Accepted Manuscript

Syntheses, structures and properties of a series of bis(benzimidazole)-based co-ordination polymers tuned by camphorate and divalent transition metal ions

Ju-Wen Zhang, Xiao-Hui Li, Chun-Hua Gong, Jin-Hui Xie, Ai-Xiang Tian, Xiu-Li Wang

PII: S0020-1693(14)00243-6DOI: http://dx.doi.org/10.1016/j.ica.2014.04.038Reference: ICA 15977

To appear in: Inorganica Chimica Acta

Received Date: 29 December 2013Revised Date: 22 April 2014Accepted Date: 30 April 2014

Please cite this article as: J-W. Zhang, X-H. Li, C-H. Gong, J-H. Xie, A-X. Tian, X-L. Wang, Syntheses, structuresand properties of a series of bis(benzimidazole)-based coordination polymers tuned by camphorate and divalenttransition metal ions, Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.04.038

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Syntheses, structures and properties of a series of bis(benzimidazole)-based coordination

polymers tuned by camphorate and divalent transition metal ions

Ju-Wen Zhang, Xiao-Hui Li, Chun-Hua Gong, Jin-Hui Xie, Ai-Xiang Tian, Xiu-Li Wang

*

Department of Chemistry, Bohai University, Liaoning Province Silicon Materials Engineering

Technology Research Center, Jinzhou 121000, P.R. China

Corresponding authors. Tel.: +86-416-3400308 (J.-W. Zhang), +86-416-3400158 (X.-L. Wang).

E-mail addresses: zhangjw@bhu.edu.cn (J.-W. Zhang), wangxiuli@bhu.edu.cn (X.-L. Wang).

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Syntheses, structures and properties of a series of bis(benzimidazole)-based coordination

polymers tuned by camphorate and divalent transition metal ions

Ju-Wen Zhang, Xiao-Hui Li, Chun-Hua Gong, Jin-Hui Xie, Ai-Xiang Tian, Xiu-Li Wang

*

Department of Chemistry, Bohai University, Liaoning Province Silicon Materials Engineering

Technology Research Center, Jinzhou 121000, P.R. China

Abstract

Five two-dimensional (2D) metal-organic coordination polymers,

[M(D-cam)(bbbm)][M(L-cam)(bbbm)] [M = Co (1), Ni (2) and Zn (3)],

[CuII(OH)(bbbm)(H2O)]2(NO3)2·2H2O (4) and [Cu

I(bbbm)2](HCOO) (5) [H2cam = camphoric

acid and bbbm = 1,1-(1,4-butanediyl)bis-1H-benzimidazole], have been hydrothermally

synthesized and structurally characterized. Complexes 1–3 are isostructural and possess

three-dimensional (3D) supramolecular architectures based on two types of homochiral layers.

Both 4 and 5 exhibit (4,4) layer structures, which are extended into 3D supramolecular

architectures by hydrogen bonding and π–π stacking interactions, respectively. The magnetic

properties of 4, as well as the thermostable and photoluminescent properties of 1–5 have been

investigated.

Keywords: Coordination polymer; Transition metal; Camphorate; Bis(benzimidazole)

1. Introduction

Chiral coordination polymers have received increasing attention owing to their potential

applications such as asymmetric catalysis and enantioselective separation [1,2]. To date, a

large number of chiral coordination polymers, such as one-dimensional (1D) helical chains

[3,4], as well as 2D layers [5,6] and 3D frameworks [7,8] with 1D helical features, have been

constructed. However, the rational design and preparation of chiral coordination polymers

Corresponding authors. Tel.: +86-416-3400308 (J.-W. Zhang), +86-416-3400158 (X.-L. Wang).

E-mail addresses: zhangjw@bhu.edu.cn (J.-W. Zhang), wangxiuli@bhu.edu.cn (X.-L. Wang).

