Structural and functional studies on coordination polymers based on 5-tert-butylisophthalic acid and N,N′-bis-(4-pyridylmethyl) piperazine

Article abstract of DOI:10.1039/c4ra03569k

By using 5-tert-butylisophthalic acid (H₂tbip) and N,N′-bis-(4-pyridylmethyl) piperazine (bpmp), three coordination polymers formulated as {[Co(bpmp)(tbip)]·H₂O}_n (1), {[Ni(bpmp)_1.5(tbip)(H₂O)]·H₂O} _n (2), and {[Cd(bpmp)_1.5(tbip)]·3H ₂O}_n (3) have been synthesized under hydrothermal conditions. All complexes were characterized by single crystal X-ray diffraction analysis, powder X-ray diffraction analysis, elemental analysis, infrared spectroscopy and thermogravimetric analysis. The results show that complex 1 presents a 3D 6-connected network. Complex 2 shows a three-fold interpenetrating framework based on subunits with {4⁶.6⁴}-bnn hexagonal BN topology while complex 3 shows a 2D → 3D polycatenation framework containing 2D bilayer motifs as the fundamental building units. The photocatalytic properties of complexes 1 and 2 have been evaluated and the results show that they are active as a catalyst for the decomposition of Rhodamine B under visible light. The photoluminescence properties of complex 3 were also assessed in the solid state at room temperature. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03569k

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PAPER
Structural and functional studies on coordination
polymers based on 5-tert-butylisophthalic acid and
N,N0-bis-(4-pyridylmethyl) piperazine†
Cite this: RSC Adv., 2014, 4, 25588
*
*
Bo Xu, Yi-Qiang Sun, Jie Li and Cun-Cheng Li
By using 5-tert-butylisophthalic acid (H2tbip) and N,N0-bis-(4-pyridylmethyl) piperazine (bpmp), three
coordination polymers formulated as {[Co(bpmp)(tbip)]$H2O}n (1), {[Ni(bpmp)1.5(tbip)(H2O)]$H2O}n (2), and
{[Cd(bpmp)1.5(tbip)]$3H2O}n (3) have been synthesized under hydrothermal conditions. All complexes
were characterized by single crystal X-ray diffraction analysis, powder X-ray diffraction analysis,
elemental analysis, infrared spectroscopy and thermogravimetric analysis. The results show that complex
1 presents a 3D 6-connected network. Complex 2 shows a three-fold interpenetrating framework based
on subunits with {46.64}-bnn hexagonal BN topology while complex 3 shows a 2D / 3D polycatenation
framework containing 2D bilayer motifs as the fundamental building units. The photocatalytic properties
of complexes 1 and 2 have been evaluated and the results show that they are active as a catalyst for the
decomposition of Rhodamine B under visible light. The photoluminescence properties of complex 3
were also assessed in the solid state at room temperature.
Received 19th April 2014
Accepted 2nd June 2014
DOI: 10.1039/c4ra03569k
www.rsc.org/advances
In the context of developing MOCPs with desired property,
several approaches have been developed including incorpora-
tion of active metal ions or cluster centers into the frameworks,4
Introduction
Over the past three decades, the design and construction of
metal–organic coordination polymers (MOCPs) have received
great attention from chemists not only due to their intriguing
variety of architectures and topologies,1 but also because of
their potential applications in areas such as gas storage, ion
exchange, sensors or catalysis.2 From the point of view of crystal
engineering, the synthetic strategies to target new hybrid
complexes typically emphasize the use of the specic local
coordination geometry of the central metal cation, as well as the
conguration and coordination geometry and size of the ligand,
to help control the topological structure and potential property
of the product that nal forms under certain reaction condi-
tion.3 By following this approach, the structure and desired
property of MOCPs can be modulated to some degree. But
precise control of the structure and property of MOCPs is still
difficult to achieve because many factors such as temperature,
pH value, counterion, solvent play important role on the
construction process. Therefore, it is undoubtedly important for
further investigation of how these factors have effect on the
assemble process.
