Anion-tuned assembly of three double cubane tetranuclear Cd(II)/Zn(II) complexes based on a 2-substituted 8-hydroxyquinoline ligand: Synthesis, crystal structures and fluorescent properties

Article abstract of DOI:10.1016/j.molstruc.2020.127770

One unique 2-substituted-8-hydroxyquinoline ligand with 2,3-dimethoxyphenyl groups was synthesized and characterized by ESI-MS, NMR spectroscopy and elemental analysis (EA). Two double cubane tetranuclear Cd(II) complexes [Cd₄(L)₆(CH₃COO)₂] (1), [Cd₄(L)₆(I)₂] (2) and one double cubane tetranuclear Zn(II) complex [Zn₄(L)₆(NO₃)₂] (3), were prepared by solvothermal reactions of Cd(CH₃COO)₂, CdI₂ and Zn(NO₃)₂. Complexes 1–3 were characterized with single crystal X-ray diffractions, powder X-ray diffractions (PXRD), thermogravimetry analyses (TGA) and EA. The building blocks exhibited unprecedented structural diversification and their self-assembly afforded tetranuclear Cd(II)/Zn(II) structures binding of different anions and solvent systems. Complexes 1–3 featured three types of supramolecular network controlled by non-covalent interactions, such as C–H?O, C–H···π, π···π stacking, hydrogen-bonding, and halogen-related interactions. Investigation of their photoluminescence properties exhibited disparate emission wavelengths and lifetimes in the solid state. This unique capability may provide a useful strategy to tune the optical properties of multinuclear materials, which could be exploited as important components for optoelectronic devices.

Full text of DOI:10.1016/j.molstruc.2020.127770

Journal Pre-proof
Anion-tuned assembly of three double cubane tetranuclear Cd(II)/Zn(II) complexes
based on a 2-substituted 8-hydroxyquinoline ligand: Synthesis, crystal structures and
fluorescent properties
Chengqiang Pan, Baixing Yang, Fei Wang, Jianxing Xu, Guowei Chen, Wenjin Ji,
Yanping Huo
PII:
S0022-2860(20)30094-6
DOI:
https://doi.org/10.1016/j.molstruc.2020.127770
MOLSTR 127770
Reference:
To appear in: Journal of Molecular Structure
Received Date: 28 November 2019
Revised Date: 14 January 2020
Accepted Date: 20 January 2020
Please cite this article as: C. Pan, B. Yang, F. Wang, J. Xu, G. Chen, W. Ji, Y. Huo, Anion-tuned
assembly of three double cubane tetranuclear Cd(II)/Zn(II) complexes based on a 2-substituted 8-
hydroxyquinoline ligand: Synthesis, crystal structures and fluorescent properties, Journal of Molecular
Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.127770.
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Table of content entry
Three double cubane tetranuclear complexeswith different spatial
structures were tuned by anion and have different photophysical
properties.

Anion-tuned assembly of three double cubane tetranuclear
Cd(II)/Zn(II) complexes based on a 2-substituted 8-hydroxyquinoline
ligand: synthesis, crystal structures and fluorescent properties
a
b
b
Chengqiang Pan , Baixing Yang , Fei Wang a, Jianxing Xu , Guowei
Chen b,Wenjin Ji b, Yanping Huo b, *
a Analysis and Test Center, Guangdong University of Technology, Guangzhou 510006,
China
b
School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, China
*Corresponding authors. Tel.: +86 20 39322236; Fax: +86 20 39322235; Mobile: +86
13798135622. E-mail addresses: yphuo@gdut.edu.cn(Y. Huo).
Abstract
One unique 2-substituted-8-hydroxyquinoline ligand with
2,3-dimethoxyphenyl groups was synthesized and characterized by
ESI-MS, NMR spectroscopy and elemental analysis (EA). Two double
cubane tetranuclear Cd(II) complexes [Cd4(L)6(CH3COO)2] (1),
[Cd4(L)6(I)2] (2) and one double cubane tetranuclear Zn(II) complex
[Zn4(L)6(NO3)2] (3), were prepared by solvothermal reactions of
Cd(CH3COO)2, CdI2 and Zn(NO3)2. Complexes 1-3 were characterized
with single crystal X-ray diffractions, powder X-ray diffractions (PXRD),
thermogravimetry analyses (TGA) and EA. The building blocks exhibited
unprecedented structural diversification and their self-assembly afforded
tetranuclear Cd(II)/Zn(II) structures binding of different anions and

