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http://pubs.acs.org/doi/abs/10.1021/jp0622381
Novel Photocatalysts for the Decomposition of Organic Dyes Based on Metal-Organic Framework Compounds
怎么给你啊？把你的邮箱告诉我吧，我已经把文献下好了。 AbstractThree novel metal-organic frameworks (MOFs) 2, 1, 2·H2O, 2, and 2, 3, with three-dimensional structures have been synthesized and characterized. The structures of the three compounds appear somewhat related, formed by the connectivity involving the metal polyhedra (Co4N trigonal bipyramids in 1, NiO4N2 octahedra in 2, and ZnO4 tetrahedra and ZnO3N2 trigonal bipyramids in 3), 4,4‘-oxybis(benzoate), and 4,4‘-bipyridine. The photocatalytic studies on 1−3 indicate that they are active catalysts for the degradation of orange G, rhodamine B, Remazol Brilliant Blue R and methylene blue. The compounds have also been characterized by powder X-ray diffraction, IR, thermogravitmetric analysis, UV−vis, photoluminescence, and magnetic studies.
Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableIntroduction
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Metal-organic framework (MOF) coordination polymers have received great attention due to their many potential applications ranging from catalysis, gas absorption, molecular recognition, optics, and so on.1-6 The continuing interest in this area is also due to their fascinating structures and many possible metal−ligand coordinations, as well as the aforementioned properties tunable through the variation of building blocks and reaction conditions. Most of the work reported in the literature, in general, concentrated on the benzene dicarboxylates7-15 or pyridine dicarboxylates16-18 of transition elements. Recently, diphenylene dicarboxylates have been shown to give rise to interesting structures with properties.19,20 We have been interested in the study of benzene dicarboxylate based coordination polymers of lanthanides and related systems, resulting in many interesting structures.21,22 In continuation of this theme, we are now studying the reactions involving 4,4‘-oxybis(benzoic acid). As a dicarboxylate ligand, the 4,4‘-oxybis(benzoate) (OBA) appears to possess the following interesting structural characteristics:  (a) There are two carboxyl groups with a 180° angle separated by an oxygen and a benzene ring, which would reduce the steric interference/hindrance. (b) This compound can form rigid frameworks with large channels. (c) This provides an opportunity to study the relative torsional displacement of the benzene rings with respect to the central oxygen atom. The investigations using the oxybis(benzoic acid) as the ligand in the construction of metal-organic framework compounds have been quite limited,23-25 which prompted us to study this ligand in detail. Photocatalysis is being used for the green ecological elimination of organic compounds or harmful pollutants,26,27 but it is also attracting increasing interest as a potentially clean and renewable source for hydrogen fuel by splitting of water into H2 and O2 by means of solar-to-chemical conversion.28-30 The typical solid photocatalysts commonly used are semiconductor metal oxide and sulfide particles such as TiO2, ZnO, WO3, CdS, ZnS, and Fe2O3; layered ni and polyoxometalates (POM), mainly of W.26,27,31-35 On the other hand, it is also of interest to search for new photocatalytic materials with improved properties. In this context, the photocatalytic activities of hybrid compounds containing carboxylate ligand are just beginning to emerge.36-38 We have now isolated three new three-dimensional (3D) framework structures, 2, 1, 2·H2O, 2, and 2, 3, employing 4,4‘-oxybis(benzoic acid), 4,4‘-bipyridine, and the corresponding metal salts. The structures of the three compounds appear somewhat related formed by the connectivity involving the metal polyhedra (CoO4N trigonal bipyramids in 1, NiO4N2 octahedra in 2, and ZnO4 tetrahedra and ZnO3N2 trigonal bipyramids in 3) and the 4,4‘-oxybis(benzoate) anions (OBA). The 4,4‘-bpy acts as a ligand connecting the metal centers in all the compounds. The photocatalytic studies, at room temperature, indicate all three compounds are active for the photodegradation of four commonly used dyes (orange G (OG), rhodamine B (RhB), Remazol Brilliant Blue R (RBBR), and methylene blue (MB)) in the textile industry. The order of photodegradation of the four dyes follows the reverse order of their band gap. In this paper, we present the synthesis, structure, and photocatalytic studies of structures 1−3. Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableExperimental Section
