Piperazine Based Guest Molecules in the Crystal

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Influence of pyrazine / piperazine based guest molecules in the crystal structures of uranyl thiophene dicarboxylate coordination polymers: structural diversities and photocatalytic activities for the degradation of organic dye Samson Jegan Jennifer, and Ajay Kumar Jana Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00826 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Crystal Growth & Design

Influence of pyrazine / piperazine based guest molecules in the crystal structures of uranyl thiophene dicarboxylate coordination polymers: structural diversities and photocatalytic activities for the degradation of organic dye Samson Jegan Jennifer* and Ajay Kumar Jana

Frameworks solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore- 560012, India.

*Corresponding Author, E-mail: [email protected]

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ABSTRACT The reaction of uranyl acetate dihydrate with 2,5-thiophenedicarboxylic acid (H2TDC) as the main ligand and pyrazine (PYZ); piperazine (PZ); 1,4 pyridyl piperazine (PYPZ); 1,4-di(pyridin-4-yl) piperazine (DPYZ); 1,2 pyrimidyl piperazine (PMPZ) as auxiliary ligand lead to the formation of five new

compounds

[UO2(TDC)(H2O)].(PYZ)0.5.H2O,

[UO2(TDC)2(H2O)].(H2DPYZ).H2O,

(III);

(I);

[(UO2)2(TDC)3].(HPZ).4(H2O), [UO2(TDC)3].(H2DPYZ),

(II); (IV);

[(UO2)2(TDC)3(H2O)].(H2PMPZ).H2O, (V). They were analysed by IR, UV-vis, thermo gravimetric analysis, X-ray diffraction analysis, powder X-ray diffraction, and fluorescence spectroscopy. Depending on the counter cation, uranyl thiophene dicarboxylate is shown to crystallise as 2D layer in (I, IV), 2D layer with two fold interpenetrated (6,3) nets in (II), 1D chains in (III) and 2D layer without two fold interpenetrated (6,3) nets in (V). The coligand DPYZ in structures of (III and IV) had not been added to the reaction but has been formed by the N-arylation of the PYPZ ligand. Compound (IV) was obtained in serendipity while trying some mixed metal compound. Furthermore their functional properties, photoluminescence and photocatalytical activity for oxidation of organic dyes have also been studied. Interestingly, compounds (II and V) with uranyl organic frameworks (UOF’s) and honeycomb (6, 3) nets possess highest efficiency in degradation of dyes.


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1. INTRODUCTION Compounds based on the uranyl ion (UO2)2+ gives rise to a number of interesting structural topologies and functional applications.1-4 Uranyl ion containing compounds have found to be useful in photocatalysis, and other related areas.5-8 Normally, the (UO2)2+ centers have been bonded with sulfonate,9-11 phosphonate,12-16 arsenate,17-19 mono- or dicarboxylates20-25 forming the structures of different dimensionalities like 1D chains,26,27 2D sheets28,29 and 3D frameworks.30, 31 In some cases, the N-donor ligands were also employed to enhance the dimensionality of the structures.

20,21,24

Also

incorporating uranyl ion with another d-block cation is a common strategy in increasing the dimensionality of the UOF’s.25 The rigid 4,4'-bipyridine along with its flexible derivatives such as 1,2-bis(4-pyridyl)ethane, and 4,4'-Trimethylene dipyridine have been employed in many of these preparations.32 The uranium(VI)-polycarboxylates framework compounds with 2D or 3D networks due to their self assembly in solid state, lead to a wide variety of fascinating interpeneterated or polycatenated structures with enchanting properties.24-26, 36-40 We have been interested in expanding the scape of the uranyl based compounds by combining the N-donor ligands and thiophene dicarboxylic acid (TDC). During the course of this study, we have employed piperazine, 1,4 pyridyl piperazine, 1,4-di(pyridin-4-yl)piperazine) and 1,2 pyrimidyl piperazine as the N-donor ligands (ESI, Scheme S1). Our studies have repelled in a number of compounds:

[UO2(TDC)(H2O)].(PYZ)0.5.H2O,

[UO2(TDC)2(H2O)].(H2DPYZ).H2O,

(III);

(I);

[(UO2)2(TDC)3].(HPZ).4(H2O), [UO2(TDC)3].(H2DPYZ),

(II); (IV);

[(UO2)2(TDC)3(H2O)].(H2PMPZ).H2O, (V). Some compounds have three-dimensional structures and we have explored the photophysical properties of the uranyl group and employed them for photocatalytic decomposition of polluting organic dye molecules. In this paper, the synthesis structure and the photocatalytic behaviour of all the compounds are presented.

