Structural Variation As a Function of pH and Structure Directing

Article pubs.acs.org/crystal

A Family of UO22+−5-Nitro-1,3-dicarboxylate Hybrid Materials: Structural Variation As a Function of pH and Structure Directing Species Paula M. Cantos and Christopher L. Cahill* Department of Chemistry, The George Washington University, 725 21st Street NW Washington, D.C. 20052, United States S Supporting Information *

ABSTRACT: Four new UO22+ compounds were hydrothermally synthesized and incorporate 5-nitro-1,3-benzenedicarboxylic acid (n-bzdc): [(UO2)(C8N1O6H3)(H2O)]·H2O (1), [(UO 2 ) 2 (C 8 N 1 O 6 H 3 ) 2 H 2 O]·TEA (2); [(UO 2 ) 2 (OH)(C 8 N 1 O 6 H 3 ) 2 ]·(TEAH)(H 2 O) (3); [(UO 2 )(C 8 N 1 O 6 H 3 )(C8N1O6H4)]·TMAH (4). Compound 1 consists of pentagonal bipyramidal uranyl monomers bridged via n-bzdc linkers to form a double-stranded chain, a structural motif that also serves as the foundation for the 2-D sheet of 2 and the 3-D framework of 3. The increasing dimensionality displayed throughout compounds 1−3 is a function of pH and the structure directing influence of TEA (triethylamine) in the case of 2 and 3. Compounds 1 and 2 exhibit similar 2-D configurations, yet each compound offers a different route toward structural assembly: supramolecular noncovalent interactions (1) and direct metal-to-ligand coordination (2). The role of TEA as a template in 2 and 3 was investigated by substituting TEA with alternative species that can potentially serve as structure directing agents: TMA (trimethylamine), TPA (tripropylamine), NaOH(aq), and CsOH(aq). The formation of compound 4 was observed in the presence of TMA and revealed a different chain motif compared with that observed in 1−3. Luminescence spectra were collected in order to ascertain whether any relationship existed between structural variation and uranyl emission.

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INTRODUCTION The continued exploration of uranyl-containing hybrid materials is largely motivated by the vast array of structural variation afforded by the combinations of organic ligands and the uranyl cation, UO22+. Beyond structural interests, uranylbearing materials also afford the opportunity to explore uranium−organic interactions, and as such there is an implied synergy between materials synthesis and studies of uranyl speciation and complexation from an analytical or environmental perspective. In general, structures of uranyl hybrid materials are influenced by the local geometry of the uranyl cation, a linear triatomic species capped with terminal oxygen atoms. This tends to restrict ligand coordination to the equatorial plane and gives rise to three primary building units (square, pentagonal, and hexagonal bipyramids) in the solid state.1 Moreover, UO22+ is susceptible to hydrolysis and therefore may promote the formation of secondary building units (dimers, trimers, sheets, etc.), which provides a metalcentered route toward increasing topological diversity of uranyl hybrid materials.2−5 Modification of the organic component of hybrid materials further contributes to structural variation. Traditional organic ligands utilized in uranyl hybrid material synthesis often contain carboxylic acid groups as per their affinity for UO22+.6−13 Although covalent metal-to-ligand bonds dominate uranyl hybrid material synthesis, noncovalent interactions from socalled “terminating” sites (halogen atoms, nitro groups, etc.) © 2014 American Chemical Society

can serve as platforms for supramolecular assembly and have been employed as alternative modes of connectivity when synthesizing uranyl compounds with extended architectures.14−17 Thus, the formation of higher dimensional structures may be promoted by pairing UO22+ with organic ligands capable of forming a combination of covalent bonds and noncovalent interactions. The organic ligand 5-nitro-1,3-benzenedicarboxylic acid (nbzdc, Figure 1a) features both aforementioned bonding sites and therefore is an attractive organic ligand for such an approach.The use of the title ligand is inspired by 1,3,5benzenetricarboxylic acid (bta, Figure 1b), wherein two (of

