Hydrothermal Synthesis of Disulfide-Containing Uranyl Compounds

Three disulfide-containing uranyl compounds, [UO2(C7H4O2S)3]·H2O (1), ... (33, 34) The introduction of flexible ligands to these systems, however, ...
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DOI: 10.1021/cg901390n

Hydrothermal Synthesis of Disulfide-Containing Uranyl Compounds: In Situ Ligand Synthesis versus Direct Assembly

2010, Vol. 10 1390–1398

Clare E. Rowland, Nebebech Belai, Karah E. Knope, and Christopher L. Cahill* Department of Chemistry, The George Washington University, 725 21st Street NW, Washington, DC 20052 Received November 5, 2009; Revised Manuscript Received December 15, 2009

ABSTRACT: Three disulfide-containing uranyl compounds, [UO2(C7H4O2S)3] 3 H2O (1), [UO2(C7H4O2S)2(C7H5O2S)] (2), and [UO2(C7H4O2S)4] (3) have been hydrothermally synthesized. Both in situ disulfide bond formation from 3- and 4mercaptobenzoic acid (C7H5O2S, MBA) to yield 3,30 - and 4,40 -dithiobisbenzoic acid (C14H8O4S2, DTBA) and direct assembly with the presynthesized dimeric ligands have been explored. While the starting materials 4-MBA and 4,40 -DTBA both yield 2 via in situ ligand synthesis and direct assembly, respectively, we observe the formation of 1 from the starting material 3-MBA via in situ ligand synthesis and of 3 from the direct assembly of the uranyl cation with 3,30 -DTBA. Concurrently with the synthesis of 1 and 2, we have observed the in situ formation of the crystalline dimeric organic species, 3,30 -DTBA, [(C7H5O2S)2] (4) and 4,40 DTBA, [(C7H5O2S)2] (5). Herein we report the synthesis and crystallographic characterization of 1-5, as well as observations regarding the utility of product formation via direct assembly and in situ ligand synthesis. Introduction The study of coordination polymers (CPs) and metal-organic frameworks (MOFs) has seen a tremendous increase in the last 10 years1-3 due to the potential practical application of these materials in such areas as gas storage,4,5 sensing,6 luminescence,7,8 and catalysis.9,10 These compounds are generally constructed from metal centers linked by bifunctional ligands, often carboxylates, which coordinate to oxophilic metal centers for polymerization in one, two, or three dimensions.11,12 We have been exploring uranyl (UO22þ) containing CP and MOF materials using many of the same synthetic and structural approaches as the more commonly studied d-metal compositions. Our interests however have focused less on the applications (above) and more on the relevance to nuclear fuel stewardship and environmental legacy of nuclear weapons production.13-15 In particular, synthesis of UO22þ-organic structures under environmentally relevant conditions is important in the understanding of the complexation of the uranyl cation with organic materials and the influence of this complexation on the migratory behavior of U(VI).16,17 Additionally, uranyl-containing materials have been studied for both their photoluminescent properties3,18-20 and photocatalytic activity.21-25 Efforts to synthesize extended networks containing UO22þ using carboxylate functionalized organic ligands have been quite successful.26-30 These materials exhibit rich structural diversity, with topologies that vary according to the coordination geometry of the uranyl cation to include monomeric and oligomeric uranyl species bridged by organic ligands,3 in addition to chains and two-dimensional sheets.31,32 Because the uranyl oxygen atoms occupy axial bonding positions and further coordination takes place in the equatorial plane, many uranyl-containing compounds are two-dimensional.33,34 The introduction of flexible ligands to these systems, however, provides a degree of freedom that may allow for increased dimensionality, making the synthesis of three-dimensional structures more tenable.35 *Corresponding author. Phone (202) 994-6959. E-mail [email protected]. pubs.acs.org/crystal

