Enclosed Chiral Environments from Self-Assembled MetalâOrganic

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 3 419-430

Review Enclosed Chiral Environments from Self-Assembled Metal-Organic Polyhedra Tamara D. Hamilton and Leonard R. MacGillivray* Department of Chemistry, University of Iowa, Iowa City, Iowa 52245 Received October 30, 2003

ABSTRACT: Enclosed chiral spaces can be achieved using self-assembled metal-organic polyhedra. Such spaces can be useful for the recognition of chiral guests and the promotion of enantioselective reactions. Here, we provide a review of recent progress in the design, synthesis, and characterization of chiral self-assembled metal-organic polyhedra, with a particular focus on symmetry and structure. Chirality may be incorporated into molecular polyhedra prior to or during the self-assembly process. Examples of both approaches are discussed. 1. Introduction The creation of chiral environments has been a longtime goal of organic chemistry because it facilitates the synthesis and isolation of enantiomerically pure compounds.1 The importance of chirality is underscored by the fact that nearly all natural products are chiral and the pharmacological action of drugs depends on recognition by chiral receptors in the body.2 In recent years, chemists have succeeded in creating chiral environments using hollow assemblies of molecules constructed by way of transition metal-mediated self-assembly. In this design, metal atoms, which typically form the vertices of a polyhedron, are held together by coordinating organic ligands, which typically form the edges and/or the faces. Indeed, excellent reviews of such polyhedral assemblies, and the rationale behind their design, appear elsewhere.3 The subject of chirality within this context has not, however, been separately addressed. Guests inside such hosts may “experience sensations of dissymmetry”,4 leading to enantioselective reactions. Indeed, a chiral metal-organic polygon, in the form of a triangle, has recently been used as a host for the asymmetric catalysis of diethylzinc additions to aromatic aldehydes, resulting in enantiomeric excesses as high as 92%.5 Research in our laboratory6 has led to an interest in the design and properties of polyhedral metal-organic assemblies. This review will focus on methods to create three-dimensional (3D) chiral environments based on metal-organic polyhedra, with a particular focus on * To whom correspondence should be addressed. E-mail: [email protected].

symmetry and structure. An ability of such chiral frameworks to act as hosts will be discussed. 2. Chiral Space From Polyhedra There are a limited number of ways in which identical chemical subunits, in the form of regular polygons, can assemble to enclose space (Figure 1).7 These arrangements consist of the five Platonic and 13 Archimedean solids, along with two infinite families, the prisms and antiprisms. Each Platonic solid possesses cubic symmetry, with all faces being identical regular polygons. Each Archimedean solid is made up of at least two different regular polygons. Prisms and antiprisms consist of two regular n-gons bisected by a perpendicular n-fold rotation axis. The n-gons of a prism are congruent, whereas the n-gons of an antiprism are twisted 180/n degrees with respect to each other. With the exception of the snub cube and the snub dodecahedron, the Platonic and Archimedean solids, as well as the prisms and antiprisms, are inherently achiral. Chirality can be introduced, however, by removing symmetry. Specifically, an object is chiral if it lacks an improper rotation axis (i.e., Sn axis).8 This implies, for example, the absence of a center of inversion or a mirror plane. Thus, symmetry operations permitted for chiral objects are proper rotations. For instance, the groups Td, Th, and T possess tetrahedral symmetry. Td and Th are achiral. T, which lacks a center of inversion and a mirror plane yet possesses a combination of 3and 2-fold rotation axes, is chiral (Figure 2). Similarly, of the groups Oh and O, which possess octahedral symmetry, O is chiral, and of Ih and I, which possess icosahedral symmetry, I is chiral.

10.1021/cg0342011 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/20/2004

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Figure 1. Regular polyhedra. The Platonic solids (red) from left to right: tetrahedron (T), cube (O), octahedron (O), dodecahedron (I), and icosahedron (I). The Archimedean solids (green) from left to right: truncated tetrahedron, cuboctahedron, truncated cube, truncated octahedron, rhombicuboctahedron, snub cube, icosidodecahedron, rhombitruncated cuboctahedron, truncated dodecahedron, truncated icosahedron, rhombicosidodecahedron, snub dodecahedron, and rhombitruncated icosidodecahedron. Prisms and antiprisms (blue) from left to right: trigonal prism, square prism, pentagonal prism, hexagonal prism, trigonal antiprism, square antiprism, pentagonal antiprism, and hexagonal antiprism.

