Kinetically controlled phenomena in dynamic combinatorial libraries

Kinetic Control of Complexity in Multiple Dynamic Libraries

Multiple dynamic libraries of compounds are generated when more than one reversible reaction comes into play. Commonly, two or more orthogonal reversible reactions are used, leading to non-communicating dynamic libraries which share no building blocks. Only a few examples of communicating libraries have been reported, and in all those cases, building blocks are reversibly exchanged from one library to the other, constituting an antiparallel dynamic covalent system. Herein we report that communication between two different dynamic libraries through an irreversible process is also possible. Indeed, alkyl amines cancel the dynamic regime on the nucleophilic substitution of tetrazines, generating kinetically inert compounds. Interestingly, such amine can be part of another dynamic library, an imine-amine exchange. Thus, both libraries are interconnected with each other by an irreversible process which leads to kinetically inert structures that contain parts from both libraries, causing a collapse of the complexity. Additionally, a latent irreversible intercommunication could be developed. In such a way, a stable molecular system with specific host-guest and fluorescence properties, could be irreversibly transformed when the right stimulus was applied, triggering the cancellation of the original supramolecular and luminescent properties and the emergence of new ones.

Neighboring Effects of Sulfur Oxidation State on Dynamic Covalent Bonds and Assemblies

The impact of a varied sulfur oxidation state (sulfide, sulfoxide, and sulfone) on imine dynamic covalent chemistry is presented. The role of noncovalent interactions, including chalcogen bonds and CH hydrogen bonds, on aldehyde/imine structures and imine exchange reactions was elucidated through experimental and computational evidence. The change in the sulfur oxidation state and diamine linkage further allowed the regulation of imine macrocycles, providing a platform for controlling molecular assemblies.

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

3D Covalent Organic Framework as a Metastable Intermediate in the Formation of a Double-Stranded Helical Covalent Polymer

The design and development of intricate artificial architectures have been pursued for decades. Helical covalent polymer (HCP) was recently reported as an unexpected topology that consists of chiral 1D polymers assembled through weak hydrogen bonds from achiral building blocks. However, many questions remained about the formation, driving force, and the single-handedness observed in each crystal. In this work, we reveal a metastable, racemic, fully covalently cross-linked, 3D covalent organic framework (COF) as an intermediate in the early stage of polymerization, which slowly converts into single-handed HCP double helices through partial fragmentation and self-sorting with the aid of a series of hydrogen bonding. Our work provides an intriguing example where weak noncovalent bonds serve as the determining factor of the overall product structure and facilitate the formation of a sophisticated polymeric architecture.

Light-Fueled Primitive Replication and Selection in Biomimetic Chemical Systems

The concept of chemically evolvable replicators is central to abiogenesis. Chemical evolvability requires three essential components: energy-harvesting mechanisms for nonequilibrium dissipation, kinetically asymmetric replication and decomposition pathways, and structure-dependent selective templating in the autocatalytic cycles. We observed a UVA light-fueled chemical system displaying sequence-dependent replication and replicator decomposition. The system was constructed with primitive peptidic foldamer components. The photocatalytic formation-recombination cycle of thiyl radicals was coupled with the molecular recognition steps in the replication cycles. Thiyl radical-mediated chain reaction was responsible for the replicator death mechanism. The competing and kinetically asymmetric replication and decomposition processes led to light intensity-dependent selection far from equilibrium. Here, we show that this system can dynamically adapt to energy influx and seeding. The results highlight that mimicking chemical evolution is feasible with primitive building blocks and simple chemical reactions.

Crystallization- and Metal-Driven Selection of Discrete Macrocycles/Cages and Their Metallosupramolecular Polymers from Dynamic Systemic Networks

Reversible imine- and metal-coordination reactions are dynamic enough to produce complex libraries of macrocycles, cages, and supramolecular polymers in solution, from which amplification effects have been identified in solution or during crystallization in response to ligand- and metal-driven selection modes. Crystallization-driven selection can lead to the amplification of unexpected metallosupramolecular architectures. The addition of Ag+ triggered the change of the optimal components, so that the crystallization process showed different ligand preferences than in solution. The most packed constituents are amplified in the solid state, taking into account the optimal coordination of metal ions together with non-specific non-covalent interactions between the macrocycle packed in dimers or trimers in the solid state.