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with expected structures and physical properties is still a remarkable subject. The common

chiral coordination polymers can be constructed using enantiopure, racemic [9,10] or achiral

[11,12] molecules as building blocks. Although chiral coordination polymers resulting from

achiral molecules can be occasionally obtained [13], the chiral molecules are still important

starting materials for the preparation of chiral coordination polymers. As a commercially

available and inexpensive chiral bridging ligand, D-(+)-camphoric acid (D-H2cam, Fig. 1) has

been extensively employed in the construction of chiral coordination polymers [14–19]. Bu

and coworkers reported a number of chiral transition-metal camphorate coordination

polymers [20–24]. Although it is possible to prepare chiral coordination polymers using only

camphorate as ligand, one effective strategy is to combine camphorate with N-donor auxiliary

ligands such as bi(pyridine)- and bis(imidazole)-based organic molecules [25,26].

As an important family of N-donor auxiliary ligands, bis(benzimidazole)-based molecules

have attracted great interest in the past decade. A number of bis(benzimidazole)-based

metal-carboxylate coordination polymers have been reported [27–30]. However, to our

knowledge, the bis(benzimidazole)-based metal-camphorate coordination polymers have not

been reported up to now. Therefore, in this work, we introduce a flexible

1,1-(1,4-butanediyl)bis-1H-benzimidazole (bbbm, Fig. 1) into metal-camphorate system and

obtain five 2D coordination polymers [M(D-cam)(bbbm)][M(L-cam)(bbbm)] [M = Co (1), Ni

(2) and Zn (3)], [CuII(OH)(bbbm)(H2O)]2(NO3)2·2H2O (4) and [Cu

I(bbbm)2](HCOO) (5). The

magnetic properties of 4, as well as the thermostable and photoluminescent properties of 1–5

are investigated.

Fig. 1.

2. Experimental Section

2.1. Materials and Measurements

Starting materials and solvents were of reagent grade and used without further purification.

The bbbm ligand was prepared by literature procedures [31]. Elemental analyses were carried

out using a Perkin-Elmer 2400 CHN elemental analyzer. IR data were recorded on a Magna

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FT-IR 560 spectrometer using KBr plate. PXRD data were collected on a Bruker AXS

D8-Advanced diffractometer with Cu-Kα (λ = 1.5406 Å) radiation. Thermal analyses were

performed on a Pyris-Diamond thermal analyzer with a heating rate of 10 °C min−1

in the

range of 30–800 °C under nitrogen. Magnetic measurements were carried out with a Quantum

Design SQUID magnetometer MPMSXL7. The data were corrected for diamagnetic

contributions calculated from the Pascal constants. Photoluminescent measurements were

performed using a Hitachi F-4500 fluorescence/phosphorescence spectrophotometer.

2.2. Syntheses of 1–5

2.2.1. Syntheses of [M(D-cam)(bbbm)][M(L-cam)(bbbm)] [M = Co (1), Ni (2) and Zn (3)]

A mixture of Co(NO3)2·6H2O (0.029 g, 0.1 mmol), D-H2cam (0.020 g, 0.1 mmol), bbbm

(0.015 g, 0.05 mmol), H2O (10 mL) and NaOH (0.1 mol L–1

, 2 mL) was stirred for 1 h in air,

and then sealed in a 25 mL Teflon-lined stainless-steel autoclave at 120 °C for 96 h. After

slowly cooling to room temperature, purple block crystals of 1 were obtained. The synthetic

procedures of green block crystals of 2 and colorless block crystals of 3 are similar to that of 1,

except that Ni(NO3)2·6H2O and Zn(NO3)2·6H2O were used, respectively. For 1, yield: 65%

(based on Co). Anal. Calcd. for C28H32CoN4O4 (547.51): C, 61.42; H, 5.89; N, 10.23%. Found:

C, 64.35; H, 5.84; N, 10.14%. IR (KBr, cm–1

): 3438 m, 2968 m, 2361 m, 1542 s, 1515 s, 1461

s, 1407 s, 1395 s, 1294 m, 1245 m, 1198 m, 1129 w, 1009 w, 917 w, 810 w, 745 s, 677 w, 611

w, 515 w, 428 w. For 2, yield: 45% (based on Ni). Anal. Calcd. for C28H32NiN4O4 (547.27): C,

61.45; H, 5.89; N, 10.24%. Found: C, 64.32; H, 5.83; N, 10.12%. IR (KBr, cm–1

): 3440 m,

2966 m, 2361 m, 1522 s, 1460 s, 1419 s, 1367 m, 1293 m, 1244 m, 1198 m, 1130 w, 1009 w,

918 w, 812 w, 744 s, 678 w, 610 w, 517 w, 428 w. For 3, yield: 50% (based on Zn). Anal.

