Syntheses, crystal structures, substituents steric effect, luminescent and moisture-stability studies of three two-dimensional zinc-organic frameworks

Article abstract of DOI:10.1016/j.ica.2012.10.006

Three new 2D zinc-organic frameworks, [Zn₃(L1) ₃(DMF)₂]_n (1), [Zn(L2)(BPP)]_n (2) and [Zn₂(L3)₂(BPP)]_n·nDMF (3), (L1 = 2,5-dimethylterephthalate, L2 = 2-aminoterephthalate, L3 = terephthalate, BPP = 1,3-bis(4-pyridyl)propane, DMF = N,N-dimethylformamide), have been prepared and characterized. Complex 1 consists of trinuclear zinc clusters connected to six L1 ligands, in which the trinuclear clusters act as pseudo-planar hexagonal nodes to give a 2D network. In complex 2, two 4⁴-sql nets interlock in parallel and give rise to a polycatenated layer (2D → 2D). Complex 3 features a 2D (4,4) net when the dinuclear [Zn₂(COO) ₂N₂O₂] unit is regarded as a quadruply-connected node. These results indicate that the steric effect of 1,4-benzenedicarboxylates substituents has an important effect on the structural topologies of such Zn(II) complexes. All of 1-3 emit the intense indigotin photoluminescence at room temperature, with lifetimes of 28.04, 3.08 and 2.43 ns, respectively. The moisture-stability analyses of 1-3 have also been investigated.

Full text of DOI:10.1016/j.ica.2012.10.006

Inorganica Chimica Acta 396 (2013) 84–91
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Syntheses, crystal structures, substituents steric effect, luminescent
and moisture-stability studies of three two-dimensional zinc-organic frameworks
a
a
a,
b,
c
De-Yun Ma a, , Kuan Lu , Liang Qin , Hai-Fu Guo , Xiong-Yi Peng , Jian-Qiang Liu
⇑
⇑
⇑
a School of Chemistry and Chemical Engineering, Zhaoqing University, Zhaoqing 526061, PR China
b School of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430073, PR China
c School of Pharmacy, Guangdong Medical College, Dongguan 523808, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 August 2012
Received in revised form 15 October 2012
Accepted 15 October 2012
Available online 29 October 2012
Three new 2D zinc-organic frameworks, [Zn3(L1)3(DMF)2]n (1), [Zn(L2)(BPP)]n (2) and [Zn2(L3)2(BPP)]n-
ÁnDMF (3), (L1 = 2,5-dimethylterephthalate, L2 = 2-aminoterephthalate, L3 = terephthalate, BPP = 1,3-
bis(4-pyridyl)propane, DMF = N,N-dimethylformamide), have been prepared and characterized. Complex
1 consists of trinuclear zinc clusters connected to six L1 ligands, in which the trinuclear clusters act as
pseudo-planar hexagonal nodes to give a 2D network. In complex 2, two 44-sql nets interlock in parallel
and give rise to a polycatenated layer (2D ? 2D). Complex 3 features a 2D (4,4) net when the dinuclear
[Zn2(COO)2N2O2] unit is regarded as a quadruply-connected node. These results indicate that the steric
effect of 1,4-benzenedicarboxylates substituents has an important effect on the structural topologies of
such Zn(II) complexes. All of 1–3 emit the intense indigotin photoluminescence at room temperature,
with lifetimes of 28.04, 3.08 and 2.43 ns, respectively. The moisture-stability analyses of 1–3 have also
been investigated.
Keywords:
Zinc-organic frameworks
Luminescent
Substituents steric effect
Moisture-stability
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
frameworks is surprisingly scarce, and much work is still necessary
to understand the coordination chemistry [6]. Furthermore, in re-
The design and synthesis of Metal–Organic Frameworks (MOFs)
have received widespread attention in recent years due to their po-
tential applications in gas separation, catalysis, magnetism, optics
and so on [1]. Studies in these fields have been focused on the de-
sign and preparation, as well as on structure–property relation-
ships. Significant progress has been achieved [2]. However, it is
still a great challenge to rationally prepare and predict their exact
structures, as many factors can affect the overall structure forma-
tion. In addition to the choice of ligands and metal ions, reaction
conditions such as solvent, pH and temperature can also affect
the final structure [3]. Rational design of ligands is usually a useful
and important way of studying the controllable synthesis of the
resulting MOF structures [4]. Based on this understanding, we have
selected three aromatic dicarboxylic anions with different substit-
uents: 2,5-dimethylterephthalic acid (L1), 2-aminoterephthalic
acid (L2), and terephthalic acid (L3), chosen according to the fol-
lowing considerations: (1) these phenyl dicarboxylates contain
two para-carboxyl groups and different substituents, which can
engender spatial effects and thus influence the structural assembly
in several different ways [5]; (2) a systemic study of the substitu-
ent effects on the generation of such carboxylate coordination
cent decades, a variety of fascinating coordination polymers have
been constructed by carboxylate ligands and N-containing ligands
as the auxiliary ligands [7]. It is widely recognized that the flexible
1,3-bis(4-pyridyl)propane ligand (BPP) is an excellent candidate
for constructing novel structures as the auxiliary ligands and
may act as a bidentate bridge with different conformations or in
a monodentate terminal mode, leading to the formation of a vari-
ety of 1D chain, 2D layer and 3D network motifs [8].
