Inclusion Compounds of Hexakis(4-cyanophenyl)benzene: Open

Jan 13, 2006 - Synopsis. Crystals of hexakis(4-cyanophenyl)benzene were grown under different conditions, and their structures were determined to asse...
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Inclusion Compounds of Hexakis(4-cyanophenyl)benzene: Open Networks Maintained by C-H‚‚‚N Interactions Kenneth E.

Maly,*,1

Thierry Maris, Eric

Gagnon,2

and James D. Wuest

De´ partement de Chimie, UniVersite´ de Montre´ al, Montre´ al, Que´ bec H3C 3J7, Canada

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 2 461-466

ReceiVed August 6, 2005; ReVised Manuscript ReceiVed NoVember 14, 2005

ABSTRACT: Hexakis(4-cyanophenyl)benzene (1) was crystallized from various solvents, and the structures were determined by X-ray crystallography. In all cases studied, inclusion compounds were obtained, and crystallization yielded networks held together primarily by C-H‚‚‚N interactions. However, the resulting structures were diverse and depended strongly on the solvent used for crystallization. In certain cases, remarkably large fractions of the volumes of the crystals (up to 58%) are accessible to guests. The diversity of the structures obtained underscores the limitations of using C-H‚‚‚N interactions in crystal engineering. Introduction Hydrogen bonds play an important role in crystal engineering, in part because they are strong and directional.3 In favorable cases, they can predetermine the pattern of molecular association in crystals, and they can organize molecules into networks with predictable architectures. Hydrogen bonding cannot always be optimized in crystalline solids at the same time that normal close molecular packing is achieved. As a result, when molecules can participate in multiple hydrogen bonds with neighbors, open networks are often generated, with significant space for including guests.4,5 Crystalline materials of this type are of interest because they are molecular analogues of zeolites and may find applications in areas such as selective separation and heterogeneous catalysis. Inclusion compounds in which a large fraction of the volume of the crystal is comprised of guests are normally rather rare.6 For this reason, molecules that can hydrogen bond to form porous networks belong to an unusual family of crystalline materials. Molecular shape is another important factor that controls the efficiency of packing in the solid state and helps determine whether inclusion compounds will be formed. For example, compounds with complex topologies such as spirobifluorenes5 and tetraarylporphyrins7 pack poorly and therefore routinely favor open structures that include solvents or other guests. The nature of the solvent itself is also thought to play a significant role in determining whether inclusion compounds are formed. In particular, solvent-host interactions can contribute to the stability of inclusion compounds.8 Recently, crystal engineers have begun to explore the potential of interactions other than classical hydrogen bonds as directors of molecular association.9 Interactions such as C-H‚‚‚O, C-H‚ ‚‚N, π-π, halogen-halogen, and C-H‚‚‚π are considerably weaker than conventional hydrogen bonds. However, they can exhibit geometric, structural, and spectroscopic features similar to those of hydrogen bonds, and they can play important roles in controlling the structure of crystals. In particular, C-H‚‚‚N interactions involving cyano groups can help direct molecular association. For example, such interactions are observed in the structures of tetrakis(4-cyanophenyl)methane, which crystallizes both as an inclusion compound and as a close-packed structure.10 In addition, recent studies have shown that 1,3,5-tris(4-cyanobenzoyl)benzene crystallizes from several different solvents * To whom correspondence may be addressed: umontreal.ca.

Figure 1. Typical C-H‚‚‚N interactions in benzonitriles (where the angle θ is variable).

as inclusion compounds in which C-H‚‚‚N interactions are present.11 In contrast, the crystal structures of various tetracyanospirobifluorenes are dominated by π-π interactions instead of C-H‚‚‚N interactions.12 These examples reveal that weak forces such as C-H‚‚‚N interactions involving nitriles can help position molecules in crystals, but they exert their effects in close competition with other weak interactions, making the resulting structures hard to predict. Another significant obstacle to using C-H‚‚‚N interactions of nitriles in crystal engineering is the variety of different types of association that can be observed (Figure 1). None of these motifs is strongly preferred, so exploiting them in the rational design of molecular networks is inherently difficult. Moreover, the observed motifs can vary with the conditions of crystallization, giving rise to different structures that depend on the choice of solvent. The presence of other functional groups and multiple molecular conformations are further variables that amplify the difficulty of using C-H‚‚‚N interactions and other weak forces in crystal engineering.

kenneth.maly@

10.1021/cg050384a CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006

462 Crystal Growth & Design, Vol. 6, No. 2, 2006

Maly et al.

