π-Metalated [15]Paracyclophanes: Synthesis and Binding to Oxo-Anions via Anion−π Interactions

Charge transfer between a metal-bound halide and a quinone through π-hole interactions leads to bulk conductivity

π-Hole interactions between a metal-bound halide and a quinoid ring are described in four novel isostructural co-crystals with the formula [Cu(terpy)ClX]·X'4Q (terpy = 2,2':6',2''-terpyridine; Q = quinone; X = Br, I; X' = Cl, Br). An unusually strong π-hole interaction between Cu-X and the quinoid ring is noted. Periodic DFT computations estimate the energy of the X⋯quinone interaction to be -20.79 kcal mol-1, indicating a very strong non-covalent interaction attributed to a higher degree of polarization along the bonding path. The black colour of the crystals originates from a cooperative intermolecular charge transfer between the [Cu(terpy)ClX] complex and the quinone π-system, with iodine playing a dominant role in this process by facilitating the π-hole interaction that enhances the charge transfer mechanism. All the compounds are considered to be weak semiconductors with the σDC magnitude ranging between 10-11 and 10-9 S cm-1. It is anticipated that by a smart choice of electron donors and electron acceptors, one can substantially enhance the effect and engineer more efficient conductive materials.

Optimizing selectivity via membrane molecular packing manipulation for simultaneous cation and anion screening

Advancing membranes with enhanced solute-solute selectivity is essential for expanding membrane technology applications, yet it presents a notable challenge. Drawing inspiration from the unparalleled selectivity of biological systems, which benefit from the sophisticated spatial organization of functionalities, we posit that manipulating the arrangement of the membrane's building blocks, an aspect previously given limited attention, can address this challenge. We demonstrate that optimizing the face-to-face orientation of building blocks during the assembly of covalent-organic-framework (COF) membranes improves ion-π interactions with multivalent ions. This optimization leads to extraordinary selectivity in differentiating between monovalent cations and anions from their multivalent counterparts, achieving selectivity factors of 214 for K+/Al3+ and 451 for NO3-/PO43-. Leveraging this attribute, the COF membrane facilitates the direct extraction of NaCl from seawater with a purity of 99.57%. These findings offer an alternative approach for designing highly selective membrane materials, offering promising prospects for advancing membrane-based technologies.

Supramolecular chemistry of liquid-liquid extraction

Liquid-Liquid Extraction (LLE) is a venerable and widely used method for the separation of a targeted solute between two immiscible liquids. In recent years, this method has gained popularity in the supramolecular chemistry community due to the development of various types of synthetic receptors that effectively and selectively bind specific guests in an aqueous medium through different supramolecular interactions. This has eventually led to the development of state-of-the-art extraction technologies for the removal and purification of anions, cations, ion pairs, and small molecules from one liquid phase to another liquid phase, which is an industrially viable method. The focus of this perspective is to furnish a vivid picture of the current understanding of supramolecular interaction-based LLE chemistry. This will not only help to improve separation technology in the chemical, mining, nuclear waste treatment, and medicinal chemistry sectors but is also useful to address the purity issue of the extractable species, which is otherwise difficult. Thus, up-to-date knowledge on this subject will eventually provide opportunities to develop large-scale waste remediation processes and metallurgy applications that can address important real-life problems.

Luminescent Ruthenium-Terpyridine Complexes Coupled with Stilbene-Appended Naphthalene, Anthracene, and Pyrene Motifs Demonstrate Fluoride Ion Sensing and Reversible Trans-Cis Photoisomerization

A new family of luminescent heteroleptic Ru(II)-terpyridine complexes coupled with stilbene-appended naphthalene, anthracene, and pyrene motifs is reported. Each of the complexes features moderately intense emission at room temperature having a lifetime of 16.7 ns for naphthalene and 11.4 ns for anthracene, while a substantially elevated lifetime of 8.3 μs was observed for the pyrene derivative. All the three complexes display a reversible couple in the positive potential window due to Ru2+/Ru3+ oxidation but multiple reversible and/or quasi-reversible peaks in the negative potential domain because of the reduction of the terpyridine moieties. All the complexes selectively sense F- among the studied anions via the intermediary of different noncovalent interactions. The interaction event is monitored through absorption, emission, and 1H and 19F NMR spectroscopy. Additionally, upon utilizing the stilbene motif, reversible trans-cis isomerization of the complexes has been undertaken upon alternate treatment of visible and UV light so that the complexes can act as potential photomolecular switches. We also carried out the anion sensing characterization of the cis form of the complexes. Theoretical calculation employing density functional theory is also executed for a selective complex (naphthalene derivative) to elucidate different noncovalent interactions that are operative during the complex-fluoride interplay.

