Coordination Polymers and Cage-Containing Frameworks in Uranyl

Article pubs.acs.org/IC

Coordination Polymers and Cage-Containing Frameworks in Uranyl Ion Complexes with rac- and (1R,2R)-trans-1,2Cyclohexanedicarboxylates: Consequences of Chirality Pierre Thuéry*,† and Jack Harrowfield*,‡ †

NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France ISIS, Université de Strasbourg, 8 allée Gaspard Monge, 67083 Strasbourg, France

‡

S Supporting Information *

ABSTRACT: Racemic and enantiopure (1R,2R) forms of trans-1,2-cyclohexanedicarboxylic acid (H2chdc and R-H2chdc, respectively) have been used in the synthesis of a series of 13 uranyl ion complexes, all obtained under solvohydrothermal conditions and in the presence of additional metal cations and/or N-donor ligands. While the homometallic complex [UO2(R-chdc)] (1) was only obtained with the enantiopure ligand, complexes [UO2(chdc)(THF)] (2), [UO2(chdc)(DMF)] (3), and [UO2(chdc)(NMP)] (4), with a coordinated solvent molecule, were obtained from the racemic form only; all crystallize as two-dimensional (2D) assemblies. The two complexes [UO2(chdc)(bipy)](5) and [UO2(R-chdc)(bipy)] (6), where bipy is 2,2′-bipyridine, are isomorphous since 5 crystallizes as a racemic conglomerate; they are both one-dimensional (1D) homochiral, helical polymers. The heterometallic complexes [UO2Cu(chdc)2(bipy)(H2O)]·H2O (7) and [UO2Cu(R-chdc)2(bipy)]·3H2O (8) crystallize as a 1D or a 2D species, respectively, while [UO2Cd(R-chdc)2(H2O)2]·H2O (9) displays a 2D arrangement with the unusual Cairo pentagonal tiling topology. The four complexes [(UO2)2Na2(chdc)3(H2O)2] (10), [(UO2)2Ag2(chdc)3(H2O)2] (11), [(UO2)2Na2(Rchdc)3(H2O)2] (12), and [(UO2)2Pb(R-chdc)3(H2O)4] (13) are closely related, all of them containing tetranuclear, pseudotetrahedral [(UO2)4(chdc/R-chdc)6]4− cage motifs, that are assembled into a three-dimensional (3D) framework by bridging counterions (Na+, Ag+, or Pb2+). These cages define a new pathway to assembly of such species based on the unique coordination geometry of uranyl ion, differing from the widely exploited use of octahedral metal ions.

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INTRODUCTION Although they constitute a minor group among the many polycarboxylates that have been used in the design of uranyl− organic coordination polymers and frameworks,1−3 those based on the cyclohexyl platform yield complexes displaying a variety of structural features arising from the nonplanarity and conformational flexibility of these molecules. While twodimensional (2D) assemblies are common in this family of uranyl ion complexes, as they are in uranyl structural chemistry generally, more unusual structures have been described, such as nanotubular forms with all-cis-1,3,5-cyclohexanetricarboxylic acid4 and all-cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid (Kemp’s acid),5 as well as octanuclear homometallic or dodeca- and hexadecanuclear uranyl−copper(II) molecular cages with the latter acid or one of its monoester derivatives.5−7 The highest member of the series, 1,2,3,4,5,6cyclohexanehexacarboxylic acid, in its all-cis form, provides rare examples of mixed uranyl−lanthanide(III) complexes,8 and it was also shown to undergo isomerization into the all-trans form to give a homometallic 2D network.9 In contrast, investigations of the dicarboxylic derivatives have been limited, with only a molecular species isolated from trans-1,4-cyclohexanedicarboxylic acid,10 and one-dimensional (1D) or 2D assemblies from cis/trans-1,3-cyclohexanedicarboxylic acid.11 trans-1,2-Cyclohexanedicarboxylic acid (H2chdc) has never been used in a similar © XXXX American Chemical Society

work, and although about 50 of its complexes with d-block metal ions are reported in the Cambridge Structural Database (CSD, Version 5.37),12 none is known with an actinide cation; only recently were some of its lanthanide ion complexes described.13 H2chdc is an alicyclic analogue of phthalic acid, a ligand which is known to give nanotubular complexes with uranyl ions,14 as well as heterometallic complexes including additional iron(II),15 copper(II),16 or lanthanide(III) cations.17 A feature lacking with phthalic acid is that of chirality, and while it is possible to create chiral crystals from achiral components, as indeed is known in the case of some uranyl ion dicarboxylate complexes,18 use of a single enantiomer of a given ligand, provided it is stable to inversion, must result in the formation of chiral crystals. Quite apart from the intrinsic interest of chiral crystals,19 the possibility that the crystal structures obtained with racemic and resolved ligands may differ renders such ligands of particular value in the exploration of lattice formation. It thus appeared worthwhile to synthesize uranyl ion complexes with both racemic, commercially available H2chdc, and its separate enantiomers under variable experimental conditions, particularly those involving different organic cosolvents in solvo-hydrothermal processes, and the now Received: October 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b02537 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 6 within 3 days (16 mg, 27% yield). Anal. calcd for C18H18N2O6U: C, 36.25; H, 3.04; N, 4.70. Found: C, 36.28; H, 3.11; N, 4.64%. The same complex was obtained in the additional presence of manganese(II) or silver(I) nitrate. [UO2Cu(chdc)2(bipy)(H2O)]·H2O (7). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Cu(NO3)2·2.5H2O (23 mg, 0.10 mmol), 2,2′-bipyridine (32 mg, 0.20 mmol), methanol (0.3 mL), and demineralized water (0.7 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light green crystals of complex 7 within 2 weeks (13 mg, 30% yield based on the acid). Anal. calcd for C26H32CuN2O12U: C, 36.06; H, 3.72; N, 3.23. Found: C, 35.94; H, 3.66; N, 3.28%. [UO2Cu(R-chdc)2(bipy)]·3H2O (8). R-H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Cu(NO3)2·2.5H2O (23 mg, 0.10 mmol), 2,2′-bipyridine (32 mg, 0.20 mmol), acetonitrile (0.3 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light blue-green crystals of complex 8 within 1 week (4 mg, 9% yield based on the acid). Anal. calcd for C26H34CuN2O13U: C, 35.32; H, 3.88; N, 3.17. Found: C, 35.95; H, 3.61; N, 3.33%. [UO2Cd(R-chdc)2(H2O)2]·H2O (9). R-H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Cd(NO3)2·4H2O (31 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.5 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 9 within 1 week (33 mg, 85% yield based on the acid). Anal. Calcd for C16H26CdO13U: C, 24.74; H, 3.37. Found: C, 24.72; H, 3.27%. [(UO2)2Na2(chdc)3(H2O)2] (10). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), NaNO3 (17 mg, 0.20 mmol), 18-crown-6 (26 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 10 within 2 days (26 mg, 69% yield based on the acid). Anal. Calcd for C24H34Na2O18U2: C, 25.45; H, 3.03. Found: C, 25.48; H, 2.99%. [(UO2)2Ag2(chdc)3(H2O)2] (11). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), AgNO3 (34 mg, 0.20 mmol), acetonitrile (0.2 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 11 within 5 days (13 mg, 30% yield based on the acid). Anal. Calcd for C24H34Ag2O18U2: C, 22.13; H, 2.63. Found: C, 22.22; H, 2.71%. [(UO2)2Na2(R-chdc)3(H2O)2] (12). R-H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), NaNO3 (17 mg, 0.20 mmol), 18-crown-6 (26 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 12 within 1 week (8 mg, 21% yield based on the acid). Anal. Calcd for C24H34Na2O18U2: C, 25.45; H, 3.03. Found: C, 25.35; H, 3.05%. [(UO2)2Pb(R-chdc)3(H2O)4] (13). R-H2chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Pb(NO3)2 (33 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 13 overnight (5 mg, 11% yield based on the acid). Anal. Calcd for C24H38O20PbU2: C, 21.68; H, 2.88. Found: C, 21.88; H, 2.90%. Crystallography. The data were collected at 150(2) K on a Nonius Kappa-CCD area detector diffractometer25 using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The crystals were introduced into glass capillaries with a protective coating of ParatoneN oil (Hampton Research). The unit cell parameters were determined from ten frames, then refined on all data. The data (combinations of φ- and ω-scans with a minimum redundancy of at least 4 for 90% of the reflections) were processed with HKL2000.26 Absorption effects were corrected empirically with the program SCALEPACK.26 The structures were solved by intrinsic phasing with SHELXT,27 expanded by subsequent difference Fourier synthesis and refined by full-matrix

