Ditopic Chiral Pineno-Fused 2,2′:6′,2″-Terpyridine: Synthesis, Self-Assembly, and Optical Properties

Article abstract of DOI:10.1021/acs.inorgchem.9b02657

The syntheses of 4′-substituted chiral 2,2′:6′,2″-terpyridine (tpy) ligands with predetermined configurations and directionalities are rather limited in the supramolecular chemistry field. In this study, a carbazole-linked ditopic chiral ligand L was synthesized using 4′-bromo-substituted pineno-fused tpy 5 as the precursor. Upon complexation with Cd(NO₃)₂·4H₂O and Zn(NO₃)₂·6H₂O, two enantiomerically pure metallosupramolecules, [Cd₃L₃] and [Zn₄L₄], have been self-assembled and characterized by NMR, electrospray ionization-mass spectrometry, traveling wave ion mobility-mass spectrometry, and DOSY analysis. In addition, their optical properties are characterized by UV-vis, fluorescence, circular dichroism, and circularly polarized luminescence, suggesting an efficiency transmission and amplification of chirality from the ligand to metal center via self-assembly.

Full text of DOI:10.1021/acs.inorgchem.9b02657

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ditopic Chiral Pineno-Fused 2,2′:6′,2″-Terpyridine: Synthesis, Self-
Assembly, and Optical Properties
Fu-Jie Zhao,† Heng Wang,‡ Kehuan Li,§ Xiao-Die Wang,† Ning Zhang,† Xinju Zhu,†
Wenjing Zhang,† Ming Wang,§ Xin-Qi Hao,*,† Mao-Ping Song,*,† and Xiaopeng Li*,‡
†College of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China
‡Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States
§State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012,
P. R. China
S
* Supporting Information
library of tpy ligands with variable functionalizations.5b,14 Since
the preparation of optical active pinene-type bipyridines15 have
been frequently utilized to accomplish various chiral (poly)-
pyridines.16,17 Meanwhile, the rigidity, steric hindrance, and
stability of pinene substituents enabled the diastereoselective
synthesis of corresponding metal complexes with potential
application in enantioselective reactions18 and multifunctional
materials.19 Despite the above progress, the syntheses of 4′-
substituted chiral tpy ligands with predetermined config-
urations and directionalities are rather limited.
ABSTRACT: The syntheses of 4′-substituted chiral
2,2′:6′,2″-terpyridine (tpy) ligands with predetermined
configurations and directionalities are rather limited in the
supramolecular chemistry field. In this study, a carbazole-
linked ditopic chiral ligand L was synthesized using 4′-
bromo-substituted pineno-fused tpy 5 as the precursor.
Upon complexation with Cd(NO3)2·4H2O and Zn-
(NO3)2·6H2O, two enantiomerically pure metallosupra-
molecules, [Cd3L3] and [Zn4L4], have been self-assembled
and characterized by NMR, electrospray ionization-mass
spectrometry, traveling wave ion mobility-mass spectrom-
etry, and DOSY analysis. In addition, their optical
properties are characterized by UV−vis, fluorescence,
circular dichroism, and circularly polarized luminescence,
suggesting an efficiency transmission and amplification of
chirality from the ligand to metal center via self-assembly.
Over the past few years, our group has developed a series of
chiral pincer metal complexes and investigated their catalytic
activities.20 Recently, we have also contributed to discrete
supramolecular fractals using multitopic tpy ligands.21 As a
continuation of our previous work, we herein reported the
synthesis of a carbazole-linked ditopic chiral ligand L with a
chiral “pineno”-fused 2,2′:6′,2″-terpyridine (ctpy) moiety,
which was introduced via a key precursor, viz., 4′-bromo-
substituted pineno-fused tpy 5. As a proof of concept, such a
ligand could readily undergo coordination with cadmium(II)
and zinc(II) to form trimeric and tetrameric metallomacro-
cycles with different geometries, symmetries, and cavities
(Figure 1). The chiral transmission along the macrocyclic
framework has been carefully investigated via circular
dichroism (CD) and circularly polarized luminescence
(CPL) spectrometries. Such a study illustrates that advance-
ment of the complexity of tpy-based supramolecules could
introduce a new property into the system.
hiral self-assembly is prevalent in nature. Inspired by
Cfascinating chiral structures with different scales in the
biological world, chemists have designed and synthesized a
number of elegant chiral molecular mimics with application in
enantioseparation, asymmetric catalysis, recognition and
sensing, pharmaceuticals, and materials.1 Meanwhile, coordi-
nation-driven self-assembly has recently emerged as a powerful
bottom-up approach to constructing nanoscale structures with
well-defined sizes, shapes, geometries, and cavities.2,3 However,
the introduction of chiral functionalities to assemble
enantiomerically pure metallosupramolecules still lagged
behind because of the lack of suitable chiral bridging ligands
and efficient synthetic methodologies.4 It is therefore highly
mandatory to explore novel ligands with chirality in specific
positions.
