Role of N-Donor Sterics on the Coordination Environment and

Article pubs.acs.org/crystal

Role of N‑Donor Sterics on the Coordination Environment and Dimensionality of Uranyl Thiophenedicarboxylate Coordination Polymers Sonia G. Thangavelu,† Ray J. Butcher,‡ and Christopher L. Cahill*,† †

Department of Chemistry, The George Washington University, 800 22nd Street, NW, Washington, DC 20052, United States Department of Chemistry, Howard University, 525 College Street, NW, Washington, DC 20059, United States

‡

S Supporting Information *

ABSTRACT: Thiophene 2,5-dicarboxylic acid (TDC) was reacted with uranyl acetate dihydrate and one (or none) of six N-donor chelating ligands (2,2′-bipyridine (BPY), 4,4′dimethyl-2,2′-bipyridine (4-MeBPY), 5,5′-dimethyl-2,2′-bipyridine (5-MeBPY), 6,6′dimethyl-2,2′-bipyridine (6-MeBPY), 4,4′,6,6′-tetramethyl-2,2′-bipyridine (4,6-MeBPY), and tetrakis(2-pyridyl)pyrazine (TPPZ) to result in the crystallization of seven uranyl coordination polymers, which were characterized by their crystal structures and luminescence properties. The seven coordination polymers, Na2[(UO2)2(C6H2O4S)3]· 4H2O (1), [(UO2)4(C6H2O4S)5(C10H8N2)2]·C10H10N2·3H2O (2), [(UO2)(C6H2O4S)(C12H12N3)] (3), [(UO2)(C6H2O4S)(C12H12N3)]·H2O (4), [(UO2)2(C6H2O4S)3]· (C12H14N2)·5H2O (5), [(UO2)3(CH3CO2)(C6H2O4S)4](C14H17N2)3·(C14H16N2)·H2O (6), and [(UO2)2(C6H2O4S)3](C24H18N6) (7), consist of either uranyl hexagonal bipyramidal or pentagonal bipyramidal coordination geometries. In all structures, structural variations in the local and global structures of 1−7 are influenced by the positions (or number) of methyl groups or pyridyl rings on the N-donor species, thus resulting in a wide diversity of structures ranging from single chains, double chains, or 2-D sheets. Direct coordination of N-donor ligands to uranyl centers is observed in the chain structures of 2−4 using BPY, 4-MeBPY, and 5-MeBPY, whereas the N-donor species participate as guests (as either neutral or charge balancing species) in the chain and sheet structures of 5−7 using 6-MeBPY, 4,6-MeBPY, and TPPZ, respectively. Compound 1 is the only structure that does not contain any N-donor ligands and thus crystallizes as a 2-D interpenetrating sheet. The luminescent properties of 1−7 are influenced by the direct coordination or noncoordination of N-donor species to uranyl centers. Compounds 2−4 exhibit typical UO22+ emission upon direct coordination of N-donors, but its absence is observed in 1, 5, 6, and 7, when N-donor species participate as guest molecules. These results suggest that direct coordination of N-donor ligands participate as chromophores, thus resulting in possible UO22+ sensitization. The lack of emission in 1, 5, 6, and 7 may be explained by the extended conjugation of the TDC ligands within their structures.

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INTRODUCTION Coordination polymers (CPs) constructed from the uranyl cation (UO22+) continue to remain attractive in the field of hybrid materials owing to their unique structural and luminescent properties.1 The UO22+ cation is a linear triatomic species that contains two axial oxygen atoms about the central uranium atom, the terminal nature of which promotes additional ligand coordination about the equatorial plane.2 This results in three types of UO22+ primary building units (PBUs) found as square, pentagonal, or hexagonal bipyramidal geometries. The uranyl cation may also undergo hydrolysis and subsequent condensation to form secondary building units (SBUs) in the form of dimers, trimers, or tetramers, etc.3,4 UO22+ hydrolysis under hydrothermal conditions is a dynamic process wherein multiple speciation products are present in a wide range of pH and concentration.5−7 This, in turn, makes it challenging to predict or control the type of uranyl building unit that will ultimately be observed in the solid state. As such, PBUs and SBUs (or a combination of both) bound by © XXXX American Chemical Society

ligands yield a diverse portfolio of uranyl CPs with rich architectures.8,9 The judicious choice of organic linkers is important in the synthesis of uranyl hybrid materials. Researchers have used mono- or dicarboxylate,10−19 phosphonate,20−25 sulfonate,26−28 arsenate,29−32 and/or cucurbit(n)uril33−37 ligands to construct uranyl CPs with diverse structural topologies. A dual donor approach (e.g., N- and O-functionality) may promote additional structural diversity via coordination preferences as with heterobimetallic CPs, or through unique local coordination geometries.25,38−43 Moreover, one may use a combination of chelating polydentate N-donors with carboxylates to promote a specific coordination geometry and assembly motifs. As such, we have recently synthesized a series of binuclear pseudo dimers and one-dimensional chains containing N-donors terpyridine (TPY) and different aromatic O-donor dicarboxylates.44 This Received: April 20, 2015

