Structures, Photoluminescence, and Photocatalytic Properties of Six

Structures, Photoluminescence, and Photocatalytic Properties of Six New Metal-Organic Frameworks Based on Aromatic Polycarboxylate Acids and Rigid Imidazole-Based Synthons Li-Li Wen,*,†,‡ Feng Wang,† Juan Feng,† Kang-Le Lv,§ Cheng-Gang Wang,† and Dong-Feng Li*,†,‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3581–3589

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, P. R. China, Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, and Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central UniVersity for Nationalities, Wuhan 430074, P. R. China ReceiVed March 18, 2009; ReVised Manuscript ReceiVed April 30, 2009

ABSTRACT: Six metal-organic coordination polymers, [Cu(bdc)(bimb)]n (H2bdc ) 1,4-benzenedicarboxylate; bimb ) 4,4′-bis(1imidazolyl)biphenyl) (1), [Cu3(btc)2(bimb)2 · (H2O)3]n (H3btc ) 1,3,5-benzenetricarboxylate) (2), [M3(btc)2(bimb)2 · (H2O)4]n (M ) Mn for 3, Co for 4, Cd for 5), and [Cd(btcH)(bimb)]n (6) were obtained under hydrothermal conditions and characterized structurally. The networks exhibit a variety of topologies: compound 1 exhibits a triply interpenetrating three-dimensional (3D) framework with a distorted primitive cubic (R-Po) single net; compounds 2-5 are isomorphic, which possess a trinodal 4-connected 3D framework; compound 6 has a two-dimensional (3,4)-connected framework. In addition, the thermal stabilities for 1-6 and the photoluminescence properties for 5 and 6 were examined. An anionic organic dye X3B was selected as a model pollutant in aqueous media to evaluate the photocatalytic effectiveness of isostructural compounds 3 and 4, and the results indicated compounds 3 and 4 represent rare examples of coordination polymers that exhibit high photocatalytic activity for dye degradation under UV light and show good stability toward photocatalysis. The difference in catalytic activity between 3 and 4 arises from the discrepancy in central metal ions of the two compounds. Introduction Multidimensional metal-organic frameworks (MOFs) have received much attention as promising candidates for gas storage, chemical separation, heterogeneous catalysis, and optical, electronic, magnetic materials.1-4 Recently, much effort has also been devoted to developing new photocatalytic materials based on MOFs, which is motivated largely by a demand for solving pollution problems in view of their potential applications in the green degradation of organic pollutants.5 A great number of organic bridging ligands have been designed and employed to prepare novel MOFs. Among them, flexible multidentate ligands with arene core and imidazole nitrogen donors have been found to be the most useful organic building blocks for constructing MOFs with versatile topologies such as cages, honeycombs, interpenetrating networks, and multidimensional frameworks architectures.6-10 Hitherto, there are only a few reports describing MOFs with rigid imidazolecontaining linkers, although the rigid ligand facilitates control in the design and assembly of the resulting metal-containing aggregates.11 Meanwhile, the organic aromatic polycarboxylate ligands, especially, 1,4-benzenedicarboxylate (H2bdc) and 1,3,5-benzenetricarboxylate (H3btc), have been extensively applied in the construction of a rich variety of MOFs because of their diverse coordination modes and high structural stability. To further understand the coordination chemistry of the rigid imidazole-based spacer 4,4′-bis(1-imidazolyl)biphenyl (bimb) * To whom correspondence should be addressed. Phone: +86 27 67862900. Fax: +86 27 67867232. E-mail: [email protected] (L.W.); dfli@mail. ccnu.edu.cn (D.L.). † Central China Normal University. ‡ Nanjing University. § South-Central University for Nationalities.

and rigid aromatic polycarboxylate acid, to evaluate the influence of different coordination geometries of metal ions on the resulting framework, and to explore new materials with beautiful architectures and good physical properties, in this contribution, H2bdc or H3btc were chosen as the starting materials together with divalent metal ions, with the aid of a rigid bidentate linker bimb. Utilizing a hydrothermal technique, six new MOFs, [Cu(bdc)(bimb)]n (1), [Cu3(btc)2(bimb)2 · (H2O)3]n (2), [Mn3(btc)2(bimb)2 · (H2O)4]n (3), [Co3(btc)2(bimb)2 · (H2O)4]n (4), [Cd3(btc)2(bimb)2 · (H2O)4]n (5), and [Cd(btcH)(bimb)]n (6), were obtained. The X-ray diffraction analysis revealed that compound 1 exhibits a triply interpenetrating three-dimensional (3D) framework with a distorted primitive cubic (R-Po) single net; the isostructural 2-5 possess trinodal 4-connected 3D frameworks; compound 6 has a two-dimensional (2D) (3,4)-connected framework. To our best knowledge, compounds 3 and 4 are among the rare examples of MOFs that exhibit photocatalytic activities under either ultraviolet or visible light irradiation; moreover, their different photocatalytic activities were also analyzed with respect to the effect of the different electronic structures of central metal atoms. Experimental Section Materials and Measurements. Reagents and solvents employed were commercially available and used as received. Ligand bimb was prepared by literature methods.11a C, H, and N microanalyses were carried out with a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded on KBr discs on a Bruker Vector 22 spectrophotometer in the 4000-400 cm-1 region. Fluorescence measurements were recorded with a Hitachi 850 fluorescence spectrophotometer. Thermogravimetric analyses were performed on a simultaneous SDT 2960 thermal analyzer under flowing N2 with a heating rate of 10 °C/min between ambient temperature and 800 °C. The powder XRD data were collected on a Siemens D5005 diffractometer with Cu KR radiation

