Disentangling Disorder in the Three-Dimensional Coordination Network of {Ag3[Tris(2-pyridylmethyl)cyclotriguaiacylene]2}(PF6)3
2005 VOL. 5, NO. 4 1321-1324
Christopher J. Sumby and Michaele J. Hardie* School of Chemistry, University of Leeds, Leeds LS2 9JT, UK Received February 18, 2005;
CRYSTAL GROWTH & DESIGN
Revised Manuscript Received April 25, 2005
ABSTRACT: The crystalline complex {Ag3[Tris(2-pyridylmethyl)cyclotriguaiacylene]2}(PF6)3 has been synthesized and shown by X-ray crystallography to have a disordered three-dimensional cubic coordination network structure. The coordination network features a back-to-back stacking of the molecular host-type ligands into tetrahedral clusters, and the PF6- anions act as both intracavity guests and lattice-type guests. Disentangling the network disorder gives a 4-fold interpenetrating (10,3)-a net. The field of coordination networks, also termed metalorganic frameworks, has become popular recently due to both the fascinating structural chemistry afforded by these complexes and their strong potential for a myriad of applications.1 Potential and realized applications include gas storage, heterogeneous catalysis, molecular separations, magnetism, and switches. Many of these applications are based on the ability of the coordination network to act as a host for other guest species, which may occur when the coordination network shows channels or cavities. All coordination networks consist of metal centers linked by multifunctional ligands, and there have been a handful of examples in which the multifunctional ligand is itself a molecular host, that is, a molecule with an intrinsic ability to complex a guest species. These molecular hosts include functionalized calixarenes with sulfonate,2 phosphate,3 and diquinone groups;4 cucurbiturils;5 crown ethers;6 and, the main focus of our research, cyclotriveratrylene.7 Cyclotriveratrylene (CTV) has three dimethoxy groups that can bind Group 1 metal cations or Ag(I) to form coordination networks, typically with chain or two-dimensional (2-D) grid structures.7 These dimethoxy groups may also be utilized as hydrogen-bond acceptors, and examples of chain, 2-D7,8 and three-dimensional (3-D)9 hydrogenbonded network structures are known. Related cavitand hosts can be synthesized with extended molecular cavities through addition of functional groups at the outer rim, with both hexa- and trisubstituted cavitands accessible.10-13 These additional groups can include ligand functionality suitable for binding to a transition metal cation.11-13 We have recently reported the synthesis of a range of pyridylfunctionalized CTV derivatives intended for use as multifunctional ligands for coordination networks.12 An example of such a network is found in complex {Ag[Tris(isonicotinoyl)cyclotriguaiacylene]2 }[Co(C 2 B 9 H 11 ) 2 ]‚9(CH 3 CN) where doubly bridged {Ag[Tris(isonicotinoyl)cyclotriguaiacylene]2}+ chains interweave into a 2-D sheet through π-π stacking and host-guest interactions.13 We report herein a cubic 3-D coordination network with the related ligand Tris(2-pyridylmethyl)cyclotriguaiacylene 1.
* To whom correspondence should be addressed. E-mail: m.j.hardie@ leeds.ac.uk.
The synthesis of ligand 1 has been previously reported and involves reacting 2-bromomethylpyridine hydrobromide with cyclotriguaiacylene in the presence of base.12 Crystals of complex [Ag3(1)2](PF6)3 2 were obtained from diffusion of ether into a 1:1 mixture of AgPF6 and 1 in acetonitrile in yields ranging from 60 to 70%, over several such experiments.14 The structure of 2 was determined by single-crystal X-ray diffraction techniques.15 The crystals were very poorly diffracting with observed data to only a 2θ value of around 43°. An isostructural species can be obtained with AgSbF6 with cubic unit cell parameters of 35.424(7) Å, which was subjected to a high-intensity X-ray source (synchrotron radiation), but this did not result in an improved data set. The structure shows significant disorder, which accounts for the poor diffraction; however, there are sufficient data to allow for the key structural features of 2 to be elucidated. Significant research efforts were expended in attempts to obtain an improved data set for the crystal of 2, but the disorder appears to be inherent to the crystals and ubiquitous to this system. Complex 2 crystallizes in a cubic unit cell, and the structure was solved in space group F43c. The asymmetric unit consists of one partially occupied Ag(I) site, a third of a ligand, and three PF6- sites, all on special positions and at low occupancy. The occupancy of the Ag was set at a half based on microanalytical data collected on a batch of hand-picked crystals of identical morphology, which indicated that the complex has a 3:2 metal-to-ligand stoichiometry. This stoichiometry was reinforced by refinement of the occupancy of the silver site, which refines to an occupancy of ca. 0.5. This composition gives the most chemically reasonable solution. Additionally, the silver site has a relatively high Ueq, indicating a degree of dynamic disorder for this position. Within complex 2, ligand 1 adopts C3 symmetry with its pyridylmethyl arms oriented such that each pyridyl group lies with its face nearly orthogonal to that of the host arene ring to which it is attached. The pyridyl group was modeled as disordered over two positions, each at 50% occupancy (Figure 1). Each ligand is surrounded by six symmetry equivalent, though half occupied, Ag(I) sites, and coordinates through the disordered pyridyl groups at Ag-N distances 2.29(4) and 2.16(4) Å. The Ag(I) centers are also perched above an oxygen atom of the host at a Ag‚‚‚O separation of 2.75 Å, which is too long to consider as a strong interaction. The Ag coordination geometry is therefore distorted linear with a N-Ag-N angle of 172(1)°. Symmetry expansion of the Ag(I) coordination results in a disordered coordination square motif within the extended structure of complex 2, as illustrated in Figure 2a. The adjacent and opposite Ag‚‚‚Ag separations within these
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Figure 1. Coordination and host-guest interactions of ligand 1 from the X-ray structure of complex [Ag3(1)2](PF6)3 2. Ag(I) centers shown as yellow spheres. Each pyridyl orientation is at 50% occupancy.
