Strategy for the Construction of Diverse Poly-NHC-Derived Assemblies and Their Photoinduced Transformations

Hierarchically Assembled Gigantic Fe/Co Cyanometallate Clusters Exhibiting Electron Transfer Behavior Above Room Temperature

The construction of large and complex supramolecular architectures through self-assembly is at the forefront of contemporary coordination chemistry. Notwithstanding great success in various systems using anionic bridges (e.g., O2- or S2-) or organic ligands (e.g., pyridine or carboxylate ligands), the assembly of large cyanide-bridged clusters with increasing nuclearity remains a formidable synthetic challenge. In this study, it is achieved in preparing two heterometallic cyanometallate clusters with unprecedented complexity, [Fe20Co20] (1) and [Fe12Co15] (2), by creating the "flexibility" through a versatile ligand of bis((1H-imidazol-4-yl)methylene)hydrazine (H2L) and low-coordinate cobalt. Complex 1 features a super-square array of four cyanide-bridged [Fe4Co4] cube subunits as the corners that are interconnected by four additional [FeCo] units, resulting in a torus-shaped architecture. Complex 2 contains a lantern-like core-shell cluster with a triple-helix kernel of [Co3L3] enveloped by a [Fe12Co12] shell. The combined structure analysis and mass spectrometry study reveal a hierarchical assembly mechanism, which sheds new light on constructing cyanometallate nanoclusters with atomic precision. Moreover, complex 1 undergoes a thermally induced electron-transfer-coupled spin transition (ETCST) between the diamagnetic {FeII LS(µ-CN)CoIII LS} and paramagnetic {FeIII LS(µ-CN)CoII HS} configurations (LS = low spin, HS = high spin) above room temperature, representing the largest molecule displaying electron transfer and spin transition characteristic.

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

Tetrakispyrazolylethene: Protonation-Induced Emission

Tetrakispyrazolylethene (1) was synthesized from pyrazole and hexachloroethane through a one-step substitution reaction. The increase of emission was detected both in solid and aqueous THF solution, compared with that in anhydrous THF. While the former originates from the crystal packing, the latter is attributed to the protonation-induced emission, independent of aggregation, based on the optical measurement under varying concentrations and particle-size distribution analysis.

Steric and Geometrical Frustration Generate Two Higher-Order CuI12L8 Assemblies from a Triaminotriptycene Subcomponent

The use of copper(I) in metal-organic assemblies leads readily to the formation of simple grids and helicates, whereas higher-order structures require complex ligand designs. Here, we report the clean and selective syntheses of two complex and structurally distinct CuI12L8 frameworks, 1 and 2, which assemble from the same simple triaminotriptycene subcomponent and a formylpyridine around the CuI templates. Both represent new structure types. In T-symmetric 1, the copper(I) centers describe a pair of octahedra with a common center but whose vertices are offset from each other, whereas in D3-symmetric 2, the metal ions form a distorted hexagonal prism. The syntheses of these architectures illustrate how more intricate CuI-based complexes can be prepared via subcomponent self-assembly than has been possible to date through consideration of the interplay between the subcomponent geometry and solvent and electronic effects.

Stereocontrolled Self-Assembly of a Helicate-Bridged CuI12L4 Cage That Emits Circularly Polarized Light

Control over the stereochemistry of metal-organic cages can give rise to useful functions that are entwined with chirality, such as stereoselective guest binding and chiroptical applications. Here, we report a chiral CuI12L4 pseudo-octahedral cage that self-assembled from condensation of triaminotriptycene, aminoquinaldine, and diformylpyridine subcomponents around CuI templates. The corners of this cage consist of six head-to-tail dicopper(I) helicates whose helical chirality can be controlled by the addition of enantiopure 1,1'-bi-2-naphthol (BINOL) during the assembly process. Chiroptical and nuclear magnetic resonance (NMR) studies elucidated the process and mechanism of stereochemical information transfer from BINOL to the cage during the assembly process. Initially formed CuI(BINOL)2 thus underwent stereoselective ligand exchange during the formation of the chiral helicate corners of the cage, which determined the overall cage stereochemistry. The resulting dicopper(I) helicate corners of the cage were also shown to generate circularly polarized luminescence.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Construction and Hierarchical Self-Assembly of a Supramolecular Metal-Carbene Complex with Multifunctional Units

