Personal Account DOI: 10.1002/tcr.201402076
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Shape-Persistent Arylene Ethynylene Organic Hosts for Fullerenes Chao Yu,† Yinghua Jin,† and Wei Zhang*[a] Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 (USA), E-mail: [email protected]
[a]
†
These two authors contributed equally.
Received: August 6, 2014 Published online: ■■
ABSTRACT: Shape-persistent host molecules that are constructed solely through covalent bonds have been extensively investigated as alternatives to supramolecular architectures, which have shown interesting applications such as guest encapsulation and release, catalysis, and molecular recognition. Similar to the supramolecular self-assembly process, recent rapid development of dynamic covalent chemistry has enabled the covalent assembly of complex organic molecules that exhibit a finite cavity of well-defined shape and size. Alkyne metathesis represents an emerging dynamic covalent reaction, which provides rigid and linear acetylene linkages. In this account, we describe the dynamic assembly of shape-persistent arylene ethynylene cages with various shapes and sizes through one-step alkyne metathesis. The controlled cage–fullerene binding and the potential application of these cages in fullerene purification are also discussed. Keywords: cage compounds, dynamic covalent chemistry, fullerenes, metathesis, receptors
1. Introduction With customizable geometry and well-defined internal cavities, shape-persistent cage compounds[1] have been widely used in host–guest chemistry and proven to be potential candidates as catalysts,[2] sensors,[3] reaction vessels,[4] and gas adsorbents.[5] For many exciting applications, the rigid and non-collapsible character of cage compounds is a critical factor, which is similar to the stiffest natural polymer, DNA, whose stiffness determines many of its features including protein–DNA binding specificity.[6] Shape persistency of cage molecules is largely determined by the backbone composition and, generally, spand sp2-hybridized carbon atoms are preferred to any saturated atoms. An important group of building blocks with inherent rigidity are arylene ethynylenes, and various shape-persistent macrocycles[7] and molecular wires[8,9] consisting of arylene ethynylenes have been reported. Our group has been interested in building shape-persistent organic cages containing solely
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rigid arylene ethynylene moieties, which can be used as host molecules for fullerenes. Two types of reactions can form the ethynylene bonds, namely, irreversible cross-coupling reactions and dynamic alkyne metathesis. Shape-persistent organic cages linked by ethynylene bonds are usually prepared in low yield via kinetically controlled reactions such as Sonogashira coupling and Glaser coupling.[10–12] Only very recently has an efficient alkyne metathesis approach[13–17] been developed to achieve the covalent assembly of cages and macrocycles from simple precursors. As alkyne metathesis is reversible, it enables self-correction during the covalent assembly process of molecular cages and thus high-yielding synthesis of the most thermodynamically stable compounds. One of the major advantages of the dynamic alkyne metathesis approach is its modularity in constructing rigid cage molecules with subtle structural changes, which allows for efficient and effective screening of candidates for fullerene binding. In this account, we will discuss the preparation of shape-persistent arylene ethynylene hosts, including monomer design, synthetic issues, and their host–guest interactions with fullerenes.
2. Synthesis of Shape-Persistent Arylene Ethynylene Cages Compared to commonly used imines and disulfides in dynamic covalent chemistry (DCvC),[18–20] alkynes are highly stable against moisture and heat, rigid and linear, and thus provide shape persistency. Since ethynylene bonds are linear, closely resembling a metal–ligand dative bond, the structural rules
Chao Yu received his Bachelor’s degree in Chemistry from Peking University in 2011. After graduation, Chao started his Ph.D. study under the supervision of Prof. Wei Zhang at the Department of Chemistry and Biochemistry, University of Colorado, Boulder. His current research interests focus on the design and synthesis of organic polyhedrons and their applications in membrane fabrication and gas separation. Yinghua (Alice) Jin received her B.S. in Chemistry from Peking University in 2000. She obtained her Ph.D. in Chemistry under the supervision of Prof. Robert M. Coates from the University of Illinois at Urbana–Champaign in 2006. Currently, she is a senior research associate at the University of Colorado, Boulder. Her research interests include the development of novel organic functional materials and their applications in energy and biomedical fields.
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governing the assembly of supramolecular cages can be applied to the covalent assembly process through alkyne metathesis. According to the energy landscape principle involved in DCvC, suitable design of building blocks can selectively lead to complex molecular architectures that are thermodynamically most stable. Unlike the popular imine bond formation, which is directional, alkyne metathesis provides symmetrical bonds that can selfexchange. One of the major benefits of using alkyne metathesis is that it requires a single type of symmetrical building block containing identical alkyne bonds, which makes the synthesis simple. The potential drawback is the difficulty in building unsymmetrical structures through alkyne metathesis. In order to solubilize the conformationally rigid frames in common organic solvents and prevent premature precipitation of intermediates, flexible alkyl chains are usually attached to monomer units. We designed building blocks with various symmetries and functional group arrangements targeting structures of different size and shape (Figure 1): C2 symmetrical bifunctional building blocks (1 and 2), C3 symmetrical trifunctional building blocks (3 and 4) with face-to-edge angles of 90° or 120°, and a C4 symmetrical tetrafunctional building block (5) with a face-toedge angle of 90°. The efficient removal of reaction byproducts (e.g., 2-butyne) from the equilibrium system is important as these can continuously participate in alkyne metathesis (nonproductive pathway) and lead to low conversion. The alkyne byproducts are usually removed by precipitation, adsorption in molecular sieves, or application of vacuum. Methyl groups (e.g., 3) or large precipitating groups (e.g., 1) are installed to facilitate the removal of alkyne byproducts.
