CuPYDC Metalloligands and Postsynthetic Rearrangement

Article pubs.acs.org/crystal

CuPYDC Metalloligands and Postsynthetic Rearrangement/ Metalation as Routes to Bimetallic Uranyl Containing Hybrid Materials: Syntheses, Structures, and Fluorescence Andrew T. Kerr and Christopher L. Cahill* Department of Chemistry, The George Washington University, 725 21st St. N.W., Washington, DC 20052, United States S Supporting Information *

ABSTRACT: Treatment of a molecular copper(II) 2,4-pyridinedicarboxylate metalloligand (CuL1) and a copper(II) 2,3pyridinedicarboxylate coordination polymer (CuL2) with the uranyl cation (UO22+) under hydrothermal conditions has yielded three new coordination polymers: [(UO2)(H2O)2(C7H3NO4)2Cu] (1), [(UO2)(H2O)2(C7H3NO4)2Cu]·H2O (2), and [(UO2)(OH)(H2O)(C7H3NO4)(C7H4NO4)Cu]·H2O (3). Compound 1 is formed upon the treatment of CuL1 with the uranyl cation to yield a one-dimensional coordination polymer. Compounds 2 and 3 are formed upon the rearrangement of the axially binding carboxylate groups in CuL2 to accommodate the uranyl cation to yield extended three-dimensional and twodimensional topologies, respectively. Attempts to prepare compounds 1−3 via direct assembly have yet to be successful, suggesting that a prebound Cu(II) metal center may be necessary in the synthesis of these bimetallic coordination polymers. Investigation of the emissive properties and the local geometries of compounds 1−3 suggests that the aromaticity of the ligand may influence energy transfer from the uranyl cation to a Cu(II) metal center, thus tuning uranyl fluorescence.

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INTRODUCTION Uranyl bearing hybrid materials have maintained significant interest owing to their rich topologies and emissive and photocatalytic properties.1−8 The uranyl cation (UO22+) is a linear, triatomic species that consists of two terminal oxygen atoms that cap a hexavalent uranium atom. As a consequence, further ligand coordination occurs primarily in the equatorial plane to give rise to three primary building units (PBUs) in the form of square, pentagonal, and hexagonal bipyramids.9 Oligomerization of the uranyl cation in aqueous media, as a consequence of hydrolysis, leads to polynuclear secondary building units (SBUs) in the form of dimers, trimers, tetramers, chains, sheets, etc.4,10−12 Ligands with an affinity to coordinate to the uranyl cation (often hard O- or N-donors) bind these PBUs and SBUs to give rise to a large number of coordination polymers (CPs) with diverse topologies.8,13−15 Both the uranyl building units and the ligands employed readily influence the properties of the resulting materials.6,16 As such, pairing the appropriate ligand with the uranyl cation becomes crucial for the synthesis of hybrid materials with desired topologies and influencing (for example) emissive properties.16,17 Whereas the © 2014 American Chemical Society

use of various O- or N-donor ligands has indeed yielded a multitude of uranium containing coordination polymers,3,13,14,18−22 there is arguably little a priori control over the topologies formed. The incorporation of a secondary metal center may be considered as an approach to directing the formation of uranyl bearing hybrid materials while simultaneously providing an opportunity to influence uranyl emission. The introduction of a second metal ion increases the diversity of possible bonding modes by offering additional coordination centers22−28 and may impart the properties of the additional metal to the overall material.29−32 In some investigations, bimetallic materials have displayed the magnetic properties of Cu(II) metal centers,33 tuned uranyl emission,24,30,34 or sensitization of a lanthanide via energy transfer.31 In our previous work, we have pointed out that the number of bimetallic uranyl containing hybrid materials comprise only ∼10% of the ∼2200 uranyl compounds Received: May 15, 2014 Revised: June 27, 2014 Published: June 30, 2014 4094

dx.doi.org/10.1021/cg5007132 | Cryst. Growth Des. 2014, 14, 4094−4103

Crystal Growth & Design

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Figure 1. Representation of CuL1 and CuL2.

