Synthesis, Structure, and Properties - American Chemical Society

DOI: 10.1021/cg100865v

Use of Polyazaheterocycles in the Assembly of New Cadmium Sulfate Frameworks: Synthesis, Structure, and Properties

2010, Vol. 10 4161–4175

Avijit Kumar Paul,† Udishnu Sanyal,‡ and Srinivasan Natarajan*,† †

Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India, and ‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India Received June 30, 2010

ABSTRACT: The reaction of cadmium sulfate in the presence of polyazaheterocyclic organic molecules gave rise to a variety of new cadmium sulfate phases in water containing solvothermal reaction. The compounds have two- (I) and three-dimensionally (II-VI) extended structures. All the compounds have structures built up by the connectivity involving the cadmium octahedra and the sulfate tetrahedra in which the heterocyclic organic molecules act as the ligand. The linkages between the Cd2þ and (SO4)2- ions form one- (II), two- (I, III, and IV), and three- (V and VI) dimensionally extended cadmium sulfate phases. The connectivity between Cd2þ ion and the heterocyclic ligand also gives rise to one- and two-dimensional structures. The interconnectivity between the two units gives rise to the observed structures. The presence of Cd-O-Cd chains and Cd-O-Cd layers in some of the structures is noteworthy. The adsorption/desorption studies suggest that the cadmium sulfate phases adsorb/desorb anionic dyes selectively in the presence of water/ethanol, respectively. The photocatalytic degradation studies on cationic dyes under UV-irradiation indicate modest activity. The cyanosilylation of imines using the present compounds as heterogeneous catalyst indicate good catalytic behavior. The various properties exhibited by the cadmium sulfate phases suggest that these compounds are versatile. All the compounds were characterized by powder X-ray diffraction, thermogravimetric analysis, infrared (IR) and UV-visible studies.

*To whom correspondence should be addressed. E-mail: snatarajan@ sscu.iisc.ernet.in.

has been attempted with reasonable success.11 It appears that a fine balance exists between the reactivity of the carboxylates and the basicity of the heterocycles in the formation of such structures. It occurred to us that it may be profitable to investigate the formation of new open-structured solids using the polyazaheterocycles along with a tetrahedral anion, such as the sulfates. It may be noted that reports of sulfate-based openframework compounds are not numerous.4a,12 The use of azaheterocycles in the formation of sulfate frameworks resulted in only limited success.13 Our current research focuses on the synthesis, structure, and possible applications of new framework materials. Recent studies on framework cadmium thiosulfate compounds indicated interesting structures and catalytic properties.8,14 The wastewater treatment is important and photocatalytic decomposition of the organic pollutants in water has attracted considerable attention in recent years.15 We have been interested in the study of new materials that could be used in wastewater treatment photocatalytically. To this end, we have made attempts to prepare a family of cadmium sulfate phases in the presence of polyazaheterocyclic ligands. During the course of our investigations, we have prepared a series of new framework cadmium sulfate compounds with two- and threedimensional structures. Thus, the compound, [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4] I, has a two-dimensional structure, and the remaining compounds [Cd(C5H5N5)(SO4)] II, [Cd3(H2O)2(C2H2N3)2(SO4)2] III, [Cd2(C2H5N5)(SO4)2] IV, [Cd5(OH)2(CH2N5)4(SO4)2] V, and [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O VI have three-dimensionally extended structures. The compounds are examined for the adsorption of the organic dyes from water. In this paper, we describe the synthesis, structure, adsorption, and preliminary studies on the possible applications of the synthesized cadmium sulfate phases as heterogeneous catalysts.

r 2010 American Chemical Society

Published on Web 08/04/2010

Introduction Compounds exhibiting open structures provide immense opportunities for exploring many important properties in the areas of catalysis, sorption, and separation processes.1 The potential as well as actual applications of the open-framework compounds provide significant opportunities for the synthetic chemists. Persistent efforts over the years, by many research groups, resulted in the incorporation of most of the elements of the periodic table as part of this family of compounds.2-5 Though the silicate and the phosphate anions have been the dominant building units in most of the synthesized openframework solids,3 recent research has demonstrated that other anions such as sulfates/sulfites,4 selenates/selinites,5 etc. can also be incorporated as part of the open structures. The open-framework compounds are, generally, synthesized in the presence of nitrogen-containing organic amine molecules, which act as the templating or structure-directing agent. It has been shown, in some cases, that the amine molecules also act as a ligand binding with the metals.6 In addition, the nitrogen-containing heterocyclic compounds can also act as ligands for the metal ions, and such ligands have been employed for enhancing the dimensionality of the open-framework compounds.7 We have recently employed this strategy for the preparation of a new family of framework compounds based on thiosulfate anions.8 Nitrogen-containing heterocyclic organic molecules have been employed for the preparation of a number of compounds in the presence of suitable metal ions.9 The resulting solids exhibit considerable variety in their structures and properties.10 Use of polyazaheterocycles along with aromatic carboxylates

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Table 1. Synthetic Composition and Conditions Employed for the Preparation of the Cadmium Sulfate Compounds, I-VIa S. no.

composition

1

0.33(3CdSO4 3 8H2O) þ 1(melamine) þ 95CH3CN þ 51EtOH þ 388H2O 0.33(3CdSO4 3 8H2O) þ 1(adenine) þ 95CH3CN þ 51EtOH þ 388H2O 0.33(3CdSO4 3 8H2O) þ 1(1,2,4-triazole) þ 95CH3CN þ 51EtOH þ 388H2O 0.33(3CdSO4 3 8H2O) þ 1(guanazole) þ 95CH3CN þ 51EtOH þ 388H2O 0.33(3CdSO4 3 8H2O) þ 1(5-aminotetrazole) þ 95CH3CN þ 51EtOH þ 388H2O 0.33(3CdSO4 3 8H2O) þ 0.5(Cd(NO3)2 3 4H2O) þ 1(NaN3) þ 95CH3CN þ 51EtOH þ 388H2O

2 3 4 5 6

time pH temp (°C) (day) (initial, final)

product

yield (%)

125

6

6.6, 6.0

[(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I

92

125

6

4.3, 5.4

[Cd(C5H5N5)(SO4)], II

88

125

6

3.5, 5.3

[Cd3(H2O)2(C2H2N3)2(SO4)2], III

91

125

6

4.1, 7.2

[Cd2(C2H5N5)(SO4)2], IV

90

125

6

2.9, 4.7

[Cd5(OH)2(CH2N5)4(SO4)2], V

85

125

6

5.2, 6.0

[Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI

89

a Elemental analysis calcd (%) for I: C 11.07, H 2.63, N 25.91; Found: C 11.12, H 2.68, N 25.97; Calcd (%) for II: C 17.46, H 1.46, N 20.38; Found: C 17.39, H 1.52, N 20.31; Calcd (%) for III: C 6.84, H 1.14, N 11.97; Found 6.78, H 1.19, N 11.91; Calcd (%) for IV: C 4.65, H 0.97, N 13.56; Found: C 4.71, H 0.91, N 13.61; Calcd (%) for V: C 4.27, H 0.89, N 24.91; Found: C 4.35, H 0.92, N 24.99; Calcd (%) for VI: C 5.05, H 2.12, N 11.79; Found: C 5.15, H 2.19, N 11.71.