Figure 1. Scheme of (a) 5-nitro-1,3-benzenedicarboxylic acid (n-bzdc (n-bzdc), (b) benzene-1,3,5-tricarboxylate (bta), and (c) 4-chloro-2,6pyridine dicarboxylic acid (chloro chelidamic acid). Received: March 2, 2014 Revised: April 3, 2014 Published: April 15, 2014 3044

dx.doi.org/10.1021/cg500304d | Cryst. Growth Des. 2014, 14, 3044−3053

Crystal Growth & Design

Article

Table 1. Synthetic Conditions for Compounds 1−4

UO2(NO3)2·6H2O (molar ratio) n-bzdc (molar ratio) DI H2O (molar ratio) TEA (μL) (2, 3) or TMA (μL) (4) pHi/pHf pure? topology U coordination

1, [(UO2)(C8N1O6H3) (H2O)]·H2O

2, [(UO2)2(C8N1O6H3)2H2O]· TEA

3, [(UO2)2(OH)(C8N1O6H3)2]·(TEAH) (H2O)

1

1

1

1

0.50 75

0.50 75 50

0.50 75 65

0.50 75 25

1.8/1.2

2.1/1.3

no chain pentagonal bipyramids

yes sheet pentagonal and hexagonal bipyramids

3.3/3.2 (4.7/4.7, 70 μL of TEA) (6.5/5.7, 75 μL of TEA) no framework pentagonal bipyramids

2.4/3.4 (3.3/3.3, 35 μL of TMA) no chain pentagonal bipyramids

4, [(UO2)(C8N1O6H3) (C8N1O6H4)]·TMAH

Table 2. Crystallographic Data compound empirical formula crystal system space group formula weight a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temp (K) Z Dcalc (g cm−1) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

1 UC8H7NO10 triclinic P1̅ 515.18 7.8201(13) 8.1694(14) 10.2669(18) 106.698(2) 103.200(2) 90.842(2) 609.38(18) 100 2 2.808 13.371 0.0451 0.0233 0.0565

2 U2C44H46N6O32 triclinic P1̅ 1880.93 9.5699(3) 11.0566(4) 14.7236(5) 94.826(1) 95.469(1) 112.878(1) 1416.30(8) 100 1 2.205 8.656 0.0415 0.0344 0.0725

three) carboxylates show direct coordination to UO22+ and one remains free to participate in supramolecular interactions.18 While not a focus of the previous effort, such interactions nonetheless serve as inspiration for the use of non-coordinating functional groups to promote supramolecular interactions by intent. Accordingly, a previous study from our laboratory investigated a series of uranyl chloro chelidamate (Figure 1c) materials that displayed a wide range of architectures due to hydrolysis and the presence of a supramolecular platform.17 Our current effort expands upon the concepts of supramolecular vs direct coordination and their influence on resulting topologies. Beyond the structural role of ligands, one may also explore secondary organic species such as TEA (triethylamine) that may affect resulting topologies either by participating in supramolecular interactions or by influencing architectures as structure directing agents.19−24 In the latter scenario, noncoordinating organic molecules may direct the formation of a given topology, as observed in the synthesis of zeolites and other 3-D framework materials.25−31 An investigation presented by Mihalcea et al. also explored the structure directing nature of two organic amines formed from the in situ decomposition of N,N-dimethylformamide within uranyl carboxylates.32 In this current study, we focused on a systematic pH variation (1 to 8), which allowed us to explore diversity in the resulting

3 U2C22H7N3O18 triclinic P1̅ 1077.37 10.6646(11) 11.0897(11) 14.729(2) 106.806(2) 95.727(2) 113.001(1) 1489.1(3) 100 2 2.434 10.948 0.0362 0.0271 0.0696

4 UC19H15N3O14 monoclinic P21/c 747.37 11.8861(3) 20.3996(6) 9.7156(3) 90 99.59 90 2322.82(12) 293 4 2.137 7.067 0.0389 0.0349 0.0779

compounds as a function of hydrolysis. It became apparent, however, that the TEA molecules used for pH adjustment demonstrated a possible structural influence. Thus, we were inspired to use other organic species, TMA (trimethylamine), TPA (tripropylamine), CsOH(aq), and NaOH(aq), and explore resulting structural motifs. Herein we present four new hydrothermally synthesized uranyl hybrid materials incorporating 5-nitro-1,3-benzenedicarboxylic acid (n-bzdc): [(UO2)(C8N1O6H3)(H2O)]·H2O (1); [(UO 2 ) 2 (C 8 N 1 O 6 H 3 ) 2 H 2 O]·TEA (2); [(UO 2 ) 2 (OH)(C8N1O6H3)2]·(TEAH)(H2O) (3); and [(UO2)(C8N1O6H3)(C8N1O6H4)]·TMAH (4). We focus on the resulting local and global structures of 1−4 as influenced by functional groups on the organic linker and to a lesser extent the effect of structure directing agents. The structural variations within these compounds also prompted a luminescence investigation to explore any relationship between topology and uranyl emission.

