Crystal Engineering with the Uranyl Cation II ... - ACS Publications

Crystal Engineering with the Uranyl Cation II. Mixed Aliphatic Carboxylate/Aromatic Pyridyl Coordination Polymers: Synthesis, Crystal Structures, and Sensitized Luminescence Lauren A. Borkowski† and Christopher L. Cahill*,†,‡

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2248-2259

Department of Chemistry, The George Washington UniVersity, Washington, D.C. 20052, and Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Northwest, Washington, D.C. 20015 ReceiVed June 3, 2006; ReVised Manuscript ReceiVed July 13, 2006

ABSTRACT: Eleven novel U(VI)-containing coordination polymers have been synthesized under hydrothermal conditions and characterized via single-crystal X-ray diffraction and fluorescence spectroscopy. These inorganic/organic hybrid materials represent an important advance in the synthesis of polymeric materials in that multiple organic species have been employed in an effort to influence topology and properties. This family of materials is thus the result of a systematic pairing of the uranyl cation, UO22+, with aliphatic dicarboxylates (see part I, this issue) and the dipyridyl species, 4,4′-dipyridyl and 1,2-bis(4-pyridyl)ethane. This second organic component assumes a number of roles in these structures, including direct coordination, charge balance, and structure direction. Distinction between which role(s) the dipyridyl species will assume appears to be correlated to the size (length) matching between it and the dicarboxylate linker used. Further, polymerization of primary building units (i.e., monomeric uranyl species) to form novel tetrameric secondary building units is observed in four compounds, possibly as a compensation for size differential between the organic species. Introduction As we have introduced in part I of this study (this issue), the uranyl cation, UO22+, is an attractive metal center from which one may construct polymeric and framework materials. The oxygen atoms in this linear, triatomic species are almost exclusively terminal, and thus additional coordination proceeds through the equatorial region of this ion. Resulting local coordination geometries include square, pentagonal, and hexagonal bipyramids. Polymerization of these primary building units may occur to form dimers, tetramers, chains, and sheets. A rich coordination chemistry of molecular species is wellknown, and as an extension, we (and others) have been exploring uranyl-containing materials with extended topologies by introducing multifunctional ligands (or perhaps more appropriately for polymeric compounds, “linkers”) with the goal of polymerizing UO22+ centers through organic backbones. Part I of this study discussed the crystal chemistry of a number of aliphatic dicarboxylates, highlighting this approach. In this contribution, we explore an alternative route in which one may influence the topologies and indeed properties of uranyl-containing coordination polymers via the introduction of a second organic linker species. Until recently, the organic linker in most metal-organic coordination polymers was restricted to a single species. Within the past few years, however, transition metal containing structures have been reported wherein two different types of organic linker species have been utilized.1-15 These studies have shown that the simultaneous use of a flexible aliphatic linker with a more rigid aromatic linker influences pore dimensions, as well as affording greater structural diversity. In an effort to employ some of this philosophy to the uranyl system, we expand upon our previous work on uranyl dicarboxylates (part I of this study, this issue) and introduce a second aromatic pyridyl functionalized linker. As we will present, via the systematic pairing of 4,4′-dipyridyl and 1,2-bis(4-pyridyl)ethane with dicarboxylic acids of varying lengths, the use of † ‡

The George Washington University. Carnegie Institution of Washington.

two linker molecules has a distinct influence not only on the topology of uranyl-containing coordination polymers but also on the luminescent properties as we have simultaneously introduced a chromophoric sensitizer species. Reported herein are the synthesis, crystal structures, and fluorescent properties of 11 novel uranium(VI) metal-organic coordination polymer structures containing both aliphatic and aromatic organic components. Experimental Section Synthesis. Caution: Whereas the uranium oxynitrate hexahydrate (UO2(NO3)2‚6H2O) used in this study contains depleted U, standard precautions for handling radioactiVe substances should be followed. UO2(NO3)2‚6H2O was produced by oxidizing UO2 (Strem) in excess concentrated HNO3.16 The organic linkers were purchased and used without any further purification. (UO2)2(C5H6O4)3‚(C10H10N2)(H2O)2 (5). Compound 5 (numbering scheme continues from part I of this contribution) was prepared by dissolving 0.25 g of UO2(NO3)2‚6H2O, 0.066 g of glutaric acid (Eastern), and 0.078 g of 4,4′-dipyridyl (Aldrich) in 1.36 g of water. The solution (pH 2.38) was prepared in a 23 mL Teflon-lined Parr bomb, and 80 µL of concentrated ammonium hydroxide (NH4OH, Fisher) was added to adjust the pH of the solution. The resulting mixture (pH 3.73, molar ratio 1:1:1:151:2) was heated statically at 180 °C for 3 days. A yellow crystalline solid was formed and washed with water and ethanol. A minor second phase, UO2(C5H6O4),17 was present in each of the preparations as determined by the analysis of the X-ray powder diffraction patterns. UO2(C6H8O4)(C10H8N2) (6). Compound 6 was prepared in the manner described above by replacing the aliphatic acid with 0.073 g of adipic acid (Aldrich) and to keep the molar ratio consistent with 5. The solution (pH 3.12) was heated statically at 180 °C for 3 days. A yellow crystalline solid was formed and washed with water and ethanol. A minor phase of UO2(C6H8O4)18 (less than 10%) was present in each preparation as determined by X-ray powder diffraction. (UO2)2(C7H10O4)2(C7H11O4)2‚(C10H8N2)(C10H10N2)(H2O)2 (7) and (UO2)4(O)2(H2O)2(C7H10O4)2(C10H8N2)‚H2O (8). Compounds 7 and 8 crystallized from the same solution, which was prepared in the manner described above by replacing the aliphatic acid with 0.080 g of pimelic acid (Eastern) and to keep the molar ratio consistent with that for 5. The solution (pH 4.31) was statically heated for 3 days at 180 °C. A

10.1021/cg060330g CCC: $33.50 © 2006 American Chemical Society Published on Web 09/01/2006

Crystal Engineering with the Uranyl Cation II

Crystal Growth & Design, Vol. 6, No. 10, 2006 2249 Table 1. Crystallographic Data for 5-11

chemical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z λ (Mo KR) Dcalcd (g cm-3) µ (mm-1) temp (K) R1 (on Fo2, I > 2σ(I)) wR2 (on Fo2, I > 2σ(I))

5

6

7

8

9

10

11

C25H28N2O18U2 1120.55 monoclinic C2/c (No. 15) 15.5406(1) 12.5765(1) 16.821(2) 90 101.024(4) 90 3226.9(6) 4 0.71073 2.306 10.106 298 0.0439 0.0713

C16H16N2O6U 570.34 trigonal P3221 (No. 154) 12.2138(8) 12.2138(8) 10.0330(6) 90 90 120 1296.17 3 0.71073 2.192 9.427 298 0.0254 0.0460

C24H32N2O11U 762.55 triclinic P1h (No. 2) 9.2194(5) 12.1003(8) 12.6351(8) 85.6120(1) 75.136(2) 78.964(2) 1336.62(1) 2 0.71073 1.895 6.134 298 0.0354 0.0602



C14H20NO10U2 838.37 triclinic P1h (No. 2) 7.2624(7) 11.4342(1) 12.3394(1) 76.767(2) 74.841(2) 84.239(2) 961.85(2) 2 0.71073 2.895 16.867 100 0.0278 0.0608

