A novel star-shaped zinc porphyrin cored [5]rotaxane

Article abstract of DOI:10.1021/ol3032686

A novel star-shaped zinc porphyrin cored [5]rotaxane with four rotaxane arms was synthesized and well characterized by ¹H, ¹³C NMR spectroscopy and HR-ESI mass spectrometry. The introduction of the zinc porphyrin core enabled the [5]rotaxane to have a fixed shape and symmetrical structure, and the simultaneous shuttling motion of four macrocycles can be driven by external acid-base stimuli. This kind of topological structure exhibits important potential in the design and construction of large sophisticated assemblies.

Full text of DOI:10.1021/ol3032686

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
A Novel Star-Shaped Zinc Porphyrin
Cored [5]Rotaxane
Hui Zhang, Qiang Liu, Jing Li, 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 November 28, 2012
ABSTRACT
1
A novel star-shaped zinc porphyrin cored [5]rotaxane with four rotaxane arms was synthesized and well characterized by H, 13C NMR
spectroscopy and HR-ESI mass spectrometry. The introduction of the zinc porphyrin core enabled the [5]rotaxane to have a fixed shape and
symmetrical structure, and the simultaneous shuttling motion of four macrocycles can be driven by external acidꢀbase stimuli. This kind of
topological structure exhibits important potential in the design and construction of large sophisticated assemblies.
With the development of topological chemistry and
dynamic covalent chemistry, more and more sophisticated
mechanically interlocked compounds with complicated
structures and highly symmetrical shapes, such as ro-
taxanes,1 catenanes,2 molecular elevators,3 Borromean
rings,4 Solomon’s knots,5 trefoil knots,6 and molecular
necklaces,7 have been reported. In all reported mechanically
interlocked molecules, rotaxanes have been extensively and
thoroughly investigated because of their excellent properties
and convenient syntheses. However, the synthesis of highly
ordered or symmetrical [n]rotaxane with predetermined
shapes still remains a challenge for chemists because of
the synthetic difficulty.8 As we know, the geometry of the
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r
10.1021/ol3032686
XXXX American Chemical Society

Figure 1. Chemical structure and schematic presentation of target star-shaped zinc porphyrin cored [5]rotaxane 1-H.
rotaxane is determined by the topology of the individual
components. It can be concluded that the utilization of
molecular building blocks with invariable and unambigu-
ous shapes and exclusive reaction sites can lead to the
formation of rotaxanes with fixed shapes. This fact inspired
us to find molecular building blocks with symmetrical
shapes and synthesize a highly symmetrical [n]rotaxane.
In this paper, we report the design, synthesis, character-
ization, and shuttling motion of a star-shaped zinc
porphyrin cored [5]rotaxane, in which each of the four
substituted dibenzo-24-crown-8 (DB24C8) macrocycles
was interlocked onto a branch of the star-shaped com-
pound with a zinc porphyrin core bearing a rigid structure
and symmetrical shape. Porphyrin was selected as the
modularbuildingblockbecauseofnotonlyitswell-defined
shape and size but also its convenient modification.9 In this
novel star-shaped zinc porphyrin cored [5]rotaxane, four
DB24C8 macrocycles can shuttle between two recognition
sites simultaneously under the external acidꢀbase stimuli,
which was confirmed using 1H NMR spectroscopy. This
kind of rotaxane will be beneficial for the construction of
more sophisticated mechanically interlocked molecules.
The syntheses, molecular structures, and schematic re-
presentations of [5]rotaxane 1-H, 2-H, and the reference
compound ferrocene-containing crown ether 5 are shown
in Figures 1 and 2. The target star-shaped zinc porphyrin
cored [5]rotaxane 1-H contains four DB24C8 macrocycles,
each of which encircles the thread of the [5]rotaxane
bearing a zinc porphyrin core with a rigid structure and
symmetrical shape. There are two distinguishable binding
sites for DB24C8, namely, dibenzylammonium (DBA)10
and N-methyltriazolium (MTA)11 recognition sites. The
N-methyltriazolium (MTA) moiety was used as a less
favorable recognition site for DB24C8 upon deprotona-
tion of a more favorable R2NH2þ binding site, and a long-
distance alkyl chain was introduced to the thread skeleton
to separate the two distinguishable binding sites. Deproto-
nationwitha basecan drive the macrocycles toreside in the
MTA stations, and it should be noted that the shuttling
movement of macrocyclesbetweenstationson the threadis
simultaneous.
The structure of alkyne 3, incorporating a DBA unit as a
primary recognition site for DB24C8 in the middle, termi-
nated at one end by a 3,5-dimethoxy benzene stopper, and
at the other end by a terminal alkyne group, is shown in
Figure 2. The syntheses of alkyne 3 and the key macro-
cycle, crown ether 5, were reported in our previous work.12
The key intermediate azide 4 has a zinc porphyrin core
and four azide functional groups at each end, which was
synthesized starting from compound P-1.13 The ester was
hydrolyzed in a 10% NaOH solution (CH3CH2OH/
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Org. Lett., Vol. XX, No. XX, XXXX

