Effect of substituent on structures and catalytic properties of cobalt(II) isophthalate coordination polymers with a semi-rigid bis(benzimidazole)

Journal of Molecular Structure 1100 (2015) 94e99

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Effect of substituent on structures and catalytic properties of cobalt(II) isophthalate coordination polymers with a semi-rigid bis(benzimidazole) Xu Zhang, Xiang-Li Meng, Cui-Miao Huang, Guang-Hua Cui* College of Chemical Engineering, North China University of Science and Technology, 46 West Xinhua Road, Tangshan, 063009, Hebei, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2015 Received in revised form 14 July 2015 Accepted 16 July 2015 Available online 17 July 2015

Three Co(II) coordination polymers based on flexible bis(2-dimethylbenzimidazole) and the derivatives of the familiar isophthalate co-ligands, namely [Co(L)(ip)]n (1), [Co2(L)2(nip)2]n (2) and [Co2(L)2(tbip)2$2H2O]n (3) (L ¼ 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene, H2ip ¼ isophthalic acid, H2nip ¼ 5-nitroisophthalic acid, H2tbip ¼ 5-tert-butylisophthalic acid) have been synthesized by hydrothermal methods and characterized by elemental analyses, IR spectra, thermogravimetric analyses and single-crystal X-ray diffraction. Both complexes 1 and 2 exhibit a 2-fold interpenetrating 3D network with 66-dia topology, whereas complex 3 is bridged by the L and tbip2 ligands to form a rarely tri-nodal (3,3,5) layer with 3,3,5L18 topology. The results indicate that Co(II) mixed coordination polymers structurally modulated by the substituent effect of isophthalate-involved co-ligands. In addition, the fluorescence and catalytic activity of the complexes for the degradation of methyl orange by the sodium persulfate in a Fenton-like process have been investigated. © 2015 Elsevier B.V. All rights reserved.

Keywords: Catalytic property Cobalt(II) coordination polymer Crystal structure Fluorescence property

1. Introduction Investigations into the synthesis, structural characterization, and properties of metal-organic coordination polymers (MOCPs) remain in intensive focus even after approximately two decades of consistent research. Fascinating properties such as gas storage and absorption, luminescence, magnetism, ion exchange, catalysis and so on certainly motivate continued research efforts [1e4]. It is well known that the most common anionic ligand choices for the construction of MOCPs are the aromatic dicarboxylate derivatives such as phthalic acid, terephthalic acid, isophthalic acid or 1,3,5-benzenetricarboxylic acid, as they all provide a rigid scaffolding and necessary charge balance [5,6]. Among them, the metacarboxylate isophthalic acid (H2ip, Scheme 1) ligand has been a preferential choice for the construction of coordination polymers [7,8]. Inclusion of a 5-position substituent on an isophthalate ring has provided the access to obtain various coordination polymer topologies. The 5-nitroisophthalic acid (H2nip, Scheme 1) can produce attractive structures due to the nitro substituent, as it could be employed as an electron-withdrawing and hydrogen-

* Corresponding author. E-mail address: [email protected] (G.-H. Cui). http://dx.doi.org/10.1016/j.molstruc.2015.07.032 0022-2860/© 2015 Elsevier B.V. All rights reserved.

bonding accepting group [9]. The sterically bulky ligand 5-tertbutylisophthalic acid (H2tbip, Scheme 1) can impose severe steric constraints on metal-organic aggregations during self-assembly, which can certainly alter the coordination/supramolecular environment and result in different topologies [10]. The coexistence of electron-donating (eC(CH3)3) or electron-withdrawing (eNO2) groups in the isophthalic acid derivatives may be a significant effect on the molecular self-assembly, and thereby different physical phenomena can be produced. La Duca and Wang groups reported several 2D and 3D Ni(II)/Cd(II)-based MOCPs incorporating an isophthalate-type dicarboxylate or its derivatives, some fantastic frameworks have been obtained [11,12]. More recently, our group has presented a series of Co(II) coordination complexes derived from flexible bis(benzimidazole) ligands with isophthalic acid and its derivatives, all of them exhibit interesting architectures and significant catalytic activities to degrade azo dyes [13,14]. The flexible bis(2-methylbenzimidazole) derivatives are considered an ideal ligand to construct extended frameworks, because it is able to adopt different conformations due to the two freely rotating methylene groups, with charming topologies, such as dia, hex and sqc3. The utilization of the flexible ligand 1,10 -(1,4butanediyl)bis(2-methylbenzimidazole) can also result in magnificent 3D frameworks [15e17]. In addition, the 2-position

