Hydrothermal Crystallization of Three Calcium-Based Hybrid Solids

Dec 15, 2007 - inorganic subnetwork and the whole structure. Three-dimensional (3D) 1 (Ca2(OH)2[ndc]) is built from layers of calcium oxo- hydroxide ...
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CRYSTAL GROWTH & DESIGN

Hydrothermal Crystallization of Three Calcium-Based Hybrid Solids with 2,6-Naphthalene- or 4,4′-Biphenyl-Dicarboxylates Christophe Volkringer,† Jérôme Marrot,† Gérard Férey,†,‡ and Thierry Loiseau*,† Institut LaVoisier (UMR CNRS 8180) and Institut UniVersitaire de France, UniVersité de Versailles St Quentin en YVelines, 45, AVenue des Etats Unis, 78035 Versailles, France

2008 VOL. 8, NO. 2 685–689

ReceiVed July 24, 2007; ReVised Manuscript ReceiVed August 30, 2007

ABSTRACT: Three new hybrid solids 1, 2, and 3 of calcium dicarboxylates have been obtained using hydrothermal conditions (24 h, 180 °C) with 2,6-naphathalenedicarboxylic acid (H2ndc) and 4,4′-biphenyldicarboxylic acid (H2bpdc) ligands. According to their single-crystal X-ray diffraction analyses, the three compounds exhibit dense networks with distinct dimensionalities for both the inorganic subnetwork and the whole structure. Three-dimensional (3D) 1 (Ca2(OH)2[ndc]) is built from layers of calcium oxohydroxide, pillared via the ndc ligands. Calcium is 7-fold coordinated with a monocapped trigonal prismatic surrounding (CaO4(OH)3). In 2 (Ca(H2O)[bpdc]), the structure consists of infinite chains of calcium polyhedra in a pentagonal bipyramidal coordination (CaO6(H2O) unit), the biphenyldicarboxylate species acting as scaffolds for the 3D structure. Complex 3 (Ca(H2O)3[bpdc]) also contains infinite chains of calcium polyhedra (with 8-fold coordination - CaO4(H2O)4 unit) but this time isolated because one of the two carboxylate functions of the ligand is terminal. The structure is therefore one-dimensional. A network of hydrogen bonds ensures the stability of this structure. Crystal data for 1: monoclinic, P21/c, a ) 12.627(2), b ) 7.8053(13), c ) 5.8555(10) Å, β ) 95.954(3)°, and V ) 573.97(17) Å3; for 2: monoclinic, P21, a ) 5.8237(4), b ) 6.9339(5), c ) 14.7231(11) Å, β ) 93.588(3)°, and V ) 593.37(7) Å3; for 3: orthorhombic, Ima2, a ) 6.5444(3), b ) 30.2468(13), c ) 7.2194(3) Å, and V ) 1429.06(11) Å3. Introduction The past decade has evidenced a growing interest in the synthesis and structural characterization of new porous crystalline architectures built from the combination of inorganic building units with rigid organic polycarboxylates molecules as linkers. This new emerging class of hybrid materials called coordination polymers or metal-organic frameworks (MOF)1–7 may potentially find many applications in the fields of gas storage, separation, catalysis, magnetism, and drug delivery. Usually, divalent and trivalent transition metals are used for the formation of such open frameworks, and a library of different motifs of inorganic building blocks3 with different nuclearities is now well established. Among the potential metal candidates, the coordination polymers based on alkaline-earth cations (group II) have been less studied, although for instance, networks incorporating magnesium8–14 may be interesting for gas storage. In this case, the use of light metallic elements should induce the formation of rather low density for these solids which could be correlated to high uptake capacity values (w/w) of small molecules (H2, CH4, CO2). Beside magnesium, calcium is the second cation that could attract the attention for the construction of porous three-dimensional (3D) networks. Moreover, both elements play significant roles in biologic processes, and such biocompatible crystalline porous networks could be of great interest for hosting and releasing pharmaceutics drugs, as it was previously reported in other MOF-type matrices involving 3d transition metals.15 The formation of hybrid solids involving polycarboxylate linkers combined with calcium was previously subject to different structural investigations, and several phases containing aliphatic16–18 or aromatic19–30 carboxylates ligands have been described. In this contribution, we focused our attention on the use of larger organic linkers containing two benzene rings [2,6* To whom correspondence should be addressed. E-mail: [email protected]. Phone: 33 1 39 254 373. Fax: 33 1 39 254 358 † Institut Lavoisier (UMR CNRS 8180). ‡ Institut Universitaire de France.

