Ni(II)-metal–organic frameworks based on 1,4-phenylenedipropionic acid: Solvothermal syntheses, structures, and photocatalytic properties

Polyhedron 119 (2016) 151–159

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Ni(II)-metal–organic frameworks based on 1,4-phenylenedipropionic acid: Solvothermal syntheses, structures, and photocatalytic properties Surapoj Sanram, Jaursup Boonmak ⇑, Sujittra Youngme Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand

a r t i c l e

i n f o

Article history: Received 24 June 2016 Accepted 24 August 2016 Available online 3 September 2016 Keywords: Metal–organic frameworks 1,4-Phenylenedipropionic acid Photocatalytic properties Solvothermal syntheses Nickel(II)

a b s t r a c t Four novel Ni(II)-metal–organic frameworks, namely [Ni(azp)(ppa)(H2O)2]n (1), [Ni(tmdp)(ppa)(H2O)2]n (2), {[Ni(bpetha)(ppa)]
1.7H2O}n (3), and {[Ni(bpy)(H2O)4](ppa)}n (4) (azp = 4,40 -azodipyridine, H2ppa = 1,4-phenylenedipropionic acid, tmdp = 4,40 -trimethylenedipyridine, bpetha = 1,2-bis-(4-pyridyl)ethane, bpy = 4,40 -bipyridine) were synthesized and structurally characterized. With the differences in the length and flexibility of N,N0 -donor coligands, Ni(II)-MOFs with the diversities of structural architectures and coordination modes of dicarboxylate ppa ligand were obtained. Compound 1 is 2D (4,4)-grid sheet whereas 2 shows 2-fold interpenetrating 2D (4,4)-layers. Compound 3 exhibits 3-fold interpenetrating coordination 3D framework with the a-Po topology while 4 shows 1D chain. Remarkably, compounds 1–4 exhibit energy band gaps of 3.27, 3.66, 3.58 and 3.36 eV, respectively. The photocatalytic activities of these materials for degradation of methylene blue (MB) were investigated with the degradation efficiencies within 90 min of 96.8%, 64.4%, 67.4%, and 75.0% for 1–4, respectively. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Functional metal–organic frameworks (MOFs) have found a great many potential applications in catalysis, adsorption, separation, magnetism, and luminescence [1–5]. Recently, the research in this field is focusing on factors affecting the final design of MOFs, which includes organic ligands [6–8], solvents [9], temperatures and reaction times [10,11], pH values [12–14], and metal centers [15]. For the effective formation of MOFs, their organic ligands must possess rigidity and flexibility in accordance with the frameworks’ applications. Together with the reaction conditions, choosing the appropriate ligands is crucial for the successful fabrication of MOFs. One of the many potential applications of MOFs is the decomposition of organic dye in wastewater because the linkers could play a role as antenna absorbing light of the appropriate wavelength and transferring energy to the metal centres. Potentially, MOFs could be a promising candidate for cleaning the polluted water by using them as photocatalysts. The diversification of the MOFs structures is part of the ongoing research, which aims to investigate the effects of their different structures resulting in different ligand-to-metal-charge-transfers (LMCT) and band gap energies on the photocatalytic processes [16–24]. ⇑ Corresponding author. Fax: +66 43 202 373. E-mail address: [email protected] (J. Boonmak). http://dx.doi.org/10.1016/j.poly.2016.08.044 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