solvent systems. Complexes 1-3 featured three types of supramolecular
network controlled by non-covalent interactions, such as C–H···O,
C–H···π, π···π stacking, hydrogen-bonding, and halogen-related
interactions. Investigation of their photoluminescence properties
exhibited disparate emission wavelengths and lifetimes in the solid state.
This unique capability may provide a useful strategy to tune the optical
properties of multinuclear materials, which could be exploited as
important components for optoelectronic devices.
Keyword: Anion-tuned; Double cubane tetranuclear; 8-hydroxyquinoline;
Crystal structure; Photophysical property
Introduction
Over the past few decades, the design and preparation of
metal–organic coordination complexes have caused widespread concerns,
not only due to their fascinating structures, such as molecular cage,
honeycomb, grid, ladder, rotaxane and catenane structures, but also their
promising applications in gas adsorption, magnetism, ion-exchange,
catalysis, lighting, display, sensing, and optical devices [1-8]. Motivated
by the success of tris(8-hydroxyquinolinato)aluminum (Alq3) in vacuum
deposited LEDs, organic chelate metal complexes based on
8-hydroxyqunoline derivatives have, in particular, attracted a lot of
attention [9].

A large number of literatures show that the coordination of different
metal ions and organic ligands involved in coordination of the ligand
atoms of type, number, and the spatial location. Therefore in the process
of design and construct target complexes, the choice of organic ligand
and metal ions are very important. The d10 metal complexes, such as
Zn(II) and Cd(II), not only have the stable electron configuration, and
when the metal conjugation organic ligand to form complexes which
cause metal to ligand or ligand to metal electron transition and transform
the luminous performance of the ligand itself. This phenomenon has
received extensive attention of the researchers [10]. In general, the
photophysical properties of 8-quinolinolate-based complexes in the solid
state are dependent on the character of the metal ion, the degree of
aggregation, and the molecular and supramolecular structures, the
intermolecular non-covalent interactions[11]. Hence, the development of
new synthetic methods and strategies for the fabrication of well-defined
heteroleptic molecular complexes or multinuclear clusters that are
supported by organic 8-hydroxyquinoline derivatives, with unique
self-assembled structures and properties, remains a big challenge for
chemists [12].
Over the past two decades, it has been demonstrated that the
self-assembly of complexes from polytopic organic linkers could be
affected by many structure-directing factors [13-18], such as the metal,

the coordinating ability of the anion, the pH value, and the reaction
temperature [19-23]. Of these factors, the anion plays a crucial role in
influencing the formation of the coordination compound and leads to their
structural and dimensional variation, supramolecular isomerism, and new
topologies [19]. With a few notable exceptions, the systematic
investigation of the effect of the anion on 8-hydroxyquinolinate-based
complexes remains unexplored [24].
In recent years, we have reported the synthesis and characterization
of a series of transition-metal complexes that involve 2-substituted
8-quinolinolate ligands with a NO donor set [25]. In this context, one
8-quinolinolate-based ligand (HL), which contained a 2, 3-
dimethoxyphenyl group, was chosen for fabricating Cd(II)/Zn(II)
complexes in different solvent systems. We studied the effects of solvent
for ligand to luminescence properties as well as the effects of anion to the
structure of complexes. By using a solvothermal method, three double
cubane tetranuclear Cd(II)/Zn(II) were constructed by the self-assembly
of Cd(CH3COO)2, CdI2 and Zn(NO3)2 with the HL ligand. In addition,
their fluorescent properties showed disparate emission wavelengths,
lifetimes, and quantum yields in the solid state. This unique capability
may provide a useful strategy for tuning the optical properties of
8-hydroxyquinoline materials, which could be exploited as important
components of optoelectronic devices.

