Synthesis, crystal structures and luminescent properties of tetranuclear Zn molecular clusters with aroylhydrazone ligand

Article abstract of DOI:10.1039/c3ce41034j

Two kinds of tetranuclear Zn(ii) molecular clusters with the designed aroylhydrazone Schiff base ligand (H₃L), [Zn₄(HL) ₄]·3(DMF)·3(H₂O) (1) and [Zn ₄(HL)₂(CH₃COO)₄]·2

Full text of DOI:10.1039/c3ce41034j

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COMMUNICATION
Synthesis, crystal structures and luminescent
properties of tetranuclear Zn molecular clusters with
Cite this: CrystEngComm, 2013, 15, 8069
aroylhydrazone ligand3
Received 6th June 2013,
Accepted 5th August 2013
a
a
a
a
a
Bei-bei Tang, Heng Ma, Guan-zheng Li, Yue-bing Wang, Gulinazi Anwar,
DOI: 10.1039/c3ce41034j
b
a
Rufei Shi and Hui Li*
www.rsc.org/crystengcomm
Two kinds of tetranuclear Zn(II) molecular clusters with the
designed aroylhydrazone Schiff base ligand (H L), [Zn (HL) ]?
(DMF)?3(H O) (1) and [Zn (HL) (CH COO) ]?2(DMF)?2(CH OH)
2), have been synthesized and studied. Molecular cluster 1
displayed a Zn boat-shaped core and 2 presented a discrete
often applied in the construction of molecular clusters combining
the two classes of ligands above.
9
3
4
4
3
2
4
2
3
4
3
We have designed and synthesized an aroylhydrazone Schiff
10
(
base ligand. A preliminary study of its transition metal
11
4
O
4
complexes and a few related literature reports have indicated
that this kind of ligand possesses the advantage of constructing
polynuclear complexes. To extend our research, another aroylhy-
drazone Schiff base ligand, (E)-N9-(2,3-dihydroxybenzylidene)-4-
linear tetranuclear Zn(II) structure. The luminescent properties of
the two molecular clusters have been investigated, which exhibit
a wide range of photo-luminescence in different organic solvents.
hydroxybenzohydrazide (H L), was designed and synthesized
3
(
Scheme 1). On comparison with the ligand in our previous
10
work, an additional hydroxyl group was introduced at the
-position of the benzylidene part, which provided suitable,
Introduction
3
Molecular clusters have received a significant amount of attention
for their structural diversity resulting from their metal-rich nature
and chemical stability combined with their fascinating potential
disposable bridging groups to control the stereochemistry of the
metallic centres as well as the number of metal ions within the
cluster motifs. Two kinds of tetranuclear Zn(II) molecular clusters
have been constructed by this ligand successfully. Interestingly,
complex 1 is the first example of a supramolecular diamondoid
structure with a boat-shaped tetranuclear molecular cluster as its
node.
1–3
application in advanced materials.
An increasing number of
4
geometrically intriguing molecular clusters, for example, linear,
5
6
7
olive, wheel, helical, cubane, rectangular, chair and boat
conformations, have been successfully obtained. The structures
of molecular clusters are dependent upon the chemical structures
of the ligand, metal ions, anions, pH value, metal-to-ligand ratio
Results and discussion
1–3,8
and solvents.
The most important factor among these that
controls molecular clusters are ligands with suitably disposed
bridging groups. Generally, there are two main classes of ligands:
one is ligands with appended potentially endogenous bridging
groups linking metal ions in a closed-cluster system (e.g. cubane,
Synthesis
As shown in Scheme S2, ESI,
3
the H
clusters are HL anions, in which the two hydroxyl groups of the
,3-dihydroxybenzaldehyde part are deprotonated. In the synthesis
3
L ligands in both molecular
22
2
8
rectangular, chair, boat cluster); the other is large ligands with
of 1, sodium hydroxide is necessary to deprotonate the hydroxyl
arrays of coordination pockets suitably organized to create specific
geometric arrangements of metal centres through bridging
connections, which can be assembled with metal ions in an
4
open-cluster (e.g. linear). The employment of auxiliary ligands is
a
Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry,
Beijing Institute of Technology, Beijing 100081, P. R. China.
