Allosteric Supramolecular Coordination Constructs

Kinetic selection of Pd4L2 metallocyclic and Pd6L3 trigonal prismatic assemblies

The self-assembly of Pd4L2 metallocylcic and Pd6L3 trigonal prismatic assemblies are described. The selection of one species over the other has been achieved by careful choice of ancilliary ligands, which switch the dynamics of the Pd-pyridine bonds such that a highly unusual and distorted smaller assembly can be kinetically trapped en route to the more energetically favourable larger species. Both assemblies provide promise as easy to access multicavity reaction vessels.

Recent developments in the construction and applications of platinum-based metallacycles and metallacages via coordination

Coordination-driven suprastructures have attracted much interest due to their unique properties. Among these structures, platinum-based architectures have been broadly studied due to their facile preparation. The resultant two- or three-dimensional (2D or 3D) systems have many advantages over their precursors, such as improved emission tuning, sensitivity as sensors, and capture and release of guests, and they have been applied in biomedical diagnosis as well as in catalysis. Herein, we review the recent results related to platinum-based coordination-driven self-assembly (CDSA), and the text is organized to emphasizes both the synthesis of new metallacycles and metallacages and their various applications.

Allosteric Regulation of DNAzyme Activities through Intrastrand Transformation Induced by Cu(II)-Mediated Artificial Base Pairing

Allosteric regulation is gaining increasing attention as a basis for the production of stimuli-responsive materials in many research areas including DNA nanotechnology. We expected that metal-mediated artificial base pairs, consisting of ligand-type nucleotides and a bridging metal ion, could serve as allosteric units that regulate the function of DNA molecules. In this study, we established a rational design strategy for developing Cu^II-responsive allosteric DNAzymes by incorporating artificial hydroxypyridone ligand-type nucleotides (H) that form a Cu^II-mediated base pair (H-Cu^II-H). We devised a new enzymatic method using a standard DNA polymerase and a ligase to prepare DNA strands containing H nucleotides. Previously reported DNAzymes were modified by introducing a H-H pair into the stem region, and the stem-loop sequences were altered so that the structure becomes catalytically inactive in the absence of Cu^II ions. The formation of a H-Cu^II-H base pair triggers intrastrand transformation from the inactive to the active structure, enabling allosteric regulation of the DNAzyme activity in response to Cu^II ions. The activity of the H-modified DNAzyme was reversibly switched by the addition and removal of Cu^II ions under isothermal conditions. Similarly, by incorporating a H-Cu^II-H pair into an in vitro-selected Ag^I-dependent DNAzyme, we have developed a DNAzyme that exhibits an AND logic-gate response to Cu^II and Ag^I ions. The rational design strategy and the easy enzymatic synthetic method presented here provide a versatile way to develop a variety of metal-responsive allosteric DNA materials, including molecular machines and logic circuits, based on metal-mediated artificial base pairing.

Construction of Supramolecular Liquid-Crystalline Metallacycles for Holographic Storage of Colored Images

Design and construction of new functionalized supramolecular coordination complexes (SCCs) via coordination-driven self-assembly strategy is highly important in supramolecular chemistry and materials science. Herein, we present a family of well-defined metallacycles decorated with mesogenic forklike dendrons through the strategy of coordination-driven self-assembly. Due to the existence of mesogenic forklike dendrons, the obtained metallacycles displayed the smectic A liquid crystal phase at room temperature while their precursors exhibited the rectangular columnar liquid crystal phase. Interestingly, by taking advantage of the electrostatic interactions between the positively charged metallacycle and the negatively charged heparin, the doping of heparin induced a significant change of the liquid-crystalline behaviors of metallacycles. More importantly, the prepared liquid-crystalline metallacycles could be further applied for holographic storage of colored images. Notably, the rhomboidal metallacycle and hexagonal metallacycle gave rise to different holographic performances although they featured a similar liquid crystal phase behavior. Therefore, this research not only provides the first successful example of supramolecular liquid-crystalline metallacycles for holographic storage of colored images but also opens a new door for supramolecular liquid-crystalline metallacycles toward advanced optical applications.

