Heterobimetallic Copper(II) Uranyl Carboxyphenylphosphonates

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Heterobimetallic Copper(II) Uranyl Carboxyphenylphosphonates Pius O. Adelani and Thomas E. Albrecht-Schmitt* Department of Civil Engineering and Geological Sciences and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

bS Supporting Information ABSTRACT: The hydrothermal reactions of uranyl nitrate with diethyl(2-ethoxycarbonylphenyl)phosphonate, 3-carboxyphenylphosphonic acid, or diethyl(3-ethoxycarbonylphenyl)phosphonate with HF result in the formation of [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1) and [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). UCuCPPE-1 is constructed from the UO22+ moiety coordinated by four additional oxygen donor atoms from the phosphonate moiety, resulting in a tetragonal bipyramidal geometry. The phosphonate groups span between the uranyl cations to create chains that are in turn linked into sheets by square planar and square pyramidal Cu(II) units. The phenyl groups separate the sheets from one another. The structure of UCuCPPE-2 consists of a three-dimensional network of tetragonal bipyramidal and pentagonal bipyramidal U(VI) centers. These units form chains that the phosphonate groups link into sheets. The sheets are then joined together by square pyramidal CuO5 units to create the three-dimensional network. In UCuCPPE-1, the phosphonate group coordinates to both uranium and copper, while the carboxylate moiety binds exclusively to the copper(II) centers. In contrast, the oxygen atoms from the phosphonate and carboxylate moieties in UCuCPPE-2 coordinate exclusively to the uranium and copper centers, respectively. Despite the presence of Cu(II) ions in these compounds, they both fluoresce, showing characteristic vibronically coupled charge-transfer based emission.

’ INTRODUCTION The chemistry of actinide phosphonates has been welldeveloped because of their importance in nuclear waste management and separation processes.1 Carboxyphosphonates represent a variation of diphosphonates in which one of the phosphonate functional groups has been substituted with a carboxylate moiety.2 Recent reports on actinide carboxyphosphonates have shown the remarkable flexibility of the phosphonoacetate and related ligands when combined with U(VI).3 The addition of transition metals into these systems vastly expands the physical properties and diversifies the topologies of these materials. Among the most complex of these heterobimetallic compounds are a series of isostructural cubic phases with large water-filled pores. These compounds show differential gas sorption.3c
e In fact, uranyl compounds display a wide range of important properties that include ion exchange, ionic conductivity, intercalation chemistry, photochemistry, nonlinear optics, and selective oxidation catalysis.4
7 The patterns of coordination of carboxyphosphonates with actinides have been investigated primarily in order to understand the migration of actinides and their behavior in biological and geological environments.8 This is important because the constituents of bacterial cell wall consist of various reactive groups, like lipopolysaccharides, peptidoglycan, or phospholipids.8c Uranyl precipitates associated with phosphorus have been isolated from bacterial cell walls, and these could have possible influence on transportation of actinides in the environment. The coordination chemistry of uranium is rich and mostly dominated by uranium in r 2011 American Chemical Society

the high oxidation state of VI. In addition, they commonly yield low-dimensional layered structures containing tetragonal, pentagonal, and hexagonal bipyramidal U(VI) units.9 Carboxyphenylphosphonate represents a variation in the carboxyphosphonates in which the alkyl substituent in the phosphonoacetate ligand has been substituted by the aryl group.2a,e,3 The structural disparity between carboxyalkylphosphonates (phosphonoacetates) and carboxyphenylphosphonates is largely attributed to the steric influence from the phenyl ring and ability to vary the position of the carboxylate (i.e., ortho-, meta-, para-substituted positions). The phosphonate oxygen atoms are harder than the carboxylates (on a Pearson scale), and the binding affinity of the U(VI) units to the phosphonate moieties is more common than to the carboxylate groups.8 The different binding preferences of the bifunctional carboxyphosphonates allow for the construction of heterobimetallic U(VI)/ transition-metals complexes.3 Transition metals have been used to connect one-dimensional uranyl phosphonoacetate networks into three-dimensional frameworks in [H3O](UO2)2Cu2(PPA)3(H2O)2,3a (UO2)2(PPA)2(HPPA)Zn2(H2O)2 3 3H2O,3g and recently in M2[(UO2)6(PPA)3O3(OH)(H2O)2] 3 16H2O3c
e (M = Mn2+/Co2+/Cd2+; PPA = phosphonoacetate). We have demonstrated in our previous reports using 4-carboxyphenylphosphonic acid the differences in bonding between Th(IV) and Received: July 28, 2011 Revised: August 29, 2011 Published: August 30, 2011 4676