Dispersive 2D Triptycene-Based Crystalline Polymers: Influence of Regioisomerism on Crystallinity and Morphology

The merging of good crystallinity and high dispersibility into two-dimensional (2D) layered crystalline polymers (CPs) still represents a challenge because a high crystallinity is often accompanied by intimate interlayer interactions that are detrimental to the material processibility. We herein report a strategy to address this dilemma using rationally designed three-dimensional (3D) monomers and regioisomerism-based morphology control. The as-synthesized CPs possess layered 2D structures, where the assembly of layers is stabilized by relatively weak van der Waals interactions between C-H bonds other than the usual π-π stackings. The morphology and dispersibility of the CPs are finely tuned via regioisomerism. These findings shed light on how to modulate the crystallinity, morphology, and ultimate function of crystalline polymers using the spatial arrangements of linking groups.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

Recent advances in DNA-encoded dynamic libraries

The DNA-encoded chemical library (DEL) has emerged as a powerful technology platform in drug discovery and is also gaining momentum in academic research. The rapid development of DNA-/DEL-compatible chemistries has greatly expanded the chemical space accessible to DELs. DEL technology has been widely adopted in the pharmaceutical industry and a number of clinical drug candidates have been identified from DEL selections. Recent innovations have combined DELs with other legacy and emerging techniques. Among them, the DNA-encoded dynamic library (DEDL) introduces DNA encoding into the classic dynamic combinatorial libraries (DCLs) and also integrates the principle of fragment-based drug discovery (FBDD), making DEDL a novel approach with distinct features from static DELs. In this Review, we provide a summary of the recently developed DEDL methods and their applications. Future developments in DEDLs are expected to extend the application scope of DELs to complex biological systems with unique ligand-discovery capabilities.

Triazine 2D Nanosheets as a New Class of Nanomaterials: Crystallinity, Properties and Applications

Based on the recent (2015–2021) literature data, the authors analyze the mutual dependence of crystallinity/amorphism and specific surface area and porosity in covalent triazine frameworks (CTFs), taking into account thermodynamic and kinetic control in the synthesis of these 2D nanosheets. CTFs have now become a promising new class of high-performance porous organic materials. They can be recycled and reused easily, and thus have great potential as sustainable materials. For 2D CTFs, numerous examples are given to support the known rule that the structure and properties of any material with a given composition depend on the conditions of its synthesis. The review may be useful for elder students, postgraduate students, engineers and research fellows dealing with chemical synthesis and modern nanotechnologies based on 2D covalent triazine frameworks.

Chemical Evolution of Natural Product Structure

Natural products are the result of Nature’s exploration of biologically relevant chemical space through evolution and an invaluable source of bioactive small molecules for chemical biology and medicinal chemistry. Novel concepts for the discovery of new bioactive compound classes based on natural product structure may enable exploration of wider biologically relevant chemical space. The pseudo-natural product concept merges the relevance of natural product structure with efficient exploration of chemical space by means of fragment-based compound development to inspire the discovery of new bioactive chemical matter through de novo combination of natural product fragments in unprecedented arrangements. The novel scaffolds retain the biological relevance of natural products but are not obtainable through known biosynthetic pathways which can lead to new chemotypes that may have unexpected or unprecedented bioactivities. Herein, we cover the workflow of pseudo-natural product design and development, highlight recent examples, and discuss a cheminformatic analysis in which a significant portion of biologically active synthetic compounds were found to be pseudo-natural products. We compare the concept to natural evolution and discuss pseudo-natural products as the human-made equivalent, i.e. the chemical evolution of natural product structure.

Interplay between chalcogen bonds and dynamic covalent bonds

A combination of chalcogen bonds, one type of emerging non-covalent bonding force, and imine bonds, allow the control of the dynamic covalent chemistry with orbital interactions and the reversal of kinetic and thermodynamic selectivity.

