Molecular Borromean Rings Based on Half-Sandwich Organometallic

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Molecular Borromean Rings Based on Half-Sandwich Organometallic Rectangles Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Ye Lu, Hai-Ning Zhang, and Guo-Xin Jin*

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State Key Laboratory of Molecular Engineering of Polymers, Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200433, China CONSPECTUS: Over the last two decades, interlocked molecular species have received considerable attention, not only because of their intriguing structures and topological importance, but also because of their potential applications as smart materials, nanoscale devices, and molecular machines. Through judicious choice of metal centers and their adjoining ligands, a range of interesting interlocked structures have been realized by coordination-driven self-assembly. In addition, researchers have extensively developed synthetic methodologies for the construction of organized self-assemblies. One fascinating and challenging synthetic target in this field is the family of molecular Borromean rings, which consist of three chemically independent rings that are locked in such a way that no two of the three rings are linked with each other. Toward this goal, we have developed a template-free self-assembly method for synthesizing molecular Borromean rings by rationally designing metal-containing precursors and organic ligands. In this Account, we present our recent work, focusing on interlocked structures comprising half-sandwich iridium- and rhodium-based organometallic assemblies obtained by rational design. We first describe a series of template-free self-assembled organometallic molecular Borromean rings, which we constructed from preorganized binuclear half-sandwich molecular clips and suitable pyridyl ligands. These molecular Borromean rings can be sorted into four types according to their different bridging ligands, including those based on metallaligands, dihalogenated ligands, naphthazarin and π-acceptor ligands. By single-crystal X-ray crystallographic analysis, NMR experiment, and DFT calculation, we discuss their driving forces and the inter-ring interactions. Furthermore, we took advantage of the dissimilarity in their interactions to realize selective, reversible conversions between molecular Borromean rings and monomeric rectangles by the use of suitable solvents or guest molecules. Subsequently, a stepwise chemoseparation method based on molecular Borromean rings was established, with the molecular Borromean rings used in the separation being recoverable and recyclable. Due to their structural complexity and difficult synthesis, useful guidelines or rules to help design complicated interlocked molecules are highly desirable. We also highlight our efforts to develop empirical guidelines to uncover the relationship between the aspect ratio of metallarectangles and the formation or stability of molecular Borromean rings. An empirical formula has further been established to show the approximate ratio of lengths of the short arm and the long arm in molecular Borromean rings based on π−π (or p-π) stacking. We then demonstrate how to use these guidelines to design new molecular Borromean rings and further lead to other interlocked structures, for example, [2]- and [3]catenane structures. Taken together, our results may lead to a promising future for the design of fascinating and useful interlocked structures by coordination-driven self-assembly.

1. INTRODUCTION

periods as a sign of strength in unity, for example, Trinity (Figure 1c).3 This kind of specific link can also be realized using three rectangles, for instance, the Kong Ming Lock, a type of traditional Chinese six-piece burr puzzle (Figure 1d). Three-rectangle-based Borromean rings also occasionally

Borromean rings, the simplest Brunnian link, are three rings linked in such a way that no two of the three rings are linked with each other, but removing any ring results in two unlinked rings (Figure 1a).1 The name “Borromean rings” originates from the coat of arms of the aristocratic Borromeo family of Northern Italy (Figure 1b).2 Because of its symbolic meaning, the design has been used in many locations and historical time © XXXX American Chemical Society

Received: May 18, 2018

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DOI: 10.1021/acs.accounts.8b00220 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

are attractive due to their predictable coordination directions, which is crucial for the design of topologically complicated interlocked molecules, for example, Borromean rings. In 2013, we serendipitously obtained a three-rectangle-based BRs by a template-free, all-in-one self-assembly.26 Since then, we have been attracted by this fascinating structure and have further explored the synthesis and application of BRs. To date, four kinds of BRs with different types of half-sandwich molecular clips have been developed, including BRs based on metallaligands, dihalogenated ligands, naphthazarin and π-acceptor ligands, respectively. Herein, we review our work on BRs based on half-sandwich organometallic rectangles. We also attempt to develop some empirical rules from our work and use them to guide the design of other interesting interlocked molecules.

