Synthetic Strategies for the Construction of Threaded and Interlocked Molecules

A Two-Stroke, Two-Cylinder Piston Rotaxane Motor

We introduce a model for a rotaxane motor. This uses two major themes of rotaxane physics, that is, the ability to change the potential of different regions of the axle (switching) and the entropy associated with free rings. The motor is based on free rings being trapped and released by potential stripes or stations. We calculate the power produced by such a motor, which is in the femtoWatt range. The power is proportional to the square of the number of rings.

Isotropic and nematic liquid crystalline phases of adaptive rotaxanes

We describe the thermodynamics of a solution of rotaxanes which can change their length from a short state of length L to a long state of length qL in response to their surrounding environment. We call these rotaxanes "adaptive." We show that such a system can exhibit both isotropic and nematic liquid crystalline phases. The system shows several interesting kinds of behaviour. First we predict that the fraction of short-length rotaxanes increases linearly with concentration and is a maximum at the critical concentration that marks the isotropic to nematic transition. Second, the critical concentration shows a minimum at a certain value of q. Our model suggests that the effect of adaptive length changes is most dramatic at small q and where the long state is slightly favoured.

Threading a Ring or Tube onto a Rod: An Entropically Rare Event

We calculate the entropy lost when a circular ring or a circular cylinder or tube is threaded onto a long rod in terms of the geometrical parameters, namely rod length and radius, the threaded ring radius, or the radius and length of a threaded tube. These formulas, constructed from a partition function, allow calculation of the fraction of rings/tubes threaded spontaneously onto rods. In all cases of practical interest, this fraction is very small and can be well represented by simple power laws depending strongly upon the geometrical parameters.

Conformation driven in situ interlock: from discrete metallocycles to infinite polycatenanes

A novel conformation driven self-assembly system, where four metallocycles with different conformations have been in situ self-assembled, has been reported. Interestingly, only square metallocycles can further interlock into polycatenanes. However, rectangular and rhombus metallocycles fail to overcome such an entropically unfavourable process, which constitutes an obstacle to the formation of polycatenanes.

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Catenanes from catenanes: quantitative assessment of cooperativity in dynamic combinatorial catenation

A series of dynamic combinatorial [2] and [3]catenanes have been prepared. Formation of the [3]catenanes occurs with positive or negative cooperativity, depending on the cyclodextrin homologue. Systems level analysis allows cooperativity to be quantified and MD simulations reveal that cooperativity derives from the extents to which hydrophobic surface area is exposed to the aqueous surroundings.

Half-rotation in a kinetically locked [2]catenane induced by transition metal ion substitution

We report on a heterocircuit [2]catenane in which a reversible half-rotation of one ring about the other can be induced, and locked in place, by switching the coordination of the interlocked rings between Pd(II) and Co(III).

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.

Molecular tweezers: concepts and applications

Taken to the molecular level, the concept of "tweezers" opens a rich and fascinating field at the convergence of molecular recognition, biomimetic chemistry and nanomachines. Composed of a spacer bridging two interaction sites, the behaviour of molecular tweezers is strongly influenced by the flexibility of their spacer. Operating through an "induced-fit" recognition mechanism, flexible molecular tweezers select the conformation(s) most appropriate for substrate binding. Their adaptability allows them to be used in a variety of binding modes and they have found applications in chirality signalling. Rigid spacers, on the contrary, display a limited number of binding states, which lead to selective and strong substrate binding following a "lock and key" model. Exquisite selectivity may be expressed with substrates as varied as C(60) , nanotubes and natural cofactors, and applications to molecular electronics and enzyme inhibition are emerging. At the crossroad between flexible and rigid spacers, stimulus-responsive molecular tweezers controlled by ionic, redox or light triggers belong to the realm of molecular machines, and, applied to molecular tweezing, open doors to the selective binding, transport and release of their cargo. Applications to controlled drug delivery are already appearing. The past 30 years have seen the birth of molecular tweezers; the next many years to come will surely see them blooming in exciting applications.

Diels-Alder active-template synthesis of rotaxanes and metal-ion-switchable molecular shuttles

A synthesis of [2]rotaxanes in which Zn(II) or Cu(II) Lewis acids catalyze a Diels-Alder cycloaddition to form the axle while simultaneously acting as the template for the assembly of the interlocked molecules is described. Coordination of the Lewis acid to a multidentate endotopic 2,6-di(methyleneoxymethyl)pyridyl- or bipyridine-containing macrocycle orients a chelated dienophile through the macrocycle cavity. Lewis acid activation of the double bond causes it to react with an incoming "stoppered" diene, affording the [2]rotaxane in up to 91% yield. Unusually for an active-template synthesis, the metal binding site "lives on" in these rotaxanes. This was exploited in the synthesis of a molecular shuttle containing two different ligating sites in which the position of the macrocycle could be switched by complexation with metal ions [Zn(II) and Pd(II)] with different preferred coordination geometries.