naphthalenedicarboxylate (ndc) and 4,4′-biphenyldicarboxylate (bpdc) ligand] with the view of generating large pore calciumbased networks, since most of the previous works reported the use of single benzene rings, and relatively dense phases have been observed. We present here the hydrothermal crystallization and the structural description of a layer-like compound 1 obtained with 2,6-naphthalenedicarboxylic acid (Ca2(OH)2[ndc]) and two chain-like phases 2 and 3 with 4,4′-biphenyldicarboxylic acid (Ca(H2O)[bpdc] and Ca(H2O)3[bpdc], respectively). Experimental Procedures Synthesis. All reactants were commercially available and used as received without further purification. Ca(OH)2 (95+%), CaCl2 · 2H2O (98%), and 4,4′-biphenyldicarboxylic acid (97%, noted H2bpdc) were purchased from Aldrich, Ca(NO3)2 · 4H2O (RPE) was from Carlo Erba, 2,6-naphthalenedicarboxylic acid (98+%, noted H2ndc) was from Alfa Aesar, and KOH was from Prolabo (Rectapur). The reactions were performed in Teflon-lined Parr autoclaves under autogenous pressure. The powdered samples were collected by filtration, washed with purified water, and dried at room temperature. With these two dicarboxylates ligands, three coordination polymers 1, 2, and 3 have been isolated using hydrothermal conditions at high pH. We present herafter the synthesis conditions that have been used for the production of well crystalline compounds containing crystals suitable for X-ray diffraction (XRD) analysis. Ca2(OH)2[ndc] (1). The compound 1 was prepared from a mixture of Ca(OH)2 (400 mg, 5.4 mmol), H2ndc (290 mg, 1.3 mmol), HNO3 4 M (1 mL, 4 mmol), and water (5 mL, 277.8 mmol). The mixture was heated at 180 °C for 24 h. The final pH value was 12. Ca(H2O)[bpdc] (2). The solid 2 was synthesized from the reaction of Ca(NO3)2 · 4H2O (505 mg, 2.1 mmol), H2bpdc (250 mg, 1.0 mmol), and KOH (622 mg, 11.1 mmol) in water (5 mL, 277.8 mmol). The mixture was heated at 180 °C for 24 h. The final pH value was 13. Ca(H2O)3[bpdc] (3). The solid 3 was synthesized from the reaction of CaCl2 · 2H2O (120 mg, 0.8 mmol), H2bpdc (100 mg, 0.4 mmol), and KOH (134 mg, 2.4 mmol) in water (5 mL, 277.8 mmol). The mixture was heated at 180 °C for 24 h. The final pH value was 12. Single-Crystal X-ray Diffraction. Suitable crystals were carefully selected and glued onto a glass fiber. The sample 1 was analyzed at room temperature on a Bruker SMART three-circle diffractometer with a CCD bidimensional detector. The crystal-to-detector distance was 45 mm allowing for the data collection up to 60° (2θ). Slightly more

10.1021/cg700685g CCC: $40.75  2008 American Chemical Society Published on Web 12/15/2007

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Table 1. Crystal Data and Structure Refinement for Calcium Dicarboxylates 1, 2, and 3 Ca2(OH)2[ndc] (1) empirical formula formula weight temperature wavelength crystal system, space group unit cell dimensions

volume (Å3) Z, calculated density (g/cm3) absorption coefficient (mm-1) F(000) crystal size (mm) theta range for data collection limiting indices reflections collected/unique refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e · Å-3)