A

DOI: 10.1021/acs.cgd.5b00549 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

is suitably above the emissive state of the uranyl cation.44 These relative positions allow for study of the influence of topology and/or chelating ligands on uranyl emission without quenching by TDC structural components. As such, we report herein the synthesis, structures, and luminescence of seven UO 2 2+ structures, Na 2 [(UO 2 ) 2 (C 6 H 2 O 4 S) 3 ]·4H 2 O (1), [(UO 2 ) 4 (C 6 H 2 O 4 S) 5 (C 10 H 8 N 2 ) 2 ]·C 10 H 10 N 2 ·3H 2 O (2), [(UO 2 )(C 6H 2 O 4 S)(C 12 H 12 N 3 )] (3), [(UO 2 )(C 6 H 2 O 4S)(C12H12N3)]·H2O (4), [(UO2)2(C6H2O4S)3]·(C12H14N2)· 5H 2O (5), [(UO2 ) 3(CH3CO 2)(C6 H2 O4 S) 4](C 14 H17N 2) 3· (C14H16N2)·H2O (6), and [(UO2)2(C6H2O4S)3](C24H18N6) (7), that have been characterized by single-crystal XRD, powder XRD, and luminescence spectroscopy.

approach, using chelating N-donor ligands, may potentially circumvent the speciation diversity of UO22+ in aqueous solutions by promoting a single type of PBU via influence of the ligand. Structural precedence for such a notion is evident in uranyl complexes containing bipyridine and terpyridine ligands, in which uranyl monomers were observed almost exclusively.45−51 In the current work, we continue to use N-donor ligands to promote the formation of a restricted uranyl-building unit profile in the solid state and subsequently allow for the systematic exploration of the influence bulky substituents (i.e., methyl or pyridyl) may have on both local and global structure. As such, the following ligands, 2,2′-bipyridine (BPY), 4,4′-dimethyl-2,2′bipyridine (4-MeBPY), 5,5′-dimethyl-2,2′-bipyridine (5-MeBPY), 6,6′-dimethyl-2,2′-bipyridine (6-MeBPY), 4,4′,6,6′-tetramethyl2,2-bipyridine (4,6-MeBPY), and tetrakis(2-pyridyl)pyrazine (TPPZ), were chosen for this study (Scheme 1).

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EXPERIMENTAL SECTION

General Synthesis of Compounds 1−7. The ligands 2,2′bipyridine (BPY), tetrakis(2-pyridyl)pyrazine (TPPZ), and thiophene 2,5-dicarboxylic acid (TDC) were purchased from Sigma-Aldrich, whereas 4,4′-dimethyl-2,2′-bipyridine (4-MeBPY), 5,5′-dimethyl-2,2′bipyridine (5-MeBPY), and 6,6′-dimethyl-2,2′-bipyridine (6-MeBPY) were purchased from VWR. All ligands were used as received. 4,4′,6,6′tetramethyl-2,2′-bipyridine (4,6-MeBPY), on the other hand, was synthesized and characterized by 1H NMR spectroscopy following a literature procedure.63 Uranyl acetate dihydrate was purchased from Fisher Scientific. Caution! Uranium acetate dihydrate (UO2(CH3COO)2·2H2O) used in this study contains depleted uranium. Standard precautions for handling radioactive and toxic substances should be followed. Compounds 2−7 were synthesized as follows: In a 25 mL Teflonlined cup, the reagents UO2(CH3COO)2·2H2O (106 mg, 0.251 mmol, 1 equiv), TDC (64.8 mg, 0.377 mmol, 1.5 equiv), N-donor ligand (0.377 mmol, 1.5 equiv), and 25 μL of 6 M NaOH were added to a solution of 2.5 mL of a H2O−2-propanol mixture (1:1.5). The Teflon cup was then placed in a stainless steel Parr bomb and heated statically at 120 °C for 5 days. The Parr bomb was then removed from the oven and allowed to cool on the benchtop overnight. Solids were isolated from the mother liquor, washed with H2O and 2-propanol, and allowed to air-dry. Single crystals were then isolated and characterized by single-crystal XRD. The molar ratios and yields for each compound are listed in Table 1. The synthesis of 1 followed a similar procedure as 2−7, except that the pH was adjusted with 50 μL of 6 M NaOH and 2.5 equiv of TDC was used (108 mg, 0.628 mmol). These synthetic parameters were modified in the case of compound 1 in order to obtain a pure phase. The synthetic conditions and molar ratios for 1−7 can be found in Table 1. We also attempted to synthesize 1−7 from water alone or at unadjusted pH using the synthetic conditions presented in Table 1, but with slight modifications. For the water only experiments, the molar ratios of reagents (as detailed in Table 1) remain unchanged, except that 2.5 mL of H2O was used in lieu of 2-propanol. For the unadjusted pH experiments, the molar ratios of reagents as presented in Table 1 remain unchanged, except that 2.5 mL of a H2O:2-propanol (1:1.5) mixture was used and the pH of the solutions was not adjusted with 6 M NaOH. In both experiments, the observed PXRD patterns did not match the calculated patterns of 1−7, nor were they indicative of any known uranium oxide phases. Given the tendency of the UO22+ to hydrolyze under these synthetic conditions (i.e., hydro(solvo)thermal synthesis), we speculate that the observed phases in our experiments may be a mixture of multiple uranium oxide or hydroxide products. Our results suggest that the synthetic conditions used to obtain 1−7 as pure or close to pure phases, using a combination of both 2-propanol and adjusted pH (i.e., Table 1), are indeed crucial synthetic parameters to form 1−7 (almost) exclusively. Powder X-ray Diffraction (PXRD). Diffraction patterns for compounds 1−7 were obtained on a Rigaku MiniFlex II Desktop Powder X-ray Diffractometer (Cu Kα, 3−60°) and analyzed using the JADE software package64 and Crystal Impact Match! (2015, version 2.4.1) software package. Purity of bulk samples of 1−7 was determined

