) ring component in the neutral state. Notably,
the fluorine nuclei resonances for the [2]rotaxane 1(0) are shifted (Figure 4c) upfield compared with
those for the dumbbell 2(0), as a result of shielding by CBPQT(0) encircling one of the two ArF rings
at any instant in time. In particular, the signal for Fb experiences an upfield shift of 2.34 ppm,
which is greater than that (0.46 ppm) of Fa, indicating that Fb resides closer to the center of the
cavity of CBPQT(0) than does Fa. It should also be noted that DNMR shows (see SI, Section 8)
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only one set of peaks are observed for the protons and fluorine nuclei on the dumbbell component
in 1(0) over a range of temperature from 193 to 298 K, demonstrating that CBPQT(0) undergoes fast
shuttling on the NMR timescale along the dumbbell in the neutral state. In the absence of peak
separations, a maximal energy barrier of 9.6 kcal mol–1 for the shuttling can be estimated.30 Since
shuttling of CBPQT4+ is slow on the NMR timescale in the fully oxidized form of the [2]rotaxane
16+, this redox-controllable energy barrier to shuttling paves the way to developing slide-ring-
based materials31-32 with switchable properties. It should also be noted that the shuttling rate of the
ring at its neutral state can be modulated by varying the binding affinity between the recognition
sites and the ring, i.e., the higher the binding affinity, the lower the shuttling rate.
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Computational Studies
We carried out DFT calculations on the level of M06-2X/6-311G**33 with zero dumping D3
correction34 as implemented in Jaguar 8.235 (more details can be found in SI) to characterize the
interactions of the CBPQT ring component with the [2]rotaxane in different charge states from its
fully oxidized (6+) to the neutral state (0) form. The inclusion of continuum solvent corrections in
the DFT is, however, not sufficient to capture the full effect of solvent and cations on the structures
and energetics, which becomes increasingly important in the case of higher charged states, i.e., the
radical and the fully oxidized states. As a result, we extracted the charges from the DFT
calculations and incorporated them into the UFF universal force field.36 Then we carried out force
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field based molecular dynamics (MD) simulations including explicit solvent with anions (PF6 ) for
several nanoseconds for each of the charge states and each of the ring positions. These calculations
were performed in a box with dimensions of ~ 3.1, ~ 3.1 and 4.1 nm in the x, y and z directions.
The final MD relaxed structures are shown in Figure 5.
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Next, we used the 2PT method37 to calculate the entropy and Gibbs free energy on the basis of the
vibrational modes extracted from the Fourier Transform of the velocity autocorrelation function.
The final free energies (Figure 5) from these MD simulations reveal that the most stable sites are
triazole units, BIPY•+ unit and ArF rings in the case of the fully oxidized, the radical and the neutral
states, respectively. The results are consistent with the experimental observations. In addition, the
energy barrier for the CBPQT ring to shuttle along the dumbbell component can be estimated by
comparing the differences in free energies for various binding sites: the energy barriers are 21.29
and 7.46 kcal mol–1 in the case of the fully oxidized and the neutral states, respectively. These
barriers are in line with the values obtained from dynamic NMR spectroscopy.
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Conclusions
We have investigated the shuttling of the CBPQT ring component in a tristable [2]rotaxane in
different oxidation states and demonstrated (Table 1) how this mechanically interlocked molecule
experiences three different kinds of molecular recognition. The reduction of the ring component
from CBPQT4+ to CBPQT2(•+) to CBPQT(0) alters its affinity toward the five recognition sites on
the dumbbell components, leading to the ring component preferentially encircling (Figure 6) the
triazole, BIPY•+ and ArF units, respectively. Significantly, the discovery of stabilizing interactions
between the CBPQT(0) ring and the p-electron-deficient ArF units¾as experimentally confirmed
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19
by H, F NMR, UV/Vis/NIR spectroscopies and electrochemical studies, and computationally
proved by DFT based MD calculations¾introduces an additional recognition site into CBPQT-
contained rotaxanes. The design of this rotaxane, which demonstrates that an otherwise very weak
interaction between the recognition unit and the ring upon reduction to the fully reduced state can
be enhanced, supports the notion of neighboring component effect and it is more than likely that
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this ubiquitous tenet of conformational analysis in the more traditional areas of stereochemistry
and physical organic chemistry should surface within the context of presumably other
mechanically interlocked molecules.
