Anion-Free Bambus[6]uril
FULL PAPER
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the reduction of Br . In sharp contrast to this, the BU6·Br
with a Bruker Avance 300 spectrometer operating at frequencies of
2
1
13
3
00.13 ( H) and 75.77 MHz ( C) and were referenced to the residual
complex does not exhibit an anodic peak within the accessi-
ble potential window. In fact, oxidation of the solvent takes
place before we can clearly observe the oxidation of the
macrocycle-bound bromide. Therefore, we conclude that the
complexation of bromide inside BU6 provides a strong dif-
ferential stabilization to bromide (versus its oxidized form,
BrC), which displaces the corresponding oxidation potential
peak of the solvent or TMS. UV/Vis spectra were obtained in 1.0 cm
quartz cuvettes. Irradiation was carried out with a 45 W Hg lamp
equipped with bandpass glass filters (l=254 or 366 nm). Diffraction data
were collected on an Oxford Diffraction Gemini CCD single-crystal dif-
fractometer equipped with a CCD detector Atlas, with mirror-collimated
CuKa radiation. The temperature during data collection was 120 K. The
electrochemical data were recorded with a BAS 100B/W workstation
(
Bioanalytical Systems), by using a single-compartment cell fitted with a
2
in the positive direction and moves it out of the accessible
potential window. We should also point out the possibility
that, in addition to the thermodynamic stabilization of bro-
mide upon binding inside BU6, bromide encapsulation may
kinetically slow down the heterogeneous rate of electron
transfer between the bound anion and the electrode surface.
One of us has already reported other examples, in which en-
capsulation has a strong kinetic effect on the electrochemis-
glassy carbon working electrode (0.071 cm ), a tungsten counter elec-
trode, and a Ag/AgCl reference electrode. The working electrode was
polished with 0.05 mm alumina powder (Buehler) on a felt surface lubri-
cated with pure water. All solutions were deoxygenated by passage of
high-purity nitrogen gas before the measurements. HRMS data were ob-
tained on a UPLC/MS-TOF apparatus equipped with an ESI interface.
4
,5-Dihydroxyimidazolidin-2-one: Urea (42.2 g; 0.70 mol) and glyoxal
(40% aqueous solution, 40.1 mL, 0.35 mol) was stirred at 808C until the
starting materials had dissolved completely. The solution was then al-
lowed to cool down and was stirred at room temperature for 1 h. The pH
of the solution was kept between 8 and 9 by additions of a concentrated
aqueous solution of NaOH during the procedure. The resulting precipi-
tate was collected by filtration; the filtrate was placed in a refrigerator
(48C) and a second portion of solid was collected after 2 days. The solid
fractions were combined and washed with ethanol to remove remaining
[41]
try of the bound substrate.
Conclusion
urea. The product was obtained as a white powder in 69% yield.
BU6 is a new macrocyclic compound, which is able to bind
1
H NMR (300 MHz, [D
d, 2H; OH); 4.61 ppm (d, 2H; CH); C NMR (75 MHz, [D
308C, TMS): d=160.7, 83.9 ppm.
2,4-Dimethylglycoluril: mixture of 4,5-dihydroxyimidazolidin-2-one
(30.0 g, 0.254 mol), 1,3-dimethylurea (22.4 g, 0.254 mol), deionized water
125 mL), and HCl (35%, 4 mL) was stirred at 1058C for 1 h. Most of
6
]DMSO, 308C, TMS): d=7.09 (s, 2H; NH); 5.86
[35]
anions with high affinity and selectivity. Until now BU6
was reported to exist only as a complex with an anion. In
this study, we demonstrated that the BU6·HI complex can
dissociate through either dark oxidation by H O or photo-
13
(
6
]DMSO,
A
2
2
(
catalytic oxidation of encapsulated iodide anion. We pro-
the liquid was then evaporated to obtain a mushlike solid, which was col-
lected by filtration. After drying under reduced pressure, a white powder
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posed that, under these conditions, the encapsulated I is
converted to I , which does not interact with the macrocycle,
1
2
was obtained in 78% yield (33.7 g). H NMR (300 MHz, [D ]DMSO,
6
thus allowing the isolation of empty BU6 in high yields. In
addition to easy and inexpensive synthesis of BU6, its recov-
ery as an anion-free receptor from a stable complex is an-
other important advantage for potential applications. We
308C, TMS): d=7.48 (s, 2H; NH); 5.12 (s, 2H; CH); 2.64 ppm (s, 6H;
1
3
CH
3 6
); C NMR (75 MHz, [D ]DMSO, 308C, TMS): d=160.9, 157.6, 67.1,
2
7.6 ppm.