Scheme 1. Chelating N-Donor Ligands Used in This Study

Beyond structural interests, luminescence of uranyl CPs is also of note considering the unique photophysical properties that are observed within these materials. Such properties include the emissive properties of UO22+,38,44,48,52,53 the π−π* transitions present within the organic ligands,21,44,54 and sometimes possible energy transfer between the metal center(s) and ligands.55−57 Energy transfer between ligands and metals often results in sensitized emission (i.e., antenna effect), as observed in lanthanide(III) systems.58−61 Analogous studies of uranyl compounds, however, are complicated by the spectral overlap of the UO22+ ligand-to-metal charge transfer (LMCT) bands and π−π* transitions within the ligands.44 Despite this limitation, we have observed that one may influence UO22+ emission via appropriate choice of O-donor dicarboxylates with an appropriately situated triplet state in a series of materials containing TPY (Cl-TPY) ligands.44 In the current work, the luminescence of uranyl CPs containing BPY and TPPZ ligands and the O-donor linker thiophene 2,5-dicarboxylic acid (TDC) was studied in an attempt to explore the influence bulky substituents may have on UO22+ emission. In the present contribution, we continue on our theme on the influence O-donor and N-donor ligands may have on both the structure and the luminescence of uranyl CPs. The O-donor linker thiophene 2,5-dicarboxylic acid (TDC) was chosen in this study because of its ability to bind to uranyl centers through carboxylate oxygens via monodentate or bidentate coordination modes. Indeed, a rich portfolio of uranyl CPs containing these linkers and related thiophene monocarboxylates has been reported.14−17,44,62 Further, as we have explored in our earlier work, the TDC triplet energy level B


yes, red block crystals, and yellow phase no

by comparing observed and calculated PXRD patterns. These patterns can be found in the Supporting Information (Figures S8−S14). Yellow solids other than crystals in the bulk samples of 5 and 7 were observed visually under a microscope. Attempts to identify these phases via PXRD were performed by comparing the powder diffraction files (PDFs) of known uranium phases via the International Centre for Diffraction Data database (ICDD, version 2.0, 2002)64 within the JADE software package. Attempts to identify these phases via PXRD were also performed by comparing the crystallographic information files (CIFs) of known uranium phases via the Crystallographic Open Database (COD, 2012)65 within the Crystal Impact Match! software package. The impurities present in 5 and 7 could not be identified definitively, suggesting that unknown or multiple uranium oxide or hydroxide phases may be present in the bulk samples. Fluorescence Measurements. Compounds 1−7 were crushed to fine powders using a mortar and pestle and placed between two glass slides. Spectra were collected on a Horbia JobinYvon Fluorolog spectrophotometer (excitation wavelengths: 365 nm, 420 nm; excitation/emission slit widths: 3 nm) using the face forward (45°) setting at 298 K. Fluorescence spectra of 1, 5, 6, and 7 (365 and 420 nm) and 2−4 (420 nm) can be found in the Supporting Information (Figures S17−S19). Interpretation of the emission spectra of 5 and 7 must proceed with some restraint as impurities present in the samples (as observed by PXRD) may affect the observed luminescence within these materials. Crystal Structure Determination. Single crystals were isolated from each bulk sample and mounted on MiTeGen micromounts. Reflections were collected using 0.5° φ and ω scans on a Bruker SMART diffractometer equipped with an APEX II CCD detector using Mo Kα radiation at 100 K. All data were integrated using the SAINT software package, and an absorption correction was applied using SADABS. Structures were determined using direct methods (either SIR-92 or SHELXS-2013) and then refined using SHELXL2013 within the WinGX software package,66−68 in which all nonhydrogen atoms were refined anisotropically. Hydrogen atoms residing on the carbon atoms of BPY, MeBPY, TPPZ, and TDC ligands were placed in calculated positions and allowed to ride on their parent atoms. Hydrogen atoms residing on the nitrogen atoms in BPY and 6-MeBPY guests were observed within difference Fourier maps, yet were placed in calculated positions regardless. In compound 7, hydrogen atoms on pyridyl nitrogen atoms N1 and N3 appear to be partially occupied and were treated with a PART command. The hydrogen component H1N represented by the PART 1 command was refined to possess 52% occupancy at nitrogen atom N1 (see the Supporting Information, Figure S16). Moreover, simultaneous occupancy is unlikely considering the nitrogen−nitrogen (N−N) distances, thus giving an overall +2 charge of the TPPZ molecule. Efforts to model hydrogen atoms on solvent waters in 1, 2, 4, 5, and 6 were unsuccessful and thus not included in the final refinement. For compounds 5−7, disorder about the pyridyl and/or thiophene rings was modeled using appropriate constraints in SHELX. The SAME command was used to model disordered pyridyl rings and/or thiophene rings to adopt the same shape as a cyclic six-membered or five-membered ring. Further, the PART command was used to model positional disorder of the aromatic rings in 6-MeBPY (5), 4,6-MeBPY (6), and TDC (7). Because of the presence of highly disordered 4,6MeBPY guests in 6, locations of the hydrogen atoms on the nitrogen atoms via Fourier electron density maps proved unsuccessful. For purposes of discussion, an assumption was made that compound 6 consists of both singly protonated and neutral 4,6-MeBPY guest species to account for charge balance in the structure. Tests for additional symmetry in all structures were done using PLATON.69 A summary of the crystallographic data for 1−7 can be found in Table 2. ORTEP representations can be found in the Supporting Information (Figures S1−S7). Crystallographic Information Files (CIFs) of compounds 1−7 were deposited to the Cambridge Crystallographic Data Centre (CCDC) and can be obtained in the Supporting Information or via http://www.ccdc.cam.ac.uk by citing deposition numbers 1060072 (1), 1060073 (2), 1060074 (3), 1060075 (4), 1060076 (5), 1060077 (6), and 1060078 (7).