Hierarchical combinations involving metal-ligand interactions and host-guest interactions can consolidate building blocks with unique functions into material properties. This study reports the construction and hierarchical self-assembly of multifunctional trinuclear AuI tricarbene complex containing three crown ether units and three ferrocene units. Host-guest interactions between the multifunctional trinuclear AuI tricarbene complex and organic ammonium salts were investigated, revealing that crown ether-based host-guest interactions can effectively regulate the electrochemical properties of the complex. Utilizing bisammonium salt as the cross-linker and multifunctional trinuclear AuI tricarbene complex as the core, a stimuli-responsive and self-healing supramolecular gel with different functional units was obtained.

Precise Modulation of Intramolecular Aggregation-induced Electrochemiluminescence by Tetraphenylethylene-based Supramolecular Architectures

The precisely modulated synthesis of programmable light-emitting materials remains a challenge. To address this challenge, we construct four tetraphenylethylene-based supramolecular architectures (SA, SB, SC, and SD), revealing that they exhibit higher electrochemiluminescence (ECL) intensities and efficiencies than the tetraphenylethylene monomer and can be classified as highly efficient and precisely modulated intramolecular aggregation-induced electrochemiluminescence (PI-AIECL) systems. The best-performing system (SD) shows a high ECL cathodic efficiency exceeding that of the benchmark tris(2,2'-bipyridyl)ruthenium(II) chloride in aqueous solution by nearly six-fold. The electrochemical characterization of these architectures in an organic solvent provides deeper mechanistic insights, revealing that SD features the lowest electrochemical band gap. Density functional theory calculations indicate that the band gap of the guest ligand in the SD structure is the smallest and most closely matched to that of the host scaffold. Finally, the SD system is used to realize ECL-based cysteine detection (detection limit=14.4 nM) in real samples. Thus, this study not only provides a precisely modulated supramolecular strategy allowing chromophores to be controllably regulated on a molecular scale, but also inspires the programmable synthesis of high-performance aggregation-induced electrochemiluminescence emitters.

Tunable Fluorescence from 2D Assemblies of LnW10 and P2 W18 Polyoxometalates Clusters with Quaternary Ammonium-type AIEgens

Two-dimensional (2D) AIE-inorganic nanocomposites exhibit high stability and high fluorescence activity. However, limited by the types and properties of 2D inorganic materials, it's challenging to realize complex structures and functions. We designed a quaternary ammonium-type AIE ligand, TPECholine. We chose TPECholine and POM clusters as building blocks and synthesized a series of single-layer nanosheets (SLNSs) by assembling LnW10 or P2 W18 POM clusters with TPECholine. By regulating the types of POM clusters and ligands, the morphology and fluorescence intensity of the SLNSs can be finely tuned. Due to the restriction of the intramolecular motions of AIEgens by the SLNSs, nanosheets can exhibit promoted quantum yield (up to 76 %). In addition, thanks to the sub-nanometric sizes and excess surface charges, SLNSs exhibit excellent solvent compatibility, including water, chloroform, ethanol, etc. And the nanosheets showed high fluorescence intensity in these solvents.

Construction of Covalent Organic Cages with Aggregation-Induced Emission Characteristics from Metallacages for Mimicking Light-Harvesting Antenna

A series of covalent organic cages built from fluorophores capable of aggregation-induced emission (AIE) were elegantly prepared through the reduction of preorganized M2 (LA )3 (LB )2 -type metallacages, simultaneously taking advantage of the synthetic accessibility and well-defined shapes and sizes of metallacages, the good chemical stability of the covalent cages as well as the bright emission of AIE fluorophores. Moreover, the covalent cages could be further post-synthetically modified into an amide-functionalized cage with a higher quantum yield. Furthermore, these presented covalent cages proved to be good energy donors and were used to construct light-harvesting systems employing Nile Red as an energy acceptor. These light-harvesting systems displayed efficient energy transfer and relatively high antenna effect, which enabled their use as efficient photocatalysts for a dehalogenation reaction. This research provides a new avenue for the development of luminescent covalent cages for light-harvesting and photocatalysis.