10.1021/cg900317d CCC: $40.75  2009 American Chemical Society Published on Web 05/26/2009

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(λ ) 1.5418 Å) over the 2θ range of 5-50° at room temperature. The solid-state diffuse-reflectance UV/vis spectra for powder samples were recorded on a Perkin-Elmer Lambda 35 UV/vis spectrometer equipped with an integrating sphere by using BaSO4 as a white standard, whereas the UV/vis spectra for solution samples were obtained on a Shimadzu UV 2450 spectrometer. Photocatalytic experiments: the UV light source was a 375 W high-pressure mercury lamp (main output 365 nm), and the visible light source was a 500 W halogen lamp with UV cutoff filter (providing visible light with λ > 400 nm). A suspension of powdered catalyst (50 mg) in fresh aqueous solution of X3B (50 mL, 6 × 10-6 mol/L) at pH 6 was first sonicated for 5 min, and shaken at a constant rate in the dark overnight (to establish an adsorption/ desorption equilibrium of X3B on the sample surface). At given irradiating intervals, a series of suspension of a certain volume were collected and filtered through a membrane filter (pore size, 0.45 µm) to remove suspended catalyst particles, and the filtrate was analyzed on the UV/vis spectrometer. The organic dye concentration was estimated by the absorbance at 510 nm, which directly relates to the structure change of its chromophore. Synthesis of [Cu(bdc)(bimb)]n (1). A mixture of Cu(NO3)2 · 3H2O (32.4 mg, 0.133 mmol), H2bdc (22.1 mg, 0.133 mmol), bimb (38.0 mg, 0.133 mmol), NaOH (10.6 mg, 0.267 mmol), and H2O (3 mL) was placed in a parr Teflon-lined stainless steel vessel (25 cm3), and then the vessel was sealed and heated at 180 °C for 3 days. After the mixture was slowly cooled to room temperature, green crystals of 1 were obtained (yield: 32% based on Cu). Anal. Calcd. for C17H11CuN2O4: C, 55.06; H, 2.99; N, 7.56%; found: C, 55.03; H, 3.04; N, 7.58%. IR spectrum (cm-1): 3420m, 3140w, 3012w, 1621s, 1517s, 1384s, 1311m, 1260w, 1239m, 1147w, 1130m, 1107m, 1061m, 1015m, 964m, 933m, 882w, 828s, 745s, 731m, 653m, 623w, 563m, 524m, 433w, 416w. Synthesis of [Cu3(btc)2(bimb)2 · (H2O)3]n (2). A mixture of Cu(NO3)2 · 3H2O (32.4 mg, 0.133 mmol), H3btc (27.9 mg, 0.133 mmol), bimb (38.0 mg, 0.133 mmol), NaOH (10.7 mg, 0.267 mmol), and deionized water (3 mL) was heated at 180 °C for 3 days in a procedure analogous to that for 1. Deep blue crystals of 2 were obtained (yield: 36% based on Cu). Anal. Calcd. For C54H40Cu3N8O15: C, 52.66; H, 3.27; N, 9.10%; found: C, 52.72; H, 3.32; N, 9.12%. IR spectrum (cm-1): 3447s, 3138s, 2985m, 1623s, 1605s, 1561s, 1519s, 1459s, 1423m, 1375s,1344s, 1320m, 1254m, 1133s, 1066s,1003m, 966m, 932w, 858m, 831m, 817s, 769s, 727s, 660m, 647m, 518m, 462m, 423w, 408w. Synthesis of [Mn3(btc)2(bimb)2 · (H2O)4]n (3). A mixture of Mn(Ac)2 · 4H2O (32.7 mg, 0.133 mmol), H3btc (28.1 mg, 0.133 mmol), bimb (37.9 mg, 0.133 mmol), NaOH (16.1 mg, 0.400 mmol), and deionized water (3 mL) was heated at 140 °C for 3 days analogous to the procedure for 1. Pale yellow crystals of 3 were obtained (yield: 41% based on Mn). Anal. Calcd. For C54H46Mn3N8O18: C, 51.48; H, 3.68; N, 8.89%; found: C, 51.44; H, 3.73; N, 8.91%. IR spectrum (cm-1): 3446s, 3122s, 2982m, 1653m, 1622s, 1576s, 1558s, 1516s, 1437m, 1378s, 1307m, 1251m, 1188m, 1120m, 1064s,1005m, 962m, 932m, 818m, 769m, 721s, 686m, 669m, 649m, 528m, 473w, 449w, 435w. Synthesis of [Co3(btc)2(bimb)2 · (H2O)4]n (4). Compound 4 was prepared in the same way as 2, using Co(Ac)2 · 4H2O (0.133 mmol) instead of Cu(NO3)2 · 3H2O at 160 °C for 3 days. Purple crystals of 4 were obtained (yield: 54% based on Co). Anal. Calcd. For C54H46Co3N8O18: C, 51.00; H, 3.65; N, 8.81%; found: C, 51.05; H, 3.71; N, 8.78%. IR spectrum (cm-1): 3438s, 3146m, 3123m, 1607s, 1572m, 1516s, 1441s, 1362s, 1317m, 1252m, 1126m, 1064s, 961m, 936m, 855m, 817m, 768m, 725m, 656m, 515m, 454m. Synthesis of [Cd3(btc)2(bimb)2 · (H2O)4]n (5). Compound 5 was prepared in the same way as 3, using Cd(NO3)2 · 4H2O (0.133 mmol) instead of Mn(Ac)2 · 4H2O at 160 °C for 3 days. Colorless crystals of 5 were obtained (yield: 36% based on Cd). Anal. Calcd. For C54H46Cd3N8O18: C, 45.29; H, 3.24; N, 7.82%; found: C, 45.33; H, 3.29; N, 7.78%. IR spectrum (cm-1): 3395s, 3130s, 1611s, 1514m, 1433m, 1360s, 1313m, 1251m, 1123m, 1060s, 958m, 934m, 818s, 765m, 727s, 649m, 516m, 452m, 416w. Synthesis of [Cd(btcH)(bimb)]n (6). Compound 6 was prepared in the same way as 2, using Cd(NO3)2 · 4H2O (0.133 mmol) instead of Cu(NO3)2 · 3H2O at 160 °C for 3 days. Colorless crystals of 6 were obtained (yield: 35% based on Cd). Anal. Calcd. For C27H18CdN4O6: C, 53.44; H, 2.99; N, 9.23%; found: C, 53.49; H, 3.05; N, 9.21%. IR spectrum (cm-1): 3409m, 3168m, 3063m, 2871m, 1712s, 1616s, 1559s,