Figure 3. General structure of the M4L6 tetrahedron.

Figure 2. Polyhedra corresponding to the point groups (a) Td and (b) T. The presence of the chiral windmill-like structure on each face in panel b reduces the symmetry.

Chemically speaking, chirality can be introduced within a metal-organic polyhedron in four ways. If the assembly is composed of achiral components, chirality may arise either at the metal center or as a consequence of the way the organic ligands are twisted in space. In these cases, symmetry can be considered to be removed in situ, during the self-assembly process. On the other hand, chirality can be introduced using enantiomerically pure starting materials that incorporate a stereogenic center into either the organic bridge or an auxiliary attached to the metal center. Here, symmetry can be considered to be removed prior to the self-assembly

process. Any combination of these four approaches may also be employed. It should be noted that Stang has identified similar ways to introduce chirality into molecular polygons9 and that the same basic principles apply here. In the following discussion, specific examples of metal-organic polyhedra derived from each approach will be addressed. We will start with chiral assemblies that form from achiral starting materials. We will then discuss chiral assemblies constructed using chiral building units. 3. Chirality From Achiral Components From a practical standpoint, an efficient route to chiral metal-organic polyhedra is via achiral starting materials. Such materials are generally less costly and more readily available than corresponding chiral counterparts. This is perhaps realized by the relatively large

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Crystal Growth & Design, Vol. 4, No. 3, 2004 421 Scheme 1

number of examples of chiral assemblies based on achiral components, as compared to those from chiral starting materials. However, the achiral approach, not surprisingly, almost always leads to a racemic mixture, both in solution and in the solid state. Also, because the origin of the chirality is largely dependent on metal coordination bonds, which can be labile in solution, enantiomers can interconvert. Consequently, the chirality can typically be considered “soft” or impermanent. Often, an optically pure chiral guest can induce formation of one host enantiomer over the other in solution. Resolved enantiomers can also be stable to racemization for extended periods of time.10 3.1. Metal Center. Tetradentate ligands with two bidentate chelating functionalities are commonly used for the assembly of cage complexes from labile metal centers and organic bridging ligands. When such ligands assemble with octahedral metal centers, the metal centers become chiral (∆ or Λ), each having three bidentate ligands. An example is an M4L6 tetrahedron (M ) metal; L ) ligand) (Figure 3). Such an assembly may possess overall T (∆∆∆∆ or ΛΛΛΛ), C3 (∆∆∆Λ or ∆ΛΛΛ), or S4 (∆∆ΛΛ) symmetry. The S4 case is a meso form and is therefore achiral. The first example of a chiral metal-organic tetrahedron was reported by Saalfrank et al. in 1990 (Scheme 1).11 The assembly, 1a, consists of four M(II) (M ) Co, Mn) ions and six tetracarboxylate ligands, which produce a tetrahedral complex with overall charge of -4. The ligand in the assembly is atropisomeric,12 with all six ligands being twisted in the same sense to link the four metal centers, giving an assembly of overall T symmetry. The X-ray crystal structure of 1a where M ) Co is shown in Figure 4. The four cobalt atoms occupy the corners of a distorted tetrahedron, with Co-Co distances ranging from 6.61 to 6.76 Å. Assemblies 1b (where M ) Co, Mg) have also been shown to host ammonium and alkylammonium cations as well as alkali metal cations exohedrally (i.e., on the outside of the cage), one above each of the four triangular faces of the tetrahedron.13,14 Both homochiral enantiomers were shown to exist in solution by 1H NMR spectroscopy. Later, T symmetric mixed valence Fe(II)/Fe(III) frameworks 1a and 1b based on the same ligands were revealed, by X-ray crystallography, to encapsulate either NH4+15 or Cs+16 as a guest. An Fe(III) tetrahedron that encapsulates a water molecule {H2O ⊂ 1b} has also been reported.16 Interestingly, 1H NMR spectroscopy reveals that the assembly 1c (M ) Mg)17 undergoes nondissociative enantiomerization in solution, converting from one enantiomer to the other without the ligands dissociating from the metal. The mechanism for the interconversion involves the synchronization of four

Figure 4. X-ray crystal structure of 1a (M ) Co): (a) the tetrahedron of metal atoms and (b) the orientation of a tetracarboxylate ligand (yellow). Hydrogen atoms have been omitted for clarity. Color scheme: pink ) Co, gray ) carbon, red ) oxygen, and yellow ) highlighted ligand.

