Supramolecular Networking of Macrocycles Based ... - ACS Publications

Supramolecular Networking of Macrocycles Based on Exo-Coordination: From Discrete to Continuous Frameworks SUNHONG PARK, SO YOUNG LEE, KI-MIN PARK, AND SHIM SUNG LEE* Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea RECEIVED ON MAY 20, 2011

CONSPECTUS

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acrocyclic ligands typically show high selectivity for specific metal ions and small molecules, and these features make such molecules attractive candidates for nanoscale chemical sensing applications. Crown ethers are macrocyclic structures with polyether linkages where the oxygen donors are often separated by an ethylene unit (OCH2CH2O). Because the oxygen lone pairs in crown-type macrocycles are directed inward, the preorganized macrocyclic cavity tends to form complexes where metals coordinate inside the cavity (endo-coordination). However, sulfur-containing macrocycles often demonstrate metal coordination outside of the cavity (exo-coordination). This coordination behavior results from the different torsion arrangements adopted by the XCH2CH2X atom sequence (X = O, gauche; X = S, anti) in these molecules. Exo-coordination is synthetically attractive because it would provide a means of connecting macrocyclic building blocks in diverse arrangements. In fact, exo-coordination could allow the construction of more elaborate network assemblies than are possible using conventional endocyclic coordination (which gives metal-in-cavity products). Exo-coordination can also serve as a tool for crystal engineering through the use of diverse controlling factors. Although challenges remain in the development of exo-coordination-based synthetic approaches and, in particular, for the architectural control of supramolecular coordination platforms, we have established several strategies for the rational synthesis of new metallosupramolecules. In this Account, we describe our recent studies of the assembly of metallosupramolecules and coordination polymers based on sulfur-containing macrocycles that employ simple and versatile exo-coordination procedures. Initially, we focus on the unusual topological products such as sandwich (1:2, metal-to-ligand), club sandwich (2:3), and cyclic oligomeric complexes as discrete network systems. The primary structures we achieve in these networked macrocycles are one to three dimensional coordination polymers based on homo- and heteronuclear metal systems. Using crystal engineering methods, we have also investigated variation in the donors, interdonor distances, ligand isomer structures, and the effect of counter anions on the chemical and physical properties of the products. Understanding the relationship between structure and function in these exo-coordination products is an important step in the design of these new supramolecules for practical applications. We investigated the photophysical properties of the exocyclic complexes and a chromogenic macrocycle, which exhibited cation-selective and anion-controlled color change depending on an exo- or endo- ligand binding mode. Overall, we suggest that the exocyclic coordination behavior provides a versatile strategy for the preparation of new molecular networks and materials.

Published on the Web 10/03/2011 www.pubs.acs.org/accounts 10.1021/ar200143n & 2011 American Chemical Society

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Supramolecular Networking of Macrocycles Park et al.

1. Introduction

coordination polymers,722 while the nonbridging exo-

Traditionally, macrocyclic ligands often show high selectivity for specific metal ions and small molecules.1 Crown ethers are representative macrocyclic ligands composed of cyclic polyether linkages, where the oxygen donors are usually separated by an ethylene unit (OCH2CH2O) or its equivalent.1,2 In this case, the torsion angle between two oxygen donors tends to have a gauche arrangement because the O 3 3 3 O attraction in the crown ring is more predominant than that of the repulsion, resulting in the formation of the endocyclic complexation (see drawing in the Conspectus). Unlike oxygen-bearing crown ethers, sulfur-bearing analogues have a tendency to show a repulsive interaction between adjacent sulfur donors stabilizes a trans (or anti) torsion arrangement.3 The presence of such a trans conformation in the thiamacrocycles tends to lead to coordination of a metal ion in an exocyclic mode relative to the macrocyclic ring.3,4 We have been involved in the synthesis of sulfur-containing mixed donor macrocycles and their supramolecular complexes for use in crystal engineering investigations.522 For ligand-directed metalorganic assembly, a rigid or semirigid bischelating dithiamacrocycle is beneficial in allowing the formation of robust and, therefore, often predictable products. In this regard, we have proposed four possible coordination modes for the category of semirigid dithiamacrocycles: endo-, exo- (bridging and nonbridging), and endo/exocyclic modes (Scheme 1).5

coordination frequently results in the formation of discrete suprastructures bearing less-common stoichiometries.6 Our initial motivations to utilize exo-coordination were twofold. First, the structures of the isolated products are readily influenced by several factors that we can easily control. Thus, a small change at the molecular level (building block reactant) in the assembly system can induce a large difference at the supramolecular level (assembled product). The second motivation was that the field of networking of macrocycles, in which exo-coordination between sulfur donors and soft metal centers directs the assembled products to form from zero- to three-dimensional (3D) topologies, had remained largely unexplored, although several exocyclic products of macrocycles, including thiacrowns and cryptands, had been reported by several groups.3,4 Thus, we have been interested in extending our research to involve a ligand-directed approach in terms of crystal engineering through employing the new macrocyclic ligand systems L1-L18 (except L5 in Scheme 2). Although not discussed in detail here, it is noted that a common synthesis of these types of thiamacrocycles has involved coupling cyclization of an appropriate benzo-dihalide with a corresponding dithiol in the presence of cesium carbonate under high-dilution conditions to boost the yield over linear polymerization.3 For these thiamacrocyclic systems, the incorporation of mono- to tribenzo groups was seen to be advantageous since their presence yielded ligands of inter-

