Toward metal complexes that can directionally walk along tracks: controlled stepping of a molecular biped with a palladium(II) foot

Fast and slow walking driven by chemical fuel

We demonstrate the fast forward and slow backward motion of a biped on a tetrahedral track using chemical fuel, cooperative binding and kinetic selectivity. Walking of the biped is based on its dibenzyl amine feet that bind to zinc porphyrin units and, upon protonation, to dibenzo 24-crown-8 sites affording pseudorotaxane linkages.

Metallo-Supramolecular Hexagonal Wreath with Four Switchable States Based on a pH-Responsive Tridentate Ligand

In biological systems, many biomacromolecules (e.g., heme proteins) are capable of switching their states reversibly in response to external stimuli, endowing these natural architectures with a high level of diversity and functionality. Although tremendous efforts have been made to advance the complexity of artificial supramolecules, it remains a challenge to construct metallo-supramolecular systems that can carry out reversible interconversion among multiple states. Here, a pH-responsive tridentate ligand, 2,6-di(1H-imidazole-2-yl)pyridine (H₂DAP), is incorporated into the multitopic building block for precise construction of giant metallo-supramolecular hexagonal wreaths with three metal ions, i.e., Fe(II), Co(II), and Ni(II), through coordination-driven self-assembly. In particular, a Co-linked wreath enables in situ reversible interconversion among four states in response to pH and oxidant/reductant with highly efficient conversion without losing structural integrity. During the state interconversion cycles, the physical properties of the assembled constructs are finely tuned, including the charge states of the backbone, valency of metal ions, and paramagnetic/diamagnetic features of complexes. Such discrete wreath structures with a charge-switchable backbone further facilitate layer-by-layer assembly of metallo-supramolecules on the substrate.

Crown Ether-Based Ion Transporters in Bilayer Membranes

Bilayer membranes that enhance the stability of the cell are essential for cell survival, separating and protecting the interior of the cell from its external environment. Membrane-based channel proteins are crucial for sustaining cellular activities. However, dysfunction of these proteins would induce serial channelopathies, which could be substituted by artificial ion channel analogs. Crown ethers (CEs) are widely studied in the area of artificial ion channels owing to their intrinsic host-guest interaction with different kinds of organic and inorganic ions. Other advantages such as lower price, chemical stability, and easier modification also make CE a research hotspot in the field of synthetic transmembrane nanopores. And numerous CEs-based membrane-active synthetic ion channels were designed and fabricated in the past decades. Herein, the recent progress of CEs-based synthetic ion transporters has been comprehensively summarized in this review, including their design principles, functional mechanisms, controllable properties, and biomedical applications. Furthermore, this review has been concluded by discussing the future opportunities and challenges facing this research field. It is anticipated that this review could offer some inspiration for the future fabrication of novel CEs-derived ion transporters with more advanced structures, properties, and practical applications.

Following a Silent Metal Ion: A Combined X-ray Absorption and Nuclear Magnetic Resonance Spectroscopic Study of the Zn2+ Cation Dissipative Translocation between Two Different Ligands

The dissipative translocation of the Zn²⁺ ion between two prototypical coordination complexes has been investigated by combining X-ray absorption and ¹H NMR spectroscopy. An integrated experimental and theoretical approach, based on state-of-the-art Multivariate Curve Resolution and DFT based theoretical analyses, is presented as a means to understand the concentration time evolution of all relevant Zn and organic species in the investigated processes, and accurately characterize the solution structures of the key metal coordination complexes. Specifically, we investigate the dissipative translocation of the Zn²⁺ cation from hexaaza-18-crown-6 to two terpyridine moieties and back again to hexaaza-18-crown-6 using 2-cyano-2-phenylpropanoic acid and its para-chloro derivative as fuels. Our interdisciplinary approach has been proven to be a valuable tool to shed light on reactive systems containing metal ions that are silent to other spectroscopic methods. These combined experimental approaches will enable future applications to chemical and biological systems in a predictive manner.

Can 2-Pyridyl-1,2,3-triazole "Click" Ligands be Used to Develop Cu(I)/Cu(II) Molecular Switches?