Bailar twists (one at each metal center) and six atropenatiomerization processes (one for each ligand).18 Raymond et al. have described a series of chiral M4L6 metal-organic tetrahedra. The first example is based on a bifunctional bis-chelating hydroxamate ligand, isophthal-di-N-(4-methylphenyl)hydroxamic acid. When this ligand is combined with Ga(III) or Fe(III), the assembly crystallizes with four encapsulated dimethylformamide (DMF) molecules.19 The average Ga-Ga distance is 8.90 Å. The assembly is achiral in the solid state, possessing S4 symmetry. Low-temperature 1H NMR spectroscopy studies, however, reveal a mixture of T, C3, and S4 isomers and mirror images in solution, which interconvert via a Bailar twist mechanism.20 Raymond has also reported a series of bis(catecholate) ligands that give M4L6 tetrahedra 2 when combined with Ga(III) or Fe(III) (Scheme 2). Specifically, the naphthyl derivative leads to tetrahedra 2a that encapsulate appropriately sized alkylammonium ions, with a preference for NEt4+ over either NPr4+ or NMe4+.21 The Fe(III) tetrahedra possess T symmetry and crystallize as a racemic homochiral mixture, with an average Fe-Fe distance of 12.8 Å.21 The volume of the cavity is

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Scheme 2

on the order of 250-350 Å3.22 Solution 1H NMR spectroscopy studies reveal that the NEt4+ guest can sense the chiral environment of the host cluster.21 Enantiomers were later resolved in the presence of a chiral guest, which was then replaced by NEt4+, leaving behind the enantiopure cluster. Remarkably, complete retention of chirality at the metal center was observed for at least 8 months, even in boiling alkaline solution. The authors attribute such behavior to “mechanical stiffness” provided by interconnection of the four labile metal centers in a rigid 3D tetrahedral arrangement.10 The X-ray crystal structure of the ∆∆∆∆-2a (M ) Ga) framework that encapsulates the NEt4+ ion is shown in Figure 5. The average Ga-Ga distance is 12.7 Å.10 The anthracyl ligand, when combined with Ga(III) or Ti(IV) in the presence of NMe4+, gives crystals that are a homochiral racemic mixture of tetrahedra 2b, with one NMe4+ ion on the inside.23 In the absence of NMe4+, a M2L3 helicate is formed, which converts to the tetrahedron upon introduction of the guest. The average Ti-Ti distance is 16.1 Å.23 The pyrene derivative gives Figure 6. X-ray crystal structure of 3a (M ) Co): (a) the tetrahedral arrangement of Co atoms at the core of the assembly and (b) the orientation of ligands around each metal center, demonstrating how chirality arises. Hydrogen atoms have been omitted for clarity. Color scheme: green ) Co, gray ) carbon, blue ) nitrogen, and red, yellow, purple ) highlighted ligand.

Scheme 3

Figure 5. X-ray crystal structure of 2a (M ) Ga): encapsulated NEt4+ cation (magenta) and the orientation of a bis(catecholate) ligand (yellow). Hydrogen atoms have been omitted for clarity. Color scheme: green ) Ga, gray ) carbon, red ) oxygen, blue ) nitrogen, and yellow ) highlighted ligand.

a tetrahedral assembly 2c when combined with Ga(III) in the presence of a suitable (e.g., alkylammonium) guest.24 The average Ga-Ga distance is 14.3 Å, and the size of the cavity is approximately 300-400 Å3. Initial solution studies suggested that chirality stemming from the metal centers was not transferred to the internal cavity. However, as the concentration of NEt4+ was increased, additional NEt4+ cations were observed to adhere to the surface of the structure, and the cavity was compressed. This compression caused the methyl-

