Synthesis, crystal structure, and spectroscopic properties of a new Zn(II) coordination polymer with 4-(2-picolinylhydrazide) methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster

Article abstract of DOI:10.1080/00958972.2012.690866

A zinc coordination polymer, {[ZnL(H₂O)₂] · 2(H₂O)(CH₃OH)}n (1) (H₂L = 4-(2-picolinylhy- drazide) methylene benzoic acid), consisting of a trapeziform chain was synthesized and characterized by elemental analysis, UV, IR, and single-crystal X-ray diffraction. Single-crystal structure of this compound reveals that the coordination geometry of zinc is a slightly distorted trigonal bipyramid and the trapeziform chains are further assembled into a 3-D network via hydrogen-bonding and π-π interactions. A water-methanol cluster, (H ₂O)₈(CH₃OH)₂, is observed in this complex. The electronic absorption spectrum was attributed to intra-ligand electronic transitions. The spectrum of 1 indicates strong fluorescence at λ_ex = 282 nm in the fluorescence spectrum at room temperature; the fluorescence gradually quenches with rising temperature. With increasing scan rate, the reversible redox process of Zn²⁺/Zn⁺ changes into a quasi-reversible one.

Full text of DOI:10.1080/00958972.2012.690866

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Synthesis, crystal structure, and
spectroscopic properties of a new
Zn(II) coordination polymer with 4-(2-
picolinylhydrazide) methylene benzoic
acid containing the (H2O)8(CH3OH)2
cluster
Shi-Liang Chen a , Zheng Liu a , Jin-Hong Xia b & Yan-Hong Li c d
a College of Chemical and Biological Engineering, Guilin University
of Technology, Guilin 541004, P.R. China
b College of Electronic Engineering, Guilin University of Electronic
Technology, Guilin 541004, P.R. China
c Guangxi Key Laboratory of Environmental Engineering,
Protection and Assessment, Guilin 541004, P.R. China
d College of Environmental Science and Engineering, Guilin
University of Technology, Guilin 541004, P.R. China
Accepted author version posted online: 04 May 2012.Published
online: 22 May 2012.
To cite this article: Shi-Liang Chen , Zheng Liu , Jin-Hong Xia & Yan-Hong Li (2012): Synthesis,
crystal structure, and spectroscopic properties of a new Zn(II) coordination polymer with 4-(2-
picolinylhydrazide) methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster, Journal of
Coordination Chemistry, 65:13, 2234-2246
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Journal of Coordination Chemistry
Vol. 65, No. 13, 10 July 2012, 2234–2246
Synthesis, crystal structure, and spectroscopic properties of a
new Zn(II) coordination polymer with 4-(2-picolinylhydrazide)
methylene benzoic acid containing the
(H2O)8(CH3OH)2 cluster
SHI-LIANG CHENy, ZHENG LIU*y, JIN-HONG XIAz and YAN-HONG LIx{
yCollege of Chemical and Biological Engineering, Guilin University of Technology,
Guilin 541004, P.R. China
zCollege of Electronic Engineering, Guilin University of Electronic Technology,
Guilin 541004, P.R. China
xGuangxi Key Laboratory of Environmental Engineering, Protection and Assessment,
Guilin 541004, P.R. China
{College of Environmental Science and Engineering, Guilin University of Technology,
Guilin 541004, P.R. China
(Received 19 November 2011; in final form 28 March 2012)
A zinc coordination polymer, {[ZnL(H2O)2] ꢀ 2(H2O)(CH3OH)}n (1) (H2L ¼ 4-(2-picolinylhy-
drazide) methylene benzoic acid), consisting of a trapeziform chain was synthesized and
characterized by elemental analysis, UV, IR, and single-crystal X-ray diffraction. Single-crystal
structure of this compound reveals that the coordination geometry of zinc is a slightly distorted
trigonal bipyramid and the trapeziform chains are further assembled into a 3-D network via
hydrogen-bonding and ꢀ–ꢀ interactions. A water–methanol cluster, (H2O)8(CH3OH)2, is
observed in this complex. The electronic absorption spectrum was attributed to intra-ligand
electronic transitions. The spectrum of 1 indicates strong fluorescence at ꢁex ¼ 282 nm in the
fluorescence spectrum at room temperature; the fluorescence gradually quenches with rising
temperature. With increasing scan rate, the reversible redox process of Zn2þ/Znþ changes into a
quasi-reversible one.
Keywords: Zinc complex; Crystal structure; Water cluster; Fluorescence spectrum;
Electrochemical analyses
1. Introduction
Zinc is a necessary trace element in biological systems and plays a very important role in
life science. Organozinc carboxylate derivatives have antitumor, antiviral, antifungal
antibacterial, anti-inflammatory activity, and rich structural chemistry [1–3]. They are
widely used in ion exchange, catalysts, gas storage, absorption, fluorescence relay, and
construction of intriguing supramolecular structures [4–7]. The presence of nitrogen
and oxygen donors makes these compounds effective and stereospecific catalysts for
*Corresponding author. Email: lisa4.6@163.com; piaocsl@163.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ꢀ 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.690866

