Hydrophobic Collapse of Foldamer Capsules Drives Picomolar-Level Chloride Binding in Aqueous Acetonitrile Solutions

Aqueous media are competitive environments in which to perform host–guest chemistry, particularly when the guest is highly charged. While hydrophobic binding is a recognized approach to this challenge in which apolar pockets can be designed to recognize apolar guests in water, complementary strategies are required for hydrophilic anions like chloride. Here, we present evidence of such an alternative mechanism, used everyday by proteins yet rare for artificial receptors, wherein hydrophobic interactions are shown to be responsible for organizing and stabilizing an aryl-triazole foldamer to help extract hydrophilic chloride ions from increasingly aqueous solutions. Therein, a double-helical complex gains stability upon burial of ∼80% of the π surfaces that simultaneously creates a potent, solvent-excluding microenvironment for hydrogen bonding. The chloride’s overall affinity to the duplex is substantial in 25% water v/v in acetonitrile (log β₂ = 12.6), and it remains strong (log β₂ = 13.0) as the water content is increased to 50%. With the rise in predictable designs of abiological foldamers, this water-assisted strategy can, in principle, be utilized for binding other hydrophilic guests

Interdigitated Hydrogen Bonds: Electrophile Activation for Covalent Capture and Fluorescence Turn-On Detection of Cyanide

Hydrogen-bonding promoted covalent modifications are finding useful applications in small-molecule chemical synthesis and detection. We have designed a xanthene-based fluorescent probe <b>1</b>, in which tightly held acylguanidine and aldehyde groups engage in multiple intramolecular hydrogen bonds within the concave side of the molecule. Such an interdigitated hydrogen bond donor–acceptor (HBD–HBA) array imposes significant energy barriers (Δ<i>G</i>^‡ = 10–16 kcal mol^–1) for internal bond rotations to assist structural preorganization and effectively polarizes the electrophilic carbonyl group toward a nucleophilic attack by CN^– in aqueous environment. This covalent modification redirects the de-excitation pathways of the cyanohydrin adduct <b>2</b> to elicit a large (>7-fold) enhancement in the fluorescence intensity at λ_max = 440 nm. A remarkably faster (> 60-fold) response kinetics of <b>1</b>, relative to its <i>N</i>-substituted (and therefore “loosely held”) analogue <b>9</b>, provided compelling experimental evidence for the functional role of HBD–HBA interactions in the “remote” control of chemical reactivity, the electronic and steric origins of which were investigated by DFT computational and X-ray crystallographic studies

Three-Stage Binary Switching of Azoaromatic Polybase

An <b>OFF–ON–OFF</b>-type three-stage binary switching was realized with an azoaniline-based polybase <b>1</b>. The optical properties of <b>1</b> and [<b>1</b>·2H]²⁺ are essentially indistinguishable to the naked eye but distinctively different from those of [<b>1</b>·H]⁺ to produce an unusual <i>bell-shaped response as a function of protonation state</i>; the underlying molecular mechanism was unraveled by a combination of experimental and DFT computational studies

A Tantalum Methylidene Complex Supported by a Robust and Sterically Encumbering Aryloxide Ligand

Treatment of [TaCl₂(CH₃)₃] with 2 equiv of NaOAr′ (OAr′ = 2,6-bis­(diphenylmethyl)-4-<i>tert</i>-butylphenoxide) yields cleanly the bis-aryloxide trimethyl complex [(Ar′O)₂Ta­(CH₃)₃] (<b>1</b>), which is isolated in 92% yield and is spectroscopically and structurally characterized. Addition of 2 equiv of HOAr′ to [TaCl₂(CH₃)₃] results in clean protonation concurrent with formation of the bis-aryloxide methyl derivative [(Ar′O)₂Ta­(CH₃)­Cl₂] (<b>2</b>), which was also fully characterized, including an X-ray structure. Despite being close derivatives, complex <b>1</b> (trigonal bipyramidal) and <b>2</b> (square pyramidal) possess very different structures, with the <i>e</i> set in a square-pyramidal molecular orbital diagram being key to their preferred geometry. Addition of excess ylide, H₂CPPh₃, to <b>2</b> results in formation of the terminal tantalum methylidene chloride complex [(Ar′O)₂TaCH₂(Cl)­(H₂CPPh₃)] (<b>3</b>) in 64% yield, which is characterized by multinuclear NMR spectroscopy and a solid-state structure determination

Hydrophobic Collapse of Foldamer Capsules Drives Picomolar-Level Chloride Binding in Aqueous Acetonitrile Solutions

Aqueous media are competitive environments in which to perform host–guest chemistry, particularly when the guest is highly charged. While hydrophobic binding is a recognized approach to this challenge in which apolar pockets can be designed to recognize apolar guests in water, complementary strategies are required for hydrophilic anions like chloride. Here, we present evidence of such an alternative mechanism, used everyday by proteins yet rare for artificial receptors, wherein hydrophobic interactions are shown to be responsible for organizing and stabilizing an aryl-triazole foldamer to help extract hydrophilic chloride ions from increasingly aqueous solutions. Therein, a double-helical complex gains stability upon burial of ∼80% of the π surfaces that simultaneously creates a potent, solvent-excluding microenvironment for hydrogen bonding. The chloride’s overall affinity to the duplex is substantial in 25% water v/v in acetonitrile (log β₂ = 12.6), and it remains strong (log β₂ = 13.0) as the water content is increased to 50%. With the rise in predictable designs of abiological foldamers, this water-assisted strategy can, in principle, be utilized for binding other hydrophilic guests