C12H8Ca2O6 328.34 monoclinic, P21/c a ) 12.627(2) Å b ) 7.8053(13) Å c ) 5.8555(10) Å β ) 95.954(3)° 573.97(17) 2, 1.900 1.017 336 0.18 × 0.18 × 0.03 1.62 to 30.02° -17 e h e 16, -10 e k e 11, -5 e l e 8 4287/1647 [R(int) ) 0.0325] full-matrix least-squares on F2 1647/0/95 1.033 R1 ) 0.0376, wR2 ) 0.1082 R1 ) 0.0454, wR2 ) 0.1151 0.688 and -0.442

than one hemisphere of data was recorded. The samples 2 and 3 were analyzed on a Bruker X8-APEX2 CCD area-detector diffractometer at room temperature. Data reduction was accomplished using SAINT. The substantial redundancy in data allowed a semiempirical absorption correction (SADABS) to be applied, on the basis of multiple measurements of equivalent reflections. The structures were solved by direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F2 data using SHELXTL. Hydrogen atoms were included in calculated positions and allowed to ride on their parent atoms. The crystal data are given in Table 1. Selected bond lengths are listed in Table 2.

Crystal Structure Description Three distinct calcium-based structures 1, 2, and 3 with different dimensionalities were isolated with the 2,6-naphthalenedicarboxylate and 4,4′-biphenyldicarboxylate ligands. The 3D crystal structure of 1 (denoted Ca2(OH)2[ndc]) is unique since it consists of layers of calcium oxo-hydroxide pillared by the naphthalene dicarboxylate moieties. The divalent cation Ca1 is coordinated to four oxygen atoms from the carboxylate groups (O1 and O3) and three hydroxyl groups (O2), which form a monocapped trigonal prism (CaO4(OH)3, Figure 1a). Indeed, the valence bond calculations31 confirms this assignment for O2 (1.08; expected value for OH: 1.2), which are linked to three calcium atoms with a specific µ3-OH bridging involving shorter bond distances (Ca1-O2 ) 2.3219(16)-2.3829(14) Å, whereas Ca1-O1 ) 2.4174(15)–2.5059(14) Å and Ca1-O3 ) 2.5274(15)– 2.5973(16) Å). Such short Ca-OH distances for a µ3-OH group seem at first glance unusual. In most of the cases, they are expected to be larger than Ca-O distances, but, in this structure, the fact that O1 and O3 belong to carboxylate groups changes the distribution of distances. Indeed, O1 and O3 are also µ3-O with two Ca and one C, the valence bond of the latter being the most important. This weakens the Ca-O bonds around O1 and O3 and correlatively increases the strength of the Ca-O(2)H bonds. This correlatively decreases the Ca-O2 distances and explains why the distances between two Ca are unusually short (3.483(3) Å). The Ca monocapped trigonal prisms share edges to form layers in the (b,c) plane with composition Ca2(OH)2 (Figure 2). In this layer, calcium and hydroxyl ions atoms strictly alternate and form a distorted 4 · 82 net. The carboxylate groups of the pillaring ligands (Figure 3) coordinate three adjacent

Ca(H2O)[bpdc] (2)

Ca(H2O)3[bpdc] (3)

C14H8CaO5 296.28 293(2) K 0.71073 Å monoclinic, P21 a ) 5.8237(4) Å b ) 6.9339(5) Å c ) 14.7231(11) Å β ) 93.588(3)° 593.37(7) 2, 1.658 0.546 304 0.35 × 0.26 × 0.20 1.39 to 25.01° -6 e h e 6, -8 e k e 8, -17 e l e 17 10079/2085 [R(int) ) 0.0894]

C14H11CaO7 331.28

1429.06(11) 4, 1.526 0.471 672 0.24 × 0.12 × 0.02 1.35 to 30.11° -9 e h e 9, -42 e k e 42, -10 e l e 10 14199/2276 [R(int) ) 0.0372]

2085/1/181 1.120 R1 ) 0.0416, wR2 ) 0.0938 R1 ) 0.0683, wR2 ) 0.1237 0.329 and -0.376

