Structural Variations of the First Family of Heterometallic Uranyl

Mar 3, 2016 - News Ed., Am. Chem. ... and Materials Science, Heilongjiang University, Harbin 150080, China ... Hydrothermal reactions of uranyl acetat...
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Structural Variations of the First Family of Heterometallic Uranyl Carboxyphosphinate Assemblies by Synergy between Carboxyphosphinate and Imidazole Ligands Weiting Yang,† Dai Wu,† Chao Liu,† Qing-Jiang Pan,*,‡ and Zhong-Ming Sun*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China ‡ Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China S Supporting Information *

ABSTRACT: Hydrothermal reactions of uranyl acetate and a series of transition metal acetates with a carboxyphosphinate and auxiliary N-donor ligands gave rise to the formation of eight heterometallic uranyl-organic assemblies, namely, Co(im)2(UO2)3(L)4 (1), Zn(bpi)(UO2)(L)2 (2), Cd(dib)(UO2)(L)2 (3), M(dib)(UO2)2(L)3 (M = Cd (4), Mn (5)), and [M(dib)2(H2O)2][(UO2)3(L)4]·nH2O (M = Co (6, n = 2), Ni (7, n = 2), Cu (8, n = 0)) [H2L = (2carboxyethyl)(phenyl)phosphinic acid (CPP), im = imidazole, bpi =1-(biphenyl-4-yl)-1H-imidazole, dib =1,4-di(1H-imidazol1-yl)benzene]. Single-crystal X-ray diffraction (XRD) analysis of 1 reveals a layered structure of UO6, UO7, and CoO4N2 units that are linked by the carboxyphosphinate ligands. Imidazole molecules modify the layer by coordinating to Co centers. Similarly, 2 is a mixed zinc-uranyl carboxyphosphinate with different topological two-dimensional structure and the decorated moiety is a bpi coligand. When in the presence of bridging dib coligands, the mixed cadmium−uranyl carboxyphosphinate sheets of 3 are pillared by dib forming a framework structure. The isostructures of 4 and 5 are also pillared frameworks constructed by a mixed heterometallic uranyl phosphinate layered subnet that is different from that of 3. The structures of 6−8 are isotype and very special in that they consist of distinct [M(dib)2(H2O)2]n2n+ cationic and [(UO2)3(L)4]n2n− anionic subnets. Such two sheets are packed alternatively and interact via hydrogen bond forming three-dimensional supramolecular structures.

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INTRODUCTION Uranyl organic coordination hybrid materials have received significant interest for their rich structural diversities and promising physicochemical properties, including photoluminescence, photocatalysis, photocurrent, and photovoltaic responses, especially for their relevance to the waste management and separation procedures of nuclear fuel.1−7 The uranyl cation (UO22+) presents a specific linear configuration with two terminal oxygen atoms seated in the axis. Thus, it favors being coordinated in the equatorial plane to yield three uranyl primary building units, including square, pentagonal, and hexagonal bipyramids. These bipyramidal units, together with their polymerized species, serve as inorganic building units that are further linked by various organic ligands, to construct the uranyl organic assemblies. Among the reported organic linkers © XXXX American Chemical Society

used for isolation of uranyl hybrids, carboxylates and phosphonates have been extensively studied.1,5,8−11 For the phosphonate ligand with common formula R-PO3H2, its spherical phosphonate group possesses three possible states of protonation, that lead to flexible ligating abilities for binding metal ions.12−14 Until now, nearly 200 uranyl phosphonates have been reported, including molecular structures, chains, ribbons, tubes, sheets, and frameworks.515 In this respect, Clearfield and co-workers made the pioneering work.16−18 Albrecht-Schmitt, Cahill, and Burns et al. have made prominent contributions to the syntheses of uranyl phosphonates.19−24 Received: November 10, 2015 Revised: January 31, 2016

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DOI: 10.1021/acs.cgd.5b01595 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Synthetic Conditions for Compounds 1−8a U-Ac (mmol)

TM (mmol)

H2L (mmol)

N-ligand (mmol)

