Self-assembly of three d10 metal coordination polymers based on a flexible bis(2-methylbenzimidazole) and dicarboxylate co-ligands

Article abstract of DOI:10.1016/j.molstruc.2014.04.037

Hydrothermal self-assembly of zinc nitrate with 1,1′-(1,3-propane) bis-(2-methylbenzimidazole) (pbmb) and different dicarboxylic acid ligands gave rise to three new coordination polymers [Zn(pbmb)(hmph)]_n (1), {[Zn₂(pbmb)(chdc)₂]·0.5H₂O}_n (2), [Zn(pbmb)(mip)]_n (3) (H₂hmph = homophthalic acid, H₂chdc = 1,4-cyclohexanedicarboxylic acid and H₂mip = 5-methylisophthalic acid). Both complexes 1 and 3 possess 2D {6³} framework and further extend into 3D supramolecular network via CH?O hydrogen bonding or π-π stack interactions. While 2 is a 1D double loop-like chain structure, which arranged into a 2D network through CH?O hydrogen bonding interactions. Three compounds all exhibit strong photoluminescence at room temperature in solid state and may be good candidates for potential luminescence materials.

Full text of DOI:10.1016/j.molstruc.2014.04.037

Journal of Molecular Structure xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Self-assembly of three d10 metal coordination polymers based on a
flexible bis(2-methylbenzimidazole) and dicarboxylate co-ligands
⇑
Jin-ming Hao, Ying-na Zhao, Rui Yang, Guang-hua Cui
College of Chemical Engineering, Hebei United University, 46 West Xinhua Road, Tangshan 063009, Hebei, People’s Republic of China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
Three d10 metal coordination
polymers have been characterized.
Effect of dicarboxylates on the
structure of complexes was
discussed.
ꢀ
ꢀ
TG analysis confirms the stability of
the coordination polymers.
Luminescence properties of
complexes have been investigated.
a r t i c l e i n f o
a b s t r a c t
Hydrothermal self-assembly of zinc nitrate with 1,10-(1,3-propane)bis-(2-methylbenzimidazole) (pbmb)
and different dicarboxylic acid ligands gave rise to three new coordination polymers [Zn(pbmb)(hmph)]n
(1), {[Zn2(pbmb)(chdc)2]ꢁ0.5H2O}n (2), [Zn(pbmb)(mip)]n (3) (H2hmph = homophthalic acid, H2chdc = 1,4-
cyclohexanedicarboxylic acid and H2mip = 5-methylisophthalic acid). Both complexes 1 and 3 possess 2D
Article history:
Received 17 January 2014
Received in revised form 10 April 2014
Accepted 11 April 2014
Available online xxxx
{63} framework and further extend into 3D supramolecular network via CAHꢁ ꢁ ꢁO hydrogen bonding or
p–
p
stack interactions. While 2 is a 1D double loop-like chain structure, which arranged into a 2D network
Keywords:
Bis(2-methylbenzimidazole)
Dicarboxylate
through CAHꢁ ꢁ ꢁO hydrogen bonding interactions. Three compounds all exhibit strong photoluminescence
at room temperature in solid state and may be good candidates for potential luminescence materials.
Ó 2014 Elsevier B.V. All rights reserved.
Zn(II) complexes
Introduction
These MOFs materials result from the reaction between organic
and inorganic species could display infinite zero-, one-, two- or
The rational design and synthesis of metal-organic frameworks
(MOFs) have received great considerable attention, not only as
their versatile intriguing architectures and topologies, but also as
their potential applications in luminescence, magnetism, porosity
and biological modeling material [1–4]. Studies in this field have
been focused on construction of novel coordination frameworks
and the relationships between their structures and properties [5].
three-dimensional structures [6,7]. However, it is still a great
challenge to predict the exact structures and compositions of the
assembly products built by coordination bonds and/or hydrogen
bonds in hydrothermal reactions, owing to the facts that the
assembly of such complexes can be easily influenced by the
geometrical and electronic properties of metal ions and ligands,
temperature, pH value of the solution, etc. [8–14].