2276/1/115 1.213 R1 ) 0.0516, wR2 ) 0.1554 R1 ) 0.0684, wR2 ) 0.1791 0.535 and -0.531

orthorhombic, Ima2 a ) 6.5444(3) Å b ) 30.2468(13) Å c ) 7.2194(3) Å

calcium cations in a tridentate bridging fashion (Figure 4). One of the carboxyl atoms bridges two adjacent calcium atoms (C3-O1 ) 1.270(2) Å), whereas the second carboxyl one is linked to one calcium only (C3-O3 ) 1.256(3) Å), which defined the specific configuration µ3-η2:η1. Stacking of the ndc ligands occurs along the [011] direction with an angle of ≈75° between the adjacent organic species. This prevents any significant π-π interactions between them. Similar layers exist with µ3-OH groups in calcium hydroxide Ca(OH)2 (natural mineral Portlandite32), but Ca is this time octahedrally coordinated and the Ca. . .Ca distances are longer (3.608(7) Å). The existence of such a layer-like arrangement is however quite uncommon since most of the carboxylate coordination polymers are built from chainlike or discrete clusters of calcium polyhedra. The structures of 2 and 3 are more conventional since they are built up from infinite polymeric chains of calcium polyhedra interconnected through the 4,4′-biphenyldicarboxylate ligands. In 2 (Ca(H2O)[bpdc]), calcium atoms (Figure 1b) are coordinated to six oxygen atoms from the carboxylate functions with typical Ca-O bond values (Ca1-O ranging from 2.282(3) to 2.618(3) Å) and one water molecule in terminal position (Ca1-O1W ) 2.395(4) Å). For the latter, valence bond calculations31 give an experimental value of 0.31 for the Ca-O bond, which is in good agreement with the expected one for a H2O moiety (0.40). The 7-fold coordination of calcium (CaO6(H2O) unit) can be defined as a pentagonal bipyramid with the water molecule located at one of the apical positions. The calcium polyhedra are edge-shared and form [010] straight chains (Figure 5) that are connected via the 4,4′-biphenyldicarboxylate ligands along the a and c axes, respectively. This particular arrangement forms a 3D compact network (Figure 6). One of the two carboxylate groups chelates and bridges three calcium atoms in a tetradentate mode with the configuration µ3-η2:η2: each oxygen atom from the C-O functions (C14-O3 ) 1.266(7) and C14-O4 ) 1.261(7) Å) is linked to two adjacent calcium atoms. The second carboxylate group bridges two distinct chains of calcium polyhedra (along the a axis) in a bidendate fashion (C13-O1 ) 1.243(5) and C13-O4 ) 1.274(6) Å). The intrachain distance Ca. . .Ca is 4.085 (2) Å, which is much longer than the value found in the layered phase

Crystallization of Calcium-Based Hybrid Solids

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Table 2. Bond Lengths [Å] for Calcium Dicarboxylates 1, 2, and 3a Ca2(OH)2[ndc] (1) Ca1-O2#1 Ca1-O2 Ca1-O2#2 Ca1-O1#3 Ca1-O1#4 Ca1-O3 Ca1-O3#3 O1-C3 O2-H2 O3-C3#5 C1-C4 C1-C5 C1-C3 C2-C2#6 C2-C6 C2-C4 C4-H4 C5-C6#6 C5-H5 C6-C5#6 C6-H6

2.3219(16) 2.3374(15) 2.3829(14) 2.4174(15) 2.5059(14) 2.5274(15) 2.5973(16) 1.270(2) 0.76(3) 1.256(3) 1.373(3) 1.423(3) 1.497(3) 1.418(4) 1.419(3) 1.420(3) 0.9300 1.369(3) 0.9300 1.369(3) 0.9300

Ca(H2O)[bpdc] (2) Ca1-O1 Ca1-O2#7 Ca1-O1W Ca1-O4#8 Ca1-O4#9 Ca1-O3#4 Ca1-O3#10 C1-C2 C1-C6 C1-H1 C2-C3 C2-H2 C3-C4 C3-C13 C4-C5 C4-H4 C5-C6 C5-H5 C6-C7 C7-C12 C7-C8 C8-C9 C8-H8 C9-C10 C9-H9 C10-C11 C10-C14 C11-C12 C11-H11 C12-H12 C13-O1 C13-O2 C14-O4 C14-O3

2.282(3) 2.305(4) 2.395(4) 2.408(4) 2.485(4) 2.498(4) 2.618(3) 1.390(7) 1.392(7) 0.9300 1.384(7) 0.9300 1.391(7) 1.508(6) 1.377(7) 0.9300 1.399(7) 0.9300 1.487(5) 1.395(7) 1.396(7) 1.394(7) 0.9300 1.385(7) 0.9300 1.388(7) 1.495(6) 1.379(7) 0.9300 0.9300 1.243(5) 1.274(6) 1.261(7) 1.266(7)