squares are 5.64 and 7.98 Å, respectively. Each ligand connects into three of these square motifs. The disordered nature of the Ag(I) centers and pyridyl groups within 2 mean that, while there are four pyridyl groups within this square motif, there are only two Ag(1) metal ions disordered over four positions. The disorder model adopted is shown schematically in Figure 2b and has two orientations of the linearly coordinated Ag(1) centers and pyridyl ligands. In each orientation, both N-Ag-N vectors are aligned in a parallel fashion. The other important structural motif within the extended structure of 2 is also tetrameric but with a tetrahedral symmetry. Ligands pack together in a back-to-back fashion to form tetrameric clusters of 1 shown in Figure 3a (note that pyridyl groups are shown at a single averaged
Communications position for the sake of clarity). The four ligands are arranged in an exact tetrahedron, and the arene groups from the core of the ligands form π-π stacking interactions with one another at centroid separations of 3.83 Å. This type of back-to-back tetramer formation has been previously reported for CTV within complicated 3-D hydrogenbonded network structures9 and has parallels with the back-to-back stacking of 12 p-sulfonatocalix[4]arenes into giant icosahedral or cuboctahedral assemblies.16 The orthogonal orientation of the pyridyl arms of 1 effectively means that the size of this [1]4 cluster is very similar to that of the previously reported [CTV]4 cluster, with both being around 16 Å across. Ligand 1 is chiral, and each ligand within an individual [1]4 cluster is of the same hand. The tetrahedral [1]4 clusters are linked together via the Ag(I) centers, which is shown for two [1]4 tetramers in Figure 3b where the linked [1]4 clusters are rotated 90° with respect to one another, and the individual ligands within them have opposite chiralities. Each [1]4 cluster is surrounded by six Ag “square motifs”. If the centroids of these square motifs are taken then they form a perfect octahedron surrounding the tetrahedral [1]4 cluster. Again, this shows a very strong similarity to the 3-D hydrogenbonded assembly of [Sr(H2O)8][(CH3CN)(CTV)]4(H2O)4[Co(C2B9H11)2]2 in which each [CTV]4 tetrahedron is surrounded by six Sr metal centers arranged in an octahedron.9b In the overall structure of 2, each [1]4 cluster links to six others through coordinate interactions identical to those shown in Figure 3b. The overall packing diagram is shown in Figure 4 and reveals a cubic 3-D coordination network with narrow channels. The molecular cavity of the ligand is occupied by a disordered PF6- counteranion which was refined at 66% overall occupancy (Figure 1). The phosphorus of this anion is sited on the same crystallographic 3-fold rotation axis
Figure 2. Coordination square motif formed by linear bridging Ag(I) centers and four ligands from the crystal structure (only a section of the ligand is shown for clarity); (a) symmetry expansion showing all positions; (b) the two disordered positions shown separately, each with 50% occupancy.
Figure 3. Tetrameric cluster of ligands 1 within complex 2. Linking the centers of each ligand within the [1]4 cluster forms a perfect tetrahedron. (a) A single [1]4 cluster; (b) linking of two [1]4 tetramers (one in gray, one in green) by linear Ag(I) centers. Disordered pyridyl groups are shown as a single averaged position for clarity.
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Figure 4. Full packing diagram of complex 2 showing the cubic [Ag3(1)2]3+ coordination network and disordered PF6- positions. Disordered pyridyl groups are shown as a single averaged position for clarity.