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Associated Content
Supporting information
Detailed synthetic procedures and characterization (HRMS and NMR ) data for all compounds,
spectroscopic (NMR and UV/Vis/NIR) studies for the rotaxane, X-ray crystallographic analysis
data (CIF) for 2•2PF6, and computational analysis for the rotaxane. This information is available
free of charge via the internet at http://pubs.acs.org.
Acknowledgements
This research is part (Project 34-945) of the Joint Center of Excellence in Integrated Nano-Systems
(JCIN) at King Abdulaziz City for Science and Technology (KACST) and Northwestern
University (NU). The authors would like to thank both KACST and NU for their continued support
of this research.
References
(1)
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Eliel, E. L., Stereochemistry of Carbon Compounds McGraw-Hill New York, NY, 1962.
Capon, B.; McManus, S. P., Neighboring Group Participation. Plenum Press: New York,
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Baasov, T.; Kohen, A. J. Am. Chem. Soc. 1995, 117, 6165–6174.
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Herdewijn, P. Chem. Eur. J. 2012, 18, 857–868.
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Brimioulle, R.; Bach, T. Science 2013, 342, 840–843.
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(11) Martinez-Cuezva, A.; Lopez-Leonardo, C.; Bautista, D.; Alajarin, M.; Berna, J. J. Am.
Chem. Soc. 2016, 138, 8726–8729.
(12) Eliel, E. L., Conformational Analysis. Interscience Publishers: New York, 1965.
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(15) Lehn, J.-M., Supramolecular Chemistry—Concepts and Perspectives. Wiley-VCH,
Weinheim: 1995.
(16) Frasconi, M.; Fernando, I. R.; Wu, Y. L.; Liu, Z. C.; Liu, W. G.; Dyar, S. M.; Barin, G.;
Wasielewski, M. R.; Goddard, W. A.; Stoddart, J. F. J. Am. Chem. Soc. 2015, 137, 11057–11068.
(17) Trabolsi, A.; Khashab, N.; Fahrenbach, A. C.; Friedman, D. C.; Colvin, M. T.; Coti, K. K.;
Benítez, D.; Tkatchouk, E.; Olsen, J.-C.; Belowich, M. E.; Carmielli, R.; Khatib, H. A.; Goddard,
W. A., III.; Wasielewski, M. R.; Stoddart, J. F. Nat. Chem. 2010, 2, 42–49.
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18)
Fahrenbach, A. C.; Barnes, J. C.; Lanfranchi, D. A.; Li, H.; Coskun, A.; Gassensmith, J. J.;
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Liu, Z.; Benítez, D.; Trabolsi, A.; Goddard, W. A., III.; Elhabiri, M.; Stoddart, J. F. J. Am. Chem.
Soc. 2012, 134, 3061–3072.
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(19) Wang, Y.; Frasconi, M.; Liu, W.-G.; Sun, J.; Wu, Y.; Nassar, M. S.; Botros, Y. Y.; Goddard,
W. A., III.; Wasielewski, M. R.; Stoddart, J. F. ACS. Cent. Sci. 2016, 2, 89–98.
(20) Wang, Y. P.; Frasconi, M.; Stoddart, J. F. ACS. Cent. Sci. 2017, 3, 927–935.
(21) Wang, Y.; Sun, J.; Liu, Z.; Nassar, M. S.; Botros, Y. Y.; Stoddart, J. F. Angew. Chem. Int.
Ed. 2016, 55, 12387–12392.
(22) Sun, J.; Wu, Y.; Wang, Y.; Liu, Z.; Cheng, C.; Hartlieb, K. J.; Wasielewski, M. R.; Stoddart,
J. F. J. Am. Chem. Soc. 2015, 137, 13484–13487.
(23) Strømgaard, K.; Saito, D. R.; Shindou, H.; Ishii, S.; Shimizu, T.; Nakanishi, K. J. Med.