Inhibition of Ligand Binding Ability of Three Porphyrins by an Organic Effector

A stimulus-responsive receptor 1 was designed and prepared to control the ligand-binding ability of three active sites, two zinc tetraphenylporphyrin units (P1) and one zinc diethynyldiphenylporphyrin unit (P2), with one effector molecule 2. Bulky hexarylbenzene units were incorporated as shielding panels in the middle of the flexible side arms of 1. Spectroscopic titrations indicated that a stable supramolecular complex 1⋅2 (K_1⋅2 =6.7×10⁶  m^-1 ) was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to P1 was inhibited by 2 in a competitive manner. Additionally, the formation of 1⋅2 brought about conformational restriction of the side arms to cover both faces of P2 with the shielding panels. The binding constant of 4-phenylpyridine with P2 in 1⋅2 decreased to 8.9 % of that in 1. Namely, the ligand-binding ability of P2 was inhibited according to an allosteric mechanism.

Coordination-driven self-assembly of discrete supramolecular double-metallacycles

This review comprehensively summarizes the recent advances in the coordination-driven self-assembly of discrete supramolecular double-metallacycles.

Exploiting complexity to implement function in chemical systems

This feature article reflects a personal overview of the importance of complexity as an additional parameter to be considered in chemical research, being illustrated with selected examples in molecular recognition and catalysis.

Substrate-dependent allosteric regulation by switchable catalytic molecular tweezers

Allosteric regulation is exploited by biological systems to regulate the activity and/or selectivity of enzymatic reactions but remains a challenge for artificial catalysts. Here we report switchable terpy(Zn-salphen)2 molecular tweezers and their metal-dependent allosteric regulation of the acetylation of pyridinemethanol isomers. Zinc-salphen moieties can both act as a Lewis acid to activate the anhydride reagents and provide a binding site for pyridinemethanol substrates. The tweezers’ conformation can be reversibly switched between an open and a closed form by a metal ion stimulus. Both states offer distinct catalytic profiles, with closed tweezers showing superior catalytic activity towards ortho substrates, while open tweezers presenting higher rate for the acetylation of meta and para substrates. This notable substrate dependent allosteric response is rationalized by a combination of experimental results and calculations supporting a bimetallic reaction in the closed form for ortho substrate and an inhibition of the cavity for meta and para substrates.

A self-assembled Ru-Pt metallacage as a lysosome-targeting photosensitizer for 2-photon photodynamic therapy

Photodynamic therapy (PDT) is a treatment procedure that relies on cytotoxic reactive oxygen species (ROS) generated by the light activation of a photosensitizer. The photophysical and biological properties of photosensitizers are vital for the therapeutic outcome of PDT. In this work a 2D rhomboidal metallacycle and a 3D octahedral metallacage were designed and synthesized via the coordination-driven self-assembly of a Ru(II)-based photosensitizer and complementary Pt(II)-based building blocks. The metallacage showed deep-red luminescence, a large 2-photon absorption cross-section, and highly efficient ROS generation. The metallacage was encapsulated into an amphiphilic block copolymer to form nanoparticles to encourage cell uptake and localization. Upon internalization into cells, the nanoparticles selectively accumulate in the lysosomes, a favorable location for PDT. The nanoparticles are almost nontoxic in the dark, and can efficiently destroy tumor cells via the generation of ROS in the lysosomes under 2-photon near-infrared light irradiation. The superb PDT efficacy of the metallacage-containing nanoparticles was further validated by studies on 3D multicellular spheroids (MCS) and in vivo studies on A549 tumor-bearing mice.

Different Modes of Anion Response Cause Circulatory Phase Transfer of a Coordination Cage with Controlled Directionality

Controlled directional transport of molecules is essential to complex natural systems, from cellular transport up to organismal circulatory systems. In contrast to these natural systems, synthetic systems that enable transport of molecules between several spatial locations on the macroscopic scale, when external stimuli are applied, remain to be explored. Now, the transfer of a supramolecular cage is reported with controlled directionality between three phases, based on a cage that responds reversibly in two distinct ways to different anions. Notably, circulatory phase transfer of the cage was demonstrated based on a system where the three layers of solvent are arranged within a circular track. The direction of circulation between solvent phases depended upon the order of addition of anions.