NH3·H2O (μL)

pHi

pHf

1 2 3, 4

0.1 0.1Co 0.05 mmol U-Zn 0.1 0.1Cd

0.15 0.15 0.15

0.2 im 0.1 bpi 0.1 dib

0 0 0−20

4 2 3−4

5 3 4−5

yellow block yellow rod yellow block

5 6 7 8

0.1 0.1 0.1 0.1

0.1 0.15 0.15 0.15

0.075 dib 0.1 dib 0.1 dib 0.1 dib

20 20 20 20

4−4.5 3.5−4 3.5 3−3.5

6 5 4.5 3.5−4

yellow rod yellow-red block yellow-green block yellow-green block

a

0.1Mn 0.1Co 0.1Ni 0.1Cu

Morphology

Pure?/yields based on uranium yes/38 mg (61%) yes/22 mg (45%) mixture with unidentified powder yes/37 mg (51%) yes/43 mg (59%) yes/38 mg (51%) with unidentified powder

UU-Ac, uranyl acetate; U-Zn, zinc uranyl acetate; TM, transition metal acetates; pHi, initial pH; pHf, final pH. phosphinic acid was synthesized according to a documented literature.35 Caution! Standard procedures for handling radioactive material should be followed, although the uranyl compounds used in the lab contained depleted uranium. Single crystals of 1 and 3−8 were synthesized as follows: A mixture of uranyl acetate, corresponding transition metal acetates, CPP ligand, imidazole auxiliary ligands, 0−20 μL 25% NH3·H2O, and 2.0 mL deionized water was loaded into a 20 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 160 °C for 3 days, and then cooled to room temperature naturally. Solid products were isolated from the mother liquor, washed with water and allowed to airdry. Single crystals were then isolated for single crystal XRD determination. The synthesis of 2 with a similar procedure only led to crystalline powder form. When Zn(UO2)2(Ac)6·7H2O was used as both uranium and zinc resources, suitable crystals for single crystal XRD analysis of 2 were obtained. The synthetic conditions, molar ratios and yields for each compound are listed in Table 1. It is noted that 3 and 4 were formed under same condition. It was difficult to obtain pure phase of 3 or 4 by modifying the synthetic parameters. Only better crystals of 4 could be isolated under modified pH by NH3· H2O. Elemental analysis observed (calcd): C 27.28% (27.21%), H 2.32% (2.39%), N 3.08% (3.02%) for 1; C 40.27% (40.44%), H 3.03% (3.08%), N 2.84% (2.86%) for 2; C 32.34% (32.49%), H 2.54% (2.59%), N 3.80% (3.89%) for 5; C 32.58% (32.61%), H 2.87% (2.92%), N 5.13% (5.07%) for 6; C 32.69% (32.61%), H 2.98% (2.92%), N 5.12% (5.07%) for 7. Characterization. Powder X-ray Diffraction (PXRD) data were collected on a D8 Focus (Bruker) diffractometer at 40 kV and 30 mA with monochromated Cu Kα radiation (λ = 1.5405 Å) with a scan speed of 5 deg/min and a step size of 0.02° in 2theta. These PXRD patterns of 1, 2, and 5−7 are displayed in the Supporting Information (Figures S1−S5). Purity of these bulk samples was confirmed by comparing experimental and calculated PXRD patterns. On the other hand, owing to the low yield and impurity of 3, 4, and 8, their PXRD patterns, elemental analysis and luminescent spectra were not characterized. Infrared spectra were collected from single crystals of these compounds using a Nicolet 6700 FT-IR spectrometer with a diamond ATR objective. The spectral patterns and assignments are shown in the Supporting Information (Table S3 and Figures S6−S13). The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-7000 luminescence spectrometer equipped with a xenon lamp of 450 W as an excitation light source. The photomultiplier tube voltage was 400 V, the scan speed was 1200 nm min−1, and both the excitation and the emission slit widths were 5.0 nm. X-ray crystal structure determination. Suitable single crystals for these compounds were selected for single-crystal X-ray diffraction analyses. Crystallographic data were collected at 293 K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo−Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined on F2 by full-matrix least-squares using SHELXTL.36 Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. All hydrogen atoms were placed by geometrical considerations and were added to the