SCHEME 1. Possible Coordination Motifs of Dithiamacrocycles

mediate flexibility and hence restricted the number of configurations that the coordinated ligand could adopt. The present Account mainly concentrates on our recent progress on the assembly of network complexes based on the exo-coordination of the above thiamacrocyclic systems. Parallel to the structural coordination chemistry for 30 representative exo-coordination-based species,13 some photoluminescence and chromogenic properties are also introduced briefly in terms of structurefunction relationships.23

Given the availability of different coordination modes provided by the sulfur-containing ligands employed, we have attempted the syntheses of a diverse array of discrete and continuous structures.622 For example, simple tuning of sulfur-to-sulfur separation in dithiamacrocycles may induce the bridging or nonbridging mode for exocyclic coordination (Scheme 1b and c).5a As proposed, such bridging exo-coordination tends to lead to the formation of 392

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2. Discrete Networking of Macrocycles 2.1. Discrete Linear Networking: Sandwich Complexes. In his early paper, Charles J. Pedersen,2 the Nobel Prize laureate in 1987, postulated that 2:1 and 3:2 crown ether (L) alkali metal cation (Mþ) complexes might exist as sandwich (L-Mþ-L) or club-sandwich (L-Mþ-L-Mþ-L) arrangements, respectively (Scheme 3).24 Unlike oxygen-bearing crown ethers, thiacrowns also bind soft metal ions by


SCHEME 2. Sulfur-Containing Macrocyclic Building Blocks Discussed in This Account

forming sandwich complexes because the S 3 3 3 S repulsion in a ring stabilizes the trans torsion about each SCH2CH2S

SCHEME 3. Diagrams of (a) Sandwich and (b) Club-Sandwich Macrocyclic Complexes

unit. Thus, the exo conformation tends to lead to exodentate coordination, rather than distortion of the ring to yield endodentate coordination (which bears an energetic cost). This allows formation of a sandwich complex through multiple MS bond formation.3 This knowledge has enabled us to prepare several sandwich and club-sandwich complexes using thiamacrocycles (Figures 1 and 2). 2.1.1. Sandwich Complexes. Ag(I) forms a sandwich complex 1, [Ag(o-L1)2]PF6, in the reaction with an orthomonobenzo-OS2-macrocycle o-L1 (Figure 1a).7 In 1, the silver(I) center sits in a distorted tetrahedral coordination sphere, with each ligand coordinated in bidentate fashion via two exodentate sulfur donors. A similar sandwich complex 2, [Hg(m-L2)2](ClO4)2, was also isolated from the reaction of the meta-monobenzo-NS2 macrocycle m-L2 with Hg(ClO4)2 (Figure 1b).8 The interaction of the 17membered O2S3-macrocycle L3 with AgClO4 in acetonitrile also afforded the sandwich complex 3, [Ag(L3)2]ClO4 (Figure 1c).9 In 3, the coordination of each L3 to the silver(I) center is via facial arrangement of its three sulfur donors, forming an approximate octahedral geometry. Comparison of the silver(I) sandwich complexes 1 and 3 suggests that the number of sulfur donors in the macrocycle is an important

factor to determine the coordination geometry. Once again, treatment of the similar 17-membered O3S2-macrocycle L8 with Cu(II) perchlorate in acetonitrile yielded the Cu(I) sandwich complex 4 (Figure 1d).10 Other examples of the spontaneous generation of Cu(I) complexes from Cu(II) in the presence of thioether ligands have also been reported.25 2.1.2. Club-Sandwich Complexes. Up to the present, only a limited number of club-sandwich complexes based on [Cs2(18-crown-6)3]2þ have been reported.24 Since dithiamacrocyclic ligand systems serve as a good motif for the preparation of the club-sandwich complexes, two examples of this type, 5 and 6, are now presented. First, the 14-membered dibenzo-O2S2 macrocycle L3 afforded a Vol. 45, No. 3

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FIGURE 1. Sandwich cationic complexes: (a) 1, [Ag(o-L1)2]PF6; (b) 2, [Hg(m-L1)2](ClO4)2; (c) 3, [Ag(L3)2]ClO4; and (d) 4, [Cu(L8)2]ClO4.

FIGURE 2. Club-sandwich complexes: (a) 5, [Ag2(L3)3] ](PF6)2 and (b) 6, [Hg2(L5)3] ](ClO4)4.

tris(macrocycle) dinuclear complex 5, [Ag2(L3)3](PF6)2, on its reaction with AgPF6 (Figure 2a).6c In more recent work involving reaction of 1,10-dithia-18-crown-6 (L5) with HgX2 (X = ClO4 and NO3), it yielded the club-sandwich complex 6, [Hg2(L5)3](ClO4)4 (Figure 2b).6d In parallel to this, two more examples of the triple-decker26 disilver € der27 and complexes have been reported by the Schro Holdt28 groups. 2.2. Discrete Cyclic Networking: Cyclic Oligomer Complexes. Cyclic oligomer complexes of type MnLn (normally n = 46) where L is a thiamacrocycle have also been prepared. For example, a cyclic tetramer complex 7, [Ag4(L11)4](PF6)4, is formed on reaction of trithiamacrocycle L11 with AgPF6 (Figure 3a).11 Each Ag atom is in a tetrahedral environment with the coordination sites occupied by three S atoms from one L11 bound in a facial manner and one S atom 394