yes yes yes pure?

a

Note: UAc represents uranyl acetate dihydrate [(UO2)(CH3COO)2]·2H2O.

yes, yellow block crystals yes, yellow block crystals yes, yellow block crystals and yellow phase yes no yes

120 5 3.75 5.31 5.77 93 mg (75%)

TPPZ (146 mg, 0.377 mmol) TDC (64.8 mg, 0.377 mmol) 120 5 3.23 5.24 4.75 156 mg

7 6

4,6-MeBPY (79.9 mg, 0.377 mmol) TDC (64.8 mg, 0.377 mmol)

UAc (106 mg, 0.251 mmol) 6-MeBPY (70.7 mg, 0.377 mmol) TDC (64.8 mg, 0.377 mmol) 120 5 3.60 5.11 5.18 95 mg UAc (106 mg, 0.251 mmol) 5-MeBPY (70.7 mg, 0.377 mmol) TDC (64.8 mg, 0.377 mmol) 120 5 3.97 5.84 5.00 123 mg (79%)

UAc (106 mg, 0.251 mmol) N-donor (mg, mmol) N/A BPY (58.8 mg, 0.377 mmol) 4-MeBPY (70.7 mg, 0.377 mmol) O-donor (mg, mmol) TDC (108 mg, 0.628 mmol) TDC (64.8 mg, 0.377 mmol) TDC (64.8 mg, 0.377 mmol) temp (°C) 120 120 120 time (days) 5 5 5 pH initial 3.43 3.81 3.55 pH adjusted 5.66 5.83 5.79 pH final 3.90 4.85 3.17 mass (% yield 118 mg (20%) 103 mg (67%) 121 mg (77%) based on U) crystals? yes, yellow block crystals yes, yellow block crystals yes, yellow block crystals UAc (106 mg, 0.251 mmol) metal (mg, mmol)

UAc (106 mg, 0.251 mmol)

[(UO2)2(C6H2O4S)3]· C12H14N2·5H2O

5 4

[(UO2)(C6H2O4S) (C12H12N3)]·H2O [(UO2)(C6H2O4S) (C12H12N3)]

3 2

[(UO2)4(C6H2O4S)5(C10H8N2)2]·C10H10N2·3H2O Na2[(UO2)2(C6H2O4S)3]· 4H2O

1

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formula

Table 1. Synthetic Conditions for Compounds 1−7a

[(UO2)3(CH3CO2) [(UO2)2(C6H2O4S)3] (C6H2O4S)4] (C24H18N6) (C14H17N2)3·(C14H16N2)·H2O UAc (106 mg, 0.251 mmol) UAc (106 mg, 0.251 mmol)


C


53 618 27 242 20 026 [R(int) 0.0675)] 5239 [(R(int) 0.0912] R1 = 0.0685, wR2 = 0.2001 R1= 0.0408, wR2 = 0.0738

17 295 9829 [(R(int) 0.0918] R1 = 0.0626, wR2 = 0.1863

volume (Å3) Z density (calcd) (Mg/m3) absorption coefficient (mm−1) reflns collected independent reflns final R indices [I > 2σ(I)]

6025.9(9) 4 2.571 11.053

β = 90.758(2)° 1871.2(2) 4 2.216 8.825

β = 91.5900(10)°

624.40 100 0.71073 monoclinic P21/n a = 8.0279(6) Å b = 13.2100(10) Å c = 17.6516(13) Å

611.84 100 0.71073 triclinic P1̅ a = 8.4850(12) Å b = 20.759 (3) Å c = 21.698(3) Å α = 97.970(3)° β = 100.426(2)° γ = 101.314(2)° 3625.2(9) 8 2.242 9.141


formula weight temp (K) wavelength (Å) crystal system space group unit cell dimensions

[(UO2)4(C6H2O4S)5(C10- [(UO2)(C6H2O4S) H8N2)2]·C10H10N2·3H2O (C12H12N2)]

