Molecular clips based on diphenylglycoluril and benzocrown ethers: Promising complexing agents for the alkali metal cations

Article abstract of DOI:10.1007/s10847-013-0357-3

The interaction of new molecular clips containing diphenylglycoluril and benzocrown ethers moieties with alkali metals ions was studied. Stability constants were determined by spectrophotometric titrations with chloride salts in methanol. Complex stabilit

Full text of DOI:10.1007/s10847-013-0357-3

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-013-0357-3
ORIGINAL ARTICLE
Molecular clips based on diphenylglycoluril and benzocrown
ethers: promising complexing agents for the alkali metal cations
•
Leonid S. Kikot’ Alexander Yu. Lyapunov Catherine Yu. Kulygina
•
•
•
•
•
Tatiana Yu. Bogaschenko Roman I. Zubatyuk Oleg V. Shishkin
Tatiana I. Kirichenko
Received: 30 May 2013 / Accepted: 30 July 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The interaction of new molecular clips con-
taining diphenylglycoluril and benzocrown ethers moieties
with alkali metals ions was studied. Stability constants
were determined by spectrophotometric titrations with
chloride salts in methanol. Complex stability and cation
binding selectivity were shown to be dependent on the size
of the crown ether moiety. The ‘‘sandwich-type’’ 1:1 (clip
to cation) complexes and the ‘‘classical’’ 1:2 complexes
were found. Their ratio varies depending on the molecular
clips nature and on the cation type. It was found an
unexpected selectivity of the molecular clip with benzo-15-
crown-5 moieties toward K? and Rb? cations. The
molecular structure of the clip complex with benzo-15-
crown-5 fragments and sodium picrate was determined by
X-ray crystallography. The crystal structure and solution-
state structure were proven to be similar.
Keywords Molecular clip ꢀ Crown ether ꢀ
Diphenylglycoluril ꢀ Host–guest complexes ꢀ Crystal
structure ꢀ Alkali metals
Introduction
The design and synthesis of receptors capable of selective
binding to metal cations is a relevant and important task in
host–guest chemistry [1–3]. The basis of these receptors, as
a rule, is the central ionophore core binding substrate both
strongly and selectively [4, 5]. The introduction of chro-
mophores or fluorophores into molecular framework can
create highly sensitive sensors for analyte measurement
in vivo [6]. The incorporation of such sensors into mem-
branes can lead to ion-selective electrodes for in vitro
research [7, 8]. Thus, the receptors can be included into
polymers through the introduction of reactive groups, fur-
thermore, they can be immobilized on the surfaces of
substrates suitable for the selective separation of mixtures
[9–11]. Many scientists are interested in searching for
receptors that can strongly and selectively bind alkali metal
cations. In particular, ionophores for Na? and K?, which
play a crucial role in maintaining the cell homeostasis, are
widely used in creation of probes that can operate directly
in the living organisms [12, 13]. Complexones which bind
137Cs strongly and selectively in acidic media can be used
at the reprocessing the spent nuclear fuel and radioactive
waste [14–17]. Studies in this field are still relevant despite
the huge number of the obtained complexones. Crown
ethers (CE) and their derivatives dominate over numerous
recently reported synthetic receptors capable to bind cat-
ions of alkali and/or alkaline earth metals [18, 19]. Last
years, biscrown ethers with two CE fragments bound by
various linkers, draw increasing interest of researchers
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-013-0357-3) contains supplementary
material, which is available to authorized users.
