Tetrahedral anion cage: Self-assembly of a (PO4) 4L4 complex from a tris(bisurea) ligand

Article abstract of DOI:10.1002/anie.201209930

A C₃-symmetric tris(bisurea) ligand L assembles with PO ₄^3- ions to form the tetrahedral cage [(PO ₄)₄L₄]^12-. The PO₄ ^3- ions form the four vertices and each forms 12 N-H?O hydrogen bonds with 6 urea groups. A unique trinuclear pinwheel-like helical complex [(SO₄)₃L₂]^6- is formed from L and SO₄^2- ions (see picture). Copyright

Full text of DOI:10.1002/anie.201209930

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DOI: 10.1002/anie.201209930
Anion Coordination
Tetrahedral Anion Cage: Self-Assembly of a (PO4)4L4 Complex from
a Tris(bisurea) Ligand**
Biao Wu,* Fengjuan Cui, Yibo Lei, Shaoguang Li, Nader de Sousa Amadeu, Christoph Janiak,
Yue-Jian Lin, Lin-Hong Weng, Yao-Yu Wang, and Xiao-Juan Yang
Anion coordination chemistry has developed rapidly in recent
years because of the important roles anions play in many
biological, environmental, and chemical processes.[1,2] Studies
have shown that the coordination behavior of the anions,
although less-well defined, is quite similar to the coordination
of transition metals; for example, anions also display “coor-
dination geometry” and “coordination number”.[2] These
analogies provide promising ideas for the self-assembly of
novel supramolecular systems based on anion coordination.
In the past few decades, zero-dimensional, aesthetically
pleasing metal coordination complexes, such as molecular
squares, capsules, tetrahedra, and other complex polyhedral
shapes, have attracted much interest.[3,4] Tetrahedral cages
that contain an isolated space have been intensely studied as
mimics of the microenvironments of bioprocesses, in stabiliz-
ing reactive intermediates, and as catalysts for chemical
transformations.[5] There are two typical approaches for
constructing tetrahedral metal cages: By using C2-symmetric
bis-chelating ligands and octahedral metal ions, M4L6-type
species (with four metal ions at the vertices and six ligands
along the edges) can be obtained.[6] The other type of
tetrahedral cages, the M4L4 compounds, is less common and
can be assembled from C3-symmetric tris-chelating ligands
which are positioned on the faces of the tetrahedron.[7] In
addition, there are also covalent organic cages comprised of
a single molecule.[8]
In contrast to the rich chemistry of metal-based systems,
supramolecular architectures driven by anion coordination
are underdeveloped, although anions can play crucial roles in
templating exciting structures (e.g. the starlike helicates).[9] In
recent years, anion-centered structures, such as catenanes,
rotaxanes, foldamers, and helices, have emerged;[10] however,
the well-defined cage complexes have not yet been explored.
Compared to the transition-metal complexes, the assembly/
disassembly process of anion-based supramolecular systems
may be controlled under mild conditions (e.g. by acid/base
modulation or by solvent polarity), which may be more
promising for biological processes.
As part of our studies on anion coordination we recently
developed a series of ortho-phenylene-bridged oligourea
ligands, which display excellent affinity and complementarity
to the tetrahedral sulfate and phosphate anions.[11] More
3À
importantly, the fully deprotonated phosphate ion (PO4
)
shows a strong tendency for “coordination saturation” (12
hydrogen bonds), a characteristic of tetrahedral anions that is
proven both theoretically and experimentally.[11,12] This prop-
erty can ensure the formation of desired structures by
hydrogen bonding six urea groups to its six edges. Hence,
we designed the tris(bisurea) ligand L (Scheme 1) by attach-
[*] Prof. B. Wu, Dr. Y. Lei, Prof. Y.-Y. Wang
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry of the Ministry of Education
College of Chemistry and Materials Science, Northwest University
Xi’an 710069 (China)
E-mail: wubiao@nwu.edu.cn
Dr. F. Cui, Dr. S. Li, Prof. X.-J. Yang
State Key Laboratory for Oxo Synthesis & Selective Oxidation,
Lanzhou Institute of Chemical Physics, CAS
Lanzhou 730000 (China)
Scheme 1. Assembly of the A4L4 tetrahedral anion cage from phosphate
anions and ligand L.