Molecular switching processes are important in a range of areas including the development of molecular machines. While there are numerous organic switching systems available, there are far less examples that exploit inorganic materials. The most common inorganic switching system remains the copper(I)/copper(II) switch developed by Sauvage and co-workers over 20 years ago. Herein, we examine if bidentate 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pytri) and tridentate 2,6-bis[(4-phenyl-1H-1,2,3-triazol-1-yl)methyl]pyridine (tripy) moieties can be used to replace the more commonly exploited polypyridyl ligands 2,2'-bypyridine (bpy)/1,10-phenanthroline (phen) and 2,2';6',2″-terpyridine (terpy) in a copper(I)/(II) switching system. Two new ditopic ligands that feature bidentate (pytri, L1 or bpytri, L2) and tridentate tripy metal binding pockets were synthesized and used to generate a family of heteroleptic copper(I) and copper(II) 6,6'-dimesityl-2,2'-bipyridine (diMesbpy) complexes. Additionally, we synthesized a series of model copper(I) and copper(II) diMesbpy complexes. A combination of techniques including nuclear magnetic resonance (NMR) and UV-vis spectroscopies, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography was used to examine the behavior of the compounds. It was found that L1 and L2 formed [(diMesbpy)Cu(L1 or L2)]²⁺ complexes where the copper(II) diMesbpy unit was coordinated exclusively in the tridenate tripy binding site. However, when the ligands (L1 and L2) were complexed with copper(I) diMesbpy units, a complex mixture was obtained. NMR and MS data indicated that a 1:1 stoichiometry of [Cu(diMesbpy)]⁺ and either L1 or L2 generated three complexes in solution, the dimetallic [(diMesbpy)₂Cu₂(L1 or L2)]²⁺ and the monometallic [(diMesbpy)Cu(L1 or L2)]⁺ isomers where the [Cu(diMesbpy)]⁺ unit is coordinated to either the bidentate or tridentate tripy binding sites of the ditopic ligands. The dimetallic [(diMesbpy)₂Cu₂(L1 or L2)](PF₆)₂ complexes were structurally characterized using X-ray crystallography. Both complexes feature a [Cu(diMesbpy)]⁺ coordinated to the bidentate (pytri or bpytri) pocket of the ditopic ligands (L1 or L2), as expected. They also feature a second [Cu(diMesbpy)]⁺ coordinated to the nominally tridentate tripy binding site in a four-coordinate hypodentate κ²-fashion. Competition experiments with model complexes showed that the binding strength of the bidentate pytri is similar to that of the κ²-tripy ligand, leading to the lack of selectivity. The results suggest that the pytri/tripy and bpytri/tripy ligand pairs cannot be used as replacements for the more common bpy/phen-terpy partners due to the lack of selectivity in the copper(I) state.

Synthetic platform for mono-functionalised tridentate macrocycles as key precursors of mechanically-linked macromolecular systems

Macrocycles bearing a variety of functional groups give access to a wide range of synthetic methods for further derivatisation or preparation of more complex structures such as mechanically interlocked molecules or polymeric materials.

Probing the Dynamics of the Imine-Based Pentafoil Knot and Pentameric Circular Helicate Assembly

We investigate the self-assembly dynamics of an imine-based pentafoil knot and related pentameric circular helicates, each derived from a common bis(formylpyridine)bipyridyl building block, iron(II) chloride, and either monoamines or a diamine. The mixing of circular helicates derived from different amines led to the complete exchange of the N-alkyl residues on the periphery of the metallo-supramolecular scaffolds over 4 days in DMSO at 60 °C. Under similar conditions, deuterium-labeled and nonlabeled building blocks showed full dialdehyde building block exchange over 13 days for open circular helicates but was much slower for the analogous closed-loop pentafoil knot (>60 days). Although both knots and open circular helicates self-assemble under thermodynamic control given sufficiently long reaction times, this is significantly longer than the time taken to afford the maximum product yield (2 days). Highly effective error correction occurs during the synthesis of imine-based pentafoil molecular knots and pentameric circular helicates despite, in practice, the systems not operating under full thermodynamic control.

