Discovery of linear receptors for multiple dihydrogen phosphate ions using dynamic combinatorial chemistry

Ferrocene: an exotic building block for supramolecular assemblies

Ferrocene (Fc), a classical organometallic complex, has found potential applications in ligand design, catalysis, and analytical, biological, medicinal and materials chemistry. In recent years, the use of Fc as a building block in supramolecular chemistry has emerged. The molecular shape, size, and hydrophobicity of Fc make it an ideal guest for a variety of macrocyclic host molecules to form stable host-guest complexes. The vertical distance (3.3 Å) between two cyclopentadienyl rings and molecular "ball bearing" property in Fc support the formation of intramolecular π-π stacking, H-bonding and metallophilic interactions between two appropriate substituents in 1,n'-disubstituted ferrocenes. Along with these molecular features, the rigidity along with rotational flexibility, redox reversibility and oxidation-triggered tunable hydrophobicity of Fc have led to its use as an exotic building block for the development of a wide range of supramolecular assemblies such as smart molecular receptors, intricate metal-organic assemblies, supramolecular polymers, and gels including out-of-equilibrium assemblies and metal nanoparticle assemblies. This review highlights the concepts behind the design and development of these assemblies, where the Fc unit has a direct and defined role in their formation and function. The use of Fc in supramolecular assembly is still a relatively young field and set to be the subject of increasing research interest towards the development of fascinating supramolecular structures with tailored properties and programmable functions towards applications in materials and biological sciences.

Dynamic Covalent Self-Assembly of Chloride- and Ion-Pair-Templated Cryptates

While supramolecular hosts capable of binding and transporting anions and ion pairs are now widely available, self‐assembled architectures are still rare, even though they offer an inherent mechanism for the release of the guest ion(s). In this work, we report the dynamic covalent self‐assembly of tripodal, urea‐based anion cryptates that are held together by two orthoester bridgeheads. These hosts exhibit affinity for anions such as Cl⁻, Br⁻ or I⁻ in the moderate range that is typically advantageous for applications in membrane transport. In unprecedented experiments, we were able to dissociate the Cs⋅Cl ion pair by simultaneously assembling suitably sized orthoester hosts around the Cs⁺ and the Cl⁻ ion.

Hydrazones in anion transporters: the detrimental effect of a second binding site

Synthetic anion transporters can be developed using anion receptors that are able to bind the anion and stabilize it in the lipophilic interior of a bilayer membrane, and they usually contain functional groups with acidic NHs, such as ureas, thioureas and squaramides. To assess the suitability of acylhydrazones as a new functional group for the preparation of anion transporters, we have studied a family of thioureas functionalized with these and related functional groups. ¹H NMR titrations and DFT calculations indicate that the thioureas bearing acylhydrazone groups behave as chloride receptors with two separate binding sites, of which the acylhydrazone binds weaker than the thiourea. Chloride transport studies show that the additional binding site has a detrimental effect on thiourea-based transporters, and this phenomenon is also observed for bis(thio)ureas with two separate binding sites. We propose that the presence of a second anion binding unit hinders the transport activity of the thiourea due to additional interactions with the phospholipids of the membrane. In agreement with this hypothesis, extensive molecular dynamics simulations suggest that the molecules will tend to be positioned in the water/lipid interface, driven by the interaction of the NHs of the thiourea and of the acylhydrazone groups with the POPC polar head groups and water molecules. Moreover, the interaction energies show that the poorest transporters have indeed the strongest interactions with the membrane phospholipids, inhibiting chloride transport. This detrimental effect of additional functional groups on transport activity should be considered when designing new ion transporters, unless these groups cooperatively promote anion recognition and transmembrane transport.

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.

Coordinating and supramolecular prospects of unsymmetrically substituted carbohydrazides

Unsymmetrically substituted carbohydrazides serve as multifunctional ligands, practicing their chelating and supramolecular roles with cis-dioxomolybdenum(vi) cationic core and the Lindqvist anion.

A tetrahedral molecular cage with a responsive vertex

Dynamic covalent chemistry (DCC) is a widely used method for the self-assembly of three-dimensional molecular architectures. The orthogonality of dynamic reactions is emerging as a versatile strategy for controlling product distributions in DCC, yet the application of this approach to the synthesis of 3D organic molecular cages is limited. We report the first system which employs the orthogonality of alkyne metathesis and dynamic imine exchange to prepare a molecular cage with a reversibly removable vertex. This study demonstrates the rational and controlled application of chemical orthogonality in DCC to prepare organic cages of expanded functionality which respond to chemical stimuli.