Site-Selective Supramolecular Complexation Activates Catalytic Ethane Oxidation by a Nitrido-Bridged Iron Porphyrinoid Dimer

Development of supramolecular methods to further activate a highly reactive intermediate is a fascinating strategy to create novel potent catalysts for activation of inert chemicals. Herein, a supramolecular approach to enhance the oxidizing ability of a high-valent oxo species of a nitrido-bridged iron porphyrinoid dimer that is a known potent molecular catalyst for light alkane oxidation is reported. For this purpose, a nitrido-bridged dinuclear iron complex of porphyrin-phthalocyanine heterodimer 3⁵⁺ , which is connected through a fourfold rotaxane, was prepared. Heterodimer 3⁵⁺ catalyzed ethane oxidation in the presence of H₂ O₂ at a relatively low temperature. The site-selective complexation of 3⁵⁺ with an additional anionic porphyrin (TPPS^4- ) through π-π stacking and electrostatic interactions afforded a stable 1:1 complex. It was demonstrated that the supramolecular post-synthetic modification of 3⁵⁺ enhances its catalytic activity efficiently. Moreover, supramolecular conjugates achieved higher catalytic ethane oxidation activity than nitrido-bridged iron phthalocyanine dimer, which is the most potent iron-oxo-based molecular catalyst for light-alkane oxidation reported so far. Electrochemical measurements proved that the electronic perturbation from TPPS^4- to 3⁵⁺ enhanced the catalytic activity.

Palladium-Based Metal-Ligand Assemblies: The Contrasting Behavior upon Addition of Pyridine or Acid

The stability of five different [Pd _n(N-donor) _m]^{2 n+} assemblies was examined by performing disassembly experiments with pyridine and with trifluoroacetic acid. Pyridine-induced disassembly was found to be most pronounced for Pd complexes containing N-donor ligands of low basicity. At the same time, these assemblies displayed high acid resistance. The contrasting stability in the presence of acid or pyridine can be used for the pH-controlled switching between different metallosupramolecular structures.

A Supramolecular Palladium Catalyst Displaying Substrate Selectivity by Remote Control

Inspired by enzymes such as cytochrome P-450, the study of the reactivity of metalloporphyrins continues to attract major interest in the field of homogeneous catalysis. However, little is known about benefitting from the substrate-recognition properties of porphyrins containing additional, catalytically relevant active sites. Herein, such an approach is introduced by using supramolecular ligands derived from metalloporphyrins customized with rigid, palladium-coordinating nitrile groups. According to different studies (NMR and UV/Vis spectroscopy, XRD, control experiments), the supramolecular ligands are able to accommodate pyridine derivatives as substrates inside the porphyrin pocket while the reactivity occurs at the peripheral side. By simply tuning a remote metal center, different binding events result in different catalyst reactivity, and this enzyme-like feature leads to high degrees of substrate selectivity in representative palladium-catalyzed Suzuki-Miyaura reactions.

Supramolecular tripodal Au(i) assemblies in water. Interactions with a pyrene fluorescent probe

The synthesis of three gold(i) tripodal complexes derived from tripropargylamine and containing the water soluble phosphines PTA (1,3,5-triaza-7-phosphaadamantane), DAPTA (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) and TPPTS (triphenylphosfine-3,3′,3′′-trisulfonic acid trisodium salt) is described here.

Polynuclear alkoxy-zinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2

The reactions between alcohols and the tetranuclear ethyl-Zn complexes of an ortho-phenylene-bridged polypyrrole macrocycle, Zn4Et4(L1) 1 and the related anthracenyl-bridged macrocyclic complex, Zn4Et4(THF)4(L2) 2 have been studied. With long-chain alcohols such as n-hexanol, the clean formation of the tetranuclear hexoxide complex Zn4(OC6H13)4(L1) 3 occurs. In contrast, the use of shorter-chain alcohols such as i-propanol results in the trinuclear complex Zn3(μ2-OiPr)2(μ3-OiPr)(HL1) 4 that arises from demetalation; this complex was characterised by X-ray crystallography. The clean formation of these polynuclear zinc clusters allowed a study of their use as catalysts in the ring-opening copolymerisation (ROCOP) reaction between cyclohexene oxide and CO2. In situ reactions involving the pre-catalyst 1 and n-hexanol formed the desired polymer with the best selectivity for polycarbonate (90%) at 30 atm CO2, whilst the activity and performance of pre-catalyst 2 was poor in comparison.