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EXPERIMENTAL SECTION

Synthesis. Caution! Whereas the uranyl nitrate hexahydrate, UO2(NO3)2·6H2O, used in these experiments contained depleted uranium, standard precautions for handling radioactive and toxic materials should be followed. Uranium nitrate was recrystallized from a mixture of uranyl nitrate and uranyl oxide dissolved in concentrated nitric acid. Powder X-ray diffraction (PXRD) confirmed the formation of uranium oxynitrate

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Figure 2. Polyhedral representation of 1 where yellow polyhedra are uranium centers, red spheres are oxygen atoms, and blue spheres are nitrogen atoms. The dashed black line represents hydrogen bonds between the nitro functional group and solvent water molecules. hexahydrate (PDF-#27-0936). Starting reagents 5-nitro-1,3-benzenedicarboxylic acid, CsOH, NaOH, TMA, and TPA were commercially available and used without any further purification. Optimal synthetic conditions yielding single crystals of compounds 1−4 are provided in Table 1. All starting materials for the syntheses were loaded into a 23 mL Teflon lined stainless steel reaction vessel and concentrated TEA, TMA, or TPA was added to adjust the pH accordingly. Additionally, 1 M aqueous solutions of CsOH and NaOH were prepared and used in the templating study. The reaction vessels were heated statically to a final temperature of 120 °C for 3 days and were allowed cooled to room temperature over 5 h on the benchtop. The mother liquor was decanted, and the remaining bulk material was rinsed with distilled water and ethanol and then left on the benchtop to air-dry. Finally, yellow crystals were collected from the bulk material. Compound 3 could also be obtained at pH values of 4.7 and 6.5 and verified by PXRD data, yet these bulk products did not contain any suitable single crystals. Compound 4 was also synthesized at different pH values, 2.4 and 3.3, both of which produced suitable single crystals. Amorphous bulk products were synthesized at pH values of 5.2, 7.3, and 8.5 with the presence of TEA and could not be identified via PXRD. It is conceivable that these are mixed hydroxide phases generated at higher pH. Characterization. X-ray Structure Determination. Single crystals were isolated from each of the bulk reaction products and mounted on a MicroMount needle (MiTeGen). Reflections were collected from 0.5° φ and ω scans at 100 or 293 K on a Bruker SMART diffractometer equipped with an APEX II CCD detector and a Mo Kα source. The data were integrated with the APEX II software suite33 and corrected for absorption using SADABS.34 The structures of compounds 1−4 were solved using direct methods and refined using SHELXL-201335 within the WinGX36 software suite. Selected crystallographic data are provided in Table 2. H atoms on aromatic carbon atoms in 1−4 were placed in calculated positions and allowed to ride on their parent atom. H atoms

on both solvent and coordinated water molecules in compound 1 were located in difference Fourier maps and refined isotropically. Compound 2 incorporated a TEA molecule and a bound water molecule. For the TEA molecule, H atoms on carbon atoms were placed in calculated positions and allowed to ride on the parent during refinement. H atoms associated with the oxygen atom of the water molecule were observed in the difference Fourier map, yet these could not be satisfactorily refined and were not modeled. In compound 3, all H atoms on carbon and nitrogen atoms of TEAH and oxygen atoms of water molecules were located in the difference Fourier maps and freely refined. Compound 4 contained a TMA molecule and the H atoms on C parent atoms were also located in the difference Fourier maps and freely refined. H atoms were observed on the N parent atom of TMA in the difference Fourier maps but could not be satisfactorily refined and were omitted from the refinement. Furthermore, bond valence summations (BVS) were calculated to identify oxygen atoms in compounds 1 and 3 as either coordinated water molecules or hydroxyl groups (Supporting Information, SI Tables 1−3).1 Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data of bulk reaction products were collected using a Rigaku Miniflex (Cu Kα, 2θ = 3−60°) and manipulated using the JADE software program.37 Powder diffraction data for compounds 1−4 are displayed in the Supporting Information (SI Figures 3−9). The PXRD data for compound 2 suggest a pure phase, however, compounds 1 and 3 both contain impurities within the bulk product. Similarly, the bulk product of 4 contained phases of both 1 and 4 as indicated by the PXRD data. Bulk Fluorescence. Solid state fluorescence data were acquired on all compounds (1−4) at ambient temperature using a Horiba JobinYvon Fluorolog-3 spectrophotometer and FluorEssence software (Supporting Information, SI Figures 10−13). The samples were excited at 365 and 420 nm, and resulting spectra were collected at room temperature using a front facing setting with excitation and emission slit widths at 3 nm. Compounds 1 and 4 generated characteristic uranyl luminescence, while spectra for 2 and 3 were 3046



