Synthesis, Structure, Photochemical [2 + 2] Cycloaddition

ARTICLE pubs.acs.org/crystal

Synthesis, Structure, Photochemical [2 + 2] Cycloaddition, Transformation, and Photocatalytic Studies in a Family of Inorganic
Organic Hybrid Cadmium Thiosulfate Compounds Avijit Kumar Paul,† Rajendran Karthik,† and Srinivasan Natarajan*,†,‡ † ‡

Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Pohang 790-784, South Korea

bS Supporting Information ABSTRACT: Three new inorganic
organic hybrid cadmium thiosulfate compounds, [Cd(C12H11N2)2(H2O)(S2O3)2] (I), [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II), and [Cd(C12H10N2)S2O3] (III), have been prepared using bispyridylethene (bpe) as a coligand. The compounds have connectivity between the Cd centers, the bpe, and the thiosulfate units, giving rise to molecular (zero) (I), two (II), and three dimensionally (III) extended structures. The formation of Cd4S4 tetrameric cluster units in III is noteworthy. The two-dimensional compound, II, was found to be a reactive intermediate and transforms to the three-dimensional compound, III, as a function of reaction time. The [2 + 2] photochemical reactions carried out on all of the compounds suggest the facile formation of the cyclized products. The photodimerization studies resulted in a one-dimensional structure (Ia) from the molecular complexes of I and rigid cyclized products in II and III. The present study illustrates the usefulness of the photochemical method as a viable alternative for the postsynthetic structural modifications in inorganic
organic hybrid compounds. The synthesized compounds also exhibit reasonable activity as a photocatalyst. The present study provides the scope and optimism for employing the thiosulfate anion as an efficient network builder in the design of inorganic
organic hybrid compounds.

’ INTRODUCTION The widespread application of extended hybrid network structures in heterogeneous catalysis, separation, sorption, and ion-exchange processes have stimulated considerable research in recent years.1 The variety and diversity of the structures are due to the coordination preferences of the central metal ions and the binding nature of the organic ligands. Other than the carboxylic acids, the nitrogen-containing organic ligands have played a crucial role in the stabilization of important hybrid structures.2 Of the many ligands, 4,40 -bipyridine (bpy) appears to be important due to its ease in binding with the metal ions, especially transition metals.3 There have been attempts to replace bpy with other similar ligands such as stilbazoles, bispyridylethenes (bpe), etc. in the stabilization of comparable structures.4 Solid-state structural transformation reactions induced by light, heat, guest removal, expansion of coordination number, oxidation of metal centers, or reaction between the ligands are very attractive and one of the challenging topics in solid-state chemistry. Among the transformations, [2 + 2] photochemical cycloaddition of CdC bonds in many organic compounds and metal complexes has been investigated over the years.5,6 The research on the solid-state photochemical [2 + 2] cycloaddition has been investigated employing in situ methods by Thomas et al. during the early 1980s.7 In recent years, there is some interest in r 2011 American Chemical Society

the use of bpe as a ligand in the stabilization of hybrid structures.8 MacGillivray and Vittal have contributed in the study of bpebased compounds and their photochemical behavior.8,9 Many of these reactions have been performed in compounds possessing lower dimensional structures. The studies suggest the facile formation of the cyclobutane units in these compounds. According to Schmidt, the photoinduced cycloaddition reaction occurs between a pair of CdC bonds only when the bonds are parallel and the separation distance is less than 4.2 Å.10 The photodimerization reactions have been investigated in organic compounds for many years, and such studies are in the nascent state in coordination polymers, especially in higher-dimensional coordination framework compounds.9 Recently, we have prepared and characterized a family of cadmium thiosulfate phases wherein bpy was used as a linker between the Cd centers.11 We wanted to explore the effects of replacing bpy using bpe. This investigation was partly motivated by the possibility of studying the photodimerization of the olefinic CdC bonds, if they obey the Schmidt's criterion.10 The movement of the coordinated bpe ligand is, generally, restricted, as the ligand is bonded with the metal ions. It is Received: October 14, 2011 Published: October 18, 2011 5741

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Table 1. Crystal Data and Structural Refinement Parameters of [Cd(C12H11N2)2(H2O)(S2O3)2] (I), [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II), and [Cd(C12H10N2)S2O3] (III)a structural parameter

I

II

III

empirical formula

[Cd(C12H11N2)2(H2O)(S2O3)2]

[Cd2(C12H10N2)3(H2O)4(S2O3)2]

[Cd(C12H10N2)S2O3]

formula weight crystal system

721.10 monoclinic

1067.82 monoclinic

406.77 orthorhombic

space group

C2/c (no. 15)

P21/n (no. 14)

Pban (no. 50)

a (Å)

9.782(3)

