From Molecular Design to Supramolecular Design: Synthesis and Size

From Molecular Design to Supramolecular Design: Synthesis and Size-Selective Coordination Chemistry of 1,2-Bis(2′-pyrazineethynyl) Benzene Nate Schultheiss,† Charles L. Barnes,‡ and Eric Bosch*,†

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 573-580

Chemistry Department, Southwest Missouri State University, Springfield, Missouri, 65804, and Chemistry Department, University of Missouri, Columbia, Missouri 65211 Received March 4, 2003;

Revised Manuscript Received April 30, 2003

ABSTRACT: The synthesis and coordination chemistry of 1,2-bis(2′-pyrazineethynyl)benzene are presented. With four Lewis base sites, the ligand may adopt a conformation in which two nitrogen atoms face inward and two nitrogens face out. In this conformation, it may complex transition metal cations either “internally” or “externally”. An “internal” 1:1 complex is formed with silver(I) nitrate, while the larger dicopper(II) tetraacetate moiety does not fit in the internal cavity and is thus externally coordinated by two separate ligands as a 2:2 coordination complex. When mixed with 1 equiv each of silver(I) cation and copper(II) acetate the ligand selectively complexes the cations and a coordination complex with two “internally coordinated” silver(I) cations and an “externally coordinated” dimeric copper(II) acetate moiety is obtained. Self-assembly with 1 equiv of silver(I) nitrate and 2 equiv of copper(II) acetate results in the formation of a mixed-metal coordination polymer with alternating silver and copper centers. In the presence of an excess of silver(I) cations, an infinite one-dimensional coordination polymer is formed with one “internal” and one “external” silver(I) cation. Introduction The desire to predictably engineer three-dimensional solids derives from the direct relationship between structure of a solid and its function or potential applications.1 The field of “crystal engineering”2 has undergone exponential growth over the past decade as evidenced by the plethora of monographs and journals devoted to the field.3 The major strategies that are employed in crystal engineering are hydrogen bonding and metal atom coordination along with a variety of other weaker intermolecular forces.4-6 It is interesting to note that the absolute prediction of three-dimensional crystal structure remains an elusive goal.7 Even so, the field has developed to the stage that some functional threedimensional network solids have been logically synthesized.8 The self-assembly of a three-dimensional coordination network is dependent on the availability of components, metal cation and organic ligand, with the appropriate compatibility. Since the coordination chemistry of metal cations is well-established, the limiting factor thus becomes the availability of a diverse set of organic ligands.9 This has led to the design and synthesis of a variety of novel polytopic ligands. For example, Yaghi and co-workers designed a series of polycarboxylic acids to serve as bridging polycarboxylate structural units in the preparation of porous metalorganic framework solids.10 Fujita11 and Stang12 independently developed pyridyl-based ligands with welldefined spatial orientation of the Lewis basic sites for the preparation of three-dimensional coordination cages and also porous solids. Robson and co-workers reported an aesthetically pleasing coordination network with large hexagonal windows on self-assembly of 2,4,6-tris(4′-pyridyl)-1,3,5-triazine with copper(II) acetate.13 * To whom correspondence should be addressed. E-mail: erb625f@ smsu.edu. † Southwest Missouri State University. ‡ University of Missouri.

Over the past several years, we have focused our attention on the design, synthesis, and coordination chemistry of a series of heterocyclic ligands.14-16 In particular, we developed a trans-coordinating bipyridyl ligand, 1,2-bis(2′-pyridylethynyl)benzene, 1, shown in Figure 1A and demonstrated the rich coordination chemistry of this conceptually simple ligand.15 Thummel independently prepared palladium(II) complexes of the same ligand17 and Bunz et al. prepared and characterized a variety of coordination complexes of the 4,5dimethoxy analogue of the ligand.18 The ligand was originally designed to be trans-coordinating as shown in the silver(I) complex in Figure 1B. It is, however, not surprisingly that the dimethoxy analogue of 1 was shown to adopt a variety of other conformations to complex larger cationic centers such as dirhodium(II) tetraacetate.18 We also demonstrated that the ligand adopted nonplanar conformations to complex the copper(I) halides.14b Clearly, ligand 1 and analogues were unselective and underwent conformational changes to complex a variety of metal cations. We recently reported the synthesis of the corresponding tetradentate ligand, tetrakis-1,2,4,5-(2′-pyridylethynyl)benzene, 2, and its complexation of metal cations and organic hydrogen bond donors.19 This ligand adopted one conformation for large guests such as resorcinol, shown in Figure 1C, and another conformation for small guests such as silver(I) cation shown in Figure 1D. In this paper, we present an extension of these ideas with our preliminary results on a new family of ligands designed to take advantage of the contrasting conformations within the same molecule. We expected that the ligand would discriminate between two metal centers based on size and are pleased to demonstrate this with the self-assembly of a predictably ordered mixed-metal coordination polymer. The ultimate goal of this research is the preparation of mixed-metal coordination polymers that incorporate a