Dynamic Covalent Chemistry Constrained Diphenylethenes: Control over Reactivity and Luminescence in both Solution and Solid State

Diarylethenes (DAEs) are an important class of building blocks in chemistry and materials science, and hence, their modulation and functionalization are of critical significance. Here we demonstrate a general strategy...

Light-Induced Formation/Scission of C-N, C-O, and C-S Bonds Enables Switchable Stability/Degradability in Covalent Systems

The manipulation of covalent bonds could be directed toward degradable, recyclable, and sustainable materials. However, there is an intrinsic conflict between properties of stability and degradability. Here we report light-controlled formation/scission of three types of covalent bonds (C-N, C-O, and C-S) through photoswitching between equilibrium and nonequilibrium states of dynamic covalent systems, achieving dual benefits of photoaddressable stability and cleavability. The photocyclization of dithienylethene fused aldehyde ring-chain tautomers turns on the reactivity, incorporating/releasing amines, alcohols, and thiols reversibly with high efficiency, respectively. Upon photocycloreversion the system is shifted to kinetically locked out-of-equilibrium form, enabling remarkable robustness of covalent assemblies. Reaction coupling allows remote and directional control of a diverse range of equilibria and further broadens the scope. Through locking and unlocking covalent linkages with light when needed, the utility is demonstrated with capture/release of bioactive molecules, modification of surfaces, and creation of polymers exhibiting tailored stability and degradability/recyclability. The versatile toolbox for photoswitchable dynamic covalent reactions to toggle matters on and off should be appealing to many endeavors.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines

A dynamic nucleophilic aromatic substitution of tetrazines (SN Tz) is presented herein. It combines all the advantages of dynamic covalent chemistry with the versatility of the tetrazine moiety. Indeed, libraries of compounds or sophisticated molecular structures can be easily obtained, which are susceptible to post-functionalization by inverse electron demand Diels-Alder (IEDDA) reaction, which also locks the exchange. Additionally, the structures obtained can be disassembled upon the application of the right stimulus, either UV irradiation or a suitable chemical reagent. Moreover, SN Tz is compatible with the imine chemistry of anilines. The high potential of this methodology has been proved by building two responsive supramolecular systems: A macrocycle that displays a light-induced release of acetylcholine; and a truncated [4+6] tetrahedral shape-persistent fluorescent cage, which is disassembled by thiols unless it is post-stabilized by IEDDA.

Activated Self-Resolution and Error-Correction in Catalytic Reaction Networks*

Understanding the emergence of function in complex reaction networks is a primary goal of systems chemistry and origin-of-life studies. Especially challenging is to create systems that simultaneously exhibit several emergent functions that can be independently tuned. In this work, a multifunctional complex reaction network of nucleophilic small molecule catalysts for the Morita-Baylis-Hillman (MBH) reaction is demonstrated. The dynamic system exhibited triggered self-resolution, preferentially amplifying a specific catalyst/product set out of a many potential alternatives. By utilizing selective reversibility of the products of the reaction set, systemic thermodynamically driven error-correction could also be introduced. To achieve this, a dynamic covalent MBH reaction based on adducts with internal H-transfer capabilities was developed. By careful tuning of the substituents, rate accelerations of retro-MBH reactions of up to four orders of magnitude could be obtained. This study thus demonstrates how efficient self-sorting of catalytic systems can be achieved through an interplay of several complex emergent functionalities.

Rapidly Adaptive All-covalent Nanoparticle Surface Engineering

Emerging nanotechnologies demand the manipulation of nanoscale components with the same predictability and programmability as is taken for granted in molecular synthetic methodologies. Yet installing appropriately reactive chemical functionality on nanomaterial surfaces has previously entailed compromises in terms of reactivity scope, functionalization density, or both. Here, we introduce an idealized dynamic covalent nanoparticle building block for divergent and adaptive post-synthesis modification of colloidal nanomaterials. Acetal-protected monolayer-stabilized gold nanoparticles are prepared via operationally simple protocols and are stable to long-term storage. Tunable surface densities of reactive aldehyde functionalities are revealed on-demand, leading to a wide range of adaptive surface engineering options from one nanoscale synthon. Analytically tractable with molecular precision, interfacial reaction kinetics and dynamic surface constitutions can be probed in situ at the ensemble level. High functionalization densities combined with rapid equilibration kinetics enable environmentally adaptive surface constitutions and rapid nanoparticle property switching in response to simple chemical effectors.