Dr. N. S. Amadeu, Prof. Dr. C. Janiak
Institut fꢀr Anorganische Chemie und Strukturchemie
Universitꢁt Dꢀsseldorf
Universitꢁtsstrasse 1, 40225 Dꢀsseldorf (Germany)
ing three bisurea moieties to a central C3-symmetric triphe-
nylamine platform. Here we report the assembly of the first
tetrahedral anion cage ([A4L4]-type (A = anion); 1) and
a unique pinwheel helical [A3L2] complex (2) from ligand L
and PO43À and SO42À ions, respectively.
Prof. Y.-J. Lin, Prof. L.-H. Weng
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Material, Department of Chemistry
Fudan University, Shanghai, 200433 (China)
Ligand L was synthesized by the reaction of p-nitro-
phenylisocyanate with N,N’,N’’-(nitrilotri-4,1-phenylene)-
tris(2-aminophenylurea). Treatment of L with an equimolar
quantity of (TMA)3PO4 (generated in situ from (TMA)OH
[**] This work was supported by the National Natural Science
Foundation of China (Grant No. 21271149).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209930.
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and H3PO4; TMA = tetramethylammonium ion) afforded
a yellow crystalline product in almost quantitative yield.
The compound is readily soluble in common solvents such as
acetone, acetonitrile, DMF, and DMSO. NMR spectroscopy,
ESIMS, and elemental analysis proved that this species
(TMA)12[(PO4)4L4] (1) was the unique product, with no
evidence for the occurrence of other stoichiometries (see
below for solution studies).
nitrophenyl group to the anion. The combination of all these
weak interactions results in
a rather stable complex
[(PO4)4L4]12À, which shows a large dissociation energy of
approximately À1709 kcalmolÀ1, as revealed by DFT stud-
ies.[14]
In the tetrahedral cage, the PO4···PO4 separation is about
15 ꢀ, and the internal volume was estimated with the
VOIDOO program to be about 181 ꢀ3.[15] The volume of
the countercation TMA was calculated to be approximately
121 ꢀ3 by DFT using the B3LYP/6-31G* method. An
optimum encapsulation of about 55% was observed in
many host–guest systems (for neutral guests),[16] and it
would be expected that the tetrahedral cage is too small to
include the TMA ion. Indeed, there is no guest inside the
cage, and in the crystal structure the TMA cations and solvent
molecules are found outside the cage.
Complex 1 crystallizes from diethyl ether/acetonitrile in
the centrosymmetric cubic space group P43n.[13] The structure
ꢀ
has the ideal T symmetry with one twelfth of the tetrahedral
molecular cage (one third of a phosphate ion, one third of the
ligand, one TMA as the counter cation) appearing in the
asymmetric unit. The four C3-symmetric ligands position on
the four triangular faces of the tetrahedron, and each vertex is
defined by a phosphate ion which is coordinated by three
3À
bisurea arms (Figure 1). The four axial-chiral PO4 centers
Treatment of L with [{K([18]crown-6)}2]SO4 afforded the
sulfate complex [{K([18]crown-6)} 6] [(SO4)3L2]·3CH3COCH3
(2). Complex 2 (C2/c) shows a pinwheel-like helical structure,
2À
in which the bisurea arms cross over the SO42À–SO4 axes
(Figure 2a).[17] The ligand adopts a conformation such that the
Figure 2. a) The “pinwheel” helical sulfate complex [(SO4)3L2]6À in the
crystal structure of 2; b,c) top and side view of the space-filling
representation. Only an M enantiomer is shown.
Figure 1. a) The tetrahedral cage [(PO4)4L4]12À in the crystal structure of
1, with the dark-blue facial ligand truncated so that the interior and
opposite PO4 corner can be seen; b) hydrogen bonds around the
PO4 ion; c,d) space-filling representation of the tetrahedron with
a view onto a corner (c) or a face (d). Only a P enantiomer is shown,
three terminal urea arms deviate from the triangular plane
and point to the same side of the plane. Two ligands stack in
3À
2À
a face-to-face fashion, with three SO4
ions between.
Notably, the structure is not C3-symmetric in the solid state.
The S···S separations are 12.7 and 14.0 ꢀ, and the central
N···N separation between two ligands is 5.50 ꢀ. Each sulfate
and non-acidic hydrogen atoms are omitted for clarity.