9.698(1)

14.933(1)

b (Å)

12.685(4)

14.314(1)

21.284(1)

c (Å)

21.187(4)

14.917(1)

10.408(1)

α (°)

90.00

90.00

90.00

β (°)

89.42(2)

101.04(1)

90.00

γ (°) V (Å3)

90.00 2628.8(12)

90.00 2032.4(1)

90.00 3307.9(4)

Z

4

4

8

D (calcd/g cm
3)

1.817

1.745

1.633

μ (mm
1)

1.203

1.316

1.578

λ (Mo Kα/Å)

0.71073

0.71073

0.71073 1.96
25.99

θ range (°)

2.63
24.98

3.11
26.00

total data collected

5541

20509

23461

unique data Rint

2196 0.0589

3940 0.0270

3177 0.0505

R indexes [I > 2σ (I)]

R1 = 0.1019; wR2 = 0.2703

R1 = 0.0392; wR2 = 0.1046

R1 = 0.0548; wR2 = 0.1306

R indexes (all data)

R1 = 0.1498; wR2 = 0.3189

R1 = 0.0607; wR2 = 0.1123

R1 = 0.0752; wR2 = 0.1407

R1 = Σ||F0|
|Fc||/Σ|F0|; wR2 = {Σ[w(F02
Fc2)2]/Σ[w(F02)2]}1/2. w = 1/[σ2(F0)2 + (aP)2 + bP], P = [max.(F02,0) + 2(Fc)2]/3, where a = 0.1878 and b = 0.0000 for I, a = 0.0521 and b = 2.7557 for II, and a = 0.0691 and b = 5.7598 for III. a

intriguing to investigate not only the structures but also the possible photodimerization studies of the framework compounds containing bpe ligands. During the course of our studies, we have prepared three new cadmium thiosulfate phases, [Cd(C12H11N2)2(H2O)(S2O3)2] (I), [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II), and [Cd(C12H10N2)S2O3] (III), with zero- (I), two- (II), and three-dimensional (III) structures formed in the presence of bpe ligand. Compound III can be prepared either from the starting materials or through the transformation of II. More importantly, in all of the compounds, the CdC bonds are oriented to facilitate facile photodimerization under UV radiation. In this paper, the synthesis, structure, the [2 + 2] cycloaddition reactions, and the photocatalytic decomposition of organic dyes using the cadmium thiosulfate phases are presented.

’ SYNTHESIS AND INITIAL CHARACTERIZATION All of the compounds were prepared under mild conditions using solvent mixtures. The reagents, Cd(NO3)2 3 4H2O (Merck, 99%), (NH4)2S2O3 (Aldrich, 99%), and bpe (Aldrich, 99%), were used as received and without any further purifications. For the synthesis of [Cd(C12H11N2)2(H2O)(S2O3)2] (I), a mixture of Cd(NO3)2 3 4H2O (0.154 g, 0.5 mM) and (NH4)2S2O3 (0.148 g, 1 mM) was dissolved in 3 mL of dimethylacetamide (DMA). The bpe ligand (0.135 g, 0.75 mM) in 3 mL of solution of DMA was poured into the metal thiosulfate solution, and the mixture was kept for 3 days at room temperature (25 °C) resulting in light yellow, blocklike crystals of I (yield = 0.252 g, 70% based on Cd). For the synthesis of [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II), a mixture of Cd(NO3)2 3 4H2O (0.154 g, 0.5 mM) and (NH4)2S2O3 (0.148 g, 1 mM) was dissolved in 3 mL of water. A solution of bpe

(0.135 g, 0.75 mM) in 3 mL of EtOH was then layered carefully on top of the aqueous solution. The reaction mixture was allowed to evaporate slowly at room temperature (25 °C), resulting in light yellow, rectangular, platelike crystals of II after ∼72 h (yield = 0.240 g, 90% based on Cd). The same reaction mixture when heated at 60 °C forms II as a pure phase within 8 h. On further heating, the reaction mixture gave the product Cd(C12H10N2)S2O3] (III) as a pure phase in 72 h (yield = 0.162 g, 80% based on Cd). Elemental analyses were carried out using a Thermo Finnigan FLASH EA 1112 CHN analyzer. Elemental analysis calcd (%) for I: C, 39.93; H, 3.33; N, 7.76. Found: C, 39.82; H, 3.23; N, 7.65. Elemental analysis calcd (%) for II: C, 40.45; H, 3.56; N, 7.86. Found: C, 40.31; H, 3.43; N, 7.75. Elemental analysis calcd (%) for III: C, 35.40; H, 2.46; N, 6.88. Found: C, 35.31; H, 2.35; N, 6.77. The compounds were characterized by powder X-ray diffraction (PXRD), infrared (IR), UV
vis, and thermogravimetric analysis (TGA) studies. The PXRD patterns (Philips, X'pertPro) were recorded in the 2θ range of 5
50° by using Cu Kα radiation (λ = 1.5418 Å). The XRD patterns indicated that the products were new, and the observed XRD patterns were entirely consistent with the simulated XRD patterns generated based on the structures determined using the single-crystal XRD studies (see Figure S1 in the Supporting Information). 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 of the compounds were comparable, and five distinct regions can be identified (see Figure S2 in the Supporting Information). (i) Compounds I and II have bands at 3600
3300 cm
1 region, dominated by a broad band centered at 3425 cm
1, indicative of the presence of water molecules; (ii) a band at 3045 cm
1, which can be assigned to aromatic υ(CH) and υ(NH) modes of the bpe ligand; (iii) a band in the region 5742