Anion-Regulated Hierarchical Self-Assembly and Chiral Induction of Metallo-Tetrahedra

The precise control over hierarchical self-assembly of superstructures relying on the elaboration of multiple noncovalent interactions between basic building blocks is both elusive and highly desirable. We herein report a terpyridine-based metallo-cage T with a tetrahedral motif and utilized it as an efficient building block for the controlled hierarchical self-assembly of superstructures in response to different halide ions. Initially, the hierarchical superstructure of metallo-cage T adopted a hexagonal close-packed structure. By adding Cl- /Br- or I- , drastically different hierarchical superstructures with highly-tight hexagonal packing or graphite-like packing arrangements, respectively, have been achieved. These unusual halide-ion-triggered hierarchical structural changes resulted in quite distinct intermolecular channels, which provided new insights into the mechanism of three-dimensional supramolecular aggregation and crystal growth based on macromolecular construction. In addition, the chiral induction of the metallo-cage T can be realized with the addition of chiral anions, which stereoselectively generated either PPPP- or MMMM-type enantiomers.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

The nature of π-hole interactions between iodide anions and quinoid rings in the crystalline state

The investigated co-crystal of 3-chloro-N-methylpyridinium iodide with tetrabromoquinone (3-Cl-N-MePy·I·Br4Q) reveals a π-hole interaction between an iodide anion and a quinoid ring involving an n → π* charge transfer. The quinoid ring has a partial negative charge (estimated to be in the range 0.08-0.11e) and a partial radical character, which is related to the black colour of the crystals (crystals of neutral tetrabromoquinone are yellow). A detailed X-ray charge density study revealed two symmetry-independent bond critical points between the iodide anions and carbon atoms of the ring. Their maximum electron density of 0.065 e Å-3 was reproduced by quantum chemical modelling. The energy of the interaction is estimated to be -11.16 kcal mol-1, which is comparable to the strength of moderate hydrogen bonding (about -10 kcal mol-1); it is dominantly electrostatic in nature, with a considerable dispersion component.

The strength and selectivity of perfluorinated nano-hoops and buckybowls for anion binding and the nature of anion-π interactions

Perfluorinated cycloparaphenylenes (F-[n]CPP, n = 5-8), boron nitride nanohoop (F-[5]BNNH), and buckybowls (F-BBs) were proposed as anion receptors via anion-π interactions with halide anions (Cl^- , Br^- and I^- ), and remarkable binding strengths up to -294.8 kJ/mol were computationally verified. The energy decomposition approach based on the block-localized wavefunction method, which combines the computational efficiency of molecular orbital theory and the chemical intuition of ab initio valence bond theory, was applied to the above anion-π complexes, in order to elucidate the nature and selectivity of these interactions. The overall attraction is mainly governed by the frozen energy component, in which the electrostatic interaction is included. Remarkable binding strengths with F-[n]CPPs can be attributed to the accumulated anion-π interactions between the anion and each conjugated ring on the hoop, while for F-BBs, additional stability results from the curved frameworks, which distribute electron densities unequally on π-faces. Interestingly, the strongest host was proved to be the F-[5]BNNH, which exhibits the most significant anisotropy of the electrostatic potential surface due to the difference in the electronegativities of nitrogen and boron. The selectivity of each host for anions was explored and the importance of the often-overlooked Pauli exchange repulsion was illustrated. Chloride anion turns out to be the most favorable anion for all receptors, due to the smallest ionic radius and the weakest destabilizing Pauli exchange repulsion.