Chem. 2002, 45, 4038–4046.
(24) Li, H.; Zhu, Z.; Fahrenbach, A. C.; Savoie, B. M.; Ke, C.; Barnes, J. C.; Lei, J.; Zhao, Y.-
L.; Lilley, L. M.; Marks, T. J.; Ratner, M. A.; Stoddart, J. F. J. Am. Chem. Soc. 2013, 135, 456–
467.
(25) The minimal energy barrier for the ring component to come across at temperature T is
estimated by using the equation ΔG‡ = aT [9.972 + log(Tc /Δn)], where Tc is the coalescence
c
temperature, Δn = 25 is the peak separation of proton a and a' in MHz, constant a = 4.575 ´ 10–3
for units of kcal mol–1. By applying T = Tc = 343 K, the minimal energy barrier is calculated as
17.5 kcal mol–1.
(26) Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990, 55, 4127–4135.
+
(27) Because of the close redox potential between the CoCp2/CoCp2 and the BIPY•+/BIPY(0)
couple, an excess amount of CoCp2 is needed for the reducing process to reach a higher conversion.
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(28) Mohammad, M. J. Org. Chem. 1987, 52, 2779–2782.
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(29) Fernando, I. R.; Frasconi, M.; Wu, Y.; Liu, W.-G.; Wasielewski, M. R.; Goddard, W. A.,
III.; Stoddartt, J. F. J. Am. Chem. Soc. 2016, 138, 10214–10225.
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(30) From the equation ΔG‡ = aT [9.972 + log(Tc /Δn)], one can see that log(Tc /Δn) is
c
considerable compared with 9.972 only when Δn is small enough. As a result, we take Δn = 25
MHz, i.e., Δd = 0.05 ppm and T = Tc = 193 K to calculate the energy barrier, which is 9.6 kcal
mol–1.
(31) Bin Imran, A.; Esaki, K.; Gotoh, H.; Seki, T.; Ito, K.; Sakai, Y.; Takeoka, Y. Nat. Commun.
2014, 5: 5124.
(32) Iwaso, K.; Takashima, Y.; Harada, A. Nat. Chem. 2016, 8, 626–633.
(33) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(34) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104.
(35) Frischmann, P. D.; MacLachlan, M. J. Chem. Soc. Rev. 2013, 42, 871–890.
(36) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem.
Soc. 1992, 114, 10024–10035.
(37) Pascal, T. A.; Lin, S. T.; Goddard, W. A. Phy. Chem. Chem. Phy. 2011, 13, 169–181.
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Captions to Figures and Scheme
Scheme 1. a) Structural Formula of the [2]Rotaxane 1•6PF6 and Its Graphical
Representation, where the Recognition Sites for the Different Redox States of the CBPQT
Ring are Shown. b) Synthetic Routes for the Preparation of the Dumbbell 2•2PF6 and the
[2]Rotaxane 1•6PF6.
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Figure 1. a) Structural formulas of the dumbbell 2•2PF6 and the [2]rotaxane 1•6PF6, with the
1
protons and fluorine atoms labelled. b) H NMR (500 MHz, CD3CN, 298 K) Spectra of 2•2PF6
and 1•6PF6. c) 19F NMR (470 MHz, CD3CN, 298 K) Spectra of 1•6PF6 and 2•2PF6. The dashed
lines are used to compare the chemical shifts of the signals for the dumbbell 2•2PF6 with those for
the [2]rotaxane 1•6PF6.
Figure 2. Cyclic voltammograms (CV) of a) 22+ and b) 16+. A glassy carbon working electrode, a
platinum counter electrode, and an Ag/AgCl reference electrode were used in the characterization
–
of 0.5 mM MeCN solutions of these compounds as their PF6 salts at 298 K with 0.1 M TBAPF6
serving as the electrolyte. A scan rate of 200 mV·s−1 was used in all the experiments.