Nickelacarborane-Supported Bis-N-heterocyclic Carbenes

Metallacarboranes have attracted significant attention due to their unique properties. Considerable efforts have been made on the reactions around the metal centers or the metal ion itself, while transformations of functional groups of the metallacarboranes have been much less explored. We presented here the formation of imidazolium-functionalized nickelacarboranes (2), their subsequent conversion to nickelacarborane-supported N-heterocyclic carbenoids (NHCs, 3), and the reactivities of 3 toward Au(PPh₃)Cl and Se powder, which resulted in the formation of bis-gold carbene complexes (4) and NHC selenium adducts (5). Cyclic voltammetry of 4 shows two reversible peaks, corresponding to the interconversion transformations Ni^II ↔ Ni^III and Ni^III ↔ Ni^IV. Theoretical calculations demonstrated relatively high-lying lone-pair orbitals, weak B-H···H-C interactions between the BH units and the methyl group, and weak B-H···π interactions between the BH groups and the vacant p-orbital of the carbene.

[2,2] Paracyclophanes-based double helicates for constructing artificial light-harvesting systems and white LED device

The construction of efficient artificial light-harvesting systems (ALHSs) is of vital importance in utilizing solar energy. Herein, we report the non-covalent syntheses of double helicates PCP-TPy1/2 and Rp,Rp-PCP-TPy1/2 by metal-coordination interaction and their applications in ALHSs and white light-emitting diode (LED) device. All double helicates exhibit significant aggregation-induced emission in tetrahydrofuran/water (1:9, v/v) solvent. The aggregated double helicates can be used to construct one-step or sequential ALHSs with fluorescent dyes Eosin Y (EsY) and Nile red (NiR) with the energy transfer efficiency up to 89.3%. Impressively, the PMMA film of PCP-TPy1 shows white-light emission when doped 0.075% NiR, the solid of double helicates (Rp,Rp-) PCP-TPy2 can be used as the additive of a blue LED bulb to achieve white-light emission. In this work, we provided a general method for the preparation of novel double helicates and explored their applications in ALHSs and fluorescent materials, which will promote future construction and application of helicates as emissive devices.

Metal-N-Heterocyclic Carbene Chemistry Directed toward Metallosupramolecular Synthesis and Beyond

ConspectusAs versatile, modular, and strongly coordinating moieties in organometallic compounds, N-heterocyclic carbenes (NHCs) have led to numerous breakthroughs in transition-metal catalysis, main group chemistry, and organocatalysis. In contrast, the chemistry of NHC-based metallosupramolecular assemblies, in which discrete individual components are held together via metal (M)-C_NHC bonds, has been underdeveloped. Integrating NHCs into supramolecular assemblies would endow them with some unforeseen functions. However, one of the most critical challenges is seeking an appropriate combination of the rigid C_NHC-M-C_NHC units with the resulting topologies and applications. Toward this goal, for the last decade we have focused on the development of M-NHC directed toward metallosupramolecular synthesis. This Account aims to summarize our contributions to the application of M-NHC chemistry toward supramolecular synthesis from structural design to postassembly modification (PAM) and their functional applications since integrating NHCs into supramolecular assemblies has garnered much attention among organometallic, photochemical, and supramolecular researchers. While presenting representative examples of NHC-based architectures, we try to illustrate the purposes and concepts behind the systems developed to aid the rational approach to the design and fabrication of complex assemblies and M-NHC-templated photochemical reactions.We present synthetic approaches for new architectures by the rational design of starting NHC precursors, including the poly-NHC-based mechanically interlocked metallacages and the heteroleptic architectures based on electronic complementary and self-sorting mechanisms. The structural regulation of poly-NHC-based architectures with increasing topological complexity is elaborated on by selective combinations of tetraphenylethylene (TPE) units, NHC backbones, and N-wingtip substituents in a controllable manner.Subsequently, we move to elucidating an M-NHC-templated PAM approach that leads to functional organic cages featuring polyimidazolium/triazolium groups of different shapes and sizes that are difficult to access using alternative organic approaches. These organic cages possess well-defined cavities, and their in situ-generated NHC sites are ideal platforms for stabilizing metal nanoparticles (MNPs) within their cavities for improved catalytic performance.Finally, we demonstrate how to design supramolecular M-NHC templates to synthesize cyclobutane derivatives in homogeneous solutions in a catalytic fashion. Selected examples of M-NHC template-dependent structural transformations and photoreactions are discussed. Their applications in molecular recognition, aggregation-induced emission (AIE), cell imaging, anticancer activity, radical chemistry, and stimuli-responsive materials are also described.Taken together, M-NHC-templated approaches have proven to be powerful methods for constructing diverse architectures with functional applications. The development of this methodology is still in its infancy, with tremendous growth potential and a promising future. We believe that this Account will guide researchers to design fascinating and valuable M-carbene species for diverse applications.