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
2235
oxidation, reduction hydrolysis, and may also show biological activities and other
transformations of organic and inorganic chemistry [8, 9]. Some drugs have more
activity as metal complexes [10, 11]. Metal complexes bridged by organic ligands are
often adopted to construct coordination polymers, which can be further assembled by
hydrogen-bonding, ꢀ–ꢀ interaction, or other intermolecular contacts into extended
networks with higher dimensionality [12–14]. Polycarboxylates as bridges are good
choices to assemble metal ions into diverse structural motifs [15–17].
We now report the syntheses of 4-(2-picolinylhydrazide) methylene benzoic acid
(H2L) and its Zn(II) coordination polymer {[ZnL(H2O)2] ꢀ 2(H2O)(CH3OH)}n (1)
characterized by crystal structures, infrared (IR), UV-Vis and fluorescence spectra and
electrochemical properties. Single-crystal X-ray analysis reveals that the trapeziform
chain structural motifs of the complex are assembled into a 3-D network via hydrogen-
bonding and ꢀ–ꢀ interactions. The stability of the supramolecular conglomeration has
a contribution of (H2O)8(CH3OH)2 clusters with a novel configuration.
2. Experimental
2.1. Materials and physical measurements
All reagents were commercially available and used without purification. Elemental
analyses (C, H, and N) were performed on a Vario ELX elemental instrument. UV-Vis
absorption spectra were obtained in DMSO using a GBC UV-Vis spectrophotometer.
IR spectra were recorded from 4000 cmꢁ1 to 400 cmꢁ1 with KBr pellets on a Shimadzu
FTIR-8400 spectrophotometer. Cyclic voltammetry was measured on a CHI860D
electrochemical analysis system (China) with a three-electrode system consisting of a
glassy carbon (GC) electrode (U ¼ 3 mm) as a working electrode, a saturated calomel
electrode as a reference electrode, and a platinum wire as an auxiliary electrode.
Emission/excitation spectra were carried out on a Hitachi RF-5301 fluorescence
spectrophotometer.
2.2. Synthesis of 2-picolinylhydrazide
2-Picolinylhydrazide was obtained by reaction of 2-picolinic acid methyl ester with
hydrazine hydrate (scheme 1) according to the literature [18]. Its m.p.: 98–100ꢂC agrees
with corresponding data in the literature.
2.3. Synthesis of H2L
As shown in scheme 1, a solution of 2-picolinylhydrazide (0.69 g, 5 mmol) in methanol
(10 mL) was added dropwise to a methanol (20 mL) solution of 4-formylbenzoic acid
(0.75 g, 5 mmol) with stirring at room temperature. The resulting mixture was refluxed
and stirred for 4 h at 70ꢂC until a white precipitate formed. The precipitate was
collected by filtration, washed with methanol, and desiccated in a loft drier. Anal. Calcd
for C14H11N3O3(%): C, 62.45; H, 4.12; N, 15.61. Found: C, 62.39; H, 4.07; N, 15.73.