Published on Web 01/29/2010

Uranyl-containing hybrid materials, and indeed many of the MOFs and CPs of current interest, have traditionally been synthesized through the direct assembly of metal cations with functionalized organic ligands that are commercially available or presynthesized. In short, what goes into the reaction vessel is subsequently observed in the structure of the reaction product. More recently however, in situ ligand formation, a process in which the intended organic linker species undergoes redox (or other) chemistry to yield a modified ligand that is then seen in the reaction product has become an attractive synthetic route. Within the context of materials synthesis, one can imagine two primary benefits. First, in situ ligand synthesis opens the possibility of “one-pot” ligand generation and simultaneous metal coordination, which is in some cases not only a simpler but also a more environmentally friendly approach. In situ ligand synthesis has also been used as a means of accessing materials that are not obtainable through direct assembly synthetic routes.36,37 In situ ligand formation has largely been a serendipitous process. As the organic starting material may undergo a variety of reactions, including C-C bond formation or cleavage, oxidation, and decarboxylation, among many others, the products of these reactions are not always intuitive.36,37 One predictable in situ reaction, however, is the formation or cleavage of disulfide bonds, an important biological process,38 which has been utilized in the synthesis of several coordination polymers.39-45 Although disulfide bond formation has been observed in other systems, to the best of our knowledge, the utility of generating these ligands in situ has not been explored in the context of accessing materials that are not available through direct assembly. To that end, we have explored the reaction of the uranyl cation with ligands containing mercapto groups that form disulfide bonds in situ. Further, we have compared these to the synthetic products generated through direct assembly with the presynthesized disulfide-containing ligand. The organic starting materials and their dimeric equivalents are shown in Scheme 1. Reported herein are the syntheses of three novel disulfidecontaining uranyl materials in which the disulfide bond has r 2010 American Chemical Society

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Scheme 1. 3- and 4-MBAs Were Dimerized in Situ to 3,30 -DTBA and 4,40 DTBA through the Oxidation of the Mercapto Group to Form a Disulfide Bond

been formed either in or ex situ. UO2(C7H4O2S)3 3 H2O (1) and UO2(C7H4O2S)2(C7H5O2S) (2), are two-dimensional U-containing coordination polymers formed through in situ dimerization of 3- and 4-mercaptobenzoic acids (MBAs), respectively. Utilizing a direct assembly approach in which we start with the preformed dithiobisbenzoic acid (DTBA), we observe a different reaction product under synthetic conditions comparable to 1, namely, UO2(C7H4O2S)4 (3). In the case of 2, however, the products formed with starting materials 4MBA and 4,40 -DTBA are identical. In addition to these Ucontaining compounds, 3,30 -DTBA (C7H5O2S)2 (4) was synthesized concurrently with 1 and 4,40 -DTBA (C7H5O2S)2 (5) concurrently with 2. These compounds were generated through in situ dimerization of 3- and 4-MBA and are the crystalline forms of the amorphous organic ligands 3,30 - and 4,40 -DTBA that were used as starting materials in the syntheses of 2 and 3. These structures, not previously reported, are presented herein to provide a more complete characterization of the reaction products of the syntheses of 1-3. Experimental Section Synthesis. Caution! While the uranyl nitrate, UO2(NO3)2, used in these experiments contained depleted uranium, standard precautions for handling radioactive material should be observed. All starting materials used in these synthetic reactions are available commercially and were used as obtained from the supplier. Compounds 1-5 were synthesized hydrothermally. In spite of a thorough exploration of synthetic pH conditions and adjustment of the metal/ligand ratio, none of these materials was obtained in pure phase. Characterizaton and identification of other products of these syntheses will be discussed below. Compound 1 was prepared via in situ ligand synthesis. Uranyl nitrate hexahydrate (Fisher Scientific; 0.124 g, 0.25 mmol), 3-MBA (Sigma-Aldrich; 0.077 g, 0.50 mmol), ammonium hydroxide (Acros Organics; 160 μL, 4.05 mmol), and distilled water (0.68 g, 38 mmol) were combined in a 23 mL Teflon-lined Parr bomb, initial pH 6.8. The reaction vessel was sealed and heated in an isothermal oven for 3 days at 180 °C, after which it was removed and allowed to cool to room temperature over approximately 8 h. The final pH of the solution was 6.8. Solid reaction products were rinsed with water and sonicated in ethanol. Single crystals of 1 were isolated from the bulk for characterization by X-ray diffraction. Other products were separated and further characterized by a variety of methods, as detailed below. Compound 2 was prepared similarly to 1. For in situ ligand generation experiments starting with the monomeric organic, uranyl nitrate hexahydrate (0.252 g, 0.50 mmol), 4-MBA (SigmaAldrich; 0.154 g, 1.0 mmol), ammonium hydroxide (160 μL, 4.05 mmol), and distilled water (1.42 g, 79 mmol) were combined for 3 days at 180 °C; pHi = 6.3, pHf = 4.2. For the direct assembly experiments using the presynthesized dimeric ligand, uranyl nitrate hexahydrate (0.124 g, 0.25 mmol), 4,40 -DTBA (Toronto Research