3

Na2[(UO2)2(C6H2O4S)3]· 4H2O

2

formula

1

Table 2. Crystallographic Data for Compounds 1−7

41 525 6114 [R(int) 0.0601)] R1 = 0.0440, wR2 = 0.1050

2174.06(14) 4 1.957 7.601

β = 94.1450(10)°


[(UO2)(C6H2O4S) (C12H12N3)]·H2O

4 [(UO2)3(CH3CO2) (C6H2O4S)4] (C14H17N2)3·(C14H16N2)·H2O 601.67 100 0.71073 triclinic P1̅ a = 11.4282(11) Å b = 20.119(2) Å c = 20.4120(2) Å α = 81.5760(10)° β = 77.9450(10)° γ = 78.0470(10)° 4463.9(8) 8 1.785 5.599

6

48 363 63 325 13 043 [R(int) = 0.0708] 23 908 [(R(int) 0.0668] R1 = 0.0656, R1= 0.0576, wR2 = 0.1403 wR2 = 0.1949

9422(5) 8 1.619 7.058

β = 94.679(5)°

1179.59 100 0.71073 monoclinic C2/c a = 7.707(2) Å b = 36.226(11) Å c = 33.863(10) Å

[(UO2)2(C6H2O4S)3]· (C12H14N2)·5H2O

5

30 462 6093 [R(int) 0.0226] R1= 0.0171, wR2 = 0.0336

4260.8(4) 8 2.246 7.823

β = 122.8670(10)°

720.46 100 0.71073 monoclinic C2/c a = 29.8727(16) Å b = 7.6518(4) Å c = 22.1924(12) Å

[(UO2)2(C6H2O4S)3] (C24H18N6)

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Crystal Growth & Design Article

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RESULTS Structural Description. C r y s t a l S t r u c t u r e o f Na2[(UO 2)2(C6H2O 4S) 3]·4H 2O (1). The structure of 1 contains one crystallographically unique uranyl cation (U1, O1, O2) bound by three bidentate TDC linkers via O3, O4, O7, O8, O11, O12 (U−Oavg, 2.468 Å) to result in a UO8 hexagonal bipyramidal primary building unit (Figure 1). The building units are linked by TDC dicarboxylates to result in honeycomb (6,3) 2-D infinite sheets consisting of 6-ring void spaces in the (010) plane. This connectivity has been observed in uranyl hybrid materials containing terephthalic acid (BDC), naphthalene 1,4-dicarboxylic acid (NDC),12 and uranyl TDC materials containing noncoordinating phenanthroline transition-metal complexes.70 Like the reported BDC structure, compound 1 shows 2-fold interpenetration of the sheets (see the Supporting Information, Figure S15). In each cavity, the sulfur atoms of alternating thiophene rings point into and out of the cavities, respectively. These cavities contain charge balancing sodium ions and solvent water guest molecules. Crystal Structure of [(UO2)4(C6H2O4S)5(C10H8N2)2]·C10H10N2· 3H2O (2). The structure of 2 contains two crystallographically unique uranyl cations (U1, O1, O2; U2, O13, O14), as seen in Figure 2. U1 is bound by one bidentate BPY ligand via N1 (2.580(1) Å) and N2 (2.680(1) Å), and two bidentate TDC linkers via O3, O4, O5, and O6 (U−Oavg, 2.469 Å) to result in a UO6N2 hexagonal bipyramidal primary building unit. U2 is

bound by three bidentate TDC linkers via O7, O8, O9, O10, O11, and O12 (U−Oavg, 2.472 Å) to result in a UO8 hexagonal bipyramidal primary building unit. U1 and U2 are linked by bridging TDC dicarboxylates to result in a double chain containing 6-ring cavities that propagate in [100], as seen in Figure 2. Each of these cavities is occupied by doubly protonated BPY guests and solvent waters. The BPY guests do not participate in π-stacking (ring centroid−centroid distance, 4.261(7) Å) and as such is not within the acceptable range of 3.3−3.8 Å between aromatic rings.71 It is noteworthy that the BPY species in 2 has a dual role in this structure as both a coordinating and noncoordinating species. Crystal Structure of [(UO2)(C6H2O4S)(C12H12N2)] (3). The structure of 3 contains one crystallographically unique uranyl cation (U1, O1), as seen in Figure 3. U1 is bound by one bidentate 4-MeBPY via N1 (2.535(5) Å), and N2 (2.580(5) Å), a bidentate TDC linker via O6 (2.491(5) Å) and O7 (2.386(5) Å), and a monodentate TDC linker via O5 (2.218(5) Å) to result in a UO5N2 pentagonal bipyramidal primary building unit. These units are linked by bridging TDC dicarboxylates to result in chains that propagate in [010]. Crystal Structure of [(UO2)(C6H2O4S)(C12H12N2)]·H2O (4). The structure of 4 contains one crystallographically unique uranyl cation (U1, O1), as seen in Figure 4. U1 is bound by one bidentate 5-MeBPY ligand via N1 (2.530(6) Å), and N2 (2.578(6) Å), a bidentate TDC linker via O3 (2.441(6) Å) and O4 (2.413(6) Å), and a monodentate TDC linker via O5

Figure 1. Crystal structure of 1. Yellow polyhedra represent uranium, whereas red and yellow spheres represent oxygen and sulfur, respectively. Sodium cations and guest water molecules have been omitted for clarity. E



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Figure 2. Crystal structure of 2. Blue spheres represent nitrogen atoms. Protonated BPY guests are shown in only one 6-ring void space for clarity, and solvent waters have been omitted.

