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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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123