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Threecoordinationcompoundsofcobaltwithorganiccarboxylicacidsand1,10-phenanthrolineasligands:syntheses,structuresandphotocatalyticproperties
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Three coordination compounds of cobalt with organic carboxylicacids and 1,10-phenanthroline as ligands: syntheses, structuresand photocatalytic properties
Chong-Chen Wang1,2 • Huan-Ping Jing1 • Yan-Qiu Zhang1 • Peng Wang1 •
Shi-Jie Gao1
Received: 21 March 2015 / Accepted: 26 May 2015 / Published online: 4 June 2015
� Springer International Publishing Switzerland 2015
Abstract Three cobalt-based coordination compounds,
[Co2(phen)4(H2qptb)](H3qptb)2 (1), [Co2(phen)4
(H2dczpb)2]�5H2O (2) and [Co2(phen)4(H2odpa)2(H2O)2]�2H2O
(3), were obtained from mixtures of quaterphenyl-4,200,500,40-tetracarboxylic acid (H4qptb), 3,4-dicarboxyl-(30,40-dicar-
boxylazophenyl) benzene (H2dczpb), 4,40-oxydiphthalic
acid (H2odpa), along with 1,10-phenanthroline (phen) and
cobalt salts by hydrothermal synthesis. Single-crystal
X-ray diffraction reveals that complexes 1–3 contain
[Co2(phen)4(L)] units, further joined into 3D frameworks via
hydrogen-bonding interactions. All three complexes display
considerable thermal stability and exhibit selective absorp-
tion in the ultraviolet region, as shown by thermogravimetric
analysis and UV–Vis diffuse reflectance spectroscopy,
respectively. Complex 1 mediates efficient photocatalytic
degradation of methylene blue under UV irradiation.
Introduction
Recently, a number of reports on the use of coordination
compounds as photocatalysts for the degradation of organic
pollutant [1–4], CO2 reduction [5–7] and Cr(VI) reduction
[8, 9] have been presented. Being compared with tradi-
tional semiconductor photocatalysts like TiO2, ZnO,
Fe2O3, CdS, GaP and ZnS, photocatalytic coordination
compounds possess several advantages; for example, the
well-defined crystalline structures of coordination com-
pounds can help to clarify the structure–property relation-
ships of these solid photocatalysts; their modular nature
allows the rational design and fine-tuning of these catalysts
at the molecular level; and their intrinsic porosity and high
surface area can facilitate fast transport of guest molecules
through the open channels, resulting in high photocatalytic
reaction efficiencies [1].
In order to gain more intricate and novel complex
structures, mixed ligands have been introduced to reach a
new level of rational design due to the synergetic coordi-
nation of different ligands with metals and subsequent
networking. Important considerations in such systems
include the solubilities of the ligands, competition of the
ligands for coordination with the metal ions, thermody-
namic and dynamic equilibria and so on [10]. Multicar-
boxylate ligands, such as quaterphenyl-4,200,500,40-tetracarboxylic acid (H4qptb), 3,4-dicarboxyl-(30,40-dicar-
boxylazophenyl) benzene (H2dczpb) and 4,40-oxydiph-
thalic acid (H2odpa), as illustrated in Scheme 1, have been
used as both multifunctional organic ligands and counte-
rions, not only because of their various coordination
modes, but also because of their ability to act as hydrogen-
bond acceptors and donors in the assembly of
supramolecular structures [11–14]. In this paper, H4qptb,
H2dczpb and H2odpa, along with 1,10-phenanthroline
(phen), a typical N-donor chelating ligand (Scheme 1),
were utilized to construct three cobalt-based coordination
compounds, namely [Co2(phen)4(H2qptb)](H3qptb)2 (1),
[Co2(phen)4(H2dczpb)2]�5H2O (2) and [Co2(phen)4
(H2odpa)2(H2O)2]�2H2O (3), respectively.
& Chong-Chen Wang
chongchenwang@126.com
1 Key Laboratory of Urban Stormwater System and Water
Environment (Ministry of Education), Beijing University of
Civil Engineering and Architecture, Beijing 100044, China
2 Beijing Engineering Research Center of Sustainable Urban
Sewage System Construction and Risk Control, Beijing
University of Civil Engineering and Architecture,
Beijing 100044, China
123
Transition Met Chem (2015) 40:573–584
DOI 10.1007/s11243-015-9950-1
Experimental
Materials and measurements
All chemicals were commercially available reagent grade
and used without further purification. Elemental analyses
were performed using an Elementar Vario EL-III instru-
ment. FTIR spectra, in the region (400–4000 cm-1), were
recorded on a Nicolet 6700 Fourier transform infrared
spectrophotometer. TGA was performed from room tem-
perature to 800 �C at a flow rate of 5 �C min-1 in an air
stream on an SDT Q600 simultaneous DSC-TGA instru-
ment (TA Instruments) using a-Al2O3 as reference mate-
rial. UV–Vis diffuse reflectance spectra of solid samples
were measured from 200 to 1200 nm using a PerkinElmer
Lambda 650S spectrophotometer, in which barium sulfate
(BaSO4) was used as the standard with 100 % reflectance.