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Chemicals; 0.154 g, 0.50 mmol), ammonium hydroxide (250 μL, 6.32 mmol), and distilled water (1.51 g, 84 mmol) were combined at 180 °C; pHi = 6, pHf = 3. Longer reaction times were found to improve the yield of single crystals in the latter synthesis, and seven days proved optimal among explored synthetic conditions. As with 1, neither synthetic route for 2 yielded a pure phase, and comparable efforts were made to identify all byproducts. Compound 3 was obtained from the reaction of uranyl nitrate hexahydrate (0.125 g, 0.25 mmol), 3,30 -DTBA (Toronto Research Chemicals; 0.154 g, 0.50 mmol), ammonium hydroxide (250 μL, 6.32 mmol), and distilled water (1.51 g, 84 mmol) at 180 °C for 3 days; pHi = 8, pHf = 1. Again, we were unable to obtain a pure phase. Further characterization efforts are detailed below. Compound 4 was obtained during the synthesis of 1. Compound 5 was obtained during the synthesis of 2. Whereas the presynthesized 3,30 - and 4,40 -DTBA starting materials purchased from Toronto Research Chemicals were amorphous, both 4 and 5 are crystalline compounds. Moreover, these disulfide bond formation reactions appear to be metal-mediated, as our efforts to obtain these compounds absent the uranyl cation proved unsuccessful. X-ray Structure Determination. Single crystals from each preparation were isolated from the bulk reaction product and mounted on a MicroMount needle (MiTeGen). Reflections were collected by 0.5° j and ω scans on a Bruker SMART diffractometer with APEXII CCD detector and Mo KR source. Data for 1 were collected at 100 K; all other data sets were collected at room temperature. The APEX II software suite46 was used to integrate the data and apply an absorption correction.47 Structures were solved using direct methods and refined with SHELX-97.48 Publication materials were prepared using the WinGX software suite,49 and figures were generated using CrystalMaker.50 H atoms were located in the Fourier electron density map, but free refinement did not prove satisfactory for any of the compounds. They were therefore placed in calculated positions with bond distance restraints of 0.82 A˚ for O-H and 0.93 A˚ for C-H. Crystallographic data for 1-5 are summarized in Table 1. Powder X-ray diffraction data were collected on a Rigaku Miniflex diffractometer (Cu KR, 3-60°) and analyzed with the Jade software package.51 Calculated powder patterns were overlaid on observed patterns to identify phases in the sample. Where possible, different phases from a single synthesis were separated prior to analysis, aiding in identification of these species. Powder X-ray diffraction patterns for compounds 1-3 and additional phases from each of these syntheses are presented in Supporting Information. Characterization. All samples were characterized by powder X-ray diffraction (PXRD). Other materials obtained during the synthesis of 1-3 (hereafter “byproducts” of 1-3) were also isolated and further characterized by infrared spectroscopy (IR), energy dispersive X-ray fluorescence (XRF) and, where relevant, fluorescence spectroscopy. IR spectra were collected on a Perkin-Elmer Spectrum RXI FT-IR system. The samples were ground with spectroscopic grade KBr and pressed into a pellet. Eight scans were run from 400 cm-1 to 4000 cm-1 with 4 cm-1 resolution. XRF data were collected in air on a Shimadzu EDX-700 energy dispersive X-ray spectrometer for 100 s at 50 kV and 100 μA with the collimator at 5 mm. Scans were collected on both the Na-Sc and Ti-U channels. Fluorescence emission spectra were collected on a Shimadzu RF-5301 PC spectrofluorophotometer at an excitation wavelength of 365 nm and emission wavelength of 400-650 nm with a slit width of 1.5 nm (excitation and emission) and a UV-35 filter.