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Table 1. Crystallographic Data for [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1) and [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2) compound formula mass

UCuCPPE-1 1590.97

color and habit

blue-green, tablet P1 (No. 2)

space group a (Å)

10.1130(11)

1.775(7)

P(1)
C(1)

1.804(10)

U(1)
O(1)

1.786(7)

P(2)
O(7)

1.522(7)

blue-green, tablet

U(1)
O(6)

2.278(7)

P(2)
O(9)

1.527(7)

P1 (No. 2)

U(1)
O(4)

2.305(7)

P(2)
O(8)

1.557(7)

9.595(3)

U(1)
O(8) U(1)
O(7)

2.366(6) 2.467(7)

P(2)
C(8) O(10)
C(7)

1.799(10) 1.282(12)

U(1)
O(8)

2.534(6)

O(11)
C(7)

1.279(11)

U(2)
O(3)

1.768(7)

O(12)
C(14)

1.270(12)

88.854(4)

U(2)
O(3)

1.768(7)

O(13)
C(14)

1.244(12)

65.345(3)

U(2)
O(9)

2.276(7)

Cu(1)
O(13)

1.938(7)

1188.0(6)

U(2)
O(9)

2.276(7)

Cu(1)
O(11)

1.957(7)

1

U(2)
O(5)

2.278(7)

Cu(1)
O(12)

1.961(7)

U(2)
O(5) P(1)
O(6)

2.278(7) 1.511(7)

Cu(1)
O(10) Cu(1)
O(14W)

1.964(8) 2.224(7)

P(1)
O(5)

1.522(7)

Cu(1)
Cu(1)

2.654(2)

P(1)
O(4)

1.528(8)

10.7154(12)

10.330(3)

10.7332(12) 60.861(1)

13.670(4) 75.702(3)

β (deg)

80.486(1)

γ (deg)

76.604(1)

V (Å)

986.37(19)

T (K)

100

100

λ (Å)

0.71073

0.71073

Fcalcd (g cm
3) μ (Mo Kα) (mm
1)

2.678 10.039

2.579 11.31

R(F) for Fo2 > 2σ(Fo2)a

0.032

0.044

Rw(Fo2)b

0.112

0.111

)

)

R(F) = ∑ Fo|
|Fc /∑|Fo|. ∑w(Fo4)]1/2 a

U(1)
O(2)

c (Å) α (deg)

1

b

distances (Å)

UCuCPPE-2 1845.46

b (Å)

Z

Table 3. Selected Bond Distances (Å) and Angles (deg) for [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2)

angles (deg)

Table 2. Selected Bond Distances (Å) and Angles (deg) for [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1) distances (Å) U(1)
O(1)

1.799(5)

P(2)—O(6)

U(1)
O(1)

1.799(5)

P(2)
O(7)

1.543(5)

U(1)
O(8) U(1)
O(8)

2.239(5) 2.239(5)

P(2)
C(8) O(9)
C(7)

1.824(8) 1.233(10)

U(1)
O(3)

2.249(5)

O(10)
C(7)

1.309(10)

U(1)
O(3)

2.249(5)

O(11)
C(14)

1.295(9)

1.525(5)

U(2)
O(2)

1.780(5)

O(12)
C(14)

1.244(10)

U(2)
O(2)

1.780(5)

Cu(1)
O(7)

1.895(5)

U(2)
O(4)

2.276(5)

Cu(1)
O(5)

1.907(5)

U(2)
O(4)

2.276(5)