Fluorescent supramolecular polymers with aggregation induced emission properties

This review summarizes the recent developments in AIE fluorescent supramolecular polymeric materials based on different types of intermolecular noncovalent interactions, and their wide ranging applications as chemical sensors, organic electronic materials, bio-imaging agents and so on.

Switchable hydrogenation with a betaine-derived bifunctional Ir–NHC catalyst

A bifunctional iridium catalyst based on the ‘uracil–abnormal NHC’ hybrid ligand platform was developed for switchable hydrogenation of quinoxalines.

A Redox-Switchable, Allosteric Coordination Complex

A redox-regulated molecular tweezer complex was synthesized via the weak-link approach. The Pt^II complex features a redox-switchable hemilabile ligand (RHL) functionalized with a ferrocenyl moiety, whose oxidation state modulates the opening of a specific coordination site. Allosteric regulation by redox agents gives reversible access to two distinct structural states-a fully closed state and a semi-open state-whose interconversion was studied via multinuclear NMR spectroscopy, cyclic voltammetry, and UV-vis-NIR spectroscopy. Two structures in this four-state system were further characterized via SCXRD, while the others were modeled through DFT calculations. This fully reversible, RHL-based system defines an unusual level of electrochemical control over the occupancy of a specific coordination site, thereby providing access to four distinct coordination states within a single system, each defined and differentiated by structure and oxidation state.

A four-state fluorescent molecular switch

Four distinct fluorescent states are achieved in a single Weak-Link Approach (WLA) construct bearing pyrene and tetraphenylethene moieties. The fluorescence of the compound in both the solution and solid phases can be manipulated through reversible coordination chemistry at the PtII center.

Hierarchical Assemblies of Supramolecular Coordination Complexes

Hierarchical self-assembly (HAS) is a multilevel organization process that first assembles elementary molecular units into ordered secondary structures via noncovalent interactions, which further act as the building blocks to form more complex multifunctional superstructures at the next level(s). The HAS strategy has been used as a versatile method for the preparation of soft-matter nanoarchitectures of defined size and morphologies, tunable luminescence, and biological importance. However, such preparation can be greatly simplified if well-defined dynamic structures are employed as the cores that upon linking form the desired nanoarchitectures. Discrete supramolecular coordination complexes (SCCs) with well-defined shapes, sizes, and internal cavities have been widely employed to construct hierarchical systems with functional diversity. This Account summarizes the prevailing strategies used in recent years in the preparation of SCC-based HASs and illustrates how the combination of dynamic metal-ligand coordination with other interactions was used to obtain hierarchical systems with interesting properties. HASs with dual orthogonal interactions involving coordination-driven self-assembly and hydrogen bonding/host-guest interaction generally result in robust and flexible supramolecular gels. Likewise, hybridization of SCCs with a suitable dynamic covalent network via a hierarchical strategy is useful to prepare materials with self-healing properties. The intrinsic positive charges of the SCCs also make them suitable precursors for the construction of HASs via electrostatic interactions with negatively charged biological/abiological molecules. Furthermore, the interplay between the hydrophilic and lipophilic characters of HASs by varying the number and spacial orientation of alkyl/oxyethylene chains of the SCC is a simple yet controllable approach to prepare ordered and tunable nanostructures. Certain SCC-cored hierarchical systems exhibit reversible polymorphism, typically between micellar, nanofiber, and vesicular phases, in response to various external perturbations: heat, photoirradiation, pH-variance, redox-active agents, etc. At the same time, multiple noncovalent interaction mediated HASs are growing in numbers and are promising candidates for obtaining functionally diverse materials. The photophysical properties of SCC-based HASs have been used in many analytical applications. For example, embedding tetraphenylethene (TPE)-based pyridyl ligands within metallo-supramolecular structures partially restricts the molecular rotations of its phenyl rings, endowing the resultant SCCs with weak emissions. Further aggregation of such HASs in suitable solvents results in a marked enhancement in emission intensity along with quantum yields. They act as sensitive sensors for different analytes, including pathogens, drugs, etc. HASs are also useful to develop multidrug systems with cooperative chemotherapeutic effects. Hence, the use of HASs with theranostic SCCs combining cell-imaging agents and chemotherapeutic scaffolds is a promising drug delivery strategy for cancer theranostics. At the same time, their responsiveness to stimuli, oftentimes due to the dynamic nature of the metal-ligand interactions, play an important role in drug release via a disassembly mechanism.