C15H20NO6U 548.35 triclinic P1h (No. 2) 8.1332(2) 9.896(2) 11.983(3) 70.663(4) 87.499(4) 67.749(4) 838.6(3) 2 0.71073 2.172 9.707 100 0.0731 0.1485

crystalline solid containing 7, 8, and a third phase (UO2(C7H12O4)) was formed and washed with water and ethanol. Efforts to crystallize the compounds as single phases proved unsuccessful. (UO2)2(O)(C8H12O4)(C10H8N2)(H2O) (9). Compound 9 was synthesized in the manner described above by replacing the aliphatic acid with 0.087 g of suberic acid (Aldrich), to keep the molar ratio established in 5. The resulting solution (pH 4.31) was heated statically at 180 °C for 3 days. A yellow crystalline solid was formed, collected, and washed with water and ethanol. Percent Yield: 24% based on uranium. Elemental analysis (Galbraith Laboratories, Knoxville, TN), observed (calculated): C 25.81% (24.96%); H 2.74% (2.4%); N 3.16% (3.1%). (UO2)4(O)2(C9H14O4)2(C10H8N2)(H2O)2 (10). Compound 10 was synthesized in the manner described above by replacing the aliphatic acid with 0.094 g of azelaic acid (Aldrich) using the molar ratio established in 5. The resulting solution (pH 3.08) was heated statically at 180 °C for 3 days. A yellow crystalline solid was formed, collected, and washed with water and ethanol. Percent Yield: 39% based on uranium. Elemental analysis, observed (calculated): C 25.46% (20.06%); H 3.12% (2.39%); N 2.28% (1.67%). Upon closer evaluation of the X-ray powder diffraction pattern, an unidentified secondary phase was determined to be present in the sample. (UO2)2(C10H16O4)2(C10H8N2) (11). Compound 11 was synthesized in the manner described above replacing the aliphatic acid with 0.101 g of sebacic acid (Aldrich) using the molar ratio established in 5. The resulting solution (pH 3.65) was heated statically at 180 °C for 3 days. A yellow crystalline solid was formed, collected, and washed with water and ethanol. Percent Yield: 53% based on uranium. Elemental analysis, observed (calculated): C 32.67% (32.85%); H 3.72% (3.65%); N 2.66% (2.55%). (UO2)2(C6H8O4)3‚(C12H12N2) (12). Compound 12 was synthesized by dissolving 0.25 g of UO2(NO3)2‚6H2O, 0.073 g of adipic acid, and 0.092 g of 1,2-bis(4-pyridyl)ethane (Aldrich) in 1.36 g of H2O. The solution (pH 2.09) was prepared in a 23 mL Teflon-lined Parr bomb, and 80 µL of concentrated NH4OH was added to adjust the pH of the solution. The resulting mixture (pH 3.25, molar ratio 1:1:1:151:2) was heated statically at 120 °C for 3 days. A yellow crystalline solid was formed and washed with water and ethanol. In each of the preparations of this compound, an unidentified secondary phase was present, and thus the elemental analysis and percent yield calculations were considered to be unreliable. (UO2)2(C7H10O4)3‚(C12H14N2) (13). Compound 13 was prepared in the same manner as 12 using the established molar ratio and replacing the aliphatic ligand with 0.080 g of pimelic acid. The final mixture (pH 2.25) was heated statically at 120 °C for 3 days resulting in a yellow crystalline solid that was collected and washed with water and ethanol. Examination of the powder XRD pattern revealed the presence of an unidentified secondary phase. A pure sample was obtained by doubling the amount of both of the organic linkers. Percent Yield: 45% based on uranium. Elemental analysis, observed (calculated): C 32.97% (33.01%); H 3.85% (3.66%); N 2.39% (2.33%). (UO2)2(O)(C8H12O4)(C12H12N2) (14). Compound 14 was prepared in the same manner as 12 using the established molar ratio by replacing the aliphatic ligand with 0.087 g of suberic acid. The final mixture

(pH 4.90) was heated statically at 120 °C for 3 days resulting in a yellow crystalline solid that was collected and washed with water and ethanol. Examination of the powder XRD pattern revealed the presence of unknown secondary phases, the presence of which suggested the elemental analysis and percent yield calculations would be unreliable, and they were thus not attempted. (UO2)2(C9H14O4)2(C12H12N2) (15). Compound 15 was prepared in the same manner as 12 by replacing the aliphatic ligand with 0.094 g of azelaic acid and using the established molar ratio. The resulting mixture (pH 4.34) was heated statically at 120 °C for 3 days resulting in a yellow crystalline solid that was collected and washed with water and ethanol. Examination of the powder XRD pattern revealed an unidentified secondary phase, again preventing reliable elemental analysis and percent yield data. X-ray Diffraction. Powder X-ray diffraction data of all of the samples were collected on a Scintag XDS 2000 diffractometer (Cu KR, 3-60°, 0.02° step, 1.0 s/step) and manipulated using the Jade software package.19 Comparison of the observed and calculated patterns either confirmed pure samples or revealed the presence of secondary phases (above). Single crystals of each of the samples were selected from the bulk and mounted on either a glass fiber (5-9 and 13-15) or a cryoloop (10-12). Reflections for 5, 6, 7, and 14 were collected at room temperature on a Bruker P4 diffractometer equipped with an Apex CCD detector using Mo KR radiation (λ ) 0.7107 Å) and 0.3° ω scans. The data for 5, 6, 7, and 14 were integrated with the SMART/SAINT software packages.20,21 Reflections for 8-13 and 15 were collected on a Bruker SMART diffractometer equipped with an Apex II CCD detector using Mo KR radiation and 0.5° ω scans. Data from single crystals of 8, 13, and 15 were collected at room temperature, whereas those for 9-12 were collected at 100 K. The data for 8-13 and 15 were integrated with the SAINT software package.22,23 The data for 5, 6, 7, 12, 13, and 15 were corrected for absorption using a semiempirical method (from equivalents), whereas the data for 8, 9, 10, 11, and 14 were corrected by using SADABS.24 Structures of compounds 5, 6, 7, 10, and 12 were solved using SIR92,25 whereas those of compounds 8, 9, 11, 13, 14, and 15 were solved using SHELXS97.26 All of the structures were refined using SHELXL9726 within the WINGX suite of software.27 Data collection and refinement details can be found in Tables 1 and 2. The heavy atoms and all other non-hydrogen atoms in each of the structures were found in Fourier-difference maps and refined anisotropically. Hydrogen atom positions for all of the structures were calculated and allowed to ride on their respective C atoms with C-H distances of 0.93-0.97 Å and Uiso(H) ) -1.2Ueq(C). Hydrogen atoms bound to water molecules or N atoms were located in the Fourierdifference map, and their distances were fixed. Disordered organic components were found in three of the structures. In 6, the center of the adipic acid molecule has two positions (C3 and C3′) where the atom is likely to be found. The presence of this disorder explains the observation of a short C-C bond distance (1.36(3) Å) between C3 and its symmetry equivalent. Each of the disorder sites has a 50% probability of being occupied, which allows for a more reasonable C-C bond distance of 1.514 Å. Disorder was also found

2250 Crystal Growth & Design, Vol. 6, No. 10, 2006

Borkowski and Cahill

Table 2. Crystallographic Data for 12-15

chemical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z λ (Mo KR) Dcalcd (g cm-3) µ (mm-1) temp (K) R1 (on Fo2, I > 2σ(I)) wR2 (on Fo2, I > 2σ(I))

12

13

14

15


C33H44N2O16U2 1200.76 triclinic P1h (No. 2) 10.6485(8) 11.6905(9) 17.1826(1) 72.419(1) 75.264(1) 81.642(1) 1966.6(3) 2 0.71073 2.028 8.295 298 0.0367 0.0922

C20H24N2O9U2 912.47 monoclinic P21/c (No. 14) 8.4931(3) 26.2436(1) 10.7482(5) 90 103.097(1) 90 2333.34(2) 4 0.71073 2.597 13.916 298 0.0301 0.0740


in the solvent water molecules of 5. The dominant position for the solvent is OW1a and has 60% probability of being occupied. This disorder prevented the modeling of the solvent hydrogen atom positions. Disorder is also observed in compound 13 where one of the pimelate molecules (C8-C14) has a dominant component with a 63% probability of occupancy. In compound 5, the H atom bound to the N atom of the 4,4′-dipyridyl molecule was located in the Fourier-difference map and was placed in a calculated position and allowed to ride on the respective atom at a N-H distance of 0.86 Å with Uiso(H) ) -1.2Ueq(N). The H atom positions for the solvent water molecules were not able to be located in a Fourier-difference map and were thus not included in the final refinement cycles. For compound 7, H1, which is attached to N1, was located in the Fourier-difference map, yet it was placed in a calculated position at 0.86 Å from N1 and allowed to ride with Uiso(H) ) -1.2Ueq(N1). H10 was placed in an idealized position at a calculated distance from O10, which was determined to be the hydroxyl part of the acid molecule by comparison of the O9 to C14 and O10 to C14 distances (1.193(8) and 1.259(9) Å, respectively). The distance between O10 and H10 was set at 0.82 Å with Uiso(H) ) -1.5Ueq(O10). The H atoms on the solvent water molecule (OW1) were located in the Fourier-difference map and were fixed at a distance of 0.80(2) Å and allowed to refine with Uiso(H) ) -1.5Ueq(OW1). In compound 8, the H atoms attached to the bound water molecules (O19 and O20) and the solvent water molecule (OW1) were found in the Fourier-difference map and fixed at a distance of 0.80(2) Å and allowed to refine with Uiso(H) ) -1.5Ueq(O). Compounds 9 and 10 each contain one bound water molecule. The H atoms for the water molecules were located in the Fourier-difference map, and the O-H bond distances were fixed at 0.80(2) Å and allowed to refine with Uiso(H) ) -1.5Ueq(O). Compounds 12 and 13 contain protonated 1,2bis(4-pryidyl)ethane molecules. The H atoms were placed in calculated positions at a distance of 0.86 Å from their respective N atoms and were allowed to ride on their positions with Uiso(H) ) -1.2Ueq(N).