Cu(1)
O(9)

1.988(5)

U(2)
O(6) U(2)
O(6)

2.307(5) 2.307(5)

Cu(1)
O(11) Cu(1)
O(13W)

2.016(5) 2.329(6)

P(1)
O(3)

1.519(5)

Cu(1)
Cu(2)

2.9288(10)

P(1)
O(4)

1.521(5)

Cu(2)
O(14W)

1.984(6)

P(1)
O(5)

1.536(5)

Cu(2)
O(14W)

1.984(6)

P(1)
C(1)

1.832(7)

Cu(2)
O(11)

1.991(5)

P(2)
O(8)

1.521(5)

Cu(2)
O(11)

1.991(5)

angles (deg) 0

O(1)
U(1)
O(1)

180

O(2)
U(1)
O(1)

R(Fo2) = [∑w(Fo2
Fc2)2/

O(2)
U(2)
O(2)0

180

U(VI) and the carboxylate portion of this ligand, and this was later extended by incorporating Cs+ and Ba2+ cations.2a,e Herein we report the syntheses, structures, and spectroscopic properties of [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6-

179.8(3)

O(2)
U(2)
O(2)0

180

H4CO2)]2 (UCuCPPE-1) and [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2).

’ EXPERIMENTAL SECTION Synthesis. UO2(NO3)2 3 6H2O (98%, International Bio-Analytical Industries), HF (48 wt %, Aldrich), cesium hydroxide hydrate (15
20%, Alfa Aesar), copper(II) acetate monohydrate (98
102%, Alfa Aesar), diethyl (2-ethoxycarbonylphenyl)phosphonate (96%, Epsilon Chimie), 3-carboxyphenylphosphonic acid (98%, Epsilon Chimie), and diethyl (3-ethoxycarbonylphenyl)phosphonate (97%, Epsilon Chimie) were used as received. Reactions were run in PTFElined Parr 4749 autoclaves with a 23 mL internal volume. Distilled and Millipore filtered water with resistance of 18.2 MΩ 3 cm was used in all reactions. Caution! While all the uranium compounds used in these studies contained depleted uranium salts, standard precautions were performed for handling radioactive materials, and all studies were conducted in a laboratory dedicated to studies on actinide elements. [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1). UO2(NO3)2 3 6H2O (100.2 mg, 0.2 mmol), Cu-

(OOCCH3)2 3 H2O (40.2 mg, 0.2 mmol), diethyl (2-ethoxycarbonylphenyl)phosphonate (57.5 mg, 0.2 mmol), 0.5 mL of water, and HF (∼10 μL) were loaded into a 23 mL autoclave. The autoclave was sealed and heated to 120 °C in a box furnace for 3
5 days and was then cooled at an average rate of 1 °C/h to 25 °C. The resulting blue-green product was washed with distilled water and methanol and allowed to air-dry at room temperature. Tablets of UCuCPPE-1 were isolated.

[H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2).

UO2(NO3)2 3 6H2O (50.6 mg, 0.1 mmol), Cu(OOCCH3)2 3 H2O (20.2 mg, 0.1 mmol), diethyl (3-ethoxycarbonylphenyl)phosphonate (40 μL), 0.7 mL of water, and HF (∼10 μL) were loaded into a 23 mL autoclave. The autoclave was sealed and heated to 200 °C in a box furnace for 3
5 days and was then cooled at an average rate of 1 °C/h to 25 °C. The resulting blue-green product was washed with distilled water and methanol and allowed to 4677

