Novel Series of Ribbon Structures in Dialkylammonium

Novel Series of Ribbon Structures in Dialkylammonium Chlorocadmates Obtained By Dimensional Reduction of the Hexagonal CdCl2 Lattice Adrienne Thorn,† Roger D. Willett,*,† and Brendan Twamley‡

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 5 1134-1142

Department of Chemistry, Washington State UniVersity, Pullman, Washington 99164 and Research Office, UniVersity of Idaho, Moscow, Idaho 83844 ReceiVed NoVember 4, 2005; ReVised Manuscript ReceiVed February 17, 2006

ABSTRACT: Small dialkylammonium cations (DMA+ ) dimethylammonium, DEA+ ) diethylammonium, and DPA+ ) dipropylammonium) were used as molecular “scissors” to obtain ribbon structures based on edge-sharing octahedra derived from the parent hexagonal CdCl2 lattice. Retro-crystal engineering concepts were used to analyze and interpret the observed structures. The chloride salts all contain coordinated solvent molecules that stabilize the ribbon structures through O-H‚‚‚Cl hydrogen bonds. The simplest of these, the R and β polymorphs of [DEA][Cd2Cl5(H2O)], are made up of double chains of edge-shared octahedra with stoichiometry [Cd2Cl5(H2O)-]∞. The more complex ribbon structures found in [DMA][Cd2Cl5(H2O)]‚H2O, [DPA]2[Cd5Cl12(H2O)2]‚H2O, and [DPA]2[Cd3Cl8(CH3OH)]‚H2O are made up of [Cd4Cl10(H2O)22-]∞, [Cd5Cl12(H2O)22-]∞, and [Cd6Cl16(CH3OH)22-]∞ moieties, each with its own distinctive packing characteristics. A wide selection of hydrogen bonding plays a crucial role in the three-dimensional stability of this unique family of structures. Introduction The synthesis of organic-inorganic materials is a growing field of research. The primary purpose of designing these new structural types is to combine the functional properties of organic and inorganic structures to create compounds with new device applications.1-5 In general, common techniques used to design these hybrid structures include the “brick and mortar” approach and the use of templating agents. Our interest has been in the design and synthesis of novel extended lattice systems based on organoammonium metal halide salts. However, due to the high variability of the metal ion coordination geometry and of the bridging capabilities of the halide ions, as well as the geometrical constraints of the hydrogen bonding capabilities of the organoammonium ions, these methods have had limited success as a useful design tool. A major exception has been in the area of layer perovskite compounds of the type (RNH3)2MX4 when M(II) ions that prefer octahedral geometry are employed.4,5 Hence, since the ability to predict these extended lattice structures a priori is still elusive, we use a “retro-crystal engineering” descriptive approach to better understand more complex structures and to develop a methodology for systematic structure design.6 Many low dimensional networks can be described as segments of higher dimensional metal halide frameworks, and thus the analysis method referred to as “dimensional reduction” has been coined.6d,6g,7 Tulsky and Long have previously applied this idea to a wide variety of inorganic salts utilizing parent lattices derived from binary MXn structures.8 In our application of this concept of dimensional reduction to the analysis of hybrid organoammonium metal(II) halide salts where the metal ions have octahedral geometries, we have generalized the Tulsky and Long approach to include the lattice networks found in the following three parent structure types: the cubic AMX3 perovskite structure containing a threedimensional (3D) array of corner-shared octahedra,9 the hex* Corresponding author. Tel: 509-335-3925. Fax: 509-335-8867. Email: [email protected]. † Washington State University. ‡ University of Idaho.

agonal MX2 structure that contains edge-shared octahedral layers,10 and the hexagonal AMX3 structure that has chains of face-shared octahedra.11 In the dimensional reduction concept, the organic cationic species, typically protonated amines, are used as “molecular scissors” to cut the M-X-M bridges of the parent lattice. As indicated above, the use of monoalkylammonium cations with the first of these parent structures leads to the (RNH3)2MX4 layer perovskite networks that have proved so useful in the study of two-dimensional (2D) magnetism and in the design of hybrid semiconducting materials. Here, the hydrogen bonding capability of the -NH3+ moiety stabilizes the cavities on the surface of the metal halide layers. The tribridged AMX3 structures are typically found when the organic cation has little or no hydrogen bonding capabilities.12 With compounds that contain edge-shared metal halides, however, no systematic synthetic approach has been developed. Instead, in our structural studies of organoammonium halocuprate(II) salts, we have used the dimensional reduction process to describe many of the resultant structures.13 Here, the effect of the Jahn-Teller distortion leads to the ferrodistortive CuX2 structure, which is conveniently viewed as stacks of infinite bibridged (CuX2)∞ chains. As a result, many of the structures contain stacks of planar bibridged CunX2n+22- anions with many different stacking arrangements of these oligomers. In our analyses of these structures, we have developed a shorthand notation and a diagrammatic representation to describe these resultant stacking arrangements. This method, while not predictive, has provided a rationale for the variety of structural networks observed. We have recently become interested in extending these ideas to the structures of organoammonium halometalate salts, which contain networks of edge-shared octahedral coordination geometry, with the long-range goal of developing systematic synthetic strategies.6g,14 In this paper, we discuss the extended halometalate ribbon structures derived by modifying the parent CdCl2 lattice using simple dialkylammonium cations (DMA+ ) dimethylammonium, DEA+ ) diethylammonium, and DPA+ ) dipropylammonium) and solvent molecules (e.g., H2O, MeOH). Distinctive arrays of hydrogen bonding are found in each structure which involves the dialkylammonium cations and/