Synthesis and Initial Characterization. All the compounds were prepared by using solvothermal conditions in the presence of water at 125 °C in a Teflon-lined autoclave. The reagents, 3CdSO4 3 8H2O (Merck, 98%), Cd(NO3)2 3 4H2O (Merck, 99%), NaN3 (SD FINE, 99%) melamine (CDH, 99%), adenine (SD FINE, 99%), 1,2,4-triazole (CDH, 99%), 3,5diamino 1,2,4-triazole or guanazole (CDH, 99%), and 5-aminotetrazole (CDH, 99%), were used as received and without any further purifications. In a typical synthesis, for [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4] I, 3CdSO4 3 8H2O (0.257 g, 0.33 mM) and melamine (0.126 g, 1 mM) were dissolved in 7 mL of distilled water. To this, 5 mL of CH3CN and 3 mL of EtOH were added under continuous stirring. The mixture was homogenized for 30 min at room temperature. The final mixture with the composition, 3CdSO4 3 8H2O/3C3H6N6/285CH3CN/ 153EtOH/1164H2O was sealed in a 23 mL PTFE-lined autoclave and heated at 125 °C for 6 days under autogenous pressure. The initial pH of the reaction mixture was 6.6 and the final pH was observed to be 6.0. The final product, containing large quantities of colorless rectangular crystals, was filtered, washed with deionized water under a vacuum, and dried at ambient conditions (yield ∼ 90% based on Cd). A similar synthesis procedure was followed for the preparation of all the other compounds (Table 1). For the preparation of [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI, we also added 0.154 g of Cd(NO3)2 3 4H2O (0.5 mM) and 0.065 g NaN3 (1 mM). In all the cases, we obtained good quality single crystals with good yields. Thus, blocklike crystals of III, IV and rodlike crystals of II, V, and VI were collected by filtration, washed with deionized water, and dried at ambient conditions. CHN analysis was performed for all the compounds using a Thermo Finnigan FLASH EA 1112 CHN analyzer (Table 1). The compounds were characterized by powder X-ray diffraction (XRD), IR, UV-vis, and TGA studies. The powder X-ray diffraction (PXRD) patterns (Philips, X’pertPro) were recorded in the 2θ range of 5-50° by using Cu-KR radiation (λ = 1.5418 A˚). The XRD patterns indicated that the products were new and the observed XRD patterns were entirely consistent with the simulated XRD pattern generated based on the structures determined using the single crystal XRD studies (see Supporting Information, Figure S1). Infrared (IR) spectroscopic studies have been carried out in the mid-IR region (4000 to 400 cm-1) on

KBr pellets (Perkin-Elmer, SPECTRUM 1000). IR spectra of all the compounds were comparable, and six distinct regions can be identified (see Supporting Information, Figure S2). (i) The compounds have bands at the 3450-3350 cm-1 region indicative of the presence of water molecules (-OH unit also), and the bands in the region 3360-3260 cm-1 can be assigned to the υ(N-H) modes of the NH2 group; (ii) a band at 3100 cm-1 can be assigned to aromatic υ(C-H) modes of the heterocyclic ring; (iii) the bands in the region 1687-1550 cm-1 can be assigned to the C-H bending vibrations and due to the asymmetric and symmetric modes of the heterocyclic ring; (iv) the bands are in the 1280-1200 cm-1 region for υs (C-C) or υs (C-N) of the heterocyclic rings; (v) 11201050 cm-1 region, with a sharp band at 1080 cm-1 is due to υs (S-O); and (vi) the sharp bands in the region of 620 cm-1 are due to the bending mode of mono- or tridentate oxygen of SO4 units. UV-vis Spectroscopy. The solid-state UV-visible spectra of all the samples were recorded in the diffuse reflectance mode at room temperature (Perkin-Elmer model Lambda 35). The powdered sample was placed inside a cuvette (75 mm 80 mm 20 mm), and the spectrum was recorded using an integrating sphere of radius 50 mm. The reflectance spectra were converted into an absorption like spectra using the Kubelka-Munk equation (see Supporting Information, Figure S3). The diffuse reflectance spectra [F(R) vs wavelength] of the compounds exhibited a sharp peak centered at the 250-300 nm region, which may be due to the intraligand charge-transfer. Compound IV exhibits an additional peak centered around 385 nm, which could be due to the charge transfer from the guanazole unit to the metal. From the UV-vis spectra, estimates for the band gaps were obtained. The band gaps for the compounds were found to be in the range of 4.13-4.96 eV. For compound IV, the band gap for the charge-transfer band was calculated to be 3.26 eV. Thermal Studies. TGA studies (Mettler-Toledo TG850) of all the compounds were carried out in an atmosphere of flowing oxygen (flow rate = 50 mL min-1) in the temperature range 30-850 °C (heating rate = 5 °C min-1) (see Supporting Information, Figure S4). The studies indicate that the compounds II, IV, and V appear to be stable up to 360 °C. The compounds I, III, and VI exhibit gradual weight loss below the 300 °C, which may be due to the loss of water molecules from the structures. The TGA studies

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Table 2. Crystal Data and Structure Refinement Parameters for I-VIa structural parameter empirical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) V (A˚3) Z D (calc/ g cm-3) μ (mm-1) λ (Mo KR/A˚) θ range (°) total data collected unique data Rint R indexes [I > 2σ(I)] R indexes (all data)

I

V

VI

[Cd2(C2H5N5)(SO4)2]

[Cd5(OH)2 (CH2N5)4(SO4)2]

343.62

[Cd3(H2O)2(C2H2N3)2 (SO4)2] 701.53

516.07

1124.49

[Cd2(OH)(H2O) (C2H3N4)(SO4)] 3 2H2O 474.95

triclinic P1 (No. 2) 6.8147(10) 10.3938(15) 14.487(2) 106.352(2) 97.049(2) 107.237(2) 916.3(2) 1 2.356

monoclinic P21/c (No. 14) 6.7010(13) 11.860(2) 11.190(2) 90.000 103.73(3) 90.000 863.9(3) 4 2.642

monoclinic P21/c (No. 14) 9.8131(4) 6.4065(3) 12.9635(6) 90.000 110.819(2) 90.000 761.77(6) 2 3.058

triclinic P1 (No. 2) 6.65880(10) 9.2325(2) 9.7163(2) 65.8430(10) 73.3020(10) 71.0830(10) 507.242(17) 2 3.379

monoclinic P21/c (No. 14) 8.8562(6) 9.5734(7) 13.3383(9) 90.000 102.9140(10) 90.000 1102.27(13) 2 3.382

orthorhombic Pna21 (No. 33) 9.6227(3) 16.7625(5) 7.1301(2) 90.000 90.000 90.000 1150.09(6) 4 2.708

2.068 0.71073 1.50-26.00 9477

2.778 0.71073 2.54-26.00 6510

4.489 0.71073 2.22-26.00 5771

4.656 0.71073 2.34-26.00 7640

5.032 0.71073 2.36-26.00 8246

3.915 0.71073 2.43-25.99 6544

3571 0.078 R1 = 0.0368; wR2 = 0.0721 R1 = 0.0516; wR2 = 0.0763

1696 0.0255 R1 = 0.0244; wR2 = 0.0557 R1 = 0.0297; wR2 = 0.0575

1478 0.0346 R1 = 0.0189; wR2 = 0.0457 R1 = 0.0205; wR2 = 0.0465

1994 0.0242 R1 = 0.0171; wR2 = 0.0423 R1 = 0.0184; wR2 = 0.0429

2156 0.0220 R1 = 0.0268; wR2 = 0.0661 R1 = 0.0297; wR2 = 0.0671

2227 0.0315 R1 = 0.0280; wR2 = 0.0597 R1 = 0.0395; wR2 = 0.0622

[(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4] 1300.14

II [Cd(C5H5N5)(SO4)]

III

IV

)

)

a R1 = Σ F0| - |Fc /Σ|F0|; wR2 = {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2. w = 1/[σ2(F0)2 þ (aP)2 þ bP], P = [max(F02,0) þ 2(Fc)2]/3, where a = 0.0271 and b = 0.9445 for I, a = 0.0289 and b = 0.6661 for II, a = 0.0194 and b = 0.5264 for III, a = 0.0178 and b = 0.7575 for IV, a = 0.0273 and b = 7.3658 for V, a = 0.0175 and b = 5.5577 for VI.