Results Structure Descriptions. The structure of UO2(C7H4O2S)3 3 H2O (1) contains the 3,30 -DTBA ligand, formed via in situ ligand synthesis, coordinated in bidentate fashion through each carboxylate to a uranyl center to form a hexagonal plane net with 63 topology as determined by TOPOS software (Figure SI1 in Supporting Information).52 The uranyl cation is formed by the crystallographically unique metal center, U1, and axial oxygen atoms, O1 and O2. The uranyl

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Table 1. Summary of Crystallographic and Structure Refinement Data from 1-5 empirical formula fw temp (K) λ (Mo KR) crystal system space group a (A˚) b (A˚) c (A˚) R (o) β (o) γ (o) V (A˚3) Z Dcalc (g 3 cm-3) μ (mm-1) Rint R1a [I > 4σ(I)] wR2a a

1

2

3

4

5

C21H14O9S3U 744.56 100 0.71073 hexagonal R3 27.4012(12) 27.4012(12) 17.2577(8) 90.00 90.00 120.00 11221.5(9) 18 1.983 6.807 0.0510 0.0328 0.0657

C21H13O8S3U 727.52 295 0.71073 monoclinic C2/c 21.8290(11) 9.9841(5) 21.7784(11) 90.00 112.2140(10) 90.00 4394.2(4) 8 2.199 7.720 0.0245 0.0246 0.0615

C28H16O12S4U2 1148.75 295 0.71073 triclinic P1 10.3354(4) 10.9653(4) 14.0982(5) 88.6010(10) 84.4200(10) 77.1690(10) 1550.47(10) 2 2.461 10.764 0.0251 0.0223 0.0486

C14H10O4S2 306.34 295 0.71073 monoclinic C2/c 14.7621(11) 5.0883(4) 17.5669(13) 90.00 92.1090(10) 90.00 1318.63(17) 4 1.543 0.413 0.0574 0.0583 0.1799

C14H10O4S2 306.34 295 0.71073 monoclinic P2/n 5.4028(6) 5.0320(6) 24.4752(28) 90.00 91.301(2) 90.00 665.26(25) 2 1.519 0.409 0.0914 0.0340 0.0599

R1 = Σ ||Fo - |Fc||/Σ|Fo|; wR2 = (Σ[w(Fo2 - Fc2)2/Σ[w(Fo2)2])1/2. Table 2. Average Bond Distances (A˚) and Angles (o)

UdO OdUdO U-O C-S S-S C-S-S C-S-S-C a

1

2

3

1.762(17) 179.65(7) 2.459(16) 1.788(3) 2.035(15) 99.56(35), 100.91(49), 103.34(31), 103.64(35), 105.94(83), 107.23(47) 84.85,a 64.71a

1.754(3) 178.14(14) 2.398(3) 1.771(4) 2.018(14) 105.47(14), 106.39(12), 106.88(13) 99.44(18), 86.28(16)

1.756(2) 178.68(11) 2.309(2) 1.782(4) 2.026(1) 102.49(12), 102.90(13), 104.84(13), 105.83(12) 74.00(18) 78.40(16)

4

5

1.777(3) 2.016(2) 105.05(11)

1.773(2) 2.017(2) 105.63(13)

109.87(15)

89.37(18)

53

Estimated standard deviations have been omitted. Values were measured manually in Mercury.

coordination geometry is a hexagonal bipyramid, with six equatorial oxygen atoms (O3-O8) contributed by carboxylate groups from one of two crystallographically unique ligands, where one is a combination of two rings bridged through a disulfide bond and the second is a single ring bound to a symmetry equivalent of itself. As shown in Figure 1, three phenyl rings (C2-C7, C12-C17, C22-C27) are coordinated to the uranyl cation through their respective carboxylate groups (C1, O3, O4; C11, O5, O6; C21, O7, O8). A sulfur is present on each ring meta to the carboxylate (S4, S14, S24). Each of these sulfur atoms forms a disulfide bond with another sulfur atom. Select bond distances and angles are summarized in Table 2. The organic components of 1 are highly disordered, as can be seen in the ORTEP illustration provided in the Supporting Information, Figure SI2. Disorder has been modeled such that a designation followed by ‘a’ (e.g., C22a) indicates the minor component, except in the case of C2-C6 and S4, where both the original atom (e.g., S4) and the disordered equivalent (e.g., S4a) are 50% occupied. One molecule of solvent water, OW1, is H-bound to O3 and O6, the details of which are summarized in Table 3. The extended structure of 1 is symmetrically complex; as shown in Figure 2, a view down [001] illustrates both 31 and 32 screw axes, as well as a 3 axis. These symmetry elements have been labeled in Figure 2b. UO2(C7H4O2S)2(C7H5O2S) (2) consists of pseudo-dimeric uranyl species bridged through carboxylate oxygens and bound to other uranyl pairs through 4,40 -DTBA formed in or ex situ. The ligand is coordinated to the uranyl cation (O1-U1-O2) through five equatorial oxygen atoms to form a pentagonal bipyramid building unit. These equatorial

