Multicomponent self-assembled metal-organic [3]rotaxanes

Article abstract of DOI:10.1021/jacs.5b07308

A set of environmentally responsive metal-organic [3]rotaxanes is described. These mechanically interlocked macromolecules may be prepared in quantitative yield via a one-pot procedure involving treatment of a flexible tetracationic macrocycle, known as the Texas-sized molecular box, with tri-1,3,5-benzenetricarboxylate anion and silver cations (Ag⁺). The use of this three-component mixture gives rise to a metal-organic [3]rotaxane via a self-assembly process that occurs under ambient conditions in DMSO-d₆ solution. The complex is stable in the presence of excess TFA. However, disassembly of the [3]rotaxane to produce anion-box associated entities may be triggered by adding a competitive counteranionic species (e.g., I^-). Adding excess Ag⁺ serves to reverse this decomplexation process. The nature of the [3]rotaxane complex could be fine-tuned via application of an external stimulus. Increasing the temperature or adding small molecules (e.g., D₂O, methanol-d₄, acetonitrile-d₃, DMF-d₇, acetone-d₆, or THF-d₈) to the initial DMSO-d₆ solution induces conformational flipping of the macrocycle within the overall complex (e.g., from limiting chair to chairlike forms). Support for the molecular stimuli responsive nature of the various structures came from solution-phase one- and two-dimensional (¹H, 1D and 2D NOESY) NMR spectroscopic studies carried out in DMSO-d₆. The core metal-linked rotaxane unit was characterized via single-crystal X-ray diffraction analysis. Initial evidence that the present self-assembly process is not limited to the use of the Ag⁺ cation came from studies involving Cd²⁺; this replacement results in formation of 2D metal-organic rotaxane-containing frameworks (MORFs).

Full text of DOI:10.1021/jacs.5b07308

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Article
Multi-Component Self-assembled Metal-organic [3]Rotaxanes
Yu-Dong Yang, Chuan-Cai Fan, Brett M. Rambo, Han-Yuan
Gong, Li-Jin Xu, Jun-Feng Xiang, and Jonathan L. Sessler
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07308 • Publication Date (Web): 17 Sep 2015
Downloaded from http://pubs.acs.org on September 19, 2015
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Multi-Component Self-assembled Metal-organic [3]Rotaxanes
YuꢀDong Yang,a ChuanꢀCai Fan,b Brett M. Rambo,c HanꢀYuan Gong*,a LiꢀJin Xu*,b JunꢀFeng Xiang*,d
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and Jonathan L. Sessler*c,e
a College of Chemistry, Beijing Normal University, Beijing, Xinjiekouwaidajie 19, 100875, P. R. China
b Department of Chemistry, Renmin University of China, Beijing, Zhongguanchundajie 59, 100872, P. R. China
c Department of Chemistry, The University of Texas at Austin, 105 E. 24th ST. STOP A5300, Austin, Texas, 78712ꢀ1224
(USA)
d Institute of Chemistry, Chinese Academy of Sciences, Beijing, Zhongguanchunbeiyijie 2, 100190, P. R. China
e Department of Chemistry, Shanghai University, Shanghai 200444, China
ABSTRACT: A set of environmentally responsive metalꢀorganic [3]rotaxanes is described. These mechanically interlocked macꢀ
romolecules may be prepared in quantitative yield via a oneꢀpot procedure involving treatment of a flexible tetracationic macrocyꢀ
cle, known as the ‘TexasꢀSized’ molecular box, with triꢀ1,3,5ꢀbenzenetricarboxylate anion and silver cations (Ag+). The use of this
three component mixture gives rise to a metalꢀorganic [3]rotaxane via a selfꢀassembly process that occurs under ambient conditions
in DMSOꢀd6 solution. The complex is stable in the presence of excess TFA. However, disassembly of the [3]rotaxane to produce
anionꢀbox associated species may be triggered by adding a competitive counter anionic species (e.g., I–). Adding excess Ag+ serves
to reverse this decomplexation process. The nature of the [3]rotaxane complex could be fineꢀcontrolled via application of an exterꢀ
nal stimulus. Increasing the temperature or adding small molecules (e.g., D2O, methanolꢀd4, acetonitrileꢀd3, DMFꢀd7, acetoneꢀd6,
THFꢀd8) to the initial DMSOꢀd6 solution induces conformational “flipping” of macrocycle within the overall complex (e.g., from
limiting chairꢀ to chairꢀlike forms). Support for the molecular stimuli responsive nature of the various structures came from solution
phase oneꢀ and twoꢀdimensional (1H, 1D and 2D NOESY) NMR spectroscopic studies carried out in DMSOꢀd6. The core metalꢀ
linked rotaxane unit was characterized via singleꢀcrystal Xꢀray diffraction analysis. Initial evidence that the present selfꢀassembly
process is not limited to the use of the Ag+ cation came from studies involving Cd2+; this replacement results in formation of 2D
metalꢀorganic rotaxaneꢀcontaining frameworks (MORFs).
However, to the best of our knowledge all the reported
methods for generating rotaxane structures have relied on a
precursor, cation, or other template, that is “inserted” in the
central cavity of the macrocyclic unit (viz., capping, clipping,
and slipping approaches).18
Introduction
Over the last three decades, the field of mechanically interꢀ
locked molecules (MIMs) has seen rapid development and
intense interest from the chemical community.1 These complex
molecular architectures are often organized or stabilized by
weak nonꢀcovalent bonding interactions and are of consideraꢀ
ble interest for use in a broad range of applications, including
molecular electronics,2 molecular switching,3 molecular devicꢀ
es,4 and sensor development.5 Rotaxanes are well known
MIMs and typically consist of a dumbbellꢀshaped molecule
threaded through the center of a macrocyclic unit.6 Rotaxanes
have been used extensively in material and biological sciences,
e.g., in the development of molecular machines,7 gels,8 drug
carriers,9 and other applications.10 These applications rely on
effective syntheses of the constituent MIMs. Known strategies
for generating rotaxanes include capping,11 clipping,12 slipꢀ
ping,13 and ion templatation,14 as depicted in Scheme 1. Howꢀ
ever, new approaches could lead to advances in the field.