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from an adjacent ligand. Other examples of cyclic tetramer silver(I) complexes with dithia- and trithiamacrocycles have also been reported both by us5a,11 and another group.29 Interestingly, an elegant flower-shaped cyclic hexamer complex 8, [Ag6(o-L2)6(PF6)](PF6)5, was isolated from the assembly of o-L2 with AgPF6 (Figure 3b).12 The Ag atom is bonded to an NS2 donor set of one o-L2 and to one S donor from one adjacent o-L2, forming a tetrahedral environment. The formation of the resulting flower-type hexamer seems to be influenced by the relatively flexible nature of the ligand together with the stabilization effect of the encapsulated PF6 ion (Figure 3b). Another cyclic tetramer complex 9, [Ag4(L3)4(BF4)2(CH3OH)2](BF4)2, was obtained fromthe reaction of the 16membered O2S2 macrocycle L3 and AgBF4 (Figure 4).13 Two Ag (Ag1 and Ag2) atoms are differentiated by their exo- and


FIGURE 3. (a) Cyclic tetramer complex 7, [Ag4(L11)4](PF6)4 and (b) flowershaped cyclic hexamer complex 8, [Ag6(o-L2)6(PF6)](PF6)5. The closest distance between Ag and F atoms is 2.987(1) Å (dashed lines).

endocyclic coordination. The Ag1 atom is facially bonded, while the Ag2 atom lies outside the cavity bridging the macrocycles via AgS bonds.

3. Continuous Networking of Macrocycles 3.1. 1D Homonuclear Network System. An infinite 1D zigzag complex 10, {[Ag(p-L1)](PF6)}n, was isolated when the rigid OS2-macrocycle p-L1 was treated with AgPF6 (Figure 5a).7 In this case, each exodentate macrocycle bridges the Ag atoms via linear coordination. This is a good example that illustrates the effect of ring rigidity in a small

macrocycle on the conformation of the resulting exocoordination product. Interestingly, one-pot reactions of mL2 with HgX2 (X = Br and I) produced isostructural 1D polymers 11, [Hg2(m-L2)Br4]n (11a, X = Br and 11b, X = I), each of which resembles ivy leaves (Figure 5b).12 For example, the unique 1D array 11a displays a HgBrHgBr chain as a backbone and a macrocyclic Hg(II) complex unit as a branch. On the other hand, the double-stranded 1D polymer 12, [Ag2(L4)2(ClO4)2]n, based on the 16-membered O2S2-macrocycle L4, was isolated from the reaction of this macrocycle with AgClO4 (Figure 5c).14 The two Ag centers (Ag1 and Ag2) that lie outside the cavity both have a tetrahedral geometry but show different coordination environments. Notably, the reaction of AgBF4 with the NO2S2-macrocycle L14 afforded a unique 1D network 13, [Ag2(L14)(CH3OH)](BF4)2}n,15 in which successive endocyclic silver(I) complexes are linked with exocyclic silver(I) units via AgS bonds. This is a rare example of a 1D network incorporating both endoand exocyclic metal ion coordination.16 3.2. 2D Homonuclear Network System: Transformation from 1D to 2D. In supramolecular assembly, the rearrangement of a kinetic product sometimes occurs to yield the corresponding thermodynamic product.30 For example, the assembly of the 16-membered O2S2-macrocycle L4 with AgClO4 in acetonitrile led to formation of the 1D polymer 14, {[Ag2(L4)2(CH3CN)](ClO4)2}n, which then transforms to the square-grid type 2D polymer 15, [Ag(L4)(ClO4)]n (Figure 6).14 The thermodynamic preference for the 2D polymeric scaffold is probably due to the flexible nature of L4 together with the stable packing mode achieved. 3.3. Heteronuclear Network System. The application of exo-coordination has enabled us to establish a hard/soft donor ligand system that was expected to influence potentially the coordinated metal ion position as well as the type of networking that forms.17 For instance, a one-pot reaction

FIGURE 4. Cyclic tetramer complex 9, [Ag4(L3)4(BF4)2(CH3OH)2](BF4)2, showing the rectangular arrangement involving the alternate silver(I) endo- and exo-coordination.

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FIGURE 5. 1D network complexes 1013: (a) zigzag-shaped 10, {[Ag(p-L1)]PF6}n; (b) ivy leaf-shaped 11a, [Hg2(m-L2)2Br4]n; (c) double-strand 12, [Ag2(L4)2(ClO4)2]n; and (d) endo/exocyclic 13, {[Ag2(L14)(CH3OH)](BF4)2}n.