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ene protons of the encapsulated NEt4+ to become diastereotopic, as evidenced by a quartet splitting into a multiplet in the 1H NMR spectrum, implying that the guest is increasingly able to sense the chiral environment of the host as the cavity becomes smaller. Interestingly, 2a exhibits “chiral memory” when an enantiopure sample of the assembly is placed in solution.25 Specifically, a phenyl derivative, which does not form a tetrahedral structure itself, replaces naphthyl ligands to produce chiral cluster 2d when introduced to the solution. The chirality of the original assembly is preserved as measured from the circular dichroism (CD) spectra, even though there is evidence that partial dissociation of the complex occurs on the time scale of seconds. Ward et al. have recently reported M4L6 tetrahedra 3 (Scheme 3) involving bidentate pyrazolyl-pyridine units connected to a central aromatic spacer. When combined with Co(II) in the presence of NaBF426 or NaClO427, the phenyl26 and 2,3-naphthyl27 derivatives produced isostructural tetrahedra 3a and 3b of T symmetry. The X-ray crystal structure of 3a is shown in Figure 6. Co-Co distances range from 8.98 to 10.1 Å, whereas in the case of 3b, the distances range from 9.30 to 10.0 Å. The assembly 3b has also been reported using Zn(II) as the metal, with Zn-Zn distances ranging from 9.35 to 10.1 Å.28 Each assembly crystallizes as a racemic homochiral mixture encapsulating either a BF4- or a ClO4- anion. Variable temperature 11B and 19F NMR spectroscopy studies showed that a BF - anion 4 is effectively trapped inside the 3a or 3b cage and does not exchange on the NMR time scale with free BF4anions in solution.29 Notably, 1H NMR spectroscopy studies of a solution with a 2:3 M:L ratio produced broad, undefined peaks until aqueous NaBF4 or NaClO4 was added. After addition of the salt, sharp peaks that matched the spectrum of redissolved crystals of the assembly appeared, suggesting that formation of the assembly is templated by the anions. A template effect was not observed with KPF6, presumably due to the large size of the PF6- anion.29 Ward has also reported that a longer biphenyl bridging ligand gives an M4L6 tetrahedral framework 3c30 that crystallizes as a homochiral racemic mixture based on C3 symmetry.30 X-ray crystallographic studies demonstrate that a guest anion is not necessary to form the assembly. Co-Co distances are relatively long, ranging from 11.5 to 12.3 Å. Solution 11B and 19F NMR spec-

Figure 7. X-ray crystal structure of assembly 4: (a) the cuboctahedral arrangement of Cu atoms at the core of the assembly and (b) orientation of a ligand, demonstrating how chirality arises. Hydrogen atoms and disorder have been omitted for clarity. Color scheme: brown ) Cu, gray ) carbon, red ) oxygen, blue ) nitrogen, and yellow ) highlighted ligand.

troscopy studies show that encapsulated BF4- anions are readily exchanged with free ions through cavities located in the center of each face. The exchange process can be frozen at -40 °C.30 The similar bis(pyrazolyl-pyridine) ligand with a 1,8naphthyl spacer, when reacted with Co(II) or Zn(II) in the presence of NaBF4 or NaClO4, produced a much larger M12L18 assembly. The crystal structure reveals a geometry consistent with a truncated tetrahedron based on T symmetry.31 Co-Co distances along each of the 18 edges range from 9.22 to 9.36 Å. All 12 metal centers of each assembly possess the same chirality, while the crystal is racemic. The polyhedron is occupied by four BF4- or ClO4- anions. It has not been determined whether the anions provide a template effect. Homochiral frameworks with chirality arising at metal centers exist for geometries other than T. For example, a structure based on a trigonal antiprism of D3 symmetry has been reported.32 Such assemblies do

