Uranyl−Organic Frameworks with 1,2,3,4-Butanetetracarboxylate and

The reaction of uranyl nitrate with 1,2,3,4-butanetetracarboxylic acid ... grid in which the BTC4− ligands act as rectangular nodes and the uranyl i...
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Uranyl-Organic Frameworks with 1,2,3,4-Butanetetracarboxylate and 1,2,3,4-Cyclobutanetetracarboxylate Ligands Pierre Thuéry*,† and Bernardo Masci‡ CEA/Saclay, DSM/IRAMIS/SCM (CNRS URA 331), Baˆt. 125, 91191 Gif-sur-YVette, France, and Dipartimento di Chimica, UniVersita` “La Sapienza”, Box 34, Roma 62, Piazzale Aldo Moro 5, 00185 Roma, Italy

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3430–3436

ReceiVed April 15, 2008

ABSTRACT: The reaction of uranyl nitrate with 1,2,3,4-butanetetracarboxylic acid (H4BTC) and 1,2,3,4-cyclobutanetetracarboxylic acid (H4CBTC) under hydrothermal conditions gives various two- and three-dimensional frameworks. The complex [(UO2)2(BTC)(H2O)4] · 4H2O (1) is a (4,4) grid in which the BTC4- ligands act as rectangular nodes and the uranyl ions as divergent, side-defining nodes. Complexes [(UO2)2(CBTC)(H2O)2] · 2H2O (2) and [(UO2)2(CBTC)(H2O)2] · H2O (3), with the ligand in the cis,trans,cis form, present two types of three-dimensional architectures, with narrow channels formed in complex 2 only. In complex 4, [H3O]2[(UO2)5(CBTC)3(H2O)6], the ligand is in the noncentrosymmetric trans,trans,trans form, which assumes a saddle shape. This peculiar geometry of the ligand results in the formation of two types of subunits: 4:4 (metal/ligand) metallacycles and 8:12 cubic boxes, which are connected to one another to form a cubic lattice containing large channels. This latter result shows that the same geometric considerations which have permitted the synthesis of uranyl-based molecular metallamacrocycles and boxes can be used for the design of porous three-dimensional frameworks based on analogous motifs. Introduction Polycarboxylates are ligands of choice for the design of uranyl-organic frameworks (UOFs)1 or molecular assemblies2 due to the high affinity of uranyl ions for oxygen donors and the almost inexhaustible variety of the resulting architectures, generally obtained under hydrothermal conditions in the case of UOFs.3 Following previous work on tetracarboxylates such as benzene-1,2,4,5-tetracarboxylate4 and O,N- or O,N,O-donors such as pyridine carboxylate derivatives,5 we have investigated recently the ligand pyrazinetetracarboxylate (PZTC), which unites the properties of both and presents a remarkable array of 10 donor atoms,6 as well as its relatives pyrazine-2,6- and -2,3-dicarboxylates.6,7 The uranyl and the mixed uranyl/alkali metal ions complexes obtained with pyrazinetetracarboxylic acid display various, two- or three-dimensional architectures comprising [UO2(H2PZTC)2]2- or linear [UO2(PZTC)]n2n- subunits, which are further assembled into higher dimensionality frameworks by bridging uranyl or alkali metal ions. However, large channels do not form in these compounds, likely as a result of the ready formation of the quite planar subunits indicated above, which is quite unsurprising, given both the planar coordination environment required by the uranyl ion and the rigid and essentially planar shape of the ligand. As previously noticed for the synthesis of uranyl-based metallamacrocycles,2 the association of uranyl to curved ligands may open the way to the formation of cavity- or, in the case of extended networks, channel-defining assemblages. Indeed, this is observed in the case of the metallacycles comprising the ligand (2R,3R,4S,5S)tetrahydrofurantetracarboxylate2b or in that of the channels formed with (1R,3S)-(+)-camphorate3e (another strategy to obtain large channels consists in using linear ditopic ligands of large size such as adipate and 4,4′-dipyridyl, to generate gridlike assemblages3f). Among the tetracarboxylic acids, both 1,2,3,4-cyclobutanetetracarboxylic acid and its acyclic counterpart 1,2,3,4-butanetetracarboxylic acid (noted H4CBTC and * To whom correspondence should be addressed. E-mail: [email protected]. † CEA/Saclay. ‡ Universita` di Roma.