widespread use of different additional metallic cations,2,3 two approaches that we recently applied to other polycarboxylic acids and that proved efficient in the isolation of a range of architectures of varying dimensionality.20−23 We thus report here the synthesis of 13 uranyl complexes, 7 with the racemic ligand and 6 with the (1R,2R) enantiomer, 7 of them including different additional cations (NaI, CuII, AgI, CdII, PbII), that have each been characterized by their crystal structure and emission spectrum in the solid state.

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EXPERIMENTAL SECTION

Syntheses. Caution! Uranium is a radioactive and chemically toxic element, and uranium-containing samples must be handled with suitable care and protection. UO2(NO3)2·6H2O (depleted uranium, R. P. Normapur, 99%), NaNO3, AgNO3, and Pb(NO3)2 were purchased from Prolabo, ractrans-1,2-cyclohexanedicarboxylic acid (H2chdc) was from Lancaster, Cu(NO3)2·2.5H2O was from Aldrich, and Cd(NO3)2·4H2O and 2,2′bipyridine (bipy) were from Fluka. Elemental analyses were performed by MEDAC Ltd. at Chobham, UK. The (1R,2R) enantiomer of H2chdc, denoted R-H2chdc, was isolated through crystallization with (R)-1-phenylethylamine as a resolving agent, as in the literature.24 [UO2(R-chdc)] (1). R-H2chdc (17 mg, 0.10 mmol), UO2(NO3)2· 6H2O (50 mg, 0.10 mmol), AgNO3 (34 mg, 0.20 mmol), acetonitrile (0.3 mL), and demineralized water (0.6 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 1 within 1 week (33 mg, 75% yield). Anal. Calcd for C8H10O6U: C, 21.83; H, 2.29. Found: C, 21.99; H, 2.29%. The same complex was obtained under similar conditions (either with or without acetonitrile cosolvent), with silver(I) nitrate being replaced by manganese(II), cobalt(II), nickel(II), copper(II), or europium(III) nitrate. [UO2(chdc)(THF)] (2). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2· 6H2O (50 mg, 0.10 mmol), Mn(NO3)2·4H2O (25 mg, 0.10 mmol), tetrahydrofuran (0.2 mL), and demineralized water (0.8 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 2 within 1 week (32 mg, 62% yield). Anal. Calcd for C12H18O7U: C, 28.13; H, 3.54. Found: C, 27.84; H, 3.50%. [UO2(chdc)(DMF)] (3). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2· 6H2O (50 mg, 0.10 mmol), Pb(NO3)2 (33 mg, 0.10 mmol), 2,2′bipyridine (16 mg, 0.10 mmol), N,N-dimethylformamide (0.2 mL), and demineralized water (0.8 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 3 overnight (10 mg, 19% yield). Anal. Calcd for C11H17NO7U: C, 25.74; H, 3.34; N, 2.73. Found: C, 25.65; H, 3.19; N, 2.70%. The same complex was obtained under similar conditions with lead(II) nitrate being replaced by cobalt(II) or nickel(II) nitrate. [UO2(chdc)(NMP)] (4). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2· 6H2O (50 mg, 0.10 mmol), N-methyl-2-pyrrolidone (0.3 mL), and demineralized water (0.7 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 4 within 1 week (21 mg, 39% yield). Hydration of the sample during workup is suggested by elemental analysis. Anal. Calcd for C13H19NO7U + H2O: C, 28.02; H, 3.80; N, 2.51. Found: C, 27.50; H, 3.42; N, 2.60%. [UO2(chdc)(bipy)] (5). H2chdc (17 mg, 0.10 mmol), UO2(NO3)2· 6H2O (50 mg, 0.10 mmol), 2,2′-bipyridine (16 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.7 mL) were placed in a 10 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 5 within 1 week (50 mg, 84% yield). Anal. calcd for C18H18N2O6U: C, 36.25; H, 3.04; N, 4.70. Found: C, 36.28; H, 3.11; N, 4.69%. The same complex was obtained in the additional presence of manganese(II) nitrate. [UO 2 (R-chdc)(bipy)] (6). R-H 2 chdc (17 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), 2,2′-bipyridine (16 mg, 0.10 mmol), acetonitrile (0.2 mL), and demineralized water (0.8 mL) B


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details chemical formula M (g mol−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F(000) reflns collcd indep reflns obsd reflns [I > 2σ(I)] Rint params refined R1 wR2 S Δρmin (e Å−3) Δρmax (e Å−3) Flack parameter chemical formula M (g mol−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F (000) reflns collcd indep reflns obsd reflns [I > 2σ(I)] Rint params refined R1 wR2 S Δρmin (e Å−3) Δρmax (e Å−3) Flack parameter

1

2

3

4

5

6

7

C8H10O6U 440.19 monoclinic P21 5.2473(2) 12.9276(8) 7.6949(5) 90 96.105(4) 90 519.02(5) 2 2.817 15.642 396 30251 3161 3050 0.017 137 0.026 0.066 1.057 −2.21 1.85 −0.002(15) 8

C12H18O7U 512.29 monoclinic P21/c 10.6489(8) 11.2476(8) 12.7554(6) 90 103.489(4) 90 1485.63(17) 4 2.290 10.952 952 47809 2815 2489 0.033 181 0.027 0.063 1.057 −1.43 0.85

C11H17NO7U 513.28 monoclinic P21/c 10.3828(6) 10.3419(6) 14.3432(5) 90 102.942(4) 90 1501.02(14) 4 2.271 10.842 952 81689 2840 2680 0.024 218 0.027 0.065 1.180 −1.16 0.89