center. Our goal was to coordinate these free O-donor sites in CuL1 and CuL2 to the uranyl cation to form bimetallic coordination polymers and study the effects of the Cu(II) metal center on uranyl fluorescence. The reaction of CuL1 with the uranyl cation resulted in the free carboxylate groups binding the uranyl cation to yield [(UO2)(H2O)2(C7H3NO4)2Cu] (1). Reaction of CuL2 with the UO22+ resulted a structural rearrangement of the axially bound carboxylate groups to accommodate the uranyl cation and yield [(UO2)(H2O)2(C7H3NO4)2Cu]·H2O (2). Adjusting the reaction conditions for the synthesis of 2 to higher pH results in the formation of uranyl dimers that likely promote further structural rearrangement of the axially bound carboxylate groups in CuL2 to yield [(UO2 )(OH)(H 2O)(C 7H 3NO 4)(C 7 H4 NO4 )Cu]·H2 O (3). Herein we report the synthesis, structural descriptions, and fluorescent properties of compounds 1−3. Moreover we discuss the use of metalloligands and postsynthetic rearrangement (as a consequence of metalation) as comprehensive approaches to the synthesis of bimetallic uranyl containing hybrid materials.

found in the Cambridge Structural Database (CSD version 5.35, November 2013).24,35,36 Many of these bimetallic coordination polymers were synthesized via direct assembly using heterofunctional multitopic ligands with both harder and softer functional groups that may have been used to selectively bind harder or softer metal centers, respectively.8,20,25,28,31,34,37 Moreover, some of these ligands include chelation (or multidentate) sites that bind one metal selectively, while leaving additional coordination sites open to bind to a secondary metal center.30,38−41 Other routes to bimetallic CPs include the use of metalloligands, species wherein a prebound metal center has additional uncoordinated ligating groups available for a secondary metal center.24,27,29,42,43 An alternative synthetic approach to the formation of bimetallic coordination polymers is postsynthetic metalation (PSMe). PSMe is the addition of a metal to a functionalized coordination polymer or framework after it has been synthesized and allows for the formation of bimetallic hybrid materials44−50 that may be otherwise inaccessible via direct assembly or through the use of metalloligands. Often functionalized CPs (once formed) tend to be robust and as a result may withstand a variety of reaction conditions and therefore provide a platform for PSMe. A potential outcome of PSMe is that the topology of the starting CP may be imparted to the resulting materials and thus provide some control over the architectures formed. As shown in this and a previous work, however, metalation may also lead to the rearrangement of a CP to result in new topologies or building units.35 The efforts reported herein make use of a previously reported molecular copper(II) 2,4-pyridinedicarboxylate,51 Cu(C7H4O4N)2(H2O)2, as a metalloligand and the postsynthetic rearrangement as a consequence of metalation of an extended copper(II) 2,3-pyridinedicarboxylate CP,52 Cu(C7H4O4N)2hereafter CuL1 and CuL2 respectively (Figure 1)to synthesize Cu2+/UO22+ hybrid materials. The Cu(II) metal center in CuL1 is bound by two water molecules and is chelated by the N,O-coordination site of two 2,4PYDC ligands to give rise to a molecular species that leaves carboxylate groups free to bind a secondary metal center. The Cu(II) metal center in CuL2 is similarly bound in the chelation site of two 2,3PYDC ligands, with the nonchelating carboxylate groups binding axially to give rise to extended chains with noncoordinating oxygen atoms free to bind a secondary metal

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EXPERIMENTAL SECTION

Caution! Whereas the uranyl nitrate hexahydrate [UO2(NO3)2]·6H2O used in these experiments contained depleted uranium, standard precautions for handling radioactive material should be observed. The [UO2(NO3)2]·6H2O was crystallized from a mixture of uranyl nitrates and oxides dissolved in boiling concentrated nitric acid. All other starting materials used in the syntheses are available commercially and were used as received. The synthesis of CuL1 was adapted from a previously reported procedure.51 In a large round-bottom flask, a warm methanolic solution of 2,4PYDC (2 g dissolved in 180 mL of MeOH) was added slowly to an aqueous solution of CuSO4·5H2O (1.50 g in 30 mL of H2O). The resulting blue precipitate was stirred for 30 min and then filtered to yield a blue solid that was washed with water and methanol and allowed to air-dry on the filter to yield 2.27 g of CuL1 (∼87% yield). CuL2 was synthesized by adding an aqueous solution of CuSO4·5H2O (0.07 g in 30 mL of H2O) to a methanolic solution of 2,4PYDC (1 g dissolved in 180 mL MeOH). The resulting solution was stirred and filtered to yield a blue solid that was washed with water and methanol. The blue solid was dried at 180 °C for 12 h to yield 0.960 g of CuL2 (∼84% yield). The scale and order of mixing in the synthesis of CuL2 appear to be important. The purity of the materials was verified via powder X-ray diffraction, as seen in the Supporting Information (SI) Figures S1 and S2. 4095



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Table 1. Synthesis Conditions and Selected Information for Compounds 1−3 compound formula ligand used molar ratio (CuPYDC/UO22+/H2O) reaction temp (°C) time (days) pH (init) topology pure?