Wei Zhang obtained his B.S. from Peking University in 2000. He then pursued his graduate study at the University of Illinois at Urbana–Champaign under the supervision of Prof. Jeff Moore. After obtaining his Ph.D. degree in 2005, he moved to MIT to conduct postdoctoral research with Prof. Tim Swager. In 2008, he started his independent career in the Department of Chemistry and Biochemistry at the University of Colorado, Boulder. His main research interest is to develop organic functional materials based on novel 2D and 3D molecular architectures that are constructed through dynamic covalent chemistry.
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Fig. 1. Building blocks of ethynylene-linked molecular cages.
Fig. 2. The structures of alkyne metathesis catalysts.
The alkyne metathesis of C2 symmetrical building blocks (1 or 2) was straightforward. Highly efficient molybdenum carbyne complexes (6 or 7) with multidentate ligands developed in our group were used as the catalyst (Figure 2).[21,22] Depending on the edge-to-face angle, cyclic dimer 8[23] or trimer 9 were obtained in good yields after stirring the substrates at 45°C overnight (Scheme 1). The equilibrium was driven by the precipitation of diarylacetylene byproduct 10. The results of alkyne metathesis of C3 symmetrical building blocks 3 and 4 were quite unexpected and surprising.[24,25] We expected a tetrahedron-shaped cage (Figure 3a) from monomer 3, since its face-to-edge angle is 60°, which is close to
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the face-to-edge angle of a tetrahedron (54.7°). The dimer (Figure 3b) was expected from the monomer 4 with a face-toedge angle of 90°. However, in both cases, we observed a tetramer with D2h symmetry, which consists of two macrocycles connected by two arms (Figure 3c). Therefore, the three originally identical alkyne groups in the monomer split into two different types (shown as yellow and orange in Figure 3c) in the cage product. The protons of the two arms forming the macrocycle with another monomer and the protons of a third arm bridging two macrocycles were observed in a 2:1 ratio in the 1H NMR spectrum of the cage 11. The structure of 11 was unambiguously determined by single-crystal X-ray analysis (Figure 4). Intrigued by this observation and to better understand the reaction progress, we monitored the metathesis of the monomer 3. We observed a significant amount of macrocyclic intermediate 12 in the early stage of the reaction (within 0.5 h), indicating that 12 could be the key intermediate towards the formation of 11 (Scheme 2). Similarly, in spite of the original design to synthesize a dimer (Figure 3b), another C3 symmetrical monomer 4 with a different face-to-edge angle of 90° from the monomer 3 (60°) also forms a tetrameric cage 13 with D2h symmetry (Scheme 2). Such cage structures with D2h symmetry are rarely observed, especially when C3 symmetric monomers, which generally
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Scheme 1. Synthesis of cyclic dimer 8 and trimer 9.
Fig. 3. Schematic representation of (a,b) expected structures and (c) observed structure from C3 symmetrical building blocks 3 and 4.
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serve as vertices or faces in the assembly of tetrahedron-shaped cages, are used as building blocks. In contrast to the failed formation of a dimer from monomer 4, a dimer 14 was successfully formed in good isolated yield (72%) from monomer 5, which also contains carbazole corner pieces arranged perpendicularly to the porphyrin ring with a face-to-edge angle of 90°, similar to monomer 4 (Scheme 3).[26] Presumably, the porphyrin ring in 5 is relatively flexible compared to the central benzene ring in
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Fig. 4. Crystal structure of cage 11.
4, thus effectively releasing the strain built up during the formation of the dimeric structure. These examples highlight the critical importance of angle strain and its significant effect on rigid alkyne bond formation during the dynamic assembly process.