Extending our previous work on self-assembly of coordination
networks as a function of pH [3a], we are studying the substituents
steric effects of the 1,4-benzenedicarboxylate ligands on frame-
work formation of the coordination complexes. Three different
Zn(II) complexes: [Zn3(L1)3(DMF)2]n (1), [Zn(L2)(BPP)]n (2) and
[Zn2(L3)2(BPP)]nÁnDMF (3), have been successfully isolated by
solvothermal conditions and characterized by elemental analysis,
IR, TGA, powder X-ray diffraction and single-crystal X-ray diffrac-
tion. The photoluminescent properties and moisture-stability of
1–3 are also reported in this paper.
2. Experimental
2.1. Materials and physical measurements
⇑
Corresponding authors. Tel.: +86 758 2716357; fax: +86 758 2716447.
E-mail addresses: mady@zqu.edu.cn (D.-Y. Ma), guohaifu@zqu.edu.cn (H.-F.
All chemicals were commercially available and used as received
without further purification. Elemental analyses for C, H, and N
Guo), peng20121111@163.com (X.-Y. Peng).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.10.006

D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
85
Table 1
Crystal data and structure refinement details of 1–3.
Compounds
1
2
3
Formula
C
36H38N2O14Zn3
C21H19N3O4Zn
442.76
orthorhombic
Pbca
11.8253(18)
17.268(3)
19.299(3)
90.00
C32H29N3O9Zn2
730.32
monoclinic
P21/m
8.4580(7)
21.6474(18)
9.4309(8)
90.00
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
918.79
monoclinic
C2/c
23.719(2)
10.7570(9)
16.0846(13)
90.00
a
(°)
b (°)
105.226(2)
90.00
3959.9(6)
4
1.541
1.870
90.00
90.00
3940.9(10)
8
1.493
109.884(2)
90.00
1623.8(2)
2
1.494
1.534
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
l
(mmÀ1
)
1.279
F(000)
1880
1824
748
Reflections collected
Independent reflections (Rint
Parameters
10793
3534 (0.0322)
255
1.039
0.0348
0.0885
0.613 and À0.398
15311
3511 (0.0690)
262
1.015
0.0641
0.1718
1.038 and À0.430
9316
2991 (0.0251)
232
1.086
0.0352
0.0976
0.469 and À0.362
)
Goodness-of-fit (GOF) on F2
R1 [I > 2
r
(I)]a
wR2 (all data)b
D
qmaximum and
(e/A3)
mininimum
P
P
a
R = (||Fo| À |Fc||)/ |Fo|.
P
P
wR = [ w(Fo2 – Fc2)2/ w(Fo)2]1/2
.
b
were carried out by using a Vario EL III Elemental Analyzer. Infra-
red spectra were recorded (4000–400 cmÀ1) as KBr disks on Shima-
dzu IR-440 spectrometer. Thermogravimetric analyses (TGA) were
performed on an automatic simultaneous thermal analyzer
(DTG-60, Shimadzu) under a flow of N2 at a heating rate of 10 °C/
min between ambient temperature and 800 °C. Powder XRD
investigations were carried out on a Bruker AXS D8-Advanced
1490(m), 1421(s), 1373(s), 1251(m), 1226(w), 1070(m), 1024(m),
957(w), 886(w), 831(m), 811(m), 773(s).
2.3.3. Synthesis of [Zn2(L3)2(BPP)]nÁnDMF (3)
Complex 3 was prepared by the same procedure as 1 except
that 2,5-dimethylterephthalic acid was replaced by terephthalic
acid. Yield 0.137 g (75%). Anal. Calc. for C32H29N3O9Zn2: C, 52.6;
H, 4.0; N, 5.8. Found C, 52.1; H, 4.4; N, 6.2%. FTIR spectra (KBr,
cmÀ1): 2933(m), 2851(m), 1680(m), 1626(s), 1590(w), 1504(w),
1479(w), 1417(w), 1388(s), 1290(w), 1222(m), 1067(w), 1037(w),
1016(w), 891(w), 828(s), 746(s).
diffractometer at 40 kV and 40 mA with Cu Ka (k = 1.5406 Å) radi-
ation. Luminescence spectra and lifetime for crystalline samples
were recorded at room temperature on an Edinburgh FLS920
phosphorimeter.
2.2. Sorption measurements
2.4. X-ray crystallographic determination
The kinetic trap effects of water on the complexes 1–3 were
monitored by TGA. Before measurements, the sample was treated
at 250 °C in flowing N2 overnight. After cooling to room tempera-
ture, the sample was exposed to a N2 flow with water vapor at
25 °C until no weight change was observed. Then the sample was
heated at a rate of 5 °C/min under a pure N2 flow.