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Figure 1. (a) Coordination environments of Cu atoms with hydrogen atoms omitted for clarity. (b) 3D single framework of 1. (c) R-Po topology structure for single net of 1 joining the centers of the binuclear copper subunits. (d) Schematic view of the 3-fold interpenetrating for 1.

New MOFs from Aromatic Polycarboxylate Acids

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Table 1. Crystal Data and Structure Refinement for 1-6

formula formula weight crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z Fcalcd/g cm-3 µ/mm-1 collected reflections unique reflections R1 [I > 2σ (I)] wR2 (all data)

1

2

3

4

5

6

C17H11CuN2O4 370.83 monoclinic P2/n 10.8900(10) 10.8796(10) 13.6720(13) 90.00 108.386(2) 90.00 1537.2(2) 4 1.602 1.445 9738 3017 0.0414 0.1058

C54H40Cu3N8O15 1231.56 monoclinic P21/n 9.0054(8) 20.5792(17) 13.5181(11) 90.00 99.438(2) 90.00 2471.3(4) 2 1.655 1.362 15673 4831 0.0496 0.1399

C54H46Mn3N8O18 1259.81 monoclinic P21/n 9.2568(8) 20.6467(16) 13.8233(11) 90.00 101.688(2) 90.00 2587.2(4) 2 1.617 0.808 19344 5616 0.0541 0.1235

C54H46Co3N8O18 1271.78 monoclinic P21/n 9.1277(9) 20.548(2) 13.6011(14) 90.00 101.689 90.00 2498.1(4) 2 1.691 1.075 13359 4855 0.0556 0.1251

C54H46Cd3N8O18 1432.22 monoclinic P21/n 9.3470(7) 20.8071(15) 13.9123(10) 90 101.5790(10) 90 2650.7(3) 2 1.794 1.278 31018 6343 0.0417 0.0750

C27H18CdN4O6 606.86 triclinic P1j 9.2397(11) 10.3411(12) 12.6811(15) 83.979(2) 85.655(2) 77.199(2) 1173.3(2) 2 1.718 0.984 7337 4320 0.0491 0.0873