Synthesis of complex 1
A mixture of CoCl2�6H2O (0.3 mmol, 0.0714 g), H4qptb
(0.3 mmol,0.1218 g)and1,10-phen(0.6 mmol,0.1189 g)witha
molar ratio of 1:1:2 was sealed in a 25-mL Teflon-lined stainless
steel Parr bomb containing deionized H2O (20 mL), heated at
160 �C for 72 h and then cooled down to room temperature. Red
block-like crystals were isolated by filtration and washed with
deionized water and ethanol as [Co2(phen)4(H2qptb)](H3qptb)2,
complex 1 (yield 90 % based on CoCl2�6H2O). Anal. Calcd. for
1, C136H84Co2N8O32: C, 66.4; N, 4.6; H, 3.4. Found: C, 66.9; N,
4.7; H, 3.4 %. IR (KBr)/cm-1: 3429 m, 3041 m, 2926 m,
2544 m, 1744 m, 1684 s, 1608 s, 1570 m, 1547 m, 1517 m,
1425 s, 1389 m, 1319 s, 1289 s, 1185 m, 1105 m, 1042w,
1015w, 865 m, 850 m, 772 m, 726 m, 572w, 552w.
Synthesis of complex 2
Pink block-like crystals of [Co2(phen)4(H2dczpb)2]�5H2O
(yield 55 % based on CoCl2�6H2O) were obtained from a
mixture of CoCl2�6H2O (0.3 mmol, 0.0714 g), H4dczpb
(0.3 mmol, 0.1074 g) and 1,10-phen (0.6 mmol, 0.1189 g)
in 1:1:2 M ratio under the same conditions as for complex
2. Anal. Calcd. for 2, C80H58Co2N12O21: C, 58.5; N, 10.2;
H, 3.5. Found: C, 59.0; N, 10.1; H, 3.6 %. IR (KBr)/cm-1:
3436 m, 3063 m, 1944w, 1702 m, 1590 s, 1518 s, 1426 s,
1378 m, 1320 m, 1270 m, 1205 m, 1102w, 1064w, 847 m,
776 m, 727 s, 655w, 640w, 590w, 513w, 424w.
Synthesis of complex 3
Orange block-like crystals of [Co2(phen)4(H2odpa)2
(H2O)2]�2H2O (3) (yield 92 % based on CoCl2�6H2O) were
synthesized from a mixture of CoCl2�6H2O (0.3 mmol,
0.0714 g), H2odpa (0.3 mmol, 0.1038 g) and 1,10-phen
(0.6 mmol, 0.1189 g) in 1:1:2 M ratio under the same
conditions as for complex 3. Anal. Calcd. for 3, C80H54-
Co2N8O21: C, 60.7; N, 7.1; H, 3.4. Found: C, 61.1; N, 7.2;
H, 3.5 %. IR (KBr)/cm-1: 3432 m, 3061 m, 1719 m,
1585 s, 1560 s, 1514 s, 1425 s, 1386 s, 1303w, 1259 m,
1226 s, 1143w, 1105 m, 970w, 853 s, 778 m, 727 s, 643w,
425w.
X-ray crystallography
X-Ray single-crystal data collection for complexes 1–3
was performed with a Bruker SMART 1000 CCD area
detector diffractometer with graphite-monochromatized
MoKa radiation (k = 0.71073 A) using u–x mode at
298(2) K. The SMART software [15] was used for data
collection and the SAINT software [16] for data process-
ing. Empirical absorption corrections were performed with
the SADABS program [17]. The structures were solved by
direct methods (SHELXS-97) [18] and refined by full-
matrix least-squares techniques on F2 with anisotropic
thermal parameters for all of the non-hydrogen atoms
(SHELXL-97) [18]. The hydrogen atoms of the organic
ligands were added according to theoretical models, and
Scheme 1 Structural formulae
of phen, H4qptb, H2dczpb and
H2odpa
574 Transition Met Chem (2015) 40:573–584
123
those of water molecules were found by difference Fourier
maps. All structural calculations were carried out using the
SHELX-97 program package [18]. Crystallographic data
and structural refinements for complexes 1–3 are summa-
rized in Table 1. Selected bond lengths and angles are
listed in Table 2.