2. SYNTHESIS BRs Based on Metallaligands

We began this work by introducing a new kind of molecular clip (Ia) based on the metallaligand [Cu(opba)]2− [opba = ophenylenebis(oxamato)], containing a coordinatively unsaturated Cu(II) ion (Scheme 1a).26 Our intention for using metallaligand [Cu(opba)]2− was to construct metallarectangles with two coordinatively unsaturated metal ions for synergistic catalysis. By choosing suitable pyridyl ligands, the distance between two catalytic centers could be conveniently adjusted according to the size of the substrate molecule. Thereby, metallarectangles 2a based on pyridyl ligand L2 (L2 = 1, 2bis(4-pyridyl)ethylene) were designed for efficiently and specifically catalyzing the acyl-transfer reaction between Nacetylimidazole (NAI) and 3-pyridylcarbinol (Scheme 1a). In order to encapsulate larger molecules for catalytic transformations, we attempted to enlarge the cavity size by using the longer pyridyl ligand L6 (L6 = N,N′-di-4pyridinyloxalamide) (Scheme 2).26 However, to our surprise, single-crystal X-ray crystallographic analysis showed that the structure of the new compound (6a-BRs) was not a simple monomeric rectangle (MR), but three metallarectangles threaded inside each other, constructing a discrete BRs structure (Figure 2a). Moreover, there are also a number of observable interactions between the Cu ions and the carbonyl groups of the pyridyl ligands, and these interactions may have had some effect on the formation of the BRs structure. To ascertain if the formation of the BRs structure is determined by weak interactions or merely by the longer pyridyl arms, the pyridyl ligand L3 (L3 = 1,4-bis(4-pyridyl)benzene), with a nearly equivalent N···N distance but without obvious functional groups, was tested (Scheme 1a). The resulting complex 3a-BRs was also found to comprise a BRs structure like that of 6a-BRs (Figure 2b), and indicated that the length of the pyridyl arms may be more important in the assembly of BRs. Nevertheless, we sought to investigate whether the length of pyridyl arms should be restricted to a certain range. Therefore, another pyridyl ligand was used, L7 (L7 = N,N′-bis(4pyridyl)terephthalamide), which is much longer than L6 (and L3) (Scheme 2), and we were glad to observe that this linker also resulted in the formation of BRs (7a-BRs; Figure 2c).27 Similar to the case of 6a-BRs, there were strong interactions between the carbonyl groups of L7 and the open copper centers of the short arms. We speculate that the length of the pyridyl arms should be restricted to a certain range for the formation of BRs. In an attempt to reach this range, an extremely long ligand was needed, and we chose L8 (L8 =

Figure 1. (a) Structure of Borromean rings; (b) the coat of arms of the aristocratic Borromeo family; (c) the symbol of Trinity; (d) a Kong Ming Lock; (e) the “Borromean Rings” chandelier (photo taken by author Y.L.); (f) Stoddart’s Borromean rings (figure generated from data from the Cambridge Crystallographic Data Centre; CCDC number 231701).

appear in our everyday lives, such as the “Borromean Rings” chandelier (Figure 1e). Over the past two decades, a wide range of topologically fascinating interlocked molecules has been prepared by coordination-driven self-assembly.4−12 One intriguing and challenging synthetic target in this field is the family of molecular Borromean rings (BRs).13 Two main synthetic strategies for BRs have been explored: ring-in-ring strategies and all-in-one strategies.14−19 In contrast with the stepwise macrocyclization process characteristic of ring-in-ring approaches, all-in-one strategies achieve the knitting together of all three rings of the Borromean link in one fell swoop.14 Obviously, the realization of this comprehensive protocol requires careful selection and elaborate design of the precursor. Using this strategy, the first molecular expression of Borromean rings was found by Seeman et al., whose Borromean rings were based on DNA strands.20 However, the landmark in this field is Stoddart’s Borromean rings, which let us unambiguously observe the topological structure of Borromean rings by X-ray crystallography (Figure 1f), and their metal-ion-templated synthesis has led to other intriguing interlocked structures.21−23 Our research group has a longstanding interest in the selfassembly of metallarectangles by two preorganized binuclear molecular clips based on half-sandwich metal (Rh or Ir) fragments and pyridyl ligands.24,25 The binuclear metal clips B


