Metalation as a Route to Bimetallic

Article pubs.acs.org/crystal

Postsynthetic Rearrangement/Metalation as a Route to Bimetallic Uranyl Coordination Polymers: Syntheses, Structures, and Luminescence Andrew T. Kerr and Christopher L. Cahill* Department of Chemistry, The George Washington University, 725 21st Street Northwest, Washington, DC 20052, United States S Supporting Information *

ABSTRACT: Postsynthetic metalation of a uranyl isonicotinate starting material (UICP) with Ag(I) and Cu(II) under hydrothermal conditions has yielded two new coordination polymers: (UO2)(OH)(C6H4NO2)2Ag (1) and (UO2)2(O)(OH)(C6H4NO2)2Cu(OH2)Cl (2). The Ag(I) in 1 directly replaces a proton in UICP and thus generates a similar topology. Compound 2, however, is formed by a topological rearrangement of the starting material upon metalation. Comparison of postsynthetic metalation and direct assembly reveals that compound 1 may be synthesized via both methods, whereas 2 was only be prepared by the former. Attempts to prepare 2 via direct assembly yielded (UO2)(OH2)2(C6H4NO2)2Cu2(C6H4NO2)4(OH2)2 (3). Use of postsynthetic metalation has proven to be a route to bimetallic uranyl-bearing coordination polymers and may yield materials otherwise inaccessible via direct assembly.

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INTRODUCTION Uranyl-bearing hybrid materials have maintained significant interest owing to their rich topologies and photophysical properties.1−7 The linear triatomic uranyl cation (UO22+) consists of a hexavalent uranium atom capped by two terminal axial oxygen atoms. The terminal nature of these oxygen atoms promotes ligand coordination about the equatorial plane of the uranyl cation to yield three common primary building units (PBUs), square, pentagonal, and hexagonal bipyramids.8 Uranyl speciation, influenced via hydrolysis, expands these PBUs to introduce secondary building units (SBUs) such as dimers, trimers, chains, etc.5,9 Ligands bind to these primary and secondary building units to yield a vast catalog of coordination polymers (CPs), which embody the properties of both the uranyl cation and the organic species.10−16 It is important, therefore, to pair the appropriate ligand with the uranyl cation in order to obtain both the desired topologies and the physical properties of the resulting materials. Whereas systematic pairing of the uranyl cation with O- or N-donor ligands using hydrothermal conditions has indeed yielded a multitude of uranium-bearing compounds,3,4,10,17−20 this method tends to offer little control over the products formed. An approach to directing formation of uranyl-bearing compounds and thus potentially offering some influence over the products formed is incorporation of a secondary metal center. Additional metal centers in bimetallic coordination polymers contribute to the topologies15,21−28 and properties (e.g., luminescent or catalytic)29−31 of the resulting materials. A search of the Cambridge Structural Database (CSD version 5.35, November 2013)32 shows that the number of bimetallic uranyl-bearing hybrid materials, compared to monometallic, is relatively low (∼10%). Previous work has employed direct © 2014 American Chemical Society

assembly and made use of heterofunctional multitopic ligands (e.g., carboxyphosphonates21,33,34 or pyridinecarboxylates22) or metalloligands23,29,35−37 to produce bimetallic uranyl-bearing compounds that have exhibited either the tuning of the UO22+ emission (via Cu(II)24,25 or Fe(II)37) or the sensitization of the secondary metal center (e.g., Sm(III))34 via energy transfer. Whereas heterofunctional multitopic ligands have indeed led to bimetallic CPs and thus offer some control over the resulting products, our experience suggests that these ligands often promote separated monometallic phases. An alternative synthetic approach to formation of bimetallic coordination polymers is postsynthetic modification (PSM). PSM, as defined by Tanabe and Cohen, is “the chemical modification of a framework after it has been synthesized”38 and allows for synthesis of materials that may be inaccessible via direct assembly or one-pot reactions.38−40 Reactive functional groups that may not survive the conditions used to synthesize a coordination polymer (e.g., a pyrizine that may hydrolyze under hydrothermal conditions) may be introduced postsynthetically and used as a site to coordinate to a secondary metal ion.38,39 Often gentler reaction conditions than those used to form the CP may be employed after the desired topologies are obtained. Beyond introduction of functional groups, one may also introduce a secondary metal center into a preformed coordination polymer via accessible N- or O-donor sites.38−40 CPs with unbound pyridyl or carboxylate groups are likely candidates for postsynthetic metalation (PSMe) as these unbound groups offer “sticky-sites” for a secondary metal Received: January 9, 2014 Revised: February 14, 2014 Published: February 19, 2014 1914

dx.doi.org/10.1021/cg500050q | Cryst. Growth Des. 2014, 14, 1914−1921

Crystal Growth & Design

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secondary metal on the resulting topologies with respect to charge balance requirements and coordination chemistry is also discussed.