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Crystal Growth & Design 1650
1200 cm
1, which can be assigned to the C
H bending vibrations and also due to the asymmetric and symmetric modes of the pyridine skeletal; (iv) 1150
1000 cm
1 region, with a sharp band at 1130 cm
1 is due to υs(S
O); and (v) the bands in the region of 805
535 cm
1 are due to the bending mode of S
O and υs(S
S) of the thiosulfate unit.

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Table 2. Important Bond Distances Observed in [Cd(C12H11N2)2(H2O)(S2O3)2] (I), [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II), and [Cd(C12H10N2)S2O3] (III)a bond

bond

distance (Å)

compound I

’ THERMAL STUDIES TGA studies (Mettler-Toledo TG850) of all of 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 Figure S4 in the Supporting Information). For compound I, the studies indicate a gradual weight loss of ∼68%, which starts at 120 °C and extends up to 500 °C, and corresponds to the loss of the coordinated water molecule and bpe units (calcd 66%). For compound II, an initial weight loss in the range 120
175 °C of ∼6% may be due to the loss of the lattice water molecules (calcd 6%). The second continuous weight loss of 54% in the temperature range 220
520 °C corresponds to the loss of bpe ligand (calcd 59%). For compound III, a gradual weight loss of 50% was observed up to 550 °C, which is due to the loss of bpe ligand (calcd 48%). In all of the cases, the calcined products were found to be poorly crystalline by PXRD, and the majority of the observed lines correspond to a mixture of CdO (ICDD no. 01-1049) and Cd3O2SO4 phases (ICDD no. 32-0140). ’ SINGLE-CRYSTAL STRUCTURE DETERMINATION 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 Kα (λ = 0.71073 Å) radiation. Data were collected with a ω scan width of 0.3°. A total of 606 frames were collected at three different settings of j (0, 90, and 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,12 and an empirical absorption correction was applied using the SADABS program.13 The structure was solved and refined using SHELXL9714 present in the WinGx suit of programs (Version 1.63.04a).15 The hydrogen position of the H2O molecule in the compound I could not be located. All of the other hydrogen positions of the bpe ligands for the compounds and the hydrogen position of the water molecules (II) 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 of the atoms, anisotropic thermal parameters for all of the nonhydrogen atoms, and isotropic thermal parameters for all of 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 of the structures are given in Table 1. The selected bond distances and angles for all of the compounds are given in the Supporting Information (Tables S1
S3). The crystallographic data for compounds I
III were deposited with the Cambridge Crystallographic Data Center (CCDC) and can be downloaded free of charge by quoting the numbers (CCDC no. 814950 for I, 814951 for II, and 814953 for III) via www.ccdc. cam.ac.uk/data_request/cif.

distance (Å)

Cd(1)
O(1)

2.238(14)

S(1)
S(2)

1.885(5)

Cd(1)
N(1) Cd(1)
N(1)#1

2.370(9) 2.370(9)

O(2)
S(2) O(3)
S(2)

1.414(9) 1.439(10)

Cd(1)
S(1)

2.639(4)

O(4)
S(2)

1.729(6)

Cd(1)
S(1)#1

2.639(4)

Cd(1)
O(1)

2.311(3)

Cd(2)
O(2)#1

2.386(3)

Cd(1)
O(1)#1

2.311(3)

Cd(2)
O(2)#3

2.386(3)

Cd(1)
N(1)

2.438(3)

Cd(2)
S(1)

2.661(1)

Cd(1)
N(1)#1

2.438(3)

Cd(2)
S(1)#2

2.661(1)

Cd(1)
S(1) Cd(1)
S(1)#1

2.616(1) 2.616(1)

S(1)
S(2) O(3)
S(2)

2.054(1) 1.387(4)

Cd(2)
N(2)

2.358(3)

O(4)
S(2)

1.371(4)

Cd(2)
N(2)#2

2.358(3)