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Figure 1. (A) 1,2-bis(2′-Pyridylethynyl)benzene, 1, in the “in-in” conformation. (B) Trans-coordination of silver(I) by ligand, 1. (C) Hydrogen-bonded complex of resorcinol with ligand 2, 1,2,4,5-tetrakis(2′-pyridylethynyl)benzene, in the “all-out” conformation. (D) Trans-coordination of two silver(I) cations by ligand 2 in the “all-in” conformation.

metal with magnetic properties in order to evaluate the modulation of the magnetic properties on inclusion of the second metal center.20 Experimental Section Synthesis. All chemicals were purchased from Aldrich and used as received. Ethynylpyrazine. Chloropyrazine (2.086 g, 18.21 mmol), trimethylsilylacetylene (2.166 g, 22.05 mmol), copper iodide (8 mg, 0.04 mmol), triphenylphosphine (60 mg, 0.23 mmol), and bis(triphenylphosphine)palladium(II) dichloride (75 mg, 0.11 mmol, 0.6 mol %) were added to a round-bottom flask. Tetrahydrofuran (25 mL) and triethylamine (8 mL) were added and nitrogen was bubbled through the resultant mixture for 15 min. A condenser was attached and the mixture was heated at 50 °C under a nitrogen atmosphere. The reaction was monitored by TLC and allowed to cool to room temperature on completion. The solution was then diluted with 100 mL of hexane/ethyl acetate (1/1), and washed with water (2 × 100 mL). The organic layer was dried over sodium sulfate. The solvent was removed on a rotary evaporator and the residue was chromatographed on silica with hexane/ethyl acetate (4/ 1) as eluant. The product was isolated as a pale yellow oil (2.357 g, 74%). 1H NMR (CDCl3) δ 8.70 (s, 1H), 8.54 (t, J ) 2.0 Hz, 1H), 8.49 (d, J ) 8.4 Hz, 1H), 0.30 (s, 9H). The product was directly deprotected. Thus, a mixture of trimethylsilylethynylpyrazine (2.137 g, 12.14 mmol) and potassium carbonate (1.5 g, 9.4 mmol) were stirred in methanol (30 mL) at room temperature for 1 h. The solution was then diluted with dichloromethane (100 mL) and washed with water (3 × 50 mL). The solvent was removed on a rotary evaporator and the residue was chromatographed on silica with pentane/ethyl ether (9:1) as eluant. Ethynylpyrazine was isolated as off-white crystals (1.014 g, 80%). 1H NMR (CDCl3) δ 8.73 (s, 1H), 8.57 (d, J ) 1.4 Hz, 1H), 8.54 (d, J ) 2.6 Hz, 1H) 3.37 (s, 1H); 13C NMR δ 148.03, 144.45, 143.59, 139.22, 81.17, 79.94. 1,2-bis(2′-Pyrazineethynyl)benzene, 3. 1,2-Diiodobenzene (2.122 g, 6.43 mmol), ethynylpyrazine (1.490 g, 14.33 mmol), copper iodide (8 mg, 0.04 mmol), triphenylphosphine (60 mg, 0.23 mmol), bis(triphenylphosphine)palladium(II) dichloride (60 mg, 0.09 mmol, 0.7 mol %) were added to a round-bottom flask. N,N-Dimethylformamide (20 mL) and triethylamine (10 mL) were added and nitrogen was bubbled through the resultant mixture for 15 min at room temperature. The reaction was stirred at room temperature under a nitrogen atmosphere and periodically monitored by TLC. On completion, the mixture was diluted with ethyl acetate (200 mL) and washed with water (3 × 100 mL). The organic layer was dried over sodium sulfate and the solvent was removed on a rotary evaporator. The residue was chromatographed on silica with hexane/ethyl acetate (3:1) as eluant and the product recrystallized from pure ethanol as colorless plates (1.67 g, 81%). 1H NMR (CDCl3) δ 8.94 (d, J ) 1.8 Hz, 2H), 8.63 (dd, J ) 1.8, 2.6 Hz, 2H), 8.52 (d, J ) 2.6 Hz, 2H), 7.70 (dd, J ) 3.4, 5.8 Hz, 2H), 7.44 (dd, J ) 3.4, 5.8 Hz, 2H); 13C NMR δ 148.17, 144.57, 143.09, 140.18, 132.42, 129.41, 124.91, 91.19, 90.38; Anal. Calcd. for C18H10N4: C, 76.58; H, 3.57; N, 19.85; Found: C,

