Construction of homochiral alkaline-lanthanide heteronuclear helicates with Na+-selective bonding in the self-assembly process

Article abstract of DOI:10.1039/c9dt02483b

A supramolecular approach to distinguish Na⁺ from other biologically important metal ions was demonstrated. By designing ligands reasonably, Na⁺-selective bonding was achieved in the construction of homochiral alkaline-lanthanide het

Full text of DOI:10.1039/c9dt02483b

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Construction of homochiral alkaline–lanthanide
heteronuclear helicates with Na+-selective
bonding in the self-assembly process†
a
Cite this: Dalton Trans., 2019, 48,
14595
Received 12th June 2019,
Accepted 6th September 2019
Ge Li,a Li Wang,b Qingxin Hanc and Weisheng Liu
*
DOI: 10.1039/c9dt02483b
rsc.li/dalton
A supramolecular approach to distinguish Na+ from other biologi- systems. The relative balance of Na+ and K+ maintains the
cally important metal ions was demonstrated. By designing ligands integrity of the function and structure of the entire cell and
reasonably, Na+-selective bonding was achieved in the construc- the imbalance of Na+ and K+ can cause many serious dis-
tion of homochiral alkaline–lanthanide heteronuclear helicates, eases.23 Therefore, it is rather important to distinguish Na+
which was further confirmed by mixed-metal self-assembly experi- and K+.
ments and 1H-NMR spectra.
Selectively distinguishing Na+ and K+ in a mixed metal self-
assembly is extremely challenging because of their very similar
coordination behavior and relies only on the difference in
their size. Furthermore, to the best of our knowledge, there are
no examples of controlling the selective-bonding of Na+ in the
self-assembly process in the presence of other biochemically
important metal ions, such as Li+, K+, Mg2+ and Ca2+. By
designing ligands reasonably, a suitable coordination environ-
ment towards special metal ions might be obtained.24,25 Some
groups have observed that several helicates with oxygen donor
ligands are capable of encapsulating alkaline metal cations in
a cryptate-type fashion.26–28 Salicylhydrazone-based ligands
with tridentate coordinating sites can satisfy the nine-co-
ordinated mode of lanthanide ions and have been used in the
synthesis of M2L3 helices.21,24,29,30 After introducing an oxygen
donor atom into the ortho position of salicylaldehyde, the
ligand can not only satisfy the coordination chemistry of the
lanthanide ions but also may provide a suitable coordination
environment to alkaline metal ions. Thus, the selective
binding of a specific alkali metal ion in the self-assembly
process can be promoted by the supramolecular cooperative
effect.
Chirality is a signature of living systems and nature.1–3 Many
efforts have been devoted to the construction of chiral lantha-
nide complexes with vital applications in asymmetric catalysis,
enantioselective discrimination, nonlinear optical materials,
single molecule magnets (especially Dy-based complexes) and
biomimetic chemistry.4–12 Introducing a predisposed point
chiral moiety in ligands is a general approach to construct
homochiral complexes.13–17 Because of the variable coordi-
nation numbers, weak stereochemical preference and kinetic
lability of lanthanide ions, the synthesis of pure enantiomeric
lanthanide complexes was particularly difficult.16,18 Therefore,
the challenge is to design a suitable point chirality to selec-
tively favor one stereoisomeric structure.
Cooperativity has vital importance in chemistry, biology
and materials science.19,20 Supramolecular self-assembly coop-
erative effects can exhibit selectivity for certain metal ions. For
example, there are a few literature studies using the coopera-
tive mechanical-coupling effect to realize the selective bonding
of lanthanide ions.21,22 Sodium ions (Na+) play vital roles in
physiological and pathological processes occurring in living
Bearing this in mind, a pair of 3-methoxysalicylhydrazone-
based homochiral tetradentate ligands ((S)-H2L1 and (R)-H2L1)
were designed and synthesized by introducing substituted
enantiopure mandelic acids (Scheme 1). Efficient self-reco-
gnition of sodium ions and chirality transfer were realized
during the self-assembly processes of heteronuclear cationic
helicates: [Dy2Na3{(S)-HL1}6]3+ (1a) and [Dy2Na3{(R)-HL1}6]3+
(1b). Compared with (S)-H2L1 and (R)-H2L1, salicylaldehyde-
based ligands (S)-H2L2 and (R)-H2L2 have no ortho-methoxy
group and hence they construct chiral homonuclear
cationic square molecules [Dy4(μ4-OH) {(S)-HL2}8]3+ (2a) and
aKey Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu
Province and State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
P. R. China. E-mail: liuws@lzu.edu.cn
bCollege of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an
710065, P. R. China
cCollege of Bioresources Chemistry and Materials Engineering, Shaanxi University of
Science and Technology, Xi’an 710021, China
†Electronic supplementary information (ESI) available. CCDC 1920443 (1a),
1920448 (1b), 1920446 (2a) and 1920447 (2b). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/C9DT02483B
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nation sites to sodium ions (Fig. 1b). Each of the Dy3+ ions
were nine-coordinated with three NO2 pockets from three
ligands and their coordination polyhedron was a distorted tri-
capped trigonal prism (Fig. S5a and b†). Each of the Na+ ions
have six-coordinated distorted triangular prism geometry from
two methoxy oxygen atoms and four phenol oxygen atoms
(Fig. S5c and d†). A notable structural feature of this cluster
was the phenol oxygen atoms of each ligand that adopt a μ3-
bridging mode to link two Na+ ions and one Dy3+ ion resulting
in an olive-shaped cavity at the center of the structure (Fig. 1b).