It is well known that the most common way to synthesize MOFs is the assemblage of metal salts with O-donor and/or N,N0 -donor linkers [25,26]. We employed 1,4-phenylenedipropionate (ppa) which is an interesting dicarboxylate O-donor linkers, mainly because of the possibility of variety of coordination sites and its flexibility. Furthermore, the geometric inclination of both of the opposing aliphatic carboxylate groups freely changes their compatibilities to get different coordination requirements of the metal center [27,28]. In addition, N,N0 -donor ligands are of great importance in the formation of various MOFs because the rigidity, as well as the flexibility affect both the coordination mode and the topology of MOFs, upon which MOFs’ applications depend [29–40]. However, there are a few of MOFs containing 1,4-phenylenedipropionato ligand have been reported and never been found in the applications of dye degradation [27,28]. In this regard, it may be possible to construct some new classes of MOFs based on ppa ligand as a photocatalytic materials through combining the two kinds of precursors. Herein, we report the preparation by self-assembly of four new mixed-ligand Ni(II)-metal–organic frameworks, [Ni(azp) (ppa)(H2O)2]n (1), [Ni(tmdp)(ppa)(H2O)2]n (2), {[Ni(bpetha)(ppa)]
1.7H2O}n (3), and {[Ni(bpy)(H2O)4](ppa)}n (4) in which one component is the flexible O-donor spacer 1,4-phenylenedipropionic acid (H2ppa) and the other flexible exoditopic N,N0 -coligands, 4,40 -azopyridine (azp), 4,40 -trimethylenedipyridine (tmdp),

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1,2-bis-(4-pyridyl)ethane (bpetha), and rigid 4,40 -bipyridine (bpy) (Scheme 1). All compounds were chemically and structurally characterized. The effects of rigidity and flexibility of ligands on the structural diversities of these compounds are discussed. In addition, photocatalytic performances in decomposing methylene blue (MB) in water under ultraviolet (UV) light for 1–4 were investigated. 2. Experimental section 2.1. Materials and general methods All reagents and solvents were purchased from commercial suppliers and used without further purification. Elemental analyses were carried out on a PerkinElmer PE 2400CHNS analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4,000–400 cm1 on a PerkinElmer Spectrum One FTIR spectrophotometer. PXRD patterns of the samples were collected on a Bruker D8 ADVANCE diffractometer using monochromatic Cu Ka radiation, and the recording speed was 0.5 s/step over the 2h range of 5–40° at room temperature. Thermogravimetric (TG) analyses were performed on a TG-DTA 2010S MAC analyzer heated from 45 to 800 °C under nitrogen gas. Diffuse reflectivity spectra were collected on finely ground samples with a UV-3101pc UV–VIS-NIR scanning spectrophotometer Shimadzu, which was measured from 400–1,100 nm using BaSO4 as a standard with 100% reflectance.

2.2. Synthesis of [Ni(azp)(ppa)(H2O)2]n (1) A mixture of Ni(NO3)2
6H2O (150 mg, 0.5 mmol), 1,4phenylenedipropionic acid (220 mg, 1 mmol), 4,40 -azodipyridine (180 mg, 1 mmol) and 9 ml of mixed H2O/DMF/MeOH (1:1:1 v/v) was placed in a 25 ml Teflon-lined stainless steel. The vessel was sealed and heated at 80 °C for 10 h and then continuously heated at 120 °C for 24 h. After the mixture was slowly cooled to room temperature, greenish brown crystals of 1 were obtained. Yield: 112 mg (44%) based on Ni salt. Anal. Calc. For C22H22N4NiO6: C, 53.00; H, 4.46; N, 11.27%. Found: C, 52.91; H, 4.50; N, 11.12%. IR spectrum (KBr, cm1): 3386s, 3210m, 2961m, 2914m, 2631s, 1956m, 1574s, 1552s, 1415s, 1383s, 1415s, 1383s, 1308s, 1227m, 1161m, 1017m, 985m, 957m, 865s, 846s, 799m, 676m, 620s, 571m, 551m, 480m. 2.3. Synthesis of [Ni(tmdp)(ppa)(H2O)2]n (2) A mixture of Ni(NO3)2
6H2O (150 mg, 0.5 mmol), 1,4phenylenedipropionic acid (220 mg, 1 mmol), 4,40 -trimethylenedipyridine (200 mg, 1 mmol) and 6 ml of mixed H2O/DMF/MeOH (1:1:1 v/v) was placed in a 25 ml Teflon-lined stainless steel. The vessel was sealed and heated at 120 °C for 24 h. After the mixture was slowly cooled to room temperature, blue crystals of 2 were obtained. Yield: 122 mg (46%) based on Ni salt. Anal. Calc. For C25H30N2NiO6: C, 58.51; H, 5.89; N, 5.46%. Found: C, 58.56; H, 5.78; N, 5.45%. IR spectrum (KBr, cm1): 3301s, 2947m, 1619m,

Scheme 1. Synthetic scheme of compounds 1–4.