More recently, our group reported a series of uranyl diphosphonates based on flexible alkyldiphosphonate ligands.25−27 In contrast to the corresponding phosphonate ligand, the phosphinate ligand, R1R2PO2H, features a less OH group, but with an additional organic function, that allows better modulation and adjustment of the substituent. As a result, more structure diversities of resultant uranyl organic hybrids would be possible. However, compared with extensively investigated uranyl phosphonates, uranyl phosphinates have received less attention and only a handful of such materials with low dimensional structures were reported.28−31 In our recent work, we used a bifuntional (2-carboxyethyl)(phenyl)phosphinic acid (CPP) as the ligand for the isolation of four homometallic uranyl phosphinate compounds.32 This ligand contains two formally analogous carboxylate and phosphinate groups, which exhibit differential coordination affinity for dblock elements and uranyl cations. Thus, the further preparation of heterometallic uranyl organic hybrids by this carboxyphophinate ligand arouses our interest. Except for the logical design and selection of the bifuntional carboxyphosphinate ligand, another effective synthetic strategy utilized for designing heterometallic uranyl organic hybrids adopted in this work is the employment of N-donor auxiliary linkers. In our previous studies, we explored the use of appropriate imidazole derivatives as the auxiliary ligands along with phosphonate main ligands to stabilize the softer zinc ions in the construction of bimetallic zinc uranyl phosphonate compounds.33,34 The synergistic effect of acidic and basic functional dual-ligands influences the connection between the metal ions and the organic ligands in the structure, as well as on charge density distribution, finally leading to various bimetallic uranyl compounds. Inspired by our previous work, in this study, we introduced different imidazole derivatives to the syntheses of heterometallic uranyl carboxyphosphinates in the presence of six different transition metal ions and obtained eight novel compounds, Co(im)2(UO2)3(L)4 (1), Zn(bpi)(UO2)(L)2 (2), Cd(dib)(UO2)(L)2 (3), M(dib)(UO2)2(L)3 (M = Cd (4), Mn (5)), and [M(dib)2(H2O)2][(UO2)3(L)4]·nH2O (M = Co (6, n = 2), Ni (7, n = 2), Cu (8, n = 0)) [H2L = (2carboxyethyl)(phenyl)phosphinic acid, im = imidazole, bpi =1(biphenyl-4-yl)-1H-imidazole, dib =1,4-di(1H-imidazol-1-yl)benzene]. Their syntheses, structures, and spectroscopic properties have been described.

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EXPERIMENTAL SECTION

Syntheses. UO2(Ac)2·2H2O, Zn(UO2)2(Ac)6·7H2O, and transition metal acetates were purchased from Aladdin, whereas imidazole, 1-(biphenyl-4-yl)-1H-imidazole, and 1,4-di(1H-imidazol-1-yl)benzene were purchased from Sinopharm. All reagents were used without further purification. On the other hand, 2-carboxyethyl(phenyl)B

DOI: 10.1021/acs.cgd.5b01595 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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U3NiC60H64N8O26P4 2209.87 Triclinic P-1 12.3337(7) 13.5585(8) 13.6142(8) 61.2180(10) 83.1540(10) 66.9540(10) 1829.35(18) 1 2.006 7.043 1.017 0.0361/0.0751 0.0565/0.0836 U3CoC60H64N8O26P4 2210.09 Triclinic P-1 12.3153(12) 13.5938(13) 13.6720(13) 61.332(2) 83.244(2) 66.8840(10) 1839.9(3) 1 1.995 6.972 0.996 0.0468/0.0862 0.0727/0.0967 U2MnC39H37N4O16P3 1441.64 Triclinic P-1 8.8770(7) 12.8064(10) 21.4838(17) 96.3440(10) 99.2210(10) 100.2960(10) 2347.5(3) 2 2.04 7.32 0.98 0.0504/0.0926 0.0882/0.1054

R1 = ∑(ΔF/∑(F0)); wR2 = (∑[w(F02 − Fc2)])/∑[w(F02)2]1/2; w = 1/σ2(F02).

