Supramolecular Ladders: Self-Assembly Fintium to Adfintium

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CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2615-2624

ReView Supramolecular Ladders: Self-Assembly Fintium to Adfintium Anatoliy N. Sokolov and Leonard R. MacGillivray* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52245-1294 ReceiVed September 14, 2006

ABSTRACT: We provide a review that describes the design and formation of supramolecular ladders in the solid state. Supramolecular ladders have been reported in the context of both metal-organic and purely organic single- and multiple-component solids. Supramolecular ladders are based on self-assembly processes propagated both finitely and infinitely. We focus on the properties that such ladders provide solids, as well as applications in the context of predictable dimensional design. Crystal Engineering One of the most important achievements of crystal engineering has been the realization that crystal structures may be viewed as networks.1 In this context, the majority of research has focused on the development of infinite one- (1D),2 two- (2D),3,4 and three-dimensional (3D)5 networks and corresponding properties; however, architectures where a finite self-assembly process occurs along with infinite self-assembly has not received as much attention. In this context, this review will focus on the synthesis, structural elements, and properties of 1D solid-state networks classified as supramolecular ladders. Such frameworks can be classified as consisting of a mix of finite and infinite self-assembly processes owing to the finite nature of a horizontal rung and an infinitely traversing vertical rail. Thus, such a framework provides an opportunity to achieve both dimensional control of supramolecular network topology and a finite selfassembly process that can generate, for example, a cavity defined by the rung and rail. Furthermore, if the rungs or rails are sufficiently close (i.e., face-to-face π-π stacking, van der Waals contact), supramolecular ladders can be considered for applications in areas such as conductivity. Indeed, organic ladders with closely spaced π-stacked surfaces can represent an important first step in realizing complex assemblies akin to ladder structures encountered in nature (e.g., DNA double helix).6 Whereas the self-assembly of molecules in biological contexts typically relies on relatively weak intermolecular forces, the interactions used to form networks within synthetic crystal lattices can be both relatively strong (i.e., metal coordination) and weak (i.e., hydrogen bonding). Moreover, reliable use of such forces for the systematic construction of ladder frameworks can eventually allow for such self-assembly processes to be employed as supramolecular synthons.7 This means that chemists have a large toolkit of intermolecular forces and can develop the proper instructions of network design through the field of crystal engineering. * To whom correspondence [email protected].

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Scheme 1. Building Units Used to Define a Supramolecular Ladder (unit in black): (a) Linear, (b) T-shaped, and (c) H-shaped

Ladder Formation in the Solid State The formation of supramolecular ladders may be supported through molecular building blocks, or tectons.8 These can include linear,9 T-shaped,10 and H-shaped11 building units that define the rung and rail components (Scheme 1). It should also be noted that these tectons can also support the formation of other framework structures (e.g., brick wall) via supramolecular isomerism.12 The self-assembly of the building units with a metal center will be predominantly dictated by the coordination bond, while, in purely organic systems, intermolecular forces (e.g., hydrogen bonds) will predominate. As we shall see, the metal-organic and purely organic approaches are currently at separate stages of development, with the former being largely focused upon controlling properties and the latter being largely focused upon dictating topology.13 For the purpose of this review, metalorganic ladders will be defined as structures that possess both covalent rung and rail components held together by coordination bonds, in which coordination bonds do not exist between ladder frameworks. On the other hand, organic ladders will be defined as structures that possess a covalent rung and rail component

10.1021/cg0606139 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006

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Figure 2. Non-interpenetrated ladder [Co(NO3)2(4,4′-bpy)1.5]∞ from (a) CH3CN and (b) CHCl3.

