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.     

A [3]rotaxane with two porphyrinic plates acting as an adaptable receptor

Following a multistep procedure, the copper(I)-templated strategy allowed preparation of a multifunctional [3]rotaxane. The dumbbell consists of a central two-bidentate chelate unit and two terminal stoppers. The two rings threaded on the rotaxane axis consist each of a 1,10-phenanthroline-incorporating macrocycle, rigidly connected to an appended zinc-complexed porphyrin. The copper(I) template can be removed, affording a free rotaxane whose two rings can glide freely along the axis and spin around it. The dumbbell being very long (approximately 85 A in its extended conformation from one stopper to the other), the porphyrin-porphyrin distance can be varied over a wide range. The two porphyrinic plates constitute the key elements of a receptor able to complex various guests between the plates. The ability of the threaded rings to move freely makes the host perfectly adjustable, allowing capture of geometrically very different guests. The copper(I)-complexed rotaxane also acts as an efficient receptor, although its adaptability is obviously more limited than that of its free rotaxane counterpart.     

A photochemically driven molecular-level abacus

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.     

Self-assembly of [2]rotaxane exploiting reversible Pt(II)- pyridine coordinate bonds

A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane.     

Stereochemistry in functionalized macrocycle complexes: control of hydroxyl substituent orientation

Use of a hydroxyl-functionalized open chain tetramine in a template reaction based on its Cu(II) complex leads, after reduction, to a new tetraaza macrocycle with both amino and hydroxyl substituents. The macrocycle is formed predominantly as its trans (anti) isomer, though the cis form is detectable and both have been structurally characterized in the form of their metal complexes. Although both the Cu(II) and the Co(III) complexes of the tetramine precursor ligand have the hydroxyl group in an axial position of a chair six-membered chelate ring, the trans macrocycle forms Co(III) complexes with this substituent in both equatorial and axial positions.     

Homo- and hetero-[3]rotaxanes with two pi-systems clasped in a single macrocycle

Here we present the first synthesis of a [3]rotaxane with two dumbbell components threaded through a single gamma-cyclodextrin macrocycle. This synthesis is carried out in two steps: first one dumbbell is synthesized threaded through the macrocycle to give a [2]rotaxane, then a second dumbbell is synthesized through the remaining cavity of the [2]rotaxane. We have synthesized a hetero- [3]rotaxane with one stilbene and one cyanine dye threaded through gamma-cyclodextrin, which exhibits quantitative energy transfer between the two encapsulated dyes. The stilbene [2]rotaxane intermediate in this synthesis has a remarkably high affinity for suitably shaped hydrophobic guests in aqueous solution, facilitating the synthesis of [3]rotaxanes and suggesting possible applications in sensors.     

Synthesis, structure, and aromaticity of the nickel(II) complex of pyricorrole, a molecular hybrid of porphyrin and corrole

Formal migration of one meso-carbon atom in the porphyrin ring into the pyrrole moiety results in an isomer "pyricorrole", a pyridine-containing corrole macrocycle. We prepared the nickel(II) complex of pyricorrole by the nickel(II)-induced cyclization of a linear precursor. Electronic absorption and proton NMR spectra of this compound revealed the presence of an 18π-electron circuit over the macrocycle, suggesting that aromaticity was retained after intensive rearrangement of the porphyrin core. X-ray crystallography of the nickel(II) complex confirmed the planar structure and demonstrated that it possesses hybrid properties of porphyrin and corrole.     