H4BTC hereafter, respectively) appear to be promising candidates for the synthesis of channel-defining UOFs, due to their divergent coordination sites and potential high-denticity. An investigation of the complexing properties of these ligands toward uranyl ions is all the more interesting that very few crystal structures of their complexes are known, these being reduced to the lanthanum8 and cadmium9 complexes of H4-xCBTCx- (x ) 3 or 4) and 3d metal ions complexes of BTC4-.10 Experimental Section Synthesis. Caution! With uranium being a radioactive and chemically toxic element, uranium-containing samples must be handled with suitable care and protection. UO2(NO3)2 · 6H2O was purchased from Prolabo and 1,2,3,4-butanetetracarboxylic (H4BTC) and 1,2,3,4-cyclobutanetetracarboxylic (H4CBTC) acids from Aldrich. 1H and 13C NMR spectra were recorded at 298 K on a Bruker AC 200 spectrometer. Elemental analyses were performed by Analytische Laboratorien GmbH at Lindlar, Germany. Conformation of H4CBTC. The presence of only one isomer in the commercial sample was checked through NMR spectra. 1H NMR showed one singlet peak, at δ 3.81 in D2O and δ 3.30 in D2O and excess KOH. 13C NMR spectra showed two peaks, at δ 41.6 and 175.7 in D2O, and at δ 45.7 and 182.4 in D2O and excess KOH. Recrystallization in water gave single crystals of the cis,trans,cis isomer, with unit cell parameters analogous to those reported in the Cambridge Structural Database (CSD, version 5.28)11,12 [monoclinic, space group P21/c, a ) 5.3783(6), b ) 12.2866(8), c ) 6.4794(8) Å, β ) 93.940(6)° at 100(2) K]. [(UO2)2(BTC)(H2O)4] · 4H2O (1). H4BTC (38 mg, 0.162 mmol), UO2(NO3)2 · 6H2O (326 mg, 0.649 mmol), NaOH (13 mg, 0.325 mmol) and demineralized water (2 mL) were placed in a 20 mL tightly closed vessel and heated at 180 °C under autogenous pressure (ca. 1.1 MPa). Light yellow crystals of complex 1 were obtained within eight days. The product was recovered after filtration and washed with water (38 mg, 26% yield on the basis of the acid). Anal. Calcd for C8H22O20U2: C, 10.51; H, 2.43. Found: C, 11.04; H, 2.48. Compound 1 was also obtained when NaOH was replaced by a 4-fold excess of NMe4OH or NEt4Cl and even in the absence of a base. [(UO2)2(CBTC)(H2O)2] · 2H2O (2). H4CBTC (18 mg, 0.078 mmol), UO2(NO3)2 · 6H2O (156 mg, 0.311 mmol), LiOH · H2O (15 mg, 0.357 mmol) and demineralized water (2.5 mL) were placed in a 20 mL tightly closed vessel and heated at 180 °C under autogenous pressure (ca. 1.1

10.1021/cg800389g CCC: $40.75  2008 American Chemical Society Published on Web 08/01/2008

Uranyl-Organic Frameworks MPa). Light yellow crystals of complex 2 were obtained within 24 h (29 mg, 44% yield on the basis of the acid). Anal. Calcd for C8H12O16U2: C, 11.44; H, 1.44. Found: C, 11.80; H, 1.66. Compound 2 was also obtained when LiOH was replaced by a 4-fold excess of NaOH. [(UO2)2(CBTC)(H2O)2] · H2O (3). H4CBTC (18 mg, 0.078 mmol), UO2(NO3)2 · 6H2O (156 mg, 0.311 mmol), NMe4OH (28 mg, 0.308 mmol) and demineralized water (1 mL) were placed in a 20 mL tightly closed vessel and heated at 175 °C under autogenous pressure (ca. 1.1 MPa). Few light yellow crystals of complex 3 were obtained within five days. [H3O]2[(UO2)5(CBTC)3(H2O)6] (4). H4CBTC (18 mg, 0.078 mmol), UO2(NO3)2 · 6H2O (156 mg, 0.311 mmol), NaOH (13 mg, 0.325 mmol), 15-crown-5 (68 mg, 0.309 mmol) and demineralized water (1.2 mL) were placed in a 20 mL tightly closed vessel and heated at 180 °C under autogenous pressure (ca. 1.1 MPa). Very small, light yellow cubic crystals of complex 4, mixed with a yellow powder which was not further characterized, were obtained within five days. Chemical analysis of the bulk was unsatisfactory, indicating that the composition of the yellow powder differs from that of the crystals. Crystallography. The data were collected at 100(2) K on a Nonius Kappa CCD area detector diffractometer13 using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The crystals were introduced in glass capillaries with a protecting “Paratone-N” oil (Hampton Research) coating. The unit cell parameters were determined from 10 frames and then refined on all data. The data (combinations of φ- and ω-scans giving complete data sets up to θ ) 25.7° and a minimum redundancy of 4 for 90% of the reflections) were processed with HKL2000.14 The structures were solved by direct methods with SHELXS-97, expanded by subsequent Fourier difference synthesis, and refined by full-matrix least-squares on F2 with SHELXL-97.15 Absorption effects were corrected empirically with the program SCALEPACK.14 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bound to oxygen atoms were found on Fourier difference maps for complexes 1 and 2 and partly for 3 (not found for the disordered water molecule), but not for compound 4; the carbon-bound hydrogen atoms were introduced at calculated positions. All hydrogen atoms were treated as riding atoms with an isotropic displacement parameter equal to 1.2 times that of the parent atom. In compound 4, the uranyl group corresponding to U2 and the oxygen atoms bound to it are disordered over two positions sharing the metal atom, which have been refined with occupancy parameters equal to 0.5 (value imposed by the symmetry). The hydronium ion (O8) has been given an occupancy parameter of 0.16667 both in order to retain an acceptable displacement factor and for charge equilibrium. Large voids in the lattice indicate the presence of unresolved water solvent molecules, probably highly disordered. Crystal data and structure refinement parameters are given in Table 1 and selected bond lengths and angles are in Table 2. The molecular plots were drawn with SHELXTL15 and Balls & Sticks.16

