Synthetic Receptors with Micromolar Affinity for Chloride in Water

Palladium-based coordination cages as dynamic crosslinks in acrylamide hydrogels

Soft polymer networks with palladium cages as crosslinks can be obtained by combining polymeric N-donor ligands with PdII salts in organic solvents. Herein, we describe an alternative procedure that enables the preparation of hydrogels with Pd n L2n -type junctions. The gels were obtained by photoinitiated copolymerization of palladium cages with acrylamide monomers in water. Cages with varying nuclearities (n = 2, 4, or 12) and different acrylates were employed. The material properties could be tuned by changing the crosslinker density. Thermoresponsive hydrogels were obtained when NIPAm was used as the monomer. The dynamic nature of the Pd-based crosslinks allows the creation of stimuli-responsive hydrogels. In particular, we were able to alter the network topology of a hydrogel by anion-induced conversion of Pd4L8 crosslinks into Pd2L4-type junctions.

Near-Quantitative Removal of Oxalate and Terephthalate from Water by Precipitation with a Rigid Bis-Amidinium Compound

A simple, readily-prepared precipitant (1⋅Cl2) precipitates oxalate or terephthalate from water with very high efficacy, removing these anions at sub-millimolar concentrations using only one equivalent of precipitant. A simple aqueous base/acid cycle can be used to regenerate 1⋅Cl2 after use. The resulting precipitates, 1⋅oxalate and 1⋅terephthalate, are anhydrous and closely-packed, with each anion receiving eight charge-assisted hydrogen bonds from amidinium N-H donors. Precipitation of oxalate and terephthalate occurs at much lower concentrations than other dicarboxylates, and direct competition experiments with the biologically/environmentally relevant divalent anions CO3 2-, HPO4 2- and SO4 2- reveal very high selectivity for oxalate or terephthalate over these competitors.

A Combined Experimental and Computational Exploration of Heteroleptic cis-Pd2L2L'2 Coordination Cages through Geometric Complementarity

Heteroleptic (mixed-ligand) coordination cages are of interest as host systems with more structurally and functionally complex cavities than homoleptic architectures. The design of heteroleptic cages, however, is far from trivial. In this work, we experimentally probed the self-assembly of Pd(II) ions with binary ligand combinations in a combinatorial fashion to search for new cis-Pd2L2L'2 heteroleptic cages. A hierarchy of computational analyses was then applied to these systems with the aim of elucidating key factors for rationalising self-assembly outcomes. Simple and inexpensive geometric analyses were shown to be effective in identifying complementary ligand pairs. Preliminary results demonstrated the viability of relatively rapid semi-empirical calculations for predicting the topology of thermodynamically favoured assemblies with rigid ligands, whilst more flexible systems proved challenging. Stemming from this, key challenges were identified for future work developing effective computational forecasting tools for self-assembled metallo-supramolecular systems.

Selective aqueous anion recognition in an anionic host

Water-soluble Fe4L4 4- cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, FeII salts, and water-soluble sulfonated formylpyridine components. The cages are soluble in purely aqueous solution and display an overall 4- charge, but are capable of binding suitably sized non-coordinating anions in the host cavity despite their anionic nature. Anions such as PF6 - or AsF6 - occupy the internal cavity, whereas anions that are too small (BF4 -) or too large (NTf2 -) are not encapsulated. The external anionic charge and sterically blocked ligand cores limit the exchange rate of bound anions, as no exchange is seen over a period of weeks with the anion-filled cages, and internalization of added PF6 - by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF6 - ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.

Observing Anion Binding in Single Charge-Neutral Metal-Organic Frameworks through C-H Hydrogen-Bonding Interactions

Achieving anion capture with metal-organic frameworks (MOFs) usually relies on anion exchange reactions. Here, we report the direct visual imaging of the anion binding process within a charge-neutral Bi-based MOF (UU-200) in water at the single-particle level using in situ dark-field optical microscopy. Notably, an unexpected anion-induced structural shrinkage of UU-200 is mapped, and concentration-dependent responses are applied to determine the association constants. The resulting anion affinity is correlated with its basicity, demonstrating that charge-dense anions such as F-, SO32-, and SO42- feature strong binding with the UU-200 framework. Moreover, the unusual anion binding processes are identified as the C-H hydrogen-bonding interactions between electron-deficient hydrogen atoms on the channel wall and negatively charged anions by combining imaging results, nuclear magnetic resonance spectroscopy, and theoretical simulation. These discoveries reshape and strengthen our fundamental understanding of the anion capture within MOFs, favoring the rational design of MOF-based anion receptors.