Coordination-driven self-assembly of discrete Ru6-Pt6 prismatic cages

The coordination-driven self-assembly of two new Ru6-Pt6 hexanuclear trigonal prismatic cages comprising arene-ruthenium(II) clips (1a(NO3)2 and 1b(NO3)2 ) and a tritopic platinum(II) metalloligand 2 has been performed in methanol at room temperature. The [3 + 2] hexanuclear cages 3a and 3b were isolated in good yields and characterized by well-known spectroscopic techniques including multinuclear NMR, mass spectrometry, UV-vis and infrared studies. Geometry optimization revealed the shapes and sizes of these hexanuclear prismatic cages. The combination of ruthenium and platinum metal center in a one-pot self-assembly reaction showcases the construction of aesthetically elegant heterometallic structures in supramolecular chemistry leading to the formation of a single major product.

Heterometallic Ru-Pt metallacycle for two-photon photodynamic therapy

As an effective and noninvasive treatment of various diseases, photodynamic therapy (PTD) relies on the combination of light, a photosensitizer, and oxygen to generate cytotoxic reactive oxygen species that can damage malignant tissue. Much attention has been paid to covalent modifications of the photosensitizers to improve their photophysical properties and to optimize the pathway of the photosensitizers interacting with cells within the target tissue. Herein we report the design and synthesis of a supramolecular heterometallic Ru-Pt metallacycle via coordination-driven self-assembly. While inheriting the excellent photostability and two-photon absorption characteristics of the Ru(II) polypyridyl precursor, the metallacycle also exhibits red-shifted luminescence to the near-infrared region, a larger two-photon absorption cross-section, and higher singlet oxygen generation efficiency, making it an excellent candidate as a photosensitizer for PTD. Cellular studies reveal that the metallacycle selectively accumulates in mitochondria and nuclei upon internalization. As a result, singlet oxygen generated by photoexcitation of the metallacycle can efficiently trigger cell death via the simultaneous damage to mitochondrial function and intranuclear DNA. In vivo studies on tumor-bearing mice show that the metallacycle can efficiently inhibit tumor growth under a low light dose with minimal side effects. The supramolecular approach presented in this work provides a paradigm for the development of PDT agents with high efficacy.

Allosteric Self-Assembly of Coordinating Terthiophene Amphiphile for Triggered Light Harvesting

Allosteric regulation is extensively employed by nature to achieve functional control of protein or deoxyribonucleic acid through triggered conformational change at a remote site. We report that a similar strategy can be utilized in artificial self-assembly to control the self-assembled structure and its function. We show that on binding of metal ions to the headgroup of an amphiphile TTC4L, the conformational change may lead to change of the dipole orientation of the energy donor at the chain end. This on the one hand leads to a drastically different self-assembled structure; on the other hand, it enables light harvesting between the donor-acceptor. Because the Forster resonance fluorescence transfer efficiency is gated by metal ions, controlling the feeding of metal ions allows switching on and off of light harvesting. We expect that using allosteric self-assembly, we will be able to create abundant structures with distinct function from limited molecules, which show prominent potential for the postorganic modification of the structure and function of self-assembled materials.