10.1021/cg050584m CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006

Ribbon Structures in Dialkylammonium Chlorocadmates

Crystal Growth & Design, Vol. 6, No. 5, 2006 1135

Figure 1. Polyhedral representations of the ribbon networks in (a) (R- and (β-[DEA][Cd2Cl5(H2O)], (b) [DMA][Cd2Cl5(H2O)]‚H2O, (c) [DPA]2[Cd5Cl12(H2O)2]‚H2O, and (d) [DPA]2[Cd3Cl8(CH3OH)]‚H2O.

Scheme 1. Diagrammatic representations of the ribbon networks in (a) r- and β-[DEA][Cd2Cl5(H2O)], (b) [DMA][Cd2Cl5(H2O)]‚H2O, (c) [DPA]2[Cd5Cl12(H2O)2]‚H2O, and (d) [DPA]2[Cd3Cl8(CH3OH)]‚H2Oa

a

Water molecules, methanol molecules, and chloride ions coordinated to a Cd atom are denoted by W, M, and O, respectively. Table 1. Summary of Bond Distance Variations (Å) compound

a

Cd-Cl (terminal)

R-[DEA][Cd2Cl5(H2O)]

2.500(2)

R-[DEA][Cd2Cl5(H2O)]

2.514(2)

[DMA][Cd2Cl5(H2O)]‚H2O

2.494(2)

[DPA]2[Cd5Cl12(H2O)2]‚H2O

2.538(1)

[DPA]2[Cd3Cl8(CH3OH)]‚H2O

2.535(2) 2.576(2) (axial)

Cd-Cl(2)a

Cd-Cl(3)a

Cd-O

2.519-2.601 (2.566) 2.544-2.591 (2.576) 2.547-2.581 (2.571) 2.524-2.635 (2.584) 2.549-2.610 (2.573)

2.636-2.804 (2.688) 2.629-2.687 (2.672) 2.614-2.755 (2.686) 2.634-2.754 (2.667) 2.634-2.750 (2.675)

2.352(5) 2.298(4) 2.337(4) 2.281(2) 2.344(4)

Values given are the range of distances, with the average distance in parentheses.

or solvent molecules, primarily H2O. These interactions play a significant role in the connectivity, packing, and stability of these complexes. Structural Results - General. The five structures reported here all contain ribbons of edge-shared octahedra that can be visualized as segments derived from the parent CdCl2 structure. These ribbons have stoichiometry [Cd2Cl5(H2O)-]∞ (in two polymorphs), [Cd4Cl10(H2O)22-]∞, [Cd5Cl12(H2O)22-]∞, and [Cd6Cl16(CH3OH)24-]∞. It is seen that, since the syntheses were carried out in aqueous or methanolic solutions, the ribbons all contain solvent molecules. Polyhedral representations of the ribbons are given in Figure 1. In addition, Scheme 1 gives diagrammatic representations of these four types of ribbons, as well as the shorthand Geiser notation.13d,14 In these representations, the longest noninfinite row of octahedra is chosen as the repeat unit to generate the diagrams.15 In general, the complexity of the ribbons increases as the size of the cation increases. Within the ribbons, the cadmium ions have distorted octahedral coordination geometry. The variations in bond distances are summarized in Table 1. These distortions are due to several

effects: perturbations due to the coordinated solvent molecules, differences in the bridging modes of the chloride ions, and the hydrogen bonding effects. Not surprisingly, the Cd-Cl bond distances increase in the order d(terminal) < d(µ2) < d(µ3). In addition to coordinated solvent molecules, several of the structures contain lattice water molecules. These play a significant role in the hydrogen bonding networks present in the overall 3D structures. A strong synergistic interaction between those hydrogen bonding networks and the metal halide scaffolding determine the nature of the metal halide ribbons. Specific details of the individual structures are given in the following sections. Structural Descriptions - Ribbons. [DEA][Cd2Cl5(H2O)]. This compound exists in two polymorphic forms, labeled the R and β phases. The R phase has the smaller volume, so it presumably corresponds to the stable low-temperature phase. Both phases belong to the monoclinic space group P21/c. These two polymorphs contain the simplest ribbon structure of the series of compounds investigated. The [Cd2Cl5(H2O)]- frameworks in both phases are nearly identical, as shown by the asymmetric units illustrated in Figure 2. The two phases differ