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2.2.2. [Co2(L)2(nip)2]n The synthetic procedure of 2 was similar to the synthesis of 1, except that H2nip (0.2 mmol, 42 mg) was used instead of H2ip. Purple block crystals were collected in 46% yield (based on Co). Anal. Calcd. for C64H50Co2N10O12 (Mr ¼ 1269.00): C 60.57, H 3.97, N 11.04%. Found: C 60.41, H 4.22, N 11.29%. IR (KBr, cm1): 3422(w), 1630(s), 1534(m), 1457(m), 1412(m), 1342(s), 739(s). 2.2.3. [Co2(L)2(tbip)2·2H2O]n (3) The synthetic procedure of 3 was similar to the synthesis of 1, except that H2tbip (0.2 mmol, 44 mg) was used instead of H2ip. Purple block crystals of 3 (yield: 35%) were obtained. Anal. Calcd. for C72H72Co2N8O10 (Mr ¼ 1327.24): C 65.16, H 5.47, N 8.44%. Found: C 66.35 H 5.68, N 8.25%. IR (KBr, cm1): 3433(s), 1623(s), 1570(w), 1456(s), 1413(m), 1364(s), 1353(m), 783(m), 746(s), 729(m). Scheme 1. The structures of the ligands.

methyl substituent on the benzimidazole ring can greatly enhance the electron-donating ability of the ligand [18]. Furthermore, when searching the Cambridge Structural Database (version 5.37, May 2015) [19] for MOCPs constructed from a semi-rigid 1,2bis(2-methylbenzimidazol-1-ylmethyl)benzene ligand (L, Scheme 1), scarcely related crystal structures were obtained. In order to examine the alterations of substituents on the dimensionality and associated properties of cobalt(II) isophthalate coordination polymers based on the bis(benzimidazole) co-ligand. Herein, we report the synthesis, single-crystal structural characterization of three coordination polymers [Co(L)(ip)]n (1), [Co2(L)2(nip)2]n (2) and [Co2(L)2(tbip)2$2H2O]n (3). Moreover, the fluorescence along with the catalytic properties of 1e3 has been investigated in detail.

2.3. X-ray crystallography Crystallographic data for 1e3 were collected on a Bruker Smart 1000 CCD diffractometer using graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å) at room temperature with u-scan mode. A semi-empirical absorption correction was applied using SADABS program [20]. The structures were solved by direct methods and refined on F2 by full-matrix least-squares using SHELXTL-97 [21]. All non-hydrogen atoms were refined anisotropically. In the structure of 3, the two lattice water molecules are disordered, thus this structure was refined by the SQUEEZE routine of PLATON program [22]. The crystallographic data for 1e3 are listed in Table 1, and selected length and angle values for 1e3 are presented in Table S1 (Supporting information). 3. Results and discussion 3.1. Crystal structure of [Co(L)(ip)]n (1)

2. Experimental 2.1. Materials and measurements All the solvents and reagents for synthesis were obtained from Jinan Camolai Trading Company and used without further purification. C, H, and N elemental analyses were performed on a PerkineElmer 240C analyzer. FT-IR spectra were recorded on an Avatar 360 (Nicolet) spectrophotometer between 400 and 4000 cm1, using the KBr pellet method. Thermogravimetric analysis (TGA) was conducted on a Netzsch TG 209 thermal analyzer from room temperature to 800  C under N2 with a heating rate of 10  C/min. The fluorescence spectra were collected with a Hitachi F-7000 spectrophotometer at room temperature. The absorptivity value of methyl orange was recorded with a Shanghai Jingke 722N spectrophotometer at the maximum wavelength of 506 nm.