O(5)
S(2)

1.413(5)

Cd(1)
N(1)

2.259(5)

Cd(2)
S(1)

2.656(1)

Cd(1)
N(1)#1

2.259(4)

Cd(2)
S(1)#2

2.656(1)

Cd(1)
S(1)

2.529(1)

S(1)
S(2)

2.083(1)

Cd(1)
S(1)#1 Cd(2)
N(2)

2.529(1) 2.262(5)

S(2)
O(1) S(2)
O(2)

1.421(5) 1.446(5)

Cd(2)
N(2)#2

2.262(5)

S(2)
O(3)

1.467(5)

compound II

compound III

a

Symmetry transformations used to generate equivalent atoms: For I: #1,
x, y,
z + 1/2. For II: #1,
x,
y + 1,
z; #2,
x + 1,
y + 1,
z; #3, x + 1, y, z. For III: #1,
x + 1/2, y,
z + 1; #2, x,
y + 3/2,
z + 1.

Figure 1. Packing diagram of the molecular cadmium thiosulfate structure of I, [Cd(C12H11N2)2(H2O)(S2O3)2].

’ STRUCTURE OF [CD(C12H11N2)2(H2O)(S2O3)2] (I) The asymmetric unit of I contains 21 nonhydrogen atoms. There is one crystallographically independent Cd2+ ion, which has a distorted square pyramidal geometry. The cadmium ion is coordinated by two sulfur atoms from the thiosulfate [S2O3]2
unit and two nitrogen atoms from the bpe ligand in the four equatorial positions, while the axial position is occupied by a terminal water molecule [CdS2N2(H2O), CN = 5]. The observed bond distances and angle are comparable to other similar cadmium thiosulfate structures reported earlier (Table 2).11 The structure of I consists of a simple molecular structure in which the thiosulfate and the bpe units are connected to the cadmium center through Cd
S and Cd
N bonds (Figure 1). 5743

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Figure 2. (a) One-dimensional cadmium thiosulfate chain in [Cd2(C12H10N2)3(H2O)4(S2O3)2] (II). (b) View of the two-dimensional layer structure of II.

Two thiosulfate units connect with the cadmium center through the sulfur atom, which is monodentate. Two bpe molecules are also connected with the cadmium centers through the nitrogen atoms, which is monodendate. This molecular structure appears to be unique as we did not find a comparable structure in the literature when the bpy ligand was employed.11 The free nitrogen atoms of the bpe ligands are protonated, and the molecular unit is stabilized by intermolecular hydrogen bond interaction through N
H 3 3 3 O hydrogen bond interactions [N(2)
H(2A) 3 3 3 O(4), N
O = 2.905(12) Å, and N
H 3 3 3 O = 170°].

’ STRUCTURE OF [CD2(C12H10N2)3(H2O)4(S2O3)2] (II) The asymmetric unit of II contains 30 nonhydrogen atoms. There are two crystallographically independent Cd2+ ions, and both occupy special positions (4c and 4b) with a site multiplicity of 0.5. The Cd2+ ions are coordinated by two sulfur atoms from two thiosulfate units, two nitrogen atoms from the bpe units, and two oxygen atoms of the water molecules forming a distorted octahedral environment [Cd(1)S2N2O2, CN = 6]. The observed bond distances and angles are in the range expected (Table 2) The structure of II consists of one-dimensional Cd
S2O3
Cd chains, cross-linked by the bpe units forming two-dimensional structure. Each [S2O3]2
ion is coordinated to the two metal centers through the terminal sulfur atom (μ2-mode). The connection

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Figure 3. (a) Cd4S4 cluster, formed by the connectivity between the Cd center and the S2O3 units, in [Cd(C12H10N2)S2O3] (III). (b) Three-dimensional structure of III. Note that the Cd4 clusters are connected in all three directions by bpe ligands.

between the octahedral Cd2+ ions and the [S2O3]2
ions gives rise to one-dimensional chains, which lie in the 21 axis (Figure 2a). The chains are connected by the bpe units, of which there are two types, while one bpe unit is terminal and freely hangs from the Cd center, the other bpe links the Cd
S
Cd chains, completing the two-dimensional layer structure (Figure 2b). To the best of our knowledge, this structural arrangement has been observed for the first time. The layer structure contains the terminal bpe units in such a way that the bpe pairs are aligned parallel face to face to have reasonable π 3 3 3 π interactions (centroid
centroid distance of the pyridyl rings = 3.75 Å).