Flexible N-containing ligands, which possess rich structural
information and free conformation, are highly attractive because
their flexibilities allow for greater structural diversity
[15,16,13,17,18]. The flexible bis(benzimidazole) derivatives in
⇑
Corresponding author. Tel.: +86 0315 2592169; fax: +86 0315 2592170.
E-mail address: tscghua@126.com (G.-h. Cui).
http://dx.doi.org/10.1016/j.molstruc.2014.04.037
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
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2
J.-m. Hao et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
which the two benzimidazole rings can freely twist around the
flexible ACH2A spacer to meet the requirements of the coordina-
tion geometries of metal atoms in the assembly process. Some
complexes with intriguing network topologies and interesting
properties based on bis(benzimidazole) ligands have been
reported, however, the coordination polymers derived from 1,10-
(1,3-propane)bis-(2-methylbenzimidazole) (pbmb) are relatively
rare. As ongoing our previous works [19–21], we report the synthe-
sis and crystal structures of three new zinc(II) coordination poly-
mers: [Zn(pbmb)(hmph)]n (1), {[Zn2(pbmb)(chdc)2]ꢁ0.5H2O}n (2),
[Zn(pbmb)(mip)]n (3), where H2hmph = homophthalic acid, H2-
chdc = 1,4-cyclohexanedicarboxylic acid and H2mip = 5-methyli-
sophthalic acid. Thermal stabilities and fluorescence properties of
these complexes are also discussed.
Synthesis of complex [Zn(pbmb)(mip)]n (3)
The procedure was similar to that of 1 except that H2mip were
used instead of H2hmph. The colorless crystals were obtained with
a yield of 52% (based on Zn). Anal. Calcd. for C28H26N4O4Zn
(Mr = 547.90): C 61.38, H 4.78 and N 10.23%. Found: C 61.16, H
4.59 and N 10.02%. IR (KBr, cmꢂ1): 2918(w), 1634(s), 1516(m),
1450(m), 1372(s), 1328(m), 1265(m), 1159(w), 1090(w), 935(w),
759(m), 465(w).
X-ray crystallography
All data were collected on a Bruker Smart 1000 CCD diffractome-
ter using graphite-monochromated Mo K
a radiation (k = 0.71073 Å)
at room temperature with -scan mode. A semi-empirical absorp-
x
tion correction was applied using SADABS program [23]. The struc-
tures were solved by direct methods and refined on F2 by full-matrix
least-squares using SHELXTL [24]. Hydrogen atoms of water mole-
cules were located on a difference Fourier map, while other hydro-
gen atoms were included in calculated positions and refined with
isotropic thermal parameters riding on the parent atoms. Crystallo-
graphic crystal data and structure processing parameters for 1, 2 and
3 are summarized in Table 1. Selected bond lengths and bond angles
of the complexes are listed in Table S1 (Supporting Information).
Experimental
Materials and measurements
All chemicals were of reagent grade, commercially available and
used without further purification. The ligand pbmb (Chart 1) was
prepared according to the literature method [22]. Elemental anal-
yses were made on a Perkin–Elmer automatic analyzer. IR spectra
were recorded on a Nicolet FT-IR Avatar 360 spectrophotometer in
4000–400 cmꢂ1 region using KBr pellets. Thermal analyses were
performed on a Netzsch TG 209 thermal analyzer from room tem-
perature at a heating rate of 10 °C/min. X-ray powder diffraction
measurement was executed on a D/MAX 2500PC X-ray diffractom-
Results and discussion
Crystal structures of 1
eter using Cu Ka radiation (k = 0.1542 nm) in the 2h range of 5–50°
with a step size of 0.02° and a scanning rate of 10° minꢂ1. The solid
fluorescence spectra were performed with a Hitachi F-7000 fluo-
rescence spectrophotometer at room temperature.