2.2. Preparation of the complexes 2.2.1. [Co(L)(ip)]n (1) A mixture of L (36 mg, 0.1 mmol), CoCl2$6H2O (48 mg, 0.2 mmol), H2ip (32 mg, 0.2 mmol), NaOH (16 mg, 0.4 mmol), and H2O (10 mL) was placed in a Teflon-lined stainless steel vessel (25 mL). The mixture was sealed and heated at 140  C for 3 days; then, the reaction system was cooled to room temperature at 5  C/ h. Purple block crystals were collected in 40% yield (based on Co). Anal. Calcd. for C32H26CoN4O4 (Mr ¼ 589.50): C 65.20, H 4.45, N 9.50%. Found: C 65.42, H 4.68, N 9.25%. IR (KBr, cm1): 3422(w), 1610(s), 1564(m), 1515(m), 1408 (s), 1352(s), 721(s).

Single crystal X-ray diffraction analysis reveals that complex 1 is in monoclinic crystal system with C2/c space group. The asymmetry unit contains crystallographically independent two Co atoms with occupancy of 0.5, one L neutral ligand and one ip ligand (Fig. 1 a.) Co(1) is four-coordinated by two oxygen atoms(O2, O2#1) of distinct ip2 anions and two nitrogen atoms (N1#2, N1#3, Symmetry code: #1 x þ 2, y, z þ 3/2; #2 x þ 3/2, y  1/2; z þ 3/2, #3 x þ 1/2, y  1/2, z) from two different L ligands, and exhibits a tetrahedron environment with the t4 parameter of 0.98, which is calculated to describe the geometry of a four-coordinated metal system [23]. Differently, the Co(2) is six coordinated by four oxygen atoms of two separate ip2 anions and two nitrogen atoms from two L ligands to furnish a distorted octahedron geometry. The CoeO/N bond lengths range from 1.960(2) Å to 2.308(2) Å, and the angles are within the limits of corresponding parameters found in the literature [24]. The adjacent metal ions are connected by L and ip2 anions to generate a single cage delimited by four cyclohexane-like windows in a chair conformation (Fig. 1b), in which the ip2 anions adopt two different kinds of coordination modes (monodentate and chelate). The distances between the CoII centers is determined by the length of L (13.3634(6) Å) or the ip2 anion (9.5382(5) Å). All the L ligands adopt trans-conformations and the dihedral angle between two benzimidazole rings is 16.404(4) . The extension of the structure into a 3D network is accomplished by connecting four linear ligands to the fourconnected Co(II) nodes. The analysis of complex 1 reveals that it is a typically 3D dia structure containing large cages and the empty space within a single diamondoid lattice allows for the mutual interpenetration of two identical frameworks (Fig. 1c), which were analyzed by the TOPOS 4.0 software [25]. The two interpenetrated

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Table 1 Crystal and refinement data for complexes 1e3.

Empirical formula Formula weight Crystal system Space group a, Å b, Å c, Å a, deg b, deg g, deg V, Å3 Z Dcalc, g/m3 m, mm1 F(000) Crystal size, mm Total reflections Unique reflections Rint GOF R1 (I > 2s(I)) wR2 (I > 2s(I)) Drmax, eÅ3 Drmin, eÅ3

1

2

3

C32H26CoN4O4 589.50 Monoclinic C2/c 12.2489(4) 32.6684(10) 14.9815(5) 90 109.351(4) 90 5656.2(3) 8 1.385 0.651 2440 0.18  0.18  0.17 34166 6238 0.0556 1.077 0.0424 0.0885 0.377 0.359

C64H50Co2N10O12 1269.00 Monoclinic P21/n 12.6486(3) 32.4245(5) 15.2540(3) 90 110.181(2) 90 5872.0(2) 4 1.435 0.639 2616 0.21  0.19  0.18 63,652 12,856 0.0576 1.029 0.0453 0.0981 0.766 0.533