postsynthetic modication of MOCPs,5 custom-designing of the
functional organic ligands such as inclusion of substituent
moieties on the organic ligand.6 For example, by using amino–
terephthalic acid Couck and co-authors reported the well-
known MIL-53 with large separation power for CO2 and CH4.6a
We have long been focusing on the construction of MOCPs with
interesting architectures and potential applications based on
exible N-coordinated pyridyl ligand and substituent aromatic
acids.7 For example, we have successfully synthesized several
compounds with interesting 2D / 3D polycatenation or inter-
penating networks based on N,N0-bis-(4-pyridyl-methyl) pipera-
zine (bpmp) ligand and 5-methylisophthalic acid ligand in our
previous studies.7a A series of coordination polymers based on
bpmp ligand have also been reported by LaDuca and co-
workers.8 These results indicate that bpmp ligand is effective
tecton in the construction of coordination complexes with
interesting structures. Bearing the aforementioned ideas in
mind, we now expand our work to bpmp ligand and rigid 5-tert-
butylisophthalic acid ligand system. The above mentioned
mixed ligand system was chosen to build entangled MOCPs
owing to the following two considerations: (a) it is well known
that mixed ligand systems provide more coordination modes
and increase structure diversity of the outcome compounds.9
Thus, they are required to realize entangled networks with
interesting topological structures. (b) The substituent isobutyl
group in the 5-tert-butylisophthalic acid ligand acting as aux-
ochromic and bathochromic group may shi the absorption
Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University
of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan,
Shandong, 250022, China. E-mail: chm_xub@ujn.edu.cn; chm_licc@ujn.edu.cn
† Electronic supplementary information (ESI) available: Powder XRD patterns, IR
spectra, UV spectra and TG curves and table of collected bonds and angles. CCDC
993066, 981096 and 981097. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ra03569k
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wavelength and promote charge transfer interactions of the slowly cooled to room temperature during 24 hours. Red block
resulting compounds, which is promising for the development crystals were recovered by ltration, washed by distilled water,
of luminescent probes and visible-light photocatalysts. (c) The and dried in air at ambient temperature. Yield: 47% (based on
5-tert-butylisophthalic acid ligand with two carboxyl groups and Co). Calcd for C28H34N4O5Co (565.53): C 59.47, H 6.06, N 9.91;
a isobutyl group has been somewhat seldom involved in the found: C 59.56, H 6.11, N 9.97. IR (KBr, cmꢀ1): 3435 (s, br),
assemble of MOCPs. Noticeably, one feature of the H2tbip 2950(w), 2815(m), 1618(s), 1534(m), 1458(m), 1371(s), 1161(m),
ligand is that it is anionic when coordinating to metal cation 1014(m), 843(m), 726(m).
and thus fullls the charge balance of the coordination
complex. This feature can prevent counter ions coordinating to
the metal center, which usually happens in a neutral ligand
system and reduce the coordination variety between metal
cation and the ligand. Now, this time we successfully synthesize
three coordination architectures with interesting topological
structures based on mixed bpmp and the 5-tert-butylisophthalic
acid ligand system. In the present work, we give a report on the
synthesis and characterization of three coordination
compounds formulated as {[Co(bpmp)(tbip)]$H2O}n (1),
{[Ni(bpmp)1.5(tbip)(H2O)]$H2O}n (2) and {[Cd(bpmp)1.5(tbip)]$
3H2O}n (3). Their structures were determined by single-crystal X-
ray diffraction analyses and further characterized by elemental
analysis, infrared spectrum (IR), thermogravimetric analysis
(TGA) and powder X-ray diffraction (PXRD). In addition, pho-
tocatalytic properties of complexes 1 and 2 for the decomposi-
tion of Rhodamine B under visible light and luminescent
Synthesis of {[Ni(bpmp)1.5(tbip)(H2O)]$H2O}n (2)
A mixture of Ni(NO3)2$6H2O (29.1 mg, 0.1 mmol), bpmp (26.8
mg, 0.1 mmol), H2tbip (22.2 mg, 0.10 mmol) and NaOH (8.00
mg 0.2 mmol) in deionized water (6 mL), was sealed in a 25 mL
ꢁ
Teon-lined stainless steel autoclave, and heated at 130 C for
72 hours, then slowly cooled to room temperature during 24
hours. Green block crystals were recovered by ltration, washed
by distilled water, and dried in air at ambient temperature.