UZnC33H30N2O10P2 979.93 Orthorhombic Pbca 23.8635(11) 9.4339(4) 30.9355(14) 90 90 90 6964.4(5) 8 1.869 5.484 1.01 0.0329/0.0606 0.0592/0.0706 U3CoC42H44N4O22P4 1853.71 Triclinic P-1 9.4464(14) 11.9944(17) 12.1970(18) 82.067(3) 87.440(3) 79.932(2) 1347.4(3) 1 2.285 9.489 1.014 0.0570/0.1159 0.1030/0.1368 Empirical formula Fw Crystal system Space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z ρcalcd/Mg/m3 μ/mm−1 gof R1/wR2 (I > 2σ(I))a R1/wR2 (all data)

a

4 3 2 1

Table 2. Crystallographic Data and Structure Refinement Parameters for Compounds 1−8

5

6

7

RESULT AND DISCUSSION Crystal Structures. The first compound Co(im)2(UO2)3(L)4 (1) possesses a layered structure in which Co-centered and U-centered polyhedra are linked by CPP ligands. There are two independent uranium sites, one cobalt site, two CPP, and one imidazole ligand in its asymmetric unit (Figure S14). U(1) is equatorially four-coordinated by oxygen atoms from four phosphinate groups. Combining the two axial oxo atoms, an uncommon square bipyramidal sphere of U(1) is formed. Whereas U(2) is in a pentagonal bipyramidal environment defined by two “yl” oxygen atoms and five equatorial oxygen atoms from two phosphinate and two carboxylate groups. U−O bond lengths and angles of 1 (as well as 2−8) are unremarkable and comparable to those in previously reported uranyl compounds.5 Co(1) is coordinated by four carboxylate oxygen atoms and two N atoms from two imidazole. Thus, a distorted octahedral geometry around Co(1) is formed. As for the two independent CPP, both of the phosphinate groups solely bind the uranyl centers as linkers between U(1) and U(2). While for the carboxylate moieties, both connect Co(1) and U(2) but through different coordination manners. One is bidentate, while the other adopts chelate fashion to ligate U(2) and uses one oxygen atom coordinating to Co(1) (Scheme 1, C and D). The uranyl square and pentagonal bipyramids as well as CoO4N2 octahedra are bridged by the phosphinate and carboxylate groups, creating a heterometallic uranyl carboxyphosphinate chain. Such chains are further linked by the ethylene moieties, therefore forming a layered structure parallel to (011) (Figure 1). The layer can be simplified as a 3,4-connected net with the Schläfli symbol {4· 62}2{42·62·82}. As shown in Figure 2, the heterometallic uranyl carboxyphosphinate sheets stack along the a axis. No significant parallel π-stacking is displayed, however CH···π interaction is present between the aromatic rings of two distinct CPP within singlet layer [H···centroid 2.96 Å, C−H···centroid 133°]. Reaction of zinc uranyl acetate with CPP in the presence of bpi under hydrothermal condition yields Zn(bpi) (UO2)(L)2 (2). This compound also features a two-dimensional (2D) assembly. Different from other heterometallic uranyl carboxyphosphinates in this work that all crystallize in the triclinic space group P-1, 2 crystallizes in the orthorhombic Pbca space group. Its asymmetric unit contains one uranium atom, one zinc atom, two CPP, and one bpi ligand (Figure S15). The uranium atom is in a common pentagonal bipyramidal geometry. Five equatorial coordinated oxygen atoms are from two phosphinate and two carboxylate groups. The zinc atom is located in a distorted tetrahedral environment defined by three oxygen atoms (one carboxylate oxygen and two phosphinate oxygen atoms) and one bpi nitrogen atom. The UO7 and ZnO3N polyhedra are linked by CPP ligands to form the layered structure of 2, which is parallel to (011) (Figure 3). In this structure, each phosphinate group of CPP binds one U and one Zn centers through bidentate coordination (Scheme 1, E and F). On the other hand, the carboxylate moiety of one distinct CPP takes similar coordination fashion with binding one U and one Zn; the other distinct carboxylate function only ligates one U through chelate coordination. The connection

U2CdC39H37N4O16P3 1499.1 Triclinic P-1 8.8205(4) 12.9021(7) 21.5932(11) 95.9500(10) 99.6270(10) 100.0800(10) 2362.9(2) 2 2.107 7.455 1.019 0.0392/0.0751 0.0546/0.0806

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UCdC30H28N4O10P2 1016.93 Triclinic P-1 9.8336(19) 12.052(2) 15.698(3) 68.548(3) 86.428(3) 77.609(3) 1690.9(5) 2 1.997 5.567 1.031 0.0525/0.1109 0.0786/0.1290

8

structure factor calculation. A summary of the crystallographic data for these heterometallic uranyl carboxyphosphinates are listed in Table 2. Selected bond lengths and angles are given in Table S1. CCDC 1430421−1430428 contain the supplementary crystallographic data for this paper.