Organometallic Borromean Rings and [2]Catenanes Featuring Di-NHC Ligands

We report herein a series of organometallic Borromean rings (BRs) and [2]catenanes prepared from benzobiscarbene ligands. The reaction of dinickel complexes of the benzobiscarbenes 1 a-1 c with a thiazolothiazole bridged bipyridyl ligand L² led by self-assembly to a series of organometallic BRs. Solvophobic effects played a crucial role in the formation and stability of the interlocked species. The stability of BRs is related to the N-alkyl substituents at the precursors 1 a-1 c, where longer alkyl substitutes improve stability and inter-ring interactions. Solvophobic effects are also important for the stability of [2]catenanes prepared from 1 a-1 c and a flexible bipyridyl ligand L³ . In solution, an equilibrium between the [2]catenanes and their macrocyclic building blocks was observed. High proportions of [2]catenanes were obtained in concentrated solutions or polar solvents. The proportion of [2]catenanes in solution could be further enhanced by lengthening of the N-alkyl substitutes.

Synthesis of triarylborane-centered N-heterocyclic carbene cages with tunable photophysical properties

Triarylborane-based discrete metal-carbene supramolecular cages [M₃(1)₂](PF₆)₃ (M = Ag, Au) were synthesized and characterized. The new hexacarbene assemblies show a significant solvatochromic effect in solvents of different polarity. Furthermore, the reversible fluoride binding property of [Au₃(1)₂](PF₆)₃ was investigated by UV-vis absorption and fluorescence titrations. This work holds promise for future developments in the area of highly emissive and stimulus-responsive NHC-metal assemblies.

Solvato-Controlled Assembly and Structural Transformation of Emissive Poly-NHC-Based Organometallic Cages and Their Applications in Amino Acid Sensing and Fluorescence Imaging

Stimuli-induced structural transformation of supramolecular cages has drawn increasing attention because of their sensitive feature to external variations as model systems to simulate biological processes. However, combining structural transformation and useful functions has remained a difficult task. This study reports the solvato-controlled self-assembly of two unique topologies with different emission characteristics, a water-soluble Ag₈ L₄ cage (A) and an Ag₄ L₂ cage (B), produced from the same sulfonate-pendant tetraphenylethene (TPE) bridged tetrakis-(1,2,4-triazolium) ligand. Both cages show interesting solvent-responsive reversible structural transformation, and the change of fluorescence signals can efficiently track the process. Additionally, water-soluble cage A exhibits unique properties in thermochromism, thiol amino acid sensing, and subcellular imaging in aqueous media.

Platinum(II)-Metallaclip-Based Theranostics for Cell Imaging and Synergetic Chemotherapy-Photodynamic Therapy

Supramolecular coordination complexes formed by coordination-induced assembly not only avoid the loss of activity of precursors but also provide an efficient way for controlled release, which can be further used in various fields of biology such as drug delivery, cell imaging, and tumor treatment. In this work, a Pt^II metallaclip (4) was prepared from 4-[4-(1,2,2-triphenylvinyl)phenyl]pyridine (1), 5,10,15-triphenyl-20-(pyridin-4-yl)porphyrin (2), 90^o Pt, and glycol-chain-modified isophthalic acid (3) in an acetone/water mixture through the "coordination-driven self-assembly" method. ³¹P and ¹H NMR spectroscopy and high-resolution mass spectrometry were used to characterize the obtained metallaclip 4. 4 can self-assemble into fluorescent nanostructures in aqueous solution because of the tetraphenylethylene unit and its amphiphilic nature. Importantly, the fluorescent nanoparticles not only can be employed for cell imaging but also can generate singlet oxygen (¹O₂) under 660 nm laser irradiation and the release of Pt drug in the tumor issue for cancer therapy. The work may provide a new way for scientists to construct functional biomaterials with multiple applications via molecular self-assembly.