indicate an initial weight loss of ∼5% in the range 200-250 °C for I, which may be due to the loss of the coordinated water molecules (calc 5%). The second continuous weight loss was observed to be 57% in the temperature range of 300-620 °C. For compound II, only one weight loss of 57% was observed in the range 420-630 °C. For III, we observed an initial weight loss of 3% in the range 100-300 °C, which may be due to the loss of the water molecules (calc 5%). The second weight loss of 33% was observed in the range 320-650 °C. For compound IV, we observed a single step weight loss of 39% in the range 420-580 °C. For compound V, we observed the total weight loss of 30% in the range 350-600 °C. For VI, we observed an initial weight loss of 10% in the range 100-320 °C, which may be due to the loss of the water molecules (calc 11%). The second weight loss of 28% was observed in the range 320-650 °C. In all the cases, the calcined products were found to be poorly crystalline by PXRD, and the majority of the lines appear to correspond to a mixture of CdO (ICDD No. 01-1049) and Cd3O2SO4 phases (ICDD No. 320140). 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 with a cyanoacrylate (superglue) adhesive. The single crystal data were collected on a Bruker AXS smart Apex CCD diffractometer at 293(2) K. The X-ray generator was operated at 50 kV and 35 mA using Mo KR (λ = 0.71073 A˚) radiation. Data were collected with ω scan width of 0.3°. A total of 606 frames were collected at three different settings of j (0, 90, 180°) keeping the sample-to-detector distance fixed at 6.03 cm and the detector position (2θ) fixed at -25°. The data were reduced using SAINTPLUS,16 and an empirical absorption correction was applied using the SADABS program.17 The structure was

solved and refined using SHELXL9718 present in the WinGx suit of programs (Version 1.63.04a).19 The hydrogen position of the μ3-OH group in the compound V and the water molecules in the compound VI could not be located from the difference Fourier maps. The possible hydrogen position was arrived based on the bond valence sum (BVS) calculations20 for the μ3-OH group. All the other hydrogen positions of the heterocyclic ring for the compounds, the hydrogen position of the lattice water molecules (I and III) were located from the difference Fourier maps and for the final refinement the hydrogen positions were placed in geometrically ideal positions and refined in the riding mode. The 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 |F2| was carried out using the WinGx package of programs. Details of the structure solution and final refinements for all the structures are given in Table 2. The selected bond distances are given in Table 3. The other geometrical parameters for all the compounds are given as Supporting Information (Table S1-S6). The crystallographic data for the compounds, I-VI, were deposited with the Cambridge Crystallographic Data Center (CCDC) and can be downloaded free of charge by quoting the numbers (CCDC: 777059-777064) via www.ccdc.cam. ac.uk/data_request/cif. Adsorption and Photocatalytic Experiments. The organic dyes for the adsorption studies, Congo red (CR), Coomassie Brilliant Blue R-250 (CBB), and Alizarin red S (ARS) (S. D. Fine-Chem. Ltd., India) were used as received. The water used was double distilled, filtered through a Millipore membrane. All the dye molecules, investigated in the present study, were stirred with the prepared compounds in the dark

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Table 3. Selected Bond Distances Observed in [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I; [Cd(C5H5N5)(SO4)], II; [Cd3(H2O)2(C2H2N3)2(SO4)2], III; [Cd2(C2H5N5)(SO4)2], IV; [Cd5(OH)2(CH2N5)4(SO4)2], V; and [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VIa bond

distance (A˚)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3) Cd(1)-O(1)#1 Cd(1)-O(2)#1 Cd(1)-O(3)#1 Cd(2)-O(4)#2 Cd(2)-O(5) Cd(2)-O(6) Cd(2)-O(7)

2.284(3) 2.281(4) 2.300(3) 2.284(3) 2.281(4) 2.300(3) 2.262(3) 2.310(4) 2.323(3) 2.332(3)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3)#2 Cd(1)-O(4)#3 Cd(1)-N(1)

2.539(3) 2.320(2) 2.433(2) 2.433(2) 2.204(3)

Cd(1)-O(1) Cd(1)-O(1)#1 Cd(1)-O(2)#2 Cd(1)-O(4) Cd(1)-N(1) Cd(1)-N(3) Cd(2)-O(4) Cd(2)-O(4)#3

2.439(2) 2.221(2) 2.375(2) 2.670(2) 2.193(3) 2.192(3) 2.294(2) 2.294(2)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3) Cd(1)-O(4) Cd(1)-O(5) Cd(1)-N(1) Cd(2)-O(4) Cd(2)-O(5) Cd(2)-O(6) Cd(2)-O(8)

2.222(2) 2.306(2) 2.346(2) 2.362(2) 2.414(2) 2.265(2) 2.349(2) 2.365(1) 2.281(2) 2.296(1)

Cd(1)-O(1) Cd(1)-O(1)#2 Cd(1)-O(5) Cd(1)-N(1) Cd(1)-N(7) Cd(1)-N(9)#1 Cd(2)-O(2) Cd(2)-O(2)#4 Cd(2)-O(5)#3 Cd(2)-O(5)#2 Cd(2)-N(2)

2.406(4) 2.471(4) 2.293(4) 2.270(5) 2.279(4) 2.359(4) 2.283(4) 2.283(4) 2.266(4) 2.266(4) 2.312(5)

Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(5) Cd(1)-O(6) Cd(1)-N(1) Cd(1)-N(4)#1 Cd(2)-O(3) Cd(2)-O(5)

2.268(4) 2.294(4) 2.269(4) 2.251(5) 2.423(15) 2.494(15) 2.283(11) 2.318(11)

bond

distance (A˚)

I Cd(2)-O(8) Cd(2)-N(1) S(1)-O(1) S(1)-O(4) S(1)-O(7) S(1)-O(8)#3 S(2)-O(3)#4 S(2)-O(6) S(2)-O(9) S(2)-O(10)

2.362(3) 2.326(4) 1.471(3) 1.466(3) 1.464(3) 1.485(3) 1.490(3) 1.480(4) 1.441(4) 1.448(4)

Cd(1)-N(2)#1 S(1)-O(1) S(1)-O(2) S(1)-O(3) S(1)-O(4)

2.239(3) 1.460(3) 1.485(2) 1.469(2) 1.477(2)

Cd(2)-O(5) Cd(2)-O(5)#3 Cd(2)-N(2) Cd(2)-N(2)#3 S(1)-O(1) S(1)-O(2) S(1)-O(3) S(1)-O(4)

2.342(3) 2.342(3) 2.246(2) 2.246(2) 1.500(2) 1.468(2) 1.436(2) 1.479(2)

Cd(2)-O(8)#2 Cd(2)-N(2)#1 S(1)-O(1) S(1)-O(2)#3 S(1)-O(5)#5 S(1)-O(6)#4 S(2)-O(3) S(2)-O(7) S(2)-O(8) S(2)-O(4)#6

2.332(1) 2.199(2) 1.471(2) 1.458(2) 1.490(2) 1.476(2) 1.472(2) 1.446(2) 1.488(1) 1.492(2)

Cd(2)-N(2)#4 Cd(3)-O(3)#5 Cd(3)-O(4) Cd(3)-O(5)#2 Cd(3)-N(3) Cd(3)-N(6) Cd(3)-N(8) S(1)-O(1) S(1)-O(2) S(1)-O(3) S(1)-O(4)

2.312(5) 2.254(4) 2.284(4) 2.291(4) 2.377(5) 2.350(4) 2.558(4) 1.480(4) 1.463(4) 1.478(4) 1.466(4)