Figure 1. The local coordination sphere of U1 shows three unique ligand halves coordinated to a hexagonal bipyramid uranyl building unit. Although disorder occurs throughout the organic component of 1, it has only been shown in S4 and S4a, where each of these positions is 50% occupied.

oxygen atoms (O3-O7) are each from carboxylate groups from one of two crystallographically unique ligands, as shown in Figure 3. One of the ligands, with aromatic ring C2-C7, is coordinated in bidentate fashion to the uranyl cation through O3 and O4. A sulfur atom (S1) is located in para position with respect to the carboxylate and is bound to symmetry-generated S3iv (iv = -x þ 1/2, -y þ 1/2, -z þ 1), which is in turn bound to a second aromatic ring, C16-C21,

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Figure 2. (a) A view of 1 down [001] illustrates packing motifs in the crystal structure. (b) An expanded view shows both 31 and 32 screw axes and a 3 inversion axis.

Figure 3. ORTEP with ellipsoids drawn at 50% probability shows the local coordination geometry of 2. H atoms have been omitted for clarity. Symmetry transformations: i = x þ 1/2, -y - 1/2, z þ 1/2; ii = -x, y, -z þ 1/2; iii = x - 1/2, -y - 1/2, z - 1/2; iv = -x þ 1/2, -y þ 1/2, -z þ 1. Table 3. Hydrogen-Bond Geometry (A˚, °) D-H 3 3 3 A

D3 3 3A 1

OW1-H1WB 3 3 3 O6ii OW1-H1WB 3 3 3 O3ii

2.894(5) 2.931(5) 2

O8-H 3 3 3 O3

2.575(4) 4

O1-H 3 3 3 O2iii

2.643(3) 5

O2-H 3 3 3 O1i

2.617(2)

Symmetry transformations: (1) ii = y þ 1, -x þ y þ 1, -z þ 1; (4) iii = -x, -y, -z þ 2; (5) i = -x þ 1/2, y, -z þ 1/2. a

with a para carboxylate group (C15, O7, O8) coordinated in monodentate fashion to the uranyl cation. H-bonding occurs between H8 on, uncoordinated O8 and O3 coordinated to the uranyl cation, with interactions summarized

in Table 3. The carboxylate of the second ligand bridges U1 and its symmetry equivalent U1ii (ii = -x, y, -z þ 1/2) through O5 and O6ii. The phenyl ring, C9-C14, has a sulfur group in the para position, which forms a disulfide bond with symmetry equivalent S2iii (iii = x - 1/2, -y - 1/2, z - 1/2). Important bond distances and angles are summarized in Table 2. As shown in Figure 4a, the ligands coordinate to the metal center through three bonding modes. Ligands that are monodentate through one carboxylate are bidentate through the other, whereas ligands that are bridging through one carboxylate are also bridging through the other. Shown in Figure 4b, pairs of uranyl cations are bound to other pairs through the length of the DTBA ligand. This results in the formation of two-dimensional net with topology 41362 as determined by TOPOS software (Figure SI3 in Supporting Information).52 UO2(C7H4O2S)4 (3) consists of one-dimensional ribbons formed by square bipyramids bridged through carboxylate groups of 3,30 -DTBA. Two crystallographically unique uranyl cations (O10-U1-O14; O20-U2-O25) are coordinated

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Figure 4. (a) The local structure of 2 shows three distinct modes of ligand coordination through two unique ligands. The bridging ligand binds two uranyl metal centers at either end. Ligands that coordinate to the uranyl cation in a bidentate fashion through one carboxylate are monodentate through the second carboxylate. (b) The packing diagram of 2, here down [010], shows pairs of bridged uranyl centers coordinated to other uranyl pairs through the length of the ligand.