Figure 1. The metal-organic ladder [Cd(1,4-bis(4-pyridylmethyl)benzene)1.5](NO3)2: (a) single framework, (b) 4-fold interpenetration, and (c) space-filling model of guest inclusion.

held together primarily by hydrogen bonds in which such forces may exist between adjacent ladder frameworks.14 This somewhat looser definition is reflective of the infancy of the development of organic ladders. In the ideal case, and similar to the metalorganic approach, interladder forces should not be present.15 While these bonding and energetic considerations are reflective of current difficulties to construct purely organic ladders, it can be argued that the organic approach can eventually lead to the development of more complex systems akin to biology.16 We will demonstrate that the reliable synthesis of ladder architectures is made difficult owing to the inherently restrictive nature of the finite assembly process. We will also show that molecular cavities arising from supramolecular ladders can be controlled via solvent templation,17 as well as interpenetration.18 Metal-Organic Ladders In 1995, Fujita et al. reported the first example of a metalorganic supramolecular ladder. The ladder was based on reaction of a pyridine functionalized ligand, 1,4-bis(4-pyridylmethyl)benzene, with a Cd(II) ion and resulted in the framework [Cd(1,4-bis(4-pyridylmethyl)benzene)1.5](NO3)2 (Figure 1). The

Figure 3. X-ray crystal structure of the molecular railroad Ni(4,4′bpy)2.5(H2O)2 (free pyridyl groups are highlighted).

framework was based upon a T-shaped geometry around the Cd(II) ion.19,20 The architecture consisted of a 4-fold interpenetrated topology that involved interlocking of the ring of one ladder with four separate rings of nearest-neighbor ladders. As a result, the cavity of the ladder, of dimensions approximately 16.4 × 16.6 Å, did not accommodate a guest molecule. A later report demonstrated that the cavities of the same ladder can include a guest molecule. The ladder was shown to enclatharate 1,4-dibromobenzene via guest templation.21 Shortly following the report of Fujita, a metal-mediated ladder motif was reported by Zaworotko22 et al. The ladder was based on non-interpenetrated cavities (Figure 2).23 Specifically, reaction of Co(NO3)2 with 4,4′-bipyridine (4,4′-bpy) in either MeOH/ CH3CN or MeOH/CHCl3 afforded [Co(NO3)2(4,4′-bpy)1.5]∞. The aromatic solvents were also shown to support the formation of the square-grid polymer [Co(NO3)2(4,4′-bpy)2].24 Both reactions showed a clear inclusion preference for hydrophobic (CH3CN, CHCl3, arenes) versus hydrophilic (H2O, MeOH) solvent guests. The shape of the cavity (approximate dimensions 11.4 × 11.4 Å) demonstrated an ability to distort from orthogonality. The responsive nature of the framework cavities, along with the

Review

Figure 4. The cavity-decorated ladders: (a) [Cd(N,N′-bis-pyridin-4ylmethylene-naphthalene-1,5-diamine)1.5(NO3)2]∞ and (b) [Co(bphd)1.5(NO3)2(CH2Cl2)2]n.

inclusion of hydrophobic solvent guests, demonstrated an emerging ability to tune and thereby create functional metalorganic ladder frameworks. A later report by Jung et al. provided evidence that a similar framework may interconvert to a 1D chain complex.25 In 1997, Yaghi et al. reported that reaction of Ni(ClO4)2‚6H2O with 4,4′-bpy produced the 1D ladder Ni(4,4′-bpy)2.5(H2O)2, which was termed a molecular railroad.26 The framework contained two axially coordinated H2O molecules and two-anda-half equatorially coordinated 4,4-bipyridines, which produced a cavity of approximate dimensions 11.4 × 11.3 Å. The cavities enclathrated ClO4- ions, H2O, and bipyridine guest molecules. As a consequence of the coordination environment, free