2,2′:6′,2″-Terpyridine (tpy) derivatives have gained much
attention because of their unique photophysical and electro-
chemical properties.5 With three N-binding sites, tpy-function-
alized analogues6 could coordinate with various metal cations
to form either metallopolymers7 or artificial supramolecular
architectures,8 which exhibit wide applications in catalysis,9
biology,10 and materials science.11 Accordingly, a number of
strategies, including traditional ring-assembly methods12 and
cross-coupling reactions,13 have been developed to access a
The preparation of L started from 4-hydroxypyridine-2,6-
dicarboxylic acid, which was converted to 4-bromo-2,6-
diacetylpyridine (3) in three steps (Figure 1a).22 A subsequent
̈
Krohnke-type reaction of N-heteropyridinium salt 4 with R-
myrtenal gave 5. It is worth noting that, although ctpy was
previously reported,23 the 4′-bromo-substituted version
remained unknown regarding synthesis. Therefore, ctpy was
mainly used to form dimeric mononuclear metal complexes
rather than discrete metallosupramolecules assembled by a
ditopic or multitopic ctpy ligand.23 As a proof of concept, we
performed a Suzuki cross-coupling of the key brominated
precursor 5 with Bpin-substituted 6 to afford ditopic ligand L
Received: September 4, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02657
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1
Figure 2. Partial H NMR spectra (500 MHz) of (a) [Cd3L3] in
CD3CN, (b) L in CDCl3, and (c) [Zn4L4] in CD3CN. # and *
represent residue solvents (CHCl3 and DMF). 2D DOSY NMR (500
MHz, CD3CN, 300 K) for (d) [Cd3L3] and (e) [Zn4L4].
Figure 1. (a) Synthesis of L. (b) Self-assembly of [Cd3L3] and
[Zn4L4]. Energy-minimized structures from molecular modeling of
trimeric [Cd3L3]: (c) top view; (d) side view. Energy-minimized
structures from molecular modeling of tetrameric [Zn4L4] macro-
cycles: (e) top view; (f) side view. The counterions were omitted for
clarity.
F and A′−F′) probably because of the different chemical
environments between the inner and outer rings caused by
high strain in the trimeric macrocycle. All of the assignments
shown in Figure 2 are readily confirmed by the 2D COSY and
2D NOESY spectra (Figures S7−S24). Furthermore, the
diffusion-ordered NMR spectra (DOSY) of the [Cd3L3] and
[Zn4L4] complexes display a narrow band at log D = −9.3 and
−9.4, respectively (Figure 2d,e), suggesting the formation of
discrete structures.
in 43% yield. L was further assembled with Cd(NO3)2·4H2O
or Zn(NO3)2·6H2O, followed by treatment with excess
NH4PF6. Finally, metallosupramolecules with molecular
compositions of [Cd3L3](PF6)6 ([Cd3L3] for short) and
[Zn4L4](PF6)8 ([Zn4L4] for short) were obtained in 90%
and 93% yields, respectively.
1
Figure 2 illustrates the H NMR spectra of L, [Cd3L3], and
Additionally, electrospray ionization-mass spectrometry
(ESI-MS) and traveling-wave ion mobility-mass spectrometry
(TWIM-MS)24 spectra were employed to provide further
structural information for the expected assembly (Figure 3).
ESI-MS of [Cd3L3] displays a series of peaks with continuous
charge states from 3+ to 6+ due to the loss of different
[Zn4L4]. After complexation, all of the proton signals at the A−
E positions of L show obviously downfield shifts because of the
decreased electron density of these protons caused by
coordination with metal ions.8a,21 On the contrary, the proton
signals at the F position are observed with an upfield shift due
to the shielding effect of the metal centers.8a,21 More
interestingly, while the [Zn4L4] complex demonstrates a
slightly broad NMR pattern in the aromatic region, the
[Cd3L3] complex exhibits a set of sharp and well-split signals
corresponding to two types of ctpy signals with a 1:1 ratio (A−
−
numbers of PF6 counterions (Figure 3a; the deconvoluted
mass is 4480 Da). The experimental isotope pattern of 5+
species is in accordance with the theoretical one of
([Cd3L3]PF6)5+. In the TWIM-MS plots, each charge state
ranging from 4+ to 6+ is observed with a narrow drift time
B
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Figure 3. (a and c) ESI-MS and (b and d) TWIM-MS plots (m/z vs
drift time) of [Cd3L3] and [Zn4L4]. The insets of parts a and c show
the theoretical and experimental isotope patterns of [Cd3L3]5+ and
[Zn4L4]5+, respectively.
