Stereochemically Dependent Synthesis of Two Cu(I) Cluster-Based Coordination Polymers with Thermochromic Luminescence

Article abstract of DOI:10.1021/acs.inorgchem.7b02123

Two Cu(I) cluster based coordination polymers, [CuI(cis-bpype)]·CH₃CN (1) and [Cu₄I₄(cis-bpype)₃(trans-bpype)]·3DMF (2), have been synthesized from cis- and trans-1,2-bis(4-(pyridin-2-yl)phenyl)ethane (cis- and

Full text of DOI:10.1021/acs.inorgchem.7b02123

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Stereochemically Dependent Synthesis of Two Cu(I) Cluster-Based
Coordination Polymers with Thermochromic Luminescence
Si−Si Zhao,† Lei Wang,*,‡ Yingjie Liu,‡ Li Chen,*,† and Zhigang Xie*,‡
†Northeast Normal University, Changchun 130024, People’s Republic of China
‡State Key Laboratory of Polymer Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
5625 Renmin Street, Changchun, Jilin 130022, People’s Republic of China
S
* Supporting Information
ABSTRACT: Two Cu(I) cluster based coordination poly-
mers, [CuI(cis-bpype)]·CH3CN (1) and [Cu4I4(cis-bpy-
pe)3(trans-bpype)]·3DMF (2), have been synthesized from
cis- and trans-1,2-bis(4-(pyridin-2-yl)phenyl)ethane (cis- and
trans-bpype) ligands and {Cu4I4(PPh3)4} as starting materials.
In compound 1, adjacent rhomboid-type {Cu2I2} units from
the decomposition of {Cu4I4(PPh3)4} starting material
connect by cis-bpype ligands to form a 1D framework.
Compound 2 also has a 1D structure, but it has a {M2L3}-
type coordinated cage constructed by three cis-bpype ligands
and two {Cu4I4} secondary building units (SBU), and these
coordination cages further link by trans-bpype to form the final
frameworks. Upon cooling from 300 to 80 K, these Cu(I)
cluster based coordination polymers exhibit interesting thermochromic behavior. In particular, compound 2 gives a chromic
process from green luminescence at room temperature to red luminescence at 80 K and its corresponding CIE coordinates shift
from green (0.34, 0.43) at 300 K to red (0.46, 0.42) at 80 K, respectively. This red shift of 124 nm (516 to 640 nm) is large
enough to ensure a color change visible to the naked eye, which can be potentially utilized as a temperature sensor with a wide
range.
polymers but also inspire us exploring new characters in this
field.
INTRODUCTION
■
Luminescent coordination polymers (CPs) or metal−organic
frameworks (MOFs) have received immense attention due to
their diverse topological structures and potential applications in
detection and sensing, bioimaging, displays, and optical
devices.1−5 One promising strategy for the preparation of
such functional materials is based on the inorganic connecting
nodes and organic linkers within the coordination polymer
framework, since those nodes or linkers could be exploited as
luminophores to generate interesting luminescence.6−8 As one
important subclass of aggregation-induced emission (AIE)
molecules, tetraphenylethylene (TPE) based ligands with
carboxyl or pyridyl sites have been designed and successfully
introduced into coordination polymer frameworks, resulting in
intriguing luminescent properties and other potential applica-
tions.9−17 A new zirconium tetraphenylethylene based MOF
with a deep blue fluorescent emission and high quantum yield
In the structural chemistry of MOFs, metal cluster based
nodes, also known as secondary building units (SBUs), may act
as anchors to ensure the overall architectural stability of the
MOF.20,21 The synthesis of MOFs has mainly relied on a one-
pot synthetic method, and the existence of metal cluster based
SBUs within the final framework is unpredictable. Notably,
utilization of these existing metal cluster compounds as starting
materials in the synthesis of MOFs represents a feasible strategy
to construct metal cluster based structures, and this strategy
may also help to predict their structure−property relationship
at the crystal engineering level.22−24 Among these existing
clusters, cubane-like {Cu4I4Lx} (L = ligand) clusters have been
extensively studied for their remarkable luminescent perform-
ance under external stimulation, such as temperature and
pressure.25−32 For the synthesis of coordination polymers,
some Cu(I) cluster-based coordination polymers containing a
{Cu4I4} SBU or other more complex SBU have already been
prepared in one pot,33−42 but utilization of {Cu4I4Lx} clusters
as starting materials has been less studied.43
(99.9
0.5%) under Ar has been obtained recently.18 In
addition, a rare earth (RE) free, three-dimensional coordination
polymer has been synthesized, and this compound contains a
blue-excitable yellow phosphor, which can be used as a
promising material for phosphor conversion WLEDs.19 These
successful examples may not only provide us with guidance in
designing and synthesizing novel luminescent coordination
Received: August 16, 2017
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of cis-bpype and trans-bpype
reported.44 4-Bromobenzophenone (5 g, 19.15 mmol) and zinc
powder (3.75 g, 57.7 mmol) were placed in a two-necked flask (250
mL) under an argon atmosphere. After the flask was immersed into an
ice bath, dry THF (60 mL) was placed in the flask through the branch.