2. EXPERIMENTAL SECTION 2.1. General synthesis of compounds (I-V) All chemical reagents for synthesis were commercial available and used as received. Uranyl acetate dihydrate [UO2(CH3COO)2·2H2O; from Fisher Scientific], thiophene 2,5-dicarboxylic acid (TDC), 3 ACS Paragon Plus Environment


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Pyrazine (PYZ), Piperazine (PZ), 1-(2-Pyrimidyl) piperazine (PMPZ) and 1-(4-Pyridyl)piperazine (PYPZ) were purchased from Sigma-Aldrich. The synthetic compositions and conditions employed for the compounds (I-V) are given in (Table 1). UO2(CH3COO)2·2H2O (105 mg, 0.250 mmol, 1 equiv), thiophene diacrboxylic acid (64.7 mg, 0.376 mmol, 1.5 equiv), pyrazine/piperazine based ligand (0.377 mmol, 1.5 equiv) and 2.5 mL of H2O were placed in a 25mL autoclave and heated at 120°C. After 3 days single crystals were formed they were isolated washed with water and characterized by single crystal XRD. Caution! Uranium acetate dihydrate UO2(CH3COO)2·2H2O used in this study contains depleted uranium. Standard precautions for handling radioactive and toxic substances should be followed.

2.2. Initial Characterization and Physical methods Fourier transform infrared spectra (FTIR) were recorded from KBr pellets using an spectrometer (PerkinElmer, model no: L125000P) (ESI, Figure S1). UV-Visible absorption (UV–vis) spectra were recorded on a Perkin-Elmer Lambda 35 spectrophotometer at room temperature (ESI, Figure S2). Room temperature solid state photoluminescence studies were carried out using PerkinElmer LS 55 luminescence spectrophotometer (ESI, Figure S3). Elemental analyses (carbon, hydrogen, and nitrogen) were performed in a Thermo Finnigan EA Flash 1112 Series. ere recorded in the 2 θ range of 5–50 ° with a scan speed of 2°/min on a PANalytical X’PERT PRO diffractometer, using Cu K α radiation ( λ =0.1542 nm, 40 kV, 40 mA). Powder X-ray diffraction patterns were recorded on PhilipsX’pert diffractometer in the 2θ range of 5-50° using Cu K α radiation (ESI, Figure S4). Purity of bulk samples of (I-V) were determined by comparing observed and simulated (from single crystal data) powder XRD patterns. These patterns can be found in the ESI (ESI, Figures S4). Thermo gravimetric analysis (TGA) was performed on (Metler-Toledo) analyzer over the temperature range 30-900°C in nitrogen atmosphere (flow rate = 40 mL/min) with a heating rate of 10°C/min (ESI, Figure S5). The data from the single crystals were collected on a Bruker AXS smart Apex CCD diffractometer.


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2.3. Photocatalytic activity measurements The photocatalysis of uranyl ions in aqueous solution have been well documented in the literature.41 Also a few water insoluble and easily recycled uranyl containing solids were reported to show photo catalysis. The photochemical reactor employed in the UV assisted catalytic study comprised of a high pressure Hg-lamp of 125W. The lamp radiated predominantly at 365nm corresponding to energy of 3.4eV. The dye solution was stirred along with the catalyst in the outer reactor to make sure that suspension is uniform. Water was circulated at 25°C through the quartz tube to avoid heating of the solution. For the degradation experiments about 40 mg catalyst powders (compounds I-V powdered) were suspended in 80 mL of dye solution (concentration : 10 ppm) and kept under Hg lamp irradiation. A sample was taken from the beaker recurrently at every 30 min interval; and was centrifuged for 5 min to remove the catalyst and then analyzed by using a PerkinElmer Lambda 35 spectrophotometer. To evaluate the stability of the as prepared photocatalysts, the IC degradation experiments of compound III were repeated up to four times under the same conditions. 2.4. Crystal Structure Determination The X-ray generator was operated at 50 kV and 35 mA using Mo−Kα (λ=0.71073 Å) radiation. The data was reduced using SAINTPLUS,42 and an empirical absorption correction was applied using the SADABS program.43 Crystal structures were solved and refined using SHELXL97, present in the WINGX package of programs (Version1.63.04a).44 The hydrogen atoms attached to the carbon atoms were fixed at calculated positions and refined using riding model. Full-matrix least-squares refinement against |F 2| were carried out using WinGx package of programs.45 The hydrogen atoms of the water molecules (O1W and O2W) for the compounds I and III and (O1W) of compound II were located in difference Fourier maps. H atom position of the uncoordinated water molecules O2W and O1W for the compounds I and V respectively could not be located. The crystal data and structure refinement details for all the compounds are given in (Table 2). The data of compound V with highly disordered solvent molecule were corrected by using the SQUEEZE option of platon program.46,47 All carbon hydrogens were positioned geometrically and refined by a riding model with Uiso 1.2 times that of attached atoms. All non H atoms were refined anisotropically. The CCDC numbers for the compounds I-V are 1551334, 1551335, 1551333, 1551331and 1551332 respectively. Selected bond 5 ACS Paragon Plus Environment