76.44; H, 3.43; N, 19.87. The ligand 3 was also prepared by reaction of chloropyrazine with 1,2-diethynylbenzene as follows: 1,2-diethynylbenzene (1.22 g, 9.68 mmol), 2-chloropyrazine (2.44 g, 21.31 mmol), copper iodide (8 mg, 0.04 mmol), triphenylphosphine (60 mg, 0.23 mmol), and bis(triphenylphosphine)palladium(II) dichloride (60 mg, 0.09 mmol, 0.4 mol %) were added to a round-bottom flask. Triethylamine (10 mL) and N,N-dimethylformamide (20 mL) were added and nitrogen was bubbled through the resultant mixture for 15 min. The reaction was stirred at room temperature under a nitrogen atmosphere and periodically monitored by TLC. The reaction mixture was diluted with 100 mL of ethyl acetate on completion and washed with water (3 × 100 mL). The organic layer was separated, dried over sodium sulfate, and the solvent removed on a rotary evaporator. The crude residue was chromatographed on silica with hexane/ethyl acetate (3:1) as the eluant and the product was recrystallized from pure ethanol as off-white crystals (1.74 g, 66%). 1:1 Complex of 1,2-bis(2′-pyrazineethynyl)benzene with silver(I) nitrate, 4. Silver(I) nitrate (5.9 mg, 0.03 mmol) and 1,2bis(2′-pyrazineethynyl)benzene (9.7 mg, 0.03 mmol) were added to a vial along with acetonitrile (1 mL). The vial was capped and the mixture heated gently until a clear homogeneous solution formed. The vial was placed in the dark and colorless rods formed after one week (5.6 mg, 36%), mp 193195 °C. Anal. Calcd. for C18H10N5O3Ag‚H2O: C, 45.98; H, 2.57; N, 14.89. Found: C, 45.73; H, 2.70; N, 15.28. 2:2 Complex of (1,2-bis(2′-Pyrazineethynyl)benzene) with copper(II) acetate, 5. Copper(II) acetate monohydrate (17.6 mg, 0.09 mmol) and 1,2-bis(2′-pyrazineethynyl)benzene (11.2 mg, 0.04 mmol) were added to a vial along with acetonitrile (1 mL). The vial was capped and gently heated until a clear homogeneous mixture resulted. It was then placed in the dark. Green rhombohedral crystals formed after 4 days (9.9 mg, 54%), mp 167 °C. Anal. Calcd. for C44H32N8O8Cu2: C, 57.01; H, 3.48; N, 12.08. Found: C, 57.34; H, 3.50; N, 12.14. 2:2:2 Complex of 1,2-bis(2′-Pyrazineethynyl)benzene with silver(I) nitrate and copper(II) acetate, 6. Silver(I) nitrate (6.4 mg, 0.04 mmol), copper(II) acetate monohydrate (8.0 mg, 0.04 mmol), and 1,2-bis(2′-pyrazineethynyl)benzene (10.0 mg, 0.04 mmol) were added to a vial along with acetonitrile (1 mL). The vial was capped and gently heated until a clear homogeneous mixture resulted. Green square crystals formed after 2 days in the dark (14 mg, 58%). mp 180 °C. Anal. Calcd. for C22H16N5O7AgCu‚CH3CN: C, 42.71; H, 2.84; N, 12.45. Found: C, 42.20; H, 2.86; N, 11.90. 1:1:2 Complex of 1,2-bis(2′-Pyrazineethynyl)benzene with silver(I) nitrate and copper(II) acetate, 7. Silver(I) nitrate (6.0 mg, 0.04 mmol), copper(II) acetate monohydrate (13.9 mg, 0.07 mmol), and 1,2-bis(2′-pyrazineethynyl)benzene (9.3 mg, 0.03 mmol) were added to a vial along with acetonitrile (1 mL). The vial was capped and heated until a clear homogeneous mixture formed. Green rhombohedral crystals formed after 7 days in the dark (21 mg, 75%). mp >280 °C. Anal. Calcd. for C26H22N5O11AgCu2‚2H2O: C, 36.68; H, 3.08; N, 8.23. Found: C, 36.89; H, 2.79; N, 8.08. 1:2 Complex of 1,2-bis(2′-Pyrazineethynyl)benzene with silver(I) nitrate, 8. Silver(I) nitrate (14.9 mg, 0.09 mmol) and 1,2-