Recently, the use of anionic precursors in the construction of
threaded molecular architectures has garnered attention. In
seminal studies, Vögtle et al., demonstrated that organic
oxoanions could be used as templates in rotaxane synthesis.15
Beer et al., expanded the scope of anion templated syntheses of
interlocked molecules and demonstrated applications in anion
sensing.16 In more recent work, Flood et al., utilized phosphate
anions as a template to create an unprecedented [3]rotaxane.17
Scheme 1. Schematic summary of rotaxane syntheses.
Herein, we report a facile oneꢀpot synthesis of metalꢀ
organic rotaxane structures. Specifically, a flexible tetracationꢀ
ic
macrocycle
(cyclo[2](2,6ꢀdi(1Hꢀimidazolꢀ1ꢀ
yl)pyridine)[2](1,4ꢀdimethylenebenzene)) known as the ‘Texas
ꢀSized’ molecular box (i.e., 14+; studied as the PF6– salt),19 was
used in combination with benzeneꢀ1,3,5ꢀtricarboxylic acid (2)
and silver cations (Ag+ as its PF6– salt) to generate a metalꢀ
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organic [3]rotaxane species ([(14+)2•53•(Ag+)5]4+) in DMSOꢀd6
solution. Interpenetrated species constructed with these three
components (i.e., 14+, 5 and Ag+), specifically a metalꢀorganic
[2]rotaxanes [14+•((Ag+)2•52•4H2O)], have been characterized
in the solid state.
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assembly strategy illustrated in Scheme 1. The cationic species
were studied as their PF6– salts, whereas the anionic forms of 2
were studied as their tetramethylammonium salts unless othꢀ
erwise indicated.
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Rotaxanes have been studied extensively as environmentally
responsive species that might have potential utility as smart
materials.20 The present MIMs were also studied in this regard.
As detailed below, chemical transformations involving the
[3]rotaxane complex, [(14+)2•53•(Ag+)5]4+, could be induced via
application of appropriate chemical stimuli. For instance, addꢀ
ing iodide anion to the preꢀassembled MIM species
[(14+)2•53•(Ag+)5]4+ led to slow decomposition and generation
of associated anionic species, such as [(14+)2•53]–, as inferred
from solution phase NMR spectroscopic studies. The subseꢀ
quent addition of excess Ag+ cation then induced recovery of
the original interpenetrated structure. This process could be
repeated several times. The rates of the individual interconverꢀ
sion steps could be fineꢀtuned by changing the temperature.
Finally, we show that either warming the DMSOꢀd6 solution or
adding small molecules (e.g., D2O, methanolꢀd4, acetonitrileꢀd3,
DMFꢀd7, acetoneꢀd6, THFꢀd8) leads to a change in the rate at
which the strutꢀthreaded macrocyclic ring undergoes conforꢀ
mational flipping.
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The synthesis proved rather invariant to the relative stoichiꢀ
ometry of the components and the order in which they were
mixed. Specifically, the same selfꢀassembled rotaxane strucꢀ
ture was obtained under a variety of solution phase reaction
conditions. This invariance leads us to postulate that the selfꢀ
assembled ensemble obtained from the mixing of the box, the
trianionic form of 2 (designated as 5), and silver cation repreꢀ
sents the thermodynamic product of the reaction. However,
preꢀcomplexation of the cation and preorganization of the aniꢀ
onic species derived from 2 outside of the central cavity of
macrocycle 14+ may serve to template the reaction in a kinetic
sense. Consistent with this latter supposition is the finding that
the rate of formation of the [3]rotaxane structure proved highly
dependent on the protonation state of the anionic precursor,
with the trianion of 2 (5) displaying the highest rate of forꢀ
mation as compared to its various other, more highly protonatꢀ
ed conjugate acid forms.
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The combination of a relatively simple preparation, in conꢀ
cert with the environmental responsive nature of the metalꢀ
organic [3]rotaxane that results from mixing 14+, 5, and Ag+,
leads us to suggest that the selfꢀassembled approach used to
create [(14+)2•53•(Ag+)5]4+ could allow access to complex moꢀ
lecular architectures with controllable switching features that
might otherwise by inaccessible through conventional synthetꢀ
ic methods. Initial support for this latter postulate comes from
the finding that replacing the Ag+ by Cd2+ leads to formation of
2D metal organic rotaxane frameworks (MORFs).
Scheme 2. Schematic representation of the key subunits used
to create rotaxane structures according to the direct selfꢀ
Scheme 3. Chemical transformations involving the metalꢀorganic [3]rotaxane [(14+)2•53•(Ag+)5]4+. Also shown are schematic repreꢀ
sentations of internal molecular motions that occur within the ensemble, specifically the chairꢀtoꢀchair conformational flipping of
the boxꢀlike macrocyclic rings.
ligand in molecular assembly.21 It was also considered likely
Results and Discussion
that, in conjunction with 14+, a flexible macrocycle that has
Initially a set of experiments was devised to probe the effect
of protonation on the interaction between 1,3,5ꢀbenzene tricarꢀ
boxylic acid (2) and its anionic forms (i.e., monoꢀanion 3, diꢀ
anion 4, and trianion 5). This particular class of substrates was
found application in the construction of MIMs,19 new frameꢀ
work structures might be produced. The present study was
undertaken in an effort to test this hypothesis.