Figure 3. Crystal structure of 3.

Crystal Structure of [(UO 2 ) 3 (CH 3 CO 2 )(C 6 H 2 O 4 S) 4 ](C14H17N2)3·(C14H16N2)·H2O (6). The structure of 6 contains two crystallographically unique uranyl cations (U1, O1, O2; U2, O11, O12), as seen in Figure 6. U1 is bound by one bidentate acetate ion via O7 and O8 (U−Oavg, 2.423 Å), and two bidentate TDC linkers via O3, O4, O5, and O6 (U−Oavg, 2.481 Å) to result in a UO8 hexagonal bipyramidal building unit. U2 is bound by three bidentate TDC linkers via O13, O14, O15, O16, O17, and O18 (U−Oavg, 2.472 Å) to result in a UO8 hexagonal bipyramidal building unit. U1 and U2 are linked by bridging TDC dicarboxylates to result in a double chain containing 6-ring and 4-ring cavities occupied by 4,6-MeBPY (protonated and neutral) guests and solvent water that propagate approximately in [100], as seen in Figure 6. Given the presence of highly disordered 4,6-MeBPY guest molecules in 6, we assumed that some of these guests are singly protonated to account for charge balance of the acetate ion. This assumption was justified via a search in the Cambridge Structural Database (CSD, version 5.34, 2013),72 in which

(2.215(7) Å) to result in a UO5N2 pentagonal bipyramidal primary building unit. These units are linked by bridging TDC dicarboxylates to result in chains that propagate in [010]. It is of note that the chain structures in 3 and 4 are analogous, yet those in 4 are planar compared to 3, and may thus imply that perhaps the positions of the methyl groups influence the planarity of the chains. Crystal Structure of [(UO2)2(C6H2O4S)3]·(C12H14N2)·5H2O (5). The structure of 5 (Figure 5) contains one crystallographically unique uranyl cation (U1, O1, O2) and displays the same local coordination environment as present in 1. U1 is bound by three bidentate TDC linkers via O8, O9, O10, O11, O12, and O13 (U−Oavg, 2.462 Å) to result in a UO8 hexagonal bipyramidal primary building unit. The UO8 units are linked by TDC dicarboxylates to result in sheets consisting of 8-ring and 4-ring cavities approximately in the (100) plane, each of which contains doubly protonated 6-MeBPY guests and solvent waters. Unlike 1, 2-fold interpenetration of the anionic [(UO2)2(C6H2O4S)3]2− sheets in 5 is not observed. F



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Figure 4. Crystal structure of 4. Solvent water molecules has been omitted for clarity.

Figure 5. Crystal structure of 5. Protonated 6-MeBPY guest species is only shown in the 8-ring void space for clarity. Solvent waters in the voids have been omitted for clarity.

Crystal Structure of [(UO2)2(C6H2O4S)3](C24H18N6) (7). The structure of 7 (Figure 7) contains one crystallographically unique uranyl cation (U1, O1, O2). U1 is bound by three

singly protonated 2,2′-BPY guest molecules within uranyl CPs containing polyfunctional aromatic dicarboxylates are relatively common.28,39,73 G



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Figure 6. Crystal structure of 6. The 4,6-MeBPY guests are shown in one 6-ring cavity for clarity. Solvent waters are not shown.

Figure 7. Crystal structure of 7. TPPZ guests have been omitted for clarity. A portion of the 2-D [(UO2)2(C6H2O4S)3]2− sheet in 7.

structural precedence of such interactions present within these species. A search in the literature and the Cambridge Structural Database (CSD, version 5.34, 2013)72 reveals that diprotonated TPPZ species form intramolecular hydrogen bonds and promote a rotation of the pyridine rings out of the pyrazine ring plane, since a planar conformation would “require an impossibly short N···N contact of ∼1.4 Å.”76 This arrangement thus allows the diprotonated TPPZ species to attain a more favorable distorted conformation to form intramolecular hydrogen bonds.76,77 These observations are consistent with those made in 7, where we observe a similar rotation of the terminal pyridyl rings relative to the pyrazine ring.

monodentate TDC ligands via O3, O4, and O7 (U−Oavg, 2.327 Å), and one bidentate TDC via O5 and O6 (U−Oavg, 2.499 Å) to result in a UO7 pentagonal bipyramidal primary building unit. These units are linked by bridging TDC dicarboxylates to result in sheets in the (001) plane, as seen in Figure 7. The interlayers within these sheets contain diprotonated TPPZ guests (Figure 8). The diprotonated TPPZ guests charge balance the anionic [(UO2)2(C6H2O4S)3]2− sheets and also form N−H···N intramolecular hydrogen bonds74,75 via N1 and N3 (2.537(3) Å) within the pyridyl rings of TPPZ (see the Supporting Information, Figure S16). The presence of intramolecular hydrogen bonds within TPPZ is a result of note, thus prompting us to explore the H



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Figure 8. Packing of 7 with protonated TPPZ guests. TPPZ is found in the interlayers between the [(UO2)2(C6H2O4S)3]2− sheets.

Figure 9. Scheme of local coordination geometries observed in 1−7.