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pyridines were present along the periphery of each rung. The noncoordinated pyridines can be considered counterintuitive and, thus, are illustrative of the delicate balance between coordination chemistry and solid-state packing forces that direct crystal formation (Figure 3), particularly as related to infinite and finite self-assembly processes. The solid sustained a loss of H2O upon heating and the partially dehydrated product supported ClO4to PF6- ion exchange. Attempts to decorate functional groups along the walls of 1D ladder frameworks have been demonstrated in several recent reports. The decoration process can be achieved in the bridging ligand, the capping ligand, or a combination thereof. For example, the introduction of a naphthalene moiety into a flexible bipyridine when coupled with a T-shaped geometry around Cd2+ resulted in the 1D ladder, [Cd(N,N′-bis-pyridin-4-ylmethylenenaphthalene-1,5-diamine)1.5(NO3)2]∞. The ladder, while structurally similar to that of Fujita et al., contained a larger cavity of approximate dimensions 20.8 × 20.8 Å (Figure 4).27 Additionally, the ladder contained an imine functionality directed into the cavities, making the solid potentially useful for pore-surface engineering applications.28 However, the ladder did not exhibit interpenetration, and the cavities did not enclathrate guest molecules. Instead, the ladders assembled offset to produce empty channels. A more recent approach by zur Loye et al. using Co(NO3)2 with 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (bphd) as a bridging ligand resulted in a similar ladder, [Co(bphd)1.5(NO3)2(CH2Cl2)2]n, with a cavity of dimensions 15.5 × 15.6 Å.29 The inclusion of two CH2Cl2 molecules into the cavity of the framework suggested that such solids may also be employed for pore-surface engineering studies (Figure 4). Organic anions that serve as capping groups have also recently been shown to decorate the interiors of related 1D ladder frameworks.30 Structural complexity in the synthesis of 1D metal-organic ladders has also been very recently exploited through selfassembly of multiple bridging ligands.31,32 For example, Cao et al. have reported an interpenetrated ladder formed by reaction of Ni(NO3)2‚6H2O, 1,3-bis(4-pyridyl)propane (bpp), and 5-hydroxyisophthalic acid (OH-BDC). The ladder, [Ni(bpp)1.5(H2O)(OH-BDC)]∞, contained a cavity of dimensions 10.2 × 14.0 Å. The cavity was interdigitated by the flexible propane rungs. An interesting feature of the solid is that the bipyridine adopted

Figure 5. The multicomponent ladders: (a) [Ni(bpp)1.5(H2O)(OH-BDC)]∞ and (b) [Cd2(bpp)(cpg)2(H2O)2]∞.

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Figure 6. 1D Ladder showing π-π stacking between the rails in (a) [{Ag(OAc)}2(µ-bpy)2]n‚6H2O, (b) [{(F3CCO2)(µ-O2CCH3)Zn}2(µ-bpe)2]n, and (c) [(Zn2(2,6-bis[N-(2-pyridylethyl)formimidoyl]-4-methylphenolate)(OH))(4,4′-bpe)2](ClO4)2‚4H2O.

Figure 7. 1D ladder {[Pb(bimb)1.5(NO3)2](DMF)}n.

two conformations, acting as both the rung and the capping group.33 Such conformational flexibility in the same solid is unique and introduces a new challenge in the design of predictable and complex architectures owing to the formation of supramolecular isomers.12 A very recent and similar report, involving 3-(4-chlorophenyl)glutaric acid (cpg) and bpp, has described the formation of the infinite 1D bilayer ladder [Cd2(bpp)(cpg)2(H2O)2]∞. Although the ladder contained a cavity of 10.2 × 14.4 Å dimensions, interdigitation of the chlorophenyl groups mitigated against host-guest properties and interpenetration (Figure 5).34 The formation of van der Waals contacts between aromatic rails of has been addressed in the construction of 1D metalorganic ladders. In particular, Lee and Moore et al. in 1996 described a Ag(I) complex with a triflate anion that enforced a face-to-face arrangement between two pyrazine rails.35 Similar solids, involving other N-heterocycles, were subsequently reported involving Ag(I),36,37 Zn(II),38,39 Cu(II),40-42 and Co(II)43,44 centers, with a variety of organic counter ions as the rungs. Notably, one Co(II) system contained a trimeric arrangement of 1,2-bis(4-pyridyl)ethane rail units.40 Very recently, ladder-like coordination polymers have been shown to incorporate reactive olefinic centers in arrangements suitable for [2 + 2] photochemical dimerizations (Figure 6).45 The reactions

