High‐Yield Diastereoselective Synthesis of Planar Chiral [2]‐ and [3]Rotaxanes Constructed from per‐Ethylated Pillar[5]arene and Pyridinium Derivatives

Planar chiral [2]‐ and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per‐ethylated pillar[5]arene ( C2 ) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)‐ C2 or (pR, pR, pR, pR, pR)‐ C2 wheels was accomplished. Furthermore, pillar[5]arene‐based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)‐ C2 , one (pS, pS, pS, pS, pS)‐ C2 and one (pR, pR, pR, pR, pR)‐ C2 (meso form), or two (pR, pR, pR, pR, pR)‐ C2 wheels. The [3]rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar[5]arene‐based interlocked molecules.     

Synthesis of [3]Rotaxanes that Utilize the Catalytic Activity of a Macrocyclic Phenanthroline–Cu Complex: Remarkable Effect of the Length of the Axle Precursor

[3]Rotaxanes, which consist of one macrocyclic phenanthroline compound and two axle components, were prepared by the oxidative dimerization of an alkyne compound with bulky tris[4′‐cyclohexyl‐(1,1′‐biphenyl)‐4‐yl]methyl blocking group. The catalytic activity of a macrocyclic phenanthroline–Cu complex was utilized to thread the two axle components inside the ring. The alkyne compound with chain of 15 or 20 methylene groups gave [3]rotaxanes in high yields, whereas the axle with a chain of six methylene groups afforded a [3]rotaxane in very poor yield. We also examined the effect of the ring size on the synthesis of [3]rotaxanes. [3]Rotaxanes were not isolated when a macrocyclic phenanthroline compound with a smaller ring size was used.     

Helicenophyrins: Expanded Carbaporphyrins Incorporating Aza[5]helicene and Heptacyclic S‐Shaped Aza[5]helicene Motifs

Incorporation of phenanthrene into a hexaphyrin(1.1.1.1.1.0) frame resulted in intramolecular ring fusion, thus giving rise to chiral helicenophyrins. These molecules contain helicene and porphyrin features by incorporating either an aza[5]helicene or heptacyclic S‐shaped aza[5]helicene.     

Nickel(II) and Copper(II) Complexes of β‐Unsubstituted 5,15‐Diazaporphyrins and Pyridazine‐Fused Diazacorrinoids: Metal–Template Syntheses and Peripheral Functionalizations

The present paper reports the first comprehensive study on the synthesis, structures, optical and electrochemical properties, and peripheral functionalizations of nickel(II) and copper(II) complexes of β‐unsubstituted 5,15‐diazaporphyrins (M‐DAP; M=Ni, Cu) and pyridazine‐fused diazacorrinoids (Ni‐DACX; X=N, O). These two classes of compounds were constructed starting from mesityldipyrromethane by a metal–template method. Ni‐DAP and Cu‐DAP were prepared in high yields by the reaction of the respective metal–bis(dibromodipyrrin) complexes with NaN₃–CuX (X=I, Br), whereas Ni‐DACN and Ni‐DACO were formed as predominant products by the reaction with NaN₃. In both cases, the metal centers change their geometry from tetrahedral to square planar during the aza‐annulation; X‐ray crystallographic analyses of M‐DAPs showed highly planar diazaporphyrin π planes. The Q band of Ni‐DAP was redshifted and intensified compared with that of a nickel–porphyrin reference, due to the involvement of electronegative nitrogen atoms at the meso positions. It was found that the peripheral bromination of Ni‐DAP and Ni‐DACO occurred regioselectively to afford Ni‐DAP‐Br₄ and Ni‐DACO‐Br, respectively. These brominated derivatives underwent Stille reactions with tributyl(phenyl)stannane to give the corresponding phenylated derivatives, Ni‐DAP‐Ph₄ and Ni‐DACO‐Ph. On the basis of the absorption spectra and X‐ray analysis, it has been concluded that the attached phenyl groups efficiently conjugate with the diazaporphyrin π system. The present results unambiguously corroborate that the β‐unsubstituted DAPs and DACXs are promising platforms for the development of a new class of π‐conjugated azaporphyrin‐based materials.     