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Figure 1. (a) A depiction of the stacking of layers in [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1). (b) A view along the b axis of [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1). UO6 tetragonal bipyramids = green, copper = blue, oxygen = red, phosphorus = magenta, carbon = black. air-dry at room temperature. UCuCPPE-2 can also be produced by replacing the ligand in the procedure above with 3-carboxyphenylphosphonic acid (20.2 mg, 0.1 mmol) and adjusting the pH (2.2
5.3) using CsOH 3 xH2O. Tablets of UCuCPPE-2 were isolated from these reactions. Crystallographic Studies. Single crystals of UCuCPPE-1 and UCuCPPE-2 were mounted on cryoloops and optically aligned on a Bruker APEXII Quazar CCD X-ray diffractometer using a digital camera. Initial intensity measurements were performed using a IμS X-ray source and a 30 W microfocused sealed tube (Mo Kα, λ = 0.710 73 Å) with a monocapillary collimator. Standard APEXII software was used for determination of the unit cells and data collection control. The intensities of reflections of a sphere were collected by a combination of four sets of exposures (frames). Each set had a different j angle for the crystal, and each exposure covered a range of 0.5° in ω. A total of 1464 frames were collected with an exposure time per frame of 30 or 40 s,

depending on the crystal size and quality. SAINT software was used for data integration including Lorentz and polarization corrections. Semiempirical absorption corrections were applied using the program SADABS.10 The program suite SHELXTL was used for space group determination (XPREP), direct methods structure solution (XS), and least-squares refinement (XL).11 The final refinements included anisotropic displacement parameters for all atoms. Selected crystallographic information are listed in Table 1
3. Atomic coordinates, bond distances, and additional structural information are provided in the Supporting Information (CIFs). UV
Vis
NIR and Fluorescence Spectroscopy. Fluorescence and absorption data were acquired for the two compounds and UO2(NO3)2 3 6H2O from single crystals using a Craic Technologies UV
vis
NIR microspectrophotometer with a fluorescence attachment. The absorption data was collected in the range of 250
1200 nm at room temperature. The fluorescence spectra were recorded in the 4678

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Figure 2. Depiction of local coordination environment in [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1). Ellipsoids are shown in the 50% probability level. range of 450
650 nm at room temperature, and excitation was achieved using 365 nm light from a mercury lamp for the fluorescence spectroscopy. Infrared Spectroscopy. Infrared spectra were collected from single crystals of the two compounds using a SensIR Technology IlluminatIR FT-IR microspectrometer. A single crystal of each compound was placed on a glass slide, and the spectrum was collected with a diamond ATR objective.

’ RESULTS AND DISCUSSION Syntheses. UCuCPPE-1 and UCuCPPE-2 can be prepared under mild hydrothermal conditions via in situ ligand syntheses. The addition of HF or CsOH 3 xH2O to the reactions is essential. The amount of HF added must be carefully controlled to avoid isolation of uranium fluorides as the predominant product. HF or CsOH 3 xH2O has substantial effects on the reactions and substantially affects crystallinity even if it is not incorporated into the final product. The absence of HF or CsOH 3 xH2O in the syntheses resulted into formation of powders and glasses as the predominant products. Attempts to increase the yield and purity of UCuCPPE-1 and UCuCPPE-2 by adjusting the reaction temperature, increasing the heating duration from 3 to 5 days, increasing the ratios of the reactants, and even increasing the pH (2
5) and substituting the phosphonic acid for the ester in UCuCPPE-2 did not result in producing a pure product. This is a very common problem in the syntheses of heterobimetallic uranyl compounds. The slow cooling of the reactions is highly critical to prevent formation of glasses. The powder X-ray diffraction patterns of the bulk products showed that the mixtures contained different phases, consisting of the isolated heterobimetallic compound and probably the unreacted reagents and uranyl phosphonates. The major products were the heterobimetallic compounds, which can be identified as blue-green tablet of crystals, forming about 60% of the reaction products. Structure of [Cu(H2O)]2Cu(H2O)2[(UO2)(PO3C6H4CO2H)(PO3C6H4CO2)]2 (UCuCPPE-1). UCuCPPE-1 adopts a layered structure that contains UO6 tetragonal bipyramids, as shown in Figure 1a. Each layer contains two uranyl centers with similar coordination environments and two crystallographically distinct

