Three Trigonal Coordination Extended Solids with Gyroid and

Three Trigonal Coordination Extended Solids with Gyroid and Lamellar Topologies Abhijit Basu Mallik, Stephen Lee,* and Emil B. Lobkovsky Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 609-616

Received June 1, 2004

ABSTRACT: In this paper, we report the syntheses and crystal structures of three silver(I)-based coordination extended solids, which are topologically similar with two minimal curvature structures, the gyroid and lamellar types. A minimal curvature structure is one in which there is a surface where the two principal curvatures are everywhere equal and opposite. 5-(4-Ethynyl pyridine)pyrimidine (1) has been crystallized with AgBF4: it forms as two interpenetrating three-dimensional networks each with [10,3] (SrSi2) topology. Such a structure bears a strong resemblance to the gyroid phase. 5-(4-Ethynyl benzonitrile)pyrimidine (2) and 5-(4-ethynyl benzonitrile)nicotinonitrile (3) were crystallized with, respectively, AgSbF6 and AgBF4. The structures contain two-dimensional honeycomb networks, which are topologically equivalent to the lamellar phase. A further Cambridge Structural Database analysis of Ag-N coordination networks revealed that the majority of the structures adopt minimal curvature topology. Out of 53 structures studied, 50 have topologies similar to the lamellar and gyroid structure types. Of these, only one of the hitherto prepared structures contains doubly interpenetrated [10,3] networks compatible with the gyroid structure. Introduction One of the major advances in recent years has been the emergence of principles governing crystal packing and the use of these same principles to make materials with desired properties.1-25 The development of these principles is often gradual. They are first applied to one family of compounds, and then, only later are they applied to other systems. This crossover of ideas not only helps the development of true universal principles of crystal topology and design but also gives chemists greater confidence in their insights into crystal packing. In this paper, we consider principles of crystal packing, hitherto, mainly applied to large length-scale organic systems such as block copolymers or liquid crystals23-30 and to inorganic2-4,31-33 or biological systems34-39 and apply them to the field of small organic molecule coordination extended solids.40 We focus on the structure types common in large length-scale systems, in this case the gyroid and lamellar topologies. As in previous work,41,42 we concentrate on the often amphiphilic nature of gyroidal and lamellar systems, but here, we consider how the local coordination environment of both metal and organic molecules can further help lead to the formation of these topologies. The lamellar and gyroid topologies adopted by block copolymers and liquid crystals are intimately connected to the amphiphilic nature of these compounds. In both topologies, an interface develops that separates the structure in two equal parts. The interface is of a particular nature: The curvature of the interface is zero everywhere. This zero curvature leads to a local minimization of the surface area. For block copolymers and liquid crystals, where there are often two immiscible components in forced contact with one another, such a minimal interface is energetically favorable. * To whom correspondence should be addressed. E-mail: sl137@ cornell.edu.

If we are to look for coordination extended solids that adopt these architectures, we need to consider coordination solids, which have two such immiscible components in forced contact with one another. In this paper, we will consider silver-aromatic ligand complexes. In such systems on one hand, there will be the low dielectric medium composed of the soft silver ions and the aromatic component and a high dielectric component due to the charged counteranions. However, choosing immiscible constituents by itself is not enough to ensure the formation of these two topologies. In this paper, we consider the local coordination environment of both the metal ions and the multitopic organic ligands as an additional design aid. In particular, we note that it is the trigonal planar coordination environment that is most compatible with the gyroid and lamellar topologies. This is so as both topologies can be derived from a trigonal coordination network. The lamellar structure is compatible with the planar honeycomb sheet; the gyroid framework can be derived from a doubly interpenetrated SrSi2 type topological network.3,4,36,38,43-46 In this paper, we report the synthesis and X-ray structure of three silver(I)-aromatic ligand extended solids. In these structures, as is often the case, the silver(I) ions adopt a trigonal planar coordination environment. The organic ligands are tritopic N-containing species. The ligands used are 5-(4-ethynyl pyridine)-pyrimidine (1), 5-(4-ethynyl benzonitrile)pyrimidine (2), and 5-(4-ethynyl benzonitrile)nicotinonitrile (3) (Scheme 1). Ligand 1 crystallizes with AgBF4 to form a three-dimensional interpenetrating network topologically similar to the gyroid phase, a topology only rarely seen in coordination extended solids. Ligands 2 and 3 form two-dimensional honeycomb networks with, respectively, AgSbF6 and AgBF4; in these structures, there is a clear resemblance to the lamellar topology.