by glutarate molecules (C1-C5) to form chains. Two chains are further connected by a second glutarate molecule (C6-C8) forming the bilayer. The dipyridyl molecules are hydrogenbonded to the bilayers though O7 and, to a lesser extent, O3 (Table 3). This H-bonding scheme may be viewed as a mode of connectivity between adjacent bilayers. Compound 6, UO2(C6H8O4)(C10H8N2), consists of individual UO2(O)4(N)2 hexagonal bipyramids that are connected by adipate and 4,4′-dipyridyl molecules to form two-dimensional sheets (Figure 3). The coordination environment of the single crystallographically unique U site (Figure 4) consists of six oxygen atoms: four from the adipate molecules that are in bidentate coordination to the UO22+ center, two uranyl oxygen

Figure 1. Polyhedral representation of 5 viewed down the [010]. The protonated 4,4′-dipyridyl molecules are shown in blue, and the solvent water molecules are represented by the blue spheres.

Results and Discussion Structural Descriptions. Compound 5, (UO2)2(C5H6O4)3‚ (C10H10N2)(H2O)2, consists of individual UO2(O)6 hexagonal bipyramids connected by glutarate molecules to form helical bilayers (Figure 1). These anionic layers are held apart from each other by protonated 4,4′-dipyridyl molecules (which serve to charge-balance the anionic layers), as well as two water molecules. The coordination of the crystallographically unique U site consists of a central UO22+ cation with six oxygen atoms from three glutarate molecules in the equatorial plane (each in a bidentate coordination, Figure 2). The uranyl oxygens (O1 and O2) are at an average distance of 1.745 Å from the U center, and the glutarate oxygens (O3-O8) are farther away at an average distance of 2.461 Å. The UO22+ centers are connected

Figure 2. ORTEP representation of 5 showing the numbering scheme. Ellipsoids are shown at 40% probability. H atoms bonded to C atoms are omitted for clarity. Atoms labeled with i, ii, or iii are at symmetry positions (-x, y, -z + 1/2), (x + 1/2, y + 1/2, z), or (-x, -y, -z), respectively.


Crystal Growth & Design, Vol. 6, No. 10, 2006 2251

Table 3. Hydrogen Bonding Distances for Selected Structuresa structure

D-H‚‚‚A

5

N1-H1‚‚‚O7 N1-H1‚‚‚O31 O10-H10‚‚‚N22 N1-H1‚‚‚O83 N1-H1‚‚‚O2 OW1-HW1‚‚‚OW12 OW1-HW2‚‚‚O7 OW1-HW1‚‚‚O6 OW1-HW2‚‚‚O4 O19-H19C‚‚‚O13 O19-H19D‚‚‚O144 O20-H20C‚‚‚O95 O20-H20D‚‚‚O18 O6-H6A‚‚‚N1 O6-H6B‚‚‚N46 N1-H1‚‚‚O67 N1-H1‚‚‚O148 N2-H2A‚‚‚O79

7

8

9 12 13

d(D-H), d(H‚‚‚A), d(D‚‚‚A), ∠(DHA), Å Å Å deg 0.86 0.86 0.82 0.86 0.86 0.80(2) 0.81(2) 0.80(2) 0.80(2) 0.79(2) 0.79(2) 0.80(2) 0.80(2) 0.78(2) 0.78(2) 0.91(1) 0.86 0.86

1.96 3.11 1.81 1.94 2.55 2.06(4) 2.07(3) 2.56(2) 2.23(8) 2.43(1) 2.02(4) 2.02(3) 2.44(1) 2.20(2) 2.20(2) 1.97(1) 1.89 1.90

2.750(1) 3.515(1) 2.624(7) 2.706(5) 3.112(6) 2.771(9) 2.852(6) 2.877(2) 2.952(1) 3.024(1) 2.802(1) 2.817(1) 2.682(1) 2.976(6) 2.968(7) 2.818(1) 2.743(8) 2.744(8)

152.4 111.5 176.3 147.2 123.5 147(7) 162(7) 105(1) 149(1) 133(1) 169(2) 170(2) 99(1) 169(7) 168(7) 155(9) 169.0 167.0

Figure 5. Polyhedral representation of 6 viewed down the [001] direction.

Symmetry transformations used to generate equivalent atoms: 1(-x + y - 1/2, -z + 1/2), 2(-x + 1, -y + 1, -z + 1), 3(-x + 1, -y + 1, -z), 4(-x + 1/ , y - 1/ , -z + 1/ ), 5(-x + 1/ , -y + 3/ , -z), 6(x - 1/ , y + 1/ , 2 2 2 2 2 2 2 z), 7(x, y + 1, z + 1), 8(x, y - 1, z), 9(x - 1, y, z). a

1/

2,

Figure 6. Polyhedral representation of 7 viewed down [010]. Neutral 4,4′-dipyridyl molecules are drawn in red, and the protonated molecules are blue. Figure 3. Representation of one of the interpenetrated sheets. The color coding is as follows: yellow for uranium, red for oxygen, blue for nitrogen, black for carbon, and gray for hydrogen.

Figure 4. ORTEP representation of 6 showing the atom numbering scheme. Ellipsoids are shown at 50% probability, and H atoms were omitted for clarity. Atoms label with an i are at symmetry positions (y, x, -z). The disorder in the adipate chain is shown in both light and dark gray with the dominant component (C3) in dark gray.

atoms, and two nitrogen atoms from the bipyridine molecules. The uranyl oxygens (O1 and its symmetry equivalent) are found at a distance of 1.751(4) Å from the uranium center. In the equatorial plane, the adipate oxygens (O2 and O3) can be found at an average distance of 2.485 Å from the uranium center, whereas the nitrogen atoms (N1 and N2) are at an average distance of 2.560 Å.

The local coordination environment of the U atom is similar to a U-adipate (UO2(C6H8O4)(H2O)2) structure previously reported by our group.18 In both 6 and UO2(C6H8O4)(H2O)2, the adipate oxygen atoms are bidentate with the UO22+ center and the carboxylate groups themselves trans to one another. This bonding allows for additional binding sites to be occupied by the 4,4′-dipyridyl molecules in 6 and water molecules in UO2(C6H8O4)(H2O)2. Both the adipate and 4,4′-dipyridyl molecules bond to U atoms in same plane forming the twodimensional sheets (Figure 3) with a rectangular “pore”. Slightly off center of this “pore” resides the U atoms of another sheet demonstrating an overall three-way interpenetration of individual sheets (Figure 5). Compound 7, (UO2)2(C7H10O4)2(C7H11O4)2‚(C10H8N2)(C10H10N2)(H2O)2, consists of individual UO2(O)6 hexagonal bipyramids that are connected by pimelate molecules to form anionic chains (Figure 6). The coordination sphere of the U atom consists of the uranyl oxygens (O1 and O2) at an average distance of 1.775 Å and six oxygen atoms (O3-O8) from three pimelate molecules at an average U-O distance of 2.459 Å (Figure 7). As with 5, there are two distinct aliphatic acid molecules. One pimelate molecule (C1-C7) is fully deprotonated and connects the UO22+ centers into the chains, whereas the second pimelate molecule (C8-C14) is only partially deprotonated and thus only bound to one U atom. The other end of the acid molecule (O9-C14-O10) is protonated and not bound to any UO22+ center yet does participate in hydrogen bonding with N2 on a guest dipyridyl molecule. The interlayer region contains both neutral and protonated 4,4′-dipyridyl molecules (Figure 8). The neutral dipyridyl molecules are



Figure 7. ORTEP representation of 7 illustrating the numbering scheme. Ellipsoids are drawn at 30% probability. H atoms bonded C atoms were omitted for clarity. Atoms labeled with i or ii are at symmetry positions (1 + x, 1 + y, 1 + z) or (-x, -y, -z), respectively. Figure 10. Polyhedral representation of 8 down [010] illustrating the placement of the solvent water molecules in the packing of the layers.