From Molecular Design to Supramolecular Design

Crystal Growth & Design, Vol. 3, No. 4, 2003 575

Table 1. Crystallographic Data for Compounds 4, 5, 6, 7, and 8 compound CCDC deposit no. formula fw cryst syst. space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z µ, mm-1 R (F0) RW (F0)

4 CCDC-208784 C20H13AgN6O3 493.23 monoclinic Cc 18.1579(10) 15.2823(9) 7.1083(4) 90 94.7180(10 90 1965.83(19) 4 1.061 0.0230 0.0519

5 CCDC-208786 C44H36Cu2N8O8 931.89 triclinic P1 11.7193(8) 11.7965(7) 15.8447(10) 69.9570(10) 87.9510(10) 88.3560(10) 2056.2(2) 2 1.100 0.0504 0.0914

Figure 2. Three essentially planar conformations of the bipyrazyl ligand 3. bis(2′-pyrazineethynyl)benzene (11.7 mg, 0.04 mmol) were added to a vial along with acetonitrile (2 mL). The vial was capped and gently heated until a clear homogeneous solution formed. The vial was placed in the dark and pale yellow crystals formed after 2 days (15 mg, 58%), mp 202 °C. Anal. Calcd. for C18H10N6O6Ag2: C, 34.76; H, 1.62; N, 13.51. Found: C, 35.07; H, 1.67; N, 13.57. Crystallography. Crystals suitable for single-crystal X-ray analysis were culled directly from the reaction mixtures. The X-ray data for each of the complexes described herein were collected on a Siemens CCD area detector-equipped diffractometer with MoKR radiation. The structures were solved using SHELXS-9721 and refined using SHELXL-97.22 Hydrogen atoms were included in the calculated positions. The crystallographic data are collected in Table 1.

Results and Discussion Ligand Design and Synthesis. The design of the ligand presented here was based on the trans-coordinating ligand 1 shown in Figure 1A. We wanted to include two more coordination centers and concluded that simply replacing the pyridyl moieties with pyrazine groups would provide both “inner” coordination sites along with two “outer” coordination sites. We were aware that the ligand may adopt any one of the three essentially coplanar conformations shown in Figure 2. These conformations are related by rotations about the two pyrazine-benzene linkages. Thus conformation A has one nitrogen atom from each pyrazine facing “in”, whereas conformation B has only one of nitrogens facing“in”. Conformation C does not have a nitrogen

6 CCDC-209788 C24H19AgCuN6O7 674.86 triclinic P1 7.1961(6) 12.5299(10) 14.7862(12) 85.1400(10) 82.932(2) 79.279(2) 1297.38(18) 2 1.630 0.0394 0.0844

7 CCDC-208787 C30H30AgCu2N7O12 915.56 triclinic P1 8.5956(11) 13.8546(18) 14.8091(19) 87.168(2) 86.264(2) 85.425(2) 1752.5(4) 3 1.827 0.0592 0.1282

8 CCDC-208785 C18H10Ag2N6O6 622.06 monoclinic P21/c 9.0048(6) 13.1443(8) 16.6792(11) 90 99.3320(10) 90 1948.1(2) 4 2.063 0.0224 0.0570