Single crystal X-ray diffraction analyses of complexes 1–3 were
performed on a Bruker Apex II CCD diffractometer operating at
50 kV and 30 mA using MoKa radiation (k = 0.71073 Å). Data col-
lection and reduction were performed using the APEX II software
[9]. Multi-scan absorption corrections were applied for all the data
sets using the APEX II program [9]. Small residual absorption effects
were treated with XABS2 [10]. The structures were solved by direct
methods and refined by least squares on F2 using the SHELXTL pro-
gram package [11]. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms attached
to carbon were placed in geometrically idealized positions and re-
fined using a riding model. The free DMF molecule in 3 is disor-
dered evenly over two positions, with occupancy ratios of
0.50(2): 0.50(2). Crystallographic data for 1–3 are listed in Table 1.
Selected bond lengths and angles are given in Table 2. H-bonding
parameters for 1–3 are given in Table 3.
2.3. Synthesis
2.3.1. Synthesis of [Zn3(L1)3(DMF)2]n (1)
A mixture of Zn(NO3)2Á6H2O (0.15 g, 0.5 mmol), 2,5-dimethylte-
rephthalic acid (0.097 g, 0.5 mmol) 1,3-bis(4-pyridyl)propane
(0.137 g, 0.5 mmol) and 10 mL DMF was sealed in a 23 mL Teflon
reactor and kept under autogenous pressure at 120 °C for 48 h. The
mixture was cooled to room temperature as a rate of 5 °C/h. Yield
0.902 g (59%). Anal. Calc. for C36H38N2O14Zn3: C, 47.0; H, 4.1; N,
3.0. Found C, 47.4; H, 3.8; N, 3.2%. FTIR spectra (KBr, cmÀ1):
2950(m), 1678(s), 1588(s), 1407(s), 1337(m), 1281(w), 1197(m),
1112(m), 1033(w), 915(m), 845(s), 796(s), 755(s), 694(m), 545(s).
3. Results and discussion
3.1. Crystal structures
2.3.2. Synthesis of [Zn(L2)(BPP)]n (2)
Complex 2 was prepared by the same procedure as 1 except
that 2,5-dimethylterephthalic acid was replaced by 2-aminote-
rephthalic acid. Yield 0.315 g (72%). Anal. Calc. for C21H19N3O4Zn:
C, 56.9; H, 4.3; N, 9.5. Found C, 57.4; H, 4.0; N, 9.2%. FTIR spectra
(KBr, cmÀ1): 3415(m), 2923(m), 2858(m), 1628(s), 1569(s),
3.1.1. Structure of complex 1
Complex 1 crystallizes in the monoclinic space group C2/c and
contains two crystallographically independent zinc atoms (Zn1
and Zn2). The perspective representations of the trinuclear com-
pound 1 is shown in Fig. 1a. The three zinc atoms in 1 form a

86
D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
Table 2
were considered as 6-connected nodes and lines, respectively.
Selected bond distances (Å) and angles (°).
The layer can be reduced to 6-connected hxl topological type with
point symbol 3.3.3.3.3.3.4.4.4.4.4.4.5(2).5(2).5(2) (Fig. 1c). The
thickness of the layer, given by the Zn-Zn distance between the ter-
minal Zn atoms in the same SBU, is 6.984(2) Å. The two DMF mol-
ecules attach in a monodentate terminal mode to coordinate a
trinuclear cluster, thereby separating two adjacent layers; there-
fore, the existence of the 2D network can be explained by the
DMF-induced steric hindrance that prevents the formation of
coordinative bonds between adjacent SBUs and in this way also
prevents the formation of a 3D network. Neighboring layered
networks are further linked into a 3D supramolecular framework
through non-classical intermolecular C–HÁÁÁO hydrogen bonding
interactions involving the methyl hydrogen atoms of DMF
molecules and carboxylate oxygen atoms (Fig. 1d, Table 3).
Moreover, intramolecular C–HÁ Á ÁO hydrogen bonds are also
observed.
Complex 1
Zn(1)–O(1)
Zn(1)–O(3)i
Zn(1)–O(4)i
Zn(1)–O(6)
Zn(1)–O(7)
Zn(2)–O(2)ii
Zn(2)–O(3)iii
Zn(2)–O(5)ii
1.931(3)
2.414(3)
2.015(3)
1.939(2)
2.005(3)
2.033(3)
2.180(3)
2.034(2)
O(1)–Zn(1)–O(6)
O(6)–Zn(1)–O(4)i
O(1)–Zn(1)–O(3)i
O(4)i–Zn(1)–O(3)i
O(2)ii–Zn(2)–O(2)
O(2)–Zn(2)–O(5)
O(5)ii–Zn(2)–O(3)i
O(2)–Zn(2)–O(3)iii
121.04(12)
126.93(13)
100.57(10)
57.56(10)
180.0(13)
92.74(11)
89.54(10)
89.42(12)
Complex 2
Zn(1)–N(2)
Zn(1)–N(3)ii
Zn(1)–O(1)i
Zn(1)–O(4)
2.086(5)
2.057(5)
1.959(6)
1.958(4)
O(4)–Zn(1)–O(1)i
O(4)–Zn(1)–N(3)ii
O(4)–Zn(1)–N(2)
O(1)i–Zn(1)–O(2)i
138.7(2)
98.8(2)
106.5(2)
56.4(2)
Complex 3
Zn(1)–O(1)
Zn(1)–O(3)
Zn(1)–O(4)
Zn(1)–N(1)
1.924(2)
1.954(2)
1.987(2)
2.004(2)
O(1)–Zn(1)–O(3)
O(1)–Zn(1)–O(4)i
O(1)–Zn(1)–N(1)
O(4)i–Zn(1)–N(1)
113.43(9)
102.28(9)
125.33(10)
98.07(9)
3.1.2. Structure of complex 2
Symmetry codes: For 1: i = x, À y, 0.5 + z; ii = 0.5 À x, 0.5 À y, 1 À z; iii = 0.5 À x,
0.5 + y, 0.5 À z; For 2: i = x, 0.5 À y, 0.5 + z; ii = 1 + x, y, z; For 3: i = 1 À x, 1 À y, 1 À z.