E-mail: lihui@bit.edu.cn; Fax: +86-10-68912667
b
Department of Chemistry, Loras College, Dubuque, Iowa 52001, USA
3
Electronic supplementary information (ESI) available: Crystal data, XRD, TG data,
IR, UV-vis, PL, bond lengths (Å) and angles (u). CCDC 877843 and 877844. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
C3CE41034J
Scheme 1 Structural formula for (E)-N9-(2,3-dihydroxybenzylidene)-4-hydroxy-
3
benzohydrazide, H L.
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group of the ligand, while in the corresponding reaction of the
synthesis of 2, it is not necessary to add a base since zinc acetate is
used instead of the zinc perchlorate in 1. Clearly, the acetate is a
ditopic ligand that can act as a base and a bridging ligand at the
same time. Inspired by the different synthesis conditions, 1 and 2
are both tetranuclear Zn clusters. Therefore, the supply of an
3
excess hydroxyl substitute on the ligand H L is an effective strategy
to construct molecular clusters. Meanwhile, the ratio of M : L, the
pH value and the auxiliary ligand play an important role in
controlling the structures of the molecular clusters in this work.
22
The ligation of hydroxyl groups in the HL ligand has different
coordination modes in 1 and 2 (Scheme S3, ESI ), which gives a
3
distinctive core structure of 1 and 2.
Crystal structure of 1
The reaction of Zn(ClO
DMF–CH OH–H O (1 : 3 : 2 volume ratio) in the presence of
NaOH produces yellow crystals of [Zn (HL) ]?3(DMF)?3(H O) (1) in
a 65% yield, the molecular structure of which was determined by
single-crystal X-ray diffraction, as depicted in Fig. 1a. The
4 2 2 3
) ?6H O with H L (1 : 1 molar ratio) in
Fig. 2 (a) The H-bonding and p–p stacking interaction linking the molecule of 1
…
3
2
with four neighbouring molecules (O8–H8 O2: green dashed line; O4–
…
4
4
2
H8 O6: pink dashed line; p–p stacking interaction: yellow dashed line); (b) the
structural representation of the distorted tetrahedral geometry of the molecule
linked with adjacent molecules; (c) the topological structure of the two-fold
interpenetrating diamondoid framework of 1.
molecular cluster 1 crystallizes in the tetragonal space group P ¯4 .
The coordination geometry of the Zn(II) centre is a distorted
12
squared-based pyramid (t = 0.23). Each ligand coordinates to a
Zn atom with the imine-nitrogen, carbonyl-oxygen and phenoxo-
oxygen atom and bridges a neighbouring Zn atom through its
1
3
sustained through coordination bonds directly. Careful exam-
ination of the crystal structure reveals that molecular cluster 1
exhibits a two-fold interpenetrated framework. The driving force to
22
phenolate group (m -O1). The HL
ligands display the
chelating–bridging mode, as shown in Scheme
Four ligands link four Zn(II) centres to form an eight-
membered ring which is folded in a boat conformation (Fig. S2,
ESI ). Thus, 1 is an unusual tetranuclear cluster with a boat-
shaped Zn core having two pairs of parallel ligand strands
arranged in a head-to-tail fashion. To best of our knowledge, this
2
…
1
2
1
1
generate the two-fold interpenetration is conventional N–H O
H-bonding, which is formed between the lattice water molecules
and the nitrogen atoms from the ligands (Fig. S4, ESI ).
3
g :g :g :g :m
S3a, ESI.
