Cucurbit[7]uril/CuCl promoting decomposition of 4-nitrobenzenediazonium in aqueous solution

Article abstract of DOI:10.1016/j.cclet.2018.03.014

4-Nitro-benzendiazonium (4-NBD) was found to form a 1:1 host-guest complex (NBD@CB) with cucurbit[7]uril (CB[7]) in aqueous solution and 4-NBD enters into the cavity of CB[7] with a binding constant of 1.28 × 10⁵ L/mol. NBD@CB can be decomposed into a nitrobenzene/4-nitrophenol mixture in a high total yield (61% + 33%) in the presence of CuCl, unlike the decomposition of 4-NBD in the presence of either CB[7] or CuCl, or both absence, where would result in significant amounts of unknown byproducts. This work might provide an economic and effective way to obtain arenes or phenols through the substitution of diazonium salts.

Full text of DOI:10.1016/j.cclet.2018.03.014

Accepted Manuscript
Title: Cucurbit[7]uril/CuCl promoting decomposition of
4-nitrobenzenediazonium in aqueous solution
Authors: Hongxing Xu, Qiaochun Wang
PII:
DOI:
Reference:
S1001-8417(18)30117-7
https://doi.org/10.1016/j.cclet.2018.03.014
CCLET 4475
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Chinese Chemical Letters
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25-2-2018
12-3-2018
Please cite this article as: Hongxing Xu, Qiaochun Wang, Cucurbit[7]uril/CuCl
promoting decomposition of 4-nitrobenzenediazonium in aqueous solution, Chinese
Chemical Letters https://doi.org/10.1016/j.cclet.2018.03.014
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Communication
Cucurbit[7]uril/CuCl promoting decomposition of 4-nitrobenzenediazonium in
aqueous solution
Hongxing Xu, Qiaochun Wang
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University
of Science & Technology, Shanghai 200237, China
 Corresponding author.
E-mail address: qcwang@ecust.edu.cn
ARTICLE INFO
ABSTRACT
Article history:
4-Nitro-benzendiazonium (4-NBD) was found to form a 1:1 host-guest complex
(NBD@CB) with cucurbit[7]uril (CB[7]) in aqueous solution and 4-NBD enters into the
cavity of CB[7] with a binding constant of 1.28×105 L/mol. NBD@CB can be
decomposed into a nitrobenzene/4-nitrophenol mixture in a high total yield
(61%+33%) in the presence of CuCl, unlike the decomposition of 4-NBD in the
presence of either CB[7] or CuCl, or both absence, where would result in significant
amounts of unknown byproducts. This work might provide an economic and effective
way to obtain arenes or phenols through the substitution of diazonium salts.
Received 17 January 2018
Received in revised form 25 February 2018
Accepted 7 March 2018
Available online
Keywords: Diazonium salts
Cucurbiturils
Host-guest complexes
Supramolecular chemistry
CuCl
Cucurbit[n]urils (CB[n]s) are a series of macrocyclic
molecules consisting of n glycoluril units bridged by 2n
methylene groups [1]. Because of their special rigid
structures of one hydrophobic cavity lying between two
polar carbonyl-lined portals, CB[n]s could include size-
matched molecules, especially those cationic species,
and have shown extensive uses in the field of
supramolecular chemistry [2]. Among these areas,
using a CB[n] as a molecular container for catalytic
synthesis [3] has been one of the key issues and many
reports have thus emerged, for example, Mock and his
co-workers have firstly reported applying CB[6] for
efficient triazole formation [4]. Tao group reported
CB[8]-catalytic oxidation of aryl alcohols to aldehydes
[5]. Kim [6] reported a facile method using CB[8] to
induce stereoselective [2+2] photoreaction. Some
other smart strategies such as photodimerization and
chemoselective photoreaction [7], catalytic alcoholysis
of epoxides [8], gas-phase reaction [9], acid catalysis
[10], esterification [11] and oxidizing reaction [12] were
also developed.
Diazonium salts are one sort of important
intermediates for further preparation of phenols,
aromatic halogens and other important organic
compounds [13], in the way of decomposing into arene
cations and then substituted by nucleophiles [14], or
into arene radicals followed by hydrogen abstraction
[15] or coupling [16]. It has been found that diazonium
salts could bind with CB[n]s to form host-guest
complexes for their positive charged nature [17],
however, there is no report about the decomposition
of a diazonium-cucubituril complex according to our
knowledge. Herein we report the encapsulation of 4-
nitrobenzendiazonium borofluoride (4-NBD) by CB[7] to
form a 1:1 complex NBD@CB (Scheme 1). NBD@CB
showed better thermo stability than 4-NBD, and its
decomposition in the presence of CuCl in aqueous
solution resulted in the formation of nitrobenzene (NB)
and 4-nitrophenol (4-NP), in the yields of 61% and 33%,
respectively, while in the absence of CuCl, it produced
4-NP in a yield of 27%. In comparison, the
decomposition of free 4-NBD produced mainly 4-NP,

