Molecular Borromean Rings Research Articles

Coordination-driven self-assembly of a molecular 818 knot and molecular borromean rings

Coordination-driven self-assembly of a molecular 818 knot and molecular borromean rings

Selective Construction of Borromean Rings andTweezer‐LikeMolecular Assembly Featuring Cp*Rh/Ir Clips forNear‐InfraredPhotothermal Conversion†

Comprehensive SummaryMaking full use of coordination‐driven self‐assembly strategy, we herein described the selective synthesis of a molecular Borromean rings and two cases of “U”‐shaped tweezer‐like molecular assemblies in high yield by using bipyridyl ligands based on biphenyl unit and half‐sandwich binuclear rhodium(III)/iridium(III) building blocks. The selective synthesis was realized by adjusting the length of dipyridyl arms. The utilization of curved U‐shaped bipyridyl ligandL1led to tweezer‐like molecular assemblies. Subsequently, olefinic bonds were introduced to elongate dipyridyl arms obtaining ligandL2. The ligandL2has two stable conformations, U‐shape and Z‐shape, which facilitated the formation of different topologies including the tetranuclear macrocycle and Borromean rings with different building blocks in this work. These structures in solid and solution all have been further confirmed by single‐crystal X‐ray diffraction, NMR analysis, and mass spectrometry. In addition, as an important driving force, π‐π stacking interactions not only played a significant role in the stability of structures but also further triggered photothermal conversion in solution. The experimental results demonstrated that compounds1aand2had good NIR photothermal conversion efficiency (11.83% and 17.76%), and further analysis found the photothermal conversion efficiency had a gradual increase in the trend with the π‐π stacking interactions increasing. This research expands the application of topological structures in materials science and provides a new idea for the synthesis of novel photothermal conversion materials.

Selective Construction of Molecular Borromean Rings and [2]Catenane Utilizing Ether Bipyridyl Ligands.

A series of Cp* Rh-based discrete architectures was constructed by selecting four ether bipyridyl ligands and three half-sandwich rhodium(III) bimetallic construction units, respectively. This study demonstrates a strategy for making the transition from a binuclear D-shaped ring to a tetranuclear [2]catenane by adjusting the length of bipyridyl ligands. In addition, changing the position on the naphthyl group of the bipyridyl ligand from 2,6- to 1,5-position substitution can realize the selective synthesis of [2]catenane and Borromean rings under similar conditions. The above-mentioned constructions have been determined via X-ray crystallographic analysis, detailed NMR techniques, electrospray ionization-time-of-flight/mass spectrometry analysis, and elemental analysis.

Cation-Templated Assembly of 613 and 623 Metalla-Links

Facilitated by multiple stacking interactions between components, two kinds of metalla-links containing molecular Borromean rings (623 links) and head-to-tail cyclic [3]catenanes (613 links), as isomers, were constructed in high yield by introducing tri-μ-methoxyl-dinuclear complexes [(Cp*M)2(μ-OCH3)3][OTf] (M = RhIII or IrIII, Cp* = η5-pentamethylcyclopentadienyl, OTf = triflate) as unusual cationic guests during coordination-driven assembly. The topology of these intricate structures was controlled by strategically selecting two dipyridyl ligands that differ in their coordination orientations, as evidenced by X-ray crystallography and electrospray ionization-time-of-flight/mass spectrometry analysis. The behavior of the abovementioned metalla-links in solution was monitored and further studied by the detailed NMR techniques.

Adaptive Self‐Assembly and Induced‐Fit Interconversions between Molecular Borromean Rings, Russian Dolls and Ring‐in‐Ring Complexes†

Main observation and conclusionHerein, we report an artificially assembled interlocked system that features adaptive self‐assembly and induced‐fit transformation properties in the presence of appropriate guest molecules. By careful choice of solvent and guests, a molecular Borromean rings complex based on organometallic half‐sandwich units could be transformed in a stepwise manner to a ring‐in‐ring complex and a monomeric rectangle, leading to a conversion cycle between these structures. This adaptive self‐assembly method was further applied to the construction of a Russian dolls complex, forming another conversion cycle composed of a molecular Borromean rings complex, a Russian dolls complex and a monomeric rectangle. A comprehensive map showing all of the structural transformations between these metallarectangles and interlocked species is drawn up, representing a recoverable and recyclable transformation mechanism.

Open versus Interpenetrated: Switchable Supramolecular Trajectories in Mechanosynthesis of a Halogen-Bonded Borromean Network

Summary Precise control over topologically intricate molecular architectures remains an open challenge for chemists due to their inherent structural complexity. We report a simple solvent-free strategy to selectively prepare two multi-component supramolecular crystalline systems based on the halogen bond, endowed with either an unknot topology or a Borromean-type entanglement. Real-time in situ monitoring of the solvent-free mechanochemical synthesis of these three-component halogen-bonded ionic co-crystals reveals that the choice of milling conditions leads to a switch in the supramolecular reaction trajectory, resulting in the selective formation of an open halogen-bonded network or a halogen-bonded Borromean-type assembly.