L. S. Kikot’ ꢀ A. Yu. Lyapunov ꢀ C. Yu. Kulygina ꢀ
T. Yu. Bogaschenko ꢀ T. I. Kirichenko (&)
Department of Fine Organic Synthesis, A.V. Bogatsky
Physico-Chemical Institute, National Academy of Sciences
of Ukraine, Lustdorfskaya Doroga 86, Odessa 65080, Ukraine
e-mail: ti-kirichenko@rambler.ru
R. I. Zubatyuk ꢀ O. V. Shishkin
Department of X-ray Diffraction Studies and Quantum
Chemistry, SSI ‘‘Institute for Single Crystals’’, National
Academy of Sciences of Ukraine, Lenina Ave. 60,
Kharkiv 61001, Ukraine
O. V. Shishkin
Department of Inorganic Chemistry, V. N. Karazin Kharkiv
National University, Svobody Sq. 4, Kharkiv 61077, Ukraine
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J Incl Phenom Macrocycl Chem
[19]. Earlier studies of some bis(crown ether) derivatives
have established that they generally exhibit larger stability
constants and higher metal cation binding selectivity than
corresponding mono-crown ethers [19–22]. The use of a
rigid molecular fragment as a linker allows obtaining pre-
organized receptor—molecular clip with pre-determined
properties (spatial arrangement of the CE moieties, the
distance from each other, etc.) [23, 24]. Glycoluril and its
derivatives are characterized by great popularity among the
large number of molecular fragments that can act as a rigid
linker [25–27]. A large number of different synthetic
receptors were obtained on the glycoluril basis, for
instance: cucurbiturils [28, 29], ‘‘tennis ball’’ and ‘‘soft
ball’’ [30, 31], molecular baskets and variety of molecular
clips [32–35]. The latter represent receptors comprising a
rigid diphenylglycoluril framework bound up with two
aromatic walls through four bridging methylene groups.
Forming pseudocavity, molecular clips can hold various
substrates inside due to a set of non-covalent interactions
[36–41].
maintained constant and the molar ratio of guest increased
with respect to the host over the range 0.2:1–20:1 during
the titration. The absorbance measurements were carried
out at four wavelengths, at which spectral changes were the
most notable (220–290 nm) simultaneously, and sets of the
obtained experimental values (4 9 30 points) were used
for joint computer processing. The data were processed
with the nonlinear least squares fitting SIRKO [45] soft-
ware (see Supplementary data).
X-ray crystallographic investigations
Crystals suitable for X-ray diffraction were grown by crys-
tallization of equimolar mixture of 2 and sodium picrate from
EtOH–Me2CO–H2O mixture (2:2:1 v/v). X-ray diffraction
study of 2 , Na picrate complex (C114H124N17O50Na3,
Mr = 2,601.27) was performed on an Xcalibur 3 diffrac-
tometer at 100 K. Crystal data: a = 17.7944(4), b =
˚
35.8060(6), c = 20.8412(4) A, b = 108.956(2)°, V =
3
˚
12,558.7(4) A , monoclinic, space group P21/n, Z = 4,
It has previously been qualitatively shown by us that
molecular clips based on the diphenylglycoluril and ben-
zocrown ethers are capable to form host–guest inclusion
complexes with alkali metal cations of different stoichi-
ometry, and also to extract them from the aqueous phase to
the organic one [42].
l(Mo Ka) = 0.118 mm-1, dcalc = 1.376 g mm-3, 72,516
reflections measured up to 2hmax = 56.96°, 28262 unique
(Rint = 0.054) which were used in all calculations. Structure
was solved by direct methods and refined in anisotropic
approximation for all non-hydrogen atoms using SHELX-97
software [46]. Hydrogen atoms refined in riding model with
Uiso = nUeq of the carrier atom (n = 1.2 for C–H and
n = 1.5 for O–H). Heavily disordered unidentified solvent
Here we report the study of the influence of alkali metal
cation size on the interactions with molecular clips con-
taining different crown ethers moieties. There are presented
the stability constants, that quantitatively reflect the com-
plexing properties of molecular clips 1–5 with alkali metal
chlorides in methanol, and the structure of complex 2 with
sodium picrate in the solid state.
3
˚
was found in the crystal structure in the cavities of 345.1 A
(4 per unit cell) with integrated number of electrons of 9.5.
The contribution from disordered solvent was removed by
BYPASS procedure [47], implemented in Olex-2 software
[48]. Refinement converged to wR2 = 0.140 (all data),
R1 = 0.056 (for 16,228 reflections with I [ 2r(I)),
S = 0.98. Crystallographic data (excluding structure fac-
tors) for the structure in given paper have been deposited
with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 910558. Copies of
the data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax:
?44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
Experimental
General
UV–Vis absorption spectra (220–800 nm) were obtained
using a Carl Zeiss Specord M40 spectrophotometer in a
10 mm path length quartz cuvette. Molecular clips 1–5
were prepared as described [42–44]. All of alkali metal
chlorides were of analytical grade.