whether CuCl existed or not, in the yields of 10% (with
CuCl) and 60% (no CuCl).
temperature reaches to 50 ºC, and then begins to drop
significantly when the heating temperature continues
to increase (ΔA50→60 = -0.009; ΔA60→70= -0.027). These
results indicate that the thermo stability of 4-NBD is
indeed considerably improved after being encapsulated
by CB[7].
To confirm that 4-NBD could combine with CB[7], the
corresponding UV-vis job plots was carried out, as
illustrated in Fig. S4 in Supporting information. The
maximum value of the vertical axle corresponds to 0.5
at horizontal axle, indicating a 1:1 complexation. High
resolution ESI-MS measurement also provides the
evidence of the formation of NBD@CB, as can be seen
in Fig. S2 in Supporting information, the peak at m/z
The decomposition of an aromatic diazonium salt
would result in the releasing of N2 molecule, and at the
same time the formation of an arene cation, which
would readily combine with nucleophile species such as
–OH in aqueous solution, forming a phenol [14]. In
comparison, the addition of Cu+ to an aromatic
diazonium salt generally favors the single electron
transfer (SET) process from Cu+ to the diazonium group
when it decomposes, and as a result generates an
arene radical [21]. The formed radical could have a
further reaction such as hydrogen abstraction and the
diazonium group is finally replaced by H atom to form
an arene.
- +
1312.3735 corresponds to the [M-BF4 ] of NBD@CB
(calcd. 1312.3739). Further 1H NMR experiments were
carried out to investigate the complexation details. As
can be seen in Fig. 1, the 1H NMR spectrum of 4-NBD in
D2O shows two groups of proton resonances (δHa = 8.63
and δHb = 8.81) with 1:1 intensity. After the addition of
CB[7] to 4-NBD solution in a molar ratio of 1:1, Hb
exhibits a distinct upfield-shifted signal (ΔδHb = -1.28)
while Ha moves slightly downfield (ΔδHa = +0.12). In
most cases, the protons within the cucurbituril cavity
undergo shielding effects and those locate near the
carbonyl portals are nearly unaffected [18]. Therefore,
the NMR results indicate that the benzene ring of 4-
NBD is encapsulated by CB[7], leaving Ha sitting near
the portal, as shown in Fig. 1B.
The decomposition of 4-NBD and NBD@CB at 0~5 °C in the
presence of CuCl was firstly monitored by UV-vis absorption. The
samples of the two solutions (C0 = 0.014 mol/L) at different
reaction time were diluted from 20 μL to 10 mL and detected by
UV-vis, the curves are shown in Fig. 4. It can be seen that the
maximum absorption band of the 4-NBD aqueous solution at 261
nm keeps nearly unchanged in the presence of 2.0 equiv. CuCl
even after standing at 0~5 °C for 44 h (Fig. 4A). In comparison,
the addition of 2.0 equiv. of CuCl to the NBD@CB solution results
in a distinct decomposition of 4-NBD guest at this temperature
(Fig. 4B), and the absorption band at 261 nm vanishes quickly
within 30 min.
ITC measurements were carried out in aqueous
solution at 25 ºC to evaluate the binding constant
between 4-NBD and CB[7]. The solutions of 0.156
mmol/L 4-NBD and 0.052 mmol/L CB[7] were placed in
the injection syringe and sample cell, respectively. The
exothermic binding isotherm was recorded with the
solution of 4-NBD being dropwise injected into the
solution of CB[7]. The obtained data were analyzed on
the basis of binding model that are given in ITC tutorial
guide. As shown in Fig. 2, the ITC results fit a 1:1
binding model, and the binding constant K is evaluated
as (1.28±0.207)×105 L/mol.
The substitution products during the decomposition were then
investigated. A solution of 4-NBD or NBD@CB in the presence
of different amounts of CuCl was decomposed at a certain
temperature. The precipitate was obtained by filtration (P1) and
the filtrate was extracted by CH2Cl2. The CH2Cl2 exact was dried
and solvent was evaporated at 30 °C on a rotary evaporator (P2).
The 1H NMR of P1 and P2 in CDCl3 were recorded (Figs. S8-S15
in Supporting information). Nitrobenzene (NB) and 4-NP were
found being the main products and the yields of NB and 4-NP were
determined during the 1H NMR measurements using DMSO as the
internal standard and the results are illustrated in Table 1.
It can be seen that CuCl enables the decomposition
of NBD@CB at around 0 °C, while at this temperature
has no effect on free 4-NBD (4-NBD keeps stable). This
is most likely caused by the complexation of the CB[7]
carbonyl rims of NBD@CB with Cu+ ion [22], which
favors the SET process. The reductive NB is the main
products (yield 61%) and 4-NP is the minor one (yield
33%) during the CuCl-catalyzed substitution of
It is well-known that diazonium salts should be
stored at low temperature because of the poor
thermostability [19, 20]. NBD@CB was expected to
have better thermo stability because of the protection
by CB[7], with the help of the electrostatic attractions
between the diazonium cation and the CB[7] portal
carbonyls, which makes the diazonium group difficult
to release a N2. The thermo-decomposition of 4-NBD
and NBD@CB were monitored by UV-vis spectra. Both
4-NBD and NBD@CB aqueous solution showed a
maximum absorption at around 260 nm. The two
solutions were then heated at different temperatures
for 1 h respectively and then the corresponding UV-vis
spectra were recorded (Figs. S5 and S6 in Supporting
information) and the absorption changes at 260 nm
were illustrated in Fig. 3. It can be seen that the
maximum absorption of 4-NBD appears accelerated
decreases when the heating temperature is higher than
20 ºC. In comparison, the maximum absorption of
NBD@CB keeps nearly unchanged even the
NBD@CB, and the total yield is satisfactory. In
comparison, the decomposition of 4-NBD in the
presence of either CB[7] or CuCl, or both absence, 4-NP
is the main product, and the total yields are relatively
low, especially in the presence of CuCl (3% NB + 10% 4-
NP) or CB[7] (4%NB + 27% 4-NP) alone. It should be
noted that there was scarcely detectable byproducts,