Calcd. for C28H32ZnN4O4 (553.95): C, 60.71; H, 5.82; N, 10.11%. Found: C, 60.55; H, 5.81;

N, 10.02%. IR (KBr, cm–1

): 3437 m, 2967 m, 2361 m, 1599 s, 1518 s, 1460 s, 1388 s, 1298 m,

1246 m, 1199 m, 1129 w, 1010 w, 920 w, 807 w, 746 s, 677 w, 612 w, 514 w, 428 w.

2.2.2. Syntheses of [CuII(OH)(bbbm)(H2O)]2(NO3)2·2H2O (4) and [Cu

I(bbbm)2](HCOO) (5)

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A mixture of Cu(NO3)2·3H2O (0.024 g, 0.1 mmol), D-H2cam (0.020 g, 0.1 mmol), bbbm

(0.015 g, 0.05 mmol), H2O (10 mL) and NaOH (0.1 mol L–1

, 2 mL) was stirred for 1 h in air,

and then sealed in a 25 mL Teflon-lined stainless-steel autoclave at 120 °C for 96 h. After

slowly cooling to room temperature, blue block crystals of 4 and yellow block crystals of 5

were obtained. For 4, yield: 25% (based on Cu). Anal. Calcd. for C18H23CuN5O6 (468.94): C,

46.10; H, 4.94; N, 14.93%. Found: C, 45.95; H, 4.91; N, 14.84%. IR (KBr, cm–1

): 3423 m,

3103 w, 2950 w, 2361 w, 1613 w, 1518 m, 1464 m, 1385 s, 1295 w, 1259 w, 1199 w, 1011 w,

754 m, 636 w, 510 w, 431 w. For 5, yield: 18% (based on Cu). Anal. Calcd. for C37H37CuN8O2

(689.29): C, 64.47; H, 5.41; N, 16.26%. Found: C, 64.35; H, 5.37; N, 16.18%. IR (KBr, cm–1

):

3441 m, 2940 w, 2361 w, 1613 w, 1531 m, 1467 m, 1385 s, 1351 s, 1323 s, 1265 w, 1230 m,

1153 w, 927 w, 860 w, 769 m, 628 w, 513 w, 426 w.

2.3. X-ray crystallographic study

X-ray diffraction data for 1–5 were collected on a Bruker Smart Apex CCD diffractometer

with Mo-Kα (λ = 0.71073 Å) radiation at 296 K. The structures of 1 and 3–5 were solved by

direct methods using the program SHELXS-97 and refined on F2 by full-matrix least-squares

methods using the SHELXL-97 crystallographic software package [32]. Pertinent crystal data

and structure refinements for 1 and 3–5 are listed in Table 1. Selected bond lengths (Å) and

angles (°) for 1 and 3–5 are given in Tables S1–S3. Crystallographic data for 1 and 3–5 have

been deposited in the Cambridge Crystallographic Data Center with CCDC Nos. 977588 (1),

977589 (3), 977590 (4) and 977591 (5).

Table 1

3. Results and discussion

3.1. Structures of 1–5

Single-crystal X-ray studies reveal that 1–3 are isostructural, hence the structure of only 1

is described for illustrative purposes. Complex 1 crystallizes in the monoclinic space group

P21/n and possesses a 3D supramolecular architecture based on two types of homochiral

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layers. Noticeably, the partial D-cam anions undergo racemization and change into the L-cam

anions under hydrothermal conditions in the preparation of 1. Thus, both D-cam and L-cam

coexist in 1. The asymmetric unit of 1 comprises one CoII ion, one cam anion and one bbbm

ligand (Fig. 2). The CoII ion is six-coordinated by four oxygen atoms from two cam anions

and two nitrogen atoms from two bbbm ligands, exhibiting a distorted octahedral coordination

geometry. The Co–O bond lengths are in the range of 2.145(6)–2.227(7) Å, and the Co–N

bond lengths are 2.061(6) and 2.074(6) Å (Table S1).