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bipyramids observed in 2. The change in the UO22+ coordination sphere seems to suggest that the presence of methyl groups may disrupt the formation of rings within the chains, thus changing the dimensionality of the structure from a double chain (2) to single chains (3, 4). The single chains, therefore, are reminiscent of the double chains except to a lesser degree of connectivity through the TDC ligands. Further, the methyl groups present in 4-MeBPY and 5-MeBPY also seem to direct the binding modes of the TDC dicarboxylates in the structure in which bidentate and monodentate coordination of the carboxylate oxygen atoms are observed, hence resulting in the UO5N2 building unit. These results suggest that sterics play an important role directing the local and global structure of compounds 1−4. Upon substitution with methyl groups at the 6- and 6′positions on BPY (6-MeBPY), a two-dimensional sheet containing UO8 hexagonal bipyramids and 8-ring and 4-ring motifs occupied with protonated 6-MeBPY guests are observed to result in 5. A side-by-side structural comparison of 5 with 1 shows similarities (two-dimensional sheets and UO8 hexagonal bipyramidal geometries) but notable differences within their structures. The cavities in 5 are occupied exclusively by 6-MeBPY, in which direct coordination to UO22+ is not observed (unlike 2−4 using BPY, 4-MeBPY, and 5-MeBPY, respectively). In fact, the positions of the methyl groups in 6-MeBPY effectively block the chelation pocket necessary for direct UO22+ coordination, thus allowing for the formation of infinite sheets similar to 1. 2-fold interpenetration of the sheets in 5 is not observed (as seen in 1), suggesting that the occupancy of the cavities with 6-MeBPY guests may preclude such formation. The introduction of four methyl groups to BPY (4,6MeBPY) further changes the structure to result in a double chain with 6-ring and 4-ring motifs occupied by 4,6-MeBPY guests, as observed in 6. A side-by-side structural comparison of 6 and 2 shows similarities as double chains with UO8 hexagonal bipyramidal geometries, yet notable differences in their

DISCUSSION Compounds 2−7 show that the presence of N-donor chelating ligands can be used to direct different structure types within uranyl CPs. These structures can be tuned depending on the number of methyl (pyridyl) groups and positions of methyl groups present on the BPY or TPPZ moieties. We observe that N-donor chelating ligands change the local coordination sphere (hexagonal versus pentagonal bipyramids) of UO22+ (Figure 9) and resulting global structures. Further, the local structure of compound 1 is similar to N-donor containing structures (Figure 9); however, its global structure is different. Compound 1 crystallizes as interpenetrating two-dimensional sheets containing 6-ring motifs, in which each uranyl center within the structure consists of UO8 hexagonal bipyramidal building units. Upon introduction of BPY, a double chain containing a 6-ring motif is observed in 2 as a consequence of direct uranyl coordination of BPY. In fact, the coordinating BPY ligands effectively terminate the structure as a double chain, which thus truncates the infinite sheets as observed in 1. This is further evident by the changes in the coordination spheres between alternating uranyl centers in 2. The uranyl center (U1) consists of a UO6N2 hexagonal bipyramidal building unit, whereas the adjacent uranyl center (U2) consists of a UO8 hexagonal bipyramidal building unit (Figure 9). Another feature of 2 is that protonated BPY guests in the structure occupy the 6-ring voids. Thus, compound 2 is the only structure within the uranyl TDC CP series wherein a N-donor capping ligand participates as both a coordinating and a noncoordinating species. The UO22+ geometry and dimensionality within 2 is further influenced upon introduction of methyl groups at the 4,4′- or 5,5′-positions of BPY (i.e., 4-MeBPY and 5-MeBPY, respectively) to result in chains as observed for 3 and 4. The UO22+ coordination spheres of 3 and 4 are changed to UO5N2 pentagonal bipyramids instead of the UO6N2 hexagonal I



Article

Figure 10. Room-temperature solid-state emission spectra of 2 (blue), 3 (green), and 4 (red). Excitation wavelength: 365 nm.

known to coordinate to UO22+.44,46−48,51,78 Direct coordination from TPPZ to UO22+ was not observed, however, and instead, protonated TPPZ guests were found to occupy the interlayers between the anionic [(UO2)2(C6H2O4S)3]2− sheets found in 7. The topology of these sheets differs from 1 and 5 wherein ring motifs are not observed (Figures 1 and 5), which seems to suggest that the presence of TPPZ guests may influence the observed sheet structure in 7. Luminescence Studies. The origin of UO22+ emission is caused by ligand-to-metal charge transfer (LMCT) electronic transitions involving relaxation of an excited electron from nonbonding 5fδ, 5fϕ uranyl orbitals to uranyl−oxygen bonding orbital(s) (σu, σg, πu, πg).79,80 Uranyl luminescence is most often observed as green emission ranging from 400 to 650 nm consisting of five or six vibronic peaks.81−83 These peaks arise from the strong coupling of the Raman active symmetric oscillations of the UO bond in the ground state with the electronic excited state.84−86 Luminescence measurements of 1−7 were taken at room temperature with excitation wavelengths of 365 and 420 nm. These wavelengths correspond to two LMCT bands observed within the absorption spectrum of UO22+ wherein the maximum peak at 365 nm represents the U-O equatorial LMCT band and the maximum peak at 420 nm represents the UO axial LMCT band.79,80 Compounds 2−4 show typical uranyl emission wherein vibronic peaks are observed between 450 and 650 nm (Figure 10; Supporting Information, Figure S19) when excited at either 365 or 420 nm, with spectra obtained at 420 nm being of higher intensity. Weak (or no) uranyl emission is observed for compounds 1, 5, 6, and 7 (Supporting Information, Figures S17 and S18) at both excitation wavelengths (365 and 420 nm).