Results and Discussion 4-

A. The BTC Complex 1. Reaction of H4BTC with uranyl nitrate under mild hydrothermal conditions, either without base or with bases such as NaOH or NMe4OH, gives complex 1, which is represented in Figure 1. The asymmetric unit comprises half a fully deprotonated tetracarboxylate ligand, two uranyl ions with the metal atoms located on symmetry centers, two coordinated and two free water molecules. Each of the two carboxylate groups in the asymmetric unit chelates an uranium atom, with an average U-O bond length of 2.487(12) Å, in agreement with the average value of 2.47(4) Å for the analogous structures reported in the CSD. Two water molecules in trans positions complete the uranium coordination sphere to give the hexagonal bipyramidal environment usual with small-bite chelating ligands. The average U-O(water) bond length, 2.451(1) Å, is unexceptional [average from the CSD 2.44(4) Å]. The mean equatorial planes of U1 and U2 are defined with root-mean-squares (rms) deviations of 0.10 and 0.13 Å, respectively, and they make a dihedral angle of 68.5(2)°. The ligand skeleton, that is, the six atoms C1, C2, C3 and their

Crystal Growth & Design, Vol. 8, No. 9, 2008 3431 Table 1. Crystal Data and Structure Refinement Details 1 empirical formula M (g mol-1) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (Mo KR) (mm-1) F(000) reflns collcd indep reflns obsd reflns [I > 2σ(I)] Rint params refined R1 wR2 S ∆Fmin (e Å-3) ∆Fmax (e Å-3)

2

3

4

C8H22O20U2 C8H12O16U2 C8H10O15U2 C24H30O42U5 914.32 triclinic P1j 7.7019(8) 7.8068(10) 9.4519(9) 77.524(8) 66.970(7) 79.762(7) 507.92(10) 1 2.989 16.018

840.24 triclinic P1j 6.6059(3) 9.8638(8) 13.1870(11) 79.317(4) 86.349(5) 83.794(5) 838.58(10) 2 3.328 19.371

822.22 monoclinic P21/n 6.5096(8) 9.2854(9) 12.1915(17) 90 92.997(7) 90 735.90(15) 2 3.711 22.064

2180.63 cubic Pm3jm 22.5710(8) 22.5710(8) 22.5710(8) 90 90 90 11 498.8(7) 4 1.260 7.067

414 23 634 1922 1453

744 37 832 3165 2894

724 32 004 1390 1156

3880 273 284 2170 1638

0.067 139 0.037 0.098 1.161 -1.33 1.76

0.035 235 0.030 0.072 1.079 -2.27 3.15

0.093 118 0.039 0.099 1.050 -1.55 1.99

0.042 95 0.044 0.125 1.052 -1.08 2.50

symmetry equivalents, is nearly planar (rms deviation 0.02 Å) and the atoms O3 and O4 are very near this plane, with displacements of 0.10(2) and -0.20(3) Å, respectively; this part of the ligand can thus be seen as an extended, planar ditopic linker. In contrast, the two central carboxylic groups are located on either side and oriented so as to make a dihedral angle of 82.1(9)° with the main plane, and they can be seen as forming together a second divergent ditopic linker subunit, with a chain length shorter than the previous one. This conformation of BTC4- is close to that in its tetraammonium salt, described as a centrosymmetric trans form with the six-carbon atoms chain in flat herringbone conformation (rms deviation of the plane lower than 0.015 Å).17 The main difference concerns the terminal carboxylate groups of the six-carbon chain, which are more tilted with respect to the chain plane in the ammonium salt (dihedral angles of 62.8 and 55.5° for the two independent molecules), whereas, as previously indicated, they are very close to the plane in 1 [dihedral angle 8(2)°]; in contrast, the other two carboxylate groups assume similar conformations in both cases (dihedral angles 84.5 and 86.0° in the ammonium salt). Each BTC4- molecule in 1 is bound to four uranyl ions forming a planar parallelogram; the longest BTC4- axis is directed along the [1 1 0] direction and it corresponds to an U1 · · · U1 separation of 11.9012(11) Å, whereas the shortest axis is directed along c and is associated to a smaller U2 · · · U2 separation of 9.4519(9) Å (equal to the c axis). The resulting assemblage is a regular (4,4) grid parallel to the (1 1j 0) plane, in which the two rows of uranyl ions are linked by BTC4rectangular nodes (the uranyl ions can be seen as divergent, side-defining 2-fold nodes). These grids are packed so that channels parallel to the a axis are formed, with a section of approximately 5 × 5 Å (about 2 × 2 Å if van der Waals radii are taken into account) and a resulting packing index of 0.63 (estimation with PLATON18); these channels are occupied by the solvent water molecules. An extended hydrogen bonding network links the water molecules, both coordinated and free, the carboxylate groups and one uranyl oxo atom to give a threedimensional assemblage. It is interesting to compare this