Tunable Orthogonal Reversible Covalent (TORC) Bonds: Dynamic Chemical Control over Molecular Assembly

Dynamic assembly of macromolecules in biological systems is one of the fundamental processes that facilitates life. Although such assembly most commonly uses noncovalent interactions, a set of dynamic reactions involving reversible covalent bonding is actively being exploited for the design of functional materials, bottom-up assembly, and molecular machines. This Minireview highlights recent implementations and advancements in the area of tunable orthogonal reversible covalent (TORC) bonds for these purposes, and provides an outlook for their expansion, including the development of synthetically encoded polynucleotide mimics.

Symmetric molecules with 1,4-triazole moieties as potent inhibitors of tumour-associated lactate dehydrogenase-A

A series of symmetric molecules incorporating aryl or pyridyl moieties as central core and 1,4-substituted triazoles as a side bridge was synthesised. The new compounds were investigated as lactate dehydro-genase (LDH, EC 1.1.1.27) inhibitors. The cancer associated LDHA isoform was inhibited with IC50 = 117-174 µM. Seven compounds exhibited better LDHA inhibition (IC50 117-136 µM) compared to known LDH inhibitor - galloflavin (IC50 157 µM).

Directional control of a processive molecular hopper

Intrigued by the potential of nanoscale machines, scientists have long attempted to control molecular motion. We monitored the individual 0.7-nanometer steps of a single molecular hopper as it moved in an electric field along a track in a nanopore controlled by a chemical ratchet. The hopper demonstrated characteristics desired in a moving molecule: defined start and end points, processivity, no chemical fuel requirement, directional motion, and external control. The hopper was readily functionalized to carry cargos. For example, a DNA molecule could be ratcheted along the track in either direction, a prerequisite for nanopore sequencing.

Threading of an Inherently Directional Calixarene Wheel with Oriented Ammonium Axles

The threading of monostoppered alkylbenzylammonium axles 7⁺ and 8⁺ with the calix[6]-wheel 3 can occur by both routes of entering the macrocycle 3 in the cone conformation: passage through the upper rim and the through the lower rim. Thus, under thermodynamic conditions, with both the axles 7⁺ and 8⁺, the two possible orientations of calix[2]pseudorotaxane, namely, endo-benzyl and endo-alkyl, are formed by a stereoselectivity controlled by the endo-alkyl rule. Interestingly, by ¹H NMR monitoring of the threading process between 8⁺ and 3, we revealed two calix[2]pseudorotaxane isomers in which the calix-wheel adopts 1,2,3-alternate and cone conformations, which represent the kinetic and thermodynamic species, respectively. Finally, the synthesis of ammonium-based oriented calix[2]rotaxane is here described.

Enzyme-Mediated Directional Transport of a Small-Molecule Walker With Chemically Identical Feet

We describe a small-molecule "walker" that uses enzyme catalysis to discriminate between the relative positions of its "feet" on a track and thereby move with net directionality. The bipedal walker has identical carboxylic acid feet, and "steps" along an isotactic hydroxyl-group-derivatized polyether track by the formation/breakage of ester linkages. Lipase AS catalyzes the selective hydrolysis of the rear foot of macrocyclized walkers (an information ratchet mechanism), the rear foot producing an (R)-stereocenter at its point of attachment to the track. If the hydrolyzed foot reattaches to the track in front of the bound foot it forms an (S)-stereocenter, which is resistant to enzymatic hydrolysis. Only macrocyclic walker-track conjugates are efficiently hydrolyzed by the enzyme, leading to high processivity of the walker movement along the track. Conventional chemical reagents promote formation of the ester bonds between the walker and the track. Iterative macrocyclization and hydrolysis reactions lead to 68% of walkers taking two steps directionally along a three-foothold track.

Artificial molecular motors

Artificial molecular motors take inspiration from motor proteins, nature's solution for achieving directional molecular level motion. An overview is given of the principal designs of artificial molecular motors and their modes of operation. We identify some key challenges remaining in the field.

