The illustrative use of thiosulfate in the formation of new three-dimensional hybrid structures

Abstract

Examples of 3D cadmium thiosulfate based inorganic–organic hybrid compounds have been shown to be active photocatalysts using sunlight.

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The extended network structures based on inorganic–organic coordination assemblies has become an integral part of chemistry as it combines the coordination versatility of the inorganic along with the functional diversity of the organic. Persistent research over the past decade or so gave rise to many interesting structures with diverse properties.¹ Most of the 3D inorganic–organic hybrid frameworks were formed employing rigid organic linkers such as aromatic carboxylates,² oxalates,³etc. The combined use of phosphates and oxalates also resulted in many hybrid structures.⁴ The use of 4,4′-bipyridine (bpy) as a rigid linker in some of the framework structures has been attempted⁵ and also in the formation of hybrid structures.⁶ We have been intrigued by the possibility of combining the common inorganic ligands with the rigid bpy linker to form new hybrid structures. To this end, we have chosen to study the use of thiosulfate [(S₂O₃)²⁻] as the inorganic anion along with bpy as the rigid linker in the formation of hybrid framework structures.⁷ Our efforts yielded three new 3D hybrid framework structures, [Cd(C₁₀H₈N₂)(H₂O)₂(S₂O₃)]·2H₂O, I, [Cd₂(C₁₀H₈N₂)₃(S₂O₃)₂], II and [Cd₂(C₁₀H₈N₂)_2.5(S₂O₃)₂], III. All three structures are closely related and possess 1D cadmium thiosulfate chains cross-linked by the bipyridine units giving rise to the 3D structures. In addition, the structures are interconvertible. The present structures appear to be the earliest examples of the formation of 3D hybrid frameworks by using thiosulfate in the synthesis mixture. In this communication, we present the synthesis, structure, transformation studies and the sunlight assisted photophysical properties of the three compounds.

All three compounds have been prepared under mild conditions and their structures determined using single crystal X-ray diffraction studies.‡ In I, the Cd²⁺ ions are octahedrally coordinated by one sulfur and one oxygen atom from the thiosulfate [S₂O₃]²⁻ unit, two nitrogen atoms from the 4,4′-bipyridine (bpy) ligand and two terminal water molecules forming (CdN₂SO{H₂O}₂, CN = 6) unit. The connection between Cd²⁺ ions and [S₂O₃]²⁻ ions gives rise to 1D chains which lie in the 2₁ axis. The bpy units link the cadmium thiosulfate in such a manner that it forms an inter-penetrated structure, as shown in Fig. 1a.

Fig. 1 Connectivity between the cadmium thiosulfate chains and the bipyridine units in (a) I, (b) II, (c) III. The differences in colour of the bipyridine rings show the different layers.

In II, there are two types of Cd²⁺ ions, both of which occupy special positions with a site multiplicity of 0.5. While, Cd(1) is octahedrally coordinated by two oxygens from the [S₂O₃]²⁻ unit and four nitrogens from the bpy ligand, Cd(2) is tetrahedrally bonded with two sulfur atoms from the [S₂O₃]²⁻ unit and two nitrogens from the bpy ligand. The strictly alternating tetrahedral and octahedral cadmium centers are connected by the [S₂O₃]²⁻ units forming a 1D cadmium thiosulfate chain. Similar to I, the bpy ligand connects the cadmium thiosulfate chains to form the 3D structure (Fig. 1b). The other way to view this structure is to consider a 2D layer formed by the connectivity between Cd and bpy units, which are pillared by the [S₂O₃]²⁻ units (see ESI).†

In III, there are two independent Cd sites, which have tetrahedral [Cd(1)] and distorted trigonal bipyramidal [Cd(2)] coordinations. Cd(1) is coordinated by two sulfur and two nitrogen atoms and Cd(2) is bonded with two oxygens and three nitrogen atoms. In III also the trigonal bipyramidal [Cd(2)] and tetrahedral [Cd(1)] ions strictly alternate and are bonded with [S₂O₃]²⁻ ions forming the 1D chains, which are cross-linked by the bpy units giving rise to the 3D structure (Fig. 1c). The absence of the octahedral coordination for cadmium in III does not favour the formation of a 2D layer arrangement when the bonding between Cd²⁺ ions and bpy ligands is considered. Instead, it forms a 1D chain-like units of molecular boxes which are bound by [S₂O₃]²⁻ ions forming the 2D and 3D connectivity (see ESI).†

