Over one century after discovery: pyrylium salt chemistry emerging as a powerful approach for the construction of complex macrocycles and metallo-supramolecules

Cage-To-Cage Transformations in Self-Assembled Coordination Cages Using "Acid/Base" or "Guest Binding-Induced Strain" as Stimuli

Controlling supramolecular systems between different functional forms by utilizing acids/bases as stimuli is a formidable challenge, especially where labile coordination bonds are involved. A pair of acid/base responsive, interconvertible 1,5-enedione/pyrylium based Pd2L4-type cages are prepared that exhibit differential guest binding abilities towards disulfonates of varied sizes. A three-state switch has been achieved, where (i) a weakly coordinating base induced cage-to-cage transformation in the first step, (ii) a strongly coordinating base triggered cage disassembly as the second step, and (iii) the third step shows acid(strong) promoted generation of initial cage, thereby completing the cycle. To our surprise, binding of a specific disulfonate guest facilitated cage-to-cage transformations by inducing strain on the cage assembly thereby opening the labile pyrylium rings of the cage. Through a competitive guest binding study, we demonstrated the superior guest binding capability of the octacationic pyrylium-based cage over a similar-sized tetracationic cage. These results provide a reliable approach to reversibly modulate the guest binding properties of acid/base-responsive self-assembled coordination cages.

Visible-Light-Driven Bifunctional Photocatalytic Radical-Cascade Selenocyanation/Cyclization of Acrylamides with KSeCN

A visible-light-induced radical-cascade selenocyanation/cyclization of N-alkyl-N-methacryloyl benzamides, 2-aryl-N-acryloyl indoles, and N-methacryloyl-2-phenylbenzimidazoles with potassium isoselenocyanate (KSeCN) was developed. The reactions were carried out with inexpensive KSeCN as a selenocyanation reagent, potassium persulfate as an oxidant, 2,4,6-triphenylpyrylium tetrafluoroborate as a bifunctional catalyst for phase-transfer catalysis, and photocatalysis. A library of selenocyanate-containing isoquinoline-1,3(2H,4H)-diones, indolo[2,1-a]isoquinoline-6(5H)-ones, and benzimidazo[2,1-a]isoquinolin-6(5H)-ones were achieved in moderate to excellent yields at room temperature under visible-light and ambient conditions. Importantly, the present protocol features mild reaction conditions, large-scale synthesis, simple manipulation, product derivatization, good functional group, and heterocycle tolerance.

Combining Multiple Photosensitizer Modules into One Supramolecular System for Synergetic Enhanced Photodynamic Therapy

Photodynamic therapy (PDT), as an emerging cancer treatment, requires the development of highly desirable photosensitizers (PSs) with integrated functional groups to achieve enhanced therapeutic efficacy. Coordination-driven self-assembly (CDSA) would provide an alternative approach for combining multiple PSs synergistically. Here, we demonstrate a simple yet powerful strategy of combining conventional chromophores (tetraphenylethylene, porphyrin, or Zn-porphyrin) with pyridinium salt PSs together through condensation reactions, followed by CDSA to construct a series of novel metallo-supramolecular PSs (S1-S3). The generation of reactive oxygen species (ROS) is dramatically enhanced by the direct combination of two different PSs, and further reinforced in the subsequent ensembles. Among all the ensembles, S2 with two porphyrin cores shows the highest ROS generation efficiency, specific interactions with lysosome, and strong emission for probing cells. Moreover, the cellular and living experiments confirm that S2 has excellent PDT efficacy, biocompatibility, and biosafety. As such, this study will enable the development of more efficient PSs with potential clinical applications.

π-Electronic ion pairs: building blocks for supramolecular nanoarchitectonics viaiπ-iπ interactions

The pairing of charged π-electronic systems and their ordered arrangement have been achieved by iπ-iπ interactions that are derived from synergetically worked electrostatic and dispersion forces. Charged π-electronic systems that provide ion pairs as building blocks for assemblies have been prepared by diverse strategies for introducing charge in the core π-electronic systems. One method to prepare charged π-electronic systems is the use of covalent bonding that makes π-electronic ions and valence-mismatched metal complexes as well as protonated and deprotonated states. Noncovalent ion complexation is another method used to create π-electronic ions, particularly for anion binding, producing negatively charged π-electronic systems. Charged π-electronic systems afford various ion pairs, consisting of both cationic and anionic π-systems, depending on their combinations. Geometries and electronic states of the constituents in π-electronic ion pairs affect the photophysical properties and assembling modes. Recent progress in π-electronic ion pairs has revealed intriguing characteristics, including the transformation into radical pairs through electron transfer and the magnetic properties influenced by the countercations. Furthermore, the assembly states exhibit diversity as observed in crystals and soft materials including liquid-crystal mesophases. While the chemistry of ion pairs (salts) is well-established, the field of π-electronic ion pairs is relatively new; however, it holds great promise for future applications in novel materials and devices.