proceeded both stereospecifically and quantitatively.46,47 Very recently, Vittal et al. have expanded on the work involving Ag(I) by showing the formation of a 1D ladder structure from the rearrangement of 1D Ag(I) chains upon dehydration.48 The 1D chains contained a photostable 4,4′-bpe as a linear spacer with an olefin-to-olefin distance of 5.1 Å. A rearrangement, which proceeded with a loss of single-crystallinity, produced a ladder that sustained a [2 + 2] photodimerization in quantitative yield. Recently, the area of extended supramolecular frameworks has turned to the development of functional materials with nonlinear optical (NLO) properties, for applications such as optical switching and image processing.49 While there are many reports of 1D coordination polymers that exhibit NLO effects, 50 ladders with rail and rung components for NLO applications have been studied to a lesser extent. In this context, a recent report by Sun et al. describes the formation of a 1D ladder involving a Pb(II) metal center with a 4,4′-bis(imidazol-1ylmethyl)biphenyl (bimb) ligand (Figure 7).51 The ladder, {[Pb(bimb)1.5(NO3)2](DMF)}n, produced very large cavities of dimensions 17.89 × 20.34 Å. The framework hosted two DMF molecules as guests. NLO measurements demonstrated weak NLO absorptive and strong NLO refractive properties.52 Metal-organic ladders have been studied in the context of magnetism.53 In an early report, a ladder-like structure based

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Figure 8. Ladder structures designed for magnetic properties: (a) [Ni(en)2]3[Fe(CN)6]2‚2H2O and (b) Co3(RL)2(hfac)6.

Figure 9. The purely organic ladder based on 4,4-bis(4′-hydroxyphenyl)-1-cyclohexanol.

on bimetallic [Ni(en)2]3[Fe(CN)6]2‚2H2O was shown by Ohba et al. to sustain magnetization at low temperatures.54 The ladders contained a meridional coordination of three cyano groups of the [Fe(CN)6]3- ion to adjacent Ni(II) ions, where the two cisNi(en)2 ions acted as the rails and the trans-Ni(en)2 ion acted as the rungs. The ladder sustained remnant magnetization upon the removal of the applied field at 18.6 K (Figure 8). More recent efforts by Lahti et al. have also focused on designing dimensional coordination complexes for magnetic applications. Specifically, a pseudo-octahedral coordination geometry around the Co(II) ion of Co(hfac)2, in combination with the radical 5-[4(N-tert-butyl-N-aminoxyl)phenyl]pyrimidine (RL), produced the 1D framework Co3(RL)2(hfac)6 (Figure 8).55 Magnetic studies showed that the solid did not behave as a ferromagnet, despite the presence of two different spin units. A very recent report, however, has shown the use of dicyanimide and tetrapyridylpyrazine or nicotinato ligands in combination with Ni(I) or Cu(II) metal to generate antiferromagnetic interactions between the metal ions.56,57 Organic Ladders The term self-assembly, strictly speaking, refers to an assembly process with identical components. Thus, it is perhaps not surprising that the earliest studies involving ladders based on purely organic components involved single-component

Figure 10. Schematic of 1D ladder based on 9,10-bis-(2,5-dihydroxy1-phenyl)anthracene (acetone as a guest).

systems. Recently, similar to the metal-organic approach, multiple-component systems (i.e., cocrystals) have been reported. As stated, the synthesis of purely organic ladders has,

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Figure 11. The 1D organic ladder based on 4,4-bis(4′-hydroxyphenyl)cyclohexanone showing (a) aniline and phenol and (b) m-bromophenol as guests.