1136 Crystal Growth & Design, Vol. 6, No. 5, 2006

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Figure 2. Illustrations of the asymmetric units of (a) the R-[DEA][Cd2Cl5(H2O)] and (b) the β-[DEA][Cd2Cl5(H2O)] structures. The octahedral coordination of each cadmium(II) atom has been completed with symmetry-related atoms (open ellipsoids). Thermal ellipsoids are shown at the 50% probability level on all asymmetric unit illustrations. Table 2. Hydrogen Bonding Interactions [Å and °]a D-H (Å) H‚‚‚A (Å) D‚‚‚A (Å) D-H‚‚‚A (°)

Figure 3. Ball and stick illustration showing a single ribbon of the R-[DEA][Cd2Cl5(H2O)] structure extending along the b-axis. Hydrogen bonding between terminal oxygen and chlorine atoms causes octahedral distortion within the ribbons while at the same serving to stabilize the ribbon structure (dashed lines).

primarily in the conformation and hydrogen bonding interactions of the DEA+ cations. In the R phase, a gauche conformation exists for each N-C bond, while in the β phase, one C-N bond has a trans conformation, while the other retains the gauche conformation. Both the R and β phases contain ribbons composed of double chains of edge-shared Cd octahedra (Scheme 1a and Figure 1a) cut from the CdCl2 parent lattice and illustrated in Figure 3 for the R phase. Cross-sections of these ribbons result in [Cd2Cl5(H2O)]-moieties, which stack together to form the infinite ribbons. These ribbons run parallel to the crystallographic b-axis in both phases. As shown in Figure 3, water molecules and chloride ions occupy alternate nonbridging terminal sites on both sides of the ribbons. This allows each water molecule to form an internal hydrogen bond to its neighboring terminal chloride ions, thus causing distortion of the octahedra within each ribbon. (See Table 2 and discussion of hydrogen bonding, vide infra, for details.) [DMA][Cd2Cl5(H2O)]‚H2O. The asymmetric unit of the previously reported monoclinic crystal structure16 contains both a coordinated and lattice water molecule in addition to two independent cadmium atoms (Figure 4). Both cadmium octahedra show significant bond length and angle distortion. However, this is more noticeable in the hydrated coordination sphere. The average Cd-Cl distance is essentially the same in the two coordination spheres: 2.628 and 2.626 Å. These octahedra aggregate through edge-sharing to form the ribbon structure shown in Figure 5. Here, the ribbon is drawn so as to emphasize the sets of four edge-shared cadmium

R-[DEA][Cd2Cl5(H2O)] O1-H1A‚‚‚Cl3A O1-H1B‚‚‚Cl3 N3-H3A‚‚‚Cl4B N3-H3A‚‚‚Cl5B N3-H3B‚‚‚O1 β-[DEA][Cd2Cl5(H2O)] O1-H1A‚‚‚Cl3 O1-H1B‚‚‚Cl3A N3-H3A‚‚‚Cl1 N3-H3A‚‚‚Cl5 N3-H3B‚‚‚Cl3A [DMA][Cd2Cl5(H2O)]‚H2O N2-H2A‚‚‚O2 N2-H2B‚‚‚O1 N2-H2B‚‚‚Cl2A O1-H1A‚‚‚O2A O1-H1B‚‚‚Cl5D O2-H2C‚‚‚Cl5B O2-H2D‚‚‚Cl5C [DPA]2[Cd5Cl12(H2O)2]‚H2O O1-H2‚‚‚Cl6B O1-H1‚‚‚O2 O2-H1‚‚‚Cl6A N4-H4A‚‚‚Cl5A N4-H4B‚‚‚Cl6A [DPA]2[Cd3Cl8(CH3OH)]‚H2O O1-H1‚‚‚Cl2A O2-H2C‚‚‚Cl3A O2-H2D‚‚‚Cl8A N4-H4A‚‚‚O2 N4-H4B‚‚‚Cl2A N11-H11A‚‚‚Cl2B N11-H11B‚‚‚Cl8B a

0.88 0.81 0.92 0.92 0.92

2.29 2.30 2.54 2.76 2.06

3.082(5) 3.101(5) 3.347(6) 3.389(6) 2.933(7)

150.0 169.5 146.6 126.9 157.6

0.86 0.86 0.92 0.92 0.92

2.42 2.45 2.48 2.76 2.29

3.259(4) 3.221(4) 3.333(5) 3.281(5) 3.204(5)

164.9 150.4 153.8 117.0 174.2

0.90 0.90 0.90 0.88 0.87 0.86 0.85

1.92 2.38 2.80 1.95 2.29 2.45 2.48

2.819(4) 3.111(6) 3.357(5) 2.818(6) 3.144(4) 3.277(4) 3.304(4)