Unusual nuclear exchange within a germanium-containing aromatic ring that results in germanium atom transfer

The delivery of single atoms is highly desirable for the straightforward synthesis of complex molecules, however this approach is limited by a lack of suitable atomic transfer reagents. Here, we report a germanium atom transfer reaction employing a germanium analogue of the phenyl anion. The reaction yields a germanium-substituted benzene, along with a germanium atom which can be transferred to other chemical species. The transfer of atomic germanium is demonstrated by the formation of well-defined germanium doped molecules. Furthermore, computational studies reveal that the reaction mechanism proceeds via the first example of an aromatic-to-aromatic nuclear germanium replacement reaction on the germabenzene ring. This unusual reaction pathway was further probed by the reaction of our aromatic germanium anion with a molecular silicon species, which selectively yielded the corresponding silicon-substituted benzene derivative.

Bi-directional geometric constraints in the construction of giant dual-rim nanorings

In the field of metallo-supramolecular assemblies, supramolecular macrocycles have attracted considerable attention due to their guest recognition and catalytic properties. Herein, we report a novel strategy for the construction of giant hollow macrocyclic structures using a bi-directional geometric constraint strategy. We investigated the structural design of two terpyridine-based tetratopic organic ligands, whose inner and outer rims have different angles. Compared to conventional strategies of self-assembly using single angular orientation building blocks that typically generate small macrocyclic objects or polymers, the mutual interaction between the different angles of the ligands could promote the formation of giant hollow macrocyclic supramolecular architectures. The self-assembly mechanism and hierarchical self-assembly of giant supramolecular macrocycles have been characterized by NMR, ESI-MS and TEM experiments. The strategy used in this study not only advances the design of giant 2D macrocycles with large inner diameters but also gives insights into the mechanism of formation of large structures.

Hourglass-Shaped Nanocages with Concaved Structures Based on Selective Self-Complementary Coordination Ligands and Tunable Hierarchical Self-Assembly

Three-dimensional (3D) structures constructed via coordination-driven self-assemblies have recently garnered increasing attention due to the challenges in structural design and potential applications. In particular, developing new strategy for the convenient and precise self-assemblies of 3D supramolecular structures is of utmost interest. Introducing the concept of self-coordination ligands, herein the design and synthesis of two meta-modified terpyridyl ligands with selective self-complementary coordination moiety are reported and their capability to assemble into two hourglass-shaped nanocages SA and SB is demonstrated. Within these 3D structures, the meta-modified terpyridyl unit preferably coordinates with itself to serve as concave part. By changing the arm length of the ligands, hexamer (SA) and tetramer (SB) are obtained respectively. In-depth studies on the assembly mechanism of SA and SB indicate that the dimers could be formed first via self-complementary coordination and play crucial roles in controlling the final structures. Moreover, both SA and SB can go through hierarchical self-assemblies in solution as well as on solid-liquid interface, which are characterized by transmission electron microscope (TEM) and scanning tunneling microscopy (STM). It is further demonstrated that various higher-order assembly structures can be achieved by tuning the environmental conditions.

Enantioselective dearomatization reactions of heteroarenes by anion-binding organocatalysis

Catalytic asymmetric dearomatization of heteroaromatic compounds has received considerable attention in the last few years, since it allows for a fast expansion of the chemical space by converting relatively simple, flat molecules into complex, three dimensional structures with added value. Among different approaches, remarkable progress has been recently achieved by the development of organocatalytic dearomatization methods. In particular, the anion-binding catalysis technology has emerged as a potent alternative to metal catalysis, which together with the design of novel, tunable anion-receptor motifs, has provided new entries for the enantioselective dearomatization of heteroarenes through a chiral contact ion pair formation by activation of the electrophilic reaction partner. In this feature, we provide an overview of the different methodologies and advances in anion-binding catalyzed dearomatization reactions of different heteroarenes.