1512s, 1369s, 1308w, 1240m, 1178m, 1118m, 1062m, 956w, 932w, 819m, 728m, 682m, 647m, 527m, 453w. X-ray Crystallography. Suitable single crystals of 1-6 were selected and mounted in air onto thin glass fibers. X-ray intensity data were measured at 293 K on a Bruker SMART APEX CCD-based diffractometer with graphite-monochromatic Mo KR radiation (λ ) 0.71073 Å). Data reductions and absorption corrections were performed with the SAINT and SADABS software packages, respectively.12 All structures were solved by a combination of direct methods and difference Fourier syntheses and refined against F2 by the full-matrix least-squares technique.13,14 Anisotropic displacement parameters were refined for all non-hydrogen atoms except for the disordered atoms. To assist the refinement, several restraints were applied: for 2, the atoms C17, C18 and disordered free water molecules (O7, O8, and O9) were restrained by ISOR, and the hydrogen atoms attached to disordered water molecules were not located but were included in the structure factor calculations. The result of thermogravimetric (TG) analysis further confirmed the existence of water molecules in 2 (see Figure S5, Supporting Information); for 3, C23 C24 were restrained by ISOR; for 4, the thermal parameters of atoms C5 and C9 (C6 and C8) in benzene rings were restrained by SIMU, and the atoms C5 and C6 were further restrained by ISOR; for 5, C5 and C6 atoms were restrained by ISOR; for 6, the disordered oxygen atom (O4, O4′) was refined using a oxygen atoms split over two equal sites, with a total occupancy of 1; the C22-O4′ and C22-O4′ bond lengths were restrained by SADI, and atoms O4 and O4′ were restrained by ISOR.

Results and Discussion Description of the Crystal Structures. [Cu(bdc)(bimb)]n (1). In compound 1, each Cu(II) ion is square-pyramidally coordinated by four carboxylate oxygen atoms at the basal positions [Cu-O (average) 1.981 Å] and one bimb nitrogen molecule [Cu-N 2.122(3) Å] at the apical position (Figure 1a). Each fully deprotonated bdc2- ligand coordinates to four Cu atoms, and both carboxylate groups adopt a µ2-η1:η1 fashion. Obviously, each pair of Cu(II) ions is bridged by four carboxylate groups to generate a well-known paddle-wheel second building unit (SBU) with a Cu · · · Cu separation of 2.682 Å, which is bridged by bdc2- dianions to form a distorted 2D square grid. These 2D square grids are further pillared by bimb, occupying the axial sites of the paddle-wheel subunits, to form a 3D porous structure with large cavities of approximately 10.88 × 18.09 × 32.64 Å3 (based on Cu · · · Cu distance), as presented in Figure 1b. If the paddle-wheel dimer can be topologically viewed as an octahedral node, meanwhile the bdc2- and bimb bridges can be viewed as linkers, the topology of single net for 1 can be best described as an uninodal six-connected distorted primitive cubic (R-Po) net (Figure 1c). The large voids formed by a single 3D network allow incorporation of two other identical networks. Therefore, the

overall structure of 1 is three identical R-Po nets, which interpenetrate each other to form a triply interpenetrating 3D framework (Figure 1d), which belongs to class Ia. The value of PICVR corresponds to the translational degree interpenetration Zt and Z ) Zt ) 3.15,16 Hence, the large pore void spaces of a single net have been reduced significantly. Upon interpenetration, the compound only retains an effective void volume of 86.1 Å3 per unit cell, which is 5.6% of the crystal volume. The most peculiar structural feature of 1 is that the framework presents a rare 3-fold inclined interpenetration.17 [Cu3(btc)2(bimb)2 · (H2O)3]n (2) and [M3(btc)2(bimb)2 · (H2O)4]n (M ) Mn (3), Co (4), and Cd (5)). Similar cell parameters with the same space group P21/n (Table 1) and the results of crystallographic analysis confirm that 2-5 are isostructural. Thus, only the structure of 2 is described in detail as a typical example, while the structures of 3, 4, and 5 are provided in Supporting Information, Figures S1, S2, and S3, respectively. As shown in Figure 2a, two types of coordination environments exist around the copper ions in the crystal structure. Cu1 lies in a distorted pyramidal coordination sphere, the equatorial plane of which comprises three carboxylate oxygen atoms (O3, O4, and O6) from two distinct btc3- anions and one bimb nitrogen atom (N1); another carboxylate oxygen atom (O2) occupies the apical coordination site. However, the Cu1-O5#3 distance is 2.692(3) Å, suggesting a non-negligible interaction with the uncoordinated carboxylate oxygen atom which can be described as a semichelating coordination mode, implying the Cu1 atom in a distorted octahedron environment.18 Cu2, situated on an inversion center with occupancy of 0.5, is coordinated to two carboxylate oxygen atoms from inequivalent btc3- and two different bimb nitrogen atoms in a distorted planar sphere. It is worth mentioning here that two free water molecules, respectively, have weak bonding interactions with Cu2 atom at the axial sites (Cu2-O9#1, Cu2-O9#4: 2.613(8) Å #4 1 - x, 1 y, 1 - z) due to the Jahn-Teller effect.19 Hence, the coordination environment of Cu2 atom may be also regarded as a octahedron sphere. In 2, the Cu-O bond lengths range between 1.967(3) and 2.209(3) Å, and the Cu-N distances are 1.957(3) and 1.961(3) Å, both of which are in the normal range. The fully deprotonated btc3- ligand coordinates to four Cu atoms, with two carboxylate groups adopting bidentate chelate modes and one adopting a µ2-η1:η1 fashion. The btc3- ligand thus links copper ions to form a 3D framework (Figure 2b). In addition, it should be noted that there are two independent sets of bimb spacers, which establish a physical bridge between Cu atoms with Cu · · · Cu separations of 16.77 and 17.43 Å. Both