Non-invasive 19F NMR analysis of a protein-templated N-acylhydrazone dynamic combinatorial library

Dynamic combinatorial chemistry (DCC) is a powerful tool to identify ligands for biological targets.

Differential metal-binding properties of dynamic acylhydrazone polymers and their sensing applications

As one of common dynamic covalent bonds, acylhydrazone bond plays an important role in developing intelligent responsive materials. In this report, we present acylhydrazone-based dynamic polymers with multi-stimuli responsiveness, particularly metal recognition behaviours and their modulation. A series of polyacylhydrazones with different metal-binding sites were designed and prepared in a modular fashion. Titration of these receptors with a diverse set of metal ions, including Cu²⁺, Zn²⁺ and La³⁺, resulted in unique optical changes, and both the sensitivity and selectivity profiles can be regulated. Moreover, the metal-binding feature was facilely modulated by changing the solvent. The addition of weakly basic anions was employed to further fine-tune the responsiveness of the polymers by taking advantage of the cooperative effect with metal coordination. Finally, the sensitive detection of 6-mercaptopurine and pyrophosphate was achieved to demonstrate the application potential of these systems.

Anion Recognition Strategies Based on Combined Noncovalent Interactions

This review highlights the most significant examples of an emerging field in the design of highly selective anion receptors. To date, there has been remarkable progress in the binding and sensing of anions. This has been driven in part by the discovery of ways to construct effective anion binding receptors using the dominant N-H functional groups and neutral and cationic C-H hydrogen bond donors, as well as underexplored strong directional noncovalent interactions such as halogen-bonding and anion-π interactions. In this review, we will describe a new and promising strategy for constructing anion binding receptors with distinct advantages arising from their elaborate design, incorporating multiple binding sites able to interact cooperatively with anions through these different kinds of noncovalent interactions. Comparisons with control species or solely hydrogen-bonding analogues reveal unique characteristics in terms of strength, selectivity, and interaction geometry, representing important advances in the rising field of supramolecular chemistry.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Symmetrical N-acylsubstituted dihydrazones containing bithiophene core--Photophysical, electrochemical and thermal characterization

Four symmetrical N-acylsubstituted dihydrazones containing bithiophene core were synthesized from condensation of 2,2'-bithiophene-5,5'-dicarboxyaldehyde with benzoic, isonicotinoyl, 2-thiophenic and 2-furoic hydrazide. The obtained compounds were characterized through the data from (1)H nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), elemental analysis, UV-vis absorption spectroscopy, photoluminescence (PL), cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements. Additionally, the electronic properties including orbital energies and resulting energy gaps were calculated by density functional theory (DFT). Their thermal behavior was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). They were thermal sable up to 320°C. The prepared N-acylsubstituted dihydrazones emitted light with λ(em) in the range of 499-530 nm in solution, whereas, in solid state as blend with PMMA blue emission was observed. They undergo quasi-reversible and irreversible electrochemical reduction and oxidation processes, respectively. Additionally, the selected compounds were tested preliminary as component of active layer in organic photovoltaic cells. The highest value of power conversion efficiency, equal to 1.68% under simulated 100 mW/cm(2) AM 1.5G irradiation was found for device with the architecture ITO/PEDOT:PSS/P3HT:PCBM:FBTH (1:2:2)/Al.

Competition-driven selection in covalent dynamic networks and implementation in organic reactional selectivity

Competition among reagents in dynamic combinatorial libraries of increased complexity leads to reactional self-sorting. This fundamental principle allowed development of selective dynamic protecting groups for controlled sequential derivatization of polyamines.

Competition among reagents in dynamic combinatorial libraries of increased complexity leads to reactional self-sorting (improved regioselectivity) in mixtures of aldehydes and oligoamines. High selectivity of a given library component is transferred to a different reacting component of low selectivity through a network of underlying equilibrating reactions which provide component exchange between all species. The selectivity of various carbonyl compounds in reactions with amines was also assessed towards the formation of defined sequences of residues along oligoamine chains. The approach was further exploited for defining selective dynamic protecting groups (DPGs), based on the reversible linkage between the substrate and the protecting group. They represent an intermediate approach between the conventional protecting groups and the protecting-group-free approach in organic synthesis. Removal of the protecting group is effected via dynamic exchange trapping by formation of a more stable product. The establishment of equilibrium eliminates the need for isolation and purification of the dynamically protected intermediate(s) and enables as well the selective sequential derivatisation of oligoamines. The DPG concept can be generalised to other reversible reactions and can thus represent a valuable alternative in the design of total synthesis of complex molecules.