center to incorporate into the structure. A uranyl isonicotinate coordination polymer,41 UO2(OH)(C6H5NO2)(C6H4NO2) (UICP), was selected as the starting material for metalation in this study owing to the presence of unbound pyridyl groups that may interact with a secondary metal center, as seen in Figure 1. Uranyl dimers in UICP are connected and decorated

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EXPERIMENTAL SECTION

Synthesis. Caution! Whereas the uranyl nitrate hexahydrate [UO2(NO3)2]·6H2O used in these experiments contained depleted uranium, standard precautions for handling radioactive material should be observed. [UO2(NO3)2]·6H2O was crystallized from a mixture of uranyl nitrates and oxides dissolved in concentrated nitric acid. All other starting materials used in the syntheses are available commercially and were used as received. The uranyl isonicotinate CP that served as our starting phase, UO2(OH)(C6H5NO2)(C6H4NO2), has been reported previously as having been synthesized via slow evaporation of a mixture of [UO2(SO4)]·2.5H2O and isonicotinic acid in a water/ethanol solution.41 For the current effort, UICP was prepared by combining [UO2(NO3)2]·6H2O with isonicotinic acid in distilled water (molar ratio 1:3:151). Reagents were sealed in a 23 mL Teflon-lined Parr bomb, heated statically at 120 °C for 3 days, and cooled to room temperature over 3 h before opening. Decanting the mother liquor from the crystalline reaction products yielded an impure mixture of UICP and unreacted isonicotinic acid as a bulk phase. The excess isonicotinic acid was removed by washing the solid with a saturated sodium bicarbonate solution to yield pure UICP, as demonstrated by powder X-ray diffraction (PXRD) as seen in the Supporting Information (Figure S1). Caution should be used to not over wash the mixture as UICP is soluble in a sodium bicarbonate solution as well. Compounds 1 and 2 were synthesized by treating UICP with Ag(I) (AgNO3), and Cu(II) (CuCl2), respectively, in water (molar ratio 1:1:300). Reagents were sealed in a 23 mL Teflon-lined Parr bomb, heated statically at 120 °C for 2 days, and cooled to room temperature over 3 h before opening. The mother liquor was decanted from the crystalline reaction products which were washed with water and ethanol. Compound 1 was not isolated as a pure phase, as unreacted UICP remained. The unreacted UICP was removed by washing the bulk sample with a saturated sodium bicarbonate solution to yield pure 1. Compound 2 coformed with a known molecular copper isonicotinate, Cu(C6H4NO2)2(H2O)445 (CuIA). Compounds 1 and 3 were synthesized via direct assembly by combining [UO2(NO3)2]·6H2O with isonicotinic acid and AgNO3 or CuCl2, respectively, in water (molar ratio 1:3:1:150). Reagents were sealed in a 23 mL Teflon-lined Parr bomb, heated statically at 120 °C for 2 days, and cooled to room temperature over 3 h before opening. Crystalline reaction products were collected after decanting the mother liquor and washed with water and ethanol. Compound 1 was obtained as an impure mixture with UICP that could be purified via washing. Compound 3 was obtained in the presence of CuIA. A summary of reaction conditions can be found in Table 1.

Figure 1. Polyhedral representation of UICP down the [001] direction. Yellow polyhedra represent uranium atoms, red spheres oxygen atoms, light blue spheres nitrogen atoms, and black lines carbon bonds. Dashed lines represent hydrogen bonding between chains.