Figure 12. Purely organic ladder based on a H-shaped building block: (a) 1,4-di[bis(4′-hydroxyphenyl)methyl]benzene and (b) the octamethyl derivative.

thus far, largely focused on topology. This also means that control of physical properties (e.g., host-guest behavior) has been less developed. Thus, the identification and development of supramolecular synthons that dictate the formation of organic ladder structures have emerged as being particularly important. Single Component. The first deliberate attempt to construct a purely organic ladder was by Desiraju et al.10 The approach involved an organic T-shaped unit decorated with hydrogen bond donor and acceptor sites. The T-shaped unit, 4,4-bis(4′hydroxyphenyl)-1-cyclohexanol, self-assembled via three intermolecular O-H‚‚‚O hydrogen bonds (Figure 9). The ladders were also linked in two dimensions through O-H‚‚‚O hydrogen

bonds. Interestingly, the structure and symmetry of the T-shaped unit allowed for the formation of a ladder with two alternating cavities (rung-to-rung distance 5.6 and 7.5 Å). This study also provided a first direct comparison between the synthesis of metal-organic and purely organic ladder frameworks. Aoyama et al., in earlier work, described the self-assembly of an H-shaped building block, 9,10-bis-(2,5-dihydroxy-1phenyl)anthracene, that produced functional 1D ladders. The study focused on multidimensional host systems that incorporated solvent molecules as guests. Specifically, the diol functionality self-assembled through intermolecular O-H‚‚‚O and O-H‚‚‚OdC solvent-ketone forces to form a 1D ladder with

Review

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Figure 13. The self-assembly of (1,4-di[bis(4′-hydroxyphenyl)methyl]benzene)‚2(4,4′-bpy) into a 1D ladder.

Figure 14. 1D ladder structure of pentaphenol with pyrazine.

Figure 15. 1D ladder structures based on (a) (4-methoxyphenylboronic acid)‚(4,4′-bpy) and (b) (phenylboronic acid)‚(4,4′-bpe)‚H2O.

anthracene units as rungs (Figure 10).58 The framework exhibited selectivity for smaller ketones that provided a better fit for smaller cavities (Figure 10).59 The organic T-shaped unit, 4,4-bis(4′-hydroxyphenyl)cyclohexanone, has also been shown to form 1D ladder host-guest frameworks in the solid state. In particular, Nangia et al. have demonstrated the ability of the ketone to form a ladder that adapts to guest size with rung-to-rung distances that vary from 7.6 to 11.3 Å. The ladder was shown to accommodate aniline, phenol, o-cresol, m-cresol, o-chlorophenol, and m-bromophenol, thus, demonstrating versatility of the tecton (Figure 11).60 Interestingly, this compound has been shown to shift to a 2D brick-type structure when either o- or m-fluorophenol was employed as the guest. The host-guest network also exhibited high selectivity for aniline, as well as a capacity to include both aniline and phenol within separate cavities that disrupt the strong aniline‚‚‚phenol hydrogen bonds. More recently, Nangia et al. have reported a variation of the H-shaped molecular tecton, involving 1,4-di[bis(4′-hydroxyphenyl)methyl]benzene and its octamethyl derivative.11 The self-

assembly of the H-shaped building unit with hydrogen-bond acceptor solvent molecules (e.g., CH3CN, dioxane) produced a 1D hydrogen-bonded ladder (Figure 12). Structures based on honeycomb-grid topologies also formed from solvents that act as both hydrogen-bond donors and acceptors (e.g., MeOH, EtOH). The octamethylated tecton behaved similarly in the presence of hydrogen-bond accepting solvents, exhibiting a small increase in the rung-to-rung distance from 11.3 to 12.2 Å. Despite the covalent modifications to the H-shaped unit, both structures accommodated two CH3CN molecules. Thus, the structures of the architectures depended more on the included solvent and less on covalent modifications to the tecton. Cocrystal. The incorporation of multiple components within a crystal lattice provides chemists an added flexibility to deliberately change the composition and, therefore, the structural complexity of a solid. Moreover, the identification of reliable molecular interactions between multiple components, or heterosynthons, can enable a “divide-and-conquer” approach to synthesis in which supramolecular components can be exchanged to control structural and physical properties of solids.

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Figure 16. 1D ladder structure for the topochemical polymerization of triacetylene.

Figure 17. The 1D ladders based on 3AP with (a) 4,4′-bpe, (b) 4,4′-bpa, and (c) 4,4′-bpy.