Crystal Growth & Design, Vol. 5, No. 4, 2005 1323 (10,3)-a net is the most common 3-D 3-connected network structure.17 The bulk material is insoluble in common nonpolar solvents but, due to the labile nature of the bonding, readily dissociates in coordinating solvents (e.g., CH3CN). Moreover, the crystals of 2 rapidly decompose upon removal from the supernatant. Consequently, no anion exchange experiments were attempted, but it is not anticipated that any exchange would be possible given that this is not a particularly porous network. Space-filling models indicate the low occupancy anions are essentially trapped within the network. The poor data also prevented the identification of any included solvate molecules (possible low occupancy solvate water molecules) that could potentially be exchanged. The structure of complex 2, while displaying significant disorder and a highly complicated packing diagram, can be understood as a 4-fold interpenetrating (10,3)-a net. This is the first example of a 3-D coordination network formed from a ligand based on the CTV host molecule. Within the structure, there are structural motifs very similar to those displayed by 3-D hydrogen-bonding networks involving CTV, namely, the back-to-back packing of host molecules into a tetrahedral cluster. Acknowledgment. We thank the EPSRC for financial support and Dr. John Warren of the Daresbury SRS for obtaining a synchrotron data set of the SbF6- analogue. Supporting Information Available: Crystallographic information file (CIF) is available free of charge via the Internet at http://pubs.acs.org.
References Figure 5. One disentangled (10,3)-a net of complex 2. Note that all disordered pyridyl positions on ligand 1 are chosen to retain the C3 axis of 1.
that the ligand is sited around; hence, the P atom is directly above the center of the -(CH2)3- plane of the ligand at a distance of 4.51 Å. The closest contact between the host ligand and guest anion is 2.57 Å between a fluorine and an arene carbon atom. A half occupied PF6- sits in a cleft of the [1]4 tetrahedral assembly. Disordered, low occupancy PF6- anions also occupy the narrow channels that run in three orthogonal directions throughout the coordination network. The disorder of the Ag(I) centers and pyridyl groups makes it nontrivial to ascertain the network topology. The network is 3-connected as that is the true connectivity of the ligand, but the extensive disorder mimics a 6-connected net. Considering the network as 3-connected, then a recognizable net can be identified provided that a single assumption is made, namely, that the C3 symmetry of the ligand holds (in other words, that the three pyridyl groups around each ligand are symmetry related rather than being two of one disorder position and one of the other). This assumption is also supported by the cubic symmetry of the structure. Once this assumption is made then the disordered network can be disentangled and gives a 4-fold interpenetrating (10,3)-a net. One such net is shown in Figure 5. The [1]4 tetrahedral clusters are where the four interpenetrating networks have the closest contact, and each ligand within a tetrahedral cluster belongs to a different (10,3)-a net. Considering the alternative position of the disorder model places the central core of the ligands in identical positions, but the Ag-pyridyl linkages run in a different direction, giving the opposite handed net. The
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Crystal Growth & Design, Vol. 5, No. 4, 2005 Chem. 2000, 39, 5768. (d) Wytko, J. A.; Boudon, C.; Weiss, J.; Gross, M. Inorg. Chem. 1996, 35, 4469. (e) Matouzenko, G.; Ve´riot, G.; Dutasta, J.-P.; Collet, A.; Jordanov, J.; Varret, F.; Perrin, M.; Lecocq, S. New. J. Chem. 1995, 19, 881. Hardie, M. J.; Mills, R. M.; Sumby, C. J. Org. Biomol. Chem. 2004, 2, 2958. Hardie, M. J.; Sumby, C. J. Inorg. Chem. 2004, 43, 6872. Synthesis of 2. A solution of Tris(2-pyridylmethyl)cyclotriguaiacylene (9.7 mg, 0.014 mmol) dissolved in hot acetone was mixed with an acetonitrile solution of silver hexafluorophosphate (10.4 mg, 0.041 mmol). Slow evaporation of the solution to dryness gave an off-white solid that was recrystallized by slow vapor diffusion of ether into a filtered acetonitrile solution of the complex. Analysis: calc. for C42H39N3O6F9P1.5Ag1.5 C 47.54, H 3.71, N 3.96; found C 45.9, 46.3, H 3.80, 3.75, N 4.05, 4.20% (samples of hand-picked crystals were dried in vacuo at 60 °C prior to analysis). X-ray data for 2. C42H39N3O6F9P1.5Ag1.5 FW ) 1061.02, cubic, F43c, a ) 35.800(4), V ) 45883(9) Å3, Z ) 32, F ) 1.229 g cm-3, µ ) 0.626 mm-1, F(000) ) 17088, colorless block 0.23 × 0.21 × 0.18 mm, 2θmax ) 43.4°, Mo KR (λ ) 0.71073 Å), T ) 150(1) K, 10409 reflections, 2232 unique, Rint ) 0.0832, 249 parameters, 105 restraints, GOF ) 2.748, wR2 ) 0.4178 for all data, R1 ) 0.1574 for 1465 data with I > 2σ(I). The pyridyl aromatic ring was modeled as disordered over two positions and restrained to be flat and have chemically reasonable bond lengths, while the other arene group was constrained with
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