Tuned Amperometric Detection of Reduced β-Nicotinamide Adenine Dinucleotide by Allosteric Modulation of the Reductase Component of the p-Hydroxyphenylacetate Hydroxylase Immobilized within a Redox Polymer

We report the fabrication of an amperometric NADH biosensor system that employs an allosterically modulated bacterial reductase in an adapted osmium(III)-complex-modified redox polymer film for analyte quantification. Chains of complexed Os(III) centers along matrix polymer strings make electrical connection between the immobilized redox protein and a graphite electrode disc, transducing enzymatic oxidation of NADH into a biosensor current. Sustainable anodic signaling required (1) a redox polymer with a formal potential that matched the redox switch of the embedded reductase and avoided interfering redox interactions and (2) formation of a cross-linked enzyme/polymer film for stable biocatalyst entrapment. The activity of the chosen reductase is enhanced upon binding of an effector, i.e. p-hydroxy-phenylacetic acid ( p-HPA), allowing the acceleration of the substrate conversion rate on the sensor surface by in situ addition or preincubation with p-HPA. Acceleration of NADH oxidation amplified the response of the biosensor, with a 1.5-fold increase in the sensitivity of analyte detection, compared to operation without the allosteric modulator. Repetitive quantitative testing of solutions of known NADH concentration verified the performance in terms of reliability and analyte recovery. We herewith established the use of allosteric enzyme modulation and redox polymer-based enzyme electrode wiring for substrate biosensing, a concept that may be applicable to other allosteric enzymes.

A high-speed network of nanoswitches for on/off control of catalysis

Copper(i) ion translocation is the key for fast and reliable communication between networked devices in the catalytic machinery.

Covalent post-assembly modification in metallosupramolecular chemistry

This review examines the growing variety of covalent reactions used to achieve the post-assembly modification of self-assembled metallosupramolecular complexes.

Mapping the Assembly of Metal-Organic Cages into Complex Coordination Networks

Structural transformations of supramolecular assemblies play an important role in the synthesis of complex metal-organic materials. Nonetheless, often little is known of the assembly pathways that lead to the final product. This work describes the conversion of cubic metal-organic polyhedra to connected-cage networks of varying topologies. The neutral cubic cage assembly of formula {Pd₃ [PO(NiPr)₃ ]}₈ (PZDC)₁₂ has been synthesized from {Pd₃ [(NiPr)₃ PO](OAc)₂ (OH)}₂ ⋅2 (CH₃ )₂ SO and 2,5-pyrazenedicarboxilic acid (PZDC-2H). This 42-component self-assembly is the largest known among the neutral cages with Pd^II ions. The cage contains twenty-four vacant carboxylate O-sites at the PZDC ligands that are available for further coordination. Post-assembly reactions of the cubic cage with Fe^II and Zn^II ions produced cage-connected networks of dia and qtz topologies, respectively. During these reactions, the discrete cubic cage transforms into a network of tetrahedral cages that are bridged by the 3D metal ions. The robustness of the [Pd₃ {[PO(NiPr)₃ }]³⁺ molecular building units made it possible to map the post-assembly reactions in detail, which revealed a variety of intermediate 1D and 2D cage networks. Such step-by-step mapping of the transformation of discrete cages to cage-connected frameworks is unprecedented in the chemistry of coordination driven assemblies.

Synthesis of stable polymetalated aromatic complexes through metal-macrocycle capsule-triggered cyclization

Polymetalated aromatic compounds are of great interest because of their intermediate roles in many organic transformations. However, they are elusive and synthetically challenging. In this study, a dynamic coordination capsule constructed by a flexible macrocycle and silver(i) ions is applied to trigger one-step or cascade cyclization reactions for various alkyne substrates, finally leading to five unprecedented polysilver heteroaromatic intermediates (including indole, quinoline, benzocarbazole and 2,2'-biindole). The acquired heteroaromatic species is doubly charged, particularly at vicinal positions, and each is surrounded by a tetrasilver aggregate. The metal-macrocycle capsule holds a great potential of flexibly adjusting its conformation to adapt different polysilver heteroaromatic species. DFT calculations further reveal that metal-perturbed aromaticity and multi-centered bonding both contribute to stabilization of the polysilver heteroaromatic complexes.