Results and discussion
UV–Vis titration experiments
Synthesis
A solution of molecular clip 1–5 (concentration about
7 9 10-5 M) in methanol was treated with increasing
amounts of alkali metal chloride solution (concentration
about 2 9 10-3 M) containing proper molecular clip of the
same concentration at 20 °C. The host concentration was
Synthesis of molecular clips was carried out through the
interaction of bisether 6 with the corresponding benzo-
crown ethers 7–11 in PPA according to the method
described previously by us (Scheme 1) [42–44].
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J Incl Phenom Macrocycl Chem
Scheme 1 Synthesis of
molecular clips 1–5
Complexation studies
and 5 despite the high constants of complexation. Notewor-
thy, in the 3–5, and to a lesser degree in 2, CE fragments can
act as independent units that results in the 1:2 complex for-
mation in essential quantities. The stability of these com-
plexes is, as a rule, several orders of magnitude lower
compared to similar 1:1 complexes, the correlation between
macrocycle/cation size is not observed. The most interesting
results have been obtained for the molecular clips 2 and 3.
Thus, the stability of the complexes 2 with K? and Rb? is
maximum in molecular clips obtained by us, and exceeds that
of the complexes with Na? and Cs? by *2.5 orders of
magnitude. Stability of the complexes 3 increases with
increasing of the cation size and reaches a maximum for Cs?.
These results are in good agreement with the data
obtained at the qualitative study of the complexation
properties of molecular clips under mass spectrometric and
picrate extraction experiments conditions [42]. Such high
values of the stability constants for the 2 and 3 indicate the
formation of sandwich type complexes when the metal
cation is located in pseudocavity (Fig. 2a) formed by two
CE fragments of molecular clip. As a result, the metal
cation interacts simultaneously with a greater number of
donor atoms that leads to a substantial increase of the
stability constants compared to the analogous CE [51–54].
Selectivity of the obtained molecular clips concerning
alkali metals cations differs also from that observed for
CE’s [51, 52]. For example, molecular clip 2 through two
ideal spatially proximate fragments of benzo-15-crown-5
forms an extremely stable complex with Rb?, what is not
typical for benzo-15-crown-5 [54]. A possible explanation
The stoichiometry and stability constants of the complexes
formed by molecular clips 1–5 and alkali metal chlorides
were determined by spectrophotometric titration using
mole ratio method in methanol at 20 °C [49, 50]. The
experimental data were processed with a non-linear least
squares method using the SIRKO software [45]. Analysis
of the experimental data revealed the presence of two types
of complexes, namely the complexes with ligand to metal
(L/M) composition equal to 1:1 and 1:2. The stability
constants were determined considering the following
equilibria:
½LMꢁ
L þ M ꢀ LM
K1 ¼
½Lꢁ½Mꢁ
½LM2ꢁ
LM þ M ꢀ LM2
K2 ¼
½LMꢁ½Mꢁ
where K1 and K2 correspond to the stability constants of the
1:1 and 1:2 complexes; [L], [M], [LM], [LM2] denote the
equilibrium concentration of the metal cations, molecular
clip, and their 1:1 and 1:2 complexes, respectively.
As can be seen from the data (Table 1; Fig. 1), the
molecular clip 1 shows very poor selectivity for the investi-
gated ions. This is probably caused by a small number of
donor oxygen atoms of molecular clip pseudocavity, and, as a
result, stability of complexes changes slightly depending on
the cavity/cation size ratio. The constants values for 1 are also
small and decrease with the increasing the cation size. Fur-
thermore, low selectivities are observed for molecular clips 4
Table 1 Stability constants of the complexes of molecular clips 1–5 with alkali metal cations in MeOH
Cation Compound
1
2
3
4
5
log K1
log K2 log K1
log K2
log K1
log K2
log K1
log K2
log K1
log K2
Na?
K?
Rb?