Figure 5. (a) A thermal ellipsoid plot with probability levels of 50% shows the local coordination environment of the uranyl centers. H atoms have been omitted for clarity. (b) A view down [100] shows the packing of chains formed by the bridging of uranyl centers through the ligand. (c) The topology of the chain shows square bipyramids linked by bridging carboxylate groups on the 3,30 -DTBA. Symmetry tranformation: i = x þ 1, y, z.

to the ligand in the equatorial plane by four O atoms (O11, O12, O13, O15; O21, O22, O23, O24), as shown in Figure 5a. Each of the equatorial O atoms in the coordination sphere of U1 is from one-half of a carboxylate group, with the other half of the carboxylate bridging to U2 and its symmetry equivalent, U2i (i = x þ 1, y, z). The phenyl ring to which the carboxylate group is bound also has a S atom located meta to the carboxylate. A disulfide bond links the two rings of the 3,30 -DTBA. Relevant bond distances and angles are summarized in Table 2. As both ends of the bifunctional ligand experience coordination to a single uranyl center, we observe the formation of wide chains that propagate down [100], as shown in Figure 5b. The topology of these chains is further illustrated in Figure 5c.

(C7H5O2S)2 (4) was generated by in situ dimerization of 3MBA during the synthesis of 1. The structure consists of two MBA units joined through a disulfide bond to form 3,30 DTBA. Shown in Figure 6a, the phenyl ring is composed of C1-C6. S1 is bound to the ring through C1. The carbonyl group is bound to the ring through C5 and consists of C7, O1, and O2, with O1 protonated. The remainder of the molecule is generated by the symmetry transformation (-x, -y, -z þ 3/2) and is bound to the crystallographically unique portion of the molecule through a disulfide bond. Important bond angles and distances are summarized in Table 2. Molecules of 4 experience head-to-head H-bonding through the carboxylate groups. These interactions are summarized in Table 3 and result in the formation of an extended network, depicted in Figure 6b.

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Figure 6. (a) The local geometry of 4, shown here as a thermal ellipsoid plot with probability levels set to 50%. H atoms have been omitted for clarity. (b) Looking down [010], the head-to-head H-bonding through carboxylate groups is evident. Symmetry transformations: i = -x, -y, -z þ 3/2; iii = -x, -y, -z þ 2.

Figure 7. (a) ORTEP with thermal ellipsoid plots at the 50% probability level shows the local structure of 5. (b) A packing diagram of 5 viewed down [010] shows head-to-head O-H 3 3 3 O bonding between adjacent carboxylate groups. Symmetry transformation: i = -x þ 1/2, y, -z þ 1/2.

(C7H5O2S)2 (5, from the in situ dimerization of 4-MBA formed during the synthesis of 2) consists of two MBA units joined through a disulfide bond to form 4,40 -DTBA. The phenyl ring is composed of C1-C6, with S1 bound to the ring at C1 and a carboxylate group (C7, O1, O2, H2) at C4. The remainder of the molecule is generated by the symmetry transformation (-x þ 1/2, y, -z þ 1/2) and is bound to the crystallographically unique portion of the molecule through a disulfide bond. Important bond angles and distances are summarized in Table 2. Molecules of 5 experience head-tohead H-bonding through the carboxylate groups. These interactions are summarized in Table 3 and result in the formation of H-bound chains that are depicted in Figure 7b. Powder X-ray Diffraction. PXRD allowed for identification of most of the crystalline phases present in our reaction products. Recall that efforts to optimize syntheses of pure phases proved ineffective, thus prompting this exhaustive inquiry. As shown in Supporting Information (Figures SI4 and SI6), we were able to isolate crystals from the preparations of 1 and 2 and match the resultant powder patterns to calculated patterns from the refined structure of each. Crystals of 3 could not be separated from the reaction products; therefore, the bulk powder pattern has been shown (Supporting Information, Figure SI9). By isolating the byproducts from the syntheses of 1 and 2, we were able to identify other crystalline species that formed concurrently with the formation of these latter two products. In the powder pattern of the byproducts of 1 (Supporting

Information, Figure SI5), we observe two known U(IV) oxides (PDF 38-0039 and 42-1215). Although we know from single crystal X-ray diffraction data that 4 forms concurrently with 1, it does not appear in the powder pattern, indicating that it is present at levels below the detection limit of the instrument. The powder patterns of the byproducts of 2 from the monomeric and dimeric starting materials (Supporting Information, Figures SI7 and SI8) include both 5 and several known U(IV) oxides (PDF 31-1421, 31-1424, 42-1215; 43-0346). Additionally, we observe a peak at low 2θ, the source of which we have been unable to identify despite an exhaustive search of sulfur allotropes and uranyl and uranium oxides, hydrates, sulfates, sulfites, sulfides, carbides, carbonates, etc. For compound 3, visual inspection of the reaction products clearly shows that it was not obtained as a pure phase, but the powder pattern does not indicate the presence of any additional crystalline material in the bulk (Supporting Information, Figure SI9). Infrared Spectroscopy. IR spectra were collected on the byproducts of 1 and 2 and compared to the spectra of 3,30 and 4,40 -DTBA, respectively. We anticipated observing DTBA in the byproducts of both syntheses because X-ray diffraction data had already confirmed the presence of these compounds. These spectra (Supporting Information, Figures SI10 and SI11) both show good correlation in the fingerprint region between the organic DTBA species and the synthetic byproducts of reactions generating 1 and 2, respectively.

