Supramolecular cages as differential sensors for dicarboxylate anions: guest length sensing using principal component analysis of ESI-MS and 1H-NMR raw data

Triply Adaptive Libraries of Dynamic Covalent Macrocycles: Switching between Sorted and Unsorted States

Dynamic noncovalent and covalent chemistries have enabled the constitutional modulation of chemical entities within chemical dynamic systems. The switching between order and disorder, i.e., self-sorted and unsorted states of constitutional dynamic libraries, remains challenging. Herein, we study the adaptive behaviors of a dynamic library of imine macrocycles generated from dialdehydes and diamines, seeking ways to exert control over sorting and unsorting processes. The distribution of constituents in the present library of dynamic macrocycles is modulated in response to internal and external effectors (e.g., time, metal cations, and chemical fuels), resulting in the transient amplification of self-sorted constituents in out-of-equilibrium states. The present study showcases higher complexity in systems subject to multiple adaptation through the dynamic interconversion between singularity/order and diversity/disorder.

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

A Carbazole-1,8-Disulfonamide-Derived Cryptand Receptor for Anion Recognition

A novel cryptand-like anion receptor 1 was synthesized in reasonable yield by a one-step condensation reaction. The UV-vis spectroscopic titrations indicated that cryptand 1 bound AcO- in preference to other monovalent anions (including its competing F- and H2PO4-) in CH3CN, generating a 1:1 binding complex with Ka = 51,000 M-1. Moreover, the crystal structures revealed that the acetate ion was encapsulated inside the cryptand's cavity in a 1:1 manner, through multiple N-H···O hydrogen bonds (although having two different crystal forms).

Site-Selective Ligand Bridging among Multiple Internal Coordination Sites of a Metallomacrocycle and Its Conformational Regulation

Metallomacrocycles with internal coordination sites have a high potential to precisely control the positions of the guest ligands and the overall shape of the assemblies by utilizing the directionality and reversibility of the coordination bonds. However, when such coordinative hosts possess multiple coordination sites, it was difficult to control to which coordination sites the incoming guest ligands bind, because such systems often result in a random, uncontrolled mixture. The metallomacrocycle that we now report, a hexanuclear palladium complex of hexapap possessing six internal coordination sites, can take two different conformations depending on the guests. One is an Alternate conformation, in which six coordination sites of pap alternatively point to Up-Down-Up-Down-Up-Down. The other is a Twisted conformation, in which the coordination sites direct Up-Middle-Down-Up-Middle-Down. Interestingly, linear ditopic α,ω-diamines are captured in three distinct cross-linking modes, and regulations between the two macrocyclic conformations have been realized by the lengths of the diamines. Furthermore, the heteroleptic site-selective bridging of two kinds of diamines has been achieved. It has been demonstrated that a slight difference in the diamine lengths leads to a significant change in the structure and selection of the produced host-guest macrocyclic complexes.

Applications of Synthetic Receptors in Bioanalysis and Drug Transport

Synthetic receptors are powerful tools for molecular recognition. They can bind to guests with high selectivity and affinity, and their structures are tunable and diversified. These features, plus the relatively low cost and high simplicity in synthesis and modification, support the feasibility of array-based molecular analysis with synthetic receptors for improved selectivity in the recognition of a wide range of targets. More attractively, host-guest interaction is reversible and guest displacement allows biocompatible and gentle release of the host-bound molecules, simplifying the stimulation designs needed to control analyte sensing, enrichment, and transportation. Here, we highlight a few recent advancements in using synthetic receptors for molecular analysis and manipulation, with the focus on macrocyclic receptors and their applications in displacement sensing, separation, imaging, and drug transport.