Cd(2)-O(4)#4 Cd(2)-O(5)#2 Cd(2)-N(2) Cd(2)-N(3)#3 S(1)-O(1) S(1)-O(2)#5 S(1)-O(3)#6 S(1)-O(4)

2.347(10) 2.205(11) 2.320(13) 2.315(13) 1.469(4) 1.471(5) 1.498(10) 1.440(11)

II

III

IV

to explore surface adsorption, if any. In a typical experiment, 200 mg of the cadmium sulfate compound was added to 100 mL of the dye solutions (conc. 100 ppm) and stirred for 90 min. The supernatant liquid samples were collected at different time intervals (0, 15, 30, 45, 60, 90 min, respectively), filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis. The concentration of the dye solution was determined using UV-vis spectroscopic investigations. The details of the experimental set up for the photocatalytic experiments have been described earlier.14 The photocatalytic degradation studies were carried out at 25 °C. The organic dyes were dissolved in double distilled Millipore filtered water. The dye solutions were collected at regular time intervals and analyzed, after filtration and centrifuging the solutions, using a UV-vis spectroscopic method. Heterogeneous Catalytic Studies. The reagents for the heterogeneous cyanosilylation experiments, N-benzilidine aniline and trimethylsilyl cyanide (Aldrich), were used as received. The reagents were taken in a 100 mL round-bottom flask with the DCM solvent. After the addition of the solid catalyst cadmium sulfate, the mixture was stirred for 2 h at 0 °C in N2 atmosphere. The product was filtered through Millipore membrane filters to remove the catalyst particles. The supernatant liquid was dried under a vacuum to remove the solvent. The product was analyzed and evaluated for the conversion of the reactants. Results

V

VI

a Symmetry transformations used to generate equivalent atoms. I: #1 -x, -y þ 1, -z þ 1; #2 -x þ 1, -y þ 2, -z þ 1; #3 x - 1, y, z; #4 -x þ 1, -y þ 1, -z þ 1. II: #1 x, -y - 1/2, z - 1/2; #2 -x þ 1, -y, -z þ 1; #3 -x þ 2, -y, -z þ 1. III: #1 -x þ 1, y - 1/2, -z þ 1/2; #2 x, y - 1, z; #3 -x, -y þ 1, -z. IV: #1 -x þ 2, -y þ 1, -z þ 2; #2 -x þ 3, -y þ 1, -z þ 1; #3 -x þ 1, -y þ 2, -z þ 1; #4 x - 1, y þ 1, z; #5 -x þ 2, -y þ 2, -z þ 1; #6 -x þ 2, -y þ 1, -z þ 1. V: #1 x, -y þ 3/2, z þ 1/2; #2 -x þ 1, -y þ 2, -z þ 1; #3 x þ 1, y, z; #4 -x þ 2, -y þ 2, -z þ 1; #5 x, -y þ 3/2, z - 1/2. VI: #1 x, y, z þ 1; #2 -x þ 1, -y þ 1, z - 1/2; #3 -x þ 1, -y þ 1, z þ 1/2; #4 x - 1/2, -y þ 1/2, z; #5 x þ 1/2, -y þ 1/2, z; #6 -x þ 3/2, y - 1/2, z þ 1/2.

Structure of [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I. The asymmetric unit of I contains 32 non-hydrogen atoms. There are two crystallographically independent Cd2þ ions and sulfate units present in the asymmetric unit. The Cd(1) is coordinated by four oxygen atom of the sulfate units and two terminal water molecules forming a distorted octahedral coordination (CdO4(H2O)2, CN = 6). The Cd(2) is coordinated by four oxygen atom of the sulfate units, one terminal water molecule, and one nitrogen atom of the melamine unit forming a distorted octahedral coordination (CdO4N(H2O), CN = 6). The average Cd-O distance is 2.301 A˚. The cadmium centers are connected with the sulfate units through a Cd-O-S bond with an average angle of 136.2°. The sulfate units have an average S-O bond distance of 1.468 A˚ and an average O-S-O angle of 109.4°. The various geometric parameters observed in I are comparable to other similar cadmium sulfate structures reported in the literature.12,13 Selected bond distances are listed in Table 3. The octahedral Cd and the tetrahedral sulfate units are connected together to form a two-dimensional cadmiumsulfate layer structure (Figure 1a). Each cadmium ion is bonded with four sulfate units. Of the two sulfate units, S(1)O4 unit binds with four cadmium ions through four oxygen atoms in a η4, μ4-coordination mode, whereas the S(2)O4 unit binds with only two cadmium ions through the two oxygen atoms in a η2, μ2-coordination mode (see Supporting Information, Figure S5). This connectivity between the Cd2þ ions and the sulfate anions gives rise to an anionic layer structure with four- and eight-membered rings. The layers are arranged in a ABAB... fashion. The axial positions of the Cd(1) octahedra have the water molecules, while a melamine and a water molecule occupy a similar position for Cd(2). The melamine unit is terminal and hangs in the interlayer spaces.

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Figure 1. (a) The two-dimensional cadmium sulfate layer observed in the structure [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I. Note the difference in the connectivity of the sulfate units. The hydrogen atoms of the water molecules are not shown. (b) View of the arrangement of the cadmium-sulfate layers in I. Note that the free and the bound melamine molecules are located in the interlamellar spaces.

The interlayer spaces also have free protonated melamine units, which act as the charge-balancing cations. Thus, the melamine units in I perform a dual role of a ligand as well as the charge-balancing cations (Figure 1b). A similar role for aliphatic amines has been observed earlier.21 The close proximity between the cations and anions gives rise to a number of N-H 3 3 3 N and N-H 3 3 3 O type hydrogen bonds (see Supporting Information, Table S7). Structure of [Cd(C5H5N5)(SO4)], II. The asymmetric unit of II contains 16 non-hydrogen atoms, of which one Cd atom and a sulfate unit is crystallographically independent. The Cd2þ ion is octahedrally coordinated by four oxygen atoms of the sulfate units and two nitrogen atoms of the adenine molecules to form a distorted octahedral environment (CdO4N2, CN = 6). The average Cd-O bond distance is 2.431 A˚. The Cd centers are connected through Cd-O-S bonds (ave: 107.1°) with the sulfate units and through Cd-N-C bonds (ave: 127.8°) with the adenine molecules. The average S-O bond distance in the sulfate unit is 1.472 A˚ (Table 3). In II, the octahedral Cd2þ ions are bonded with the three different sulfate units, and each sulfate unit is connected with

Figure 2. (a) View of the one-dimensional cadmium-sulfate ladder in [Cd(C5H5N5)(SO4)], II. (b) View of the two-dimensional layer formed by the connectivity between the cadmium-sulfate ladder and the adenine unit. (c) View of the three-dimensional structure in the “bc” plane.

the three Cd2þ ions. The connectivity of the sulfate units can be considered to be a η4, μ3-coordination mode (see Supporting Information, Figure S6a). The CdO4N2 and SO4 units are connected to form one-dimensional ladder-like chains (Figure 2a). The adenine unit connects the one-dimensional ladders through the nitrogen atoms forming the two-dimensional structure (Figure 2b). The layers are further connected through another nitrogen atom of the adenine ligand to form the three-dimensional structure (Figure 2c). The connectivity between the adenine ligand and the cadmium centers forms a

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Figure 3. (a) View of the two-dimensional Cd-O-Cd layer observed in [Cd3(H2O)2(C2H2N3)2(SO4)2], III. (b) The two-dimensional cadmiumsulfate layer observed in III. The 21 axis is highlighted (cyan colored bonds). (c) The cadmium triazolate corrugated layer observed in III. (d) View of the connectivity between the cadmium sulfate layers and the cadmium triazolate layers.