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Table 2. Environment of the Uranium Atoms in Compounds 1-4: Selected Bond Lengths (Å) and Angles (deg) 1 U1-O1 U1-O3 U1-O4 U1-O7 U2-O2 U2-O5 U2-O6 U2-O8

1.762(7) 2.483(8) 2.502(7) 2.452(7) 1.763(8) 2.471(7) 2.493(7) 2.450(7)

O1-U1-O1′ O3-U1-O4 O4-U1-O7 O7-U1-O3′ O2-U2-O2′′ O5-U2-O6 O6-U2-O8 O8-U2-O5′′

180 51.6(2) 65.5(2) 63.4(2) 180 52.0(2) 64.6(2) 64.4(2)

U1-O1 U1-O2 U1-O5 U1-O7 U1-O9 U1-O13 U1-O14 U2-O3 U2-O4 U2-O10 U2-O11 U2-O6′ U2-O8′′ U2-O12#

1.753(6) 1.770(6) 2.353(6) 2.349(6) 2.378(6) 2.501(6) 2.399(6) 1.773(6) 1.749(6) 2.423(5) 2.399(5) 2.377(5) 2.409(5) 2.355(5)

O1-U1-O2 O5-U1-O7 O7-U1-O13 O13-U1-O9 O9-U1-O14 O14-U1-O5

177.3(3) 72.7(2) 66.69(19) 73.0(2) 73.4(2) 74.8(2)

O3-U2-O4 O10-U2-O11 O11-U2-O8′′ O8′′-U2-O6′ O6′-U2-O12# O12#-U2-O10

177.3(3) 72.36(18) 72.70(19) 69.3(2) 74.1(2) 70.88(18)

U-O1 U-O2 U-O3 U-O5 U-O4′ U-O6′′ U-O7

1.763(8) 1.742(8) 2.375(7) 2.403(9) 2.361(7) 2.388(8) 2.453(7)

O1-U-O2 O3-U-O5 O5-U-O4′ O4′-U-O7 O7-U-O6′′ O6′′-U-O3

U1-O1 U1-O2 U1-O5 U2-O3 U2-O4 U2-O6A U2-O6B U2-O7A U2-O7B

1.774(12) 1.773(13) 2.460(4) 1.805(7) 1.723(8) 2.492(13) 2.417(16) 2.543(14) 2.392(17)

O1-U1-O2 O5-U1-O5′ O5′-U1-O5′′ O3-U2-O4 O6A-U2-O6B% O6B%-U2-O7B$ O7A-U2-O6A

2

3 179.7(4) 71.5(3) 70.0(3) 71.6(3) 71.9(2) 75.3(3)

4 180 52.4(2) 67.6(2) 174.1(8) 51.4(3) 65.2(2) 63.81(19)

Symmetry codes: 1: ′ ) 1 - x, 2 - y, -z; ′′ ) -x, 1 - y, 1 - z. 2: ′ ) x - 1, 1 + y, z; ′′ ) x, 1 + y, z; # ) x - 1, y, z. 3: ′ ) 1 + x, y, z; ′′ ) 1/2 - x, y - 1/2, 1/2 - z. 4: ′ ) z, y, x; ′′ ) z, x, y; % ) x, -y, z. $ ) 1 - x, -y, z.

structure with those of the BTC4- complexes with Mn(II), Co(II) and Ni(II), which are the only complexes reported with this ligand.10 Whereas all carboxylate groups are chelating in 1 (and none is bridging), they are either monodentate or bridging (synsyn bis-monodentate) with the 3d metal ions and two or four oxygen atoms are left uncoordinated in some ligands. The polymeric network in the latter case is nonetheless also described as a (4,4) grid, which includes bis(µ-carboxylate)-µ-aqua-bridged dinuclear subunits, but each square or rectangular unit in the grid is built from two metal centers and two ligands, instead of four of each in 1. Both the 3d ions and BTC4- ligands are thus 4-fold nodes, whereas, in 1, the ligands are 4-fold, but the uranyl ions 2-fold connectors. All these results with metal ions very different in their coordination preferences show that BTC4- in its planar conformation (and no other conformation was ever characterized) has a strong propensity to build two-dimensional frameworks as a 4-fold node, which may form extended channels, as in the present case. B. The cis,trans,cis-CBTC4- Complexes 2 and 3. These complexes both involve the ligand in its original, cis,trans,cis conformation (see Experimental Section). Complex 2 was