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153

1575s, 1512m, 1398s, 1330m, 1223m, 1067m, 1024m, 886m, 842m, 823s, 681s, 530m.

crystal data, selected bond lengths and angles, and hydrogen bonds for 1–4 are listed in Tables 1, S1 and S2, respectively.

2.4. Synthesis of {[Ni(bpetha)(ppa)]
1.7H2O}n (3)

3. Results and discussion

Compound 3 was synthesized in the same way as 2, excepted that 1,2-bis-(4-pyridyl)ethane (180 mg, 1 mmol) was used instead of 4,40 -trimethylenedipyridine. Green crystals of 3 were obtained. Yield: 96 mg (37%) based on Ni salt. Anal. Calc. For C24H26N2NiO5: C, 58.14; H, 5.98; N, 5.65%. Found: C, 59.17; H, 5.61; N, 5.70%. IR spectrum (KBr, cm1): 3386s, 3120m, 2961m, 2914m, 2631m, 1956m, 1574s, 1552s, 1415s, 1383s, 1308s, 1227s, 1161m, 1017m, 985s, 957m, 865s, 846s, 799m, 676s, 620s, 571m, 551m, 480m.

3.1. Crystal structures

2.5. Synthesis of [[Ni(bpy)(H2O)4](ppa)]n (4) Compound 4 was synthesized in the same way as 2, excepted that 4,40 -bipyridine (180 mg, 1 mmol) was used instead of 1,2-bis-(4-pyridyl)ethane. Blue crystals of 4 were obtained. Yield: 102 mg (39%) based on Ni salt. Anal. Calc. For C22H28N2NiO8: C, 59.35; H, 6.79; N, 6.29%. Found: C, 59.27; H, 6.61; N, 6.40%. IR spectrum (KBr, cm1): 3329s, 3072s, 1670m, 1610s, 1543s, 1455m, 1403s, 1273m, 1223m, 1163m, 1067m, 1028m, 910m, 842m, 810s, 729m, 671m, 635m, 499m. 2.6. Photocatalytic experiment The photocatalytic decomposition of methylene blue was studied for all products as well as that of ZnO. The UV light source used was a 400 W high-pressure mercury lamp. A suspension containing MOFs catalyst (60 mg) and 100 mL of MB (40 ppm) solution was stirred in the dark for about an hour to ensure that no effect of its absorption on the material surfaces. Then, the mixture which contains MOF and MB was stirred continuously under UV irradiation. A sample solution (2 ml) was taken every 15 min. and separated through a centrifuge to remove suspended MOF particles. After filtration, the samples were analyzed by the UV–vis spectrophotometry at 665 nm. On the contrary, the simple photolysis experiment was also investigated under the same conditions without any catalyst. After photocatalytic, the powders were obtained by powder X-ray diffractometer (PXRD) to check the stability of all compounds. In addition, compound 1 was selected to evaluate the reusability. 2.7. X-ray crystallography X-ray diffraction intensity data of 1–4 were measured on a Bruker D8 Quest PHOTON100 CMOS detector with graphitemonochromated Mo Ka radiation using the APEX2 program [41]. The diffraction data were integrated with SAINT [41]. Lorentz and polarization effect and empirical absorption corrections were applied with SADABS [41]. The structures were solved by direct methods and then refined with full matrix least-square methods based on F2 (SHELXTL) [42]. Anisotropic displacement parameters were refined for all non-hydrogen atoms. All hydrogen atoms were placed in calculated positions and refined isotropically. The hydrogen atoms on the coordinated water molecule in 1 could not be located. The C2 and C3 atoms in the aliphatic groups of ppa ligand for 3 are disordered with the occupancies of 0.4 and 0.6 for C2 and C3, respectively. The disordered lattice water molecules in 3 could not be located. In order to give detailed information of water molecules in the lattice for 3, we removed the disordered water molecules using the SQUEEZE instruction. After squeezing, there are about 16.7 electrons corresponding to 1.7 lattice water molecules which is consistent with the TGA data. The details of