Scheme 1. Synthesis of complexes 1– 3 from ligand HL
Experimental
As outlined in Scheme 1, bidentate ligand HL was prepared by a
similar procedure from the cheap commercial available
8-hydroxyquinaldine and characterized by ESI-MS, NMR spectroscopy
and elemental analysis. Single crystals of complexes were readily
obtained in good yields by heating Cd(CH3COO)2, CdI2 and Zn(NO3)2
and HL in a mixture of DMF and MeOH.
Materials and methods
All solvents and reagents were purchased from commercial sources
and used as received. Elemental analyses were performed with an
1
Perkin-Elmer 2400 elemental analyzer. HNMR experiments were
performed on a MERCURYplus 400 spectrometer. All UV-Vis

absorption spectrum were carried out on a Lambda 20 UV-Vis
Spectrometer (Perkin Elmer, Inc., USA). The IR (KBr pellet) spectrum
was recorded (400-4000 cm-1 region) on a NICOLET380 FT-IR
spectrometer. Powder X-ray diffraction (PXRD) data were collected on a
DMAX2500 diffractometer using Cu Kα radiation. Thermogravimetric
analyses (TGA) were performed using a NETZSCH STA409P thermal
analyzer under N2 atmosphere at a heating rate of 15 °C·min−1 in the
temperature region of 25～1000 °C.
Preparation of 2-[2-(2, 3-dimethoxyphenyl)vinyl]-8-acetoxyquinoline
To a solution of 2, 3-dimethoxybenzaldehyde(1.33 g, 8 mmol) in
acetic anhydride (15 ml) was added 1.1 equiv of 8-hydroxyquinaldine
(1.4 g). The mixture was refluxed at 145 °C under N2 for 30 h. The
reaction mixture was extracted with dichloromethane. After removal of
solvent under reduced pressure, the residue was purified on silica gel by
column using acetone/hexane as the eluent to afford the target compound
(1.15 g, 41 %). mp 90 °C, 1H NMR (400 MHz, CDCl3) δ: 8.17 – 8.04 (m,
2H), 7.67 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 12.3 Hz, 2H), 7.39 – 7.31 (m,
2H), 7.10 (t, J = 8.0 Hz, 1H), 6.91 (d, J = 8.0 Hz, 1H), 3.91 (d, J = 7.0 Hz,
6H), 2.58 (s, 3H); ESI-MS m/z: 350.1 ([M+H+]); Anal. Calcd (%) for
C21H19NO4 : C 72.19, H 5.48, N 4.01; Found: C 72.21, H 5.42, N 4.04.
Preparation of Ligand HL: 2-[2-(2, 3-dimethoxyphenyl)vinyl]-
8-quinolinol

A solution of 2-[2-(2, 3-dimethoxyphenyl)vinyl]-8-acetoxyquinoline
(1.0 g, 2.9 mmol) in pyridine(15 mL) was refluxed for 30 min, then water
(10 mL) was added. The reaction mixture was refluxed at 130 °C for 5 h.
After cooled, the solution was extracted with dichloromethane. After
removal of solvent under reduced pressure, the crude product was
purified with column chromatography over silica gel using acetone/petro
1
-leum ether as the eluent to give HL (0.81 g, 91 %). mp 104 °C ; H
NMR (400 MHz, DMSO) δ: 9.60 (s, 1H), 8.26 (d, J = 8.6 Hz, 1H), 8.05
(d, J = 16.5 Hz, 1H), 7.81 (d, J = 8.6 Hz, 1H), 7.45 (d, J = 16.5 Hz, 1H),
7.35 (d, J = 7.1 Hz, 3H), 7.19 – 6.97 (m, 3H), 3.80 (d, J = 7.8 Hz, 6H);
13C NMR (101 MHz, CDCl3) δ: 154.19, 153.22, 152.04, 137.95, 136.28,
130.52, 129.59, 128.63, 127.26, 124.25, 120.10, 118.35, 117.66, 112.40,
110.09, 61.33, 55.84; ESI-MS m/z: 308.1 ([M+H+]); Anal. Calcd (%) for
C19H17NO3 : C 74.25, H 5.58, N 4.56; Found: C 73.97; H 5.61; N 4.38.
Preparation of complex 1
A mixture of Cd(OAc)2·2H2O (5.4 mg, 20 µmol), HL (2.4 mg, 7.8
µmol), DMF (0.5 mL), and EtOH (1.5 mL) in a capped vial was heated at
80 °C for three days. Yellow block-like crystals of 1 were collected,
washed with EtOH, and dried at 60 °C. Yield: 2.5mg (81 %, based on
HL); IR (KBr, cm-1): 3431.11(m), 1625.9(m), 1578.36(m), 1553.04(m),
1504.15(m), 1477.86(m), 1445.04(s), 1424.34(s), 1372.34(m),
1339.00(m), 1304.56(m), 1276.92(s), 1222.47(m), 1100.59(m),