BU6·HI: BU6·HCl (2 g, 1.77 mmol) was dissolved in chloroform (50 mL)
and methanol (50 mL), and an aqueous solution of HI (56%, 0.237 mL)
was added. The liquid was then evaporated. The precipitate was washed
were able to determine the K values for the complexes of
a
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BU6 with anions, such as BF , NO , or CN , in acetoni-
with water and acetone, and dried under reduced pressure. The yield was
4
3
5
ꢀ1
1
trile/water (1:1). The highest Ka(I) value (8.9ꢀ10 m ) was
found for BU6·HI. X-ray diffraction was also used to dem-
onstrate that BU6 has a flexible structure when the glycolur-
il building blocks adapt their position to the anion size. Fur-
thermore, the crystals obtained from the solution of BU6
and TBA salts showed an unusual arrangement of macrocy-
cles into the layers, which are separated by more than 6 ꢃ
by the intervening space filled with solvent molecules and
95%. H NMR (300 MHz, CD
3
OD/CDCl
3
(2:1), 308C, TMS): d=5.62 (s,
).
1
2H; CH), 5.12 (s, 12H; CH
2
), 3.17 ppm (s, 36H; CH
3
Empty BU6: Method A: BU6·HI (1 g, 0.819 mmol) was dissolved in
methylene chloride (150 mL) and methanol (150 mL). An aqueous solu-
tion of hydrogen peroxide (30%, 0.25 mL) was added dropwise and the
resulting mixture was stirred for 1 h at 208C, heated at reflux for 5 min,
and cooled to 208C. The solvent was evaporated and the resulting precip-
itate was washed with a mixture of methanol/dichloromethane (1:1) and
acetone. Empty BU6 was obtained as a white powder in 92% yield.
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Method B: BU6·HI (0.515 g, 0.422 mmol) was dissolved in water/acetoni-
trile (1:1, 75 mL). An aqueous solution of hydrogen peroxide (30%,
cations. Cyclic voltammetry of BU6·Br showed that the
anodic peak corresponding to bromide oxidation is shifted
substantially in the positive direction.
0
.25 mL) was added dropwise. The mixture was stirred for 24 h at 208C.
The solvent was evaporated and the resulting precipitate was washed
four times with water/acetonitrile (1:1, 75 mL) and acetone (50 mL).
Empty BU6 was obtained as a white powder in 90% yield. The product
was insoluble in any of the tested solvents. It was therefore characterized
as a BU6·TBACl complex, which was formed after addition of TBACl to
the suspension of anion-free BU6 in methanol/chloroform (2:1).
The presented results indicate that BU6 has great poten-
tial in many fields, such as removal and sensing of anions, or
crystal engineering. Further studies, which may reveal more
exciting properties of BU6, are currently under way in our
laboratory.
1
H NMR (CD OD/CDCl (2:1), 300 MHz); d=5.41 (s, 12H), 4.20 (s,
3
3
1
(
2H), 3.17 (t, 8H), 3.03 (s, 36H), 1.62 (m, 8H), 1.39 (m, 8H), 0.99 ppm
t, 12H).
Photocatalysis: Titanium dioxide powder (Aeroxide P25, typically
.1 mg) was suspended in a solution of BU6·HI (5.0 mm) in acetonitrile/
Experimental Section
0
water (1:1, 0.5 mL) in a standard quartz UV cuvette. The mixture was
purged with oxygen for 5 min and irradiated at either l=254 or 366 nm.
UV/Vis and NMR spectra were recorded periodically at the given time
General: Starting materials were purchased from commercial suppliers
and were used without further purification. NMR spectra were recorded
Chem. Eur. J. 2011, 17, 5605 – 5612
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5611