3.1.1. Crystal structure of [Ni(azp)(ppa)(H2O)2]n (1) The X-ray structural analysis reveals that compound 1 crystal
space group. The asymmetric unit comprises lizes in the triclinic P1 a half of Ni(II) ion, azp, and ppa, and one coordinated water molecule (Fig. S1). Each Ni(II) ion is the axially elongated octahedral coordination geometry. The equatorial plane is defined by two oxygen atoms from two ppa ligands and two symmetry-related coordinated water molecules (Ni–O = 2.080(2) and 2.0889(18) Å). The axial positions are occupied by two nitrogen atoms from two bpp ligands (Ni–N = 2.113(2) Å). (Fig. 1a). Both carboxylates from a ppa ligand covalently bind to two Ni(II) ions in the monodentate fashion and the carbon atoms in aliphatic carboxylate (C–CH2–CH2–C) of ppa adopted a cis-form with a dihedral angle of 70.7° (Scheme 2), generating a [Ni(ppa)]n zigzag chains with Ni


Ni distance of 12.330(2) Å (Fig. S2a). In addition, the azp acting as a N,N0 -bidentate bridge in trans-conformation (T) (Scheme S1) links the neighboring Ni(II) chains with Ni


Ni distance of 13.186(2) Å forming a (4,4) 2D grid layer of 1 (Fig. 1b and c). Both pyridine rings of trans-azp are symmetry-related perfectly planar through the inversion center. These neighboring layers are finally extended into a 3D supramolecular network through inter-layer weak hydrogen bonds (C–H. . .O) between the pyridine carbon atoms from azp ligands and oxygen atoms from coordinated water molecules (Fig. 1d and Table S2) as well as C–H. . .p interactions between the electron cloud in pyridine rings of azp and aliphatic CH2 group from ppa ligands (2.862(2) and 3.331(2) Å) (Fig. S2b). 3.1.2. Crystal structure of [Ni(tmdp)(ppa)(H2O)2]n (2) The X-ray structural analysis reveals that compound 2 crystallizes in the monoclinic C2/c space group. The asymmetric unit consists of one Ni(II) ion, one azp, one ppa and two coordinated water molecules (Fig. S1). Each Ni(II) ion is in six-coordinated distorted octahedral geometry, constructed by two oxygen atoms from two ppa ligands and two nitrogen atoms from two tmdp ligands in the basal plane, as well as, two coordinated water molecules in the axial positions (Fig. 2a). The bond distances for Ni(II)–O and Ni(II)–N are in the range of 2.094(2)–2.110(2) Å, and 2.091(3)– 2.098(3) Å, respectively. The ppa anion bridges two adjacent Ni (II) ions in the monodentate fashion with Ni


Ni separation of 15.355(2) Å, similar fashion as in 1. However, the carbon atoms in aliphatic carboxylate (C–CH2–CH2–C) of ppa adopted anti-form with a dihedral angle of 169.6° and 175.5° (Scheme 2). The tmdp adopts a trans–trans (TT) conformation (Scheme S1) connecting between Ni(II) centres with Ni