1072.31(m), 1006.81(m); Anal. Calcd (%) for C118H102Cd4N6O22 : C
58.91, H 4.27, N 3.49; Found: C 58.94, H 4.26, N 3.51.
Preparation of complex 2
A mixture of CdI2 (7.3 mg, 27 µmol), HL (2.4 mg, 7.8 µmol), DMF
(0.5 mL), and MeOH (30 drops) in a capped vial was heated at 80 °C for
two days. Orange-yellow block-like crystals of 2 were collected, washed
with MeOH, and dried at 60 °C. Yield: 1.4 mg (86 %, based on HL); IR
(KBr, cm-1): 3440.39(m), 1596.40(m), 1578.32(m), 1550.39(m),
1503.95(m), 1443.84(s), 1424,83(s), 1371.62(m), 1337.49(m),
1303.06(m), 1276.05(s), 1099.09(m), 1071.26(m); Anal. Calcd (%) for
C57H48Cd2 I N3 O9 : C 53.88, H 3.81, N 3.31; Found: C 53.84, H 3.79, N
3.35.
Preparation of complex 3
A mixture of Zn(NO3)2·6H2O (9 mg, 30 µmol), HL (4.8 mg, 15.6 µmol),
DMF (0.5 mL), and EtOH (40 drops) in a capped vial was heated at 65 °C
for three days. Yellow-green block-like crystals of 3 were collected,
washed with EtOH, and dried at 60 °C. Yield: 4.6 mg (78 %, based on
HL); IR (KBr, cm-1): 3437.02(m), 1627.18(m), 1598.13(m), 1579.07(m),
1557.89(m), 1503.85(m), 1478.87(s), 1448.14(s), 1380.42(s), 1335.24(s),
1274,97(s), 1223.83(m), 1102.72(s), 1071.61(s), 1006.00(m); Anal. Calcd
(%) for C114 H99 N8 O26 Zn4 : C 60.62, H 4.42, N 4.96; Found: C 60.71, H
4.37, N 5.03.

Crystal data collection and refinement
The single crystal X-ray data of the three polymers were collected on
a Bruker APEX area-detector X-ray diffractometer with MoK radiation
(λ = 0.71073 Å). The raw data frames were integrated with SAINT+,
which were also corrected for Lorentz and polarization effects.
Absorption corrections were applied by using the multi-scan program
SADABS [18]. The respective structures were solved by direct methods,
and refined with a full-matrix least-squares technique based on F2 (G. M.
Sheldrick, SHELXTL97, program for crystal structure refinement,
University of Göttingen, Germany, 1997). The non-hydrogen atoms were
refined anisotropically and the H-atoms were geometrically included in
the final refinement. Crystallographic crystal datas and structural
parameters for 1-3 are given in table 1, whereas the selected bond
distances and angles of 1-3 are presented in table S1-S3. The CCDC
numbers of 1-3 are 1402718, 1402719 and 1402720, respectively.
Table 1 Crystallographic data and structure refinement details for 1- 3
1
2
3
C118 H102 Cd4 N6 O22
C114 H96 Cd4 I2 N6 O18
C114 H99 N8 O26 Zn4
Empirical formula
Mr /g mol–1
T /K
2405.66
293(2)
2541.37
298(2)
2258.49
296(2)
Crystal system
Space group
a/Å
Monoclinic
P21/c
Triclinic
P1
Monoclinic
C2/c
13.5052(16)
22.217(2)
20.391(2)
90
12.4248(13)
14.9354(16)
29.935(3)
89.802(2)
82.238(2)
74.6920(10)
27.7095(15)
11.8305(5)
31.9391(17)
90
b/Å
c/Å
o
α
β
γ
/
o
99.090(2)
90
91.548(3)
90
/
o
/

