Sandwich phosphate complexes of macrocyclic tris(urea) ligands and their rotation around the anion

Article abstract of DOI:10.1039/c6cc03144g

Four heteroditopic macrocyclic ligands incorporating both anion coordination sites (tris-urea units) and a cation binding fragment (polyether) were designed for possible application in molecular devices. Sandwich-type phosphate complexes were formed, which display a reversible rotation around the anion upon protonation/deprotonation of phosphate and binding of the cation (Emim⁺).

Full text of DOI:10.1039/c6cc03144g

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Sandwich phosphate complexes of macrocyclic
tris(urea) ligands and their rotation around the
anion†
Cite this:DOI: 10.1039/c6cc03144g
Received 14th April 2016,
Accepted 27th April 2016
Liguo Ji,a Zaiwen Yang,b Yanxia Zhao,a Meng Sun,a Liping Cao,a Xiao-Juan Yang,a
Yao-Yu Wanga and Biao Wu*a
DOI: 10.1039/c6cc03144g
www.rsc.org/chemcomm
Four heteroditopic macrocyclic ligands incorporating both anion possibility of molecular motion about an anion center. A series of
coordination sites (tris-urea units) and a cation binding fragment macrocyclic, heteroditopic ligands incorporating the tris(urea)
(polyether) were designed for possible application in molecular scaffold (as a highly complementary binding site for a phosphate
devices. Sandwich-type phosphate complexes were formed, which anion)10 and a polyether moiety (well-established cation-binding
display a reversible rotation around the anion upon protonation/ site) were synthesized (L1–L4, Scheme 1), which were expected
deprotonation of phosphate and binding of the cation (Emim+).
to form sandwich structures for molecular rotation upon anion/
cation binding. Herein we describe the assembly of the phosphate
Molecular rotation is ubiquitous in molecular dynamics and is complexes and their rotation around the anion axle triggered by
fundamental to some molecular machines, e.g. rotors, gears, pro- cation binding and protonation/deprotonation of phosphate.
pellers, switches, brakes and turnstiles.1 In the past few decades,
Ligands L1–L4 were synthesized by the reaction of 1,3-bis(2-
efforts have been devoted to synthetic systems that can rotate about aminophenyl)urea with various oligomeric ethylene glycol bis(4-
an axle in the molecule. In practice, such rotation may be controlled nitrophenyl-N-(4-phenyl)carbamate) derivatives (for details see the
by external stimuli such as light, temperature, pH, and chemical ESI†). Assembly of these ligands with phosphate was tested by using
reactions.2 There is a class of molecular rotors, which have the a series of organic and inorganic salts, such as (TBA)3PO4 (in situ
sandwich-type structure, such as metallocenes,3 metallacarboranes,4 generated from (TBA)H2PO4 and (TBA)OH, TBA = tetrabutyl-
and porphyrin and phthalocyanine complexes.5 In these systems the ammonium), Na3PO4, K3PO4, and [K([18]crown-6)]3PO4. Interestingly,
intramolecular rotation occurs around a metal ion or a positively single crystals of complex [K([18]-crown-6)]2[HPO4(L3)2] (1) were
charged center. In contrast, rotation about an anionic axle has obtained in moderate yield from L3, K3PO4, and [18]crown-6 via
not been explored.
diffusion of ether into an acetone solution of the mixture.
2ꢀ
Recent progress in anion coordination chemistry has demon-
Complex 1 contains a divalent anion HPO4 that is sand-
strated the promising coordination properties of anions, which wiched by the two tris(urea) moieties of two macrocyclic
largely resemble the transition metals in terms of the coordination
number and geometry.6 We have obtained a series of novel
supramolecular structures, e.g. the first anion triple helicates7
and tetrahedral anion cages,8 based on the coordination of
phosphate ions by oligourea ligands, which have been proven to
be excellent building blocks for the assembly of supramolecular
architectures.9,10 To further explore the potential application of
anion coordination in molecular devices, we set out to study the
a 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
b College of Chemistry and Chemical Engineering, Xi’an University of Science and
Technology, Xi’an 710054, China
† Electronic supplementary information (ESI) available: Experimental details,
including synthesis, spectral data, and crystal data in the cif format. CCDC
Scheme 1 Structure of L1–L4 and schematic illustration of the formation
1456031–1456038. For ESI and crystallographic data in CIF or other electronic
of sandwich anion complexes and their rotation around the phosphate ion.