Figure 8. Polyhedral representation of 7 viewed down (100). Neutral 4,4′-dipyridyl molecules are drawn in red, and the protonated molecules are blue.

Figure 9. Polyhedral representation of a layer of 8 viewed down [001].

involved in hydrogen bonding with O10 connecting the chains into pseudolayers in the (001) plane. The layers are separated by alternating groups of protonated dipyridyl and water molecules. The protonated dipyridyl molecules balance the charge of the anionic chains resulting in a neutral structure overall. The structure of 8, (UO2)4(O)2(H2O)2(C7H10O4)2(C10H8N2)‚ H2O, consists of (UO2)4(O)12(N)2 tetramers linked by pimelate and 4,4′-dipyridyl molecules to form layers (Figure 9). The layers are in a sense “coupled” because water molecules reside between every other layer (Figure 10). There are four crystallographically unique uranium sites in the tetramer building unit (Figure 11), three of which have a pentagonal bipyramid local coordination sphere, whereas the fourth has hexagonal bipyramid coordination geometry. All of the uranyl oxygen atoms (O3-O10) reside at an average distance of 1.773 Å from their respective U atoms. Two more oxygen atoms (O1 and O2) are essential to the tetramer unit and are bound to three U atoms in

Figure 11. ORTEP representation of 8 illustrating the numbering scheme. H atoms bonded to C atoms were omitted for clarity. Atoms labeled with i or ii are at symmetry positions (-x, y, -z + 1/2) or (x + 1 /2, y + 1/2, z), respectively.

a trigonal planar geometry. Bond valence calculations for these oxygen sites (2.10 and 1.99 vu for O1 and O2, respectively) suggest that they are not hydroxyl groups.28,29 U2 and U4 have similar pentagonal bipyramid coordination spheres. Both are connected to five oxygen atoms each in the equatorial plane resulting in eight unique oxygen atoms, two of which are O1 and O2 (above); four O atoms (O12, O13, O16, and O18) are part of four distinct acid molecules, and the remaining two (O19 and O20) are bound water molecules. U3 is also found in pentagonal bipyramidal geometry with one nitrogen atom from a dipyridyl molecule, four oxygen atoms from two distinct acid molecules, and the tricoordinated O2 in the equatorial plane. The carboxylate groups from one acid molecule (O13 and O14) are found in bridging tridentate coordination with the tetramer building unit; O13 is shared between U2 and U3, and O14 bound only to U2. Trans to O13 and O14 is O17, which, with O18, bridges U3 to U4. Bonded to U3 opposite O2 is N2 from the dipyridyl molecule. The other end of the dipyridyl molecule, N1, is connected to U1 completing a chain of tetramers. U1 is found in hexagonal bipyramidal coordination with N1 and O1 trans to one another. There are two carboxylate groups found in bridging tridentate coordination (O11, O12 and O15, O16) trans to one another completing the hexagonal bipyramidal coordination. Compound 9, (UO2)2(O)(C8H12O4)(C10H8N2)(H2O), consists of (UO2)4O12N2 tetramer units linked by suberate and 4,4′dipyridyl molecules to form layers (Figure 12). The tetramer units are constructed from three crystallographically unique UO22+ centers in two coordination spheres, hexagonal and



Figure 14. Polyhedral representation of 10 viewed down the [201h] direction illustrating the chains. Figure 12. Polyhedral view of 9 viewed down the [001] direction illustrating a layer.

Figure 13. ORTEP of 9 showing the numbering scheme. H atoms bonded to C atoms were omitted for clarity. Ellipsoids are shown at 60% probability. Atoms labeled with i, ii, or iii are at symmetry positions (-x, y, -z + 1/2), (x + 1/2, y + 1/2, z), or (-x + 1/2, y + 1/2, -z + 1/2), respectively.

pentagonal bipyramidal coordination. All of the uranyl oxygens (O1-O4) reside at an average distance of 1.779 Å from their respective U atoms. Two more oxygen atoms (O5) are essential to the tetramer unit and are bound to three U atoms in a trigonal planar geometry (Figure 13). Bond valence calculations28,29 for this oxygen site (2.09 vu) suggest that they are not hydroxyl groups. U1 is found in hexagonal bipyramid coordination with the equatorial plane consisting of four oxygens from two suberate molecules trans to each other and a water molecule trans to O5. The suberate molecules are found in bridging tridentate coordination with the tetramer units, further stabilizing the tetramer units. U2 and U3 have similar coordination spheres that consist of four O atoms from the trigonal planar oxygen atom (O5) and from the bridging tridentate suberate group (O7 or O9), and one N atom from the dipyridyl molecule (N2 or N3). The suberate molecules link tetramer units into layers, while the dipyridyl molecules are bound to the tetramer units at one end (N2 and N3) and hydrogen bonded to the bound water molecules (O6) at the other end (N1 and N4) (Table 3). Compound 10, (UO2)4(O)2(C9H14O4)2(C10H8N2)(H2O)2, is composed of (UO2)4O12N2 tetramer units linked by azelate and 4,4′-dipyridyl molecules to form chains (Figure 14). There are two crystallographically unique U sites found in pentagonal bipyramidal geometry. The uranyl oxygen atoms (O1-O4) are found at an average distance of 1.782 Å from their respective U atoms (Figure 15). O5 is bound to U1 and U2 in trigonal planar coordination and is essential to the tetramer formation. A value of 1.98 vu for the bond valence summation of this site suggests that O5 is not a hydroxyl group. Although U1 and U2 are both found in pentagonal bipyramid geometry, the equatorial plane of U2 is different from that of U1. The equatorial plane of U1 consists of O5 (2.217(4) Å), three carboxylate oxygen atoms (O6-O8) at an average distance of 2.462 Å associated with two azelate molecules, and

Figure 15. ORTEP of 10 showing the numbering scheme. H atoms bonded to C atoms were omitted for clarity. Ellipsoids are shown at 60% probability. Atoms labeled with i, ii, or iii are at symmetry positions (-x, -y, -z), (x + 1, y + 1, z + 1), or (-x + 1, -y + 1, -z + 1), respectively.

Figure 16. Polyhedral representation of 11 illustrating the crystallographically unique layer of this compound.

one water molecule (Ow1, 2.400(4) Å). U2 is bound to two carboxylate oxygen atoms (O6 and O9, 2.482 Å) from two azelate molecules, O5 (and its symmetry equivalent) at average distance of 2.286 Å, and N1 (2.559(5) Å) from the 4,4′-dipryidyl molecule. Compound 11, (UO2)2(C10H16O4)2(C10H8N2), consists of individual UO2(O)4N pentagonal bipyramids connected by sebacate and 4,4′-dipyridyl molecules to form layers (Figure 16). The uranyl oxygen atoms (O1 and O2) are found at an average distance of 1.715 Å from the crystallographically unique U atom. The equatorial plane of the UO22+ moiety consists of one N atom from a 4,4′-dipyridyl molecule and four O atoms from three sebacate molecules (Figure 17). The equatorial O atoms are found at an average distance of 2.35 Å from the UO22+ moiety, while the U-N distance is slightly longer at a distance of 2.545(1) Å. The carboxylate groups of the sebacate molecules are found in two different coordination modes: the group containing O5 and O6 is bound to the UO22+ cation in a bidentate fashion, whereas at the opposite end of the sebacate


Figure 17. ORTEP of 11 showing the numbering scheme. Ellipsoids are shown at 40% probability, and H atoms were omitted for clarity. Atoms labeled with i or ii are at symmetry positions (-x, -y, -z) or (x + 1, y + 1, z + 1), respectively.

Figure 18. Polyhedral representation of 12 illustrating the [(UO2)2(C6H8O4)3]2- layers and the placement of the protonated 1,2-bis(4pyridyl)ethane molecules.