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Accounts of Chemical Research Scheme 1a

a

(a) Synthesis of BRs based on [X(opba)]2− (X = Cu or Pd) and pyridyl ligands containing aromatic hydrocarbon groups. (b) Synthesis of BRs based on [Cu(pba)]2−. (c) Synthesis of BRs based on [X (opba)]2− (X = Cu or Pd). Reproduced with permission from ref 28. Copyright 2018 Wiley-VCH.

Scheme 2. Synthesis of BRs Based on [X(opba)]2− (X = Cu or Pd) and Pyridyl Ligands Containing Carbonyl Groups

Reproduced with permission from ref 28. Copyright (2018) Wiley-VCH.

further Cu(II) metallaligand, [Cu(pba)]2− [pba = 1,3propylenebis(oxamato)] (Scheme 1b).27 Due to the paramagnetic Cu(II) nuclei, some important information about BRs in solution cannot be obtained. Thus, we attempted to use diamagnetic Pd(II) nuclei instead of paramagnetic Cu(II) nuclei for NMR characterization. Thereby, another kind of molecular clip (Ib) based on metallaligand [Pd(opba)]2− was used to construct metallarectangles under the same conditions (Scheme 1a).28 When the short pyridyl arms L1 were used, the MR complex 1b was obtained, but when longer pyridyl arms L3, L4, L6, and L7 were used, a series

N,N′-([1,1′-biphenyl]-4,4′-diyl)diisonicotinamide). As expected, 8a was found to be a MR (Scheme 2). Because of the large inner space of 8a, one additional L8 ligand was found to coordinate with the open copper center inside the short arms. In order to probe the importance of Cu···O interactions in the formation of the BRs 7a-BRs, sodium p-toluenesulfonate (NaTs) was included to inhibit intramolecular Cu···O interactions between the rectangles, with the expectation of obtaining MR instead of BRs. By applying this method, the expected L7-bridged MR 7a was obtained. This synthetic strategy for BRs was further confirmed by the application of a C


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Figure 3. Single-crystal X-ray structures of 14a (a) and 15a-BRs (b) (N, blue; O, red; C, gray; Rh, orange; Cl, green); hydrogen atoms and counteranions are omitted. Reproduced with permission from ref 29. Copyright 2017 Cell Press.

the same conditions as those described above (Scheme 3). According to NMR spectroscopy in methanol, at low concentrations the new complex 15a also exists as a discrete tetranuclear metallarectangle, similar to 14a. However, upon increasing the reaction concentration, new peaks were observed in the NMR spectrum, along with peaks from 15a, that indicated the formation of a new compound (15a-BRs). Single-crystal X-ray crystallographic analysis established the structure of 15a-BRs to be a discrete BRs structure (Figure 3b). In this structure, every p-dichlorobenzene segment from chloranilic acid was found to be stacked with the alkynyl group of a nearby pyridyl ligand. The distance from each chlorine atom to the corresponding alkynyl group is ca. 3.5 Å, which is accordance with the conventional distance of p−π interactions.32 A similar situation was observed in the Cp*Ir-based metallarectangles under the same reaction conditions (Scheme 3). These results demonstrate that the chlorine atoms play a crucial role in forming BRs. In order to further explore the effect of halogen atoms in forming BRs, chloranilic acid was replaced with fluoranilic acid and bromanilic acid, respectively, under identical reaction conditions, resulting in the formation of new dirhodium precursor VI (based on fluoranilic acid) and VII (based on bromanilic acid) (Scheme 3).29 The precursors VI and VII react, in turn, with L4 to provide 16 and 17 at low concentrations. Accordingly, two BRs structures, 16-BRs and 17-BRs, were formed at high concentrations, which were confirmed by X-ray crystallography and DFT binding energy calculations. Though 15a-BRs, 16-BRs, and 17-BRs all display BR structures, the strengths of the inter-ring interactions in the three BRs is different. According to NMR experiments and DFT calculations, the inter-ring interactions of 17-BRs (based on bromanilic acid) were the strongest among the three BRs, followed by 15a-BRs (based on chloranilic acid), with 16-BRs (based on fluoranilic acid) displaying the weakest inter-ring interactions of the three BRs. This implies that the inter-ring interaction in BRs can be adjusted by changing the halogen atoms on the precursor.