C13H19NO7U 539.32 monoclinic P21/c 11.6986(7) 11.8302(5) 13.1803(7) 90 112.903(3) 90 1680.31(16) 4 2.132 9.691 1008 57746 3189 2783 0.026 227 0.023 0.058 1.047 −1.20 0.91

C18H18N2O6U 596.37 trigonal P31 16.7704(7) 16.7704(7) 18.8976(6) 90 90 120 4602.8(4) 9 1.936 7.968 2520 98400 11632 10221 0.059 731 0.036 0.069 1.023 −1.69 1.39 −0.021(5) 11

C18H18N2O6U 596.37 trigonal P32 16.7746(7) 16.7746(7) 18.8965(5) 90 90 120 4604.9(5) 9 1.936 7.965 2520 113186 11615 11115 0.036 731 0.035 0.080 1.078 −1.34 0.94 −0.006(7) 12

C26H32CuN2O12U 866.10 monoclinic P21/n 6.4368(3) 19.1229(12) 23.0130(12) 90 97.250(3) 90 2810.0(3) 4 2.047 6.580 1676 84653 5332 4078 0.051 416 0.032 0.061 0.987 −0.87 0.64

C26H34CuN2O13U 884.12 orthorhombic P21212 19.1847(8) 23.8680(11) 6.4934(2) 90 90 90 2973.3(2) 4 1.975 6.224 1716 67862 7676 6926 0.031 407 0.033 0.063 1.049 −0.89 1.11 −0.008(7)

9

10

C16H26CdO13U 776.80 orthorhombic P212121 8.8464(3) 10.5820(2) 22.7846(10) 90 90 90 2132.93(12) 4 2.419 8.646 1464 53955 5476 4952 0.028 281 0.027 0.074 1.027 −1.25 0.91 −0.003(6)

C24H34Na2O18U2 1132.55 tetragonal I41/a 18.0521(8) 18.0521(8) 19.4747(13) 90 90 90 6346.4(7) 8 2.371 10.301 4224 60186 3013 2478 0.028 245 0.045 0.099 1.135 −1.31 0.56

least-squares on F2 with SHELXL-2014.28 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms bound to oxygen atoms were retrieved from difference Fourier maps (except when indicated below), and the carbon-bound hydrogen atoms were introduced at calculated positions; all hydrogen atoms were treated as riding atoms with an isotropic displacement parameter

C24H34Ag2O18U2 1302.31 tetragonal I41/a 18.1546(5) 18.1546(5) 19.5356(8) 90 90 90 6438.7(4) 8 2.687 11.307 4800 101020 3037 2956 0.027 209 0.026 0.069 1.090 −1.40 0.66

C24H34Na2O18U2 1132.55 tetragonal I41 17.8219(4) 17.8219(4) 19.9154(7) 90 90 90 6325.5(4) 8 2.378 10.335 4224 68525 8121 6373 0.034 416 0.033 0.088 0.941 −1.20 0.79 −0.003(12)

13 C24H38O20PbU2 1329.79 tetragonal P43212 16.9724(3) 16.9724(3) 23.6721(5) 90 90 90 6819.0(3) 8 2.591 14.483 4864 126225 6486 5790 0.042 426 0.053 0.154 1.097 −2.07 3.19 0.018(6)

equal to 1.2 times that of the parent atom (1.5 for CH3, with optimized geometry). Special details are as follows. Complex 3. The DMF molecule is disordered over two positions sharing the oxygen atom, which were refined with occupancy parameters constrained to sum to unity. Restraints on displacement parameters were applied for some atoms in the cyclohexyl ring and the disordered DMF molecule. C


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Inorganic Chemistry Complex 4. The NMP molecule is disordered over two positions sharing four atoms, which were refined with occupancy parameters constrained to sum to unity and restraints on displacement parameters. Complexes 5, 6, and 11. Two-component twinning was detected with TwinRotMat (PLATON29). Taking into account further racemic twinning in 5 appeared unnecessary since the corresponding refined batch scale factor was very close to 0. Complexes 7 and 10. The cyclohexyl ring of one ligand is disordered over two positions sharing two carbon atoms, which have been refined with occupancy parameters constrained to sum to unity and restraints on displacement parameters and some bond lengths. Complex 8. Two of the lattice water molecules are disordered over four sites that were refined with occupancy parameters of 0.5 and restraints on displacement parameters. The hydrogen atoms of these molecules were not found. Complex 13. The hydrogen atoms of one water molecule (O20) were not found. Three-component twinning was detected with TwinRotMat and taken into account. Crystal data and structure refinement parameters are given in Table 1. The molecular plots were drawn with ORTEP-3,30 and the polyhedral representations with VESTA.31 The topological analyses were conducted with TOPOS.32 Luminescence Measurements. Emission spectra were recorded on solid samples using a Horiba-Jobin-Yvon Fluorolog spectrofluorometer. The powdered complex was pressed between two silica plates which were mounted such that the faces were oriented vertically and at 45° to the incident excitation radiation. An excitation wavelength of 420 nm, a commonly used point although only part of a broad manifold, was used in all cases and the emission was monitored between 450 and 650 nm.

Table 2. Chirality of the Ligands and Compounds compound 1 2 3 4 5 6 7 8 9 10 11 12 13

ligand chirality

space group

compound chirality

(1R,2R) enantiomorph racemic racemic racemic racemic

Sohncke

enantiopure

centrosymmetric centrosymmetric centrosymmetric chiral

(1R,2R) enantiomorph racemic (1R,2R) enantiomorph (1R,2R) enantiomorph racemic racemic (1R,2R) enantiomorph (1R,2R) enantiomorph

chiral

racemic racemic racemic racemic conglomerate enantiopure

centrosymmetric Sohncke

racemic enantiopure

Sohncke

enantiopure

centrosymmetric centrosymmetric Sohncke

racemic racemic enantiopure

chiral

enantiopure

species of different dimensionalities, as will be described below (the different cosolvents used during the syntheses of these two compounds, although not retained in the final species, may however play a role here; unfortunately, we could not grow crystals from both ligands with the same cosolvent or without an organic solvent in the UO2/Cu/bipy case). Overall, these observations point to a genuine effect of enantiopurity on the nature of the isolated solids, as previously encountered, for example, in complexes formed by manganese(II) halides with Land DL-proline33 or in salts of [Tb(dipic)3]3− (dipic = pyridine2,6-dicarboxylate) with Δ,Λ- and Δ-[Co(en)3]3+.34 A significant aspect of the present observations is that R-chdc2− appears to be stable to inversion under the reaction conditions employed, unlike related species with higher degrees of substitution on the cyclohexane ring. Crystal Structures. The structures of compounds 1−13 display a general increase in complexity and will be described in this order, regardless of their racemic or enantiopure nature. Complex 1, [UO2(R-chdc)], is the simplest species possible, with one uranyl ion and one fully deprotonated ligand in the asymmetric unit (Figure 1), and it crystallizes in the P21 Sohncke group. Somewhat unusually in carboxylate complexes, the uranyl ion is bound to only four equatorial donors pertaining to four different ligands, and the uranium coordination polyhedron is thus square pyramidal. The uranyl U−O(oxo) bond lengths are 1.762(6) and 1.767(6) Å; the U− O(carboxylate) bond lengths, in the range 2.279(6)−2.331(6) Å [average 2.31(2) Å], are unexceptional, the average value for the 29 comparable six-coordinate uranium carboxylate complexes reported in the CSD being 2.33(6) Å. The dicarboxylate ligand, with the cyclohexyl ring assuming the chair conformation with the functional groups equatorial, as in all other present complexes, is bound to four metal cations in a bis(μ2-η1:η1) coordination mode. This gives rise to the formation of a 2D assembly parallel to (0 0 1), with the point (Schläfli) symbol {44.62} of the common tetragonal Shubnikov lattice. The cyclohexyl rings protrude on both sides of the sheets, the latter being packed with no significant free space left, as shown by the Kitaigorodski packing index (KPI) value of 0.69 (estimation with PLATON29), usual values for nonporous packings being of the order of 0.65. Cyclohexyl