Then titanium tetrachloride (3.2 mL, 29.2 mmol) was slowly added
with rapid stirring over approximately 30 min. The mixture was
refluxed for 16 h at 90 °C. Then, the product was filtered and then
vacuum-evaporated. A 50 mL portion of chloroform was added to the
black oil and then recrystallized with methanol (400 mL) to give a
white powder.
Herein, {Cu4I4(PPh3)4}26 has been selected as the starting
material due to its external stimuli luminescence feature and the
weak coordination bonds between PPh3 groups and Cu(I) sites.
Two organic ligands with a pyridyl function and novel
luminescent properties, cis-1,2-bis(4-(pyridin-2-yl)phenyl)-
ethane and trans-1,2-bis(4-(pyridin-2-yl)phenyl)ethane (cis-
bpype and trans-bpype), have been synthesized in two steps
according to previous works, but with some modification
(Scheme 1).19,44 The solvothermal reaction of cis-bpype and
{Cu4I4(PPh3)4} starting material in acetonitrile yields a one-
dimensional coordination polymer with {Cu2I2} nodes, [CuI-
(cis-bpype)]·CH3CN (1). In the reaction of cis-bpype and
trans-bpype ligands with {Cu4I4(PPh3)4} starting material in
N,N-dimethylformamide (DMF), another one-dimensional
coordination polymer with {Cu4I4} nodes, [Cu4I4(cis-bpy-
pe)3(trans-bpype)]·3DMF (2), has been obtained. The
structural and thermochromic luminescence of the as-
synthesized Cu(I) cluster based coordination polymers have
been well studied.
Synthesis of cis-1,2-Bis(4-(pyridin-2-yl)phenyl)ethane and
trans-1,2-Bis(4-(pyridin-2-yl)phenyl)ethane (cis-bpype and
trans-bpype). The preparation of the ligands cis-bpype and trans-
bpype (Scheme 1 and Figure S1 in the Supporting Information) was
modified from the original reported procedure.19 The cis-/trans-bbpe
mixture (0.84 mmol, 0.411 g), pyridine-4-boronic acid (6.72 mmol,
1.209 g), tetrakis(triphenylphosphine)palladium (Pd(PPh3)4; 0.0588
mmol, 68 mg), and potassium carbonate (25.2 mmol, 3.48 g) were
placed in the 100 mL flask. A 30 mL portion of dimethylformamide
(DMF) and 8 mL of water were then also added, and the solution was
refluxed at 130 °C for 48 h. After a Schiffs base reaction, the product
was first vacuum-evaporated and then extracted three times with
dichloromethane. After it was washed with water and dried with
anhydrous Na2SO4, the product was recrystallized in petroleum ether
to give a pale yellow powder. The final products of cis-bpype and trans-
bpype were purified by column chromatography with CH2Cl2/MeOH
30/1 mobile phase; the product ratio of these ligands was 1:1. trans-
EXPERIMENTAL SECTION
■
Materials and Methods. The C, H, and N elemental analyses
were performed on a Euro vector EA3000 elemental analyzer. FT-IR
spectra were obtained on a Mattson Alpha Centauri spectrometer. The
thermal analyses were measured using a gravimetric analyzer (Pyris
Diamond TGA/DTA) under nitrogen. 1H NMR spectra were
measured in DMSO-d6 at 20 1 °C by an AV-400 NMR spectrometer
from Bruker. The UV−vis diffuse reflectance spectra (DRS)
measurements were carried out on a UV-2600 spectrophotometer
(Shimadzu) with BaSO4 as a reference. Powder X-ray diffraction
(PXRD) patterns were determined on a Rigaku Dmax 2000 X-ray
diffractometer with Cu Kα radiation (λ = 0.154 nm). The
photoluminescent properties of compounds 1 and 2 were conducted
utilizing a PerkinElmer FLSP920 edinburgh fluorescence spectrometer
with cooling from 300 to 80 K. Photoluminescence (PL) measure-
ments of 1,2-bis(4-(pyridin-2-yl)phenyl)ethane (bpype) were carried
out on a Hitachi F-7000 spectrophotometer.