All three structures described herein are all related as they have similar building units and comparable connectivities. The average Cd–O bond distance is 2.33 Å, the Cd–N bond distance is 2.31 Å and the Cd–S bond distance is 2.52 Å. The obvious differences in the coordination environments of the Cd²⁺ ions coupled with the differences between the Cd: the bpy ratio among structures I–III creates the observed differences between the three structures. Compound I with a 4-fold connectivity between Cd²⁺, S₂O₃ and bpy (Cd possesses two terminal water molecules) gives rise to an interpenetrated structure, while II and III have closely related structures. All three structures can be derived from the primitive cubic net (pcu), which is based on an octahedrally connected structure (Fig. 2a). In the structure of II, the presence of a tetrahedral center creates distortions in the original pcu forming a net described as fgs (Fig. 2b).⁸ A similar structure with fgs topology has been observed in Mn₂(dca)₃(NO₃)(Mepyz)₂.⁹ The structure of III is more distorted compared to II as the octahedral positions are replaced by trigonal bipyramidal ones (Fig. 2c). Thus, compound III creates further distortions in the fgs net. Compound I has only 4-connections due to the presence of two terminal water molecules and makes further distortions in the structure (Fig. 2d).

Fig. 2 (a) Schematic representation of the fully ordered pcu net. The octahedral metal centre (light blue) is shown as the sphere and the organic linker as the blue line). (b) Schematic representation of the structure of II where every alternate octahedral centre is replaced by a tetrahedral centre forming the fgs net. (c) Schematic representation of the structure of III showing the replacement of the octahedral centre in the fgs net by a trigonal bipyramidal centre. (d) Schematic representation of I showing the effect of the loss of another coordination. Color code: green sphere - Cd; yellow line - thiosulofate bond; blue and red line - bpy linkages.

It is to be noted that the Cd : S₂O₃ ratio among the three structures is the same, but the Cd : bpy ratios are different. In order to understand and to establish the close relationship between the structures, we carried out simple transformation reactions by taking an appropriate quantity of bpy with I. Thus 0.25 and 0.5 mol of bpy were reacted with 1 mol of I. Compounds II and III were obtained in 20 h after heating at 45 and 60 °C, respectively. The products of the reactions were analysed using PXRD (see ESI).† Our studies clearly indicated that it is, indeed, possible to transform I to II and III by these reactions and we obtained II and III in high yield (∼85% based on I). It is a little premature to envision a possible mechanism for these transformation reactions, but a dissolution and recrystallization route would be more likely.

It has been shown recently that the bipyridine bridged MOFs are attractive candidates for studying photophysical behaviour, especially the UV-assisted decomposition of organic dyes.¹⁰ Our preliminary studies on the UV-assisted (λ_max = 365 nm) decomposition of simple dyes using I–III were quite encouraging. It has been our aim to look for newer photocatalysts that could show reasonable activity in the presence of natural sunlight. To this end, compounds I–III were used as photocatalysts for the sunlight assisted decomposition of Methylene Blue (MB) in water. Our studies show that the present compounds exhibit reasonable stability and activity for the sunlight assisted decomposition of MB. To the best of our knowledge, this is the first time MOF compounds have been shown to be active as photocatalysts in the presence of sunlight. The results are shown in Fig. 3. For comparison, the performance of commercial TiO₂ (Degussa P-25) was also assessed under the same experimental conditions. It is clear that the present compounds are active photocatalysts under sunlight and their activity is comparable with TiO₂ catalyst.

Fig. 3 The sunlight assisted photocatalytic degradation profiles (close symbol) for 25 ppm MB with 2.0 g L⁻¹ of I–III and Degussa P-25 and in the presence of UV light (open symbol). Colour code: orange for III, olive for II, blue for I and red for Degussa P-25.