C72H72Co2N8O10 1327.24 Triclinic P
ı 12.2561(3) 15.0220(4) 20.1165(4) 104.026(2) 99.531(2) 109.978(2) 3249.30 2 1.357 0.577 1388 0.18  0.16  0.15 69,236 14,303 0.0688 1.027 0.0520 0.1100 0.927 0.797

2.2 1/2 . R1 ¼ SjjFoj  jFcjj/SjFoj; wR2 ¼ S[w F2o  F2.2 c ]/S [w Fo ]

nets are related by a single translational vector (Class IIa) [26], with Zt ¼ 1 and Zn ¼ 2, where Zt represents the number of interpenetrated nets related by translation and Zn denotes the number of interpenetrated nets related by crystallographic symmetry [27]. Intriguingly, a void volume of 402.8 Å3 is also left after interpenetration and the pore volume ratio was calculated to be 7.1% by the PLATON program.

3.2. Crystal structure of [Co2(L)2(nip)2]n (2) Complex 2 crystallizes in the monoclinic space group P21/n. The fundamental building unit of 2 contains two individual Co(II) atoms, two nip2 anion and two L ligand. As illustrated in Fig. 2a, the CoII(1) ion is six coordinated by two nitrogen atoms (N1, N5) from two L ligands and four oxygen atoms (O3, O4, O7, O8) from the carboxyl groups of two nip2 anions in a distorted octahedron geometry. Dissimilar to Co(1), the Co(2) center is four coordinated by two oxygen atoms (O2, O11#1) of two independent nip2 anions and two nitrogen atoms (N8#2, N4#3, Symmetry code: #1 x  1, y, z  1; #2 x þ 3/2, y  1/2, z þ 1/2; #3 x þ 3/2, y  1/ 2, z þ 3/2) of two L ligands in a distorted tetrahedron environment. As shown in Fig. 2b, there exist two kinds of helical chains in 2. The L ligands serve as bridges linking Co2þ ions to build 1D [CoL]n infinite helical chains along the a axis, in which the dihedral angle between the mean planes of the two benzimidazole rings is 16.098(4) and the Co1/Co2 distance is 13.3702(5) Å. Besides, the nip2 anions connect adjacent Co2þ ions to shape another helical chain along the b direction and the separation of Co1/Co2 is 9.6797(4) Å. The coordination mode of L and nip2 are similar to that in complex 1. The neighboring two parallel [Co-L]n chains are further cross-linked by [Co-nip2-]n chain to generate a 3D network (Fig. 2c). Two identical frameworks interpenetrate each other, giving a diamond 2-fold interpenetrating framework (Fig. S1, Supporting information). The Co(II) atom is simplified as a 4connected node, and L, nip2 ligands are considered as linkers. Accordingly, the 3D complex framework of 2 can be simplified to a 4-connected dia topology in Class IIa with the topological notation of 66. Interestingly, calculations from the X-ray structure data