10.1021/cg0300679 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/14/2004

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Experimental Section General Procedure. Unless otherwise indicated, all commercially available reagents were purchased from Aldrich and used without further purification. Analytical grade solvents were obtained from commercial suppliers (Aldrich, Fisher Scientific, and Mallinckrodt). All atmosphere sensitive reactions were conducted under nitrogen using a Schlenk vacuum line. 1H NMR and 13C NMR were performed on a Bruker AF-400 spectrometer at 25 °C. High-resolution mass spectra (HRMS) were collected on a Micromass 70-4F spectrometer. For the crystallization experiments, Teflon-lined screw caps were used to seal the vials. No additional precautions were employed to exclude oxygen or moisture during crystallization. For X-ray powder analysis, the crystalline samples were sealed in special 0.5 mm glass capillary tubes with small amounts of the mother liquid to prevent the degradation of crystallinity. Single crystal X-ray data were collected on a Bruker SMART diffractometer equipped with a CCD area detector using Mo KR radiation. Single crystal diffraction data were collected at 173 K. The structure solution was obtained by direct method and refined using full-matrix least squares with Shelxl 97. All nonhydrogen atoms were refined anisotropically. Tables of bond distances, bond angles, anisotropic thermal factors, observed and calculated structure factors, and powder data appear in the Supporting Information. Powder X-ray diffraction data were recorded on an INEL MPD diffractometer (XRG 3000, CPS 120 detector) at 25 mA and 35 kV for CuKR1; λ ) 1.54056 Å, with external silver behenate and elemental silicon as standards. The lattice constants were fit, and powder data were indexed with a leastsquares method. 5-(4-Ethynyl pyridine)pyrimidine (1). 5-Bromopyrimidine (0.1 g, 0.628 mmol), 4-ethynyl pyridine (78 mg, 0.754 mmol), bis(triphenylphosphine)palladium(II) chloride (9 mg, 0.012 mmol), copper(I) iodide (1.2 mg, 0.006 mmol), and triphenylphosphine (16.5 mg, 0.06 mmol) were combined with triethylamine (5 mL) in a dry, heavy-walled tube sealed with a Teflon screw cap. The resulting mixture was degassed and back-filled with nitrogen three times and left under nitrogen at room temperature. The tube was then sealed with the Teflon screw cap and placed in an oil bath. Heating at 70 °C for 12 h produced an orange solution with a precipitate, which was presumed to be triethylammonium bromide. The mixture was cooled to room temperature diluted with dichloromethane (10 mL) and filtered to remove the precipitate. The orange filtrate was concentrated in vacuo to yield crude product as a dark red liquid. Column chromatography (2/1 hexane/ethyl acetate) afforded 1 as a light yellow oil, which was crystallized as a white solid upon standing under vacuum for 1 h (0.102 g, 80% yield): Rf 0.3 (2/1 hexane/ethyl acetate). 1H NMR (400 MHz, CDCl3): δ 9.15 (s, 1H), 8.85 (s, 2H), 7.7-7.6 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 160.33, 158.58, 150.218, 130.03, 128.42, 119.06, 93.65, 86.67. HRMS (EI, 70 eV) m/z 182.0705 [(M + H)+; calcd for C11H8N3, 182.0718]. 5-(4-Ethynyl benzonitrile)pyrimidine (2). Compound 2 was synthesized in 79% yield in the same procedure as for 1 except that 4-ethynyl benzonitrile was used instead of 4-ethynyl pyridine [5-bromopyrimidine (0.1 g, 0.546 mmol), 4-ethynyl benzonitrile (83 mg, 0.655 mmol), bis(triphenylphosphine)palladium(II) chloride (8 mg, 0.011 mmol), copper(I) iodide (1 mg, 0.005 mmol), triphenylphosphine (14.3 mg, 0.054 mmol), and triethylamine (5 mL)]. The eluent for column chromatog-

