Metal(II) Complexes: Molecular Building Blocks that Generate

Oct 14, 2005 - complexes 1-Co, 1-Ni, 1-Cu, and 1-Zn are reported and compared to those of a related family of bis(imidazolium 2,6- dicarboxypyridine) ...
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Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II) Complexes: Molecular Building Blocks that Generate Isomorphous Hydrogen-Bonded Frameworks Mehmet V. Yigit,† Kasim Biyikli,† Brian Moulton,‡ and John C. MacDonald*,†

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 63-69

Department of Chemistry & Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, and Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912 ReceiVed March 7, 2005; ReVised Manuscript ReceiVed September 9, 2005

ABSTRACT: The supramolecular chemistry and crystal structures of four bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) trihydrate complexes, where M ) Co, Ni, Cu, or Zn (1-Co, 1-Ni, 1-Cu, and 1-Zn, respectively) are reported. Complexes 1-Co, 1-Ni, 1-Cu, and 1-Zn have similar molecular structures that crystallize to give the same packing pattern. 2,4,6-Tricarboxypyridine anions and imidazolium cations form strong N-H‚‚‚O and O-H‚‚‚O hydrogen bonds that dominate crystal packing by forming a three-dimensional network of molecules. These complexes form an isomorphous series with a single crystalline architecture that fits at least four different transition metals without changing molecular packing. Thus, these complexes are a family of modular materials in which the organic components define a single packing arrangement that persists over a range of structures and in which the metal serves as a component that can be changed. The molecular and crystal structures of bis(imidazolium 2,4,6-tricarboxypyridine) M(II) complexes 1-Co, 1-Ni, 1-Cu, and 1-Zn are reported and compared to those of a related family of bis(imidazolium 2,6dicarboxypyridine) M(II) complexes 2-Mn, 2-Co, 2-Ni, 2-Cu, and 2-Zn reported previously. We show that complexes 1-Co, 1-Ni, 1-Cu, and 1-Zn form layers of cations and anions similar to those in 2-Mn, 2-Co, 2-Ni, 2-Cu, and 2-Zn and that introducing a CO2H group at the C4 position on the pyridine ring promotes additional acid-acid interactions that link and align adjacent layers. Introduction We are utilizing molecular self-assembly to construct open frameworks of molecules in an effort to fabricate porous crystalline materials. Recently advances in the development of porous crystalline materials based on hydrogen-bonded and metal-organic frameworks have produced a variety of solids that display porous behavior.1-39 Porous crystals based on metal-organic frameworks in particular exhibit large pore volumes, permanent porosity, high thermal stability and have pores with well defined structures and sizes that show great promise for application in storage and separation of gases, catalysis and ion exchange, which traditionally have been the domain of zeolites. Numerous examples of crystals that derive their porosity from hydrogen-bonded frameworks also have been reported recently.40-51 Although hydrogen bonds generally are less robust than metal-ligand bonds, the reversible nature of hydrogen-bond formation, structural tolerance for range of geometries between donors and acceptors, and the structural flexibility of hydrogen-bonded frameworks offer advantages for designing porous crystalline materials that are unique to hydrogen-bonded solids. Development of porous materials based on single crystals has lagged behind that of other materials such as gels, plastics and zeolites for commercial applications because molecular crystals traditionally have proven difficult to prepare and process and have properties unique to the solid state that can limit their utility. For example, exchanging or modifying the structure of molecular components frequently alters crystal packing on switching from one set of components to another.52-56 Molecules that assemble in different relative orientations or that have flexible conformations that are close in energy often form polymorphs by crystallizing in more than one packing arrangement.57-59 Since the physical properties of crystals are * Corresponding author: [email protected]. † Worcester Polytechnic Institute. ‡ Brown University.