In this context, heterosynthons that produce cocrystalline ladders have recently emerged, while modularity has been reported.7 In particular, Nangia et al. have demonstrated that the introduction of a bipyridine spacer unit with an H-shaped tecton forms the 1D ladder (1,4-di[bis(4′-hydroxyphenyl)methyl]benzene)‚2(4,4′-bpy) (Figure 13).11 The ladders self-assembled through O-H‚‚‚N forces between the bipyridines and the H-shaped units to produce a cavity. The introduction of the linear spacer increased the rung-to-rung distance from 11.3 to 20.2 Å. This report demonstrated that such organic ladders possess an ability to attain the modularity encountered in analogous metal-organic solids via synthon development. The modularity of cocrystalline supramolecular ladders also has been explored in the context of condensed structures.61 In particular, Valiyaveettil et al.62 have shown that cocrystallization of a pentaphenol with a variety of N-heterocycles, namely, pyrazine (pyz), 4,4′-bpy, trans-1,2-bis(4-pyridyl)ethylene (4,4′bpe), and 1,2-bis(4-pyridyl)ethane, produced both 1D and 2D frameworks. The 1D ladders formed solely in the case of pyrazine, giving rise to (pentaphenol)‚(pyz)0.5 (Figure 14). The ladders were propagated through O-H‚‚‚O hydrogen bonds between the pentaphenols, with neighboring pentaphenols forming two O-H‚‚‚N hydrogen bonds to a pyrazine to form the rungs. In addition, two phenols of neighboring pentaphenols participated in an offset face-to-face π-stacking arrangement with the pyrazines (Figure 14). The bipyridine derivatives, however, formed 2D frameworks. Peddireddi et al. have shown that π-stacking can be engineered into cocrystalline condensed ladders using boronic acids.63 In particular, boronic acids were cocrystallized with two bipyridines, namely, 4,4′-bpy and 4,4′-bpe. Cocrystallization of 4-methoxyphenylboronic acid with 4,4′-bpy produced the 1D infinite ladder (4-methoxyphenylboronic acid)‚(4,4′-bpy). The structure contained both syn and anti conformations of the hydroxyl groups, with O-H‚‚‚O hydrogen bonds between nearest-neighbor boronic acids. The rungs were sustained with O-H‚‚‚N type hydrogen bonds (Figure 15). Cocrystallization

of 4,4′-bpy with phenylboronic acid produced a finite threecomponent assembly. The assemblies contained only syn hydroxyl conformations and were held together via two O-H‚‚‚N bonds. The finite structures were subsequently linked into molecular tapes through weak C-H‚‚‚N forces. Cocrystallization of 4,4′-bpe with phenylboronic acid was shown to produce the 1D infinite ladder, (phenylboronic acid)‚(4,4′bpe)‚H2O. The structure, composed of only syn hydroxyl groups, was propagated through O-H‚‚‚O hydrogen bonds. A water molecule participated in forming the rungs of the ladder (Figure 15). The fortuitous incorporation of the water molecule addresses a further need to be able to reliably construct ladders using a boronic acid-based synthon. A supramolecular ladder achieved via a cocrystallization approach has recently afforded a solution to a long standing problem, specifically, the polymerization of triacetylenes.64 In particular, Lauher et al. demonstrated that a 1D hydrogenbonded ladder may form through the cocrystallization of a pyridine and a carboxylic acid derivative. The ladder served to preorganize triacetylenes into appropriate geometries for a topochemical polymerization (Figure 16). The modularity of the approach also allowed for the interchange of the pyridine and carboxylic acid functionalities as a means to resolve issues of solubility between the host and guest systems. Thus, the “divideand-conquer” approach of cocrystallization produced a functional supramolecular ladder in the solid state (Figure 16). Our Work. Recently, our research group has turned to developing a robust synthon that sustains the reliable formation of condensed cocrystalline ladders in the solid state.65 In particular, we have reported the ability of 3-aminophenol to produce isoreticular ladders based on a variety of bipyridine rungs, namely, 4,4′-bpy, 4,4′-bpe, and 1,2-bis(4-pyridyl)acetylene (4,4′-bpa). The self-assembly of the phenols with each bipyridine through a combination of N-H‚‚‚O interactions between neighboring 3-aminophenols and O-H‚‚‚N and N-H‚‚‚N forces produced ladders with structures tolerant to changes in the shape of the framework (Figure 17). The