Yield: 40% (based on Ni). Calcd for C36H46N6O6Ni (717.48): C
60.26, H 6.46, N 11.71; found: C 60.36, H 6.51, N 11.68. IR (KBr,
cmꢀ1): 3440 (s, br), 2943(w), 2815(m), 1619(s), 1537(m),
1427(m), 1371(s), 1295(m), 1012(m), 846(m), 784(w), 726(m),
496(w).
Synthesis of {[Cd(bpmp)1.5(tbip)]$3H2O}n (3)
property of compound 3 has also been explored and discussed Compound 3 was prepared in the same way as that for 2 but
in detail.
using Cd(NO3)2$6H2O (30.8 mg, 0.1 mmol). Colorless block
crystals were recovered by ltration, washed by distilled water,
and dried in air at ambient temperature. Yield: 44% (based on
Cd). Calcd for C36H48N6O7Cd (789.21): C 54.79, H 6.13, N 10.65;
found: C 54.87, H 6.18, N 10.73. IR (KBr, cmꢀ1): 3440 (w, br),
2963(w), 2813(w), 1626(s), 1567(m), 1432(m), 1366(s), 1160(w),
1012(w), 838(w), 786(w), 734(w), 490(w).
Experimental details
Materials and general methods
All commercially available reagents and starting materials were
of reagent-grade quality and used without further purication.
A piperazine–pyridine ligand, 1,4-bis-(4-pyridyl-methyl) pipera-
zine (bpmp), was prepared according to a previously reported
procedure.10 Elemental analyses (C, H, N) were carried out on an
Elementar Vario EL III analyzer. Infrared (IR) spectra were
recorded on PerkinElmer Spectrum One as KBr pellets in the
range 4000–400 cmꢀ1. Thermogravimetric analysis was recor-
ded with a Perkin-ElmerꢀD1 iamond TG/DTA instrument at a
Photodegradation experiments
Photocatalytic activity of complexes 1 and 2 was evaluated by
photocatalytic degradation of RhB under visible light at
ambient temperature. A tungsten lament lamp was used as a
visible light source. In the typical experiment, 100 mL glass
reactor was charged with 50 mL aqueous RhB solution (10 mg
Lꢀ1) and 50 mg photocatalyst (1 or 2) was dispersed in it for
30 min in the absence of light to attain adsorption equilibrium.
Subsequently, 2 mL of 30% M H2O2 solution was added and the
pH value was adjusted to 3 with sulfuric acid (0.5 mol Lꢀ1). Then
the reaction was illuminated by a tungsten lament lamp and
aliquots of the reaction mixture were periodically taken and
analyzed with a UV-vis spectrophotometer at an absorption
wavelength of 552 nm. For comparison, the photodegradation
process of RhB without any photocatalyst has also been studied
under the same conditions.
ꢁ
heating rate of 10 C min under nitrogen atmosphere. X-ray
powered diffraction (XRPD) patterns of the samples were
recorded by an X-ray diffractometer (Rigaku D/Max 2200PC)
with a graphite monochromator and CuKa radiation at room
temperature while the voltage and electric current are held at 40
kV and 20 mA. UV-visible absorption spectra were collected in
Hitachi U-4100. Solid-state emission and excitation spectra were
carried out on an Edinburgh FLS920 phosphorimeter equipped
with a continuous Xe-900 xenon lamp and an nF900 ns ash
lamp.