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RESULTS AND DISCUSSION Synthesis. All compounds 1−13 were synthesized under solvo-hydrothermal conditions (140 °C), with the organic cosolvent being either acetonitrile (1, 5, 6, 8−13), tetrahydrofuran (2), N,N-dimethylformamide (3), N-methyl-2-pyrrolidone (4), or methanol (7). Only in complexes 2−4 is this organic solvent retained as a coligand in the final compound, but this does not preclude an effect on the complexes formed in the other cases also, since the cosolvent modifies the solubility of the species present and the pH of the solution (particularly when it is liable to undergo hydrolysis, such as with N,Ndimethylformamide). While 7 out of the 13 complexes reported include additional metal cations present in the solution, this is not so in compounds 1−3, which are homometallic in spite of the d-block or PbII cations being present (see the Experimental Section). For each ligand, numerous combinations of solvent/ additional metal cation/N-donor (bipy or 1,10-phenanthroline) were tested systematically (and in some cases 18-crown-6 as Odonor as well), but only those reported here provided crystalline materials suitable for structure determination. It is notable that no complex contains oligomeric secondary building units with oxo- or hydroxo-bridged uranyl ions. While the present series comprises about as many achiral complexes based on the racemic dicarboxylate ligand chdc2− as homochiral ones containing the enantiopure form R-chdc2− (Table 2), it is remarkable that only in a few instances (compounds 5/6 and 10/12 in particular) were complexes of identical composition obtained with both ligands. The simplest complex 1 is readily obtained from R-chdc2− under varying experimental conditions (see the Experimental Section), but its counterpart of the same composition with the racemic ligand could not be isolated under any of the conditions tested. In contrast, the achiral complexes 2−4 incorporating coordinated cosolvents have no equivalent in the chiral series. Complexes 7 and 8, with overall formulas close to one another, crystallize as D


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Inorganic Chemistry

bipyramidal geometry. Considering all three compounds, the U−O(oxo) bond lengths are in the range 1.759(4)−1.767(4) Å [average 1.763(3) Å], the U−O(carboxylate) bond lengths are 2.439(2)−2.477(3) Å [average 2.452(12) Å] for chelating, and 2.274(3)−2.403(4) Å [average 2.34(5) Å] for monodentate groups, and the bond lengths with the solvent molecules are 2.447(3), 2.383(4), and 2.372(3) Å for THF, DMF, and NMP, respectively, consistent with the order of their Gutmann donor numbers of 20.0, 26.6, and 27.3 kcal mol−1, respectively.36 The ligand is bound to three metal cations in η2 and μ2-η1:η1 fashion, so that both metal and ligand are 3-fold nodes in the 2D network formed. The latter is parallel to (1 0 0) and displays a tessellation of 4- and 8-membered rings (point symbol {4.82}, fes topological type). Here also, the cyclohexyl rings point outward from the sheets, the latter being stacked in a bump-to-hollow fashion leaving no significant free space (KPI 0.66−0.68). Calculation of the Hirshfeld surface for 2, the only solvate in which the bound solvent displays no disorder, indicates that there are no interactions beyond dispersion involving the methylene units of the cyclohexane or THF rings. The complexes [UO2(chdc)(bipy)] (5) and [UO2(R-chdc) (bipy)] (6) provide one instance of identical compounds obtained from the racemic and enantiopure forms of the dicarboxylate ligand. While 6 crystallizes unsurprisingly in a chiral space group (P32), 5 crystallizes unexpectedly as a racemic conglomerate, the particular crystal chosen for structure determination corresponding to the (1S,2S) enantiomer (space group P31), so that, in this case, the complexes with both enantiomeric forms have been characterized. Apart from the different chirality, both display the same structure, with three crystallographically independent uranyl ions and ligands in the asymmetric unit. The uranium atom is chelated by two carboxylate groups and the bipy molecule, and its eightcoordinate environment is thus hexagonal bipyramidal (Figure 3). The U−O(oxo) bond lengths [1.726(13)−1.779(17) Å, average 1.746(16) Å, including both compounds], and the U− O(carboxylate) bond lengths [2.432(12)−2.478(11) Å, average 2.458(12) Å], are unexceptional, as well as the U−N bond lengths [2.614(13)−2.658(17) Å, average 2.634(14) Å (2.63(4) Å from the CSD)]. As usual in eight-coordinate uranyl complexes with chelating N-donors such as 2,2′bipyridine, 1,10-phenanthroline or their derivatives, the six donor atoms deviate strongly from the uranyl equatorial plane, in such manner that the uranium environment is chiral.37 Together with the uranium atom, the four carboxylate oxygen atoms define a mean UO4 plane approximately perpendicular to the uranyl ion axis, with root-mean-square (rms) deviations in the range 0.036−0.058 Å (considering both complexes 5 and 6). The two rings of the bipy molecule are tilted with respect to one another by 13.4(19)−16.8(4)°, and the mean plane of the bipy molecule makes a dihedral angle of 37.1(8)−41.0(3)° with the average UO4 plane [corresponding to out-of-plane displacements of the nitrogen atoms of 0.48(3)−0.87(3) Å]. The terminal nature of the bipy ligand limits the dimensionality of the assembly and results in the formation of a 1D homochiral, helical polymer running along the c axis of the trigonal cell. When viewed down the chain axis, the cyclohexyl groups and bipy ligands point outward as three propeller blades, and the packing is such that no π-stacking or CH···π interactions are present, although the Hirshfeld surface displays evidence that CH(bipy)···O interactions provide links between the helices; the KPI of 0.62 is slighly smaller than in the former complexes, but no solvent-accessible space is present.