Multiresponsive Dynamic Covalent Assemblies for the Selective Sensing of Both Cu(2+) and CN(-) in Water

The use of dynamic assembly for molecular sensing is an intensive area of research in supramolecular chemistry. However, the development of self-assembly architectures for the detection of multiple signals remains challenging. Here, we present dynamic covalent assemblies with multiresponsive properties that also show unique selectivity profiles in water. The receptors were generated in a single step with modularly designed building blocks through acylhydrazone linkages, and their orthogonal assembly with a series of external stimuli was investigated. Notably, the system exhibits responses toward cations, anions, solvents, pH, and amphiphilicity. The discrimination of Cu(2+) from other divalent metal ions was achieved by simply changing the solvent and was evidenced by a Cu(2+)-induced pKa shift. The selective recognition of CN(-) in pure aqueous media was also accomplished through a cooperative effect in conjunction with Zn(2+). Furthermore, the assembly and its responsiveness are functional both in solution and the solid state. The aggregation ability of these dynamic covalent systems supports their binding and sensing properties.

Ferrocene-based polyethyleneimines for burning rate catalysts

Synthesis, electrochemical behavior, burning rate catalytic performance, and burning rate catalytic and migration mechanism of ferrocene-based polyethyleneimines.

Rationalization of chirality induction and inversion in a zinc trisporphyrinate by a chiral monoalcohol

The host–guest system formed between a zinc trisporphyrinate and a chiral monoalcohol (1-phenylethylalcohol) shows the chirality induction and inversion processes. These processes are controlled by the corresponding 1 : 1 and 1 : 2 complexes.

From simplicity to complex systems with bioinspired pseudopeptides

This feature article highlights some of the recent advances in creating complexity from simple pseudopeptidic molecules. The bioinspired approaches discussed here allowed an increase in the structural, chemical and interactional complexity (see figure).

Convergent heteroditopic cyclo[6]aramides as macrocyclic ion-pair receptors for constructing [2]pseudorotaxanes

A strategy of using amide groups as the only functionality was developed to construct convergent heteroditopic cyclo[6]aramides that are able to strongly bind dibutylammonium chloride in chloroform (>10(5) M(-1)), leading to the formation of [2]pseudorotaxanes.

Low-migratory ionic ferrocene-based burning rate catalysts with high combustion catalytic efficiency

1-(Ferrocenylmethyl)alkylimidazolium-based ionic compounds with low-migration, high thermal stability and catalytic activity in the thermal degradation of ammonium perchlorate were synthesized and characterized.

Introducing a static receptor to compete with a dynamic combinatorial library in template binding

Association constants can be obtained from HPLC analysis of a system comprising a dynamic combinatorial library and a static host.

Discovery of potent inhibitors of human β-tryptase from pre-equilibrated dynamic combinatorial libraries

Pre-equilibrated combinatorial libraries based on multivalent peptide acyl hydrazones were used to find potent inhibitors of β-tryptase. The best inhibitors bind to the protein surface, and inhibit β-tryptase with nanomolar affinity (K_ica. 10 nM) and high selectivity in a reversible and non-competitive way.

Pre-equilibrated dynamic combinatorial libraries based on acyl hydrazone interchange of peptide-derived hydrazides and di- and tri-aldehydes have been used to discover potent inhibitors with nanomolar affinities for β-tryptase. To identify potent inhibitors the activity of the full library containing 95 members was compared with those of sub-libraries in which individual building blocks were missing. The most active library members contain a rigid central aromatic scaffold with three cationic peptide arms. The arms of the best inhibitors also contained a tailor-made GCP oxoanion binding motif attached to a lysine side chain. The most potent tri-armed hydrazones with peptide arms GKWR or GKWK(GCP) were shown to inhibit β-tryptase (K_ica. 10–20 nM) reversibly, non-competitively and selectively (compared to related serine proteases, e.g. trypsin and chymotrypsin), most likely by binding to the protein surface, also in agreement with molecular modelling calculations. These new inhibitors are one order of magnitude more efficient than related tetravalent inhibitors obtained from previous work on a split-mix-combinatorial library and were identified with significantly less effort, demonstrating the usefulness of this approach for the identification of enzyme inhibitors in general.