Synthesis of {[Co(bpmp)(tbip)]$H2O}n (1)
Crystallographic details
A mixture of Co(NO3)2$6H2O (29.1 mg, 0.1 mmol), bpmp (26.8 Data collection were performed on an Xcalibur, Eos, Gemini
mg, 0.1 mmol) and H2tbip (22.2 mg, 0.10 mmol) was dispersed diffractometer with graphite-monochromated Mo-Ka (l ¼
˚
in mixed N,N-dimethylformamide (DMF) and deionized water (9 0.71073 A) radiation at room temperature. Data reduction is
mL, v/v ¼ 2 : 1), and then sealed in a 25 mL Teon-lined accomplished by the CrysAlisPro (Oxford Diffraction Ltd.,
stainless steel autoclave, and heated at 150 ꢁC for 72 hours, then version 1.171.33.55) program.11 Empirical absorption
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Table
1 Crystal data and structure refinement parameters for
coordination mode while the other one holds a bidentate
chelating mode as shown in Scheme 1a. These tbip2ꢀ ligands
link the Co(II) ions into 1D binuclear chains as shown in Fig. 1b.
Further, each such chain is bridged by bpmp ligands to four
adjacent identical motifs forming a 3D framework as shown in
Fig. 1c. Topologically, each binuclear [Co2O4] cluster is con-
nected to six other [Co2O4] clusters through four bpmp mole-
cules and two tbip2ꢀ anions and is 6-connected. Both the bpmp
molecule and two tbip2ꢀ anion are 2-connected. Therefore,
complex 1 shows 3D 6-connected network with point symbol of
{33.46.55.6} as shown in Fig. 1d. If the 2-connected nodes are
transformed as links, the structure of 1 can be simplied as a 6-
connected 3D net with the pcu topology.
compoundsa
Complex
1
2
3
Empirical formula C28H34N4O5Co C36H46N6O6Ni C36H48N6O7Cd
Formula weight
Crystal system
Space group
565.53
Triclinic
P1
717.48
Monoclinic
P21/c
789.21
Monoclinic
P21/c
˚
a (A)
10.192(4)
12.0704(19)
12.0780(15)
91.254(12)
98.01(3)
106.54(3)
1407.5(6)
2
11.0054(3)
17.5194(5)
19.5049(5)
90
105.175(3)
90
17.0669(10)
10.2827(7)
21.854(2)
90
95.664(5)
90
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
3629.57(17)
4
3816.4(5)
4
Z
Dcalc. (g cmꢀ3
F(000)
)
1.330
1.306
1.363
Description of structure 2
590
1504
1616
Single crystal X-ray diffraction analysis reveals that complex 2
crystallizes in the monoclinic space group P21/c and the asym-
metric unit of 2 is constructed from one NiII ion, one fully
deprotonated tbip2ꢀ ligand, one and a half bpmp ligands, one
coordinated water molecule and one guest water molecule
(Fig. 2a). The NiII ion is coordination by two carboxylic oxygen
atoms from two tbip2ꢀ ligands, three pyridyl nitrogen atoms
from three bpmp ligands and one oxygen atom from water
molecule in a slightly distorted [NiO3N3] octahedral geometry.
The two coordinated carboxyl O atoms together with two pyridyl
N atoms dene the equatorial positions, while the axial posi-
tions are occupied by one pyridyl N atom and the coordinated
water molecule. The Ni–O bond distances are in the range of
Rens collected/
13760/5599
36444/7399
29564/8670
unique
m (mmꢀ1
)
0.653
1.056
0.0453
0.1231
0.587
1.058
0.0433
0.1204
0.626
1.057
0.0513
0.1423
GOF on F2
R1a(I > 2s (I))
wR2b (all data)
a
2
R ¼ S||Fo| ꢀ |Fc||/S|Fo|. b wR(F2) ¼ [Sw(Fo ꢀ Fc2)2/Sw(Fo2)2]1/2
.