by isonicotinate ligands to form chains which hydrogen bond to one another via protonated pyridyl groups to form 2-D sheets. Our goal was to replace these protons with metal ions in order to form a bimetallic uranyl-bearing hybrid material. Treatment of UICP with Ag(I) results in a simple replacement of the pyridyl protons with silver ions to yield (UO2)(OH)(C6H4NO2)2Ag (1). Reaction with Cu(II), on the other hand, results in a structural rearrangement of UICP accompanied by metalation, similar to the results of ion exchange in uranyl-bearing CPs,42−44 to yield (UO2)2(O)(OH)(C6H4NO2)2Cu(OH2)Cl (2). A comparison of PSMe to direct assembly was performed to explore the formation mechanisms for the two approaches. In the case of Ag(I), products obtained by direct assembly were the same as those yielded by PSMe. The products formed by direct assembly with Cu(II), however, resulted in another phase (UO 2 )(OH2)2(C6H4NO2)2Cu2(C6H4NO2)4(OH2)2 (3). The synthesis, structural descriptions, and fluorescent properties of compounds 1−3 are presented herein. Moreover, the effect of a

Table 1. Synthesis Conditions and Selected Information for UICP and Compounds 1−3

formula secondary metal (Mn+) molar ratio direct assembly (U:isonicotinic acid:Mn+:H2O) molar ratio PSMe (UICP:Mn+:H2O) reaction temp time (days) pH (init.) tpology pure?

UICP

1

2

3

UO2(OH)(C6H5NO2) (C6H4NO2) NA 1:3:NA:151

(UO2)(OH) (C6H4NO2)2Ag Ag(I) 1:3:1:151

(UO2)2(O)(OH) (C6H4NO2)2Cu(OH2)Cl Cu(II) NA

(UO2) (OH2)2(C6H4NO2)2Cu2(C6H4NO2)4(OH2)2 Cu(II) 1:3:1:151

NA 120 °C 3 1.58 1-D polymer

1:1:151 120 °C 2 1.21 2-D coordination polymer no: coforms with UICP41

1:1:151 120 °C 2 2.57 2-D polymer

NA 120 °C 2 1.26 2-D polymer

no: coforms with CuIA45

no: coforms with CuIA45

no: coforms with isonicotinic acid

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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 scans on a Bruker SMART diffractometer with APEXII CCD detector and Mo Kα source. The APEX II software suite46 was used to integrate the data and apply an absorption correction.47 Structures were solved by direct methods using SIR9248 or SHELXS-2013.49 All structures were refined using SHELXL-201349 within the WINGX software suite.50 All figures were prepared with CrystalMaker.51 Crystallographic data for compounds 1−3 can be found in Table 2.

verify the identity of select oxygen atoms as water molecules, hydroxide anions or oxide anions, as hydrogen atoms on these species were not located in the difference Fourier map. 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.53 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. PXRD spectra of 1−3 that highlight the products formed under different reaction conditions can be found in the Supporting Information (Figures S2− S4). Fluorescence Studies. Room-temperature solid state emission spectra were acquired for compounds 1−3 on a Horiba JobinYvon Fluorolog-3 spectrophotometer using FluorEssence software. Samples were ground and placed between two quartz slides and excited at 365 and 420 nm. Resulting spectra were collected between 450 and 600 nm using a front face setting with excitation and emission slit widths at 2.5 nm.

Table 2. Crystallographic Data for Compounds and 1−3 1 empirical formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temp (K) Z λ (Mo Ka) Dcalcd (g cm−3) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

C18H9 N2O7AgU 639.11 monoclinic P21/n 8.7921(3) 18.1758(6) 8.944(3) 90 98.52 90 1413.54(8) 293 4 0.71073 3.003 12.867 0.0321 0.0217 0.0507

2 C12H12N2O11ClCuU2 935.25 triclinic P-1 8.6901(11) 10.6622(14) 10.7588(14) 71.659(2) 80.212(2) 84.494(2) 931.5(2) 100 2 0.71073 3.335 18.680 0.0557 0.0422 0.0947

3 C36H32 N6O18Cu2U 1201.78 orthorhombic Cmca 27.122(4) 11.1223(15) 13.5430(18) 90 90 90 4085.4(9) 100 4 0.71073 1.954 5.071 0.0637 0.0368 0.1088

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RESULTS AND DISCUSSION Structural Descriptions. Compound 1, (UO2)(OH)(C6H4NO2)2Ag, consists of one crystallographically unique uranyl cation (U1, O1, and O2) bound by two hydroxide ions (O4 and O4#2) and three unique monodentate isonicotinate anions via O6, O3#3, and O5 to result in a pentagonal bipyramidal coordination geometry about U1, as seen in Figure 2. The pentagonal bipyramids edge share with their symmetry equivalents via hydroxide anions to form dimers, which are further connected to one another via bridging bidentate isonicotinate units to form anionic chains that propagate along the [100] direction (Figure 3). These chains are further