A ferrocene based switchable molecular folding ruler

A bis(ferrocene) three tiered molecular folding ruler can be induced to undergo a large scale extension and contraction process using either chemical or better electrochemical methods.

Five-State Rotary Nanoswitch

In our quest to develop artificial multistate devices, we synthesized the nanomechanical switch 1 that is characterized by a tetrahedral core equipped with four pending arms. The rotary arm with its azaterpyridine terminal is intramolecularly coordinated to a zinc(II) porphyrin station that is the terminus of another arm in 1. The two other arms carry identical sterically shielded phenanthroline stations. The 2-fold alternate addition of a copper(I) ion and [1,10]-phenanthroline (1 equiv each) results in the formation of five different switching states (State I→ State II→ State III→ State IV→ State V → State I), which force the toggling arm to move back and forth between the zinc(II) porphyrin and phenanthroline stations separated by a distance of 25 Å. All switching states constitute clean single species, except for State III, and thus are fully characterized by spectroscopic methods and elemental analysis. Finally, the initial state of nanoswitch was reset by addition of cyclam for complete removal of the copper(I) ions.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

Redox-configurable ambidextrous catalysis: structural and mechanistic insight

A helically chiral copper complex is used as a switchable asymmetric catalyst capable of delivering either enantiomer of a Michael addition reaction.

A ligand capable of adopting two pseudo-enantiomeric helically chiral states when bound to copper has been applied as an asymmetric catalyst in the Michael addition of malonate substrates to nitrostyrenes. The absolute configuration of the helically chiral ligand is inverted upon oxidation/reduction of the copper center. In this way, the handedness of the Michael addition product (R/S) can be selected based on the handedness of the catalyst (Λ/Δ). Exciton coupled circular dichroism (ECCD) was used to identify which of the two pseudo-enantiomeric forms the catalyst adopted after reduction/oxidation, with additional support from X-ray crystallographic data. The synthesis of the ligand was achieved in five steps with an overall 61% yield. Enantiomeric excesses of the Michael addition products of up to 72% (S) and 70% (R) were obtained in acetonitrile. The ability to choose the handedness of the product based on the chiral state of the catalyst has been demonstrated with several different solvents, bases, nitrostyrene/malonate substrates, and prochiral malonate substrates. A combination of molecular modelling, crystal structure and kinetic data suggest that one urea moiety of the catalyst ligand likely binds the nitrostyrene substrate while blocking the Re face of the nitrostyrene in the transition state.

The eternal youth of azobenzene: new photoactive molecular and supramolecular devices

The development of multicomponent chemical systems that can perform predetermined functions under external control – i.e., molecular devices – is a challenging task in chemistry and a fascinating objective in the frame of a bottom-up approach to nanostructures. Photochromic units undergo profound changes in their chemical and/or electronic structure upon light excitation, and are highly interesting for the construction of photocontrollable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene – owing to its high efficiency, excellent reversibility and significant physico-chemical differences between the two forms – is a highly useful reaction in this regard. Azobenzene photoisomerization has been known for almost 80 years and has been exploited to implement light-induced functionalities with a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our recent investigations highlighting how this outstanding photochrome can be utilized to develop (supra)molecular systems with valuable light-induced functionalities.

Directional Dynamic Covalent Motion of a Carbonyl Walker on a Polyamine Track

Controlled directional displacement of a molecular group has been achieved based on dynamic covalent motions implementing the reactional features of the imine bond. ortho-Carboxybenzaldehyde derivatives are able to form stable adducts with both primary and secondary amines as imines or as amino lactones, respectively, depending on the acidity of the medium. They may thus perform pH-driven intramolecular "walking" along a non-symmetric polyamine chain, in which an imine serves as the terminus under basic conditions on one end of the chain and a lactone formed on a secondary hydroxylamine nitrogen on the other end serves as the terminal site upon addition of acid. The displacement between the termini occurs stochastically through reversible change in valency at the carbon site of the carbonyl group between imine, aminal, iminium and amino lactone form. On the other hand, the directionality results from the stabilisation of the terminal products under given pH conditions. By its ability to undergo interconversion between C=N and O-C-N moieties, the ortho-carboxybenzaldehyde group extends the realm of dynamic covalent chemistry of imines to secondary amines and opens new perspectives in this field.