Effects of Axle‐Core,Macrocycle, and Side‐Station Structures on the Threading and Hydrolysis Processes of Imine‐Bridged Rotaxanes

Imine‐bridged rotaxanes are a new type of rotaxane in which the axle and macrocyclic ring are connected by imine bonds. We have previously reported that in imine‐bridged rotaxane 5 , the shuttling motion of the macrocycle could be controlled by changing the temperature. In this study, we investigated how the axle and macrocycle structures affect the construction of the imine‐bridged rotaxane as well as the dynamic equilibrium between imine‐bridged rotaxane 5 and [2]rotaxane 7 by using various combinations of axles ( 1 A , B ), macrocycles ( 2 a – e ), and side‐stations (XYL and TEG). In the threading process, the flexibility of the macrocycle and the substituent groups at the para position of the aniline moieties affect the preparation of the threaded imines. The size of the imine‐bridging station and the macrocyclic tether affects the hydrolysis of the imine bonds under acidic conditions.     

Reversible Photoswitching of Rotaxane Character and Interplay of Thermodynamic Stability and Kinetic Lability in a Self‐Assembling Ring–Axle Molecular System

We have designed, synthesized, and investigated a self‐assembling system that can be reversibly interconverted between thermodynamically stable (pseudorotaxane) and kinetically inert (rotaxane) forms by light irradiation. The system is composed of a dibenzo[24]crown‐8 ring and an axle comprised of a dibenzylammonium recognition site and two azobenzene end groups. The isomeric form of the azobenzene units of the axle has a little influence on the stability constants of the respective pseudorotaxanes but greatly affects the threading–dethreading rate constants. In fact, equilibration of the ring and the axle in its EE isomeric form occurs within seconds in acetonitrile at room temperature, whereas the ZZ axle threads–dethreads the ring at least four orders of magnitude slower. Moreover, we show that a change in the stability of the complex, achieved by deprotonating the dibenzylammonium recognition site on the axle, affects its kinetic behavior. We compare the results of these experiments with those observed upon dethreading the (pseudo)rotaxane by using a competitive guest for the ring, an approach which does not inherently destabilize the ring–axle interaction. This study outlines a general strategy for the reversible photochemical control of motion kinetics in threaded and interlocked compounds and constitutes a starting point for the construction of multicomponent structures that can behave as photochemically driven nanomachines.     

Anion-induced shuttling of a naphthalimide triazolium rotaxane

The anion-templated synthesis of a rotaxane structure, incorporating the new naphthalimide triazolium motif, is described and the interlocked host shown to exhibit selective, uni-directional, anion-induced shuttling. Initial pseudorotaxane investigations demonstrate the ability of a naphthalimide triazolium threading component to form interpenetrated assemblies with counter-anion-dependent co-conformations. (1)H NMR studies reveal that the shuttling behaviour of the analogous rotaxane host system is controlled by selective anion binding and by the nature of the solvent conditions. Complete macrocycle translocation only occurs upon the recognition of the smaller halide anions (chloride and bromide). The rotaxane solid-state crystal structure in the presence of chloride is in agreement with the solution-phase co-conformation. The sensitivity of the axle naphthalimide absorbance band to the position of the macrocycle component within the interlocked structure enabled the molecular motion to be observed by UV/Vis spectroscopy, and the chloride-induced shuttling of the rotaxane was reversed upon silver hexafluorophosphate addition.     

A [2]catenane constructed around a Ru(diimine)(3)(2+) complex used as a template

A [2]catenane has been constructed using an octahedral complex of the Ru(diimine)( 2+)(3) family as a scaffold. Two diimine chelates have been incorporated in a ring prior to the ruthenium(II) complexation reaction. The macrocyclic complex thus obtained has been subsequently threaded by a long linear fragment containing the third chelate. The rutheniuml(II)-complexed catenane, cyclized by ring-closing metathesis, is the first example of an interlocking ring system built around an octahedral tris-chelate complex.     