Figure 2. Single-crystal X-ray structures of 6a-BRs (a), 3a-BRs (b), 7a-BRs (c), and 4b-BRs (d). The three different colors denote the three rings of the BRs; hydrogen atoms and counteranions are omitted.

of BRs, 3b-BRs, 4b-BRs (Figure 2d), 6b-BRs, and 7b-BRs was achieved, respectively. Even though they are all BRs, their states in different solvents are different. According to NMR spectroscopy, 3b-BRs and 4b-BRs displayed high stability in methanol solutions, and their BRs structure is immune to both dilution and addition of DMSO. However, for 6b-BRs and 7bBRs, their BRs structures change to the corresponding MR structures by adding DMSO. Metallarectangles based on [Cu(opba)]2− are promising catalysts. However, by lengthening the distance between the two Cu(II) ions, BRs are formed, impeding catalysis with larger substrates. Thus, we considered how it might be possible to maintain the distance between the two open Cu(II) centers and achieve an empty MR. We attempted to realize this goal by increasing the width of the linker ligand.28 L5 (L5 = 1,4-bis(4pyridyl)naphthalene) was chosen to construct a metallarectangles (5) with precursor Ia as L5 is the same length as L3, but significantly wider (Scheme 1a). As predicted, the structure of 5 was a discrete MR. A similarly result was observed upon increasing the width of the metallaligand. When the wider metallaligands [Pd(nabo)]2+ and [Cu(nabo)]2+ [nabo = 2, 3-naphthalenebis(oxamato)] were used to construct binuclear precursors IIIa and IIIb, two MR complexes based on L3 were obtained, 12a and 12b (Scheme 1c). Subsequently, 5 and 12a were used as catalysts for the acyl transfer reaction between N-acetylimidazole and (4-(pyridin-4-yl)phenyl)methanol.28 If the pyridyl arm was further lengthened, the BRs structure was again obtained, e.g., 13a-BRs (Scheme 1c). BRs Based on Dihalogenated Ligands

BRs structure can also be constructed using dirhodium or diiridium precursors based on dihalogenated ligands.29 A solution of dirhodium precursor IVa, based on 2,5-dihydroxy1,4-benzoquinone, was treated with L4, leading to exclusive formation of discrete tetranuclear metallarectangle 14a (Figure 3a). Similar to IVa, chloranilic acid has frequently been used as a bridging ligand to construct the dirhodium precursor Va.30,31 The difference between the two precursors Va and IVa, is in their aromatic substituents, being Cl and H, respectively. We combined chloranilic-acid−based precursor Va with L4 under

BRs Based on Naphthazarin and Its Derivatives

Because of their similar coordination mode to the above dihalogenated ligands, naphthazarin and its derivatives are often used as bridging ligands to construct binuclear precursors. These linkers provide a metal···metal distance between binuclear metal clips of ca. 8.3 Å, which is very close to the aforementioned binuclear precursors based on dihalogenated ligands (7.9 Å).33 The precursor VIII based on naphthazarin was thus combined with L4 under the same D


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Accounts of Chemical Research Scheme 3. Synthesis of BRs Based on Dihalogenated Ligands

Reproduced with permission from ref 29. Copyright 2017 Cell Press.