À
have the same configuration (DDDD or LLLL) in the
tetrahedron. However, as in most M4L4 tetrahedra with
achiral ligands,[7] both P- and M-handed [A4L4] enantiomers
are present in the crystal lattice of complex 1.
The tetrahedral cage 1 represents the first example of
a new class of anion-coordination supramolecular hosts. Each
phosphate ion is bound to six urea groups from three different
ion is bound by four urea groups through eight N H···O
hydrogen bonds (see Figure S8 and Table S3 in the Support-
À
ing Information). Moreover, C H···O hydrogen bonds also
occur with the anions (see Figure S8 in the Supporting
Information), and T-shaped CH···p interactions are formed
between the terminal nitrophenyl rings and the o-phenylene
planes, with CH···plane distances of 3.67–3.98 ꢀ (see Fig-
ure S7b in the Supporting Information).
Notably, although the tetrahedral sulfate and phosphate
ions can both potentially accept a maximum of 12 hydrogen
bonds,[12] we found that they lead to significantly different
solid-state structures. The trianionic phosphate ion tends to
accept 12 hydrogen bonds and, thus, can coordinate with three
bisurea fragments, while the dianionic sulfate ion does not
À
ligands through 12 strong N H···O hydrogen bonds (N···O
distances range from 2.811(5) to 2.827(5) ꢀ, average 2.817 ꢀ;
À
N H···O angles range from 157 to 1688, average 1638; see
Figure 1 and Table S2 in the Supporting Information).
À
Altogether there are 48 N H···O hydrogen bonds. In
À
À
addition, there is also a C H···O hydrogen bond (C H:
3.340(5) ꢀ; ]CHO: 1338) from one o-CH proton of each
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reach saturated coordination. This difference was also
observed in the assembly of the triple anion helicate in the
presence of a phosphate ion, whereas the sulfate ion only
leads to a single-stranded 1:2 complex.[11d] The reason might
be the higher negative charge density and stronger basicity of
the phosphate ion which requires more hydrogen bonds for
efficient binding.
Furthermore, 2D-DOSY was also performed in
[D6]DMSO at 298 K. The DOSY spectrum of complex
1 (see Figure S14 in the Supporting Information) showed
that all of the signals that correlate to the chemical shifts of
ligand L are in a horizontal line, with a diffusion coefficient
(D) of 9.0 ꢁ 10À11 m2 sÀ1. The hydrodynamic radius (rs) of this
species was calculated from the Stokes–Einstein equation to
be about 12.0 ꢀ, which is slightly smaller than that found in
the crystal structure (see Figure S15 in the Supporting
Information). The high-resolution ESI mass spectrum of
(TMA)12[(PO4)4L4] exhibited intense signals for [A4L4] spe-
cies [(TMA)x(H)y(PO4)4L4]zÀ at m/z = 1068.707 (x = y = 4, z =
5), 1083.326 (x = 4, y = 5, z = 5), 1354.409 (x = y = 4, z = 4),
and 1372.679 (x = 5, y = 3, z = 4; see Figure S17 and Table S4
in the Supporting Information). The observed isotopic
patterns agree well with the simulated ones. These results
suggest that the tetrahedral structure also persists in solution.
The phosphate ion can be protonated or deprotonated to
different states (PO43À, HPO42À, H2PO4À, and H3PO4) when
the external environment is changed, which could greatly
influence the binding affinity and regulate the structure of the
anion complex. Thus, a reversible formation of complex 1 was
established by acid/base modulation and monitored by
1H NMR spectroscopy ([D6]DMSO/5% H2O). The addition
of HClO4 to a solution of complex 1 resulted in the PO43À ion
being protonated gradually and the tetrahedral structure was
disassembled (Figure 4). In contrast, when a base (TMAOH)
was introduced, the protonated anions were deprotonated
again, and the tetrahedral structure was reassembled (see
Supporting Information for details). These results demon-
strate that the assembly/disassembly of the anion-coordina-
tion-based cage can be readily controlled by the addition of
acid or base. Moreover, the effects of other conditions such as
temperature, solvent, and concentration on complex 1 were
also examined, and the results revealed the complex to be
The assembly of L and PO43À in solution was investigated.