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Materials. The compounds needed for the synthesis of the three coordination polymers, Co (NO3)2·6H2O, Ni (NO3)2·6H2O, and Zn (NO3)2·6H2O (Ranbaxy, 99%), 4,4‘-oxybis(benzoic acid) (Lancaster, 99%), 4,4‘-bipyridine (98%), piperazine (Aldrich, 99.99%), NaOH (S. D. Fine-Chem. Ltd, India, 99%) and the organic dyes for the photocatalytic experiments, orange G, rhodamine B, Remazol Brilliant Blue R, and methylene blue (Merck, India), were used as received. The water used was double distilled filtered through a Millipore membrane. Synthesis and Initial Characterization. The two compounds were synthesized by employing hydrothermal methods. In a typical synthesis, for 1, Co(NO3)2·6H2O (0.146 g, 0.5 mM) was dissolved in 8 mL of 0.125 M NaOH solution. 4,4‘-oxybis(benzoic acid) (OBA, 0.1304 g, 0.5 mM) and 4,4‘-bipyridine (4,4‘-bpy, 0.08 g, 0.5 mM) were then added, with continuous stirring, and the mixture was homogenized at room temperature for 30 min. The final mixture was sealed in a 23 mL PTFE-lined stainless steel acid digestion autoclave and heated at 180 °C for 72 h. The initial and final pH values of the reaction mixture were 5.5 and 5, respectively. The final product, containing large quantities of violet rectangular crystals only, was filtered, washed with deionized water under vacuum, and dried at ambient conditions (yield 70% based on Co). For 2, Ni(NO3)2·6H2O (0.075 g, 0.25 mmol) was dissolved in 3 mL of water along with 4,4‘-oxybis(benzoic acid) (4,4‘-OBA, 0.0652 g, 0.25 mmol), piperazine (0.0218 g, 0.25 mmol), and 4,4‘-bipyridine (4,4‘-bipy, 0.04 g, 0.25 mmol). The reaction mixture was heated in a 7 mL autoclave at 180 °C for 72 h. The initial pH of the reaction mixture was 5.5, and there was no change of pH in the final solution. The green color, cubic-like crystals were filtered, washed with water, and dried at ambient conditions (yield 75% based on Ni). For 3, Zn(NO3)2·6H2O (0.152 g, 0.5 mM) was dissolved in 8 mL of water. OBA (0.1304 g, 0.5 mM), NaOH (0.04 g, 1 mM), and 4,4‘-bipyridine (0.08 g, 0.5 mM) were added subsequently, and the mixture was sealed in 23 mL PTFE-lined acid digestion autoclaves and heated at 150 °C for 120 h. The initial pH of the reaction mixture was 5.5, and no appreciable change in pH was noted after the reaction. The product contained large quantities of colorless, rodlike crystals only. The crystals were filtered, washed with water, and dried at ambient conditions (yield 75% based on Zn). The powder X-ray diffraction (XRD) patterns were recorded on crushed single crystals in the 2θ range 5−50° using Cu Kα radiation. The XRD patterns indicated that the product the patterns were entirely consistent with the simulated XRD patterns generated based on the structures determined using single-crystal XRD. Thermogravimetric analysis (TGA) has been carried out (Metler-Toledo) in air (flow rate = 50 mL/min) in the temperature range 30&# °C (heating rate = 5 °C/min). The studies indicate that all the compounds show one major weight loss in the range 350&# °C. The total observed weight losses of 81.5 and 77%, in the cases of 1 and 3, correspond well with the loss of the OBA anion and 4,4‘-bpy (calcd 82%, 80%). For 2, the total observed weight loss of 83% corresponds to the loss of OBA anion, 4,4-bpy, and lattice water (calcd 84.5%). The final calcined product was found to be crystalline by powder XRD and correspond to Co3O4 (JCPDS:), NiO (JCPDS:	), and ZnO (JCPDS:), respectively. IR spectroscopic studies have been carried out in the mid-IR region as a KBr pellet (Bruker IFS-66v). The results indicate characteristic sharp lines with almost similar bands. Minor variations in the bands have been noticed between the compounds (Table 1).
Table 1:  Observed IR bands for 1, {2}, 2, {2·H2O}, and 3, {2}
bands 1 (cm-1) 2 (cm-1) 3 (cm-1)&&
νas(O−H)&¬ observed&&;(s)&¬ observed&&
νs(C−H)aromatic&&;(w)&&;(m)&&;(w)&&
νs(CO)&&;(m)&&;(m)&&;(s)&&
δ(H2O)&¬ observed&&;(s)&¬ observed&&
νs(C−C)skeletal&&;(m)&&;(m)&&;(m)&&
δ(COO)&&;(s)&&;(s)&&;(s)&&
δ(CHaromatic)in-of-plane&&;(s)&&;(m)&&;(m)&&
δ(C−N)skeletal&&879 (s)&&879 (m)&&879 (s)&&
δ(CHaromatic)out-of-plane&&780 (s)&&780 (w)&&780 (s)
Single-Crystal Structure Determination. A suitable single crystal of each compound was carefully selected under a polarizing microscope and glued to a thin glass fiber. Crystal structure determination by X-ray diffraction was performed on a Siemen's Smart-CCD diffractometer equipped with a normal focus and 2.4 kW sealed tube X-ray source (Mo Kα radiation, λ = 0.71073&A) operating at 40 kV and 40 mA. An empirical absorption correction based on symmetry equivalent reflections was applied using the SADABS program.39 The structure was solved and refined using the SHELXTL-PLUS suite of programs.40 All the hydrogen atoms of the carboxylic acids were initially located in the difference Fourier maps, and for the final refinement, the hydrogen atoms were placed in geometrically ideal positions and held in the riding mode. The hydrogen atoms of the water molecule were not located in the difference Fourier maps. Final refinement included atomic positions for all the atoms, anisotropic thermal parameters for all the non-hydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. Full-matrix least-squares refinement against |F|2 was carried out using the SHELXTL-PLUS suite of programs.40 Details of the structure solution and final refinements for 1, 2, and 3 are given in Table 2.