one-dimensional wirelike structure (see Supporting Information, Figure S6b). Thus, the three-dimensional structure of II is formed by the cross-coupling between the cadmiumsulfate ladders and the cadmium-adenine wires. The free amino group of the adenine unit participates in N-H 3 3 3 O hydrogen bonding interactions (see Supporting Information, Table S7). Structure of [Cd2(H2O)(C2H2N3)(SO4)], III. The asymmetric unit of III contains 13 non-hydrogen atoms. There are two crystallographically independent Cd2þ ions and one sulfate unit present in the asymmetric unit. Of the two cadmium atoms, Cd(2) occupies a special position (2c) with a site multiplicity of 0.5. Both the Cd atoms have octahedral coordination. While the Cd(1) is coordinated by the four oxygen atoms from three sulfate units and two nitrogen atoms from two different triazolate units, the Cd(2) is coordinated by two oxygen atoms from sulfate unit, two nitrogen from two different triazolate units, and two terminal water molecules. The average Cd-O and Cd-N bond distances are 2.334 and 2.210 A˚, respectively. The oxygen atoms, O(1) and O(4) were found to have μ3-connectivity, binding with two Cd and a sulfur. The Cd centers are bonded with the sulfate units through Cd-O-S linkages (ave:

125.4°) and with the triazole units through Cd-N-C/N linkages (ave: 126.6°). The sulfate unit has an average S-O bond distance of 1.470 A˚ (Table 3). The three-dimensional structure of III can be considered to be a cross-linking between two independent layered structures, viz., the cadmium sulfate and the cadmium triazolate layers. The sulfate unit binds with four cadmium ions through a η3, μ4-coordination mode (see Supporting Information, Figure S7). The cadmium centers are connected through the μ3-oxygen, [O(1)], of the sulfate unit, giving rise to infinite Cd-O-Cd one-dimensional (1D) chains. The Cd-O-Cd chains are arranged along the 21 screw axis and are also connected with the Cd(2) centers through another μ3-oxygen, [O(4)], forming a two-dimensional Cd-O-Cd layer with apertures bound by 8-T (T = Cd) atoms (Figure 3a). The sulfate units are bonded with the Cd-O-Cd layers in such a way that it forms a cadmium sulfate layers with three- and six-membered apertures (Figure 3b). The formation of Cd-O-Cd layers in this structure is unique and noteworthy. The triazolate units are monoanionic and are connected to three cadmium centers forming a corrugated cadmium-triazolate layer (Figure 3c). The cadmium sulfate layers and the cadmium triazolate layers

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are interconnected forming the three-dimensional structure (Figure 3d). O-H 3 3 3 O type hydrogen bond interactions have been observed due to the interaction between the coordinated water molecules and the oxygen atom of the sulfate units (see Supporting Information, Table S7). Structure of [Cd2(C2H5N5)(SO4)2], IV. The asymmetric unit of IV contains 19 non-hydrogen atoms, of which two cadmium and sulfate units are crystallographically independent. Both the cadmium ions are octahedrally coordinated by five oxygen atoms from the sulfate units and one nitrogen atom of the guanazole unit. The average Cd-O and Cd-N bond distances are 2.327 and 2.232 A˚, respectively. The Cd centers are connected with the sulfate units through CdO-S linkages (ave: 128.4°) and with the guanazole unit through Cd-N-C/N linkages (ave: 127.6°). The sulfate units have average S-O bond distances of 1.474 A˚ (Table 3). The oxygen atoms, O(4), O(5), and O(8), exhibit μ3-coordination connectivity connecting Cd(1) and Cd(2) centers. This connectivity gives rise to a tetrameric unit (see Supporting Information, Figure S8c). The rather complex three-dimensional structure of IV can be understood by considering simpler building units involving the connectivity between the cadmium and the sulfate as well as the cadmium and the guanazole units. In IV, each cadmium center is connected to five sulfate units and vice versa. The sulfate units exhibit differences in their bonding. Thus, S(1)O4 unit binds with five cadmium ions in a η4, μ5coordination mode, and S(2)O4 unit binds with a η3, μ5coordination mode (see Supporting Information, Figure S8). The connectivity between Cd2þ ions and (SO4)2- units forms the secondary building unit, SBU-4 (Figure 4a).22 The SBU4 units are separated and connected by the four-membered cadmium sulfate rings (Figure 4b). To the best of our knowledge, this structural feature has been observed for the first time in a cadmium sulfate structure. This arrangement gives rise to a one-dimensional columnar unit, which are connected together through the terminal oxygens forming a twodimensional cadmium sulfate layers (Figure 4c). Each cadmium center of the cadmium sulfate layer has one bonded nitrogen atom of the guanazole unit. Thus, the guanazole units link the two layers to form the three-dimensional structure, which can be considered to be a pillared layer arrangement (Figure 4d). The hydrogen atoms of the terminal amino groups from the guanazole unit participate in the N-H 3 3 3 O type hydrogen bond interactions (see Supporting Information, Table S7). Structure of [Cd5(OH)2(CH2N5)4(SO4)2], V. The asymmetric unit of V contains 21 non-hydrogen atoms, of which three Cd atoms and one sulfate unit are crystallographically independent. One of the cadmium atoms, Cd(2), occupies a special position (2c) with a site multiplicity of 0.5. All the three cadmium centers have octahedral coordination. Cd(1) and Cd(3) have a chemically similar environment formed by one μ3-OH unit, two oxygen atoms of the sulfate units and three nitrogen atoms of the 5-aminotetrazolate units. Cd(2) is coordinated with the two μ3-OH units, two oxygen atoms from the sulfate units, and two nitrogen atoms of the 5-aminotatrazolate units. The average Cd-O and Cd-N bond distances are 2.309 and 2.352 A˚, respectively. The oxygen atoms, O(1) and O(5), have μ3-connectivity. While O(1) binds to two Cd atoms [two Cd(1), symmetry generated] and one SO4 unit, O(5) binds three Cd centers [Cd(1), Cd(2), and Cd(3)]. In addition, BVS calculations20 (see Supporting Information, Table S8) indicate that O(5) oxygen is, in fact, a -OH


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Figure 4. (a) The SBU-4 unit observed in the structure [Cd2(C2H5N5)(SO4)2], IV. (b) Figure shows the connectivity between the SBU-4 units through the four-membered rings. (c) The two-dimensional cadmium sulfate layer observed in the structure IV. (d) Figure shows the connectivity between the cadmium sulfate layers and the guanazole units forming the three-dimensional structure in IV.

group. The presence of the three-coordinated oxygen atoms in V gives rise to one-dimensional Cd-O-Cd chains. The Cd atoms are connected to the sulfate units via Cd-O-S bonds