Photocatalytic degradation of methylene blue
Methylene blue (MB) with molecular formula of
C16H18N3SCl (FW 319.85 g/mol), as illustrated in
Scheme 2, which is difficult to degrade in wastewater, was
used as model organic dye pollutant to evaluate the photo-
catalytic activities of complexes 1–3 at room temperature
and under 500 W Hg lamp irradiation in a photocatalytic
assessment system (Beijing Aulight Co. Ltd.). The distance
between the light source and the beaker containing the
reaction mixture was 5 cm. A powdered sample of each
complexes (50 mg) with particle size\147 lm, obtained by
grinding the as-prepared single crystals of complexes 1–3,
was put into 200 ml of MB (10 mg/L) aqueous solution in a
300-ml flask. During the photodegradation reaction, stirring
was maintained to keep the mixture in suspension. Aliquots
of volume 1 mL were extracted at regular intervals using a
0.45-lm syringe filter (Shanghai Troody) for analysis.
A Laspec Alpha-1860 spectrometer was used to monitor the
changes of the dye absorbance in the range of 200–800 nm in
a 1-cm path length spectrometric quartz cell. The MB con-
centration was estimated by the absorbance at 664 nm.
Results and discussion
Infrared spectra
Broad bands observed at 3429, 3436 and 3432 cm-1 for 1,
2 and 3, respectively, are assigned to the carboxyl groups
of the corresponding carboxylate ligands. The C–H
stretching mode for the phen ring is relatively weak and
observed at about 3041, 3063 and 3061 cm-1 for 1, 2 and
3, respectively. Sharp bands at 1608 and 1389 cm-1 (for
1), 1590 and 1378 cm-1 (for 2), and 1585 and 1386 cm-1
(for 3) are attributed to the asymmetric and symmetric
vibrations of the carboxylate groups, respectively. The
characteristic absorption peaks of the 1,10-phenanthroline
ligand are observed at 1425, 850 and 726 cm-1 for 1, 1426,
847 and 727 cm-1 for 2, and 1425, 853 and 727 cm-1 for
3.
Table 1 Details of X-ray data
collection and refinement for the
compounds 1–3
1 2 3
Formula C136H84Co2N8O32 C80H58Co2N12O21 C80H54Co2N8O21
M 2459.97 1641.24 1581.18
Crystal system Triclinic Triclinic Triclinic
Space group P-1 P-1 P-1
a, (A) 12.4210 (11) 12.6863 (11) 11.1800 (9)
b, (A) 15.8209 (13) 15.9865 (13) 12.5709 (11)
c, (A) 16.0291 (15) 19.4680 (17) 13.0761 (12)
a, (o) 109.515 (2) 88.011 (2) 102.655 (2)
b, (o) 98.1240 (10) 74.7970 (10) 105.764 (2)
c, (o) 105.8660 (10) 74.8280 (10) 92.5920 (10)
V, (A3) 2760.2 (4) 3674.9 (5) 1714.9 (3)
Z 1 2 1
l (Mo, Ka) (mm-1) 0.392 0.538 0.572
Total reflections 13896 18,764 8586
Unique 9542 12,714 5916
F (000) 1266 1688 812
Goodness of fit on F2 1.038 1.087 1.024
Rint 0.0282 0.1024 0.0278
R1 0.0517 0.0972 0.0638
xR2 0.1051 0.2069 0.1577
R1 (all data) 0.0992 0.1966 0.1049
xR2 (all data) 0.1185 0.2313 0.1899
Largest diff. peak and hole (e/A3) 0.713, -0.358 0.681, -0.957 0.842, -0.677
Transition Met Chem (2015) 40:573–584 575
123
Table 2 Selected bond lengths and angles for the compounds 1–3 (A and o)
(1)
Bond lengths (A)
Co(1)–N(3) 2.087 (3) Co(1)–N(2) 2.090 (3) Co(1)–N(4) 2.091 (3)
Co(1)–N(1) 2.126 (3) Co(1)–O(1) 2.162 (2) Co(1)–O(2) 2.173 (2)
Bond angles (o)
N(3)–Co(1)–N(2) 99.27 (10) N(3)–Co(1)–N(4) 79.80 (11)