Hexaphenyltriphenylene-Based Multicomponent Metallacages: Host-Guest Complexation for White-Light Emission

A hexaphenyltriphenylene-based hexatopic pyridyl ligand is designed and used to prepare three hexagonal prismatic metallacages via metal-coordination-driven self-assembly. Owing to the planar conjugated structures of the hexaphenyltriphenylene skeleton, such metallacages show good host-guest complexation with a series of emissive dyes, which have been further used to tune their emission in solution. Interestingly, based on their complementary emission colors, white light emission is achieved in a mixture of the host metallacages and the guests.

Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste

The role of N-heterocyclic carbene, a well-known reactive site, in chemical catalysis has long been studied. However, its unique binding and electron-donating properties have barely been explored in other research areas, such as metal capture. Herein, we report the design and preparation of a poly(ionic liquid)-derived porous organic polycarbene adsorbent with superior gold-capturing capability. With carbene sites in the porous network as the "nanotrap", it exhibits an ultrahigh gold recovery capacity of 2.09 g/g. In-depth exploration of a complex metal ion environment in an electronic waste-extraction solution indicates that the polycarbene adsorbent possesses a significant gold recovery efficiency of 99.8%. X-ray photoelectron spectroscopy along with nuclear magnetic resonance spectroscopy reveals that the high performance of the polycarbene adsorbent results from the formation of robust metal-carbene bonds plus the ability to reduce nearby gold ions into nanoparticles. Density functional theory calculations indicate that energetically favourable multinuclear Au binding enhances adsorption as clusters. Life cycle assessment and cost analysis indicate that the synthesis of polycarbene adsorbents has potential for application in industrial-scale productions. These results reveal the potential to apply carbene chemistry to materials science and highlight porous organic polycarbene as a promising new material for precious metal recovery.

Synthesis of Carborane-Backbone Metallacycles for Highly Selective Capture of n-Pentane

The specific recognition and separation of alkanes with similar molecular structures and close boiling points face significant scientific challenges and industrial demands. Here, rectangular carborane-based metallacycles were designed to selectively encapsulate n-pentane from n-pentane, iso-pentane, and cyclo-pentane mixtures in a simple-to-operate and more energy-efficient way. Metallacycle 1, bearing 1,2-di(4-pyridyl) ethylene, can selectively separate n-pentane from these three-component mixtures with a purity of 97%. The selectivity is ascribed to the capture of the preferred guest with matching size, C-H···π interactions, and potential B-H^δ-···H^δ+-C interactions. Besides, the removal of n-pentane gives rise to original guest-free carborane-based metallacycles, which can be recycled without losing performance. Considering the variety of substituted carborane derivatives, metal ions, and organic linkers, these new carborane-based supramolecular coordination complexes (SCCs) may be broadly applicable to other challenging recognition and separation systems with good performance.

Synthesis of N-Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2-Chloroazolium Salts Using Electron-Rich Phosphines

N‐Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2‐chloroazolium salts using electron‐rich phosphines. PPh₃, PCy₃, and PtBu₃ are unsuitable for Cl⁺ abstraction, while the sterically encumbered tris(1,3‐tert‐butylimidazolidin‐2‐ylidenamino)phosphine 1 selectively removes Cl⁺ from 2‐chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via in situ Cl⁺ abstraction from 2‐chloroazolium salts. The dechlorination was employed for the site‐selective monometallation with Ir^I, Ir^III, Rh^I, Rh^III, and Ru^II of a bis‐NHC precursor composed of a 2‐chlorobenzimidazolium and a 2‐chlorobenzimidazole group, followed by the preparation of the heterobimetallic Ir^III/Pd^II complex [18](BF₄)₂ by a dechlorination/oxidative addition reaction sequence.

Bi- and trimetallic complexes with macrocyclic xanthene-4,5-diNHC ligands

Three different types of bimetallic NHC-metal complexes were synthesized, whose NHC units are attached at the 4,5-positions of xanthene. The NHC units are in close proximity and are designed such that each carbene coordinates one ML unit, while the chelation of one metal by two NHC is not possible. Several xanthene-((NHC)ML)₂ complexes with ML = RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, AuCl, AgCl, CuCl and Pd(allyl)Cl were synthesized and investigated.

