Studies Toward the Synthesis of Phenylacetylene Macrocycle Based Rotaxane Precursors as Building Blocks for Organic Nanotubes

From π-Conjugated Rods to Shape-Persistent Rings, Wheels, and Ladders: The Question of Rigidity

ConspectusRigid-rod oligomers and polymers are mostly based on (hetero)aromatic rings connected with each other, either directly or via ethynylene or butadiynylene linkers, or by a combination of both structural elements. Although they are much more rigid than vinyl polymers, they exhibit considerable structural flexibility, often more than would be expected merely from their chemical structure. This disparity holds for both linear as well as for cyclic structures. The flexibility of rigid-rod polymers, which is directly observable for defined oligomers of different lengths at the solid-liquid interface by means of scanning-tunneling microscopy, also impacts their optical and electronic properties. The flexibility can be used, for example, to control whether an oligomer with two different fluorescent end-groups emits from either the one or the other. The flexibility of shape-persistent macrocycles also has an impact on the overall thermal stability of mechanically interlocked molecular architectures. However, the degree of flexibility can be reduced when rigid struts are covalently mounted into the inside of the rings, leading to the formation of so-called molecular spoked wheels. The combination of these two elements─rings and rods─stiffens both of them: the ring perimeter is prevented from collapsing and the internal rods from bending. These compounds have been further developed as platform molecules, where three spokes stiffen the ring and together form a tripod-like platform, while a fourth arm points─after adsorption to a solid substrate─above the plane of the molecule. This pillar makes it possible to decouple a functional group at the end of the arm from the surface. Rigidity enhancement by the introduction of rigid spacer elements can also be applied to the case of rigid-rod polymers and is visualized by sophisticated molecular dynamics simulations. In this case, formation of single-stranded oligomers and polymers, and a subsequent zipping reaction to form ladder-like structures, directly allows, by means of single-molecule fluorescence spectroscopy, a comparison of the single- and double-stranded molecules. In particular in the case of the polymers, which can be up to 100 nm in length, the enhancement of rigidity is quite remarkable. Overall, the covalent connection of two or more rigid molecular entities has a self-reinforcing effect: all parts of the molecule gain rigidity. Since overall synthetic yields for such complex high-molecular weight covalently bound shape-persistent structures can still be low, scanning tunneling microscopy and single-molecule fluorescence spectroscopy are the methods of choice for structural analyses. Preliminary results illustrate how these compounds can serve as versatile sources of deterministic single photons on demand, since rigidity also enhances the intramolecular flow of excitation energy, and suggest a range of applications in optoelectronic devices.

Modular Synthesis of Improbable Rotaxanes with All-Benzene Scaffolds

"Improbable" rotaxanes consisting of interlocked conjugated components represent non-trivial synthetic targets, not to mention those with all-benzene scaffolds. Herein, a modular synthetic strategy has been established using an isolable azo-linked pre-rotaxane as the core module, in which the azo group functions as a tracelessly removable template to direct mechanical bond formations. Through versatile connections of the pre-rotaxane and other customizable modules, [2]- and [3]rotaxanes derived from all-benzene scaffolds have been accomplished, demonstrating the utility and potential of the synthetic design for all-benzene interlocked supramolecules.

Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of AgI -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state 13 C NMR of PdII -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution 1 H NMR studies indicate complete and immediate macrocycle formation upon the introduction of PdII to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the 1 H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

Covalently Templated Syntheses of Mechanically Interlocked Molecules

Mechanically interlocked molecules (MiMs), such as catenanes and rotaxanes, exhibit unique properties due to the mechanical bond which unites their components. The translational and rotational freedom present in these compounds may be harnessed to create stimuli-responsive MiMs, which find potential application as artificial molecular machines. Mechanically interlocked structures such as lasso peptides have also been found in nature, making MiMs promising albeit elusive targets for drug discovery. Although the first syntheses of MiMs were based on covalent strategies, approaches based on non-covalent interactions rose to prominence thereafter and have remained dominant. Non-covalent strategies are generally short and efficient, but do require particular structural motifs which are difficult to alter. In a covalent approach, MiMs can be more easily modified while the components may have increased rotational and translational freedom. Both approaches have complementary merits and combining the unmatched efficiency of non-covalent approaches with the scope of covalent syntheses may open up vast opportunities. In this review, recent covalently templated syntheses of MiMs are discussed to show their complementarity and anticipate future developments in this field.