Figure 1. (a) Calcium coordination polyhedron (CaO4(OH)3) in Ca2(OH)2[ndc] (1) showing the monocapped trigonal prism. O2 corresponds to a hydroxyl group. (b) Calcium coordination polyhedron (CaO6(H2O)) in Ca(H2O)[bpdc] (2) showing the pentagonal bipyramid. O1W corresponds to water and is in terminal position. (c) Calcium coordination polyhedron (CaO4(H2O)4) in Ca(H2O)3[bpdc] (3) showing the distorted dodecahedron. O5, O3, and O4 are water molecules in µ2- and terminal positions, respectively (ORTEP-type drawing with 50% probability of ellipsoids).

Figure 2. Polyhedral view (left) and representation of the network (right) of a layer of calcium oxo-hydroxide 1 in the plane (b,c). Calcium cations are indicated by dark grey circles, and µ3-OH groups are indicated by light grey circles in the layer, defining a distorted 4 · 82 net.

Ca(H2O)3[bpdc] (3) Ca1-O1#11 Ca1-O1#12 Ca1-O4 Ca1-O3 Ca1-O1#13 Ca1-O1 Ca1-O5#14 Ca1-O5 O1-C1 O2-C10 C1-O1#13 C1-C2 C2-C3#13 C2-C3 C3-C4 C3-H3 C4-C5 C4-H4 C5-C4#13 C5-C6 C6-C7#13 C6-C7 C7-C8 C7-H7 C8-C9 C8-H8 C9-C8#13 C9-C10 C10-O2#13

2.338(2) 2.338(2) 2.390(4) 2.417(6) 2.516(2) 2.516(2) 2.580(3) 2.580(3) 1.256(3) 1.257(3) 1.256(3) 1.497(5) 1.360(4) 1.360(4) 1.399(5) 0.9300 1.367(5) 0.9300 1.367(5) 1.482(5) 1.354(4) 1.354(4) 1.397(5) 0.9300 1.349(4) 0.9300 1.349(4) 1.508(5) 1.257(3)

a Symmetry transformations used to generate equivalent atoms: #11 -x + 1, -y + 2, -z; #12 -x + 1, y - 1/2, -z + 1/2; #13 x,-y + 3/2, z + 1/2; #4 -x + 1, y + 1/2, -z + 1/2; #5 x, -y + 3/2, z - 1/2. #6 -x, -y + 1, -z; #7 x - 1, y, z; #8 -x + 1, y - 1/2, -z + 2; #9 x 1, y, z - 1; #10 -x + 1, y + 1/2, -z + 2; #11 -x + 1,-y, z; #12 x + 1/2, -y, z; #13 -x + 3/2, y, z; #14 x -1/2, -y, z; #5 -x + 2, -y.

Figure 3. View of the structure along the c axis showing the connection of the naphthalate ligands between the inorganic layers.

1. No significant π-π interactions are observed between the biphenyl groups since these molecules are stacked along the b axis with an angle close to ≈75° (similar stacking of the aromatic ring in the compound 1). The phase 3 (Ca(H2O)3[bpdc]) is an augmented version of calcium terephthalate22,25 Ca(H2O)3[bdc] (bdc ) 1,4-benzenedicarboxylate), bpdc replacing bdc. It is built from chains of calcium polyhedra, but, as only one of the carboxylate functions of bpdc is connected to the calcium atoms, the structure is onedimensional, hydrogen bonds ensuring the cohesion of the structure. Calcium is this time (Figure 1c) 8-fold coordinated with a distorted dodecahedral geometry (CaO4(H2O)4 unit). It is linked to four oxygen atoms from the carboxylate groups (2

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Figure 4. Detailed view of the connection of the naphthalate ligand with the calcium polyhedra in a tridendate bridging mode (µ3-η2:η1) and positions of the µ3-hydroxo group bridging three calcium atoms.

Figure 7. View of the arrangement of chains with the 4,4′-biphenyldicarboxylate building blocks in 3 (Ca(H2O)3[bpdc]). View of a single chain of calcium polyhedra running along the a axis and showing the connection of the polyhedra via triangular faces.

