A switchable ferrocene-based [1]rotaxane with an electrochemical signal output

Article abstract of DOI:10.1021/ol302826g

A [1]rotaxane, in which a linear rod is attached to one cyclopentadienyl (Cp) ring of a ferrocene unit and threaded into a dibenzo-24-crown-8 connected to the other Cp ring, was prepared. The mechanical motion of the rod-like part relative to the macrocycle has been demonstrated using ¹H NMR spectroscopy. Cyclic voltammetry (CV) showed that the system can be chemically switched between electrochemically reversible and irreversible states, depending on the inclusion and exclusion of the ammonium/amine group from the macrocycle.

Full text of DOI:10.1021/ol302826g

ORGANIC
LETTERS
2012
Vol. 14, No. 23
5900–5903
A Switchable Ferrocene-Based
[1]Rotaxane with an Electrochemical
Signal Output
Hong Li, Hui Zhang, Qiong Zhang, Qi-Wei Zhang, and Da-Hui Qu*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, P. R. China
dahui_qu@ecust.edu.cn
Received October 14, 2012
ABSTRACT
A [1]rotaxane, in which a linear rod is attached to one cyclopentadienyl (Cp) ring of a ferrocene unit and threaded into a dibenzo-24-crown-8
connected to the other Cp ring, was prepared. The mechanical motion of the rod-like part relative to the macrocycle has been demonstrated using
1H NMR spectroscopy. Cyclic voltammetry (CV) showed that the system can be chemically switched between electrochemically reversible and
irreversible states, depending on the inclusion and exclusion of the ammonium/amine group from the macrocycle.
Bistable rotaxanes, as the most common species of me-
chanically interlocked molecules (MIMs),1 have received con-
siderable attention because of their applications in molecular
devices and as components of molecular machinery.1ꢀ3
Until now, there has been much work aimed at the design
and construction of multifunctional [2]rotaxanes4 with two
distinguishable states that can be recognized by various
output signals, such as UV/vis absorption,4,5 fluorescence,5,6
electrochemical signals,7 and circular dichroism.8
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44, 5296. (c) Wang, Q.-C.; Ma, X.; Qu, D.-H.; Tian, H. Chem.;Eur. J.
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r
10.1021/ol302826g
Published on Web 11/12/2012
2012 American Chemical Society