Single-crystal X-ray diffraction analysis reveals that compound
2 has wave-like (4,4) layered structure, crystallizing in the space
group Pbca. An ORTEP diagram view of 2 is shown in Fig. 2a. In
the asymmetric unit of 2, there is one Zn(II) cation, one L2 anion
and one BPP ligand. The Zn(II) centre is four-coordinated by two
oxygen atoms from two L2 ligands, two nitrogen atoms from two
BPP ligands, and displays a distorted tetrahedral coordination
sphere. The Zn–O, Zn–N bond lengths and O–Zn–O, O–Zn–N bond
angles range from 1.958(4) to 2.086(5) Å and 92.9(2)° to
138.7(2)°, respectively, which is within the reasonable range of ob-
served values for other four-coordinated Zn(II) complexes with
oxygen and nitrogen donating ligands [14]. The BPP ligand has
TG conformation (the two pyridyl groups of BPP ligand show dif-
ferent trans(T) and gauche(G) orientations) [8] and adopts two
end pyridyl N atoms that link to two Zn(II) atoms with a ZnÁ Á ÁZn
separation of 11.825 Å (Scheme 1f). A linear chain is formed by di-
rect connection of Zn(II) atoms bridged by BPP ligands. The L2 li-
gands also bridge the Zn(II) atoms to form a zigzag chain, and
the ZnÁ Á ÁZn separation between the L2 ligands is 10.871 Å
(Scheme 1c). Such parallel linear and parallel zigzag chains are per-
pendicularly intersected by the Zn(II) atoms at the nodes, resulting
in a wave-like 2D (4,4) network (Fig. 2b). Each 4-membered ring
consists of two L2 ligands, two BPP ligands and four Zn(II) atoms.
The wave shape and large voids with dimensions of 8.3 Â 7.3 Å
for each layer allow the formation of catenation between adjacent
two layers in a parallel manner.
perfectly linear [Zn3(l2-OOCbridging)4(l2-OOCchelating-bridging)2] unit.
Zn1 is five-coordinated by four carboxylate oxygen atoms from
three different L1 ligands and one oxygen atom of a DMF molecule,
adopting a distorted trigonal biyramidal geometry; Zn2 is six-coor-
dinated by six carboxylate oxygen atoms from six different L1 li-
gands and displays octahedral coordination. The Zn–O distances
and O–Zn–O bond angles ranging from 1.931(3) to 2.414(3) Å
and from 57.56(10)° to 180.00° respectively, all of which are within
the range of those reported in other trinuclear zinc(II) compounds
involving carboxylate ligands [12]. The [Zn3(l2-OOCbridging)4(l2-
OOCchelating-bridging)2] unit can be regarded as a secondary building
unit (SBU). The trinuclear zinc clusters in 1 are connected by rigid
organic linkers (L1). Similar to [Mg3(ndc)3(dif)4] [13], the L1 li-
gands in compound 1 adopt two coordination modes that connect
two Zn(II) atoms: a bidentate bridging mode and an asymmetric
chelating bridging mode (Scheme 1a and b). Consequently, a 2D
layered Metal–Organic Framework (MOF) with triangular meshes
is formed (Fig. 1b). If the trinuclear zinc clusters and L1 ligands
Table 3
Hydrogen bonds of 1–3.
As shown in Fig. 2c, two 44-sql nets interlock in parallel and
give rise to a polycatenated layer (2D ? 2D). Such parallel (4,4)
layered interlocking modes are well know to belong to the paral-
lel–parallel (p–p) class [15]. There are two crystallographically
independent 4-membered rings that a 4-membered ring simply
catenated by two rings of a layer (Fig. 2d). Neighboring polycate-
nated layers are linked into a 3D suparomolecular framework via
D–HÁ Á ÁA
1
d(D–H)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
\(DHA)
C(3)–H(3)Á Á ÁO(3)
C(3)–H(3)Á Á ÁO(5)iv
C(10)–H(10C)Á Á ÁO(4)
C(15)–H(15B)Á Á ÁO(6)i
C(16)–H(16)Á Á ÁO(4)i
C(17)–H(17C)Á Á ÁO(6)v
C(18)–H(18A)Á Á ÁO(7)i
0.93
0.93
0.96
0.96
0.93
0.96
0.96
2.40
2.37
2.43
2.40
2.56
2.53
2.28
2.756(4)
3.220(5)
2.839(6)
2.905(5)
3.074(5)
3.461(8)
2.706(8)
103
152
105
113
115
164
106
C–HÁ Á Á
p
stacking interactions and C–HÁ Á ÁO, C–HÁ Á ÁN hydrogen
bonds (Fig. 2e, Table 3). The H-to-centroid distance of
H10Á Á ÁCg1i = 2.99(2) Å [Cg1 is the centroid of the C1–C6 ring. Sym-
metry Code: (i) À0.5 + x, 0.5Ày, 1Àz)].