3. Shape-Persistent Molecular Cages as Fullerene Receptors Since their discovery,[27] fullerenes have received tremendous attention due to their unique properties, such as high electron affinity, superior electron transport and radicalscavenging capability.[28–30] However, their poor solubility in common solvents largely impedes the purification and modification process and thus the widespread application of fullerenes in the fields of photovoltaics,[31] medicine and cosmetics.[32] The development of synthetic host molecules that can non-covalently interact with fullerenes has been actively pursued in the hope of enhancing the solubility of fullerenes and facilitating their purification and application. In 1994, Atwood reported the efficient purification of C60 from fullerene soot through selective complexation of fullerenes with calixarenes.[33,34] In the past two decades, various organic host molecules[35–39] have been developed, including calixarene derivatives,[33,40,41] cyclotriveratrylenes (CTVs),[42,43] cycloparaphenyleneacetylenes (CPPAs),[44,45] and porphyrinbased receptors.[39,46,47] It has been reported that the flexibility of host molecules is important in order to maximize host–guest interactions through conformational change and achieve higher binding affinity. For example, the zinc porphyrin cyclic dimer (15) linked by linear C6H12 chains shows good binding interaction with C60 in benzene at 25°C (KC60 = 6.7 × 105 M−1), while the similar dimer (16) linked by rigid diacetylenic linkers does not show any sign of complexation with fullerenes (Figure 5).[48] On the other hand, there also exist many examples of hosts and receptors that have benefited from their shape-persistent struc-
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tural features, which include the influence of DNA stiffness on the specificity of protein–DNA complexes.[49,50] The rigidity of shape-persistent host molecules can provide preorganization and size-based recognition, and thus achieve high binding affinities and reinforce the size selectivity between guests. Anderson and co-workers have reported a rigid zinc porphyrin trimer (17, Figure 5),[51] which shows strong binding interactions with C60 and C70 in toluene with association constants of 2 × 106 and 2 × 108 M−1, respectively. It should be noted that such a rigid trimer structure provides exceptionally high binding selectivity toward C70 over C60 (i.e., KC70/KC60 ≈ 100), which surpasses the C70/C60 selectivity of most flexible host molecules (typically KC70/KC60 ≈ 10). By contrast, a flexible zinc porphyrin trimer (18, Figure 5) shows a considerably lower binding affinity for C60 (K = 2.7 × 103 M−1),[52] which could be attributed to the lower level of preorganization of porphyrin moieties. Various shape-persistent cages prepared through dynamic alkyne metathesis were evaluated for fullerene encapsulation, and we revealed that some of these cages have high binding affinities for fullerenes and preferentially bind a certain fullerene over others. The porphyrin dimer 8 shows a high binding selectivity for C84 over the lower fullerenes C70 and C60.[23] As shown in Figure 6, computer calculations on the energy-minimized models show that the free 8 adopts a somewhat collapsed conformation (Figure 6a) with a distance of 4.8 Å between the top and bottom porphyrin panels. However, upon the addition of fullerene guests, the dimer 8 undergoes conformational change with increased internal cavity size from 4.8 Å to 13.7 Å to accommodate the guest molecules (Figure 6b). UV–vis titration experiments revealed that 8 forms 1:1 complexes with C60, C70, and C84 with association constants of 1.3 × 104 M−1, 2.0 × 106 M−1, and 2.2 × 107 M−1, respectively, in toluene at room temperature. The dimer 8 exhibits remarkably high binding selectivities for larger fullerenes over C60 (e.g., KC70/KC60 > 100 and KC84/ KC60 > 1500). By contrast, the more flexible dimer 15 shows moderate selectivity (KC70/KC60 > 30) for C70 over C60.
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Scheme 2. Synthesis of tetrameric cages (11 and 13) with D2h symmetry from C3 symmetrical building blocks.
Importantly, such dimer 8 and fullerene complexes undergo reversible association and dissociation under acid/base stimuli for multiple cycles without noticeable decomposition and responsiveness decay. Another porphyrin dimer 14, with a rectangular prism shape, also shows strong binding interactions with fullerenes. According to the UV–vis titration experiment and the Job plot,
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cage 14 forms 1:1 complexes with C60, C70, and C84 with association constants of 1.4 × 105 M−1, 1.5 × 108 M−1, and 2.4 × 107 M−1 in toluene at room temperature,[26] which are among the highest fullerene binding constants reported so far.[36] It is interesting to note that the cage 14 binds most strongly with C70, showing an unprecedentedly high binding selectivity for C70 over C60 (>1000/1). This is in great contrast
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Scheme 3. Synthesis of rectangular prism 14.
to 8, which shows the highest binding affinity toward C84. Presumably, due to the high shape persistency of 14, C70 fits perfectly inside the cage cavity, while C60 fits more loosely and C84 fits a little bit too tight. A computational modeling study revealed that the four-arm dimer 14 is much more rigid compared to the two-arm dimer analogue 8. Owing to the high degree of shape persistency of the cage, there is only a small conformational change upon fullerene binding: the height of 14 is slightly increased to 12.1–12.2 Å in C60@14 and C70@14 from the initial 11.9 Å in the unoccupied 14; the diameters of the cavities are decreased slightly to 17.6–17.8 Å from 18.3 Å (Figure 7). Since the porphyrins in 14 are non-metalated, the binding between the cage and fullerene is fully reversible under acid/base stimuli. Through the “selective complexation– decomplexation” strategy, efficient separation of C70 from a C60-enriched fullerene mixture (C60/C70, 10/1, mol/mol) was achieved as shown in Figure 8. After only one cycle of separation, C70 abundance in the fullerene mixture was increased
Fig. 5. The structures of previously reported fullerene receptors.
Fig. 6. Energy-minimized models of (a) compound 8 (side view) and (b) C84@8. For simplicity, methyl groups were used in the calculation instead of hexadecyl chains. The height of 8 was defined as the distance between the top and bottom porphyrin panels.
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Fig. 7. Energy-minimized models of (a) cage 14 (side view) and (b) C60@14. For simplicity, methyl groups were used in the calculation instead of hexadecyl chains.