Self-assembly of metalla[3]catenanes, Borromean rings and ring-in-ring complexes using a simple π-donor unit.

Despite extensive research and several stunning breakthroughs in the synthesis of interlocked molecular species, [3]catenanes, Borromean rings and ring-in-ring complexes are exceedingly rare and their targeted synthesis remains a formidable challenge. Herein, a series of Cp*Rh-based homogeneous and heterogeneous interlocked structures have been prepared by coordination-driven self-assembly, not only including metalla[2]catenanes and molecular Borromean rings, but also linear metalla[3]catenanes and ring-in-ring complexes. The interlocked structures are all based on bithiophenyl groups. The bithiophenyl groups effectively enhance the strength of the inter-ring interactions and play a crucial role in the formation of these interlocked structures. By taking advantage of the strong interaction between π-donor (D) and π-acceptor (A) groups, the electron-deficient methylviologen cation was introduced into a cationic metallarectangle based on bithiophenyl groups. Taking inspiration from these results, a cationic metallarectangle based on A units was threaded into a metallarectangle based on D units, leading to a heterogeneous D–A ring-in-ring structure.

Towards the Stepwise Assembly of Molecular Borromean Rings. A Donor-Acceptor Ring-in-Ring Complex

The assembly of molecular Borromean Rings from constitutionally independent rings in a stepwise manner depends on the preparation of robust “ring-in-ring” complexes. The π-electron rich macrocycle bis-1,5-dinaphtho[50]crown-14 (1) is shown to form a donor-acceptor ring-in-ring complex with the π-electron poor cyclophane cyclobis(paraquat-4,4'-biphenylene) (24+) in solution. In the crystal superstructure of [1⊂2]·4PF6, CH···O interactions between the polyether loops of 1 and the bipyridinium units of 24+ could disfavor the threading of dialkylammonium components of a third ring.
 

Molecular Borromean Rings Based on Half-Sandwich Organometallic Rectangles.

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.

Cover Picture: Highly Stable Molecular Borromean Rings (Chin. J. Chem. 2/2018)

The cover picture shows a series of Cp*Rh‐based molecular Borromean rings (BRs), 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. Interestingly, the structure of BRs is very similar to Kong Ming Lock (孔明锁), a kind of Chinese traditional six‐piece burr puzzle. In this work, some of as‐synthesized BRs display high stability in solution. The reason is related to the length ratio of the long‐arm linker and short‐arm linker, where smaller aspect ratios of the metallarectangles promote improved stability of the BRs in solution. Increasing the width of the metallaligand or pyridyl ligand hinders the formation of BRs and leads to unoccupied monomeric rectangles, which were further used as catalysts. More details are discussed in the article by Jin et al. on page 106–111. image

Stacking-interaction-induced host-guest chemistry and Borromean rings based on a polypyridyl ligand.

Template-free molecular Borromean rings (BRs) and open-ended molecular capsules were realized by strategic selection of the polypyridyl ligand tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j] phenazine (TPPHZ) with a large surface area and specific geometry. The topology of these intricate architectures was constructed via precisely controlled stacking interactions, as evidenced from single-crystal X-ray analysis.

Highly Stable Molecular Borromean Rings

A series of Cp*Rh‐based molecular Borromean rings (BRs) are prepared from naphthazarine or metallaligand. Some of as‐synthesized BRs display high stability and are formed in high yields in solution. The reason is related to the length ratio of the long‐arm linker and short‐arm linker, where smaller aspect ratios of the metallarectangles promote improved stability and yields of the BRs in solution. Increasing the width of the metallaligand or pyridyl ligand hinders the formation of BRs and leads to unoccupied monomeric rectangles, which were further used as catalysts for the acyl transfer reaction between N‐acetylimidazole and (4‐(pyridin‐4‐yl)phenyl) methanol.

Molecular Borromean Rings Based on Dihalogenated Ligands

Summary A series of Cp*Rh- and Cp*Ir-based molecular Borromean rings (BRs) have been prepared with dihalogenated ligands (fluoranilic acid, chloranilic acid, and bromianilic acid) and 4,4′-bipyridylacetylene. The halogen atoms play a crucial role in forming BRs. The strength of the interaction among the rings for different BRs is not the same and can be adjusted by changes in the halogen atoms on the precursor. We took advantage of this dissimilarity in the interactions to realize selective reversible conversion between BRs and a monomeric rectangle by the use of different p -dihalobenzenes. Subsequently, a stepwise separation method for p -dichlorobenzene, p -dibromobenzene, and p -diiodobenzene at ambient temperature was developed. The BRs used in the separation are recoverable and recyclable.

Molecular Borromean Rings: From Controlled Construction to Potential Applications

In this issue of Chem , Jin and coworkers have reported the efficient synthesis of a series of Borromean rings (BRs), through which they achieved template-free BR formation with high yields in a highly controllable manner. They further show that these BRs can be utilized to separate halogenated aromatic compounds efficiently.

Stacking Interactions Induced Selective Conformation of Discrete Aromatic Arrays and Borromean Rings.