chosen for its ability to serve potentially as a threeꢀcoordinate
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No evidence of interaction was observed when the fully proꢀ
As a complement to the Job plot analyses, isodesmic titraꢀ
tions were performed in DMSOꢀd6. Here, the changes in the
chemical shift corresponding to the imidazole CꢀH signal (e.g.,
H(1); cf. Scheme 2; Supporting Information) were monitored
as the concentration of guests 3, 4, or 5 was increased while
leaving that of host 14+ unchanged. Based on fits of the titraꢀ
tion data, association constants of Ka1 = (2.7 ± 0.1) × 103 Mꢀ1
for the 1:1 complex ([14+•3]3+) and Ka2 = (3.6 ± 0.2) × 102 Mꢀ1,
corresponding to the subsequent conversion to [14+•32]2+, were
calculated. For the formation of [14+•4]2+ in solution, a value of
Ka = (1.0 ± 0.1) × 105 Mꢀ1 was obtained. In the case of the triꢀ
anion 5 and 14+ association constants of Ka1 = (3.5 ± 0.2) × 106
Mꢀ1 and Ka2 = (1.3 ± 0.1) × 1015 Mꢀ2 were calculated; based on
the Job plot analysis above, these binding constants were conꢀ
sidered to reflect formation of stronglyꢀbound 1:1 and 2:3
hostꢀguest complexes, respectively.
tonated form 2 was combined with 14+ in a 1:1 molar ratio in
DMSOꢀd6 (the solvent used for all studies unless otherwise
noted), as inferred from the lack of spectral shifts associated
with 14+ in the associated 1H NMR spectra. To probe whether
the deprotonated forms of 2 might interact with 14+ more
strongly, tetramethylammonium hydroxide (TMA+•OH–) was
used to produce the corresponding monoꢀ, diꢀ and trianions 3,
4, and 5, respectively. Direct titration of these latter preformed
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species into 14+ gave rise to distinct changes in the H NMR
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spectrum of 14+ (most notably in the imidazole C–H resoꢀ
nance) (cf. Supporting Information). This was taken as initial
evidence that hydrogen bonding and/or other weak intermoꢀ
lecular bonding interactions take place between 14+ and the
anionic forms of 2 in DMSOꢀd6 solution.
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To analyze further the interactions between 14+ and 3, 4, or 5
in DMSOꢀd6 solution, twoꢀdimensional nuclear Överhauser
effect spectroscopic (NOESY) analyses were carried out. No
observable cross peaks were seen in these studies, leading us
to suggest that in solution the anionic guests (G) 3, 4, or 5 are
located “outside” of the cationic host (H) 14+ (cf. Scheme 4).
Further evidence for the proposed complexes came from
single crystal Xꢀray diffraction analyses. Diffraction grade
single crystals were obtained from mixtures of 14+ (5 mM) and
5 molar equiv of the anionic species 3, 4, or 5 via slow evapoꢀ
ration from solution using mixtures of water/DMF (v/v = 1:1)
(cf. Supporting Information). The structures corresponding to
these crystals (i.e., [14+•34•12H2O], [14+•42•12H2O] and
[14+•5•OH–•2DMF•17H2O]; cf. Figure 1) were solved and used
to confirmed the outside binding mode proposed from the
NMR solution studies. As seen in previous studies,18a the
“box” 14+ proved to have a high degree of structural flexibility,
and demonstrates the ability to conform its shape and size to
accommodate guest binding. In fact, several different conforꢀ
mations, including a “completeꢀchair”, “partial chair”, and
“twist chair”, were seen in the complexes formed with 3, 4, or
5, respectively (cf. Supporting Information)
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Job plots, constructed from H NMR spectral analyses (cf.
Supporting Information), revealed maximum values of 0.67,
0.50, and 0.60 ([G]/([H]+[G])) in the case of guest 3, 4, or 5,
respectively. These values are consistent with the formation of
complexes with net 1:2, 1:1, and 2:3 (H:G) binding stoichiomꢀ
etry between 14+ and guests 3, 4, or 5, respectively. The excess
positive charge in these complexes is balanced by the residual
PF6¯ anions.
Additional support for the proposed formation of multicomꢀ
ponent supramolecular complexes came from ESIꢀMS analꢀ
yses, which revealed peaks corresponding to [14++(3)2ꢀ2H]+•
(m/z= 1047.3040), [14++3ꢀ2H]+• (m/z = 837.2882), [14++(4)2]+•
(m/z = 1047.3067), [14+ + 4ꢀH]+• (m/z = 837.2909), and
[14++(5)2+2H]+• (m/z = 1047.3064), [14+ + 5]+• (m/z =
837.2899) in the gas phase (cf. Supporting Information).
Scheme 4. Schematic representation of the interactions between 14+ and guest species 3, 4, and 5, as inferred from 1H NMR spectroscopic
analyses carried out in DMSOꢀd6.
(c1,2), as seen within three independent complexes,
[14+•34•12H2O],
[14+•42•12H2O],
and
[14+•5•OHꢀ
•2DMF•17H2O], respectively, as determined by single crystal
Xꢀray diffraction analysis. Some of the counter ions and solꢀ
vent molecules have been omitted for clarity. Note the “comꢀ
pleteꢀchair”, “partial chair”, and “twist chair” conformation of
14+ present in these three structures, respectively.
Taken in concert, the solution and solid state studies deꢀ
scribed above provide evidence that 14+ is capable of interactꢀ
ing with anionic guest 3, 4, and 5 via an outside binding mode
prior to addition of any metal cationic species. The nature of
these interactions led us to consider the possibility of actual
Figure 1. Top (a1, b1, or c1) and side (a2, b2, or c2) views of the
1:2 (H:G) subunits formed from 14+ and various anionic guest
species, namely [14+•32]2+ (a1,2), [14+•42] (b1,2), and [14+•52]2ꢀ
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threading based on the use of an appropriate metal cation to
again, at a 2:3 molar ratio of 14+ and 5 essentially all of the
host and guest (i.e., 14+ and 5) were coꢀbound in solution.
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stabilize the formation of mechanically interlocked structures.
To test this hypothesis, Ag+ (as its PF6– salt) was titrated into a
DMSOꢀd6 mixture containing one molar equiv each of 14+ and
5. Based on the observation of a new set of proton signals in
On the basis of the conversion stoichiometries observed in
the above experiments, we suggest that a 2:3 complex between
14+ and 5 represents the dominant species formed in DMSOꢀd6
solution in the presence of Ag+ and that the final thermodyꢀ
namic product contains two molecules of 14+, three molecules
of 5, and five Ag+ cations under conditions where none of the
components is limiting.