Ni separation of 12.610(2) Å. The ppa and tmdp spacers link adjacent Ni(II) ions generating a 2D layers. Further investigation indicates that each 2D layer of 2 involves two types of one-dimensional helical infinite chains, where the right-handed [Ni(ppa)]n and left-handed [Ni(tmdp)]n2+ helical chains are in an alternate array by sharing both ppa and tmdp bridges (Fig. 2b) which is different from the previously reported compound {[Ni(ppa)(mdp)2].2H2O} (mdp = methylenebis(3,5-dimethylpyrazole)) [43]. The [Ni(ppa)]n moiety shows wave-like chains while the [Ni(mdp)]n2+ moiety exhibits double helical chains. In 2, each Ni(II) ion links with four neighboring Ni(II) ions via two azp and two ppa, thus Ni(II) can be defined as a 4-connected node forming a (4,4)-grid layer. Moreover, each (4,4)-grid layer of 2 is interwoven giving rise to a 2-fold interpenetrating 2D layer (Fig. 2c), which is stabilized by interlayer

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Table 1 Crystallographic data for 1–4.

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) F(0 0 0) Rint Goodness-of-fit (GOF) on F2 R1a[I > 2r(I)] wR2b (all data) a b

1

2

3

4

C22H24N4NiO6 497.11 triclinic
P1

C25H30N2NiO6 513.20 monoclinic C2/c 22.2958(15) 11.0733(6) 20.2133(12) 90 105.965(3) 90 4797.7(5) 8 1.421 2160 0.0298 1.0590 0.0577 0.1755(4890)

C24H27.4N2NiO5.7 493.14 triclinic
P1

C22H28N2Ni O8 507.15 triclinic
P1

10.0385(5) 11.0359(5) 11.2362(5) 95.033(1) 101.489(1) 92.445(1) 1212.84(10) 2 1.268 484 0.0640 1.0390 0.0474 0.0990(4976)

7.1314(2) 7.6641(2) 10.3275(3) 91.9385(11) 101.6045(11) 95.9648(10) 549.06(3) 1 1.534 226 0.0236 1.049 0.0288 0.0751(2734)

5.7272(11) 7.9098(17) 12.330(2) 73.328(9) 76.702(8) 80.515(9) 517.80(17) 1 1.588 256 0.0439 1.068 0.0366 0.1438(2527)

P P R = ||Fo|  |Fc||/ |Fo|. P P Rw = { [w(|Fo|  |Fc|)]2/ [w|FO|2]}1/2.

Fig. 1. (a) Coordination environment of Ni(II) ion in 1. (i) = 1  x, 1  y, 1  z. The ellipsoids are shown at 50% probability level. (b) 2D single framework of 1 in bc plane. (c) (4,4)-connected net of 1. (d) Stacking layers in 1, projected on c axis.

hydrogen bonding interactions between coordinated water molecules and carboxylate oxygens of ppa ligands (Fig. S3, Table S2). The centroid of pyridine ring of tmdp ligands can interact with another centroid of adjacent tmdp with the centroid to centroid distance of 3.974 Å stabilizing an overall 3D supramolecular structure of 2 (Fig. 2d).

3.1.3. Crystal structure of {[Ni(bpetha)(ppa)]
1.7H2O}n (3) Single-crystal X-ray analysis reveals that 3 crystallizes in tri
space group. The asymmetric unit consists of one Ni(II) clinic P1 ion, a half of two independent ppa, one bpetha and 1.7 lattice water molecules (Fig. S1). The coordination environment of 3 is depicted in Fig. 3a. The Ni(II) ion is six-coordinated by four carboxylate oxy-

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Scheme 2. The orientation of aliphatic carboxylate groups in the flexible ppa linker in 1–4 and coordination modes in 1–3.

Fig. 2. (a) Coordination environment of Ni(II) ion in 2 with hydrogen atoms omitted for clarity. The ellipsoids are shown at 50% probability level. (b) 2D single layer of 2 in ab plane with the left-handed and right-handed one-dimensional helical chains (blue = tmdp, yellow = ppa). (c) The twofold interpenetrating (4,4)-connected sheets for 2. (d) 3D packing diagram of 2 via p–p interaction. (Colour online.)

