Strategies To Assemble Catenanes with Multiple Interlocked

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Strategies To Assemble Catenanes with Multiple Interlocked Macrocycles Ho Yu Au-Yeung,* Chi-Chung Yee, Antony Wing Hung Ng, and Keling Hu Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: As a major class of mechanically interlocked molecules, not only are catenanes topologically intriguing targets that challenge the chemical synthesis to the efficient formation of mechanical bonds, but also the mechanical properties arising from the topology offer unique and attractive features for the development of novel functional molecular materials. Despite advancements in templated methods for different types of interlocked architectures, [n]catenane possessing multiple numbers of interlocked macrocycles still remains a difficult synthetic target with very few reported examples. If the unique mechanical properties of catenanes are to be fully exploited, reliable, controllable, and efficient strategies for accessing [n]catenanes will be necessary. In this Viewpoint, challenges, considerations, and strategies to [n]catenanes are discussed.

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INTRODUCTION Catenane synthesis is always challenging the imagination and skills of synthetic chemists. As a class of topologically nontrivial molecules containing mechanical bonds, catenanes represent a unique problem in chemical synthesis, and the formation of a mechanical bond has been one of the early focuses in catenane chemistry.1 From a statistical approach to the directed synthesis to templated methods, significant progress in the formation of a mechanical bond has been made over the past decades. The Hopf link, the simplest catenane with two macrocycles interlocked with two molecular crossings (Figure 1), may now be considered as a straightforward synthetic target.1,2 Nonetheless, the creation of a single mechanical bond is just a start, and there are various challenges in catenane chemistry awaiting ahead, with the

synthesis of catenanes more complicated than the Hopf link being one of them.3 Other unmet challenges in catenane chemistry and chemical topology include, for example, control of the number and stereochemistry of the intertwining points and incorporation of the mechanical bond into materials.4,5 In this Viewpoint, we will discuss the challenges and strategies in assembling [n]catenane with multiple numbers of mechanically interlocked macrocycles. The number n in the prefix of [n]catenane indicates the total number of interlocked components in the catenane. For example, the Hopf link, Solomon link, and Star of David catenanes are all [2]catenanes with two interlocked macrocycles but with two, four, and six crossings, respectively (Figure 1).1 In the following discussion, we will primarily focus on examples of [n]catenanes with n ≥ 4. More detailed discussions on other high-order mechanically interlocked structures such as knots or links with multiple crossing points can be found in other reviews.6−8

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RESULTS AND DISCUSSION Basics of Templated Catenane Synthesis. Early examples of catenane syntheses from a statistical approach and a directed synthesis are often too low yielding to yield the interlocked products in enough quantity for further studies.9 It is only after the application of templated synthesis that catenane and related interlocked structures can be made in significant quantity to reveal the properties and potentials of the mechanical bond.10,11 Preorganization and macrocyclization are the two major steps in the templated synthesis of a catenane. Taking the templated synthesis of a [2]catenane (the Hopf link) as an example, the Figure 1. (a) Cartoon representations of the Hopf link, Solomon link, and Star of David catenanes, which are all [2]catenanes with two, four, and six crossings (blue asterisks), respectively. (b) Cartoon representation of a linear [n]catenane.

© XXXX American Chemical Society

Special Issue: Self-Assembled Cages and Macrocycles Received: October 9, 2017

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DOI: 10.1021/acs.inorgchem.7b02523 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Representative strategies of the templated synthesis of (a) a [2]catenane and an [n]catenane from (b) cyclization of a [n](pseudo)rotaxane or (c) oligomerization of [2](pseudo)rotaxane.