’ STRUCTURE OF [CD(C12H10N2)S2O3] (III) The asymmetric unit of III contains 21 nonhydrogen atoms, of which there are two types of Cd2+ ions. Both of the Cd2+ ions occupy special positions (4j and 4 h) with a site multiplicity of 0.5 and are tetrahedrally coordinated by two sulfur from the [S2O3]2
unit and two nitrogen atoms from the bpe ligand [CdS2N2, CN = 4]. The observed bond distances and angles are in the expected range (Table 2) The structure of III is unique and consists of a Cd4S4 cluster core, connected by the bpe ligand. The Cd centers connect with the sulfur atom of the S2O3 units, through Cd
S bonds, forming [CdS2O3]4 tetrameric clusters (Figure 3a). Since the Cd center is bonded with two nitrogen atoms, each of the cluster, [CdS2O3]4, is connected with eight nitrogen atoms of the bpe ligand with four 5744

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Figure 4. Figure shows the 4-fold interpenetration observed in III.

nearest neighbor [CdS2O3]4 clusters. Thus, the clusters are connected by the bpe ligands in all of the three directions, giving rise to the three-dimensional structure (Figure 3b). This assembly resembles four parallel rail connections with four [CdS2O3]4 units. We have earlier reported the formation of cadmium thiosulfate compounds by the use of 4,4-bipyridine ligands.11 Presently, we have employed bpe ligands for the stabilization of cadmium thiosulfate phases with molecular (zero), two, and three dimensionally extended structures. The bpe ligands are longer as compared to the rigid bpy ligand employed in our previous study, which resulted in larger channels for the three-dimensional structure. The flexible nature of the ligand, however, resulted in having a 4-fold interpenetrated structure for III. Here, the Cd centers being tetrahedral and the bpe ligand linear, one could expect to have a diamondoid-related structure. Indeed, we observed a diamondoid structure, and the networks are interpenetrated, resulting in a 4-fold interpenetration (Figure 4). The 4-fold interpenetration arises due to the perpendicular interpenetration of two 2-fold interpenetrated structures (see Figure S6 in the Supporting Information).

’ TRANSFORMATION STUDIES The preparation of III was achieved by heating the synthesis mixture at 60 °C for 72 h. As can be noted, the same synthesis composition also produced II within 8 h. We desired to investigate this reaction mixture by carrying out a time-dependent study. To this end, we have stopped the reaction periodically (every 8 h) and examined the products using PXRD. The results are summarized in Figure 5a. We observed only two products, compounds II and III. Compound II appears to be the majority phase up to 32 h, after which we start to observe the formation of III. Compound III forms as a pure phase after 56 h. From this study, it appears that II could be a reactive intermediate in the formation of III. To verify and confirm this observation, we have carried out another time-dependent study by taking compound II as the starting source and heating in a water
ethanol mixture (2 mL of each) at 60 °C for varying periods of time. The results of this study are shown in Figure 5b. Compound II starts to become amorphous at ∼16 h and then transforms in to a new phase. Unfortunately, we did not obtain suitable single crystals to analyze this product further. At 32 h, however, the pure phase of III was obtained. We did not observe any other phase after the

Figure 5. Powder XRD patterns of the formation of III. (a) From the starting reaction mixture. (b) Starting from compound II.

Figure 6. Synthesis of III as a function of time: 0, 4 = starting from the reaction mixture; 9, 2 = starting from II.

formation of III. This suggests that compound II is a reactive intermediate in the synthesis of III. The transformation is facile and possibly proceeds via a dissolution and recrystallization mechanism. The observation of II becoming amorphous as a function of time and the appearance of a hitherto unknown phase 5745

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Scheme 1. Schematic Showing the Formation of the Cyclobutane Unit in the Compounds (a
c) I, (d) II, and (e) III

during the transformation lend credence to this argument. Similar mechanistic pathways for the formation of higher dimensional structure from structures of lower dimensions have been proposed earlier.16 The entire transformation reaction is summarized in Figure 6.

’ PHOTOCHEMICAL [2 + 2] CYCLOADDITION STUDIES All of the structures in the present study have a bpe ligand. A careful analysis of the structures (I
III) indicates that the bpe ligands are aligned parallel to each other as well as the CdC bond distances between two different bpe ligands in the range to satisfy the Schmidt's criteria (I = 3.72 Å, II = 4.00 Å, and III = 3.75 Å).10 It occurred to us that it may be possible to investigate the [2 + 2] cycloaddition reactions in these compounds. It may be noted that the dimensionality of compounds II and III is already determined by the bonding between the Cd centers and the bpe ligands, and compound I possesses a simple molecular complex structure. It is likely that the [2 + 2] cycloaddition reactions, in the present compounds, would either lead to a higher dimensional structure for I and to more rigid structures in the case of II and III. The [2 + 2] cycloaddition reactions were carried out with the solid samples using a wavelength of λ = 365 nm. Typically, the photodimerization experiments were carried out for 15
20 h. The products were analyzed using 1H NMR investigations in d6DMSO solvent (see Figure S9 in the Supporting Information). In addition, we have made attempts to investigate a single-crystal to single-crystal (SCSC) photodimerization study using the single crystals. The investigations indicate that the single crystalline