2
3
3
Crystal structure of 2
The reaction of Zn(CH
CH OH–DMF (2 : 2 volume ratio) produces yellow crystals of
Zn (HL) (CH COO) ]?2(DMF)?2(CH OH) (2) in an 82% yield, the
4 4
O
3
2 2 3
COO) ?2H O with H L (4 : 1 molar ratio) in
8e
is the second example of a boat tetranuclear cluster, while most
3
8
of the clusters are rectangular or cube-core structures. The
[
4
2
3
4
3
peripheral O–M–O angles are ca. 105.1u and 111.7u, therefore the
molecular structure of which was determined by single-crystal
X-ray diffraction, as depicted in Fig. 1b. The molecular cluster 2
crystallizes in a triclinic space group and is linear. Two
asymmetrical units [Zn (HL)(CH COO) ] in 2 are bridged through
overall arrangement of the [Zn
geometry (Fig. S3a, ESI ). The Zn Zn separations in 1 are
.4534(11) and 3.7167(2) Å, respectively. Taking the hydrogen-
bonding function of H L into account, [Zn (HL) ] is a self-
4 4
(HL) ] unit is close to tetrahedral
…
3
3
2
3
2
2
2
3
4
4
the hydroxyl group at the 3-position in the HL ligand. In each
22
complementary moiety in which the uncoordinated oxygen atoms
in the terminal –OH groups in the ligands form strong H-bonding
with coordinated –OH groups from an adjacent moiety (Fig. 2a
and 2b). These moieties are self-assembled through the
H-bonding into a 3D diamondoid network (Fig. 2c). It was noted
that many diamondoid networks of coordination complexes are
asymmetrical unit, the HL ligand coordinates to two Zn atoms
with imine-nitrogen, carbonyl-oxygen and bridging phenoxo-
oxygen atoms combined with two bridging acetate ligands (syn–
syn mode). The Zn1–Zn2 distance in the asymmetrical unit is
3.2805 Å and that of Zn2–Zn2# between the two asymmetrical
22
units is 3.1576 Å. The coordination mode of the HL ligand in 2
2
2
1
1
displays a g :g :g :g :m
3
chelating–bridging mode (Scheme S3b,
ESI ), which differs from what is observed in 1. The Zn(II) centres
3
are also five coordinated with slightly different coordination
environments of Zn1–NO4 and Zn2–O5, respectively. Alternatively,
the coordination geometry of Zn(II) in 2 can be regarded as an
intermediate between a squared-based pyramid and a trigonal
bipyramid (Zn(1): t = 0.48; Zn(2): t = 0.49). As expected, the ligand/
acetate binary ‘‘blend’’ strategy in 2 means the molecular structure
of 2 differs from that of 1. The acetate anion acts as a bridging
14
ligand in the [Zn
2 3 2
(HL)(CH COO) ] fragment. Based on the
fragmentation info, 2 can be classified as a dimer-of-dimers
15
Fig. 1 View of the coordination environments of Zn and the ligands in (a) 1 and
structure. The linear Zn
4 2
(HL) core is almost coplanar (Fig. S5,
(
b) 2. Solvent molecules and hydrogen atoms are omitted for clarity.
ESI ). The coordinated acetate ligands are located up and down
3
8
070 | CrystEngComm, 2013, 15, 8069–8073
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Conclusions
Based on this ‘‘key’’ ligand, two tetranuclear Zn(II) molecular
clusters have been synthesized successfully. Molecular cluster 1 is
a boat-shaped, closed-cluster and assembled into a diamondoid
supramolecular framework through strong hydrogen bonding,
which is quite unusual. Molecular cluster 2 is a linear open-
cluster. The UV-vis and photoluminescent properties of both
clusters were characterized in the solid state and ethanol. Within a
wide range of wavelengths, the clusters can emit different colours,
implying the solvent polarity and H-bonding donor (HBD) has an
effect on their fluorescence properties.
Experimental
Materials and instrumentation
All the starting materials and solvents used in this work were
commercially available and of analytical grade from Alfa Aesar
Chemical Company without further purification. Organic solvents
of analytical purity were supplied by commercial sources and were
used as received. Elemental analyses (C, H, N) were performed on
a Flash EA1112 microanalyzer at the Beijing Institute of
Technology. The FT-IR spectrum was recorded in a Nicolet-360
3
Fig. 3 (a) UV-vis spectra and (b) normalized emission spectra of the ligand H L,
1
and 2 in the solid state at room temperature.
21
FT-IR spectrometer as KBr pellets in the 4000–400 cm region.
The UV-vis absorption spectra were examined on a JASCO UV-1901
spectrophotometer in the wavelength range of 200–800 nm. The
photoluminescent (PL) spectra were recorded by a Hitachi F-7000
luminescence spectrophotometer equipped with a 450 W xenon
lamp as the excitation source. Measurements were collected at
room temperature. The photomultiplier tube voltage was 700 V,
the plane. The molecules are linked from a 1D chain through
unconventional H-bonding between the acetate anions (Fig. S6,
3
ESI ). Then, the 1D chain is extended to a 2D layer by hydrogen
bonding between solvent CH OH and the amide nitrogen atom
3
located in the chain (Fig. S7b, ESI
3
).