distribution, excluding the formation of other structural
conformers or isomers (Figure 3b). Meanwhile, ESI-MS of
[Zn4L4] (Figure 3c) shows a dominate set of peaks with
continuous charge states from 4+ to 8+, featured with well-
resolved isotope patterns. The measured mass of [Zn4L4] is
5778 Da, supporting its chemical composition. Narrowly
distributed signals of each charge state in TWIM-MS (Figure
3d) suggest the formation of a rigid and shape-persistent
structure.
Figure 4. (a) UV−vis, (b) FL, (c) CD, and (d) CPL spectra and
theoretical (e) UV−vis and (f) CD spectra of L, [Cd3L3], and
[Zn4L4]. Note: The absorption coefficient of L in part f was plotted at
20% of their actual values for an easier comparison. The
concentrations of the repeating unit (L) in the solution of [Cd3L3]
and [Zn4L4] were used to evaluate the corresponding absorption
coefficient and [θ]. The quantum yields of L, [Cd3L3], and [Zn4L4]
were measured as 0.85, 0.43, and 0.65, respectively.
We reasoned that the weaker coordination strength of
cadmium(II) might introduce more flexibility and coordina-
tion deviation in the formation of a trimeric structure instead
of a tetramer formed by zinc(II) because of the larger ionic
radius of cadmium(II) (109 pm) than that of zinc(II) (88
pm).25 Further evidence was provided by a molecular
modeling study, and the results were summarized in Table
S1. Distortions of the carbazole groups in the modeling
structures of [Cd3L3]6+ and [Zn3L3]6+ were observed (shown
in Figures 1c,d and S27−S29). In comparison, the four-
membered-ring modeling structures ([Cd4L4]8+ and
[Zn4L4]8+) did not display significant distortion via arrange-
ment of the ligands in a herringbone way (Figures 1e,f and
S26). Because of the distortion, the torsion energies per unit of
[Cd3L3]6+ and [Zn3L3]6+ were much higher than those of
[Cd4L4]8+ and [Zn4L4]8+ (Table S1). As a result, the lower
torsion energy of [Zn4L4]8+ contributed to its lower total
energy per unit compared with [Zn3L3]6+, suggesting that it is
the enthalpically favorable structure. However, the selective
formation of [Cd3L3]6+ is determined by electrostatic
repulsion, in which [Cd4L4]8+ shows much higher electrostatic
energy per unit.
To characterize the optical properties of L and the chiral
metallomacrocycles, the UV−vis absorption spectra (Figure
4a) were collected first and analyzed with the help of
theoretical calculation (Figure 4e). The calculation was
performed based on density functional theory with 6-31G
basis sets (see the details in the Supporting Information).26
The experimental results were consistent well with the
theoretical calculations with respect to the locations and
relative intensities of the absorption peaks. In detail, L in
Figure 4a shows a set of peaks at 264, 295, and 340 nm
(shoulder), which refers to the electron transition (ET) from
HOMO−7 to LUMO (local pyridinyl π−π* transition),
HOMO−1 to LUMO+1 (intramolecular charge transfer,
ICT, from the carbazole to pyridinyl group), and HOMO to
LUMO (π−π* transition on the carbazole group), respectively
(Figure S2). Upon complexation, a new peak centered at ca.
400 nm appears in both parts a and e of Figure 4 and is
assigned to ET from HOMO to LUMO+12 (Figure S2;
ligand-to-ligand charge transfer, LLCT).27
In CD spectra (Figure 4c), L displays a weak positive
Cotton effect (CE) at 268 nm. It indicates that only the
pyridinyl rings attached to the chiral moieties are settled in
some asymmetric environment, and no other long-ranged
chiral induction exists because of the backbone flexibility.
However, sophisticated CD signals on both spectra of [Cd3L3]
and [Zn4L4] are observed, which show negative (ca. 380 nm),
negative couplet (ca. 340 nm), and negative (ca. 295 nm) CEs.