Figure 1. Top: View of complex 1. Solvent water molecules have been omitted. Hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: ′ ) 1 - x, 2 - y, -z; ′′ ) -x, 1 - y, 1 - z; # ) -x, 1 - y, -z. Bottom: View of the assemblage down the a axis with the hydrogen atoms and water solvent molecules omitted. The uranium coordination polyhedra are represented and the other atoms are shown as spheres of arbitrary radii.

readily obtained in the presence of either LiOH or NaOH, whereas complex 3 was obtained in very small yield in the presence of NMe4OH. Although their overall chemical formulas differ by only one solvent water molecule, their crystal structures are quite different, which evidence the high sensitivity of the assemblages formed to rather small variations in experimental conditions. In both 2 and 3, the centrosymmetric ligand CBTC4is bound to six metal atoms, two in chelating fashion through two monodentate carboxylate groups in cis position and the other four through the four monodentate carboxylate groups (Scheme 1). In complex 2 (Figure 2), the asymmetric unit comprises two independent uranyl ions in different environments and two CBTC4- halves located on inversion centers; the first uranyl ion (containing U1) is chelated by atoms O5 and O7 from two cis-carboxylate groups of one ligand and bound to the atom O9 of the second ligand and to two trans coordinated water molecules (O13 and O14) located between the two CBTC4ligands, which gives the usual pentagonal bipyramidal uranium environment. Atom U2 is in an environment of similar geometry, but it is chelated by the second ligand (O10 and O11) and further bound to three carboxylate oxygen atoms from three different ligands (O6, O8 and O12). The average U-O(carboxylate) bond length (including both uranium atoms) of 2.38(3) Å and the average U1-O(water) bond length of 2.45(5) Å are unexcep-

Uranyl-Organic Frameworks

Crystal Growth & Design, Vol. 8, No. 9, 2008 3433

Scheme 1. Coordination Mode of CBTC4- in Compounds 2 and 3 (cis,trans,cis Conformation), and 4 (trans,trans,trans Conformation)

tional. The mean equatorial planes of the two cations (rms deviations 0.103 and 0.030 Å, respectively) make a dihedral angle of 79.05(10)°. All the carboxylate groups are bound to two metal atoms with a syn-anti conformation. Both cyclobutane rings are planar, as in the free form of H4CBTC,12 but not perfectly square since the C-C bonds between atoms bearing cis-carboxylate groups are slightly larger than the others, with average bond lengths of 1.571(5) and 1.542(17) Å, respectively, and the intracyclic C-C-C angles are in the range 89.5(6)90.5(6)°. The same was observed in cis,trans,cis-cyclobutanetetracarboxylic acid tetramethyl ester.19 A search of the CSD shows that the cis,trans,cis-cyclobutanetetracarboxylic acid derivatives can be either planar or puckered, whereas the trans,trans,trans form is puckered; this is in agreement with the rule stating that noncentrosymmetrically substituted rings are puckered whereas centrosymmetrically substituted ones may be planar or puckered.19 The bonding of each ligand in 2 to six metal ions and, conversely, of the metal atoms to two (U1) or four (U2) CBTC4ligands, results in the formation of a three-dimensional framework in which channels of approximate dimension 6.8 × 4.5 Å (about 3.8 × 1.5 Å when van der Waals radii are subtracted) run down the a axis. The packing index of 0.66 is comparable to that in complex 1. These channels are occupied by the solvent water molecules which are hydrogen bonded to one another, to one coordinated water molecule and one carboxylate group; other hydrogen bonds are formed between the coordinated water molecules and oxo (uranyl) and carboxylate groups. The simplest subunit which can be discerned in the assemblage comprises the centrosymmetric chelate binuclear moiety [(UO2)2(CBTC)]. Two such subunits are present, corresponding to U1 and U2, which are differently connected to their neighbors. Atom U1 is a simple node between two different ligands and its associated, chelating ligand is further bound to four atoms of the U2 family whereas atom U2 links four subunits (two of each kind) and the associated ligand is further bound to two atoms in both the U1 and U2 families. The asymmetric unit in complex 3 comprises one uranyl ion, half a CBTC4- ligand, one coordinated and half a solvent water molecules. As in complex 2, the cation is chelated by the ciscarboxylate groups and it is further bound to two carboxylate

Figure 2. Top: View of complex 2. Hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: ′ ) x - 1, 1 + y, z; ′′ ) x, 1 + y, z; # ) x 1, y, z; * ) 1 - x, 1 - y, 1 - z; $ ) 1 - x, 2 - y, 2 - z. Middle and bottom: Views of the assemblage down the a and b axes with the hydrogen atoms and water solvent molecules omitted. The uranium coordination polyhedra are represented and the other atoms are shown as spheres of arbitrary radii.