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 the design of crystals composed of coordination complexes that contain interchangeable transition metal components in an effort to develop porous materials with structural and optical properties that can be controlled. Our goal is to identify families of molecular building blocks (1) that form a single type of hydrogen-bonded framework that persists across a range of different crystal structures in order to minimize the incidence of multiple crystal forms, (2) that allow one or more transition metal components to be exchanged without significantly altering the molecular structure of the building block or altering the connectivity and geometry of the framework and crystal packing, and (3) that feature an array of functional groups that promote divergent assembly in three dimensions as a means to create frameworks with open channels. Our strategy to meet these goals is to work with building blocks that vary in chemical composition but that have similar molecular structures. An advantage of this strategy is that replacing one component with an isomorphous component that has different electronic or optical properties allows the properties of the material to be varied systematically while maintaining the same crystal structure. This strategy has been utilized recently to prepare isosteric sets of molecules that are unique chemically that mimic the centrosymmetric packing behavior of racemates when cocrystallized.60-62 We aim to use this modular approach to generate porous crystalline materials from libraries of components that can be exchanged as a means to manipulate the electronic or optical properties of the material without altering or destroying the porous properties. We are developing these and related modular porous materials to fabricate optical sensors with pore dimensions that can be customized. Toward this end we are investigating structurally related families of building blocks based on coordination complexes

10.1021/cg050084t CCC: $33.50 © 2006 American Chemical Society Published on Web 10/14/2005

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between first-row transition metals, 2,6-dicarboxypyridine 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 ) Mn, Co, Ni, Cu and Zn (2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn).63,64 These solids were designed to assemble into layers with an open framework by forming +N-H‚‚‚O- hydrogen bonds between the imidazolium cations and metal(II) anions. The molecular structures and crystallographic packing in all five solids was nearly identical as anticipated. The ability of this series of complexes to assemble predictably into open twodimensional frameworks of molecules is a feature that we take advantage of in the present work to create three-dimensional frameworks by stacking and linking two-dimensional frameworks. 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.64 The ability to form composite and mixed crystals established that complexes such as 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn have similar molecular structure that is not perturbed significantly when one metal ion is replaced by another, and that two or more different metal complexes can be positioned in the same crystal by mixing different complexes in solution or by serial epitaxial growth. This study confirmed that optical properties of mixed crystals such as color and refractive index could be varied by changing the relative molar ratio of the different metal complexes present in the crystal. Loss of water from the crystalline lattice of these complexes at approximately 50°C, however, renders this family of complexes impractical as a materials where thermal stability is desired. To address this problem, we recently reported that bis(2methylimidazolium 2,6-dicarboxypyridine) M(II) complexes also form an isomorphous series with hydrogen bonding and crystal packing similar to that of the bis(imidazolium 2,6-dicarboxypyridine) M(II) complexes.65 This study showed that inclusion of water in the crystal lattice could be prevented by replacing imidazole with a more sterically demanding component, 2-methylimidazole, thereby increasing the thermal stability of the resulting crystals. More recently, we have shown that one type of metal complex can be patterned on a substrate of a different type of metal complex via solvent-mediated self-assembly to generate crystals with complex composition.66 In this paper, we describe the molecular and crystal structures of four bis(imidazolium 2,4,6-tricarboxypyridine) M(II) complexes, where M ) Co, Ni, Cu, and Zn (1-Co, 1-Ni, 1-Cu and 1-Zn), as shown in Figure 1. We have replaced 2,6-dicarboxypyridine with 2,4,6-tricarboxypyridine based on the following assumptions: (1) the open hydrogen-bonded layers generated by bis(imidazolium 2,6-dicarboxypyridine) M(II) complexes also will be generated by bis(imidazolium 2,4,6-tricarboxypyridine) M(II) complexes; (2) introducing an additional CO2H group onto the pyridine ring will cause the layers to stack with the anions aligned via acid-acid hydrogen-bonding interactions; (3) alignment of the holes in adjacent layers will create open channels that result in crystals that exhibit porous behavior. In addition, we aim to establish that complexes of 1-Co, 1-Ni, 1-Cu and 1-Zn form a modular isomorphous series with a persistent molecular structure does not change significantly when one metal ion is replaced by another. Toward this end, we have synthesized and crystallized each complex by reacting the

Yigit et al.