atom facing“in”. Of these conformations, B has an internal weak C-H‚‚‚N hydrogen bond whereas conformation A would suffer from repulsion of the proximal lone pairs on the nitrogen atoms. We expected the ligand to discriminate between small transition metal cations that favor linear (or square planar) geometry and larger cationic units. In accord with our14 and Thummel’s17 earlier results with the trans-coordinating bipyridyl ligand 1,2-bis(2′-pyridylethynyl)benzene, 1, shown in Figure 1B, we reasoned that transition metal cations such as silver(I) and palladium(II) would be accommodated between the “inner” nitrogen atoms in conformation A in Figure 2. The synergistic intramolecular binding is expected to be favored over any other possible binding modes. In contrast larger cations, or cationic units, may be bound by the “outer” facing nitrogen atoms in any one of the three conformations as shown in Figure 3 with conformation A. The ligand 3 was prepared in moderate yield by the palladium-catalyzed Sonogashira coupling of a slight excess of ethynylpyrazine with 1,2-diiodobenzene in triethylamine solvent,23 as shown in Scheme 1. The ethynyl pyrazine itself was obtained in excellent yield by the Sonogashira coupling of chloropyrazine with trimethylsilyl acetylene followed by base promoted desilylation. The ligand was also prepared by coupling of 1,2-diethynylbenzene with chloropyrazine. Discrete Coordination Complexes. To test the hypothesis outlined in Figure 3, we first attempted to prepare an example of each of the two complex types. We therefore prepared a homogeneous equimolar solution of the ligand 3 and silver(I) nitrate in acetonitrile and allowed the complex to crystallize over a week. Colorless crystals were isolated, and elemental analysis indicated that a 1:1 complex had been formed. A single crystal was analyzed by X-ray crystallography, and the structure of the complex is shown in Figure 4.

Figure 3. Proposed coordination of cations such as silver(I) and larger cationic moieties for example the dimeric copper(II) acetate unit.

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Schultheiss et al. Scheme 1a

a (i) Me SiCCH, Cl Pd(PPh ) , NEt , thf; (ii) K CO , MeOH; (iii) 3 2 3 2 3 2 3 1,2-Diiodobenzene, Cl2Pd(PPh3)2, NEt3, dmf.

Figure 4. View of the 1:1 complex of the ligand 3 with silver(I) nitrate along with the included solvent, acetonitrile.24

Figure 5. View along c of the packing of the 1:1 complex of ligand 3 with silver(I) nitrate and the solvent of crystallization, acetonitrile.

The pyrazine’s complex the silver in a linear fashion with a N(1)-Ag(1)-N(2) angle of 168.05(8)° and bond distances of 2.194(2) and 2.204(2) Å for Ag(1)-N(2) and Ag(1)-N(1). The nitrate anion complexes the silver atom in a bidentate fashion with silver-oxygen distances of 2.616 and 2.686 Å for Ag(1)-O(1) and Ag(1)-O(2), respectively. These values are very similar to those reported for the coordination polymer formed between pyrazine and silver(I) nitrate as reported by Vranka and Amma.25 Interestingly, the triangular-shaped silver

complexes π-stack on top of each other with interplanar distances of approximately 3.4 Å as shown in Figure 5. To investigate the complexation of larger units, we decided to use copper(II) acetate since it is known to form a “paddlewheel” dimeric unit in which four acetates bridge the two copper centers.26 This unit is far larger than a single cation but still favors linear coordination by nitrogen ligands. Thus, reaction of copper(II) acetate with pyrazine has been shown to form a one-dimensional coordination polymer with alternate pyrazines and dimeric copper units.27 Accordingly, we prepared a homogeneous equimolar solution of ligand 3 and copper(II) acetate in acetonitrile and isolated green block-shaped crystals suitable for X-ray crystallography. Elemental analysis of the bulk solid indicated a 1:1 ligand/copper stoichiometry. The structure comprises two independent 2:2 complexes, as shown in Figure 6. For each complex, the asymmetric unit comprised one ligand and one copper(II) diacetate moiety. In both independent complexes, the ligand is essentially planar with torsional angles of 5 to 10° between the central benzene ring and the pendant pyrazines. There is a weak intramolecular C-H‚‚‚N interaction in both independent ligand molecules. The C‚‚‚N distances are 3.599 and 3.720 Å with H‚‚‚N distances of 2.700 and 2.821 Å for the independent complexes. To accommodate this weak intramolecular hydrogen bond, the outer alkyne, C(12)-C(13)C(14)-C(15), is bent in by about 6°. The nitrogen copper bond distances of 2.192(3) and 2.201(3) Å are typical for pyrazine-dicopper(II) tetraacetate complexes.27 The copper-copper separation of 2.6075(9) and 2.6031(9) Å for the two independent complexes is similar to the 2.583 Å reported for the copper(II) acetate-pyrazine coordination polymer.27 We have drawn this as a copper-copper bond, although this assignment remains

Figure 6. Perspective view showing both of the independent 2:2 complexes of ligand 3 with copper(II) acetate with complex A drawn as ball-and-stick model and labeled. Complex B is drawn as a stick image.