173.0 138.5 121.4 173.4 167.6 163.6 164.1

1.00 0.94 0.83 0.92 0.92

2.17 1.85 2.59 2.48 2.34

3.172(4) 2.783(6) 3.356(5) 3.311(5) 3.223(5)

178.5 171.7 154.2 150.5 160.4

0.84 0.88 0.88 0.92 0.92 0.92 0.92

2.39 2.61 2.27 1.92 2.43 2.48 2.34

3.209(4) 3.322(4) 3.140(4) 2.804(6) 3.309(5) 3.256(5) 3.227(5)

163.6 138.9 174.6 159.9 161.1 141.7 161.5

See figures for atom labeling.

octahedra. The plane of this tetrameric unit has a [Cd4Cl9(H2O)-] stoichiometry, with the water molecule occupying a nonbridging, terminal site. In contrast to the DEA polymorphs, the water molecule is not involved in hydrogen bonding to chloride ions within the ribbon. To complete the repeat unit of the ribbon, the coordination sphere of the end cadmium atoms are capped by a chloride ion on one side and by a water molecule on the other. These ribbons can be illustrated by the stacking diagram in Scheme 1b where “W” indicates the location of the equatorial coordination water molecules. [DPA]2[Cd5Cl12(H2O)2]‚H2O. The asymmetric unit for this monoclinic structure contains three edge-shared octahedra, as


Figure 4. Asymmetric unit of the [DMA][Cd2Cl5(H2O)]‚H2O structure. The coordination of each cadmium(II) atom is completed with symmetry-related Cl atoms (open ellipsoids).

Figure 5. Ball and stick representation of the [DMA][Cd2Cl5(H2O)]‚ H2O structure showing the extended ribbons that run parallel to the c-axis.


Figure 7. Ball and stick representation of the centrosymmetric ribbon in the [DPA]2[Cd5Cl12(H2O)2]‚H2O structure as viewed parallel to the c-axis.

Figure 8. Asymmetric unit of the [DPA]2[Cd3Cl8(CH3OH)]‚H2O structure, augmented with symmetry-related atoms (open ellipsoids) to show octahedral coordination of each cadmium atom.

Figure 9. Ball and stick illustration of the Cd6Cl16(MeOH)2]n4n- ribbons that run parallel to the a-axis in the [DPA]2[Cd3Cl8(CH3OH)]‚H2O structure. The black dots represent the carbon atoms of each methanol group.

Figure 6. Illustration of the asymmetric unit of the [DPA]2[Cd5Cl12(H2O)2]‚H2O structure showing the disorder in one arm of the DPA+ cation having a 70:30 site occupancy. The coordination of each cadmium(II) atom has been completed with symmetry-related Cl atoms (open ellipsoids).

shown in Figure 6. The presence of a coordinated water molecule again causes distortions in Cd-Cl bond lengths and angles much like in the previous structure of [DMA][Cd2Cl5(H2O)]‚H2O. This leads to an average bond distance of 2.641 Å in the hydrated unit and 2.629 Å in the CdCl6 units. Also present in the asymmetric unit are a lattice water molecule and a dipropylammonium cation. The C1-C2-C3-N4 arm has the energetically favorable trans conformation. However, the other arm is disordered, with two different gauche conformations, as seen in Figure 6. The B sites have a 30% occupancy. The ribbons of [DPA]2[Cd5Cl12(H2O)2]‚H2O are illustrated in Figures 1c and 7. The cross-sectional slices of these ribbons yield planar bibridged [Cd5Cl10(H2O)2]2- units (Scheme 1c), with water molecules occupying the nonbridging terminal positions. These cross-sectional slices are augmented with nonbridging chloride ions in an axial position on the terminal cadmium ions to complete their octahedral coordination. The water molecules are not involved in internal hydrogen bonds to chloride ions in the ribbons.

[DPA]2[Cd3Cl8(CH3OH)]‚H2O. The widest member of the ribbon family is found in this structure. Three cadmium atoms are contained in the asymmetric unit of this triclinic structure (Figure 8). However, a coordinated methanol molecule, rather than a water molecule, is included in one of the coordination spheres. Surprisingly, the range of Cd-Cl distances only differs from the other structures by a small amount, despite the presence of a larger solvent molecule incorporated in the cadmium halide framework. The average Cd-Cl distance is 2.621 Å for the solvated unit and 2.627 Å for the CdCl6 units. There are two crystallographically inequivalent DPA ions, one in the energetically favorable all-trans conformation the other with a gauche conformation in one arm. The ribbons of this structure contain planar [Cd6Cl142-] crosssectional slices (Scheme 1d), as seen in Figures 1d and 9. To complete the coordination spheres of the cadmium atoms, methanol molecules occupy axial positions in the coordination sphere of the terminal cadmium atoms, while terminal chloride ions are coordinated in an axial positions of the octahedra in the proximal cadmium ions. This arrangement is shown diagrammatically in Scheme 1d whereby M indicates the location of the methanol molecules. Internal hydrogen bonding between the methanol molecules and the adjacent terminal chloride ions cause the distortions of the octahedra that are visible in Figure 9. (See Table 2 and the discussion in the following section.)