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(ave: 125.2°) and to the 5-aminotetrazole units through Cd-N-C/N bonds (ave: 125.1°). The sulfate units have an average S-O bond distance of 1.471 A˚ (Table 3). The structure of V is different from the other cadmium sulfate phases described here as it has a three-dimensionally extended cadmium sulfate structure in which cadmium tetrazolate layers are also present. The cadmium sulfate framework can be understood by considering simple building units. Thus, the Cd-O-Cd chains, formed by the μ3-OH groups, are connected by the sulfate units to form a layer (see Supporting Information, Figure S9). The layers are crosslinked through the sulfate oxygens giving rise to the threedimensional cadmium sulfate structure (Figure 5a). Considering the connectivity of the sulfate units, we observed that each sulfate unit binds five cadmium ions through a η4, μ5coordination mode (see Supporting Information, Figure S9a). The structure of V contains two different monoanionic tetrazolate units, of which one is coordinated to three Cd centers while the other binds with four Cd centers. The connectivity between the cadmium octahedra and the tetrazolate units gives rise to a two-dimensional cadmium-tetrazolate layer (Figure 5b). The cadmium-tetrazolate layer and the three-dimensional cadmium sulfate units interpenetrate to form the overall, rather complex, three-dimensional cadmium sulfate structure (Figure 5c). We have observed N-H 3 3 3 O and N-H 3 3 3 N type hydrogen bond interactions in V (see Supporting Information, Table S7). Structure of [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI. The asymmetric unit of VI contains 17 non-hydrogen atoms, of which two Cd2þ ions and one sulfate unit are crystallographically independent. Both the cadmium centers have octahedral coordination. Cd(1) is coordinated to one μ3-OH unit, one terminal water molecule, two oxygen atoms of the sulfate units, and two nitrogen atoms of the 5-methyltetrazolate units. Cd(2) is coordinated to the two μ3-OH units, two oxygen atoms from the sulfate units, and two nitrogen atoms of the 5-methyltatrazolate units. One of the oxygen atoms, O(5), connects to three different cadmium centers and also is found to be a μ3-OH group. The presence of the μ3-OH units gives rise to one-dimensional Cd-O-Cd chains. The average Cd-O and Cd-N bond distances are 2.279 and 2.388 A˚, respectively. The presence of a μ3-OH group in VI was also confirmed by the bond valence sum calculations (see Supporting Information, Table S8).20 The Cd atoms are connected to the sulfate units via Cd-O-S bonds (ave: 128.2°) and to the 5-methyltetrazole units through CdN-C/N bonds (ave: 126.3°). The sulfate unit has an average S-O bond distance of 1.469 A˚ (Table 3). Similar to V, the structure of VI also has a three-dimensional cadmium sulfate structure and the cadmium tetrazolates have formed a simple one-dimensional ladder structure instead of the cadmium tetrazolate layers observed in V. All the sulfate units have a similar connectivity of binding with four cadmium centers through a η4, μ4-coordination mode (see Supporting Information, Figure S10a). The threedimensional structure of VI can be understood by considering simpler building units. Thus, the connectivity involving the μ3-OH unit forms a one-dimensional Cd-O-Cd chain (see Supporting Information, Figure S10b). The sulfate tetrahedra bonds with the Cd-O-Cd chains to form a cadmium sulfate layer, which are connected through the oxygen atoms to form the three-dimensional cadmium sulfate structure (Figure 6a). A closer examination of the cadmium sulfate structure reveals the presence of a well-established building

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Figure 5. (a) View of the three-dimensional cadmium-sulfate structure observed in [Cd5(OH)2(CH2N5)4(SO4)2], V in the “ac” plane. (b) Figure shows the two-dimensional layer formed by the connectivity between the 5-aminotetrazolate units and the Cd2þ ions. Trz3 and Trz4 imply the 3-connected and 4-connected aminotetrazolate units (see text). (c) The three-dimensional structure of V in the “bc” plane.

unit, Spiro-5, known in traditional tetrahedral frameworks such as the alumino-silicate zeolites.23 The Spiro-5 unit is formed by the connectivity involving Cd(2)O4N2, Cd(1)O4N2

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dimensional structure (Figure 6b). The one-dimensional cadmium tetrazolate units viewed individually exhibit a herringbone arrangement, which are linked by the sulfate tetrahedra (see Supporting Information, Figure S10f). The cross coupling between the cadmium tetrazolate one-dimensional units and the cadmium sulfate structures gives rise to the framework of VI (Figure 6c). Discussion

Figure 6. (a) The three-dimensional cadmium-sulfate structure observed in [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI. Note the formation of the Spiro-5 units (see text). (b) Figure shows the cadmiumtetrazolate structure observed in VI. Note that the μ3-OH connects the Cd centers forming Cd-O-Cd chains. (c) The three-dimensional structure of VI in the “bc” plane.

and S(1)O4 tetrahedral units (see Supporting Information, Figure S10e). The spiro-5 units are connected together through the sulfate units forming the cadmium sulfate structure. This is indeed a unique structure as such building units are not common in compounds having both the octahedral as well as the tetrahedral units. To the best of our knowledge, this is the first observation of spiro-5 units in a cadmium sulfate phase. The 5-methyltetrazole units connect with four different cadmium centers giving rise to a one-

Six different cadmium sulfate phases with two- (I) and threedimensional (II-VI) structures have been prepared under mild solvothermal conditions in the presence of water. All the compounds were prepared using cadmium sulfate as the starting material (see Supporting Information, Figure S11), which acts as the source for both the cadmium as well as the sulfate species. It is also possible to prepare the present compounds using other salts of cadmium along with a source for sulfate - such as sulphuric acid. We, however, observed that good quality single crystals were formed only when cadmium sulfate was used as the combined source for cadmium and sulfate. In all the cases, the heterocyclic amine molecules were found to behave as a ligand, but the connectivity exhibits differences among the structures. During the preparations, we consistently observed an increase in the pH value. This suggests that the amine molecule was not fully utilized in the formation of the final solid product, whereas more cadmium sulfate would have been consumed. There appears to be no particular correlation between the starting composition and the final product, which is typical of solvothermal preparations.24 Structurally, all the compounds are unique, being formed by the connectivity between the octahedral Cd centers, tetrahedral sulfate, and the heterocylic ligands. A detailed analysis of the structures indicates that in most of the compounds the fundamental building unit, four-membered ring, is present. In addition, the observation of two-dimensional Cd-O-Cd layers in III and Cd-O-Cd chains in V and VI is noteworthy. Formation of a metal-oxide layer is not common and only a few reports are known.25 In VI, the 5-methyltetrazole ligand was formed in situ, generated from the starting materials, azide anions and acetonitrile. The possible mechanism of the formation of 5-methyltetrazole is given in Scheme 1.26 As can be noted, the reaction proceeds via a [2 þ 3] dipolar cycloaddition reaction in the presence of the Lewis acid, (cadmium center). Similar in situ formation of 5-methyltetrazole has been observed during the preparation of other cadmium sulfate phases.13c The observation of SBU-4 units (IV) and spiro-5 units (VI) indicates a rich diversity in the present cadmium sulfate structures. From the various structures observed in the present study, one can visualize and simplify the formation of the different frameworks schematically (Scheme 2). The present compounds also exhibit another feature wherein the cadmium sulfate itself forms one- (II), two- (I, III, and IV), and three-dimensionally (V and VI) extended structures. The heterocyclic ligand molecules bind with the cadmium and give rise to the observed higher dimensional structures except in I, where the amine molecules hang in the interlayer spaces (Table 4). Increasing the dimensionality of a particular phase using heterocyclic as well as aliphatic amines has been shown elegantly earlier.6,7,27 The situation in the present compounds, however, is different as the metal-amine connectivity also gives rise to a one- (II, IV, VI) and two- (III, V) dimensional

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arrangement. This type of interpretation of two different sublattices has been observed for the first time in sulfate-based open structures. A careful study of the present structures reveals that the structural features observed in the present compounds may be compared with other related reported structures. The inorganic cadmium sulfate ladder, observed in II, has a close Scheme 1. Schematic of the Possible Pathway for the Formation of 5-Methyltetrazole Ligand in [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI. (i) a Two-Step Mechanism (ii) a Concerted Mechanism

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resemblance to many similar structures reported in the literature.3a,b In the iron-sulfate structure, [C2N2H10][FeF3(SO4)],28 the FeF3O3 octahedra share a vertex with triply bridging SO4 tetrahedra to form a one-dimensional ladder structure (Figure 7a). In the present compound II, we observed a similar ladder, but the connectivity of the SO4 tetrahedra is different (Figure 7b). In compound III, we have the unusual formation of Cd-O-Cd layers. If we consider the sulfate connectivity also, one can simplify the structure into a Table 4. The Observed Coordination Behavior of the Heterocyclic Rings in the Cadmium Sulfate Compounds, I-VI compound [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I [Cd(C5H5N5)(SO4)], II [Cd3(H2O)2(C2H2N3)2(SO4)2], III [Cd2(C2H5N5)(SO4)2], IV [Cd5(OH)2(CH2N5)4(SO4)2], V [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI

amine

no. of ring nitrogen Cd2þ/Na

melamine

3

1/3 þ 0

adenine 1,2,4-triazole

4 3

2/4 3/3

guanazole 5-aminotetrazole

3 4

2/3 3/4 þ 4/4

5-methyltetrazole

4

4/4

a The ratio is calculated by the number of ring nitrogen atoms attached to the metal centers; for example, in structure I, one melamine unit (3 ring nitrogens) is coordinated to the one metal center (i.e., 1/3) and one unit is free from coordination (i.e., 0).