U3CuC60H60N8O24P4 2178.67 Triclinic P-1 12.5951(7) 13.2866(7) 13.4446(7) 61.4510(10) 85.8070(10) 68.0200(10) 1817.17(17) 1 1.991 7.12 1.001 0.0374/0.0806 0.0542/0.0885

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DOI: 10.1021/acs.cgd.5b01595 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Scheme 1. Coordination Modes of the CPP Ligand in 1−8

Figure 2. View of the packing with alternate layers in 1.

Figure 3. Top: View of the layered assembly in 2. The coordinated bpi molecules were omitted for clarity. Bottom: Nodal representation of the layered assembly.

Figure 1. Top: View of the layered assembly in 1. The coordinated imidazole molecules were omitted for clarity. Bottom: Nodal representation of the layered assembly.

coordinated by five oxygen atoms from two phosphinate groups, one chelate and one bidentate carboxylate groups, producing a common pentagonal bipyramid. The cadmium ion is six-coordinated by four oxygen atoms (from two phosphinate and two carboxylate groups) and two nitrogen atoms (from two dib molecules). Similar to 1 and 2, the UO7 and CdO4N2 polyhedra are bridged and chelated by CPP ligands forming a 2D assembly parallel to (101) (Figure 5). However, the CPP ligands adopt type E and G coordination models (Scheme 1) that both phosphinate and carboxylate groups bind U and Cd centers. This layered structure features the same topology of 1. Such heterometallic uranyl carboxphosphinate layers are

between the inorganic and organic building units in the layer can be topologically simplified as 3,3,3,3,4-connected net with total point (Schläfli) symbol {4·62}3{4·63·82}{63}. Figure 4 displays the packing of the alternate heterometallic uranyl carboxphosphinate layers along the [100] direction. There is π···π interaction between bpi coligands of adjacent layers with the intercentroid distance of 3.81 Å. Compound Cd(dib) (UO2)(L)2 (3) forms a three-dimensional (3D) framework structure. The asymmetric unit involves one uranium ion, one cadmium ion, two CPP, and one dib ligand (Figure S16). The uranyl cation is equatorially D

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Figure 4. Packing model of alternate layers along the a axis in 2.

Figure 6. Top: View of the framework in 3 along the [100] direction. Bottom: Nodal representation of the network.

topology with point symbol {52·6 × 106·12}{52·6}3{53·6· 82}2{53}2 (Figure 8). The structures of the Co(II), Ni(II), and Cd(II) variants of [M(dib)2(H2O)2][(UO2)3(L)4]·nH2O (6−8) are isotypic, and only 6 is discussed as an example in detail in this work. It is a very interesting 2D assembly in that the heteroatom is not incorporated within the uranyl carboxyphosphinate network but as a separated subnet sheet connected by dib molecules. The anionic [(UO2)3(L)4]n2n− part displays a 2D arrangement parallel to (011) (Figure 9). Both uranyl square and pentagonal bipyramids are embodied, where 6-fold U(1) is surround by four phosphinate groups, and the sphere of U(2)O22+ is completed by two phosphinate and two carboxylate groups. Two types of coordination A and B are displayed by CPP in this layer (Scheme 1, Figure S18), where all the phosphinate moieties are bidentate with binding two uranium centers, and the two distinct carboxylate parts are monodentate and chelate, respectively, with each ligating one uranium center. Topological simplification of the uranyl carboxphosphinate layer reveals a 3,3,4-connected net with point symbol {42·6}4{42·82.102}{43·62· 8}2. As for the cationic part, [Co(dib)2(H2O)2]n2n+, the cobalt cation is in an octahedral environment with four dib nitrogen atoms in the plane [Co−N bond lengths: 2.122(6) ∼ 2.153(7) Å] and two axial water molecules [Co−O bond length of 2.137(6) Å]. It is noted that for the Cu-bearing analogue, a square bipyramidal geometry around the copper center is displayed due to the Jahn−Teller distortion [Cu−N bond lengths: 1.993(5) ∼ 2.046(5) Å; Cu−O bond length: 2.502(6) Å]. The transition metal centers are linked by dib forming a frequently encountered 2D square grid assembly with sql net