the NMR spectroscopic data of 19-BRs in methanol was obviously different from the results of the reported work. The single-crystal X-ray crystallographic analysis further established that 19-BRs adopts a BRs structure, not a MR structure (Figure 4c). We speculate that 19-BRs may be unstable in nitromethane and therein transform to the corresponding MR (19) similar to the behavior of 18-BRs in DMSO. In 2016, the group of Chi reported BRs (20-BRs and 21BRs) based on the (cymene)Ru fragment, which was constructed from the (cymene)Ru precursor based on 6,11dihydroxy-5,12-naphthacenedione, a derivative of naphthazarin, and longer pyridyl ligands L10 and L11, respectively (Figure 4d and Scheme 4).35 20-BRs presented the lower stability in solution than 18-BRs or 19-BRs, whose BR structure could be destroyed by dilution, nitromethane or guest molecule. The group of Chi later reported BRs based on the Cp*Ir fragment, constructed using the same bridging ligand and pyridyl ligand.36

conditions, leading to formation of a discrete BRs structure, 18-BRs (Figure 4a and b).28 The BRs based on naphthazarin

BRs Based on π-Acceptor Ligands

In chemical and biological processes, the electrostatic interactions between electron-rich (donor, D) and electrondeficient (acceptor, A) aromatics are important driving forces that are used in the self-assembly of structurally and topologically nontrivial structures.37−41 Inspired by this idea, we attempted to use D−A π interactions to construct BRs architectures (Scheme 5a).42 A naphthalenediimide (NDI)based ligand was chosen to prepare a dirhodium precursor as its planar, electron-deficient aromatic surface can engender favorable aromatic D−A π interactions. The reaction of [Cp*RhCl2]2 with DHNDI (DHNDI = 2,7dihydroxybenzo[lmn][3,8] phenanthroline-1,3,6,8(2H,7H)tetraone) produced dirhodium precursor XI, which has two types of structures: the cis form and the trans form (Scheme 5a). Following our usual synthetic strategy, precursor XI was treated with rigid linear pyridyl ligands (L1 and L3) in a 1:1 molar ratio, leading to formation of MR complexes 22 and 23. As a result of random combinations of two conformations of XI and different orientations of the NDI groups, a number of isomers may be formed upon the self-assembly of metallarectangles in the absence of any biasing interactions. However, when the π-electron-rich molecule pyrene, with a similar backbone to the NDI units, was used as a guest

Figure 4. Single-crystal X-ray structures of 18-BRs (a), the metallarectangle of 18-BRs (b) (N, blue; O, red; C, gray; Rh, orange; Cl, green), 19-BRs (c), and 20-BRs (d). The three different colors denote the three rings of the BRs; hydrogen atoms and counteranions are omitted.

present better stability than the BRs based on dihalogenated ligands in solution, and there is no observable equilibrium between BRs and MR in methanol. However, such BRs structure was found to transform to the corresponding MR upon addition of DMSO (Scheme 4). Metallarectangles constructed using naphthazarin and L4 were reported by Kang, Stang, and Chi in 2011, with the difference between the two metallarectangles (18-BRs and 19-BRs) being in the nature of the metal corner: Cp*Rh and (cymene)Ru, respectively.34 On the basis of NMR spectroscopy in nitromethane and ESI-MS, Chi at el. presumed their assynthesized assembly to be a MR. This result confused us, as it seemed unlikely that the BRs structure would be impeded only by changing the metal corner. So, we checked this reaction using the precursor IX based on (cymene)Ru, and found that E


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Accounts of Chemical Research Scheme 4. Synthesis of BRs Based on Naphthazarin (and a Derivative Thereof)

Scheme 5a

a

(a) Synthesis of BRs based on DHNDI; (b) synthesis of BRs based on TPPHZ.

plus double the Rh−N distance (2.1 Å × 2). Thus, the length of the pyridyl arm should be 14.7 Å. Diamide pyridyl ligand L7 was chosen for this purpose (Scheme 5a). As expected, the resulting BRs structure 24-BRs was formed (Figure 5a, 5b and 5c). According to NMR spectroscopy, the BRs structure of 24BRs is transformed to the corresponding MR 24 in low concentration methanol solutions or by addition of DMF. This synthetic strategy based on D−A π interactions was further probed using another electron-deficient bridged ligand, TPPHZ (TPPHZ = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2‴,3‴j]phenazine) (Scheme 5b).43