A Charge-Neutral Self-Assembled L2Zn2 Helicate as Bench-Stable Receptor for Anion Recognition at Nanomolar Concentration

The field of anion recognition chemistry is dominated by two fundamental approaches to design receptors. One relies on the formation of covalent bonds resulting in organic and often neutral host species, while the other one utilizes metal-driven self-assembly for the formation of charged receptors with well-defined nanocavities. Yet, the combination of their individual advantages in the form of charge-neutral metal-assembled bench-stable anion receptors is severely lacking. Herein, we present a fluorescent and uncharged double-stranded hydroxyquinoline-based zinc(II) helicate with the ability to bind environmentally relevant dicarboxylate anions with high fidelity in dimethyl sulfoxide (DMSO) at nanomolar concentrations. These dianions are pinned between zinc(II) centers with binding constants up to 145 000 000 M^-1. The presented investigation exemplifies a pathway to bridge the two design approaches and combine their strength in one structural motif as an efficient anion receptor.

Chiroptical Enhancement of Chiral Dicarboxylic Acids from Confinement in a Stereodynamic Supramolecular Cage

The fundamental implications that chirality has in science and technology require continuous efforts for the development of fast, economic, and reliable quantitative methods for enantiopurity assessment. Among the different analytical approaches, chiroptical techniques in combination with supramolecular methodologies have shown promising results in terms of both costs and time analysis. In this article, a tris(2-pyridylmethyl)amines (TPMA)-based supramolecular cage is able to amplify the circular dichroism (CD) signal of a series of chiral dicarboxylic acids also in the presence of a complex mixture. This feature has been used to quantify tartaric acid in wines and to discriminate different matrixes using principal component analysis (PCA) of the raw CD data.

Picking on Carbonate: Kinetic Selectivity in the Encapsulation of Anions

Supramolecular hosts bind to inorganic anions at a fast rate and select them in proportion with thermodynamic stability of the corresponding [anion⊂host] complexes, forming in a reversible manner. In this study, we describe the action of hexapodal capsule 1 and its remarkable ability to select anions based on a large span of rates by which they enter this host. The thermodynamic affinity of 1 toward eighteen anions extends over eight orders of magnitude (0<K_a <10⁸  M^-1 ; ¹ H NMR spectroscopy). The capsule would retain CO₃ ^2- (K_a =10⁷  M^-1 ) for hours in the presence of eleven competing anions, including stronger binding SO₄ ^2- , HAsO₄ ^2- and HPO₄ ^2- (K_a =10⁷ -10⁸  M^-1 ). The observed selection resulted from 1 possessing narrow apertures (ca. 3×6 Å) comparable in size to anions (d=3.5-7.1 Å) slowing down the encapsulation to last from seconds to days. The unorthodox mode of action of 1 sets the stage for creating hosts that pick anions by their ability to access the host.

Helicity control of a perfluorinated carbon chain within a chiral supramolecular cage monitored by VCD

Confinement within supramolecular systems is the leading technology to finely tune guest functional properties. In this communication we report the synthesis of a chiral supramolecular cage able to bias the helicity of a perfluorinated carbon chain hosted within the cage. We monitor the phenomenon of chiral induction by Vibrational Circular Dichroism (VCD) experiments complemented by DFT calculations over the possible conformers.

Expanding structural diversity in a library of disulfide macrocycles through in-situ imide hydrolysis

We describe here an unorthodox approach to dynamic covalent chemistry in which the initially-unexpected in-situ hydrolysis of a bis-imide is employed to control the composition of a library of structurally diverse macrocycles. A single building block is used to generate a library of numerous disulfide-based architectures in a one-pot single-step process. The dual-stimuli method is based on simultaneous changes in pH and DMSO concentration to expand the structural diversity of the macrocyclic products. Mechanistic details of this complex process are investigated by the kinetics analysis. We delivered a facile strategy for the synthesis of water-soluble, multicomponent and dynamic macrocycles equipped with number of different functional groups, thus giving a prospect of their application in guest-driven phase transfer.

