Oleksandr

Selective anion extraction is useful for the recovery and purification of valuable chemicals, and in the removal of pollutants from the environment. Here we report that Fe^II₄L₄ cage 1 is able to extract an equimolar amount of ReO₄⁻, a high-value anion and a nonradioactive surrogate of TcO₄⁻, from water into nitromethane. Importantly, the extraction was efficiently performed even in the presence of 10 other common anions in water, highlighting the high selectivity of 1 for ReO₄⁻. The extracted guest could be released into water as the cage disassembled in ethyl acetate, and then 1 could be recycled by switching the solvent to acetonitrile. The versatile solubility of the cage also enabled complete extraction of ReO₄⁻ (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.

A Fe^II₄L₄ coordination cage enabled complete extraction of ReO₄⁻, a nonradioactive surrogate of TcO₄⁻, from water into an organic phase. In the presence of 10 other anions, 97 % of ReO₄⁻ was selectively removed. The extracted ReO₄⁻ could be released and the cage extractant recycled by a solvent-switching strategy. Rendering the cage water-soluble also allowed complete extraction of ReO₄⁻ from an organic phase into water.

The control of the connectivity between nodes of synthetic networks is still largely unexplored. To address this point we take advantage of a simple dynamic chemical system with two exchange levels that are mutually connected and can be activated simultaneously or sequentially. Dithioacetals and disulfides can be exchanged simultaneously under UV light in the presence of a sensitizer. Crossover reactions between both exchange processes produce a fully connected chemical network. On the other hand, the use of acid, base or UV light connects different nodes allowing network rewiring.

Network rewiring: Molecular networks with markedly different structures can be rewired through the simultaneous or sequential activation of dithioacetal and disulfide exchanges.

Understanding how the constitutional dynamics of a dynamic combinatorial library (DCL) adapts to surfaces (compared to bulk solution) is of fundamental importance to the design of adaptive materials. Submolecular resolved scanning tunneling microscopy (STM) can provide detailed insights into olefin metathesis at the interface. Analysis of the distribution of products has revealed the important role of environmental pressure, reaction temperature, and substituent effects in surface-confined olefin metathesis. We also report an unprecedented preferred deposition and assembly of linear polymers, and some specific oligomers, on the surface that are hard to obtain otherwise.

Surface-confined olefin metathesis was studied by scanning tunneling microscopy to determine the effects of substituents, reaction temperature, and pressure. Surface confinement significantly enhances the formation of linear polymers and allows selective synthesis of specific oligomers that are difficult to obtain by solution-phase synthesis. DCC=Dynamic covalent chemistry.

The transient activation of function using chemical fuels is common in nature, but much less in synthetic systems. Progress towards the development of systems with a complexity similar to that of natural ones requires chemical fuel selectivity. Here, we show that a self-assembled nanosystem, composed of monolayer-protected gold nanoparticles and a fluorogenic peptide, is activated for transient signal generation only in case the chemical fuel matches the recognition site present at the nanoparticle surface. A modification of the recognition site in the nanosystem completely changes the chemical fuel selectivity. When two nanosystems are simultaneously present, the selectivity expressed by the system depends on the concentration of nucleotide added.

Matching fuel for action: The transient activation of function by using chemical fuels is common in nature, but much less in synthetic systems. In a newly developed system transient signal generation is only observed when the chemical fuel matches the recognition site in monolayer-protected gold nanoparticles.

Axial chirality is a prevalent and important phenomenon in chemistry. Herein we report a combination of dynamic covalent chemistry and axial chirality for the development of a versatile platform for the binding and chirality sensing of multiple classes of mononucleophiles. An equilibrium between an open aldehyde and its cyclic hemiaminal within biphenyl derivatives enabled the dynamic incorporation of a broad range of alcohols, thiols, primary amines, and secondary amines with high efficiency. Selectivity toward different classes of nucleophiles was also achieved by regulating the distinct reactivity of the system with external stimuli. Through induced helicity as a result of central-to-axial chirality transfer, the handedness and ee values of chiral monoalcohol and monoamine analytes were reported by circular dichroism. The strategies introduced herein should find application in many contexts, including assembly, sensing, and labeling.

Talking sense: A convergence of the concepts of dynamic covalent chemistry and axial chirality enabled the reversible incorporation and chirality sensing of multiple classes of mononucleophiles (see picture). Selectivity toward different classes of nucleophiles could also be induced by regulating the distinct reactivity of the system with external stimuli.

Invited for the cover of this issue is the group of Michael D. Ward at the Universities of Warwick and Sheffield. The image depicts the ‘hydrophobic effect’ that is responsible for guest binding in the host cavity. Read the full text of the article at 10.1002/chem.201704163.

“Using a combination of X-ray crystallography and NMR studies, we have demonstrated the extensive host/guest properties of our cubic cage.” Read more about the story behind the cover in the Cover Profile and about the research itself on page ▪▪ ff. (DOI: 10.1002/chem.201704163).

Borinic acids have typically not been considered as hydrogen bond donor groups in molecular recognition. Described herein is a bifunctional borane/borinic acid derivative (2) in which the two functionalities are connected by a 1,8-biphenylenediyl backbone. Anion binding studies reveal that 2 readily binds a fluoride anion by formation of a unique B−F⋅⋅⋅H−O−B hydrogen bond. This hydrogen bond is characterized by a short H-F distance of 1.79(3) Å and a large coupling constant (¹J_HF) of 57.2 Hz. The magnitude of this interaction, which has also been investigated computationally, augments the fluoride anion binding properties of 2, thus making it compatible with aqueous environments.

A Brønsted and Lewis handshake: The borinic acid functionality of a new diboron electrophilic host reaches across the binding pocket to engage a borane-bound fluoride anion in a strong hydrogen-bonding interaction. The resulting B−F⋅⋅⋅H−O−B interaction stabilizes the complex in aqueous solutions, thereby illustrating the role that borinic acids may play as hydrogen bond donor groups.

Catalyst discovery from systems of potential precursors is a challenging endeavor. Herein, a new strategy applying dynamic chemistry to the identification of catalyst precursors from C−H activation of imines is proposed and evaluated. Using hydroacylation of imines as a model reaction, the selection of an organometallic reactive intermediate from a dynamic imine system, involving many potential directing group/metal entities, is demonstrated. The identity of the amplified reaction intermediate with the best directing group could be resolved in situ by ESI-MS, and coupling of the procedure to an iterative deconvolution protocol generated a system with high screening efficiency.

In the director′s chair: A new strategy, applying dynamic chemistry to the selection of catalyst precursors from C−H activation of imines is presented. Using hydroacylation as a model reaction, the strategy enabled identification of an organometallic reactive intermediate from a dynamic imine system, where the best directing group could be resolved in situ.