Figure 19. ORTEP of 12 showing the numbering scheme. Ellipsoids are shown at 60% probability, and the H atoms bonded to C atoms are omitted for clarity. Atoms labeled with an i are at symmetry position (-x, -y, -z).

molecule, the group containing O3 and O4 is coordinated in a bridging bidentate mode. O3 and O4 bridge the individual pentagonal bipyramids into pseudodimers that are connected by the sebacate molecules to form chains running in the [11h1] direction. The coordination of the 4,4′-dipyridyl molecules to the UO22+ further connects the UO2-sebacate chains into layers. Compound 12, (UO2)2(C6H8O4)3‚(C12H12N2), consists of UO2(O)6 hexagonal bipyramids connected by adipate molecules to form anionic chains that are charge-balanced by protonated 1,2-bis(4-pyridyl)ethane molecules (Figure 18). The coordination environment of the crystallographically unique U atom consists of two uranyl oxygen atoms (O1 and O2) found at an average distance of 1.775 Å from the U atom and six equatorial O atoms from three adipate chains found at an average distance of 2.48 Å (Figure 19). Each of the adipate molecules is coordinated in a bidentate fashion to the UO22+ center. One adipate molecule and its symmetry equivalent link the UO22+ into pseudodimers,


Figure 20. Polyhedral representation of 13 illustrating the interdigitation of the [(UO2)2(C7H10O4)3]2- layers.

Figure 21. ORTEP representation of 13 illustrating the numbering scheme. Ellipsoids are at 45% probability, and H atoms attached to C atoms as well as the disordered atoms (C10a, C11a, and C12a) were omitted for clarity. Atoms labeled with i or ii are at symmetry positions (-x, -y, -z) or (x + 1, y + 1, z + 1), respectively.

while the second crystallographically unique adipate molecule polymerizes the pseudodimers into chains running in the [111] direction. The [(UO2)2(C6H8O4)3]2- chains are separated by protonated 1,2-bis(4-pyridyl)ethane molecules that are hydrogen bonded to the chains through an interaction between N1-H1 and O6 (Table 3). Compound 13, (UO2)2(C7H10O4)3‚(C12H14N2), consists of individual UO2(O)6 hexagonal bipyramids linked by pimelate molecules to form anionic layers that are charge-balanced by protonated 1,2-bis(4-pyridyl)ethane molecules (Figure 20). There are two crystallographically unique U sites that have the same coordination environments that consist of two uranyl oxygen atoms and six equatorial oxygen atoms from three pimelate molecules. The uranyl oxygens (O1-O4) are found at an average distance of 1.76 Å from their respective U atoms (Figure 21). There are three crystallographically unique pimelate molecules coordinated in a bidentate fashion to the UO22+ centers that serve two distinct roles in the overall structure. Two of the pimelate molecules link the UO22+ centers into chains, while the third molecule connects the chains into layers. The [(UO2)2(C7H10O4)3]2- layers interdigitate, and thus “trap” the protonated pyridyl molecules within the structure. Last, there are two crystallographically unique pyridyl molecules that are hydrogen bonded to the layers (Table 3). Compound 14, (UO2)2(O)(C8H12O4)(C12H12N2), consists of (UO2)4O10N4 tetramer units linked by suberate and 1,2-bis(4pryidyl)ethane molecules to form layers (Figure 22). The tetramer units are constructed of two crystallographically unique



Figure 24. Polyhedral representation of 15 illustrating a crystallographically unique layer. Figure 22. Polyhedral representation of 14 illustrating a layer of this material.

Figure 23. ORTEP representation of 14 showing the numbering scheme. Ellipsoids are shown at 50% probability, and the H atoms were deleted for clarity. Atoms labeled with i, ii, or iii are at symmetry positions (-x, y + 1/2, -z + 1/2), (-x, -y, -z), or (x, -y + 1/2, z + 1/ ), respectively. 2

U sites each found in pentagonal bipyramidal geometry. The uranyl oxygens (O1-O4) are found at an average distance of 1.773 Å from their respective U atoms (Figure 23). Two more oxygen atoms (O5 and its symmetry equivalent) that are essential to the tetramer units are found coordinated to three U atoms, and bond valence calculations (2.05 vu) suggest that O7 is not a hydroxyl oxygen. U1 and U2 have essentially the same coordination spheres consisting of an equatorial plane containing one nitrogen atom and four oxygen atoms, with the source molecule of coordinate oxygen atoms being the primary difference between them. U1 is bound to two carboxylate oxygens from two suberate molecules (O6 and O9) at an average distance of 2.415 Å and O5 (and its symmetry equivalent) at an average distance of 2.287 Å. U2 is bound to three carboxylate oxygens from two suberate molecules (O6, O7, and O8) at an average distance of 2.514 Å and O5 (2.166(4) Å). The 1,2-bis(4-pryidyl)ethane molecule is bound to U1 through N1 (2.601(5) Å) and U2 through N2 (2.603(5) Å). Compound 15, (UO2)2(C9H14O4)2(C12H12N2), consists of individual pentagonal bipyramids connected by azelate and 1,2bis(4-pyridyl)ethane molecules to form layers (Figure 24). There is one crystallographically unique U site with its uranyl oxygens (O1 and O2) found at an average distance of 1.755 Å (Figure 25). The equatorial plane consists of four oxygen atoms associated with three azelate molecules and one nitrogen atom (N1) from the 1,2-bis(4-pyridyl)ethane molecule. The N atom is found at a slightly longer distance (2.566(5) Å) from the UO22+ moiety than the equatorial O atoms. The carboxylate group containing O3 and O4 is coordinated in a bidentate fashion to the UO22+ center, and these oxygens are at an average distance of 2.437 Å. The remaining carboxylate oxygens (O5

Figure 25. ORTEP representation of 15 showing the numbering scheme. Ellipsoids are shown at 50% probability, and the H atoms were deleted for clarity. Atoms labeled with i are at symmetry positions (-x, -y, -z).

and O6, average distance 2.312 Å) bridge the UO22+ centers into pseudodimers. The pseudodimers are connected by the azelate molecules forming chains running in the [11h0] direction that are further connected by the 1,2-bis(4-pyridyl)ethane molecules to form the overall 2D topology. Structural Systematics. The exploration and synthesis of the structures reported herein began as an effort to influence the pore size and dimensionality of an existing three-dimensional framework material, UO2(C6H8O4).18 A more rigid molecule (4,4′-dipyridyl) was added, in theory, to expand the pores contained within UO2(C6H8O4) and perhaps “template” a novel topology in an approach similar to some of our other studies that is well-known from zeolite chemistry and has been applied to Ln-MOF systems.30,31 Rather than serving strictly as a guest molecule in this system, however, the bipyridine molecules instead actually play three distinct roles: ligand (6, 8-11, 14-15), charge balancing cation (5, 7, 12, 13), and potential structure directing agent (7). In compounds 5-11, the 4,4′-dipyridyl molecule is most frequently found bound to the central UO22+ ion. In all of these compounds, the 4,4′-dipyridyl molecule is essential in the formation of the overall topology. In 6, the dipyridyl molecule is relatively flat with a torsion angle of 2.4(4)° as found by measuring the angle between C5, C6, C7, and C8. A torsion angle of this size for 6 can be attributed to the steric hindrance caused by the interpenetration. In structure 8, the torsion angle (C4-C5-C6-C10) is measured at 19.9(2)°. A lack of steric hindrance as well as weak interactions between the dipyridyl molecules and the uranyl oxygens may allow for the increase in torsion angle.32 In structures 5 and 7, the dipyridyl molecules remain uncoordinated, although they are H-bonded to hexagonal bipyramid building units. Table 4 contains the observed torsion angles for 5-11, and in all but two structures (8 and 9), the 4,4′-dipyridyl molecules remain relatively flat regardless of coordination to the UO22+ centers. A survey of the torsion angles

2256 Crystal Growth & Design, Vol. 6, No. 10, 2006 Table 4. Torsion Angles for 5-11a structure 5 6 7 8 9 10 11

angle (deg) C11-C12-C121-C131 C5-C6-C7-C8 C26-C27-C272-C282 C21-C22-C223-C233 C4-C5-C6-C10 C5-C44-C34-C24 C8-C9-C10-C11 C4-C3-C35-C25 C4-C3-C32-C22