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Figure 3. Polyhedral representation of (a) the extended sheet and (b) local geometry of compound 2.

Figure 4. Polyhedral representation of the hydrogen bonding in 2 observed between C22 and O9 (a) and O12 and O15 (b). more complex possibly due multiple uranyl sites in 2 and 3. Only the bulk products of compound 2 consisted of a pure phase and emission spectra from 1, 3, and 4 should therefore be interpreted with caution.

molecule (O3), as well as oxygen atoms (O4, O5, O6, and O7) from carboxylate groups of the title ligand. U−O bond lengths and angles in 1 (as well as 2−4) are unremarkable and similar to those in previously reported uranyl compounds. U1 connects to its symmetry equivalent along [001] through mono- and bidentate coordination of O6−C8−O7 and O4−C1−O5 of nbzdc creating a chain (Figure 2). Furthermore, O6−C8−O7 acts as a bidentate bridge between symmetry equivalent UO22+ sites, which doubles the chain width along [100]. Nitro group

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RESULTS Structural Description. Compound 1 ([(UO 2 )(C8N1O6H3)(H2O)]·H2O) contains one crystallographically unique uranyl cation, which adopts a pentagonal bipyramidal geometry with a coordination sphere consisting of a water 3047



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Figure 5. Polyhedral and ball and stick representation of the chain motif as well as the uranyl dimers within 3.

Figure 6. Polyhedral representations of (a) the local geometry of uranyl dimers and (b) the overall 3-D framework in 3. TEAH+ and H2O molecules omitted for clarity.

Information, SI Table 4, provides other selected π-based interactions observed in compounds 1−4. Compound 2 ([(UO2)2(C8N1O6H3)2H2O]·TEA) incorporates the chain motif observed in 1 as a foundation or “backbone” for a 2-D sheet (Figure 3a). The key difference in 2 is a uranyl linkage that directly connects the parallel chains to one another (Figure 3b). This linker is a crystallographically unique uranyl cation (U2) adopting a hexagonal bipyramid geometry with a coordination sphere consisting of two water molecules and two bidentate n-bzdc ligands. O4 of the n-bzdc ligand is connected to the “backbone” via U1 to form an infinite sheet. The stacked sheets form an interlayer, which is occupied

oxygen atoms O8 and O9 hydrogen bond to solvent water molecule OW1 yielding D−H···O distances of 2.066(6) and 2.625(7) Å, respectively. This hydrogen bonding network ultimately forms a pseudo-2-D sheet viewed down the [010] direction in Figure 2. The stacked “sheets” then give rise to π−π interactions (4.044(2) Å), which are measured from calculated centroids (Cg). The displacement angle (β) between the centroids was also measured (30.85°). Additionally, −NO2 functional groups interact with π-systems at distance of 3.428(4) Å. These π-based interactions contribute additional connectivity to the pseudo-2-D topology and have been investigated in other uranyl compounds.38−40 Supporting 3048



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Figure 7. Polyhedral representation of 4 featuring the chain motif (a) and a hydrogen bond (b) observed in 4.