Chemically induced contraction and stretching of a linear rotaxane dimer

Copper(I)-induced assembly of two self-complementary identical units, which consist of a ring that incorporates a 1,10-phenanthroline group attached to a small filament containing a second 1,10-phenanthroline (phen) group, leads quantitatively to a doubly threaded complex. Each copper(I) center is four-coordinate and is located inside a ring and bound to a phen from the macrocyle. The two other coordination sites are occupied by a phen from the filament connected to the other ring. An X-ray structure of the dicopper(I) complex unambiguously demonstrates the doubly threaded nature of the system. The molecule has C(2) symmetry in the crystal. This is an extended form with a Cu small middle dot small middle dot small middle dotCu separation of 18.3 A and an overall length close to 40 A. Further synthetic work, which utilizes the two terminal phenolic functions of the previous dicopper(I) complex, gives rise to a more complex system in which both filaments have been prolonged in opposite directions by 2,2':6',2"-terpyridine (terpy) motifs and bulky stoppers. The organic backbone is that of a rotaxane dimer. Although redox cycling of Cu(I) to Cu(II) did not lead to intramolecular rearrangement, simple chemical reactions induced large conformational changes. The rotaxane dimer was set in motion as follows. The dicopper(I) complex, which is in an extended conformation, was demetallated by using KCN. From the free ligand, the dizinc complex was formed quantitatively at room temperature. (1)H NMR data show that a new conformation is obtained: each Zn(II) is five-coordinate (phen + terpy), and the molecule is in a contracted conformation. This process is reminiscent of biological muscles in the sense that the two filaments of this system can be moved along one another in a gliding motion that keeps the whole system together, but which converts a stretched compound (overall length approximately equal to 83 A) into a contracted species (overall length approximately equal to 65 A, according to CPK models). The motion is quantitatively reversed by the addition of an excess of copper(I) to the dizinc complex; this regenerates the extended starting form. Although the motivation of the present contribution was to illustrate that a muscle-like molecule may be stretched or contracted using electrochemistry and coordination chemistry, the main body of the work is organic synthesis. This is testified by the fact that the dicopper(I) rotaxane dimer was obtained in 23 steps from commercially available compounds.     

Synthesis of polyyne rotaxanes

Active-metal templating has been used to synthesize rotaxanes consisting of a phenanthroline-based macrocycle threaded around a C8, C12, or C20 polyyne chain. The crystal structure of the C12 rotaxane has been determined. In the rhenium(I) carbonyl complex of this rotaxane, with Re(CO)(3)Cl coordinated to the phenanthroline macrocycle, the proximity of the polyyne chain quenches the luminescence of the rhenium. These rotaxanes offer a new approach to controlling the environment and interactions of a polyyne chain.     

Macrocyclic Dinuclear Palladium Complex as a Novel Doubly Threaded [3]Rotaxane Scaffold and Its Application as a Rotaxane Cross-Linker

A dinuclear Pd^II complex possessing a cyclic ligand was developed as a novel doubly threaded [3]rotaxane scaffold and applied as a rotaxane cross-linker reagent. The dinuclear complex (PdMC)₂ was prepared by one-step macrocyclization followed by the double palladation reaction. ¹H NMR analysis and UV/Vis measurements revealed the formation of a doubly threaded pseudo[3]rotaxane by the complexation of (PdMC)₂ with 2 equivalents of 2,6-disubstituted pyridine 3 through double metal coordination. The treatment of (PdMC)₂ with 2 equivalents of 4-vinylpyridine (VP) afforded a doubly threaded [3]rotaxane cross-linker (PdMC-VP)₂ . Radical co-polymerization of VP and t-butylstyrene in the presence of (PdMC-VP)₂ afforded a stable rotaxane cross-linked polymer (RCP). An elastic RCP was also prepared by using n-butyl acrylate as a monomer. The obtained RCPs exhibited higher swelling ability and higher mechanical toughness compared with the corresponding covalent cross-linked polymers.     