Figure 5. Layered assembly separated from 3 by disregarding bridging dib.

furthered linked by dib molecules through Cd−N connection forming a pillared framework structure (Figure 6). Through topological analysis, the framework can be simplified as 3,4,6connected net with point symbol {4·62}4{42·62·810·10}{42·62· 82}. Compounds M(dib) (UO2)2(L)3 (M = Cd (4), Mn (5)) are isotypic and 4 is taken as an example to depict the structure in detail. Although 4 coforms with 3, it possesses different composition and structure. Two crystallographically different uranium atoms, one cadmium atom, three distinct CPP and two half dib molecules exist in the asymmetric unit (Figure S17). U(1) is surround by two uranyl oxygen atoms and four equatorial oxygen atoms from three phosphinate and one carboxylate groups. U(2) is seven coordinated by two oxo atoms in the axis, as well as five oxygen atoms in the equatorial plane from two phosphinate and two carboxylate groups. Cd(1) is in a trigonal bipyramidal geometry defined by three oxygen atoms from two phosphinate and one carboxylate groups as well as two nitrogen atoms from two dib molecules. Two of the three different CPP adopt type C coordination moles (Scheme 1) and the last in type F fashion. The linkage of UO6 square bipyramids, UO7 pentagonal bipyramids and CdO3N2 trigonal bipyramids through CPP creates a 3,3,3,3,4-connected layered assembly parallel to (110) (Figure 7). Similar to 3, the neighboring layers are connected by dib to form the whole pillared structure of 4, which can be simplified as a six nodal E

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Figure 9. Top: View of the uranyl phosphinate sheet in 6. Bottom: Nodal representation of the sheet.

Figure 7. Top: The Cd−U layered assembly separated from 4 by disregarding bridging dib. Bottom: Nodal representation of the layered assembly.

(point symbol {44·62}) (Figure 10). As shown in Figure 11, the layered cobalt- and uranyl-containing parts are parallel and

Figure 10. View of the cobalt sheet via linkage of dib in 6.

alternately arranged along the a axis. The two hybrid layers interact through electrostatic cation/anion interactions. Hydrogen bonding interaction (O···O distance of 2.84 Å) links the uranyl carboxyphosphinate units and the transition metal subnets, resulting in the formation of a supramolecular framework assemblage. Discussion. Compared with phenylphosphonic acid, CPP ligand features one less P−O connection, but with an additional carboxylate group connected by a flexible −CH2CH2− spacer to the phosphinate moiety; thus, the coordination diversities of the ligand can be displayed and consequently are embodied in the structures of the resultant metal complexes. Insight into the coordination modes of the CPP ligands in these bimetallic uranyl compounds, the phosphinate moieties of the seven coordination fashions all adopt bidentate modes bridging two uranium centers or one transition metal and one uranium centers. Whereas the carboxylate groups exhibit versatile

Figure 8. Top: View of the framework in 4 along the [100] direction. Bottom: Nodal representation of the network. F

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As for our compounds 6−8, they are the first examples of uranyl complex that both the cationic transition metallic part and the anionic uranyl-organic part are layered networks. Luminescent Spectroscopy. The synthesized pure uranyl carboxyphosphinates, 1, 2, and 5−7 were studied by photoluminescent spectroscopy (Figure 12 and Figures S19−S22).

Figure 11. View of the packing of uranyl phosphinate and cobalt layers alternately in 6 along the a axis.