Design of a D3h-symmetry prismatic tris-(ferrocene-1,1ʹ-diyl) molecular cage bearing boronate ester linkages

This paper presents a simple, highly selective, and efficient (isolated yield 68%) synthesis of a novel D3h-symmetry prismatic tris-(ferrocene-1,1ʹ-diyl) organic cage (FcB-cage) by incorporating the boronate ester as a linkage...

Development of "Imprint-and-Report" Dynamic Combinatorial Libraries for Differential Sensing Applications

Sensor arrays using synthetic receptors have found great utility in analyte detection, resulting from their ability to distinguish analytes based on differential signals via indicator displacement. However, synthesis and characterization of receptors for an array remain a bottleneck in the field. Receptor discovery has been streamlined using dynamic combinatorial libraries (DCLs), but the resulting receptors have primarily been utilized in isolation rather than as part of the entire library, with only a few examples that make use of the complexity of a library of receptors. Herein, we demonstrate a unique sensor array approach using "imprint-and-report" DCLs that obviates the need for receptor synthesis and isolation. This strategy leverages information stored in DCLs in the form of differential library speciation to provide a high-throughput method for both developing a sensor array and analyzing data for analyte differentiation. First, each DCL is templated with analyte to give an imprinted library, followed by in situ fluorescent indicator displacement analysis. We further demonstrate that the reverse strategy, imprinting with the fluorescent reporter followed by displacement with each analyte, provides a more sensitive method for differentiating analytes. We describe the development of this differential sensing system using the methylated Arg and Lys post-translational modifications (PTMs). Altogether, 19 combinations of 3-5 DCL data sets that discriminate all 7 PTMs were identified. Thus, a comparable sensor array workflow results in a larger payoff due to the immense information stored within multiple noncovalent networks.

Self-assembly of chiral BINOL cages via imine condensation

Condensation of an (S)- or (R)-BINOL-derived dialdehyde and tris(2-aminoethyl)amine produced chiral [2+3] imine cages, which were further reduced to furnish more stable chiral amine cages and applied in the enantioselective recognition of (1R,2R)- and (1S,2S)-1,2-diaminocyclohexane.

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)₃Mn(μ-SeR)₂Mn(CO)₃}₂(μ-L)₂] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

Molecular recognition and sensing of dicarboxylates and dicarboxylic acids

The recognition and detection of dicarboxylic acids and dicarboxylates is of significance for a wide variety of applications, including medical diagnosis, monitoring of health and of environmental contaminants, and in industry. Hence small molecule receptors and sensors for dicarboxylic acids and dicarboxylates have great potential for applications in these fields. This review outlines the challenges faced in the recognition and detection of these species, strategies that have been used to obtain effective and observable interactions with dicarboxylic acids and dicarboxylates, and progress made in this field in the period from 2014 to 2020.

Tripodal gold(i) polypyridyl complexes and their Cu+ and Zn2+ heterometallic derivatives. Effects on luminescence

Three gold(i) tripodal complexes containing the tris(2-pyridylmethyl)amine (TPA) ligand coordinated to Au-PR₃ moieties (PR₃ = 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane, PTA (1), 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane, DAPTA (2) and triphenylphosphane (3)) were prepared together with a cage-like structure containing triphosphane 1,1,1-tris(diphenylphosphinomethyl)ethane (4). The luminescence of these complexes has been studied and they show a red shift upon the formation of heterometallic complexes by reaction with Zn(NO₃)₂, CuCl and [Cu(CH₃CN)₄]BF₄. The different coordination motifs of the Zn²⁺ and Cu⁺ heterometallic species and the resulting changes in the recorded absorption, emission and NMR spectra were analysed and supported by TD-DFT calculations.