V/Å3
6041.4(11)
2
5305.8(9)
2
10466.4(9)
4
Z
1.322
6.105
1.591
1.442
27131
18365
0.1226
1.033
0.0966
0.1829
1.433
0.987
33688
9427
ρ
c /g cm–3
/mm–1
µ
Reflections collected 21099
Unique reflections
10465
0.1322
1.011
Rint
0.0575
1.004
0.0434
0.1208
S
R1a (I > 2σ(I))
wR2b (all data)
0.1337
0.2758
Results and discussion
Spectroscopic properties
In order to investigate the interaction mode of the HL ligand with
the Cd2+ ion, the coordination reaction of HL with the Cd(OAc)2 was
monitored through a UV-vis spectroscopic titration. The absorption bands
of HL in MeOH dramatically changed upon the addition of the Cd2+ ions.
As depicted in Fig.1a, the band of 1 showed a significant bathochromism
shift from 297 to 311 nm. The peak at 338 nm was found to become weak
and disappeared in the presence of increasing amounts of the Cd2+ ion.
The new band at 424 nm was attributed to the charge transfer from the
metal to ligand band. No differences were observed in the absorption
spectra after the 2 : 3 ratio of Cd2+/HL, so this experiment clearly
indicates the formation of (Cd2L3)n. By following the same procedure, the
formation of (Cd2L3)n and (Zn2L3)n for 2 and 3 was also determined by
UV-vis spectroscopic titrations (Fig. 1b and 1c).

Fig. 1 UV-vis titrations of HL (0.1 mM) in methanol with Cd(OAc)2 (a), CdI2 (b) and
Zn(NO3)2 (c), respectively. Each spectrum was acquired 5 min after the Cd2+addition.

Fig. 2 Fluorescence emission changes of HL in methanol solutions upon addition of
Cd(OAc)2(a), CdI2(b) and Zn(NO3)2(c), respectively.
Fig. 3 Fluorescence emission spectra of HL in different solvents
Photophysical properties
Results from the UV-vis spectrophotometric titrations were
corroborated by the fluorescence spectroscopic titration. Fluorescence
photographs were obtained under irradiation 340 nm. As for ligand HL,
addition of Cd2+ produced a clear growing orange band at 598 nm and
emission band at 469 nm decreased simultaneously (Fig. 2a). By

following the same procedure, the formation of (Cd2L3)n and (Zn2L3)n for
2 and 3 was also determined by fluorescence spectroscopic titration (Fig.
2b and 2c).
We also investigated the steady-state spectroscopic properties of
ligand. This strengthened our understanding and arguments regarding the
relaxation behavior of the system in the same kind of media, in which the
dynamical properties have been investigated. Figure 3 showed the
fluorescence emission spectra of ligand in the 400-550 nm region in
seven nonaqueous solvents, namely n-hexane, methylene chloride,
tetrahydrofuran, ethanol, methanol, N,N-dimethyl formamide and
acetonitrile. From the figure 3, with increase of polarity of the solvent,
fluorescence emission of the ligand showed significant red-shift. In
nonpolar aprotic n-hexane, the fluorescence emission intensity was
weakest and the wavelength was shortest. The reason for this
1
phenomenon may be that the n-π* state and π-π* state of ligand were
1
very similar and the lowest excited singlet belongs to n-π* in the
hydrocarbon solvent. But in polar solvent (protic or aprotic ), the
fluorescence emission of ligand was enhanced, probably because of
1n- π* state and π-π* state reversed, π-π* state became the lowest excited
singlet [26,27]. In addition, the emission peak of ligand in both ethanol
and methanol solvent was found to move, which may be attributed to the
hydrogen bond interaction between ligand molecule and solvent