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Review Scheme 5

Scheme 6

not contain a guest, however, and therefore will not be discussed here. 3.2. Twisting of Ligands. Robson et al. have described a cubelike assembly 4 derived from a 2:3 ratio of a tris-bidentate ligand with copper(II) (Scheme 4).33 The resulting homochiral capsule (Figure 7) crystallizes as a racemic mixture of M12L8 cubes where the ligand serves as the corners and the metal as the edges. Alternatively, one can view the copper atoms as forming a cuboctahedron, with the eight triangular faces being provided by the ligand. Chirality arises from the sense in which the ligands are twisted. Free rotation about the single C-N bonds allows the phenyl “arms” to twist in either a clockwise or counterclockwise direction. The hollow shell has a volume of ca. 816 Å3, a sufficient size to contain approximately six DMF molecules. Electron density data from single-crystal X-ray diffraction are consistent with 5-6 DMF molecules within the shell, two of which are well-resolved. Fujita et al. have reported a M3L2 complex with a structure that conforms to a trigonal prism. The complex consists of Pd and a tris(pyridyl)benzene ligand and, upon including 1,3,5-benzenetricarboxylic acid, is distorted into chiral cage 5 (Scheme 5).34 The asymmetry of the cage was confirmed by 1H NMR spectroscopy. When the flat acid was replaced by a more spherical guest, such as CBrCl3, an achiral cage formed. Molecular modeling revealed that the asymmetric cage possesses a flat cavity, while that of the symmetric cage is spherical, which accounts for the acid inducing assembly of the former and the halocarbon the latter. Competition experiments between p-xylene and CBrCl3 revealed a slight preference for asymmetric 5. Two diastereomeric complexes were observed in solution when (R)-mandelic acid was added, suggesting that racemization of one diastereomer to the otherswhich would require cleav-

age of two Pd-N bondssdoes not take place on the NMR time scale. When racemic mandelic acid was added, diastereomers were not observed. This suggests that the R and S forms of the guest exchange quickly from one host enantiomer to the other, showing the host enantiomers to be spectroscopically identical. Robson et al. have reported a polyhedral M12L4 assembly 6 where four tris-chelating ligands form the faces of a tetrahedron.35 When a tris-bidentate ligand is combined with Cd in the presence of NEt4OH, 6 is

Figure 8. X-ray crystal structure of assembly 6: (a) ball-andstick view showing the ligand (pink) and (b) spacefilling view. Hydrogen atoms have been omitted for clarity. Color scheme: green ) Cd, gray ) carbon, red ) oxygen, blue ) nitrogen, and yellow ) chlorine.

Review


Scheme 8

formed (Scheme 6). Two oxo-bridged Cd atoms make up each edge of the tetrahedron. Each Cd center adopts a square pyramidal coordination environment with a Clanion in the apical position, giving the assembly an overall charge of -8. Each cage, in contrast to the crystal, is chiral, due to the twisting of ligands on each face of the polyhedron in a clockwise or counterclockwise direction about the C-N single bonds. The structure of 6 is shown in Figure 8. 13C MAS NMR spectroscopy revealed a single NEt4+ ion to occupy the interior of the structure. Related assemblies were isolated with Brin place of Cl- or NMe4+ in place of NEt4+ by making the appropriate substitutions in the assembly process. Assembly 7, designed by Ikeda, Shinkai et al., is a trigonal antiprism composed of two bowl-shaped homooxacalix[3]arene units with 3-pyridyl substituents at the para positions, which are held together by three Pd(II) centers (Scheme 7).36 Two Na+ ions are complexed on the outsides of the calixarene bowls. As speculated by the authors, the Na+ ions force the phenyl rings in 7 to “stand up”. Therefore, the 3-pyridyl rings can no longer arrange perpendicular to the phenyl rings, to adopt a

suitable angle to bind to the Pd. Consequently, the top and bottom calixarenes twist relative to each other, into either a right- or a left-handed helical form. 1H NMR spectroscopy and CD studies revealed that the ratio of the enantiomers can be controlled by the inclusion of chiral guests [e.g., the (S)-2-methylbutylammonium ion]. A similar chiral M3L2 assembly involving a pyridylbased cyclotriveratrylene and cis-protected Pd units has also been reported.37 The cage was obtained quantitatively within seconds upon mixing the components, as evidenced by 1H NMR spectroscopy. The structure of the assembly was also supported by CSI-MS and CD spectroscopy. Kimura et al. have reported cuboctahedral assembly 8 based on a trimeric Zn(II) cyclen complex and trithiocyanuric acid (Scheme 8).38 The exterior of the cage twists right or left, depending on the sense in which the C-S-Zn linkages are bent. The interior is described as a truncated tetrahedron, screwed clockwise or counterclockwise. X-ray analysis (Figure 9) showed that each individual crystal of the assembly contained only one enantiomer. In fact, crystals spontaneously resolved into

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Figure 10. X-ray crystal structure of assembly 9: (a) spacefilling and stick view showing the twisting of the helicates and the PF6- anions on the inside of the assembly (dark green) and (b) spacefilling and stick view showing the CH-π interactions between the methyl groups of one triangular helicate and the phenyl rings of another, which hold the assembly together. Hydrogen atoms have been omitted for clarity.