1

2

3

[(UO2) (H2O)2(C7H3NO4)2Cu] CuL1 1:1:300

[(UO2)(H2O)2(C7H3NO4)2Cu]· H2O CuL2 1:1:300

[(UO2)(OH)(H2O)(C7H3NO4)(C7H4NO4)Cu]· H2O CuL2 1:1:300

120 2 1.74 1-D chains yes

120 2 1.49 3-D framework yes

120 2 5.75 2-D sheet yes

Table 2. Crystallographic Data for Compounds and 1−3 compound

1

2

3

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temperature (K) Z λ (Mo Kα) Dcalc (g cm−3) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

C14H10N2O12CuU 699.81 triclinic P1̅ 5.3553(10) 7.6850(15) 10.951(2) 104.448(3) 90.485(3) 100.058(3) 429.11(14) 100 1 0.710 73 2.708 10.738 0.0310 0.0249 0.0511

C14H12N2O13CuU 717.83 orthorhombic Pca21 8.4363(11) 13.1485(17) 17.833(2) 90 90 90 1978.1(4) 293 4 0.710 73 2.410 9.324 0.0704 0.0411 0.1022

C14H12N2O13CuU 919.2 triclinic P1̅ 9.344(4) 10.249(4) 11.295(5) 90.275(6) 113.058(6) 110.556(6) 919.2(7) 293 2 0.710 73 2.509 10.033 0.0312 0.0242 0.0478

Figure 2. Local structure of 1. Dark blue spheres represent copper atoms, red spheres oxygen atoms, light blue spheres nitrogen atoms, black spheres carbon atoms, and yellow spheres uranium atoms. Fully labeled ORTEP representations can be found in the Supporting Information. products which were subsequently washed with water and ethanol. A summary of reaction conditions can be found in Table 1. Single Crystal X-ray Diffraction. Single crystals of each compound were isolated from bulk samples and mounted on a MicroMount needle (MiTeGen). Reflections were collected from 0.5 ω and φ scans on a Bruker SMART diffractometer with APEXII CCD detector and Mo Kα source. The APEX II software suite53 was used to integrate the data and to apply an absorption correction.54 Structures

Compounds 1−3 were synthesized by treating either CuL1 or CuL2 with [UO2(NO3)2]·6H2O in water (molar ratio 1:1:300). The pH was adjusted using concentrated NH3 for compound 3, whereas the pH remained unadjusted for compounds 1 and 2. The reagents were sealed in a 23 mL Teflon-lined Parr bomb and heated statically at 120 °C for 3 days and cooled to room temperature over 3 h before opening. The mother liquor was decanted from the crystalline reaction 4096



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Figure 3. Polyhedral representation of 1 shown down the [001] direction. The color scheme is the same as Figure 2. Dashed lines represent hydrogen bonding.

Figure 4. Local structure of 2. Solvent water molecules have been omitted for clarity. were solved by direct methods using SIR9255 or SHELXS-2013.56 All structures were refined using SHELXL-201356 within the WINGX software suite.57 All figures were prepared with CrystalMaker.58 Crystallographic data for compounds 1−3 can be found in Table 2. One disordered water molecule (Ow3) in compound 2 was modeled satisfactorily via the PART command. The hydrogen atoms associated with Ow3 and the two bound water molecules (Ow1 and Ow2) were located in the difference Fourier map and subsequently restrained to appropriate bond lengths and refined isotropically. Bond valence summations were performed for all compounds (see Supporting Information Tables S1−S3) to verify the identity of the oxygen atoms as either water molecules or hydroxide ions. Powder X-ray Diffraction. Powder X-ray diffraction data were collected on a Rigaku Miniflex diffractometer (Cu Kα, 3−60°) and analyzed with the Jade software package.59 PXRD data were used to verify reproducibility as well as to explore purity and verify that the single crystals were representative of the bulk sample. It is of note that the PXRD data for compound 1 does not match well with the