Table 1. Crystallographic Data for Inclusion Compounds of Hexakis(4-cyanophenyl)benzene (1) solvent

CH3OH

CH3CH2OH

CH3COOC2H5

DMF (+H2O)

DMSO (+H2O)

dioxane

THF/CH3CN

formula crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z D (g cm-3) T (K) µ (mm-1) R1 (%) ωR2 (%) porosity (%)

C50H32N6O2 tetragonal P42212 15.118(2) 15.118(2) 9.1050(18) 90 90 90 2081.0(6) 2 1.195 276 0.592 5.38 15.05 15

C52H36N6O2 tetragonal P42212 15.2168(3) 15.2168(3) 9.0188(3) 90 90 90 2088.31(9) 2 1.235 100 0.608 5.15 16.21 16

C60H48N6O6 monoclinic P21/c 9.0763(7) 14.7937(12) 19.3722(14) 90 91.237(4) 90 2600.5(3) 2 1.212 100 0.637 7.98 18.99 40

C66H72N12O9 trigonal R3hm 23.5720(14) 23.5720(14) 10.2698(13) 90 90 120 4941.8(8) 3 1.187 100 0.655 6.33 15.96 57

C60H64N6O8S6 trigonal R32 23.338(9) 23.338(9) 10.842(9) 90 90 120 5114(5) 3 1.159 280 2.272 7.32 16.81 58

C72H72N6O12 trigonal R3hm 24.165(2) 24.165(2) 9.9242(12) 90 90 120 5018.8(8) 3 1.204 100 0.670 7.86 24.55 50

C78H81N9O6 trigonal R3hm 23.6533(2) 23.6533(2) 10.1601(2) 90 90 120 4922.80(11) 3 1.255 100 0.637 6.57 16.19 50

To further assess the potential of C-H‚‚‚N interactions in crystal engineering, we decided to study the behavior of hexakis(4-cyanophenyl)benzene (1). This compound has several noteworthy features, including a periphery rich in cyano groups and a low degree of conformational flexibility, associated with rotation around the six aryl-aryl single bonds. In addition, there are no heteroatomic functional groups that can compete with the cyano groups to direct molecular association. The parent compound, hexaphenylbenzene (2), crystallizes as two polymorphic close-packed structures13 and as an inclusion compound with anisole in which only 11% of the volume of the crystals is occupied by solvent.14 As a result, the hexaphenylbenzene core does not have an intrinsically awkward topology that prevents efficient packing. In contrast, derivatives of hexaphenylbenzene that are substituted with hydrogen-bonding groups, including hexaphenol 3,15 hexacarboxylic acid 4,16 and hexacarboxamide 5,17 all crystallize to form open networks with significant volume occupied by guests. The wide difference between the behavior of hexaphenylbenzene (2) itself and that of substituted derivatives 3-5 underscores the ability of hydrogen bonds to compete successfully with other intermolecular interactions and thereby program the construction of open networks. As a result, we expected that a parallel study of 1 would be a valuable source of information about the relative ability of C-H‚‚‚N interactions to direct the formation of porous networks.

conformation with torsional angles between the central and outer rings ranging from approximately 69 to 73°.20 As shown in Figure 2, each molecule of compound 1 interacts with each of eight neighboring molecules of compound 1 via a C-H‚‚‚N interaction of Motif A (see Figure 1), giving a network held together by a total of eight primary C-H‚‚‚N interactions per molecule (N‚‚‚H distance 2.75(1) Å, C-H‚‚‚N angle 150.2(5)°). Other N‚‚‚H distances in the structure exceed 2.9 Å. Four