Tetraphenylethylene-embedded [15]paracyclophanes: AIEgen and macrocycle merged novel supramolecular hosts used for sensing Ni2+ ions

Transformation of [1₅]paracyclophanes ([15]PCP) into fluorophores has been achieved by embedding tetraphenylethene (TPE) units into their skeletons at the meso-positions. The obtained two hosts demonstrated distinct aggregation-induced emission (AIE) properties and their fluorescence could be selectively quenched by Ni²⁺ ions.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Flexible Vertex Engineers the Controlled Assembly of Distorted Supramolecular Tetrahedral and Octahedral Cages

Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.

Watching Hydrogens Migrate: Step by Step from [ReI(η6-C6H6)2]+ to [ReIII(η3-C6H9)(η6-C6H6)(NCCH3)2]2

Arene substitution reactions in [M(η⁶-arene)₂]^0/2+ are well documented for Groups 6 and 8 but are essentially unknown for the manganese triad. Aiming to replace benzene in [Re^I(η⁶-C₆H₆)₂]⁺, we altered the hapticity of one coordinated benzene, which we found to be tunable stepwise from an η⁶ to an η³-allyl coordination mode. Reduction of [Re^I(η⁶-C₆H₆)₂]⁺ with hydrides gives [Re^I(η⁵-C₆H₇)(η⁶-C₆H₆)]. Subsequent addition of acid yields [Re^IIIH(η⁵-C₆H₇)(η⁶-C₆H₆)]⁺, which converts to [Re^I(η⁴-C₆H₈)(η⁶-C₆H₆)NCCH₃]⁺ in acetonitrile. Further protonation gives the title complex [Re^III(η³-C₆H₉)(η⁶-C₆H₆)(NCCH₃)₂]²⁺ by a rhenium-mediated, intramolecular hydride shift. Herein, we present a full mechanistic elucidation of these transformations based on NMR studies, isolation of reaction intermediates, and their full characterizations. The structural feature {Re^III(η⁶-C₆H₆)} is unprecedented. Direct arene exchange from [Re^I(η⁶-C₆H₆)₂]⁺ to [Re^I(η⁶-arene)(η⁶-C₆H₆)]⁺ was found only under strongly acidic conditions in neat arene. The analogous chemistry of the lighter homologue technetium (⁹⁹Tc) is distinctly different. Treatment of [Tc^I(η⁵-C₆H₇)(η⁶-C₆H₆)] with acid in acetonitrile yields only mixtures of [Tc^I(η⁶-C₆H₆)₂]⁺ and [Tc^II(NCCH₃)₆]²⁺.

Investigating the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry

RATIONALE

Exploring the formation mechanism of the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry (ESI-MS) is crucial for further understanding the physicochemical properties of Ru(II)-arene complexes in solution.

METHODS

All mass spectra were collected with an AB SCIEX TripleTOF 5600⁺ mass spectrometer in positive mode. Theoretical calculations were carried out using the density functional theory method at the B3LYP level with a hybrid basis set consisting of 6-31G(d,p) and LanL2DZ in the Gaussian 03 program.

RESULTS

When ruthenated [1₅ ]paracyclophanes (Ru-[1₅ ]PCPs) and Ru(II)-arene dimers were dissolved in alkyl alcohol solvents, the adducts of alkoxides with Ru(II)-arene cations were observed under positive ion mode ESI-MS, which resulted from the coordination of alkyl alcohol molecules with the Ru(II)-arene cations followed by the deprotonation of O-H bonds of the coordinated alcohols. Furthermore, the number of alkoxides binding to Ru-[1₅ ]PCPs was regulated by the number of ruthenium atoms. Attributed to good solubility and small steric hindrance, the signal intensity of the adducts of methoxides with Ru(II)-arene cations was the strongest among the three alkyl alcohols used in this study.

CONCLUSIONS

The characteristic adducts of alkoxides with Ru(II)-arene cations were pervasively present in positive ion mode ESI-MS of nine Ru(II)-arene complexes dissolved in alkyl alcohol solvents. Taking into consideration the solubility and signal response, methanol is the most suitable solvent for the ESI-MS experiments with Ru(II)-arene complexes among the solvents studied, where almost only the diagnostic adducts of methoxides with Ru(II)-arene cations are present.