2236
S.-L. Chen et al.
Scheme 1. i: 80% NH2NH2 ꢀ H2O, 10 h refluxing; ii: 4-formylbenzoic acid, CH3OH, 3 h refluxing.
2.4. Synthesis of {[ZnL(H2O)2] ꢀ 2(H2O)(CH3OH)}n (1)
Zn(CH3COO)2 hydrate (0.11 g, 0.5 mmol) in methanol (5 mL) was slowly added to a
methanol solution (25 mL) of triethylamine (0.5 mL) and H2L (0.15 g, 0.5 mmol) with
stirring at 70ꢂC. The mixture was refluxed and stirred for 2 h until a yellowish
precipitate formed. The yellowish turbid solution was filtered and the filtrate slowly
evaporated at room temperature resulting in yellow single crystals suitable for X-ray
diffraction. Anal. Calcd for C15H21N3O8Zn (%): C, 41.25; H, 4.85; N, 9.62. Found:
C, 41.34; H, 4.21; N, 9.66.
2.5. X-ray crystal structure determination
A block single crystal with dimensions 0.19 ꢃ 0.16 ꢃ 0.15 mm3 was mounted on a
goniometer and data collection was performed on a Bruker APEX-II CCD
diffractometer by the ’ and ! scan technique using graphite-monochromated Mo-Ka
˚
radiation (ꢁ ¼ 0.71073 A) at 296(2) K. The intensity symmetries indicate a monoclinic
P21/c space group. A total of 13,341 reflections (3217 unique) were performed within ꢂ
range 2.63–25.50ꢂ. SHELXS-97, SHELXL-97, and SHELXTL [19] were used for
structure determination and refinement. The structures were solved by direct methods
and all non-hydrogen atoms were obtained from the difference Fourier map and
subjected to anisotropic refinement by full-matrix least-squares on F2. All hydrogen
atoms of 1 were added geometrically or positioned by difference Fourier synthesis and
refined isotropically. Details concerning crystal data and refinement are given in table 1.
3. Results and discussion
3.1. Crystal structure
X-ray structural determination of 1 confirms the structural assignments based on
spectroscopic data. Compound 1 crystallizes in P21/c space group and consists of a 3-D
polymeric structure of {[ZnL(H2O)2] ꢀ 2(H2O)(CH3OH)}n where the zincs are
connected by two discrete water molecules and one L2ꢁ. The fundamental unit is

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
Table 1. Crystal data and structure refinements for 1.
2237
CCDC reference no.
Empirical formula
Formula weight
825719
15H21N3O8Zn
436.72
C
Temperature (K)
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions (A,
296(2)
0.71073
Monoclinic
P21/c
ꢂ
˚
)
a
b
c
ꢃ
ꢄ
ꢅ
7.2068(8)
21.877(3)
11.1010(13)
90.00
99.178(2)
90.00
1727.8(3), 4
1.679
1.473
3
˚
Volume (A ), Z
Calculated density (Mg mꢁ3
)
Absorption coefficient (mmꢁ1
F(000)
)
904
0.19 ꢃ 0.16 ꢃ 0.15
Crystal size (mm3)
ꢂ range for data collection (ꢂ)
Limiting indices
2.63–25.50
ꢁ8 ꢄ h ꢄ 8; ꢁ26 ꢄ k ꢄ 26; ꢁ13 ꢄ l ꢄ 13
13,341
Reflections collected
Independent reflections
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I 4 2ꢆ(I)]
R indices (all data)
3217 [R(int) ¼ 0.0393]
Full-matrix least-squares on F2
3217/6/231
0.933
R1 ¼ 0.0491, wR2 ¼ 0.1333
R1 ¼ 0.0697, wR2 ¼ 0.1489
0.592 and ꢁ0.718
ꢁ3
˚
Largest difference peak and hole (e A
)
Figure 1. Molecular structure of 1. The hydrogen atoms were omitted.
Symmetry code: A: ꢁx þ 1, y þ 1/2, ꢁz þ 3/2.
shown in figure 1, in which displacement ellipsoids are drawn at the 30% probability
level. Selected bonds, lengths, and angles are listed in table 2.
The crystal structure shows that this complex contains crystalline water molecules
and methanols. There is one crystallographically unique Zn in the crystal unit of 1.