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Fig. 3. (a) Coordination environments of Ni(II) ion in 3 with hydrogen atoms omitted for clarity. The ellipsoids are shown at 50% probability level. (b) View of the linkage of the SBUs by ppa and bpetha ligands. (c) The 3D motif with voids shown by yellow spheres in 3. (d) Schematic representation of 3-fold interpenetrating a-Po net of 3. (Color online.)

gen atoms from three ppa anions and two nitrogen atoms of two bpetha to give a {NiN2O4} distorted octahedron. The bond distances for Ni(II)–N and Ni(II)–O are in the ranges of 2.092(2)–2.095(2) Å, and 1.999(2)–2.158(2) Å, respectively. Unlike compounds 1 and 2, both carboxylate groups of two independent ppa anions in 3 adopt different coordination modes: one is a triatomic-bridging mode {(j1-j1)-(j1-j1)-l4-ppa2} and the another is a chelating coordination mode {(j2)-(j2)-l2-ppa2} (Scheme 2). Moreover, the triatomic carboxylate bridges from two ppa anions link two adjacent Ni(II) ions in (syn–syn)-fashion constructing a dinuclear Ni(II) secondary building unit (SBU) with Ni


Ni distance of 4.305(5) Å. Each SBU is connected to the neighboring six SBUs by four ppa anions and two pairs of bpetha ligands giving rise to a 3D covalent framework (Fig. 3b and c). The Ni


Ni distances

via ppa and bpetha ligands are 15.3129(7) and 13.4941(6) Å, respectively. The bpetha coligands adopts a trans-bidentate bridging fashion between the neighboring SBUs. From the topological view, if the SBU serves as a 6-connected node, ppa and bpetha ligands act as linear linkers, the final 3D motif of 3 represents a a-Po topology. Moreover, to minimize the voids and consolidate the coordination framework, three identical 3D frameworks are interpenetrated with each other, generating 3-fold interpenetrated a-Po 3D architecture of 3, as shown in Fig. 3d.

3.1.4. Crystal structure of {[Ni(bpy)(H2O)4](ppa)}n (4) Single-crystal structure analysis reveals that compound 4 crys
space group. The asymmetric unit comprises a tallize in triclinic P1

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half of Ni(II) ion, bpy, and ppa, and two coordinated water molecules (Fig. S1). The crystal structure of 4 consists of cationic [Ni (bpy)(H2O)4]n2+ chains with lattice ppa2- anions in which the Ni (II) ions are bridged via l2-bpy ligand with Ni


Ni separation of 11.308(3) Å. Each Ni(II) ion is six-coordinated distorted octahedral geometry constructed by four oxygen atoms of four coordinated water molecules in the equatorial plane with NiO distances of 2.059(2) and 2.056(2) Å, while the axial positions are located by the bpy nitrogen atoms with Ni–N distance of 2.107(2) Å (Fig. 4a). In 4, the carbon atoms in aliphatic carboxylate (C–CH2–CH2–C) of free ppa anion adopted an anti-conformation with a dihedral angle of 177.2° (Scheme 2). In the packing diagram, the chains of 1 are assembled by hydrogen bonds between the carboxylate groups of ppa, and the coordinated water molecules, generating an overall 2D supramolecular network (Fig. 4b, Table S2). As we know, the flexible exoditopic ligands were used as the auxiliary ligands to modify diverse structures accompanied with the flexible dicarboxylate ligand for their strong coordination ability to transition metal ions [29–40]. In compound 1, azp coligand with trans-conformation (T) linking adjacent Ni(II) ions generates (4,4) 2D network of 1 (Scheme S1). When the longer and more flexible tmdp ligand was applied in 2, the 2-fold interpenetrating 2D network was obtained with trans–trans-conformation (TT) of tmdp. Interestingly, when the shorter bpetha was used in 3, a 3-fold interpenetrating 3D a-Po type framework was generated with trans-conformation (T) of bpetha. In contrast, when the small and rigid bpy ligand was applied in 4, this gives extended 2D supramolecular network containing 1D chain structure with free carboxylate ppa anions in the lattice. For compounds 2 and 3, they