A [15]paracyclophenone and its fluorenone-containing derivatives: syntheses and binding to nerve agent mimics via aryl-CH hydrogen bonding interactions

A [1₅]paracyclophenone and its fluorenone-containing derivatives were synthesized. The novel macrocyclic host I binds nerve agent mimics through the ‘non-traditional’ aryl-CH hydrogen bonding interactions.

Novel co-crystals with π-hole interactions between iodide anions and quinoid rings involving charge transfer

Six novel co-crystals of tetrabromoquinone with iodide salts of organic cations displaying short contacts between iodide anions and the quinoid rings have been structurally characterised.

Diversity and uniformity in anion-π complexes of thiocyanate with aromatic, olefinic and quinoidal π-acceptors

Despite the progress in the study of anion-π interactions, there are still inconsistencies in the use of this term and the experimental data about factors affecting the strength of such bonding are limited. To shed light on these issues, we explored supramolecular associations between NCS^- anions and a series of aromatic, olefinic or quinoidal π-acceptors. Combined experimental and computational studies revealed that all these complexes were formed by an attraction of the anion to the face of the π-system, and the arrangements of thiocyanate followed the areas of the most positive potentials on the surfaces of the π-acceptors. The stabilities of the complexes increased with the π-acceptor strength (reflected by their reduction potentials), and were essentially independent of the magnitudes of the maximum electrostatic potentials on their surfaces. The complexes showed intense absorption bands in the UV-Vis range, and the energies of these bands were correlated with the difference of the redox potentials of the anions and π-acceptors. Such features, as well as results of atoms-in-molecules and non-covalent index analyses suggested that besides electrostatics, molecular orbital interactions play a substantial role in the formation of these complexes. The unified trends in variations of the characteristics of the complexes between thiocyanate and a variety of π-acceptors point to their common nature. To embrace diversity and uniformity of the anion-π associates, we suggest (following the halogen bond's definition) that anion-π bonding occurs when there is evidence of a net attraction between the anions and the face of the electrophilic π-system.

Amphiphilic Organic Cages: Self-Assembly into Nanotubes and Enhanced Anion-π Interactions

An amphiphilic organic cage was synthesized and used as self-assembly synthon for the fabrication of novel functional supramolecular structures in solution. The transmission electron microscopy (TEM) results showed that this amphiphilic cage self-assembled in aqueous solution into unilamellar nanotubes with a diameter of 29±4 nm at a concentration of 0.05 mg mL^-1 . Interestingly, the self-assembly of this cage significantly enhanced the anion-π interactions as indicated by a remarkable increasement of association constant (K_a ) between Cl^- and this amphiphilic cage after self-assembly. In specific, K_a was increased from 223 M^-1 for discrete cages in methanol to 6800 M^-1 for aggregated cages after self-assembly in water at the same concentration of 2.26×10^-5  M. A mechanism based on a synergistic effect was proposed in order to explain this self-assembly process through enhanced anion-π interactions.

n → π* interactions as a versatile tool for controlling dynamic imine chemistry in both organic and aqueous media

The imine bond holds a prominent place in supramolecular chemistry and materials science, and one issue is the stability of imines due to their electrophilic nature. Here we introduced ortho-carboxylate groups into a series of aromatic aldehydes/imines for dictating imine dynamic covalent chemistry (DCC) through n → π* interactions, one class of widespread and yet underused non-covalent interactions. The thermodynamically stabilizing role of carboxylate-aldehyde/imine n → π* interactions in acetonitrile was elucidated by the movement of the imine exchange equilibrium and further supported by crystal analysis. Computational studies provided mechanistic insights for n → π* interactions, the strength of which can surpass that of CH hydrogen bonding and is dependent on the orientation of interacting sites based on natural bond orbital analysis. Moreover, the substituent effect and the combination of recognition sites allowed additional means for modulation. Finally, to show the relevance of our findings ortho-carboxylate containing aldehydes were used to regulate imine formation/exchange in water, and modification of the N-terminus of amino acids and peptides was achieved in a neutral buffer. This work represents the latest example of weak interactions governing DCC and sets the stage for assembly and application studies.