precursors will first be spatially oriented to favor the formation of a mechanical bond. This step is known as preorganization. This preorganization step overcomes the entropic cost in interlocking the macrocycles and is usually achieved through the use of noncovalent interactions (e.g., hydrogen bonding, metal− ligand coordination, π−π interactions, hydrophobic effects, halogen bonding, etc.).1,3,12−16 When a macrocycle is used as a precursor, a (pseudo)rotaxane intermediate will be obtained. Further ring-closing reaction(s) at the ends of the linear precursor(s) then interlock(s) the rings and give(s) [2]catenane (Figure 2a). Logical extensions of this templated [2]catenane synthesis to include more interaction sites in a single building block to interlock more macrocycles (Figure 2b) and/or to promote the formation of larger macrocycles from more building blocks (Figure 2c) are common strategies to prepare [n]catenanes. [2]Catenane versus [n]Catenane. There are a few additional considerations to be taken into account when extending the templated [2]catenane synthesis to the templated [n]catenane synthesis. For a [2]catenane synthesis, one major competitive reaction pathway will be macrocyclization of the unthreaded precursors, which gives noninterlocked products. If kinetically labile, reversible interactions are used to template the precursors, the yield of [2]catenane will then be influenced by the equilibrium concentration of the preorganized intermediate under the reaction conditions. For an [n]catenane synthesis involving n precursors, there will then be n − 1 of these equilibria, and only one of these equilibrium species will give the desired [n]catenane upon macrocyclization (Figure 2b). The remaining equilibrium species will give other topological isomers that will lower the

yield of the desired [n]catenane. Purification may also be more difficult. In addition, the number of bond-forming reactions will also increase, in general, when the number n of the target [n]catenane increases (Figure 2c). Maximizing the efficiency of both the preorganization and macrocyclization will therefore be increasingly important for [n]catenane synthesis. Topological isomerism is another important consideration for [n]catenanes. To describe nontrivial topology, the Alexander− Briggs notation (Xyz) is commonly used, where x, y, and z are the number of crossings, number of discrete components, and order of the structures, respectively.17 For example, the isomeric Hopf link, Solomon link, and Star of David [2]catenanes can be represented by the notations 221, 421, and 621, respectively. Because these [2]catenane isomers are all prime links (i.e., links that cannot be decomposed to simpler knots or links), they can be described and differentiated by different Alexander−Briggs notations. When the number n in an [n]catenane increases, more topological isomers will become synthetically accessible. For example, the radial and linear [4]catenanes are two topological isomers with both four discrete macrocycles and six crossings (Figure 3). Because the radial and linear [4]catenanes are not prime links and they differ only by the interlocking pattern of the rings, the Alexander−Briggs notation is not applicable to differentiate these isomers. Additional classification and notation systems will be necessary. Of note, the radial [n]catenane with one large central macrocycle interlocked with n − 1 rings is the most common type among all of the reported [n]catenanes to date. Other possible interlocking patterns include the linear and branched [n]catenanes, which are of much fewer examples. B


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Inorganic Chemistry

linear compound to give the [3]catenane 3 in 6% yield (Figure 4).19 Compared with the diiodo compound in the synthesis of the [2]catenane 2, the shorter α,ω-dibromo compound favored [1 + 1] macrocyclization less and gave the [3]catenane 3 as a result of [2 + 2] macrocyclization. Using the copper(I) complex 4, cyclic oligomerization from oxidative couplings gave a mixture of catenane homologues containing 3 to 7 interlocked macrocycles (5−9), which were primarily characterized by mass spectrometry (Figure 4b).20 In this example, various degrees of cyclic oligomerization were possible to give different [n]catenane products. The highest order [7]catenane 9 was obtained in roughly 5% yield. Starting from the porphyrin [2]rotaxane 10 containing two terminal alkynes, Anderson and co-workers reported the templated synthesis of the radial [4]catenane 12 via oxidative alkyne couplings.21 The use of the hexatopic template 11 favored cyclotrimerization, and 12 was obtained in 62% yield (Figure 5). Gel permeation chromatographic analysis of the reaction mixture revealed another [7]catenane 13, which was formed from an unusual Vernier template route, in 6% yield.

Figure 3. Some topological isomers of a [4]catenane with different numbers of crossings (blue asterisks).