groups from different molecules, separated in the equatorial plane by the coordinated water molecule (Figure 3). The average U-O(carboxylate) bond length of 2.382(16) Å and the U-O(water) bond length of 2.453(7) Å, as well as the pentagonal bipyramidal environment geometry, are unremarkable. The cyclobutane ring is planar, but not perfectly square, as in 2 [C-C bond lengths of 1.573(16) and 1.524(15) Å for atoms bearing cis and trans carboxylate groups, respectively, and angles of 91.4(9) and 88.6(9)° around C2 and C3, respectively]. All carboxylate groups are bound to two metal atoms in the syn-anti mode. The resulting three-dimensional arrangement in complex 3 is more compact than that in 2, with a packing index of 0.76, and no channel of significant size is present. C. The trans,trans,trans-CBTC4- Complex 4. This complex was obtained in an experiment involving NaOH and 15-crown5, which was aimed at possibly include a sodium/crown ether complex in the uranium-organic framework. In fact, none of

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Figure 3. Top: View of complex 3. Displacement ellipsoids are drawn at the 30% probability level. Symmetry codes: ′ ) 1 + x, y, z; ′′ ) 1/2 - x, y - 1/2, 1/2 - z; # ) -x, 1 - y, -z. Bottom: View of the assemblage down the a axis with the hydrogen atoms and water solvent molecules omitted. The uranium coordination polyhedra are represented and the other atoms are shown as spheres of arbitrary radii.

these species is present in the final product, but at least part of the CBTC ligand appears to have undergone isomerization into the trans,trans,trans form (little crystalline material was however obtained, see Experimental Section). A possible mechanism for this transformation involves the formation of small amounts of carbanions through acid/base equilibria and their pyramidal inversion. Such an isomerization of CBTC in the presence of a base under hydrothermal conditions has previously been reported, but the initial form does not seem to have been checked in this case.8 Complex 4 crystallizes in the cubic space group Pm3jm with two independent uranium atoms located on sites of symmetry 3m (U1) and mm2 (U2) and a CBTC4- ligand with its center located on an mm2 symmetry site (Figure 4). As in complex 1, all carboxylate groups are chelating (Scheme 1). Atom U1 is bound to three chelating carboxylate groups pertaining to three ligands related to one another by the ternary axis and it is thus in an hexagonal bipyramidal environment. Atom U2 has an environment of similar geometry, but it is bound to only two chelating carboxylate groups in trans relative positions and to two water molecules [average U-O(water) bond length 2.47(8) Å]. The carboxylate oxygen atoms, oxo groups and coordinated water molecules bound to U2 are disordered over two positions related by a mirror plane (see Experimental Section) and the coordination polyhedron of U2 thus assumes two positions rotated with respect to one another by 46.1(2)° along the line defined by the carbon atoms of the carboxylate groups. The

Figure 4. Top: View of complex 4. Only one position of the disordered environment of U2 is represented. Displacement ellipsoids are drawn at the 30% probability level. Symmetry codes: ′ ) z, y, x; ′′ ) z, x, y; ′′′ ) x, z, y; ′′′′ ) y, z, x; # ) y, x, z; % ) x, -y, z; * ) 1 - x, y, z; $ ) 1 - x, -y, z; ^ ) z, -y, x. Bottom: View of the assemblage in the ab plane with the counterions and hydrogen atoms omitted. The uranium coordination polyhedra are represented in yellow (U1) or green (U2, only one position represented) and the other atoms are shown as spheres of arbitrary radii.

average U-O bond length of 2.46(3) Å is in agreement with the values in complex 1 and the CSD (the large esd for this experimental value arises from the lack of precision for bond lengths in the disordered part). The mean equatorial planes of U1 and U2 make a dihedral angle of 40.8(2)°. The cyclobutane ring is in a puckered conformation, as expected for the noncentrosymmetric trans,trans,trans form,19 with a displacement of the carbon atoms out of the mean plane by ( 0.122(7) Å and all the carboxylate groups in equatorial position. This conformation confers a double curvature to the CBTC4- ligand, with the carboxylate groups pointing on the same side of the mean plane defining a ditopic subunit with an angle of 109.5(7)° (angle between the two COO planes not affected by disorder). The whole ligand can thus be described as saddle-shaped, which, considering the angle not too far from 90°, is a quite perfect geometry for the design of contiguous and perpendicular uranyl-containing metallamacrocycles con-

Uranyl-Organic Frameworks

Figure 5. Two views of the cubic box subunit in compound 4. The carboxylate groups not involved in the box formation have been omitted for clarity. The bottom view is down the [1 1 1] direction.