Figure 1. Reaction of MX2 (M ) Co, Ni, Cu, or Zn; X ) Cl or Br) with 2,4,6-pyridinetricarboxylic acid and imidazole in 9:1 H2O/DMSO gives the corresponding bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) complexes (1-Co, 1-Ni, 1-Cu, and 1-Zn). Bis(imidazolium 2,6-dicarboxypyridine) metal(II) complexes (2-Mn, 2-Co, 2-Ni, 2-Cu, and 2-Zn) are shown for comparison (bottom).

Figure 2. Crystals of bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) complexes 1-Co, 1-Ni, 1-Cu, and 1-Zn.

appropriate metal halide with 2,4,6-tricarboxypyridine and imidazole in aqueous DMSO (Figure 1). Here we compare the supramolecular behavior of bis(imidazolium 2,4,6-tricarboxypyridine) M(II) complexes 1-Co, 1-Ni, 1-Cu and 1-Zn to that of the bis(imidazolium 2,6-dicarboxypyridine) M(II) complexes 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn that we reported previously.63,64 Results and Discussion Molecular Structures of Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II) Complexes. Reaction of 2,4,6-pyridinetricarboxylic acid and imidazole with Co, Ni, Cu, or Zn gave the corresponding bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) complexes, 1-Co, 1-Ni, 1-Cu and 1-Zn. These complexes form crystals in which the metal ions are coordinated octahedrally by two ligands of 2,4,6-pyridinetricarboxylic acid. The resulting salts contain imidazolium cations that form when imidazole deprotonates two carboxylic acid groups on the anion. The remaining two carboxylic acid groups lose protons to balance the +2 charge of the metal ion. All of the complexes form rhombic crystals as shown in Figure 2. The crystal structures of 1-Co, 1-Ni, 1-Cu and 1-Zn are nearly identical. Comparison of the molecular structures shows that the geometry of the anions is similar across the series as shown in Figure 3. For example, the N-M-N angle (M ) Co, Ni, Cu or Zn) involving the nitrogen atoms on the two pyridyl ligands and the central metal atom is fixed at 180° by 2-fold rotation symmetry within the complexes. The angle between the mean

Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II)

Crystal Growth & Design, Vol. 6, No. 1, 2006 65

Figure 3. Comparison of the molecular geometries of the anions in 1-Co, 1-Ni, 1-Cu, and 1-Zn. Differences in structure are illustrated by viewing the pyridyl rings from the side (a) and top (b). The cobalt, nickel, copper, and zinc complexes are represented in orange, green, blue, and yellow, respectively.

planes of the two pyridyl rings varies by no more than 1.4° (91.1°-92.5°) across the series. In contrast, the N-M-N angle of the bis(imidazolium 2,6-dicarboxypyridine) M(II) complexes 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn that we reported previously 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)°].64 Crystal Packing of Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II) Complexes. Complexes 1-Co, 1-Ni, 1-Cu and 1-Zn have similar crystal packing in addition to molecular structure. All of the metal complexes crystallize in the monoclinic space group C2/c and have unit cells with dimensions and volumes that differ at most by 0.155 Å (1.0%) and 35.5 Å3 (1.3%), respectively. Crystallographic data is given in Table 1. Packing in these solids is dominated by hydrogen bonding between bis(2,4,6-tricarboxypyridine) metal(II) anions and neighboring anions or imidazolium cations. The anions are joined to the cations by ionic +N-H‚‚‚O- hydrogen bonds.67 The four carboxylate groups on the anions are oriented in a flattened tetrahedral arrangement around the metal. Consequently, each anion participates in the formation of two perpendicular chains of +N-H‚‚‚O- interactions that intersect