2
N(1)–H(1A)Á Á ÁO(2)
C(6)–H(6)Á Á ÁO(1)
C(9)–H(9)Á Á ÁN(1)iii
C(16)–H(16A)Á Á ÁO(2)iv
0.86
0.93
0.93
0.97
2.04
2.46
2.55
2.51
2.666(7)
2.777(9)
3.438(6)
3.245(6)
129
100
161
132
3.1.3. Structure of complex 3
3
X-ray crystallography shows that complex 3 crystallizes in the
monoclinic space group P21/m, with one Zn atom, half a BPP ligand,
two halves of an L3 ligand and one DMF molecule in the asymmet-
ric unit. The four-coordinated Zn(II) center is surrounded by three
oxygen atoms from three L3 ligands and one nitrogen atom from a
BPP ligand, which exhibits a distorted tetrahedral coordination
geometry (Fig. 3a), with Zn–O, Zn–N distances and O–Zn–O, O–
Zn–N bond angles ranging from 1.924(2) to 2.004(2) Å and
C(3)–H(3A)Á Á ÁO(1)
C(7)–H(7A)Á Á ÁO(2)ii
C(10)–H(10A)Á Á ÁO(1)iii
C(13)–H(13A)Á Á ÁO(2)ii
C(18)–H(18A)Á Á ÁO(5)
0.93
0.93
0.93
0.93
0.96
2.45
2.59
2.55
2.53
2.33
2.758(5)
3.272(1)
3.264(8)
3.198(6)
2.768(4)
100
131
134
129
107
Symmetry codes: For 1, i: x, Ày, 0.5 + z; iv: 0.5Àx, À 0.5 + y, 0.5 À z; v: Àx, 1 À y, 1Àz.
For 2, iii: À0.5 + x, 0.5 À y, 1 À z; iv: 1.5 À x, À y, 0.5 + z. For 3, ii: 1 À x, 1 À y, À z; iii:
2 À x, 1 À y, 1Àz.

D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
87
Fig. 1. View of structure 1. (a) ORTEP diagram showing the coordination environment for Zn(II) ions in 1. All H atoms are omitted for clarity (symmetry codes: (ii) 0.5Àx,
0.5Ày, 1Àz); (b) View of the layered network of 1; (c) View of the hxl-type 6-connected architecture of 1 (Zn cations, dark red balls; L1 ligands, yellow sticks); (d) View of the
3D framework of 1 formed by C–HÁ Á ÁO hydrogen bonds (pink dash lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.).
98.07(9)° to 125.33(10)°, respectively, all of which are within the
range of those found in other four-coordinated Zn(II) complexes
with oxygen and nitrogen donating ligands [16]. In the polymeric
structure of 3, the L3 ligands adopt two different bridging modes:
one acts as bridging mode l2 to link two Zn(II) ions, whereas the
other has bridging mode l4 connecting four Zn(II) ions
(Scheme 1d, e). The BPP ligand has TG conformation [8] and acts
as a trans bidentate bridging ligand to link two Zn(II) ions, with
3.2. Discussion
3.2.1. Effect of the 1,4-benzenedicarboxylate ligands on the grometries
of 1–3
From the structures descriptions above, we can see that the
choice of 1,4-benzenedicarboxylate anions is critical in determin-
ing the structures of the resultant complexes. In this work, the role
of 1,4-benzenedicarboxylate anions can be explained in terms of
their different substituents. Complexes 1–3 show the effects of
substituents of the 1,4-benzenedicarboxylate ligands on the struc-
tures. Although the 1,4-benzenedicarboxylate anions L1, L2 and L3
have the same carboxylate groups, L1 has two substituents
(methyl), whereas L2 and L3 have one (amino) and none, respec-
tively. As a result, [Zn3(L1)3(DMF)2]n (1), in which the Zn atom
was only coordinated by L1 and DMF, shows a 2D layer structure
with trinuclear units, [Zn(L2)(BPP)]n (2) displays a 2D polycatenat-
ed layer structure constructed by L2 and BPP ligands, whereas
[Zn2(L3)2(BPP)]nÁnDMF (3) has a different 2D structure involving
L3 and BPP ligands with dinuclear units. Obviously, the substitu-
ents of the 1,4-benzenedicarboxylate anions are in large part
responsible for the structural differences of 1–3.