Figure 9. X-ray crystal structure of assembly 8: (a) the adamantane guest inside the cavity, (b) the environment of a Zn(II) cyclen complex, and (c) the environment of a trithiocyanuric acid molecule. Hydrogen atoms have been omitted for clarity. Color scheme: dark gray ) Zn, gray ) carbon, blue ) nitrogen, yellow ) sulfur, and red ) highlighted ligand.

a conglomerate racemic mixture. Encapsulation studies, conducted using 1H NMR spectroscopy, suggested that encapsulation of an appropriate chiral guest could be used to control the chirality of the assembly. Hannon et al. have reported an M12L12 chiral ball 9 with T symmetry assembled from Cu(I) and a bisbidentate ligand (Scheme 9).39 The truncated tetrahedron 9 consists of four bowl-shaped triangular circular helicates held together by CH-π interactions. Each triangle of the ball is of identical chirality, meaning that the aggregate is chiral. The chirality is a consequence of the twisting arrangement of the pyridyl groups of each triangle, which twist in the same direction to permit the triangles to fit together. Assembly 9 was

characterized by X-ray crystallography (Figure 10). The crystals resolve spontaneously such that the balls of a single crystal are of identical chirality. The diameter of each ball is 25 Å, and the internal cavity contains four PF6- anions. 4. Chirality From Chiral Components There is a paucity of examples of chiral metal-organic polyhedra based on chiral starting materials. This is perhaps due to difficulties in synthesizing enantiomerically pure bridging ligands and/or auxiliaries. An advantage of the approach is that by using an enantiopure chiral bridging ligand, the chirality of the resulting polyhedral structure is predetermined. Additionally, because the chirality is typically based on covalent bonds, it can be considered permanent or “hard”. Thus, in contrast to the achiral case, such a polyhedron is unlikely to racemize. 4.1. Organic Bridging Units. To our knowledge, the only example of a cavity-containing polyhedron with metal centers bridged by an enantiopure chiral organic linker was reported by Stang et al. Specifically, a bis(platinum) complex was shown to assemble with a chiral tritopic linker to produce a M6L4 adamantanoid 10

Scheme 9

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Scheme 11

(Scheme 10).40 Optical rotation studies confirmed optical activity, due to the four stereogenic centers along the exterior of the polyhedron. As a result, 10 possesses D2 symmetry, as opposed to the Td symmetry expected for an achiral adamantanoid. The assembly 10 was also characterized by 1H and 31P NMR spectroscopy, as well as electrospray mass spectrometry. Unfortunately, crystals suitable for single-crystal X-ray diffraction could not be obtained. To support the structure of 10, MM2 force field simulations were conducted and showed that the internal cavity approaches 3.0 nm in diameter. Lin et al. have reported a M4L6 adamantoid of T symmetry 11 obtained upon assembly of a racemic mixture of 2,2′-hydroxy-1,1′-binaphthalene-6,6′-dicarboxylic acid (H2BDA) ligand with lanthanide ions, such as Gd3+ or La3+ (Scheme 11).41 The crystal structure (M ) Gd) is shown in Figure 11. The BDA ligands of each adamantoid possess the same chirality, giving the T symmetric structure. Single crystals of 11 are racemic, with each assembly of a given handedness being surrounded by assemblies of opposite handedness. The GdGd distance is 15.6 Å. Each adamantoid has an open cavity with a volume of ca. 1700 Å3, which is occupied in the solid state by six DMF molecules. The material is porous, and the assemblies display face-to-face intercluster hydrogen bonds and assemble to form two types of rhombus-shaped zigzag channels of dimensions ca. 11 Å × 15 Å and ca. 6 Å × 8 Å. Enemark and Stack have reported a M4L6 tetrahedral cluster, based on Ga and a bis(catecholate) ligand, with two stereogenic centers.42 When an enantiopure form of the ligand was used, a single homochiral cluster of T symmetry formed. This contrasts an enantiomeric pair of homochiral tetrahedral clusters that formed from a

racemic mixture of the ligand.42 The structure of the single enantiomers was characterized by 1H NMR

Figure 11. X-ray crystal structure of assembly 11: (a) balland-stick view and (b) spacefilling view. Hydrogen atoms have been omitted for clarity. Color scheme: green ) Gd, gray ) carbon, and red ) oxygen.