Results and Discussion 118

Hexanitrile was prepared from hexakis(4-bromophenyl)benzene (6)19 by a new method and was crystallized from seven different solvents. The resulting structures were determined by X-ray crystallography, and the crystallographic data are summarized in Table 1. In all cases studied, compound 1 crystallized as inclusion compounds, with open networks maintained primarily by C-H‚‚‚N interactions. Significant features of the structures are summarized in the following paragraphs. Inclusion Compound 1‚2 CH3OH. Crystals of hexanitrile 1 were obtained by slow evaporation of a solution in CH3OH at 25 °C. The resulting crystals were found to belong to the tetragonal space group P42212 at 25 °C and to have the composition 1‚2 CH3OH. The molecules of included solvent are disordered at 25 °C but become ordered at low temperature. This transition is accompanied by a change in symmetry from tetragonal (space group P42212) to monoclinic (space group P21/ n).20 In inclusion compound 1‚2 CH3OH, approximately 15% of the volume of the crystal is occupied by molecules of solvent.21,22 The phenyl arms of the core adopt a chiral propeller

Figure 2. (a) View of the structure of crystals of the inclusion compound 1‚2 CH3OH showing a central molecule of hexanitrile 1 (blue) and the eight neighboring molecules of compound 1 linked by primary C-H‚‚‚N interactions involving cyano groups. Included molecules of CH3OH are omitted for clarity. The central molecule acts as a donor of hydrogen to cyano groups of the four neighbors shown in yellow and as an acceptor to the four neighbors in red. (b) Detailed view of two neighboring molecules linked by a C-H‚‚‚N interaction of Motif A (Figure 1), shown as a broken line.

Inclusion Compounds of Hexakis(4-cyanophenyl)benzene

Figure 3. (a) View of the structure of crystals of the inclusion compound 1‚2 CH3CH2OH showing a central molecule of hexanitrile 1 (blue) and the eight neighboring molecules of compound 1 linked by primary C-H‚‚‚N interactions involving cyano groups. Included molecules of CH3CH2OH are omitted for clarity. The central molecule acts as a donor of hydrogen to cyano groups of the four neighbors shown in yellow and as an acceptor to the four neighbors in red. (b) Detailed view of two neighboring molecules linked by double C-H‚ ‚‚N interactions of Motif A (Figure 1), shown as broken lines.

of the neighbors are in a plane above the central molecule, while the other four neighbors are in a plane below the central molecule. Curiously, only four of the six (4-cyanophenyl) arms of compound 1 participate in significant C-H‚‚‚N interactions with the eight principal neighbors. Four of the neighbors (red in Figure 2) act as donors of hydrogen to cyano groups of the central molecule (blue), while the cyano groups of the other four neighbors (yellow) act as acceptors of hydrogen donated by the central molecule. Inclusion Compound 1‚2 CH3CH2OH. Slow evaporation of a solution of 1 in CH3CH2OH at 25 °C proceeded as in CH3OH to give crystals belonging to the tetragonal space group P42212. The composition is 1‚2 CH3CH2OH, and 16% of the volume of the crystals is accessible to ordered molecules of solvent. As in the previous case, the phenyl arms of the core adopt a chiral propeller conformation with torsional angles between the central and outer rings ranging from approximately 69 to 73°.20 Although the lattice parameters and architecture of the resulting network are virtually the same as those observed for

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Figure 4. (a) View of the structure of crystals of the inclusion compound 1‚3 CH3COOC2H5 showing a central molecule of hexanitrile 1 (blue) and the eight neighboring molecules of compound 1 linked by primary C-H‚‚‚N interactions involving cyano groups. Included molecules of CH3COOC2H5 are omitted for clarity. The central molecule interacts (1) with the four neighbors in yellow according to dimeric Motif B; (2) with the two neighbors in red by serving as an acceptor of hydrogen in two C-H‚‚‚N interactions of Motif A; and (3) with the two neighbors in green by serving as a donor of hydrogen in two C-H‚ ‚‚N interactions of Motif A. (b) Detailed view of the central molecule (blue) interacting with one neighbor (yellow) according to dimeric Motif B and with a second neighbor (red) by two single C-H‚‚‚N interactions of Motif A. In both cases, C-H‚‚‚N interactions are represented by broken lines.