at the anions. The resulting 2-D grid of cross-linked chains forms layers that are permeated with large pores. Shown on the left in Figure 4a,b are two stacked layers (green and magenta) from the crystal structure of 1-Co viewed from the top and side. Corresponding views of stacked layers from the crystal structure of 2-Co are shown on the right in Figure 4 for comparison.64 The hydrogen-bonded layers formed in crystals of 1-Ni, 1-Cu and 1-Zn are similar in structure to those in 1-Co (as are those in 2-Mn, 2-Ni, 2-Cu and 2-Zn to 2-Co) and are not shown. Figure 4 shows that layers of bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) complexes have connectivity and topology similar to that of bis(imidazolium 2,6-dicarboxypyridine) metal(II) complexes, and that introducing a CO2H group at the para position on the pyridine ring does not interfere with hydrogenbonding or disrupt the formation of layers. As shown in Figure 4b, the para CO2H groups of 1-Co protrude outward on both sides of a given layer. These groups form O-H‚‚‚O interactions with CO2H groups on neighboring complexes and promote stacking of layers. We expected the CO2H groups to form hydrogen-bonded acid-acid dimers with graph set R22(8).68-70 Instead, the OH group on one acid forms a hydrogen bond to the CdO group on the second acid (graph set D), and the OH group on the other acid forms a hydrogen bond to a molecule of water (graph set D). This connectivity generates channels along the 100 direction in the crystal by connecting metal centers and aligning the pores (Figure 4a). Layers in 2-Co also stack to form channels along the 001 direction in a manner comparable to that in 1-Co despite the absence of para CO2H groups and O-H‚‚‚O interactions. Consequently, packing of the layers in 1-Co is similar to that in 2-Co except that the distance between metal centers in adjacent layers increases from 12.4 Å to 16.2 Å to accommodate the additional hydrogen bonding in 1-Co. The invariant molecular structure and geometry of the anions and the similarity of crystal packing across the series demonstrate that the crystal structures of 1-Co, 1-Ni, 1-Cu and 1-Zn are isomorphous. Interpenetration of Networks. Although we were successful in aligning pores in adjacent stacked layers via hydrogen bonding between CO2H groups to form channels (Figure 4a), the goal to generate porous networks of molecules was not achieved. On the left in Figure 4c is a view showing four independent hydrogen-bonded networks of complexes in green, magenta, blue and orange. These networks intertwine such that a given network (e.g., green) has three other networks (e.g., magenta, blue and orange) connected through it. Thus complexes 1-Co, 1-Ni, 1-Cu and 1-Zn exhibit 4-fold interpenetration of networks. Occupation of the channels by the molecular components of interpenetrated networks effectively removes any void space that might result in porous or host-guest behavior.

Table 1. Crystallographic Data and Refinement Information for 1-Co, 1-Ni, 1-Cu, and 1-Zn metal complex formula formula weight crystal system space group crystal color a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalc (g/cm3) no. reflns µ (mm-1) R wR2

1-Co C22H22N6O15Co 667.37 monoclinic C2/c purple 16.172(3) 18.456(4) 9.900(2) 112.172(4) 2736.4(10) 4 1.620 3195 0.713 0.058 0.147

1-Ni C22H22N6O15Ni 667.15 monoclinic C2/c green 16.0967(13) 18.4218(15) 9.8346(8) 111.6090(10) 2711.3(4) 4 1.634 3062 0.801 0.054 0.122

1-Cu C22H22N6O15Cu 671.98 monoclinic C2/c blue-green 16.0819(15) 18.6487(17) 9.8993(9) 112.3000(10) 2746.8(4) 4 1.608 3109 0.879 0.052 0.128

1-Zn C22H22N6O15Zn 673.81 monoclinic C2/c colorless 16.2364(12) 18.3043(13) 9.9220(7) 111.9710(10) 2734.6(3) 4 1.637 3198 0.989 0.044 0.109