Mallik et al. raphy was hexane/ethyl acetate (2:1). 1H NMR (400 MHz, CDCl3): δ 9.21 (s, 1H), 8.90 (s, 2H), 7.70-7.62 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 161.10, 158.58, 151.76, 132.51, 132.48, 126.77, 118.39, 112.96, 94.45, 86.47. HRMS (EI, 70 eV) m/z 206.0725 [(M + H)+; calcd for C13H8N3, 206.0718]. 5-(4-Ethynyl benzonitrile)nicotinonitrile (3). Compound 3 was synthesized in 79% yield in the same procedure as for 1 except that 5-bromonicotinonitrile and 4-ethynyl benzonitrile were used instead of 5-bromopyrimidine and 4-ethynyl pyridine, respectively [5-bromonicotinonitrile (0.1 g, 0.628 mmol), 4-ethynyl benzonitrile (96 mg, 0.754 mmol), bis(triphenylphosphine)palladium(II) chloride (9 mg, 0.012 mmol), copper(I) iodide (1.2 mg, 0.006 mmol), triphenylphosphine (16.5 mg, 0.06 mmol), and triethylamine (5 mL)]. The eluent for column chromatography was hexane/ethyl acetate (2:1). 1H NMR (400 MHz, CDCl3): δ 9.21 (s, 1H), 8.90 (s, 1H), 7.88 (s, 1H), 7.71-7.63 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 155.34, 151.50, 141.56, 132.60, 132.52, 126.51, 118.51, 117.75, 113.15, 93.36, 87.45. HRMS (EI, 70 eV) m/z 230.0730 [(M + H)+; calcd for C15H8N3, 230.0718]. X-ray Quality Single Crystals of 1. AgBF4 (4). A mixture of 1 (5 mg, 0.027 mmol) and silver(I) tetrafluoroborate (5.2 mg, 0.027 mmol) in benzene (5 mL) was placed in a clean vial with a Teflon-lined screw cap. A white precipitate formed immediately. Ethanol (2 mL) was added to the precipitate. After 10 min of heating in an oil bath at 60 °C, a homogeneous solution was observed. The solution was cooled to room temperature overnight. Subsequent evaporation of ethanol with the screw cap loosely attached produced colorless plates after 2 days. The X-ray powder diffraction data taken on the bulk sample indicate that the primary product of this procedure is 4 as major peaks in the powder data correspond to the peaks calculated from the solved single crystal structure. However, an additional impurity phase is also revealed by the presence of an additional diffraction peak in the bulk sample powder pattern. The additional peak is marked by an asterisk in Figure 1 of the Supporting Information. X-ray Quality Single Crystals of 2. AgSbF6 (5). The same method was applied as for 4. Ligand 2 and AgSbF6 were used instead of 1 and AgBF4 [2 (5 mg, 0.024 mmol), silver(I) hexafluoroantimonate (8.3 mg, 0.024 mmol), benzene (5 mL), and ethanol (2 mL)]. Colorless needle-shaped single crystals were obtained as the final product. The X-ray powder diffraction pattern of the bulk sample corresponds to the pattern generated by the solved single crystal structure of 5; see the Supporting Information. No second phase was detected in this diffraction pattern. X-ray Quality Single Crystals of 3. AgBF4 (6). The same method was applied as for 4. Ligand 3 was used instead of 1 [3 (5 mg, 0.022 mmol), silver(I) tetrafluoroborate (4.2 mg, 0.022 mmol), benzene (5 mL), and ethanol (2 mL)]. Colorless needleshaped single crystals were obtained as the final product. The X-ray powder diffraction pattern of the bulk sample corresponds to the pattern generated by the solved single crystal structure of 6; see the Supporting Information. No second phase was detected in this diffraction pattern.

Results The crystallographic descriptions of the three structures are given below. Crystal data and details of the structure determinations for complexes 4-6 are given in Table 1. Crystal Structure of 1. AgBF4 (4). The crystal structure of complex 4 is shown in Figure 1a. Each silver(I) center, shown as red and yellow large spheres in the figure, connects three ligands of 1. Similarly, each ligand molecule is connected to three silver atoms. The nitrogen atoms in the figure are shown as small spheres. The coordination geometry around each silver(I) center is distorted trigonal planar with N-Ag-N bond angles of 94.2, 122.5, and 143.0°. The Ag-N bond distances