Large residual electron density remained in the difference Fourier map of compound 2 despite attempts to perform an absorption correction using SADABS.47 As such, other correction methods were explored and the ellipsoidal option within XPREP46 yielded the most satisfactory results. After the refinement converged however, relatively large electron density peaks still remained near the heaviest atoms (U1 and U2), which is not unusual for compounds containing such highly absorbing species. The structure was checked for missing symmetry using PLATON,52 and having none, it was concluded that the residual electron density was not indicative of an incorrect space group choice. Atomic displacement parameters for three carbon atoms (C2, C1, and C7) and one oxygen atom (OW1) were constrained using the ISOR command to prevent them from becoming nonpositive definite. Compound 3 has one disordered water molecule (OW2) which was modeled satisfactorily via the PART command. Attempts to model this water molecule as partially occupied resulted in Ow2 becoming a nonpositive definite atom. Bond valence summations were performed for all compounds (see Table S1−S3, Supporting Information) to

Figure 3. Polyhedral representation of 1 shown down the [001] direction. The color scheme is the same as Figure 1.

decorated by monodentate isonicotinate units that coordinate to the uranyl cation. The chains are linked to one another via a crystallographically unique Ag(I) site in linear coordination

Figure 2. Local structure of 1. The same color scheme for Figure1 is used, with gray spheres representing silver atoms. Fully labeled ORTEP representations can be found in the Supporting Information. Symmetry equivalents: (#1) −x + 2, −y, −z + 1; (#2) −x + 3, −y + 1, −z + 1; (#3) −x + 2, −y + 1, −z + 1. 1916



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edge share via the hydroxide anions with their symmetry equivalents to form chains that propagate along the [100] direction and are decorated with bridging bidentate isonicotinate anions (Figure 5). Similar to compound 1, the

with two pyridyl groups on parallel chains (Ag1−N1, 2.129(3) Å and Ag1−N2, 2.132(3) Å) to form sheets in the (110) plane. These sheets are stabilized via C−O···π and π−π interactions. O7 is oriented such that it points toward the center of the aromatic ring containing N1 (Cg1) to form a C−O···π interaction. The C−O···π bond lengths and angles were determined by measuring the distance between the lone pair donor atom (O) and the acceptor (a calculated centroid, Cg, corresponding to the center of aromatic ring), as similarly conducted in previous studies:54,55 C−O···Cg 3.287(3) Å; ∠C−O···Cg 144.2(2)°. The two isonicotinate anions containing N1 (Cg1) and N2 (Cg2) form π−π interactions where the center of the aromatic ring of one anion overlaps with the periphery of the next in four different orientations. Centroids were calculated in the center of the aromatic isonicotinate rings participating in these interactions in order to report the linear distance (Cg···Cg) between the centroids: Cg1···Cg2 4.821(2) Å, Cg1···Cg2 5.419(2) Å, Cg1···Cg2 4.127(2) Å, and Cg1··· Cg2 5.468(2) Å. The angle (β) formed by the intersection of the line between centroids and the perpendicular displacement to the place of the cation were measured and are β = 51.23°, 38.00°, 21.43°, and 47.58°, respectively. (UO2)2(O)(OH)(C6H4NO2)2Cu(OH2)Cl (2) consists of two crystallographically unique uranyl cations (U1, O1, O2 and U2, O3, O4) as seen in Figure 4. U1 is bound by one hydroxide

Figure 5. Polyhedral representation of 2 shown down the [010] direction.

isonicotinate anions bind the secondary metal, in this case the Cu(II), via the nitrogen binding sites to form sheets that propagate in the (101) plane. These sheets are further held together by π−π interactions between the two aromatic isonicotinate anions containing N1 (Cg1) and N2 (Cg2) in two different orientations. Relevant distances and angles are Cg1···Cg2 4.253(5) Å, β = 5.14 and Cg1···Cg2 4.438(5) Å, β = 6.57. Compound 3, (UO2)(OH2)2(C6H4NO2)2Cu2(C6H4NO2)4(OH2)2, consists of one crystallographically unique uranyl cation (U1, O1, O1#4) which is bound by two water molecules (Ow1 and Ow1#4) and two bidentate isonicotinate anions via O2, O2#1, O2#4, and O2#5 to form an overall hexagonal bipyramidal coordination geometry (Figure 6). The crystallo-

Figure 4. Local structure of 2. The same color scheme is followed as in Figure 1, with dark blue spheres representing copper(II) cations and green spheres representing chloride anions. Symmetry equivalents: (#2) −x + 1, −y + 1, −z + 2; (#3) −x, −y + 1, −z + 2; (#5) −x + 1, −y + 2, −z; (#6) −x, −y + 2, −z.