Fig. 8. Schematic representation of the C70 isolation process using cage 14. Step (I): To a solution of the C60 and C70 mixture in CS2 (C60/C70 = 10/1, molar ratio) was added a small amount of 14 (equal to or less than the stoichiometric amount of C70), resulting in the favored formation of C70@14. After separating the unbound free fullerenes by precipitation in CHCl3 (precipitate shown in photograph (b)), cage–fullerene complexes (mostly C70@14) were collected in the solution phase. Step (II): Upon the addition of 100 equivalents of trifluoroacetic acid (TFA) to the solution collected in step I, fullerene guest molecules (mostly C70) were released as black precipitates and removed to complete one cycle of the isolation process (shown in photograph (c)). Step (III): Regeneration of 14 was accomplished by the addition of 100 equivalents of triethylamine (TEA) to the above solution.
from the initial 9 mol % to 79 mol % (ca. ninefold increase). Such size-selective complexation of a particular fullerene from fullerene mixtures could be an interesting alternative for separation and purification of fullerenes.[33,34,53] Interestingly, we found that the C60@14 complex can bind to the surface of single-walled carbon nanotubes (SWCNTs) in a “side-to-face” fashion through π–π stacking interactions. Such a ternary nanohybrid system consisting of SWCNTs and the C60@14 complex serves as a light-harvesting photoactive layer in solar cell devices.[54] With 1 wt % SWCNT loading, a short-circuit current of 1.4 mA/cm2, an open-circuit voltage of 0.47 V, a fill factor of 24%, and an efficiency of 0.16% were
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measured under AM1.5 conditions. Although the hybrid material shows modest light-harvesting performance, it opens up new possibilities for fabricating efficient photoactive molecular devices and shows the great potential of such nanohybrid systems in photovoltaic applications. Although it has been proven that increasing the number of porphyrin units within one host could increase its affinity toward fullerenes,[51] porphyrin trimer 9 did not show any sign of complexation with C60, C70, or C84 under UV–vis titration conditions, likely due to the high rigidity of the host and its size mismatch with fullerene guests. A similar detrimental effect of the extreme rigidity of a host on fullerene binding has been reported.[55] One such example is the “nanobarrel” containing four porphyrin units, which shows an association constant (KC60 = 5.3 × 105 M−1 in toluene at room temperature) not as large as expected. As the research on multiporphyrinic cages expands,[56] we are expecting more and more shape-persistent porphyrin-based cages to be developed as fullerene hosts. In addition to widely recognized porphyrin-based fullerene hosts, other aromatics have also been utilized,[35–39] such as tetrathiafulvalene[57–59] and solely carbon-based CPPAs.[44,45] Tetrameric cage 11, with triphenylbenzene units connected by ethynyl linkers, can also bind fullerenes (Figure 9), albeit much more weakly than the porphyrincontaining host molecules previously discussed.[24] A 1H NMR titration experiment revealed that cage 11 has favorable interactions with C70 over C60. It forms a 1:1 complex with C70 (KC70 = 3.9 × 103 M−1), whereas no noticeable binding with C60 was observed. This result was supported by computer modeling, which showed the binding energy of 11 with C70 to be 10 kcal/mol lower than that with C60 (−48.7 vs −38.4 kcal/ mol). It is postulated that such a difference is largely the result of the relatively larger size of C70 and its ellipsoidal shape, thus providing a better fit inside the cavity of cage 11. Interestingly, when the peripheral phenyl units of cage 11 were replaced by carbazole units, the analogous tetrameric cage 13 showed much weaker binding interactions with C70 and the binding constants could not be obtained due to the weak interactions.
4. Summary With the rapid development of alkyne metathesis in DCvC research, shape-persistent organic cages with different sizes, functional groups, and binding affinities toward fullerenes can be obtained in a modular fashion from simple precursors. Provided delicately designed shape and size, shape-persistent cages possess advantages over flexible ones, such as preorganization and increased size selectivity, thus leading to comparable or even higher fullerene-binding affinity and selectivity. In host–guest systems, shape complementarity, such as the lock-and-key principle, preorganization, and the induced-fit
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Fig. 9. (a) The chemical shift changes in 1H NMR spectra obtained in toluene-d8 during the titration of the cage 11 (0.14 mM) with an increasing amount of C70 (0–8.6 equiv). (b) The computational model (side view) of C70@11. Methyl groups are used for simplification.
concept have been deemed important for designing effective hosts.[35] It has been shown that the porphyrin unit, which is generally known as a planar moiety, can slightly distort from planarity and adopt a concave structure to maximize the short contacts between hosts and fullerene guests.[60] In addition to shape complementarity, intermolecular interactions, including van der Waals interaction, π–π interaction, and electrostatic force, also play important roles in forming a stable host–guest complex.[34,35] Shape-persistent organic cages consisting of electron-enriched aromatic groups with a tunable non-polar cavity of appropriate size have a great potential to serve as selective complexation agents to enhance the solubility of fullerenes and assist their purification and further applications.
Acknowledgements
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We thank the National Science Foundation (DMR-1055705) and the Alfred P. Sloan Foundation for their financial support of the research reported in this account.