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the H NMR spectrum (Figure 2) and integrations of the reꢀ
spective peak intensities, it is concluded that roughly 67% of
14+ is transformed into a new stable product when the Ag+
cation is added in excess (i.e., 3 ꢀ 20 molar equiv relative to
1
14+). A H NMR spectroscopicꢀbased Job plot was carried out
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Operating within the context of this assumption, efforts
were made to obtain insights into the degree of complexation
between the various species involved in the equilibrium. On
the basis of independent 1H NMR spectral titrations carried out
in DMSOꢀd6 (cf. Supporting Information), the binding conꢀ
stants corresponding to the interactions between 5 and Ag+
were found to be Ka1 = (1.4 ± 0.1) × 105 Mꢀ1 and Ka2 = (6.1 ±
0.6) × 1015 Mꢀ2 for complexes with 1:1 and 2:3 stoichiometry,
respectively. For the full conversion to [(14+)2•53•(Ag+)5]4+ an
association constant Ka ≥ 1058 Mꢀ9 could be calculated (cf.
Supporting Information). However, the various assumptions
underlying the derivation of this value, including those associꢀ
ated with speciation, lead us to suggest that this association
constant be used only as a qualitative indicator of the strong
interactions that occur when 14+, 5, and Ag+ are mixed in
DMSOꢀd6 under conditions where the concentration of no
individual components is limiting.
while maintaining the total concentration of 14+ and 5 equal to
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–
2 mM in the presence of 24 mM Ag+•PF6 in DMSOꢀd6. The
maximum complex concentration was seen when the value of
[5]/([14+]+[5]) was 0.6. Such a finding is consistent with the
formation of a supramolecular complex, with a 2:3 stoichiomꢀ
etry (14+ relative to 5), upon mixing the components under
these solution phase conditions.
With the ratio between 14+ and 5 set as 2:3, another Jobꢀplot
analysis was carried out, wherein the overall concentration of
–
[(14+)2•53]– and Ag+•PF6 was 0.2 mM. In this case, the peak
maximum was seen at a [Ag+]/([14+]+[Ag+]) ratio of 0.71. This
observation leads us to suggest that the final product contains
5 Ag+ centers, 2 equivalents of 14+, and 3 molar equivalents of
the anionic precursor 5.
To obtain further insights into the stoichiometry, a second
set of 1H NMR spectral titration studies was carried out whereꢀ
in the molar ratio of 14+ and 5 was held constant at 2:3 while
Ag+ was titrated into the solution until 4 molar equiv of Ag+
(relative to 14+) had been added. On the basis of the observed
1H NMR spectral changes, it is concluded that approximately
2.5 molar equiv of Ag+ (relative to 14+) were needed to effect
essentially complete conversion to the product. The spectral
features of the resulting complex were identical to those proꢀ
duced above (cf. Figure 2).
In contrast to what is seen for the ternary mixture, control
1H NMR spectroscopic studies, in which Ag+ is added directly
to a solution of 14+ in DMSOꢀd6 in the absence of 5, revealed
no evidence of interaction, as inferred from the lack of disꢀ
cernible shifts in the imidazolium protons peaks of 14+. We
thus conclude that no appreciable insertion of Ag+ into the
macrocyclic cavity of 14+ occurs in the absence of trianion 5.
We thus do not believe that the silver cation per se is serving
to “gather” and “thread” directly anion 5, as seen in more traꢀ
ditional, metalꢀbased rotaxane syntheses.
In a third experiment, the trianion 5 was titrated into a mixꢀ
ture containing one molar equiv of 14+ and 2.5 molar equiv of
Ag+ (cf. Figure 2). The resulting NMR spectrum showed that
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ꢀ
Figure 2. (a) 1H NMR spectroscopic titration corresponding to the addition of Ag+•PF6 to a mixture of 1 molar equiv of 14+•4PF6–
(2.00 × 10ꢀ4 M) and 1.5 molar equiv of the triꢀ1,3,5ꢀbenzenetricarboxylate anion (5) in the form of its TBA+ salt; (b) 1H NMR specꢀ
troscopic titration corresponding to the addition of the triꢀ1,3,5ꢀbenzenetricarboxylate anion (5) in the presence of 1 molar equiv of
14+•4PF6ꢀ (1.00 × 10ꢀ3 M) and 2.5 molar equiv of Ag+•PF6 . All spectra were recorded in DMSOꢀd6 at 300 K (600 MHz) 1 h after the
–
various components were mixed.
Insights into how [(14+)2•53•(Ag+)5]4+ might be forming unꢀ
the classic metalꢀbased “gathering” and “threading” apꢀ
der conditions where the components are mixed in DMSOꢀd6
came from time dependent 1H NMR spectral analyses wherein
the mixing order of the three components was changed. When
the concentration of receptor 14+ was kept constant at 0.2 mM
and the ratio [14+]:[5]:[Ag+] was set at 2:3:5, we found that
three disparate mixing procedures yielded the associated comꢀ
plex [(14+)2•53•(Ag+)5]4+ in nearꢀquantitative yield. The first
two of these procedures consisted of 1) mixing 14+ and Ag+
and then adding the anionic precursor 5, or 2) adding Ag+ into
the complex of 14+ and 5, respectively. Both methods gave the
[3]rotaxane product at similar rates (54% five minutes after
mixing). In contrast, slower formation kinetics were seen
when 14+ was added into a mixture of 5 and Ag+. In this case,
only 30% conversion is seen at 5 min after all three compoꢀ
nents were mixed. In all three cases, complete conversion was
seen at longer times. On the basis of these observations, we
conclude that the preꢀorganized “outside” complex formed
between 14+ and 5 (vide supra) abets kinetically the selfꢀ
proach used to make cationꢀcoordinated MIMs.
