The self-complexation of mono-urea-functionalized pillar[5]arenes with abnormal urea behaviors

Article abstract of DOI:10.1039/c3cc47823h

Mono-urea-functionalized pillar[5]arenes were prepared which exhibited abnormal urea behaviors. They were shown to form pseudo[1]rotaxanes with poor anion binding abilities in solution and [c2]daisy chains with an abnormal urea motif in the solid state. T

Full text of DOI:10.1039/c3cc47823h

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The self-complexation of mono-urea-functionalized
pillar[5]arenes with abnormal urea behaviors†
Cite this: Chem. Commun., 2014,
50, 1317
Mengfei Ni, Xiao-Yu Hu,* Juli Jiang* and Leyong Wang
Received 11th October 2013,
Accepted 8th November 2013
DOI: 10.1039/c3cc47823h
www.rsc.org/chemcomm
Mono-urea-functionalized pillar[5]arenes were prepared which
exhibited abnormal urea behaviors. They were shown to form
pseudo[1]rotaxanes with poor anion binding abilities in solution
and [c2]daisy chains with an abnormal urea motif in the solid state.
Mechanically interlocked molecules (MIMs) have attracted the
attention of supramolecular chemists in recent years owing to
their characteristic topological structures and potential applica-
tions in molecular machines.1 Among the various types of MIMs,
[1]rotaxanes2 (or pseudo[1]rotaxanes) and [c2]daisy chains3 have
been intensively studied. However, the construction of [1]rotax-
anes (or pseudo[1]rotaxanes) and [c2]daisy chains is usually
based on traditional macrocycles such as crown ethers and
cyclodextrins. Pillar[5]arenes4 are a new type of macrocycle, which
possess unique equilateral pentagonal structures and column-
shaped cavities. Recently, Ogoshi and coworkers5 reported a
pseudo[1]rotaxane in chloroform which was formed by a mono-
Fig. 1 (a) Structures of a mono-urea-functionalized pillar[5]arene 1a
reported in our previous work, and two reference compounds 1b and
1c. (b) Structures of two mono-urea-functionalized pillar[5]arenes 2a and
2b, and illustration of the self-complexation of them with abnormal urea
behaviors.
guest-functionalized pillar[5]arene. Huang and coworkers6 pillar[5]arene 1a (Fig. 1a), which not only acted as an effective
reported the first pillar[5]arene-based [c2]daisy chain formed by anion receptor, but also showed an excellent ability for ion pair
a copillar[5]arene with a long bromo-substituted alkyl chain,6a recognition of alkylammonium salts in chloroform.10 Herein,
and based on this work they subsequently developed a solvent- we synthesized two mono-urea-functionalized pillar[5]arenes 2a
driven spring.6b However, there are few examples of the construc- and 2b (Fig. 1b), which were initially designed for ion pair
tion of pillar[5]arene-based [1]rotaxanes (or pseudo[1]rotaxanes) recognition, however surprisingly they showed unique self-
and [c2]daisy chains until now.7
complexation properties and abnormal urea behaviors. It was
The aggregation of urea is generally via N–HꢀꢀꢀOQC hydrogen demonstrated that 2a and 2b formed pseudo[1]rotaxanes in
bonding, which forms a typical urea a-tape motif, which was named solution with no urea aggregation and poor anion binding
a Velcro type hook-and-loop fastener by Steed.8 In addition, ureas are abilities in chloroform. Furthermore, X-ray analysis of 2a
also well known for their strong anion binding ability and showed the formation of a [c2]daisy chain with an abnormal
their derivatives have been widely used as anion receptors.9 In urea motif which could be the first example in urea chemistry.
our previous work, we reported a mono-urea-functionalized
Mono-urea-functionalized pillar[5]arenes 2a and 2b (Fig. 1b)
were prepared in a facile way by reaction of amino pillar[5]arenes
with isocyanates (see ESI†). Compounds 1b and 1c were
designed and synthesized as two reference ureas (see ESI†).