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angle and bond lengths for compound I-V and intermolecular interactions found in crystal structures are listed in (Table S1 and S2) respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Center (CCDC) via www.ccdc.cam. ac.uk/data_request/cif.

3. RESULTS AND DISCUSSION 3.1.

Crystal structures of {[UO2(TDC)(H2O)].(PYZ)0.5.H2O}, (I). The asymmetric unit consists

of an uranyl ion coordinated to a thiophene dicarboxylate as well as a water molecule (ESI, Figure S6a). A half pyrazine molecule and a water of crystallisation are found at the lattice. The central uranium atom is seven fold coordinated showing an ideal pentagonal bipyramidal geometry (ESI, Figure S6b), with U=O bond lengths of 1.751(5) Å, 1.756(5) Å and O=U=O bond angle of 178.3(2)°. The equatorial coordination sphere of the (UO2)2+ cation contain five oxygen atoms from one bidentate carboxylate group, two bridging carboxylate groups and an aqua molecule respectively. Each of the TDC ligand is arranged in such a way to connect three uranyl building blocks [(UO2)O5] through a bidentate and two bridging modes, while each of the [(UO2)O5] unit links three other TDC ligands (ESI, Figure S6b). As a consequence a neutral infinite 2D framework the [(UO2)(TDC)]n is with larger rings is formed (Figure 1a). The internal cavity of the formed ring has a side length of 14.97 X 4.43Å. This 2D layer structure is further stabilised by a pair of hydrogen bonds between the coordinated and the uncoordinated water molecules as shown in (Figure 1b). These uncoordinated water molecules are slightly above or below the plane of the layer and these O-H···O hydrogen bonds leads to a R22(4) ring motif (Figure 1b). The layers are stacked one over the other in an ABAB ··· fashion (Figure 2). The uncoordinated pyrazine molecule connects two of the 2D layers by O-H···N hydrogen bonds and plays a major role in expansion of the three dimensional structure. This O-H···N hydrogen bonding is found in between coordinated water molecule of one layer and pyrazine molecule (Figure 2).

3.2.

Crystal structures of {[(UO2) 2(TDC)3].(HPZ).4(H2O)}, (II). The asymmetric unit of (II)

consists of an uranyl ion coordinated to a TDC as well as a half TDC molecules (ESI, Figure S7a). The electro neutrality of the compound is maintained by the uncoordinated protonated piperazine 6 ACS Paragon Plus Environment

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molecule (HPZ) (ESI, Figure S7b). There are two more aqua molecules that lie on the lattice. The UO(carboxylate oxygen) bond length is longer than the axial distances and range from 2.460(2)Å to 2.493(2)Å, these values are also comparable to the relevant compounds.9-25 The terminal U=O bond lengths are 1.756(3)Å and 1.761(3)Å, the O=U=O bond angle is 179.16(12)° which is a typical and most commonly found in similar typed hexavalent uranium compounds.24-26 Each of the uranyl ion is bound to three bidentate chelating carboxylate groups from three different TDC ligands in the equatorial plane showing a hexagonal bipyramidal geometry and extends in to an infinite anionic 2D honey comb layers (Figure 3a). Similar 2D honey comb layers have been observed in uranyl compounds containing terephthalic acid, naphthalene dicarboxylic acid, benzene dicarboxylic acid and thiophene dicarboxylic acid. These honey comb like motifs posses a large cavity/channel of about to 16.002(1) X 17.262(1)Å length. In the [(UO2)6(TDC)6] honey comb, the alternate TDC ligands adopt two configurations of endo and exo with S atom pointing toward or outward respectively to the hexagon center.34 The average equatorial O-U-O bond angle in the is 67°, which is comparable to the ideal 60° for a regular hexagon. The layer is actually not flat but it is slightly bent (bend angle 140.04°), adopting a chair configuration (Figure 3b). As in reported uranyl- benzene dicarboxylate structure, compound (II) shows 2-fold interpenetration of the sheets.36 The bending of the 2D layer makes it easy to accommodate the second layer through interpenetration (Figure 4a). Two opposite edges of a honey comb structure in a layer pass through the centre of a honey comb of the other layer as illustrated in (Figure 4a). These adjacent layers with two fold interpenetrated (6, 3) nets are stacked together with the guest HPZ cations (N1H22B···O6 and N1-H22B···O7), and versatile weak interactions among them (Figure 4b).