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Figure 4. Views of crystal packing and hydrogen bonding for the bis(imidazolium 2,4,6-tricarboxypyridine) Co(II) complex 1-Co (left) and the bis(imidazolium 2,6-dicarboxypyridine) Co(II) complex 2-Co (right): (a) top-down view of two stacked hydrogen-bonded layers (green and magenta); (b) side view of two stacked hydrogen-bonded layers (green and magenta)sanions in the green and magenta layers in 1-Co (left) are linked by hydrogen bonds between the para CO2H groups that project outward above and below each layer; (c) top-down view showing 4-fold interpenetration of four independent hydrogen-bonded networks (green, magenta, blue, and orange).

Interpenetration also occurs in crystals of 2-Co, 2-Ni, 2-Cu and 2-Zn. On the right in Figure 4c is a top-down view of four independent hydrogen-bonded layers in green, magenta, blue and orange. Two-fold interpenetration occurs between green and blue layers and between magenta and orange layers. Alternating stacking of intertwined green-blue and magenta-orange pairs results in crystal packing similar to that observed in 1-Co, 1-Ni, 1-Cu and 1-Zn.

Inclusion of Water. Our previous studies of 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn showed that the bis(imidazolium 2,6dicarboxylate) complexes form dihydrates.64 Therefore, it is not surprising to find water included in the structures of 1-Co, 1-Ni, 1-Cu and 1-Zn, especially considering that the crystals were prepared by slow evaporation from aqueous solutions of DMSO left open to the atmosphere. In addition to forming an acid‚‚‚ acid O-H‚‚‚O hydrogen bond (O‚‚‚O ) 2.556-2.589 Å) to a

Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II)

CO2H group in an adjacent layer, the para CO2H group also forms an O-H‚‚‚O hydrogen bond (O‚‚‚O ) 2.698-2.702 Å) to a molecule of water. These acid‚‚‚water interactions occur on both ends of the anion in 1-Co, 1-Ni and 1-Cu and have identical geometry because the anion lies centered on a 2-fold rotation axis. The geometry of the hydrogen bond from CO2H to water in the crystal structure of 1-Zn is uncertain because the water is disordered. The two molecules of water bonded to each anion form weaker O-H‚‚‚O hydrogen bonds (O‚‚‚O ) 2.881-2.955 Å) to a third molecule of water that sits on a 2-fold rotation axis in the structures of 1-Co, 1-Ni, 1-Cu and 1-Zn. This weakly bound molecule of water is disordered in the structure of 1-Zn. In summary, each metal complex is associated with a total of three molecules of water (1 1/2 molecules of water in the asymmetric unit), of which two sit in general positions related by 2-fold rotation (1 molecule of water in the asymmetric unit), and one sits on the 2-fold rotation axis (1/2 of a molecule of water in the asymmetric unit). Disorder in the structure of 1-Zn gives four regions of electron density in the difference maps that correspond to one and a half molecules of water in the asymmetric unit. The disordered water in 1-Zn appears in the same general region as the ordered water in the structures of 1-Co, 1-Ni and 1-Cu. Although the hydrogen atoms on water were not located in the electron density difference maps during refinement of the crystal structures, the geometry of hydrogen bonds involving molecules of water was determined with confidence based on the O‚‚‚O distances. This pattern of connectivity differs significantly from that found previously in the bis(imidazolium 2,6-dicarboxylate) complexes, where water bonds to the carboxylate groups on the anions. While carboxylate groups commonly act as acceptors and form hydrogen bonds with the acidic OH donor of water,71 it is less common to find carboxylic acids acting as donors by bonding to the weakly basic oxygen of water.72 In these structures, molecules of water are not involved as an integral structural component within the 3-D network of hydrogen bonds that defines connectivity between the cations and anions. Considering the relatively long O‚‚‚O distances, particularly between molecules of water, it follows that 1-Co, 1-Ni, 1-Cu and 1-Zn lose water readily at relatively low temperature. Although we anticipated that water or solvent would be included in the structures of 1-Co, 1-Ni, 1-Cu and 1-Zn simply to fill void space and maximize close packing had the complexes formed open channels with little or no interpenetration, inclusion of water in these crystals likely is driven by formation of energetically favorable hydrogen bonding to the carboxylic acid groups. Characterization of samples by melting point and thermogravimetric analysis (TGA) revealed that crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn turn cloudy and lose 2-3% of total mass between 60 and 120 °C. This behavior is illustrated for 1-Cu by the blue curve in Figure 5. The TGA traces for 1-Co, 1-Ni and 1-Zn are similar and are not shown. Although we did not analyze effluent during heating, the reduction in mass corresponded consistently to a loss of approximately one equivalent of water from the crystal lattice between 60 and 120 °C. Upon further heating, the crystals lose approximately 24% of total mass and decompose between 220 and 280 °C. TGA analysis of crystals that were preheated to 150 °C for 1 h to drive off water, then cooled to RT and allowed to stand in an open container for up to two weeks showed no loss of mass between 60 and 120 °C (magenta curve in Figure 5) and decomposed above 220 °C. Similar behavior was observed with preheated crystals that were placed in water at RT in a sealed vial for 1 day. These results suggest that loss of water from the lattice is