Spectroscopic properties of H L, 1 and 2
21
3
the scan speed was 1200 nm min and the slit width was 2.5 nm
Electronic absorption spectra of H
absorption spectra of molecular clusters 1 and 2 measured on a
3
L, 1 and 2. In the
(5.0 nm). Thermogravimetric analyses (TGA) were carried out on a
SEIKO TG/DTA 6200 thermal analyzer from room temperature to
800 uC at a ramp rate of 10 uC min in a flowing 150 mL min
21
21
BaSO plate, three peaks at 233, 309 and y425 nm for 1, and 240,
4
nitrogen atmosphere. X-ray powder diffraction (XPRD) of the
samples were measured using a Rigaku D/max c A X-ray
diffractometer equipped with graphite-monochromatized Cu–Ka
radiation (l = 0.154060 nm). ESI-mass spectra were recorded on a
3
13 and y425 nm for 2, respectively, are observed. The H
on an BaSO plate is observed to absorb at 234 and 322 nm. The
33 and 309 nm absorptions for 1 (240 and 313 nm for 2) may be
3
L ligand
4
2
assigned as p–p* charge transfer, similar to the 234 and 322 nm
peaks of the free ligand, and the relatively lower peak (y425 nm)
is a result of MLCT transition (Fig. 3a).
Photoluminescent properties of H L, 1 and 2. Zinc complexes
with Schiff base ligands are known for their fascinating
luminescence properties, with emission bands currently located
between 390 and 590 nm. The photoluminescent behaviours of
1
13
Bruker Apex IV FTMS. HNMR and C NMR spectra were
16
recorded on a Bruker ARX400 spectrometer (400 and 75 MHz,
6
respectively) instrument in DMSO-d with Me
4
Si as the internal
3
standard.
17
X-ray crystallographic refinement details
1
, 2 and the free ligand H L were studied in the solid state at room
3
Suitable single crystals with dimensions of 0.16 6 0.16 6 0.10
mm and 0.42 6 0.18 6 0.10 mm for molecular clusters 1 and 2
were selected for single-crystal X-ray diffraction analysis, respec-
tively. Diffraction intensities for the two molecular clusters were
collected on a Rigaku RAXIS-RAPID CCD diffractometer equipped
with graphite-monochromatic Mo–Ka radiation (l = 0.71073 Å)
using the v scan mode at 153 K. The diffraction intensity data
were collected in the h range of 3.00–27.47u for 1 and 2.72–25.02u
for 2. The collected data were reduced using the SAINT software
and empirical absorption corrections were performed using the
temperature (Fig. 3b). Under l = 309 nm excitation, the emission
for molecular cluster 1 is at 556 nm, accompanied by a shoulder
peak at 402 nm. Compared to the emission of the free ligand (lem
=
471 and 550 nm, lex = 322 nm), the peaks at 402 and 556 nm can
be attributed to ligand-to-metal charge transfer (LMCT) and p–p*
intraligand (IL) transitions of the ligand. Molecular cluster 2
exhibits green luminescence with a peak maximum centered on
lem = 540 nm when lex = 312 nm. The electronic absorption
spectra and photoluminescence properties of the ligand H
and 2 in solution were studied to further understand the solution
behaviour (section 6, ESI ).
3
L, 1
18
SADABS software. The two structures were solved by direct
19
3
methods and refined using full-matrix least square techniques
2
18
on F (ref. 20) with the program SHELXL-97. All non-hydrogen
atomic positions were located in difference Fourier maps and
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2
1
4
3
8.28; H, 4.11; N, 9.57. Selected IR (KBr pellet, cm ): n(Ar–O) 3186,
Table 1 Crystal data of molecular cluster 1 and 2
+
048 (w); n(CLO) 1656 (s); n(CLN) amide 1607, 1584 (m).
Synthesis of the molecular cluster [Zn (HL) (CH
(DMF)?2(CH OH) (2). Zn(CH COO) ?2H O (44 mg, 0.2 mmol) in
1
2
3 4
COO) ]?