Figure 3. a) UV/Vis/NIR Spectra of 22+ and its reduced forms recorded in MeCN solutions (50
µM) at room temperature. In order to generate 2(•+) and 2(0), 1 and 6 equiv of CoCp2 were added,
respectively, to MeCN solutions of 2•2PF6. b) UV/Vis/NIR Spectra of 16+ and its reduced forms
recorded in MeCN solutions (50 µM) at room temperature. In order to generate 13(•+) and 1(0), 3
and 11 equiv of CoCp2 were added, respectively, to MeCN solutions of 1•6PF6. c) Enlargement of
the region between 350 and 650 nm of the spectra in b), showing the strong charge transfer band
centered on 550 nm. d) An interpretation of the charge transfer processes that appear in the spectra
of 2(0) and 1(0).
Figure 4. a) Structural formulas of the the dumbbell 2(0) and the [2]rotaxane 1(0), with the protons
1
and fluorine atoms labelled. b) H NMR (500 MHz, CD3C6D5, 298 K) Spectra of 1(0) and 2(0). c)
19F NMR (470 MHz, CD3C6D5, 298 K) Spectra of 1(0) and 2(0). The dashed lines are used to
compare the chemical shifts of the signals for the dumbbell 2(0) with those of the [2]rotaxane 1(0).
Figure 5. The simulated co-conformations of the [2]rotaxane at different oxidation states and their
relative energy level. In each oxidation state, the energy of the most stable co-conformation is
assigned to be 0 kcal mol–1.
Figure 6. A graphical summary of the actuating processes with the energy levels involved in the
[2]rotaxane as a result of changes in its redox state. a) In the fully oxidized state, the CBPQT4+
ring encircles the triazole unit and does not shuttle along the dumbbell component on either the 1H
or the 19F NMR timescales. In the radical state, the CBPQT2(•+) ring interacts with the BIPY•+ unit
and gives rise to the formation of a trisradical tricationic species. In the neutral state, the CBPQT(0)
ring recognizes the electron-deficient ArF recognition sites, forming a charge transfer complex.
The ring shuttles quickly along the dumbbell component because of the removal of the Coulombic
repulsion present in the fully oxidized state. b) The energy profiles for the [2]rotaxane in different
redox states when the ring shuttles along the dumbbell component. The lines in pink, blue, purple,
red and green correspond to the recognition units shown in a).
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Scheme 1
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Figure 1
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a)
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5 µA
– 244
– 631
– 610
+ 114
b)
– 149
15 µA
– 217
– 143
– 686
– 800
– 400
– 200
0
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E / mV
Figure 2
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Figure 3
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Figure 4
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ArF ring
21.29
Redox State Triazole unit
6+
BIPY•+ unit
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⎼
1
ΔG (kcal mol )
0.00
19.86
3(●+)
⎼
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ΔG (kcal mol )
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0.00
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7.46
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ΔG (kcal mol )
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Figure 5
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Figure 6
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Table 1. Summary of the Key Spectral Features of the [2]Rotaxane in Its Different Oxidation
States, in Comparison with Those of the Dumbbell Compound, which Reveals the Interactions
between the CBPQT Ring and the Corresponding Recognition Site(s).
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Oxidation State of
the [2]Rotaxane
Recognition
Site(s)
Key Spectral Features of
the [2]Rotaxane
Corresponding Spectral Features of
the Dumbbell
d(HTR') = 3.72 ppm
d(Ha') = 6.21 ppm
d(Fa') = ⎼135.3 ppm
d(HTR) = 8.36 ppm
d(Ha) = 6.16 ppm
d(Fa) = ⎼140.0 ppm
6+
Triazole rings
BIPY●+ unit
Reduction potentials at
⎼143 and ⎼217 mV
UV/Vis/NIR absorption band at
1091 nm
Reduction potential at
⎼244 mV
UV/Vis/NIR absorption bands at
400 and 605 nm
3(●+)
Strong UV/Vis/NIR absorption bands at Weak UV/Vis/NIR absorption bands at
0
ArF units
376 and 550 nm
Reduction potential at ⎼686 mV
d(Fb) = ⎼149.1 ppm
371 and 520 nm
Reduction potential at ⎼631 mV
d(Fb) = ⎼146.7 ppm
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