N(2)–Co(1)–N(4) 97.93 (11) N(3)–Co(1)–N(1) 96.36 (10)
N(2)–Co(1)–N(1) 78.91 (12) N(4)–Co(1)–N(1) 174.64 (10)
N(3)–Co(1)–O(1) 162.45 (9) N(2)–Co(1)–O(1) 97.10 (9)
N(4)–Co(1)–O(1) 91.74 (10) N(1)–Co(1)–O(1) 92.96 (9)
N(3)–Co(1)–O(2) 104.99 (9) N(2)–Co(1)–O(2 153.43 (10)
N(4)–Co(1)–O(2) 96.87 (9) N(1)–Co(1)–O(2) 87.71 (9)
O(1)–Co(1)–O(2) 60.47 (8)
(2)
Bond lengths (A)
Co(1)–O(4)#1 2.077 (6) Co(1)–O(3) 2.097 (5) Co(1)–N(1) 2.146 (7)
Co(1)–N(4) 2.151 (6) Co(1)–N(2) 2.157 (6) Co(1)–N(3) 2.161 (7)
Co(2)–O(12)#2 2.066 (6) Co(2)–O(11)2.123(6) Co(2)–N(7) 2.133 (7)
Co(2)–N(10) 2.146 (7) Co(2)–N(9) 2.174 (8) Co(2)–N(8) 2.177 (7)
Bond angles (o)
O(4)#1–Co(1)–O(3) 90.2 (2) O(4)#1–Co(1)–N(1) 171.9 (2)
O(3)–Co(1)–N(1) 95.6 (2 O(4)#1–Co(1)–N(4) 91.1 (2)
O(3)–Co(1)–N(4) 89.1 (2) N(1)–Co(1)–N(4) 94.7 (2)
O(4)#1–Co(1)–N(2) 95.6 (3) O(3)–Co(1)–N(2) 98.9 (2)
N(1)–Co(1)–N(2) 77.9 (3) N(4)–Co(1)–N(2) 169.5 (3)
O(4)#1–Co(1)–N(3) 85.5 (2) O(3)–Co(1)–N(3) 166.1 (2)
N(1)–Co(1)–N(3) 90.1 (2) N(4)–Co(1)–N(3) 77.8 (3)
N(2)–Co(1)–N(3) 94.6 (2) O(12)#2–Co(2)–O(11) 90.6 (2)
O(12)#2–Co(2)–N(7) 101.4 (2) O(11)–Co(2)–N(7) 96.2(3)
O(12)#2–Co(2)–N(10) 88.9 (3) O(11)–Co(2)–N(10) 91.1 (2)
N(7)–Co(2)–N(10) 167.3 (3) O(12)#2–Co(2)–N(9) 166.0 (2)
O(11)–Co(2)–N(9) 85.5 (2) N(7)–Co(2)–N(9) 92.3 (3)
N(10)–Co(2)–N(9) 77.9 (3) O(12)#2–Co(2)–N(8) 94.8 (2)
O(11)–Co(2)–N(8) 172.6 (3) N(7)–Co(2)–N(8) 77.7 (3)
N(10)–Co(2)–N(8) 94.1 (3) N(9)–Co(2)–N(8) 90.5 (3)
Symmetry transformations used to generate equivalent atoms: #1 -x ? 1, - y?1, - z?1; #2 -x?1 - y?2, - z
(3)
Bond lengths (A)
Co(2)–O(1) 2.063 (3) Co(2)–N(4) 2.129 (4) Co(2)–N(1) 2.130 (4)
Co(2)–N(3) 2.138 (4) Co(2)–O(10) 2.146 (4) Co(2)–N(2) 2.150 (4)
Bond angles (o)
O(1)–Co(2)–N(4) 91.83 (14)
O(1)–Co(2)–N(1) 94.81 (14) N(4)–Co(2)–N(1) 172.00 (16)
O(1)–Co(2)–N(3) 86.04 (14) N(4)–Co(2)–N(3) 78.14 (16)
N(1)Co(2)–N(3) 97.85 (16) O(1)–Co(2)–O(10) 90.01 (14)
N(4)–Co(2)–O(10) 91.78 (15) N(1)–Co(2)–O(10) 92.68 (15)
N(3)–Co(2)–O(10) 169.03 (16) O(1)–Co(2)–N(2) 171.92 (14)
N(4)–Co(2)–N(2) 95.94 (15) N(1)–Co(2)–N(2) 77.63 (15)
N(3)–Co(2)–N(2) 97.70 (15) O(10)–Co(2)–N(2) 87.54
576 Transition Met Chem (2015) 40:573–584
123
Crystallographic analysis of complex 1
The analysis of the crystal structure reveals that
[Co2(phen)4(H2qptb)](H3qptb)2 (1) is built up from discrete
cationic [Co2(phen)4(H2qptb)]2? units and partly deproto-
nated H3qptb- anions. The Co(II) centers, in an octahedral
geometry, are six-coordinated by four nitrogen atoms from
two different phen ligands and two oxygen atoms from the
same H2qptb2- ligand, such that one nitrogen and oxygen
atom (N2 and O2) occupy the axial positions, and the
remaining three nitrogen atoms (N1, N3 and N4) and one
oxygen atom (O1) occupy the four sites of equatorial plane,
as shown in Fig. 1a and Scheme 3a. The Co–O and Co–N
bond distances are all as expected [19, 20]. In the equa-
torial plane, the bond angles of N3–Co1–N2, N3–Co1–N4,
N2–Co1–N4 and N3–Co1–N1 are 99.27 (1), 79.80 (11),
97.93 (11) and 96.36 (10)o, respectively, and the bond
angle of N2–Co1–O2 is 153.43 (1)o, showing that the co-
centered coordination octahedron is seriously distorted.