Figure 8. Polyhedral representation of segments of the chain motif observed in 1 (a) and 4 (b). The bold purple line highlights voids A and B in both compounds.

by TEA molecules (Figure 3a). This arrangement also promotes a hydrogen bond (2.723(3) Å) between bound water molecule O15 of U2 and the “free” carboxylic oxygen atom O12 (Figure 4b). Additionally, another hydrogen bond is observed between nitro group oxygen O9 and the hydrogen atom of C22 of TEA at a distance of 2.469(2) Å. The latter

hydrogen bond in 2 is perhaps anticipated since nitro groups protrude into interlayer voids, thus allowing interaction with TEA molecules. The structure of compound 3, [(UO2)2(OH)(C8H3N1O6)2]· (TEAH)(H2O), also contains the now familiar uranyl-n-bzdc chain (Figure 5). Similar to compound 2, neighboring chains in 3049



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Figure 9. The structure of [UO2(C9O6H4)(H2O)]·H2O18 highlighting the chain motif and hydrogen bonding (dashed lines).

3 are also connected through a linking unit: a uranyl dimer composed of hydroxyl groups (O13 and O13′) and mono- and bidentate coordinated n-bzdc ligands (Figure 6a). Carboxylate group O7−C16−O8 forms a bidentate bridge from the uranyl dimer to U1 of the chain structure, joining neighboring chains to create a sheet. This same chain−dimer−chain connectivity is repeated via U2′ and stitches together chains in the plane above, thus forming a 3-D framework opposed to the more common 1- and 2-D uranyl architectures (Figure 6b). As a consequence of the architecture, the aromatic ligands form π−π interactions (5.9782(7) and 4.8564(6) Å and β = 33.54° and 45.54°, respectively). Charge balancing TEAH+ cations reside within the channels (Supporting Information, SI Figure 1). A TEAH+ cation also donates a hydrogen atom (H13) to nitro group oxygen O16 at a DH···O distance of 2.704(3) Å (Figure 6a). Similar to 2, nitro groups project into the void space and participate in hydrogen bonds with the secondary organic species (TEAH+ cations). Compound 4, [(UO2)(C8N1O6H3)(C8N1O6H4)]·TMAH, represents a departure from the structural theme developed thus far, as it does not feature the same chain motif observed in the previous three compounds. The final structure in this series is composed of a single crystallographically unique pentagonal bipyramid bound to its symmetry equivalents through two unique n-bzdc ligands (Figure 7a). Carboxylate group O6− C1−O7 acts as a bidentate bridge between two neighboring uranyl cations whereas O10−C8−O9 remains uncoordinated. The second n-bzdc ligand features both mono- and bidentate coordinating carboxylic acids (O5−C16−O8 and O3−C9− O4) connecting two symmetry equivalent uranyl metal centers. The overall arrangement of the uranyl chains in 4 creates voids wherein TMAH+ cations reside (Supporting Information, SI Figure 2). The formation of a hydrogen bond between a nitro group and the secondary organic species has been previously observed in compounds 2 and 3 as well as 4. In compound 4, nitro group oxygen O11 extends from the chain to a TMAH+

cation and forms a hydrogen bond measured at 2.437(5) Å (Figure 7b). Interestingly, nitro group O13−N2−O14 does not participate in any interactions perhaps due to its lack of proximity to any potential hydrogen donating species. Similar to compounds 1 and 2, compound 4 also contains π-based interactions, details of which are provided in Supporting Information, SI Table 4. Upon closer inspection, the chain motif of compound 4 bears some structural similarities to the chain motif observed in compound 1. Figure 8b displays a segment of the chain of compound 4 wherein A represents a void formed from the bridging bidentate coordination mode of −COO− groups between U1 and U1′. Meanwhile, void B is produced from a combination of mono- and bidentate coordination of carboxylic acid groups to U1 and its symmetry equivalent. As a result, the A and B sites alternate throughout the extended structure of 4. Compound 1 also exhibits the same alternating voids, yet the A and B sites are “connected” in a perpendicular fashion compared with 4 (Figure 8a). This connective zigzag arrangement of voids in 1 is due to the bridging bidentate coordination of O7−C8−O6 between uranyl metal centers. The analogous −COO− functional group in 4 (O5−C16−O8) remains unbound, rearranging the chain motif, and thus voids A and B remain separated.