the analogous inclusion compound 1‚2 CH3OH, there are noteworthy differences in the interactions that maintain the network. Again, a central molecule of hexanitrile 1 interacts with eight neighboring molecules of compound 1 by forming primary C-H‚‚‚N interactions (Figure 3). As in the analogous inclusion compound 1‚2 CH3OH, four of the neighbors (red in Figure 3) act as donors of hydrogen to form C-H‚‚‚N interactions corresponding to Motif A (N‚‚‚H distance 2.68(1) Å, C-H‚‚‚N angle 151.2(5)°). In addition, however, a hydrogen atom meta to the cyano group of the central molecule then acts as a donor to a cyano group provided by an adjacent (4cyanophenyl) arm of a red neighbor (N‚‚‚H distance 2.87(1) Å, C-H‚‚‚N angle 169.1(5)°). The other four neighbors (yellow in Figure 3) interact similarly with the central molecule. Inclusion Compound 1‚3 CH3COOC2H5. Crystals of hexanitrile 1 were also obtained by slow evaporation of a solution in CH3COOC2H5. These crystals were found to belong to the monoclinic space group P21/c and to have the composition 1‚3 CH3COOC2H5. The included solvent is partly disordered. As suggested by the stoichiometry, the fraction of the volume of the crystal accessible to solvent (40%) is larger than in the cases of the inclusion compounds of CH3OH and CH3CH2OH (1516%).21,22 In crystals obtained from CH3COOC2H5, the hexaphenylbenzene core adopts an achiral conformation in which two

464 Crystal Growth & Design, Vol. 6, No. 2, 2006

Figure 5. Representations of the structure of crystals of the inclusion compound 1‚3 CH3COOC2H5, viewed along the a axis (a) and along the b axis (b). Both views show a 2 × 2 × 2 array of unit cells. Included molecules of solvent are omitted for clarity, and atoms are represented by spheres of van der Waals radii to show the cross sections of the channels.

para-oriented phenyl arms are nearly parallel to each other and perpendicular to the central phenyl ring, while the other two pairs of para-oriented arms are parallel but tilted in the same direction with respect to the central ring. The resulting structure consists of a network maintained by C-H‚‚‚N interactions of Motif A, as well as by interactions of the dimeric Motif B (Figure 1). Again, each molecule of hexanitrile 1 interacts with eight neighboring molecules of compound 1 (Figure 4). Two neighbors (red in Figure 4) act as hydrogen-bond donors according to Motif A (N‚‚‚H distances 2.51(1) and 2.78(1) Å, C-H‚‚‚N angles 174.7(5)° and 126.8(5)°, respectively), and two other neighbors (green) act as hydrogen-bond acceptors with the same lengths and angles. The remaining four neighbors (yellow) each interact with the central molecule by forming the dimeric Motif B (N‚‚‚H distances 2.53(1) and 2.69(1) Å, C-H‚‚‚N angles 170.1(5)° and 141.4(5)°, respectively). Other N‚‚‚H distances in the structure exceed 2.8 Å. In the inclusion compond 1‚3 CH3COOC2H5, all six cyanophenyl arms of compound 1 engage in significant C-H‚ ‚‚N interactions. Small channels occupied by included molecules of solvent can be observed along both the a and b axes of the crystal (Figure 5). Inclusion Compound 1‚6 DMF‚3 H2O. New crystals of hexanitrile 1 were obtained by slowly cooling a hot solution in DMF. These crystals proved to belong to the trigonal space group R3hm and to have the composition 1‚6 DMF‚3 H2O. A highly open network is formed, with approximately 57% of the volume of the crystal accessible to molecules of solvent,21,22

Maly et al.

Figure 6. (a) View of the structure of crystals of the inclusion compound 1‚6 DMF‚3 H2O showing a central molecule of compound 1 (blue) and the six neighboring molecules of compound 1 (red) linked by C-H‚‚‚N interactions involving cyano groups. Included molecules of DMF and H2O are omitted for clarity. The central molecule interacts with each neighbor in red by forming two pairs of C-H‚‚‚N interactions according to dimeric Motif B. (b) Detailed view of the central molecule (blue) and one of its neighbors (red), with C-H‚‚‚N interactions represented by broken lines.