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Figure 5. TGA curve for crystals of 1-Cu showing loss of water (∼23% mass) between 60 and 120 °C followed by decomposition between 220 and 280 °C (blue). TGA curve for crystals of 1-Cu preheated to 150 °C showing no loss of water between 60 and 120 °C (magenta).

irreversible. These experiments demonstrate clearly that crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn exhibit low thermal stability. This behavior is somewhat similar to that observed for bis(imidazolium 2,6-dicarboxylate) dihydrate complexes 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn, which lose water at 50 °C, and thus exhibit even lower thermal stability.64 One notable difference is that crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn lose only one equivalent of water between 60 and 120 °C with retention of two equivalents of water until the crystals decompose at higher temperatures (220-280 °C); both equivalents of water are lost from crystals of 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn upon heating above 50 °C. Loss of water at two different temperature ranges suggests that the strength of the intermolecular interactions to water plays an important role in the stability of water within the lattice. For example, two of the molecules of water form stronger hydrogen bonds with the acid groups (O‚‚‚O ) 2.698-2.702 Å), while the third molecule of water forms weaker hydrogen bonds with another molecule of water (O‚‚‚O ) 2.881-2.955 Å). It stands to reason that crystals will lose one equivalent of the weakly bound water in a lower temperature range and two equivalents of strongly bound water in a higher temperature range upon heating. This hypothesis is consistent with the thermal behavior exhibited by crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn. Although it is possible and even likely that a new structure with different molecular packing results upon losing one equivalent of water, we were not able to collect X-ray powder diffraction traces of samples before and after heating to determine if a change in crystal packing occurred. Conclusions We have shown that bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) trihydrate complexes 1-Co, 1-Ni, 1-Cu and 1-Zn 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 crystalline architecture that accommodates a range of transition metals. Consequently, these complexes provide a paradigm for engineering 3-D frameworks through hydrogen bonding. The crystalline solids of 1-Co, 1-Ni, 1-Cu and 1-Zn represent a 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. Developing strategies to implement 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