nature of the compounds was lost after prolonged exposure to UV radiation (∼10
20 h). It is likely that the considerable lattice strain involved in the cyclization would have resulted in the loss of single crystalline nature of the compounds. In all of the cases, we observed the photodimerized product with a rctt-tpcb conformation [1,2,3,4-tetrakis(4-pyridyl)cyclobutane] (Scheme 1), analyzed with 1H NMR spectra. A similar conformation for the photodimerized product was reported by Vittal and co-workers.8a The molecular structure of I has the bpe units in such a way that they are aligned parallel with face-to-face π 3 3 3 π interactions (the distance between the centroid of the pyridyl group is 3.60 Å). We have carried out the [2 + 2] cycloaddition reaction on powdered sample of the compounds for a longer time (20 h). The photodimerization experiment was monitored using 1H NMR studies (Figure S9 in the Supporting Information). The 1 H NMR spectrum in d6-DMSO exhibited peaks at 8.32, 7.21, and 4.64 ppm in addition to signals corresponding to some unreacted I. From the integration of the signals, it was observed that the double bonds have photodimerized with a yield of 66%. Compound II has the hanging bpe ligands, which are prone to photodimerize into 4,40 -tpcb ligand (rctt conformation). The NMR spectra exhibited peaks at 8.35, 7.24, and 4.67 ppm, in addition to the signals corresponding to some unreacted II. From the integration of the signals, it was observed that the photodimerization yielded 68% of the rctt-tpcb derivative (IIa). Further exposure to UV irradiation did not improve the yield of IIa. The photodimerization studies on powdered III also indicated the formation of rctt-tpcb derivative with ∼80% yield. The 1H NMR spectrum of the compound in d6-DMSO obtained shows peaks at 8.31, 7.23, and 4.61 ppm in addition to signals corresponding 5746

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Figure 7. Solid-state UV
visible absorbance-like spectra of compounds I
III and their photodimerized products (Ia
IIIa).

to some unreacted III. It is generally known that higher-dimensional structures are not conducive for photodimerization as the movements of the bridging bpe ligands are restricted due to the bonding with the metal centers. In III, the sulfur atom of the S2O3 unit has μ2-binding mode (connecting two Cd centers), which is more rigid. In spite of this rigidity, we did observe reasonable photodimerization in III (yield ∼80%). The observation of only one (rctt-tpcb) isomer may be due to the pedal-like motion of the bpe ligands. Similar pedal-like motion has been proposed recently.6a,17 The bulk PXRD pattern of II does not exhibit any profound changes during the cycloaddition studies, but the overall crystallinity appears to be affected (Figure S1 in the Supporting Information). The entire photodimerization process is summarized in Scheme 1. Such photodimerization reactions have been observed in organic as well as coordination polymer systems, and to the best of our knowledge, this is the first such observation in thiosulfate-based inorganic
organic hybrid compounds. More studies are required to understand the mechanism of such photochemical transformation in solid state. The photodimerized compounds were also subjected to investigations using TGA, IR, and UV
visible spectroscopic studies. The IR spectra indicated a shift (5
10 cm
1) in the photodimerized products as compared to the original one (Figures S2 and S3 in the Supporting Information). Similar shifts in the IR frequencies have been observed before.8c The TGA analysis (Figures S4 and S5 in the Supporting Information) indicated that the total weight losses for the compounds I and III (68 and 50%, respectively) were similar to those observed for the photodimerized products (69 and 49%, respectively). Compound II, however, exhibited a small increase in the weight loss after the photodimerization (60 and 63%, respectively), which can be due to the partial loss of water molecules.

’ UV
VIS SPECTROSCOPIC STUDIES The solid-state UV
visible spectra of all of 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 spectra were recorded using an integrating sphere of radius 50 mm. The reflectance spectra were converted into absorption-like spectra using the Kubelka
Munk equation (Figure 7). The diffuse reflectance spectra [F(R) vs wavelength] of the compounds

Figure 8. Plot of the time vs concentration for the dissociation of organic dyes (a) MR and (b) RBL under UV light in the presence of the compounds II and III and their photodimerized compounds (IIa and IIIa).