taining four uranyl ions and four ligands or of box-shaped cages containing eight metal atoms.2 Indeed, this is what happens since the three-dimensional framework in 4 is an assemblage of such boxes and metallacycles at right angles to one another. As can be seen in the projection of the structure on the ab plane represented in Figure 4, each of the two atoms U1 and U2 is involved with its symmetry equivalents in the formation of 4:4 metallacycles. The situation is more simple for U2, which is bound to two CBTC4- ligands in trans positions in the uranyl equatorial plane, and thus forms simple cycles with uranyldefined sides and ligand-defined corners, which are further connected to eight U1 atoms through their four ligands; each U2 atom is thus part of only one cycle. In contrast, atom U1, being bound to three ligands, is comprised in three orthogonal metallacycles which define an octanuclear cubic box subunit (Figure 5). The whole assembly is thus built from U1-containing boxes connected to one another by U2-containing metallacycles. Each metallacycle is bound to 4 boxes (one at each corner) and each box is bound to 12 metallacycles corresponding to the 12 ligands. Complex 4 thus unites in a single framework two motifs

Crystal Growth & Design, Vol. 8, No. 9, 2008 3435

which had previously been obtained separately as molecular uranyl polynuclear complexes involving di- or tetracarboxylates2 or bis-catechol20 ligands. Channels are present along the three directions of the cubic lattice; those centered on the [x 0 0] axes correspond to the main axes of the cubic subunits and those along the [x 1/2 1/2] axes to the metallacycles, with dimensions of about 7 × 7 and 9 × 9 Å (about 4 × 4 and 6 × 6 Å after subtraction of the van der Waals radii), respectively. Each channel is connected to those of its own family to give rise to a continuous porous structure, with a global packing index of about 30%. The size of these channels is comparable to the cavity size in the molecular box based on a monoester derivative of the cis,trans epimer of Kemp’s triacid and µ-peroxo ligands [10.8 × 7.6 Å (about 7.8 × 4.6 Å after subtraction of the van der Waals radii)],2a and the tetranuclear metallacycle built from (2R,3R,4S,5S)-tetrahydrofurantetracarboxylate (THFTA) [7 × 7 Å (about 4 × 4 Å after subtraction of the van der Waals radii)].2b The U · · · U separations between successive atoms bound to the same ligand in the cycles are also quite similar [8.7 and 7.4 Å in the latter two compounds, respectively, and 9.2 (U1) or 8.6 (U2) Å in 4], which is due to the ligands having comparable dimensions. It should be noted however that the carboxylate coordination mode is not the same in all cases, since chelation by each group is found in the Kemp’s acid derivative and in the present case, whereas chelation by adjacent monodentate groups is present in the THFTA complex. The bending angles of the ligands (angles between the divergent complexing groups) are 83.5 and 97.4° in the previous cases and 109.5° in 4, which shows that deviations as large as 20° from the ideal, right angle geometry, are still compatible with the formation of cyclic species, owing to the possibility of small rotations of the ligand out of the uranyl equatorial plane, as exemplified most clearly by the trinuclear metallacycle which can also be obtained with THFTA.2b The large voids in 4 are occupied by the counterions and likely also by numerous solvent molecules which could not be located properly and which, considering the available space, could amount to 3-4 water molecules per asymmetric unit (the counterion was assumed to be a hydronium ion, which is possible considering the acidic medium in which the crystals were grown; however, the alternative presence of a disordered proton on the carboxylate groups cannot be ruled out). Conclusions The first crystal structures of uranyl complexes (and more generally of 5f element complexes) with the ligands BTC4and CBTC4- reported herein evidence the interest of these polycarboxylates for the synthesis of UOFs. In all the cases, all the eight oxygen atoms are coordinated, the ligand being thus bound to four (1 and 4) or six (2 and 3) metal atoms, depending on the bonding mode, with each carboxylate chelating one uranyl ion in 1 and 4 and chelation by proximal carboxylates associated to monodentate coordination in 2 and 3. These coordination modes are different from those in the other two complexes reported with CBTC in the trans,trans,trans form and the more isotropic La(III)8 or Cd(II)9 ions, in which bidentate, bridging oxygen atoms are present. It may be noted that complexes 2 and 3 are the first to be structurally characterized which involve cis,trans,cis-CBTC4-. A two-dimensional assemblage is generated with BTC4-, which is a regular (4,4) grid with BTC4- rectangular nodes (complex 1), whereas threedimensional frameworks (2 and 3) are obtained with cis,trans,cis-CBTC4-. Narrow channels only are present in 1 and 2, while 3 is a quite compact architecture with no significant

3436 Crystal Growth & Design, Vol. 8, No. 9, 2008

void. The more interesting compound in terms of porosity has been obtained accidentally with the CBTC4- ligand in the trans,trans,trans form; complex 4 is a three-dimensional framework of cubic symmetry with a network of large channels. Two kinds of subunits can be discerned in 4: metallacycles built from 4 metal atoms and 4 ligands and cubic boxes built from 8 metal atoms and 12 ligands, with each box being connected to the neighboring ones by metallacycles. This result shows that polycarboxylate ligands with the proper angle between the complexing groups, which had previously been used to synthesize uranyl-based molecular cages or metallamacrocycles, can also be of interest for the hydrothermal synthesis of highly porous frameworks based on analogous motifs. Supporting Information Available: Tables of crystal data, atomic positions and displacement parameters, anisotropic displacement parameters, and bond lengths and bond angles in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