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Energy Dispersive X-ray Fluorescence. XRF confirmed the presence of U and S in the byproducts that formed concurrently with 1-3. These signals can be attributed to both uranium oxides and monomeric or dimerized ligand. Findings are summarized in Supporting Information, Figure SI12. Fluorescence. Typical uranyl fluorescence is absent or extremely weak in these materials, both in 1-3 and in their synthetic byproducts. The byproducts of 1 exhibited minor fluorescence, but the emission spectrum of this sample closely resembles the emission spectrum of 3,30 -DTBA (Supporting Information, Figure SI13), the presence of which was confirmed by PXRD. It is therefore likely that the fluorescence we observe from this material is in fact simply that of the dimeric organic species. Discussion In situ ligand synthesis has been explored in various systems for two primary purposes, both as a simple means of making a ligand that might be difficult to synthesize otherwise - socalled “one-pot” syntheses - and as a means of accessing materials that are not available through direct assembly routes.36,37 While in situ disulfide bond formation has been previously studied in the synthesis of coordination polymers, the reaction products of these studies have not been compared to the products of direct assembly synthetic efforts.39-41,43,45 Until this point, therefore, we were unable to say decisively whether these materials were in fact inaccessible via direct assembly or if this route was an alternative (and perhaps simpler) path to obtaining an identical product. In this study, we hoped to determine which argument for the utility of in situ ligand synthesis held true for this system involving disulfide bond formation. In fact, we see both of these purposes realized. With the starting material 4-MBA, we observe in situ formation of 4,40 -DTBA and the synthesis of 2. The direct assembly synthetic route, with 4,40 -DTBA as the starting material, gives us the same product. In this case, the benefit of a one-pot synthesis is significant: not only is this ligand expensive, but its ex situ synthesis is also difficult, having proven elusive after several attempts following published procedures for its preparation.54-57 In fact, our inability to synthesize the dimeric ligand ultimately prompted our purchase of the material. On the other hand, with 3-MBA as a starting material, we observe in situ dimerization to 3,30 DTBA in the crystalline reaction product, 1. Beginning with the presynthesized dimeric 3,30 -DTBA, however, results in the formation of 3. The fact that both the monomeric and dimeric starting material yield the same product in the case of 2 and different products in the instance of 1 and 3 can perhaps be attributed to the likely order of bond formation. Because of our own difficulties in forming a disulfide bond between aromatic thiols to dimerize the MBA and because of the known affinity of the uranyl cation for oxygen, we suppose that over the course of these syntheses the carboxylate groups coordinate to the metal prior to disulfide bond formation. Furthermore, if disulfide bond formation occurred first, the reaction conditions of in situ ligand synthesis and direct assembly should ultimately be identical: the metal cation and the DTBA together in solution at similar pH and temperature for an identical length of time. One can therefore imagine in the case of the MBA starting materials that metal-ligand species might exist as discrete entities in solution until the conditions