FIGURE 6. 2D square-grid network 15, [AgL4(ClO4)]n, transformed from 1D network 14, {[Ag2(L4)2(CH3CN)](ClO4)2}n.

of the O4S2-macrocycle L10 with CuI in the presence of KI afforded the endo-coordinated potassium(I) coordination polymer 16, {[K(L10)(CH3CN)2][Cu4.25I5.25](CH2Cl2)0.75}n, linked with a ribbonlike copper iodide cluster (Figure 7).17 Consequently, the used of potassium(I) iodide induced not only the endocyclic complexation of potassium(I) but also the rearrangement of the exo-coordinated copper(I) iodide cluster via the additional coordination of I. Another strategy for the preparation of heterobimetallic complexes involves the use of metal complexes as ligands. For example, in one study of this type, the exo-coordinated complex 17, [cis-Cl2Pd(L6)], in which the Pd atom lies outside the cavity, was prepared from the reaction of the O2S3 macrocycle L6 with K2PdCl4 (Figure 8).18 Interestingly, subsequent reactions using 17 and silver salts gave two types of complexes 396

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(18, ClO4 and 19, NO3), whose topologies and stoichiometries vary with the anion. In marked contrast to 17, the Pd atom in 18, [Pd(L6)(CH3OH)](ClO4)2, occupies the cavity, forming an endodentate environment. On the other hand, 19, [Pd(L6)Ag(NO3)2.5](NO3)0.5, crystallizes as a 2D network involving a heterobinuclear Pd(II)/Ag(I) complex unit. The structure shows an unusual coordination arrangement, with the Ag atom bound exo to the ring while the Pd atom is bound via three S donors in an endofashion.

4. Crystal Engineering of Networking: Controlling Factors 4.1. Influence of Ligands. One strategy for macrocyclic ligand-based crystal engineering is to tune the structural and electronic features of a given macrocycle toward the metal


ion of interest. In this approach, the type and number of donor atoms are considered as variables. Ligand isomers formed by changing the substitution position in the aromatic subunit are also covered in this approach. 4.1.1. Donor Effect. Coupled with exo-coordination, the introduction of heterodonors into homoleptic macrocycles often results in dramatic changes in both binding ability and geometry of the complexes.31 In accord with this, a single donor (X) variation in the dibenzo-O2S2X macrocycles (L7-L9) has been shown to trigger sudden changes in the corresponding coordination geometries (Figure 9).9 The sandwich complex 3 formed from L7 (X = S) and AgClO4 has already been discussed (Figures 1c and 9a). In contrast to the formation of this complex, similar reaction of L8 (X = O) and AgClO4 afforded two new complex types 20 and 21 (Figure 9b,c), in which the binding behavior varies with the solvents used. In acetonitrile, 20 crystallizes a cofacial dimer arrangement, with two [Ag(L8)(CH3CN)] units held together (Ag 3 3 3 Ag

FIGURE 7. Endocyclic potassium(I) and exocyclic copper(I) iodide heteronuclear complex 16, {[K(L10)(CH3CN)2][Cu4.25I5.25](CH2Cl2)0.75}n, showing the 1D network.

distance 3.329(1) Å, dashed line in Figure 9b). In methanol, 21 crystallizes in a 1D network consisting of [Ag4(L8 )2(CH3OH)2 ](ClO4 )4 units, in which two different ligands encircle Ag1 and Ag2 atoms in an endo-manner (Figure 9c). While Ag3 and Ag4 atoms bridge the endodentate Ag1 and Ag2 coordination spheres, resulting in a [Ag4(L8)2(CH3OH)2]4þ repeating unit. A related investigation to the above was carried out for the corresponding macrocycle L9 (X = NH). Reaction with AgClO4 yielded the unique cyclic tetramer complex 22, [Ag4(L9)4(μ-ClO4)2](ClO4)2(CH3CN)2, in which each mononuclear unit is linked via both AgS bonds as well as bidentate perchlorate ions (Figure 9d). 4.1.2. Isomer Effect. The tribenzo-macrocycle isomers, ortho (o-L13), meta (m-L13), and para (p-L13), each contain two sulfur donors in the crown rings that may act as possible bridging sites for exo-coordination network formation.19 Their conformational ring flexibility can be assumed to be o-L > m-L > p-L.7 Hence, we reasoned that the use of these positional isomers might induce the formation of different products. Further, we have coupled this approach with the use of thiocyanate as a bridging anion (Figure 10). Ligands o-L13 and p-L13 afforded 1D linear (23, Figure 10a) and 1D zigzag (25, Figure 10c) coordination polymer networks, respectively, whereas m-L13 gave a brick wall type 2D network polymer (24, Figure 10b). Structural comparison of 2325 reveals that even small isomeric structural change may result in a dramatic impact on the topology of the resulting exo-coordinated framework. 4.1.3. Interdonor Distance Effect. We proposed that tuning of sulfur-to-sulfur separation in dithiamacrocycles represents a method for controlling the structures of resulting soft metal complexes.5 This approach means that if the separation between two S donors is small in the macrocyclic cavity, the

FIGURE 8. Exocyclic palladium(II) complex 17, [cis-Cl2Pd(L6)], and its reaction products with silver(I) salts; 18, [Pd(L6)(CH3OH)](ClO4)2, with an endocyclic palladium(II) and 19, {[Pd(L6)Ag(NO3)2.5](NO3)0.5 }n, showing endocyclic Pd(II) and exocyclic silver(I) with a 2D network structure.