1
bpype: H NMR (DMSO-d6, 400 MHz) δ 8.59 (dd, 4H), 7.69−7.62
1
(m, 8H), 7.23−7.05 (m, 14H). cis-bpype: H NMR (DMSO-d6, 400
MHz) δ 8.58 (4H, dd), 7.70−7.64 (m, 8H), 7.21−7.1 (m, 10H), 7.03
(dd, 4H) (Figure S1).
Synthesis of {Cu4I4(PPh3)4} Starting Material.26 CuI (1.9 g, 10
mmol), triphenylphosphine (2.65 g, 0.1 mmol), and 250 mL of
toluene were placed in a 500 mL flask, and the solution was refluxed at
120 °C for 16 h. Then, the product was filtered through double-
layered filter paper. The precipitates were dried under vacuum at room
temperature, yielding {Cu4I4(PPh3)4} as a white powder.
Solvothermal Synthesis of 1 and 2. Typically, {Cu4I4(PPh3)4}
starting material (19 mg, 0.025 mmol), cis-bpype (8 mg, 0.016 mmol),
and acetonitrile (6 mL) were sealed in a Teflon reactor (15 mL) and
heated at 120 °C for 3 days. Yellow block crystals of 1 were obtained
in a 40% yield based on Cu. Anal. Calcd for C38H29N3CuI (Mr =
718.08): C, 63.56; H, 4.07; N, 5.85. Found: C, 63.51; H, 4.01; N, 5.87.
IR data (KBr, cm−1): 3054 (s), 3025 (s), 2994 (s), 2250 (s), 1681 (s),
Synthesis of cis-/trans-1,2-Bis(4-bromophenyl)-1,2-dipheny-
lethene (cis-/trans-bbpe). cis-bbpe and trans-bbpe were prepared via
the McMurry coupling of 4-bromobenzophenone, and the product
was a mixture with a cis/trans isomer ratio of 44/56, as previously
B
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1605 (w), 1535 (m), 1485 (w), 1441 (w), 1420 (w), 1399 (w), 1326
(m), 1274 (s), 1216 (w), 1182 (s), 1154 (s), 1111 (m), 1069 (m),
1031 (m), 1005 (m), 975 (s), 958 (s), 916 (s), 858 (m), 836 (m), 803
(w), 769 (m), 748 (w), 733 (w), 700 (w), 671 (w), 643 (w), 625 (m),
586 (s), 573 (m), 548 (m), 529 (s), 498 (m), 470 (m), 415 (s).
For 2, {Cu4I4(PPh3)4} starting material (19 mg, 0.025 mmol), cis-
bpype (4 mg, 0.008 mmol), and trans-bpype (4 mg, 0.008 mmol) were
dissolved in DMF (6 mL). Then the mixture was heated in a Teflon
reactor (15 mL) at 120 °C for 3 days. Dark yellow crystals of 2 were
obtained in a 50% yield based on Cu. Anal. Calcd for
C153H125O3N11Cu8I8 (Mr = 3689.15): C, 49.81; H, 3.42; N, 4.18.