The self-complexation behavior of 2a and 2b in solution were
first studied by 1H NMR spectroscopy in different solvents.
Similar signal changes of the protons on the alkyl chains of 2a
and 2b were observed when comparing their 1H NMR spectra in
nonpolar solvents and polar solvents (Fig. 2 and Fig. S22, ESI†).
Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular
Chemistry, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, China. E-mail: huxy@nju.edu.cn, jjl@nju.edu.cn;
Fax: +86 025 83317761; Tel: +86 025 83597090
† Electronic supplementary information (ESI) available: synthetic procedures,
characterizations, crystal data, NMR and IR spectra. CCDC 947531. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc47823h
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did not show significant changes, suggesting that 2a and 2b did not
form intermolecular complexes in CDCl3.14 The aggregation of an
N, N0-dialkyl urea in nonpolar solvents is generally concentration-
dependent,15 which can be confirmed by the variable concentration
1H NMR spectra of 1c in CDCl3 (Fig. S30, ESI†). Therefore, it could
be concluded that 2a and 2b form pseudo[1]rotaxanes in CDCl3 with
no urea aggregation. Furthermore, the absence of N–HꢀꢀꢀOQC
hydrogen bonding in 2a and 2b in solution was demonstrated by
FTIR measurements, since a set of exclusively weak and sharp NH
stretching bands at 3417–3427 cmꢁ1 (non-hydrogen-bonded NH
stretch) were observed for CHCl3 solutions of 2a and 2b (Fig. S40,
ESI†).16 We also performed theoretical calculations for the self-
complexation geometries of 2a, and the results confirmed the
formation of pseudo[1]rotaxanes as expected (Fig. S51, ESI†).
It had been mentioned that 2a and 2b formed pseudo[1]-
rotaxanes in chloroform but the pseudo[1]rotaxanes were destroyed
in polar solvents. Therefore it was possible to tune the equilibrium
between uncomplexed species and pseudo[1]rotaxanes by changing
the polarity of the solvent. Considering the weaker broadening
effects, 2b was chosen for the further study. By increasing the
Fig. 2 1H NMR spectra (400 MHz, 298 K) of 2b (8 mM) in different percentage of DMSO-d6 in the mixture with CDCl3 the solvent
solvents: (a) acetone-d6, (b) CD3CN, (c) DMSO-d6, and (d) CDCl3.
polarity could be increased. With this increase the peaks for
H5–H9, Ha, and Hb of 2b shifted downfield gradually, and the peaks
for H7–H11 overlapped while the peaks for H2–H3 split (Fig. S41,
Considering the less remarkable broadening effects,11 2b was ESI†). These phenomena indicated that the equilibrium shifted from
chosen for detailed description. It is obvious that in CDCl3 the the pseudo[1]rotaxane to the uncomplexed species. In addition, the
signals of H5–H10 and Hb are shifted upfield, indicating the pseudo[1]rotaxane was so stable that even in CDCl3–50% DMSO-d6
inclusion of the ureido group and alkyl chain into the cavity of (v/v) it was not totally destroyed.