Single crystal X-ray diffraction analysis shows that complex 1
crystallizes in the monoclinic space group P21/n and is a 2D network.
In the asymmetric unit of 1, there is one Zn center, one pbmb and one
hmph2ꢂ ligands. As shown in Fig. 1a, each Zn(II) atom sits in a
Synthesis of complex [Zn(pbmb)(hmph)]n (1)
Table 1
A mixture of Zn(NO3)2ꢁ6H2O (0.1 mmol), pbmb (0.1 mmol), H2-
hmph (0.1 mmol), NaOH (0.2 mmol) and H2O (10 mL) was placed
in a 25 mL Teflon-lined stainless steel vessel. The mixture was
sealed and heated at 140 °C for 3 days. After the mixture was
cooled to room temperature at a rate of 5 °C/h, colorless block-
shaped crystals of 1 were obtained with a yield of 41% (based on
Zn). Anal. Calcd. for C28H26ZnN4O4 (Mr = 547.90): C 61.38, H 4.78,
and N 10.23%. Found: C 60.86 H 4.53 and N 9.84%. IR (KBr,
cmꢂ1): 2922(w), 1644(s), 1608(s), 1513(w), 1461(m), 1416(s),
1247(w), 1151(m), 1074(m), 937(w), 748(m), 670(w), 451(m).
Crystal and refinement data for complexes 1, 2 and 3.
1
2
3
Empirical
formula
Formula
weight
Crystal
system
Space group
a (Å)
C
28H26N4O4Zn
C35H41N4O8.50Zn2
C28H26N4O4Zn
547.90
784.46
547.90
Monoclinic
Triclinic
Monoclinic
ꢀ
P21/n
P21/n
P1
11.368(2)
17.489(3)
13.533(2)
10.3024(10)
11.9362(11)
13.6767(13)
84.1570(10)
85.3840(9)
83.8400(10)
1659.3(3)
2
12.090(2)
17.439(3)
12.629(2)
b (Å)
c (Å)
a
(deg)
Synthesis of complex {[Zn2(pbmb)(chdc)2]ꢁ0.5H2O}n (2)
b (deg)
109.706(2)
108.314(3)
c
(deg)
The procedure was similar to that of 1 except H2chdc were used
instead of H2hmph. The colorless crystals were obtained with a
yield of 36% (based on Zn). Anal. Calcd. for C35H41N4O8.50Zn2
(Mr = 784.46): C 53.59, H 5.27 and N 7.14%. Found: C 53.24, H
5.02 and N 6.92%. IR (KBr, cmꢂ1): 3420(m), 3039(w), 2932(m),
1610(s), 1575(s), 1520(w), 1455(m), 1415(s), 1295(w), 924(w),
749(m), 656(w), 454(w).
V (Å3)
2533.0(8)
4
1.437
1.012
1136
2527.9(8)
4
1.440
1.014
1136
Z
Dcalc (g/m3)
1.570
1.507
814
l
(mmꢂ1
)
F(000)
Crystal size
(mm)
0.25 ꢃ 0.22 ꢃ 0.21 0.26 ꢃ 0.23 ꢃ 0.22 0.26 ꢃ 0.23 ꢃ 0.23
Total
15,079
5768
8507
5779
15,369
5848
reflections
Unique
reflections
Rint
0.0727
0.968
0.0251
1.024
0.0466
1.008
GOF
R1 (I > 2
wR2 (I > 2
r
(I))
0.0499
0.0406
0.1024
0.434
0.0460
0.1198
0.421
N
N
r(I)) 0.0949
D
D
q
max
(eÅꢂ3
min (eÅꢂ3
0.369
)
N
N
q
)
ꢂ0.271
ꢂ0.401
ꢂ0.343
2
1/2
R1
=
R
||Fo| ꢂ |Fc||/
R
|Fo|; wR2
=
R
[wF2o ꢂ F2c 2]/
R
[wF2o
]
.