[n]Catenanes from (Pseudo)Rotaxane Oligomerization. One of the first examples of templated [3]catenane synthesis was given by Sauvage and Weiss. Similar to their very first templated [2]catenane synthesis,18 the tetrahedral copper(I) phenanthroline complex 1 was ring-closed with an α,ω-dibromo

Figure 4. Sauvage’s syntheses of (a) [2]- and [3]catenanes templated by copper(I). (b) A series of [3]-, [4]-, [5]-, [6]-, and [7]catenanes from [n + n] oxidative coupling cyclization.18−20 C


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Inorganic Chemistry

Figure 5. Anderson’s templated synthesis of [4]- and [7]catenanes.21

Figure 6. Radial [4]- and [5]catenanes assembled by Kim.22

In addition to being a template for precursor preorganization, metal−ligand coordination has also been used to ring-close the

precursors in the catenane synthesis. Kim and co-workers have reported the assembly of radial catenanes via platinum(II) D


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Inorganic Chemistry

Figure 7. Radial [4]- and [7]catenanes from a molecular triangle and a hexagon by Stang and co-workers.24

pyridine coordination.22 In the assembly, a [2]pseudorotaxane (14) was first prepared from a bispyridyl 1,4-butyldiammonium that formed a highly stable inclusion complex with cucurbit[6]uril (CB[6]).23 Heating of an aqueous solution of 14 and cisPt(en)2(NO3)2 under refluxing conditions gave the radial [4]catenane 15 in near-quantitative yield. When the assembly was conducted at room temperature, a mixture of 15 and the [5]catenane 16 was obtained (Figure 6). It was proposed that the steric hindrance between CB[6] molecules in 15 could not be easily overcome at room temperature, and therefore 16 was formed. By using an isomeric 3-pyridyl precursor, a similar mixture of [4]- and [5]catenanes was also obtained.22 By combining organic building blocks of well-defined geometry and a linear trans-platinum(II) center, Stang and co-workers reported the assembly of the radial [4]catenane 20 and the [7]catenane 21 that respectively consist of a central molecular triangle and a hexagon. The two alkynes in building blocks 18 and 19 are at respectively 60° and 120°, which directed

the assembly to [3 + 3] and [6 + 6] macrocyclization exclusively. Subsequent inclusion of the diammonium units in dibenzo-24crown-8 (DB24C8) gave the [4]- and [7]catenanes in quantitative yield (Figure 7).24 Because of the weak ammonium− crown ether binding (K ≈ 200),25,26 an excess of DB24C8 was used to favor the formation of [n]catenanes. It can be seen from the above examples that one advantage of synthesizing [n]catenanes by oligomerizing (pseudo)rotaxane is the relatively straightforward adaptation of the building block design and reaction conditions from a related catenane synthesis. With the proper choice of preorganization and macrocyclization conditions, the mechanical bond in the preorganized intermediates can be preserved and transferred to [n]catenane. One key consideration in this approach is controlling the degree of cyclic oligomerization. If the degree of cyclic oligomerization is not well controlled and the dispersity is wide, a mixture of [n]catenanes of different numbers of interlocked units will be obtained. In addition, this strategy limits the topological variety E


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Inorganic Chemistry

Figure 8. Aromatic donor−acceptor templated synthesis of (a) [2]- and [3]catenanes.27,28 (b) Mixture of [5]-, [6]-, and [7]catenanes by Stoddart and co-workers.29

macrocycle 22 templated the formation and interlocking of the π-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) formed from building blocks 23 and 24 (Figure 8a). Replacing 23 and 24 with the biphenylene-derived 25 and 26 expanded the π-deficient macrocycle for accommodating two dioxynaphthalene units and gave the [3]catenane 28 (Figure 8b).28 On the other hand, the use of the expanded π-rich macrocycle 29 gave the [3]catenane 30, which has a larger cavity and additional π-rich sites for interlocking additional CBPQT4+ to give a mixture of [5]-, [6]-, and [7]catenanes (31−34; Figure 8c).29 A high pressure of 12 kbar

of the products to only radial [n]catenane. Other strategies will be necessary if [n]catenanes of other interlocking patterns are desired. [n]Catenanes from Building Blocks with Multiple Interlocking Sites. Another logical extension from a [2]catenane synthesis to an [n]catenane synthesis is to expand the number and/or size of the recognition sites in the precursor building blocks. One representative example is the synthesis of a series of [5]-, [6]-, and [7]catenanes by Stoddart and co-workers. In their synthesis of the donor−acceptor [2]catenane 27,27 the π-rich F