Iodide Recognition and Sensing in Water by a Halogen‐Bonding Ruthenium(II)‐Based Rotaxane

The synthesis and anion‐recognition properties of the first halogen‐bonding rotaxane host to sense anions in water is described. The rotaxane features a halogen‐bonding axle component, which is stoppered with water‐solubilizing permethylated β‐cyclodextrin motifs, and a luminescent tris(bipyridine)ruthenium(II)‐based macrocycle component. ¹H NMR anion‐binding titrations in D₂O reveal the halogen‐bonding rotaxane to bind iodide with high affinity and with selectively over the smaller halide anions and sulfate. The binding affinity trend was explained through molecular dynamics simulations and free‐energy calculations. Photo‐physical investigations demonstrate the ability of the interlocked halogen‐bonding host to sense iodide in water, through enhancement of the macrocycle component’s Ru^II metal–ligand charge transfer (MLCT) emission.     

A Flexible Copper(I)‐Complexed [4]Rotaxane Containing Two Face‐to‐Face Porphyrinic Plates that Behaves as a Distensible Receptor

A new cyclic [4]rotaxane composed of two flexible bis‐macrocycles and two rigid axles is described. Each bis‐macrocycle consists of two rings attached to antipodal meso positions of a central Zn porphyrin through single C? C bonds. Each ring incorporates a 2,9‐diphenyl‐1,10‐phenanthroline chelation site. The axles contain two coplanar bidentate sites derived from the 2,2′‐bipyridine motif. The building blocks were assembled by using a one‐pot threading‐and‐stoppering reaction, which afforded the [4]rotaxane in 50 % yield. The “gathering‐and‐threading” effect of copper(I) was utilised in the formation of a [4]pseudorotaxane, which was immediately converted to the corresponding [4]rotaxane by a quadruple CuAAC stoppering reaction. The rotaxane contains two face‐to‐face zinc porphyrins, which allowed the coordination of ditopic guest substrates. The rotaxane host showed remarkable flexibility and was able to adjust its conformation to the guest size. It can be distended and accommodate rod‐like guests of 2.6 to 15.8 Å in length.     

A new type of insulated molecular wire: a rotaxane derived from a metal-capped conjugated tetrayne

The platinum butadiynyl complex trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(2)H and a CuI adduct of a 1,10-phenanthroline based 33-membered macrocycle react in the presence of K(2)CO(3) and I(2) or O(2) to give a rotaxane (ca. 9%) in which the macrocycle is threaded by the sp carbon chain of trans,trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(4)Pt(Pp-tol(3))(2)(C(6)F(5)). The crystal structure and macrocycle/axle electronic interactions are analyzed in detail.     

Reversing a rotaxane recognition motif: threading oligoethylene glycol derivatives through a dicationic cyclophane

An already well-established recognition motif-namely one in which the NH2+ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH2OCH2- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (1H and 13C) spectroscopy in nitromethane-d3. The [2]rotaxane exhibits quite dramatic changes in the 1H chemical shifts of the signals for its CH2N+ and CH2O protons compared with those in the free dumbbell compound. The 1H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH2O probes give an average Ka value of 2900 +/- 750 M-1 in nitromethane-d3. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.     

Dynamic Control of Chiral Space Through Local Symmetry Breaking in a Rotaxane Organocatalyst

We report on a switchable rotaxane molecular shuttle that features a pseudo‐meso 2,5‐disubstituted pyrrolidine catalytic unit on the axle whose local symmetry is broken according to the position of a threaded benzylic amide macrocycle. The macrocycle can be selectively switched (with light in one direction; with catalytic acid in the other) with high fidelity between binding sites located to either side of the pyrrolidine unit. The position of the macrocycle dictates the facial bias of the rotaxane‐catalyzed conjugate addition of aldehydes to vinyl sulfones. The pseudo‐meso non‐interlocked thread does not afford significant selectivity as a catalyst (2–14 % ee), whereas the rotaxane affords selectivities of up to 40 % ee with switching of the position of the macrocycle changing the handedness of the product formed (up to 60 % Δee).     

Introducing negative charges into bis-p-phenylene crown ethers: a study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO(-) or SO(3)(-). The structure of the resulting charge-compensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1'-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ mol(-1) at 25 degrees C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1'-diethyl-3,3'-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.