(I
III) exhibited three peaks: (i) one sharp peak centered at ∼260 nm due to the charge transfer from the thiosulfate ion to the metal, (ii) one sharp peak centered at ∼345
355 nm due to the intraligand (bpe) electron transfer, and (iii) one broad peak centered at ∼440
490 nm region due to the charge transfer from the bpe ligand to the metal. The solid-state UV
visible spectroscopic studies on the photodimerized compounds (Ia, IIa, and IIIa) exhibit some differences as compared to the original compounds (Figure 7). The broad hump, observed in the parent compounds in the region of 440
490 nm, were not observed in the photodimerized 5747

dx.doi.org/10.1021/cg2013655 |Cryst. Growth Des. 2011, 11, 5741–5749

Crystal Growth & Design compounds. This is likely due to the loss of the conjugation of the aromatic bpe ligands during the cyclizatioin forming the aliphatic cyclobutane ring. This would result in a reduced charge transfer from the ligand to the metal.

’ PHOTOCATALYTIC STUDIES We have evaluated the optical band gaps from the differtial plot of the original diffuse reflectance spectroscopic data on the 2D and 3D (II and III) compounds, which were found to be 2.53 eV (corresponding to the ligand-to-metal charge transfer band in the range 440
490 nm). The band gap value suggested that these compounds may be useful as a photocatalyst using the UV radiation (λ = 365 nm). For comparison, we have also carried out the photocatalytic behavior of the dimerized compounds (IIa and IIIa). As part of the study to evaluate the efficacy of these compounds as possible photocatalysts, we examined the degradation of two dyes, viz., methyl red (MR) and rhodamine blue (RBL). In a typical experiment, 100 ppm solution of the dyes was taken along with the catalyst (II, III, IIa, and IIIa) (2 g/L) and exposed to UV light. The experimental details have been described elsewhere.18 The compounds, II and III, exhibited reasonable activity as a photocatalyst, and we observed ∼50
60% decomposition of the dyes (Figure 8). The photodimerized compounds, IIa and IIIa, were found to be much less active under identical experimental conditions (typically, ∼ 24
31% photo decomposition). We believe that the loss of conjugation would be responsible for the reduced photo catalytic behavior in the dimerized compounds (IIa and IIIa). The compounds (II, III, IIa, and IIIa) were examined for possible deterioration after the photocatalytic studies using PXRD, which indicated that the compounds were stable under the photocatalytic experimental conditions, although a small reduction in the crystallinity was observed (Figures S7 and S8 in the Supporting Information). It is also likely that the compounds are less stable as compared to the earlier reported cadmium thiosulfate phases.18 ’ CONCLUSIONS Synthesis, structure, photodimerization, and photocatalytic studies have been carried out on a family of cadmium thiosulfate compounds with varying dimensionalities. The observation of terminal bpe ligands, unique Cd4S4 clusters, and 4-fold interpenetrated diamondoid structures in the three-dimensional compound is noteworthy. We have shown that the two-dimensional compound, II, is reactive and transforms readily to the more stable three-dimensional compound, III, as a function of time. The formation of a one-dimensional structure (Ia) from the molecular complexes of I and rigid cyclized products in II and III suggest that the photochemical reactions in the present compounds are facile. The photochemical method provides a viable alternative for the structural modification through postsynthetic route in inorganic
organic hybrid compounds. The prepared compounds exhibit reasonable activity as a photocatalyst. The present study along with our earlier studies11 clearly suggest that the chemistry of the tetrahedral thiosulfate ions is rich and provides avenues for further research. ’ ASSOCIATED CONTENT

bS

Supporting Information. Tables for bond angles for compounds I
III (Tables S1
S3); scheme for the possible