the ZnÁ Á ÁZn separation being 10.921 Å. In this manner, each l4
-
L3 ligand links four Zn atoms in 2-bridging bidentate coordination
l
mode to result in an interesting neutral chain of linear [Zn2(L3)2]n
with ZnÁ Á ÁZn separation of 10.306 Å. The adjacent neutral chains
are bridged by flexible
l2-BPP and rigid l2-L3 spacers to form a
2D framework with bimetallic [Zn2(COO)2N2O2] building units in
which the Zn1Á Á ÁZn1i distance is 3.603 Å (Fig. 3b). The 2D frame-
work can be topologically recognized as a (4,4) net where the dinu-
clear [Zn2(COO)2N2O2] unit is regarded as quadruply-connected
node. In these 2D sheets each bimetallic [Zn2(COO)2N2O2] building
unit is connected to six others, two of them by
of them by two 2-L3 and two 2-BPP spacers. Adjacent 2D net-
works connect to form a 3D framework through C–HÁ Á ÁO hydrogen
bonds and a stacking interaction between the parallel phenyl
Á Á Á
ring of a 4-L3 ligand and a pyridyl ring of 2-BPP ligand of neigh-
boring complexes, with centroid to centroid distance of
3.563(4) Å (Fig. 3c, Table 3).
l4-L3 ligands, four
l
l
p
p
l
l
3.2.2. IR spectra
a
FT-IR spectra of complexes 1–3 were recorded as KBr pellets
(Fig. S1). In the IR spectrum, the moderate absorption band

88
D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
Scheme 1. The coordination modes of L1, L2, L3 and BPP ligands in 1–3.
Fig. 2. View of structure 2. (a) ORTEP diagram of the asymmetry unit of 2. All H atoms omitted for clarity (Symmetry codes: (i) x, 0.5Ày, 0.5 + z; (ii) 1 + x, y, z); (b) View of the
(4,4) layer in 2; (c) 2D parallel polycatenation in 2; (d) The type of interlocking mode in 2; (e) View of the 3D framework of 2 formed by C–HÁ Á Á stacking interactions and
hydrogen bonds (pink dash lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
p
observed at 2950 cmÀ1 for 1 is attributed to the
tion of L1 ligand. The moderate absorption band centered at
3415 cmÀ1 is assigned to the
(N–H) vibration of N–H group of
L2 ligand. The moderate absorption bands at 2923, 2858 cmÀ1 for
2, 2933, 2851 cmÀ1 for 3 may be assigned to the
(Cmethenyl–H)
vibration of BPP ligands. The strong and sharp bands at 1678 and
1680 cmÀ1 are assigned to the carbonyl group of the DMF
molecule. The features at 1588 and 1407 cmÀ1 for 1, 1628 and
m
(Cmethyl–H) vibra-
1569, 1421 cmÀ1 for 2, 1626 and 1479, 1417 cmÀ1 for 3, are asso-
ciated with the asymmetric (C-O-C) and symmetric (C–O–C)
m
stretching vibrations. The Dm(m m
as(COOÀ)– s(COOÀ)) values are
181 cmÀ1 for 1, 59 and 207 cmÀ1 for 2, 147 and 209 cmÀ1 for 3,
indicating the coordination of L1, L2, L3 with zinc in bridging, uni-
dentate and chelating, bridging and unidentate modes, respec-
tively [17], which is well consistent with the X-ray diffraction
structural analysis.
m

D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
89
Fig. 3. View of structure 3. (a) ORTEP diagram showing the coordination environment for Zn(II) ions in 3. All H atoms are omitted for clarity (symmetry codes: (i) 1Àx, 1Ày,
stacking interactions and hydrogen bonds (pink dash lines). (For interpretation of the
1Àz); (b) View of the (4,4) layer in 3; (c) View of the 3D framework of 3 formed by pÁ Á Áp
references to color in this figure legend, the reader is referred to the web version of this article.)
3.2.3. Thermal analyses
The thermogravimetric anslyses (TGA) of complexes 1–3 were
performed in a N2 atmosphere wherein the sample was heated to
800 °C at a rate of 10 °C/min. The TG curves are shown in Fig. 4.
Complex 1 shows two weight loss steps. The first corresponding
to the release of two DMF molecules is observed from 250 °C to
300 °C (obsd 16.1%, calcd 15.9%). The weight-loss step occurred
above 400 °C which corresponds to the decomposition of the
framework structure. Complex 2 has thermal stability as no strictly
clean weight loss step occurs below 360 °C. Then a sharp weight
loss occurred above 360 °C due to the decomposition of the struc-
ture. Complex 3 also shows two weight loss steps. The first corre-
sponding to release of one free DMF molecule is observed from
30 °C to 120 °C (obsd 10.2%, calcd 10%). The compound begins to
decompose when the temperature is raised to 350 °C.
Fig. 4. Thermogravimetric curves for 1–3.
3.2.4. Luminescent properties
The solid-state photoluminescent properties of the free ligands
The photoluminescent spectra of L1, L2 and L3 show emission
maxima at 383 nm (kex = 330 nm), 345 nm (kex = 317 nm) and
329 nm (kex = 296 nm), respectively. The emission bands of these
and complexes 1–3 have been investigated at room temperature.