H2O = 4/1) to afford compound P-2, which was further
esterificated with 2-bromoethanol under the condition of
EDCI and DMAP in CH2Cl2 to obtain compound P-3.
In order to avoid the nitrogen atom on the porphyrin
core influencing subsequent reactions, we decided to treat
the porphyrin core with zinc acetate. After that, the zinc
porphyrin compound P-4 reacted with sodium azide to
produce the azide intermediate 4.
The selective and effective interaction between a diben-
zylammonium ion (R2NH2þ) and a dibenzo-24-crown-8
(DB24C8) ring was chosen as the binding motif in the
design of the rotaxane. The widely used “click chemistry”,
namely, Cu(I)-catalyzed azideꢀalkyne cycloaddition,14
was chosen as the end-capping method in the preparation
of rotaxane 2-H due to its functional group tolerance and
high yield. As shown in Figure 2, alkyne 3 and crown ether
5 were mixed in dry CH2Cl2 at room temperature, after
which azide 4 and Cu(CH3CN)4PF6 as the catalyst were
added to the solution, and the mixture was stirred for two
daystoformthe rotaxanes 2-H ina 50% isolated yield. The
methylation of the triazole unit in rotaxane 2-H followed
by the anion exchange of the CH2Cl2 solution with satu-
rated NH4PF6 solution can produce the target rotaxane
1-H in a 50% yield. It should be noted that rotaxane 2-H
has only one kind of recognition site (DBA site), while
rotaxane 1-H has two kinds of recognition sites (DBA site
and MTA site), as shown in Figure 2.
1
Rotaxanes 1-H, 2-H were well characterized using H,
13C NMR spectroscopy and HR-ESI mass spectrometry
(Supporting Information (SI)). The 2D COSY NMR
spectra of 2-H and 1-H were measured to assign the
protons.15 The reversible shuttling motion of the macro-
cycles between the two different recognition sites in rotax-
ane 1-H was also confirmed by the 1H NMR spectroscopy,
as discussed below.
Figure 2. Syntheses of azide 4 and rotaxanes 2-H, 1-H.
1
First, we analyzed the H NMR spectrum of rotaxane
1
1-H (Figure 3b). The comparison between the H NMR
symmetrical. The HR-ESI mass spectrum of the target
rotaxane 1-H (SI) also confirmed the structure as shown in
Figure 1, which has the peak at m/z 840.8027, correspond-
spectra of rotaxane 1-H and our previously synthesized
rotaxane12 revealed that the macrocycle 5 predominately
resides around the DBA recognition station in rotaxane
ꢀ
ing to the species that lost eight PF6 counterions, i.e.,
1
1-H.15 On the other hand, compared with the H NMR
[M-8PF6ꢀZn]8þ. The calculated value for rotaxane 1-H
(C368H428N20O72Fe88þ) is 840.8155. The mass spectrum of
rotaxane 1-H also has the peaks emerged at m/z 981.6267,
1169.4509, and 1432.2815, corresponding to the species
that lost seven, six, and fivePF6ꢀ counterions, respectively.
All this evidence confirmed that the target [5]rotaxane 1-H
was synthesized successfully as we have designed.
spectrum of rotaxane 2-H (Figure 3a), there was no signifi-
cant change that was observed in the peaks corresponding
1
to the DBA recognition site in the H NMR spectrum
of [5]rotaxane 1-H. Nevertheless, the peaks of protons
H4, H5, H7, H8 on or neighboring the triazole unit were
shifted downfield (Δδ = 0.45, 0.21, 0.96, and 0.56 ppm,
respectively) due to the methylation of the triazole
unit; meanwhile, the signal for the methyl proton H6 in
N-methyltriazolium part emerged at 4.50 ppm. The single
peak of H7 also confirmed the shape of rotaxane 1-H is
Next, we investigated the shuttling motion of the macro-
cycles between the two distinguishable recognition sites
1
in the molecular thread of rotaxane 1-H using H NMR
spectroscopy. After the addition of 4 equiv of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) to the CD3COCD3
solution of rotaxane 1-H, the ammonium moiety was
deprotonated; thus the DB24C8 macrocycles migrated
from the DBA stations to the MTA recognition sites
(Figure 1). As shown in Figure 3c, the methylene protons
H14 and H15 in the DBA station moved upfield signifi-
cantly (Δδ = ꢀ1.01 and ꢀ0.87 ppm, respectively) be-
cause of the deprotonation and the movement of the
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Org. Lett., Vol. XX, No. XX, XXXX
C