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have large voids in the framework due to the flexibility of ligand constituents, thus, these voids induce the interpenetration to form 2-fold and 3-fold interpenetrated frameworks, respectively. With different N,N0 -spacers, ppa ligand shows a variety of orientations and coordination modes, as shown in Scheme 2. In 1, the carbon atoms in the aliphatic carboxylate ppa adopted a cis-form with (j1)-(j1)-l2-ppa2- bridging mode, while 2–4 exhibited an anti-conformation with coordination modes of (j1)-(j1)-l2-ppa2- for 2, and (j1- j1)-(j1-j1)-l4-ppa2-, (j2)-(j2)-l2-ppa2 for 3. These results imply that the length, rigidity, flexibility, the orientation and coordination modes of both dicarboxylate ligand and auxiliary N,N0 -coligands play an important role on the structural topologies of Ni(II)MOFs. 3.2. Thermal analysis The thermal stability of 1–4 was studied by TG-DTA technique, as shown in Fig. 5. At the temperature ranges 45–160 °C, all compounds exhibit the weight loss of all water molecules, which are 7.20%, 6.98%, 6.25%, and 13.74% for 1–4, respectively, corresponding to the loss of two coordinated water molecules for 1 and 2, 1.7 lattice water molecules for 3, and four coordinated water molecules for 4 (calcd 7.19% for 1, 6.99% for 2, 6.19% for 3, and 14.61% for 4). Then, the structural frameworks start to collapse at around 300 °C, except that of 2 (200 °C). Finally, the decomposition residue species was assumed to be metal oxides. Compound 2 is the 2-fold interpenetrating 2D framework which is stabilized by hydrogen bonding interactions between each single layer through oxygen atoms of coordinated water molecules. After the removal of

Fig. 4. (a) Coordination environments of Ni(II) ion in 4. The ellipsoids are shown at 50% probability level. (b) 3D packing diagram of 4 with hydrogen bonds between the chains and lattice ppa molecules (blue = [Ni(bpy)(H2O)4]2+, green = lattice ppa). (Colour online.)

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Fig. 5. TGA curves of compounds 1–4.

Fig. 7. Degradation percentage within 90 min of ZnO and cycling test of photocatalytic activity of 1.

The solid-stated diffuse reflectance UV–vis spectra reveal the absorption features of all compounds (Fig. S4). Compounds 1–4 show sufficiently similar characteristic electronic absorption bands for the three spin-allowed transitions for octahedral Ni(II) centres: 3A2g(3F) ? 3T2g in the range of 10,000–10,400 cm–1, 3 A2g(3F) ? 3T1g(3F) in the range of 15,128–15,528 cm–1, and, 3 A2g(3F) ? 3T1g(3P) in the range of 25,316–27,397 cm–1. The energy band gap (Eg) of 1–4 can be calculated corresponding to the formula: ahm2 = K(hm  Eg)1/2 (where hm is the discrete photo energy, a is the absorption coefficient, Eg is the band gap energy, and K is a constant) [44] thus, the band gap energy are 3.27, 3.66, 3.58 and 3.36 eV for 1–4, respectively.