The emission peaks of L1, L2, L3 and complexes 1–3 are shown in
Fig. 5.

90
D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
Fig. 7. TGA of 1–3 with water absorbate.
Fig. 5. Solid-state emission spectra of L1–L3 and compounds 1–3 at room
temperature.
sion peaks occur at 535 nm (kex = 421 nm) for 1, 487 nm
(kex = 340 nm) for 2, 502 nm (kex = 407 nm) for 3, all of which are
clearly red-shifted compared with the free ligands. The emissions
of 1–3 probably originate from a ligand-to-metal charge transfer
(LMCT) [18] excited state, because the L1, L2, L3, and BPP ligands
with their extended aromaticity are regarded as good candidates
for enhancement of emissive properties. The luminescent lifetimes
of solids 1–3 measured using an Edinburgh FLS920 phospho-
rimeter with a 450 W xenon lamp as excitation source were,
28.04, 3.08, 2.43 ns, respectively (Fig. S2).
3.2.5. Stability of the complexes
To investigate the stability of complexes 1–3, the kinetic trap ef-
fects of water on the samples were monitored by TGA. The TGA
curve shows that the partially decomposed 3 still adsorbed large
numbers of water molecules, and the structure mostly collapsed
with the removal of water upon heating (Fig. 7). 2 exhibits en-
hanced hydrophobicity as compared to 3 and adsorbed up to
10 wt.% water at room temperature. Increasing the water repellent
groups, 1 displays a different TG curve from 2. 1 adsorbed up to
7.5 wt.% water at 25 °C, and there is no additional weight loss upon
heating up to 300 °C. The weight-loss step occurred below 105 °C
which corresponds to the release of free water molecules inside
the pores (ꢀ3 wt.%) and two coordinated water molecules replac-
ing the two DMF molecules of the unit cell (obsd 4.5%, calcd 4.4%).
The samples of 1–3 were treated under high vacuum at 120 °C
for 12 h, then were exposed to air (relative humidity of 80–90%)
and examined using PXRD to provide insight on the structural
integrity of the materials under ambient conditions. PXRD shows
that 3 was unstable in air and its structure mostly collapsed after
moisture exposure for 7 days (Fig. 6c). After exposing to air 7 days,
no new peaks appear for 2, but all of the peak intensities for 2 de-
crease (Fig. 6b). In contrast, PXRD patterns of 1 remain essentially
unchanged up to 30 days, with no new peaks and apparent loss in
peak intensity (Fig. 6a). Compared to 3, the results indicate that the
incorporation of the water repellent groups of the ligand (methyl
for L1, amino for L2) enhance the stability of 1 and 2 under stan-
dard atmospheric conditions. It has been demonstrated that the
introduction of water repellent functional groups (e.g. methyl)
within the frameworks can largely enhance the hydrophobic prop-
erties of MOFs [1c].
4. Conclusion
Fig. 6. PXRD patterns for (a) 1, (b) 2 and (c) 3.
In conclusion, three new zinc-organic frameworks based on
structurally related 1,4-benzenedicarboxylate with or without
1,3-bis(4-pyridyl)propane have been prepared under solvothermal
conditions. These compounds display interesting and versatile 2D
coordination features. The structural diversities of the complexes
free ligands are probably attributable to the p⁄–n or p⁄
tion. Upon complexation of these ligands with Zn(II) ions, the emis-
–p transi-

D.-Y. Ma et al. / Inorganica Chimica Acta 396 (2013) 84–91
91
45 (2006) 616;
show that the organic anions play important roles in the formation
(c) L.F. Ma, L.Y. Wang, M. Du, S.R. Batten, Inorg. Chem. 49 (2010) 365;
(d) L.F. Ma, Q.L. Meng, C.P. Li, B. Li, L.Y. Wang, M. Du, F.P. Liang, Cryst. Growth
Des. 10 (2010) 3036;
(e) L.F. Ma, X.Q. Li, Q.L. Meng, L.Y. Wang, M. Du, H.W. Hou, Cryst. Growth Des.
11 (2011) 175;
(f) L.F. Ma, B. Liu, L.Y. Wang, C.P. Li, M. Du, Dalton Trans. 39 (2010) 2301;
(g) L.F. Ma, J.W. Zhao, M.L. Han, L.Y. Wang, M. Du, Dalton Trans. 41 (2012)
2078.
of the MOFs. Complexes 1–3 exhibit strong blue photolumines-
cence in the solid state at room temperature, which may have po-
tential applications as luminescent materials in blue light-emitting
devices. The moisture-stability results of 1–3 reveal that the intro-
duction of water repellent groups (e.g. methyl, amino) of the or-
ganic ligands in a MOF could efficiently enhance the water
resistance of MOF materials.
[3] (a) D. Ma, K. Lu, H. Guo, Y. Pan, J. Liu, J. Mol. Struct. 1021 (2012) 179;
(b) G. Zhang, G. Yang, Q. Chen, J.S. Ma, Cryst. Growth Des. 5 (2005) 661;
(c) R.W. Saalfrank, I. Bernt, M.M. Chowdhury, F. Hammpel, G.B.M. Vaughan,
Chem. Eur. J. 7 (2001) 2765.