Table 2:  Crystal Data and Structure Refinement Parameters for 1, {2}, 2, {2·H2O}, and 3, {2}a
structure parameter 1 2 3&&
empirical formula&&C38H24N2O10Co2&&C48H34N4O11Ni2&&C38H24N2O10Zn2&&
formula weight&&786.46&&960.2&&799.335&&
crystal system&&monoclinic&&triclinic&&triclinic&&
space group&&C2/c (no.15)&&P1̄ (no.2)&&P1̄ (no.2)&&
a (&A)&&22.493(3)&&11.269(4)&&11.292(3)&&
b (&A)&&13.6914(19)&&11.270(4)&&12.339(3)&&
c (&A)&&12.7681(18)&&20.711(7)&&14.413(3)&&
α (deg)&&90.0&&77.608(5)&&94.857(4)&&
β (deg)&&115.723(3)&&77.608(5)&&109.711(4)&&
γ (deg)&&90.0&&64.727(4)&&108.016(4)&&
volume (&A3)&&)&&)&&)&&
Z&&4&&2&&2&&
T (K)&&293(2)&&293(2)&&293(2)&&
ρcalc (g cm-3)&&1.475&&1.383&&1.511&&
μ (mm-1)&&0.998&&0.882&&1.427&&
θ range (deg)&&1.80 to&#&&1.02 to&#&&1.78 to&#&&
λ (Mo Kα) (&A)&&0.773&&0.71073&&
R indexes &&R1 =, wR2 =&&R1 =, wR2 =&&R1 =, wR2 =&&
R (all data)&&R1 =�, wR2 =&&R1 =, wR2 =&&R1 =, wR2 =
a R1 = Σ ||F0| − |Fc||/Σ |F0|; wR2 = {Σ /Σ }1/2; w = 1/; P = /3, where a = 0.0564 and b =0.000 for 1, a = 0.0638 and b = 0.7061 for 2, and a =0.0563 and b = 0.0837 for 3.
Photocatalytic Experiments. The photochemical reactor employed in this study was comprised of a jacketed quartz tube of 3.4 cm inner diameter (i.d.), 4 cm outer diameter (o.d.), and 21 cm length and an outer Pyrex glass reactor of 5.7 cm i.d. and 16 cm length. A high-pressure mercury vapor lamp (HPML) of 125 W (Philips, India) was placed inside the jacketed quartz tube after removal of the outer shell. The ballast and capacitor were connected in a series with the lamp to avoid fluctuations in input supply. Water was circulated through the annulus of the quartz tube to avoid heating of the solution due to dissipative loss of UV energy. The solution was taken in the outer reactor and continuously stirred to ensure that the suspension of the catalyst was uniform. The lamp radiated predominantly at 365 nm corresponding to an energy of 3.4 eV, and the photon flux is 5.8 × 10-6 moles of photons per second. Further details of the experimental setup can be found elsewhere.34 For the degradation studies, the dyes were dissolved in double distilled Millipore filtered water. The effect of the initial concentration of the dyes and the effect of catalyst loading on the reaction rates were investigated. The reactions were carried out at 40 °C, which was maintained by circulating water in the annulus of the jacketed quartz reactor. Samples were collected at regular intervals, filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis, as described below. Samples were analyzed with a Perkin-Elmer UV−vis spectrophotometer (model Lambda 32). The calibration for OG, RhB, RBBR, and MB was based on the Beer−Lambert law at its maximum absorption wavelengths, λmax, of 481, 554, 590, and 661 nm, respectively. Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableResults and Discussion
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Structure of 1, { 2}. The asymmetric unit of 1 consists of 26 non-hydrogen atoms, of which one cobalt atom is crystallographically independent. The Co2+ ions have a distorted trigonal biprismatic geometry formed by four carboxylate oxygen atoms and a nitrogen atom of the 4,4‘-bipyridine. The Co−O bond distances have an average value of 2.024 &A, and the Co−N distance has a value of 2.103(3) &A. The O/N−Co−O/N bond angles are in the range 85.8 (1)&#(1)°. There is only one OBA anion present in the structure, and the carboxylates have bis-bidentate connectivity with the Co2+ cations forming a eight-membered ring. The bond distances and angles associated with the OBA anions are in the ranges expected for this type of bonding. The selected bond distances 1 are listed in Table 3. The connectivity between Co2+, OBA anions, and 4,4‘-bpy gives rise to a 3D structure. The complex structure of 1 can be understood in terms of simpler building units. Thus, the Co2+ ions are linked to the carboxylate units giving rise to an eight-membered ring, which are further connected through their corners forming a 1D chain structure. The chain structure resembles the commonly observed corner-shared 1D phosphate structure.41
Table 3:  Selected Bond Distances in 1, {2}
bond distance, &A bond distance, &A&&
Co(1)−O(1)&&2.012(3)&&Co(1)−O(4)&&1.973(3)&&
Co(1)−O(2)&&2.085(3)&&Co(1)−N(1)&&2.103(3)&&
Co(1)−O(3)&&2.024(3)& && &
The connectivity between the corner-shared eight-membered ring chains and the OBA anions forms a layer (Figure 1a), which is cross-linked by the 4,4‘-bpy molecules giving rise to the 3D structure. The 4, 4‘-bpy molecules have both in-plane and out-of-plane connectivity. Viewing down the c axis, the linkages between the Co2+ ions and the OBA anions are cross-linked by the 4,4‘-bpy molecules (Figure 1b). The entire structure appears to have an efficient packing for optimum π···π interactions between the OBA anions and 4,4‘-bpy molecules.