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Figure 2. (a) Coordination environments of Cu atoms with hydrogen atoms omitted for clarity. (b) 2D layer constructed from btc3- and Cu2+. (c) The complicated 3D structure of 2. (d) A schematic representation of the trinodal 4-connected network structure of 2: green spheres represent Cu1 nodes, blue spheres represent Cu2 nodes, purple spheres represent btc3- nodes, and bimb bridges are shown as black bonds.

sets reside at a crystallographic inversion center, resulting in the formation of a complicated 3D structure with channels along the a- and c-axes, which are occupied by water molecules (Figure 2c). The effective free volume of 2 was calculated by PLATON analysis as 8.6% of the crystal volume (211.5 out of the 2471.5 Å3 unit cell volume), where disordered water molecules reside as labile ligands. A better insight into the nature of this intricate framework can be acquired by using topological analysis. In 2, each btc3anion is four-connected by linking to four Cu(II) ions and the bimb groups can be simplified to be linear connectors; Cu1 and Cu2 were also considered as distinct nodes; therefore, the combination of nodes and connectors suggests an uncommon trinodal 4-connected network with a Scha¨fli symbol of

(65 · 10)2(62 · 8 · 103)3(73 · 8 · 12) for (Cu1)2(btc)3(Cu2). One of the nodes (Cu2) has a square planar geometry, while the other two (Cu1 and btc) have tetrahedral arrangement. This is very unusual topology, and very different from other typical 4-connected nets.20-22 [Cd(btcH)(bimb)]n (6). The CdII center is in a slightly distorted octahedral environment, the equatorial plane of which comprises four carboxylate oxygen atoms from three distinct btcH2- anions and two nitrogen atoms from two different bimb ligands occupying the apical site. The Cd-O bond lengths range from 2.266(4) to 2.460(3) Å, and the Cd-N distances are 2.286(5) and 2.278(4) Å, respectively. In 5, the partially deprotonated ligand coordinates to three Cd atoms, with two full deprotonated carboxylate groups



Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 1-6

Cu1-O3 O3-Cu1-O4#2

1a 1.983(2) Cu1-N1 2.122(3) 166.83(10) O4#2-Cu1-N1 97.10(11)

Cu1-O2#1 O1-Cu1-N1

1.981(2) 97.27(9)

Cu1-O4#2 O2#1-Cu1-O3

Cu1-O2 2.209(3) Cu2-O1#2 1.983(3) O3#1-Cu1-O4#1 64.19(11)

Cu2-O1 Cu1-O4#1 O2-Cu1-N1

2b 1.983(3) Cu1-N1 1.957(3) 2.030(3) Cu2-N3#2 1.961(3) 104.60(11) O1-Cu2-O1#2 180.00

Cu2-N3 Cu1-O6#3 N3-Cu2-N3#2

1.961(3) 1.967(3) 180.00

Cu1-O3#1 2.077(3) O4#1-Cu1-O6#3 155.11(11) O1-Cu2-N3#2 93.18(11)

Mn1-O5 Mn1-O3#2 Mn2-O2#3 O1#1 -Mn1-O6 O2 -Mn2-N1#3

2.221(2) 2.279(2) 2.192(2) 148.17(9) 87.01(9)

Mn1-O6 Mn2-O2 Mn2-O7#3 O3#2-Mn1-N3 O7-Mn2-N1#3

2.368(2) 2.192(2) 2.192(3) 155.28(9) 90.73(10)

Co1-O1 Co1-O4#2 Co2-O7#3 O1-Co1-O5#1 O2-Co2-N3

2.020(3) 2.165(3) 2.101(4) 105.66(12) 86.54(13)