4
2
2
3
3
2
2
Empirical formula
C
65
H
67
N
11
O
22Zn
4
C
44 54 6 4
H N O20Zn
4
mL of methanol was added to a 10 mL CH
3
OH–DMF mixture
Formula weigh
Crystal system
Space group
a (Å)
b (Å)
c (Å)
1615.86
Tetragonal
P¯4
14.789(2)
14.789(2)
17.044(3)
90
90
90
3727.9(11)
2
153.1500
1.439
1.350
1658
3.00, 27.47
1.089
0.0544
0.0641, 0.1685
0.0848, 0.182
0.960, 20.489
1248.41
Triclinic
P¯1
(2 : 3 volume ratio) of H
3
L (14 mg, 0.05 mmol) with continuous
stirring ca. 30 minutes. The resultant solution was then filtered and
left at room temperature as a transparent yellow solution. Yellow
single crystals suitable for X-ray analysis were produced by slow
evaporation for one day. Yield: 82%. Anal. calcd for
10.508(2)
10.684(2)
12.276(3)
112.10(3)
104.75(3)
90.29(3)
1671.0(6)
1
153.1500
1.690
0.981
640
2.72, 25.02
0.981
a (u)
b (u)
44 54 6 4
C H N O20Zn : C, 42.33; H, 4.36; N, 6.73. Found: C, 42.28; H,
c (u)3
21
4.35; N, 6.77. Selected IR (KBr pellet, cm ): n(Ar–O) 3304, 3204 (w);
V (Å )
+
Z
n(CLO) 1658 (s); n(CLN) amide 1608, 1588 (m); n as (CO2) 1548 (m);
n as (CO2) 1383 (m).
T (K)
2
3
D
c
(mg m
)
2
1
m (mm
F(000)
)
Acknowledgements
h
S
min, hmax (u)
We thank the National Science Council of the People’s
Republic of China for supporting this research (No.
20771014, 21071018, 21271026) and the Specialized Research
Fund for the Doctoral Program of Higher Education, State
Education Ministry of China (20091101110038).
R
int
0.0282
Final R
, wR
Peak and hole (e Å
1
, wR
(all data)
2
[I . 2s(I)]
0.0263, 0.0630
0.0351, 0.0657
0.321, 20.314
R
1
2
2
3
)
Notes and references
refined anisotropically. Some of the hydrogen atoms were placed
in their geometrically generated positions and other hydrogen
atoms were located from the difference Fourier map and refined
isotropically. The crystallographic data are given in Table 1.
1
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3
CCDC 877843 (1) and CCDC 877844 (2) contain the supplementary
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Synthesis of the H L ligand
An ethanol (20 mL) solution of 2,3-dihydroxybenzaldehyde (1.381
g, 0.01 mol) was added dropwise to a stirring ethanol (20 mL)
solution of 4-hydroxybenzoylhydrazine (1.522 g, 0.01 mmol). The
addition of 1 drop of concentrated hydrochloric acid induced a
color change along with immediate precipitation. Then the
mixture was refluxed for 8 hours with vigorous stirring and
subsequently cooled to room temperature. The gray precipitation
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diethyl ether, and dried in air (Scheme S1, ESI
calcd for C14 : C, 61.76; H, 4.41; N, 10.29. Found: C, 61.69;
H, 4.38; N, 10.32. Selected IR (KBr pellet, cm ): n(O–H) 3328 (m);
3
). Yield: (72%). Anal.
12 2 4
H N O
21
+
1
n(CLO) 1654 (s); n(CLN) amide 1615, 1587 (m). HNMR (400 MHz,
6
DMSO-d ): d (ppm) 8.44 (s, 1H), 7.84 (s, 1H), 7.82 (m, 2H), 6.90 (m,
13
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H), 6.80 (m, 1H), 6.78 (m, 1H), 6.76 (m, 1H), 6.19 (s, 3H). CNMR
6
(75 MHz, DMSO-d ) d (ppm) 162.50, 160.94, 148.37, 146.06, 145.59,
4
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1
29.76, 123.25, 120.21, 119.12, 118.81, 117.29, 115.17. ESI-MS: m/z
+
(100%) 273.1 [M + 1] .
5
Synthesis of the molecular cluster [Zn
4
(HL)
4
]?3(DMF)?3(H
L (28
mg, 0.1 mmol) in 12 ml DMF–CH OH–H O (1 : 3 : 2 volume ratio)
2
O)
5
6
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(
4 2 2 3
1). A solution of Zn(ClO ) ?6H O (38 mg, 0.1 mmol) and H
3
2
was stirred into a methanol solution of NaOH (0.1 mmol) for 15
minutes. The resulting yellow solution was left unperturbed to
allow slow evaporation. Yellow single crystals suitable for X-ray
diffraction analysis were formed after three days. Yield: 65%. Anal.
233–235.
67 11 4
calcd for C65H N O22Zn : C, 48.27; H, 4.15; N, 9.53. Found: C,
8
072 | CrystEngComm, 2013, 15, 8069–8073
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