2238
S.-L. Chen et al.
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1.
N(1)–Zn(1)
N(2)–Zn(1)
O(3)–Zn(1)#1
N(2)–N(3)
C(14)–O(3)
C(14)–O(2)
C(10)–C(11)
C(11)–C(14)
C(5)–C(6)
2.082(5)
2.099(5)
2.054(4)
1.397(6)
1.246(8)
1.244(9)
1.398(9)
1.514(8)
1.505(8)
O(4)–Zn(1)
O(5)–Zn(1)
Zn(1)–O(3)#2
C(6)–O(1)
C(6)–N(2)
C(7)–N(3)
C(1)–N(1)
C(5)–N(1)
2.018(5)
2.045(4)
2.055(4)
1.255(7)
1.322(7)
1.270(7)
1.346(8)
1.338(8)
N(1)–Zn(1)–N(2)
O(3)#2–Zn(1)–N(1)
O(4)–Zn(1)–N(2)
O(5)–Zn(1)–N(2)
O(4)–Zn(1)–O(3)#2
O(5)–Zn(1)–O(3)#2
O(4)–Zn(1)–O(5)
C(5)–N(1)–Zn(1)
C(7)–N(3)–N(2)
N(1)–C(5)–C(6)
O(1)–C(6)–C(5)
78.81(18)
90.34(19)
98.37(18)
92.23(18)
95.44(19)
89.38(18)
111.0(2)
114.4(4)
116.3(5)
116.5(5)
118.4(5)
114.7(5)
O(3)#2–Zn(1)–N(2)
O(4)–Zn(1)–N(1)
O(5)–Zn(1)–N(1)
C(14)–O(3)–Zn(1)#1
C(6)–N(2)–Zn(1)
N(3)–N(2)–Zn(1)
C(1)–N(1)–Zn(1)
C(6)–N(2)–N(3)
O(3)–C(14)–O(2)
O(3)–C(14)–C(11)
O(2)–C(14)–C(11)
N(3)–C(7)–C(8)
164.48(19)
107.5(2)
141.3(2)
128.7(4)
115.5(4)
131.9(4)
127.4(5)
112.3(5)
125.3(6)
116.9(6)
117.8(6)
121.1(5)
N(2)–C(6)–C(5)
Symmetry codes: #1: ꢁx þ 1, y þ 1/2, ꢁz þ 3/2; #2: ꢁx þ 1, y ꢁ 1/2, ꢁz þ 3/2.
Each L2ꢁ provides two nitrogen atoms from pyridine ring and C–N, respectively, to
coordinate with Zn(II) forming a five-membered chelate ring of ZnNC2N. The
geometry of Zn(1) is a slightly distorted trigonal bipyramid with two nitrogen atoms
from L2ꢁ, one O from another carboxyl of L2ꢁ, and two water molecules. In the
ZnN2O3 slightly distorted trigonal bipyramid, N(1), O(4), and O(5) are located in
the equatorial plane (plane equation: ꢁ1.273x þ 19.177y ꢁ 4.592z ¼ ꢁ5.1928), the
˚
distance is 0.0445 A between Zn(1) and the plane, while O(3)A and N(2) occupy axial
positions.
The Zn–N distances of 2.082(5)–2.099(5) A fall in the reported range of 1.970(2)–
˚
˚
˚
2.105(2) A in [Zn2(pzdc)(im)3]n [20]. The Zn–O bond lengths of 2.018(5)–2.055(4) A are
very close to those of [Zn2(pzdc)(im)3]n [20]. Bond angles O(4)–Zn(1)–N(1), O(5)–
Zn(1)–N(1), and O(4)–Zn(1)–O(5) are 107.5(2)ꢂ, 141.3(2)ꢂ, and 111.0(2)ꢂ, respectively.
The sum of these angles of 359.8ꢂ is close to 360ꢂ suggesting planar N(1), O(4), O(5),
and Zn(1). The coordination geometry of Zn(1) is a slightly distorted trigonal
bipyramid with a ꢇ-value of 0.39 (the structure index is defined as ꢇ ¼ (ꢄ–ꢃ)/60, where ꢃ
and ꢄ are the largest coordination angles, ꢄ ¼ O(3)#2–Zn(1)–N(2) 164.48ꢂ and
ꢃ ¼ O(5)–Zn(1)–N(1) 141.3ꢂ; symmetry code: #2 ꢁ x þ 1, y ꢁ 1/2, ꢁ z þ 3/2). This type
of slightly distorted trigonal bipyramid was also observed in the reported Cu(II)
complex [21]. The N(1)–Zn(1)–N(2) angle of 78.81(18)ꢂ (in the chelate ring) is less than
90ꢂ and the O(5)–Zn(1)–N(2) angle of 92.23(18)ꢂ (in the inter-ring) is larger than 90ꢂ,
perhaps due to the constraint of the chelating ligand. These values are 1ꢂ greater than
those in hexahedral zinc complexes [20, 22] showing that the ring flexibility resulting
from rotatable single bonds makes this ligand more adjustable to different geometrical
requirements. The chelate ring adopts an approximately planar conformation and the
observed torsion angles (table 3) (the torsion angles of C(6)–N(2)–Zn(1)–N(1), N(1)–
C(5)–C(6)–N(2), and C(5)–C(6)–N(2)–Zn(1) are 0.1(4)ꢂ, ꢁ2.6(8)ꢂ, and 1.2(6)ꢂ,