Tris-pyridylmethylamine (TPMA) complexes functionalized with persistent nitronyl nitroxide organic radicals

The chance to have persistent organic radicals in combination with metals has attracted much interest since it offers the possibility of having new functional molecules with multiple open-shell elements. In this study, we report the synthesis of two tripodal tris(2-pyridyl)methylamine ligands (TPMA) functionalized with nitronyl nitroxide persistent radicals. The newly formed ligands have been used to coordinate zinc(ii), copper(ii), iron(ii) and cobalt(ii). The resulting complexes have been investigated by means of electron paramagnetic resonance (EPR), ESI-MS, FT-IR spectroscopy and X-ray diffraction. An electron reduction of the N-O radical moiety has been observed, depending on the metal used for the formation of the complex and the reaction conditions. We have observed small differences in the EPR spectra depending on the meta or para position of the radical moiety in the complex structure and some antiferromagnetic interactions between the paramagnetic M(ii) ions and the radical species.

Template effects of vesicles in dynamic covalent chemistry

Vesicle lipid bilayers have been employed as templates to modulate the product distribution in a dynamic covalent library of Michael adducts formed by mixing a Michael acceptor with thiols. In methanol solution, all possible Michael adducts were obtained in similar amounts. Addition of vesicles to the dynamic covalent library led to the formation of a single major product. The equilibrium constants for formation of the Michael adducts are similar for all of the thiols used in this experiment, and the effect of the vesicles on the composition of the library is attributed to the differential partitioning of the library members between the lipid bilayer and the aqueous solution. The results provide a quantitative approach for exploiting dynamic covalent chemistry within lipid bilayers.

Conformationally adaptable macrocyclic receptors for ditopic anions: analysis of chelate cooperativity in aqueous containing media

The effect of chelate cooperativity on the binding of several ditopic anions to two tetrathiourea macrocycles has been analysed in competitive solvent mixtures (H2O : DMSO 1 : 9 v/v). The semi-flexible receptors bind dicarboxylates with high affinity dependent on the length and flexibility of the guest. Chemical double mutant cycle (DMC) analysis allowed the chelate cooperativity effects to be measured in detail and revealed both positive and negative cooperativity effects which were dependent on guest size, flexibility and spacer interactions between guest and macrocycle. 1H NMR and crystallographic studies confirmed the macrocycle hosts are adaptable, changing conformation to match their pore size to a selected guest.

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array

The development of synthetic receptors for the selective binding and discrimination of anions in water requires an understanding of how anions interact with these synthetic receptors. Molecules designed to differentiate nucleoside phosphate anions (e.g. ATP, ADP, GTP, GDP, UDP) under physiological conditions could underpin exciting new sensing tools for biomedical research and drug discovery, but it is very challenging due to the similarities in anion structure, size and charge. We present a series of lanthanide-based anion receptors and establish key structural elements that impact on nucleoside phosphate anion binding and sensing. Structural evidence of anion binding using X-ray crystallographic and NMR data, supported by DFT calculations indicate the binding modes between the lanthanide complexes and certain phosphoanions, revealing a bidentate (α-, γ-) binding mode to ATP. We further use four of the receptors to allow discrimination of eight nucleoside phosphate anions in the first array-based assay using lanthanide complexes, taking advantage of the multiple emission bands and long emission lifetimes associated with luminescent lanthanide complexes.

Singlet oxygen stimulus for switchable functional organic cages

Molecular cages 1a and 2a incorporating a 9,10-diphenylanthracene (DPA) chromophore were synthesized through a templated ring-closure metathesis approach that allows variation in cavity size through the introduction of up to three different pillars. Reversible Diels–Alder reaction between the DPA moiety and photogenerated singlet oxygen smoothly converted 1a and 2a to the corresponding endoperoxide cages 1b and 2b, which are converted back to 1a and 2a upon heating. Endoperoxide formation constitutes a reversible covalent signal that combines structural changes in the interior of the cage with introduction of two additional coordination sites. This results in a large modulation of the binding ability of the receptors attributed to a change in the location of the preferred binding site owing to the added coordination by the endoperoxide oxygen lone pairs. Cages 1a and 2a form complexes with sodium and cesium whose association constants are modified by 4–20 fold for Na⁺ and 200–450 fold for Cs⁺ upon conversion to 1b and 2b. DFT calculations show that in the anthracene form, cages 1a and 2a can bind 2 metal cations in their periphery so that each cation is coordinated by 4 oxygens and one amine nitrogen, whereas the endoperoxide cages 1b and 2b bind cations centrally in a geometry that favors coordination to the endoperoxide oxygens.