Sterics and Hydrogen Bonding Control Stereochemistry and Self-Sorting in BINOL-Based Assemblies

Here we demonstrate how the hydrogen-bonding ability of a BINOL-based dialdehyde subcomponent dictated the stereochemical outcome of its subsequent self-assembly into one diastereomeric helicate form when bearing free hydroxy groups, and another in the case of its methylated congener. The presence of methyl groups also altered the self-sorting behavior when mixed with another, short linear dialdehyde subcomponent, switching the outcome of the system from narcissistic to integrative self-sorting. In all cases, the axial chirality of the BINOL building block also dictated helicate metal center handedness during stereospecific self-assembly. A new family of stereochemically pure heteroleptic helicates were thus prepared using the new knowledge gained. We also found that switching from Fe^II to Zn^II, or the incorporation of a longer linear ligand, favored heteroleptic structure formation.

By-design molecular architectures via alkyne metathesis

Shape-persistent purely organic molecular architectures have attracted tremendous research interest in the past few decades. Dynamic Covalent Chemistry (DCvC), which deals with reversible covalent bond formation reactions, has emerged as an efficient synthetic approach for constructing these well-defined molecular architectures. Among various dynamic linkages, the formation of ethynylene linkages through dynamic alkyne metathesis is of particular interest due to their high chemical stability, linearity, and rigidity. In this review, we focus on the synthetic strategies of discrete molecular architectures (e.g., macrocycles, molecular cages) containing ethynylene linkages using alkyne metathesis as the key step, and their applications. We will introduce the history and challenges in the synthesis of those architectures via alkyne metathesis, the development of alkyne metathesis catalysts, the reported novel macrocycle structures, molecular cage structures, and their applications. In the end, we offer an outlook of this field and remaining challenges.

The recent synthesis of novel shape-persistent 2D and 3D molecular architectures via alkyne metathesis is reviewed and the critical role of catalysts is also highlighted.

Metal Cation-Driven Dynamic Covalent Formation of Imine and Hydrazone Ligands Displaying Synergistic Co-catalysis and Auxiliary Amine Effects

Optimizing C=N bond formation and C/N component exchange has major significance in Dynamic Covalent Chemistry (DCC). Imine and hydrazone generation from their aldehyde, amine and hydrazine components showed large accelerations in presence of AgOTf or Zn(OTf)₂ , up to 10⁴ for the Zn(II)-(p-anisidine)imine complex. Zn(OTf)₂ and auxiliary p-anisidine together accelerated 630 times the formation of the Zn(II)-hydrazone complex, revealing a strong synergistic effect, traced to very fast initial formation of the reactive Zn(II)-imine complex presenting a C=N bond metallo-activated towards reaction with the hydrazine component. Reactions involving more entities showed kinetically faster and thermodynamically simpler outputs due to dynamic competition within a mixture of higher complexity. Catalytic amounts of metal salts and auxiliary amine gave similar marked rate accelerations and turnover, indicating true catalysis. The synergistic effect achieved by combining metallo- and organo-catalysis points to a powerful co-catalysis strategy of bond-formation in DCC through interconnected chemical transformations.

Simultaneous Formation of a Fully Organic Triply Dynamic Combinatorial Library

Here we report the simultaneous formation of doubly and triply dynamic libraries as a result of exchange reactions between functionalized organic building blocks. A combination of three different reversible covalent linkages involving a boronate ester transesterification along with an imine and disulfide exchange was employed to generate a new type of fully organic triply dynamic molecular assembly.

Self-sorting of porous Cu4L2L'2 metal-organic cages composed of isomerisable ligands

We report the self-sorting of a dynamic combinatorial library (DCL) of metal-organic cages composed of a rotationally isomerisable ligand. Convergence of the DCL occurs upon crystallisation and leads to low-symmetry Cu4L2L'2 cages that display differing porosities based on their overall shape and ligand configuration.