Streptocyanine as an activation mode of amine catalysis for the conversion of pyridine rings to benzene rings

Amine catalysts have emerged as an invaluable tool in organic synthesis. Iminium, enamine, and enamine radical cation species are representative activation modes of amine catalysis. However, the development of new amine catalysis activation modes that enable novel synthetic strategies remains highly desirable. Herein, we report streptocyanine as a new amine catalysis activation mode, which enables the skeletal editing of pyridine rings to benzene rings. N-Arylation of pyridines bearing an alkenyl substituent at the 3-position generates the corresponding N-arylpyridiniums. The resulting pyridinum reacts with a catalytic amount of piperidine to afford a streptocyanine intermediate. Catalytically generated streptocyanine forms a benzene ring via a ring-closing reaction, thereby releasing the amine catalyst. Consequently, the alkene moiety in the starting pyridines is incorporated into the benzene ring of the products. Pyridiniums bearing various alkene moieties were efficiently converted to formyl-substituted benzene derivatives. Mechanistic studies support the postulation that the present catalytic process was intermediated by streptocyanine. In this reaction system, streptocyanine could be regarded as a new activation mode of amine catalysis.

Precise Synthesis of Concentric Ring, Helicoid, and Ladder Metallo-Polymers with Chevron-Shaped Monomers

Molecular geometry represents one of the most important structural features and governs physical properties and functions of materials. Nature creates a wide array of substances with distinct geometries but similar chemical composition with superior efficiency and precision. However, it remains a formidable challenge to construct abiological macromolecules with various geometries based on identical repeating units, owing to the lack of corresponding synthetic approaches for precisely manipulating the connectivity between monomers and feasible techniques for characterizing macromolecules at the single-molecule level. Herein, we design and synthesize a series of tetratopic monomers with chevron stripe shape which serve as the key precursors to produce four distinct types of metallo-macromolecules with well-defined geometries, viz., the concentric hexagon, helicoid polymer, ladder polymer, and cross-linked polymer, via platinum-acetylide couplings. Concentric hexagon, helicoid, and ladder metallo-polymers are directly visualized by transmission electron microscopy, atomic force microscopy, and ultra-high-vacuum low-temperature scanning tunneling microscopy at the single-molecule level. Finally, single-walled carbon nanotubes (SWCNTs) are selected as the guest to investigate the structure-property relationship based on such macromolecules, among which the helicoid metallo-polymer shows high efficiency in wrapping SWCNTs with geometry-dependent selectivity.

Atom Swapping on Aromatic Rings: Conversion from Phosphinine Pincer Metal Complexes to Metallabenzenes Triggered by O2 Oxidation

Herein, we report a novel method for the synthesis of metallabenzenes by swapping the phosphorus atom in an aromatic phosphinine ring with transition metal fragments. The oxidation of a phosphine-phosphinine-phosphine pincer iridium complex by O₂ triggered the replacement of the phosphorus atom of the phosphinine ring by an iridium fragment to afford iridabenzene. Dianionic rhodabenzene was also synthesized from a phosphinine rhodium complex by oxidation of the phosphorus atom, followed by subsequent reduction using metallic potassium. The aromaticity of the newly synthesized irida- and rhoda-benzenes was evaluated both experimentally and theoretically.

Design and Self-Assembly of Macrocycles with Metals at the Corners Based on Dissymmetric Terpyridine Ligands

Terpyridine-based discrete supramolecular architectures with metal ions in the corners have rarely been reported. Herein, we report two dissymmetric terpyridyl ligands LA and LB decorated at the 5-position and 4-position of terpyridine respectively. The complexes constructed by the self-assembly of LA and LB with Zn(II) exhibit hand-circle-like structures. Moreover, all Zn(II) are successfully fixed in the corners. A series of dimeric to hexameric macrocycles is obtained by head-to-tail connections with changing concentration. This work will pave the way for preparation of more elaborate self-assembled structures based on dissymetric ligands.

Terpyridine-Based 3D Discrete Metallosupramolecular Architectures

Terpyridine (tpy)-based 3D discrete metallosupramolecular architectures, which are often inspired by polyhedral geometry and the biological structures found in nature, have drawn significant attention from the community of metallosupramolecular chemistry. Because of the linear tpy-M(II)-tpy connectivity, the creation of sophisticated 3D metallosupramolecules based on tpy remains a formidable synthetic challenge. Nevertheless, with recent advancement in ligand design and self-assembly, diverse 3D metallosupramolecular polyhedrons, such as Platonic solids, Archimedean solids, prims as well as Johnson solids, have been constructed and their potential applications have been explored. This review summarizes the progress on tpy-based discrete 3D metallosupramolecules, aiming to shed more light on the design and construction of novel discrete architectures with molecular-level precision through coordination-driven self-assembly.

Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me₄enPd)_2n(ML)_n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me₄en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me₄enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me₄enPd)₄(ML)₂-type cage, all other M(ii) ions led to isostructural (Me₄enPd)₆(ML)₃-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me₄enPd)₆(ZnL)₃ host an effective catalyst for the CO₂ to CO₃²⁻ conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO₃²⁻ and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.

A series of coordination hosts were prepared and their applications in CO₂ fixation were studied.

Condensation of pyrylium salts with mixed anhydrides: aryl ethers, aryl amines and sterically congested aromatics

Condensation of 2,4,6-triaryl pyrylium salts with mixed anhydrides, formed in situ from α-functionalized sodium acetates and an anhydride solvent, leads in good yields to the corresponding 2,4,6-triaryl benzenes functionalized at their 1-position.

Efficient collection of perrhenate anions from water using poly(pyridinium salts) via pyrylium mediated transformation

Poly(pyridinium salts) composed of cationic pyridinium groups with benzene-rich motifs demonstrated high efficiency and selectivity in the capture of ReO4− from SO42− containing water.

Heteroatom-Embedded Approach to Vinylene-Linked Covalent Organic Frameworks with Isoelectronic Structures for Photoredox Catalysis

Embedding heteroatoms into the main backbones of polymeric materials has become an efficient tool for tailoring their structures and improving their properties. However, owing to comparatively harsh heteroatom-doping conditions, this has rarely been explored in covalent organic frameworks (COFs). Herein, upon aldol condensation of a trimethyl-substituted pyrylium salt with a tritopic aromatic aldehyde, a two-dimensional oxonium-embedded COF with vinylene linkages was achieved, which was further converted to a neutral pyridine-cored COF by in situ replacement of oxonium ions with nitrogen atoms under ammonia treatment. The two heteroatom-embedded COFs are conceptually isoelectronic with each other, featuring similar geometric structures but different electronic structures, rendering them capable of catalyzing the visible-light-promoted multi-component synthesis of tri-substituted pyridine derivatives with good recyclability.

Perylene Bisimide-Cored Supramolecular Coordination Complexes: Interplay between Ensembles, Excited State Processes, and Aggregation Behaviors

Manipulating the optical properties of fluorescent species is challenging owing to complicated and tedious synthetic works. Herein, the photophysical properties of perylene bisimide (PBI) were effectively tuned by varying the geometrical arrangement of PBI moieties within supramolecular coordination complexes (SCCs), where a PBI-based dicycle (2) and a trigonal prism (3) were generated via using a typical 90° Pt(II) reagent, cis-(PEt₃ )₂ Pt(OTf)₂ -based coordination-driven self-assembly approach. The ligand, an ortho-tetrapyridiyl-PBI (1), exhibits a moderate fluorescence quantum yield (∼13 %) and efficient inter-system crossing (ISC). 2, however, is much more emissive with a fluorescence quantum yield of ∼41 %, and the relevant ISC process is significantly hindered. The fluorescence quantum yield of 3 is merely ∼6 % due to the observed symmetry-breaking charge separation (SB-CS), which turns to triplet state upon charge recombination. Interestingly, 3 could be fully transformed into 2 by simply adding a suitable amount of a 90° Pt(II)-based neutral triangle. Moreover, 2 tends to form discrete dimers both in crystal and solution states, but 3 does not show the property. Therefore, controlling geometrical arrangement of fluorophores through coordination-driven self-assembly could be taken as another effective way to tune their excited state relaxation pathways and construct high-performance optical molecular materials, which generally have to be prepared via organic synthesis.

Unveiling the role of pyrylium frameworks on π-stacking interactions: a combined ab initio and experimental study

A multidisciplinary study is presented to shed light on how pyrylium frameworks, as π-hole donors, establish π-π interactions. The combination of CSD analysis, computational modelling (ab intitio, DFT and MD simulations) and experimental NMR spectroscopy data provides essential information on the key parameters that characterize these intereactions, opening new avenues for further applications of this versatile heterocycle.

Sequential Ring-Opening and Ring-Closing Reactions for Converting para-Substituted Pyridines into meta-Substituted Anilines

Herein we report a method for converting para-substituted pyridine rings into meta-dialkylamino-substituted benzene rings through sequential ring-opening and ring-closing reactions. The nitrogen atom in the pyridine rings was replaced with a methine group, and a dialkylamino substituent was introduced onto the original unsubstituted carbon atom in the pyridine rings. This process can be formally regarded as a hybrid of the skeletal editing and C-H amination of pyridine rings.