Co1-N1 Co2-O2 Co2-N3#3 O3#2-Co1-O5#1 O2-Co2-O7

2.065(4) 2.129(3) 2.096(4) 100.02(11) 91.80(13)

Cd1-O1 Cd1-O6#2 Cd2-O7#3 O3-Cd2-O3#3

2.297(3) 2.301(3) 2.317(3) 180.00

Cd1-O2 Cd2-O3 Cd2-N3#3 O7-Cd2-O7#3

2.523(3) 2.310(3) 2.279(3) 180.00

Cd1-O1 Cd1-O6#2 O5#1-Cd1-N3

2.460(3) 2.287(4) 94.11(14)

Cd1-O2 N1 -Cd1-N3 O2-Cd1-N3

6f 2.380(3) Cd1-N1 2.286(5) 173.72(14) O1-Cd1-O6#2 95.41(12) 95.96(13)

Cu1-O1 O1-Cu1-O2#1

1.971(2) 167.02(9)

1.989(2) 90.05(9)

3c Mn1-N3 Mn2-O7 O5-Mn1-O6 O2-Mn2-O2#3

2.186(3) 2.192(3) 56.99(9) 180.00

Mn1-O4#2 Mn2-N1 O1#1-Mn1-O4#2 O7-Mn2-O7#3

2.199(3) 2.219(3) 107.38(9) 180.00

Mn1-O1#1 Mn2-N1#3 O4#2-Mn1-O5 N1-Mn2-N1#3

2.083(2) 2.219(3) 155.21(10) 180.00

4d Co1-O5#1 Co2-O7 O6#1-Co1--N1 O2-Co2-O2#3

2.072(3) 2.101(4) 158.83(13) 180.00

Co1-O6#1 Co2-N3 O1-Co1-O3#2 O7-Co2-O7#3

2.260(3) 2.096(4) 146.60(11) 180.00

Co1-O3#2 Co2-O2#3 O4#2-Co1-O5#1 N3-Co2-N3#3

2.228(3) 2.129(3) 158.75(11) 180.00

5e Cd1-N1 Cd2-O7 O1-Cd1-O2 N3-Cd2-N3#3

2.244(3) 2.317(3) 54.26(9) 180.00

Cd1-O4#1 2.212(2) Cd2-N3 2.279(3) O5#2-Cd1-O6#2 56.20(9)

Cd1-O5#2 Cd2-O3#3 O4#1-Cd1-N1

2.363(3) 2.310(3) 106.02(9)

Cd1-N3 O2-Cd1-O5#1

Cd1-O5#1 O6#2-Cd1-N3

2.266(4) 90.56(14)

2.278(4) 91.45(12)

a #1 x, -1 + y, z; #2 3/2 - x, y, -1/2 - z. b #1 1 + x, y, z; #2 -x, 1 - y, 1 - z; #3 1/2 + x, 3/2 - y, 1/2 + z. c #1 -1/2 + x, 3/2 - y, -1/2 + z; #2 1/2 + x, 3/2 - y, -1/2 + z; #3 1 - x, 2 - y, 2 - z. d #1 1 + x, y, z; #2 1/2 + x, 3/2 - y, 1/2 + z; #3 2 - x, 2 - y, 2 - z. e #1 -1/2 + x, 1/2 y, -1/2 + z; #2 1/2 + x, 1/2 - y, -1/2 + z; #3 1 - x, 1 - y, 1 - z. f #1 x, -1 + y, z; #2 -x, 2 - y, 2 - z.

adopting µ2-η1:η1 and bidentate chelate modes. The btcH2 anions thus link cadmium ions to form a one-dimensional (1D) ladderlike chain along the b axis (Figure 1b). The 1D chain is further stacked with bidentate bimb spacers, with a Cd · · · Cd distance of 17.90 Å, thus giving rise to the formation of a 2D (3,4)connected framework with a Scha¨fli symbol of (42 · 6)(42- · 67 · 8) (Figure 2c). The architecture of 2D coordination polymers is based on both dinuclear cluster SBU [Cd2(COO)2] with a Cd · · · Cd distance of 4.276 Å (Figure 2d). By closer inspection of the structure 6, if the dinuclear cluster [Cd2(COO)2] acts as nodes and the bimb bridge serves as linkers, compound 6 exhibits a (4,4) net topology structure. The resulting 2D structure is cross-linked by hydrogen-bond interactions between C-H groups from bimb (btcH2-) and carboxylate oxygen atoms, thus leading to the formation of a 3D supramolecular architecture. Obviously, it is still a challenge to obtain the desired MOFs because they highly depend on the combination of several factors, such as the coordination geometry of metal ions, the nature of ligands,23,24 the ratio between metal salt and ligand,25 and reaction conditions. For example, in our work, the compounds 2-5 are isostructural with each other though they possess different metal centers under same conditions, while for 5 and 6, the raw materials are the same, but distinct structures are obtained for the different reaction temperatures (140 °C for 5 and 180 °C for 6) and the different pH values (adjusted by the molar ratio of aromatic acid/NaOH: 1/3 for 5 and 1/2 for 6). Thermal Analysis. Compound 1 is stable up to 316 °C where the organic groups start to decompose. For 2, the weight loss of 4.43% below 285 °C (calcd 4.38%) corresponds to the loss of three free aqua molecules per formula. For 3, the weight loss of 8.28% below 353 °C (calcd 8.57%) corresponds to the loss of two coordinated aqua molecules and four free water