2(2) 2.4(4) -0.3(1) 0.5(1) 19.9(2) 3.8(4) -32.4(4) 1.2(1) 5(4)

a Symmetry transformations to generate equivalent atoms: 1(-x, -y + 1, -z), 2(-x, -y + 2, -z), 3(-x, -y + 1, -z + 1), 4(x + 1/2, y + 1/2, z), 5(-x + 1, -y + 1, -z + 1).

of coordinated and uncoordinated 4,4′-dipyridyl molecules found that the majority of the molecules had torsion angles in the range of 0-4° or 20-40° and our structures are consistent with the 4,4′-dipyridyl survey.33 The role that the bipyridine molecule plays in the construction of the structures appears to depend on its length relative to the length of the aliphatic linker. In structure 6, it is conceivable that both the adipate and dipyridyl molecules act as linkers because they are relatively similar in length (7.550 and 7.109 Å),34,35 allowing the dipyridyl molecule to act as an additional linker. Throughout this discussion, the lengths of free aliphatic acid molecules were determined by measuring the distance from a carbonyl O at one end to a hydroxyl O at the other end in pure crystalline samples reported by Thalladi et al.34 These molecules are found in roughly idealized conformations for aliphatic chains. The length of the bipyridine molecules was measured from N to N in pure crystalline samples.35,36 We are of the understanding that these length measurements are just an estimate, and the free acid and bipyridine molecules may have different conformations in solution and the solid state, yet we do see some utility in this comparison. In our previous report of U-adipate structures,18 we speculated that the change in dimensionality of the building unit from hexagonal bipyramids (UO2(C6H8O4)(H2O)2) to dimeric pentagonal bipyramids (UO2(C6H8O4)) was a result of the decrease of solvent water, which caused a decrease in the overall amount of ligands available to bond to the UO22+ ion. Structure 6 contains the same building unit as UO2(C6H8O4)(H2O)2 with dipyridyl molecules replacing the bound water molecules (above). This substitution can be seen as a result of an increase in the amount of organic species available to bond to the UO22+ ion. In structure 5, the dipyridyl molecules maintain a charge balancing role. In this case, the length of the glutarate molecule (6.486 Å)34,64 is slightly shorter than the average length of the 4,4′-dipyridyl molecule (7.109 Å).35 By comparing the relative differences between the adipic acid and 4,4′-dipyridyl (0.525 Å) and glutaric acid and 4,4′-dipyridyl (0.539 Å), one could conclude that the dipyridyl molecules should act as a ligand in 5. They do not, however, because the longer of the two ligands (4,4′-dipyridyl) in 5 is the more rigid of these and thus cannot bend to decrease its overall length to match that of the glutarate molecule. A similar argument can explain why the 4,4′-dipyridyl molecules are found in nonbonding roles in structure 7. The relative length of a pimelic acid molecule is 8.997 Å,34 which is almost 2 Å longer than the 4,4′-dipyridyl molecule. In this case the longer ligand is the flexible aliphatic ligand, but the difference between them is too large to be compensated by bends in the pimelate molecules. We hesitate to bring structure 8 into this discussion because its fundamental building unit, the


(UO2)4(O)12(N)2 tetramer, is distinct from the hexagonal bipyramids found in 5-7. With this caveat, however, it appears that the size-pairing argument is applicable here as well. Despite the differences in length, the tetramer provides two distinct binding locations as illustrated by the coordination environment of U1 (Figure 11). As stated above U1, is found in hexagonal bipyramidal coordination with the acid molecules trans to one another. This leaves two open bonding sites, one being used to form the tetramer by coordination between U1 and O1 and the other being open to bind to another organic species. The increase in the dimensionality of the building unit may offset the differences in the organic lengths and allow for the 4,4′-dipyridyl molecule to bond to the tetramer building unit. In structures 9 and 10, the difference in the relative lengths of the linkers is too large to be compensated by the conformation of the aliphatic linkers, and the building units polymerize to create larger secondary building units (Table 5). Structure 11, however, does not follow this trend, and instead both the sebacate and 4,4′-dipyridyl molecules are coordinated to the UO22+ center without a polymerization of the building unit. We attribute this observation as a result of the length and inherent “flexibility” of the sebacate molecules. The sebacate molecule in this structure has several “kinks” in the chains that decrease its overall length, and this decrease can be seen by comparing the lengths of the pure sebacic acid (12.633 Å)34,64 and a sebacate molecule in 11 (10.397 Å, the average of the distance between O3-O5 and O4-O6). The same size comparison can be extended to the structures containing 1,2-bis(4-pyridyl)ethane (12-15). Using 1,2-bis(4pyridyl)ethane, we have been able to synthesize structures utilizing adipic, pimelic, suberic, azelaic, and sebacic acids, although to date, we have been unsuccessful in crystallizing a coordination polymer containing both glutaric acid and 1,2-bis(4-pyridyl)ethane. Since the reaction conditions, including the molar ratios, were kept constant throughout this study, the difference in size between adipic (and pimelic) acid and 1,2bis(4-pyridyl)ethane (7.550 and 8.997 vs 9.278 Å, respectively)34,36 could be seen as the reason the pyridyl molecule is not coordinated to the UO22+ center in 12 and 13. Adding an ethane bridge to a 4,4′-dipyridyl molecule, resulting in 1,2-bis(4-pyridyl)ethane, imparts a degree of flexibility to the aromatic species. This effect can be seen in structure 14, where the pyridine rings are at a 74.53° angle to each other (determined by measuring the torsion angle among C13-C14C15-C16). This rotation about the ethane bridge could be seen as an effect of the difference between the lengths of the suberic acid (10.073 Å) and 1,2-bis(4-pyridyl)ethane (9.278 Å) and the formation of the tetramer units. For structure 15, which contains the longest aliphatic linkers in this study, one would expect from a size comparison that the 1,2-bis(4-pyridyl)ethane would be found in a nonbonding role in the structures; instead the bipyridine molecule is bound to the UO22+ centers in both structures. Although the aliphatic linker is longer than the bipyridine species by at least 2 Å in 15, the difference in the sizes appears to be compensated by rotations in the azelate molecule as seen by the difference in torsion angles calculated for the ends of the molecule (C7-C8-C9-C10, 67.6(8)°, and C12-C13-C14-C15, 149(3)°). We thus conclude that an appropriately paired combination of organic molecules (in terms of size) is required for each to serve as a linking species. In structures where this is not observed, the bipyridine is subject to protonation. Considering the variety of secondary building units observed in this family of materials, it is desirable to consider the aqueous



Table 5. Summary of Structural Features and Fluorescent Behavior linker(s)a

formula (UO2)2(C5H6O4)3‚ (C10H10N2)(H2O)2 UO2(C6H8O4)(C10H8N2) (UO2)2(C7H10O4)2(C7H11O4)2‚ (C10H8N2)(C10H10N2)(H2O)2 (UO2)4(O)2(H2O)2(C7H10O4)2 (C10H8N2)‚H2O (UO2)2(O)(C8H12O4)(C10H8N2)(H2O) (UO2)4(O)2(C9H14O4)2(C10H8N2)(H2O)2 (UO2)2(C10H16O4)2(C10H8N2) (UO2)2(C6H8O4)3‚(C12H14N2) (UO2)2(C7H10O4)3‚(C12H14N2) (UO2)4(O)2(C8H12O4)(C12H12N2) (UO2)2(C9H14O4)(C12H12N2) a

primary building unitb

secondary building unit

carboxylate coordinationc

fluorscenced

glutarate

HBP

b

blue shift

adipate, 4,4′-dipyridyl pimelate

HBP HBP

b b

blue shift red shift

pimelate, 4,4′-dipyridyl

1 HBP, 3 PBP

tetramer

bb, bt

red shift

suberate, 4,4′-dipyridyl azelate, 4,4′-dipyridyl sebacate, 4,4′-dipyridyl adipate pimelate suberate, bpe azelate, bpe

2 HBP, 2 PBP 4 PBP PBP HBP HBP 2 PBP, 2 SBP PBP

tetramer tetramer

bt bb, bt b, bb b b bb b, bb

red shift red shift red shift

b

tetramer

blue shift red shift c

1,2-Bis(4-pyridyl)ethane is shortened to bpe. HBP, PBP, and SBP represent hexagonal, pentagonal, and square bipyramids, respectively. b represents bidentate, bb represents bridging bidentate, and bt represents bridging tridentate. d All compounds were excited at 365 and 424 nm and compared to a UO2(NO3)2‚6H2O spectrum.