Found: C, 49.87; H, 3.45; N, 4.23. IR data (KBr, cm−1): 3641 (s),
3050 (s), 3025 (s), 2923 (s), 1672 (w), 1605 (w), 1533 (m), 1486
(w), 1440 (w), 1421 (w), 1400 (w), 1382 (w), 1254 (m), 1218 (w),
1154 (m), 1112 (m), 1084 (m), 1069 (m), 1032 (m), 1005 (m), 858
(s), 837 (s), 817 (w), 804 (w), 771 (m), 747 (m), 730 (w), 701 (w),
672 (m), 644 (s), 623 (s), 576 (s), 553 (s), 525 (s), 492 (m).
X-ray Crystallography. Crystallographic data of 1 were collected
at 193 K on a Bruker APEX CCD area detector using Mo Kα radiation
(λ = 0.71073 Å). Single-crystal X-ray data of 2 were measured on an
Oxford Diffraction Gemini R Ultra diffractometer using Mo Kα
radiation (λ = 0.71073 Å) at 293 K. The starting structural models
were solved by direct methods and refined by full-matrix least-squares
techniques using SHELXL-2014 within WINGX,45 and all non-
hydrogen atoms were refined anisotropically. The SQUEEZE function
in PLATON was applied to remove residual electron density by
disordered solvent molecules.46 Further crystallographic details and
structure refinements for 1 and 2 are provided in Table 1. Selected
bond lengths and angles for 1 and 2 are given in Tables S1 and S2 in
the Supporting Information.
Figure 1. Asymmetric unit of compound 1. Symmetry codes: (#1) −x,
−y + 1, −z; (#2) −x − 1, −y, −z.
structures.16,47 Two Cu1 sites connect each other by a μ2-
bridging I1 atom to form a rhomboid-like {Cu2I2} unit coming
from decomposed {Cu4I4(PPh3)4} starting material, and the
Cu−Cu bond distance is about 2.71 Å, within the region of
metallophilic interactions. Adjacent {Cu2I2} units connect by
cis-bpype ligands to form a 1D chain (Figure 2a). Moreover,
adjacent chains are further interlinked by C−H···π interactions
between a hydrogen of benzene and pyridine into a 3D
supramolecular structure (Figure 2b).
Table 1. Crystal Data and Structure Refinement for 1 and 2
1
2
formula
Mr
C38H29N3CuI
718.08
C153H125O3N11Cu8I8
3689.15
monoclinic
P21
cryst syst
space group
a (Å)
monoclinic
P21/n
10.1530(7)
11.6380(9)
26.7750(19)
90
16.8555(12)
17.5982(12)
25.041(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
95.8520(14)
90
99.550(7)
90
3147.3(4)
4
7324.7(9)
2
Dcalc (g cm−3
F(000)
Rint
)
1.515
1.673
1440
3360
0.0651
0.0816
GOF on F2
1.092
1.088
Flack factor
0.00(5)
0.1126
a
R1 (I > 2σ(I))
0.0650
b
wR2 (all data)
0.1305
0.2951
a
b
Figure 2. (a) View of the 1D chain in compound 1. (b) View of the
3D supramolecular structure of compound 1. The CH···π interactions
are illustrated by black dotted lines.
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(Fo − Fc )2]/
2
∑w(Fo )2]}1/2
.
RESULTS AND DISCUSSION
Structural Description of 1 and 2. Compound 1
Compound 2 crystallized in the space group P21, and its
asymmetric unit includes three cis-bpype ligands, one trans-
bpype ligand, eight crystallographically independent Cu(I) ions,
and eight crystallographically unique I atoms (Figure 3). Four
Cu(I) ions are coordinated to four adjacent μ3-bridging iodine
ions to build up a cubane-shaped {Cu4I4} SBU. Similar to the
case for compound 1, all of the Cu(I) sites within the {Cu4I4}
SBU adopt a distorted-tetrahedral coordination mode but can
be divided into two types according to the coordinated N
■
crystallizes in the monoclinic space group P21/n and its
asymmetric unit (Figure 1) is composed of one Cu(I) cation,
one cis-bpype ligand, and one lattice acetonitrile molecule. Cu1
sites are coordinated with two iodine atoms and two N atoms
of cis-bpype ligands to form an approximate tetrahedron.