Figure 1 (a) Connectivity between the Co2+ and OBA anions. (b) Connectivity along the c axis.
Structure of 2, { 2·H2O}. The asymmetric unit of 2 consists of 66 non-hydrogen atoms, of which two nickel atoms are crystallographically independent. Both the Ni2+ ions have distorted octahedral coordination formed by four carboxylate oxygen atoms and two nitrogen atoms of the 4,4‘-bipyridine ligand. The Ni−O distances are in the range 2.009(3)&#(3) &A (av. Ni−O = 2.072), and the Ni−N distances have an average value of 2.094 &A. The O/N−Ni(1)−O/N bond angles are in the range 61.94(12)&#(14)°. There are two OBA anions present in the structure, and the carboxylates bond with the Ni2+ ions to form eight-membered rings. The bond distances and angles associated with the OBA anions are in the ranges expected for this type of bonding. The selected bond distances for 2 are listed in Table 4.
Table 4:  Selected Bond Distances in 2, {2·H2O}
bond distance, &A bond distance, &A&&
Ni(1)−O(1)&&2.114(3)&&Ni(2)−O(5)a&&2.009(3)&&
Ni(1)−O(2)&&2.127(3)&&Ni(2)−O6)b&&2.036(3)&&
Ni(1)−O(3)&&2.009(3)&&Ni(2)−O(7)&&2.129(3)&&
Ni(1)−O(4)&&2.037(3)&&Ni(2)−O(8)&&2.116(3)&&
Ni(1)−N(1)&&2.099(3)&&Ni(2)−N(4)&&2.100(3)&&
Ni(1)−N(2)&&2.088(3)&&Ni(2)−N(21)&&2.087(3)
a Symmetry transformations used to generate equivalent atoms:  x + 2, y − 2, z.b Symmetry transformations used to generate equivalent atoms:  −x + 1, −y + 1, −z + 1.
The 3D structure of 2 is built-up from the connectivity involving Ni2+, OBA anions, and 4,4-bpy ligands. As in 1, the connectivity between the Ni2+ cations and OBA anions gives rise to eight-membered rings. This is the simplest building unit in 2. The overall structure of 2, though complex, can be understood by considering smaller fragments and taking connectivity into consideration. Thus, the connectivity between the Ni2+ ions and OBA anions forms a 2D porous grid in the (110) plane (Figure 2a), which is cross-linked by the 4,4‘-bpy ligands forming the 3D structure (Figure 2b).
Figure 2 (a) Connectivity between Ni2+ ions and OBA anions forming a two-dimensional grid in the (110) plane. (b) Three-dimensional structure.
Structure of 3, { 2}. The asymmetric unit of 3 consists of 52 non-hydrogen atoms, of which two zinc atoms are crystallographically independent. While the Zn(1) has a distorted trigonal biprismatic geometry formed by three carboxylate oxygen atoms and two nitrogen atom of the 4,4‘-bipyridine, Zn(2) has a distorted tetrahedral geometry connected by four carboxylate oxygen atoms. The Zn−O distances are in the range 1.932(2)&#(2) &A (av Zn(1)−O = 2.0774 and Zn(2)−O = 1.945 &A), and Zn−N distances have an average value of 2.109 &A. The O/N−Zn(1)−O/N bond angles are in the range 87.13(8)&#(9)°, and the distorted tetrahedral O−Zn(2)−O bond angles have the average value of 109.3°. There are two OBA anions present in the structure, and the carboxylates bond with the Zn2+ ions to form two types of eight-membered rings. The bond distances and angles associated with the OBA anions are in the ranges expected for this type of bonding. The selected bond distances for 3 are listed in Table 5.
Table 5:  Selected Bond Distances in 3, {2}
bond distance, &A bond distance, &A&&
Zn(1)−O(1)&&2.093(2)&&Zn(1)−N(1)&&2.119(2)&&
Zn(1)−O(2)&&2.0891(19)&&Zn(1)−N(2)&&2.098(2)&&
Zn(1)−O(3)&&2.0501(19)&&Zn(2)−O(4)b&&1.958(2)&&
Zn(2)−O(6)a&&1.932(2)&&O(4)−Zn(2)b&&1.958(2)&&
Zn(2)−O(7)&&1.939(2)&&O(6)−Zn(2)d&&1.932(2)&&
Zn(2)−O(5)&&1.952(2)& && &
a Symmetry transformations used to generate equivalent atoms:  x, y, z − 1.b Symmetry transformations used to generate equivalent atoms:  −x, −y + 2, −z + 1.c Symmetry transformations used to generate equivalent atoms:  −x, −y + 1, −z.d Symmetry transformations used to generate equivalent atoms:  x, y, z + 1.