ARTICLE

pathway for the formation of III from II; powder XRD, IR, and TGA (before and after photodimerizataion); 1H NMR spectra for the photodimerized compounds (Ia, IIa, and IIIa); and PXRD patterns before and after photocatalysis. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the Department of Science and Technology (DST), Government of India, and the Council of Scientific and Industrial Research (CSIR), Government of India, for the award of research grants. S.N. thanks DST for the award of a RAMANNA Fellowship. A.K.P. and R.K. thank IISc for the Fellowship. ’ REFERENCES (1) (a) Kitagawa, S.; Kitaura, S.; Noro, S. I. Angew. Chem., Int. Ed. 2004, 43, 2334. (b) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. (c) Maspoch, D.; Ruiz-Molina, D.; Veciana, J. Chem. Soc. Rev. 2007, 36, 770. (d) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem., Int. Ed. 2004, 43, 1466. (2) (a) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.; O'Keeffe, M.; Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186. (b) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939. (c) Xu, G.-C.; Hua, Q.; Okamura, T.-a.; Bai, Z.-S.; Ding, Y.-J.; Huang, Y.-Q.; Liu, G.-X.; Sun, W.-Y.; Ueyama, N. CrystEngComm 2009, 11, 261. (d) Paul, A. K.; Sachidananda, K.; Natarajan, S. Cryst. Growth Des. 2010, 10, 456. (e) Zhu, A.-X.; Lin, J.-B.; Zhang, J.-P.; Chen, X.-M. Inorg. Chem. 2009, 48, 3882. (3) (a) Chang, W.-J.; Jiang, Y.-C.; Wang, S.-L.; Lii, K.-H. Inorg. Chem. 2006, 45, 6586. (b) Chang, W.-K.; Chiang, R.-K.; Jiang, Y.-C.; Wang, S.-L.; Lee, S.-F.; Lii, K.-H. Inorg. Chem. 2004, 43, 2564. (c) Lin, H.; Maggard, P. A. Inorg. Chem. 2008, 47, 8044. (4) Nagarathinam, M.; Peedikakkal, A. M. P.; Vittal, J. J. Chem. Commun. 2008, 5277 and references therein. (5) (a) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, CA, 2010. (b) Synthetic Organic Photochemistry; Griesbeck, A. G., Mattay, J., Eds.; Marcel Dekker: New York, 2005. (6) (a) Bhogala, B. R.; Captain, B.; Parthasarathy, A.; Ramamurthy, V. J. Am. Chem. Soc. 2010, 132, 13434. (b) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433. (c) Friscic, T.; Drab, D. M.; MacGillivray, L. R. Org. Lett. 2004, 6, 4647. (7) Nakanish, H.; Jones, W.; Thomas, J. M.; Hursthouse, M. B.; Motevalli, M. J. Chem. Soc. Chem. Commun. 1980, 611. (8) (a) Nagarathinam, M.; Vittal, J. J. Chem. Commun. 2008, 438. (b) MacGillivray, L. R. CrystEngComm 2004, 6, 77. (c) Peedikakkal, A. M. P.; Koh, L. L.; Vittal, J. J. Chem. Commun. 2008, 441. (d) Michaelides, A.; Skoulika, S.; Siskos, M. G. Chem. Commun. 2004, 2418. (e) Gao, X.; Friscic, T.; MacGillivray, L. R. Angew. Chem., Int. Ed. 2004, 43, 232. (f) Friscic, T.; MacGillivray, L. R. Chem. Commun. 2003, 1306. (9) (a) Mir, M. H.; Koh, L. L.; Tan, G. K.; Vittal, J. J. Angew. Chem., Int. Ed. 2010, 49, 390. (b) Dushyant, B.; Papaefstathiou, G. S.; MacGillivray, L. R. Chem. Commun. 2002, 1964. (c) Peedikakkal, A. M. P.; Peh, C. S. Y.; Koh, L. L.; Vittal, J. J. Inorg. Chem. 2010, 49, 6775. (d) Papaefstathiou, G. S.; Georgiev, I. G.; Friscic, T.; MacGillivray, L. R. Chem. Commun. 2005, 3974. (e) Varshney, D. B.; Gao, X.; Friscic, T.; MacGillivray, L. R. Angew. Chem., Int. Ed. 2006, 45, 646. (10) (a) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. (b) Schmidt, G. M. J.; et al. Solid State Photochemistry; Ginsberg, D., Ed.; Verlag Chemie: New York, 1976. 5748

dx.doi.org/10.1021/cg2013655 |Cryst. Growth Des. 2011, 11, 5741–5749

Crystal Growth & Design

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(11) (a) Paul, A. K.; Madras, G.; Natarajan, S. CrystEngComm 2009, 11, 55. (b) Paul, A. K.; Madras, G.; Natarajan, S. Dalton Trans. 2010, 39, 2263. (12) SMART (V 5.628), SAINT (V 6.45a), XPREP, SHELXTL; Bruker AXS Inc.: Madison, WI, 2004. (13) Sheldrick, G. M. Siemens Area Detector Absorption Correction Program; University of G€ottingen: G€ottingen, Germany, 1994. (14) Sheldrick, G. M. SHELXL-97 Program for Crystal Structure Solution and Refinement; University of G€ottingen: G€ottingen, Germany, 1997. (15) Farrugia, J. L. J. Appl. Crystallogr. 1999, 32, 837. (16) (a) Murugavel, R.; Walawalkar, M. G.; Dan, M.; Roesky, H. W.; Rao, C. N. R. Acc. Chem. Res. 2004, 37, 763. (b) Walton, R. I.; Millange, F.; Bail, A. L.; Loiseau, T.; Serre, C.; O'Hare, D.; Ferey, G. Chem. Commun. 2000, 203. (c) Choudhury, A.; Neeraj, S.; Natarajan, S.; Rao, C. N. R. J. Mater. Chem. 2001, 11, 1537. (17) Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2001, 123, 10884. (18) Paul, A. K.; Madras, G.; Natarajan, S. Phys. Chem. Chem. Phys. 2009, 11, 11285.

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