Cs?
a
3.27 0.04
3.28 0.03
2.78 0.06
2.59 0.04
–
–
–
–
4.13 0.04 2.90 0.02 4.51 0.05 2.09 0.07 N/Da
N/Da
N/Da
N/Da
6.25 0.07
6.34 0.08
3.74 0.02
–
–
–
4.96 0.04 1.64 0.06 4.44 0.06 1.20 0.05 4.64 0.02 2.89 0.01
5.23 0.01 3.32 0.06 4.51 0.10 1.28 0.09 4.05 0.05 1.53 0.06
5.92 0.13 1.48 0.07 4.43 0.04 3.63 0.05 4.93 0.08 2.28 0.03
The changes in spectrum were too small and it was impossible to calculate the binding constant
123

J Incl Phenom Macrocycl Chem
Molecular and crystal structure
The X-ray diffraction study of molecular clip 2 complex
(Fig. 3) has demonstrated its dimeric character. The CE
fragment of one clip is inserted into cavity formed by the
CE fragments of the other molecule. In the complex each
Na? cation is located within its own CE cavity and coor-
dinated by all six oxygen atoms of the CE macrocycle, as
well as, by the oxygen atoms of water molecules and
picrate anions. Both outer CE rings contain the Na? cations
coordinated additionally by the oxygen atoms of picrate
anion, and one of the Na? cations is coordinated also by the
water molecule located at the same side as the anion. As a
˚
result, cations are shifted by 1.09–1.19 A from the mean
plane of etheric oxygen atoms. One of the inner CE rings
contains the Na? cation in the ring center coordinated in
both axial directions by water molecules, and the other
inner CE ring is empty and hydrogen bonded with two
water molecules in axial directions. Similar coordination
pattern has been found in the crystal of 15-crown-5 with
lithium chlorochromate complex [56]. Due to symmetric
environment, the inner CE ring adopts an almost perfect
boat conformation and cation is located almost within the
Fig. 1 The log K values for interactions of molecular clips 1–5 with
alkali metal cations
for the selectivity of 2 is that the binding of the larger
cation (e.g. Cs?) causes substantial rearrangement of
pseudocavity and somewhat of the diphenylglycoluril
frame (angle between the phenyl rings).
This should lead to the tension of 1,3-benzodiazepine
fragment in glycoluril frame, making the process less
favorable (Fig. 2b). Higher conformational flexibility of
polyether chains in 4 and 5 allows ‘‘host’’ to realize con-
formational changes with less energetic losses, what leads
to a notable selectivity decrease, while the high values of
the stability constants remain almost unchanged. Molecular
clip 3 is in intermediate position by both the efficiency and
selectivity of complexation.
˚
mean plane of the CE oxygen atoms (deviation is 0.07 A).
Two inner CE rings are nearly parallel (the angle between
mean planes is 4.5°). The outer rings are inclined related to
inner one by 43–46°. Two clip molecules in the complex
have similar conformation and are oriented approximately
centrosymmetrically with respect to each other. Clip mol-
ecules are linked with each other by the O–HꢀꢀꢀO hydrogen
bonds with water molecules. Thus, the complex could be
considered as hydrogen-bonded clipped dimer of two 1:1
and 1:2 complexes.
Stability of the 1:2 complexes of described clips is
significantly lower than that of 1:1 ones. Ratio 4K2/K1 \ 1
[55] clearly indicates anticooperative effect due to elec-
trostatic repulsion of the two cations (Fig. 2c) located
nearby. Complexes of such stoichiometry are not formed in
the case of molecular clip 1, and in the case of 2 they are
registered for Na? only. In the solid state, the molecular
clip 2 forms the 3:2 complex with sodium picrate.
X-ray diffraction data are in good agreement with the
fact that whereas K1 for Na? is relatively small, K2 value is
quite high. This confirms that there is equilibrium in the
solution between the particles of the both 1:1 and 1:2
stoichiometry observed also in the solid state.
Fig. 2 Schematic
representation of cooperative
(a), anticooperative (c), effects
for complexes with different
stoichiometry and possible
structural changes in host
molecule (b)
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J Incl Phenom Macrocycl Chem
Fig. 3 Molecular structure of
molecular clip 2 complex with
sodium picrate 2:3 according to
X-ray diffraction data.
Displacement ellipsoids are
shown at 50 % probability level
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In summary, molecular clips obtained by us show selec-
tivity to certain alkali metal ions. Clips 4 and 5 form quite
stable complexes with K?, Rb? and Cs? cations, whereas
the stability of their complexes with Na? is much lower.
This can be used to separate K?, Rb? and Cs? cations from
the excess of Na?. The obtained clips can be prospective as
a base for the design of new chemosensors through the
‘‘indicator groups’’ introduced in the existing frame for
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