A [1]rotaxane,9 in which the macrocycle and the axle part
are bound covalently and closely, is also an important
member of the family of MIMs. The architecture of this
kind of interlocked system is rather unique and appealing,
and their syntheses and functions also merit extensive atten-
tion. However, until now, only few [1]rotaxane systems9 have
been reported, most of which do not employ two well-
separated recognition sites.10 We believe that introduction
of two recognizable stations in such a structure is the key for
the design of a switchable system that can convert between
two different states with remarkable output changes.
Scheme 1. Switching Process and Schematic Representation of
the bistable Ferrocene-Based [1]Rotaxane 1-H That Can Switch
between Two Different States
In this paper, we report the synthesis and switching
property of a bistable ferrocene-based [1]rotaxane 1-H,
which can switch between two different states, namely, an
electrochemically reversible state and an electrochemically
irreversible state, caused by chemically driven inclusion
and exclusion of an ammonium/amine group by the
dibenzo-24-crown-8 (DB24C8) macrocycle. As shown in
Scheme 1, [1]rotaxane 1-H employs a ferrocene unit7 as an
axle, and an electrochemically active center is covalently
attached with a DB24C8 macrocycle and a rod-like
thread at each cyclopentadienyl (Cp) ring, respectively.
The rod part, which contains two distinguishable recogni-
tion sites, namely, a dibenzylammonium site (DBA) and a
N-methyltriazolium (MTA)11 site, was threaded into the
macrocycle with a bulky 3,5-dimethoxybenzene stopper
situated at the end of the rod part to form an interlocked
[1]rotaxane structure. In the original state, the DB24C8
macrocycle encircles the DBA station, which makes the
amine group “included”; as a result, cyclic voltammetry
(CV) showed a reversible oxidationꢀreduction curve of
the free ferrocene unit. After deprotonation with a base,
the macrocycle allowed the amine group to be “excluded”
because of its movement and its location at the MTA
station, which showed anirreversible oxidationꢀreduction
curve. Thus, the system can switch between an electro-
chemically reversible state and an electrochemically irre-
versible state, which gives insight for the design of smart
materials with switchable properties.
CH2Cl2, which was converted into [1]rotaxane 2-H in a
moderate yield through the well-known copper(I)-cata-
lyzed Huisgen 1,3-dipolar cycloaddition reaction with 3,5-
dimethoxybenzyl azide 5. [1]Rotaxane 2-H was treated
with methyl iodine, which was then followed by anion
exchange with saturated NH4PF6 to yield [1]rotaxane 1-H
with primary DBA and secondary MTA recognition sites
for the DB24C8 macrocycle. The noninterlocked reference
compound 3-H was prepared using two different strate-
gies, as shown in Schemes 2 and S3, respectively. The first
strategy, shown in Scheme 2, contains a “click” reaction
between alkyl 4 and azide 5 in 54% yield to generate
compound 3-Boc, which was treated with TFA to remove
the Boc group, and then followed by anion exchange to
afford compound 3-H in 76% yield. In the other strategy,
compound 4 was first treated with TFA to remove the Boc
group to obtain an intermediate with a DBA station that
has a noncomplexing structure in polar solvent such as
DMF, which was converted into compound 3-H in a
moderate yield (40%) through the “click” reaction with
azide 5. The two different synthetic strategies can yield the
The syntheses of [1]rotaxane 1-H, reference compound
3-H, and the key intermediates are shown in Scheme 2. We
designed compound 4, containing a dibenzo-24-crown-8-
based macrocycle and a Boc-protected DBA station with a
terminal alkyne attached to each Cp ring of the ferrocene
unit, respectively. After removal of the Boc group by
treatment with TFA, it can form a predominantly self-
complexing [1]pseudorotaxane in less polar solvent such as
1
same product, as evidenced by the same H NMR spec-
trum and the same molecular weight.
[1]Rotaxane 2-H and compound 3-H were all character-
1
ized by H NMR spectroscopy and high-resolution elec-
trospray ionization (HR-ESI) mass spectrometry. The
HR-ESI mass spectra of 2-H and 3-H showed the major
peaks at m/z 1293.4583 and 1293.4580, respectively, which
can be assigned to the loss of PF6ꢀ, i.e. [M-PF6]þ. This
result supports the different isomers of 2-H and 3-H with
the same molecular weight. However, the 1H NMR spectra
(Figure 1) of 2-H and 3-H showed different patterns in
CD3COCD3. Compared with those of compound 3-H
(Figure 1a), the peaks for the methylene protons H15 and
H16 on the DBA recognition site (Figure 1b) in rotaxane 2-H
are integrated and shifted downfield (Δδ = 0.20, 0.29 ppm).
Meanwhile, the chemical shifts for the phenyl protons H13,
H14, H17 adjacent to the DBA center are also changed with
Δδof ꢀ0.24, 0.14, and ꢀ0.20 ppm, respectively. The protons
(8) (a) Zhu, L.-L.; Zhang, D.; Qu, D.-H.; Wang, Q.-C.; Ma, X.; Tian,
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Tian, H. Chem. Commun. 2007, 1409. (b) Ma, X.; Wang, Q.-C.; Tian, H.
Tetrahedron Lett. 2007, 48, 7112. (c) Franchi, P.; Fani, M.; Mezzina, E.;
Lucarini, M. Org. Lett. 2008, 10, 1901. (d) Zheng, X.; Michael, F. M.
J. Am. Chem. Soc. 2010, 132, 3274.
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4107.
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3741. (b) Coutrot, F.; Busseron, E. Chem.ꢀEur. J. 2008, 14, 4784. (c)
Zhang, H.; Kou, X.-X.; Zhang, Q.; Qu, D.-H.; Tian, H. Org. Biomol.
Chem. 2011, 9, 4051. (d) Zhang, H.; Hu, J.; Qu, D.-H. Org. Lett. 2012,
14, 2334.
Org. Lett., Vol. 14, No. 23, 2012
5901