corrections were applied to the data using the SADABS
program.12 The structures were solved by direct methods and
rened by the full-matrix least-squares on F2 using the
SHELXTL-97 program.13 All non-hydrogen atoms were rened
with anisotropic displacement parameters. The positions of
hydrogen atoms attached to carbon atoms were generated
˚
2.0436(15)–2.1039(17) A while the Ni–N bond distances are in
˚
the range of 2.0961(19)–2.1474(18) A, which are all in good
agreement with those reported for other nickel–oxygen and
nickel–nitrogen donor compounds.16 The tbip2ꢀ ligand holds a
h1–h1 coordination mode with each of the two carboxylate
group bridging to one NiII ion as shown in Scheme 1b. The NiII
centers are rst linked by the tbip2ꢀ linkers to form 1D chain as
shown in Fig. 2b. Meanwhile, the NiII centers are also bridged by
bpmp ligands to form 2D layers with (6, 3) topology as shown in
Fig. 2c. Then these chains and layers are cross-connected with
each other to form ve-connected 3D framework with {46.64}-
bnn hexagonal BN topology in which the NiII ions act as unin-
odal center (Fig. 2d). Further investigation into the topology of
complex 2 reveals that there are three identical independent 3D
networks that interpenetrate with each other (Fig. 2e). The three
3D subunits are related to each other by one translation, placing
the interpenetration into Class Ia (only one translation).17 In a
word, the structure of 2 exhibits a 3D coordination network with
three-fold interpenetration of subunits with {46.64}-bnn hexag-
onal BN topology.
˚
geometrically (C–H bond xed at 0.99 A). Idealized positions of
H atoms belonging to nitrogen atoms and water molecules were
located from Fourier difference maps and rened isotropically.
Crystallographic data and structure determination summaries
are listed in Table 1. Selected bond lengths and angles of
complexes 1–3 are listed in Table S1.† The topological analysis
and some diagrams were produced using TOPOS program.14
Results and discussion
Description of structure 1
The single-crystal X-ray analysis reveals that complex 1 crystal-
lizes in the triclinic P1 space group and exhibits a three-
dimensional framework. As shown in Fig. 1a, the asymmetric
unit of complex 1 contains one Co(II) cation, one fully depro-
tonated tbip2ꢀ anion, two haves of two crystallographically
distinct bpmp units (marked by N1–N3 and marked by N2–N4,
respectively) and one lattice water molecule. The local coordi-
nation geometry around the Co(II) center adopts a typical octa-
hedron. It is coordinated by four oxygen atoms of two tbip2ꢀ
Description of structure 3
anions and two nitrogen atoms of two bpmp molecules. The Determination of the structure of compound 3 by single crystal
Co–O and Co–N bonds are in the range of 2.0858(19)–2.3327(18) X-ray diffraction analysis indicates that compound 3 crystallizes
A and 2.137(2)–2.193(2) A respectively (Table S1†), of which are in the monoclinic space group P21/c with one CdII ion, one fully
similar to those reported for other Co(II) compounds.15 The deprotonated tbip2ꢀ ligand, one and a half bpmp ligands and
tbip2ꢀ ligand adopts the h2m2–h2 coordination mode with one three guest water molecules in the asymmetric unit (Fig. 3a).
of the two carboxylate groups displays a bidentate bridging The CdII centers display distorted [CdN3O3] octahedral
˚
˚
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Scheme 1 Coordination modes of H2tbip in 1–3.
mode with one of the two carboxylate groups adopts chelating
coordination mode while the other one holds a monodentate
bridging mode as shown in Scheme 1c. The coordination bond
lengths that involve the Cd(II) center are in the range of
˚
˚
2.361(4)–2.416(4) A for Cd–N bonds and 2.260(3)–2.571(3) A for
Cd–O bonds, which are all in good agreement with those typi-
cally observed (Table S1†).18 The CoII centers are rst linked by
the bpmp linkers in trans-conguration to form 1D ladder chain
with (4,4) grids consist of cationic {[Cd(bpmp)]4}8+ squares with
˚
Cd/Cd distances of 17.0669(11) and 17.2073(15) A. These
square units are then joined by anionic tbip2ꢀ bridges to give
two-dimensional bilayer motif with cuboidal box consists of
eight Cd(II) atoms at the corners, showing large 1D channel
(Fig. 3b). The large channel in the structure results in a fasci-
nating and peculiar structural feature of 3, that is each layer
motif is polycatenated with the two adjacent (the upper and the
lower) identical motifs in a parallel fashion to give rise to a 3D
polycatenated framework (Fig. 3c). The resulting 3D entangle-
ment network is somewhat uncommon as only several examples
of the species that comprised of two-level layers polycatenated
with two adjacent ones have been reported.19
Thermal analysis and PXRD results
Thermal stabilities of these new crystalline materials have been
measured under nitrogen atmosphere to check the stability as
shown in Fig. S2.† Complexes 1, 2 and 3 exhibit two main weight
loss stages. In the TGA curve of complex 1, there are two
continuous weight-loss steps. The rst weight loss of 3.06% in
the temperature range of 40 to 135 ꢁC is equivalent of losing one
lattice water molecule with a calculated value of 3.18%. The
second weight-loss in the temperature from about 320 ꢁC to
600 ꢁC can be attributed to the collapse of the structure. For
complex 2, a weight loss of about 4.93% occurs between 130 ꢁC
to 220 ꢁC corresponds to the removal of the one coordinated
water molecule and the one guest water moleculeꢁ(calcd 5.02%).