Figure 7. Two orthogonal views of the overlapping “steplike packing” of complex A.

Figure 8. Perspective view of the 2:2:2 complex formed between ligand 3, silver(I) nitrate and copper(II) acetate. The acetonitrile solvent of crystallization has been omitted for clarity.

controversial.28 The bulky copper(II) moiety prevents simple π-stacking, so the complexes stack in the offset manner shown in Figure 7. Mixed-Metal Coordination Complexes. We were intrigued by the logical and predictable formation of mixed-metal complexes in which one (smaller) metal is internally coordinated as shown in Figure 4 and effectively locks the ligand into conformation A (Figure 2), so that a second (larger) metal center can be complexed externally between two complexes. We hoped that the ligand would thus selectively complex, or selfassemble, with different cations from a mixture of the cations. This expectation was realized when green blockshaped crystals formed from the homogeneous solution containing equimolar amounts of the ligand, silver(I) nitrate and copper(II) acetate. Elemental analysis confirmed the formation of a 1:1:1 mixed crystal and X-ray crystallographic analysis revealed the structure of the complex as 2:2:2 as shown in Figure 8. This complex is clearly related to each of the individual complexes; thus, the silver(I) cation is also complexed in a trans fashion with a weakly bound bidentate nitrate anion and the copper(II) acetate moiety is complexed by two “external” nitrogen atoms of different ligands. In this complex, the ligand is slightly twisted from planarity with torsional angles of about 13° and 22° for the two pyrazine rings with respect to the central benzene ring. This slight twist is accompanied by a correspondingly bent N(1)Ag(1)-N(3) angle of 152.75(11)°. The silver-nitrogen distances, Ag(1)-N(1) and Ag(1)-N(3), of 2.216(3) and 2.2190(3) Å respectively, and the silver-oxygen distances, Ag(1)-O(1) and Ag(1)-O(3), of 2.646 and 2.622 Å, respectively, are similar to the distances in the 1:1 complex of the ligand with silver(I) nitrate. The coppernitrogen distance of 2.199(3) Å and the copper-copper separation of 2.5751(8) Å are also similar to the corresponding distances observed in the ligand-copper(II) acetate complex.

Figure 9. Two orthogonal views showing the packing of the discrete 2:2:2 complex.

As noted in the ligand-copper(II) acetate complex the copper(II) acetate moiety prevents direct π-stacking, and therefore the complexes stack in an offset manner as shown in Figure 9. We further expected to self-assemble a mixed metal coordination polymer using the same strategy with an additional copper(II) acetate to link the subunits shown in Figure 8 together. This specifically requires a 1:1:2 ratio of ligand 3/silver(I) nitrate/copper(II) acetate. We were gratified to note that green block-crystals grew from a homogeneous solution of the components with this modified ratio. Elemental analysis of the bulk solid confirmed that a complex of 1:1:2 stoichiometry was indeed formed. Single-crystal X-ray analysis indicated that the infinite mixed-metal coordination polymer shown in Figure 10 was formed. The labeled repeating unit is shown in Figure 11. In this mixed-metal coordination polymer the ligand is essentially planar with torsional angles of approximately 2.5 and 6° between the central benzene ring and the pendant pyrazine rings. The silver cation is accordingly bound in a more linear fashion with a N(1)-

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Figure 10. Portion of the mixed-metal coordination polymer formed between ligand 3 and copper(II) acetate and silver(I) nitrate.

Figure 11. Perspective view of the repeating unit of the mixed metal coordination polymer formed between ligand 3, silver(I) nitrate and copper(II) acetate with the solvent of crystallization, acetonitrile, and adventitious water omitted for clarity.

Figure 12. Two orthogonal views showing packing of adjacent ribbons of the mixed silver-copper one-dimensional polymer. The included acetonitrile solvent, water molecule, nitrate anion, and hydrogens have been omitted for clarity.

