Supramolecular Behavior of an Isomorphous Series of Five Bis (2

Jul 31, 2004 - John C. MacDonald,*,† Tzy-Jiun M. Luo,‡ and G. Tayhas R. Palmore*,‡,§. Department of Chemistry and Biochemistry, Worcester Polyt...
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Supramolecular Behavior of an Isomorphous Series of Five Bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) Complexes John C. MacDonald,*,† Tzy-Jiun M. Luo,‡ and G. Tayhas R. Palmore*,‡,§

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1203-1209

Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, and Divisions of Engineering and Biology and Medicine, Brown University, Providence, Rhode Island 02912 Received January 9, 2004

ABSTRACT: The supramolecular chemistry and crystal structures of five bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) complexes, where M ) Zn, Cu, Ni, Co, and 7:3 Mn/Cu (1-5, respectively), are reported. These complexes form building blocks with nearly identical molecular structures that crystallize in the same packing pattern. Anions of 2,6-dicarboxypyridine and cations of 2-methylimidazole form N-H‚‚‚O and O-H‚‚‚O hydrogen bonds that dominate crystal packing by forming linear ribbons of molecules. Thus, complexes 1-5 form an isomorphous series with a single robust crystalline architecture that accommodates five different transition metals without altering molecular packing. The growth of crystals from solutions that contain two different metal complexes produces mixed crystals in which mixtures of the different metal complexes are incorporated in the same relative molar ratio present in solution. This technique was used to grow crystals of 5 with Mn and Cu complexes in a 7:3 molar ratio. Complexes 1-5 form crystalline solids that represent a novel class of modular materials in which the organic ligands serve as a structural component that defines a single packing arrangement that persists over a range of structures and in which the metal serves as an interchangeable component with which to vary the physical properties of the material. The molecular and crystal structures of bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) complexes 1-5 are reported and compared to those of a related family of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes 1′-5′ (M ) Zn, Cu, Ni, Co, and Mn) reported previously. We show that complexes 1-5 and 1′-5′ have similar packing arrangements and that introducing a methyl substituent (similar in size to water) at the C2 position of imidazole displaces water and prevents it from being incorporated into the lattice of 1-5. Introduction We are exploring the use of molecular self-assembly as a means to fabricate crystalline materials with complex composition and morphology in an effort to develop modular materials for nanoscale design in the solid state.1-5 Molecular self-assembly currently is of interest as a method for nanofabrication because molecules spontaneously assemble into aggregates with discrete or extended architectures that can be predicted.6-21 Further assembly of these aggregates generates crystals that range in size from nanometers to centimeters.22 Consequently, molecular self-assembly via crystallization provides a potential means to fabricate structural features with dimensions several orders of magnitude smaller than current photolithographic techniques allow.23 The development of materials based on single crystals or arrays of crystals, however, has lagged behind that of other materials such as plastics for commercial applications because crystals traditionally have proven difficult to prepare and process. In addition, crystals have properties unique to the solid state that can limit their utility. For example, changing molecular components or modifying their structure through synthesis generally alters crystal packing on switching from one set of components to another.2,5,24-27 Molecules that can adopt several conformations or pack * To whom correspondence should be addressed. J.C.M. E-mail: [email protected]. G.T.R.P. E-mail: [email protected]. † Worcester Polytechnic Institute. ‡ Division of Engineering, Brown University. § Division of Biology and Medicine, Brown University.