Dynamic covalent bond constrained ureas for multimode fluorescence switching, thermally induced emission, and chemical signaling cascades

A combination of organic ureas and dynamic covalent chemistry was demonstrated for multistate switching, thermally induced fluorescence, and signaling cascades.

Hydrogen-Bond Catalysis of Imine Exchange in Dynamic Covalent Systems

The reversibility of imine bonds has been exploited to great effect in the field of dynamic covalent chemistry, with applications such as preparation of functional systems, dynamic materials, molecular machines, and covalent organic frameworks. However, acid catalysis is commonly needed for efficient equilibration of imine mixtures. Herein, it is demonstrated that hydrogen bond donors such as thioureas and squaramides can catalyze the equilibration of dynamic imine systems under unprecedentedly mild conditions. Catalysis occurs in a range of solvents and in the presence of many sensitive additives, showing moderate to good rate accelerations for both imine metathesis and transimination with amines, hydrazines, and hydroxylamines. Furthermore, the catalyst proved simple to immobilize, introducing both reusability and extended control of the equilibration process.

Kinetic and Thermodynamic Modulation of Dynamic Imine Libraries Driven by the Hexameric Resorcinarene Capsule

The composition of dynamic covalent imine libraries (DCL) adapts to the presence of the hexameric resorcinarene capsule. In the presence of the self-assembled capsule, a kinetic and thermodynamic modulation of the imine constituents of the DCLs was observed, which was induced by an unusual predatory action of the capsule on specific imine constituents. More complex 2 × 2 DCLs also adapt to the presence of the hexameric capsule, showing a thermodynamic and kinetic modulation of the constituents induced by the predatory action of the capsule. By cross-referencing experimental data, a good selectivity (up to 66%) for one constituent can be induced in a 2 × 2 DCL.

Three-Way Chemoselectivity Switching through Coupled Equilibria

Controlling the chemoselectivity of reactions operating on complex mixtures, including those found in biological and petrochemical feedstocks or in the primordial soup from which life emerged, is generally challenging. The selectivity of imine oxidation can be controlled in dynamic combinatorial libraries, wherein coupled equilibria of imine exchange and the diaza-Cope rearrangement determine whether and when the oxidizable precursor is made available to the oxidant. Adjusting the rate of oxidant addition allows the isolation of three dominant products.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

Self-Assembly, Adaptive Response, and in,out-Stereoisomerism of Large Orthoformate Cryptands

We report on triethylene glycol-based orthoformate cryptands, which adapt their bridgehead configurations in response to metal templates and intramolecular hydrogen bonding in a complex manner. In contrast to smaller 1.1.1-orthoformate cryptands, the inversion from out,out-2.2.2 to in,in-2.2.2 occurs spontaneously by thermal homeomorphic isomerization, i. e., without bond breakage. The global thermodynamic minimum of the entire network, which includes an unprecedented third isomer (in,out-2.2.2), could only be reached under conditions that facilitate dynamic covalent exchange. Both inversion processes were studied in detail, including DFT calculations and MD simulations, which were particularly helpful for explaining differences between equilibrium compositions in solvents chloroform and acetonitrile. Unexpectedly, the system could be driven to the in,out-2.2.2 state by using a metal template with a size mismatch with respect to the out,out-2.2.2 cage.

Adaptive Covalent Networks Enabled by Dual Reactivity: The Evolution of Reversible Covalent Bonds, Their Molecular Assemblies, and Guest Recognition

Adaptive chemistry allows transformation and selection within molecular networks, and adaptive systems composed of different types of dynamic covalent reactions (DCRs) are challenging. Herein, we demonstrate dual reactivity-based covalent networks encompassing the regulation of and switching between C-N- and C-S-based reversible covalent assemblies. The creation and exchange of C-N- or C-S-derived assemblies exhibiting diverse architectures, including linear structures, macrocycles, and cages, were achieved. The shift of reactivity then permitted the interconversion between C-N- and C-S-containing assemblies. Moreover, the adaption of intramolecular and intermolecular scaffolds was feasible via linker design. The latent hemiaminal chirality center offered a pathway for the induction of chirality within assemblies. Finally, switchable structural change and controlled extraction of ions were realized with Hg²⁺ as a guest for macrocycles. The remarkable complexity of networks described herein could open the door for the utility in sophisticated functional systems.