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
Table 3. Selected torsion angles (ꢂ) and hydrogen bonds.
2239
C(1)–N(1)–Zn(1)–N(2)
C(6)–N(2)–Zn(1)–O(4)
N(3)–N(2)–Zn(1)–O(4)
C(6)–N(2)–Zn(1)–O(5)
N(3)–N(2)–Zn(1)–O(5)
C(6)–N(2)–Zn(1)–O(3)#2
N(3)–N(2)–Zn(1)–O(3)#2
C(6)–N(2)–Zn(1)–N(1)
N(3)–N(2)–Zn(1)–N(1)
Zn(1)–N(2)–N(3)–C(7)
C(5)–C(6)–N(2)–Zn(1)
C(6)–N(2)–N(3)–C(7)
C(6)–C(5)–N(1)–Zn(1)
C(2)–C(1)–N(1)–Zn(1)
ꢁ178.1(5)
ꢁ106.1(4)
80.3(5)
142.2(4)
ꢁ31.3(5)
46.5(9)
O(2)–C(14)–O(3)–Zn(1)#1
C(11)–C(14)–O(3)–Zn(1)#1
C(5)–N(1)–Zn(1)–O(4)
C(1)–N(1)–Zn(1)–O(4)
C(5)–N(1)–Zn(1)–O(5)
C(1)–N(1)–Zn(1)–O(5)
C(5)–N(1)–Zn(1)–O(3)#2
N(1)–C(5)–C(6)–N(2)
C(5)–N(1)–Zn(1)–N(2)
C(8)–C(7)–N(3)–N(2)
O(1)–C(6)–N(2)–N(3)
C(5)–C(6)–N(2)–N(3)
N(1)–C(5)–C(6)–O(1)
O(1)–C(6)–N(2)–Zn(1)
2.6(12)
ꢁ176.1(4)
93.8(4)
ꢁ82.7(5)
ꢁ81.0(5)
102.5(6)
ꢁ170.4(4)
ꢁ2.6(8)
ꢁ127.0(7)
0.1(4)
ꢁ173.4(5)
ꢁ18.2(7)
1.2(6)
168.1(5)
2.7(6)
176.8(5)
ꢁ1.6(4)
178.4(5)
ꢁ4.6(8)
176.0(5)
178.0(5)
ꢁ179.5(5)
D–H ꢀ ꢀ ꢀ A
d(D–H)
d(H ꢀ ꢀ ꢀ A)
d(D ꢀ ꢀ ꢀ A)
ff(DHA)
O(5)–H(3 W) ꢀ ꢀ ꢀ O(2)#2
O(5)–H(4 W) ꢀ ꢀ ꢀ N(3)#3
O(5)–H(4 W) ꢀ ꢀ ꢀ O(1)#3
O(4)–H(1 W) ꢀ ꢀ ꢀ O(1)#4
O(4)–H(2 W) ꢀ ꢀ ꢀ O(8)#5
O(6)–H(6 W) ꢀ ꢀ ꢀ O(6)#6
O(6)–H(5 W) ꢀ ꢀ ꢀ O(1)#7
0.83
0.83
0.83
0.84
0.83
0.83
0.86
1.90
2.06
2.58
1.88
1.91
2.32
1.98
2.661(7)
2.861(7)
3.169(7)
2.703(6)
2.704(16)
2.76(2)
151.0
161.3
128.8
168.2
158.1
114.1
141.0
2.706(9)
Symmetry codes: #1: ꢁx þ 1, y þ 1/2, ꢁz þ 3/2; #2: ꢁx þ 1, y ꢁ 1/2, ꢁz þ 3/2; #3: ꢁx þ 1, ꢁy, ꢁz þ 1; #4: ꢁx, ꢁy, ꢁz þ 1;
#5: x, y ꢁ 1, z; #6: ꢁx, ꢁy þ 2, ꢁz; #7: x, y þ 1, z.
respectively) are close to the ideal 0ꢂ pattern owing to the intra-molecular conjugate.
L2– is non-planar with an inclination angle between pyridine from 2-picolinylhydrazide
and phenyl from 4-formylbenzoic acid of 2.5ꢂ. The carboxyl of L2ꢁ is monodentate with
the carbonyl coordinating to Zn(II) [23].
The lattice water clusters (H2O)4 assemble by hydrogen-bonding in 1. As shown in
figures 2 and 3, the angles of each (H2O)4 cluster motifs of O5a–O7b–O5c, O7b–O5c–
O7d, O5c–O7d–O5a, and O7d–O5a–O7b are 154.5ꢂ, 25.5ꢂ, 154.5ꢂ, and 25.5ꢂ (symmetry
codes: a: ꢁx þ 1, y þ 1/2, ꢁz þ 3/2; b: ꢁx þ 1, y ꢁ 1/2, ꢁz þ 3/2; c: x, ꢁy þ 1/2, z ꢁ 1/2;
d: x, ꢁy þ 3/2, z ꢁ 1/2), respectively. Adding up these angles gives 360.0ꢂ, which
indicates a planar structure of four oxygen atoms (O5a, O7b, O5c, and O7d). The O5a–
O7d–O7b and O5c–O7b–O7d angles have the same value of 39.8ꢂ, resulting in a parallel
quadrilateral consisting of two triangles with four oxygen atoms at the vertices. As
shown in figure 3, the (H2O)8(CH3OH)2 cluster originates from (H2O)4 linking two
methanols in a centrosymmetric tablet chair mode. The most interesting feature of the
crystal structure is the intermolecular hydrogen-bonding and ꢀ–ꢀ stacking interaction
˚
˚
(the centroid distances are 3.5725 A and 3.6366 A) (figure 4). The intermolecular
hydrogen bonds (as shown in table 3) between water and L2ꢁ (the distances of O ꢀ ꢀ ꢀ O
ꢂ
˚
are in the range 2.661(7)–3.169(7) A, range of O–H ꢀ ꢀ ꢀ O angles, 114.1–168.2 . The
˚
separation of O(5)–H(4 W) ꢀ ꢀ ꢀ N(3)#3 is 2.861(7) A (symmetry codes: #3 ꢁx þ 1, ꢁy,
ꢁz þ 1) and the angle is 161.3ꢂ) fall into the normal range of hydrogen-bonding
interactions [24]. Water clusters, providing insight into the structure and characteristics
of liquid water or ice [25], have been extensively studied both theoretically and
experimentally due to their unusual properties and importance in many physical,
chemical, and biological processes [26]. The water cluster in 1 is particularly interesting.