Chart 1. The ligand pbmb.
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J.-m. Hao et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
3
Fig. 1. (a) The coordination environment of Zn(II) ions in complex 1 with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity (symmetry code: #1 x + 1/2, ꢂy + 1/2,
z + 1/2; #2 ꢂx + 1, ꢂy + 1, ꢂz + 1); (b) 1D zig–zag chain constructed by hmph2ꢂ anions; (c) view of the 2D butterfly-shaped layer structure of 1; (d) view of the 2D 63 network
of 1; (e) simplified view of the 3D binodal (4,4)-connected net of {3.4.5.6.72}2{3.6.72.82} topology in complex 1.
four-coordinate tetrahedral geometry with the value of the
being 0.84 [25], defined by two nitrogen atoms N3 and N1#2 from
two different pbmb ligands and two oxygen atoms (O1 and O3#1)
s
4 factor
from two independent hmph2ꢂ anions (symmetry code #1: x + 1/
2, ꢂy + 1/2, z + 1/2; #2: ꢂx + 1, ꢂy + 1, ꢂz + 1). The ZnAN bond dis-
tances are 2.031(3) Å (Zn1AN3) and 2.084(3) Å (Zn1AN1#2) and
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J.-m. Hao et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
ZnAO lengths are 1.931(2) (Zn1AO3#1) and 1.955(2) Å (Zn1AO1),
which are comparable to those of the similar zinc complexes [26].
In complex 1, the carboxylate groups of hmph2ꢂ ligands adopt
bis-monodentate coordination mode bridge adjacent Zn atoms giv-
ing rise to a 1D zig–zag chain [Zn(hmph)] (Fig. 1b), in which the
Znꢁ ꢁ ꢁZn separation is 7.703(8) Å. The pbmb ligands exhibit anti-
conformation with the dihedral angle between the two benzimid-
azole planes of 83.870(8)° and the Znꢁ ꢁ ꢁZn distance across a pbmb
ligand is 10.648(1) Å. The chains are connected by pbmb ligands to
form a 2D butterfly-shaped network (Fig. 1c). From a topological
perspective, if Zn(II) ion is simplified as a 3-connected node and
the ligands all are considered as linkers, the 2D structure can be
described as a 63 topology (Fig. 1d). In addition, the adjacent 2D
layers are further stacked into a 3D supermolecule framework by
three kinds of H-bonding interactions in which the
C8AH8Aꢁ ꢁ ꢁO3#3, C9AH9Bꢁ ꢁ ꢁO4#2 and C11AH11Aꢁ ꢁ ꢁO4#2 (sym-
metry code #3: 1/2 ꢂ x, 1/2 + y, 1/2 ꢂ z) bond distances are
3.190(6), 3.191(5) and 3.076(4) Å and the bond angles are 150°,
146° and 118°. To better understand the supermolecule framework
of 1, the topological analysis approach is employed. All Zn(II)
centers acting as four-connecting nodes, connecting two pbmb
ligands and two hmph2ꢂ anions. And the pbmb ligands implified
to be 4-connected nodes bridging two hmph2ꢂ anions and two
Zn(II) centers and then the hmph2ꢂ are simplified as 4-connected
nodes. As a result, the 3D supermolecule network can be reduced
as a new binodal (4,4)-connected net by TOPOS 4.0 program [27],
with a point symbol of {3.4.5.6.72}2{3.6.72.82} (Fig. 1e).