Average bond lengths of Cu−I and Cu−N are 2.66 and 2.12 Å,
respectively, which are close to those in similar reported
C
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Figure 3. Asymmetric unit of compound 2. Symmetry codes: (#1) x −
2, y, z − 1; (#2) x + 2, y, z + 1.
Figure 4. Structural presentation and illustration of as-synthesized
compound 2. {M2L3}-type coordination cages formed by {Cu4I4}
SBUs and three cis-bpype ligands are drawn with balls for clarity. trans-
bpype is simplified as a “gold stick” for clarity.
atoms from the cis-bpype ligand or trans-bpype ligand. The
average bond lengths of Cu−I, Cu−N, Cu−Cu, and I−I in the
{Cu4I4} SBUs are 2.69, 2.03, 2.70, and 4.49 Å, respectively,
which are in agreement with those in isolated {Cu4I4} SBUs.
Notably, the Cu−Cu distances all fall in the range 2.63−2.78 Å,
shorter than the sum of the van der Waals radii of Cu atoms
(2.80 Å),48 suggesting effective metallophilic interactions in the
{Cu4I4} SBU of 2. Two {Cu4I4} SBUs (simplified as M) and
cis-bpype ligands (simplified as L) within the {Cu4I4}2{cis-
bpype}3 coordination cage connect with each other to form a
{M2L3}-type cage (Figure S2 in the Supporting Informa-
tion).49,50 These coordination cages link with trans-bpype
ligands to construct a 1D chain with a “candied gourd” shape.
Adjacent chains are joined together by π···π stacking between
pyridine and pyridine groups, resulting in a 3D supramolecular
architecture (Figures 4 and 5). PLATON calculations reveal
that the solvent-accessible volume for 2 is 17.8%,46 which is
occupied by three DMF molecules on the basis of elemental
and thermogravimetric analysis (Figure S3 in the Supporting
Information).
Figure 5. View of the π···π stacking mode of compound 2.
Characterizations of 1 and 2. The crystal structure and
photophysical properties of compounds 1 and 2 have been well
characterized by various measurements at room temperature.
Powder XRD patterns for samples 1 and 2 are nearly identical
to their simulated patterns, which confirmed the phase purity of
as-synthesized samples (Figure S4 in the Supporting
Information). Compounds 1 and 2 show similar characteristic
frequencies within all measured regions in infrared spectra
(Figure S5 in the Supporting Information). The UV absorption
bands (Figure S6 in the Supporting Information) of samples 1
and 2 range from 225 to 650 cm−1 and from 225 to 550 cm−1,
respectively. Upon excitation at 372 nm, the emission peak of
bpype is blue-shifted from 485 nm at 298 K to 457 nm at 77 K,
which could be attributed to the nature of the AIE ligands at
low temperature (Figure 6a). At 298 K, compound 1, which is
composed of rhombic {Cu2I2} units, gives light green emission
centered at 500 nm on excitation at 340 nm (Figure 6b). For
compound 2 with cubane-type {Cu4I4} cores as SBUs, its
room-temperature solid-state emission spectrum (Figure 6c)
also shows a green emission at 516 nm (λex 380 nm). In
contrast to the emission of the free bpype ligand at 485 nm,
both emissions of compounds 1 and 2 exhibit slight red shifts,
giving approximate wavelengths of 15 and 31 nm, respectively.
This phenomenon may be assigned to ligand to metal charge
transfer (LMCT) from the bpype ligands to the Cu(I) centers.
The excited-state lifetimes (τ) (Figure S7 and Table S3 in the
Supporting Information) of the as-synthesized samples are 0.88
ns for 1 and 0.45 ns for 2, respectively.
Thermochromic Luminescence Properties. One of the
interesting features for Cu(I) cluster-based coordination
polymers is their novel emission shifts and color changes
under external stimuli.36−38 Hence, the significant structural
diversity of compounds 1 and 2 may lead to different
thermochromic behavior. To further study the thermochromic
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Figure 6. Solid-state emission spectra of bpype at 298 and 77 K (a) and of 1 (b) and 2 (c) at room temperature.