molecule, the formation of the metallarectangles was observed, leading to discrete quintet or nonet aromatic stacking structures. The binding of electron-rich pyrene within rectangles based on NDI prompted us to explore the synthesis of molecular BRs by employing a bridging ligand with a suitable electron-rich central group.42 We speculated that favorable D−A stacking interactions between NDI and a suitable pyridyl ligand may enable the self-assembly of BRs assemblies. We thus carefully selected bridging ligands of suitable length. The length of the short-arm linker (trans-XI) is ca. 11.9 Å, which is large enough to allow the bridging ligand with a phenyl group to pass through. The ideal NDI-NDI parallel distance (long-arm length) is ca. 11.9 + 3.5 Å (π−π stacking distance) × 2 = ca. 18.9 Å, which corresponds to the length of the bridging ligand

3. APPLICATIONS OF BRs The strength of the inter-ring interactions in the three BRs based on dihalogenated ligands (15a-BRs, 16-BRs, and 17F


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1, p-diiodobenzene can be extracted from Mixture I by an excess of 17-BRs, and tetrahydrofuran (THF) was used to further precipitate 17-BRs and provide p-diiodobenzene in the filtrate. The precipitated 17-BRs can be recycled with a high recovery rate. In Step 2, the p-dibromobenzene in Mixture II is further extracted by 15a-BRs via a similar method and again separated by THF. Like 17-BRs, 15a-BRs can also be recycled. Then, only p-dichlorobenzene was found to remain in Mixture III.

4. GUIDELINES FOR CONSTRUCTION Throughout the work described in the previous sections, the dimensions of the long and short arms of the assemblies have played very important roles in the formation and properties of BRs. By considering all the data thus far obtained, we hope to compile some empirical rules for BRs based on rectangular structure and present constructive suggestions for designing the interlocked rectangles.

Figure 5. Single-crystal X-ray structure of the short (a) and long arms (b) of 24-BRs (N, blue; O, red; C, gray; Rh, orange; H, light blue), 24-BRs (c), and 27-BRs (d). The three different colors denote the three rings of the BRs; hydrogen atoms and counteranions are omitted. Reproduced with permission from ref 42. Copyright 2017 American Chemical Society.

The Formation of BRs

The binuclear precursors and pyridyl ligands used in the reported BRs are compiled in Charts 1 and 2. To form BRs, three criteria are necessary: (1) the short arms need to be long enough to allow the long-arm linker to pass through; (2) the long arms need to be long enough accommodate the entire short arms; (3) the ratio of long arm and short arm should be appropriate for the production of ring−ring interactions.27 We attempt to describe this ratio by empirical formula. For accuracy, the length of the long and short arms is defined as the metal···metal distance, and these components are denoted Al (l = long arm) and As (s = short arm), respectively. In the reported BRs, the π−π (or p−π) stacking is the most common driving force used to construct BRs. The conventional distance of π−π (or p−π) interactions is about 3.5 Å. Hence, under ideal conditions, the length of the long arm minus the length of short arm is equal to the double of the conventional distance of π−π (or p−π) interactions (7 Å) (Figure 8). However, this ideal situation is rarely found in practice. If the length of the long arm minus the length of the short arm is more than 7 Å, the long arm often curves outward to achieve the optimal distance for π−π interactions. Similarly, while the length of the long arm minus the length of the short arm is less than 7 Å, the

BRs) could be adjusted by altering the halogen atom of the precursor. This prompted us to use various p-dihalobenzenes as guests to induce the transformation of BRs to MR (Figure 6).29 NMR spectroscopic experiments showed that pdibromobenzene and p-diiodobenzene induce the transformation of 15a-BRs to 15a, while p-dichlorobenzene does not. In contrast, only p-diiodobenzene was found to force transformation of 17-BRs to its corresponding MR, while pdichlorobenzene and p-dibromobenzene do not. This selective transformation was further confirmed in a mixed solution of H2O and CH3OH (CH3OH/H2O = 10:10), and the MR as a host was found to dramatically improve the solubility of pdihalobenzenes in water. BRs were regenerated after removing p-dihalobenzene using diethyl ether. Based on this work, we took advantage of this selective and reversible interconversion to design a stepwise, ambienttemperature separation method for p-dichlorobenzene, pdibromobenzene, and p-diiodobenzene (Figure 7).29 In Step

Figure 6. Selective interconversion between BRs and MR based on p-dihalobenzenes. Reproduced with permission from ref 29. Copyright 2017 Cell Press. G


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Figure 7. Stepwise separation method for p-dichlorobenzene, p-dibromobenzene, and p-diiodobenzene. Reproduced with permission from ref 29. Copyright (2017) Cell Press.