format see DOI: 10.1039/c6cc03144g
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Fig. 2 Crystal structure of the ‘‘double-core’’ sandwich 2 containing the
PO4
Fig. 1 Crystal structure of the [HPO4(L3)2]2ꢀ core in complex 1. (a) Side
view; (b) top view; (c) hydrogen bonds between the monohydrogen
phosphate (HPO42ꢀ) and urea groups. Countercations ([K([18]crown-6)]+)
were omitted for clarity.
anion and the [Emim]+ cation. (a) Side view; (b) top view; (c)
3ꢀ
phosphate coordination by twelve hydrogen bonds from six urea groups.
for the PO43ꢀ ion. Most remarkably, the two polyether moieties
ligands (Fig. 1a). There are only a few examples of such anion of the macrocycles are arranged in the same direction, stacking
sandwich structures in the literature.11 The appearance of face-to-face to sandwich one [Emim]+ cation (Fig. 2a). Thus,
phosphate in the monoprotonated form may arise from the complex 2 has essentially an ‘‘ion pair’’ sandwich structure. It
3ꢀ
hydrolysis of PO4 and can be confirmed by the number of can be viewed as that one of the ligands in complex 1 has rotated
countercations. Alternatively, complex 1 can also be obtained from for about 1801 around the anion, resulting in a syn-conformation
2ꢀ
[K([18]crown-6)]2HPO4 and L3. The HPO4 anion is coordinated of the two polyether parts. The imidazolium plane is almost
by twelve hydrogen bonds (NꢁꢁꢁO distances range from 2.726 to perpendicular to the polyethers, with the methyl group crossing
3.230 Å, N–HꢁꢁꢁO angles from 127 to 1671; Fig. 1c and Table S3, one of them, forming a total of eight C–Hꢁ ꢁ ꢁO hydrogen bonds
ESI†) from the six urea groups of two ligands. Meanwhile, (Fig. 2a and Table S4, ESI†). The other two countercations, one
the HPO4 ion also functions as a hydrogen bond donor and [K([18]crown-6)]+ and one [Emim]+, are located outside of the
2ꢀ
forms an O–HꢁꢁꢁO bond (OꢁꢁꢁO: 3.026 Å, O–HꢁꢁꢁO: 1341) with one sandwich structure (Fig. S2, ESI†).
carbonyl oxygen atom in another [HPO4(L3)2]2ꢀ (AL2, A = anion)
The interconversion between complexes 1 and 2 was inves-
fragment, and a pair of such O–HꢁꢁꢁO bonds link two AL2 units tigated by 1H NMR spectroscopy. The spectrum of complex 1 in
into a dimer (Fig. S1, ESI†). This binding mode is similar to the acetone-d6/5% H2O solution (Fig. 3 and Fig. S8, ESI†) reveals
phosphate binding proteins (PBP), in which the monoprotonated significant downfield shifts of the urea NH signals (Ddmax
=
HPO42ꢀ anion is bound preferentially and the proton enables it to 1.88 ppm) compared to the free ligand. For a mixture of
act as a hydrogen bond donor, thus discriminating from sulfate or 0.5 equiv. of HPO4 ions (as [K([18]crown-6)]+ salt) and L3,
2ꢀ
other fully ionized, tetrahedral oxoanions despite their similar the NMR spectrum is quite similar to that of complex 1 (Fig. 3).
structures.12 Notably, the polyether parts of the two ligands in 1 No further change was observed with the addition of more
are located on opposite sides (anti-orientation) of the anion HPO42ꢀ ions, indicating a 2 : 1 binding mode (Fig. S8, ESI†). In
core and do not bind any cation (Fig. 1b). Instead, the K+ ion is the spectrum of complex 2, the urea NH signals disappeared,
coordinated by the external [18]crown-6 and a urea carbonyl
oxygen of the ligand (Fig. S1, ESI†).