The second weight loss step start at about 220 C and end at
560 ꢁC can be ascribe to the collapse of the structure. For
complex 3, the weight loss attributed to the release of the three
uncoordinated water molecule is observed from 40 to 95 ꢁC
(obsd 6.80%, calcd 6.85%). The decomposition of the residual
composition occurs from 285 to 560 ꢁC. The nal thermal
decomposition products are unidentied because they are
Fig. 1 (a) The coordination environment of Co(II) ion in 1; (b) view of
the 1D chain of Co(II) linked by tbip2ꢀ; (c) view of the 3D framework in
1; (d) simplified 6-connected net in 1. Co, tbip2ꢀ, and bpmp ligands are
simplified into red, blue, and green nodes, respectively. Hydrogen
atoms and guest molecules are omitted for clarity. Symmetry codes:
(A) 1 + x, 1 + y, 1 + z; (B) ꢀx, 1 ꢀ y, 1 ꢀ z.
geometry with three carboxylic oxygen atoms from two tbip2ꢀ amorphous. The powder X-ray diffraction analyses of complexes
ligands and one pyridyl nitrogen atom from a bpmp ligand 1–3 have also been carried out at room temperature. The
dene the equatorial positions and two pyridyl nitrogen atoms experimental PXRD are in good agreement with the simulated
from another two bpmp ligands occupies the axial positions. patterns from the single-crystal structures as shown in Fig. S3,†
The tbip2ꢀ ligands in complexes 3 exhibit h2–h1 coordination which reveals the phase purity of the bulk crystalline materials.
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Fig. 3 (a) The coordination environment of Cd(II) ion in 3; (b) view of
the 2D bilayer subunit in 3; (c) schematic illustration of the poly-
catenation framework in 3. Hydrogen atoms and guest molecules are
omitted for clarity. Symmetry codes: (A) x, 2 + y, 1 + z; (B) ꢀ1 + x, 1 + y,
1 + z.
UV-vis absorbance properties and photocatalytic activity
The solid-state UV-vis spectra of complexes 1–3 were measured
at room temperature and the results are shown in Fig. S4.† The
bands in the wavelength range 200–300 nm for complexes 1–3
could be assigned to the excitation of p-electrons of the
aromatic system.20 Complex 1 and 2 show broad absorption
bands in the visible light region (from about 450 nm to 550 nm
for 1 and from 560 nm to 700 nm for 2), which are probably
attributable to the d / d transitions of the central metal ions in
the octahedron coordination eld.21 For complex 3, no absorp-
tion were found in the visible light region due to the closed-shell
electron conguration of d10 for Cd(II) ion.
Fig. 2 (a) The coordination environment of Ni(II) ion in 2; (b) view of
the 1D chain of Ni(II) linked by tbip2ꢀ in 2; (c) view of the (6, 3) layer of
Ni(II) bridged by bpmp; (d) view of the 3D framework with {46.64}-bnn
hexagonal BN topology; (e) schematic illustration of the three-fold
interpenetration network in 2. Hydrogen atoms and guest molecules
are omitted for clarity. Symmetry codes: (A) ꢀ1 + x, y, z; (B) 1 ꢀ x, 1/2 +
y, ꢀ1/2 ꢀ z.