Thorn et al.

Figure 10. Illustration of the hydrogen bonded layers of ribbons in the (R-[DEA][Cd2Cl5(H2O)] (left) and (β-[DEA][Cd2Cl5(H2O)] (right) structures. In both cases, the structures are projected onto the corresponding ac-planes. The hydrogen atoms on the ethyl groups have been omitted for clarity, and the c-axis lies horizontally in both cases.

Figure 11. Illustration of the hydrogen bonding in the (R-[DEA][Cd2Cl5(H2O)] (left) and (β-[DEA][Cd2Cl5(H2O)] (right) structures. The hydrogen atoms on the ethyl groups have been omitted for clarity.

Hydrogen Bonding and Supramolecular Structure. The size differences in the dialkylammonium cations, the presence of solvent molecules, as well as lattice water molecules, within the ribbon structures give rise to distinctive hydrogen bonding features and structural packing in each of the structures. The important hydrogen bonding contacts are listed in Table 2. In the two DEA+ polymorphs, O-H‚‚‚Cl bonds lead to face-toface stacking of the ribbons, with the DEA+ cations sheathing these layers. In contrast, the ribbons in the DMA+ structure are linked via their corners, forming a 3D network. In the first of the DPA+ salts, coordinated and lattice water molecules combine to link the ribbons into layers via edge-on contacts. The DPA+ cations lie between these layers and hydrogen bond adjacent layers together. The presence of the methanol molecules on the edges of the ribbons in the second DPA+ salt prohibits the formation of these edge-on layers. Rather, one set of DPA+ cations helps link the ribbons into alternating cation/ribbon stacks, with the second set of DPA+ cations sheathing these stacks. [DEA][Cd2Cl5(H2O)]. The ribbons in both of these polymorphs are linked together into layers via hydrogen bonds, as illustrated in Figure 10. In both cases, the layers lie parallel to the (1 0 0) planes. The close relationship between the two phases is clearly seen by a comparison of the two packing illustrations in Figure 10. Indeed, the packing between adjacent layers is

essentially identical in the two phases. Examination of Figure 10 shows that the difference in the choice of the a-axis direction between the two phases corresponds to a difference in translation of adjacent layers by 1/2c. With the absence of lattice waters of hydration, complementary O-H‚‚‚Cl hydrogen bonds exist between the coordinated water molecules and the terminal chloride ions on adjacent ribbons in both structures. Thus, this portion of the structure is identical in the two polymorphs. These inorganic layers are sheathed by hydrogen bonded DEA+ cations, forming alternating hydrophobic and hydrophilic layers. It is the manner in which the cations hydrogen bond to the ribbons that is the distinguishing factor between the two polymorphs, as shown in Figure 11 and detailed in Table 2. In the R phase, each DEA+ cation forms a bifurcated hydrogen bond to two chloride ions on one ribbon and a normal hydrogen bond to a water molecule on the adjacent ribbon. In contrast, each cation in the β phase maintains the same type of bifurcated hydrogen bond; however, the N-H‚‚ ‚O bonds have been replaced by N-H‚‚‚Cl bonds. In both structures, the coordinated water molecules form one internal O-H‚‚‚Cl bond and one O-H‚‚‚Cl bond to a terminal chloride ion on an adjacent ribbon. [DMA][Cd2Cl5(H2O)]‚H2O. In this structure, the ribbons lie parallel to the ac-plane and are held together via a series of hydrogen bonding contacts, as seen in Figure 12a. However, in



Figure 12. (a) Packing diagram of the [DMA][Cd2Cl5(H2O)]‚H2O structure showing hydrogen bonding between ribbons as projected onto the ab-plane. (b) Detailed diagram of the [DMA][Cd2Cl5(H2O)]‚H2O structure showing hydrogen bonding.