molecules per formula; consecutive decompositions suggest the total destruction of the framework, and then a residue of MnO (obsd 18.02%, calcd 16.89%) remained. Compound 4 exhibits thermal decomposition behavior similar to that of compound 3. For 5, the weight loss of 7.54% below 321 °C (calcd 7.31%) corresponds to the loss of two coordinated aqua molecules and four free water molecules per formula. A plateau region is observed for 5 from 321 to 403 °C. Compound 6 also has demonstrated a high thermal stability, which is stable up to 284 °C, where the framework structures begin to collapse. Photoluminescence Properties. The solid-state excitation emission spectra of 5 and 6 were studied at room temperature (Figure 4). The strongest emission peak for the free H3btc is at 370 nm with the excitation peak at 334 nm, which is attributed to the π*-n transitions.26 The strongest excitation peaks for 5 and 6 are at 334 nm, and their emission spectra mainly show strong peaks at 356 and 385 nm, 357 and 388 nm, respectively. The emissions of 5 and 6 are similar to each other, which may chiefly originate from ligand-centered electronic transitions perturbed by the coordination to metal ions. Photocatalytic Activity. Herein, we selected an anionic organic dye X3B as a model pollutant in aqueous media to evaluate the photocatalytic effectiveness of compounds 3 and 4, considering that X3B is commonly used as a representative of widespread organic dyes that are very difficult to decompose in waste streams under UV or visible light irradiation.27 The photodegradation experiment under UV or visible light irradiation was carried out after the dark adsorption equilibrium was achieved. In addition, control experiments on the photodegradation of X3B were carried out. No significant change in the degradation of X3B was observed in the following reaction conditions: (1) in the dark; (2) without catalyst. The distinctly shortened degradation time compared with the control experiments indicates that both catalysts 3 and 4 are active for the decomposition of X3B in the presence of UV or visible light

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Figure 3. (a) Coordination environments of Cd atoms with hydrogen atoms omitted for clarity. (b) 1D ladder chain constructed from btcH2- and Cd2+. (c) The 2D layer structure of 6. (d) A schematic representation of network structure of 6: green spheres represent Cd nodes, pink spheres represent C25 nodes, and bimb bridges are shown as yellow bonds. (e) 2D (4,4) topology structure for 6: gray spheres represent [Cd2(COO)2] nodes and bimb bridges are shown as green linkers.


Figure 4. Fluorescent emission spectra of complexes 5 and 6 in solid state at room temperature.

Scheme 1. Molecular Structure of X3B


Figure 6. Control experiments on the photodegradation of X3B. (a) X3B/compound 4/dark; (b) X3B/UV light (without catalyst); (c) X3B/ compound 4/tert-butyl alcohol/UV light; (d) X3B/compound 4/visible light; (e) X3B/compound 4/ UV light.

Scheme 2. A Simplified Model of Photocatalytic Reaction Mechanism of X3B on Catalyst 3 and 4

irradiation. When pseudo-first-order kinetics was fitted with the experimental data, for 3, the rate constant under UV light irradiation was found to be 11 × 10-2 h-1, and the rate constant under visible light irradiation was 7.3 × 10-2 h-1; for 4, the rate constant under UV light and visible light irradiation was 26 × 10-2 and 13 × 10-2 h-1, respectively. Notably, the photocatalytic efficiency of 4 under either ultraviolet or visible light is higher than that of 3. Compound 4 is able to degrade X3B almost completely in 10 h of UV irradiation. To study the photocatalytic reaction mechanism in detail, the photodegradation of X3B was carried out in the presence of t-butyl alcohol (TBA), a widely used · OH scavenger.28 The presence of TBA greatly depressed the photodegradation rate of X3B on catalyst 3 (Figure 5, curves d) and 4 (Figure 6, curves c); that is, the relevant rate constant for 3 and 4 was sharply individually decreased from 11 × 10-2 to 0.8 × 10-2 h-1 and