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the material. We have demonstrated the utility of this modular approach previously through the design of mixed crystals, composite crystals and complex crystals.64,66 In this work we have shown that building blocks that promote 2-D assembly of hydrogen-bonding layers with pores can be modified by adding an acid donor group to form building blocks that link these layers and promote 3-D assembly. This study also shows that the pores in adjacent layers to be aligned to form large channels. The resulting crystalline solids do not exhibit porous behavior, however, because 4-fold interpenetration of hydrogen-bonded networks eliminates void space in the channels. Experimental Section General Techniques. Cobalt(II) bromide, nickel(II) bromide, copper(II) bromide, zinc(II) bromide, imidazole and 2,4,6-trimethylpyridine were purchased from Aldrich or Acros. All chemicals were used as received without further purification. 2,4,6-Pyridinetricarboxylic acid was synthesized by oxidation of 2,4,6-trimethylpyridine with KMnO4 as reported in the literature.73 IR samples were prepared as powders. IR spectra were obtained in a Nexus 870 FT-IR 360 instrument with an ATR accessory. Melting point data were collected with a Meltemp instrument and are uncorrected. Thermogravimetric analysis data were collected using a TGA 2950 thermogravimetric analyzer from TA instruments. Synthesis of 2,4,6-Pyridinetricarboxylic Acid.73 2,4,6-Trimethylpyridine (15.66 g, 0.129 mol) was added to 250 mL of water followed by 163 g (1.035 mol) of KMnO4. The red slurry was stirred for 14 h at RT and then refluxed for 15 h at 50°C until the solution turned black. The solution was cooled to RT and a black precipitate was removed by filtration to yield a clear solution that was concentrated to 125 mL. The pH was adjusted to 2 with concentrated HCl to yield a white precipitate. The precipitate was filtered, washed with water, and dried under vacuum to give 2,4,6-pyridinetricarboxylic acid: yield 13.91 g (51%); mp > 400 °C; 1H NMR (hot D2O) δ 8.53 (s 2H, Ar); 13C NMR (hot D2O) δ 127.36, 148.99, 165.65, 167.09, 168.23; IR (cm-1) 3415, 3111, 3096, 2500 (br), 1900 (br), 1722, 1711, 1587, 1548, 1428, 1402, 1243, 1195, 1169, 999, 927, 898, 772, 758, 706, 683, 615. General Method To Synthesize and Crystallize Bis(imidazolium 2,4,6-tricarboxypyridine) Metal(II) Complexes (1-Co, 1-Ni, 1-Cu, and 1-Zn). The following general procedure was used to synthesize and grow crystals of bis(imidazolium 2,4,6-tricarboxypyridine) metal(II) complexes with Co, Ni, Cu, and Zn. 2,4,6-Pyridinetricarboxylic acid (0.050 g, 0.237 mmol) and imidazole (0.016 g, 0.027 mmol) were dissolved in 5 mL of a mixture of 9:1 H2O/DMSO with stirring at 50 °C for 20 min. The metal chloride or bromide salt (0.118 mmol) was dissolved in the same solvent system at 40 °C in a separate container until the solution turned clear and then cooled to RT. The solution with the metal halide was gently layered on top of the solution with 2,4,6-pyridinetricarboxylic acid and imidazole using a pipet. The layered solution was left uncovered. Single crystals of bis(imidazolium 2,4,6tricarboxypyridine) metal(II) trihydrate formed as clear rhombic crystals in solution after several days. Single crystals were removed from solution and dried on filter paper. It should be noted that formation of metal complexes and subsequent growth of single crystals was achieved by mixing 2,4,6-pyridinetricarboxylic acid, imidazole and metal in 9:1 H2O/DMSO solvent system in a 2:2:1 molar ratio, respectively. This protocol was established on the basis of experiments involving synthesis and crystallization of the complexes by slow evaporation from the solutions of DMSO in which the relative concentrations of three components were varied systematically. Large, single crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn formed when the relative ratio of 2,4,6-pyridinetricarboxylic acid to imidazole was equal. The optimal relative ratio of 2,4,6-pyridinetricarboxylic acid, imidazole and metal was determined empirically to be 2:2:1, respectively. In contrast, our previous work with bis(imidazolium 2,6dicarboxylate) complexes 2-Mn, 2-Co, 2-Ni, 2-Cu and 2-Zn showed the optimal relative ratio of 2,6-pyridinedicarboxylic acid, imidazole and metal to be 4:4:1.64,66 Determination of X-ray Crystal Structures. Single-crystal X-ray diffraction data were collected on a Siemens SMART/CCD diffractometer with graphite monochromated Mo KR radiation and equipped with an LT-II low-temperature device. Diffracted data were corrected