Thuéry and Masci

(4)

(5)

(6) (7) (8) (9) (10)

References (1) Cahill, C. L.; de Lill, D. T.; Frisch, M. CrystEngComm 2007, 9, 15. (2) (a) Thuéry, P.; Nierlich, M.; Baldwin, B. W.; Komatsuzaki, N.; Hirose, T. J. Chem. Soc., Dalton Trans. 1999, 1047. (b) Thuéry, P.; Villiers, C.; Jaud, J.; Ephritikhine, M.; Masci, B. J. Am. Chem. Soc. 2004, 126, 6838. (3) See, for example: (a) Kim, J. Y.; Norquist, A. J.; O’Hare, D. Dalton Trans. 2003, 2813. (b) Borkowski, L. A.; Cahill, C. L. Inorg. Chem. 2003, 42, 7041. (c) Frisch, M.; Cahill, C. L. Dalton Trans. 2005, 1518. (d) Thuéry, P. Chem. Commun. 2006, 853. (e) Thuéry, P. Eur. J. Inorg. Chem. 2006, 3646. (f) Borkowski, L. A.; Cahill, C. L. Cryst. Growth Des. 2006, 6, 2241. and 2248. (g) Go, Y. B.; Wang, X.; Jacobson, A. J. Inorg. Chem. 2007, 46, 6594. (h) Frisch, M.; Cahill, C. L. J. Solid State Chem. 2007, 180, 2597. (i) Knope, K. E.; Cahill, C. L. Inorg. Chem. 2007, 46, 6607. (j) Thuéry, P. Polyhedron 2007, 26, 101. (k) Thuéry, P. Inorg. Chem. 2007, 46, 2307. (l) Thuéry, P.

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CrystEngComm 2007, 9, 358. (m) Thuéry, P. CrystEngComm 2008, 10, 79. (a) Yu, Z. T.; Liao, Z. L.; Jiang, Y. S.; Li, G. H.; Chen, J. S. Chem. Eur. J. 2005, 11, 2642. (b) Jiang, Y. S.; Yu, Z. T.; Liao, Z. L.; Li, G. H.; Chen, J. S. Polyhedron 2006, 25, 1359. See, for example: (a) Immirzi, A.; Bombieri, G.; Degetto, S.; Marangoni, G. Acta Crystallogr., Sect. B 1975, 31, 1023. (b) Chen, W.; Yuan, H. M.; Wang, J. Y.; Liu, Z. Y.; Xu, J. J.; Yang, M.; Chen, J. S. J. Am. Chem. Soc. 2003, 125, 9266. (c) Masci, B.; Thuéry, P. Polyhedron 2005, 24, 229. (d) Zheng, Y. Z.; Tong, M. L.; Chen, X. M. Eur. J. Inorg. Chem. 2005, 4109. (e) Harrowfield, J. M.; Lugan, N.; Shahverdizadeh, G. H.; Soudi, A. A.; Thuéry, P. Eur. J. Inorg. Chem. 2006, 389. (f) Jiang, Y. S.; Li, G. H.; Tian, Y.; Liao, Z. L.; Chen, J. S. Inorg. Chem. Commun. 2006, 9, 595. (g) Frisch, M.; Cahill, C. L. Dalton Trans. 2006, 4679. (h) Thuéry, P. Acta Crystallogr., Sect. C 2008, 64, m50. Masci, B.; Thuéry, P. Cryst. Growth Des. 2008, 8, 1689. Masci, B.; Thuéry, P. CrystEngComm, in press. Kim, Y. J.; Jung, D. Y. Inorg. Chim. Acta 2002, 338, 229. Luo, J.; Jiang, F.; Wang, R.; Hong, M. Inorg. Chem. Commun. 2004, 7, 638. Can˜adillas-Delgado, L.; Fabelo, O.; Pasán, J.; Delgado, F. S.; Lloret, F.; Julve, M.; Ruiz-Pérez, C. Inorg. Chem. 2007, 46, 7458. While this paper was in press, another report on 3d-group metal complexes of BTC appeared: Liu, Y. Y.; Ma, J. F.; Yang, J.; Ma, J. C.; Su, Z. M. CrystEngComm 2008, 10, 894. Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380. Braga, D.; Benedi, O.; Maini, L.; Grepioni, F., private communication (CSD code TAJPAW) 2003. Hooft, R. W. W. COLLECT; Nonius BV: Delft, The Netherlands, 1998. Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. Ozawa, T. C.; Kang, S. J. J. Appl. Crystallogr. 2004, 37, 679. Barnes, H. A.; Barnes, J. C. Acta Crystallogr., Sect. C 1996, 52, 731. Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. Margulis, T. N. J. Am. Chem. Soc. 1971, 93, 2193. Thuéry, P.; Masci, B. Supramol. Chem. 2003, 15, 95.

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