1 Introduction

2 Tetrahedral Templates

2.1 A Carbonate Template for Non-Rusty Catenanes

2.2 All-Benzene Catenanes on a Silicon Template

2.3 Backfolding from Quaternary Carbon

3 Planar Templates

3.1 Rotaxanes Constructed in a Ring

3.2 Hydrindacene as a Dynamic Covalent Template

3.3 Templating on Tri- and Tetrasubstituted Benzenes

4 Conclusion

A Nanosized Phenylene-Ethynylene-Butadiynylene [2]Catenane

In a convergent, template-directed synthesis, an efficient route to a phenylene-ethynylene-butadiynylene based [2]catenane is described. The key step is performed by the aminolysis of the corresponding precatenane, which is obtained by a sequence of metal-catalyzed cross-coupling and desilylation reactions. The cyclization reaction leads besides the [2]precatenane to a variety of larger precatenanes and offers an attractive approach to mechanically interlocked structures of different size.

Dynamic or undynamic chirality generated by helical arrangement of a shape-persistent ring and rod doubly bridged in a molecule

Dynamic or undynamic chirality is generated in either threaded or unthreaded form of an assembly of phenylene–ethynylene-based ring and rod.

Synthesis of spiro quasi[1]catenanes and quasi[1]rotaxanes via a templated backfolding strategy

Due to their well-defined three-dimensional geometry, spiro compounds are widely utilized in drug research. From the central tetrahedral carbon atom, besides the regular structure, an inverted spiro connectivity may be envisioned. Here we disclose the synthesis of this molecule class that we have coined quasi[1]catenanes. Next to their fascinating and aesthetic shape, the higher compactness as compared to regular spiro bicycles is noteworthy. To enable synthetic access to compact entangled multimacrocyclic molecules, we have developed a new strategy. The key element is a template, which is covalently connected to the linear precursors, and spatially directs the sterically congested backfolding macrocyclizations that are required to give quasi[1]catenanes. Similarly, quasi[1]rotaxanes are made.

Spiro compounds contain two or more rings linked together through one common atom. Here the authors provide a method to backfold both rings, producing spiro quasi[1]catenanes, via a strategy of temporarily linking the linear intermediates with covalent bonds.

A Short Covalent Synthesis of an All-Carbon-Ring [2]Rotaxane

While the current supramolecular syntheses of [2]rotaxanes are generally efficient, the final product always retains the functional groups required for non-covalent preorganization. A short and high-yielding covalent-template-assisted approach is reported for the synthesis of a [2]rotaxane. A terephthalic acid template core preorganizes the covalently connected ring precursor fragments to induce a clipping-type cyclization over the thread moiety. Cleavage of the temporary ester bonds that connect the ring and thread fragments liberates the [2]rotaxane.

A stable open-shell redox active ditopic ligand

Herein we describe the synthesis, structure and electronic properties of an unusual redox-active ditopic ligand with a stable open-shell configuration. This stable phenoxyl radical features intense and very low energy electronic transitions in the near infrared (NIR) part of the spectrum and is structurally set up to strongly spin couple coordinated transition metal ions in [2 × 2] grid-type structures.

Synthesis and Dynamics of Nanosized Phenylene-Ethynylene-Butadiynylene Rotaxanes and the Role of Shape Persistence

Phenylacetylene‐based [2]rotaxanes were synthesized by a covalent‐template approach by aminolysis of the corresponding prerotaxanes. The wheel and the bulky stoppers are made of phenylene–ethynylene–butadiynylene macrocycles of the same size. The stoppers are large enough to enable the synthesis and purification of the rotaxane. However, the wheel unthreads from the axle at elevated temperatures. The deslipping kinetics and the activation parameters were determined. We described theoretically the unthreading by state‐of‐the‐art DFT‐based molecular‐mechanics models and a string method for the simulation of rare events. This approach enabled us to characterize in detail the unthreading mechanism, which involves the folding of the stopper during its passage through the wheel opening, a process that defies intuitive geometrical considerations. The conformational and energetic features of the transition allowed us to infer the molecular residues controlling the disassembly timescale.

A heterotrimetallic Ir(iii), Au(iii) and Pt(ii) complex incorporating cyclometallating bi- and tridentate ligands: simultaneous emission from different luminescent metal centres leads to broad-band light emission

Di- and tri-nuclear metal complexes incorporating Au(iii), Ir(iii) and Pt(ii) units linked via a 1,3,5-triethynylbenzene core.