Figure 13. (a) Ball and stick packing diagram showing hydrogen bonding between ribbon layers of the [DPA]2[Cd5Cl12(H2O)2]‚H2O structure as projected on the ac-plane. (b) Detailed diagram showing hydrogen bonding characteristics of the DPA+ cation and water molecules.

contrast to the previous structures, the coordinated water molecules do not form internal O-H‚‚‚Cl bonds. Rather, the coordinated and lattice water molecules collaborate to link adjacent ribbons in a corner-to-corner fashion. Each coordinated water molecule forms one O-H‚‚‚Cl bond to a terminal chloride on an adjacent ribbon and one O-H‚‚‚O bond to the lattice water molecule. In addition, the lattice water molecules are involved in hydrogen bonds to the terminal chloride ions on these same two adjacent different ribbons. Thus, a 3D network of ribbons is generated, rather than the layer networks in the previous structure. Each DMA+ cation lies on the face of the ribbon and forms a bifurcated hydrogen bond to that ribbon as well as a N-H‚‚‚O bond to a lattice water molecule. In this manner, the faces of each ribbon are sheathed by a hydrophilic layer of cations. Details of the hydrogen bonding scheme are shown in Figure 12b and Table 2. [DPA]2[Cd5Cl12(H2O)2]‚H2O. This structure also demonstrates the importance of water molecules in the structure because it uses both coordination and lattice water molecules to link adjacent ribbons into layers that lie parallel to the [1 01h] plane (Figure 13a). The coordinated water molecules maintain normal O-H‚‚‚Cl bonds to terminal chlorine atoms of an adjacent layer and O-H‚‚‚O bonds to lattice water molecules. The lattice water molecules lie on 2-fold axes midway between pairs of ribbons and connect these ribbons via O-H‚‚‚Cl bonds while accepting the O-H‚‚‚O hydrogen bonds from the corresponding coordinated water molecules. The

dipropylammonium cations form a hydrophobic layer between the faces of the ribbons in these layers. However, in contrast to the [DEA] [Cd2Cl5(H2O)] polymorphs, the DPA+ cations serve to connect these ribbon/water layers via hydrogen bonds, thus forming a 3D structure. One N-H‚‚‚Cl bond connects a terminal chlorine atom on a ribbon in one layer, and the other N-H‚‚‚ Cl bond connects to a bridging chlorine atom of an adjacent layer. Figure 13b shows a detailed diagram of the cation hydrogen bonding and is augmented by Table 2. [DPA]2[Cd3Cl8(CH3OH)]‚H2O. The presence of the coordinated methanol molecules, rather than coordinated water molecules, dramatically changes the nature of the hydrogen bonding network, and thus the role of the solvent water molecules. The hydrogen atom of each methanol is involved in an internal hydrogen bond to the proximal nonbridging chloride ion. Thus, it is not favorable to connect the ribbons in a lateral fashion, as observed in the previous two structures. Rather, a facial hydrogen bonding pattern, involving one type of cation and lattice water molecules, links the ribbons into layers. These layers are sheathed by a second set of cations. A detailed illustration of the hydrogen bonding present in this structure is shown in Figure 14b. Discussion The synthetic strategy employed involved the reactions of the dialkylammonium cations with excess CdCl2 in aqueous


Thorn et al.

Figure 14. (a) Packing diagram of the [DPA]2[Cd3Cl8(CH3OH)]‚H2O structure as viewed along the a-axis. (b) Detailed diagram to show the hydrogen bonding of each DPA+ cation.

solutions. In general, this has yielded a novel series of hybrid salts based on edge-sharing of the CdCl6 octahedra. When excess CdCl2 was not used, salts with smaller Cd:Cl ratios were obtained. Although no systematic attempts were made to characterize these species, the structures of two new compounds were obtained, those of [DMA]CdCl3 and [DPA]2CdBr4. The most common feature observed in the edge-shared systems of this series is the presence of coordinated water, or other solvent, molecules in the crystal structures. In addition, lattice water molecules are incorporated in three of the five structures reported here. The coordinated solvent molecules along the edges of the excised cadmium chloride arrays serve to reduce the charge on each network. The lattice water molecules provide overall stability to the structures via hydrogen bonding as well as allowing for cation hydrogen bonding flexibility within the lattice. It is this hydrogen bonding capability that allows for the development of the novel ribbon networks observed in the structures reported here. These strong electrostatic interactions help compensate for the relatively large charge density on the chloride ions. In strictly anhydrous inorganic systems, the primary mechanism for this charge compensation is through the formation of µ-chloro bridges. However, the N-H‚‚‚Cl and O-H‚‚‚Cl hydrogen bonds can compete with these bridging interactions. The essential role of these interactions is seen by the fact that the majority of these hydrogen bonds are directed to terminal chloride ions on the ribbons. The choice of molecular scissors (i.e., organic cation) roughly determines the width of the excised, or dimensionally reduced, cadmium chloride arrays. In general, as the alkyl chain length of the dialkylammonium cation is reduced, the dimensional reduction of the CdCl2 ribbon increases. More specifically, in the cases in which the parent CdCl2 lattice has been cut into ribbons, a decrease in cation length corresponds to a decrease in ribbon width. For example, the structure containing DPA+ cations is made up of ribbons that are six cadmium octahedra in width, while the structures containing DMA+ or DEA+ cations contain narrow ribbons that are one to three octahedra wide. Furthermore, the hydrogen bonding ability of these cations also provides necessary charge compensation for the excised pieces and provides 3D stability to the lattice as well. While there are numerous examples of neutral ribbon structures in the Cambridge Database,17 only a few structures of compounds containing anionic ribbons have been reported.