in different relative orientations that are close in energy frequently form polymorphs when crystallized.28-31 Because the physical properties of crystals are determined in large part by the arrangement in which molecules pack, either situation can give rise to crystals with structures and properties that cannot be predicted. We currently are investigating crystals that contain one or more interchangeable molecular components in an effort to develop crystalline materials that are modular. Our goal is to identify families of molecular building blocks that promote self-assembly into a single packing arrangement that persists across a range of different crystal structures in order to minimize the incidence of multiple crystal forms when components are switched. Several requirements must be satisfied to successfully design modular crystals with the potential for commercial applications. Components must have the requisite structure to impart the desired physical property (e.g., molecular dipole, conjugated π-system, hydrogen-bonding group, etc.) at the molecular level. The resulting bulk crystals must have the correct composition, packing arrangement, morphology, and mechanical as well as thermal stability to render them useful as optical, electrooptic, photorefractive, and magnetic materials.32-34 Our strategy to meet these requirements is to work with molecules that vary in chemical composition but that have similar structures. Replacing one molecular component with an isomorphous component that has different electronic or optical properties will allow the properties of the material to be varied systematically while maintaining the same

10.1021/cg049974j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/31/2004

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Figure 1. Reaction of MX2 (M ) Zn, Cu, Ni, Co, or Mn; X ) Cl or Br) with 2,6-dicarboxypyridine and 2-methylimidazole gives the corresponding bis(2-methylimidazolium 2,6-pyridinedicarboxylate) M(II) complexes (1-5). Bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes 1′-5′ are shown below for comparison.

crystal structure. This strategy has been demonstrated recently by Wheeler, who prepared isosteric sets of molecules that are unique chemically but have opposite stereochemical handedness, which enables them to mimic the centrosymmetric packing behavior of racemates when cocrystallized.35-37 Toward this end, we are investigating structurally related families of building blocks based on coordination complexes between first row transition metals, 2,6dicarboxypyridine, and imidazoles. We demonstrated previously that these components react with unsubstituted imidazole to form complexes of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate, where M ) Cu, Ni, Co, Zn, and Mn (1′-5′).38,39 These solids were designed to assemble into layers with an open framework by forming +N-H‚‚‚O- hydrogen bonds between the imidazolium cations and the metal(II) anions. The molecular structures and crystallographic packing in all five solids were nearly identical as anticipated. We subsequently showed that epitaxial growth of crystalline layers that contained other metal(II) ions was possible on selected faces of seed crystals. The resulting composite crystals featured different metal complexes segregated in different regions of the crystals. We also showed that crystals composed of mixtures of two or more different metal complexes could be prepared in which the different metals were disordered in the crystal lattice.38 This study confirmed that physical properties of mixed crystals such as color and refractive index can be varied by changing the relative molar ratio of the different metal complexes present in the crystal. Moreover, it showed that large crystals could be grown with rhombic prismatic morphology in the presence of a 2-fold excess of imidazole and 2,6-dicarboxypyridine. Loss of water from the crystalline lattice of 1′-5′ at approximately 50 °C, however, renders this family of complexes impractical as a materials where thermal stability is desired. We recently reported a method to control the morphology of crystals of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrates 1′-5′ that was used to fabricate composite crystals with complex shapes.40 This work showed that multicomponent crystals with specific morphologies and compositions could be fabricated when

supersaturation, chemical additives, and mixing were used in combination. In another recent paper, we studied the kinetics of growth and morphology for a series of crystals comprised of bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II), where M ) Zn, Cu, Ni, Co, and Mn (1-5).41 We showed that templated epitaxial growth of these complexes occurred on a substrate crystal of 2′ and that stencils or stamps could be used to pattern crystals of 1-5 on the substrate. This work was significant in establishing that the crystal packing of the two different families of metal complexes is sufficiently similar to permit the growth of crystals from one family in predefined patterns on the surface of crystals of the other family. In this paper, we describe the molecular and crystal structures of five bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) complexes, where M ) Zn, Cu, Ni, Co, and 7:3 Mn/Cu (1-5). We compare the supramolecular behavior of these building blocks to that of the bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes 1′-5′ reported previously. The investigation of 1-5 was carried out initially to test whether we could develop a new family of metal complexes with a substituted imidazole that would (i) exclude water during crystallization, (ii) exhibit molecular structures, crystal packing, and physical behaviors similar to those of 1′-5′, and (iii) form large single crystals with crystal packing and morphologies that render them compatible for mixed and epitaxial growth with 1′-5′. Toward this end, we carried out a one-pot synthesis and crystallization of each complex by reacting the appropriate metal halide with 2,6-dicarboxypyridine and 2-methylimidazole in aqueous dimethyl sulfoxide (DMSO) (Figure 1). Molecular modeling of the crystal structures of 1′-5′ revealed that molecules of water occupied sites next to C2 of the imidazolium cations. We chose to replace imidazole with 2-methylimidazole based on the hypothesis that the methyl group would prevent water from being included in the lattice during crystallization. We demonstrate that complexes 1-5 form an isomorphous series with hydrogen-bonded connectivity that differs from that of 1′-5′. Despite these differences, crystal packing is similar in the two families of complexes.

Bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II)

Figure 2. Crystals of bis(2-methylimidazolium 2,6-dicarboxypyridine) metal(II) complexes 1-4.

Results and Discussion Structures of Bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) Salts. The reaction of 2,6dicarboxypyridine and 2-methylimidazole with Zn, Cu, Ni, Co, or 7:3 Mn:Cu gave the corresponding bis(2methylimidazolium 2,6-dicarboxypyridine) metal(II) complexes, 1-5. These complexes form crystals in which the metal ions exhibit octahedral coordination via two ligands of 2,6-dicarboxypyridine. The resulting salts contain two 2-methylimidazolium cations that form when 2-methylimidazole deprotonates two carboxylic acid groups on the anion. These complexes form rhombic prisms with similar morphology as shown in Figure 2. The crystal structures of 1-5 are nearly identical. Comparison of the molecular structures of the anions shows that the geometry of the ligands bound to the metal ion is similar across the series. For example, the N-M-N angle (M ) Zn, Cu, Ni, Co, or 7:3 Mn:Cu) involving the nitrogen atoms on the two pyridyl ligands and the central metal atom varies by no more than 3.4° (174.4-178.8°). The angle between the mean planes of the two pyridyl rings varies by no more than 1.4° (74.676.0°) across the series. In contrast, the N-M-N angle of the bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes 1′-5′ varies from bent in the Zn, Co, and Mn complexes [e.g., N-Zn-N ) 166.2(2)°] to nearly linear in the Cu and Ni complexes [e.g., N-Cu-N ) 178.5(7)°]. The invariant molecular structure and geometry of the anions in 1-5 indicate that these complexes form an isomorphous series. Crystal Packing and Hydrogen Bonding. In addition to having the same molecular structure, complexes 1-5 have similar crystal packing. Views of crystal packing and hydrogen bonding from the [100], [01 h 0], and [101] directions in the crystal structure of the bis(2-methylimidazolium 2,6-dicarboxypyridine) Ni(II) complex 3 are shown on the left in Figure 3. Crystallographic data and refinement parameters are shown in Table 1. The crystal packing observed in 3 is representative of all of the metal complexes; therefore, packing in 1, 2, 4, and 5 is not shown. Views of packing along the [100], [01 h 0], and [001] directions in the crystal structure of the bis(imidazolium 2,6-dicarboxypyridine) Ni(II) dihydrate complex 3′ (similarly representative of