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Inorganic Chemistry

Figure 9. Hajime’s stepwise synthesis of a linear [5]catenane.31

Figure 10. An [n]catenane synthesis from an intramolecular cyclization of an [n]rotaxane.32

Figure 11. Wu’s synthesis of a radial [5]catenane.33

accommodate two dialkylammonium units.31 The [2]catenane 37 was first formed from the inclusion of 1 equiv of 35 in BPP34C10, intramolecular alkene metathesis, and hydrogenation. Further inclusion of another dialkylammonium (38) unit in 37 gave the [3]pseudorotaxane 39. Because of the relatively short alkyl linkers in 38, further alkene metathesis gave the linear [5]catenane 40 via an intermolecular [1 + 1] cyclization (Figure 9). The yield of the metathesis step in the formation of the linear [5]catenane 40 was 12%.

was employed in the synthesis to overcome steric and electrostatic repulsion between π-deficient macrocycles to favor the formation of [n]catenanes. The [7]catenane 34 was obtained in 26% yield. A similar macrocycle expansion strategy was also adopted by Vögtle and co-workers for the [n]catenane synthesis based on hydrogen bonds.30 More recently, Hajime and co-workers have reported the stepwise synthesis of a linear [5]catenane featuring bis(p-phenylene)-34-crown (BPP34C10), which has a cavity large enough to G


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Inorganic Chemistry

Figure 12. Synthesis of [4]- and [6]catenanes from building blocks that can engage in orthogonal supramolecular interactions.34

Figure 13. Synthesis of a [4]catenane from CB[6] and β-CD.

Macrocyclization of an [n](pseudo)rotaxane to give an [n]catenane was reported by Okada and Harada. An anthracene-stoppered [n]rotaxane (41) consisting of poly(ethylene glycol) (Mw = 3350) threaded through about 19 α-cyclodextrin (α-CD) units was subjected to light irradiation to trigger photodimerization of the anthracene stoppers (Figure 10).32 Size exclusion chromatographic analysis of the photoreaction products revealed two peaks, in which one of them had a retention time similar to that of 41 and was proposed to be the [n]catenane 42 formed from an intramolecular photodimerization.

Dasgupta and Wu have also described a radial [5]catenane synthesis via an intramolecular cyclization of a [5]pseudorotaxane. Threading of the tetrakris(dialkylammonium) 43 through DB24C8 and subsequent ring-closing alkene metathesis gave the radial [5]catenane 44 in 20% yield (Figure 11).33 Of note, an excess of DB24C8 was used (20 eq. in total, 5 eq. for each dialkylammonium) to favor the complete threading of the crown on all four dialkylammoniums in 43. Our laboratory has explored the use of orthogonal interactions to preorganize and interlock multiple macrocycles on a single catenane. By the combined used of Cu(I)-phenanthroline H


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Inorganic Chemistry

Figure 14. Assembly of a [3]catenane from a disulfide dynamic library.35

Figure 15. Self-assembly of a cyclic [3]catenane from polypyridyl building blocks.36

Figure 16. Self-assembly of a [4]catenane with 12 crossings.37

coordination and CB[6]-ammonium binding, the branched [6]catenane 48 and the radial [4]catenane 49 were obtained from the CB[6]-catalyzed azide−alkyne cycloaddition of building block 45 and Cu(I) complexes 46 and 47 respectively (Figure 12).34 Of note, CB[6] was both a macrocyclic component and a cycloaddition catalyst in the syntheses. The CB[6] were interlocked upon triazole formation and hence other topological isomers with less interlocked CB[6] were not formed. Further interlocking of β-cyclodextrin (β-CD) on the dioxynaphthalene unit of 45 via hydrophobic effect gave another radial [4]catenane (51) in 82% yield, suggesting that other higher order [n]catenanes can be prepared by using a combination of these orthogonal interactions (Figure 13). Other [n]Catenanes from Self-Assembly. Self-assembly is a powerful strategy to gain access to highly complicated,