Copper(II) Complexes of a Hexadentate Mixed‐Donor N3S3 Macrobicyclic Cage: Facile Rearrangements and Interconversions

The potentially hexadentate mixed‐donor cage ligand 1‐methyl‐8‐amino‐3,13,16‐trithia‐6,10,19‐triazabicyclo[6.6.6]eicosane (AMME‐N₃S₃sar; sar=sarcophagine) displays variable coordination modes in a complex with copper(II). In the absence of coordinating anions, the ligand adopts a conventional hexadentate N₃S₃ binding mode in the complex [Cu(AMME‐N₃S₃sar)](ClO₄)₂ that is typical of cage ligands. This structure was determined by X‐ray crystallography and solution spectroscopy (EPR and NIR UV/Vis). However, in the presence of bromide ions in DMSO, clean conversion to a five‐coordinate bromido complex [Cu(AMME‐N₃S₃sar)Br]⁺ is observed that features a novel tetradentate (N₂S₂)‐coordinated form of the cage ligand. This copper(II) complex has also been characterized by X‐ray crystallography and solution spectroscopy. The mechanism of the reversible interconversion between the six‐ and five‐coordinated copper(II) complexes has been studied and the reaction has been resolved into two steps; the rate of the first is linearly dependent on bromide ion concentration and the second is bromide independent. Electrochemistry of both [Cu(AMME‐N₃S₃sar)]²⁺ and [Cu(AMME‐N₃S₃sar)Br]⁺ in DMSO shows that upon reduction to the monovalent state, they share a common five‐coordinated form in which the ligand is bound to copper in a tetradentate form exclusively, regardless of whether a six‐ or five‐coordinated copper(II) complex is the precursor.     

Control over the Redox Cooperative Mechanism of Radical Carbene Transfer Reactions for the Efficient Active-Metal-Template Synthesis of [2]Rotaxanes

A 5,15-bis(1,1′-biphenyl)porphyrin-based molecular clip covalently connected to a ditopic aliphatic ester loop moiety yields a semi-rigid macrocycle with a well-defined cavity. The resulting macrocycle fits the structural requirements for the preparation of porphyrinates capable of promoting formation of C−C bonds. To demonstrate the usefulness of porphyrin-based macrocycles, an active-metal-template synthesis of rotaxanes through a redox non-innocent carbene transfer reaction is described. Coordination of Co^II ions into the porphyrin subunit followed by addition of appropriate monodentate nitrogen-based additives to function as axial ligands enables the radical carbene transfer reactions to styrene derivatives to occur exclusively through the cavity of the macrocycle to afford cyclopropane-linked rotaxanes in excellent 95 % yield. Investigation of the product distribution afforded from the rotaxane assembly reaction reveals how the redox cooperative action between the carbene species and the Co^II ions can be manipulated to gain control over the radical-type mechanism to favor the productive rotaxane forming process.     

Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes

The interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron-donor and electron-acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers. To this aim, the photoinduced electron transfer processes involving a 1,8-naphthalimide chromophore, embedded in several rotaxane-based dyads, were investigated by steady-state and time-resolved absorption and luminescence spectroscopic experiments in the 300 fs–10 ns time window. Different rotaxanes built around the dialkylammonium/ dibenzo[24]crown-8 ether supramolecular motif were designed and synthesized to decipher the relevance of key structural factors, such as the chemical deactivation of the ammonium-crown ether recognition, the presence of a secondary site for the ring along the axle, and the covalent functionalization of the macrocycle with a phenothiazine electron donor. Indeed, the conformational freedom of these compounds gives rise to a rich dynamic behavior induced by light and may provide opportunities for investigating and understanding phenomena that take place in complex (bio)molecular architectures.     

Olefin Cyclopropanation by Radical Carbene Transfer Reactions Promoted by Cobalt(II)/Porphyrinates: Active‐Metal‐Template Synthesis of [2]Rotaxanes

A Co^II/porphyrinate‐based macrocycle in the presence of a 3,5‐diphenylpyridine axial ligand functions as an endotopic ligand to direct the assembly of [2]rotaxanes from diazo and styrene half‐threads, by radical‐carbene‐transfer reactions, in excellent 95 % yield. The method reported herein applies the active‐metal‐template strategy to include radical‐type activation of ligands by the metal‐template ion during the organometallic process which ultimately yields the mechanical bond. A careful quantitative analysis of the product distribution afforded from the rotaxane self‐assembly reaction shows that the Co^II/porphyrinate subunit is still active after formation of the mechanical bond and, upon coordination of an additional diazo half‐thread derivative, promotes a novel intercomponent C?H insertion reaction to yield a new rotaxane‐like species. This unexpected intercomponent C?H insertion illustrates the distinct reactivity brought to the Co^II/porphyrinate catalyst by the mechanical bond.     