Fig. 2.

The D-cam anions bridge the CoII ions to form a left-handed helical chain (Fig. 3a). Such

chains are connected by the bbbm ligands to generate a layer A (Fig. 4). In the layer A, the

bbbm ligands link the [Co2(D-cam)]2+

cations to give the second left-handed helical chain

(Fig. 3b). Hence A is a homochiral layer containing two types of left-handed helical chains

(Fig. 4). Similarly, the L-cam anion results in the formation of homochiral layer B containing

two types of right-handed helical chains (Figs. S1 and S2). The A and B layers are alternately

arranged and connected through the π–π interactions between the benzimidazole rings,

forming a 3D supramolecular architecture (Fig. S3). Hence 1 undergoes racemization and

crystallizes in the achiral space group P21/n. Compared with the previous several 2D

transition-metal camphorate coordination polymers containing homochiral layers constructed

from both camphorate and N-donor auxiliary ligands [33–35], complex 1 consists of two

types of such homochiral layers and undergoes racemization.

Fig. 3.

Fig. 4.

Complex 4 is a 2D coordination polymer. The asymmetric unit consists of one CuII ion, one

bridging hydroxyl group, one bbbm ligand, one coordinated water, one nitrate counterion and

one interstitial water (Fig. 5a). The CuII ion is five-coordinated by three oxygen atoms from

two bridging hydroxyl groups and one coordinated water, as well as two nitrogen atoms from

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two bbbm ligands, displaying a distorted square-pyramidal geometry. The Cu–O bond

distances vary from 1.954(2) to 2.329(2) Å, and the Cu–N bond distances are 1.996(3) and

2.011(2) Å (Table S2). Two hydroxyl groups bridge two CuII ions to form a dinuclear cluster

with a non-bonding Cu∙∙∙Cu distance of 2.9719(5) Å. Each dinuclear cluster is linked by four

bbbm ligands to generate a (4,4) layer (Fig. 5b). The adjacent layers are further extended by

the hydrogen bonding interactions into a 3D supramolecular architecture (Fig. S4).

Fig. 5.

Complex 5 is a 2D coordination polymer. The asymmetric unit contains one CuI ion, one

bbbm ligand and one formate counterion (Fig. 6a). The CuI ion is four-coordinated by four

nitrogen atoms from four bbbm ligands, showing a quadrilateral geometry. The Cu–N bond

distance is 1.954(14) Å (Table S3). Each CuI ion is connected by four bbbm ligands to give a

(4,4) layer (Fig. 6b). The adjacent layers are further extended through the π–π interactions

between the benzimidazole rings into a 3D supramolecular architecture (Fig. S5). The (4,4)

layer structures of 4 and 5 are similar to those of the bbbm-based complexes reported by Hou

and coworkers [36,37].

Fig. 6.

Complexes 1–5 were synthesized under identical hydrothermal conditions, except that

different divalent transition metal ions were used. When M(NO3)2·6H2O (M = Co, Ni and Zn)

were used, three isostructural 2D coordination polymers containing two types of homochiral

layers 1–3 were obtained. It is noticeable that partial D-cam changes into L-cam in the

preparation of 1–3. Such a racemization under hydrothermal conditions has been observed in

metal-camphorate systems [38–40]. Interestingly, when Cu(NO3)2·3H2O was employed, two

2D coordination polymers without camphorate 4 and 5 were afforded in a Teflon-lined

stainless-steel autoclave. The oxidation states of copper are different in 4 and 5, which may be

attributed to the reduction of bbbm towards partial CuII ions under hydrothermal conditions.

Such a phenomenon has been observed in hydrothermal systems containing both N-donor

ligands and CuII ions [41,42]. In addition, the formate anion in 5 may be resulted from the

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decomposition of camphorate. Therefore, the camphorate and divalent transition metal ions

dominate the structures of 1–5 together (Scheme 1).