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Shape-Persistent Arylene Ethynylene Organic Hosts for Fullerenes Chao Yu,† Yinghua Jin,† and Wei Zhang*[a] Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 (USA), E-mail: [email protected]
[a]
†
These two authors contributed equally.
Received: August 6, 2014 Published online: ■■
ABSTRACT: Shape-persistent host molecules that are constructed solely through covalent bonds have been extensively investigated as alternatives to supramolecular architectures, which have shown interesting applications such as guest encapsulation and release, catalysis, and molecular recognition. Similar to the supramolecular self-assembly process, recent rapid development of dynamic covalent chemistry has enabled the covalent assembly of complex organic molecules that exhibit a finite cavity of well-defined shape and size. Alkyne metathesis represents an emerging dynamic covalent reaction, which provides rigid and linear acetylene linkages. In this account, we describe the dynamic assembly of shape-persistent arylene ethynylene cages with various shapes and sizes through one-step alkyne metathesis. The controlled cage–fullerene binding and the potential application of these cages in fullerene purification are also discussed. Keywords: cage compounds, dynamic covalent chemistry, fullerenes, metathesis, receptors
1. Introduction With customizable geometry and well-defined internal cavities, shape-persistent cage compounds[1] have been widely used in host–guest chemistry and proven to be potential candidates as catalysts,[2] sensors,[3] reaction vessels,[4] and gas adsorbents.[5] For many exciting applications, the rigid and non-collapsible character of cage compounds is a critical factor, which is similar to the stiffest natural polymer, DNA, whose stiffness determines many of its features including protein–DNA binding specificity.[6] Shape persistency of cage molecules is largely determined by the backbone composition and, generally, spand sp2-hybridized carbon atoms are preferred to any saturated atoms. An important group of building blocks with inherent rigidity are arylene ethynylenes, and various shape-persistent macrocycles[7] and molecular wires[8,9] consisting of arylene ethynylenes have been reported. Our group has been interested in building shape-persistent organic cages containing solely
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rigid arylene ethynylene moieties, which can be used as host molecules for fullerenes. Two types of reactions can form the ethynylene bonds, namely, irreversible cross-coupling reactions and dynamic alkyne metathesis. Shape-persistent organic cages linked by ethynylene bonds are usually prepared in low yield via kinetically controlled reactions such as Sonogashira coupling and Glaser coupling.[10–12] Only very recently has an efficient alkyne metathesis approach[13–17] been developed to achieve the covalent assembly of cages and macrocycles from simple precursors. As alkyne metathesis is reversible, it enables self-correction during the covalent assembly process of molecular cages and thus high-yielding synthesis of the most thermodynamically stable compounds. One of the major advantages of the dynamic alkyne metathesis approach is its modularity in constructing rigid cage molecules with subtle structural changes, which allows for efficient and effective screening of candidates for fullerene binding. In this account, we will discuss the preparation of shape-persistent arylene ethynylene hosts, including monomer design, synthetic issues, and their host–guest interactions with fullerenes.
2. Synthesis of Shape-Persistent Arylene Ethynylene Cages Compared to commonly used imines and disulfides in dynamic covalent chemistry (DCvC),[18–20] alkynes are highly stable against moisture and heat, rigid and linear, and thus provide shape persistency. Since ethynylene bonds are linear, closely resembling a metal–ligand dative bond, the structural rules
Chao Yu received his Bachelor’s degree in Chemistry from Peking University in 2011. After graduation, Chao started his Ph.D. study under the supervision of Prof. Wei Zhang at the Department of Chemistry and Biochemistry, University of Colorado, Boulder. His current research interests focus on the design and synthesis of organic polyhedrons and their applications in membrane fabrication and gas separation. Yinghua (Alice) Jin received her B.S. in Chemistry from Peking University in 2000. She obtained her Ph.D. in Chemistry under the supervision of Prof. Robert M. Coates from the University of Illinois at Urbana–Champaign in 2006. Currently, she is a senior research associate at the University of Colorado, Boulder. Her research interests include the development of novel organic functional materials and their applications in energy and biomedical fields.
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governing the assembly of supramolecular cages can be applied to the covalent assembly process through alkyne metathesis. According to the energy landscape principle involved in DCvC, suitable design of building blocks can selectively lead to complex molecular architectures that are thermodynamically most stable. Unlike the popular imine bond formation, which is directional, alkyne metathesis provides symmetrical bonds that can selfexchange. One of the major benefits of using alkyne metathesis is that it requires a single type of symmetrical building block containing identical alkyne bonds, which makes the synthesis simple. The potential drawback is the difficulty in building unsymmetrical structures through alkyne metathesis. In order to solubilize the conformationally rigid frames in common organic solvents and prevent premature precipitation of intermediates, flexible alkyl chains are usually attached to monomer units. We designed building blocks with various symmetries and functional group arrangements targeting structures of different size and shape (Figure 1): C2 symmetrical bifunctional building blocks (1 and 2), C3 symmetrical trifunctional building blocks (3 and 4) with face-to-edge angles of 90° or 120°, and a C4 symmetrical tetrafunctional building block (5) with a face-toedge angle of 90°. The efficient removal of reaction byproducts (e.g., 2-butyne) from the equilibrium system is important as these can continuously participate in alkyne metathesis (nonproductive pathway) and lead to low conversion. The alkyne byproducts are usually removed by precipitation, adsorption in molecular sieves, or application of vacuum. Methyl groups (e.g., 3) or large precipitating groups (e.g., 1) are installed to facilitate the removal of alkyne byproducts.