Since the alkali metal phosphate salts did not yield any
products (in which the alkali metal ion Na+ or K+ was expected
to be bound by the polyether part of the ligand), some other
(organic) cations were induced. Encouragingly, addition of
1-ethyl-3-methylimidazolium iodide (EmimꢁI)13 to a mixture of
[K([18]crown-6)]3PO4 and L3 in acetone yielded complex
[K([18]crown-6)](Emim)2[PO4(L3)2] (2). The phosphate appears as
3ꢀ
2ꢀ
the trivalent PO4 ion (rather than HPO4 in 1), which is also
sandwiched by two tris(urea) groups of two ligands through twelve
hydrogen bonds (NꢁꢁꢁO distances range from 2.735 to 2.990 Å,
and N–HꢁꢁꢁO angles from 139 to 1781; Fig. 2 and Table S4, ESI†).
This represents the typical saturated coordination (12 H-bonds)
Fig. 3 1H NMR spectra of ligand L3, complexes 1 and 2, and the corres-
ponding mixture in acetone-d6/5% H2O (400 MHz, 298 K).
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1
and protons b0 and c0 of the entrapped [Emim]+ ion display in acetone-d6/5% H2O).14 In parallel to the H NMR experiments,
obvious upfield shifts (Ddmax = ꢀ0.48 ppm) relative to free Emim+ 0.5 equiv. of EmimꢁI salt was added to a solution of complex 1,
ions in acetone-d6/5% H2O solution (Fig. 3). The mixture of which caused no obvious change. However, upon addition of the
3ꢀ
0.5 equiv. of PO4 ions (as K([18]crown-6)]+ salt), 0.5 equiv. of base (TMA)OH, a slow exchange process was established, with
2ꢀ
EmimꢁI, and L3 shows NMR features that are very close to gradual weakening of the peak of HPO4
and appearance
complex 2, implying the same species.
(and increase) of a new peak corresponding to the coordinated
Subsequently, to investigate the transformation of 1 into 2, PO43ꢀ ion (d = 10.75 ppm), which clearly indicates the conversion of
0.5 equiv. of EmimꢁI salt was added to an acetone-d6/5% H2O the HPO42ꢀ to the PO43ꢀ complex.8,9 Again, subsequent addition of
solution of L3 and 0.5 equiv. of HPO42ꢀ (complex 1), and upfield acid (HClO4) reversed the rotation process as in the case of 1H NMR.
shifts (Dd = ꢀ0.31 ppm) of the CH protons b0 and c0 on the
An analogous ‘‘double-core’’ sandwich [Emim]2ꢁTBAꢁ[PO4(L4)2]
Emim+ ring were observed, which are smaller than the changes (3) was also obtained by using the larger ligand L4 (Fig. S3, ESI†).
in complex 2 (Fig. 4). However, when (TMA)OH was introduced Complex 3 displays similar structural features to 2, with the
to deprotonate HPO42ꢀ to PO4 , further upfield shifts of b0 and inclusion of both a phosphate anion and an Emim+ cation
3ꢀ
c0-protons occurred, which reached the maximum when using within two face-to-face stacked ligand deckers. A difference is
0.5 equiv. of (TMA)OH (to completely deprotonate HPO42ꢀ), and that in complex 3 the other countercation is TBA+ instead of
the NMR spectrum resembles that of 2. These results indicate [K([18]crown-6)]+ (Fig. S3, ESI†).
that complex 1 can transform into 2 upon binding of the Emim+
Moreover, some other anions were examined for possible
cation and deprotonation of monohydrogen phosphate. The use in molecular rotation. For a sulfate ion, two isostructural
synergism of both functions is necessary, as complex 1 cannot sandwich complexes 4 and 5 were obtained from L1 or L3 with
be transferred into 2 with the addition of only (TMA)OH base but K2SO4 and [18]crown-6, and complex 4 is discussed here.
2ꢀ
3ꢀ
without the Emim+ cation (Fig. S9, ESI†).