2240
S.-L. Chen et al.
Figure 2. Perspective of the crystal packing and (H2O)8(CH3OH)2 clusters in the unit cell along the a-axis
for 1. Dashed lines indicate hydrogen bonds. Some of the hydrogen atoms were omitted for clarity.
Figure 3. Detailed view of (H2O)8(CH3OH)2 trapped through hydrogen-bonding in 1. Dashed lines indicate
hydrogen bonds.
Symmetry codes: a: ꢁx þ 1, y þ 1/2, ꢁz þ 3/2; b: ꢁx þ 1, y ꢁ 1/2, ꢁz þ 3/2; c: x, ꢁy þ 1/2, z ꢁ 1/2; d: x, ꢁy þ 3/
2, z ꢁ 1/2; e: ꢁx þ 1, y ꢁ 1/2, ꢁz þ 1/2.
A series of configurations of water clusters such as book, boat, and chair with cyclic
conformation [24], as well as ring [27–30], tape [31, 32], folded-chain [33–35], helical
chains [36], hanging-ladder [37], cage [38], column [39], water–methanol intermix
clusters [40], and prism polyhedral structures have been reported [41–43]. However, to
the best of our knowledge, such a chair-type water cluster consisting of triangles or
parallel quadrilateral has not been reported.
In 1, the polymer backbone is along the crystallographic b-axis. The 1-D chains are
consecutively linked by O(3)–Zn(1)#1 bridges (figure 5 L) to produce a ladder
coordination polymer (figure 5 T). The metal–metal distances across each polymer
˚
backbone are 11.6711(17) A, which are longer than those in [Zn(ip)(im)2]n ꢀ 3nH2O (the
˚
metal–metal distances of 10.391(14) A) [44]. The 2-D layers stack in an ABABAB

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
2241
Figure 4. 2-D layer of 1 via hydrogen-bond interactions and face-to-face ꢀ–ꢀ stacking interactions. Some of
the hydrogen atoms were omitted for clarity.
Figure 5. Perspectives of (T) 3-D ladder and (L) 1-D chain.
sequence of the ladder coordination polymer. There are water clusters and ꢀ–ꢀ
stacking interactions between the 2-D layers (figure 4), forming a complicated 3-D
supramolecular network. These extensive weak intermolecular interactions make an
important contribution to the stability of the coordination compound.
3.2. UV-Vis spectroscopy
Electronic absorption spectra of the free ligand and 1 were measured in DMSO
solution. UV-Vis spectrum of 1 was recorded with 5.0 ꢃ 10ꢁ5 mol Lꢁ1 sample.