The connectivity in 3, between Zn2+, OBA anions, and 4,4‘-bpy, gives rise to a 3D structure. There are two types of OBA anions:  acid-1 is connected to two Zn(1) atoms and one Zn(2) atom and acid-2 is connected to three Zn(2) atoms and one Zn(1) atom, through the carboxylate oxygen atoms. This gives rise to one of the carboxylate oxygen atoms of acid-2 being free. The Zn2+ ions are linked to the carboxylate units giving rise to two different eight-membered rings, which are further connected by the OBA anions giving rise to a layerlike structure in the bc plane (Figure 3a). Unlike 1, the eight-membered rings are not connected together in 3. The layers are further connected by both the OBA anions and the 4,4‘-bpy ligands. The arrangement of the structure in the ac plane is shown in Figure 3b. As can be noted, the Zn(1) atoms are connected by two sets of 4,4‘-bpy ligands forming a 1D structure, which are connected by the OBA ligands with the Zn(2) eight-membered rings.
Figure 3 (a) Layer structure formed by the connectivity between Zn2+ and OBA. (b) Three-dimensional structure formed by the connectivity involving the 4,4‘-bpy units.
Three new compounds, 1, 2, and 3, have been prepared, which have 3D connectivity resulting from the expected building units of OBA anions, 4,4‘-bpy ligands, and the central M2+ ion. There are, however, subtle differences in the coordination geometry around the central M2+ ions. In 1, the Co2+ ions have distorted trigonal bipyramidal geometry (four oxygens and a nitrogen), in 2, the Ni2+ ions have octahedral geometry (four oxygens and two nitrogens), and the Zn2+ ions have both the tetrahedral (four oxygens) and the trigonal bipyramidal (three oxygens and two nitrogens) geometries in 3. The formation of tetrahedral Zn is not common in MOF compounds. In many transition metal benzenecarboxylates, the octahedral coordination is most often encountered. The formation of five-coordinated Co2+ and Zn2+ ions, in 1 and 3, is probably due to the differences in the reaction conditions.2,4,5 The OBA anion can be considered similar to the water molecule. The angle between H−O−H is 104.5°, and the free acid has an angle of 122.5° suspended between the two benzene rings. The large angles in the free OBA acid might be due to hydrogen bond interactions. In compounds 1−3, the angles between the OBA anions with respect to the central oxygen atom are 118.1° (1), 117.6 and 117.3° (2), and 115.4 and 118.5° (3). The observed angles in the compounds are similar to the angles observed in diphenyl ether and other related compounds. The role of π···π interactions in the stability of supramolecularly engineered crystal structures have been well established.42,43 In recent years, in the MOFs formed using benzenecarboxylates, the role of π···π interactions has been a topic of much interest. In the present compounds, we find two types of π···π interactions:  one homo interaction involving similar species, for example, the two OBA anions or two 4,4‘-bpy ligands, and a hetero interaction involving the OBA anions and 4,4‘-bipyridine ligand. In 1, we find only the hetero interaction, and in 2, we find only the homo interaction, and both homo and hetero interactions have been observed in 3. The role of π···π interactions for the stability of organic compounds has been discussed in the literature with particular emphasis on the centroid−centroid distances (d) and the angles (θ) suspended between the benzene rings.42,43 A correlation between the two (d and θ) has been discussed in terms of a phase diagram with the distances and angles having a unique relationship. From this, the following interactions appear to be important for different values of the centroid−centroid distance (d) and angles (θ):  (i) d = 0−3 &A and θ = 50−90°, (ii) d = 3−7 &A and θ = 0−50°, (iii) d = 4&# &A and θ = 140&#°, and (iv) d = 6−7 &A and θ = 0&#°. The centroid−centroid distance (d) and the interplanar angles (θ) between the ligands in all the compounds are shown in Figure 4, which clearly indicates favorable π···π interactions in all the compounds. For 1, we find interactions between OBA and 4,4-bpy with d = 3.9 &A and θ = 12°, in 2, the two 4.4‘-bpy ligands have d = 4.5 &A and θ = 25.5°, which are less favorable, and in 3, we find two distances and angles due to the positioning of the OBA anions and 4,4‘-bpy. Thus, d = 3.8 and 4 &A with θ = 15 and 19° have been observed (Figure 4c). In addition, homo interactions between 4,4‘-bpy and the OBA anions have also been encountered in 3 (parts d and e of Figure 4). The distances and angles between the two OBA anions and the 4, 4‘-bpy ligands in 3 are as follows:  for OBA, d = 3.88 &A and θ = 4° and for 4,4‘-bpy, d = 4.13 &A and θ = 1.7°.
Figure 4 Possible π···π interactions between the various participating ligands:  (a) 1, (b) 2, and (c)−(e) 3.