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Table 1. Crystal Data and Structure Refinements for Compounds 4-6 formula mol wt crystal size (mm3) crystal color T (K) wavelength (Å) system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Fcalcd (g/cm3) absorption coeff (mm-1) θ range (°) limiting indices data/restraints/parameters measured reflns unique reflns absorption correction GOF on F2 Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] largest difference peak and hole (e Å-3)

4

5

6

C11H7AgBF4N3 375.88 0.30 × 0.20 × 0.30 colorless 173(2) 0.71073 orthorhombic Pbcm 20.1558(10) 6.9654(4) 21.9501(12) 90 90 90 3081.6(3) 8 1.620 1.340 2.11-24.77 -23 e h e 23 -8 e k e 8 -25 e l e 25 2634/124/216 18220 2634 SADABS 1.167 0.0565 0.0763 0.0960 1.598 and -1.017

C44H32Ag2F12N6Sb2 205.22 0.40 × 0.30 × 0.20 colorless 173(2) 0.71073 monoclinic P21/c 19.1637(8) 12.8407(4) 19.2025(7) 90 101.402(2) 90 4632.0(3) 4 1.910 2.074 1.37-33.14 -28 e h e 25 -19 e k e 18 -20 e l e 27 14375/0/595 46571 14375 SADABS 1.015 0.0276 0.0305 0.0715 1.095 and -1.143

C15H7AgBF4N3 423.92 0.40 × 0.20 × 0.10 colorless 173(2) 0.71073 monoclinic P2/n 12.5302(12) 8.4288(8) 14.9355(14) 90 109.308(2) 90 1488.7(2) 4 1.891 1.399 2.42-30.51 -17 e h e 17 -12 e k e 12 -21 e l e 21 3507/0/256 4540 3507 SADABS 1.025 0.0284 0.0390 0.1017 0.741 and -0.771

are 2.194, 2.256, and 2.385 Å. These trigonal coordination environments are seen canted in Figure 1a. The combination of tritopic 1 and tricoordinated silver atoms leads to the formation of two interpenetrating threedimensional networks. These networks are shown in red and yellow in Figure 1a. In Figure 1b, we show a schematic view of this double network. The networks are again shown in red and yellow. In this figure, the ligand molecules are represented by large spheres, while silver cations are shown as small spheres. These large spheres are located at the centroids of the ligand pyrimidine rings. As each ligand is coordinated to three silver atoms, these large spheres are three-coordinate nodes. The resultant view renders more clearly the topology of the overall system. The shortest closed loop of silver ions and ligand nodes is a decagon with five silver atoms and five pyrimidine rings (one such decagon is shown in black in Figure 1b; the decagon is nearly perpendicular to the plane of the paper). In the network notation put forward by Wells, both the red and the yellow threedimensional frameworks are [10,3] nets (10-membered rings and three-coordinate nodes).46-59 This [10,3] type network has the topology of the Si net in SrSi2. One key difference between structure 4 and SrSi2, however, is that in the latter, there is a single [10,3] net whose void space comfortably fits the Sr counterions, while in structure 4, the void spaces are large enough to contain not only the tetrafluoroborate counteranions but also a whole second [10,3] net as well. These two [10,3] nets are not translationally equivalent; they are of opposite chirality. This chirality is most clearly seen if one looks at the helices running in the crystallographic b-direction. One such helix is shown in the center of Figure 1b. (The black spheres labeled 1-5

belong to one full loop of one of these helices.) As this figure shows, the helices are 4-fold. The aforementioned helix has a counterclockwise pitch. However, to its right in the figure is a crystallographically equivalent red helix with a clockwise pitch. These two helices are related to one another by an inversion center. These two [10,3] nets are interdigitated. The closest contacts between the two nets involve the pyridine rings. Some of these overlapping pyridine rings can be seen slightly above the center of Figure 1a. They are shown here as interdigitated red and yellow aromatic rings. As this figure shows, the pyridine rings are in a staggered face-to-face arrangement. The C-C distances between adjacent pyridine atoms range from 3.49 to 3.61 Å, the closest distance between any adjacent aromatic carbon atoms. Crystal Structure of 2. AgSbF6 (5). The crystal structure of 5 is illustrated in Figure 2a. The silver atoms are shown as red and yellow large spheres whereas the nitrogen atoms are shown as small spheres. There are two crystallographically independent silver atoms and both adopt an approximately T-shaped coordination involving two pyridine and one nitrile nitrogen atom, the N-Ag-N bond angles being 94.4, 98.2, and 167.0° for Ag(1) and 95.9, 95.9, and 160.8° for Ag(2). The interatomic Ag-N distances vary substantially; they range from 2.19 to 2.54 Å. Once again, each silver atom is coordinated to three ligand molecules and each molecule is connected to three silver atoms. However, unlike complex 4, the alternation between silver atoms and ligand molecules leads to the formation of two-dimensional honeycomb [6,3] networks (sixmembered rings and three-coordinate nodes). Each sixmembered ring is composed of three silver ions and three organic molecules. Two such networks are shown in Figure 2a. The upper layer is shown in red, and the