Silver(I)-Carbene Bond-Directed Rigidification-Induced Emissive Metallacage for Picric Acid Detection

A new triphenylamine-based tetraimidazolium salt L was developed for silver(I)-carbene bond-directed synthesis of tetranuclear silver(I) octacarbene ([Ag₄(L)₂](PF₆)₄) metallacage 1. Interestingly, after assembly formation, metallacage 1 showed a nine-fold emission enhancement in dilute solution while ligand L was weakly fluorescent. This is attributed to the rigidity induced to the system by metal-carbene bond formation where the metal center acts as a rigidification unit. The enhanced emission intensity in dilute solution and the presence of the triphenylamine core made 1 a potential candidate for recognition of picric acid (PA). This recognition can be ascribed to the dual effect of ground-state charge-transfer complex formation and resonance energy transfer between the picrate and metallacage 1. For metallacage 1, a considerable detection limit toward PA was observed. The use of such metal-carbene bond-directed rigidification-induced enhanced emission for PA sensing is noteworthy.

Stimuli-Responsive and Structure-Adaptive Three-Dimensional Gold(I) Cluster Cages Constructed via "De-aurophilic" Interaction Strategy

Self-assembly of three-dimensional (3D) metallosupramolecular cages has drawn increasing attention for their potential to interconvert between different architectures due to the dynamic and reversible features of the coordination bond. These supramolecular transformations can provide unique approaches for the construction of stimuli-responsive supramolecular model systems to mimic biological transformation processes. While gold(I) clusters have attracted much interest due to their propensity to exhibit aurophilic interactions, the construction of 3D gold(I) cluster cages has remained a challenging and daunting task. Here, we proposed a "de-aurophilic" interaction strategy, which involves the prevention of aurophilic interaction formation between the basic [(μ₃-S)Au₃]⁺ units, to construct 3D gold(I) cluster cages. Through the judicious design of diphosphine ligands, an unprecedented class of gold(I) cluster cages with adaptive structures has been constructed. These gold(I) cluster cages are found to show intriguing stimuli-responsive structure transformation and interconversion. This work not only provides a strategy for the design and construction of novel 3D supramolecular cages based on cluster nodes but also offers a paradigm to study the stimuli-responsive structural interconversion between the unique structures of these gold(I) cluster cages.

Light-emitting self-assembled metallacages

Coordination-driven self-assembly of metallacages has garnered significant interest because of their 3D layout and cavity-cored nature. The well-defined, highly tunable metallacage structures render them particularly attractive for investigating the properties of luminophores, as well as for inducing novel photophysical characters that enable widespread applications. In this review, we summarize the recent advances in synthetic methodologies for light-emitting metallacages, and highlight some representative applications of these metallacages. In particular, we focus on the favorable photophysical properties—including high luminescence efficiency in various physical states, good modularity in photophysical properties and stimulus responsiveness—that have resulted from incorporating ligands displaying aggregation-induced emission (AIE) into metallacages. These features show that the synergy between carrying out coordination-driven self-assembly and using luminophores with novel photophysical characteristics like AIE could stimulate the development of supramolecular luminophores for applications in fields as diverse as sensing, biomedicine and catalysis.

Photoinduced E to Z isomerization of tetraphenylethylene derivatives within organometallic supramolecular assemblies

Isolation of E-1,2-bis(4-bromophenyl)-1,2-diphenyl-ethylene from the E/Z isomer mixture obtained by a McMurry coupling reaction and reaction of this isomer with imidazole followed by N-alkylation with nBuBr and anion exchange yielded the bisimidazolium tetraphenylethylene (TPE) derivative H2-E-1(PF6)2. The reaction of H2-E-1(PF6)2 with Ag2O yielded the di-nuclear metallarectangle [Ag2(E-1)2](PF6)2 where the two bis-NHC donors E-1 bridge two silver atoms. Irradiation of [Ag2(E-1)2](PF6)2 leads to E/Z isomerization of the di-NHC ligand and formation of Z-1 in the mononuclear complex [Ag(Z-1)]PF6. Demetallation of the di-NHC ligand with NH4Cl/NH4PF6 yielded bisimidazolium salt H2-Z-1(PF6)2. The unique isomerization of the E-TPE derivative into its Z-isomer via metal complex formation/irradiation/demetallation cannot be achieved by irradiation of the individual imidazolium salt. The emissive properties of the TPE complexes [Ag2(E-1)2](PF6)2 and [Ag(Z-1)]PF6 have been investigated.