besides NB and 4-NP, could be found during CuCl-
catalyzed substitution of NBD@CB while a large
amount of unknown byproducts were obtained during
the latter three substitutions (Figs. S10, S12 and S14 in
Supporting information). Thus the CuCl-catalyzed
substitution of 4-NBD/CB[7] complex to NB/4-NP is an
more effective and economic way than common
substitutions, and 4-NP can be easily separated from
NB by being simply basified to phenol salt.
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Acknowledgments
This work was financially supported by the National
Natural Science Foundation of China (No. 21572063
and 21372076), the Science Fund for Creative Research
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0.50
0.45
0.40
0.35
0.30
4-NBD
0.25
NBD@CB
0.20
0.15
20
30
40
50
60
70
80
T (oC)
Fig. 3. The maximum absorptions of 4-NBD (4.22×10-5 mol/L) and
NBD@CB (C4-NBD = 4.22×10-5 mol/L, CCB[7] = 1.12×10-3 mol/L)
aqueous solutions after being heated at different temperatures for
1 h, respectively.
Fig. 1. The 1H NMR spectra (400 MHz, D2O, 298K) of free 4-NBD (A)
0.6
(A)
4-NBD
and NBD@CB (B) (CNBD = 0.0106 mol/L, CCB[7] = 0.0108 mol/L).
0.5
4-NBD+CuCl 1h
4-NBD+CuCl 20h
0.4
4-NBD+CuCl 44h
0.3
0.2
0.1
0.0
200
250
300
350
400
450
Wavelength (nm)
0.5
(B)
NBD@CB
0.4
0.3
0.2
0.1
0.0
NBD@CB+CuCl 30 min
NBD@CB+CuCl 60 min
200
250
300
350
400
450
Wavelength (nm)
Fig. 4. UV-vis spectra of 4-NBD (A) and NBD@CB (B) aqueous solutions
after the addition of 2.0 equiv. of CuCl at 5 °C, both diluted from 20
μL of the reacted aqueous solutions (C0 = 0.014 mol/L) to 10 mL.
Fig. 2. ITC results for the complexation of 4-NBD with CB[7] in aqueous
solution at 25 ºC.

Scheme 1. The synthesis of 4-NBD and the formation of NBD@CB.