Wei Zhang obtained his B.S. from Peking University in 2000. He then pursued his graduate study at the University of Illinois at Urbana–Champaign under the supervision of Prof. Jeff Moore. After obtaining his Ph.D. degree in 2005, he moved to MIT to conduct postdoctoral research with Prof. Tim Swager. In 2008, he started his independent career in the Department of Chemistry and Biochemistry at the University of Colorado, Boulder. His main research interest is to develop organic functional materials based on novel 2D and 3D molecular architectures that are constructed through dynamic covalent chemistry.
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Fig. 1. Building blocks of ethynylene-linked molecular cages.
Fig. 2. The structures of alkyne metathesis catalysts.
The alkyne metathesis of C2 symmetrical building blocks (1 or 2) was straightforward. Highly efficient molybdenum carbyne complexes (6 or 7) with multidentate ligands developed in our group were used as the catalyst (Figure 2).[21,22] Depending on the edge-to-face angle, cyclic dimer 8[23] or trimer 9 were obtained in good yields after stirring the substrates at 45°C overnight (Scheme 1). The equilibrium was driven by the precipitation of diarylacetylene byproduct 10. The results of alkyne metathesis of C3 symmetrical building blocks 3 and 4 were quite unexpected and surprising.[24,25] We expected a tetrahedron-shaped cage (Figure 3a) from monomer 3, since its face-to-edge angle is 60°, which is close to
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the face-to-edge angle of a tetrahedron (54.7°). The dimer (Figure 3b) was expected from the monomer 4 with a face-toedge angle of 90°. However, in both cases, we observed a tetramer with D2h symmetry, which consists of two macrocycles connected by two arms (Figure 3c). Therefore, the three originally identical alkyne groups in the monomer split into two different types (shown as yellow and orange in Figure 3c) in the cage product. The protons of the two arms forming the macrocycle with another monomer and the protons of a third arm bridging two macrocycles were observed in a 2:1 ratio in the 1H NMR spectrum of the cage 11. The structure of 11 was unambiguously determined by single-crystal X-ray analysis (Figure 4). Intrigued by this observation and to better understand the reaction progress, we monitored the metathesis of the monomer 3. We observed a significant amount of macrocyclic intermediate 12 in the early stage of the reaction (within 0.5 h), indicating that 12 could be the key intermediate towards the formation of 11 (Scheme 2). Similarly, in spite of the original design to synthesize a dimer (Figure 3b), another C3 symmetrical monomer 4 with a different face-to-edge angle of 90° from the monomer 3 (60°) also forms a tetrameric cage 13 with D2h symmetry (Scheme 2). Such cage structures with D2h symmetry are rarely observed, especially when C3 symmetric monomers, which generally
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Scheme 1. Synthesis of cyclic dimer 8 and trimer 9.
Fig. 3. Schematic representation of (a,b) expected structures and (c) observed structure from C3 symmetrical building blocks 3 and 4.
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serve as vertices or faces in the assembly of tetrahedron-shaped cages, are used as building blocks. In contrast to the failed formation of a dimer from monomer 4, a dimer 14 was successfully formed in good isolated yield (72%) from monomer 5, which also contains carbazole corner pieces arranged perpendicularly to the porphyrin ring with a face-to-edge angle of 90°, similar to monomer 4 (Scheme 3).[26] Presumably, the porphyrin ring in 5 is relatively flexible compared to the central benzene ring in
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Fig. 4. Crystal structure of cage 11.
4, thus effectively releasing the strain built up during the formation of the dimeric structure. These examples highlight the critical importance of angle strain and its significant effect on rigid alkyne bond formation during the dynamic assembly process.