To understand the nature and role of the π···π interactions, we have performed preliminary calculations using the AM1 parametrized Hamiltonian available in the Gaussian set of codes.44,45 Using the crystal structure geometry, we have made an evaluation of the strength of the π···π interactions in 1 and 2 based on single-point energy calculation without the symmetry constraints. The calculated π···π interaction energies between 4,4‘-bpy and OBA were found to be 4.3 kcal/mol and 4.8 kcal/mol, respectively, for 1 and 3. The π···π interaction energy between the two 4, 4‘-bpy compounds was found to be 1.83 kcal/mol in 2. The π···π interaction energies, in 3, between the two 4,4‘-bpy and the two OBA anions were found to be 4.12 and 1.5 kcal/mol. The calculated energies suggest that they are comparable to the energies encountered in systems involving weak hydrogen bonds (3−6 kcal/mol).46 It is interesting to note that, in 1 and 2, only one type of π···π interaction (between OBA&#‘-bpy) and (between two 4,4‘-bpy) has been observed. In the case of 3, however, three distinct types have been encountered (OBA&#‘-bpy, OBA−OBA, and 4,4‘-bpy&#‘-bpy). It is becoming clear that the π···π interactions appear to play an important role, as the weak hydrogen bonds in some of the supramolecularly arranged structures and in the structural arrangement and the stability of MOF compounds. The geometry of 4,4‘-bipyridine in the three compounds is different with varying degrees of twist angles between the two pyridine rings. The twist angles are 0.23°, 25.60°, and 4.26°, respectively, for 1, 2, and 3. The calculated twist angle between the two pyridine rings in 4,4‘-bpy, in the gas phase, is 35.56°. This shows the subtle influence affected by the participating OBA ligands and the M2+ ions. The diffuse reflectance UV−vis spectra at room temperature were recorded for the sodium salt of OBA and for the as-synthesized coordination polymers (Figure S6). In all cases, the main absorption bands are at 311, 294, 290, and 275 nm, for 1, 2, 3, and Na−OBA, respectively. The bands can be assigned to ligand-to-metal charge transfer (LMCT). As can be noted, the absorption maximum of the LMCT band of the Na−OBA is blue shifted compared with the LMCT band of the three compounds. This may be due to the fact that the Na+1 ion is more electropositive than the Zn2+, Ni2+, and Co2+ ions and would require more energy for the transfer of the charge from the ligand to the metal. It is well established that the polarizability of the four ions are in the order Co2+ > Ni2+ > Zn2+ > Na+1, and hence, the energy required for the LMCT would be in the reverse order.47 In the case of 1, we also observe two clear additional peaks at 480 and 575 nm and a broad one at a higher wavelength. The peaks at 480 and 575 nm, probably, originate from the d−d transition of the d7 (Co2+) ion. The Co2+ ions in 1 are in a trigonal bipyramidal environment, and the peaks at 480 and 575 nm can be assigned to 4A‘2(F) → 4E‘ ‘(P) and 4A‘2(F) → 4A‘2(P), and the broad peak at higher wavelength could be due to 4A‘2(F) → 4E‘(F) and 4A‘2(F) → 4E‘ ‘(F) transitions.48 Similarly, the other peaks in 2 may also originate from the d−d transition of the d8(Ni2+) ion. Ni2+ ions are in an octahedral environment so the peaks at 361 and 656 nm can be assigned to 3A2g(F) → 3T1g(P) and 3A2g(F) → 3T1g(F), respectively.48 To obtain the precise values of band gap from the absorption edges, the point of inflection determined from the minimum in the first derivative of the absorption spectrum was used. The values of the band gap, thus obtained corresponding to the LMCT transitions from the UV−vis spectra, are 3.11, 3.89, and 4.02 eV for 1, 2, and 3, respectively. Room-temperature solid-state photoluminescence studies were carried out on powdered samples (Perkin-Elmer, U.K.) (Figure S7). Photoluminescence studies of the MOF compounds here have been investigated in great detail during the past few years.49-52 In our present study, we found that all three MOF compounds 1, 2, and 3 and Na−OBA exhibit photoluminescence. To understand the nature of the emission bands, we have confirmed that both free 4,4‘-oxybis(benzoic acid) and 4,4‘-bipyridine ligands do not emit any luminescence in the range 400&# nm.24 Compound 1 exhibits a broad emission band at 430 nm and two more peaks at 483 nm and at 525 nm, when excited at 235 nm. The emission peak at 430 nm for 1 may be due to the ligand-to-metal charge transfer (LMCT) and the peaks at 483 nm and at 525 nm can be assigned to the 4A‘2(F) → 4E‘ ‘(P) and 4A‘2(F) → 4A‘2(P) transitions coming from the d−d transition of the d7(Co2+) system.48 Compound 2exhibits a broad emission peak at 443 nm and two more peaks at 483 nm and at 525 nm, when excited at 235 nm. The emission peak at 443 nm may be due to the LMCT band and the peaks at 483 nm and at 524 nm can be assigned to the 