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FIGURE 9. Effect of donor atom set on the structures of silver(I) perchlorate complexes: (a) 3, (b) 20, (c) 21, and (d) 22.

two S donors will cooperate to chelate to one metal ion (Type A in Scheme 4). On the other hand, if the separation is larger, each S donor tends to coordinate to different metal ion centers, leading to the formation of a networked species (Type B). SCHEME 4. Interdonor Distance Effect on Networking

the four CuS bonds between the cubane core and sulfur donors arising from four different ligands occupy well-separated positions, such that the adjacent ligand molecules are also arranged spaciously in Type B fashion. 4.2. Influence of Counter Anions. Clearly, controlling the exo- or endo-coordination behavior discussed so far is a challenging task. Since the nature of the anion is one of the important factors in directing the formation of coordination polymers, we suggested that endo- versus exo-coordination behavior might be controlled by the choice of the anion used.21 This strategy enabled us to prepare two distinct silver(I) complexes of the O3S2 macrocycle L12 such that

These predictions were confirmed experimentally. For example, formation of the discrete complexes 26, [Cu2I2(L15)2], and 27, [CuI(L16)], from CuI and the corresponding macrocycles (see Figure 11) can be explained by the shorter sulfur-to-sulfur distances leading to the Type A arrangement, whereas the same reaction with L17 incorporating a greater sulfur-to-sulfur distance, under identical conditions, leads to the formation of the double-stranded 1D polymer 28, [Cu4I4(L17)2]n. In this, the macrocycles are linked by a Cu4I4 cubane-tetramer. In 28, 398

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their coordination modes were anion-controllable as depicted in Scheme 5.

SCHEME 5. Anion Effect on Exo- and Endocyclic Coordination Modes


FIGURE 10. Isomer effect on the structures of mercury(II) thiocyanate complexes: (a) ID looped network, [Hg2(o-L13)(SCN)4]n (23); (b) 2D brick-wall type network, [Hg2(m-L13)(SCN)4]n (24); and (c) ID zigzag network, [Hg(p-L13)(SCN)2]n (25).

AgPF6 and AgClO4 were used for complexation to examine the anion effect. Employing these respective salts, two complexes (29, PF6 and 30, ClO4) were prepared and characterized (Figure 12).21 In the hexafluorophosphate complex 29, [Ag2(L12)2](PF6)2, the metal ion is accommodated inside the cavity (Figure 12a). The perchlorate complex 30, however, is an exo-coordinated species because of the anion coordination which induces the metal ion locate outside the cavity (Figure 12b).

5. Physical Properties and Applications In this section, we present the photophysical properties of the endo/exocyclic complexes based on thiamacrocycles in terms of structurefunction relationships for network complexes of thiamacrocycle with often exhibition photoluminescence in solid dependent on their structure d10 metal halide such as mercury(II) and copper(I). A dye-attached chromogenic macrocycle exhibiting cation-selective and anion-controlled color change is also introduced. 5.1. Solid-State Photoluminescence. One example of the broad category of photoluminescence complexes is presented

by mercury(II) halide complexes of the isomeric monobenzo NS2-macrocycle L2: (Figure 13a). The isostructural mercury(II) complexes of m-L2, 11a (Br-form) and 11b (I-form) shown in Figure 5b, exhibit emissions at 360 and 366 nm, respectively. These may be attributed to halide-to-ligand charge transfer (XLCT).32 In the copper(I) iodide complexes of some tribenzothiamacrocycles L15-L17, the discrete complexes 26 and 27 (Figure 11) were found to be nonemissive. In contrast, the network complex 28 linked by a cubane-type Cu4I4 cluster exhibits a bright-green emission (553 nm) which is likely due to the presence of a cluster-centered excited state with mixed halideto-metal charge transfer character (Figure 13b).12,15,16b,23,33 Although they are not discussed in detail here, it is noted that a range of the cubane-type Cu4I4 network complexes of exocoordinated thiamacrocycles including calix[4]-bis-thiacrown derivatives based on exo-coordination reported by us16a,b,20b also show similar photoluminescence properties. 5.2. Dye-Attached Macrocycle and Its Complexes: StructureColor Relationships. A number of chromoionophoric systems based on metal-ion induced charge-transfer phenomena have been reported, including systems employing Vol. 45, No. 3

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FIGURE 11. Interdonor distance effect on the copper(I) iodide complexes 2628: [Cu2I2(L15)2] 3 3CH2Cl2 (26), [CuI(L16)] 3 3CH2Cl2 (27), and {[Cu4I4(L17)2] 3 3CH2Cl2}n (28).

FIGURE 12. (a) Endocyclic structure of 29, [Ag2(L12)2](PF6)2, and (b) exocyclic structure of 30, [Ag2(L12)2(ClO4)2].

macrocyclic rings as the metal-binding site.22 Several research groups reported NS2-macrocycle-based donoracceptor chemosensors for silver(I) and/or mercury(II).34 Inspired by its exo/endo-coordination behavior, we synthesized the N-azo-coupled NO2S2-macrocycle L18, which shows Hg2þ selectivity.22a L18 exhibits an intense absorption at 480 nm (red). The largest cation-induced hypochromic shift was observed for Hg2þ (as nitrate) and resulted in a change from red to pale-yellow (Figure 14a). Interestingly, the color change for L18 with Hg2þ was dependent on the anion present; the addition of NO3 and ClO4 resulted in hypochromic shifts to 367 and 350 nm (pale-yellow), respectively (Figure 14b), 400

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but no color changes were observed upon the addition of other anions. We assumed that the observed anion-dependent behavior discussed above reflected structures of the respective mercury(II) complexes. Thus, the isolation and crystal structure of each colored species due to the different of anions (shown in Figure 14b) were undertaken. The pale-yellow (31, Figure 15a) and red (32, Figure 15b) complexes were successfully isolated as single crystals from solutions of L18 with Hg(ClO4)2 and HgI2, respectively. Pale-yellow 31 was revealed to be a 1:1 [Hg(L18)2(ClO4)]ClO4 species in which Hg2þ is coordinated in an endo fashion (Figure 15a). One ClO4


is weakly bound to Hg(II) at a fourth binding site. The presence of a direct HgN bond appears to be reflected by the color change to pale-yellow. That is, it suggests that the presence of the less coordinating ClO4 allows (“Push”) the Hg(II) to engage the N lone pair and thereby results in direct HgN bonding in endo mode, resulting in the observed color change from red to pale yellow, both in solution and the solid state.