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Scheme 13

Scheme 14

spectroscopy and X-ray crystallography. The cavity, however, was not large enough to contain a guest. 4.2. Auxiliaries. Stang et al. have utilized Pt(II) or Pd(II) with R-(+)-BINAP chelating ligands as shapedefining corner units for the self-assembly of optically active M6L4 adamantoid cages 12 with T symmetry (Scheme 12).43 In these cases, the chirality stems from ligands bound to the exterior of each cage. Each as-

sembly was characterized by 1H NMR spectroscopy and mass spectrometry. Host-guest behavior was not reported, although a related achiral cage, reported by Fujita et al., was shown to include four adamantylcarboxylate guests.44 Fujita et al. have used (R,R)-diaminocyclohexane as an auxiliary for a M6L4 cage with a structure that conforms to a truncated tetrahedron, based on Pd(II)

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and a D2h symmetric pyridyl ligand.45 The assembly in the absence of the auxiliary was characterized by X-ray diffraction, CSI-MS, and 1H NMR spectroscopy, which showed two ligand environments, with each ligand being desymmetrized. When (R,R)-diaminocyclohexane was used as the shape-defining corner unit of the Pd(II) assembly, 13 was obtained (Scheme 13), with two diastereomers being observed by 1H NMR spectroscopy (ratio 1.3:1). A positive Cotton effect was observed in the CD spectrum, while a negative Cotton effect was observed for the (S,S)-diaminocyclohexane analogue. Solution studies revealed that aromatic guests (e.g., p-dichlorobenzene, p-xylene) were encapsulated by 13. Ikeda et al. have reported that Zn-based porphyrins that bear four pyridyl groups combine with Pd centers that bear the (R)-(+)-BINAP cis-protecting group to yield a chiral M4L2 assembly 14 (Scheme 14). The assembly is a square prism, possessing D4h symmetry. Chirality arises from the chiral auxiliaries, as well as from a twisting motion within the assembly induced by the auxiliaries.46 The assembly was characterized by 1H NMR, CSI-MS, UV-vis, and CD spectroscopy. Studies are under way to determine whether the assembly encapsulates guests. Notably, an achiral analogue, reported by the same group, was shown to accommodate bipyridine as a guest.47 5. Chiral Organic Assemblies Purely organic self-assembled chiral polyhedra have also been reported. Although a review will not be presented here,48 it should be noted that such assemblies, which are typically held together by hydrogen bonds, can also be derived from either achiral49,50 or chiral51,52 starting materials. When chiral starting materials are employed, chirality constitutes either the walls51 or the periphery52 of the polyhedra. Such pathways to chiral self-assembled hosts are analogous to those of the chiral metal-organic polyhedra. 6. Summary and Outlook Chemists are beginning to design polyhedral metalorganic assemblies that exhibit chirality. Such assemblies can act as hosts for achiral or chiral guests. Two approaches have been employed to design and construct such polyhedra. A “soft” approach employs achiral building blocks to produce chirality at a metal center or due to twisting of ligands, while a “hard” approach employs optically pure chiral building blocks to produce hosts of predetermined chirality. In the former, racemization can occur in solution or in the solid state while in the latter, enantiopurity is largely ensured. A next step will be to design hosts around a given chiral guest. Clearly, enantioselective chemical reactions are another goal in this area.53 The examples discussed here encompass pathways to achieve these and related goals in a host-guest chemistry based on self-assembly.54 References (1) Scheffer, J. R. Can. J. Chem. 2001, 79, 349-357. (2) Seyden-Penne, J. In Chiral Auxiliaries and Ligands in Asymmetric Synthesis; Wiley and Sons: New York, 1995.

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