Tetraphenylethylene-Based Multicomponent Emissive Metallacages as Solid-State Fluorescent Materials

The construction of solid-state fluorescent materials with high quantum yield and good processability is of vital importance in the preparation of organic light-emitting devices. Herein, a series of tetraphenylethylene (TPE)-based multicomponent emissive metallacages are prepared by the coordination-driven self-assembly of tetra-(4-pyridylphenyl)ethylene, cis-Pt(PEt₃ )₂ (OTf)₂ and tetracarboxylic ligands. These metallacages exhibit good emission both in solution and in the solid state because the coordination bonds and aggregation restrict the molecular motions of TPE synergistically, which suppresses the non-radiative decay of these metallacages. Impressively, one of the metallacages achieves very high fluorescence quantum yield (Φ_F =88.46 %) in the solid state, which is further used as the coatings of a blue LED bulb to achieve white-light emission. The study not only provides a general method to the preparation of TPE-based metallacages but also explores their applications as solid-state fluorescent materials, which will promote the future design and applications of metallacages as useful emissive devices.

Supramolecular Coordination Cages Based on N-Heterocyclic Carbene-Gold(I) Ligands and Their Precursors: Self-Assembly, Structural Transformation and Guest-Binding Properties

The incorporation of functional groups into the cavity of discrete supramolecular coordination cages (SCCs) will bring unique functions and applications. Here, three dicarboxylate ligands (H₂ L1Cl, H₂ L2Cl and H₂ L3Cl) containing N-heterocyclic carbene (NHC) precursors as linkers were introduced to construct SCCs by combining with two C₃ -symmertic (CpZr)₃ (μ₃ -O)(μ₂ -OH)₃ clusters as three-connect vertices, resulted in a series of rugby-like V₂ E₃ (V=vertex, E=edge) type homoleptic cages (SCC-1, SCC-2 and SCC-3). However, V₄ E₆ -type tetrahedral cages (SCC-4 and SCC-5), incorporating six Au-NHC moieties, were obtained when the corresponding NHC-gold(I) functionalized ligands (H₂ L1^Au , H₂ L2^Au ) were applied. For the first time, we present a trackable CpZr-involved cage to cage conversion to generate a heteroleptic V₂ E₃ cage (SCC-6) from two homoleptic cages (SCC-2 and SCC-5) with different geometries of V₂ E₃ and V₄ E₆ . The heteroleptic assembly SCC-6 can also be formed upon a subcomponent displacement strategy. The structural transformation and reassembly processes were detected and monitored by ¹ H NMR spectroscopy and electrospray-ionization mass spectrometry. The formation of heteroleptic assembly was further supported by single crystal X-ray diffraction analysis. Moreover, homoleptic cage SCC-2 possesses a trigonal bipyramidal cationic cavity allowing the encapsulation of a series of sulfonate anionic guests.

Self-assembled metallasupramolecular cages towards light harvesting systems for oxidative cyclization

Designing artificial light harvesting systems with the ability to utilize the output energy for fruitful application in aqueous medium is an intriguing topic for the development of clean and sustainable energy. We report here facile synthesis of three prismatic molecular cages as imminent supramolecular optoelectronic materials via two-component coordination-driven self-assembly of a new tetra-imidazole donor (L) in combination with 180°/120° di-platinum(ii) acceptors. Self-assembly of 180° trans-Pt(ii) acceptors A1 and A2 with L leads to the formation of cages Pt4 L 2(1a) and Pt8 L 2(2a) respectively, while 120°-Pt(ii) acceptor A3 with L gives the Pt8 L 2(3a) metallacage. PF6 - analogues (1b, 2b and 3b) of the metallacages possess a high molar extinction coefficient and large Stokes shift. 1b-3b are weakly emissive in dilute solution but showed aggregation induced emission (AIE) in a water/MeCN mixture as well as in the solid state. AIE active 2b and 3b in aqueous (90% water/MeCN mixture) medium act as donors for fabricating artificial light harvesting systems via Förster resonance energy transfer (FRET) with organic dye rhodamine-B (RhB) with high energy efficiency and good antenna effect. The metallacages 2b and 3b represent an interesting platform to fabricate new generation supramolecular aqueous light harvesting systems with high antenna effect. Finally, the harvested energy of the LHSs (2b + RhB) and (3b + RhB) was utilized successfully for efficient visible light induced photo-oxidative cross coupling cyclization of N,N-dimethylaniline (4) with a series of N-alkyl/aryl maleimides (5) in aqueous acetonitrile with dramatic enhancement in yields compared to the reactions with RhB or cages alone.