which are partly disordered. In the structure, hexanitrile 1 adopts a conformation in which all of the phenyl arms are nearly perpendicular to the central ring. The network is maintained by C-H‚‚‚N interactions according to dimeric Motif B (N‚‚‚H distance 2.61(1) Å, C-H‚‚‚N angle 152.7(5)°). As shown in Figure 6, each molecule of hexanitrile 1 interacts with six neighboring molecules of compound 1 to form a network in which adjacent (4-cyanophenyl) arms of the central molecule (blue in Figure 6) interact with adjacent arms of a neighbor (red) according to Motif B (Figure 6b), giving a total of 12 dimeric interactions per molecule of compound 1 or 24 single C-H‚‚‚N interactions. In this way, the nitrogen atom of each cyano group acts simultaneously as an acceptor in two C-H‚‚‚N interactions. The resulting network has small channels aligned with the c axis, with more significant space between layers of compound 1 perpendicular to the c axis (Figure 7). Figure 8 provides a stereoview of the channels themselves.23 Included molecules of solvent are partly disordered, but show a novel segregation, with H2O occupying spaces between the layers and DMF located in the channels. Inclusion Compound 1‚6 DMSO‚2 H2O. Crystals of hexanitrile 1 were also obtained by slowly cooling a hot solution in DMSO. These crystals were found to belong to the trigonal space group R32 and to have the composition 1‚6 DMSO‚2 H2O. The lattice parameters, molecular conformation, and structural organization are virtually the same as those observed in crystals

Inclusion Compounds of Hexakis(4-cyanophenyl)benzene

Figure 7. Representation of the structure of crystals of the inclusion compound 1‚6 DMF‚3 H2O, viewed along the c axis. The image shows a 2 × 2 × 2 array of unit cells, with included molecules of solvent omitted for clarity, and atoms represented by spheres of van der Waals radii to show the cross sections of the channels.

Figure 8. Stereoscopic representation of channels within the network formed by hexanitrile 1 when crystallized from DMF. The image shows a 2 × 2 × 2 array of unit cells viewed along the c axis. The outsides of the channels appear in light gray, and dark gray is used to show where the channels are cut by the boundaries of the array. The surface of the channels is defined by the possible loci of the center of a sphere of diameter 3.5 Å as it rolls over the surface of the ordered network.23

of inclusion compound 1‚6 DMF‚3 H2O. A similar fraction of the volume of the crystal (58%) is accessible to included solvent,21,22 and the network is again maintained by C-H‚‚‚N interactions according to dimeric Motif B (N‚‚‚H distance 2.69(1) Å, C-H‚‚‚N angle 156.8(5)°). As in the analogous inclusion compound of DMF and H2O, included molecules of solvent are partly disordered and cleanly segregated, with H2O again found in spaces between the layers and DMSO filling the channels. Inclusion Compound 1‚6 Dioxane. Crystals of hexanitrile 1 obtained by cooling a solution in hot dioxane proved to belong to the trigonal space group R3hm and to have the composition 1‚6 dioxane. The resulting network of C-H‚‚‚N interactions is very similar to that observed in the inclusion compounds of DMF and DMSO (N‚‚‚H distances 2.93(1) and 2.96(1) Å, C-H‚‚‚N angles 137.5(5)° and 138.0(5)°, respectively). Again, a significant fraction (50%) of the volume of the crystal is accessible to included solvent.21,22 The (4-cyanophenyl) arms of the inclusion compound of dioxane are statistically disordered, unlike those of the other inclusion compounds studied. Another noteworthy feature of the structure is that the C-N‚‚‚H interactions considered to maintain the network are much longer than those observed in the other inclusion compounds of compound 1, and the C-H‚‚‚N angles are smaller. This may