An X-ray analysis of 32 confirms that this red product has a dimeric structure with a formula of [Hg2(L18)2I4)] (Figure 15b). Importantly, in this case, the Hg(II) is coordinated in an exo fashion and the N donor of L18 is not bound to Hg2þ. This result can be rationalized in terms of the strong coordination of the iodides acting to prevent HgN bond formation and thus resulting in no color change. In summary,

FIGURE 13. Solid-state photoluminescence spectra of (a) [Hg2(m-L2)2Br4]n (11a) and [Hg2(m-L2)2I4]n (11b), and (b) {[Cu4I4(L17)2] 3 3CH2Cl2}n (28).

FIGURE 14. (a) UV/vis spectra of L18 in the presence of metal nitrates and (b) anion-dependent shift in the spectrum of L18 for various mercury(II) salts in acetonitrile.

FIGURE 15. (a) Endo-coordinated structure of 31, [Hg(L18)(ClO4)](ClO4) (pale-yellow crystals) and (b) exo-coordinated dimeric structure of 32, [Hg2(L18)I4] (red crystals).

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the “Push-n-Pull process” serves to illustrate how the different coordinating ability of anions can be used to control the observed color change through formation of endo- or exocoordinated complexes.

6. Conclusion/Outlook This Account describes our current progress in the development of interesting discrete and continuous metallosupramolecules based on the exocyclic coordination of macrocyclic ligand systems. The exo-coordinated products presented are attractive not only because of their unusual topologies but also because of their potential application as new photophysical sensor systems. For example, the discussed anion-controlled heavy metal sensor system can be understood in terms of endo/exocoordination behavior. In the past, exocyclic coordination behavior has tended to receive less attention in mainstream macrocyclic chemistry because this unusual phenomenon has often been considered as not having a direct relation with selective recognition or self-assembly processes, both of which are key concepts in supramolecular science. In the authors' experience, one of the most challenging tasks in this field comes from how endo- and exocyclic coordination might be fully controlled and how such behavior might be applied. Nevertheless, controllable endo/exo-coordination as discussed in this Account should be a good starting point for developing nanoresponsive materials. As mentioned, exo-coordination related research remains a relatively unexplored area, with more extensive studies being necessary to more fully understand structurefunction relationships and also to develop associated novel nanomaterials that include sensors, switches, catalysts, and storage devices. We thank all of the co-workers who have contributed to this research as cited. This work was supported by World Class University (WCU) program (R32-20003). BIOGRAPHICAL INFORMATION Sunghong Park completed his chemistry degree at Gyeongsang National University (GNU) in 2005. He received his Ph.D. in 2011 under the supervision of professor Shim Sung Lee. He is currently working as a postdoctoral researcher in the World Class University (WCU) Center for NanoBio Chemical Materials at GNU. His scientific interests are in the field of the macrocycle-based metallosupramolecules and chemosensors.

So Young Lee studied chemistry at GNU and received her Ph.D. in 2010 under the supervision of professor Shim Sung Lee. She is currently working as a postdoctoral researcher in the WCU Center for NanoBio Chemical Materials at GNU. During her Ph.D. course, she was awarded the prestigious Young Scientists Fellowships from KBS and 402

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KOSEF. Her scientific interests are crystal engineering and application metalorganic frameworks with supramolecular structures.