This cavity was composed of two lanthanide atoms, three
sodium atoms and six μ3-O bridges. Three pairs of chiral
ligands twine orderly around the triangular biconical cavity
along the fixed spiral direction and yield a triple-stranded M
helicate. The distances of Dy1⋯Dy2, Na1⋯Na2 and Na1⋯Dy1
were 6.38 Å, 3.35 Å and 3.77 Å, respectively. To the best of our
knowledge, this structure is the first example of a chiral triple-
stranded alkali–lanthanide heteronuclear helicate.
Scheme 1 Self-assembly of Na3Dy2 heterometallic helicates and Dy4
homonuclear square structures from chiral ligands.
Compared with H2L1, H2L2 has no ortho-methoxy from
o-vanillin and cannot simultaneously satisfy the coordination
geometry of lanthanide ions and alkali metal ions. Although
lanthanide ions and alkali metal ions were present in the
system, only a homonuclear complex of lanthanide ions was
obtained. The structures of 2a and 2b are the same, except for
the differences in the surrounding solvent molecules. For the
sake of simplicity, complex 2a was selected as an example to
describe their crystal structure. X-ray crystallographic analysis
demonstrated that complex 2a crystallized in the chiral space
group P212121. Complex 2a was composed of four Dy3+ atoms,
eight deprotonated ligands and a μ4-OH to form a Dy4 square
molecule (Fig. 1d–f). Each pair of chiral ligands coordinate
with two Dy3+ atoms in an antiparallel mode through acylhy-
drazone groups and deprotonated phenol oxygen atoms. Each
Dy3+ atom was nine-coordinated resulting in a distorted tri-
capped trigonal prismatic geometry (Fig. S6†). Furthermore,
all the deprotonated phenol groups act as μ2 bridges to bond
two Dy3+ atoms along each edge of the square plane and result
in a Dy4O8 central core.
[Dy4(μ4-OH) {(R)-HL2}8]3+ (2b) in the presence of sodium and
lanthanide ions, respectively. These results confirmed the sig-
nificant role of O-methoxyl of vanillin in the construction of
alkaline–lanthanide heteronuclear helicates and provided the
basis for selective bonding of Na+.
Single-crystal X-ray diffraction analyses revealed that the
structures of complexes 1a and 1b were the same and crystal-
lized in the chiral space group C2221. Therefore, complex 1a
was selected as a representative example to describe their
crystal structure. Complex 1a was composed of six monodepro-
tonated chiral ligands, two Dy3+ ions and three Na+ ions to
form a triple-stranded M helicate (Fig. 1a and c). For each
ligand, the acylhydrazone group along with the phenol moiety
provided tridentate coordination sites to Dy3+ ions and the
phenol moiety and methoxy oxygen provided bidentate coordi-
Complexes 1a, 1b, 2a and 2b were characterized by IR,
PXRD, TGA and HRESI-MS, respectively. The IR spectra
(Fig. S7 and S8†) of the complexes experienced significant
changes compared with those of their corresponding ligands,
indicating the coordination interaction between the ligands
and the metal ions. Powder X-ray diffraction patterns (PXRD)
of these complexes were recorded to ensure the phase purity of
these complexes (Fig. S9†). It was found that the experimental
spectra were basically consistent with the simulated spectra.
The thermal stability of the complexes was revealed by thermal
gravimetric analysis (TGA). As shown in Fig. S10 and S11,† all
the complexes remained stable up to 240 °C.
High-resolution electrospray ionization mass spectrometry
(HRESI-MS) confirmed the molecular composition of the crys-
tals in solution. As shown in Fig. S14,† the HRESI-MS spec-
trum of complex 1a features a series of peaks that could be
assigned to charged species with progressive loss of H+: peaks
at m/z = 729.8128, 1094.2084 and 2188.3183 belong to charged
Fig. 1 (a) Molecular structure of complex 1a. (b) Coordination mode of
(S)-H2L1 and the olive-shaped cavity at the center of complex 1a. (c)
Space-filling representation of complex 1a. (d) Molecular structure of
complex 2a. (e) Coordination mode of (S)-H2L2 and the Dy4O8 central
core of complex 2a. (f) Space-filling representation of complex 2a.