Scheme 2. Schematic of the Structures I-VI Depicting the Connectivity of the Organic Ligand Part with the Inorganic Cadmium Sulfate Parta

a

Two different colors used for the cadmium sulfate framework and the organic ligand part.

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Figure 7. (a) View of the one-dimensional ladder observed in the structure of [C2N2H10][FeF3(SO4)]. (b) View of the one-dimensional cadmium sulfate ladder observed in the structure of [Cd(C5H5N5)(SO4)], II.

Figure 9. (a) The T-atom connectivity observed in K4[Cd3(HPO4)4(H2PO4)2]. Note that the layers are formed by the Spiro-5 units (see text). (b) The Spiro-5 units observed in the three-dimensional cadmium sulfate structure [Cd2(OH)(H2O)(C2H3N4)(SO4)] 3 2H2O, VI. Note the difference between the Spiro-5 units.

Figure 8. (a) Figure shows the connectivity between the sulfate and cadmium nodes in the cadmium sulfate layer of [Cd3(H2O)2(C2H2N3)2(SO4)2], III. Note the formation of edge-shared fourmembered rings. (b) Figure shows the cadmium sulfate layer observed in [Cd(Htrz)(SO4)(H2O)]. Note the close resemblance between the structures. Also note the formation of corner-shared four-membered rings.

one-dimensional ladder connected by Cd centers. The nodal representation between the Cd centers and the sulfate units gives rise to a layer with four- and eight-membered apertures (Figure 8a). This arrangement has close resemblance to the cadmium sulfate structure, [Cd(Htrz)(SO4)(H2O)].29 The four-membered rings in the reported structure, [Cd(Htrz)(SO4)(H2O)],29 share the corners forming different chain units, which are connected by the Cd centers forming a similar arrangement (Figure 8b). Though the layers have a close similarity between the two structures, the connectivity involving the triazole ligand between the structures is different. In [Cd(Htrz)(SO4)(H2O)],29 the triazole ligand is neutral, whereas in III the triazole units are anionic. The Spiro-5 unit observed in VI can be compared to the Spiro-5 unit present in the twodimensional cadmium phosphate structure, K4[Cd3(HPO4)4(H2PO4)2].30 Both the compounds have similar Spiro-5 building units connected through the terminal MO4 (M = S, P) units (Figure 9). In VI, the connectivity between the Spiro-5 units forms a three-dimensional structure, whereas it has only a two-dimensional structure in K4[Cd3(HPO4)4(H2PO4)2].30 Adsorption-Desorption Studies. Preliminary studies on the adsorption/desorption behavior using organic dyes were carried out on a few cadmium sulfate compounds prepared in the present study. For this, we took both the anionic (e.g.,

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Figure 10. (a) Adsorption profile of Congo red (CR) by compounds I-VI (2 g/L in each case). (b) Adsorption profile of Coomassie Brilliant Blue R-250 (CBB) by compounds III, IV, and VI (2 g/L in each case). (c) Adsorption profile of Alizarin red S (ARS) by compounds III, IV, and VI (2 g/L in each case).

Congo Red, Coomassie Brilliant Blue R-250 and Alizarin Red S) as well as the cationic (e.g., Methylene Blue and Methyl Violet) dyes. The dyes were taken in aqueous solution and the studies were performed in the dark in the presence of compounds III, IV, and VI. In all the cases, we observed that the adsorption of the cationic dyes were negligible compared to the anionic dyes. The initial calibration for the anionic dyes, Congo Red (CR), Coomassie Brilliant Blue R-250 (CBB), and Alizarin Red S (ARS), was based on the Beer-Lambert law at its maximum absorption wavelength, λmax of 493, 555, and 428 nm, respectively. The UV spectrum of the dye solution exhibited a continuous decrease in the peak intensity, which corresponded to the decrease in the concentration of the dye in the solution. We

have not observed any additional peaks in the UV-vis spectra, which suggests that there are no detectable intermediates formed during the adsorption of the organic dyes. The adsorption studies indicated that the Congo red (CR) adsorbs more efficiently compared to the other two dyes. We have observed a similar behavior in cadmium thiosulfate compounds recently.14 The studies also indicated that the compounds III and VI exhibit reasonable adsorption of the CR dye (Figure 10a). The adsorption of the CR dye resulted in a reduction of the concentration from 100 to 5 ppm in 1 h by the compounds III and VI, while it decreased from 100 to 15 ppm in the case of IV. Since compounds III and VI exhibited reasonable adsorption behavior, we investigated the adsorption studies with other related anionic dyes, CBB

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and ARS. The studies reveal that the rate of adsorption of CBB is slower than that for CR and ARS. The concentration of CBB is reduced from 100 ppm to 30 ppm after 1 h, while the concentration of ARS is reduced from 100 to 12 ppm after 1 h (Figure 10). The present studies show that the rate of adsorption follows the order CR > ARS > CBB. The dyes are physically adsorbed on the compounds, and such physisorption of the anionic dyes on the cadmium centers could depend on many factors. For example, the physisorption depends on the contact time between the dye and the adsorbent and the availability of a free-site on the surface of the solid adsorbent. It is known that the cadmium can possess coordination numbers that could be more than six.31 Thus, the approaching anionic dye could, then, loosely bind with the cadmium centers on the surface of the compounds. The compounds III and VI, which exhibit reasonable physisorption, have water molecules coordinated with the cadmium centers. The coordinated water molecules would make spaces for the adsorption of the anionic dye molecule at the surface. This mechanistic view could be plausible as physisorption of dye molecules is a surface phenomenon and depends on the type and nature of the medium employed for the adsorption studies. If this assumption is true, then it may be possible to desorp the adsorbed dye molecules by modifying the experimental conditions. Thus, desorption studies were carried out using an alcoholic medium. Studies of this nature are required to understand the adsorption/desorption behavior as well as to examine the reusability of the same solid compound repeatedly. The desorption of the dyes was studied by treating the dyeadsorbed compounds with ethanol. For the desorption experiments, 100 ppm of the dyes (initial concentration) were taken with compound III and stirred for 2 h. The contact time between the dye and the solid phase was enhanced to 2 h, as the adsorption of the dye molecules reaches a constant value at ∼90 min of exposure (Figure 10a). The increase in time of exposure is to ascertain that the dye molecules have adsorbed and equilibrated fully on the cadmium sulfate compounds. After the adsorption was completed, the solutions were filtered and the dye adsorbed solids were recovered and dried at ambient conditions. Then, the compound was taken in ethanol for the desorption experiments. When the dye-adsorbed compounds were stirred in ethanol for 2 h at room temperature, the desorption of the dye molecules were observed (Figure 11). The desorption behavior was also investigated similar to the adsorption studies using the UV-vis spectroscopy. Similar desorption studies were also carried out for compounds IV and VI. The desorption studies suggested that the CR desorbs only 63% (total adsorbed = 95%), while ARS desorbs 80% (total adsorbed = 88%) and CBB desorbs 81% (total adsorbed = 84%) after 2 h of stirring in ethanol (Figure 11). Compound III, after the adsorption/desorption, were also examined using PXRD. No change in the structure was observed indicating that the prepared compounds are stable under experimental conditions (see Supporting Information, Figure S12). The changes in the color of the solutions during the adsorption and the desorption can also be observed visually (see Supporting Information, Figure S13). Photocatalytic Studies. The adsorption studies indicated that the sulfonated dyes (anionic) exhibited a reasonable physisorption behavior. It occurred to us that the nonsulfonated dyes (cationic) could undergo photocatalytic decomposition under the exposure of the UV radiation.