The partly deprotonated H2qptb2- acts as both bis-chelat-
ing ligand to link two Co(II) centers and counterion to
compensate the charge of [Co(phen)2]2?. The neighboring
[Co2(phen)4(H2qptb)](H3qptb)2 units are further linked into
a 3D framework by hydrogen-bonding interactions
involving the partly deprotonated H2qptb2- and H3qptb-
as well as p–p stacking interactions with centroid–centroid
distances ranging from 3.600 (2) to 3.760 (3) A, as illus-
trated in Fig. 1b; Tables 3 and 4.
Crystallographic analysis of complexes 2
Similar to the co center environment in complexes 1, in
[Co2(phen)4(3,4-dczpb)2]�5H2O (2), both Co1 and Co2
centers are six-coordinated by four nitrogen atoms from
two phen ligands and two oxygen atoms from two 3,4-
Scheme 2 Structure of methylene blue (MB)
(a)
(b)
Fig. 1 a Coordination environment of the binuclear [Co2(phen)4(H2-
qptb)] units in [Co2(phen)4(H2qptb)] (H3qptb)2 (1). H atoms are
omitted for clarity. b Packing view of the 3D framework built from
[Co2(phen)4(H2qptb)] (H3qptb)2 and uncoordinated H4qptb via abun-
dant hydrogen-bonding interactions along the c-axis for complex 1
Transition Met Chem (2015) 40:573–584 577
123
dczpb ligands to produce slightly distorted octahedral
geometries, as illustrated in Fig. 2a and Table 2. The partly
deprotonated 3,4-H2dczpb2- acts as both a bis-monoden-
tate ligand to link two Co(II) centers via a COO- group [as
shown in Fig. 2a and Scheme 3b] and as a counterion to
compensate the charge of [Co(phen)2]2?, to form a zero-
dimensional binuclear complex [Co2(phen)4(3,4-dczpb)2].
Finally, a three-dimensional framework is built up from
[Co2(phen)4(3,4-dczpb)2] units and lattice water molecules
via hydrogen-bonding and p–p stacking interactions with
centroid–centroid distances of 3.763 (3) A, as listed in
Fig. 2b; Tables 3 and 4. In the structure of complex 2, the
oxygen atoms (O21 and O22) and their corresponding
hydrogen atoms in two lattice water molecules are disor-
dered over two positions, with a site occupancy factor ratio
of 0.5/0.5.
Crystallographic analysis of complex 3
As illustrated in Fig. 3a and Scheme 3c, in [Co2(phen)4
(H2odpa)2(H2O)2]�2H2O (3), the Co(II) center, in an octa-
hedral geometry, is six-coordinated by four nitrogen atoms
from two different phen ligands, one oxygen atom from a
H2odpa ligand and one oxygen atom from a lattice water
molecule. As a mononuclear complex, the crystal structure
of 3 is quite different from binuclear 1 and 2, while
hydrogen-bonding and p–p stacking interactions with
centroid–centroid distances of 3.577 (5) A also play a
crucial role in the construction of the three-dimensional
framework of 3, as shown in Fig. 3b; Tables 3 and 4. It was
worth noting that the carbon atoms (C9, C10, C11, C12, 13,
C14, C15, C16) of the benzene ring and the oxygen atoms
of the two attached COOH groups (O4, O5, O6, O7, O8
and H6) are disordered over two sites, and their corre-
sponding site occupancy factor ratio is 0.629(8)/0.371(8),
respectively. The oxygen atom (O11) and its corresponding
hydrogen atoms in two lattice water molecules are also
disordered over two positions, with a site occupancy factor
ratio of 0.5/0.5.