coordination modes including monodentate (binding one U), bidentate (binding one U and one transition metal), chelate (binding one U) and tridentate (binding one transition metal and chelating one U). These variations in coordination of ligand contribute to the topologically structural diversity of the resulting complexes. Besides, the auxiliary imidazole ligands play an important role in structure dimensions. In 1 and 2, the bimetallic uranyl carboxyphosphinate layers are coordinated by imidazole and bpi, respectively, both of which possess one N donor, thus leading to final 2D arrangements. While in 3−5, dib with two N donors link the adjacent layers forming pillared framework structures. Another important feature is that 6-fold coordinated uranium cation was encountered in 1 and 4−8, which is less common compared with commonly occurred 7fold coordinated geometry. As shown in Table S2, the average U = O bond lengths of 1.729−1.776 Å in the six-coordinated state are comparable to that in the seven-coordinated case (1.715−1.768 Å). It is noted that these U = O bond distances are slightly shorter than the typical average value of 1.79 Å.1 While the U−O bonds in the equatorial plane have average distances of 2.268−2.291 Å, that are typical for the sixcoordinated uranium polyhedra (2.28 Å in literature) and significantly shorter than those of 2.373−2.396 Å for the sevencoordinated ones (2.37 Å in literature).37 In addition, the variability of the U = O bond lengths are also indicated by their vibrations in the IR spectra. Bond lengths related to the antisymmetric stretching vibration ν3 for uranyl groups in these compounds based on Veal’s empirical relationship RU−O(pm) = 8120ν3−2/3 + 89.5 are calculated.38 As shown in Table S4, the predicted U = O bond lengths are in good agreement with the values determined from the single crystal XRD analyses. It is noted that three isostructures of 6−8 are different from most reported heterometallic uranyl coordination polymers in that the transition cationic part does not directly interact with the uranyl part. Instead, it serves as countercation, compensating the negative charge of the uranyl-organic assembly. As far as we know, discrete hydrated transition metal species, dinuclear metal cluster and infinite chains of metal centers have been reported intercalate between the low-dimensional uranylorganic assemblies.39−41 Only one case that a layered subnet of transition metal part incorporates copper centers connected by 4,4′-bipyridine was known.42 Dianionic uranyl citrate dimers are encapsulated in the interlayer spaces of the cationic sheets.

Figure 12. Excitation and emission spectra of 2.

We also investigated the luminescence of the CPP ligand and imidazole coligands (Figure S23−S26). Among these compounds, 2 exhibits characteristic emission of uranly cation when excited at both 338 and 437 nm, which correspond to the direct excitation of oraganic ligand and UO22+ (Figure 12). The two emission spectra only differ in resolusion and both contain four prominent peaks located at 498, 520, 542, and 567 nm. These four peaks are correspond to the electronic and vibronic transitions S11−S00 and S10−S0v (v = 0−4) of uranyl cation. Such spectra are typical for uranyl-bearing compounds, which exhibit green light centered near 520 nm and often consist of several peaks.43 On the other hand, the luminescent spectra of 5−7 are similar (Figure S20−S22), that display fluorescent emission band with two intensive peaks (408 and 434 nm) and a shoulder peak (458 nm). Such emission behaviors can be attributed to the dib coligands-based emission according to the examination of the luminescence of CPP and dib (Figure S23 and S26). While 1 exhibits two bands centered at 307 and 395 nm in its luminescent spectrum, which are assigned to the imidazole (Figure S19 and S24). No characteristic uranyl emission of these compounds were observed even when excited at the excitation length of uranyl cation (410−440 nm). This is a common phenomenon, because not all uranyl compounds exhibit luminescent properties, which are associated with many factors, such as the interior nature in bonding, size and quality of the crystals, etc.44

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CONCLUSION We have presented the first examples of heterometallic uranyl carboxyphosphinates which were synthesized hydrothermally in the presence of different imidazole auxiliary ligands. This contribution provides a forum for studying the influence of ligand modification, secondary metal incorporation, and imidazole coligands on the architectures of the final heterometallic uranyl organic hybrid materials. The bifunctional carboxyphosphinate ligand displayed varied coordination modes for binding uranyl and transition metal centers, thus leading to different layered assemblies. Further judicious G

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Crystal Growth & Design

Article

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selection of auxiliary imidazole ligands with one or two N donors can interrupt or extend the dimensions of the resultant structures (1 and 2 are 2D, and 3−5 are 3D). Lastly, in some cases (6−8), separated subnet sheets of transition metal connected by dib molecules contribute toward producing distinct heterometallic uranyl carboxyphosphinate structures. The eight compounds capitalize on collaboration between metal centers and organic ligands toward synthesizing and expanding the catalog of uranyl organic assemblies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01595. Bond lengths and angles; PXRD patterns, ORTEP representations, and IR and luminescent spectroscopy. (PDF) Accession Codes

CCDC 1430421−1430428 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of this work by National Natural Science Foundation of China (No. 21571171, U1407101, 21301168, 21273063).

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REFERENCES

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DOI: 10.1021/acs.cgd.5b01595 Cryst. Growth Des. XXXX, XXX, XXX−XXX