Allosteric switchable organic cages allow variability in cation recognition.

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.

Louis LE, Waters ML. Using changes in speciation in a dynamic combinatorial library as a fingerprint to differentiate the methylation states of arginine

A dynamic combinatorial library was shown to provide a direct method of sensing methylated arginine and lysine due to differences in speciation. This provides the first sensor array for all the methylation states of arginine.

Tuning the Solubility of Self-Assembled Fluorescent Aromatic Cages Using Functionalized Amino Acid Building Blocks

We previously reported novel fluorescent aromatic cages that are self-produced using a set of orthogonal dynamic covalent reactions, operating simultaneously in one-pot, to assemble up to 10 components through 12 reactions into a single cage-type structure. We now introduce N-functionalized amino acids as new building blocks that enable tuning the solubility and analysis of the resulting cages. A convenient divergent synthetic approach was developed to tether different side chains on the N-terminal of a cysteine-derived building block. Our studies show that this chemical functionalization does not prevent the subsequent self-assembly and effective formation of desired cages. While the originally described cages required 94% DMSO, the new ones bearing hydrophobic side chains were found soluble in organic solvents (up to 75% CHCl3), and those grafted with hydrophilic side chains were soluble in water (up to 75% H2O). Fluorescence studies confirmed that despite cage functionalization the aggregation-induced emission properties of those architectures are retained. Thus, this work significantly expands the range of solvents in which these self-assembled cage compounds can be generated, which in turn should enable new applications, possibly as fluorescent sensors.

Anion Pairs Template a Trigonal Prism with Disilver Vertices

Here we describe the formation of a trigonal prismatic cage, utilizing 2-formyl-1,8-naphthyridine subcomponents to bind pairs of silver(I) ions in close proximity. This cage is the first example of a new class of subcomponent self-assembled polyhedral structures having bimetallic vertices, as opposed to the single metal centers that typically serve as structural elements within such cages. Our new cage self-assembles around a pair of anionic templates, which are shown by crystallographic and solution-phase data to bind within the central cavity of the structure. Many different anions serve as competent templates and guests. Elongated dianions, such as the strong oxidizing agent peroxysulfate, also serve to template and bind within the cavity of the prism. The principle of using subcomponents that have more than one spatially close, but nonchelating, binding site may thus allow access to other higher-order structures with multimetallic vertices.

Poly(ionic liquid)s as a distinct receptor material to create a highly-integrated sensing platform for efficiently identifying numerous saccharides

A highly-integrated sphere-based sensing platform for directly identifying numerous saccharides very efficiently is developed.

Saccharides have strong hydrophilicities, and are complex molecular structures with subtle structure differences, and tremendous structural variations. The creation of one sensing platform capable of efficiently identifying such target systems presents a huge challenge. Using the integration of unique multiple noncovalent interactions simultaneously occurring in poly(ionic liquid)s (PILs) with multiple signaling channels, in this research an aggregation-induced emission (AIE)-doped photonic structured PIL sphere is constructed. It is found that such a sphere can serve as a highly integrated platform to provide abundant fingerprints for directly sensing numerous saccharides with an unprecedented efficiency. As a demonstration, 23 saccharides can be conveniently identified using only one sphere. More importantly, by using simple ion-exchanges of PIL receptors or/and increasing the AIE signaling channels, this platform is able to perform, on demand, different sensing tasks very efficiently. This is demonstrated by using it for the detection of difficult targets, such as greatly extended saccharides as well as mixed targets, in real-life examples on one or two spheres. The findings show that this new class of platform is very promising for addressing the challenges of identifying saccharides.