Optical energy gap and thermal properties
In order to investigate the conductivities of these com-
plexes, the UV–visible diffuse reflectance spectra were
recorded for powder samples to get their band gap Eg
values [21, 22]. The Eg values were confirmed as the
intersection point between the energy axis and the line
extrapolated from the linear portion of the adsorption edge
in a plot of Kubelka–Munk function F versus energy
E [23]. The Kubelka–Munk function, F = (1-R)2/2R, was
Scheme 3 Coordination environment of Co2? in complexes 1–3
578 Transition Met Chem (2015) 40:573–584
123
transformed from the recorded UV–visible diffuse reflec-
tance spectra data, in which R is the reflectance of an
infinitely thick layer at a given wavelength [24]. The
F versus E plots for the complexes are illustrated in Fig. 4,
where steep absorption edges are exhibited and the Eg
values of complexes 1, 2 and 3 are 3.2, 3.0 and 2.9 eV,
respectively, indicating that all three complexes show
selective absorption in the ultraviolet region [25, 26].
The thermal properties of the three complexes were
investigated by thermogravimetric analysis (TGA) under
air, as shown in Fig. 5. For complexes 1, the decomposition
of the organic ligands occurs from a temperature of 320 �C,
Table 3 Hydrogen bonds for
compounds 1–3 (A and �) D–H d (D–H) d(H…A) \DHA d(D…A) A
(1)
O4–H4 0.820 1.860 168.52 2.669 O3 [-x, -y, -z ? 2]
O6–H6 0.820 1.805 164.73 2.605 O9
O6–H6 0.820 2.654 120.48 3.150 O10
O7–H7 0.820 1.973 175.55 2.792 O11 [x, y-1, z]
O12–H12 0.820 1.826 154.36 2.590 O9 [-x ? 1, -y ? 1, -z ? 2]
O13–H13 0.820 1.770 165.89 2.573 O15 [x-1, y-1, z-1]
O16–H16 0.820 1.859 171.13 2.672 O14 [x ? 1, y ? 1, z ? 1]
(2)
O2–H2 0.820 1.821 166.56 2.626 O17
O6–H6 0.820 1.540 166.00 2.344 O7
O9–H9 0.820 1.822 171.73 2.636 O20 [-x ? 1, -y ? 1, -z]
O14–H14 0.820 1.647 157.76 2.426 O15
O17–H17C 0.850 2.117 167.49 2.952 O13
O17–H17D 0.850 1.934 167.07 2.769 O16 [-x, -y ? 2, -z ? 1]
O18–H18C 0.850 2.484 135.82 3.150 O13 [x ? 1, y, z]
O18–H18D 0.850 2.495 136.06 3.163 O14 [x ? 1, y, z]
O19–H19B 0.850 2.497 110.49 2.906 O11 [-x ? 1, -y ? 1, -z]
O19–H19C 0.850 2.497 110.52 2.906 O11 [-x ? 1, -y ? 1, -z]
O19–H19C 0.850 2.682 161.84 3.499 N9 [- x?1, -y ? 1, -z]
O20–H20C 0.850 2.030 167.80 2.866 O5 [-x ? 1, -y ? 1, -z]
O20–H20D 0.850 1.952 167.25 2.787 O8 [x-1, y, z]
O20–H20D 0.850 2.494 118.45 2.994 O21 [-x ? 1, -y ? 1, -z ? 1]
O21–H21C 0.850 2.159 178.64 3.009 O10 [x, y, z ? 1]
O21–H21D 0.850 1.852 178.61 2.702 O8 [-x ? 2, -y ? 1, -z ? 1]
O22–H22C 0.850 1.473 171.42 2.317 O21
O22–H22D 0.850 2.358 171.07 3.201 O17 [x ? 1, y, z]
(3)
O3–H3 0.820 1.895 144.97 2.609 O50_b [-x, -y ? 1, -z ? 1]
O3–H3 0.820 2.443 146.73 3.161 O10 [-x ? 1, -y ? 1, -z ? 2]
O6–H6_a 0.820 1.565 176.14 2.384 O7_a
O60–H60_b 0.820 1.622 148.72 2.361 O70_b
O10–H10B 0.850 1.832 154.02 2.623 O2
O10–H10C 0.850 1.700 138.39 2.403 O50_b [x ? 1, y, z ? 1]
O10–H10C 0.850 1.993 155.99 2.791 O8_a [x ? 1, y, z ? 1]
O10–H10C 0.850 2.609 161.76 3.427 O60_b [x ? 1, y, z ? 1]
O11–H11C 0.850 1.970 163.14 2.794 O7_a [-x, -y ? 2, -z ? 1]
O11–H11C 0.850 2.141 170.67 2.983 O70_b [-x, -y ? 2, -z ? 1]
O11–H11D 0.850 1.435 134.58 2.120 O5_a [x ? 1, y, z]
O11–H11D 0.850 2.162 173.97 3.008 O80_b [x ? 1, y, z]
O11–H11D 0.850 2.531 166.19 3.362 O6_a [x ? 1, y, z]
Transition Met Chem (2015) 40:573–584 579
123
Table
4D
efin
edri
ng
and
rela
tiv
ep
aram
eter
so
fth
ep–
pin
tera
ctio
ns
inco
mp
ou
nd
s1
–3
Co
mp
ou
nd1
Cg
(1):
N(1
)?