calculated pattern due to a shift caused by an asymmetric contraction of the unit cell at 100 K (see Supporting Information Figure S3). This shift is observed specifically with peaks corresponding to the (011̅ ) family of planes. Comparison of the PXRD data to a pattern calculated from a room temperature data collection results in a better match and suggests that the sample is a pure phase (see Supporting Information Figure S4). Fluorescence. Room temperature solid state emission spectra were acquired for compounds 1−3 on a Horiba JobinYvon Fluorolog3 spectrophotometer using FluorEssence software. The samples were ground and placed between two quartz slides, and excited at 420 nm. The resulting spectra were collected between 450 and 600 nm using a front face setting with excitation and emission slit widths at 2.5 nm.

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RESULTS AND DISCUSSION Structural Description. Compound 1, [(UO2)(H2O)2(C7H3NO4)2Cu], consists of one crystallographically unique uranyl cation (U1, O1, and O1′) that is bound by two water 4097



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Figure 5. Polyhedral representation of 2 shown down the [100] direction. Solvent water molecules have been omitted for clarity.

is bound by two axial oxygen atoms, O1 and O2, two water molecules, Ow1 and Ow2, and three monodentate 2,3PYDC ligands via O3, O5, and O10 to result in a pentagonal bipyramidal coordination geometry (Figure 4). One crystallographically unique copper atom is contained in the asymmetric unit and is chelated by two 2,3PYDC ligands via N1, N2, O6, and O7 (Cu1−N1, 1.974(10) Å; Cu1−N2, 1.963(10) Å; Cu1− O6, 1.944(9) Å and Cu1−O7, 1.944(9) Å) and bound by a carboxylate oxygen (O9; Cu1−O9 2.319(10) Å) to result in a distorted square pyramidal coordination geometry similar to the Jahn−Teller-distorted Cu(II) coordination geometry in CuL2. Each square pyramidal Cu(II) building unit is connected to the next via an oxygen atom on the carboxylate that is not involved in chelation to give rise to infinite chains that propagate in the [100] direction. These chains are then connected to one another via the pentagonal bipyramidal uranyl centers to result in a 3-D coordination polymer with channels that run along the [100] direction (Figure 5). Also present in this three-dimensional CP are C−O···π and π−π stacking interactions. O4 and O8 are oriented such that they point toward the center of aromatic rings containing N1 (Cg1) and N2 (Cg2) respectively to form C−O···π interactions: C− O4···Cg1 3.061(11) Å; ∠C−O4···Cg1 122.5(7)° and C−O8··· Cg2 3.287(3) Å; ∠C −O8···Cg2 68.2(9)°.61,62 The 2,3PYDC ligand containing N1 (Cg1) overlaps with the periphery of a

molecules (Ow1 and Ow1′) and two bidentate 2,4 pyridine dicarboxylate (2,4PYDC) ligands via O2, O2′, O3, and O3′ to result in a hexagonal bipyramidal coordination geometry (Figure 2). The asymmetric unit also contains one crystallographically unique copper(II) cation, Cu1, that is bound by two chelating 2,4PYDC ligands via N1, N1′, O5, and O5′ (Cu1− N1, 1.964(3) Å and Cu1−O5, 1.954(3) Å) to result in a square planar geometry. Tables of selected bond lengths and angles can be found in the Supporting Information. The hexagonal bipyramidal uranyl units are connected to one another via CuL1 metalloligands to give rise to infinite chains that propagate approximately along the [111] direction (Figure 3). These chains hydrogen bond to one another via Ow1 and carboxyl oxygen atoms of the PYDC (O4) and Ow1 and the uranyl oxygen atom O1 to form an extended layered topology: Ow1−H···O4 2.687(5) Å and Ow1−H···O1 2.908(4) Å. These chains are further stabilized via weak π−π stacking interactions of two symmetry equivalent 2,4PYDC ligands wherein the center of one aromatic ring (Cg) overlaps with the next: Cg··· Cg 5.355(2) Å; β = 57.5 and Cg···Cg 5.176(2) Å; β = 52.2.60 Comprehensive tables of supramolecular interaction distances and angles for all compounds can be found in the Supporting Information. Compound 2, [(UO2)(H2O)2(C7H3NO4)2Cu]·H2O, consists of one crystallographically unique uranium atom, U1, that 4098



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Figure 6. Local structure of 3. Solvent water molecules have been omitted for clarity.