reduced gradually with the increase of reaction time. Moreover, control experiments toward the degradation of MB molecule were also carried out under UV light. The results show that all compounds possess high photodegradation efficiencies for MB contaminant under UV light irradiation. C/C0 versus irradiation time plot suggests that 1 exhibits the best photocatalytic activity with the MB degradation efficiency of 96.8% while 2–4 give 64.4, 67.4, and 75.0%, respectively, within 90 min. The differences of catalytic activity under UV light irradiation for 1–4 may result from the diverse structures of the four compounds. As calculated, the band gaps of compounds 1–4 are 3.27, 3.66, 3.58 and 3.36 eV, respectively. The Eg of 1–4 follows the sequence of 1 < 4 < 3 < 2, and the reverse sequence of band gaps agrees with the decomposition efficiency of MB [45,46]. In addition, the aromaticity of N,N0 -donor ligands which is in the order of 1 > 4 > 3 > 2, also agrees with the decomposition efficiency of MB. However, the degradation of MB in the presence of compound 1 (96.8% of degradation efficiency) appears to be slightly higher than that of the ZnO catalyst under the same condition (94.1% of degradation efficiency within 90 min). Among these results, 1 shows the highest efficiency as aromaticity in 1 can enhance the mobility of p-electrons in the organic aromatic ligand, which could facilitate the ligand-to-metal-chargetransfer (LMCT) transitions and decrease the electronic band gap of the MOF, thereby enhancing the photocatalytic efficiency [24,46– 48]. These results could be demonstrated that the presence of different bridging ligands results in different LMCT transitions which make compounds 1–4 potentially tunable photocatalysts. Compound 1 with the highest efficiency was chosen to explore the reusability of photocatalyst in the photodegradation of MB. As shown in Fig. 7, compound 1 was recycled for five rounds with the degradation efficiencies of 89.9%, 93.4%, 92.9%, 90.4%, and 90.1%, respectively. The cycling results indicate that 1 can be reused for photodegradation of MB. Additionally, the catalyst powders of all compounds were obtained by filtration after the photocatalytic reaction, and their PXRD patterns (Fig. S6) are identical to those of the original compounds indicating that these MOFs are stable during photocatalysis.

3.4. Photocatalytic activities

4. Conclusion

As shown in Fig. 6 and S5, using compounds 1–4 and ZnO as photocatalysts, the absorption intensities of methylene blue (MB)

In summary, we have synthesized and characterized four new Ni(II)-metal–organic frameworks based H2ppa with various

Fig. 6. Fractional decomposition of aqueous MB with UV light irradiation in the presence of compounds 1–4 within 90 min.

these coordinated water molecules, the structure of 2 is, therefore, destabilized and starts to decompose much faster than the other compounds. 3.3. Optical band gaps

S. Sanram et al. / Polyhedron 119 (2016) 151–159

N,N0 -linkers under solvothermal condition. All Ni(II)-MOFs display diverse structural architectures due to a variety of the length and flexibility of N,N0 -linkers and the varied coordination modes of ppa. The 2D layer for 1, 2-fold interpenetrating 2D framework for 2 with the same (4,4)-connected topology were observed. Compound 3 containing a medium length and flexibility of bpetha coligand shows a 3-fold interpenetrating 3D framework with a-Po topology. While the small and rigid bpy coligand in 4 gives rise to 1D chain structure with the uncoordinated dicarboxylate ppa anion. The photocatalytic activities of 1–4 are investigated and compared to the ZnO catalyst. The photo-decomposition of the MB under UV irradiation indicates that 1–4 are photo-catalytically active for degrading MB, especially compound 1 which could be the excellent candidate for decomposing the pollutant organic molecules and dyes. Acknowledgements Funding for this work is provided by Science Achievement Scholarship of Thailand (SAST), The Thailand Research Fund: Grant No. MRG5980237 (for J.B.) and BRG5980014 (for S.Y.), the Higher Education Research Promotion and National Research University Project of Thailand, through the Advanced Functional Materials Cluster of Khon Kaen University, and the Center of Excellence for Innovation in Chemistry (PERCH–CIC), Office of the Higher Education Commission, Ministry of Education, Thailand. Appendix A. Supplementary data CCDC 1483695–1483698 contains the supplementary crystallographic data of 1–4. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2016.08.044. References [1] [2] [3] [4] [5] [6]

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