Figure 5. Control experiments on the photodegradation of X3B. (a) X3B/compound 3/dark; (b) X3B/UV light (without catalyst); (c) X3B/ compound 3/visible light; (d) X3B/compound 3/t-butyl alcohol/UV light; (e) X3B/compound 3/UV light.

from 26 × 10-2 to 1.4 × 10-2 h-1 in the presence of TBA under UV light. The · OH quenching experiment result suggests that the photodegradation of X3B on catalysts 3 and 4 is predominately through attack of · OH radicals, rather than direct hole oxidation. Therefore, a simplified model of photocatalytic reaction mechanism was proposed as depicted in Scheme 2. Because the HOMO is mainly contributed to by oxygen and (or) nitrogen 2p bonding orbitals (valence band) and the LUMO by empty Mn(Co) orbitals (conduction band), charge transfer actually takes place from oxygen and (or) nitrogen to Mn(Co) on photoexcitation. The HOMO strongly demands one electron to return to its stable state. Therefore, one electron was captured from water molecules, which was oxygenated into the · OH active species. Then the · OH radicals could cleave X3B effectively to complete the photocatalytic process. To exclude the possibility that the photocatalytic properties of 3 and 4 result from dissolved molecular or oligomeric fragments of solid catalysts in the photocatalytic process, other control experiments were conducted. The reaction suspensions after 10 h of irradiation were filtered to remove the solid catalyst particles, and fresh X3B was added into the respective filtrates for catalysis testing. Without solid catalyst in the reaction system, the fresh X3B was not degraded during another 10 h of irradiation under Hg lamp, which indicates that the solution contains no photocatalytically active fragments. Clearly, the photocatalytic activities arise solely from the solid 3 and 4. In addition, the stability of compounds 3 and 4 was monitored using powder X-ray diffraction (PXRD) during the course of photocatalytic reactions (see Figure S10 and S11, Supporting Information). After photocatalysis, compounds 3 and 4 display

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application in heterogeneous photocatalysis. Remarkably, compound 4 represents a rare example of coordination polymers that exhibit high photocatalytic activity for dye degradation under UV light and showed good stability toward photocatalysis. Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (Nos. 20801021 and 20802022), the Natural Science Foundation of Hubei Province of China (2008CDZ039), Open Fund of Hubei Key Laboratory of Catalysis and Materials Science (CHCL 08005), and China Postdoctoral Science Foundation (20070420985). Supporting Information Available: X-ray crystallographic files in CIF format, figures of crystal packing, TG curves. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. UV/vis diffuse-reflectance spectra of compounds 3 and 4 with BaSO4 as background.

a powder XRD pattern nearly identical to that of the original compound; in other words, its stability toward photocatalysis is good. Although catalysts 3 and 4 possess the same topology structures, different central metal ions between 3 and 4 may lead distinct bandgap sizes, which give rise to the discrepancy in their photocatalytic activity. The diffuse-reflectance UV/vis spectra reveal that solid 3 and 4 have different absorption features (Figure 4). Both spectra consist of absorption components in the UV and visible regions. In both cases, the main UV absorption bands are at 307(262), 333(265) nm for 3 and 4, which can be assigned to ligand-to-metal charge transfer (LMCT). In the case of 4, two clear additional peaks were observed at 547 and 721 nm, which probably originate from the d-d spin-allowed transition of the d7 (Co2+) ion. The absorption of 3 in the visible region is not as distinct as that of 4, which may result from the d-d spin-forbidden transition of the d5 (Mn2+) ion. To obtain the precise values of band gap from the absorption edges, the point of inflection in the first derivatives of the absorption spectrum was used. The values of the band gap obtained from corresponding LMCT transitions are 4.04 and 3.72 eV for 3 and 4, respectively. Clearly, the band gaps of 3 and 4 follow the order 4 < 3 and the degradation rate of X3B follows the reverse order. In a sense, the difference in catalytic activity under UV or visible light irradiation between 3 and 4 arises from the UV/vis absorption properties, which may trace back to discrepancy in central metal atoms of the two compounds. In this regard, we have explored new water-insoluble and easily recycled materials with photocatalytic activities. Especially, compound 4 represents a rare example of coordination polymers that exhibit high photocatalytic activity for dye degradation under UV light. Conclusions In summary, we have synthesized and characterized six metal-organic frameworks based on aromatic polycarboxylate acids and a rigid imidazole-based bidentate ligand, which show rich structural features. Compound 1 exhibits a triply interpenetrating 3D framework with distorted primitive cubic (R-Po) single net; the isostructural 2-5 possess a trinodal 4-connected 3D framework; compound 6 has a 2D (3,4)-connected framework. Remarkably, the encouraging photophysical properties for 3 and 4 excite our interest in exploring their useful

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CG900317D

Structures, Photoluminescence, and Photocatalytic Properties of Six

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