[4] Y.Q. Lan, S.L. Li, J.S. Qin, D.Y. Du, X.L. Wang, Z.M. Su, Q. Fu, Inorg. Chem. 47
(2008) 10600.
[5] B. Kesanli, Y. Cui, M.R. Smith, E.W. Bittner, B.C. Bockrath, W. Lin, Angew. Chem.,
Int. Ed. 44 (2005) 72;
(b) X.M. Chen, G.F. Liu, Chem. Eur. J. 8 (2002) 4811.
[6] C.B. Ma, C.N. Chen, Q.T. Liu, D.Z. Liao, L.C. Li, L.C. Sun, New J. Chem. 27 (2003)
890.
Acknowledgments
We are thankful for financial support of this work provided by
The Foundation for Distinguished Young Talents in Higher Educa-
tion of Guangdong Province (50304004), the Zhaoqing University
with doctoral start-up fund (611268) and the Science and Technol-
ogy Planning Project of Zhaoqing City (2012G013). Thanks to Dr.
Sean Parkin for all his help with the suggestions for the paper.
[7] (a) F.P. Huang, J.L. Tian, G.J. Chen, D.D. Li, W. Gu, X. Liu, S.P. Yan, D.Z. Liao, P.
Cheng, CrystEngComm 12 (2010) 1269;
(b) F.W. Zhang, Z.F. Li, T.Z. Ge, H.C. Yao, G. Li, H.J. Lu, Y.Y. Zhu, Inorg. Chem. 49
(2010) 3776.
Appendix A. Supplementary material
[8] (a) L. Carlucci, G. Ciani, D.M. Proserpio, S. Rizzato, CrystEngComm 4 (2002)
121;
(b) M.L. Zhang, D.S. Li, J.J. Wang, F. Fu, M. Du, K. Zou, X.M. Gao, Dalton Trans. 38
(2009) 5355;
(c) C. Li, D.S. Li, J. Zhao, Y.Q. Mou, K. Zou, S.Z. Xiao, M. Du, CrystEngComm 13
(2011) 6601;
(d) C.X. Meng, D.S. Li, J. Zhao, F. Fu, X.N. Zhang, L. Tang, Y.Y. Wang, Inorg. Chem.
Commun. 12 (2009) 793.
CCDC Nos. 884819–884821 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2012.10.006.
[9] Bruker, APEXII software, version 6.3.1, Bruker AXS Inc., Madison, Wisconsin,
USA, 2004.
[10] S. Parkin, B. Moezzi, H. Hope, J. Appl. Crystallogr. 28 (1995) 53.
[11] G.M. Sheldrick, Acta Crystallogr., A 64 (2008) 112.
[12] (a) C.Y. Niu, X.F. Zheng, Y. He, Z.Q. Feng, C.H. Kou, CrystEngComm 12 (2010)
2847;
References
(b) A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem. Commun. 9 (2006) 1169.
[13] I. Senkovska, J. Fritsch, S. Kaskel, Eur. J. Inorg. Chem. (2007) 5475.
[14] (a) E. Schon, D.A. Plattner, P. Chen, Inorg. Chem. 43 (2004) 3164;
(b) J. Li, J.H. Zhou, Y.Z. Li, L.H. Weng, X.T. Chen, Z. Yu, Z. Xue, Inorg. Chem.
Commun. 7 (2004) 538.
[1] (a) H. Deng, S. Grunder, K.E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F.
Gandara, A.C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki,
J.F. Stoddart, O.M. Yaghi, Science 336 (2012) 1018;
(b) D. Bradshaw, J.B. Claridge, E.J. Cussen, T.J. Prior, M.J. Rosseinsky, Acc. Chem.
Res. 38 (2005) 273;
[15] L. Carlucci, G. Ciani, D.M. Proserpio, Coord. Chem. Rev. 246 (2003) 247.
[16] (a) O.R. Evans, W. Lin, Chem. Mater. 13 (2001) 2705;
(b) G. Li, Z. Li, H. Hou, X. Meng, Y. Fan, W. Chen, J. Mol. Struct. 694 (2004) 179.
[17] Y. Zheng, D.M. Xu, S.X. Liu, Inorg. Chim. Acta 294 (1999) 163.
[18] Z. Li, M. Li, X.P. Zhou, T. Wu, D. Li, S.W. Ng, Cryst. Growth Des. 7 (2007) 1992.
(c) D. Ma, Y. Li, Z. Li, Chem. Commun. 47 (2011) 7377;
(d) D. Zacher, O. Shekhah, C. Woll, R.A. Fischer, Chem. Soc. Rev. 38 (2009) 1418;
(e) B.Q. Ma, K.L. Mulfort, J.T. Hupp, Inorg. Chem. 44 (2005) 4912.
[2] (a) A.Y. Robin, K.M. Fromm, Coord. Chem. Rev. 250 (2006) 2127;
(b) L. Pan, D.H. Olson, L.R. Ciemnolonski, R. Heddy, J. Li, Angew. Chem., Int. Ed.