show that the framework possesses a solvent-accessible volume of 292 Å3, corresponding to 5.0% of the unit cell. 3.3. Crystal structure of [Co2(L)2(tbip)2·2H2O]n (3) X-ray structural analysis reveals that complex 3 is a 2D network in triclinic crystal system with P1 space group. The asymmetric unit of compound 3 contains two cobalt atoms, two L ligand, two tbip ligands. As shown in Fig. 3a, there are two Co(II) centers with different coordination environment. The Co(1) is five-coordinated with a trigonal bipyramid coordination geometry by one N-donor atom from the benzimidazole group of L ligand, and four oxygen atoms derived from four tbip2 groups, which is completed by O1#4, O2#3, N4 located at the basal plane and O7#2, O8#1 at the apex (Symmetry code: #1 x þ 1, y þ 1, z; #2 x þ 2, y þ 1, z þ 1; #3 x þ 2, y þ 2, z þ 1; #4 x þ 1, y, z). However, Co(2) is fourcoordinated with two nitrogen atoms (N1, N5) of two L ligands and two oxygen atoms from two individual tbip2 anions in a distorted tetrahedron environment. The CoeN distances are within the range of 2.029e2.058 Å, while the CoeO bond lengths range from 1.932 to 2.067 Å. The tbip2 ligand in 3 adopts the m3-h1:h1:h1 coordination mode and four tbip2 ligands bridge adjacent cobalt atoms to form a 32-membered macrocycle [Co4(tbip)4] with the size of 11.45  13.66 Å (Fig. 3b). It is noteworthy that the two crystallographically independent L ligands exhibit distinct coordination behaviors when coordinated to cobalt atoms, one acts as a terminal ligand (N5eN8) and the other one acts as a linker (N1eN4), in which the dihedral angle between the mean planes of the two benzimidazole rings is 7.652(85) and 10.841(61) , respectively. The L ligands bridge neighboring [Co4(tbip)4] units to construct a 2D layer. The m3tbip2- bridge ligands can be simplified as topologically equivalent 3connected nodes, each Co1 is linked by four tbip2 ligands and one L ligand acting as a 5-connected node and Co2 can be abstracted as a 3connected node to give a rarely 3,3,5-connected 3,3,5L18 network (Fig. 3c), a point symbol of {4.6.8}2{46.62.82}{62.8} acquired by using the TOPOS4.0 software. To the best of our knowledge, this structural topology has not been documented hitherto. This is the first example of metal coordination polymer with 3,3,5L18 topology.

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Fig. 1. (a) The coordination environment of Co(II) ion in complex 1 with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) Single adamantanoid cage in 1. (c) View of the full 2-fold interpenetrated framework in complex 1. Individual networks are displayed in different colors, and ligands are represented by straight lines.

3.4. Effect of the dicarboxylate anions on the structures of the complexes More relevant to our purpose, the different structures of three MOCPs demonstrate the influence of a substituent group of isophthalic acid and its derivatives on the assembly of MOCPs. In the case of the coordinated cobalt complexes 1 and 2, both the isophthalate and 5-nitroisophthalate derivatives showed a 3D dia topology. However, complex 3 exhibits a quite different 2D layer structure. In these three complexes, L adopts the same transconfiguration in 1 and 2 acting as bridge ligands, while in 3, it employs diverse coordination behaviors when coordinated to central atoms. When it comes to the carboxylate ligand, it employs the m3-h1:h1:h1 bridging mode in 3, which is different from the other two complexes. 1 and 2 are almost same in molecular arrangement and topologies except the void volume, which may

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Fig. 2. (a) The coordination environment of Co(II) ion in complex 2 with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) The helical chain constructed by nip2 anions and Co(II) ions. (c) Schematic representation of the 3D framework in complex 2.

attribute to the inclusion of a 5-position electron-withdrawing nitro substituent. The diversity of 3 compared to 1 and 2 in dimensionality and properties is probably due to the different coordination mode of L ligands. In the other hand, the participation of the tertiary butyl can certainly alter the coordination and/or supramolecular environment during self-assembly of coordination polymers, resulting in substantially different topologies when compared with less sterically hindered congeners.

3.5. Thermogravimetric analysis The decomposition behaviors of complexes 1e3 were examined by thermogravimetric analysis (TGA) with a heating rate of 10  C/ min in the temperature range of 25e800  C, as shown in Fig. 4. For complex 1, the curve shows that chemical decomposition starts at

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Fig. 4. Thermogravimetric curves for complexes 1e3.