Similarly to the HPO4 and PO4 ions in complexes 1–3, the
On the other hand, when HClO4 was added to change the sulfate ion is coordinated by two macrocyclic tris(urea) ligands
2ꢀ
PO43ꢀ back to HPO4 , protons b0 and c0 moved back gradually through twelve N–Hꢁ ꢁ ꢁO hydrogen bonds (Fig. 5 and Table S6,
(Fig. 4), indicating the recovery of complex 1 again. Hence, the ESI†). Noticeably, the polyether half-cycles of the two ligands
rotation between complexes 1 and 2 is reversible and is possibly are in the syn-arrangement, but there is no encapsulated cation
driven by protonation/deprotonation of phosphate and Emim+ between. Instead, there are a pair of C–Hꢁ ꢁ ꢁp interactions of the
cation binding. Titration of EmimꢁI into an acetone-d6/5% H2O aryl rings that connect the anion- and cation-binding sites
solution of L3/0.5 equiv. of PO43ꢀ (Fig. S11, ESI†) indicated that the (Fig. 5a and Table S7, ESI†).15 The polyether ‘‘crescent’’ in
binding of the organic cation is relatively weak (log K = 5.22 Mꢀ1 one of the macrocycles is slightly folded to facilitate two weak
based on protons b0 and c0 of Emim+; Fig. S11, ESI†).
C–Hꢁ ꢁ ꢁO contacts (Fig. 5 and Table S6, ESI†).16 It can be seen
2ꢀ
The dynamic rotation between complexes 1 and 2 was further that this complex is significantly different from HPO4
confirmed by 31P NMR in acetone-d6/5% H2O (Fig. S10, ESI†). The complex 1, although they are both divalent tetrahedral anions,
spectrum of complex 1 shows a single peak at 6.58 ppm for the which might demonstrate the importance of the proton in
coordinated HPO42ꢀ ion (PPh3 as internal standard, d = ꢀ4.15 ppm HPO4 for the anti-arrangement of the two ligands, at least
2ꢀ
Fig. 4 1H NMR studies on the rotation of the sandwich phosphate
complexes upon acid/base modulation and Emim+ ion binding (acet-
one-d6/5% H2O, 400 MHz, 298 K).
Fig. 5 Crystal structure of the sulfate complex [SO4(L1)2]2ꢀ in 4. (a) Side
view; (b) top view; (c) sulfate encapsulation by twelve hydrogen bonds
from six urea groups.
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2ꢀ
in the solid state. The binding of Ln with the SO4 anion in
solution was investigated by 1H NMR in acetone-d6/5% H2O,
which revealed a 2 : 1 binding mode (Fig. S12, ESI†) as in the
crystal structure. Furthermore, the sulfate sandwich 5 can
also accommodate the Emim+ cation between the polyether
moieties (Fig. S13 and 14, see the ESI†).
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On the other hand, the sandwich structure was not observed
when the halide anions were used. For a chloride ion, two
analogous 1 : 1 complexes (TPA)[Cl(L2)] (6) and (TEA)[Cl(L3)] (7) were
isolated. The chloride anion was bound by two urea groups of one
ligand and a TPA+ or TEA+ countercation (Fig. S5, S6 and Tables S9,
S10, ESI†). The other urea group of the ligand is involved in the
infinite urea a-tape linked by N–Hꢁ ꢁ ꢁO hydrogen bonds (Fig. S5b
and S6b, ESI†)17 to form a herringbone structure.18 In the case of
an iodide ion (as Emim+ salt), complex (Emim)2[I2(L4)] (8) was
formed, which contains two Emim+ cations and two iodide
anions bound by one L4 ligand (Fig. S7 and Table S11, ESI†).
Notably, one of the Emim+ cations is located in the cavity of the
polyether part. However, it does not link a second ligand to form
a cation sandwich as in 2 and 3. In contrast, two ligand planes
stack to each other in a ‘‘head-to-tail’’ manner (Fig. S7, ESI†).
From these results, it is clear that neither the halide anions nor
the Emim+ cation alone can hold a sandwich structure due to
their relatively weak binding, which was also indicated by
solution binding studies (see the ESI†).
In conclusion, we have synthesized four macrocyclic ligands
containing both the tris(urea) anion binding site and polyether
cation receptor. Sandwich-type anion complexes (with free cation
binding sites) were formed by the ligands and HPO4 anion,
which can rotate reversibly around the anion axle upon cation
binding and protonation/deprotonation of phosphate. In contrast,
2ꢀ
2ꢀ
other anions including SO4 and halide ions cannot template
such a molecular rotor. The results confirm again the excellent
coordination ability of phosphate. Moreover, the interconversion
of different protonated states of phosphate provides versatile
chances for its use in molecular devices.
This work was supported by the National Natural Science
Foundation of China (21271149 and 21325102).
´
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Chem. Commun.
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