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packing in 1′, 2′, 4′, and 5′) are shown for comparison on the right in Figure 3. Complexes 1-5 crystallize in the same space group (P2/n) and have unit cells with dimensions and volumes that differ at most by 0.088 (0.74%) and 3.79 Å (0.33%), respectively. Packing in these solids is dominated by hydrogen bonding between the 2-methylimidazolium cations and the carboxylate anions. Each anionic metal complex is joined to four different cations by ionic N+-H‚‚‚O- hydrogen bonds that form hydrogen-bonded chains of alternating cations and anions {basic binary graph set C22(10)}.42-44 Pairs of chains intertwine to form a series of rings {complex binary graph set R22(20) along the [100] direction as highlighted in pink on the left in Figure 3c}. This intermolecular connectivity differs significantly from that of complexes with imidazole (1′-5′) despite the fact that the cations and anions reside in the same general positions in the crystal structures of both series. For example, while each anionic complex in 3′ also is joined to four different cations by ionic N+-H‚‚‚O- hydrogen bonds, the resulting hydrogen-bonded chains {basic binary graph set C22(12)} are linear with a mutually perpendicular orientation as highlighted in pink in Figure 3c. This arrangement forms a two-dimensional grid of large rings {complex binary graph set R22(48)} that generates layers instead of linear ribbons. A sideby-side comparison of the different views of 3 and 3′ clearly shows that the cations and anions occupy similar positions within the lattices of the two structures. The primary difference in packing and hydrogen-bonded connectivity arises because the 2-methylimidazolium and imidazolium cations pack in different orientations; the cations in 3 are flipped with respect to those in 3′ about the 2-fold rotation axis along the [01h 0] direction. The relative orientations of the cations are visualized most easily by comparing the left and right packing diagrams shown in Figure 3a,c. Hydrogen bonding that is present along the [01 h 0] direction (y-axis) in 3′ is eliminated in 3 as a consequence of this change in orientation of the cations, indicated by the dashed red lines in Figure 3. Exclusion of Water in the Crystalline Lattices of 1-5. Our previous work with bis(imidazolium 2,6dicarboxypyridine) M(II) complexes demonstrated that 1′-5′ crystallize as dihydrates (Figure 3) and that the presence of water in crystals of 1′-5′ renders them thermally labile with loss of water and subsequent loss of crystallinity at temperatures above 50 °C. This low thermal stability makes crystals of 1′-5′ unsuitable as materials for applications that involve heat. We chose to investigate bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) complexes 1-5 in part with the goal of finding a family of metal complexes with crystal packing isomorphous or similar to that of 1′-5′ that exclude water during crystallization and thus increase the thermal stability of the resulting crystals. As reported recently, we also wanted to identify a new family of complexes with molecular structures and crystal packing similar to those of 1′-5′ to develop new strategies to fabricate crystals with complex compositions, morphologies, and surface structures.40,41 Computer models of the crystal structures of 1′-5′ revealed that water sits at positions approximately the length of a C-H‚‚‚ O hydrogen bond from C2 of the imidazolium cations

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Figure 3. Views of crystal packing and hydrogen bonding for the bis(2-methylimidazolium 2,6-dicarboxypyridine Ni(II) complex 3 (left) and the bis(imidazolium 2,6-dicarboxypyridine Ni(II) complex 3′ (right). (a) View along the [100] direction showing two segregated hydrogen-bonded layers edge-on in 2 and a continuous network of hydrogen bonds in 3′. (b) View along the [01 h 0] direction in 3 and 3′ showing a single hydrogen-bonded layer. (c) View along the [101] direction showing two segregated hydrogenbonded ribbons edge-on in 3 and along [001] showing a layered network of hydrogen bonds in 3′. Two intertwining chains and two perpendicular chains are highlighted in purple in 3 and 3′, respectively.

Bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II)

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Table 1. Crystallographic Data and Refinement Information for 1-5 metal complex formula formula weight crystal system space group crystal color a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g/cm3) no. of reflections µ (mm-1) R wR2 a

1 C22H20N6O8Zn 561.82 monoclinic P2/n colorless 11.845(1) 8.276(1) 13.073(1) 116.700(1) 1144.85 2 1.630 3134 1.14 0.029 0.080

2 C22H20N6O8Cu 559.98 monoclinic P2/n blue 13.071(1) 8.244(1) 11.906(1) 117.20(1) 1141.14 2 1.630 3679 1.02 0.026 0.076

3 C22H20N6O8Ni 555.13 monoclinic P2/n green 11.826(1) 8.295(1) 13.046(1) 116.691(1) 1143.43 2 1.612 3427 0.91 0.024 0.072