molecules.
To confirm that the crystal structures of complexes 1–3 were truly
representative of their bulky materials as employed in the photochemical
studies, powder X-ray diffraction (PXRD) experiments were carried out
on the resultant samples. The simulation curves were to simulate the cif
into XRD by mercury software. As shown in figure 4, the peak positions
of the experimental pattern was in good agreement with its simulated
PXRD pattern, respectively. The differences in intensity may be due to
the preferred orientation of the crystal samples. The luminescence
behavior of 1-3 and the corresponding free HL ligand were investigated
in the solid state at room temperature (Figure 5). The free HL ligand
shows a weak emission band at λmax= 452 nm upon being excited at 346
nm, whereas complexes 1-3 exhibit photoluminescences with maximum
emission peaks at 575, 590 and 545 nm, respectively. Relative to that of
the free ligand, the bright and extensive yellow emissions of complexes
1-3 predominantly originate from a ligand-to-metal charge-transfer
(LMCT) transition and exhibit a remarkable red-shift emission, which
may be ascribed as the incorporation of Zn (II) and Cd(II) effectively
increasing the conformational rigidity of the ligand and decreasing
HOMO-LUMO energy gaps of those compounds [28]. Meanwhile, the
enhancement of emission intensity for the Zn(II) complex was probably
due to inhibition of PET (photoinduced electron transfer) process, as the

lone pairs involved in PET process get localized due to relatively strong
interaction with the Zn(II). With Cd(II), where the metal ligand bonds
were much longer compared to those of the Zn(II) complex, the lone pairs
are less effectively localized [29]. Furthermore, differences in the optical
properties of 1-3 are probably attributed to the different central metal ions
or anion effects. The enhanced fluorescence also showed corresponding
asignificant changes in emission lifetime, as illustrated in Fig. 6 (1.91,
1.83, 5.15 ns for 1, 2, 3, respectively). In addition, we study the
fluorescence properties of complexes 1-3 in ethyl acetate. As shown in
Fig. 7, the fluorescence emission intensities of these complexes were also
relatively stronger than the HL ligand and there were almost no apparent
emission peaks of the HL ligand, which indicates that the three
compounds possess stable structures.
Fig. 4 PXRD patterns of complexes 1-3
Fig. 5 Emission spectra for HL, 1, 2 and
3 in the solid state at λex = 346 nm.

Fig. 6 Time-resolved fluorescence decay of 1-3 Fig. 7 The one-photon fluorescence
of 1-3
spectra of HL, 1, 2, 3(1µM in
ethyl acetate) at λex =380.
Thermal stability
The thermal stability of complexes 1-3 was carried out by
thermogravimetric analysis (TGA) from 27 to 1000 °C under a nitrogen
atmosphere with a heating rate of 10 °C·min−1, as shown in Fig. 8. For
complex 1, there was no obvious change until 311 °C. With further
heating, the first weight loss of 22.48 % from 311 to 386 °C. This may be
due to the loss of one Cd (II) atom, one L ligand and two acetate anions
(calcd: 22.30 %). The second mass loss occuring from 386 to 651 °C was
17.26 % and was assigned to the decomposition of one organic ligand L
and one Cd (II) atom (calcd. 17.39 %). Then the backbone of complex 1
started to decompose gradually with the increasing temperature. TGA
profile of complexes 1 and 2 was nearly comparable. However, for 2, a
small weight loss about 2.22 % was observed below 185 °C, which can
be attributed to the loss of the remaining DMF. The next weight loss of
21.40 % occurred from 257 to 405 °C could be attributed to the removal

of one Cd (II) atom, one L ligand and one iodine atom (calc. 21.45 %).
The further weight loss about 5.08 % until 534 °C was observed,
attributable to the loss of the other coordinated iodine atom. Then the
framework of compound 2 began to collapse gradually with the
increasing temperature. For complex 3, the first weight loss of 19.13 %
(calc. 19.05 %) occurred at around 289 °C might be attributed to the
removal of one L ligand and two nitrate anions. Next, the backbone of
compound 3 started to collapse with the residue of about 16.66 % (calc.
16.44 %), corresponding to one L ligand and one Zn (II) atom.
Fig. 8 TGA scans for 1-3
Structural description
Structure of [Cd4(L)6(OAc)2](1).
Single crystals of coordination polymer 1 were readily obtained in