These include a single chain system in a hydrazinium salt, C2H9CdCl2IN4S,18a a double chain in 2-amino-4,5-dihydro-3H+thiazolium trichlorocadmate,18b and a triple chain in [(C6H5)4P]4Pb15I34(dmf)6.18c In addition, ribbons with widths of four and five cadmium octahedra are found in (N-ethylethylenediammonium)2Cd4Cl12‚H2O and (C7H16N)2[Cd5Cl12(H2O)2]‚H2O, respectively.18d,e Given this limited number of anionic structures, continued systematic exploration of these systems will yield a wide variety of new ribbon structures. Dimensional reduction is an extremely useful tool for describing hybrid organic-inorganic compounds via “retrocrystal engineering”.6g By illustrating structures as if they were cut from a simple parent lattice, it is easier to develop a methodology for understanding the effects that cations, solvents, and halides have on crystal arrangement and packing abilities. The ultimate goal of this technique is to systematically document trends and noticeable effects with the intent of using this knowledge to design new and useful structures. However, the application of these principles is still in its infancy. There are still a variety of factors not yet fully understood that make it difficult to control the actual crystalline structure obtained. For example, small differences in halide-halide, van der Waals, and steric interactions play a significant role in hydrogen bonding, packing, etc., despite the ability of the Cd2+ ion to provide a strong directional motif to each structure. Experimental Section General Procedure. All solvents and reagents were used as received. Unless otherwise mentioned, the organic salts were prepared by adding approximately 3 mL of amine (DMA ) dimethylammonium, DEA ) diethylammonium, DPA ) dipropylammonium) to 3 mL of deionized water in a small, chilled beaker. The solution was then acidified with the appropriate concentrated acid and allowed to evaporate at room temperature (23 °C). R-[DEA][Cd2Cl5(H2O)]. [DEA]Cl (0.109 g, 1 mmol) and CdCl2 (0.553 g, 3 mmol) were dissolved in a warm solution of deionized water (10 mL) and methanol (10 mL). Small, clear, colorless needles of diethylammonium aquapentachlorodicadmate(II) formed after 5 days. β-[DEA][Cd2Cl5(H2O)]. This was prepared as above using [DEA]Cl (0.114 g, 1 mmol) and CdCl2 (0.543 g, 3 mmol) dissolved in deionized water (10 mL), methanol (10 mL), and HCl (1 M, 5 drops) and slowly evaporated over a period of two weeks. Clear, colorless, rodlike crystals formed. [DMA][Cd2Cl5(H2O)]‚H2O. [DMA]Cl (0.141 g, 1.7 mmol) and CdCl2 (0.546 g, 3.0 mmol) were dissolved in a solution of deionized water (10 mL) and HCl (4 M, 8 drops). The solution was heated gently



Table 3. Crystallographic Data Collection and Refinement Data empirical formula formula weight temp (K) description size (mm3) system space group Z a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) exposure (s) D calc (Mg m-3) µ (mm-1) reflections used Rint GOF R indices [I > 2σ(I)]a R indices (all data)a a

R-[DEA][Cd2Cl5(H2O)] C4H14Cd2Cl5NO

β-[DEA][Cd2Cl5(H2O)] C4H14Cd2Cl5NO

[DMA][Cd2Cl5(H2O)]‚H2O C2H12Cd2Cl5NO2

[DPA]2[Cd5Cl12(H2O)2]‚H2O C12H38Cd5Cl12N2O3

[DPA]2[Cd3Cl8(CH3OH)]‚H2O C13H38Cd3Cl8N2O2

492.21

494.21

484.18

1245.84

875.25

86(2) colorless needle 0.20 × 0.11 × 0.03 monoclinic P2(1)/c 4 12.672(1) 7.6220(6) 16.4766(9) 90.00 122.282(4) 90.00 1345.49(17) 10 2.440

86(2) colorless needle 0.44 × 0.07 × 0.06 monoclinic P2(1)/c 4 10.7505(8) 7.5495(6) 16.8913(13) 90.00 96.412(1) 90.00 1362.34(18) 5 2.410

292(2) colorless needle 0.18 × 0.09 × 0.03 monoclinic Cc 4 11.089(5) 21.674(4) 6.5210(4) 90.00 125.44(3) 90.00 1276.85(13) 5 2.519

86(2) colorless needle 0.19 × 0.01 × 0.01 monoclinic C2/c 4 30.1626(18) 6.7051(4) 19.176(1) 90.00 117.874(1) 90.00 3428.3(3) 60 2.414