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Figure 11. Desorption profile of (i) CR, (ii) ARS, and (iii) CBB by compound III in ethanol (2 g/L in each case).

For this, we employed compound IV as it exhibited an additional band in the UV spectra (see Supporting Information, Figure S3), which corresponds to 3.26 eV. The 385 nm (3.26 eV) band could be due to the charge transfer from guanazole ligand to the cadmium center (LMCT) in the solid phase. Since this band is near the UV region, we explored the photodecomposition of the organic dyes. Two cationic dyes, viz., methylene blue (MB) and methyl violet (MV), were investigated in the presence of UV light. The photocatalytic studies indicated that the concentration of MB was reduced from 100 to 42 ppm, while the concentration of MV was reduced from 100 to 50 ppm after 90 min (Figure 12). The control experiment on the pure dye solutions (without the catalyst) indicates that the degradation of the dyes under UV radiation is negligible. The cadmium sulfate compound was examined after catalytic studies using PXRD, the unchanged PXRD pattern suggests that the compound is stable and can be used repetitively (see Supporting Information, Figure S14). The degradation studies were also compared with Degussa P-25 (TiO2) catalyst, which exhibits a complete degradation of the dyes (100 to 5 ppm) within 1 h. The present study indicates that the cadmium sulfate (IV) compounds exhibit marginal photocatalytic activity when exposed to the UV-light. The photocatalytic activity of the compound IV can be explained based on our earlier observation of cadmium thiosulfates.14 It is likely that the additional band at 385 nm (3.26 eV) observed in our UV spectra could be responsible for the observed photocatalytic behavior. The other cadmium sulfate phases in the present study did not exhibit any appreciable photocatalytic activity as there are no such bands in the UV-region, which can be used for UV-assisted photocatalytic purposes. From the molecular orbital point of view, the HOMO has filled d-orbital (d10 in Cd2þ) and the

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in CH2Cl2 solution (10 mL) of imine (A, 0.09 g, 0.5 mmol). Trimethylsilyl cyanide (0.075 g, 0.75 mmol) was added at 0 °C, and the reaction mixture was stirred for 2 h. Similar molar concentrations have been employed for other cadmium sulfate phases. The product, aminonitrile (B), was isolated and analyzed by 1H NMR spectra. From the catalytic studies, we observed that compounds III, V, and VI exhibited a near quantitative conversion of the imine to the aminonitrile (∼98%). We observed poorer conversion (∼50%) for the other compounds (see Supporting Information, Table S9). The control experiments in the absence of cadmium sulfate compounds have also been carried out to ensure that the reaction proceeds catalytically. We have also carried out the reaction in the presence of Cd(NO3)2 3 4H2O, which gave a conversion of ∼20%. The present study suggests that the sulfate compounds are active for the heterogeneous catalytic cyanosilylation of the imines. It is likely that the presence of the Lewis acidity may be responsible for the observed catalytic behavior. Similar catalytic conversion has also been made earlier, though the reaction times were shorter.33

Figure 12. Degradation profile of methyl violet (MV) and methylene blue (MB) under UV illumination. (a) and (b) without the catalyst, (c) and (d) with the catalyst (2 g/L of IV in each case).

LUMO (free s-orbital) is vacant in IV. In the presence of UV light there would be an electron transfer from the HOMO to the LUMO. The transferred electron of the LUMO can be easily lost from the excited state, whereas the HOMO would require the electron to return to its original state. Thus, the excited Cd2þ center would decay to the ground state quickly. During the decay process, if any organic molecule is present within a reasonable energy range and appropriate orientation, then it can form an activated complex, which could be transient. From the structure of the organic dye, one can state that the electron-withdrawing group attached to the carbon center can receive the electron. During this electron transfer process, the hydrogen atom of the methyl group in MB can sacrifice its electron and leave as Hþ species. The electron could be abstracted by the excited Cd2þ, which results in the cleavage of the C-N bond in a stepwise manner to the total degradation of MB. The other dye, MV, has similar groups which offer the cleavage as C-O, N-N, and C-N bonds in the structure. Similar photoassisted decomposition mechanisms have been proposed earlier.32 Heterogeneous Catalytic Studies. Heterogeneous catalysis using open-framework compounds are being investigated continuously over the years.1a Catalytic studies on compounds with metal ion having filled d10 shell have not been examined, though the open metal sites could provide the Lewis acidic sites. Recent studies on cadmium thiosulfate framework compounds showed that the Lewis acidity arising from coordinatively unsaturated cadmium centers can be exploited fruitfully in the cyanosilylation of imines.8b It occurred to us that it may be feasible to carry out a similar study on these cadmium sulfate phases. The heterogeneous catalytic studies were performed on all the compounds prepared in the present study. In a typical experiment, powdered catalyst (0.07 g of III, 0.1 mmol) was suspended

The cadmium sulfate compounds were examined after the catalytic studies using PXRD, which did not exhibit observable differences. This indicates that the cadmium sulfate phases are stable under the experimental conditions. We have also repeated the cyanosilylation studies on the used catalyst, which also indicated that the heterogeneous catalytic conversion of imines were comparable to the studies performed using the fresh catalyst. This study suggests that the cadmium sulfate phases are stable and can be used recyclably. Conclusions The synthesis, structure, and characterization of a family of sulfate phases have been accomplished. The polyazaheterocyclic organic molecules act as the ligand linking one- (II), two- (I, III, and IV), and three- (V and VI) dimensional cadmium sulfate units within the structures. The observation of Cd-O-Cd two-dimensional layers in compound III is unique and noteworthy. Some of the structures possess well-established secondary building units such as SBU-4 and Spiro-5. The adsorption studies indicate that the cadmium sulfates selectively adsorb/desorb anionic dyes, the behavior depending on the reaction medium. This suggests that the cadmium sulfates can be employed for the selective scavenging of the anionic dyes in a reaction mixture. The photocatalytic studies indicate a modest activity in the photodegradation of the cationic dyes under UV irradiation. The cyanosilylation heterogeneous catalytic studies suggest that the cadmium sulfate phases can be employed as an effective heterogeneous catalyst. The diversity of the structures along with the variety in the properties exhibited by the cadmium sulfate phases suggests that this area has considerable potential for further investigations.

Article


Acknowledgment. The authors thank Prof. B. R. Jagirdar, Department of Inorganic and Physical Chemistry, Indian Institute of Science, for help with the catalytic studies. The authors thank the Department of Science and Technology (DST), Government of India, for the award of a research grant. The Council of Scientific and Industrial Research (CSIR), Government of India, is thanked for the award of a research grant (S.N.) and a fellowship (A.K.P.). S.N. also thanks DST for the award of a RAMANNA Fellowship. Supporting Information Available: Selected bond angles in I-VI; hydrogen bond interactions in I-VI; selected bond distances and bond valance sums in I-VI; product obtained from the cyanosilylation reaction catalyzed by compounds I-VI; powder XRD (CuKR) patterns of compounds I-VI; IR spectra of I-VI; diffuse reflectance spectra of compounds I-VI; TGA studies of I-VI; various compound structures; powder XRD patterns of III and IV; dye solution images. This material is available free of charge via the Internet at http://pubs.acs.org.

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