Scheme 2. Preparation of [1]Rotaxane 1-H, 2-H and
Compound 3-Ha
Figure 1. Partial 1H NMR spectra (400 MHz, CD3COCD3,
298 K) of (a) 3-H and (b) 2-H.
DBA station resides inside the DB24C8 ring. Using HR-
1
ESI mass spectrometry, H NMR spectroscopy, and 2D
Roesy NMR spectroscopies, we confirmed compound 1-H
existing as an interlocked [1]rotaxane structure as shown in
Scheme 1.
Next we focused on the reversible acidꢀbase-driven
mechanical motion of 1-H. Addition of 1.0 equiv of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) to the CD3COCD3
solution of rotaxane 1-H can deprotonate the DBA moi-
ety, generating [1]rotaxane 1 (Scheme 1); as a result, the
macrocycle was changed to reside over the MTA station,
and remarkable 1H NMR spectral changes were observed.
As shown in Figure 2b, the protons H22 and H27 on the
MTA recognition site are dramatically shifted (Δδ = 0.55
and ꢀ0.45 ppm, respectively), which indicates the migra-
tion of the MTA recognition site into the DB24C8 ring.
a The assignments for protons of 2-H or 3-H are the same as these for
rotaxane 1-H shown in Scheme 1.
H12 neighboring the DBA station in the rod part emerged as
a doublet in [1]rotaxane 2-H, in comparison with a singlet in
compound 3-H. All the evidence confirmed that compound
2-H has an interlocked structure, and the DB24C8 ring
encircles at the DBA recognition site.
The structure of the target compound [1]rotaxane 1-H
obtained by the methylation of [1]rotaxane 2-H was also
confirmed. The HR-ESI mass spectrum of [1]rotaxane 1-H
showed that the most intense peak emerges atm/z 654.2414
as a doubly charged peak, corresponding to the consecu-
tive loss of two PF6ꢀ counterions. The 1H NMR spectrum
(Figure 2a) of [1]rotaxane 1-H also proved that the
DB24C8 macrocycle was encircling at the DBA binding
site, similar to the structure of 2-H (Figure 1). Moreover,
2D Roesy NMR spectrum of rotaxane 1-H (Supporting
Information (SI), Figure S3) provided good evidence for
the formation of the rotaxane 1-H and the fact that the
Figure 2. Partial 1H NMR spectra (400 MHz, CD3COCD3,
298 K) of (a) 1-H, (b) deprotonation with addition of 1.0 equiv
of DBU to sample a, and (c) reprotonation with addition of
2.5 equiv of TFA to sample b.
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Org. Lett., Vol. 14, No. 23, 2012