3.3.

Crystal structures of {[UO2(TDC)2(H2O)]. (H2DPYZ).H2O}, (III). Crystallographic

analysis shows that the compound (III) crystallises in triclinic P-1 space group containing one uranyl cation, two TDC anions, an aqua ligand and a lattice water molecule in its asymmetric unit. As shown in (ESI, Figure S8a), the U atom is coordinated by two uranyl oxygen atoms, four oxygen atoms of four different TDC anions and one oxygen atom from an aqua ligand forming a pentagonal bipyramidal chromophore. 7 ACS Paragon Plus Environment


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The (TDC)2- anions are in monodentate fashion (ESI, Figure S8b) and they connect two adjacent U atoms leading to a 1D chain (Figure 5a). These anionic chains [UO2(TDC)2]n2- are neutralised by protonated 1,4-(dipyridinium) piperazine cations (H2DPYZ) (Figure 5b). The presence of H2DPYZ not only accounts for charge balance in the crystal structure but its formation itself is quite interesting because it was not used in the reaction. It has been formed during the reaction conditions by the dimerization of its precursor 1-(4-pyridyl)piperazine which was used in the reaction. This is the first report of this kind transformation reaction in an uranyl compound (Scheme 1). As shown in (Scheme 1) the H2DPYZ cation in III shows a boat conformation. Two of these 1D chains are linked together by 1,4-(dipyridinium) piperazine cations. The uncoordinated oxygen atom of the TDC is further connected to the TDC oxygen atom of next chain by a pair of N-H···O interactions by the H2DPYZ cations (Figure 5b). 3.4.

Crystal structures of {[UO2(TDC)3].(H2DPYZ)}, (IV). Compound (IV) is a pseudo-

polymorph of compound (III). These two pseudo-polymorphs display interesting but different molecular packing and are notably distinct in their unit cell parameters (Table 2).The compound (IV) crystallizes in the orthorhombic Pccn space group. The asymmetric unit of the compound (IV) consists of one uranyl cation, a TDC and half TDC anions, and a half H2DPYZ cation (ESI, Figure S9a). As depicted in (Scheme 1), the H2DPYZ cation in IV shows a chair conformation. Each of the uranyl centre is bound equatorially by four distinct TDC anions to form an overall pentagonal-bipyramidal geometry. Two of the UO7 polyhedra units are bridged through two carboxylate edges of two (TDC)2- ligands, in a bidentate bridging fashion (ESI, Figure S9b). The next arm of the TDC anion connects to the adjacent dinuclear unit in a chelating mode. These infinite chains {[(UO7)2(TDC)]n} resulting from TDC molecules linking two adjacent UO7 polyhedra were developed extending along the a-axis (Figure 6a). These chains further connect through one of the monodentately coordinated carboxylate arms of a second type of TDC linkers in order to propagate additional connectivity and generates a 2D sheet in the (a, b) plane (Figure 6a). The thiophene rings of the bridging and the chelating TDC anions are all oriented almost in the same plane but the monodentately coordinated TDC anions are oriented in a near perpendicular plane and exhibits tilt angle of 85.41°. The uncoordinated oxygen of the monodentate carboxylate atom that does not 8 ACS Paragon Plus Environment

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involve in any coordination serves as a bridge to extend the network. Two of these oxygens of the nearby 2D sheets are further connected to form a supra molecular 3D network structure by a pair of N-H···O hydrogen bonds (Figure 6b). 3.5.

Crystal structures of {[(UO2)2(TDC)3]. (H2PMPZ).H2O}, (V). Compound (V) crystallizes

in the triclinic P-1 space group and its large asymmetric unit contains two crystallographically distinct uranyl centers, three (TDC)2- anions, an uncoordinated water molecule and a counter H2PMPZ ion. The coordination sphere around the uranium atom in compounds (V) and (II) are identical (ESI, Figure S10). The central uranium atom shows slightly distorted hexagonal-bipyramidal geometry with the uranyl (UO2)2+ oxygen atoms occupying the apical position. The U-O axial bond lengths are 1.761(3) Å and 1.767(3)Å are in good agreement with the reported average uranyl bond length of 1.758(4) Å. The uranyl centre is coordinated by three different carboxylate from three chelating TDC ligands. The arrangement of the uranyl polyhera generates a honey comb like 2D network, which have a large cavity (Figure 7). This honey comb structure in (V) seems similar to that of (II), but the 2D layer in (V) is almost flat with a bend angle of 177.28°. The 2D layers are coplanar, thus the parallel interpenetration of the layers like in (II) is completely absent in (V) (Figure 8a). The inter layer spaces in the honey comb structure is partially occupied by H2PMPZ cations and by the guest water molecules (Figure 8b). Adjacent double layers which lie parallel are stacked together by the guest H2PMPZ cations. The layers are stacked one over the other in an ABAB ··· fashion (Figure 8a). The inter layer spacing available for interpenetration is limited ⁓3.4Å, and the layers are closely packed and prefer weak hydrogen bonding to stabilize the overall framework. Thus the stability of the closely packed framework in compound (V) is achieved by weak interactions rather by interpenetration in (II).