The stable product obtained upon mixing 14+, 5, and Ag+,
([(14+)2•53•(Ag+)5]4+), was characterized by split (doubled)
1
signals for protons H(7`a) and H(7`b) in the H NMR specꢀ
trum. These two protons are on the same carbon. However, in
a locked conformation, one points outside of the cavity, and
the other inward. The observed splitting is thus consistent with
limited dynamic motion in the complex product,
[(14+)2•53•(Ag+)5]4+. The other protons (i.e., H(2`), H(3`), H(4`),
H(5`) and H(6`)) on 14+ are characterized by one set of signals,
a finding that is consistent with free rotation of the imidazoliꢀ
um groups occurring around the singleꢀbonds within the comꢀ
plex (cf. Supporting Information).
Diffusionꢀordered spectroscopic (DOSY) analyses revealed
that in the presence of Ag+ all the protons on each individual
organic species (i.e., 14+ and 5) showed similar diffusion coefꢀ
ficients in DMSOꢀd6 solution (cf. Supporting Information).
Although not a proof of either structure or stoichiometry, the
DOSY analysis does provide further evidence that a stable
complex was constructed from the mixture of host 14+, guest
anion 5, and Ag+.
assembly
process
leading
to
the
[3]rotaxane
[(14+)2•53•(Ag+)5]4+, while preꢀcomplexation between 5 and
Ag+, to the extent it occurs, retards complex formation. The
present selfꢀassembly process thus differs dramatically from
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Figure 3. (a) Oneꢀdimensional NOE NMR spectral studies and (b) twoꢀdimensional NOESY NMR spectroscopic analysis of comꢀ
plex [(14+)2•53•(Ag+)5]4+ recorded in DMSOꢀd6 at 300 K (500 MHz). The red arrow in (a) indicates irradiation at the frequency of
H(5`a) on anion 5, while the small red circles in (b) designate the cross signals between H(5`a) on 5 and H(2`), H(3`), H(4`), H(5`),
H(6`), H(7a`), and H(7b`) on 14+.
To probe in greater detail the molecular complex formed in
solution between 14+, 5, and Ag+, twoꢀdimensional nuclear
Överhauser effect spectroscopic (NOESY) studies were carꢀ
ried out. In these studies, cross signals between H(5`a) on
guest anion 5 and H(2`) , H(3`), H(4`), H(5`), H(6`), H(7a`),
and H(7b`) on 14+ were observed. Such findings are consistent
with guest 5 being inserted inside macrocycle 14+, as would be
expected for a metalꢀorganic [3]rotaxane (cf. Figure 3). Oneꢀ
dimensional nuclear Överhauser effect (NOE) NMR spectroꢀ
scopic analyses, which are more sensitive than twoꢀ
dimensional NOESY, showed correlations between the signals
on 5 and 14+. This latter analysis thus supports the conclusions
drawn from the 2DꢀNOESY spectral analysis (cf. Supporting
Information).
anionic guest and (2) labile protons serve to compete directly
with cation Ag+ for the Lewis basic binding sites in 14+.
The multicomponent nature of complex [(14+)2•53•(Ag+)5]4+
led us to consider that it might display environmentally reꢀ
sponsive behavior in solution. In an effort to understand the
relative importance of the individual components play in stabiꢀ
lizing the threeꢀcomponent complex of [(14+)2•53•(Ag+)5]4+, I–
(in the form of its tetrabutylammonium (TBA+) salt) was addꢀ
ed to a solution of the [3]rotaxane species in DMSOꢀd6. It was
found that the addition of 2 molar equiv of I– (relative to Ag+)
to a 1 mM solution of [(14+)2•53•(Ag+)5]4+, followed by storage
under ambient conditions for 168 h, resulted in only about
20% of the complex undergoing decomposition, as inferred
1
from H NMR spectroscopic analyses (cf. Supporting Inforꢀ
The effect of adding Ag+ to 14+ in the presence of either 3 or
4 (the mono and dianion of 2, respectively) was also studied.
mation). In contrast, the addition of 1 molar equiv of I– (relaꢀ
tive to Ag+) served to effect full and immediate decomposition
of the complex formed from 5 (0.3 mM) and Ag+ (0.45 mM).
Taken in concert, these two results provide evidence that the
presence of 14+ in the [3]rotaxane [(14+)2•53•(Ag+)5]4+ serves to
1
In DMSOꢀd6, similar H NMR spectral shifts were obtained
(cf. Supporting Information). However, it was found that the
rate of conversion to the final complex was highly dependent
on the protonation state of 2. For instance, in the presence of
3, 4, or 5 equilibrium times of roughly 10, 3, or 1 h, respecꢀ
“protect” the Ag+ cation from precipitating out as AgI (Ksp
=
8.52 × 10ꢀ17 Mꢀ1 in water22,23).
1
tively, were required before a reproducible set of H NMR
To explore the above chemistry in greater detail, an addiꢀ
tional molar equiv of Ag+ was added to the solution of
[(14+)2•53•(Ag+)5]4+ that had been subject to partial, iodideꢀ
spectra were obtained following mixing of the components (cf.
Supporting Information). It was also found that the protonaꢀ
tion state of 2 directly impacted the yield of the resulting hostꢀ
guest Ag+ꢀcontaining complexes. It was found that when the
same amount of Ag+ (i.e., 2.5 molar equiv relative to 14+) was
added to a mixture of 14+ and 1.5 molar equiv of 3, 4, or 5, the
yield of the products (all of which gave rise to identical proton
signals for 14+ in the 1H NMR spectra) was found to be 27, 65,
and 100%, respectively. Increasing the anionic character of the
trifunctionalized carboxylic acid substrate appears to increase
the rate of the reaction as well as improving the yield.
1
induced dissociation. On the basis of H NMR spectroscopic
monitoring, it was concluded that the original [3]ꢀrotaxane
structure was fully reformed in 48 hours.