2242
S.-L. Chen et al.
The ligand shows two bands at 327 nm and 370 nm in the ultraviolet region assigned
to a conjugated B absorption band of ꢀ-bond from benzene and an n–ꢀ* transition of
conjugation between lone-pair electron of p orbital of nitrogen in C ¼ N [45]. The peaks
at 291 and 314 nm are attributed to ꢀ–ꢀ* and n–ꢀ* transitions of C¼N of the complex,
respectively. The ꢀ–ꢀ* transition of benzene usually appears at 245 nm [46]. The
absorption spectrum of 1 displays reduced oscillator strengths and blue shifts compared
with the ligand, related to the molecular structure of the complex, i.e. the planar
geometry of the ligand is hampered by steric hindrance of the complex [47]. The lack of
conjugation is consistent with changes observed in intensity and energy of the electronic
spectra. The shift of the characteristic band indicates the formation of 1 [48].
3.3. IR spectrum analysis
The characteristic IR spectra of L2ꢁ and 1 are presented in ‘‘Supplementary material.’’
The IR spectrum of 1 shows significant differences from the free ligand. The absence of
bands of methanamide ꢈ(N–H) and ꢈ(O–H) from the carboxyl in the complex suggests
that deprotonated N–H and O–H coordinate with zinc [49, 50]. The absorption bands
of the ligand at 1280 cmꢁ1 for ꢈ(C–N) shift to 1262 cmꢁ1 in the complex indicating
formation of metal–ligand bonds. Other evidence that the nitrogen of methanamide is
involved in coordination is the ꢈ(N–N) shifts 39 cmꢁ1 from free ligand to complex. The
difference of Dꢈ(ꢈas(COOꢁ)–ꢈsym(COOꢁ)) is often used to determine the bonding of
carboxylate complexes [51] with Dꢈ less than 200 cmꢁ1, indicating bidentate carbox-
ylate, while larger than 200 cmꢁ1 means monodentate carboxylate [52]. The ꢈas(COOꢁ)
and ꢈsym(COOꢁ) vibrations are 1617 cmꢁ1 and 1385 cmꢁ1 for 1 and the difference
Dꢈ(ꢈas(COOꢁ)–ꢈsym(COOꢁ)) between these frequencies is close to those found in
monodentate carboxylate (232 cmꢁ1). This observation is consistent with X-ray
structural analyses. The Zn–N stretching vibration at 517 cmꢁ1 is in agreement with
that of a related complex [53]. The stretching frequency of O–H bonds of the water
cluster is observed as a wide weak multiple absorption at 3429 cmꢁ1. The IR spectrum
of ice shows the O–H stretch at 3220 cmꢁ1 while that in liquid water appears at
3490 cmꢁ1 and 3280 cmꢁ1 [54–57]. Thus the O–H stretch in the water cluster of 1 is
similar to that of liquid water.
3.4. Fluorescence spectrum
Photoluminescence of 1 was measured with 5.0 ꢃ 10ꢁ5 mol Lꢁ1 DMSO solution at room
temperature. The photoluminescence spectrum of 1 is depicted in figure 6, where an
intense emission at 338 nm (ꢁex ¼ 282 nm) is exhibited. As a d10 ion, Zn(II) does not
contribute to the electronic transition of the complex, which reconciles with the UV-Vis
analysis. The fluorescence properties at different temperatures were determined
(figure 7), showing that the fluorescence efficiency of 1 is influenced by temperature
change from 25ꢂC to 65ꢂC with increment of 5ꢂC. The spectra show larger fluorescence
intensity of 235.209 a.u. at 25ꢂC. Fluorescence intensity gradually lowers with
increasing temperature, reaching 206.672 a.u. at 65ꢂC. The increasing temperature
may increase fluorescence quenching.