between an electrochemically reversible state and an elec-
trochemically irreversible state. Based on electrochemical
properties of ferrocene (Fc), the cyclic voltammetric
(CV) curves of [1]rotaxane 1-H in CH3CN containing 0.1 M
[(n-Bu)4N]PF6 as a supporting electrolyte (Figure 3) were
investigated. In the original state, [1]rotaxane 1-H has a
reversible CV curve of the Fc unit. The half-wave potential
(E1/2) of 1-H is detected by a shift in E1/2 = 0.879 V
(Figure 3a) to a more positive value (ΔE1/2 = þ0.455 V),
compared with that of Fc (E1/2 = 0.424 V) (SI, Figure S5a),
and by a decrease in the peak currents (Ip). After deproto-
nated with DBU, the CV curve (Figure 3b) of [1]rotaxane
112 exhibits an irreversible feature in that the oxidation and
reduction peaks gradually became lower. We deduced that
this CV signal change in the two states of [1]rotaxane 1-H is
ascribed to the conformation change, i.e., the inclusion or
exclusion of the DBA site by the macrocycle. Control
experiments with [1]rotaxane 2-H and compound 3-H were
performed to confirm this deduction. [1]Rotaxane 2-H in
which the DBA site is also included showed a very similar
CV curve (Figure 3c) with that of rotaxane 1-H, whereas
compound 3-H showed an irreversible CV curve (Figure 3d).
Further control experiments showed that the addition of 1.0
equiv of dibenzylamine to 1-H or Fc (SI, Figure S5b-S5c) also
yielded irreversible CV curves, indicating an interaction
between the amine and the oxidized or reduced ferrocene
species, which results in the irreversible oxidation and reduc-
tion processes of ferrocene.13
Figure 3. Evolution of the cyclic voltammetric curves of (a) 1-H,
(b) 1, (c) 2-H, and (d) 3-H in CH3CN with [(n-Bu)4N]PF6 (0.1 M)
as supporting electrolyte, scanned at 20 mV sꢀ1. The contentra-
tion of each compound is 1 ꢁ 10ꢀ3 M.
Moreover, a key change was observed in the peak of the
protons H12 neighboring the DBA station, which was
changed from one doublet into two doublets with Δδ of
ꢀ0.31 and ꢀ0.09 ppm, respectively. The 2D Roesy NMR
spectrum of rotaxane 1-H after addition of DBU (ESI,
Figure S4) showed clear correlation peaks between the
phenyl protons H5, H7, H6, H8, and H9 on the DB24C8
ring, and the protons H27 (peak A), H26 (peak B), H21
(peak C), H23 (peak D), and H14 (peak E) on the thread
also confirmed that the MTA recognition site resides in the
DB24C8 ring. Upon addition of 2.5 equiv of CF3COOH,
the original 1H NMR spectrum was regenerated (Figure 2c),
which suggested that the system was recovered completely
to its original state. In short, using 1H NMR and 2D Roesy
NMR spectroscopies, we proved the reversible translational
motion of the molecular rod relative to the macrocycle. It
should be noted that the structural difference between 2-H
(lack of a methyl group in the triazole unit) and 1-H makes
2-H a system that has only one recognition site, which can
also be deprotonated with DBU; however, no obvious
shuttling motion is observed (SI, Figure S2).
In conclusion, we have designed and constructed a
ferrocene-based [1]rotaxane with two recognition sites,
which can perform reversible shuttling motion and show
distinguishable electrochemical signals in the two states.
The system can switch between an electrochemically re-
versible state and an electrochemically irreversible state,
which holds potential for the design and construction of
switchable molecular systems.
Acknowledgment. We thank the NSFC/China (20902024,
21272073 and 21190033), National Basic Research 973
Program (2011CB808400), the Foundation for the Author
of National Excellent Doctoral Dissertation of China
(200957), the Fok Ying Tong Education Foundation
(121069), the Fundamental Research Funds for the Central
Universities (WJ1014001, WJ1213007), and the Innovation
Program of Shanghai Municipal Education Commission
for financial support.
In this system, the reversible inclusion and exclusion of
the macrocycle on the amine group in the rod-like thread
can be adjusted by external acidꢀbase stimuli, which can
be used to construct a molecular device that can be switched
Supporting Information Available. Full experimental
procedures and characterization data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(12) [1]Rotaxane 1 was obtained in a following procedure:
[1]Rotaxane 1-H was treated with excess DBU in CH2Cl2 and then
(13) Rabie, U. M. Spectrochim. Acta, Part A 2009, 74, 746.
washed with deionized water five times to remove DBU, followed by
flash column chromatography to yield [1]rotaxane
measurement.
1 for CV
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 23, 2012
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