Figure 6. Local structure of 3. Symmetry equivalents: (#1) x, −y, −z + 1; (#2) x, y + 1/2, −z + 3/2; (#3) x, y − 1/2, −z + 3/2; (#4) −x + 1,−y, −z + 1; (#5) −x + 1, y, z.

anion (O5), two oxide anions (O6, and O6#3), and two monodentate isonicotinate anions via O8 and O9 which results in a pentagonal bipyramidal primary building unit. Similarly, U2 is bound by two hydroxide anions (O5 and O5#2), one oxide anion (O6#2), and the two isonicotinate anions (O7 and O10) to adopt a pentagonal bipyramidal coordination geometry. The asymmetric unit contains two crystallographically unique copper(II) cations, Cu1 and Cu2. Cu1 is bound by two isonicotinate anions via N1 and N1#5 and two chloride anions Cl1 and Cl1#5 and adopts a square planar molecular geometry. Cu2 is bound by two isonicotinate anions via N2 and N2#6, two water molecules (OW1 and OW1#6), and two chloride anions Cl1#5 and Cl1#6 in elongated bonds to result in a Jahn−Teller-distorted octahedral geometry. The uranyl units

graphically unique copper(II) cation (Cu1) is bound by one disordered water molecule (Ow2) and five isonicotinate anions via N1, N2, N2#1, O3, and O3#1 to result in a pseudooctahedral coordination geometry. The uranyl unit is bound by two bidentate isonicotinate anions via bidentate carboxylate groups which go on to bind two copper(II) centers via the nitrogen atom in the pyridine ring (Figure 7). Each Cu(II) is further connected to four other Cu centers via isonicotinate anions to form an extended “sheet-like” topology in the (011) plane. The Cu−isonicotinate sheets are connected to one another via the uranyl cation to form “slabs”. These slabs interpenetrate one another to form a network as shown in Figure 8. The slabs interact with one another via C−H···π and 1917



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5.423(3) Å, β = 52.50°; Cg1···Cg2 4.944(3) Å, β = 59.01°; and Cg1···Cg2 5.097(3) Å, β = 5.45°. Postsynthetic Rearrangement/Metalation. Compound 1 can be obtained via both metalation of UICP with Ag(I) (Scheme 1) as well as via direct assembly of Ag(I), isonicotinic acid, and UO22+ (Scheme 2). Upon comparison of 1 (Figure 3) and UICP (Figure 1) there is very little difference in the structures of the two materials. Both contain uranyl dimers that are connected by bridging bidentate isonicotinate anions to form chains decorated with monodentate isonicotinic units. One of the two crystallographically unique isonicotinate pyridyl groups is protonated in UICP to yield neutral chains that hydrogen bond to one another. Compound 1, however, contains a silver cation that takes the place of the charge balancing pyridyl proton to form 2-D sheets from 1-D chains. Compound 2 could only be prepared via reaction of UICP with Cu(II). Attempts to synthesize 2 via direct assembly (changing molar ratios, time, and temperature) resulted consistently in compound 3 and CuIA. These observations suggest that 2 forms upon metalation of rearranged UICP. Despite formation of 2 (Figure 5) from a rearranged UICP, the two compounds do share some structural similarities. The secondary building units adopted by the uranyl cation in 2 are no longer dimers but are instead infinite chains. The SBUs in both compounds however, form 1-D backbones that are decorated by isonicotinate anions which form hydrogen bonds (UICP) or bind a secondary metal center (2) to form similar extended topologies. The ligand to uranyl ratio, however, changes between UICP (2:1) and 2 (1:1), suggesting a ligand loss may be followed by “fusion” of the dimer building units to form the chains, thus offering a possible mechanism for the rearrangement (Scheme 3). The ligand that is released during the rearrangement may react with Cu(II) to form the coproduct, CuIA. Although the uranyl SBUs in 1 and 2 are dimers and infinite chains, respectively, a similar sheet topology is present in both materials. Each compound has a 1-D uranyl backbone that is decorated by isonicotinate anions that further coordinate to Ag(I) and Cu(II) ions via two pyridyl nitrogen atoms. The similarities between compounds 1 and 2 demonstrate that the use of UICP as a starting material may influence the topology of the resulting compounds. Rearrangement of the UICP topology and incorporation of Cu(II) results in 2, whereas 3 results from direct assembly. Compound 2 maintains the uranyl backbone with Cu(II) ions bound by two pyridyl nitrogen atoms, whereas 3 adopts monomeric uranyl building units, and the Cu(II) metal centers are bound by five isonicotinate anions.