Crystal structures of 2
Complex 2 crystallizes in the triclinic system with Pı space
¯
group. As shown in Fig. 2a, the fundamental structural unit of 2
contains two crystallographic independent Zn(II) centers (Zn1
and Zn2), one bridging pbmb, two chdc2ꢂ anions and half a lattice
water molecule. Each Zn1 is five-coordinated by one N1 atom and
four oxygen atoms (O3#1, O4, O5#1 and O6) (symmetry code #1:
ꢂx + 1, ꢂy + 1, ꢂz + 1) from four chdc2ꢂ anions in a monodentate
mode to conform the square pyramid coordination geometry,
which is pointed out by a s5 value of 0.009. A pair of Zn1 centers
with a separation of 3.021(6) Å are bridged by four carboxylate
groups from four chdc2ꢂ anions lying about 90° apart about each
other to form a paddle wheel-like unit of [Zn2(chdc)4], leaving
the apical sites at both terminals of the Znꢁ ꢁ ꢁZn axis occupied by
the nitrogen atoms. The Zn1AN bond distance is 2.037(3) and
the Zn1AO bond lengths are in the range of 2.027(3)–2.085(2) Å,
which are comparable to those of the similar zinc complexes
[26]. While Zn2 centers are also five-coordinated to one chelating
and two monodentate carboxylate groups (O1#3, O2#3, O7#2
and O8) (symmetry code #2: ꢂx, ꢂy + 1, ꢂz + 2; #3: x ꢂ 1, y,
z + 1) belonging to three chdc2ꢂ anions and one nitrogen atom
(N4), which have different coordination configuration with Zn1
Fig. 2. (a) The coordination environment of Zn(II) ions in complex 2. Hydrogen
atoms and the free water molecule are omitted for clarity (symmetry code: #1:
ꢂx + 1, ꢂy + 1, ꢂz + 1; #2: ꢂx, ꢂy + 1, ꢂz + 2; #3: x ꢂ 1, y, z + 1); (b) 1D loop-like
chain generate by two kinds of dinuclear units; (c) the 1D double loop-like chain of
2; (d) simplified view of the trinodal (3,4,6)-connected network topology of 2.
of chdc2ꢂ and pbmb ligands (C(11)AH(11B)ꢁ ꢁ ꢁO(2) = 3.189(5) Å,
169°), respectively. In order to more fully understand the architec-
ture of complex 2, the topological method was used to simplify and
analyze the 2D supramolecular framework. The first kind of dinu-
clear units implified to be a 6-connected node bridging four pbmb
ligands and two dinuclear units, the second dinuclear units bound
by two pbmb ligands and two dinuclear units acting as a 4-con-
nected node and then the pbmb ligands are simplified as topolog-
ically equivalent 3-connected nodes. The succeeding topology
analysis by TOPOS 4.0 program suggests the 2D coordination
framework can be simplified as a trinodal (3,4,6)-connected net-
work with a point symbol of {3.4.5}2{32.42.52.62.74.82.9}{32.62.72}
(Fig. 2d).
units, the value of the s5 factor being 0.516 demonstrating a distort
trigonal bipyramidal structure. Two Zn2 atoms linked by four
chdc2ꢂ anions to form a dinuclear unit Zn2(COO)6 with a 8-member
ring and the non-bonding Zn2ꢁ ꢁ ꢁZn2 distance is 3.798(7) Å. And
then, two kinds of dinuclear units [Zn2(chdc)4] and Zn2(COO)6
arranged alternately to generate a 1D loop-like chain (Fig. 2b).
Interestingly, the dinuclear units are further bridged by the pbmb
ligands with anti-conformations to form a double loop-like chain
(Fig. 2c), where the Zn1ꢁ ꢁ ꢁZn2 distance across pbmb ligand is
7.936(9) Å and the dihedral angles between the mean planes of
the two benzimidazole rings are 77.053(7)°. Furthermore, the adja-
cent 1D chains are constructed to a 2D supermolecule framework
via intermolecular CAHꢁ ꢁ ꢁO hydrogen-bonding interactions
between the lattice water molecules and pbmb ligands
(C(19)AH(19C)ꢁ ꢁ ꢁO(1 W) = 2.91(2) Å, 123°) and carboxyl groups
Crystal structures of 3
Single-crystal X-ray analysis shows that complex 3 is a 2D
lamella structure and it crystallizes in monoclinic space group
P21/n. The asymmetric unit contains a crystallographically distinct
Zn(II) cation, one completely deprotonated mip2ꢂ anion and one
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J.-m. Hao et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
5
flexible pbmb adopts anti conformation with the dihedral angles
between two benzimidazole rings being 84.033(8)°. Furthermore,
the adjacent 2D layers are extended into 3D supramolecular frame-
work by the
p–p stacking interactions with the center-to-center
separations of 3.502 Å between neighboring imidazole rings and
benzene rings (Fig. 3d).