n → π* interactions as a versatile tool for controlling dynamic imine chemistry in both organic and aqueous media

The imine bond holds a prominent place in supramolecular chemistry and materials science, and one issue is the stability of imines due to their electrophilic nature. Here we introduced ortho-carboxylate groups into a series of aromatic aldehydes/imines for dictating imine dynamic covalent chemistry (DCC) through n → π* interactions, one class of widespread and yet underused non-covalent interactions. The thermodynamically stabilizing role of carboxylate-aldehyde/imine n → π* interactions in acetonitrile was elucidated by the movement of the imine exchange equilibrium and further supported by crystal analysis. Computational studies provided mechanistic insights for n → π* interactions, the strength of which can surpass that of CH hydrogen bonding and is dependent on the orientation of interacting sites based on natural bond orbital analysis. Moreover, the substituent effect and the combination of recognition sites allowed additional means for modulation. Finally, to show the relevance of our findings ortho-carboxylate containing aldehydes were used to regulate imine formation/exchange in water, and modification of the N-terminus of amino acids and peptides was achieved in a neutral buffer. This work represents the latest example of weak interactions governing DCC and sets the stage for assembly and application studies.

Noncovalent and Dynamic Covalent Chemistry Strategies for Driving Thermoresponsive Phase Transition with Multistimuli and Controlled Encapsulation/Release

We report the development of multiresponsive thermally sensitive polymers through both supramolecular and reversible covalent strategies as well as their use in controlled encapsulation and release. Novel acylhydrazone-based dynamic covalent polymers displaying lower critical solution temperature (LCST) or upper critical solution temperature (UCST) were synthesized. A remarkable control over thermal phase transition can be tuned through multimodes, such as anions, cations, solvent, pH, and competing components. In particular, anion recognition allowed disassembly and thus led to a significant decrease of UCST in dimethyl sulfoxide, and the combination of anion and solvent effects offered additional handle for control. Moreover, the use of anions, cations, as well as pH change was employed for the modulation of LCST-type polymer in water. Furthermore, switching on/off thermoresponsiveness was readily achieved by dynamic covalent exchange. Mechanistic studies also shed light on stimuli-induced changes in aggregation behaviors. Finally, thermally controlled encapsulation and release of hydrophobic and hydrophilic dyes were realized with great repeatability and reversibility, respectively, showing potential in delivery and sensing. The results and strategies described should provide opportunities for many aspects, including dynamic assemblies, complex systems, and adaptive materials.

Self-sorting of two imine-based metal complexes: balancing kinetics and thermodynamics in constitutional dynamic networks

A major hurdle in the development of complex constitutional dynamic networks (CDNs) is the lack of strategies to simultaneously control the output of two (or more) interconnected dynamic processes over several species, namely reversible covalent imine bond formation and dynamic metal–ligand coordination. We have studied in detail the self-sorting process of 11 constitutional dynamic libraries containing two different amines, aldehydes and metal salts into two imine-based metal complexes, having no overlap in terms of their compositions. This study allowed us to determine the factors influencing the fidelity of this process (concentration, electronic and steric parameters of the organic components, and nature of the metal cations). In all 11 systems, the outcome of the process was primarily determined by the ability of the octahedral metal ion to select its pair of components from the initial pool of components, with the composition of the weaker tetrahedral complex being imposed by the components rejected by the octahedral metal ions. Different octahedral metal ions required different levels of precision in the “assembling instructions” provided by the organic components of the CDN to guide it towards a sorted output. The concentration of the reaction mixture, and the electronic and steric properties of the initial components of the library were all found to influence the lifetime of unwanted metastable intermediates formed during the assembling of the two complexes.