good yield by heating Cd(OAc)2 and HL in the mixture of DMF/EtOH.
X-ray diffraction analysis reveals that complex 1 crystallizes in the
monoclinic space group P21/c with Z = 2. The asymmetric unit contains
two Cd(II) atoms, three L ligands and one coordinated acetate anion and
it can be seen that there are two crystallographically independent Cd(II)
atom (Fig. 9a). Cd1 is six-coordinated to four phenolato oxygen atoms
(one O1, one O7, and two O4) and two nitrogen atoms (N1 and N3) from
four L ligands displaying a distorted octahedron geometry. Cd2 is also
six-coordinated with a distorted octahedron geometry containing O1, O4,
O7 and N2 from three L ligands, and the other two coordination atoms
for Cd2 are O10 and O11 from one acetate anion. The bond lengths and
angles around Cd(II) are 2.35(2)–2.38(2)
Å
for Cd–N,
2.173(15)–2.430(15)Å for Cd–O, 55.8(6)–169.9(6)° for O–Cd–O,
74.0(7)–146.3(7)° for O–Cd–N and 102.4(7)° for N–Cd–N, respectively.
However, the bridging of the six oxygen atoms of the six L ligands makes
2Cd1 and 2Cd2 involved in a double-open cubane-like unit(Cd4O6)(Fig.
9a).
There are intramolecular nonclassical C–H···O (between the oxygen
atoms of acetate anion and the ethenyl group, C–H···O = 2.374–3.477Å,
12.494–67.142°) hydrogen bondings or C–H···π (between the –CH3
group of acetate anion and the adjacent dimethoxyphenyl ring, C–H···π =
3.688Å) interactions, which play a vital role in the construction of the

tetranuclear Cd(II) units (Fig. 10a). As shown in Fig. 10c, through
C–H···O = 2.750–3.094Å, the neutral molecular units in 1 are linked into
a 2-D network structure in ab plane, giving a 3-D supramolecular
structure (Fig. 10d) via an interlayer hydrogen bond C–H···O (between
the C–H group of the dimethoxyphenyl ring and O atoms of –OCH3
groups in another adjacent HL ligand, 2.750–2.817Å) interaction (Fig.
10b), which also generate a topological structure of 1 in bc plane(Fig.
10f), which can be simplified into a ball-stick space array, that is, each
purple ball represent for a tetranuclear “diamond unit” of 1, which is
considered as a node, and each brown stick represent for C–H···O
intermolecular interactions(Fig. 10e).
(a)
(b)
(c)

Fig. 9 Perspective drawing of 1(a), 2(b), 3(c) showing the local coordination
environment around Cd(II) and Zn(II) (H atoms omitted for clarity).
(e)
(f)
(a)
(b)
(c)
(d)

Fig. 10 (a) The intramolecular C–H···O and C–H···π interactions of complex 1; (b)
the 2D network of 1 mediated by the nonclassical C–H···O hydrogen bonds in bc
plane; (c) The 2D net structure through the nonclassical C–H···O hydrogen bonds in
ab plane; (d) the 3D network of 3; (e) Space array of a “diamond unit” of 1 in bc
plane ; (f) Topological structure of 1 in bc plane. Hydrogen atoms have been omitted
for clarity. Color codes: green, Cd; red, O; blue, N; gray, C.
Structure of [Cd4(L)6I2](2)
The crystal structure analysis indicates that 2 crystallizes in the
triclinic P1 space group. The crystal structure of 2 is similar to complex 1,
but the asymmetric unit contains two Cd(II) atoms, three L ligands and
one iodine atom insteading of one coordinated acetate anion in 1. As
shown in Fig. 9b, there are also two crystallographically independent
Cd(II) in the tetranuclear centrosymmetric complex 2. The two Cd1
centers adopt a distorted octahedral geometry with the equatorial plane
occupied by the NO3 donors of three L ligands and the apical position
occupied by one phenol oxygen atom and one pyridyl nitrogen atom of
two ligands, respectively. While the two terminal Cd2 atoms are
five-coordinated by one iodine atom, one nitrogen atom and three
hydroxyl oxygen atoms from the three L ligands. The bond lengths and
angels around Cd(II) are 2.281(15)–2.370(13)Å for Cd–N,
2.186(12)–2.560(9)Å for Cd–O, 73.2(4)–167.3(4)° for O–Cd–O,
70.1(4)–152.9(4)° for O–Cd–N and 98.0(4)° for N–Cd–N, respectively.
Similar to complex 1, six oxygen atoms of six ligands HL link 2Cd1 and
2Cd2 into a double-open cubane-like unit(Cd4O6) (Fig. 9b).
There are intramolecular C–H···I (between the cooperated iodine