'Frågor'

Electropolymerization of tris­(dioximate) cage complexes furnished metal-containing conducting polymers (MCPs) that deposit directly onto the electrode surface as uniform films. The injection of electrons into, or removal of electrons from, these electroactive materials proceeds via different pathways with different rates, the underlying molecular mechanisms of which were investigated by a combination of electrochemical, spectroscopic, and focused-ion-beam–scanning electron microscopy (FIB-SEM) cross-section analysis studies. For cobalt-containing polymers, both the metal centers and π-conjugated organic backbone work cooperatively as hopping stations for migrating holes, whereas the reduced polymer utilizes less-efficient self-exchange between cobalt­(II) and cobalt­(I) centers for electron transport. A small molecule model of such reductively doped polymer was prepared independently, which provided compelling electrochemical and spectroelectrochemical evidence to support the structural integrity of the metal centers upon redox switching. A well-defined metal-to-ligand charge transfer (MLCT) band of the <i>n</i>-doped polymer was exploited further as a straightforward spectroscopic tool to quantify the number of redox-active metal centers directly and to estimate the lower distance limit of diffusional charge transport across the bulk material

Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

A bulky tris­(carbene)­borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron­(II) complex [PhB­(MesIm)₃Fe­(CO)₃]­[B­(C₆F₅)₄] (<b>1</b>) ([PhB­(MesIm₃)]⁻ = phenyltris­(1-mesitylimidazol-2-ylidene)­borate) yields the low-spin (<i>S</i> = 1/2) iron­(I) complex PhB­(MesIm)₃Fe­(CO)₂ (<b>2</b>), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K]­[PhB­(MesIm)₃Fe­(CO)₂] (<b>3</b>), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of <b>3</b> yields the weakly acidic iron hydride PhB­(MesIm)₃Fe­(CO)₂H (<b>4</b>), which can also be isolated by treating the κ³-coordinated alkylborohydrido complex PhB­(MesIm)₃­Fe­(κ³-BH­(CH₂CH₃)₃) (<b>5</b>) with CO. The strong donor ability of the tris­(carbene)­borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy

Charge Injection and Transport in Metal-Containing Conducting Polymers: Spectroelectrochemical Mapping of Redox Activities

Electropolymerization of tris­(dioximate) cage complexes furnished metal-containing conducting polymers (MCPs) that deposit directly onto the electrode surface as uniform films. The injection of electrons into, or removal of electrons from, these electroactive materials proceeds via different pathways with different rates, the underlying molecular mechanisms of which were investigated by a combination of electrochemical, spectroscopic, and focused-ion-beam–scanning electron microscopy (FIB-SEM) cross-section analysis studies. For cobalt-containing polymers, both the metal centers and π-conjugated organic backbone work cooperatively as hopping stations for migrating holes, whereas the reduced polymer utilizes less-efficient self-exchange between cobalt­(II) and cobalt­(I) centers for electron transport. A small molecule model of such reductively doped polymer was prepared independently, which provided compelling electrochemical and spectroelectrochemical evidence to support the structural integrity of the metal centers upon redox switching. A well-defined metal-to-ligand charge transfer (MLCT) band of the <i>n</i>-doped polymer was exploited further as a straightforward spectroscopic tool to quantify the number of redox-active metal centers directly and to estimate the lower distance limit of diffusional charge transport across the bulk material

A Nitrido Salt Reagent of Titanium

Deprotonation of the parent titanium imido (^tBunacnac)­TiNH­(Ntolyl₂) (^tBunacnac^– = [ArNC^tBu]₂CH; Ar = 2,6-ⁱPr₂C₆H₃) with KCH₂Ph forms a rare example of a molecular titanium nitride as a dimer, {[K]­[(^tBunacnac)­TiN­(Ntolyl₂)]}₂. From the parent imido or nitride salt, the corresponding aluminylimido–etherate adduct, (^tBunacnac)­TiN­[AlMe₂(OEt₂)]­(Ntolyl₂), can be isolated and structurally characterized. The parent imido is also a source for the related borylimido, (^tBunacnac)­TiNBEt₂(Ntolyl₂)

Structural Elucidation of the Illustrious Tebbe Reagent

The Tebbe reagent, [Cp₂Ti­(μ₂-Cl)­(μ₂-CH₂)­AlMe₂] (<b>1</b>), has finally been structurally characterized due to the fortuitous formation of cocrystals of <b>1</b> and [Cp₂Ti­(μ₂-Cl)₂AlMe₂] (<b>2</b>). Single crystals of <b>1</b> and <b>2</b>, despite being extremely reactive and forming an amorphous white coat, can be mounted and data collected to high resolution, thereby providing for the first time a solid-state representation of a titanium methylidene adduct with diphilic AlClMe₂