C(1
)?
C(2
)?
C(3
)?
C(4
)?
C(5
)?
Cg
(3):
N(3
)?
C(1
3)?
C(1
4)?
C(1
5)?
C(1
6)?
C(1
7)?
Cg
(5):
C(4
)?
C(5
)?
C(6
)?
C(7
)?
C(1
2)?
C(1
1)?
Cg
(6):
C(1
6)?
C(1
7)?
C(1
8)?
C(1
9)?
C(2
4)?
C(2
3)?
Cg
(I)
Cg
(J)
Dis
t.ce
ntr
oid
s(A
)D
ihed
ral
ang
le(o
)P
erp
.d
ist.
(IJ)
(A)
Per
p.
dis
t.(J
I)(A
)
Cg
(1)?
Cg
(5)i
3.7
60
(3)
1.9
(2)
3.5
16
3(1
7)
3.4
90
(2)
Cg
(3)?
Cg
(6)ii
3.6
00
(2)
0.1
6(1
9)
3.4
13
7(1
5)
3.4
11
2(1
7)
Cg
(6)?
Cg
(6)ii
3.6
81
(2)
03
.41
00
(17
)3
.41
01
(17
)
Sy
mm
etry
cod
es:
(i)
2-
x,1-
y,1-
z;(i
i)1-
x,-
y,1-
z
Co
mp
ou
nd2
Cg
(5):
N(3
)?
C(2
9)?
C(3
0)?
C(3
1)?
C(3
2)?
C(3
3)?
Cg
(10
):C
(32
)?
C(3
3)?
C(3
4)?
C(3
5)?
C(4
0)?
C(3
9)?
Cg
(I)
Cg
(J)
Dis
t.ce
ntr
oid
s(A
)D
ihed
ral
ang
le(o
)P
erp
.d
ist.
(IJ)
(A)
Per
p.
dis
t.(J
I)(A
)
Cg
(5)?
Cg
(10
)3
.76
3(3
)3
.4(3
)3
.41
7(2
)3
.38
9(2
)
Sy
mm
etry
cod
es:
(i)1
-x,
1-
y,1
–z
Co
mp
ou
nd3
Cg
(7):
C(4
)?
C(5
)?
C(6
)?
C(7
)?
C(1
2)?
C(1
1)?
Cg
(9):
C(2
7)?
C(2
8)?
C(2
9)?
C(3
0)?
C(3
1)?
C(3
2)?
Cg
(I)
Cg
(J)
Dis
t.ce
ntr
oid
s(A
)D
ihed
ral
ang
le(o
)P
erp
.d
ist.
(IJ)
(A)
Per
p.
dis
t.(J
I)(A
)
Cg
(7)?
Cg
(9)
3.5
77
(5)
2.0
(4)
3.3
04
(4)
3.3
07
(3)
580 Transition Met Chem (2015) 40:573–584
123
and the final residue, Co2O3, is 7.04 % (calculated:
6.99 %). The TGA curve for 2 shows an initial weight loss
below 250 �C, which can be ascribed to the removal of
lattice water molecules (observed: 5.7 %, calculated:
5.5 %). Further weight loss above 250 �C indicates
decomposition of the coordination framework. The final
(a) (b)
Fig. 2 a Asymmetric unit of [Co2(phen)4(3,4-dczpb)2]�5H2O (2). Lattice water molecules and H atoms are omitted for clarity. b. 3D framework
built from [Co2(phen)4(3,4-dczpb)2] and lattice water molecules via hydrogen-bonding interactions along the a-axis for complex 2
(a)
(b)
Fig. 3 a Asymmetric unit of [Co2(phen)4(H2odpa)2(H2O)2]�2H2O
(3). Lattice water molecules and H atoms are omitted for clarity.
b. 3D framework built from [Co2(phen)4(H2odpa)2(H2O)2]�2H2O and
lattice water molecules via abundant hydrogen-bonding interactions
along the a-axis for complex 3
Transition Met Chem (2015) 40:573–584 581
123
residue, Co2O3, is 10.6 % (calculated: 10.8 %). For com-
plex 3, the first weight loss below 255 �C can be assigned
to the loss of both lattice and coordinated water assigned
(observed: 3.6 %, calculated: 3.4 %). The second loss from
255 to 475 �C is assigned to the removal of organic
ligands, leaving the final residue, Co2O3, as 11.3 % (cal-
culated: 11.2 %). The final residue products of all three
complexes were Co2O3, which can be assigned to the
oxidation of Co(II) under air, comparable to similar com-
pounds reported previously [27–29]. The results show that
all three compounds exhibit good thermal stabilities.