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Figure 1. Crystal structure of 4. (a) View showing the stacking arrangement of the two interpenetrated Ag-1 nets. Silver atoms are shown as large spheres; nitrogen atoms are shown as small spheres. (b) A schematic representation as [10,3] nets where only trigonal nodes are shown. Large spheres represent the pyrimidine rings; small spheres represent silver atoms. The vertices of a 10-membered ring are numbered and shown in black. In both figures, the interpenetrating networks are shown in red and yellow. The counterions are not shown for the sake of clarity.

layer beneath is shown in yellow. As Figure 2a shows, the upper layer is rotated and shifted with respect to the layer below. The distance of approximately 6.4 Å between the neighboring layers precludes any substantial interaction between them. Instead, the hexafluoroantimonate counterions and benzene solvent molecules, both shown in green, fill up the space between the layers. This intercalation of the counterions and solvent molecules is more evident in Figure 2b. Here, we show a perpendicular view of the same structure. Once again, the honeycomb nets are shown in red and yellow while the counterions and solvent molecules are shown in green. As one can see, the SbF6- ions and benzene molecules are sandwiched between the honeycomb layers. Crystal Structure of 3. AgBF4 (6). The crystal structure of 6 is illustrated in Figure 3a. The silver atoms, shown as red and yellow large spheres, are in a slightly distorted trigonal planar geometry involving one

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Figure 2. Crystal structure of 5. (a) Stacking arrangements of two adjacent honeycomb nets along with solvent molecules and counterions. (b) Perpendicular view showing the intercalation of hexafluoroantimonate anions and benzene solvent molecules between the layers. Silver atoms are shown as large spheres; nitrogen atoms are shown as small spheres. The coordination nets are shown in red and yellow. The solvent molecules and counterions are shown in green.

pyridine and two nitrile nitrogen atoms. The nitrogen atoms in the figure are shown as small spheres. The N-Ag-N bond angles are 110.0, 117.5, and 131.2°. The Ag-N distances are 2.281, 2.188, and 2.159 Å. The ligand molecules are not strictly planar (the dihedral angle between the planes of pyrimidine and benzene rings is 9.7°), and each ligand binds to three silver atoms. Similar to complex 5, the network topology of 6 is also of the honeycomb [6,3] type. Two adjacent graphitic honeycomb networks are shown in Figure 3a in red and yellow. The silver atoms in the red and yellow networks are in close proximity. The Ag-Ag distances are 3.105 Å. With these argentophilic Ag‚‚‚Ag interactions, the two honeycomb nets dimerize into double sheets. The tetrafluoroborate counterions, shown in green, lie between pairs of dimerized sheets. The intercalation of the counterions can be seen more clearly in Figure 3b. In this figure, we show a perpendicular view of the same structure with the BF4- ions (in green) occupying the space between pairs of dimerized networks.


Figure 3. Crystal structure of 6. (a) Stacking arrangements of two adjacent honeycomb nets and tetrafluoroborate counterions. (b) Perpendicular view showing the intercalation of the tetrafluoroborate counterions between the dimerized sheets. Silver atoms are shown as large spheres; nitrogen atoms are shown as small spheres. The coordination nets are shown in red and yellow. The counterions are shown in green.