Expedient Decagram-Scale Synthesis of Robust Organic Cages That Bind Sulfate Strongly and Selectively in Water

Selective anion recognition remains a key challenge in supramolecular chemistry: only a very small number of systems that can function in water are known, and these nearly always preferentially bind hydrophobic anions. In this work, we report three robust hexa-cationic cages that can be prepared on scales up to 14 g in two simple and high-yielding steps from commercially available materials. One of these cages displays unusually strong sulfate binding in water (Ka = 12,000 M-1), and demonstrates high selectivity for this anion over H2PO4-/HPO42- in DMSO/buffer mixtures. These results demonstrate that relatively large, three-dimensional supramolecular hosts can be prepared in high yields and on large scales, and can be highly potent receptors.

Nanogels with Metal-Organic Cages as Functional Crosslinks

A dinuclear metal-organic cage with four acrylate side chains was prepared by self-assembly. Precipitation polymerization of the cage with N-isopropylacrylamide yielded a thermoresponsive nanogel. The host properties of the cage were retained within the gel matrix, endowing the nanogel with the capability to serve as a sorbent for chloride ions in water. Moreover, a heteroleptic cage with the drug abiraterone as co-ligand was integrated into a nanogel. The addition of chloride ions induced a structural rearrangement of the metal-ligand assembly, resulting in the gradual release of abiraterone.

Modular Strategy for Constructing para-Cage[n]arenes, meta-Cage[n]arenes, and meta-Bimacrocyclic-Arenes

Here, we present a versatile modular strategy for crafting novel covalent organic cages (para-cage[n]arenes and meta-cage[n]arenes, n = 3,4) and bimacrocycles (meta-bimacrocyclic-arenes) with stable backbones and modifiable rims. These structures can be synthesized from commercially available aromatic multialdehydes in a three-step process: quantitative bromination, Suzuki-Miyaura reaction (yielding over 60%), and a rapid one-pot Friedel-Crafts reaction with paraformaldehyde. Notably, the cage[n]arenes exhibit a well-defined prismatic shape, and the bimacrocyclic-arenes display both dimeric and monomeric configurations.

Orientational Self-Sorting in Octahedral Palladium Cages: Scope and Limitations of the "cis Rule"

Octahedral coordination cages of the general formula [Pd6L12](BF4)12 were obtained by combining [Pd(CH3CN)4](BF4)2 with heteroditopic N-donor ligands. Four different ligands were employed. These ligands have 3-pyridyl donor groups at one end and 4-pyridyl, imidazolyl, or triazolyl donor groups at the other end. According to a geometric analysis, cages with a cis configuration at the six metal centers should be preferred ("cis rule"). This prediction was corroborated by spectroscopic data and crystallographic analyses. Limitations of the "cis rule" were also encountered, and possible explanations are discussed.

Selective Supramolecular Recognition of Nitroaromatics by a Fluorescent Metal-Organic Cage Based on a Pyridine-Decorated Dibenzodiaza-Crown Macrocyclic Co(II) Complex

Two isomorphous fluorescent (FL) lantern-shaped metal-organic cages 1 and 2 were prepared by coordination-directed self-assembly of Co(II) centers with a new aza-crown macrocyclic ligand bearing pyridine pendant arms (L_py). The cage structures were determined using single-crystal X-ray diffraction analysis, thermogravimetric, elemental microanalysis, FT-IR spectroscopy, and powder X-ray diffraction. The crystal structures of 1 and 2 show that anions (Cl^- in 1 and Br^- in 2) are encapsulated within the cage cavity. 1 and 2 bear two coordinated water molecules that are directed inside the cages, surrounded by the eight pyridine rings at the "bottom" and the "roof" of the cage. These hydrogen bond donors, π systems, and the cationic nature of the cages enable 1 and 2 to encapsulate the anions. FL experiments revealed that 1 could detect nitroaromatic compounds by exhibiting selective and sensitive fluorescence quenching toward p-nitroaniline (PNA), recommending a limit of detection of 4.24 ppm. Moreover, the addition of 50 μL of PNA and o-nitrophenol to the ethanolic suspension of 1 led to a significant large FL red shift, namely, 87 and 24 nm, respectively, which were significantly higher than the corresponding values observed in the presence of other nitroaromatic compounds. The titration of the ethanolic suspension of 1, with various concentrations of PNA (>12 μM) demonstrated a concentration-dependent emission red shift. Hence, the efficient FL quenching of 1 was capable of distinguishing the dinitrobenzene isomers. Meanwhile, the observed red shift (10 nm) and quenching of this emission band under the influence of a trace amount of o- and p-nitrophenol isomers also showed that 1 could discriminate between o- and p-nitrophenol. Replacement of the chlorido with a bromido ligand in 1 generated cage 2 which was a more electron-donating cage than 1. The FL experiments showed that 2 was partially more sensitive and less selective toward NACs than 1.