The rate of the stimulusꢀinduced decomplexation and
reformation sequence proved temperature dependent. Carrying
out the sequential addition procedure at 343 K rather than
298K, the overall cycle time decreased from 96 h to 3 h. The
kinetics could be further fineꢀtuned by carrying out the deꢀ
complexation step at 298K and reassembly at 343K, the first
part of the switching cycle required roughly 48, whereas the
reformation step required only 1.5 h. The decompositionꢀ
reconstruction cycles could be repeated a number of times
without appreciable degradation (cf. Figure 4).
We rationalize these findings in terms of two limiting exꢀ
planations: (1) Increased protonation reduces the number of
stabilizing hydrogen bonding interactions between 14+ and the
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Figure 4. Plot showing the percentage of intact complex inferred from H NMR spectroscopic analyses as [(14+)2•53•(Ag+)5]4+ is
subject to cycles of iodide anionꢀinduced decomposition and silver cationꢀpromoted reassembly in DMSOꢀd6 at 298K (a), 343K (b)
or decomposition at 298K and reassembly at 343K (c).
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Further support for the proposed responsive nature of
[(14+)2•53•(Ag+)5]4+ was provided by a series of temperature
dependent 1H NMR spectroscopic measurements. It was found
that upon increasing the temperature, the signal of H(3`) on 14+
shifted to lower field. Warming the solution was also found to
result in splitting of the H(7`) signal. These findings lead us to
suggest that the complex [(14+)2•53•(Ag+)5]4+ becomes less
tightly bound as the temperature is raised and that macrocycle
14+ becomes more conformationally mobile as thermal energy
is added to the original DMSOꢀd6 solution.
The presumed conformational changes involving macrocyꢀ
cle 14+ within the complex [(14+)2•53•(Ag+)5]4+ were evaluated
from 298K to 388K in DMSOꢀd6 using a twoꢀsite exchange
model. The kinetic parameters for the inferred molecular moꢀ
1
tion of 14+, derived from H NMR spectroscopy, were then
calculated using methods reported in the literature.24 These
analyses gave values of ꢀ2.1 ± 0.1 kJ•Mꢀ1 for H≠ and ꢀ195.2 ±
∆
0.2 J•Kꢀ1•Mꢀ1 for
∆
S≠. The activation energy at 298 K was
calculated to be 56.0 ± 0.2 kJ•Mꢀ1. Based on these values, a
potential energy profile for the underlying processes could be
constructed; it is shown in Figure 5.
Figure 5. Potential energy diagram for the formation of different species relative to 14+, 5, and Ag+ at 298 K and the inferred conꢀ
formational motions involving 14+ within the [3]rotaxane complex [(14+)2•53•(Ag+)2]4+.
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It was also found that upon increasing the molar fraction of
Page 8 of 27
1
2
3
4
D2O in the original DMSOꢀd6 solution, the chemical shift difꢀ
ference between H(7`a) and H(7`b) became increasingly small.
Such a finding is consistent with a conformational motion that
involves
chairꢀtoꢀchair
“flipping”
of
14+
within
[(14+)2•53•(Ag+)5]4+, rather than “ring around axis” rotation (cf.
Figure 6). This motion becomes increasingly fast in the presꢀ
ence of D2O (cf. Figure 7). Specifically, increasing the molar
percentage of added D2O from 0% to 80% serves to increase
the rate constant, kc, from 943 ± 10 Hz to 1043 ± 10 Hz at
298K. One possible explanation for this inferred increase in
rate is that the faster exchange of the waters molecules bound
to the Ag+ cation in the presence of D2O allows for a more
facile “flipping” of macrocycle 14+. Support for the proposed
interaction between the bound Ag+ cation and water molecules
came from a single crystal diffraction study discussed below.
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Figure 7. Plot of kc versus molar percentage of added small
molecules (D2O, methanolꢀd4, acetonitrileꢀd3, DMFꢀd7, aceꢀ
toneꢀd6, THFꢀd8
) into an original DMSOꢀd6 solution of
[(14+)2•53•(Ag+)5]4+. These additions are thought to modulate
the rate of the chairꢀtoꢀchair conformational “flipping” of 14+
within the [3]rotaxane [(14+)2•53•(Ag+)5]4+. A schematic repreꢀ
sentation of this flipping process is shown in Figure 6.
Evidence that a metalꢀorganic interpenetrated structure is
formed in the solid state came from a single crystal Xꢀray difꢀ
fraction
analysis
of
single
crystals
of
[14+•(Ag+•4•5)]•[14+•((Ag+)2 •52•4H2O)]•29H2O. The crystals
used in the data analysis were prepared by slow diffusion usꢀ
ing a threeꢀlayer solution setup. Specifically, 2.0 molar equiv
of Ag+•PF6– (based on 14+•4PF6–) were dissolved in an aqueous
solution and placed in a small vial. A mixture of DMF and
water (1:1, v/v) was added as the second layer, and a mixture
consisting of 14+•4PF6– (1.0 molar equiv.), 2.0 molar equiv. of
2, and 6.0 molar equiv of TMA+•OH– dissolved in DMF and
water (1:1, v/v) was subsequently added as the third layer.
Single crystals suitable for Xꢀray diffraction analyses were
obtained within seven days (cf. Supporting Information).
+
Figure 6. Proposed flipping motion involving 14+ that occurs
within the overall complex [(14+)2•53•(Ag+)5]4+.
Structural analysis of the above crystals revealed the presꢀ
ence of two independent [2]rotaxane structures. The first of
The effect of other solvents on the rate of macrocycle flipꢀ
ping within [(14+)2•53•(Ag+)5]4+ was probed by adding other
small molecules to an initial solution of this [3]rotaxane in
DMSOꢀd6. Based on 1H NMR spectroscopic analyses, the
addition of methanolꢀd4 or acetonitrileꢀd3 mirrors what is
found upon adding D2O and serves to increase the rate of conꢀ
formational motion. In contrast, adding DMFꢀd7 did not
change the rate of this flipping process appreciably, while
adding acetoneꢀd6 and THFꢀd8 served to retard the process (cf.