Discussion In this paper, we report the crystal structures of coordination polymers, which are reminiscent of crystal structures commonly seen in block copolymers and liquid crystals. As we will show, the three structures described in the results section bear strong similarities to the block copolymer lamellar and gyroid structure types. That one would have been able to make such an analogy is not utterly apparent (but please note the relation described below is well-known in the literature).32,45,60-64 For in block copolymer crystal structures, the backbone of the polymer is itself not often determined. Instead, the crystal structure is concerned only with the interface between the two copolymer components. By contrast, in coordination extended solids, one’s initial attention is drawn to the connectivity between the organic and the inorganic parts and consequently to the network that these two components form together. This dichotomy is illustrated in Figure 4. In Figure 4a,c, we show stereoviews of the gyroid structure. Emphasized in these figures is the interface itself, a complex three-dimensional surface. What makes this surface so interesting and what presumably makes this interface so stable are that every point on the interface is a saddle point; at every point, the curvature along the principal axes is equal and opposite. Mathemati-


cians term this a zero mean curvature surface.65 It makes sense that block copolymers adopt this topology. For as some thought shows, in a minimal curvature surface, one locally minimizes the surface area itself. If the two components of the block copolymer are immiscible in one another, such a minimization could stabilize the overall system. To make a connection between the gyroid structure and the coordination extended solid networks, we need to make a connection between this surface and a net itself. We do so by considering those points that lie furthest from the gyroid surface itself. These points are shown in Figure 4b,d. As this figure shows, these points form into two separate sets, one shown in red and the other in yellow. As Figure 4b shows, the red and yellow sets each form in a separate network. These networks contain 4-fold helices. In the center of this figure, two of these helices are seen side by side. The yellow helix runs in a counterclockwise direction, and the red one is clockwise. We begin to see how silver complexes of molecules 1-3 could potentially form in a gyroid structure. As we noted previously, in all of these molecules, the organic molecules are trigonally planar tritopic ligands coordinated to the silver ion, and the silver ions themselves are trigonally planar. Furthermore, as we noted previously, the Ag-1 complex, 4, is composed of two separate networks, each with 4-fold helices such as those we have just seen in the gyroid structure. A comparison of structure 4 to the gyroid structure shows that structure 4 has the same network topology as the gyroid structure. In Figure 5, we redraw structure 4 in a cell and orientation analogous to Figure 4b. If 4 has adopted a gyroid topology, then what role does the gyroid surface, shown in Figure 4a,c, play in its structure? A hint to the answer of this question is shown in Figure 6a. In this figure, we show the gyroid surface where it has been metrically adapted to the orthorhombic unit cell of complex 4. Shown also in this figure are the BF4- counterions in magenta. For reasons of clarity, the red and yellow net atoms have been omitted. As this stereoview shows, there is a relationship between the location of the BF4- anions and the gyroid surface. The BF4- ions seem to trace out the pattern of the gyroid surface itself. This relationship is drawn in an even more clear fashion in Figure 6b. In Figure 6b, we show the portion of the gyroid surface lying at the center of the unit cell (see Figures 4 and 5). As this figure shows, this surface has an undulating form. The BF4- ions are also shown in this picture. They lie in the crevasses of this surface. Interestingly, one can find a relation between the symmetry of the surface and the Td symmetry of the BF4- ions. This is so as the gyroid surface contains two sets of points of higher local point group symmetry: ones with D3d and ones with D2d symmetry. D2d is a subgroup of Td while D3d is not. Interestingly, the BF4- ions lie just off the gyroid special positions of D2d symmetry. We now turn to complex 5. In the previous section, we showed a view of this structure emphasizing the π-π stacking arrangements of the coordination nets (see Figure 2). As this figure shows, the silver ions and the tritopic organic ligands form in nearly perfect planar networks. These networks are given in red and yellow;

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Figure 4. (a,c) Stereoview showing the gyroid surface. (b,d) Stereoview showing the gyroid surface along with the trigonal nets. The surface is shown in green. The trigonal networks are shown in red and yellow.