Figure 7. Emission changes and the commission internacionaled eclairage (CIE) color space chromaticity diagram of samples 1 (a) and 2 (b) from
80 to 300 K. The excitation wavelength for samples 1 and 2 are 340 and 380 nm, respectively.
properties of 1 and 2, their solid-state emission spectra have
been monitored from 300 to 80 K by dipping those samples
into liquid nitrogen. For compound 1, emission intensities are
gradually enhanced upon cooling, and its emission maximum
blue-shifts by about 12 nm when the temperature is lowered to
80 K (Figure 7a). This slight color change can hardly be
distinguished by the naked eye, which is in agreement with the
result of the 1931 Commission Internationale d’Eclairage
(CIE) chromaticity from (0.331, 0.44) at 300 K to (0.294,
0.485) at 80 K. This result is similar to the observed
phenomenon of the ligand at 298 and 77 K, essentially
belonging to ligand-dominated luminescence.51,52
However, compound 2 exhibits a chromic process from
green luminescence at room temperature to red luminescence
at 80 K at an excitation wavelength of 380 nm. At 298 K, it also
has ligand-dominated luminescence. As shown in Figure 7b, in
addition to the existence of green luminescence at 516 nm, a
new red luminescence with a maximum wavelength of around
640 nm at low energy (LE) appears during the temperature
decrease from 300 K to about 240 K. Under further continual
cooling to 80 K, the continuing enhancement of emission
intensities and the blue shift of maximum wavelengths for both
green and red luminescence have been observed, but the
emission intensity of red luminescence increases more rapidly
and the LE band plays a dominant role. Its corresponding CIE
coordinates shift from green (0.34, 0.43) at 300 K to red (0.46,
0.42) at 80 K, respectively. This red shift of 124 nm (516 to
640 nm) is large enough to ensure a color change visible to the
naked eye. Unlike those reported {Cu4I4} cluster-based
frameworks with a new band at high energy (HE) at low
temperature coming from a halide to ligand charge-transfer
(3XLCT) excited state,36 the probable explanation for
compound 2 may be ascribed to the structure composition
and coordination mode of the {Cu4I4} SBU and the two types
E
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of bpype ligands, which may lead to more complex charge
transfer processes among Cu(I) sites, I atoms, and cis- or trans-
bpype, such as halide to metal charge transfer and copper-
centered d → s, p transitions.25,48 Moreover, these results also
demonstrate that the existence of higher nuclearity Cu(I)
clusters is essential for the design and synthesis of functional
coordination polymers with stimuli-responsive properties.
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CONCLUSION
■
In summary, two Cu(I) cluster based coordination polymers
with cis-bpype and trans-bpype ligands have been successfully
prepared. Compound 1 exhibits a classical 1D chain with a
single cis-bpype ligand, while 2 shows a 1D chain with a
{M2L3}-type coordination cage containing both cis-bpype and
trans-bpype ligands. Remarkably, compound 2 exhibits
interesting thermochromic behavior during cooling from 300
to 80 K, giving a large 124 nm red shift and a visible color
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ASSOCIATED CONTENT
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Accession Codes
CCDC 1563659−1563660 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail for L.W.: leiwang@ciac.ac.cn.
*E-mail for L.C.: chenl686@nenu.edu.cn.
*E-mail for Z.X.: xiez@ciac.ac.cn.
ORCID
Lei Wang: 0000-0003-4395-5002
Li Chen: 0000-0001-7922-0599
Zhigang Xie: 0000-0003-2974-1825
Notes
(18) Wei, Z.; Gu, Z. Y.; Arvapally, R. K.; Chen, Y. P.; McDougald, R.
N., Jr.; Ivy, J. F.; Yakovenko, A. A.; Feng, D.; Omary, M. A.; Zhou, H.
C. Rigidifying Fluorescent Linkers by MetalOrganic Framework
Formation for Fluorescence Blue Shift and Quantum Yield Enhance-
ment. J. Am. Chem. Soc. 2014, 136, 8269−8276.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was kindly provided by the National Natural
Science Foundation of China (Project Nos. 21401188,
51522307, 21474012, and 51273037).
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F
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