Among these rich and strong CEs, CEs at the LLCT (400 nm)
absorption bands are characteristic and reveal that the chirality
is transferred from chiral carbon centers to the coordination
sites. Also, [Zn4L4] with a more intense signal suggests that the
chiral induction efficiency of the [Zn4L4] macrocycle is higher
than that of [Cd3L3]. The similar CD spectra of [Cd3L3] and
[Zn4L4] suggest their similar chirality. Also, the calculated CD
spectra based on the models of the complexes show signals
similar to those of the experimental results (Figure 4f). In
addition, the specific rotations of L, [Cd3L3], and [Zn4L4] were
25
investigated. Both [Cd3L3] ([α]D = −135°; c = 0.218 in
CH3CN) and [Zn4L4] ([α]D25 = −131°; c = 0.234 in CH3CN)
revealed a larger negative specific rotation than that of L
C
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([α]D25 = −70°; c = 0.220 in CHCl3), further suggesting chiral
transmission from L to the macrocyclic backbone.
1
oretical data, and H and 13C NMR spectra for all new
compounds (PDF)
We then investigated the fluorescence (FL) properties of L,
[Cd3L3], and [Zn4L4]. As shown in Figure 4b, L emits light
around 390 nm. After coordination, the light emission of
[Cd3L3] and [Zn4L4] has an extraordinary red shift (∼120 nm)
to the green-light region (peaked at ca. 505 nm), which is
assigned as a LLCT transition.7b Compared with CD
spectrometry, which characterizes the chiroptical property in
the ground state, CPL spectrometry provides more chiral
properties of the analytes in the excited state.7b In addition,
CPL has great potential application in photonic devices, such
as 3D displays and energy-efficient backlights for LCD
displays.28 As a result, we investigated the CPL properties of
L, [Cd3L3], and [Zn4L4]. As shown in Figure 4d, L displays a
positive CPL peak at ca. 400 nm. After coordination, [Zn4L4]
shows an obviously negative CPL peak at its emission region
(centered at ca. 530 nm), which indicates that, in the excited
state, the [tpy-Zn-tpy] luminophores are settled in a distinct
chiral environment with that of the ligand. By comparison,
[Cd3L3] exhibits a much weaker positive CPL signal because of
its lower quantum yield (Φ, 0.43) compared with Φ of L
(0.85) and [Zn4L4] (0.65). This also might be attributed to the
combination of the positive signal from L and a much weaker
negative signal from tpy-CdII. After deconvolution (the
detailed method is summarized in the Supporting Informa-
tion), the luminescence dissymmetry factors (|glum|),29 which
are used to evaluate the magnitude of CPL of L, [Cd3L3], and
[Zn4L4] are around 8.8 × 10−4 (λex = 400 nm), 4.0 × 10−4 (λex
= 530 nm), and 18.0 × 10−4 (λex = 530 nm), respectively. The
higher |glum| of [Zn4L4] indicates a higher efficiency in the
chiral transmission and amplification from the ligand to metal
center in the case of [Zn4L4] than in [Cd3L3]. Notably,
compared with the chiroptical properties of [Zn4L4], the much
weaker CPL signal of [Cd3L3] is consistent with its weaker CE
at the ICT absorption band in the ground state possibly
because the larger ionic radius of cadmium(II) affected the
chiral-transferring efficiency. These values are of the same
order of magnitude as the values of other examples of
biomacromolecules and chiral macromolecules.30
AUTHOR INFORMATION
Corresponding Authors
*E-mail: xqhao@zzu.edu.cn (X.-Q.H.).
*E-mail: mpsong@zzu.edu.cn (M.-P.S.).
*E-mail: xiaopengli1@usf.edu (X.L.).
■
ORCID
Xinju Zhu: 0000-0003-1966-3480
Wenjing Zhang: 0000-0001-7964-6792
Ming Wang: 0000-0002-5332-0804
Xin-Qi Hao: 0000-0003-1942-8309
Mao-Ping Song: 0000-0003-3883-2622
Xiaopeng Li: 0000-0001-9655-9551
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 21672192, 21803059, and 21929101), the
China Postdoctoral Science Foundation (Grants
2016M602254 and 2016M600582), the Program for Science
& Technology Innovation Talents in Universities of Henan
Province (Grant 17HASTIT004), the Aid Project for the
Leading Young Teachers in Henan Provincial Institutions
(Grant 2015GGJS-157), and the Natural Science Foundation
of Henan Province (Grant 182300410255).
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02657.
General experimental methods, synthetic procedures,
photophysical, electrochemical, self-assembly, and the-
D
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