Constitutional dynamic networks have been developed to simultaneously control the output of two interconnected dynamic processes over several species of the same reaction mixture.

Dynamic Covalent Switches and Communicating Networks for Tunable Multicolor Luminescent Systems and Vapor-Responsive Materials

Molecular switches are an intensive area of research, and in particular, the control of multistate switching is challenging. Herein we introduce a general and versatile strategy of dynamic covalent switches and communicating networks, wherein distinct states of reversible covalent systems can induce addressable fluorescence switching. The regulation of intramolecular ring/chain equilibrium, intermolecular dynamic covalent reactions (DCRs) with amines, and both permitted the activation of optical switches. The variation in electron-withdrawing competition between the fluorophore and 2-formylbenzenesulfonyl unit afforded diverse signaling patterns. The combination of switches in situ further enabled the creation of communicating networks for multistate color switching, including white emission, through the delicate control of DCRs in complex mixtures. Finally, reversible and recyclable multiresponsive luminescent materials were achieved with molecular networks on the solid support, allowing visualization of different types of vapors and quantification of primary amine vapors with high sensitivity and wide detection range. The results reported herein should be appealing for future studies of dynamic assemblies, molecular sensing, intelligent materials, and biological labeling.

Mutualistic benefit in the self-sorted co-aggregates of peri-naphthoindigo and a 4-amino-1,8-naphthalimide derivative

Photoluminescence enhancement for all the members of a self-sorted co-aggregate was observed for the first time by successfully amalgamating AIEE and social self-sorting. Intermolecular H-bonding and π-π stacking were utilised to prepare several co-aggregates of peri-naphthoindigo (PNI) and a 4-amino-1,8-naphthalimide derivative dye, NH₂-NMI. In the heteromeric aggregates, photoluminescence intensities were increased by 28% for the imide and more than 400% for PNI. Due to spectral overlap between the emission of the imide and the absorption of PNI, energy transfer took place from the former to the latter. The heteromeric aggregates are dual emissive and the relative intensities of the emissions can easily be tuned by varying the stoichiometry of the dyes.

Regulation of Axial Chirality through Dynamic Covalent Bond Constrained Biaryls

A strategy of dynamic covalent chemistry within constrained biaryls was developed for the modulation of axial chirality. The ring fusion partners of amide and aldehyde allowed the manipulation of ring/chain equilibrium and chirality transfer within cyclic diastereomeric hemiaminal. Dynamic covalent reactions (DCRs) with alcohols, thiols, and secondary amines further enabled the reversal of chirality relay and thereby regulation of axial chirality. Moreover, a combination of NMR, X-ray, and density functional theory results shed light on the structural basis of chirality transfer, exhibiting modest to excellent diastereoselectivity under thermodynamic control. The critical role of the amide unit in the modulation of axial chirality was also corroborated. Finally, the chiroptical signal was controlled through changing solvents, DCRs, and stimuli-responsive switching of DCRs.

Interplay between n→π* Interactions and Dynamic Covalent Bonds: Quantification and Modulation by Solvent Effects

Orbital donor-acceptor interactions play critical roles throughout chemistry, and hence, their regulation and functionalization are of great significance. Herein we demonstrate for the first time the investigation of n→π* interactions through the strategy of dynamic covalent chemistry (DCC), and we further showcase its use in the stabilization of imine. The n→π* interaction between donor X and acceptor aldehyde/imine within 2-X-2'-formylbiphenyl derivatives was found to significantly influence the thermodynamics of imine exchange. The orbital interaction was then quantified through imine exchange, the equilibrium of which was successfully correlated with the difference in natural bond orbital stabilization energy of n→π* interactions of aldehyde and its imine. Moreover, the examination of solvent effects provided insights into the distinct feature of the modulation of n→π* interaction with aprotic and protic solvents. The n→π* interaction involving imine was enhanced in protic solvents due to hydrogen bonding with the solvent. This finding further enabled the stabilization of imine in purely aqueous solution. The strategies and results reported should find application in many fields, including molecular recognition, biological labeling, and asymmetric catalysis.