Figure 7). The differential response of [(14+)2•53•(Ag+)2]4+ to
different added small molecules highlights an interesting enviꢀ
ronmental response that does not involve either translational
motion along the threading strut or disassembly of the underꢀ
lying MIM.
these proved to be
a
neutral rotaxane unit,
[14+•(Ag+)2•52•4H2O)], formed from one molecule of 14+, two
molecules of 5, and two Ag+ cations with two coordinated
water ligands per cation. Of particular interest in this structure
is the fact that the “rod” or “strut” is composed of an AgꢀAg
dimer, which is stabilized via coordination to one of the carꢀ
boxylate groups on each molecule of 5, as well as two comꢀ
plexed water molecules. This coordination mode results in a
stabilized metalꢀorganic strut threaded through the center of
14+. Based on the geometric parameters, it is inferred that inꢀ
termolecular hydrogen bonding interactions between the water
ligands bound to the Ag+ cations and to 14+, as well as πꢀπ
donorꢀacceptor interactions between the anion 5 and cation
14+, contribute to the stability of the overall structure (cf. Figꢀ
ure 8).
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The second core rotaxane subunit ([14+•(Ag+•4•5)] within
solvent molecules and counter anions have been omitted for
clarity.
The observation of two distinctly different rotaxane strucꢀ
1
2
3
4
5
6
7
8
the overall coꢀcomplex with an overall formula
[14+•(Ag+•4•5)]•[14+•((Ag+)2•52•4H2O)]•29H2O) is characterꢀ
ized by the presence of threading subunits in two different
protonation states. The existence of these two protonated
forms is ascribed to Ag+ acting as a Lewis acid and promoting
a hydrolysis process that is presumed to occur under the condiꢀ
tions of crystallization. In contrast to what was seen in the first
structure, this particular [2]rotaxane contains only one Ag+
cation in the rotaxaneꢀdefining “rod” that serves to link the
anionic species 4 and 5 through the cavity of macrocycle 14+
(cf. Figure 9). The absence of coordinating water ligands on
the Ag+ center leads us to suggest that weak hydrogen bonds
along with πꢀπ donorꢀacceptor interactions between the anionꢀ
ic subunit (either 4 or 5) and 14+ serve to stabilize the overall
rotaxane structure.
tures (cf. Figures 8 and 9) within the same datum crystal leads
us suggest that the nature of the metalꢀorganic [2]rotaxanes
stabilized by the imidazolium box 14+ is influenced by the
specific protonation states of 2 involved in the threading proꢀ
cess. This inference is fully consistent with the solid state and
1
solution phase H NMR studies carried out in the absence of
Ag+, wherein very different outside binding modes are obꢀ
served, as discussed above.
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Conclusions
This study serves to underscore the notion that metalꢀ
organic rotaxane structures can be prepared directly via the
simple mixing of appropriately chosen components without
the need for metalꢀbased “gathering” followed by “threading”.
In the present case, this paradigm is expressed in the context
of a oneꢀpot method that allows for the generation of interꢀ
locked species in nearly quantitative yield. Three components
are used, namely a flexible tetraimidazolium molecular box
(14+), a trianion, and a source of Ag+ cations. Their use gives
rise to rotaxane species that are stable in solution and in the
solid state. In spite of this stability, the nature of the system
can be controlled. For instance, sequential treatment with I–
followed by Ag+ ion in DMSOꢀd6 solution allows the sequenꢀ
tial partial decomposition and subsequent reconstruction of the
core [3]rotaxane entity (complex [(14+)2•53•(Ag+)5]4+). Increasꢀ
ing the temperature or adding certain small molecular species
(e.g., D2O, methanolꢀd4, acetonitrileꢀd3) serves to increase the
rate of internal motion within the overall complex. Other addiꢀ
tives, namely acetoneꢀd6 and THFꢀd8 serve to retard the conꢀ
formational flipping process.
Figure 8. (a) Schematic representation showing the proposed
formation of the metalꢀorganic [2]rotaxane unit consisting of
[14+•((Ag+)2•52•4H2O)]. Top (b) and side (c) views of the
[14+•((Ag+)2•52•4H2O)] complex present in [14+•(Ag+•4•5)]•
[14+•((Ag+)2•52•4H2O)]•29H2O, as derived from a single crystal
Xꢀray diffraction analysis. Some of the solvent molecules and
counter anions have been omitted for clarity.
The versatility and simplicity of the current approach leads
us to propose that the use of multiꢀcomponent assembly inꢀ
volving anionic guests, cationic hosts, and appropriately choꢀ
sen metal centers may allow for the facile synthesis of comꢀ
plex metalꢀorganic MIMs that are not easily accessed via othꢀ
er, more classic synthetic methods.
In preliminary work designed to test the above proposition,
an effort was made to replace the Ag+ cation by Cd2+ (as the
nitrate salt). In this case, when a oneꢀpot preparation analoꢀ
gous
to
that
used
to
obtain
crystals
of
[14+•(Ag+•4•5)]•[14+•((Ag+)2•52•4H2O)]•29H2O was employed,
single crystals of a complex, [(14+)2•54•(Cd2+)4•(HOꢀ)4•4H2O]
•19H2O were obtained. Xꢀray diffraction analysis revealed a
metal organic rotaxane framework (MORF) structure, wherein
the Cd2+ cations act as bridging units for the anionic compoꢀ
nents (5), which are organized as 2D networks (c.f. Figure 10).
The macrocyclic component, 14+, acts as edges about 5 within
the overall polyrotaxane structure. The interactions between
14+ and the threading polyanionic guest 5 are characterized by
CHꢀπ, anionꢀπ, and intermolecular hydrogen bonding interacꢀ
tions, as inferred from an atomic distance between C(52) and
C(101) that is less than 3.8 Å, an atomic distance between O(9)
and C(47) ≤ 3.4 Å, and atomic distance between O(2) and
C(20) that is less than 3.0 Å, respectively. On this basis we
think it should be possible to use a variety of anions and catiꢀ
ons in conjunction with box 14+ to create new interpenetrated
structures. Studies along these lines are currently in progress.