Ki-Min Park received his B.Sc. in chemistry at Pusan National University and M.Sc. in applied chemistry at Pukyung National University in 1986 and 1989, respectively. He received a Ph.D. in 1992 at Tokyo University. He spent 19971998 at POSTEC with professor Kimoon Kim. During 19992009, he was a research professor at the Research Institute of Natural Sciences in GNU. He is currently working in the WCU Center for NanoBio Chemical Materials at GNU as a research professor. Shim Sung Lee studied chemistry at Korea University, where he also completed Ph.D. work. He joined GNU as a full-time lecturer in 1984 and has been a professor since 1994. He spent the year 19901991 at Cornell University with professor David B. Collum and the year 19971998 at Sydney University with professor Leonard F. Lindoy as a visiting scholar. During 20062009, he was a director of both Brain Korea (BK)21 Graduate School for Molecular Materials & Nanochemistry, and Research Institute of Natural Sciences at GNU. He is presently a director of the WCU Center for NanoBio Chemical Materials at GNU. He currently serves as an associate editor of the Journal of Korean Chemical Society. He received the Inorganic Chemistry Award (2006) and Sigma-Aldrich Chemist Award (2008) from Korean Chemical Society, Special Award (2008) from GNU, and Scientist Award of This Month (2010) from Korea Ministry of Education, Science & Technology. He is an author of about 220 peer reviewed publications. His research interests include metallosupramolecular chemistry of tailorable macrocyclic receptors including calixarene derivatives and their applications. FOOTNOTES *To whom correspondence should be addressed. E-mail: [email protected]. REFERENCES 1 (a) Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH Verlagsgesellschaft: Weinheim, 1993. (b) Lindoy, L. F. The Chemistry of Macrocyclic Complexes; Cambridge University Press: Cambridge, U.K., 1989. 2 Pedersen, C. J. Cyclic Polyethers and Their Complexes with Metal Salts. J. Am. Chem. Soc. 1967, 89, 7017–7036. 3 (a) Wolfe, S. Gauche Effect. Stereochemical Consequences of Adjacent Electron Pairs and Polar Bonds. Acc. Chem. Res. 1972, 5, 102–111. (b) Wolf, R. E., Jr.; Hartman, J. R.; Storey, J. M. E.; Foxman, B. M.; Cooper, S. R. Crown Thioether Chemistry: Structural and Conformational Studies of Tetrathia-12-crown-4, Pentathia-15-crown-5, and Hexathia-18crown-6. Implications for Ligand Design. J. Am. Chem. Soc. 1987, 109, 4328–4335. (c) Buter, J.; Kellog, R. M.; van Bolhuis, F. Synthesis, Complexation Behaviour and Reactions of Thia-crown Ethers Incorporating Propan-2-one Units. J. Chem. Soc., Chem. Commun. 1991, 910–912. (d) Geue, R.; Jacobson, S. H.; Pizer, R. Cryptand Conformational Analysis and Its Mechanistic Implications. Molecular Mechanics Calculations on Cryptands [111] and [222]. J. Am. Chem. Soc. 1986, 108, 1150–1155. 4 Heller, M. A Novel Huge Diamond-like Three-fold Interpenetrated Network of CuI and Crown Ether. Z. Anorg. Allg. Chem. 2006, 632, 441–444. 5 (a) Lee, S. Y.; Seo, J.; Yoon, I.; Kim, C.-S.; Choi, K. S.; Kim, J. S.; Lee, S. S. A Poly(bicyclic dimmer) and a Cyclic Tetramer: Ligand Isomerism of S2O2 Macrocycles During the Assembly of Supramolecular Silver(I) Complexes. Eur. J. Inorg. Chem. 2006, 3525–3531. (b) Lee, S. Y.; Park, S.; Kim, H. J.; Jung, J. H.; Lee, S. S. Ligand- and Anion-Directed Assembly of Exo-Coordinated Mercury(II) Halide Complexes with O2S2-Donor Macrocycles. Inorg. Chem. 2008, 47, 1913–1915. 6 (a) Habata, Y.; Seo, J.; Otawa, S.; Osaka, F.; Noto, K.; Lee, S. S. Synthesis of Diazahexathia24-crown-8 Derivatives and Structures of Agþ Complexes. Dalton Trans. 2006, 2202– 2206. (b) Kim, H. J.; Yoon, I.; Lee, S. Y.; Seo, J.; Lee, S. S. A Flexible Dibenzo-O4S2Macrocycle: Twist-and-Squeeze Type Metal Binding via Synergic Action of Metal-Ligand and Metal-π Interactions. Tetrahedron Lett. 2007, 48, 8464–8467. (c) Lee, S. Y.; Park, S.;