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Cu(II) centers. The Cu(II) ions are in two different coordination environments: square pyramidal Cu(1) and square planar Cu(2) (see Figure 2). A view along the [ac] plane reveals that the uranyl chains are bridged exclusively through the phosphonate moiety, while the copper atoms are connected into chains by both phosphonate and carboxylate moieties. The uranium centers, U(1) and U(2), are bound to two oxo atoms at the axial positions, resulting in linear uranyl cations. The [OdUdO]2+ units have average UdO bond distances of 1.799(5) and 1.780(5) Å for U(1) and U(2), respectively. Four oxygen atoms are coordinated to these uranyl centers along the equatorial plane, leading to U
O bonds that range from 2.239(5) to 2.307(5) Å. These distances result in bond-valence sums of 6.01 for U(1) and 5.89 for U(2), which are consistent with U(VI).12 The P
O bond distances range from 1.519(5) to 1.543(5) Å, while the C
O bonds within the carboxylate moiety range between 1.233(10) and 1.309(10) Å. Protonation of the C
O group is indicated by the slight asymmetry of the C
O bonds. Cu
O bond distances range from 1.895(5) to 1.991(5) Å, and the distance between Cu(1)
Cu(2) is 2.9288(10) Å. Cu(1) is coordinated by five oxygen atoms, two from carboxylate moieties, two from phosphonates, and one from a water molecule. For the Cu(2) center, two of the four oxygen atoms are from carboxylate moieties and the remaining two from water molecules. Bond-valence sum calculations for Cu(1) and Cu(2) are 2.14 and 1.74, respectively. These confirm the presence of Cu2+ (as do the UV
vis data vide infra).12 Selective bond distances are provided in Table 2. Structure of [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). UCuCPPE-2 adopts a three-dimensional network structure that contains both UO6 and UO7 tetragonal bipyramid and pentagonal bipyramid coordination environments. The UO7 polyhedra share an edge to form dimers that are connected to the UO6 polyhedra via the PO3 moiety, as shown in Figure 3b. These chains of uranyl phosphonate polyhedra are interconnected through the Cu(II) centers by the carboxylate moiety to form a three-dimensional structure (Figure 3a
c). The Cu(II) coordination environment is square pyramidal (Figure 4). There are disordered cocrystallized water molecules and an hydronium ion for charge balance. The structure of UCuCPPE-2 consists of two crystallographically distinct uranyl centers, U(1) and U(2), that are coordinated by two nearly linear oxo atoms, UO22+ units, and the OdUdO bond angles are 179.8(3)° and 180° with normal average UdO bond distances of 1.781(7) and 1.768(7) Å, respectively. The U
O bond distances for U(1) and U(2) range from 2.278(7) to 2.534(6) Å and 2.276(7) to 2.278(7) Å, respectively. The bondvalence sums for U(1) and U(2) are 6.01 and 6.04, respectively. They are both consistent with the formal oxidation state of U(VI).12 The P
O bond distances range from 1.511(7) to 1.557(7) Å, while the C
O bonds range between 1.244(12) and 1.282(12) Å. The Cu(II) center coordinates exclusively to four carboxylate moieties with Cu
O bond distances that range from 1.938(7) to 1.964(8) Å and bound water molecule with a bond distance of 2.224(7) Å. The distances between Cu(II) centers make a short Cu(1)
Cu(1)0 distances of 2.654(2) Å. Bondvalence sum calculations for Cu(II) is 2.13, in agreement with the Cu(II) oxidation state.12 Selective bond distances are given in Table 3. Comparison of the Coordination Environments of Uranyl Carboxyphenylphosphonates. Some interesting comparisons 4679

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Figure 3. (a) A depiction of the packing diagram in [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). (b) A polyhedral representation of the uranyl phosphate chain showing the UO7 dimers and UO6 coordination environment in [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). (c) Illustration of [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2) as viewed along the [bc] plane. UO6 tetragonal bipyramids and UO7 pentagonal bipyramids = green, copper = blue, oxygen = red, phosphorus = magenta, carbon = black.