Scheme 1.

The as-synthesized PXRD patterns of 1 and 3–5 are in agreement with those simulated

from the single-crystal data for 1 and 3–5 (Figs. S6–S9), indicating the phase purity of bulk

crystal products. The as-synthesized PXRD pattern of 2 is consistent with those of 1 and 3

(Fig. S10). Meanwhile, the single-crystal data for 2 have been obtained, although they can not

be successfully solved and refined. The unit cell parameters of 2 (a/b/c,

12.6015/10.7137/19.7667 Å; α/β/γ, 90/92.7810/90°) are also similar to those of 1 and 3 (Table

1). Hence 1–3 should be isostructural.

3.2. Thermogravimetric analyses of 1–5

The thermal stabilities of 1–5 were investigated in the range of 30–800 °C (Fig. S11).

Complexes 1–3 and 5 maintain stability up to 310, 300, 370 and 260 °C, respectively, and

then the frameworks begin to collapse, corresponding to the decomposition of the cam and

bbbm ligands. For 4, a weight loss of 7.4% in the range of 70–180 °C is consistent with the

removal of one interstitial and one coordinated water molecules (calcd. 7.7%). The collapse of

the framework starts at 270 °C and corresponds to the decomposition of bbbm. The final

decomposition products of 1–5 are CoO (calcd. 13.7%, found 11.8%), NiO (calcd. 13.6%,

found 13.5%), ZnO (calcd. 14.7%, found 13.2%), CuO (calcd. 17.0%, found 17.3%) and Cu2O

(calcd. 10.4%, found 10.2%), respectively.

3.3. Magnetic properties of 4

In view of the dinuclear CuII

2 clusters within 4, the variable-temperature magnetic

susceptibilities of 4 were measured in the range of 2–300 K at an applied field of 1000 Oe

(Fig. 7). The χmT value is 0.76 emu mol–1

K at 300 K, which is close to the spin-only value of

0.75 emu mol–1

K for two magnetically uncoupled CuII ions with S = 1/2 and g = 2.0. Upon

cooling, χmT decreases gradually to 0.03 emu mol–1

K at 2 K, suggesting antiferromagnetic

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interactions between the CuII ions within the Cu

II2 cluster. The χm value increases gradually

from 0.0025 emu mol–1

at room temperature to a maximum value of 0.0034 emu mol–1

at 85

K, and then decreases practically to 0.0028 emu mol–1

at 30 K before increasing sharply to

0.0123 emu mol–1

at 2 K. Such a behavior further indicates a typical antiferromagnetic

coupling and implies the presence of a paramagnetic impurity [43,44].

Fig. 7.

3.4. Photoluminescent properties of 1–5

The photoluminescent behaviors of bbbm and 1–5 were contrastively investigated in the

solid state at room temperature (Fig. 8). When excited with 320 nm light, two emission bands

of bbbm are observed at 375 (weak) and 445 (strong) nm, which are assigned to the

intra-ligand π*→π transitions. Complexes 1–5 exhibit two emission bands at 375 and 465 nm

(λex = 320 nm), 366 and 472 nm (λex = 320 nm), 397 and 469 nm (λex = 300 nm), 396 and 468

nm (λex = 310 nm), as well as 385 and 463 nm (λex = 310 nm), respectively. Compared with

bbbm, although the emission maxima of 1–5 are either blue-shifted or red-shifted, they can be

attributed to the intra-ligand fluorescent emission [45,46].

Fig. 8.

4. Conclusions

Five new 2D bis(benzimidazole)-based transition-metal coordination polymers were

successfully synthesized under hydrothermal conditions. The racemization of partial D-cam

leads to the formation of two types of homochiral layers in 1–3. The reduction of partial CuII

results in the formation of 4 and 5 without camphorate in an autoclave. Complex 4 possesses

an antiferromagnetic property. Complexes 1–5 show thermal stabilities and photoluminescent

properties.