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Lee, S. S. Copper(I), Siver(I), and Palladium(II) Complexes of a Thiaoxamacrocycle Displaying Unusual Topologies. Inorg. Chem. 2009, 48, 11335–11341. (d) Park, I.-H.; Park, K.-M.; Lee, S. S. Preparation and Characterization of Divalent Hard and Soft Metal (M = Ca, Co, Cu, Zn, Cd, Hg and Pb) Complexes of 1,10-Dithia-18-crown-6: Structural Versatility. Dalton Trans. 2010, 39, 9696–9704. Seo, J.; Song, M. R.; Lee, J.-E.; Lee, S. Y.; Yoon, I.; Park, K.-M.; Kim, J.; Jung, J. H.; Park, S. B.; Lee, S. S. Exo-Coordination Based Supramolecular Silver(I) Complexes of S2O Macrocycles: Effect of Ligand Isomerism on the Structural Diversity. Inorg. Chem. 2006, 45, 952–954. Park, S.; Lee, S. Y.; Lee, S. S. Discrete Mercury(II) Complexes, 1D and Palladium(II)Mediated 2D Silver(I) Coordination Polymers of NS2-Macrocycle: Synthesis and Structural Characterization. Inorg. Chem. 2010, 49, 1238–1244. Yoon, I.; Seo, J.; Lee, J.-E.; Song, M. R.; Lee, S. Y.; Choi, K. S.; Jung, O.-S.; Park, K.-M.; Lee, S. S. Donor Effect on Supramolecular Structures of Silver(I) Perchlorate Complexes of Macrocycles with O2S2X (X = S, O and NH) Donor Sets. Dalton Trans. 2005, 2352–2354. Jo, M.; Seo, J.; Seo, M. L.; Choi, K. S.; Cha, S. K.; Lindoy, L. F.; Lee, S. S. Donor Set Induced Coordination Sphere and Oxidation State Switching in the Copper Complexes of O2S2X (X = S, O and NH) Macrocycles. Inorg. Chem. 2009, 48, 8186–8189. Kim, H. J.; Yoon, I.; Lee, S. Y.; Choi, K. S.; Lee, S. S. Exo-Coordinated Discrete and Continuous Supramolecular Silver(I) Complexes with Thiaoxa-Macrocycles. New J. Chem. 2008, 32, 258–263. Park, S.; Lee, S. Y.; Lee, J. Y.; Lee, S. S. Molecular Botanical Garden: Assembly of Supramolecular Silver(I) and Mercury(II) Complexes of NS2-Donor Macrocycles with Flower-, Leaf- and Tree-Shaped Structures. CrystEngComm 2009, 11, 43–46. Lee, S. Y.; Lee, S. S. Anion-Directed Supramolecular Silver(I) Complexes of an O2S2Macrocycle: Cyclic Oligomer and 2-D Coordination Polymers. CrystEngComm 2010, 12, 3471–3475. Lee, S. Y.; Jung, J. H.; Vittal, J. J.; Lee, S. S. Solvent-Dependent Networking of O2S2Macrocycle with Silver Perchlorate into 1-D and 2-D Coordination Polymers. Cryst. Growth Des. 2010, 10, 1033–1036. Kang, E.-J.; Lee, S. Y.; Lee, H.; Lee, S. S. Sulfur-Containing Mixed-Donor TribenzoMacrocycles and Their Endo- and Exocyclic Supramolecular Silver(I) and Copper(I) Complexes. Inorg. Chem. 2010, 49, 7510–7520. (a) Lee, J. Y.; Lee, S. Y.; Park, S.; Kwon, J.; Sim, W.; Lee, S. S. Unsymmetrical Calixcrowns Incorporating Hard and Soft Loops as a New Scaffold for Multinuclear Endo/Exocyclic Complexation and Networking. Inorg. Chem. 2009, 48, 8934–8939. (b) Lee, J. Y.; Kim, H. J.; Jung, J. H.; Sim, W.; Lee, S. S. Networking of Calixcrowns: From Heteronuclear Endo/ Exocyclic Coordination Polymers to A Photoluminescence Switch. J. Am. Chem. Soc. 2008, 130, 13838–13839. (c) Kim, H. J.; Sultana, K. F.; Lee, J. Y.; Lee, S. S. Endo- and/or Exocyclic Silver(I) and Mercury(II) Complexes of an NO2S2-Macrocycle: Effect of Ligand Ratio and Anion. CrystEngComm 2010, 12, 1494–1500. Jin, Y.; Kim, H. J.; Lee, J. Y.; Lee, S. Y.; Shim, W. J.; Hong, S. H.; Lee, S. S. Hard/Soft Heterometallic Network Complex of Macrocycle with Endo/Exocyclic Coordination. Inorg. Chem. 2010, 49, 10241–10243. Yoon, I.; Seo, J.; Lee, J.-E.; Park, K.-M.; Kim, J. S.; Lah, M. S.; Lee, S. S. Assembly of Heterobinuclear 2-D Network: A Rare Example of Endo- and Exocyclic Coordination of Pd(II)/ Ag(I) in a Single Macrocycle. Inorg. Chem. 2006, 45, 3487–3489. Kim, H. J.; Lindoy, L. F.; Lee, S. S. Networking of Tribenzo-O2S2-Macrocycles with Mercury Thiocyanate: Effect of Macrocyclic Isomers. Cryst. Growth Des. 2010, 10, 3850–3853. (a) Lee, J. Y.; Kim, H. J.; Park, C. S.; Sim, W.; Lee, S. S. A Calix-bis-crown with Hard and Soft Crown Cavities: Heterobinuclear Kþ/Agþ Complexation in Solid and Solution States. Chem.;Eur. J. 2009, 48, 8989–8992. (b) Lee, J. Y.; Lee, S. Y.; Sim, W.; Park, K.-M.; Kim, J.; Lee, S. S. Temperature-Dependent 3-D CuI Coordination Polymers of Calix[4]-bisdithiacrown: Crystal-to-Crystal Transformation and Photoluminescence Change on Coordinated Solvent Removal. J. Am. Chem. Soc. 2008, 130, 6902–6903. Kim, H. J.; Lee, S. S. Anion-Controlled Endo- and Exocyclic Disilver(I) Complexes of An S2O3 Macrocycle. Inorg. Chem. 2008, 47, 10807–10809. (a) Lee, S. J.; Jung, J. H.; Seo, J.; Yoon, I.; Park, K.-M.; Lindoy, L. F.; Lee, S. S. A Chromogenic Macrocycle Exhibiting Cation-Selective and Anion-Controlled Color Change: An Approach to Understanding Structure-Color Relationships. Org. Lett. 2006, 8, 1641– 1643. (b) Lee, H.; Lee, S. S. Thiaoxaaza-Macrocyclic Chromoionophores as Mercury(II) Sensors: Synthesis and Color Modulation. Org. Lett. 2009, 11, 1393–1396. (c) Kim, H. J.; Han, J. H.; Kim, M. K.; Lim, C. S.; Kim, H. M.; Cho, B. R. Dual-Color Imaging of Sodium/ Calcium Ion Activities with Two-Photon Fluorescent Probes. Angew. Chem., Int. Ed. 2010, 49, 6786–6789.

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