Crystal Growth & Design, Vol. 6, No. 2, 2006 465

be explained by the fact that dioxane is larger than DMF and DMSO, and that the network is distorted somewhat to accommodate the extra volume. Inclusion Compound 1‚6 THF‚3 CH3CN. Hexanitrile 1 could also be crystallized by allowing THF to diffuse into a solution in CH3CN. The resulting crystals were also found to belong to the trigonal space group R3hm and to have the composition 1‚6 THF‚3 CH3CN. The resulting network is closely similar to those of the inclusion compounds of DMF, DMSO, and dioxane. Again, the structure is maintained by C-H‚‚‚N interactions of dimeric Motif B (N‚‚‚H distance 2.64(1) Å, C-H‚‚‚N angle 146.7(5)°), and a similarly large fraction (50%) of the volume of the crystal is accessible to included solvent.21,22 The (4-cyanophenyl) arms are again partly disordered. Conclusions Hexakis(4-cyanophenyl)benzene (1) was prepared by a new method and crystallized from various solvents. In all cases, inclusion compounds were obtained, and the resulting structures feature open networks held together in part by multiple C-H‚ ‚‚N interactions. The specific structure formed depends on the solvent used for crystallization, and three distinctly different types of networks were observed. Crystals obtained from CH3OH and CH3CH2OH incorporated very similar networks maintained by C-H‚‚‚N interactions of Motif A (Figure 1), with modest volumes occupied by included molecules of solvent. In contrast, the structures obtained by crystallization from DMF, DMSO, dioxane, and CH3CN/THF revealed the presence of multiple C-H‚‚‚N interactions according to dimeric Motif B. In these crystals, the fraction of the volume accessible to included molecules of solvent lies in the ranges 50-58%. These values are notable for crystals maintained by weak interactions, and they are similar to those of networks maintained by much stronger interactions such as conventional hydrogen bonds. The inclusion compound obtained by crystallizing hexanitrile 1 from CH3COOC2H5 has an intermediary structure maintained both by simple C-H‚‚‚N interactions (Motif A) and cyclic dimers (Motif B). The fraction of volume accessible to solvent (40%) is also intermediate in value. The recurrence of C-H‚‚‚N interactions in stuctures of 1 obtained under various conditions confirms their potential in crystal engineering as directors of molecular association, at least in cases in which other weak intermolecular interactions cannot compete effectively. However, detailed examination of the structures of 1 has shown that (1) C-H‚‚‚N interactions can exhibit multiple motifs with diverse geometries and (2) the resulting networks depend critically on the choice of solvent. These observations underscore the difficulty of using C-H‚‚‚ N interactions to position molecules predictably in crystals, especially when the molecules are flexible and contain other functional groups able to form weak interactions. Experimental Section Hexakis(4-cyanophenyl)benzene (1).18 A mixture of hexakis(4bromophenyl)benzene19 (1.07 g, 1.06 mmol) and CuCN (0.88 g, 9.8 mmol) in DMF (20 mL) was heated at reflux under N2 for 22 h. The mixture was then cooled to 25 °C and diluted with H2O. The resulting precipitate was separated by filtration, suspended in H2O, and treated with ethylenediamine (2 mL). The mixture was stirred at 25 °C for 30 min, and the solid was collected by filtration. The solid was again suspended in H2O and treated with ethylenediamine. Finally, the remaining solid was separated by filtration and extracted with CH3CN. Solvent was removed from the extracts by evaporation under reduced pressure to give crude product (0.72 g), which was recrystallized from dioxane to give hexakis(4-cyanophenyl)benzene (1; 0.46 g,

466 Crystal Growth & Design, Vol. 6, No. 2, 2006 0.67 mmol, 63%) as nearly colorless needles; mp > 250 °C; 1H NMR (400 MHz, CD3CN) δ 7.07 (d, 3J ) 8.4 Hz, 12H), 7.32 (d, 3J ) 8.4 Hz, 12H); 13C NMR (100 MHz, CD3CN) δ 110.5, 118.7, 131.5, 132.2, 139.4, 144.1. HRMS (APCI) calcd for C48H24N6+Na m/e 707.19547, found 707.19544. X-ray Crystallographic Studies. X-ray diffraction data were collected with Cu KR radiation using a Bruker SMART 2000 CCD or a Bruker Smart 6000 CCD diffractometer equipped with an FR591 rotating anode generator. The structures were solved by direct methods using SHELXS-97 and refined with SHELXL-97.24 All non-hydrogen atoms were refined anisotropically, except for disordered molecules of included solvent. Hydrogen atoms were placed in ideal positions and refined as riding atoms.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Ministe`re de l’EÄ ducation du Que´bec, the Canada Foundation for Innovation, and the Canada Research Chairs Program for financial support. In addition, we thank Prof. Jurgen Sygusch for providing access to a Bruker SMART 6000 CCD diffractometer equipped with a rotating anode. Supporting Information Available: Further structural details, including ORTEP drawings and tables of crystallographic data, atomic coordinates, anisotropic thermal parameters, and bond lengths and angles (including weak interactions) for compound 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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