atom and the C–H group of ethenyl, 3.264Å) and π···π interactions
(face-to-face distance of 2,3-dimethoxyphenyl ring: 3.824Å) in 2(Fig.11
a), which contribute to the construction of the tetranuclear Cd(II) units.
As shown in Fig.11b, via C–H···O(2.642–3.056Å) and C–H···I (3.125Å)
intermolecular interactions, the neutral molecular units in 2 are linked
into 2D layers in the ab plane (Fig. 12). The two planes formed by the
cooperated iodine atoms are parallel to the ab plane.
(a)
(b)
Fig. 11 (a) Two types of intramolecular interaction π···π(dashed red lines) and
C–H···I(dashed yellow lines) in compound 2. (b) the 2D supramolecular structure of
2 mediated by C–H···O and C–H···I interactions
Fig. 12 2D layers in the ab plane of 2

Structure of [Zn4(L)6(NO3)2](3)
X-ray single crystal diffraction analysis shows that 3 crystallizes in
the monoclinic crystal system of C2/c space group, with one half of a
formula unit in the asymmetric unit, that is, two Zn(II) atoms, three L
ligands and one coordinated nitrate anion. As shown in Fig. 9c, there
exists to be two crystallographically independent Zn(II) in the
tetranuclear centrosymmetric complex 3. Zn1 is six-coordinated to four
phenolato oxygen atoms ( two O1, one O7, and one O4) and two nitrogen
atoms (N2 and N3) from four L ligands displaying a distorted octahedron
geometry. Zn2 is also six-coordinated with a distorted octahedron
geometry. In addition to O1, O4, O7 and N1 from three L ligands, the
other two coordination atoms for Zn2 are O10 and O12 from one nitrate
anion. The bond distances and angels around Zn(II) are 2.10(2)–2.20(2)Å
for Zn–N, 1.992(19)–2.215(19)Å for Zn–O, 73.2(4)–167.3(4)° for
O–Zn–O, 70.1(4)–152.9(4)° for O–Zn–N and 98.0(4)° for N–Zn–N,
respectively.
There exists weak non-classical C–H···O intramolecular hydrogen
bonds in the complex 3. As shown in Fig. 13a, the intramolecular
interactions between the oxygen atoms(nitrate anion) and the ethenyl
group(C–H···O = 2.586–3.254Å) play a critical role in the consolidation
of the tetranuclear structure. Through C–H···O(2.751–3.342Å)
intermolecular interactions, the neutral molecular units in 3 are linked

into 2D layers in the ab plane (Fig. 13c), which are further assembled into
a 3D supramolecular network (Fig. 13d) via an interlayer hydrogen bond
C–H···O (between the oxygen atoms (nitrate anion) and the –CH3 group
(methoxyl) of the adjacent HL, 3.005–3.038Å) interaction (Fig. 13b) ,
contributing to the additional stability of the structure.
(a)
(b)
(c)
(d)
Fig. 13 (a) Perspective views of the intramolecular C–H···O hydrogen bondings in 3;
(b) View of the 1D supramolecular chain fabricated via C–H···O hydrogen bondings;
(c) the 2D network of 3 mediated by C–H···O interactions; (d) the 3D network of 3.
Conclusion
In conclusion, one unique ligand HL containing 2-substituted-
8-hydroxyquinoline were synthesized, and three double cubane

tetranuclear complexes 1-3 were prepared by solvothermal reactions. The
aggregation behavior of zinc or cadmium salt and ligands in different
solution was investigated by several techniques, including UV-vis and PL.
We have shown that the anion of the metallic salt solution take part in
coordination and lead to different crystal system and space group of
complexes 1-3. As a result, there are obviously different to the absorption
wavelength, the emission wavelength, fluorescence lifetime, and
thermostability of complexes. In addition, the polarity of the solvent
effect fluorescence emission of the ligand. The photophysical properties
of complexes 1-3 correlate with the values of the HOMO and LUMO of
the respective substituents calculated. This strategy offers a powerful tool
for the preparation of complexes with predictable photophysical
properties. These complexes have potential applications in the OLED.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (61671162 and 21372051) and Technology Plan of
Guangdong Province (2016A010103031).
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