4 C22H20N6O8Co 555.43 Monoclinic P2/n brown 11.818(1) 8.286(1) 13.062(1) 116.564(1) 1144.08 2 1.612 3440 0.81 0.025 0.074

5a C22H20N6O8Mn/Cu 553.95 monoclinic P2/n green 11.882(1) 8.246(1) 13.076(1) 116.905(1) 1142.54 2 1.628 3646 0.83 0.029 0.096

Complex contains a 7:3 ratio of Mn(II) and Cu(II) ions.

Figure 4. Space-filling views of a single 2-methylimidazolium cation (left) and an imidazolium cation and molecule of water (right) from the crystal structures of 3 and 3′, respectively.

(C‚‚‚O ) 3.18 Å). Moreover, water is not involved in the networks of strong N+-H‚‚‚O- hydrogen bonds between the cations and the anions. Consequently, we tested the hypothesis that water could be excluded by steric displacement during crystallization when imidazole carried a methyl substituent at C2. The growth of crystals of 1-5 was achieved in the presence of water by slow evaporation of solutions in aqueous DMSO (90% v/v) that were left open to the atmosphere. Notably, water is absent in the structures of 1-5. The methyl group on the 2-methylimidazolium cations occupies sites where water was present in the crystal structures of dihydrates 1′-5′. Figure 4 shows a side-by-side comparison of space-filling views of a single 2-methylimidazolium cation and an imidazolium cation and molecule of water from the crystal structures of 3 and 3′, respectively. This series of structures (1-5) demonstrates that steric displacement is a viable strategy that can be used to prevent the formation of hydrated crystals under the right circumstances. Possible limitations of this general strategy include the following: (i) having prior knowledge of the crystal structure of the hydrated compound of interest; (ii) identifying a molecular component in close proximity to water onto which a methyl or other isosteric substituent (e.g., a halogen) can be introduced; and (iii) avoiding structures in which water is an integral component of a hydrogen-bonded

network such that steric displacement of water results in an entirely new hydrogen-bonded network or packing arrangement. Bis(2-methylimidazolium 2,6-dicarboxypyridine) Mn(II) Forms Crystals Only When Mixed with Other Metal Complexes. Single crystals of pure bis(2-methylimidazolium 2,6-dicarboxypyridine) metal(II) complexes with Zn, Cu, Ni, and Co (1-4) were grown as described in the Experimental Section. Briefly, an aqueous DMSO solution (90% v/v) was prepared that contained a 1:4:4 ratio of metal(II) ion, 2,6-dicarboxypyridine, and 2-methylimidazole, respectively. Crystals in the shape of rhombic prisms could be harvested within 1 or 2 days. Although the ratio of the molecular components in the crystals of the metal(II) complexes is 1:2:2 (metal:2,6-dicarboxypyridine:imidazole), excess imidazole and 2,6-dicarboxypyridine (e.g., 1:4:4) were added to the growth solution to reduce the overall rate of growth of the crystals. We have shown previously that reducing the overall rate of growth in this manner improves the quality of the crystals and avoids the formation of multiple nuclei.38-41 Attempts to crystallize the Mn complex using this procedure, however, inexplicably failed to produce any crystals of the Mn complex. Crystals of excess 1:1 2-methylimidazolium 2,6-dicarboxypyridine salt eventually crystallized from solution as colorless needles. The yellow color of the remaining mother liquor indicated that the Mn complex was present but too soluble at a concentration of 168 mM to nucleate and crystallize. Increasing the concentration of the stock solutions as well as varying the relative ratio of the three components similarly failed to produce crystals of the Mn complex. While unexpected, this behavior is somewhat consistent with that of bis(imidazolium 2,6-dicarboxypyridine) Mn(II) dihydrate (5′). Compound 5′ was difficult to crystallize and usually came out of solution as fine yellow precipitates after many days or weeks. Crystals of 5′ formed infrequently during numerous attempts at crystallization and only after crystals of the imidazolium 2,6-dicarboxypyridine salt appeared first in solution. The few crystals that did grow generally were smaller (less than 0.1 mm in the largest dimension) than those of 1′-4′ and grew attached to crystals of the 1:1 imidazolium 2,6-dicarboxypyridine salt. This result suggests that crystals of the imidazolium 2,6-dicarboxypyridine salt may be necessary to promote nucleation and growth of