Ag(1)-N(3) angle of 171.0(2)° and Ag(1)-N(1) and Ag(1)-N(3) distances of 2.192(6) and 2.177(6) Å and silver-oxygen distances of 2.615 and 2.741 Å for Ag(1)O(1) and Ag(1)-O(2), respectively. The copper-nitrogen distances are 2.219(6) and 2.202(5) Å for Cu(1)-N(4) and Cu(2)-N(2)#1, respectively. Adjacent ribbons of the coordination polymer stacked in the offset manner shown in Figure 12. While our strategy for the formation of mixed-metal coordination polymers is based on size selectivity it is interesting to note that zur Loye recently reported the use of a pyrazine with a pendant carboxylate group to

control the formation of alternating copper(II)-silver(I) coordination polymers. Thus, the self-assembly of 2carboxy-5-methylpyrazine with copper(II) and silver(I) salts yielded alternating mixed metal coordination polymers.29 In those examples, the carboxylate and the adjacent N-atom complex the copper(II) center while the second N-atom complexes the silver(I) cation. Silver Coordination Networks. We speculated that coordination networks, for example, one-dimensional coordination polymers, could also be formed with a single metal cation by simply changing the stoichiometry of the ligand and metal (silver or copper) before self-assembly. Thus, self-assembly of silver(I) nitrate with the ligand 3 in a 2:1 ratio resulted in the formation of clear rod-shaped crystals with the desired 2:1 ratio as determined by elemental analysis of the bulk solid. Crystals suitable for X-ray crystallographic analysis were selected directly from the mother liquor. The structure was revealed to be a three-dimensional coordination network comprising ribbons of one-dimensional silver-linked ligand-silver complexes interconnected in three dimensions by bridging multidentate nitrate anions. As shown in Figure 13 there are two distinct silver atoms, one internally complexed and one externally complexed cation. The internal silver(I) is complexed in the usual pseudolinear fashion with a N(1)-Ag(1)-N(2) angle of 151.13(7)° and silver-nitrogen distances of 2.2004(19) and 2.2213(19) Å for Ag(1)-N(1) and Ag(1)N(2), respectively. The adjacent nitrate anion has a weak bidentate connection to the silver with distances of 2.656(2) and 2.715(2) Å for Ag(1)-O(1) and Ag(1)O(2), respectively. A second nitrate is also coordinated with a Ag(1)-O(6) distance of 2.490(2) Å. The second silver atom, Ag(2), is bound to two external nitrogen atoms as shown in with an angle of N(3)-Ag(2)-N(4) of 132.80(7)° and slightly longer bond distances of Ag(2)-N(3) and Ag(2)-N(4) of 2.282(2) and 2.3130 Å, respectively. This silver is also bound to three adjacent nitrate oxygen atoms. The bond distances vary from 2.459 to 2.854 Å. The interconnectivity between the nitrates and adjacent silver atoms actually sets up a fairly rigid threedimensional network. Thus, one nitrate anion, N(5), binds the internal silver, Ag(1), in a bidentate mode through oxygens O(1) and O(2) but also binds a Ag(2) cation from an adjacent ligand silver strand through O(1). In contrast, the second nitrate, N(6), has a bidentate bond to the external silver, Ag(2), through oxygen atoms O(4) and O(5) and a single bond to the internal silver, Ag(1), through O(6) as shown in Figure



Figure 13. The ligand-silver coordination network formed on self-assembly of ligand 3 with 2 equiv of silver(I) nitrate with the bridging nitrate anions omitted for clarity.

Scheme 2

14. A partial view of the three-dimensional network is shown in Figure 15. We were unsuccessful in preparing an extended coordination network between ligand 3 and copper(II) acetate. All attempts at forcing a second copper moiety to complex at the open site shown in Figure 6 were unsuccessful. We believe that the slightly less accessible nature of the second nitrogen atom renders coordination unfavorable. Our brief survey of the silver(I) and copper(II) coordination chemistry of the ligand 1,2-bis(2′-pyrazineethynyl)benzene is summarized in Scheme 2.

Figure 14. View of the coordination geometry about the two distinct silver(I) atoms in the 2:1 complex formed on selfassembly of ligand 3 with 2 equiv of silver(I) nitrate. (A) The internally bound silver, Ag(1); and (B) The externally bound silver, Ag(2).

Figure 15. Two orthogonal views of a portion of the nitrate bridged 2:1 three-dimensional network. Only the pyrazinesilver(I) nitrate connectivity is shown and carbons C(5) to C(14) are omitted for clarity.