In conclusion, we have shown, for the first time, that thiosulfate can be used for the preparation of new 3D hybrid structures. The transformation studies show that the structures are interconvertible under suitable experimental conditions. The photocatalytic studies indicate that the cadmium thiosulfate-based compounds are attractive candidates for the study of sunlight assisted decomposition of organic dyes.

The authors thank DST and CSIR India for the award of a research grant and a fellowship. S.N. thanks DST for the award of Ramanna Fellowship.

References

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Footnotes

† Electronic supplementary information (ESI) available: Photocatalytic experiment (Fig. S1–S18). CCDC reference numbers 697711–697713. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b817150p

‡ A mixture of Cd(NO₃)₂·4H₂O (0.3085 g, 1 mM) and Na₂S₂O₃·5H₂O (0.496 g, 2 mM) were dissolved in 3 ml of water and 4,4′-bipyridine (0.156 g, 1 mM) was taken in 3 ml EtOH. The alcoholic solution was layered carefully on top of the water mixture and kept for slow evaporation at room temperature. The rod-like crystals of I were obtained after 60 h. Compounds II and III were obtained by reacting a mixture of Cd(NO₃)₂·4H₂O (0.3085 g, 1 mM), Na₂S₂O₃·5H₂O (0.496 g, 2 mM), 4,4′-bipyridine (0.312 g, 2 mM) and 3 ml water. NH₄OH was slowly added to the mixture to maintain the pH at 8. Colourless plate-like crystals of II and colourless rectangular block-like crystals of III were obtained in high yield (>75%) at 75 °C in 24 and 60 h, respectively. The compounds were characterized by TGA, IR and UV studies. The structures were determined using a Bruker AXS smart Apex CCD diffractometer at 293(2) K. The structures were solved and refined using SHELXL97 present in the WinGx suite of program (Version 1.63.04a).¹¹ The hydrogen positions were initially located in the difference Fourier maps and for the final refinement, the hydrogen atoms were placed in geometrically ideal positions and refined in the riding mode. The hydrogen atoms attached in the water molecule in I, O(200), could not be located. Restraints for the bond distances were used to keep the hydrogen atoms bound with the oxygens for the water molecules. Final refinement included atomic positions for all the atoms, anisotropic thermal parameters for all the non-hydrogen atoms and isotropic thermal parameters for the hydrogen atoms.Crystal data for I: monoclinic, space groupP2₁/n (no. 14), a = 7.1760(13), b = 10.5145(18), c = 20.899(4) Å, β = 99.30(1)°, V = 1556.1(5) ų, Z = 4, ρ_cal = 1.924 g cm⁻³, µ(Mo_kα) = 1.705 mm⁻¹, 12147 reflections, 3160 unique (R_int = 0.025), 2855 observed I > 2σ(I), R₁ = 0.0374, wR₂ = 0.0823 and GOF =1.109 for 223 parameters. For II: monoclinic, space groupC2/c (no. 15), a = 12.628 (2), b = 15.081(2), c = 17.362(3) Å, β = 107.076(3)°, V = 3160.8(9) ų, Z = 8, ρ_cal = 1.982 g cm⁻³, µ(Mo Kα) = 1.665 mm⁻¹, 13654 reflections, 3729 unique (R_int = 0.0305), 3172 observed I > 2σ(I), R₁ = 0.0433, wR₂ = 0.0742 and GOF =1.090 for 218 parameters. For III: monoclinic, space groupP2₁/c (no. 14), a = 11.0343(17), b = 15.894(2), c = 15.823(17) Å, β = 91.969(3)°, V = 2773.3(7) ų, Z = 4, ρ_cal = 2.011 g cm⁻³, µ(Mo Kα) = 1.887 mm⁻¹, 23968 reflections, 6541 unique (R_int = 0.0310), 5120 observed I > 2σ(I), R₁ = 0.0558, wR₂ = 0.0705 and GOF =1.099 for 379 parameters. CCDC Nos: 697711–697713.

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