Figure 7. Polyhedral representation of 3 shown down the [111] direction. Solvent water molecules have been omitted for clarity.

second ring containing N2 (Cg2) to result in a weak π−π stacking interaction: Cg1···Cg2 5.942(8) Å; β = 59.6.60 The symmetry equivalent of each 2,3PYDC ligand overlaps with another to form further π−π interactions: Cg1···Cg1 4.532(8) Å; β = 44.7 and Cg2···Cg2 4.477(8) Å; β = 38.0. Compound 3, [(UO2)(OH)(H2O)(C7H3NO4)(C7H4NO4)Cu]·H2O, consists of one crystallographically unique uranyl cation (U1, O1, and O2) that is bound by one water molecule Ow1, two hydroxide anions O3 and O3′, and two monodentate 2,3PYDC molecules via O4 and O5 to result in a pentagonal bipyramidal coordination geometry (Figure 6). The asymmetric unit contains one crystallographically unique copper(II) cation, Cu1, that is bound by two chelating 2,3PYDC ligands via N1, N2, O7, and O10 (Cu1−N1, 1.972(3) Å; Cu1−N2, 1.960(3) Å; Cu1−O7, 1.980(3) Å and Cu1−O10, 1.946(3) Å) and one monodentate carboxylate via O8 (Cu1 −O8 2.420(3) Å) to result in a distorted square pyramidal coordination geometry

similar to the Jahn−Teller-distorted Cu(II) coordination geometry in CuL2. Each pentagonal bipyramidal uranyl building unit is connected to another via hydroxide anions to form dimers (Figure 7). These dimers are connected to one another via bridging bidentate 2,3PYDC molecules to form chains that propagate in the [111] direction. These chains are connected to one another via copper “pseudodimers” (two Cu(II) metal centers linked to one another via 2,3PYDC ligands to form a molecular species) to yield a sheet topology in the (111) plane. This two-dimensional CP forms a threedimensional network via U = O···π, C−O···π and π−π stacking interactions. O1 and O11 are oriented such that they point toward the center of aromatic rings containing N1 (Cg1) and N2 (Cg2) respectively to form U = O···π, C−O···π interactions: U = O1···Cg1 3.883(4) Å; ∠U = O1···Cg1 111.72(13)° and C−O11···Cg2 3.627(4) Å; ∠C−O11···Cg2 71.9(3)°. The 2,3PYDC ligand containing N1 (Cg1) overlaps 4099



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Figure 8. Local structures of [(UO2)(C7H3O4)]39 and [(UO2)3(OH)2(O)(H2O)2(C7H3O4)]·H2O63 shown to highlight the coordination mode of the uranyl cation in the N,O-chelating environment of 2,4PYDC. The solvent water molecules have been omitted for clarity in the latter.

Scheme 1. Representation of the CuL Extended Chain and Pseudo-Dimer Formed upon the Treatment of CuL2 with UO22+ at Low and High pH in Compounds 2 and 3 Respectivelya

a

Carboxylate groups that remain axially bound are green and groups that reoriented to bind the uranyl cation are yellow.

Cu(II) metal center may have played a role in influencing the resulting topology. In monometallic U-PYDC compounds39,63 (Figure 8) and materials formed with analogous ligands64−70 (e.g., pyridine-2-carboxylate and pyrazine-2-carboxylate), the N,O-site is typically occupied by UO22+. Although there are examples of other metals occupying an N,O-environment in the presence of the uranyl cation,30,40,41 it is of note that in most cases the uranyl cation has hydrolyzed to yield more bulky SBUs that may not easily fit in the this site due to sterics. We surmise that the Cu(II) remains bound in the N,Ocoordination site as a result of being prebound as a metalloligand. This idea offers a possible explanation as to

with a second ring containing N2 (Cg2) in two orientations to form weak π−π interactions: Cg1···Cg2 4.751(3) Å; β = 20.2 and Cg1···Cg2 5.291(3) Å; β = 43.3. The symmetry equivalent of one 2,3PYDC ligand overlaps with the periphery another to form another π−π stacking interaction: Cg2···Cg2 4.243(3) Å; β = 34.4.60 Metalloligands. In this study, compound 1 was obtained via the treatment of a metalloligand, CuL1, with (UO2)2+. The uranyl cation was bound by the free carboxylate groups of the metalloligand to form extended chains. Upon the comparison of 1 to the monometallic uranyl bearing hybrid materials formed with 2,4PYDC,39,63 we find that the “prepackaged” 4100