Figure 7. Polyhedral representation of 3 shown down the [001] direction.

Figure 8. Polyhedral representation of 3 down the [001] direction. Each color represents a different “slab” interpenetrating with the next.

π−π interactions. One hydrogen atom off of an aromatic ring, H3, aligns with the center of a neighboring aromatic ring. The C−H···π distances and angles were determined by measuring the distance between the hydrogen-bond donor atom (C) and the acceptor:56−58 C−H···Cg 2.98 Å; ∠C−O···Cg 157.00°. The isonicotinate anion containing N1 (Cg1) overlaps with the periphery of the ring of a second anion containing N2 (Cg2) in three different orientations. The relevant distances and angles associated with these interactions are as follows: Cg1···Cg2

Scheme 1. Representation of the Products Formed via Metalation of UICP with Ag(I) and Cu(II)

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Scheme 2. Representation of the Products Formed via Direct Assembly of (UO2)2+ and Isonicotinic Acid with Ag(I) and Cu(II)

Scheme 3. Representation of the Rearrangement/Metalation of UICP To Form 2 and CuIA

Satisfying Charge Balance Requirements. We speculate that the charge balance requirements of CPs are an additional factor in promoting the resulting topologies. Metalation requires incorporation of a charged metal ion into a coordination polymer while maintaining overall charge neutrality. Compound 1 is formed by direct replacement of the pyridyl proton in UICP with a Ag(I) ion. The charge of the coordination polymer is therefore maintained. Cu(II), however, cannot directly replace the pyridyl proton and maintain the charge neutrality of the resulting CP. During formation of 2, UICP is treated with CuCl2 and a chloride anion is incorporated along with the Cu(II) in order to maintain charge balance. These results suggest that the charge of a metal ion may have an additional influence on the CP topologies formed. Fluorescence. Fluorescence studies were carried out on UICP, compound 1, and the bulk products of compounds 2 and 3, despite the CuIA impurity as it is a nonluminescent species and thus should not interfere with uranyl emission. UICP exhibits typical uranyl emission when excited at 420 nm, whereas compound 1 does not (Figure 9), as the Ag(I) likely quenches the uranyl fluorescence by providing a nonradiative relaxation pathway. Compounds 2 and 3, however, do exhibit weak and poorly resolved uranyl emission upon excitation at 420 nm despite the presence of Cu(II) metal centers. The vibronic structure that is typically observed with uranyl

Figure 9. Emission spectra of compounds 1−3 and UICP excited at 420 nm.

emission is diminished in both (Figure 9) and likely due to reabsorption by the Cu(II) metal centers as supported by an absorption band in 2 that begins at 522 nm (Figure S9, Supporting Information). We as well as others have observed that uranyl emission may be quenched in the presence of Cu(II).24,25,31,59 Upon a search of the CSD,32 7 of the 34 known (UO22+)−Cu(II) coordination polymers provide or comment on uranyl emission data with 2 of those 7 reporting no emission and the remaining 5 exhibiting similar diminished 1919



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vibronic uranyl emission.24,25 Compounds 1 and 2 exhibit no emission when excited at 365, whereas 3 displays similar yet weaker emission as compared to that obtained when excited at 420 nm.

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CONCLUSIONS Metalation of a uranyl-bearing coordination polymer (UICP) has proved to be a successful route to bimetallic materials. Both the charge balancing requirements of the starting CP and the coordination chemistry of the secondary metal influence the topologies of the resulting materials. The solid state absorption data of 2 and 3 suggest that the uranyl center transfers energy to the Cu(II) center, thus diminishing the characteristic uranyl emission. Future selection of uranyl-bearing CPs with sites open for metalation may yield hybrid materials with desirable topologies and properties that are otherwise inaccessible via direct assembly.

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ASSOCIATED CONTENT

* Supporting Information S

Thermal ellipsoid plots of all compounds, PXRD spectra of selected compounds, and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CIF files 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 977473−977475 for compounds 1−3.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (202) 994 6959. Fax: (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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REFERENCES

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Metalation as a Route to Bimetallic

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