Figure 1. (top) View of complex 1. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: i = x + 1, y, z; j = 1 − x, y − 1/2, 2 − z; k = −x, y − 1/2, 2 − z; l = x − 1, y, z; m = 1 − x, y + 1/2, 2 − z; n = −x, y + 1/2, 2 − z. (middle) View of the 2D assembly with uranium coordination polyhedra colored yellow. (bottom) Packing with layers viewed edge-on. Hydrogen atoms are omitted in all views.

units from separate sheets are sufficiently close for dispersion interactions to be significant, and the calculation of the Hirshfeld surface with CrystalExplorer 3.0,35 for an asymmetric unit of the polymer, indicates that CH···O interactions also play a role in linking sheets. The three complexes [UO2(chdc)(L)], with L = THF (2), DMF (3), or NMP (4), crystallize in the same centrosymmetric space group (P21/c) and with rather close unit cell parameters. The content of the asymmetric unit differs from that in 1 by the addition of the coordinated solvent, as shown in Figure 2 for complex 4 (views of complexes 2 and 3 are given in Figures S1 and S2, Supporting Information). The uranyl ion is chelated by one carboxylate and bound to two carboxylate donors from two more ligands and the solvent molecule, so that the uranium atom is in a seven-coordinate environment with pentagonal E


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Inorganic Chemistry

Figure 2. (top left) View of complex 4. Displacement ellipsoids are drawn at the 40% probability level. Symmetry codes: i = x, 3/2 − y, z + 1/2; j = 1 − x, y − 1/2, 1/2 − z; k = x, 3/2 − y, z − 1/2; l = 1 − x, y + 1/2, 1/2 − z. (top right) View of the 2D assembly with uranium coordination polyhedra colored yellow. (bottom left) Packing with layers viewed edge-on. (bottom right) Nodal representation of the 2D network: yellow, uranium; red, oxygen; blue, dicarboxylate ligand; dark red, NMP. Only one position of the disordered NMP molecule is shown and hydrogen atoms are omitted in all views.

each of them a homometallic 1D subunit in its own right, and connected to one another by the bridging carboxylate groups. An intrachain parallel-displaced π-stacking (dispersive) contact is probably present between successive bipy units [centroid··· centroid distance 3.588(3) Å, dihedral angle 4.3(2)°] and chains in adjacent sheets are linked by a CH···O interaction involving bipy and the uranyl oxo atom O2 (2.44 Å). When viewed down their main axis, these chains show a very irregular shape, and they are also linked to one another through hydrogen bonding between the water ligand and the uncoordinated carboxylate oxygen atom [O11···O10 k 2.738(5) Å, H···O10k 1.85 Å, O11−H···O10k 159°; symmetry code k = x − 1/2, 3/2 − y, z + 1/2], further bridging between these chains related by a glide plane being mediated by the lattice water molecule, and resulting in the formation of hydrogen-bonded layers parallel to (0 1 0). Note also that the separations between methylene units of the cyclohexane rings in adjacent sheets are similar to those commonly found in bilayers formed by amphiphilic molecules and may be indicative of significant dispersion interactions. The KPI of 0.72 is indicative of a compact packing. The unique uranium atom in complex 8 [U−O(oxo) bond lengths of 1.777(5) and 1.796(5) Å] is chelated by one carboxylate group [U−O bond lengths of 2.419(5) and 2.455(4) Å] and it is also bound to three carboxylate oxygen atoms from three more chdc2− anions [2.309(5)−2.364(5) Å], being thus in a pentagonal bipyramidal environment (Figure 5).

Two complexes including both copper(II) cations and bipy coligands were obtained, with the racemic form of the ligand, [UO2Cu(chdc)2(bipy)(H2O)]·H2O (7), or with the enantiopure one, [UO2Cu(R-chdc)2(bipy)]·3H2O (8). In this case, there is no resolution of the two enantiomers in the racemic form and the structures of the two complexes are different. In complex 7, the unique uranium ion is chelated by two carboxylate groups of one chdc2− ligand, thus forming a sevenmembered chelate ring; it is also bound to three more carboxylate oxygen atoms from three other ligands, and it is thus in a pentagonal bipyramidal environment; the copper(II) ion is chelated by the bipy molecule and bound to two carboxylate oxygen atoms from two chdc2− ligands and a more distant water molecule, being thus in a square pyramidal environment (Figure 4). The U−O(oxo) bond lengths are 1.767(4) and 1.772(4) Å and the U−O(carboxylate) bond lengths are in the range 2.306(3)−2.395(3) Å [average 2.37(3) Å]. The Cu−O(carboxylate) [1.966(3) and 1.982(3) Å] and Cu−N [1.985(4) and 2.001(4) Å] bond lengths are shorter by more than 0.2 Å than the Cu−O(water) bond length [2.233(3) Å]. The two carboxylate groups of one of the two independent ligands are bound to one uranium and one copper atom each, in μ2-η1:η1 fashion, while the other ligand is bound to two uranium atoms only, one of the carboxylate oxygen atoms (O10) being uncoordinated. The coordination polymer formed is 1D and directed along the a axis, and it displays two parallel rows of either uranyl or copper(II) ions running side-by-side, F


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Inorganic Chemistry

Figure 3. (top) Views of one of the independent complex units in 5 (left, (1S,2S) ligand enantiomer from a racemic conglomerate) and 6 (right, pure (1R,2R) ligand enantiomer). Displacement ellipsoids are drawn at the 30% probability level. Symmetry codes: i = y − x + 1, 2 − x, z − 1/3; j = 2 − y, x − y + 1, z + 1/3 for 5; i = y − x, − x, z + 1/3; j = −y, x − y, z − 1/3 for 6. (middle) View of the 1D assembly in 5 with the uranium coordination polyhedra colored yellow. (bottom) Packing in 5, with chains viewed end-on. Hydrogen atoms are omitted in all views.

with respect to that in 7, which involves bonding in a fourmembered chelate ring, results in the coordination polymer formed here being 2D. The network is parallel to (0 1 0) and has the point symbol {4.62}{43.63}, with the first symbol being for the 3-fold nodes (CuII and one of the R-chdc2− ligands) and the second for the 4-fold nodes (UVI and the other ligand). The layers display alternate rows of uranyl and copper(II) cations directed along the c axis, these rows being analogous to those in complex 7, the increase in dimensionality arising from the uncoordinate carboxylate oxygen atom in 7 replacing water in the copper(II) coordination sphere in 8 (the close relationship

The copper(II) atom is bound to the chelating bipy molecule [Cu−N bond lengths of 1.987(6) and 1.996(6) Å] and two carboxylate oxygen atoms [1.957(5) and 1.974(5) Å], the difference with its counterpart in 7 being the replacement of the water ligand in the axial position by a third, more distant carboxylate oxygen atom at 2.250(5) Å. One of the R-chdc2− ligands is thus bound to two uranyl and one copper(II) cations with η2 and μ2-η1:η1 coordination modes, and the other to two uranyl and two copper(II) cations in bis(μ2-η1:η1) mode, the latter ligand having thus the same connectivity as its counterpart in 7. The higher connectivity of the former ligand G