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DISCUSSION Recent studies in our group have focused on supramolecular interactions and their contributions to the structures of uranyl hybrid materials.14,41 We can combine this supramolecular approach with traditional structural assembly methods, such as utilizing carboxylate functional groups, by selecting organic linkers (n-bzdc) that offer both directly coordinating sites and platforms for supramolecular interactions.15,17 Compounds 1− 3 produced different structural manifestations of a reoccurring chain motif as a function of pH and TEA molecules. As such, 3050



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Compound 3 also incorporates multiple chains connected through a dimeric uranyl linkage. Whereas this uranyl dimer presented in 3 contributes to structural diversity, it is perhaps more importantly an indicator of uranyl hydrolysis at these higher pH conditions: 3.2, 4.7, and 6.5 (Table 1). Since TEA molecules reside within channels of the 3-D framework, the same notion of TEA as a possible structure directing agent may also be applied to the formation of compound 3. Further exploration of this concept prompted syntheses with TMA, TPA, CsOH, and NaOH. From these efforts, only TMA resulted in an identifiable crystalline compound (4) and indicated a structure directing role of the organic species. The structural disparities between the chain motifs of 1 and 4 could be attributed to the replacement of TEA with TMA. Beyond the formation of compound 4 with TMA however, the results from this avenue of inquiry did suggest a structural influence of TEA within the syntheses of compounds 2 and 3.

one may observe an overall architectural evolution within these three materials. Compound 1 is a chain composed of pentagonal bipyramidal uranyl monomers connected by n-bzdc ligands. Nitro functional groups protrude from the chains and hydrogen bond to solvent water molecules and stitch together neighboring chains, ultimately forming a pseudo-2-D sheet. The topology of 1 highlights the structural utility of such supramolecular interactions. The double-stranded chain motif was also observed in [UO2(C9O6H4)(H2O)]·H2O (Figure 9), which incorporated the carboxylic acid analogue of our title ligand, 1,3,5-benzene tricarboxylic acid.18 The hydrogen bonding network displayed in [UO2(C9O6H4)(H2O)]·H2O is composed of a head-to-head arrangement between unbound carboxylate groups of neighboring chains; whereas in 1 hydrogen bonds (2.066(6) and 2.625(7) Å) are formed between donor solvent H2O molecules and -NO2 functional groups. In addition to 1, compounds 2−4 also contain hydrogen bonds or π-based interactions (or both), which provide supplemental structural connectivity (Table 3).38−40

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Table 3. Hydrogen Bonding within Compounds 1-4 compound

interaction

distance (Å)

1

N1−O8···H (Ow1) N1−O9···H (Ow1) N1−O9···H (C22) C9−O12···H (O15, bound H2O) N1−O16···H (C18) N1−O11···H (C18)

2.066(6) 2.625(7) 2.469(2) 2.723(3) 2.704(3) 2.437(5)

2 3 4

LUMINESCENCE STUDIES

The uranyl cation is a strongly emissive entity and as such it is interesting to explore this property as a function of its surrounding environment.39,42−44 The compounds presented here offer an opportunity to observe an association between structural topology and uranyl emission. The bulk products of 1−4 were excited at 365 and 420 nm, yet only compounds 1 and 4 generated well-resolved uranyl emission spectra (Supporting Information, SI Figures 10 and 13). The diminished uranyl emission of 2 may be consistent with the multiple uranyl sites within the compound (Supporting Information, SI Figure 11). The atypical uranyl luminescence spectra of 3 may be explained by a previous emission study of UO22+ compounds, which suggests the occurrence of a selfquenching mechanism, thus inhibiting uranyl emission.45 While speculative, the structure of 3 contains both uranyl monomers and dimers connected through a conjugated organic species, which may account for uncharacteristic uranyl emission spectra (Supporting Information, SI Figure 12). The PXRD data for 1 and 3 indicated impurities in each bulk product. Accordingly we could not confidently attribute uranyl fluorescence exclusively to these individual compounds. Similarly, we could not determine the true emissive nature of compound 4 as the PXRD data displayed the coformation of 1 and 4 in the same bulk sample. The luminescence spectra for 1, 3, and 4 are therefore included in the Supporting Information section for reference only (Supporting Information, SI Figures 10, 12, and 13). The PXRD data of 2, however, displayed a single pure phase (Supporting Information, SI Figure 4).