chemistry and speciation within the hydrothermal UO22+ system. Speciation studies have been conducted on UO22+ solutions to explore the hydrolysis of the UO22+ ion in aqueous media.37-41 These studies have typically been conducted on dilute UO22+ solution in noncomplexing media at temperatures below that of hydrothermal conditions. Although the studies were not conducted at conditions similar to those in this study (specifically temperature and UO22+ concentration), it can be inferred that oligomeric species (dimers, trimers, tetramers, etc.) can be formed at higher UO22+ concentrations and higher pH values. Since the concentration of the UO22+ ion (0.368 M) was maintained throughout these studies, it can be concluded that there are additional factors, besides the speciation of the UO22+, that play a role in the formation and ultimately the crystallization of oligomeric species. With respect to U(VI) coordination chemistry, there are three commonly observed primary building units: square, pentagonal, and hexagonal bipyramids. As we have seen in this study, these may polymerize into higher dimensional secondary building units. The majority of known U(VI) coordination polymers have monomeric building units42,43 (also as evidenced by a cursory search of the Cambridge Structural Database44), although we have previously synthesized a series of structures containing pentagonal bipyramid dimers.17,18 In the current series of structures, we have seen a polymerization of the primary building units in 8, 9, 10, and 14 to tetrameric secondary building units (SBUs). Each of the tetramer units reported herein are unique when compared to each other, yet there are a few features common to all. Tetrameric SBUs are not entirely common, and a search of the Cambridge Structural Database (CSD, v5.27)44 revealed only seven structures that contain such building units consisting of U4O10 units.45-51 For the tetramers presented herein, each is constructed from an essential U4O10 unit and contains at least two UO22+ centers in pentagonal bipyramidal coordination. The distinction between them can be seen in the coordination of the remaining two UO22+ centers as shown in Figure 26. Similarities can be drawn between the tetramers found in 9 and 14 (panels B and D in Figure 26, respectively). Distorted octahedra in panel D (U2) can be seen as “missing” the fifth and sixth equatorial bonds to form pentagonal or hexagonal bipyramids. Closer inspection of the U-O distances in the equatorial plane of compound 14 indicates that these “missing” bonds are slightly longer (U2-O6 2.749; U2-O9 3.015 Å) than typical equatorial U-O bonds, which usually range from 2.166 to 2.656 Å with an average length of 2.416 Å (based on a CSD search of U(VI) polymeric com-

Figure 26. Polyhedral representation of the tetrameric building units of 8 (A), 9 (B), 10 (C), and 14 (D).

pounds). Since these bonds are slightly longer than what is typically observed, we have opted to not include them as U-O equatorial bonds in an effort to highlight subtleties between these materials. Inclusion of these distances as bonds would in fact make the tetramers found in 9 and 14 identical. Fluorescence. The luminescence properties of U(VI) are of interest due to potential applications including photocatalysis.52-57 The utilization of UO22+ ions in coordination polymers arranges the metal centers into an extended topology as well as allows for the introduction of chromophoric organic linkers, which could sensitize the UO22+ luminescence. As with part I of this study, fluorescence experiments were conducted on a Shimadzu RF-5301 PC spectrofluorophotometer using a xenon lamp (emission wavelengths 400-800 nm; excitation slit width 1.5 nm; emission slit width 3.0 nm; high sensitivity). Fluorescence measurements were taken of 5-11, 13, and 15, although there were secondary phases present in 5, 6, and 15. We assumed that these secondary phases were sufficiently minor and, in the case of 5 and 6, consisted of uranyl-aliphatic carboxylates that contained no aromatic species to affect sensitization experiments. Since 7 and 8 were produced by the same reaction, pure samples were obtained by manual separation. All of the samples were explored via direct excitation of the UO22+ centers at 365 and 424 nm.58,59 Samples 5-11 were also examined via excitation of the 4,4′-dipyridyl molecules at 236 nm in order to investigate the effect of ligand to metal energy transfer (the “antenna effect”).53,57,60-63 All of the


spectra were compared to a spectrum of UO2(NO3)2‚6H2O excited at 365 nm for comparison of the peak positions. The spectra can be found in the Supporting Information, yet we have summarized the results in Table 5. Uranium(VI) fluorescence typically has a characteristic six peak spectrum relating to the S11 f S00 and S10 f S0ν, where ν ) 0-4, electronic transitions,64 and for UO2(NO3)2‚6H2O the most intense peak (S10 f S00) is positioned at 508 nm. Comparison of the observed spectra to the uranium oxynitrate reference showed an overall shift in each of the patterns that was independent of the excitation wavelength. While the majority of the compounds displayed a red shift ranging from 24 to 35 nm, three compounds were blue shifted by a maximum of 7 nm. Although it is understood that shifts in uranyl emission spectra can be influenced by a number of factors, including coordination environment, hydration level, and crystal packing,54,59,65 we are delaying a full discussion of these phenomena at present and instead are working on a more comprehensive publication detailing fluorescent behavior in uranyl-containing MOFs and coordination polymers. Some preliminary observations, however, suggest a correlation between degree of polymerization of primary building units and shift direction. For example, compounds 8-10 are all red shifted with respect to UO2(NO3)2‚6H2O and each contain tetrameric SBUs. Further, compounds 1 and 2, as well as UO2(C5H6O4) and UO2(C6H8O4) (see part I of this study, this issue), are also each red shifted and contain either dimers or chains. Consistent with these observations are structures that contain isolated or monomeric building units, such as 5, 6, 13, and UO2(C6H8O4)(H2O)2, each of which is blue shifted. Notable exceptions to this trend are compounds 7, 11, 15, and UO2(C4H4O4)(H2O), yet more subtle or synergistic factors may be influencing this behavior. Conclusions We have successfully been able to crystallize a series of UO22+ structures constructed from two linkers of differing functional groups. Although two linkers have been used with transition metals,1-15 this study is the first time, to our knowledge, two linkers were used to connect UO22+ centers. We synthesized all of the structures using an aliphatic dicarboxylate, containing between 5 and 10 carbon atoms, and one of two bipyridine species (4,4′-dipyridyl or 1,2-bis(4-pyridyl)ethane). In all cases, the carboxylate groups coordinate to the uranyl centers in either bidentate, bridging bidentate, or bridging tridentate coordination. The dipyridyl species, on the other hand, display a variety of roles including direct coordination, charge balancing, and space filling. There appears to be a synergistic relationship between observed solid-state UO22+ speciation and ligand characteristics that promote diversity in this class of materials. Coupled to this, we have been able to determine reaction conditions that promote the polymerization of the primary building units into higher order secondary building units that can been seen in both parts I and II. Finally, preliminary fluorescence studies suggest a possible relationship between building unit polymerization and shifts in emission spectra. Acknowledgment. This work was supported by the National Science Foundation (Grant DMR-0348982, CAREER Award to C.L.C. and NSF-MRI Grant DMR-0419754, diffractometer acquisition). Supporting Information Available: Fluorescence spectra of compounds 5-11, 13, and 15 and crystallographic data in CIF format for compounds 5-15. This material is available free of charge via the Internet at http://pubs.acs.org. In addition, CIF files have been deposited

Borkowski and Cahill with the Cambridge Crystallographic Data Centre (http://www.ccdc.cam.ac.uk) and may be obtained by citing reference numbers 614228614238.