Chart 1. Metal···Metal Distance of Binuclear Half-Sandwich Molecular Clips Used in BRs

Chart 2. N···N Distance of Pyridyl Ligand Used in BRs

unit can be curved inward or outward. Hence, the length of the long arm is approximately equal to the N···N distance of the pyridyl ligand (Ap, p = pyridyl ligand) plus two metal−N coordination bond lengths. The Rh−N and Ir−N coordination bond lengths are ca. 2.1 Å. Thus, Al could be further described as

long arm has to curve inward (Figure 8). Thus, the relationship between the lengths of the long and short arms can be approximately described as Al − A s ≈ 7 Å For the short arm, the plane of the precursor is relatively rigid and hard to curve. However, the pyridyl ligand long arm H


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with an aspect ratio intermediate in the series, presented stronger stability than the former, with no observable equilibrium between the BRs and the corresponding MR in methanol solution. However, 18-BRs was found to be unstable in DMSO. 4b-BRs had the smallest aspect ratio of the three assemblies and also showed the strongest stability among them, being not only immune to dilution in methanol but also to addition of DMSO. This higher stability in solution may be attributed to the length ratio of the long-arm and short-arm linkers. In order to confirm this, the aspect ratios of the metallarectangles were decreased further. 3b-BRs is constructed from the same precursor unit as 4b-BRs but with the shorter pyridyl ligand L3. Like 4b-BRs, 3b-BRs was also found to be immune to dilution and addition of DMSO; however, DFT calculations showed that the binding energy between the three MRs of 3b-BRs was much lower than that of 4b-BRs. These results further underscore our contention that metallarectangles with smaller aspect ratios could help to increase the stability of the BRs structure. This rule not only applies to BRs based on π−π (or p−π) stacking, but also BRs based on interactions between metal ions and carbonyl groups, as shown by 6b-BRs and 7b-BRs.

Figure 8. Relationship between the aspect ratio of the metallarectangles and the formation of BRs; only two rings of BRs have been displayed for clarity.

Al ≈ A p + (2.1 Å × 2)

Combining the two formulas: A p + (2.1 Å × 2) − A s ≈ 7 Å A p − A s ≈ 2.8 Å

This empirical formula shows the approximate ratio of lengths of the short arm and the pyridyl ligand in BRs based on π−π (or p−π) stacking. Some points must be underlined. First, this empirical formula is only for the BRs based on π−π (or p−π) stacking. Second, this ratio is just an approximate value: its range depends on the flexibility of the pyridyl ligand. Third, this does not mean BRs can only be formed following this empirical formula, as there is another possibility: ring-in-ring structures. Ring-in-ring structures can be considered as substructures of BRs, whereby one ring is removed from BRs. The first (and thus far only) reported ring-in-ring structure based on coordination self-assembly is from Stang and Chi, and is constructed by combining precursor X with L4.44 We speculate that the formation of this ring-in-ring structure is related to the width of precursor, as shortening only the width of the precursor led to BRs (19-BRs). However, ring-in-ring structures are so rare that this class of assembly cannot be discussed in detail.