Figure 5. Representation of the structure of Ca(H2O)[bpdc] (2) showing the connection of the infinite chains of the calcium polyhedra running along b, with the 4,4′-biphenyldicarboxylate ligand.

Figure 8. View of the hydrogen bond scheme occurring between the carboxylate functions and µ2-bridging and terminal aquo ligands in 3 (Ca(H2O)3[bpdc]).

Figure 6. Representation of the structure of Ca(H2O)[bpdc] (2) showing the connection of the infinite chains of the calcium polyhedra with the 4,4′-biphenyldicarboxylate ligands along b.

× Ca1-O1 ) 2.338(2) and 2 × Ca1-O1 ) 2.516(2) Å), two µ2-aquo groups (2 × Ca1-O5 ) 2.580(3) Å), and two others (Ca1-O4 ) 2.390(4) and Ca1-O3 ) 2.417(6) Å) in terminal position. Valence bond calculations31 confirm the assignment of the aquo groups (calculated values of 0.38 and 0.32 and 0.30, respectively). The calcium atoms are linked to each other via two carboxylate-type oxygen atoms and one bridging µ2-aquo

ligand to generate straight chains (Figure 7) running along the a axis with Ca. . .Ca interactions of 3.619(1) Å. For this chain, each calcium polyhedron shares two opposite triangular faces with the adjacent ones. Only one of the carboxylate functions is connected to the calcium atoms and acts as a tetradentate linkage with both chelating and bridging modes (µ3-η2:η2) with typical C1-O1 distances of 1.256(3) Å. A similar C-O-Ca connection fashion was observed in the phase 2. The remaining carboxylate anion, not connected to calcium, interacts through strong hydrogen bonds with the bridging and terminal water molecules (Figure 8). Typical interactions distances are observed between the carboxyl oxygen O2 and the bridging µ2-aquo ligand O5 (2.739(3) Å), also with the terminal aquo ones O3 (2.930(5) Å) and O4 (2.748(4) Å). This specific hydrogen scheme occurring between the water molecules and carboxylate groups ensures the 3D cohesion of the structure. Morever, this molecular assembly is also strengthened by the π-π interactions

Crystallization of Calcium-Based Hybrid Solids

between the aromatic rings since typical distances C. . .C in the range of 3.6–3.7 Å occur in this structure. Concluding Remarks Using 2,6-naphtalenedicarboxylate (ndc) and 4,4′-biphenyldicarboxylate (bpdc) ligands, three new calcium-based coordination polymers have been isolated using hydrothermal conditions. Two of them correspond to new structure types, and one is related to the previously described calcium terephtalate.22,25 The three structures consist of compact molecular assemblies based on calcium polymeric backbones. They do not reveal any porosity. With ndc, uncommon layers of calcium oxo-hydroxide with specific µ3-OH bonding and short Ca. . .Ca distances of (3.483(3) Å) are observed. Another point is the influence of pH on the reaction. At variance to the immense majority of hybrid solids, which form in acidic medium, the system with calcium implies strongly basic conditions (pH g 12). For a given ligand (here bpdc), pH influences both the dimensionality of the structure and the degree of hydration. First, at high pH, 3D structures appear (phase 2), whereas decreasing the pH leads to a 1D material (phase 3) in similar conditions of concentration. Similar conclusions, but in acidic medium, were previously reported for the formation of calcium coordination polymers with 3,5-pyrazoledicarboxylic acid.23 With this specific linker, the effect of pH was investigated and two layer-like and one 3D networks have been isolated at pH 2.5 and 6, respectively. This change of the acidity of the medium shows also that not only the intrinsic acido-basic characteristics of Ca cation are involved in the formation of hybrids but also those of the linker. Finally, pH affects the coordination surrounding of calcium in relation with the hydration degree of the coordination polyhedra: the more acidic the medium, larger the number of molecules around calcium and correlatively larger its coordination. The present study shows for 2 (pH ) 13), one H2O in the 7-fold coordination of Ca; for 3 (pH ) 12), four H2O in its 8-fold coordination. Acknowledgment. The authors would like to thank A. Martin, K. Chaoui, W. Xuan, and W. Bouïta for their help in the synthesis work. Supporting Information Available: Crystallographic information files (CIF) are available free of charge via the Internet at http:// pubs.acs.org.

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