3.6. IR spectroscopy study The synthesised uranyl compounds (I-V) were characterised by IR spectra as shown in (ESI, Figure S1). The broad peak occurring in the 3250-3500 cm−1 confirms the presence of H2O molecules.48 Compounds (I-V) contain bands in the region between 880-1000 cm-1 which can be attributed to the 9 ACS Paragon Plus Environment


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asymmetric stretching vibrations of (O=U=O) group. The U=O double bond shows a characteristic IR band in between 900-905 cm-1.

3.7. Optical Studies The UV-vis spectra of compounds (I-V) are shown in (ESI, Figure S2). All these compounds show characteristic band around 325 nm and 430 nm which are the characteristic equatorial U-O and axial U=O charge transfer bands respectively for UVI compounds.5 The solid state emission spectra for the bulk products under excitation wavelength of 320 nm for compound I, III-V, 345 nm for compound II, were recorded at room temperature. It has been well studied that most of the uranyl containing compounds emit green light centered near 520nm, with strong vibronic coupling yielding well resolved five peak pattern.49-51 For compounds I, II and IV the conspicuous characteristic emission of green light for uranyl cations (439, 487, 507, 529 and 551 nm for I; 470, 488, 509, 529 and 552 nm for II; 405, 451, 497, 523 and 540 nm for IV respectively are observed in the spectra) (ESI, Figure S3). These emission peak correspond to the electronic and vibronic transitions of S11→S00, S10→S0υ(υ=0-2) and S11→S00, S10→S0υ(υ=0-3) respectively.5 The most intense peak is positioned at 507, 509, and 451 nm for I, II and IV respectively. Compared to the bench mark compound UO2(CH3COO)2.2H2O, compounds I, II and IV exhibited a slight blue shift.52 Compounds III and V exhibited broad bands around 450-550 nm with no fine structures were observed, however these are also attributed to the charge transfer of the uranyl cations.53 3.8. Thermogravimetric Analysis The TG curve for compound I {[UO2(TDC)(H2O)].(PYZ)0.5.H2O}, showed the first weight loss of ∼7.5% observed in the temperature range of 30-110°C, which is associated with the loss of the lattice and coordinated water molecules (calc. 6.9%). The collapse of the framework of I started at 208°C (ESI, Figure S5). For compound II {[(UO2)2(TDC)3].(HPZ).4(H2O)}, a small weight loss of ∼5.1% was observed in the temperature range of 30-160°C, which are attributable to the loss of four crystalline water molecules (calc.5.9 %). The second loss in weight started at 235°C and ended at 305°C which was due to the loss of the HPZ ligands. The collapse of the framework of II was observed at 407°C (ESI, Figure S5). A small loss in weight of ∼4.3% observed was observed from 10 ACS Paragon Plus Environment