Dynamic combinatorial/covalent chemistry: a tool to read, generate and modulate the bioactivity of compounds and compound mixtures

Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.

"Choose-a-size" approach in dynamic combinatorial chemistry: a single substrate dynamic combinatorial library of oligomacrocycles that adapts to the size and shape of carboxylates

A neutral anion binding receptor based on dipicolinic acid diamide was equipped with thiol groups in the amidic side arms. After the thiol was oxidized to disulfide groups with I2, a mixture of cyclic oligomers (a library) was obtained. The distribution of macrocycles can be controlled kinetically during the oxidation process or thermodynamically at basic conditions via disulfide bond exchange. The library proved to be very sensitive to templation with various carboxylates in DMSO. The amplification pattern reflects the structural features of the anionic template and is sensitive to changes in the template's geometry. The application of carboxylates with multiple functional groups resulted in very strong amplification of the large penta- and hexameric macrocycles. The thermodynamic parameters of some templation effects were rationalized using a simple model and confirmed using competitive NMR titration.

Advances in anion supramolecular chemistry: from recognition to chemical applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under-exploited anion-π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion-templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.

Transient substrate-induced catalyst formation in a dynamic molecular network

In biology enzyme concentrations are continuously regulated, yet for synthetic catalytic systems such regulatory mechanisms are underdeveloped. We now report how a substrate of a chemical reaction induces the formation of its own catalyst from a dynamic molecular network. After complete conversion of the substrate, the network disassembles the catalyst. These results open up new opportunities for controlling catalysis in synthetic chemical systems.

Synthesis, spectroscopic, coordination and biological activities of some organometallic complexes derived from thio-Schiff base ligands

Two series of mono- and binuclear complexes cyclic or acyclic thio-ferocine Schiff base ligands, derived from the condensation of 2-aminobenzenthiol (L) with monoacetyl ferrocene in the molar ratio 1:1 or in the molar ratio 1:2 for diacetyl ferocine have been prepared. The condensation reactions yield the corresponding Schiff Base ligands, HLa-Maf and H2Lb-Daf. The chelation of the ligands to metal ions occurs through the sulfur of the thiol group as well as the nitrogen atoms of the azomethine group of the ligands. HLa-Maf acts as monobasic bidentate or dibasic tetradentate, while H2Lb-Daf behaves as twice negatively cargend tetradentate ligand. The structures of these ligands were elucidated by elemental analysis, infrared, ultraviolet-visible spectra, as well as (1)H NMR spectra. Reactions of the Schiff bases ligands with ruthenium(III), oxovanadium(IV) and dioxouranium(VI) afforded the corresponding transition metal complexes. The properties of the newly prepared complexes were analyse by elemental analyses, infrared, electronic spectra, (1)H NMR as well as the magnetic susceptibility and conductivity measurement. The metal complexes exhibits different geometrical arrangements such as octahedral and square pyramidal coordination. Schiff base ligands and their metal complexes were tested against two pathogenic bacteria as Gram-positive and Gram-negative bacteria as well as one kind of fungi to study their biological activity. All the complexes exhibit antibacterial and antifungal activities against these organisms.

Catenanes from catenanes: quantitative assessment of cooperativity in dynamic combinatorial catenation

A series of dynamic combinatorial [2] and [3]catenanes have been prepared. Formation of the [3]catenanes occurs with positive or negative cooperativity, depending on the cyclodextrin homologue. Systems level analysis allows cooperativity to be quantified and MD simulations reveal that cooperativity derives from the extents to which hydrophobic surface area is exposed to the aqueous surroundings.

Synthesis of novel ferrocenyl N/O-heterocycles, chiral P,N-ligand and α-dehydro-β-amino acid derived short peptides from Morita–Baylis–Hillman adducts of ferrocenealdehyde

The golden triangle of Fc, OH/NH, COO moieties created by classical/aza-MBH reaction of ferrocenealdehyde has been exploited for the first time for the synthesis of the title compounds.

Anion receptor chemistry: highlights from 2011 and 2012

This review covers advances in anion complexation in the years 2011 and 2012. The review covers both organic and inorganic systems and also highlights the applications to which anion receptors can be applied such as self-assembly and molecular architecture, sensing, catalysis and anion transport.