Figure 5. Stereoview along the 4-fold helices in structure 4. The interpenetrating nets are shown in red and yellow. Silver atoms are shown as large spheres; nitrogen atoms are shown as small spheres. The tetrafluoroborate counterions are shown in green.

layers of SbF6 counterions and solvent molecules are shown in green. The connection between this structure and the block copolymer lamellar structure is clear. As we found in the gyroid structure, the green-colored solvent and counterion moieties can be taken to lie in the interfacial surfaces of a lamellar structure. Just as in the case of the gyroid, we now have to consider those points that lie furthest from the interfacial surface itself. For the gyroid, this set of points led to the appearance of [10,3] three-dimensional networks (see Figure 1b). In the case of parallel lamellar interfaces, the points furthest away from the interface do not form a network but form instead a planar sheet halfway between the interfacial sheets. Nonetheless, we can still draw a connection between this set of furthest

Figure 6. (a) Stereoview showing the gyroid surface in 4 along with the tetrafluoroborate counterions. (b) Stereoview showing a portion of the center region in panel a. The surface is shown in green, and the tetrafluoroborate counterions are shown in magenta.


points and the silver:ligand network. Just as in 4, the silver:ligand network lies on this collection of points. We can draw a similar analogy between the crystal structure of 6 and the block copolymer lamellar structure. Here, as Figure 3 showed, the lamellar structure is composed of double planes of Ag:6 [6,3] networks with layers of BF4- in between. Again, the counterions lie at the set of points the furthest distant from the Ag:organic layers; again, we have a lamellar structure. Taken together, 4-6 tell the same story. We find a common analogy between their structures and the gyroid and lamellar structures found in block copolymer systems. In this analogy, the counteranions lie on the interfaces of either the gyroid or the lamellar topology while the metal:ligand network lies in positions physically separate from this interface. The ionic coordination networks discussed in this paper, like the block copolymers, adopt structures with zero mean curvature surfaces. That this is so is not fully suprising. Indeed, structural inorganic chemists have noted for a long time the role zero mean curvature surfaces play in a number of heterogeneous (and especially two component) intermetallic systems.2-4,66,67 Conclusion In this paper, we have reported three examples where the presence of a local three-coordinate planar environment in a two component microseparable chemical system leads to the adoption of the lamellar and gyroid topologies, topologies frequently seen in block copolymers and liquid crystals. These results suggest that we examine the structural literature to see if other threecoordinate extended solids adopt these same topologies. We therefore examined the Cambridge Structural Database for tritopic ligands coordinated to trigonal planar coordinate silver ions. To limit the scope of this search, we limited ourselves to ligands that were in a C3v or D3h arrangement and where there were three nitrogen atoms coordinated to each Ag ion. We considered only systems where the three-dimensional crystal structure was reported. This search revealed that 53 compounds complied with the stipulated requirements. Of these, 45 proved to contain honeycomb nets; four contained networks that had the SrSi2 topology.49-51,68 Of these latter four, only one had double interpenetrated SrSi2 networks.50 Analysis of this last compound showed, as in our structure, that the anions adhere to a gyroid surface. For the remaining three SrSi2 network systems, there was one and not two SrSi2 networks. However, analysis of these three structures showed that in each case a correspondence could be found between the anion positions and a second SrSi2 network. Thus, one can again make an analogy between these crystal structures and the gyroid topology. Here, the two components, the metal-ligand network and the arrangement of the anions, each form separately one of the SrSi2 networks of the gyroid structure. The gyroid surface in this case interposes itself between these two components. Of the remaining four (out of 53) systems, one was in a layer structure containing a planar net of Ag-organic ligands different from the commonly observed honeycomb pattern.69 In this structure, the metal-ligand network adopted a pattern of squares and octagons, an


arrangement sometimes seen in bathroom tiling.70 This structure is therefore lamellar in the same way that the honeycomb pattern is lamellar. A second of the remaining four formed in a ladder structure;71 the silver ions were in a T-shaped environment rather than the trigonal planar environment discussed in this paper. An analogy may be drawn in this case to the rod arrangement, another of the common topologies seen in large length-scale systems. Thus, 51 of the 53 structures adopt topologies related to the large length-scale minimal surface topologies. The suggestion is clear. For the case of silver tritopic N ligands, the lamellar and gyroid topologies found in block copolymers and liquid crystals are fully relevant to the understanding of these coordination solids. It would be of interest to examine other ligand types and other metal ions to see if this same analogy holds true. Acknowledgment. This work was supported by the National Science Foundation (Grants DMR-0104267 and CHE-0209934). Supporting Information Available: Tables of crystal refinement data, bond distances, bond angles, anisotropic thermal factors for compounds 4-6, and experimental and calculated powder diffraction data for the above compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG0300679

Three Trigonal Coordination Extended Solids with Gyroid and

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