structures are observed. The double chain structure in 6 is terminated by an acetate ion (capped by BPY in 2), which may preclude the formation of infinite sheets, as observed in 1 and 5. Unlike 2, direct coordination of 4,6-MeBPY in 6 is not observed yet rather participates as protonated and neutral guests in the cavities exclusively, which suggests that steric hindrance of the methyl groups in 4,6-MeBPY may block the coordination sites available for direct UO22+ coordination. It is noteworthy that the size of the ring motifs in 2, 5, and 6 may be influenced by the presence of methyl groups on BPY. For reference, in the absence of N-donors, the ring size in compound 1 is a 6-ring motif. Upon introduction of BPY, the ring size remains unchanged (6-ring motif); however, the presence of two methyl groups at the 6- and 6′-positions on BPY, respectively, in 5 results in alternating 8-ring and 4-ring moieties that differ from both 1 and 2. Moreover, the presence of four methyl groups in 4,6-MeBPY in compound 6 further influences the cavity size wherein alternating 6-ring and 4-ring moieties are observed. These results suggest that substituent groups present on the N-donor noncoordinating species influence cavity sizes within the global structures. The evolution of structures in 1−6 allowed us to explore another N-donor chelating ligand, TPPZ. This ligand was chosen because the additional pyridyl rings present on pyrazine may have a greater influence on the UO22+ local and global structure than the BPY ligands because of its size. Further, the availability of six nitrogen atoms within TPPZ may allow for the possibility of multiple coordination chelating sites to distinct metal centers. We hypothesized that TPPZ will directly coordinate to UO22+ because bidentate and tridentate chelation pockets using similar BPY and terpyridine (TPY) analogues are J



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Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00549.

In light of these observations, specifically that compounds 2−4 show comparatively enhanced uranyl emission and that each contains N-donor species coordinated directly to uranyl centers, one may speculate on the potential for sensitization within these materials. Indeed, it may be that BPY, 4-MeBPY, and 5-MeBPY act as chromophores to transfer energy to uranyl centers and promote enhanced emission thereof. Additional support for this conclusion stems from the weak emission observed in compounds 1, 5, 6, and 7 where N-donors are either absent or serve simply as guests. Moreover, the triplet states of BPY (23 419 cm−1),87 4-MeBPY (23 310 cm−1),87 and 5-MeBPY (22 831 cm−1)87 with respect to the emissive UO22+ state (21 321 cm−1)44 are well suited for possible energy transfer (in lanthanide systems, a “safe” energy difference between 2500 and 3500 cm−1 allows for efficient transfer).88 That said, given that the UO22+ coordination spheres in 1, 5, 6, and 7 are saturated with TDC linkers (Figure 9), one may also speculate that their extended conjugation may cause the uranyl cations to be electronically indistinct, as observed in a similar study using 1,2-(4-pyridyl)-ethylene and 1,2-(4-pyridyl)ethane, respectively,21 and thus results in weakened or quenched uranyl emission. From a structural perspective, the coordination of N-donors may possibly disrupt this conjugation and allow uranyl emission, perhaps in conjunction with possible energy transfer as mentioned previously. Interestingly, although compound 2 consists of both coordinating and noncoordinating BPY species, the emission within this compound further confirms that direct coordination of N-donors indeed influences the observed uranyl luminescence. As is often the case when discussing uranyl emission from coordination polymers, many factors are at play and it may be difficult to isolate variables, especially when considering the overlap of the LMCT regions with each other as well as with the absorption regions of the ligands. Moreover, other factors such as structure dimensionality (sheets, chains) and UO22+ coordination geometries (hexagonal, pentagonal)16 may also influence emission. Nonetheless, the TDC system with or without coordinated N-donors provides a platform for at least delineating these issues and ideally prompting more quantitative studies going forward.

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CONCLUSION

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*Phone: (202) 994-6959. E-mail: [email protected] (C.L.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Basic Energy Science, under Award Number DE-SC0001089.

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In summary, seven crystal structures containing the O-donor linker TDC and/or N-donor ligands BPY, 4-MeBPY, 5-MeBPY, 6-MeBPY, 4,6-MeBPY, and TPPZ (1−7) were synthesized and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and luminescence spectroscopy. The steric groups present within the N-donors influence the structural and luminescent properties within these materials. We observe a systematic change in the local and global structure within 1−7 as a consequence of changing the positions of the methyl groups or increasing the number of methyl (pyridyl) groups on the N-donor ligands. Subsequently, these structural changes also influence the observed luminescence in 1−7 wherein coordination or noncoordination of N-donor chelating ligands results in either weak uranyl emission or strong emission, possibly as a consequence of sensitization.

S Supporting Information *

CIFs of 1−7 at 100 K (CCDC 1060072−1060078), ORTEPs, PXRD patterns, and luminescence spectra. The Supporting K



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