References (1) Zheng, Y.-Q.; Ying, E.-B. Polyhedron 2005, 24, 397-406. (2) Zheng, Y.-Q.; Kong, Z.-P. Z. Anorg. Allg. Chem. 2003, 629, 14691471. (3) Zheng, Y. Q.; Lin, J. L.; Kong, Z. P. Inorg. Chem. 2004, 43, 25902596. (4) Xie, C.; Zhang, B.; Liu, X.; Wang, X.; Kou, H.; Shen, G.; Shen, D. Inorg. Chem. Commun. 2004, 7, 1037-1040. (5) Zhao, X.-L.; Mak, T. C. W. Polyhedron 2005, 24, 940-948. (6) Zhang, Q.-Z.; Lu, C.-Z.; Xia, C.-K. Inorg. Chem. Commun. 2005, 8, 304-306. (7) Rather, B.; Zaworotko, M. J. Chem. Commun. 2003, 830-831. (8) Li, J.; Zeng, H.; Chen, J.; Wang, Q.; Wu, X. Chem. Commun. 1997, 1213-1214. (9) Li, J. M.; Zhang, Y. G.; Chen, J. H.; Rui, L.; Wang, Q. M.; Wu, X. T. Polyhedron 2000, 19, 1117-1121. (10) Rodriguez-Martin, Y.; Ruiz-Perez, C.; Sanchiz, J.; Lloret, F.; Julve, M. Inorg. Chim. Acta 2001, 318, 159-165. (11) Rodriguez-Martin, Y.; Hernandez-Molina, M.; Sanchiz, J.; RuizPerez, C.; Lloret, F.; Julve, M. Dalton Trans. 2003, 2359-2365. (12) Mukherjee, P. S.; Konar, S.; Zangrando, E.; Mallah, T.; Ribas, J.; Chaudhuri, N. R. Inorg. Chem. 2003, 42, 2695-2703. (13) Maji, T. K.; Sain, S.; Mostafa, G.; Lu, T.-H.; Ribas, J.; Monfort, M.; Chaudhuri, N. R. Inorg. Chem. 2003, 42, 709-716. (14) Ying, S.-M.; Mao, J.-G.; Sun, Y.-Q.; Zeng, H.-Y.; Dong, Z.-C. Polyhedron 2003, 22, 3097-3103. (15) Lopez, J. P.; Kraus, W.; Reck, G.; Thuenemann, A.; Kurth, D. G. Inorg. Chem. Commun. 2005, 8, 281-284. (16) Halasyamani, P. S.; Francis, R. J.; Walker, S. M.; O’Hare, D. Inorg. Chem. 1999, 38, 271-279. (17) Borkowski, L. A.; Cahill, C. L. Acta Crystallogr., Sect. E 2005, E61, m816-m817. (18) Borkowski, L. A.; Cahill, C. L. Inorg. Chem. 2003, 42, 7041-7045. (19) JADE, v. 6.1; Materials Data Inc.: Livermore, CA, 2002. (20) SAINTPLUS, v. 6.01; Bruker AXS Inc.: Madison, WI, 1998. (21) SMART, v. 5.053; Bruker AXS Inc.: Madison, WI, 1998. (22) APEXII, v. 1.27; Bruker AXS, Inc.: Madison, WI, 2005. (23) SAINTPLUS, v. 7.20; Bruker AXS, Inc.: Madison, WI, 2005. (24) Sheldrick, G. M. SADABS, v. 2.03; University of Gottingen: Germany, 2002. (25) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343-350. (26) Sheldrick, G. M. SHELXS97 and SHELXL97; University of Gottingen: Germany, 1997. (27) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838. (28) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, B47, 192-197. (29) Burns, P. C.; Ewing, R. C.; Hawthorne, F. C. Can. Mineral. 1997, 35, 1551-1570. (30) de Lill, D. T.; Gunning, N. S.; Cahill, C. L. Inorg. Chem. 2005, 44, 258-266. (31) de Lill, D. T.; Bozzuto, D. J.; Cahill, C. L. Dalton Trans. 2005, 21112115. (32) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford Science Publications: Oxford, U.K., 1999. (33) A survey of the Cambridge Structural Database was conducted with search criteria of 4,4′-dipyridyl molecule in the presence of any metal center. (34) Thalladi, V. R.; Nuesse, M.; Boese, R. J. Am. Chem. Soc. 2000, 122, 9227-9236. (35) Boag, N. M.; Coward, K. M.; Jones, A. C.; Pemble, M. E.; Thompson, J. R. Acta Crystallogr., Sect. C 1999, C55, 672-674. (36) Ide, S.; Karacan, N.; Tufan, Y. Acta Crystallogr., Sect. C 1995, C51, 2304-2305. (37) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976. (38) Ahrland, S. In The Chemistry of the Actinide Elements, 2nd ed.; Katz, J. J., Seaborg, G. T., Morss, L. R., Ed.; Chapman and Hall: London, 1986; Vol. 2, pp 1480-1546. (39) Clark, D. L.; Conradson, S. D.; Donohoe, R. J.; Keogh, D. W.; Morris, D. E.; Palmer, P. D.; Rogers, R. D.; Tait, C. D. Inorg. Chem. 1999, 38, 1456-1466.

Crystal Engineering with the Uranyl Cation II (40) Moulin, C.; Decambox, P.; Moulin, V.; Decaillon, J. G. Anal. Chem. 1995, 67, 348-353. (41) Zanonato, P.; Di Bernardo, P.; Bismondo, A.; Liu, G.; Chen, X.; Rao, L. J. Am. Chem. Soc. 2004, 126, 5515-5522. (42) Cahill, C. L.; Borkowski, L. A. In Structural Chemistry of Inorganic Actinide Compounds; Krivovichev, S. V., Burns, P. C., Tananaev, I. G., Eds.; Elsevier: Amsterdam, in press. (43) Leciejewicz, J.; Alcock, N. W.; Kemp, T. J. Struct. Bonding 1995, 82, 43-84. (44) Allen, F. H. Acta Crystallogr., Sect. B 2002, B58, 380-388. (45) Yu, Z.-T.; Li, G.-H.; Jiang, Y.-S.; Xu, J.-J.; Chen, J.-S. Dalton Trans. 2003, 4219-4220. (46) Zozulin, A. J.; Moody, D. C.; Ryan, R. R. Inorg. Chem. 1982, 21, 3083-3086. (47) Thuery, P.; Nierlich, M.; Souley, B.; Asfari, Z.; Vicens, J. Dalton Trans. 1999, 2589-2594. (48) Van den Bossche, G.; Spirlet, M. R.; Rebizant, J.; Goffart, J. Acta Crystallogr., Sect. C 1987, C43, 837-839. (49) Gerasko, O. A.; Samsonenko, D. G.; Sharonova, A. A.; Virovets, A. V.; Lipkowski, J.; Fedin, V. P. Russ. Chem. Bull. 2002, 51, 346-349. (50) Rose, D.; Chang, Y.-D.; Chen, Q.; Zubieta, J. Inorg. Chem. 1994, 33, 5167-5168. (51) Turpeinen, U.; Hamalainen, R.; Mutikainen, I.; Orama, O. Acta Crystallogr., Sect. C 1996, C52, 1169-1171. (52) Zheng, Y.-Z.; Tong, M.-L.; Chen, X.-M. Eur. J. Inorg. Chem. 2005, 4109-4117.

Crystal Growth & Design, Vol. 6, No. 10, 2006 2259 (53) Frisch, M.; Cahill, C. L. Dalton Trans. 2005, 1518-1523. (54) Burrows, H. D.; Kemp, T. J. Chem. Soc. ReV. 1974, 3, 139-165. (55) Wittberger, D.; Berens, C.; Hammann, C.; Westhof, E.; Schroeder, R. J. Mol. Biol. 2000, 300, 339-352. (56) Das, S.; Madhavaiah, C.; Verma, S.; Bharadwaj, P. K. Inorg. Chim. Acta 2006, 359, 548-552. (57) Harrowfield, J. M.; Lugan, N.; Shahverdizadeh, G. H.; Soudi, A. A.; Thuery, P. Eur. J. Inorg. Chem. 2006, 389-396. (58) Almond, P. M.; Talley, C. E.; Bean, A. C.; Peper, S. M.; AlbrechtSchmitt, T. E. J. Solid State Chem. 2000, 154, 635-641. (59) Rabinowitch, E.; Belford, R. L. Spectroscopy and Photochemistry of Uranyl Compounds; MacMillan: New York, 1964. (60) Klonkowski, A. M.; Lis, S.; Pietraszkiewicz, M.; Hnatejko, Z.; Czarnobaj, K.; Elbanowski, M. Chem. Mater. 2003, 15, 656663. (61) Bunzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048-1077. (62) Bunzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53-61. (63) Jiang, Y.-S.; Yu, Z.-T.; Liao, Z.-L.; Li, G.-H.; Chen, J.-S. Polyhedron 2006, 25, 1359-1366. (64) Brachmann, A.; Geipel, G.; Bernhard, G.; Nitsche, H. Radiochim. Acta 2002, 90, 147-153. (65) Valeur, B. Molecular Fluorescence, 1st ed.; Wiley-VCH: Weinheim, Germany, 2002.

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