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30°C to 160°C for compound III, {[UO2(TDC)2(H2O)].(H2DPYZ).H2O}, is attributable to the loss of the crystalline and coordinated water molecules (calc. 4.05 %). The next step involved in the loss of weight from 160°C to 206°C is attributable to the loss of the H2DPYZ ligands (ESI, Figure S5). The decomposition of III appears to start after 290°C. The TG curve of [UO2TDC)3].(H2DPYZ) (IV) illustrates that the initial weight loss observed at 250°C and completed at 630°C, corresponds to the loss of H2DPYZ and TDC ligands. For [(UO2)2(TDC)3(H2O)].(H2PMPZ).H2O (V) the first weight loss ∼2.3% was observed at 30°C and completed at 100°C, and is associated with the loss of one crystalline and one coordinated water molecules (calc.2.9 %). The next loss in weight started at 295°C and ended at 565°C, and is associated with the loss of H2PMPZ and TDC ligands. The end products after thermal analysis for all the compounds I-V were found to be uranium oxide (U3O8) (JCPDS:761851). 3.9. Photocatalytic Performance In this work, utilizing a series of organic dyes such as methylene blue (MB), RhodamineB (RhB), indigo carmine (IC) and methyl orange (MO); we are able to compare the photo catalytic performances of uranyl bearing compounds (I-V). The photocatalytic performance of (I-V) were estimated from the wavelength and absorption intensity changes with the lengthening of irradiation time, which directly relates to the structural change of chromophore along with degradation of dye. The degradation curves (Figure 9) show the distinct photocatalytic activities of compounds (I-V). It can be observed that II, V exhibit highest degradation efficiency of all the dyes. For V, the full degradation time for IC is about 230 min. III is also efficient in degradation of IC for an irradiation time of 240 min. It is obvious that the PXRD patterns of recovered samples are nearly identical with those of the as prepared samples indicating that catalysts can be reused (ESI, Figure S11). Generally, two photodegradation mechanisms hydrogen abstraction and electron transfer have been proposed for the uranyl catalyzed photo-oxidation of organic dyes.34, 54-56 The uranyl ion would be excited when the compound is irradiated under light, and this leads to the generation of an excited [UO22+]* species by the promotion of electrons from HOMO (2p bonding orbitals of oxygen) to LUMO (empty uranium 5f orbitals) (Figure 10). The excited electron in the LUMO is unstable since the energy level of LUMO is higher and it tries to come back to ground state instantly. However, in 11 ACS Paragon Plus Environment


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the presence of organic dyes with reasonable concentration range and proper orientation, an electron from the nucleophile group of the dye molecule may be abstracted by the excited [UO22+]*. This electron occupies the HOMO and yields an intermediate and a proton. The excited electron remains in the LUMO until they are abstracted by electronegative substances such as O2 in the solution, generating highly reactive peroxide anion radical. The peroxide anions further oxidize and decompose the intermediates formed in the solution and results complete degradation of dyes.

4. CONCLUSION In this work, five new uranyl compounds were synthesized and their structures were investigated. We observe various architectures as a consequence of counter cations, like 2D layer in (I,IV), 2D layer with two fold interpenetrated (6,3) nets in (II), 1D chains in (III) and 2D layer without two fold interpenetrated (6,3) nets in (V). The ligand 1,4 pyridyl piperazine (PYPZ) undergoes N-arylation reaction to form 1,4-di(pyridin-4-yl)piperazine) (DPYZ), which is the first example of its kind in uranyl compounds. Compound (III) is a pseudo-polymorph of compound (IV). Also the H2DPYZ cations of III and IV show two different conformations. Subsequently the structural changes in (I-V) also have their influence in luminescence studies and revealed that the compounds showed either a blue or red shift, when compared with the characteristic emission of the uranyl center. Meanwhile the photoluminescence and photocatalytical activity for degradation of organic dyes by these compounds have been studied. Moreover we found that compounds II and V posses highest efficiency in degradation of dyes which may be due to their structural differences (with (6,3) nets) compared to that of other compounds.

ACKNOWLEDGEMENTS The authors are grateful to Professor S. Natarajan of SSCU, IISc Bangalore, India for his suggestions and manuscript corrections. S.J.J. thanks the University Grants Commission of the Government of India for Dr. D. S. Kothari Post Doctoral Fellowship and IISc Bangalore for the research facilities. Council of Scientific and Industrial Research (CSIR), Government of India is thanked for a research fellowship (A.K.J.). 12 ACS Paragon Plus Environment

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Figure Captions. Figure 1. (a) 2D layer in (I), yellow polyhedra represent uranium, whereas red and yellow spheres represent oxygen and sulfur, respectively. (b) 2D layer stabilised by O-H···O hydrogen bonds and enlarged view of the ring formed. Figure 2. PYZ is found in the interlayers between the [UO2(C6 H2 O2 S)]2- layers in (I). 2D layers linked by O-H···N interactions by neutral PYZ. Figure 3. (a) Polyhedral representation of a single 2D layer. (b) Schematic representation of single 2D layer; the green balls represent uranyl units and the blue line represent the TDC. HPZ cations and guest water molecules have been omitted for clarity. Figure 4. (a) Schematic representation of two fold interpenetrated (6,3) nets; the green balls represents the uranyl units and the blue sticks represents TDC of one layer and pink sticks represents TDC of next layer. (b) Two of the two fold interpenetrated (6,3) nets stacked by hydrogen bonding interactions; pink and blue represent TDC of a interpenetrated net while red and gold represent TDC of adjacent interpenetrated net. The solvent / guest HPZ molecules are omitted for clarity. Figure 5. (a) A portion of the 1D chain formed in (III). (d) Two of the adjacent chains linked by NH···O hydrogen bonding by the H2DPYZ cations (symmetry code ( i): x,y,z; (ii): 4-x,-1-y,1-z). Figure 6. (a) 2D sheet in compound (IV) propagating along the in the (a, b) plane. (b) Linkage of the the H2DPYZ cations by the hydrogen bond N-H···O in between the 2D layers to form a 3D structure (symmetry code i: x, 0.5-y, 0.5+z). The thiophene rings of the TDC anions are omitted for clarity. Figure 7. Single layer of the structure of (V). Hydrogen atoms, charge balancing H2PMPZ cations, and guest water molecules are omitted for clarity. Figure 8. (a) Schematic representation of the layer stacking without interpenetration showing the view of a honeycomb with the guest H2PMPZ cations linking two of the adjacent layers by C-H···O interactions. (b) Hydrogen bonding interactions (N-H···O and C-H···O) in between the H2PMPZ cations and uranyl oxygen atoms, connecting the layers.


