An (8,3)-a 3D Coordination Network and Concomitant Three-Connected Supramolecular Isomers Brian S. Luisi,† Victor Ch. Kravtsov,‡ and Brian D. Moulton*,† Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02906, and Institute of Applied Physics of the Academy of Sciences of R. MoldoVa, Academy Street 5, MD2028 Chisinau, R. MoldoVa
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2207-2209
ReceiVed August 9, 2006
ABSTRACT: An unprecedented (8,3)-a 3D coordination network was synthesized using crystal engineering strategies as one of three concomitant supramolecular isomers of the general formula [Cu2(tae)(bipy)2]‚2NO3-‚xS (bipy ) 4,4′-bipyridine; tae ) dibasic tetraacetylethane; S ) solvent: H2O/MeOH). The (8,3)-a network yields optically active single crystals with 58% solvent-accessible volume, despite 4-fold interpenetration. The design and synthesis of molecular materials having specific topologies and advantageous bulk physical properties remains an important challenge.1 A common approach is to exploit the wellunderstood coordination geometry of transition-metal cations via coordination to multifunctional ligands.2 Varying the geometry and functionality of the ligands has led to the formation of many different topologies, which have been shown to exhibit important bulk properties such as porosity,3 chirality,4 and magnetism.5 Indeed, many of the uniform nets described by Wells6 have been observed in molecular materials. In addition, materials with the same composition (i.e., metal and ligand) can possess different topologies via supramolecular isomerism.7 In this communication, we report a design strategy for, and the successful preparation of, a material that possesses the (8,3)-a network topology, which also resulted in the concomitant synthesis of two supramolecular isomers possessing the (10,3)-a and (10,3)-b topologies. To the best of our knowledge, this is the first reported example of a molecular (8,3)-a net. Previously, Cieren et al.8 reported on the synthesis of a solid-state (8,3)-c net, and the synthesis of an (8,3)-b structure has also been recently reported.9 Figure 1 illustrates an ideal (8,3)-a network (P6222; 6(i), x)2/5; c/a)3x2/5), which is intrinsically chiral. Unlike the related three-connected chiral (10,3)-a network that commonly interpenetrates as a racemate,10 the (8,3)-a networks described herein do not interpenetrate with an enantiomer; rather, networks with the same handedness interpenetrate, necessarily yielding an optically active single crystal. Furthermore, the cross-sectional area of the [001] pore in an (8,3)-net is nearly two times larger than the corresponding pore in a (10-3)-a net having the same edge length, despite having less vertexes in its smallest circuit. The (8,3)-a net can be most simply described as a network of connected trigonal planar vertexes. The synthetic challenge is that more than 10 of Wells’ 3D nets can also be theoretically formed by the connection of trigonal planar vertexes,11 as can the twodimensional (6,3)-net; the only difference lies in the torsion angles between the vertexes. For the ideal (8,3)-a net, the angles are ca. 70.5° for two of the connections, and ca. 38.9° for the other. We, therefore, decided to exploit a coordination chromophore that had a 2:1 ratio of two different ligands. Specifically, we chose to take advantage of the bis(pyridyl)/acetylacetonate chromophore.12 A search of the Cambridge Structural Database13 (CSD) reveals 26 structures having this chromophore, 25 of which utilize copper(II); however, there are no examples that contain bifunctional ligands exclusively. There have been a few closely related coordination polymers reported: 1D chains composed of a bis(acetylacetonate)-bis(4,4′-bipyridine) chromophore of zinc(II)14; 1D * To whom correspondence should
[email protected]. † Brown University. ‡ Academy of Sciences of R. Moldova.
be
addressed.
E-mail:
Figure 1. Ideal 8,3-a net.
coordination ladder15 composed of copper(II) bonded to tetraacetylethane and 4,4′-bipyridine; a molecular square synthesized with cobalt(II) and tetraacetylethane was reported with end-capping ligands that prevented polymer formation.16 [Cu2(tae)(bipy)2]‚2NO3-‚xS (bipy ) 4,4′bipyridine; tae ) dibasic tetraacetylethane; S ) solvent: H2O/MeOH), 1, and two concomitant supramolecular isomers (i.e., same formula), 2 and 3, were prepared by slow diffusion of a methanolic solution of bipy into a methanolic solution of tae and copper(II) nitrate across an inverted U-tube bridge. Blue-green, prismatic crystals of each isomer were collected after 3 days from distinct regions of the bridge. 1 crystallizes in P6122 and features 4-fold interpenetrated networks of the same handedness; however, examination of multiple crystals reveals that the bulk material is racemic due to a mixture of P6122 and P6522 crystals. The networks, illustrated in Figure 2, can be described as Cu(II) nodes located at the vertexes of an (8,3)-a network. The copper(II) chromophore is in a distorted octahedral environment, with each copper chelated by one tae and coordinated by two bipy ligands around the equatorial positions and disordered nitrate and water at the axial positions. Thermogravimetric analysis of 1 shows a weight loss of ca. 23% at 118 °C, attributed to loss of the disordered lattice solvent, presumed to be methanol. This is in good agreement with the loss of methanol from [Cu2(tae)(bipy)2]‚2NO3-‚7MeOH (22.8% MeOH by mass). The onset of thermal decomposition is observed at ca. 175 °C. As is evident by the large amount of lattice solvent, despite 4-fold interpenetration, the networks comprise only ca. 42% of the unitcell volume.17 Indeed, there is a large volume of solvent-accessible channels that run though the structure that can also be described as an (8,3)-a net having the same handedness as the parent coordination networks. However, this space cannot accommodate another coordination network, as it does not have congruent dimensions (i.e., [001] pore diameter is 50% smaller).