can be drawn from the coordination environments of U(VI) and Cu(II) centers in these two structures. The different binding preferences of the phosphonate and carboxylate groups around the uranium and the Cu(II) centers, respectively, in these two novel structures are not strictly based on Pearson’s principle of hard/soft acid/base.13 The U(VI) metal centers in both UCuCPPE-1 and UCuCPPE-2 show exclusive affinity for phosphonate functional group, while the Cu(II) metal ions preferentially coordinate to carboxylate moiety. In UCuCPPE-1

structure, the Cu(II) metal ions also coordinate to phosphonate moiety; a similar trend was observed in most heterobimetallic uranyl phosphonoacetates incorporating various transition metals.3 As expected, the structure of UCuCPPE-2 show exclusive coordination of phosphonate group to UO22+ units and carboxylate group to Cu(II) metal ions, the same trend was observed in the coordination chemistry of 4-carboxyphenylphosphonic acid with uranyl and alkali metal cations, except at high pH.2e The steric influence from the phenyl ring is 4680

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Figure 4. Local coordination environment in [H3O]2[Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). Ellipsoids are shown in the 50% probability level.

Figure 5. UV
vis
NIR of UCuCPPE-1, UCuCPPE-2, and UO2(NO3)2 3 6H2O.

the major difference and is responsible for various coordination patterns in these two novel compounds compared to U(VI)/ transition metal complexes of phosphonoacetates. The difference observed within these two structures around Cu(II) metal ions was due to a chelating effect from the close arrangement of the phosphonate and carboxylate groups in 2-carboxyphenylphosphonates compared to 3-carboxyphenylphosphonates (ortho- and para-substituted effects). UV
Vis
NIR and Fluorescence Spectroscopy. The absorbance spectra for these two novel compounds and uranyl nitrate hexahydrate were collected. As shown in Figure 5, the characteristic equatorial U
O charge transfer band and axial UdO charge transfer band (vibrionic coupling) were observed around 325 and 421 nm, respectively, for UO2(NO3)2 3 6H2O. For the two novel Cu(II) uranyl carboxyphenylphosphonate compounds, additional absorbance peaks were also observed along with the characteristic uranyl bands at values around 687
778 nm for d
d transition in the Cu(II) ions.

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Figure 6. Fluorescence spectra of UCuCPPE-1, UCuCPPE-2, and UO2(NO3)2 3 6H2O showing the emission of green light with welldefined charge-transfer vibronic transitions.

The fluorescence of uranyl compounds centered near 520 nm has been known for centuries. The charge-transfer-based emission is vibronically coupled to both bending and stretching modes of the uranyl cation and typically consists of a five-peak spectrum, although far more lines can be observed at low temperature.14 Denning and his group members have examined in detail these transitions for the solid samples containing [UO2Cl4]2
anion owing to its crystallization in a cubic space group.15 However, not all uranyl compounds possess luminescence properties, and the mechanisms of the emission from uranyl compounds are most often difficult to explain. Clearfield and his co-workers demonstrated a considerable variance in the luminescence properties of two closely related uranyl phenylphosphonate compounds, [UO2(HO3PC6H5)2(H2O)]2 3 8H2O and UO2(HO3PC6H5)2(H2O) 3 2H2O; the structural difference was based on conformational symmetry.3c The addition of Cu(II) has been known to eliminate emission from uranyl complexes because of the overlap of emission from uranyl cations with the d
d absorption band of Cu(II), yielding energy transfer and nonradiative decay.3a,16,17 As we also reported here, unusual luminescence properties have been documented recently in a copper(II) uranyl phosphonate.18 The fluorescence spectra for these two novel carboxyphenylphosphonate compounds and the benchmark compound UO2(NO3)2 3 6H2O were collected under the same experimental conditions and procedures and are shown in Figure 6. The luminescence spectra of UCuCPPE-1 and UCuCPPE-2 differ only in the peak resolution; this is likely due to the sizes and quality of the crystals examined, and their peak maxima positions differ from thise of the benchmark compound. Four prominent peaks are clearly resolved at 488, 543, 585, and 611 nm, which correspond to electronic and vibronic transitions S11
S00 and S10
S0v (v = 0
4). The most intense peak is positioned at 543 nm for these two compounds. As for the benchmark compound, the luminescent spectrum showed well-resolved sharp vibronic peaks at 487, 509, 532, 558, 586, and 612, and the most intense peak (S10
S00) is positioned at 509 nm. These compounds exhibit a slight red shift of 35 nm along with lesser intensity for the emission bands compared to the benchmark compound. The slight difference from these two compounds and the benchmark 4681