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bipyridine rungs exhibited a face-to-face π-π stacking based on close van der Waals contacts. The robustness of this synthon is currently under further investigation. Challenges and Future Outlook In this review, we have described supramolecular ladders based on metal-organic and purely organic components. While a challenge exists to control the formation of finite and infinite geometries, the identification of reliable synthons for both metal-organic and purely organic systems has produced solids that exhibit host-guest behavior, reactivity, NLO, and magnetic properties. From a topological standpoint, it is intriguing to consider the future design of ladders based solely upon finite self-assembly processes. A pioneering example has been reported by Lehn et al., in which a ladder with a discrete number of horizontal rungs formed through self-assembly.66 An important extension has also been recently demonstrated by Drain et al. in the finite arrangement of porphyrin derivatives.67 Here, the self-assembly of two or three molecular units, encoded with the proper “molecular information” and “read” by a metal center,68 resulted in the formation of 3 × 3 grids and fourcomponent tape structures. The development of such systems in the context of pure organics has yet to be reported. Such multicomponent systems, however, demonstrate the degree of molecular recognition and structural complexity required to form ladders based on finite assembly processes and direct chemists to further strive to achieve structural complexity in supramolecular systems. Acknowledgment. We are grateful to the University of Iowa Mathematical and Physical Sciences Funding program for support of this work. References (1) Sharma, C. V. K. Cryst. Growth Des. 2002, 2, 465. (2) (a) MacDonald, J. C.; Whitesides, G. M. Chem. ReV. 1994, 94, 2383. (b) Zerkowski, J. A.; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 5473. (c) Zerkowski, J. A.; Seto, C. T.; Wierda, D. A.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 9025. (d) Reddy, D. S.; Panneerselvam, K.; Pilati, T.; Desiraju, G. M. J. Chem. Soc. Chem. Commun. 1993, 661. (e) Khlobystov, A.; Blake, A. J.; Champness, N. R.; Lemenovskii, D. A.; Majouga, A. D.; Zyk, N. V.; Schro¨der, M. Coord. Chem. ReV. 2001, 222, 155. (3) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629. (4) Yaghi, O. M.; O’Keeffe, K.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kin, J. Nature 2003, 423, 705. (5) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107. (6) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman and Company: New York, 1988; p 71. (7) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311. (8) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc. 1991, 113, 4696. (9) Barnett, S. A.; Champness, N. R. Coord. Chem. ReV. 2003, 246, 145. (10) Aitipamula, S.; Thallapally, P. K.; Thaimattam, R.; Jaskolski, M.; Desiraju, G. R. Org. Lett. 2002, 4, 921. (11) Aitipamula, S.; Nangia, A. Supramol. Chem. 2005, 17, 17. (12) Hennigar, T. L.; MacQuarrie, D. C.; Losier, P.; Rogers, R. D.; Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 972. (13) Desiraju, G. R. Science 1997, 287, 404. (14) Although this review emphasizes the formation of ladders via relatively strong noncovalent bonds, ladders based on relatively weak interactions have been reported, see (a) Thodupunoori, S. K.; Alamudun, I. A.; Cervantes-Lee, F.; Gomez, F. D.; Carrasco, Y. P.; Pannell, K. H. J. Organomet. Chem. 2006, 691, 1790; (b) Koizumi, T.-A.; Tanaka, K. Inorg. Chim. Acta 2004, 357, 3666. (15) For related step-like structures, see Nguyen, V. T.; Ahn, P. D.; Bishop, R.; Scudder, M. L.; Craig, D. C. Eur. J. Org. Chem. 2001, 4489. (16) Zeng, H.; Miller, R. S.; Flowers, R. A.; Gong, B. J. Am. Chem. Soc. 2000, 122, 2635.

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