Organic dyes such as MB and RhB are extensively used in
industry elds and cause great environment pollution. Some
coordination polymers have been reported showing photo-
degradation capacity for organic dyes under ultraviolet
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irradiation.22 Absorption of complexes 1 and 2 in visible light
region indicated they may show absorption responses to visible
light and may be used as heterogeneous photocatalysts for waste
water treatment. Thus, the photocatalytic activity of as-prepared 1
and 2 was tested by the degradation of RhB solution under visible
light irradiation. Analogous experiment was performed without 1
and 2 as described in the experimental section. The degradation
experiment of RhB was tracked by visible spectroscopy and the
results are depicted in Fig. 4. It was found that decomposition
percentage of RhB within 6 h is about 76% in the presence of 1
and is about 69% in the presence of 2. For comparison, the
Fig. 5 Solid-state fluorescence spectra of compound 3.
photodegradation process of RhB without any photocatalyst has
also been studied and the result suggests that about 12% RhB
decomposition under the same condition (see also Fig. S5†). The
above results clearly show that complexes 1 and 2 are effective
catalysts for the decomposition of RhB under visible light in the
presence of H2O2. The possible photocatalytic mechanism for the
above degradation reactions can be explained based on some of
the earlier observations.23 d / d transitions of the central metal
ions in the octahedron coordination eld occur upon visible light
irradiation. Charge transfer promote electrons from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO). The electrons of the excited state in
the LUMO are usually much activated and easily oxygenate H2O2
molecules to generate the cOH radicals which is known to have
high activity to decompose the organic dyes.
Photoluminescent properties
Luminescent complexes are of great interest due to their
potential application in chemical sensors, optical devices and
various other elds.24 The solid state photoluminescent spec-
trum of complex 3 has been investigated at room temperature
and the results are shown in Fig. 5. The emission spectrum has
broad peaks with a maximum at 427 nm for complex 3 under
excitation of 315 nm. There are typical two types of electronic
excited state transition: ligand-to-ligand charge transfer (LLCT)
and ligand-to-metal charge transfer (LMCT) in d10 metal coor-
dination compounds.25 According to literature, the bpmp ligand
show a uorescent emission band at 386 nm under an excita-
tion of 300 nm, while the free H2tbip ligand have emission band
with a maximum of 345 nm with excitation upon 315 nm, both
of which are probably attributable to the intra-ligand p* / p or
p* / n transitions.21 Thus, the signicant red-shis of complex
3 compared to the emission spectra of the two free ligands may
be tentatively ascribed to ligand-to-metal charge transfer
(LMCT) that is in good agreement with literature examples on
this class of coordination compounds.26
Fig. 4 Absorption spectra of a solution of RhB in the presence of
compounds 1 and 2 under exposure to visible light (a and b); the
photodegradation of RhB by compounds 1 and 2 monitored as
the normalized change in concentration as a function of irradiation
time (c).
Conclusion
In summary, three new coordination complexes have been
successfully synthesized under hydrothermal condition. Single
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crystal X-ray diffraction analyses show that complex 1 presents a
3D 6-connected network, complex 2 holds three-fold interpen-
etration framework based on subunits with {46.64}-bnn hexa-
gonal BN topology while complex 3 presents 2D / 3D
polycatenation framework containing 2D bilayer motifs as the
fundamental building units. The photocatalytic activities of 1–2
were evaluated by the decomposition of RhB in aqueous solu-
tions under visible light irradiation. Photoluminescent property
of complex 3 has been studied in solid state at room
temperature.
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Acknowledgements
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This work was nancially supported by the National Natural
Science Foundation of China (no. 21301069) and the Shandong
Provincial Natural Science Foundation of China (no.
ZR2012BQ004). C. Li, as a Taishan Scholar Endowed Professor,
acknowledges the support from Shandong Province and UJN.
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