Effect of dicarboxylate coligands on the structures of 1–3
As it is shown in the descriptions above, three d10 coordination
polymers 1–3 with the pbmb ligand and different dicarboxylates
were successfully synthesized and characterized. Based on the X-
ray analysis results, the pbmb ligand uniformly behave as the bis-
monodentate linkers to connect the metal centers in 1–3, but exhibit
different bending and rotating ability, which lead to generating dif-
ferent non-bonding Znꢁ ꢁ ꢁZn distances (10.648(1) Å for 1, 7.936(9) Å
for 2, 10.155 Å for 3) and the different dihedral angles (83.870(8)° in
1, 77.053(7)° in 2, 84.033(8)° in 3) between two benzimidazole rings
with one pbmb ligand. The coordination behavior of dicarboxylate
anions also has essential influence on the structures of complexes.
In complex 1, hmph2ꢂ anions adopt bis-monodentate coordination
mode bridge adjacent Zn atoms giving rise to a 1D zig–zag chain,
which is extended by pbmb ligands resulting in a 2D (6, 3) network.
While in 2, chdc2ꢂ anions show two kinds of coordination mode
fashions. In one fashion, the ligand contains one chelating and one
bridging bidentate carboxylate groups; in the other, the ligand con-
tains two bidentate carboxylate groups. The Zn centers are coordi-
nated by chdc2ꢂ anions in two fashions mentioned above to form
1D loop-like chain with paddlewheel-like building blocks [Zn2(-
chdc)4] and dinuclear units Zn2(COO)6 arranged alternately. The
loop-like chain is further bridged by the pbmb ligands to form a dou-
ble loop-like chain. For 3, mip2ꢂ anions only act as bridging ligand to
link two neighboring Zn(II) centers to form a 1D zig–zag-shaped
chain, which are linked by flexible pbmb ligands to produce a 2D
{63} undulated layer. These results demonstrate that the coordina-
tion behavior of dicarboxylate co-ligands have significant contribu-
tions to the conformation of complexes and provide the potential for
directing the coordination of flexible ligands with metal ions during
the two-ligand assembly system.
IR spectra
In IR spectra of 1, 2 and 3, no band in the region 1690–1730 cmꢂ1
,
indicates complete deprotonation of the carboxyl groups. The broad
band center at around 3420 cmꢂ1 for 2, relates to the OAH stretching
vibration modes of water molecule. The absorption bands observed
at 2922 cmꢂ1 for polymer 1, 2932 cmꢂ1 for polymer 2 and
2918 cmꢂ1 for polymer 3 could be associated with the stretching
vibrations of ACH2A. The bands at 1513 cmꢂ1 for 1, 1520 cmꢂ1 for
2 and 1516 cmꢂ1 for 3 can be assigned to the mC@N absorption of
benzimidazole rings of the ligand pbmb. The asymmetric and sym-
metric stretching vibrations of carboxyl groups are observed at
1644, 1461 and 1416 cmꢂ1 for 1, 1610, 1575, 1455 and 1415 cmꢂ1
for 2 and 1634 and 1372 cmꢂ1 for 3, respectively. The separations
Fig. 3. (a) The coordination environment of Zn(II) ions in complex 3. Hydrogen
atoms are omitted for clarity (symmetry code: #1: x + 1/2, ꢂy + 1/2, z + 1/2; #2:
ꢂx + 2, ꢂy, ꢂz); (b) 1D zig–zag-shaped chain bridged by mip2ꢂ anions along the c
axis; (c) 2D {63} undulated layer in complex 3; (d) view of the 3D supramolecular
framework by p–p interactions.