The 1H NMR spectrum of complex 1 in [D6]DMSO/5% water
(v/v) revealed very large downfield shifts of the signals
corresponding to all the urea NH protons (Dd = 2.75–
3.58 ppm) compared to the free ligand, thus indicating
strong hydrogen bonds to the PO43À ions. When 1.0 equivalent
of PO43À ions (as the [K([18]crown-6)]+ salt) were titrated into
a solution of L (5 ꢁ 10À3 m), similar downfield shifts of the NH
protons were observed (Figure 3 and see Figure S9 in the
Figure 3. 1H NMR titration of L (5ꢂ10À3 m) with [{K([18]crown-6)}3]PO4
in [D6]DMSO/5% H2O and the spectrum of complex 1 in [D6]DMSO
(400 MHz, 298 K).
Supporting Information). No further changes appeared with
more PO43À, thus indicating that the 1:1 (4:4) binding mode is
persistent in solution. In addition, a clear upfield shift of the
signal corresponding to the H8 proton (Dd = À0.80 ppm)
occurred because of the shielding effects in the anion
complex. Notably, the TMA countercation showed only one
signal in the normal range (2.99 ppm), with no upfield-shifted
peak, which implies that no TMA ion is encapsulated within
the cage, and is in agreement with the crystal structure.
The high-symmetry structure of 1 is reflected in its simple
1H NMR spectrum. In the NOESY spectrum (see Fig-
ure S11a,b in the Supporting Information), cross-peaks are
formed between all adjacent NH protons as well as between
H6–H7/H8, H4/H5–H7, and H4/H5–H8, which are not found
in the TOCSY spectrum (see Figure S12a in the Supporting
Information) and, thus, are caused by the through-space
coupling in 1. These interactions are also indicated by the
spatial proximity of the nitrophenyl and o-phenylene groups
in the crystal structure (see Figure S7a in the Supporting
Information). Moreover, the 31P NMR spectrum of 1 in
[D6]DMSO (see Figure S13 in the Supporting Information)
showed one signal that is substantially downfield shifted to
8.03 ppm relative to the free PO43À ion (2.96 ppm) because of
the strong hydrogen bonding, and there is no other phosphate
species in solution.
1
Figure 4. Details of H NMR detection of the gradual assembly and
disassembly of the tetrahedral cage 1 upon acid/base modulation in
[D6]DMSO/5% H2O (400 MHz, 298 K, TBA=tetra-n-butylammonium).
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1
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The 1H NMR spectrum of the sulfate complex 2 recorded
in [D6]DMSO revealed that all of the resonances of the urea
NH groups underwent downfield shifts (Dd = 0.46–1.62 ppm)
relative to the signal of free ligand. The NOESY spectrum
also demonstrated the formation of complex 2 (see Fig-
ure S11c,d in the Supporting Information). The DOSY results
gave a diffusion coefficient of D = 9.8 ꢁ 10À11 m2 sÀ1, which
corresponds to an Rh value of about 11.0 ꢀ (see Figures S14
and S15 in the Supporting Information). The evolution of the
spectrum was easy to follow in the 1H NMR titration experi-
ments (see Figure S10 in the Supporting Information).
Detailed discussion of the titration process is given in the
Supporting Information. Notably, the titration revealed both
2:3 and 1:3 (host to guest) binding modes. This difference
between the binding stoichiometry in the solid state and
solution is common and has been observed for many anion-
binding receptors.
In conclusion, a C3-symmetric tris(bisurea) ligand (L) has
been designed. Self-assembly of L and phosphate anions
(PO43À) affords the expected highly negatively charged
[A4L4]-type tetrahedral cage [(PO4)4L4]12À (complex 1) held
together by 48 hydrogen bonds. In the complex, the phos-
phate ions occupy the vertices and the ligands lie on the faces.
This conceptual study represents the first example of the
successful assembly of tetrahedral cages through anion
coordination, further proving the resemblance of metal
coordination and anion coordination behavior in terms of
the coordination number and geometry. Notably, the rever-
sible assembly/disassembly of the tetrahedral cage 1 can be
readily modulated under mild conditions by the addition of
acid and base. Extension to bigger cages and higher polyhedra
as well as investigation of their host–guest properties are
currently underway.
Received: December 12, 2012
Published online: April 8, 2013
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Keywords: anion coordination · oligourea · phosphate anion ·
self-assembly · tetrahedral cage
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[15] The cavity volume was calculated using the VOIDOO program,
with a 1.4 ꢀ diameter rolling ball on the crystal structure of
complex 1.
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