3. Shape-Persistent Molecular Cages as Fullerene Receptors Since their discovery,[27] fullerenes have received tremendous attention due to their unique properties, such as high electron affinity, superior electron transport and radicalscavenging capability.[28–30] However, their poor solubility in common solvents largely impedes the purification and modification process and thus the widespread application of fullerenes in the fields of photovoltaics,[31] medicine and cosmetics.[32] The development of synthetic host molecules that can non-covalently interact with fullerenes has been actively pursued in the hope of enhancing the solubility of fullerenes and facilitating their purification and application. In 1994, Atwood reported the efficient purification of C60 from fullerene soot through selective complexation of fullerenes with calixarenes.[33,34] In the past two decades, various organic host molecules[35–39] have been developed, including calixarene derivatives,[33,40,41] cyclotriveratrylenes (CTVs),[42,43] cycloparaphenyleneacetylenes (CPPAs),[44,45] and porphyrinbased receptors.[39,46,47] It has been reported that the flexibility of host molecules is important in order to maximize host–guest interactions through conformational change and achieve higher binding affinity. For example, the zinc porphyrin cyclic dimer (15) linked by linear C6H12 chains shows good binding interaction with C60 in benzene at 25°C (KC60 = 6.7 × 105 M−1), while the similar dimer (16) linked by rigid diacetylenic linkers does not show any sign of complexation with fullerenes (Figure 5).[48] On the other hand, there also exist many examples of hosts and receptors that have benefited from their shape-persistent struc-
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tural features, which include the influence of DNA stiffness on the specificity of protein–DNA complexes.[49,50] The rigidity of shape-persistent host molecules can provide preorganization and size-based recognition, and thus achieve high binding affinities and reinforce the size selectivity between guests. Anderson and co-workers have reported a rigid zinc porphyrin trimer (17, Figure 5),[51] which shows strong binding interactions with C60 and C70 in toluene with association constants of 2 × 106 and 2 × 108 M−1, respectively. It should be noted that such a rigid trimer structure provides exceptionally high binding selectivity toward C70 over C60 (i.e., KC70/KC60 ≈ 100), which surpasses the C70/C60 selectivity of most flexible host molecules (typically KC70/KC60 ≈ 10). By contrast, a flexible zinc porphyrin trimer (18, Figure 5) shows a considerably lower binding affinity for C60 (K = 2.7 × 103 M−1),[52] which could be attributed to the lower level of preorganization of porphyrin moieties. Various shape-persistent cages prepared through dynamic alkyne metathesis were evaluated for fullerene encapsulation, and we revealed that some of these cages have high binding affinities for fullerenes and preferentially bind a certain fullerene over others. The porphyrin dimer 8 shows a high binding selectivity for C84 over the lower fullerenes C70 and C60.[23] As shown in Figure 6, computer calculations on the energy-minimized models show that the free 8 adopts a somewhat collapsed conformation (Figure 6a) with a distance of 4.8 Å between the top and bottom porphyrin panels. However, upon the addition of fullerene guests, the dimer 8 undergoes conformational change with increased internal cavity size from 4.8 Å to 13.7 Å to accommodate the guest molecules (Figure 6b). UV–vis titration experiments revealed that 8 forms 1:1 complexes with C60, C70, and C84 with association constants of 1.3 × 104 M−1, 2.0 × 106 M−1, and 2.2 × 107 M−1, respectively, in toluene at room temperature. The dimer 8 exhibits remarkably high binding selectivities for larger fullerenes over C60 (e.g., KC70/KC60 > 100 and KC84/ KC60 > 1500). By contrast, the more flexible dimer 15 shows moderate selectivity (KC70/KC60 > 30) for C70 over C60.
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Scheme 2. Synthesis of tetrameric cages (11 and 13) with D2h symmetry from C3 symmetrical building blocks.
Importantly, such dimer 8 and fullerene complexes undergo reversible association and dissociation under acid/base stimuli for multiple cycles without noticeable decomposition and responsiveness decay. Another porphyrin dimer 14, with a rectangular prism shape, also shows strong binding interactions with fullerenes. According to the UV–vis titration experiment and the Job plot,
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cage 14 forms 1:1 complexes with C60, C70, and C84 with association constants of 1.4 × 105 M−1, 1.5 × 108 M−1, and 2.4 × 107 M−1 in toluene at room temperature,[26] which are among the highest fullerene binding constants reported so far.[36] It is interesting to note that the cage 14 binds most strongly with C70, showing an unprecedentedly high binding selectivity for C70 over C60 (>1000/1). This is in great contrast
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Scheme 3. Synthesis of rectangular prism 14.
to 8, which shows the highest binding affinity toward C84. Presumably, due to the high shape persistency of 14, C70 fits perfectly inside the cage cavity, while C60 fits more loosely and C84 fits a little bit too tight. A computational modeling study revealed that the four-arm dimer 14 is much more rigid compared to the two-arm dimer analogue 8. Owing to the high degree of shape persistency of the cage, there is only a small conformational change upon fullerene binding: the height of 14 is slightly increased to 12.1–12.2 Å in C60@14 and C70@14 from the initial 11.9 Å in the unoccupied 14; the diameters of the cavities are decreased slightly to 17.6–17.8 Å from 18.3 Å (Figure 7). Since the porphyrins in 14 are non-metalated, the binding between the cage and fullerene is fully reversible under acid/base stimuli. Through the “selective complexation– decomplexation” strategy, efficient separation of C70 from a C60-enriched fullerene mixture (C60/C70, 10/1, mol/mol) was achieved as shown in Figure 8. After only one cycle of separation, C70 abundance in the fullerene mixture was increased
Fig. 5. The structures of previously reported fullerene receptors.
Fig. 6. Energy-minimized models of (a) compound 8 (side view) and (b) C84@8. For simplicity, methyl groups were used in the calculation instead of hexadecyl chains. The height of 8 was defined as the distance between the top and bottom porphyrin panels.
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Fig. 7. Energy-minimized models of (a) cage 14 (side view) and (b) C60@14. For simplicity, methyl groups were used in the calculation instead of hexadecyl chains.