86(2) colorless needle 0.20 × 0.05 × 0.02 triclinic P-1 2 10.1322(6) 10.8346(6) 14.7444(9) 79.134(1) 84.094(1) 63.586(1) 1423.31(14) 20 2.042

4.120 3204

4.069 5126

4.344 3491

4.006 3899

2.985 5165

0.0352 1.098 R1 ) 0.0480

0.0342 1.095 R1 ) 0.0406

0.0293 1.024 R1 ) 0.0257

0.0705 0.978 R1 ) 0.0354

0.0438 1.026 R1 ) 0.0448

wR2 ) 0.0877 R1 ) 0.0617

wR2 ) 0.0623 R1 ) 0.0510

wR2 ) 0.0516 R1 ) 0.0283

wR2 ) 0.0680 R1 ) 0.0713

wR2 ) 0.0746 R1 ) 0.0658

wR2 ) 0.0928

wR2 ) 0.0656

wR2 ) 0.0525

wR2 ) 0.0790

wR2 ) 0.0826

R1 ) ∑|Fo| -|Fc|/∑|Fo|; wR2 ){∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

to ensure dissolution and then allowed to evaporate slowly at room temperature. Clear, colorless, needle-shaped crystals of dimethylammonium aquapentachlorodicadmate(II) hydrate were obtained after 2 days. [DPA]2[Cd5Cl12(H2O)2]‚H2O. Prepared as described above using [DPA]Cl (0.165 g, 1 mmol) and CdCl2 (0.597 g, 3 mmol) dissolved in a solution of methanol (10 mL) and deionized water (4 mL). Suitable crystals could not be obtained from the solution. After complete evaporation of the solvent, the precipitate was redissolved in equal amounts of methanol and deionized water and heated gently. Small, clear, colorless needles of bis(dipropylammonium) aquadodecachloropentacadmate(II) hydrate formed after 24 h. [DPA]2[Cd3Cl8(CH3OH)]‚H2O. Bis(dipropylammonium) octachloromethanoltricadmate(II) hydrate was prepared as described above using [DPA]Cl (0.150 g, 1 mmol) and CdCl2 (0.556 g, 3 mmol) dissolved in methanol (35 mL) and acetone (5 mL). The solution was heated gently and then allowed to cool and evaporate slowly. Clear, colorless hexagonal plates with occasional rod-shaped crystals formed in the solution a day later. Only the rod-shaped crystals were suitable for X-ray structure analysis. X-ray Structure Analysis. Crystals were removed from solution and immediately placed in hydrocarbon oil to prevent the crystalline material from possible deterioration due to solvent loss. A suitable crystal was selected, attached to a glass fiber, and placed immediately in the low-temperature nitrogen stream.19 Data were collected at ca. 86(2) K (except for [DMA][Cd2Cl5(H2O)]‚H2O, 292(2)K) using a Bruker/Siemens SMART APEX instrument (Mo KR radiation, λ ) 0.71073 Å) equipped with a Cryocool NeverIce low-temperature device. Data were measured using omega scans of 0.3° per frame for various exposures, and a full sphere of data was collected in each case. The first 50 frames were re-collected at the end of the data collection to monitor for decay. Cell parameters were retrieved using SMART20 software and refined using SAINTPlus21 on all observed reflections. Data reduction and correction for Lp and decay was performed using the SAINTPlus21 software. Absorption corrections were applied using SADABS.22 The structures were solved by direct methods and refined by the least-squares method on F2 using the SHELXTL program package.23 No decomposition was observed during data collection. In [DPA]2[Cd5Cl12(H2O)]‚H2O, one arm of the DPA+ group was disordered. The disordered atoms were modeled in two positions with a 70:30 occupancy ratio and the thermal displacement parameters were held isotropic. All other atoms were refined anisotropically. Hydrogen

atoms associated with water molecules were located and then fixed in position. Details are given in Table 3.

Acknowledgment. The Bruker (Siemens) SMART APEX diffraction facility was established at the University of Idaho with the assistance of the NSF-EPSCoR program and the M. J. Murdock Charitable Trust, Vancouver, WA. Research is supported in part by ACS-PRF Grant 34779-AC. Supporting Information Available: Tables S1-S10 containing positional parameters and bond distances and angles for the five compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Crystal data in CIF format for the five compounds reported here, as well for the additional structures of [DMA]CdCl3 and [DPA]2CdBr4, have been deposited in the Cambridge Data Base (R[DEA][Cd2Cl5(H2O)], CCDC No. 275805; β-[DEA][Cd2Cl5(H2O)], CCDC No. 275806; [DMA][Cd2Cl5(H2O)]‚H2O, CCDC No. 275808; [DPA]2[Cd5Cl12(H2O)2]‚H2O, CCDC No. 275809; [DPA]2[Cd3Cl8(CH3OH)]‚H2O, CCDC No. 275810).

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CG050584M

Novel Series of Ribbon Structures in Dialkylammonium

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