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or log(β2) = 14.75 for two ligands) is larger than that reported for UO22+ (log(K1) = 4.51),71 which suggests that the N,Ochelation site has a higher affinity for copper(II) ions. Indeed the results reported by Frisch and Cahill further support the idea that the Cu(II) has a higher affinity for a N,O-site compared to the uranyl cation as reflected by the resulting topology formed via direct assembly.30 The work presented herein, however, has shown that Cu(II) may require prebinding in the N,O-site as the same products have yet to be formed via direct assembly, presumably due to competition between the uranyl and copper(II) cations for the binding site. For this argument, we caution that these reported stability constants were obtained at room temperature and at varied ionic strengths. We speculate, however, that the concept of the structural stability imparted via N,O-chelation may still be relevant under hydrothermal conditions. Luminescence. Fluorescence studies were carried out on the bulk phase of compounds 1−3. Compound 1 exhibits weak and poorly resolved uranyl emission upon excitation at 420 nm (Figure 9). This diminished uranyl emission is likely due to

why attempts to synthesize 1 via direct assembly have yet to be successful. Multiple attempts to synthesize 1 via direct assembly of UO22+, 2,4PYDC, and a Cu2+ over a range of reaction conditions (e.g., various temperature, time, and pH values) resulted in yet unidentified crystalline (as indicated via PXRD) reaction products. Postsynthetic Rearrangement/Metalation. Compounds 2 and 3 were produced from the treatment of a coordination polymer (CuL2) with UO22+. CuL2 undergoes a rearrangement in order to incorporate the uranyl cation in both compounds. Upon comparison of CuL2 with the extended Cu(II) network in 2 (Scheme 1), we see that one of the carboxylic acid groups previously bound axially to a copper metal center has reoriented to bind a uranyl cation exclusively, while the other carboxylate bridges the UO22+ and Cu2+ cations. This rearrangement results in infinite Cu(II) chains, similar to those in CuL2, in which the Cu metal centers are connected to one another via the axially binding carboxylate groups. Compound 3 is produced upon increasing the pH of the reaction mixture, which presumably results in the hydrolysis of the uranyl cation to yield dimeric SBUs. We surmise that these SBUs are bound only by the most accessible carboxylate groups of the CuL2 chain to result in a reorientation. The rearrangement of each accessible carboxylate group leads to the truncation of the extended Cu CP and instead a “metalloligand-like” pseudodimer remains (Scheme 1). Attempts to synthesize 2 and 3 via direct assembly resulted in poorly crystalline reaction products that have yet to be identified. Reactive Jahn−Teller Distorted Species. Similar to compound 1, the Cu(II) metal centers in 2 and 3 remain in the N,O-chelating environment of the PYDC ligands, while the uranyl cation is bound by the free O-donor groups. These results are likely due to the Cu(II) metal center being prebound in both the metalloligand (CuL1) and the CP (CuL2). The question is raised as to why the axially coordinated carboxylate groups of the Cu(II) in CuL2 reorient to bind to the uranyl cation in compounds 2 and 3. This reorientation is likely a consequence of longer and presumably weaker Cu−O bonds at the axial positions of the Jahn−Teller-distorted Cu(II) metal center in CuL2. These oxygen atoms are less tightly bound by the Cu(II) metal center and may result in a reactive species. Upon treatment of CuL2 with the uranyl cation, the longer (and presumably weaker) Cu−O bonds (2.660(3) Å)52 are broken in favor of the formation of shorter (and presumably stronger) U−O bonds (between 2.327(3) and 2.432(3) Å). Also, the axial Cu−O bonds observed in 2 and 3 become more stable than those in CuL2 as reflected by shorter bond lengths (2.319(10) and 2.420(3) respectively). A rearrangement in which the uranyl cation replaces the Cu(II) in the PYDC binding pockets, however, does not occur. We surmise that the Cu(II) metal centers have little energetic incentive to leave the N,O-binding sites, and thus the equatorial coordination geometry about each copper(II) ion remains unchanged. This idea is supported by the significantly large stability constants of Cu(II) with 2,3PYDC and 2,4PYDC (log(β2) = 14.15 and 14.55, respectively),71 which are reflective of the high affinity between the N,O-chelation site and Cu(II) ions. As the stability constants of the uranyl cation with 2,3PYDC and 2,4PYDC are not reported, we have compared the binding affinities of Cu(II) or UO22+ with pyridine-2carboxylate (an analogous ligand with a N,O-site). The stability constant of pyridine-2-carboxylate with Cu(II) (log(K1) = 7.9