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Inorganic Chemistry

only three carboxylate oxygen atoms from two ligands [2.380(5)−2.387(5) Å], its coordination sphere being completed by two water molecules [2.396(5) and 2.445(5) Å] (Figure 6). The cadmium(II) cation is chelated by two η2carboxylate groups (one of them very unsymmetrical) and it is bound to two more carboxylate oxygen atoms from two different anions, with Cd−O bond lengths in the range 2.225(5)−2.630(5) Å [Cd−O(carboxylate) bond lengths reported in the CSD vary widely, with the largest part of the distribution in the interval ∼2.10−2.70 Å and an average value of 2.34(11) Å]. Two longer contacts with atoms O3i and O5j, pertaining to the same carboxylic groups as the coordinated atoms O4i and O6j, at 2.880(5) and 2.847(5) Å, respectively, are probably not indicative of bonding interactions. Each of the two R-chdc2− ligands is bound to one UVI and two CdII cations, albeit with a different coordination mode: the two carboxylate groups of one ligand are bound in the μ2-η1:η1 mode (and they share the uranium atom), while those of the other ligand correspond to the μ2-η2:η1 and η2 modes. While uranium atoms are simple links in the coordination polymer, both ligands are 3-fold nodes and the cadmium atom a 4-fold one. The 2D network formed, parallel to (0 0 1), has the point symbol {53}2{54.82} characteristic of the Cairo pentagonal tiling topological type (or MacMahon’s net). However, the metrics of the (nonplanar) pentagons here is of course different from that in the classical Cairo tiling and the network is visually closer to the topologically equivalent basketweave tiling since, while the four edges ending in a CdII vertex have close to the same length, that containing the uranyl cation linker is nearly twice as large. Few examples of the Cairo tiling topological type have been reported in metal−organic assemblies,38,39 and, to the best of our knowledge, none in a uranyl compound. As in complexes 1−4 and 8, the cyclohexyl rings protrude on both sides of the layers, and the packing does not contain free spaces (KPI 0.72, or 0.69 with solvent excluded). The water ligands are hydrogen bonded to carboxylate groups and the lattice water molecule, the latter being itself a hydrogen bond donor toward carboxylate or oxo groups [O···O distances 2.637(8)− 3.017(7) Å, O−H···O angles 126−162°, the weakest bond being that with the uranyl oxo group]; all these hydrogen bonds are intralayer ones. The last four complexes, [(UO2)2Na2(chdc)3(H2O)2] (10), [(UO 2 ) 2 Ag 2 (chdc) 3 (H 2 O) 2 ] (11), [(UO 2 ) 2 Na 2 (Rchdc)3(H2O)2] (12), and [(UO2)2Pb(R-chdc)3(H2O)4] (13), are very closely related and will be discussed together. The structures of complexes 10 and 11, both containing the racemic form of the ligand, are isomorphous (tetragonal space group I41/a), while complex 12, with the enantiomerically pure ligand, crystallizes in the Sohncke, polar space group I41, with unit cell parameters very close to those of 10 and 11. Complex 13 crystallizes in the tetragonal chiral space group P43212, with unit cell parameters different from those of 10−12, but its structure nevertheless displays features analogous to theirs. In this, it provides another example of how differences in the coordination preferences of metal ions of the same charge, here PbII and CdII, can lead to very different structures of their mixed uranyl ion complexes. The asymmetric unit in complexes 10 and 11 contains one UVI and one NaI or AgI cations in general positions, and two chdc2− ligands, one of them having 2-fold rotation symmetry (Figure 7 for 11, Figure S3 for 10, Supporting Information); in complexes 12 and 13, it contains two UVI and either two NaI or one PbII cations in general position, and four R-chdc2− ligands among which two have 2-

Figure 4. (top) View of complex 7. Displacement ellipsoids are drawn at the 40% probability level. Symmetry codes: i = x + 1, y, z; j = x − 1, y, z. (middle) The 1D coordination polymer with the uranium coordination polyhedra colored yellow and those of copper(II) blue. (bottom) Packing with the chains viewed end-on. Solvent molecules and hydrogen atoms are omitted in all views (except for the water protons in the top view), and only one position of the disordered atoms is shown.

between the two arrangements manifesting itself in close unit cell axes lengths). The hydrogen bonding of the three water solvent molecules can only be partly determined since two out of these molecules are disordered and their hydrogen atoms were not found (see Experimental Section); the well-resolved water molecule is hydrogen bonded to a uranyl oxo group and a carboxylate oxygen atom pertaining to the same layer. An intralayer parallel-displaced π-stacking contact links successive bipy units along the c axis [centroid···centroid distance 3.610(4) Å, dihedral angle 3.9(3)°] and again there is at least one CH···O interaction, here involving one bipy unit and disordered water. The KPI value is 0.69, or 0.63 with solvent excluded. The cadmium(II)-containing heterometallic complex [UO2Cd(R-chdc)2(H2O)2]·H2O (9) could only be obtained from the enantiopure ligand. Although it is here also in a pentagonal bipyramidal environment, the uranyl cation [U− O(oxo) bond lengths of 1.761(5) and 1.763(5) Å] is bound to H


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Inorganic Chemistry

Figure 5. (top left) View of complex 8. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: i = x, y, z − 1; j = x + 1/2, 3/2 − y, 1 − z; k = x, y, z + 1; l = x − 1/2, 3/2 − y, 1 − z. (top right) View of the 2D assembly with uranium coordination polyhedra colored yellow and those of copper(II) blue. (bottom left) Packing with layers viewed edge-on. (bottom right) Nodal representation of the 2D network: yellow, uranium; light blue, copper; red, oxygen; dark blue, dicarboxylate ligand; dark red, bipy. Solvent molecules and hydrogen atoms are omitted in all views.

are known,45 this seems to be the first instance of tetranuclear cages. The uranium atoms are vertices of a distorted tetrahedron, the edges of which are defined by the ditopic ligands, with the cyclohexyl rings protruding outward. In compounds 10 and 11, the cage admits a 4-fold roto-inversion axis as symmetry element and thus contains both enantiomers of the ligand, while a 2-fold rotation axis is present in the enantiopure cages in 12 and 13. As a consequence of this nonideal tetrahedral geometry, all the intracage U···U distances are not equal, and, overall, they range from 5.9766(6) to 6.9450(6) Å, the distortion with respect to the ideal geometry being minimal in complex 13. As evidenced by the spacefilling representations shown in Figure S5 (Supporting Information) in the case of compound 13, the internal space of the cages is negligible and unsuitable for inclusion purposes, even though four uranyl-O atoms are directed toward the center of the tetrahedron and could have been considered appropriate for binding to a 4-fold hydrogen bond donor species such as ammonium ion (in a receptor where such binding of [NH4]+ is known to occur,46 the N···acceptor atom distance is close to 3 Å, whereas here the distance uranyl-O···centroid is only 2.1− 2.3 Å). The cages are assembled through the bridging NaI, AgI, or PbII cations into a compact 3D framework, with further hydrogen bonding of the water molecules to uranyl oxo and carboxylate groups (KPI values 0.73, 0.70, 0.74, and 0.69 for 10−13, respectively). It may be noted that, while no crystals of sufficient quality for proper structure refinement could be obtained in these cases, analogous compounds with discrete, separate cage moieties involving R-chdc2− ligands crystallized with either Ni(bipy)32+ or (R)-1-phenylethylammonium cations (the latter synthesized from the crude product obtained during