14596 | Dalton Trans., 2019, 48, 14595–14599
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species [(HL)6Na3Dy2]3+, [(HL)6Na3Dy2-H+]2+ and [(HL)6Na3Dy2-
2H+]+, respectively. Furthermore, the experimental isotope pat-
terns were in good agreement with the theoretical isotopic dis-
tribution which further confirmed the assignments. As shown
in Fig. S15,† the HRESI-MS spectrum of complex 2a shows
three peaks at m/z = 940.1566, 1409.7321 and 2828.4453,
which were attributed to [(HL)8(μ4-OH) Dy4]3+, [(HL)8 (μ4-OH)
Dy4-H+]2+and [(HL)8(μ4-OH) Dy4-2H+]+, respectively, and the
experimental isotope patterns were also in good agreement
with the theoretical distribution.
The optical activity and enantiomeric nature of complexes
1a, 1b, 2a and 2b were confirmed by circular dichroism (CD)
spectroscopy in methanol (Fig. 2). The CD spectrum of
complex 1a exhibited a negative Cotton effect at 310 and
380 nm, while complex 1b showed a positive Cotton effect at
the same wavelengths to form a clear mirror image, indicating
that complexes 1a and 1b are enantiomeric complexes. The CD
spectra of complexes 2a and 2b were also mirror images:
complex 2a shows a negative Cotton effect in a wide range
from 260 nm to 370 nm, while complex 2b shows a positive
Cotton effect peak in the same range. These results demon-
strated that the point chiralities of ligands were effectively
transferred to the overall structure and two pairs of enantio-
meric complexes were obtained, respectively.
Fig. 3 Chirality transfer from ligands to the corresponding complexes.
X-ray crystallographic analyses also confirmed the effective ligands were also successfully transferred: they have chiral
chirality transfer from ligands to complexes in self-assembly metal centers and form square molecular structures, which are
processes. The point-chirality of the ligands was transferred to mirror-symmetrical to each other.
give chiral metal centers and hence leads to the chirality of the
In order to provide deep insights into the construction
overall assemblies. As shown in Fig. 3, the point-chiralities of mechanism of alkaline–lanthanide helicates, Na+-selective
(S)-H2L1, (R)-H2L1, (S)-H2L2 and (R)-H2L2 were derived from bonding was examined through the one-pot mixed-metal self-
S-mandelic acid and R-mandelic acid. After coordinating with assembly experiments in detail. Herein, (S)-H2L1 was selected
metal ions, (S)-H2L1 gave Λ lanthanide centers and (R)-H2L1 as a representative to discuss. The highly selective crystalliza-
gave Δ lanthanide centers and finally formed M-helicity and tion behavior of Na+ in mixed-metal systems was strong evi-
P-helicity, respectively. In the self-assembly processes of com- dence for highly selective bonding of Na+ in the self-assembly
plexes 2a and 2b, the point chiralities of the corresponding process. By slow evaporation of acetonitrile and methanol solu-
tions containing deprotonated (S)-H2L1 and various metal ions
(including Li+, Na+, K+, Mg2+, Ca2+ and Dy3+) at room tempera-
ture, single crystals with good quality were obtained. X-ray crys-
tallographic analysis revealed that only complex 1a with the
cationic helicate [Dy2Na3{(S)-HL1}6]3+ was obtained. For com-
parison, the above experiment was also carried out with (S)-
H2L2. As a result, only single crystals of a pure lanthanide
complex were obtained and the sodium ion was not selectively
captured. In order to further confirm Na+-selective bonding in
the mixed metal ion self-assembly process, we separated the
crystals from the solution and washed their surface with aceto-
nitrile. Then, the crystals were characterized by XPS (X-ray
photoelectron spectroscopy) and EDX (energy dispersive X-ray
spectroscopy) to analyze the presence of metallic elements.
As shown in the full-scan XPS spectrum (Fig. 4a), the
binding energies of Na 1s and Na 2s peaks were 1070.6 eV and
63.6 eV and there were no significant values for Li+, K+, Ca2+
and Mg2+. It is reasonable to deduce that Li+, K+, Ca2+ and
Mg2+ were not involved in the formation of heteronuclear com-
plexes. The EDX spectrum (Fig. 4b) also exhibited clear signals
that belong to Na+ and Dy3+ and no peaks for other metal ions.
Fig. 2 CD spectra of (a) complexes 1a and 1b and (b) complexes 2a and
2b in methanol solution (5 × 10−5 M).
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supramolecular approach to distinguish Na+ from other bio-
logically important metal ions, which provides a new method
to sense Na+.
Conflicts of interest
There are no conflicts to declare.
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