Crown ether–tert-ammonium salt complex fixed as rotaxane and its derivation to nonionic rotaxane

Molecular entanglement can strongly increase basicity

Brønsted basicity is a fundamental chemical property featured by several kinds of inorganic and organic compounds. In this Review, we treat a particularly high basicity resulting from the mechanical entanglement involving two or more molecular subunits in catenanes and rotaxanes. Such entanglement allows a number of basic sites to be in close proximity with each other, highly increasing the proton affinity in comparison with the corresponding, non-entangled counterparts up to obtain superbases, properly defined as mechanically interlocked superbases. In the following pages, the development of this kind of superbases will be described with a historical perusal, starting from the initial, serendipitous findings up to the most recent reports where the strong basic property of entangled molecular units is the object of a rational design.

Controlling molecular shuttling in a rotaxane with weak ring recognition sites

We describe a rotaxane molecular shuttle encompassing triazolium and tertiary ammonium units as weak recognition sites for the ring. Such a design, which differs from that of typical controllable rotaxanes, allows the precise tuning of the ring distribution among the two sites - i.e., the coconformational equilibrium - by changing the solvent polarity or the nature of the counteranions. Shuttling of the ring between the two stations can also be toggled by acid-base stimuli. Such an approach is paradigmatic to obtain rotaxanes that can sense environmental changes and transduce them into a coconformational response and opens avenues for novel applications in sensing and stimuli-responsive materials.

Rotaxanes with dynamic mechanical chirality: Systematic studies on synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion

Dynamic mechanical chirality of [2]rotaxane consisting of a C s symmetric wheel and a C 2v symmetric axle is discussed via the synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion. This [2]rotaxane is achiral and/or prochiral when its wheel locates at the center of the axle, but becomes chiral when the wheel moves from the center of the axle. These were proved by the experiments on the enantiomer separation and racemization. The racemization energy of the isolated single enantiomers was controlled by the bulkiness of the central substituents on the axle. Furthermore, the chiral-prochiral interconversion was achieved by relative positional control of the components. The present systematic studies will provide new insight into mechanically chiral interlocked compounds as well as the utility as dynamic chiral sources.

Ferrocene-Containing Pseudorotaxanes in Crystals: Aromatic Interactions with Hammett Correlation

Single crystals of pseudorotaxanes, [(FcCH₂NH₂CH₂Ar)(DB24C8)][PF₆] (DB24C8 = dibenzo[24]crown-8, Fc = Fe(C₅H₄)(C₅H₅), Ar = -C₆H₃-3,4-Cl₂, -C₆H₃-3,4-F₂, -C₆H₄-4-F, -C₆H₄-4-Cl, -C₆H₄-4-Br, -C₆H₃-3-F-4-Me, -C₆H₄-4-I) and [(FcCH₂NH₂CH₂C₆H₄-4-Me)(DB24C8)][Ni(dmit)₂] (dmit = 1,3-dithiole-2,4,5-dithiolate), were obtained from solutions containing DB24C8 and ferrocenylmethyl(arylmethyl)ammonium. X-ray crystallographic analyses of the pseudorotaxanes revealed that the aryl ring of the axle moiety and the catechol ring of the macrocyclic component were at close centroid distances and parallel or tilted orientation. The structures with parallel aromatic rings showed correlation of the distances between the centroids to Hammett substituent constants of the aryl groups.

Reversible cyclic-linear topological transformation using a long-range rotaxane switch

A reversible linear-cyclic topological transformation of polymers facilitated by a long-range rotaxane switch.

Template-Free Synthesis of a Phenanthroline-Containing [2]Rotaxane: A Reversible pH-Controllable Molecular Switch

The synthesis of symmetric and asymmetric rotaxanes consisting of neutral axle and ring components without ionic templates is necessary for applications in molecular sensors and molecular switches. A phenanthroline-containing symmetric [2]rotaxane was newly synthesized by inducing hydrogen bonding and π-interaction using a template-free threading-followed-by-stoppering method. The obtained rotaxane serves as a reversible pH-controllable molecular switch.

Isoselective Lactide Ring Opening Polymerisation using [2]Rotaxane Catalysts

Polylactide (PLA) is a fully biodegradable and recyclable plastic, produced from a bio-derived monomer: it is a circular economy plastic. Its properties depend upon its stereochemistry and isotactic PLA shows superior thermal-mechanical performances. Here, a new means to control tacticity by exploiting rotaxane conformational dynamism is described. Dynamic achiral [2]rotaxanes can show high isoselectivity (Pi =0.8, 298 K) without requiring any chiral additives and enchain by a chain end control mechanism. The organocatalytic dynamic stereoselectivity is likely applicable to other small-molecule and polymerization catalyses.