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crystals of 5′. Although we did not observe crystals of the Mn complex with 2-methylimidazole growing on crystals of 2-methyimidazolium 2,6-dicarboxypyridine after several days, it is possible that crystals would form if the solutions were left undisturbed for a longer period of time. Despite the fact that the Mn complex will not crystallize, crystals containing the Mn complex as the major component form when complexes containing metals other than Mn are added to the growth solution as a minor component in at least 30 mole percent. Crystals of 5, which contained a mixture of 70% Mn complex (major component) and 30% Cu complex (minor component), were obtained using a stock solution of metal that contained a mixture of Mn and Cu in a 7:3 molar ratio. Attempts to grow crystals with Mn and Cu that contain less than 30 mole percent of the Cu complex failed to yield crystals presumably because of greater solubility at higher relative concentrations of the Mn complex. We have shown previously with complexes 1′5′ that crystallization from solutions containing two or more metal complexes produces single crystals that incorporate both complexes.38,40 This type of material, which we refer to as a mixed crystal, contains a mixture of several different types of transition metals distributed randomly within the same crystal. Analysis by flame atomic absorption spectroscopy or inductively coupled plasmon mass spectrometry of mixed crystals shows that the relative ratio of the metals in mixed crystals is the same as that placed in solution initially. Mixed crystals exhibit mechanical and physical behaviors similar to those of pure crystals of the individual components. For example, mixed crystals that contain combinations of 1′-5′ extinguish light uniformly under a polarizing stereomicroscope and have thermal stabilities (i.e., melting point) similar to those of pure crystals of 1′-5′. We reported previously that crystallographic refinement of the X-ray data for mixed crystals with 1′5′ gave crystal structures with N-M bond lengths that varied between that of the two parent complexes as a function of the relative molar ratio of the two metals.38 Although we could not determine the crystal structure of the pure bis(2-methyimidazolium 2,6-dicarboxypyridine) Mn(II) complex, the increase in the N-M bond distance (where M ) Cu or 7:3 Mn:Cu) from 1.931 Å in the structure of 2 to 1.967 Å in the structure of 5 is consistent with the behavior of mixed Mn:Cu crystals of 5′ with 2′. Formation of mixed crystals is possible because of the similarity of the molecular structures and associated packing energies of the different metal complexes. The hydrogen-bonded network of organic ligands serves effectively as a host lattice in which one metal ion can be substituted by different metal ions without disturbing the lattice. Mixed crystals represent a novel class of materials that exhibit properties that can be altered systematically as a function of the types and relative ratios of metals that are present. For example, the index of refraction and color of mixed crystals composed of different combinations of 1′-5′ can be controlled predictably. The Mn complex also formed mixed crystals when metals other than Cu were added, although 5 is the only example of a crystal structure of a mixed crystal that is reported here. Thus, the formation of mixed crystals is a general phenomenon with the