fold rotation symmetry (Figure 7 for 13, Figure S4 for 12, Supporting Information). In all cases, the uranium atoms, in a hexagonal bipyramidal environment, are bound to two oxo atoms [U−O bond lengths 1.722(18)−1.775(19) Å, average 1.759(14) Å, all complexes included], and they are chelated by three carboxylate groups [2.416(8)−2.578(8) Å, average 2.48(4) Å]. The NaI cations in 10 and 12 are bound to three or four carboxylate oxygen atoms pertaining to three or four different ligands [2.274(9)−2.634(9) Å] and two bridging water molecules [2.326(10)−2.418(7) Å], thus forming a Na2(H2O)2 dimeric motif with the latter. AgI in 11 is bound to three carboxylate groups [2.390(6)−2.496(5) Å] and two water molecules [2.402(5) and 2.460(5) Å], its environment being of very irregular geometry. The Ag···Ag separation in the Ag2(H2O)2 dimer, 3.1736(12) Å, is smaller than twice the van der Waals radius of silver (1.72 Å) and indicative of a possibly significant argentophilic interaction. The PbII cation in 13 is bound to four carboxylate groups pertaining to four ligands [2.95(2)−3.09(2) Å] and four water molecules (none of them bridging) [2.32(2)−2.50(2) Å], its eight-coordinate environment being of very irregular geometry. Apart from bridging two uranyl ions, the (R-)chdc2− ligands in these four complexes are bound to one to three additional cations, the carboxylate groups being coordinated in the η2, μ2-η2:η1 or μ3η1:η2:η1 modes. The remarkable feature common to these four complexes appears when only uranyl cations are considered. Four uranyl cations and six chdc2− ligands assemble into a [(UO2)4(chdc)6]4− closed, cage-like assembly of tetrahedral geometry (Figure 8). Several cases of octanuclear uranylcontaining cages with carboxylate5−7,40−43 or catechol44 ligands have been described, but, although tetranuclear metallacycles I


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Inorganic Chemistry

Figure 6. (top left) View of complex 9. Displacement ellipsoids are drawn at the 50% probability level. Carbon-bound hydrogen atoms are omitted and hydrogen bonds are shown as dashed lines. Symmetry codes: i = x − 1, y, z; j = −x, y − 1/2, 3/2 − z; k = −x, y + 1/2, 3/2 − z; l = x + 1, y, z. (top right) View of the 2D assembly with uranium coordination polyhedra colored yellow and cadmium(II) cations shown as green spheres. (bottom left) Packing with layers viewed edge-on. (bottom right) Nodal representation of the 2D network: yellow, uranium; green, cadmium; red, oxygen; dark blue, dicarboxylate ligand. Solvent molecules and hydrogen atoms are omitted in the last views.

as well as cyclobutanetetracarboxylate, are edge-defining, so that cages with either cubic (a) or tetrahedral (c) geometries are formed. In contrast, Kemp’s triacid and p-carboxylatocalix[4]arene are 4-fold nodes and thus analogous to a face of the uranyl cubic arrangement (b). The smaller size of the tetranuclear cage (c) with respect to the octanuclear form (a) is probably a result of the shorter separation between the complexation sites of the ligand, with three intermediate alicyclic carbon atoms in camphorate and cyclobutanetetracarboxylate (groups in 1 and 3 positions) and only two in (R-)chdc2−. As a consequence, the shortest U···U separations, of ∼6.0−6.9 Å in (c), are much larger in (a), at ∼9.2−9.8 Å. However, an even shorter U···U separation of ∼5.0−5.6 Å is observed with Kemp’s acid (b) as a result of the different geometry and the more pronounced outward displacement of the ligands. Luminescence Properties. The emission spectra of all complexes in the solid state were recorded at room temperature under excitation at a wavelength of 420 nm, a value suitable for excitation of the uranyl chromophore.56 Representative examples of the spectra obtained are given in Figure 10, and the others are shown in Figure S6 (Supporting Information). In all cases, the vibronic progression corresponding to the S11 → S00 and S10 → S0ν (ν = 0−4) electronic transitions57 is apparent, with four or five peaks being intense and well-resolved. It is well-known that the positions of the emission bands are dependent on the nature and number of donor atoms in the uranyl equatorial plane.58,59 Such displacement of the emission maxima is observed in the present series. Complex 1, in which

enantiomer separation). While there is no marked selectivity in regard to the ligand enantiomers bridging the edges of the tetrahedral clusters, there is the expected selectivity of ligand binding47 in that only four- and not seven-membered chelate rings are formed on UVI, though this is certainly not a universal observation in the complete present series of complexes. Although there is a general tendency for uranyl ion complexes with polycarboxylic acids or other ligands to crystallize as 1D or 2D coordination polymers close to planarity due to equatorial arrangement of the ligands, as exemplified by most of the complexes reported here, polytopic ligands with a suitable geometry are potentially able to form discrete, closed annular,7,45 cage-like,5−7,18,41−44 or nanotubular species.4,5,48−53 Even very simple ligands such as peroxides have proven suitable for cage formation.54,55 Up to now and considering only polycarboxylate ligands, discrete homometallic octanuclear cages have been obtained from the monoester derivative of the cis,trans epimer of Kemp’s triacid (including also peroxo bridges),6 Kemp’s triacid itself,5,7 (1R,3S)-(+)-camphoric acid,41,43 and p-carboxylatocalix[4]arene,42 while cages being part of a 3D framework have been obtained from trans,trans,trans-1,2,3,4-cyclobutanetetracarboxylic acid (for which two functional groups only are involved in the building of the cage).40 The three forms of homometallic, homoleptic cages known, including that reported here, are represented in Figure 9. The uranyl ion is in all cases a trigonal node, being chelated by three carboxylate groups pertaining to three ligands (or bound in a more irregular manner in the case of Kemp’s triacid). The dicarboxylate ligands camphorate and (R-)chdc2−, J


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Inorganic Chemistry

and those of the NMP-containing one being red-shifted by 2 nm with respect to the values for the THF derivative. Uranyl luminescence in the copper(II)-containing complexes 7 and 8 appears to be largely quenched, a quite common phenomenon probably due to d-block metal cations providing nonradiative relaxation pathways.61 The maxima positions for the cadmium(II)-containing complex 9, with five equatorial donors, are redshifted by ∼8 nm with respect to those for complex 4. Maxima positions identical to those in complex 9 have previously been observed in uranyl ion complexes with different polycarboxylate ligands and a pentagonal bipyramidal uranium environment. 20,62 Among the four complexes containing the tetranuclear uranyl cage motif, 10, 11, and 13 give nearly identical spectra, while luminescence in 12 is less intense and with maxima positions red-shifted by ∼3 nm. The maxima positions in 10, 11, and 13 are blue-shifted with respect to all the previous complexes in this series, with main maxima at 480−482, 501−503, 523−525, and 547−549 nm. Similar values have been found in other complexes with six equatorial carboxylate donors (tris-chelation),4,9,20−23,63,64 although exceptions can be found.65 Overall, there is a tendency, here as well as in previous work, for the complexes with uranyl ions chelated by three carboxylate groups to be associated with the most blue-shifted emission maxima positions. Replacement of one chelating carboxylate by a chelating N-donor may result in a red-shift of ∼5 nm, as observed here for complexes 5 and 6 and also for other complexes,23,63 but this shift is absent or reduced in some cases involving 1,10-phenanthroline.37 The average vibronic splitting energies for the S10 → S0ν transitions are in the range 825−865 cm−1 for the whole series of complexes 1−13, as usual for uranyl complexes with carboxylates.20,22,57,60,65,66

■

Figure 7. (top) View of complex 11. Displacement ellipsoids are drawn at the 30% probability level. Symmetry codes: i = 3/4 − y, x − 1/4, 3/4 − z; j = 3/2 − x, 1/2 − y, 1/2 − z; k = 5/4 − y, x − 1/4, z − 1/4; l = y + 1/4, 5/4 − x, z + 1/4; m = y + 1/4, 3/4 − x, 3/4 − z; n = 1 − x, 1/2 − y, z. (bottom) View of complex 13. Displacement ellipsoids are drawn at the 40% probability level. Symmetry codes: i = 1 − y, 1 − x, 3/2 − z; j = 1/2 − x, y − 1/2, 7/4 − z; k = 1/2 − x, y + 1/2, 7/4 − z. Carbon-bound hydrogen atoms are omitted in both views and the hydrogen bond is shown as a dashed line.