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2208 Crystal Growth & Design, Vol. 6, No. 10, 2006
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Figure 2. (a) Orthographic projection down [001] for the (8,3)-a network in 1 (100% VDW radii). The large channels have an effective diameter of ca. 23.5 Å, but porosity is reduced because of 4-fold interpenetration. (b) depiction of the local connectivity around Cu(II) centers in 1, and similarly in supramolecular isomers 2 and 3.
Figure 3. (a) Orthographic projection down [001] of one of the (10,3)-a networks in 2; (b) orthographic projection down [110] of one of the (10,3)-b networks in 3 (solvent molecules omitted for clarity). Table 1. Expected and Observed Torsion Angles for 1-3 compd
topology
angle
expected (deg)
observed (deg)
1
(8,3)-a
2
(10,3)-a
1 2, 3 1-3
38.9 70.5 70.5
3
(10,3)-b
71.5 89.3 80.8 (1) 54.6 (2, 3) 78.9 180
1 2, 3
90 180
2 and 3 form as concomitant supramolecular isomers in the reaction that yielded 1 and are depicted in Figure 3. 2 crystallizes in P42/nbc and is comprised of 4-fold interpenetrated (10,3)-a networks (two of each enantiomer), whereas 3 crystallizes in P21/n and is comprised of 4-fold interpenetrated (10,3)-b networks. Copper(II) nodes, which occupy the vertexes of both networks, are in a distorted octahedral environment in each structure. In 2, Cu(II) is coordinated to one tae and two bipy ligands around the equatorial positions and two methanol molecules at the axial positions. Charge-compensating nitrates are disordered over 2-fold rotation axes and are located in lattice voids, with the shortest close contacts to coordinated methanol (O‚‚‚O ca. 2.7 Å). In 3, the Cu(II) nodes are connected to one tae and two bipy ligands around the equatorial positions and water at the axial sites. Chargecompensating nitrates can be found in the lattice voids, with the shortest close contacts to the coordinated water molecules (O‚‚‚O ca. 2.9-3.0 Å). From a design perspective, the primary structural difference between the networks in compounds 1-3 should be the torsion angles between the copper vertexes. However, as can be seen in Table 1 (which indicates the expected and observed torsion angles for the structures reported herein), an added difficulty arises from
the fact that the network connectivity can withstand significant distortion from the expected torsion angles calculated from the ideal network geometry. This is, in part, a response to the different edge lengths that necessarily result from coordination to the different ligands: the copper-copper distance bridged by bipy is nearly 38% longer (11 Å) than the copper-copper distance bridged by tae (8 Å). Synthetically, our approach was to increase the diffusion length typically used in coordination polymer syntheses from ca. 5 to ca. 30 cm in order to generate larger regions of the necessary solute concentrations in which the various supramolecular isomers nucleate. Indeed, four distinct zones of crystal growth were present after only 3 days. The crystals located in the area of highest tae concentration (i.e., closest to the copper nitrate/tae feedstock) were determined to be compound 1, with compounds 3 and 2 located further along the diffusion axis, respectively. Additionally, in a region of relatively high bipy concentration, blue, prismatic crystals that are isostructural with the previously reported M(bipy)2X2 (M ) transition metal, X ) anion) square-grid coordination polymers18 were collected. In summary, we have synthesized the first example of an (8,3)-a network topology and have shown this to be an advantageous synthetic target with respect to other three-connected nets, as it has the largest primary pore diameter. Furthermore, materials that have maximally interpenetrated networks maintain a low density, and generate a well-structured chiral channel structure. This relationship between the channel structure and the networks appears to favor the formation of optically active materials, i.e., networks of the same handedness interpenetrate. We have also described how lengthening the diffusion axis offers opportunities to identify new supramolecular isomers and, specifically, new topologies. These
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Crystal Growth & Design, Vol. 6, No. 10, 2006 2209
results underscore how the principles of crystal engineering can be applied to the synthesis of new molecular materials, and the importance of identifying new molecular network topologies. Acknowledgment. This work has been supported in part by MRDA (Moldova)-CRDF (United States), Award MRDA-008: BGP-III, MOC-3063-CS-03. Supporting Information Available: Crystallographic and thermogravimetric data for 1-3; illustration of channel structure. This material is available free of charge via the Internet at http://pubs.acs.org
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