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family of bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) complexes as it is with the family of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes. Conclusions We have shown that bis(2-methylimidazolium 2,6dicarboxypyridine) M(II) complexes 1-5 serve as a family of building blocks that have a single, persistent molecular structure in the solid state. These complexes pack in a predictable arrangement to form an isomorphous series with a robust crystalline architecture that accommodates a variety of transition metals. Consequently, these complexes provide a platform for engineering the structures and, ultimately, the properties of crystals. The crystalline solids of 1-5 represent a novel class of modular materials in which the organic ligands function as a structural component that defines a single packing arrangement that persists over a range of structures and in which the metal serves as an interchangeable component. This type of modularity is crucial in the design of crystalline materials because it provides a handle with which to alter physical properties without changing the structure of the material. We have demonstrated the utility of this modular approach previously through the design of mixed crystals. In this work, we have shown that formation of mixed crystals provides the only means to prepare crystals that incorporate the bis(2-methylimidazolium 2,6-dicarboxypyridine) Mn(II) complex (5). We also have shown that the crystal packing of 1-5 is similar to that of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes 1′-5′ and that introducing a methyl substituent at C2 of imidazole results in the steric displacement of water from the lattice in 1-5. These results are consistent with a recent report in which we demonstrate that 1-5 are structurally compatible with 1′-5′ such that mixed crystals result when the two different types of complexes are mixed in solution.41 Our current studies include new types of crystalline materials constructed with ligands that contain as many as four 2,6-dicarboxypyridine groups rather than a single 2,6-dicarboxypyridine group. Use of these poly(2,6-dicarboxypyridyl) ligands in conjunction with transition and lanthanide metals will provide the means to fabricate novel architectures with two- and threedimensional connectivity. We expect that this strategy will allow for complete control over molecular packing in three dimensions and will provide a convenient means to fabricate crystalline materials with open frameworks for the design of porous materials. We also are utilizing 2,6-dicarboxypyridine-metal interactions in media other than crystals as a structural motif to construct multilayer assemblies of molecules attached noncovalently to surfaces as a means to fabricate photovoltaic devices45 and to control surface wettability.46 Experimental Section Reagents. All reagents and solvents were purchased from Aldrich and used without further purification. General Procedure for Growing Crystals of 1-5. Stock solutions that contained a 1:4 molar ratio of metal(II) halide (168 mM) and 2,6-dicarboxypyridine (672 mM) dissolved in

Bis(2-methylimidazolium 2,6-dicarboxypyridine) M(II) aqueous DMSO (90% v/v) were prepared for Zn, Cu, Ni, Co, and Mn. A stock solution of 2-methylimidazole (672 mM) dissolved in aqueous DMSO (90% v/v) also was prepared. Equal volumes of each stock solution were mixed at room temperature, resulting in a growth solution comprised of metal(II) ion:2,6-dicarboxypyridine:2-methylimidazole in a 1:4:4 molar ratio, respectively. The mixed solution was transferred to 24 well plates or small beakers. The containers were covered to prevent the entry of air-borne contaminants and left undisturbed overnight, after which the crystals were harvested by filtration. Crystals of 5, which contained a mixture of 70% Mn complex and 30% Cu complex, were obtained from a solution prepared by mixing stock solutions with Mn and Cu in a 7:3 ratio and then adding an equal volume of stock solution of 2-methylimidazole to give a growth solution comprised of Mn/Cu(II) ion:2,6-dicarboxypyridine:2methylimidazole in a 1:4:4 molar ratio, respectively. Crystallography. All X-ray data were collected at 93 K on a Siemens Smart diffractometer equipped with a CCD area detector. Lattice parameters were determined from leastsquares analysis of 36 reflections, and the reflection data were integrated using the program SAINT. The structures were solved by direct methods and refined by full matrix leastsquares on F2 using SHELXTL-97.47 All atoms except hydrogen atoms were refined anisotropically. Hydrogen atoms involved in hydrogen bonding were refined after location on a difference map with isotropic temperature factors. The remaining hydrogen atoms were placed in idealized positions with assigned isotropic thermal parameters.

Acknowledgment. This research was supported by grants from the National Science Foundation, the ACS Petroleum Research Fund, the Whitaker Foundation, and the Office of Naval Research. Supporting Information Available: X-ray crystallographic data for 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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