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
2243
Figure 6. Excitation and emission spectra of 1.
Figure 7. Fluorescence emission spectra of 1 at different temperatures and fluorescent intensity with the
change in the temperature on the top right.
3.5. Electrochemistry character
‘‘Supplementary material’’ depicts the cyclic voltammogram of 1 in DMSO containing
0.05 mol Lꢁ1 tetrabutylammonium bromide at different scan rates. The electrochemical
data of 1 at different scan rates are listed in table 4, where the anodic and cathodic
peaks correspond to Zn2þ/Znþ [58]. The separation of the cathodic peaks of ꢁ0.529 V
and anodic peak of ꢁ0.460 V (DE is 0.069 V) indicates that the electrochemical behavior
of 1 on the GC electrode is electrochemically reversible. Values of oxidation and
reduction peaks are more positive than those of {[Zn(ip)(Eim)2] ꢀ H2O} (Epa ¼ ꢁ1.161 V,
Epc ¼ ꢁ1.345 V, DE ¼ 0.184 V) and Zn(ip)(Mim)2 (Epa ¼ ꢁ1.141 V, Epc ¼ ꢁ1.282 V,
DE ¼ 0.141 V) [59]. The cathodic peaks Epc gradually become more negative and the
cathodic peak currents (Ipc) increase with increasing scan rate, while the anodic peaks
Epa shift to more positive and anodic peak currents (Ipa) increase with increasing scan
rate. These changes result in increasing DE separations and the transformation of the
redox process from a reversible process to a quasi-reversible one.

2244
S.-L. Chen et al.
Table 4. Electrochemical data at different scan rates.
Cathodic peaks
Anodic peaks
Separation
DE (V)
Scan rate (V sꢁ1
)
Epc (V)
Ipc (mA)
Epa (V)
Ipa (mA)
0.005
0.01
0.03
0.06
0.09
0.12
0.15
0.18
0.21
0.24
0.30
0.40
ꢁ0.529
ꢁ0.532
ꢁ0.543
ꢁ0.547
ꢁ0.556
ꢁ0.560
ꢁ0.566
ꢁ0.575
ꢁ0.576
ꢁ0.578
ꢁ0.582
ꢁ0.597
2.979
3.955
6.160
8.134
9.803
11.280
12.830
13.300
13.460
13.480
13.520
14.050
ꢁ0.460
ꢁ0.460
ꢁ0.478
ꢁ0.452
ꢁ0.450
ꢁ0.451
ꢁ0.450
ꢁ0.449
ꢁ0.445
ꢁ0.435
ꢁ0.433
ꢁ0.435
2.308
3.146
4.837
5.785
7.285
8.623
9.843
9.990
9.435
10.200
9.339
10.530
0.069
0.072
0.065
0.095
0.106
0.109
0.116
0.126
0.131
0.143
0.149
0.162
DE ¼ Epa – Epc.
This suggests that donor such as unsaturated oxygen and nitrogen of a ligand can
stabilize a low oxidation state, such as Znþ, more than a saturated one, by ꢀ-back
bonding between metal and oxygen and nitrogen [60].
4. Conclusion
A metal coordination polymer, {[ZnL(H2O)2] ꢀ 2(H2O)(CH3OH)}n, has been synthe-
sized and characterized. Crystal structure shows that there is one ligand and one
crystallographically unique Zn in the symmetrical unit. Zn(1) with a slightly distorted
trigonal bipyramid geometry is coordinated by two nitrogen atoms of L2ꢁ and three
oxygen atoms of another L2ꢁ and two water molecules. The trapeziform chains are
further assembled into a 3-D metal–organic framework via water clusters through
hydrogen-bonding and ꢀ–ꢀ interaction. The electronic absorption spectra are attributed
to intra-ligand electronic transitions. Experimental fluorescence measurements show
that the complex emits strong fluorescence at ꢁex ¼ 282 nm and rising temperature leads
to fluorescence quenching. With increasing scan rate, the redox process gradually
changes to quasi-reversible.
Supplementary material
Crystallographic data in this article have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication No. CCDC–825719
for 1. Copies of the data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Tel: þ44 (0) 1223 762-911 or E-mail:
deposit@ccdc.cam.ac.uk).

Methylene benzoic acid containing the (H2O)8(CH3OH)2 cluster
Acknowledgments
2245
The support from the Key Laboratory of Environmental Engineering, Protection and
Assessment of Guangxi, the National Water Pollution Control and Management of
China (No. 2008ZX07317–02) are greatly appreciated.
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