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Crystal Growth & Design compound may be attributed to the coordination environment around the uranium centers, UO6 and UO7, and ligand field effects. The fluorescence spectra from these two novel compounds are different from those reported for uranyl carboxyphenylphosphonate, UO2(PO3HC6H4CO2H)2 3 2H2O.2a Five prominent peaks of intense emission with vibronic coupling were reported with peak maxima positioned at 528.03 nm. These two novel heterobimetallic compounds exhibit a slight red shift of 15 nm compared to UO2(PO3HC6H4CO2H)2 3 2H2O. The fluorescence spectra observed here are similar to those reported for Cs+/Ba2+ uranyl carboxyphenylphosphonates.2e Therefore, we can infer that the alkali and transition metals could be playing important roles as well. Infrared Spectroscopy. The low wavenumber region of the IR spectra, from 660 to 747 cm
1, is dominated by the O
P
O bending, phenyl ring, and P
C stretching vibrations. The asymmetric and symmetric stretching modes of the uranyl cation, UO22+, range from about 811 to 919 cm
1. The group of peaks around 1000
1200 cm
1 is at expected values for P
O and PdO symmetric and asymmetric stretching modes of phosphonates. The bands between 1320 and 1427 cm
1 are due to phenyl ring stretching vibrations. The ν(CdO) of the carboxylate groups is observed between 1478 and 1700 cm
1. The peaks around 2900 cm
1 are indicative of the C
H stretching in the phenyl ring. The broad bands around 3500 cm
1 are associated with free water molecules and are observed in UCuCPPE-2 only (see the Supporting Information).19
21

’ CONCLUSIONS Although there are previous examples of uranyl/transition metals coordinated to carboxyalkylphosphonic acid,3a
f the present study is a novel extension to carboxyphenylphosphonic acid ligands. The purpose of this work was to compare the coordination chemistry of uranium compounds that contain the bifunctional groups 2- or 3-carboxyphenylphosphonate ligands (meta- and ortho-position). The structural disparity between these materials and the phosphonoacetate is largely attributed to the steric influence from the carbon residues. The use of the phenyl spacer tends to acts as a pillaring agent linking the adjacent layers. We have observed in our previous work that in 4-carboxyphenylphosphonate,2e the carboxylate moiety exclusively binds the softer metal cations and only coordinates to uranium centers at high pH. Attempts were made to incorporate transition metal in the 4-carboxyphenylphosphonate ligand without success. When 2- or 3-carboxyphenylphosphonic acid is used, novel Cu(II) uranyl carboxyphenylphosphonate compounds are isolated. The ligands’ chelating effects coupled with its steric influences are playing vital roles here. The major difference in the coordination pattern of these two compounds is around the Cu(II) metal ions. In UCuCPPE-1, the Cu(II) center coordinates to both carboxylate and phosphonate moieties of the 2-carboxyphenylphosphonate ligand in similar pattern to complexes from phosphonoacetates,3a
f whereas in UCuCPPE-2, only carboxylate moieties are involved in coordination to the Cu(II) metal ions; this trend is similar to what we reported before.2e However, the central issue we intend to address in the ongoing studies is to incorporate other transition metals and lanthanide cations in order to probe further the structural variations and electronic properties.

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’ ASSOCIATED CONTENT Supporting Information. X-ray crystallographic files in CIF format for [Cu(H 2 O)]2 Cu(H 2 O)2 [(UO2 )(PO 3 C 6 H4 CO 2 H)(PO 3 C6 H4 CO2 )]2 (UCuCPPE-1) and [H 3 O]2 [Cu(H2O)]2[(UO2)3(PO3C6H4CO2)4] 3 3H2O (UCuCPPE-2). This material is available free of charge via the Internet at http:// pubs.acs.org/.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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