Figure 9. Emission spectra of compounds 1−3 and uranyl nitrate hexahydrate excited at 420 nm.

reabsorption by the Cu(II) metal center, as has been observed in other Cu2+/UO22+ compounds.26,30,34,35,72 Compounds 2 and 3 exhibit stronger and well-defined uranyl emission, however, despite the presence of Cu(II) metal centers. The emission spectra for both 2 and 3 display a series fine vibronic bands in the ∼450 to 600 nm range that correspond to the well-known S11 → S00 and the S10 → S0ν (ν = 0−4) electronic transitions of the uranyl cation. These results indicate that although the Cu(II) metal center may reabsorb uranyl emission, the local geometries about the metal ions and the extended topologies of the structures may play a role in energy transfer. As one may expect, an aromatic (or more conjugated) system may provide a route for energy transfer from one metal center to another. In the case of compound 1, each uranyl cation is connected to Cu(II) cations via aromatic 2,4PYDC ligands (Figure 3), and thus energy may be transferred to the Cu(II) metal center effectively. In compounds 2 and 3, however, the uranyl-bound carboxylate is no longer planar and thus reduces the aromaticity of the 2,3PYDC ligands. As a result, energy transfer to the Cu(II) may be qualitatively less efficient. Other examples of bimetallic Cu(II)−uranyl hybrid materials with aromatic ligands have typically demonstrated quenched uranyl emission,30,72 yet Cu2+/UO22+ compounds in which aromaticity is disrupted or the ligand employed is aliphatic, however, do exhibit uranyl emission.26,34,35 These 4101



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results suggest that uranyl emission may be influenced by the aromaticity of ligands as well as the presence of secondary metals. Indeed, the nonplanar isonicotinate anions in reported Cu2+/UO22+ hybrid materials35 may result in less effective energy transfer to the Cu(II) metal center, and thus uranyl emission is observed. It has also been demonstrated that increasing the conjugation of ligands (e.g., substituting 1,2bis(4-pyridyl)ethane with 1,2-bis(4-pyridyl)ethylene) within monometallic UO22+ bearing hybrid materials may lead to the self-quenching of uranyl luminescence.15

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CONCLUSIONS Pairing of a metalloligand (CuL1) and rearrangement as a consequence of metalation of a coordination polymer (CuL2) with the uranyl cation have proven to be successful routes to bimetallic coordination polymers. CuL1 binds the uranyl cation to form an extended chain topology (1). CuL2 undergoes a rearrangement of the axially binding carboxylate groups about the Cu(II) to accommodate uranyl PBUs or SBUs at various pH values to result in extended topologies (2 and 3). The emission spectra of compounds 1−3 suggest that the local geometries and extended topologies about the metal ions may influence emission of the uranyl cation by facilitating energy transfer. Future studies with metalloligands and coordination polymers that may be metalated with the uranyl cation may lead to a controlled approach in the synthesis of hybrid materials with desirable topologies and properties.

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ASSOCIATED CONTENT

S Supporting Information *

Thermal ellipsoid plots, PXRD data, solid state absorption spectra, tables of selected bond lengths and angles, and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CIFs have also been deposited at the Cambridge Crystallographic Data Centre and may be obtained from http://www.ccdc.cam.ac.uk by citing deposition numbers 1003493−1003495 for compounds 1−3 respectively.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (202) 994 6959. Phone: (202) 994 5873. E-mail: cahill@ gwu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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.

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CuPYDC Metalloligands and Postsynthetic Rearrangement

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