Yigit et al. for absorption using the SADABS program. SHELXS-86 and SHELXL93 software were used to solve and refine structures on an SGI O2 UNIX platform. Refinement was based on F2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on heteroatoms were located and refined with isotropic thermal parameters. Hydrogen atoms on oxygen atoms in molecules of water were not located or refined. The remaining hydrogen atoms were fixed in calculated positions and refined isotropically with thermal parameters based upon the corresponding attached carbon atoms [U(H) ) 1.2 Ueq (C)]. The two crystallographically unique molecules of water in the structure of 1-Zn were disordered over four regions of electron density in the difference maps. The four regions corresponded to one and a half molecules of water in the asymmetric unit since one molecule sat on a 2-fold axis. The disordered water in 1-Zn appears in the same general region as the ordered water in the structures of 1-Co, 1-Ni and 1-Cu. Refinement was carried out with SHELXL using the SUMP command to restrain the free variables that represent the occupancies of the oxygen atoms of water to give a total occupancy of 1.5 over the four positions. Thermal Behavior of 1-Co, 1-Ni, 1-Cu, and 1-Zn. Upon heating in a Meltemp melting point instrument, crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn turned opaque between 65 and 85 °C and appeared to become polycrystalline, followed by decomposition above 230 °C. Thermogravimetric analysis of crystals heating at a rate of 10 °C per minute showed that crystals of 1-Co, 1-Ni, 1-Cu and 1-Zn lost 2-3% of total mass between 60 and 120 °C, which is consistent with the loss of one molar equivalent of water. Further heating resulted in decomposition above 220 °C. TGA analysis of crystals that were preheated to 150 °C for 1 h, then cooled to RT and allowed to stand in an open container for up to two weeks showed no loss of mass between 60 and 120 °C and decomposed above 220 °C. Bis(imidazolium 2,4,6-tricarboxypyridine) Cobalt(II) Trihydrate (1-Co). Crystals: dark red prisms. IR (cm-1): 3555, 3358, 3111, 2952, 2835, 2566, 1712, 1659. TGA (weight loss %): 2% weight loss between 60 and 120 °C. Bis(imidazolium 2,4,6-tricarboxypyridine) Nickel(II) Trihydrate (1-Ni). Crystals: clear green prisms. IR (cm-1): 3646, 3375, 3139, 2954, 2641, 1712, 1658. TGA (weight loss %): 2% weight loss between 60 and 120 °C. Bis(imidazolium 2,4,6-tricarboxypyridine) Copper(II) Trihydrate (1-Cu). Crystals: clear blue prisms. IR (cm-1): 3519, 3108, 2967, 2825, 2562, 1722, 1640. TGA (weight loss %): 2% weight loss between 60 and 120 °C. Bis(imidazolium 2,4,6-tricarboxypyridine) Zinc(II) Trihydrate (1-Zn). Crystals: colorless prisms. IR (cm-1): 3571, 3111, 2964, 2825, 2647, 1721, 1659, 1640. TGA (weight loss %): 3% weight loss between 60 and 120 °C.

Acknowledgment is made to the U. S. Army Research Office (Grant No. W911NF-05-0293) and to the donors of the Petroleum Research Fund (Grant No. 34604-B3), administered by the American Chemical Society, for support of this research at Worcester Polytechnic Institute. Supporting Information Available: Crystallographic CIF files for 1-Co, 1-Ni, 1-Cu, and 1-Zn. This material is available free of charge via the Internet at http://pubs.acs.org.

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