5. OUTLOOK Here we will use a simple example to display how we use these guidelines to design new BRs and other interlocked structures. L12 was chosen as a pyridyl ligand to construct BRs because of its narrow width and electron-rich central unit. The N···N distance of L12 is ca. 12.4 Å, thus, according to the above rules, precursor VIII was selected as the binuclear clip in view of its suitable length (8.3 Å). As expected, BRs 29-BRs was obtained by reaction of precursor VIII and L12, which was confirmed by X-ray crystallography (Scheme 6). Overall, if we want to construct BRs, we should follow these guidelines, however, if we want to produce other interlocked structures, we should break these rules. As mentioned before, construction of metallarectangles with smaller aspect ratios is adverse to the formation of BRs, thus we attempted to deliberately decrease the aspect ratio to avoid the formation of BRs. Thereby, we keep the long-arm linker, but shorten the short arm by using IVa instead of VIII. After reaction under the same conditions, single-crystal X-ray crystallographic analysis confirmed the product 29-catenane to have a [2]catenane structure (Scheme 6). Encouraged by these results, we then tried to greatly lengthen the pyridyl ligands to realize a decrease of the aspect ratio, leading to L13 (L13 = 2,5-bis(pyridin-4-ylethynyl)thieno-

The Stability of BRs

In addition to their formation, the aspect ratio of the metallarectangles also affected the stability of the BRs.28 To test this, we used three BRs based on π−π (or p−π) stacking as a comparison, 15a-BRs, 18-BRs, and 4b-BRs, because they are constructed from the same pyridyl ligand (L4) but short-arm linkers of different length (Figure 9). Of these, 15a-BRs had the largest aspect ratio and showed modest stability and yield in methanol solution, which was reduced by dilution. 18-BRs,

Figure 9. Relationship between aspect ratio of metallarectangles and the stability of BRs. I


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Accounts of Chemical Research Scheme 6. Synthesis of BRs and Other Interlocked Structures

Biographies

[3,2-b]thiophene). By reacting with precursor IVa, the obtained complex 30-catenane presented another interesting trimeric structure, a [3]catenane, which was also confirmed by X-ray crystallography (Scheme 6).

Ye Lu was born in 1985 in Shanghai, China. He obtained his Ph.D. in chemistry from Tongji University in 2015. After two years of postdoctoral research at Fudan University with Professor Guo-Xin Jin, he joined the Department of Chemistry of Fudan University as Assistant Research Fellow. His research interests are primarily in the area of molecular architectures via self-assembly.

6. CONCLUDING REMARKS In this Account, we have described our recent work on the rational design of organometallic assemblies and on the development of efficient methods to construct BRs structures. Furthermore, we have also emphasized the potential application of BRs in selective chemoseparations. By appraisal of the overall results of this research, the relationship between the aspect ratio of metallarectangles and the formation or stability of BRs was summarized in useful empirical guidelines. We further demonstrated how to use these guidelines to design new BRs and other interlocked structures. We hope that our findings will help advance the field of organometallic assemblies as follows: (i) By choosing suitable bridging ligands and pyridyl ligands, a series of discrete BRs structures based on a range of different interactions could be developed. (ii) The establishment of a stepwise chemoseparation method based on BRs will offer interesting possibilities for the application of interlocked structures. (iii) The review and summary of reported BRs is not only important in and of itself, more importantly, the real goal is to provide helpful guidelines for the design of more complicated, attractive and useful interlocked structures. We believe our work summarized herein is just a beginning.

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Hai-Ning Zhang obtained his B.S. from Anhui Normal University in 2016 and is currently a Ph.D. candidate at the Department of Chemistry of Fudan University, supervised by Prof. Guo-Xin Jin. Guo-Xin Jin received his Ph.D. from Nanjing University in 1987. After postdoctoral work as an Alexander von Humboldt Fellow at the University of Bayreuth, Germany, he joined Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, in 1996 as a professor. In 2001, he moved to Shanghai and has since held the position of Chair Professor (Cheung Kong Scholarship) of Inorganic Chemistry at Fudan University. His research interests are in organometallic chemistry, particularly in organometallic macrocyclic architecture and catalysts for olefin polymerization.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (21531002, 21720102004) and the Shanghai Science Technology Committee (13JC1400600). G.X.J. thanks the Alexander von Humboldt Foundation for a Humboldt Research Award.

AUTHOR INFORMATION

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Corresponding Author

*E-mail: [email protected]. ORCID

Guo-Xin Jin: 0000-0002-7149-5413 Notes

The authors declare no competing financial interest. J


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Molecular Borromean Rings Based on Half-Sandwich Organometallic

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