3A2g(F) → 3T1g(P) and 3A2g(F) → 3T1g(F) transitions coming from the d−d transition of the d8(Ni2+) system.48 Compound 3, on the other hand, exhibits a single broad emission band at 436 nm, when excited at 235 nm. This peak can be assigned to the LMCT band. The single peak for compound 3 also confirms that the two low intensity peaks observed for compounds 1 and 2 are due to the d−d transitions not from the ligands. The Na−OBA also exhibits one broad emission band at 410 nm, when excited at 235 nm, which is due to the LMCT. The different positioning of the main emission peaks in 1−3 indicates that the band is due to the LMCT and not from the intraligand fluorescence. Temperature-dependent magnetic susceptibility measurements for 1 and 2 have been performed on powder samples using a SQUID magnetometer (Figure 5). In the case of Co2+ and Ni2+ compounds, the first-order spin−orbital interactions and the orbital contribution have a significant contribution to the exchange magnetic coupling parameter due to the orbitally degenerate electronic ground state of Co2+ and Ni2+.53 These contributions are completely suppressed for octahedral geometry around Ni2+, but for Co2+ in the trigonal bipyramidal geometry, the orbital contribution has significant input to the magnetic moment at room temperature. The observed effective magnetic moment of compound 1 at room temperature (300 K) is 4.44 μB, which corresponds to the value expected for noninteracting paramagnetic Co2+ ions with significant orbital contribution, as the spin-only moment of the Co2+ ion is 3.87 μB. A plot of 1/χM vs T for 1 was found to be liner in the range 50&# K with a Weiss temperature (θP) of −11 K and a Curie constant (C) of 2.5 emu/mol. The negative Weiss temperature (θP) indicates weak antiferromagnetic interactions in 1.54,55 The effective magnetic moment of 2 at room temperature (300 K) is 2.97 μB which corresponds to the value expected for noninteracting paramagnetic Ni2+ ions, a value that is very close to the Ni2+ spin-only moment of 2.83 μB. A plot of 1/χM vs T for 2 was found to be linear in the range 50&# K with a Weiss temperature (θP) of −15 K and a Curie constant (C) of 1.13 emu/mol. The negative θP values, again, indicate the presence of weak antiferromagnetic interactions in 2.54,55
Figure 5 Temperature variation of the molar magnetic susceptibility (χM) for 1 (a) and for 2 (b). The inset shows the corresponding 1/χM vs T plots.
Photocatalytic Investigations. Photocatalytic degradation of four commonly used dyes, orange G (OG), rhodamine B (RhB), Remazol Brilliant Blue R (RBBR), and methylene blue (MB), was investigated. These four dyes were selected as model dye pollutants in aqueous media to demonstrate the efficacies of the photocatalytic behavior of the three polymers. The photocatalytic performance of the three polymers was estimated from the variation of the color in the reaction system by monitoring the absorbance (at λ = 482, 554, 590, and 661 nm) characteristic of the targets OG, RhB, RBBR, and MB which directly relate to the structural changes of their chromophore. For comparison, the photocatalytic performance of commercial TiO2 (Degussa P-25) was also assessed under the same experimental conditions. Control experiments without catalyst, UV, and both were also carried out:  while no degradation was observed with catalyst only in the absence of UV, degradation was observed in the absence of catalyst in the presence of UV. The degradation profiles of all the four dyes with an initial concentration of 100 ppm by using three compounds, TiO2 (Degussa P-25) and dyes with out catalysts are shown in Figure 6 where the catalysts concentration are constant (2 kg/m3) for each case.
Figure 6 Degradation profiles of all four dyes with an initial concentration of 100 ppm without catalyst (a), with Degussa P-25 (b), with compound 1 (c), with compound 2 (d), and with compound 3 (e).
The degradation of four different dyes was investigated with a loading of 2 kg/m3. To quantify the reactions, the kinetics was determined by the Langmuir−Hinshelwood (L−H) kinetic. This can be written as r0 = k0C0/(1 + K0C0), where r0 is the initial rate, C0 is the initial concentration of the dyes, k0 is the kinetic rate constant, and parameter K0 represents the equivalent adsorption coefficient. The plots of the reciprocal initial degradation rate (1/ro) with the reciprocal of the initial dye concentration (1/C0) by using 1, 2, and 3 are shown in Figure 7. The values of the parameters, k0 and K0, for the photocatalytic degradation of the dyes, obtained from the slope and the intercept of the linear plot, are shown in Table 6.
Figure 7 Variation of the initial photodegradation rate with the initial concentration of the four dyes for (a) 3, (b) 2, and (c) 1. Note that in all the cases a linear correlation with a regression coefficient >0.99 has been observed.