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are appropriate for the mercapto groups of two properly aligned ligands to bond. Looking at the structures of 1-3, and particularly at the C-S-S bond angles, we see that the compound that deviates most from the angles we observe in the free ligand (4 and 5) is undoubtedly 1. While in 4 and 5, we observe a C-S-S angle of 105°, in 1, these bond angles range from 100° to 107°. A search of the Cambridge Structural Database for typical C-S-S bond angles shows a fairly narrow observed range, where angles of 100° and 107° are relatively rarely reported (Supporting Information, Figure SI14).58,59 That the presynthesized dimeric ligand would form this bond angle in its coordination to the uranyl cation seems unlikely when it could assume a less strained geometry by adopting another coordination mode (as in 3). Moreover, we may suppose that in the synthesis of 1 the carboxylate groups coordinate first to the uranyl and saturate the available binding sites, thus precluding the formation of 3, a less coordinatively saturated structure. The subsequent formation of disulfide bonds in 1 apparently occurs in spite of the deviation in bond angle from what we observe in the free ligand. In other words, we propose that disulfide bond formation and the subsequent crystal packing introduces a strain in 1 that does not exist in 3. With respect to 2, however, no impediment appears to exist that would prevent the formation of the same product from both monomer and dimeric starting materials. These comments are admittedly speculative, yet we offer them at this point for comparison to future studies as in situ ligand synthesis increases in both frequency and maturity. In addition to the formation of different products in one instance and the same products in the other, this system has further proven complicated in our inability to achieve phase purity for any of our materials. Moreover, identification of other phases present as byproducts of 1-3 has been challenging. We initially believed that a black powder present as a byproduct of the syntheses of 1 and 2 might be a U(IV) oxide, reduced as a result of the oxidation of the mercapto group to a disulfide. To identify these species with certainty, we performed a battery of tests, including XRF, PXRD, and IR, on both the black powder from the syntheses of 1 and 2 and the white powder that was a formed during the synthesis of 3. XRF confirmed the presence of U and S in all of these synthetic byproducts. The use of PXRD allowed us to identify the crystalline components of these byproducts. Among the materials in the byproducts of 1 and 2 that we have been able to identify by PXRD are several U(IV) oxides (Supporting Information Figure SI5: PDF 38-0039, 42-1215; SI7: 31-1421, 31-1424, 42-1215; SI8: 43-0346). Indeed, as we had initially expected, the U(VI) of our starting material appears to have been reduced over the course of the reaction. The redox potential for the U(VI) to U(IV) transition has been experimentally established as 267.5 mV.60 The redox potential for disulfide bond formation between aromatic mercaptans is -245 mV.61 Thus, we suppose that the oxidation of the mercaptan drives the reduction of U(VI) to U(IV). We were also able to identify 5 as one of the phases present as a byproduct of the synthesis of 2 (Supporting Information, Figures SI7 and SI8). Efforts to identify the phase responsible for one additional peak in the byproducts of 2 have proven unsuccessful. Finally, although in the case of 2 we were able to identify the organic species in the PXRD pattern (5), in 1, the crystalline organic species (4) is present at levels below the detection level of the instrument.

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Lastly, IR spectra of the byproducts of 1 and 2 match closely with the spectra of the corresponding DTBA (Supporting Information, Figures SI10 and SI11), confirming the presence of these organic species. Conclusion Three novel uranium-containing materials have been presented here, including two 2-dimensional compounds synthesized through in situ generation of a disulfide bond and one 1dimensional compound synthesized with the preassembled disulfide-containing ligand. We have seen demonstrated here the dual utility of in situ ligand synthesis - the simplicity of a one-pot synthesis to generate an otherwise difficult-to-obtain ligand and the ability to access materials that are inaccessible through direct assembly. One possible explanation for our observation of two different products from 3-MBA and 3,30 DTBA may be the bond strain present in 1, which the product from starting material 3,30 -DTBA is able to avoid through adoption of a different coordination geometry (3). Where this bond strain is absent (2), we observe the formation of the same product through both synthetic routes. Finally, we have crystallographically characterized two organic compounds that were generated through the same in situ disulfide bond formation that we observed in the formation of compounds 1 and 2. Acknowledgment. This material is based upon work supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089 (C.E.R. & K.E.K.). Additional support (C.L.C. & N.B.) was from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Program, U.S. Department of Energy, under Grant DEFG02-05ER15736 at GWU. X-ray diffraction equipment was purchased with National Science Foundation funding (DMR-0348982 and DMR-0419754). The authors are grateful to Dr. Peter M€ uller (MIT) for crystallographic assistance given during the 2009 ACA Summer Course in Small Molecule Crystallography. We are also grateful to the reviewer who suggested using TOPOS to augment our structural descriptions. Supporting Information Available: ORTEP representation of 1, powder X-ray diffraction spectra for 1-3 and the synthetic byproducts of 1-3, IR spectra of the byproducts of 1 and 2, XRF data from the byproducts of 1-3, fluorescence emission spectrum of the byproducts of 1, and a histogram of disulfide bond angles found in the CSD; X-ray crystallographic information files (CIF) for compounds 1-5. This information is available free of charge via the Internet at http://pubs.acs.org/. CIFs may also be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif by referencing CCDC 755429-755433.

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