sometimes unexpected, topologies. For example, Sanders and co-workers discovered the [3]catenane 54 from an aqueous disulfide dynamic library.35 The [3]catenane 54 was thermodynamically favored by a combination of aromatic donor−acceptor, hydrophobic, and electrostatic interactions. Interestingly, the polycationic spermine was found to effectively stabilize and template the anionic 54 to 60% yield (Figure 14). Nitschke and co-workers reported the self-assembly of polypyridyl building blocks into the cyclic [3]catenane 59 (a 633 link) via dynamic imine exchange and metal coordination (Figure 15).36 Careful analysis of the assembly conditions revealed that a delicate balance of various intermolecular interactions between the building blocks is necessary for the successful assembly of 59. I


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Inorganic Chemistry More recently, Fujita and co-workers have reported the self-assembly of the [4]catenane 61 with 12 crossings from coordination of a peptide-based pyridine building block to Ag+ (Figure 16).37 The tripeptide building block 60 contains a Pro-Gly-Pro sequence, which was crucial in directing the folding and entwining of the building blocks. Similar Ag+ coordination of an isomeric Gly-Pro-Pro building block was found to result in a noninterlocked helical assembly.38

His current research covers several areas in supramolecular chemistry, focusing on catenane chemistry, small-molecule recognition, and responsive luminescent probes. He was recently awarded the Thieme Chemistry Journals Award 2016, Croucher Innovation Award, and Graeme Hanson Early Career Researcher Award (2016).

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CONCLUSIONS A flourish of templated syntheses has led to various beautiful molecular examples of nontrivial topology. The continuing development of efficient templated methodologies, selfassembly, and covalent bond formation has allowed different strategies to realize mechanically interlocked compounds with increasing topological complexity. Yet, the chemistry of catenane is just a start, and only a few types of catenanes, with a few interlocked macrocycles, have been successfully synthesized. Efficient interlocking of multiple rings, efficient macrocyclization strategies, control of the interlocking patterns and topological isomerism of [n]catenane are all unmet challenges in catenane chemistry within the greater unexplored area in chemical topology that await exciting and innovative solutions.

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Chi-Chung Yee received his B.Sc. in Applied Chemistry from City University of Hong Kong in 2011. He began his Ph.D. study at The University of Hong Kong in 2014 under the supervision of Ho Yu Au-Yeung. His work focuses on the synthetic studies of high-order catenanes.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02523. Synthesis and characterization data of 51 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ho Yu Au-Yeung: 0000-0002-7216-7921 Notes

The authors declare no competing financial interest. Biographies Antony Wing Hung Ng received his B.Sc. in Chemistry from The University of Hong Kong in 2017 and is continuing on his Ph.D. study in the laboratory of Ho Yu Au-Yeung. His research interests concern [n]catenanes and related interlocked structures.

Ho Yu Au-Yeung obtained his B.Sc. (1st Hons) and M.Phil. degrees in Chemistry from The Chinese University of Hong Kong. He was awarded a Croucher Foundation Scholarship to pursue his Ph.D. with Prof. Jeremy K. M. Sanders at the University of Cambridge and received his Ph.D. in 2010. After postdoctoral research in the group of Prof. Christopher J. Chang at the University of California, Berkeley, he returned to Hong Kong and joined the Department of Chemistry at The University of Hong Kong as an Assistant Professor in Sept 2013.

Keling Hu comes from Jining, the hometown of Confucius and Mencius, Shandong Province, P. R. China. He received his Ph.D. from Nankai University in June 2016, working on the synthesis and characterization of biomass-derived polyesters. He then joined the group of Ho Yu J


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Inorganic Chemistry

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Au-Yeung as a postdoctoral fellow, studying mechanically cross-linked polymeric materials.

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ACKNOWLEDGMENTS We are thankful for support from the Croucher Foundation and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Early Career Scheme; Project HKU 27300014). C.-C.Y. and A.W.H.N. are recipients of the Postgraduate Scholarship from The University of Hong Kong. We acknowledge UGC funding administered by The University of Hong Kong for support of the Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry Facilities under support for Interdisciplinary Research in Chemical Science.

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