Furthermore, the preservation of the chain motif after functional group substitution (−NO2 for −COOH) suggests a proclivity for this architectural manifestation. Compound 1 was synthesized at an unadjusted pH and therefore these chains represent the “benchmark” structure among compounds 1−3. Despite the different chain motif of 4, all the compounds contain terminating −NO2 sites. These −NO2 groups present an ancillary role of supramolecular interactions within 2−4 as compared with 1. Within 2−4, these functional groups extend from the chain and hydrogen bond to TEA or TMA molecules within void spaces (2 and 4, respectively) or TEA species in channels (3). As discussed later, the templating study suggests a structural influence of TEA in 2 and 3 and TMA in 4. This secondary function of supramolecular sites should not be overlooked as they may be replaced with halogen or −NH2 groups and demonstrate their utility with respect to expanding the breadth of uranyl-bearing topologies. Hence, future investigations may continue to modify platforms for supramolecular interactions via the secondary organic species or perhaps the noncoordinating functional group. The same chain from 1 is also observed in compound 2, yet the hydrogen bonding network observed in 1 is replaced by a linking unit composed of a uranyl center coordinated to an nbzdc ligand (Figure 3b). This new uranyl linkage in 2 directly connects neighboring chains and produces an infinite sheet containing TEA molecules in the interlayer. The inclusion of TEA molecules is not entirely unexpected since synthetic conditions (Table 1) for compound 2 include concentrated TEA. As the addition of TEA is the only modified synthetic condition among the three compounds, this highlights the potential structure directing function of this organic molecule.

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CONCLUSION Four new uranyl compounds were hydrothermally synthesized and display structural variation as a function of pH and structure directing organic guest molecules, TEA and TMA. The title ligand, n-bzdc, was selected in an effort to continue our exploration of the influence of supramolecular interactions and traditional metal−ligand coordination on uranyl hybrid material assembly. Compounds 1−3 presented a previously observed chain motif, the formation and common inclusion of which in these three compounds remain noteworthy since architectural diversity is therefore a function of which species “link” the chains as observed in 2 and 3. In compounds 2 and 3, direct coordination through additional n-bzdc−uranyl−n-bzdc linkers gave rise to 2-D sheets in 2 and a 3-D framework in 3051



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compound 3. Compound 4 displayed a rearrangement of the chain motif observed in compound 1, which may be attributed to the presence of TMA. In this series of compounds, supramolecular interactions either adopt a structural function or serve ancillary roles. Of these four coordination polymers, only compound 1 demonstrates the presence of supramolecular interactions via nitro groups to form a pseudo-2-D sheet. Interestingly, a similar supramolecular network was observed previously in uranyl compound [UO2(C9O6H4)(H2O)]·H2O, which also produced a pseudo-extended structure.18 The nitro groups in compounds 2−4, however, demonstrate a different supramolecular interaction scheme from 1. Within 2, 3, and 4, hydrogen bonding is observed within void spaces occupied by both −NO2 groups and secondary organic species TEA (2), TEAH (3), or TMAH (4). Hence, future platforms for supramolecular interactions can be modified in terms of noncoordinating functional groups or secondary organic species, thus increasing the diversity of extended uranyl topologies. Alternative structure directing species (TMA, TPA, NaOH(aq), and CsOH(aq)) were employed in an effort to evaluate the structure directing ability of TEA in 2 and 3. The results indicated that 2, 3, or 4 could not be formed with the alternative organic species, thus suggesting a structural influence from TEA (2 and 3), as well as TMA (4). Luminescence studies were conducted and revealed complex spectra, especially in the case of compounds 2, 3, and 4. The PXRD data displayed purity issues within the individual bulk products of 1, 3, and 4 and prohibited a conclusive correlation between uranyl topology and emission. Overall, these four hybrid materials offer insight into structural assembly as influenced by supramolecular interactions, direct metal-toligand coordination, and structure directing agents. Future studies utilizing these diverse assembly pathways will likely enhance our understanding of formation criteria with uranyl hybrid materials.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Bond valence summations, PXRD patterns, luminescence spectra, thermal ellipsoid plots, and crystallographic data in CIF format. This material is free of charge via the Internet at http://pubs.acs.org. CIF files have also been deposited at the Cambridge Crystallographic Database Centre and may be obtained from http://www.ccdc.cam.ac.uk by citing reference codes 989428, 989429, 989430, and 989431 for compounds 1− 4, respectively.

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Article

AUTHOR INFORMATION

Corresponding Author

*Christopher L. Cahill. Phone: (202)994-6959. Fax: (202)9945873. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work supported by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (Grant DESC0001089). 3052



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