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Figure 9. Concentration change of IC, MB, OG and RhB under UV lamp as a function of irradiation time with or without presence of compounds (I-V), Ct and C0 stand for the dye concentrations before and after irradiation. Figure 10. Proposed photocatalytic reaction mechanism of RhB in the presence of a uranyl complex.



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Table 1: Synthetic compositions and conditions employed for the compounds (I-V). Compou nds

(I)

(II)

(III)

(IV)

Composi tion

UO2(CH3COO) 2 ·2H2O + TDC + PYZ

UO2(CH3COO) 2 ·2H2O + TDC + PZ

UO2(CH3COO) 2·2H2O + TDC + PYPZ

UO2(CH3COO) 2·2H2O + TDC + PYPZ

(V) UO2(CH3COO) + TDC + PMPZ

2·2H2O

Temp. 120 120 120 60 120 (°C) Time (h) 96 96 96 48 96 Yield 82 65 75 70 69 (%) Elemental analysis: Anal. Calcd (%) for I: C 18.61, H 1.56, N 2.71, S 6.21; found: C 19.16, H 1.99, N 3.79, S 5.84; for II C 21.90, H 1.84, N 2.32, S 7.97; found: C 20.85, H 1.59, N 2.06, S 7.63; for III: C 35.14, H 2.95, N 6.30, S 7.22; found: C 35.97, H 2.71, N 6.86, S 6.97; for IV: C 29.73, H 1.87, N 4.33, S 7.44; found: C 30.24, H 1.55, N 4.95, S 7.42; for V: C 25.33, H 1.64, N 4.55, S 7.80; found: C 26.38, H 1.36, N 4.89, S 7.67.


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Table 2: Crystal data and structure refinement information for compounds (I-V). Compound I

Compound II

Compound III

Compound IV

Compound V

[UO2(TDC)( H2O)].(PYZ) 0.5.H2O

[(UO2)2(TDC)3 ].(HPZ).4(H2O )

[UO2(TDC)2(H2 O)].(H2DPYZ). H2O

[UO2TDC)3].( H2DPYZ)

[(UO2)2(TDC)3 (H2O)].(H2PMP Z).H2O

516.24

1208.67

888.66

1292.79

1234.72

Monoclinic

Orthorhombic

Triclinic

Orthorhombic

Triclinic

Space group

P21/c

Pccn

P-1

Pccn

P-1

A (Å)

8.638(4)

11.2802(7)

9.4658(18)

18.2950(5)

10.8376(15)

B (Å)

13.182(6

15.8870(9)

10.375(2)

14.1382(4)

12.8214(18)

C (Å)

11.701(5)

18.4345(12)

17.544(3)

13.9113(4)

17.239(2)

α(º)

90

90

100.663(6)

90

103.158(4)

Β (º)

94.184(17)

90

97.488(6)

90

104.095(4)

γ (º)

90

90

114.388(5)

90

106.020(4)

V (Å3)

1328.8(10)

3303.6(4)

1500.4(5)

3598.27(18)

2118.5(5)

Z

4

4

2

4

2

Density (gcm-3)

2.580

2.430

1.967

2.754

2.463

Mu [ /mm ]

12.405

10.068

5.618

13.883

11.659

F(000)

940

2248

860

2700

1436

0.0289

0.0223

0.0296

0.0395

0.0270

0.0615

0.0423

0.0781

0.0860

0.0701

1.00

1.03

1.15

0.908

1.05

-0.79, 0.65

-0.64, 0.67

-0.87, 2.16

-1.957, 2.873

-0.714, 0.922

Empirical Formula Formula weight Crystal system

Final R1 index [I> 2σ(I)] Wr2 (all data) Goodness of fit on F2 Largest difference in peak and -3 hole (e Å ) a