Figure 9. (a) Schematic representation of the proposed forꢀ
mation of the metalꢀorganic [2]rotaxane unit [14+•(Ag+•4•5)].
Top (b) and side views (c) of the core rotaxane unit
[14+•(Ag+•4•5)] seen in the single Xꢀray crystal structure of
[14+•(Ag+•4•5)]•[14+•((Ag+)2•52•4H2O)]•29H2O. Some of the
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Figure 10. (a) Schematic representation of the proposed formation of the the 2D metal organic rotaxane framework (MORF) conꢀ
sisting of [(14+)2•54•(Cd2+)4•(HOꢀ)4•4H2O]n . (b) Front and (c) side views of the core rotaxane unit [(14+)2•54•(Cd2+)4•(HOꢀ)4•4H2O].
(d) The 2D metal organic rotaxane structure found within the single Xꢀray crystal structure of [(14+)2•54•(Cd2+)4•(HOꢀ
)4•4H2O]•19H2O. Some of the solvent molecules and counter anions have been omitted for clarity.
Cao, D.; Stoddart, J. F. Acc. Chem. Res. 2012, 45, 1581. (e)
ASSOCIATED CONTENT
Supporting Information
Supporting information is provided that includes experimental
details, NMR spectroscopic analysis, ESIꢀMS results and single
crystal Xꢀray diffraction studies. This material is available free of
charge via the Internet at http://pubs.acs.org.
McGonigal, P. R.; Stoddart, J. F. Nat. Chem. 2013, 5, 260. (f) Li, S.
J.; Huang, J. Y.; Cook, T. R.; Pollock, J. B.; Kim, H.; Chi, K. W.;
Stang, P. J. J. Am. Chem. Soc. 2013, 135, 2084. (g) Campbell, C. J.;
Leigh, D. A.; VitoricaꢀYrezabal, I. J.; Woltering, S. L. Angew. Chem.
Int. Ed. 2014, 53, 13771. (h) De Bo, G.; Kuschel, S.; Leigh, D. A.;
Lewandowski, B.; Papmeyer, M.; Ward, J. W. J. Am. Chem. Soc.
2014, 136, 5811. (i) Zhu, K. Loeb, S. J. “Organizing Mechanically
Interlocked Molecules to Function inside Metalꢀorganic Frameꢀ
works” in Molecular Machines and Motors: Topics in Current Chemꢀ
istry. SpringerꢀVerlag, Berlin Heidelberg, 2014. (j) Vukotic, V. N.;
Loeb, S. J. Chem. Soc. Rev. 2012, 41, 5896–5906. (k) Qu, D. H,;
Wang, Q. C.; Zhang, Q. W.; Ma, X.; Tian, H. Chem. Rev. 2015, 115,
7543. (l) Yang, L.; Tan, X.; Wang, Z.; Zhang, X. Chem. Rev. 2015,
115, 7196.
AUTHOR INFORMATION
Corresponding Authors
hanyuangong@bnu.edu.cn; xulj@chem.ruc.edu.cn;
jfxiang@iccas.ac.cn; sessler@cm.utexas.edu
Notes
(2) (a) Hansen, S. W.; Stein, P. C.; Sørensen, A.; Share, A. I.; Witꢀ
licki, H. W.; Kongsted, J.; Flood, A. H.; Jeppesen, J. O. J. Am. Chem.
Soc. 2012, 134, 3857. (b) Amabilino, D. B. Nat. Chem. 2013, 5, 365.
(c) Serreli, V.; Lee, C. F.; Kay, E. R.; Leigh, D, A. Nature. 2007, 445,
523.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
H.ꢀY.G. is grateful to the National Natural Science Foundation of
China (21202199), National Basic Research Program of China
(973 Program 2015CB856502), the Young OneꢀThousandꢀTalents
Scheme, the Fundamental Research Funds for the Central Univerꢀ
sities, the Beijing Municipal Commission of Education, and the
Beijing Normal University for financial support. J.L.S. thanks the
National Science Foundation (grant no. CHEꢀ1402004) and the
Robert A. Welch Foundation (grant Fꢀ1018) for financial support.
(3) (a) Fahrenbach, A. C.; Zhu, Z. X.; Cao, D.; Liu, W. G.; Li, H.;
Dey, S. K.; Basu, S.; Trabolsi, A.; Botros, Y. Y.; Goddard III, W. A.;
Stoddart, J. F. J. Am. Chem. Soc. 2012, 134, 16275. (b) Grunder, S.;
McGrier, P. L.; Whalley, A. C.; Boyle, M. M.; Stern, C.; Stoddart, J.
F. J. Am. Chem. Soc. 2013, 135, 17691.
(4) (a) Coskun, A.; Spruell, J. M.; Barin, G.; Dichtel, W. R.;
Flood, A. H.; Botros, Y. Y.; Stoddart, J. F. Chem. Soc. Rev. 2012, 41,
4827. (b) Kay, E. R.; Leigh, D, A. Angew. Chem. Int. Ed. 2015, 54,
10080.
(5) (a) Caballero, A.; Zapata, F.; Beer, P. D. Coord. Chem. Rev.
2013, 257, 2434. (b) Spence, G. T.; Beer, P. D. Acc. Chem. Res. 2013,
46, 571. (c) Allain, C.; Beer, P. D.; Faulkner, S.; Jones, M. W.; Kenꢀ
wright, A. M.; Kilah, N. L.; Knighton, R. C.; Sørensen, T. J.; Tropiꢀ
ano, M. Chem. Sci. 2013, 4, 489. (d) Collins, C. G.; Peck, E. M.;
Kramera, P. J.; Smith, B. D. Chem. Sci. 2013, 4, 2557.
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fiths, K. E. Synthesis of Mechanically Interlocked Molecules Using
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1
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