Photochromic rotaxanes and pseudorotaxanes

Among stimulus-responsive molecular ring-on-thread rotaxanes and pseudorotaxanes, variants incorporating photochromic sub-units are attracting considerable attention as their properties and structure can be remotely and precisely controlled, additionally without producing chemical waste. The focus herein is on photoswitching-driven assembly/disassembly and modulation of properties resulting from light-activated isomerization or changes in electronic properties.

Cationic and Neutral Rotaxanes Having Different Functional Groups in the Axle Molecule and Their Coordination to PtII

Dibenzo[24]crown-8 (DB24C8) forms rotaxanes with a linear molecule having a dialkylammonium group and a triazole group as well as with the acetylation product of a cationic axle molecule. The former cationic rotaxane is stabilized by multiple intermolecular hydrogen bonds between the NH₂⁺ and oxyethylene groups. The neutral rotaxane contains the macrocycle in the vicinity of the terminal aryl group. The co-conformation of both the cationic and neutral rotaxanes can be fixed by coordination of the triazole group of the axle molecule to PtCl₂ (dmso)₂ . A ¹ H NMR spectroscopic study on the thermodynamics of the Pt coordination revealed a larger association constant for the rotaxanes than for the corresponding axle molecules and a larger value for the neutral rotaxane than for the cationic rotaxane.

Size effects in the alkali metal ion-templated formation of oligo(ethylene glycol)-containing [2]catenanes

The most suitable alkali metal ions for templating the assembly of various homo- and hetero-[2]catenanes from the diamines containing central di-, tri-, and tetra(ethylene glycol) motifs, and isophthalaldehyde are investigated.

Reversible polyphenylacetylene helix conversion driven by a thermoresponsive rotaxane switch in the solid state

The reversible helical pitch change of polyphenylacetylenes by a thermoresponsive trichloroacetate-containing rotaxane switch inserted into the side chain was demonstrated through an accompanying clear color change.

Linear–cyclic polymer structural transformation and its reversible control using a rational rotaxane strategy

A new strategy for “polymer structural transformation” was developed. One [1]rotaxane unit was introduced at the chain end of a linear polymer and the wheel component position was defined by controlling the attractive interaction between the polymer ends. Thus, the reversible linear–cyclic structural transformation was demonstrated.

Macromolecular [2]Rotaxanes: Effective Synthesis and Characterization

Macromolecular [2]rotaxanes, which consist of a polymer chain threading into a wheel component, were synthesized in high yield and with high purity. The synthesis was achieved by the ring-opening polymerization (ROP) of δ-valerolactone (VL) using a hydroxyl-terminated pseudorotaxane as an initiator with diphenyl phosphate as a catalyst in dichloromethane at room temperature. The ¹H NMR, gel permeation chromatography (GPC), and MALDI-TOF-MS measurements of the resulting poly(δ-valerolactone)s clearly indicate the presence of the rotaxane structure with the polymer chain, confirming that the diphenyl phosphate-catalyzed ROP of VL proceeds without deslippage of the wheel component. The obtained macromolecular [2]rotaxane was acetylated to afford a nonionic macromolecular [2]rotaxane, in which only one wheel component is movable from one end to another along the polymer chain.

Reversible helix-random coil transition of poly(m-phenylenediethynylene) by a rotaxane switch

Pendant rotaxane switch-tethering poly(m-phenylene diethynylene) was synthesized by the polyoxidative coupling of a rotaxane containing an axle-terminal m-diethynylbenzene group and an optically active crown ether. The reversible helix-random coil transition of the polymer was successfully performed by the positional switching of the rotaxane wheel.

(Anthracen-9-ylmethyl)benzylammonium chloride

In the title compound, C₂₂H₂₀N⁺·Cl⁻, the anthracene system makes a dihedral angle of 72.65 (4)° with the benzene ring. The C—N—C—C torsion angles in the chain connecting the benzene ring and anthracene system are 52.24 (15) and −170.73 (11)°. The crystal structure is stabilized by inter­molecular N—H⋯Cl and C—H⋯Cl hydrogen bonds, which link the mol­ecules into tetra­mers about inversion centers.

Recent advances in the synthesis of ammonium-based rotaxanes

The number of synthetic methods enabling the preparation of ammonium-based rotaxanes has increased very rapidly in the past ten years. The challenge in the synthesis of rotaxanes results from the rather weak interactions between the ammonium-containing rod and the crown ether macrocycle in the pseudorotaxane structure that rely mostly on O*H hydrogen bonds. Indeed, no strong base or polar solvent that could break up H-bonding can be used during the formation of rotaxanes because the two components will separate as two distinct entities. Moreover, most of the reactions have to be performed at room temperature to favor the formation of pseudorotaxane in solution. These non-trivial prerequisites have been taken into account to develop efficient reaction conditions for the preparation of rotaxanes and those are described in detail along this review.