A Simple and Highly Effective Ligand System for the Copper(I)‐Mediated Assembly of Rotaxanes

A [2]rotaxane was produced through the assembly of a picolinaldehyde, an amine, and a bipyridine macrocycle around a Cu^I template by imine bond formation in close‐to‐quantitative yield. An analogous [3]rotaxane is obtained in excellent yield by replacing the amine with a diamine, thus showing the suitability of the system for the construction of higher order interlocked structures. The rotaxanes are formed within a few minutes simply through mixing the components in solution at room temperature and they can be isolated through removal of the solvent or precipitation.     

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.     

Four‐State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali‐Metal Cation Stimuli

In this study, we synthesized [2]rotaxanes possessing three recognition sites—a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations—in their dumbbell‐like axle component and dibenzo[24]crown‐8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four‐state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali‐metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali‐metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly.     

Energy Transfer and Concentration‐Dependent Conformational Modulation: A Porphyrin‐Containing [3]Rotaxane

A zinc porphyrin‐containing [3]rotaxane A was synthesized through a copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Energy donors and acceptor porphyrin were introduced to dibenzo[24]crown‐8 (DB24C8) and dibenzyl ammonium (DBA) units of [3]rotaxane A to understand the intramolecular energy transfer process. Investigations of the photophysical properties of [3]rotaxane A demonstrated that the intramolecular efficient energy transfer readily occurred from the donors on the wheels to the porphyrin center on the axis. The fluorescence of energy donors in the region of 400 to 450 nm was efficiently absorbed by the porphyrin acceptor under irradiation at 345 nm, and finally a red light emission at about 600 nm was achieved. Further investigation indicated that the conformation of [3]rotaxane A was self‐modulated by changing its concentration in CH₂Cl₂. The triazole groups on the wheel coordinated or uncoordinated to Zn²⁺ through intramolecular self‐coordination with the change in the concentration of [3]rotaxane A in CH₂Cl₂. Therefore, this conformational change was reversible in a non‐coordinating solvent such as CH₂Cl₂ but inhibited in a coordinating solvent such as THF. Such interesting behaviors were rarely observed in porphyrin derivatives. This self‐modulation feature opens up the possibility of controlling molecular conformation by varying concentration.     

Langmuir and Langmuir–Blodgett Films from Amphiphilic Pillar[5]arene‐Containing [2]Rotaxanes

Amphiphilic pillar[5]arene‐containing [2]rotaxanes have been prepared and fully characterized. In the particular case of the [2]rotaxane incorporating a 1,4‐diethoxypillar[5]arene subunit, the structure of the compound was confirmed by X‐ray crystal structure analysis. Owing to a good hydrophilic/hydrophobic balance, stable Langmuir films have been obtained for these rotaxanes and the size of the peripheral alkyl chains on the pillar[5]arene subunit has a dramatic influence on the reversibility during compression–decompression cycles. Indeed, when these are small enough, molecular reorganization of the rotaxane by gliding motions are capable of preventing strong π–π interactions between neighboring macrocycles in the thin film.     

Heteroleptic Copper(I)‐Based Complexes for Photocatalysis: Combinatorial Assembly,Discovery, and Optimization

A library of 50 copper‐based complexes derived from bisphosphines and diamines was prepared and evaluated in three mechanistically distinct photocatalytic reactions. In all cases, a copper‐based catalyst was identified to afford high yields, where new heteroleptic complexes derived from the bisphosphine BINAP displayed high efficiency across all reaction types. Importantly, the evaluation of the library of copper complexes revealed that even when photophysical data is available, it is not always possible to predict which catalyst structure will be efficient or inefficient in a given process, emphasizing the advantages for catalyst structures with high modularity and structural variability.