A ring-flippable sugar as a stimuli-responsive component of liposomes

For the development of a liposome that takes in and out a drug in response to stimuli, 2,4-diaminoxylose (Xyl), which allows stimuli-responsive conformational switches between (4)C1 and (1)C4, was incorporated into a lipid structure: Xyl derivatives with C8 and C16 methylene chains at the 1,3-positions (C8Xyl and C16Xyl) were synthesized. (1)H NMR spectroscopy indicates that the addition of Zn(2+) and then H(+) induces conformational switches from the chair ((4)C1) to the reverse chair ((1)C4) and (1)C4-to-(4)C1, respectively, at Xyl; this leads to transformation of the lipids between linear and bent structures. Osmotic pressure and electron microscopy studies demonstrate that C8Xyl in water forms spherical solid aggregates (C8Xyl-Zn), which are converted into liposomes (C8Xyl+Zn) upon the addition of Zn(2+), and C16Xyl forms liposomes regardless of the presence of Zn(2+). The aggregates of C8Xyl±Zn incorporated a fluorophore and only C8Xyl+Zn released the content upon the addition of HCl. This study shows that Xyl could be a stimuli-responsive component of a liposome.

The electronic effects of ligands on metal-coordination geometry: a key role in the visual discrimination of dimethylaminopyridine and its application towards chemo-switch

Visual discrimination of p-DMAP has been demonstrated by alternation of the geometry of Cu(ii)–terpyridine complexes after selective ligand coordination, and their feasibility towards chemo-switch has also been realized.

Chemically and electrochemically induced expansion and contraction of a ferrocene rotor

A 2,2′-bipyridine-appended ferrocene rotor can be switched, upon treatment with [Cu(CH₃CN)₄](PF₆) and 6,6′-dimesityl-2,2′-bipyridine, from the stacked (syn) conformation to the unstacked (anti) conformation. The switching was completely reversible and could be triggered either chemically, or electrochemically.

A switchable bis-branched [1]rotaxane featuring dual-mode molecular motions and tunable molecular aggregation

A multifunctional bis-branched [1]rotaxane containing a perylene bisimide (PBI) core and two identical bistable[1]rotaxane arms terminated with ferrocene units was prepared and characterized by (1)H NMR, (13)C NMR, and 2D ROESY NMR spectroscopies and by HR-ESI spectrometry. The system is shown to possess several key features: (1) In acetone solution, external acid-base stimuli can result in relative mechanical movements of its ring and thread, which can induce extension and contraction movements of the whole system accompanied by a rotational movement of the ferrocene units, thus realizing dual-mode molecular motions, and the optimized conformations at different states are obtained through molecular dynamics simulations employing the general Amber force field. (2) The introduction of PBI enables the system fluorescence encoding through distance-dependent photoinduced electron transfer process from the ferrocene units to the PBI fluorophore. (3) The addition of Zn(2+) can increase the degree of aggregation of the system, while adding base hinders aggregation because of the movement of the macrocycle. The tunable aggregated nanostructural morphologies of [1]rotaxane were examined by scanning electron microscopy. These results can pave the way to achieve precise control of integrated and coupling nanomechanical motions at a single-molecule level and provide more insight into controlling the aggregate behavior of switchable mechanically interlocked molecules.

Processive catalysis

Nature's enzymes are an ongoing source of inspiration for scientists. The complex processes behind their selectivity and efficiency is slowly being unraveled, and these findings have spawned many biomimetic catalysts. However, nearly all focus on the conversion of small molecular substrates. Nature itself is replete with inventive catalytic systems which modify, replicate, or decompose entire polymers, often in a processive fashion. Such processivity can, for example, enhance the rate of catalysis by clamping to the polymer substrate, which imparts a large effective molarity. Reviewed herein are the various strategies for processivity in nature's arsenal and their properties. An overview of what has been achieved by chemists aiming to mimic one of nature's greatest tricks is also included.