induced reversible shuttling motion of the macrocycles
along the rotaxane thread in [5]rotaxane 1-H has been
demonstrated. In addition, from the spectra, it can be
concluded that the movement of four macrocycles is
simultaneous.
Finally, the photophysical properties of rotaxane 1-H
were also investigated.15 In our previous work,12b,c we
have demonstrated that the movement of the ferrocene-
functionalized macrocycle16 can result in a dramatic
decrease in fluorescence intensity of the fluorophore.
However, in this system, the fluorescence intensity of the
porphyrin increased. This weird phenomenon may be
explained by the fact that the zinc porphyrin core can
coordinate with the nitrogen atoms on the DBU molecule.
To confirm this explanation, DBU was added to the solu-
tion of zinc tetraphenylporphyrin (zinc-TPP), the fluores-
cence intensity of zinc-TPP was observed to also increase
(Figure S3).
In summary, a novel star-shaped zinc porphyrin cored
[5]rotaxane, with four DB24C8 macrocycles interlocked
onto the thread component containing a zinc porphyrin
core was synthesized and well characterized. The simulta-
neous shuttling motion of the macrocycles between the
DBA station and the MTA station can be driven by
external acidꢀbase stimuli. By introducing a zinc porphyr-
in core into the center of the rotaxane, the shape of the
rotaxane becomes very symmetrical. This kind of por-
phyrinic rotaxane with a highly symmetrical geometry
exhibits important potential for the construction of
large sophisticated assemblies. Most importantly, the syn-
thetic principle described in this paper should be general-
ized so as to synthesize more complicated rotaxanes in the
future.
Figure 3. Partial 1H NMRspectra(400MHz, CD3COCD3, 298 K)
(a) [5]rotaxane 2-H. (b) [5]rotaxane 1-H. (c) deprotonation with
addition of 4 equiv of DBU to sample b. (d) Reprotonation with
addition of 4.1 equiv of TFA to sample c. The assignments
correspond to the structures as shown in Figure 1.
macrocycles. Meanwhile, the peaks for the N-methyltria-
zolium protons were shifted due to association with the
macrocylic compound 5, for H6 with a Δδ of ꢀ0.54 ppm
and H4, H5, H7, and H8 with Δδ of 0.37, 0.19, 0.31, and
ꢀ0.36 ppm, respectively. All these observations indicated
that the DB24C8 macrocycles moved to the MTA recogni-
tion sites. The 2D NOESY NMR spectrum of1-H after the
Acknowledgment. We thank the NSFC/China (20902024,
21272073, and 21190033), the National Basic Research
973 Program (2011CB808400), the Foundation for the
Author of National Excellent Doctoral Dissertation of
China (200957), the Fok Ying Tong Education Founda-
tion (121069), the Fundamental Research Funds for the
Central Universities, and the Innovation Program of
Shanghai Municipal Education Commission for financial
support.
1
addition of DBU also confirmed the location.15 The H
NMR spectrum was completely recovered after the repro-
tonation of the ꢀNHꢀ center with the addition of 4.1
equiv of CF3CO2H (TFA) (Figure 3d), indicating the
return of the DB24C8 rings to the DBA stations. Thus,
by 1H NMR spectroscopic measurements, the acidꢀbase
Supporting Information Available. Experimental pro-
cedures and characterizations for all intermediates and
compounds; 1H,13C NMR spectra; HR-ESI mass spectra
of all compounds; 2D COSY and NOESY NMR spectra
of 2-H and 1-H; UVꢀvis absorption spectra; fluorescence
spectra of compounds 1-H and zinc-TPP. This material
is available free of charge via the Internet at http://pubs.
acs.org.
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The authors declare no competing financial interest.
D
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