(Dm[mas(COO)Ams(COO)]) between these bands indicate the pres-
ence of monodentate (228 cmꢂ1 for 1 and 262 cmꢂ1 for 3), chelating
(183 cmꢂ1 for 1 and 120 cmꢂ1 for 2) and bridging (195 cmꢂ1 for 2)
coordination modes of carboxyl groups [28].
pbmb molecule, as shown in Fig. 3a. Each Zn(II) ion is four-coordi-
nated by two carboxylate oxygen atoms from two different mip2ꢂ
anions (Zn1AO1#1 1.925(2) Å, Zn1AO3 1.928(2) Å) (symmetry
code #1: x + 1/2, ꢂy + 1/2, z + 1/2) and two nitrogen atoms from
two pbmb ligands (Zn1AN1 2.018(2) Å, Zn1AN3 2.074(2) Å),
showing a trigonal pyramidal geometry with the s4 value of
0.834. In 3, each mip2ꢂ anion acts as a bridging ligand to link
two neighboring Zn(II) centers, a zig–zag-shaped chain can be
formed (Fig. 3b). The neighboring Znꢁ ꢁ ꢁZn separation across one
mip2ꢂ is 7.967(9) Å. These 1D chains are further linked by flexible
pbmb ligands to produce a 2D {63} undulated layer (Fig. 3c). The
X-ray powder diffraction (XRPD) and thermogravimetric analysis
(TGA)
Complexes 1, 2 and 3 were also characterized by X-ray powder
diffraction (XRPD) at room temperature (Fig. S1 in the Supporting
Information). It is clear that the peak positions in the experimental
patterns are well matched to the corresponding simulated patterns
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J.-m. Hao et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
emission since very similar emission is also observed for free pbmb
ligand. The red shift of emission occurs in complex 1, compared
with free H2hmph (kem = 325 nm and kex = 274 nm), which is
because of the fact that the ligands are not allowed to relax along
the torsional mode on photoexcitation [30]. Complex 2 exhibits a
strong emission with 369 nm (kex = 330 nm) and the observed red
shift of ca. 46 nm between 2 and the 1,4-H2chdc (kem = 323 nm
and kex = 264 nm) ligand can be assigned to may be tentatively
assigned to ligand-to-metal charge transfer (LMCT) as reported
for other ZnII complexes with N-donor ligands [31]. As to 3, the
weak emission of 365 nm (kex = 310 nm) with a small red shift as
compared with the free pbmb and H2mip (kem = 357 nm and
kex = 315 nm) ligands may be tentatively attribute to the
p ?
p*
transition of the coordinated ligands since the Zn2+ ion is difficult
to oxidize or to reduce due to its d10 configuration [32–34].
In addition, further investigation indicates that the fluorescent
intensities of the three complexes increase as compared with the
free pbmb ligand. The varying degrees of red shift may be attrib-
uted to different coordination environments of N-containing ligand
and the different structures caused by different organic dicarboxyl-
ates in this work.
Fig. 4. TGA curves of the three complexes.
Conclusion
In summary, three new d10 coordination polymers based on
bis(2-methylbenzimidazole) ligand have been successfully
obtained under hydrothermal conditions. The structural analysis
demonstrates that the configuration of ligand, carboxylate anions
and supramolecular interantions play an important role in leading
to the different structures of resulting coordination and supramo-
lecular architectures. Furthermore, the three complexes are stable
and have particular fluorescence properties, and they may have
promising application in photophysical chemistry.
Appendix A. Supplementary material
Fig. 5. The solid-state photoluminescent spectra of free pbmb ligands and the three
complexes.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.
04.037.
calculated from the single-crystal X-ray diffraction data, confirm-
ing the phase purities of 1, 2 and 3. The differences in peak inten-
sive for the simulations may be due to the preferred orientation of
the powder samples.
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