Photocatalytic activities
Coordination compounds have already potential as photo-
catalysts for the degradation of organic pollutants [1]. The
photocatalytic performances of complexes 1–3 for the
photocatalytic degradation of MB were carried out under
UV irradiation. Control experiments for the photodegra-
dation of MB were also performed. The photocatalytic
activities were monitored by measuring the maximum
absorbance intensity at k = 664 nm, characteristic of MB.
As seen from Fig. 6a, b and c, when solutions of MB
were irradiated under UV light, the maximum absorption
peaks of MB decreased with the reaction time in the
presence of all three complexes. Furthermore, no new
peaks were observed in Fig. 6a, b and c during the process
of degradation.
The efficiencies of MB degradation for three photocat-
alysts are shown in Fig. 6d. All data for degradation effi-
ciencies are the average values of three parallel tests. It can
be seen that the photocatalytic activities of MB degradation
increased from 40.5 (control experiment without any
photocatalyst) to 97.0, 77.5 and 56.8 % for complexes 1, 2
and 3, respectively, under irradiation for 120 min. The
photodegradation of MB mediated by complex 1 followed
pseudo-first-order kinetics, evidenced by the linear plot of
ln(C/C0) versus reaction time t. The pseudo-first-order rate
constants (k) and the corresponding correlation coefficient
(R2) for the photocatalytic degradation of MB with 1 as
photocatalyst were -0.0299 min-1 and 0.993,
respectively.
Under UV light irradiation, some electrons will be
transferred from the HOMO to the LUMO [1, 30–33]. The
HOMO is derived mainly from O and/or N 2p bonding
orbitals, while the LUMO is mainly constructed from
empty metal orbitals. In general, the excited electrons in
the LUMO are easily lost [1, 2]. Therefore, such excited
electrons can be captured by water molecules, which then
decompose into OH active species [1, 33–35]. Hence,
the �OH radicals are probably the species for MB decom-
position [4, 33–35].
Some coordination compounds are considered to be
semiconductors based on their optical transition properties
and electrochemical and photochemical activities [1, 2,
32]. However, Gascon and coworkers pointed out in their
recent report that such semiconducting behavior only
occurs in a very limited subset of coordination compounds
[32]. In general, photocatalysts based on coordination
compounds should be treated as molecular catalysts rather
than as typical semiconductors. To understand the photo-
catalysis mechanisms of coordination compounds, the ter-
minology of HOMO–LUMO gap is most useful to describe
the discrete character of the light-induced transitions [32].
This model explains why complex 1 exhibits good photo-
catalytic performance for MB degradation, even though
complexes 1, 2 and 3 have nearly identical optical energy
gaps (Eg = 3.2, 3.0 and 2.9 eV, respectively) [2].
Fig. 4 Kubelka–Munk—transformed diffuse reflectance spectra of
complexes 1–3
Fig. 5 TGA curve of complexes 1–3
582 Transition Met Chem (2015) 40:573–584
123
Conclusions
All three cobalt(II) coordination compounds reported here
are constructed from phen plus organic polycarboxylates.
The latter act as both ligands and counterions, increasing
the dimensionality of the crystal structures and further
helping to extend the crystal structures via intermolecular
interactions. The thermogravimetric analyses showed that
the frameworks of all three compounds are stable under
250 �C. All three complexes have nearly identical optical
energy gaps, but show different photocatalytic degradation
of MB under UV light irradiation, implying that complex 1
can be regarded as a molecular photocatalyst.
Supplementary material
CCDC 1042967, 1042980 and 1042972 contain the sup-
plementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Acknowledgments We thank the financial support from the Beijing
Natural Science Foundation & Scientific Research Key Program of
Beijing Municipal Commission of Education (KZ201410016018), the
Training Program Foundation for the Beijing Municipal Excellent
Talents(2013D005017000004), the Importation & Development of
High-Caliber Talents Project of Beijing Municipal Institutions
(CIT&CD201404076), the Scientific Research Common Program of
Beijing Municipal Commission of Education (KM201510016017),
Special Fund for Cultivation and Development Project of the Scien-
tific and Technical Innovation Base (Z141109004414087) and Open
Research Fund Program of Key Laboratory of Urban Stormwater
System and Water Environment (Ministry of Education).
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