Herein, we describe how to utilize stacking interactions to achieve selective supramolecular transformation and molecular Borromean rings (BRs). By using a dinuclear naphthalenediimide (NDI)-based Cp*Rh acceptor and linear bipyridyl ligands, organometallic rectangles featuring dynamic behavior have been constructed. Unique discrete aromatic stacking arrays were formed by inducing pyrene units as guest molecules. The topology of the BRs was realized by the use of a strategically chosen ligand which was capable of participating in D-A interactions and hydrogen bonding, as evidenced from single-crystal X-ray analysis and computational studies. These self-assembly processes underline the advantages of dynamic bonding and π-π stacking interactions, and serve to illustrate a new approach to generating structurally and topologically nontrivial supramolecular architectures.

Selective synthesis of iridium(iii)-derived molecular Borromean rings, [2]catenane and ring-in-ring macrocycles via coordination-driven self-assembly.

This paper reports the formation of unprecedented iridium(iii)-derived topological macrocycles. Discrete molecular Borromean rings 1 and 3 in pure form are synthesized via coordination-driven self-assembly of an acceptor [(Cp*Ir)2(OO∩OO)](OTf)2 (Cp* = pentamethylcyclopentadienyl, OO∩OO = 6,11-dioxido-5,12-naphthacenedione) (A) with dipyridyl donors 1,4-bis(4-pyridinylethynyl)benzene (L1) and 2,5-bis(4-pyridinylethynyl)thiophene (L2) respectively in methanol. Self-assembly using the same acceptor under similar conditions with two other donors 9,10-bis(4-pyridinylethynyl)anthracene (L3) and 1,4-di(4-pyridinylethynyl)buta-1,3-diyne (L4) resulted in [2]catenane 5 and non-catenane ring-in-ring topological macrocycle 7 respectively. Rectangular macrocycles 2, 4, 6, and 8 were respectively obtained when the self-assembly of acceptor A with one of the donors L1-L4 was carried out under dilute conditions in nitromethane or methanol. All these new macrocycles were characterized by 1H and 13C NMR, 2D NMR, ESI-MS and elemental analyses. Single crystal X-ray structures of Borromean rings 1 and 3, and ring-in-ring macrocycle 7 revealed that the length and functionality of donors enabling CHπ and ππ interactions govern the topology.

Selective Synthesis of Molecular Borromean Rings: Engineering of Supramolecular Topology via Coordination-Driven Self-Assembly.

Molecular Borromean rings (BRs) is one of the rare topology among interlocked molecules. Template-free synthesis of BRs via coordination-driven self-assembly of tetracene-based Ru(II) acceptor and ditopic pyridyl donors is reported. NMR and single-crystal XRD analysis observed sequential transformation of a fully characterized monomeric rectangle to molecular BRs and vice versa. Crystal structure of BRs revealed that the particular topology was enforced by the appropriate geometry of the metallacycle and multiple parallel-displaced π-π interactions between the donor and tetracene moiety of the acceptor. Computational studies based on density functional theory also supported the formation of BRs through dispersive intermolecular interactions in solution.

Tapestry Woven From Organic Threads

In an extension of chemical domesticity, researchers have shown that organic chemistry is good not only for cooking but for weaving as well. Several types of supramolecular structures—catenanes, rotaxanes, molecular knots, and molecular Borromean rings—have interwoven two- and three-dimensional parts. Now, Osamu Terasaki of Stockholm University; Omar M. Yaghi of the University of California, Berkeley; and coworkers have designed and constructed the first structures in which 1-D molecular “threads” are interwoven, forming a crystalline covalent organic framework (Science 2016, DOI: 10.1126/science.aad4011). The researchers first synthesized a copper(I) bisphenanthroline complex in which the two organic groups adopt an interlaced inverted-U orientation. They then added benzidines to the ends of the organic groups and linked the benzidines by forming imine bonds, creating a woven material they call COF-505. The copper ions can be removed and reinserted reversibly. The demetalation increases the material’s el...

Self‐Assembly of Molecular Borromean Rings from Bimetallic Coordination Rectangles

AbstractIn contrast to conventional stepwise synthesis of molecular Borromean rings, a self‐assembly synthetic method which proceeds without the aid of a template has been developed. In the formation of molecular rectangles, by adjustment of the long‐arm length of the rectangles, a series of size‐dependent Borromean‐link frameworks were constructed. Both the shortest length of two arms and the relative proportion of the length of the long arm to that of the short arm play a key role in the formation of Borromean rings. DFT calculations were used to provide theoretical support for the formation of discrete interlocked frameworks.

Self-assembly of molecular Borromean rings from bimetallic coordination rectangles.

In contrast to conventional stepwise synthesis of molecular Borromean rings, a self-assembly synthetic method which proceeds without the aid of a template has been developed. In the formation of molecular rectangles, by adjustment of the long-arm length of the rectangles, a series of size-dependent Borromean-link frameworks were constructed. Both the shortest length of two arms and the relative proportion of the length of the long arm to that of the short arm play a key role in the formation of Borromean rings. DFT calculations were used to provide theoretical support for the formation of discrete interlocked frameworks.