Table 6:  Kinetic Parameters for the Degradation of Dyes Using Three Compounds
compound dye k0 (min-1) K0 (ppm-1)&&
1&&OG&&0.031&&0.0022&&
1&&RhB&&0.013&&0.0035&&
1&&RBBR&&0.033&&0.0007&&
1&&MB&&0.032&&0.0064&&
2&&OG&&0.029&&0.0049&&
2&&RhB&&0.008&&0.0023&&
2&&RBBR&&0.029&&0.0015&&
2&&MB&&0.027&&0.0027&&
3&&OG&&0.02&&0.0029&&
3&&RhB&&0.007&&0.002&&
3&&RBBR&&0.028&&0.0069&&
3&&MB&&0.023&&0.0029
As the parameter, K0, represents the adsorption equilibrium coefficient, the low value of K0 can be attributed to negligible adsorption. This is also confirmed by powder XRD of the catalyst after the photocatalytic degradation experiments, which clearly indicated that the structure remained the same and no adsorption was observed. For the degradation of all four dyes, the order of degradation is 1 > 2 > 3. The band gaps of 1, 2, and 3 follow the order 1 < 2 < 3; the degradation of all dyes follow the reverse order. The photocatalytic activity of the present compounds can be explained based on some of the earlier observations. Several Co complexes with chelating nitrogen ligands catalyze oxidation reactions, for example, 2+ activates O2 and oxidizes N-methyl-anilines, benzyl alcohols, and aldehydes.56 Polydentate CoII complexes with ligands capable of intercalation into DNA strands are capable of inducing DNA cleavage under photochemical conditions.57 The activation of O2 in the above systems occurs through the ligand-to-metal charge transfer. The oxidations of organic compounds by using other transition metal compounds with chelating ligand occur through the same principle in the presence of light. In the present compounds, we have a similar situation as metal exists in M+2 states. From our UV−vis spectroscopic study, we clearly see two types of transition:  the most intense peak originates due to the LMCT and the other peaks belong to the d−d spin allowed transition. Although the photocatalytic experiments on 1, 2, and 3 were carried out in heterogeneous conditions, it is believed that the photocatalytically active metal center would behave in a way similar to that in solution. Of the two types of electronic states observed in the compounds, the LMCT effect, which is in the UV region, probably is responsible for the photocatalytic activity.53 To understand the photocatalytic degradation of the organic dyes, we employ a simple approach based upon highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) considerations. Accordingly, the HOMO and LUMO of the charge transfer state, in the absence of the UV light, would have two electrons in the HOMO and the LUMO would be vacant. In the presence of UV light, there is an electron transfer from the HOMO to LUMO. The electron of the excited state in the LUMO is usually very easily lost, while the HOMO strongly demands one electron to return to its stable state. Generally, the excited M+2 center decays to its ground state quickly. However, if some molecules are within a reasonable range and have an appropriate orientation, for example, RhB in this case, transitional active complexes can be formed. Thus, one α-hydrogen atom of the methylene group bonded to the electron-withdrawing nitrogen atom of RhB, which will give up its electron and leave as H+ later, is abstracted by metal species, and this results in the cleavage of the C−N bond and stepwise N-deethylation of RhB. Since the HOMO is then reoccupied, the excited electron must remain in the LUMO until it is captured by electronegative substances such as molecular oxygen in solution, which would transform into highly active peroxide anion and subsequently accomplish further oxidation and total degradation of RhB. A similar mechanism has been proposed recently for the degradation of organic dyes in the presence of metal carboxylates.37 Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableConclusions
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The synthesis and structure of three new coordination polymers has been accomplished by employing hydrothermal methods. The samples have been characterized using a variety of techniques such as powder XRD, single-crystal XRD, IR, UV−vis, photoluminescence, magnetic, and photocatalytic studies. All the compounds appear to be active for the photocatalytic decomposition of common organic dyes (OG, RhB, RBBR, and MB) used in textile industries, with activities better than Degussa P-25 TiO2 catalysts. A possible mechanism, through an activated complex involving M+2, has been proposed. This study clearly reveals that it is profitable to investigate metal carboxylates for photocatalytic activity. Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableAcknowledgment
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S.N. thanks the Department of Science and Technology (DST), Government of India, for the award of a research grant, and P.M. thanks the Council of Scientific and Industrial Research (CSIR), Government of India, for the award of a research fellowship.
Top of PageIntroductionExperimental SectionResults and DiscussionConclusionsSupporting Information AvailableSupporting Information Available
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Powder X-ray patterns of (a) simulated and (b) experimental of 1 (Figure S1); powder X-ray patterns of (a) simulated and (b) experimental of 2 (Figure S2); powder X-ray patterns of (a) simulated and (b) experimental of 3 (Figure S3); TGA studies of the three compounds (Figure S4); IR spectra of the three compounds (Figure S5); UV−vis spectra of the three compounds and Na salt of OBA (Figure S6); Photoluminescence spectra of the three compounds and Na salt of OBA (Figure S7); Bond angles of the three compounds (Table S1−S3). CCDC:&#&# contain(s) the supplementary crystallographic data for this paper (Cambridge Crystallographic Data Centre, www.ccdc.cam.ac.uk/data_request/cif). This material is available free of charge via the Internet at http://pubs.acs.org. This article references 57 other publications.
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AbstractThree novel metal-organic frameworks (MOFs) 2, 1, 2·H2O, 2, and 2, 3, with three-dimensional structures have been synthesized and characterized. The structures of the three compounds appear&&... 三楼真狠，膜拜中，你上传到网盘给他个网址就行了吧~ : Originally posted by pypypy-317 at
怎么给你啊？把你的邮箱告诉我吧，我已经把文献下好了。 我的邮箱，，谢谢啦！ : Originally posted by meizi0314 at
我的邮箱，，谢谢啦！... 因为我不是常在线，刚看到你的消息，文献已经给你发过过去了，请注意查收。
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