Acknowledgements

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This work was supported by the Program for New Century Excellent Talents in University

(NCET-09-0853), the National Natural Science Foundation of China (Nos. 21171025,

21101015 and 21201021) and the Program of Innovative Research Team in University of

Liaoning Province (LT2012020).

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Table

Table 1

1 3 4 5

formula C28H32CoN4O4 C28H32ZnN4O4 C18H23CuN5O6 C37H37CuN8O2

formula weight 547.51 553.95 468.94 689.29

crystal system monoclinic monoclinic monoclinic tetragonal

space group P21/n P21/c P21/c P4/n

a (Å) 12.5921(17) 12.4597(9) 5.8718(4) 14.3011(7)

b (Å) 10.7571(14) 11.0092(8) 14.2721(15) 14.3011(7)

c (Å) 19.818(3) 22.7429(13) 24.6470(18) 8.7456(8)

α (°) 90 90 90 90

β (°) 93.110(3) 119.934(3) 95.5810(10) 90

γ (°) 90 90 90 90

V (Å3) 2680.6(6) 2703.5(3) 2055.7(3) 1788.7(2)

Z 4 4 4 2

Dc (g cm–3

) 1.357 1.361 1.505 1.280

μ (mm–1

) 0.681 0.948 1.108 0.654

Rint 0.0628 0.0301 0.0359 0.0570

F(000) 1148 1160 960 720

R1a (I > 2σ(I)) 0.0928 0.0866 0.0432 0.1439

wR2b (all data) 0.2769 0.2325 0.1234 0.4094

GOF on F2 1.038 1.009 1.150 0.999

a R1 = Σ||Fo| – |Fc||/Σ|Fo|.

b wR2 = [Σw(Fo

2 – Fc

2)2/Σw(Fo

2)2]1/2

.

- 16 -

Scheme

Scheme 1.

- 17 -

Figures

Fig. 1.

Fig. 2.

- 18 -

Fig. 3.

Fig. 4.

- 19 -

Fig. 5.

- 20 -

Fig. 6.

Fig. 7.

- 21 -

Fig. 8.

- 22 -

Captions of scheme, table and figures

Fig. 1. Two types of ligands in this work.

Table 1 Crystal data and structure refinements for 1 and 3–5.

Fig. 2. Coordination environment of the CoII ion in 1. All hydrogen atoms are omitted for

clarity. Symmetry codes: A 3/2 – x, –1/2 + y, –1/2 – z; B 1 + x, y, z; C 1/2 + x, 1/2 – y, 1/2 + z;

D 2 – x, 1 – y, –z; E 3/2 + x, 1/2 – y, 1/2 + z.

Fig. 3. Two types of left-handed helical chains in 1.

Fig. 4. Homochiral layer A in 1.

Fig. 5. (a) Coordination environment of the CuII ions in 4. All hydrogen atoms are omitted for

clarity. Symmetry codes: A 2 – x, –y, 1 – z; B 1 + x, 1/2 – y, 1/2 + z; C 1 – x, –1/2 + y, 1/2 – z.

(b) (4,4) layer in 4.

Fig. 6. (a) Coordination environment of the CuI ion in 5. All hydrogen atoms are omitted for

clarity. Symmetry codes: A x, 1/2 – y, z; B 1/2 – x, 1/2 – y, z; C 1/2 – x, y, z. (b) (4,4) layer in

5.

Scheme 1. Syntheses of 1–5.

Fig. 7. Temperature dependence of χm and χmT for 4.

Fig. 8. Photoluminescent spectra of bbbm and 1–5.

- 23 -

Graphical abstract

- 24 -

Graphical abstract synopsis

Five 2D bbbm-based transition-metal coordination polymers are synthesized. Racemization

of D-(+)-camphoric acid and reduction of CuII are observed. Magnetic, thermostable and

photoluminescent properties are investigated.

- 25 -

Highlights

Five 2D bbbm-based transition-metal coordination polymers are obtained.

The racemization of D-(+)-camphoric acid and the reduction of CuII are observed in the

preparation of five polymers.

The magnetic, thermostable and photoluminescent properties of five polymers are

investigated.