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We are currently exploring the use of the palladium dichloride complex of ligand 3 as a catalyst in the Suzuki, Heck, and Sonogashira coupling reactions. Acknowledgment. We thank the Petroleum Research Fund administered by the ACS (Grant No. 37506-B3) and the Graduate College at SMSU for partial funding of this research and the NSF for a GK12 Fellowship for Nate Schultheiss (Grant No. DGE0086335). Supporting Information Available: X-ray crystallographic information files (CIF) for all five structures 4-8. This material is available free of charge via the Internet at http://pubs.acs.org.

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Schultheiss et al. (9) (a) Zaworotko, M. J. Angew. Chem. Int. Ed. Engl. 2000, 39, 3052. (a) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. (10) (a) Chae, H. K.; Eddaoudi, M.; Kim, J.; Hauck, S. I.; Hartwig, J. F.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 11482; (b) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (11) (a) Umemoto, K.; Yamaguchi, K.; Fujita, M. J. Am. Chem. Soc. 2000, 122, 7150; (b) Fujita, M. Acc. Chem. Res. 1999, 32, 53. (12) (a) Leininger, S.; Fan, J.; Schmitz, M.; Stang, P. J. Proc. Nat. Acad. Sci. U.S.A. 2000, 97, 1380; (b) Olenyuk, B.; Leininger, S.; Stang, P. J. Chem. Rev. 2000, 100, 853; (c) Olenyuk, B.; Whiteford, J. A.; Fechtenko¨tter, A.; Stang, P. J. Nature 1999, 398, 796; (d) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (13) Batten, S. R.; Hoskins, B. F.; Moubaraki, B.; Murray, K. S.; Robson, R. Chem. Commun. 2000, 1095. (14) (a) Bosch, E.; Barnes, C. L. Inorg. Chem. 2001, 40, 3097. (b) Bosch, E.; Barnes, C. L. J. Coord. Chem. 2003, 56, 329. (15) (a) Bosch, E.; Barnes, C. L. Inorg. Chem. 2001, 40, 3234. (b) Bosch, E.; Barnes, C. L. Inorg. Chem. 2002, 41, 2543. (16) Georgiev, I.; Barnes, C. L.; Bosch, E. J. Supramol. Chem. 2001, 153. (17) Hu, Y. Z.; Chamchoumis, C.; Grebowicz, J. S.; Thummel, R. P. Inorg. Chem. 2002, 41, 2296. (18) Fiscus, E. J.; Shotwell, S.; Layland, R. C.; Smith, M. D.; zur Loye, H.-C.; Bunz, U. H. F. Chem. Commun. 2001, 2674. (19) Bosch, E.; Schultheiss, N.; Rath, N.; Bond, M. Cryst. Growth Des. 2003, 3, 263. (20) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (21) Sheldrick, G. M. SHELXS-97, Crystal Structure Solution, University of Go¨ttingen, Germany, 1997. (22) Sheldrick, G. M. SHELXL-97, Crystal Structure Solution, University of Go¨ttingen, Germany, 1997. (23) Sonogashira, K. In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F.; Stang, P. J., Eds.; Wiley-VCH: New York, 1998; pp 203-229. (24) All figures drawn using X-seed. See Atwood, J. L.; Barbour, L. J. Cryst. Growth Des. 2003, 3, 3. (25) Vranka, R. G.; Amma, E. L. Inorg. Chem. 1966, 5, 1020. (26) van Niekerk, J. N.; Schoening, F. R. L. Acta Crystallogr., 1953, 6, 227. See also Stephens, J. C.; Khan, M. A.; Houser, R. P. Inorg. Chem., 2001, 40, 5064. (27) Morosin, B.; Hughes, R. C.; Soos, Z. G. Acta Crystallogr., 1975, 31, 762. Coordination polymers of other carboxylates with copper(II) acetate are also known; see for example: (a) Nukada, R.; Mori, Takamizawa, S.; Mikuriya, M.; Handa, M.; Naono, H. Chem. Lett. 1999, 367. (b) Mikuriya, M.; Nukada, R.; Morishita, H.; Handa Chem. Lett. 1995, 617. (28) See (a) Royer, D. J. Inorg. Chem. 1965, 4, 1830. (b) Cotton, F. A. Acc. Chem. Res. 1969, 2, 240. (29) (a) Dong, Y. B.; Smith, M. D.; zur Loye, H.-C. Inorg. Chem. 2000, 39, 1943. (b) Dong, Y., B.; Smith, M. D.; zur Loye, H.-C. Solid State Sci. 2000, 2, 335.

CG034036P

From Molecular Design to Supramolecular Design: Synthesis and Size

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