Fig. 8. Schematic representation of the C70 isolation process using cage 14. Step (I): To a solution of the C60 and C70 mixture in CS2 (C60/C70 = 10/1, molar ratio) was added a small amount of 14 (equal to or less than the stoichiometric amount of C70), resulting in the favored formation of C70@14. After separating the unbound free fullerenes by precipitation in CHCl3 (precipitate shown in photograph (b)), cage–fullerene complexes (mostly C70@14) were collected in the solution phase. Step (II): Upon the addition of 100 equivalents of trifluoroacetic acid (TFA) to the solution collected in step I, fullerene guest molecules (mostly C70) were released as black precipitates and removed to complete one cycle of the isolation process (shown in photograph (c)). Step (III): Regeneration of 14 was accomplished by the addition of 100 equivalents of triethylamine (TEA) to the above solution.
from the initial 9 mol % to 79 mol % (ca. ninefold increase). Such size-selective complexation of a particular fullerene from fullerene mixtures could be an interesting alternative for separation and purification of fullerenes.[33,34,53] Interestingly, we found that the C60@14 complex can bind to the surface of single-walled carbon nanotubes (SWCNTs) in a “side-to-face” fashion through π–π stacking interactions. Such a ternary nanohybrid system consisting of SWCNTs and the C60@14 complex serves as a light-harvesting photoactive layer in solar cell devices.[54] With 1 wt % SWCNT loading, a short-circuit current of 1.4 mA/cm2, an open-circuit voltage of 0.47 V, a fill factor of 24%, and an efficiency of 0.16% were
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measured under AM1.5 conditions. Although the hybrid material shows modest light-harvesting performance, it opens up new possibilities for fabricating efficient photoactive molecular devices and shows the great potential of such nanohybrid systems in photovoltaic applications. Although it has been proven that increasing the number of porphyrin units within one host could increase its affinity toward fullerenes,[51] porphyrin trimer 9 did not show any sign of complexation with C60, C70, or C84 under UV–vis titration conditions, likely due to the high rigidity of the host and its size mismatch with fullerene guests. A similar detrimental effect of the extreme rigidity of a host on fullerene binding has been reported.[55] One such example is the “nanobarrel” containing four porphyrin units, which shows an association constant (KC60 = 5.3 × 105 M−1 in toluene at room temperature) not as large as expected. As the research on multiporphyrinic cages expands,[56] we are expecting more and more shape-persistent porphyrin-based cages to be developed as fullerene hosts. In addition to widely recognized porphyrin-based fullerene hosts, other aromatics have also been utilized,[35–39] such as tetrathiafulvalene[57–59] and solely carbon-based CPPAs.[44,45] Tetrameric cage 11, with triphenylbenzene units connected by ethynyl linkers, can also bind fullerenes (Figure 9), albeit much more weakly than the porphyrincontaining host molecules previously discussed.[24] A 1H NMR titration experiment revealed that cage 11 has favorable interactions with C70 over C60. It forms a 1:1 complex with C70 (KC70 = 3.9 × 103 M−1), whereas no noticeable binding with C60 was observed. This result was supported by computer modeling, which showed the binding energy of 11 with C70 to be 10 kcal/mol lower than that with C60 (−48.7 vs −38.4 kcal/ mol). It is postulated that such a difference is largely the result of the relatively larger size of C70 and its ellipsoidal shape, thus providing a better fit inside the cavity of cage 11. Interestingly, when the peripheral phenyl units of cage 11 were replaced by carbazole units, the analogous tetrameric cage 13 showed much weaker binding interactions with C70 and the binding constants could not be obtained due to the weak interactions.
4. Summary With the rapid development of alkyne metathesis in DCvC research, shape-persistent organic cages with different sizes, functional groups, and binding affinities toward fullerenes can be obtained in a modular fashion from simple precursors. Provided delicately designed shape and size, shape-persistent cages possess advantages over flexible ones, such as preorganization and increased size selectivity, thus leading to comparable or even higher fullerene-binding affinity and selectivity. In host–guest systems, shape complementarity, such as the lock-and-key principle, preorganization, and the induced-fit
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Fig. 9. (a) The chemical shift changes in 1H NMR spectra obtained in toluene-d8 during the titration of the cage 11 (0.14 mM) with an increasing amount of C70 (0–8.6 equiv). (b) The computational model (side view) of C70@11. Methyl groups are used for simplification.
concept have been deemed important for designing effective hosts.[35] It has been shown that the porphyrin unit, which is generally known as a planar moiety, can slightly distort from planarity and adopt a concave structure to maximize the short contacts between hosts and fullerene guests.[60] In addition to shape complementarity, intermolecular interactions, including van der Waals interaction, π–π interaction, and electrostatic force, also play important roles in forming a stable host–guest complex.[34,35] Shape-persistent organic cages consisting of electron-enriched aromatic groups with a tunable non-polar cavity of appropriate size have a great potential to serve as selective complexation agents to enhance the solubility of fullerenes and assist their purification and further applications.
Acknowledgements
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We thank the National Science Foundation (DMR-1055705) and the Alfred P. Sloan Foundation for their financial support of the research reported in this account.
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