began at 410  C, which corresponds to the loss of 88.4% (calcd. 88.2%). The TGA curve of complex 3 exhibits three main steps of weight losses. The lattice water molecules were lost from 133 to 178  C (calcd.: 2.7%, found: 2.5%). The second one started at 285  C, which attributes to the release of the tbip2 ligand. The observed weight loss of 33.1% is close to the calculated values 33.3%. The last step covers from 426 to 635  C, during which the L ligands are burned, the weight loss of 55.3% was observed (calcd. 55.4%), the remaining weight corresponds to CoO. 3.6. Fluorescence properties In order to investigate the photoluminescence properties of the coordination polymers, fluorescence spectra were recorded for complexes at room temperature. As shown in Fig. 5, the free L ligand shows fluorescence in solid state with the emission peak at 366 nm upon excitation at 275 nm. MOCPs 1e3 show different fluorescent emissions bands at 398 nm for 1 (lex ¼ 260 nm), 396 nm for 2 (lex ¼ 328 nm), and 400 nm for 3 (lex ¼ 260 nm), respectively. Comparing with the emission of L, a red-shift of calcd. 32 nm, 30 nm and 34 nm have been observed separately in complexes 1e3, respectively. As previously reported [28], the main emission band can be assigned to the p*p transition of the coordinated L ligand. The red-shift emission peak may be related to the intraligand fluorescent emission [29]. The different emission intensity of complexes 1, 2 and 3 may be caused by the coordination

Fig. 3. (a) The coordination environment of Co(II) ion in complex 3 with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) A [Co4(tbip)4] unit in 3. (c) Topological view of the tri-nodal 3,3,5L18 network in 3.

443  C with the weight loss of 87.1%, equivalent to the loss of total organic ligands (calcd. 87.3%); the remaining weight corresponds to CoO. The decomposition of complex 2 was similar to that of complex 1, but showed lower thermal stability. Here mass decrease

Fig. 5. Fluorescence spectra of polymers 1e3 and free L ligand.

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steric bulky groups afforded coordination polymers with lower dimensionality, indicating potential for deliberately tuning their structures. Moreover, complex 1 is found to be a more positive catalyst for the degradation of methyl orange dye than 2 and 3 in a Fenton-like process. Acknowledgments The project was supported by the National Natural Science Foundation of China (51474086), Natural Science Foundation e Steel and Iron Foundation of Hebei Province (B2015209299). Appendix A. Supporting information Supporting information related to this article can be found at http://dx.doi.org/10.1016/j.molstruc.2015.07.032. Fig. 6. Experimental catalytic degradation of methyl orange.

References model of aromatic dicarboxylate ligands and the different coordination environment of the metal center [30].

[1] [2] [3] [4]

3.7. Catalytic properties Azo dyes are difficult to degrade in the environment, and most of them are toxic, non-biodegradable and potentially carcinogenic in nature [31]. It is urgent to find an efficient way to degrade these organic pollutants. Advanced oxidation technologies such as the Fenton-like method utilize oxidation processes, which generate sulphate radicals that are very effective in degrading organic pollutants because of their strong oxidant power [32,33]. In order to investigate the catalytic activity of the complex, degradation experiments of methyl orange with persulfate were investigated, the experiment was prepared according to the literature [34]. As depicted in Fig. 6, when the compound 1 (2 or 3) is added as a catalyst, this leads to a color removal of the dye. The degradation efficiencies can reach 91.40%, 84.15% and 72.42% after 150 min, for 1, 2 and 3, respectively. It can be explained that the presence of  catalyst can catalyze persulfate to produce SO4 radicals and can further influence the efficiency of the pollutant degradation significantly. However, in the absence of catalyst, the degradation efficiency of control experiments is low, reaching 18.20% after 150 min under the same conditions, which indicates that CoCl2 alone was unable to decompose the azo dye effectively. The different catalytic performances of the three complexes may be due to the distinct coordination environment around the metal centers [35]. 4. Conclusion Variation of isophthalate-type dicarboxylate substituents, altering electronic effects and steric hindrance, has exerted a tremendous structure directing effect in a series of divalent Co(II) coordination polymers containing the bis(2-methylbenzimidazole) ligand. Both complex 1 and 2 show a 3D 4-connected dia topology, while in 3, the 2D layer can be abstracted as a tri-nodal 3,3,5connected 3,3,5L18 network. The present work also follows some previously reported trends where enhanced dicarboxylates with

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