R1 = Σ∥F0| − |Fc∥/Σ|F0 |; wR2 = {Σ|w(Fo2 − Fc2)]/Σ[w(F02)2]}1/2. w= 1/[ρ2(F0)2 + (aP)2 + bP]. P = [max (F0,O) + 2(Fc)2]/3, where a= 0.0057 and b=0.00 for I, a=0.0145 and b=4.8481 for II, a=0.0285 and b= 5.9440 for III, a=0.0531 and b=4.5233 for IV and a=0.0342 and b=0.7847 for V. 19 ACS Paragon Plus Environment


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H N

H N N C/ 125°

N

N

N

s 3day

Boat

UO2(CH3COO)2.2H2O + H2TDC 60° C

/ 2 d ay s

N H

HN

N

Chair

N

NH

1-(4-Pyridyl)piperazine 1,4-di(pyridinium)piperazine) Scheme 1. Transformation of the ligand during the reaction with two different conformational isomers.


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Figure 1. (a) 2D layer in (I), yellow polyhedra represent uranium, whereas red and yellow spheres represent oxygen and sulfur, respectively. (b) 2D layer stabilised by O-H···O hydrogen bonds and enlarged view of the ring formed.

Jennifer et al. Figure 1.



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Figure 2. PYZ is found in the interlayers between the [UO2(C6 H2 O2 S)]2- layers in (I). 2D layers linked by O-H···N interactions by neutral PYZ.



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Figure 3. (a) Polyhedral representation of a single 2D layer. (b) Schematic representation of single 2D layer; the green balls represent uranyl units and the blue line represent the TDC. HPZ cations and guest water molecules have been omitted for clarity. Jennifer et al. Figure 3.



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Figure 4. (a) Schematic representation of two fold interpenetrated (6,3) nets; the green balls represents the uranyl units and the blue sticks represents TDC of one layer and pink sticks represents TDC of next layer. (b) Two of the two fold interpenetrated (6,3) nets stacked by hydrogen bonding interactions; pink and blue represent TDC of a interpenetrated net while red and gold represent TDC of adjacent interpenetrated net. The solvent / guest HPZ molecules are omitted for clarity. Jennifer et al. Figure 4.


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Figure 5. (a) A portion of the 1D chain formed in (III). (d) Two of the adjacent chains linked by NH···O hydrogen bonding by the H2DPYZ cations (symmetry code ( i): x,y,z; (ii): 4-x,-1-y,1-z).




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Figure 6. (a) 2D sheet in compound (IV) propagating along the in the (a, b) plane. (b) Linkage of the the H2DPYZ cations by the hydrogen bond N-H···O in between the 2D layers to form a 3D structure (symmetry code i: x, 0.5-y, 0.5+z). The thiophene rings of the TDC anions are omitted for clarity.



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Figure 7. Single layer of the structure of (V). Hydrogen atoms, charge balancing H2PMPZ cations, and guest water molecules are omitted for clarity.

Jennifer et al. Figure 7. 27 ACS Paragon Plus Environment


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Figure 8. (a) Schematic representation of the layer stacking without interpenetration showing the view of a honeycomb with the guest H2PMPZ cations linking two of the adjacent layers by C-H···O interactions. (b) Hydrogen bonding interactions (N-H···O and C-H···O) in between the H2PMPZ cations and uranyl oxygen atoms, connecting the layers.


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Figure 9. Concentration change of IC, MB, OG and RhB under UV lamp as a function of irradiation time with or without presence of compounds (I-V), Ct and C0 stand for the dye concentrations before and after irradiation.




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Figure 10. Proposed photocatalytic reaction mechanism of RhB in the presence of a uranyl complex.


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For Table of Contents Use Only Influence of pyrazine / piperazine based guest molecules in the crystal structures of uranyl thiophene dicarboxylate coordination polymers: structural diversities and photocatalytic activities for the degradation of organic dye

Samson Jegan Jennifer* and Ajay Kumar Jana

UO2(CH3COO)2.2H2O with 2,5-thiophenedicarboxylic acid (H2TDC) as the main ligand and pyrazine (PYZ); piperazine (PZ); 1,4 pyridyl piperazine (PYPZ); 1,4-di(pyridin-4-yl)piperazine) (DPYZ); 1,2 pyrimidyl piperazine (PMPZ) as auxiliary ligand lead to the formation of five new uranyl compounds with different structural architectures and their photocatalytical activity have been used for degradation of organic dyes.


Piperazine Based Guest Molecules in the Crystal

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