CONCLUSIONS Seven of the 13 complexes presently described involve the racemic form of the ligand and six its pure (1R,2R) enantiomer, but this does not reflect equal ease in isolation of species of identical composition. In their homometallic complexes built from uranyl and dicarboxylate only (1), or involving one coordinated solvent molecule (2−4), both forms of the ligand give 2D coordination polymers, but it is notable that the analogue of 1 was not obtained from the racemic ligand and, conversely, that of 2−4 from the enantiopure form, probably indicating significant differences in solubility. The homometallic complexes containing an additional 2,2′-bipyridine ligand (5 and 6) are isomorphous, with that obtained from the racemic ligand crystallizing as a racemic conglomerate; in this case, it appears that one enantiomer is incompatible with the presence of the other in the same lattice and this may well be true of 1 also. Another chirality-related effect is apparent in the heterometallic complexes containing additional copper(II) cations, since a 1D polymer is formed with racemic chdc2− (7) and a 2D network with R-chdc2− (8), these two arrangements being however closely related to one another. Given that (R-)chdc2− ligands bound within the coordination sphere of a given metal center are quite remote from one another, it seems unlikely that the differences in the lattices of 7 and 8 arise from differences in direct R,R-S,S and R,R-R,R interactions and thus that they are associated with more subtle aspects of the lattice structures; indeed, there are differences in the interactions of the 2,2′-bipyridine ligands with themselves and with oxygen atoms of two types, and possibly in the dispersive interactions of cyclohexyl entities as well. A complex

the uranyl cation has only four equatorial donors, displays main maxima at 501, 524, 548, and 575 nm, but the most intense peaks appear to be the superposition of at least two components. It may be noted that a recent study of luminescence from square-bipyramidal UVI complexes at low temperature has shown that the relatively broad peaks seen at room temperature result from the superposition of two distinct vibronic components.60 Nearly identical positions (499, 522, 546, and 573 nm) are found in the uranium−cadmium heterometallic complex 9, in which the uranyl cation has five equatorial ligands, showing that there is no simple relationship between the number of donors and maxima positions, even when a single ligand is considered. The closely related homometallic complexes 2−4, with coordinated solvent molecules and five equatorial donors, and complexes 5 and 6, with coordinated bipy and six equatorial donors, display nearly superimposable spectra, with main maxima at 486−492, 506− 514, 528−538, and 553−564 nm, the lowest and highest values of each pair corresponding to complexes 5 and 4, respectively. The slight position shifts in the 2−4 series probably reflect differences in the neutral oxygen donor strength, with the DMF-containing complex maxima being blue-shifted by 2 nm K


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Inorganic Chemistry

Figure 8. Comparison of the tetranuclear uranyl cages and the 3D frameworks in complexes 11 (left column) and 13 (right column). Uranium coordination polyhedra are colored yellow and silver and lead atoms are represented as blue or green spheres, respectively. Hydrogen atoms are omitted in all views.

involving cadmium(II) was obtained only with R-chdc2−, and it crystallizes as a 2D assembly displaying the unusual Cairo pentagonal tiling topology (9); that no complex was obtained from reaction with the racemic ligand may, as for complex 1, be explained by the subtle factor that for a given quantity of ligand, the concentration of a given enantiomer in the racemate solution is half that in the pure enantiomer solution. Four heterometallic complexes containing either sodium(I), silver(I) or lead(II) cations, and involving either chdc2− (10 and 11) or R-chdc2− (12 and 13) display the same motif of a tetranuclear, pseudotetrahedral [(UO2)4(chdc/R-chdc)6]4− cage, these units

being assembled into a 3D framework by the additional cations. While the differences in coordination between NaI, AgI, and PbII appear to be inconsequential in this series, the opposite is true when PbII is compared with CdII (in complex 9), when different stoichiometry results. It appears that, apart from crystallization in either centrosymmetric or Sohncke/chiral space groups, the effect of the chirality of the ligand in these last four complexes is unimportant. The tetranuclear cage is a common feature in the uranyl/chdc2− system, built as easily from the racemic as from the enantiopure ligand, and it provides an example of an alternative mode of construction of a L


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Inorganic Chemistry

Figure 10. Emission spectra of complexes 1, 2, 9, and 10 in the solid state. The excitation wavelength was 420 nm.

to include a guest such as ammonium ion and thus that photocatalytic reactions of a guest species must be excluded. Nevertheless, formation of cage compounds in uranyl− polycarboxylate systems appears to be more common than would have been thought considering the coordination preferences of this linear cation, although it is much less studied than that of cages based on d-block transition metal cations. Further exploration of the present tetrahedral species and its formation under varying conditions is presently under way.

Figure 9. Simplified views of the octanuclear cages obtained with (1R,3S)-(+)-camphoric acid or trans,trans,trans-1,2,3,4-cyclobutanetetracarboxylic acid (a), Kemp’s triacid or p-carboxylatocalix[4]arene (b) and tetranuclear cage formed with trans-1,2-cyclohexanedicarboxylic acid (c): yellow, uranium; red, oxygen; blue, carboxylate ligand.

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ASSOCIATED CONTENT

* Supporting Information

tetrahedral cage to that based on octahedral metal ions first described and developed by Raymond et al.67 and more recently extended by others.68 Integration of this somewhat globular species in an intricate network appears to be insensitive to its chirality. In terms of composition, the effect of chirality appears to be more marked in the simplest complexes containing the smallest number of different components, such as 1−4. The manner in which (R-)chdc2− binds to UVI clearly depends upon coligands and the absence or presence of a second metal cation, but interaction exclusively through dihapto coordination of the carboxylates results in the novel tetranuclear cage-like species that is conserved in the presence of several metal ions with rather different coordination preferences. The observation that emission from the complexes containing the tetranuclear unit is strong is interesting in that this emission occurs even in the presence of AgI, which is known, in other systems,23 to sometimes quench luminescence, although it is clear that the tetrahedral cavity is too small even

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02537. Views of the crystal structures of complexes 2, 3, 10, 12; spacefill views of the tetranuclear cage in complex 13; emission spectra of complexes 3−8 and 11−13 (PDF) X-ray crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.T.). *E-mail: harrowfi[email protected] (J.H.). ORCID

Pierre Thuéry: 0000-0003-1683-570X Notes

The authors declare no competing financial interest. M


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Inorganic Chemistry

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Coordination Polymers and Cage-Containing Frameworks in Uranyl

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