1
the pillar[5]arene (Fig. 2). Comparison of the H NMR spectra
Urea derivatives have been widely used as anion receptors due to
between 2b and a reference urea 1c (a building subunit of strong hydrogen bonding between ureas and anions.9 In our
pillar[5]arene 2b, Fig. 1a) at the same concentration in CDCl3 previous work, pillar[5]arene 1a was proven to be an effective anion
confirmed the inclusion phenomenon of 2b (Fig. S21, ESI†). receptor for anions such as Clꢁ, Brꢁ, and CF3COOꢁ.10 However, the
Moreover, the self-complexation of 2a and 2b in CDCl3 could 1H NMR spectra of 2b with the addition of different equivalents of
also be observed by 2D NOESY analysis based on the NOE tetrabutylammonium chloride (TBACl) showed that the chemical
correlation signals between the protons on the linear chain shifts of Ha (4.64 ppm) and Hb (2.89 ppm) shifted slightly downfield
and pillar[5]arene cavity (Fig. S26 and S27, ESI†). In contrast, to 4.68 ppm and 2.95 ppm, respectively when 2.5 equiv. of Clꢁ were
1
there no inclusion phenomenon was seen for reference com- added (Fig. 3). The weak response of 2b displayed in the H NMR
pound 1b (Fig. 1a) based on the 1H NMR spectra (Fig. S23, ESI†), spectra also occurred when other anions such as Brꢁ, AcOꢁ,
presumably for the reason that the aromatic unit is too bulky to CF3COOꢁ and NO3 were added (Fig. S44–S47, ESI†). In contrast,
ꢁ
be included in the pillar[5]arene cavity.12
reference compounds 1c and 1b showed dramatic signal changes
Subsequently, we attempted to obtain more information on for their urea protons (Dd = 0.52 ppm for 1c, and Dd = 0.79 ppm and
the aggregation sizes of 2a and 2b by ESI-MS. The results showed 0.86 ppm for Ha and Hb of 1b, respectively) with the addition of Clꢁ
that only the peaks corresponding to the monomers could be (Fig. S42 and S43, ESI†). From these observations, we could
observed in the spectra (Fig. S31 and S32, ESI†). In addition, conclude that in CDCl3 2b was not capable of anion binding, which
diffusion-ordered 1H NMR spectroscopy (DOSY)13 experiments was probably because the ureido group was partly included in the
were carried out on 2a and 2b in CDCl3 with heptakis(2,3,6-tri- pillar[5]arene cavity due to the formation of pseudo[1]rotaxane.
O-methyl)-b-cyclodextrin (CD) or 1,4-dimethoxypillar[5]arene (DMP5)4a
We also wished to confirm the formation of pseudo[1]rotaxanes
as the internal reference. The diffusion coefficients of the of 2a and 2b by X-ray analysis. Fortunately, we obtained a single
aggregates of both 2a and 2b were close to that of DMP5, but crystal of 2a by slow diffusion of isopropyl ether into a solution of 2a
much larger than that of CD, implying that 2a and 2b formed in a dichloromethane–ethyl acetate mixture. However, X-ray analysis
pseudo[1]rotaxanes in CDCl3 (Fig. S33–S39 and Table S1, ESI†). of 2a revealed that 2a formed a [c2]daisy chain in solid state (Fig. 4),
The self-complexation behavior of 2a and 2b in CDCl3 was which was not consistent with the pseudo[1]rotaxane formed in
further studied by variable concentration 1H NMR spectroscopy. solution. The conformation of the [c2]daisy chain could only be
The aggregates of 2a and 2b were very stable and formed in even found the pS–pR form.6a,7b,17 There were multiple non-covalent
very dilute CDCl3 solution (Fig. S28 and S29, ESI†). As the concen- interactions such as C–Hꢀꢀꢀp, C–HꢀꢀꢀOQC, and van der Waals
tration increased to 100 mM, the proton resonances of 2a and 2b interactions, which played an important role in the formation of the
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in chloroform, which was very different from the reference
pillar[5]arene 1b. Moreover, we also succeeded in tuning the
equilibrium between uncomplexed species and the pseudo[1]-
rotaxane by changing the polarity of the solvent. In contrast, the
crystal structure of 2a revealed a [c2]daisy chain formation, with
an abnormal urea motif in which two ureido groups were head
to head and possible CQOꢀ ꢀ ꢀOQC secondary bonding was
observed. This study provided examples of abnormal urea beha-
viors and the existence of a special non-covalent interaction that
is difficult to achieve by common routes.
We gratefully acknowledge the financial support of the
National Natural Science Foundation of China (Nos. 21202083,
21102073), National Basic Research Program of China
(2011CB808600). We thank Prof. Dunru Zhu (Nanjing University
of Technology) for the collection of X-ray data. We also thank Dr
Yangfan Guan, Dr Chao Deng, Dr Wei Xia, and Mr Shuhan Xiong
for their help on this work.
Notes and references
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Chem. Commun., 2014, 50, 1317--1319 | 1319