Russian-Doll-Like Molecular Cubes

Nanosized cage-within-cage compounds represent a synergistic molecular self-assembling form of three-dimensional architecture that has received particular research focus. Building multilayered ultralarge cages to simulate complicated virus capsids is believed to be a tough synthetic challenge. Here, we synthesize two large double-shell supramolecular cages by facile self-assembly of presynthesized metal-organic hexatopic terpyridine ligands with metal ions. Differing from the mixture of prisms formed from the inner tritopic ligand, the redesigned metal-organic hexatopic ligands bearing high geometric constraints that led to the exclusive formation of discrete double-shell structures. These two unique nested cages are composed of inner cubes (5.1 nm) and outer huge truncated cubes (12.0 and 13.2 nm) with six large bowl-shape subcages distributed on six faces. The results with molecular weights of 75 232 and 77 667 Da were among the largest synthetic cage-in-cage supramolecules reported to date. The composition, size and shape were unambiguously characterized by a combination of ¹H NMR, DOSY, ESI-MS, TWIM-MS, TEM, AFM, and SAXS. This work provides an interesting model for functional recognition, delivery, and detection of various guest molecules in the field of supramolecular materials.

The transformations of a methylene-bridged bis-triazolium salt: a mesoionic carbene based metallocage and analogues of TCNE and NacNac

Unusual and unexpected chemical transformations often provide access to completely new types of functional molecules. We report here the synthesis of a methylene-bridged bis-triazolium salt designed as a precursor for a new bis-mesoionic carbene (MIC) ligand. The direct metalation with silver oxide led to the isolation and crystallographic characterization of a cationic tetranuclear octacarbene-silver(i) complex. During metalation the formal bis-MIC precursor undergoes significant structural changes and chemical transformations. A combined synthetic, crystallographic and (spectro-)electrochemical approach is used to elucidate the mechanistic pathway: starting from the methylene-bridged bis-triazolium salt a single deprotonation leads to a NacNac analogue, which is followed by a redox-induced radical dimerization reaction, generating a new tetra-MIC ligand coordinated to silver(i) central atoms. Decomplexation led to the isolation of the corresponding tetratriazoliumethylene, a profoundly electron-poor alkene, which is an analogue of TCNE.

Biotinylated platinum(ii) metallacage towards targeted cancer theranostics

A biotinylated metallacage was prepared via metal-coordination-driven self-assembly, combining fluorescence, anti-cancer and targeted properties into a single entity. The metallacage was successfully employed for targeted cell imaging and therapy, offering a platform for cancer theranostics.

Coordination-Induced Emission from Tetraphenylethylene Units and Their Applications

Thanks to the potential of aggregation-induced emission (AIE) phenomena, improved stabilities, and the good selectivity and sensitivity of the chemical responses exhibited by the products, coordination-driven self-assembly with tetraphenylethylene (TPE) units has recently received much attention and has been widely investigated for application in chemical sensors, cell imaging agents, light-harvesting systems, and others. Several reviews have emerged on the topics of AIE chemistry and aggregation-induced emission luminogen (AIEgen)-based supramolecular assembles, however, there is still a distinct lack of full overviews of emission enhancement from the viewpoint of metal-coordination effects. Thus, this minireview offers recent advances that have been made in the design and application of TPE-based metallacycles, metallacages, metal-organic frameworks (MOFs) and coordination polymers (CPs).

Self-Assembly of Benzimidazole-Derived Tris-NHC Ligands and AgI -Ions to Hexanuclear Organometallic Cages and Their Unusual Transmetalation Chemistry

Multi-ligand self-assembly to attain the Ag^I -N-heterocyclic carbene (NHC)-built hexanuclear organometallic cages of composition [Ag₆ (3 a,b)₄ ](PF₆ )₆ from the reaction of benzimidazole-derived tris(azolium) salts [H₃ -3 a,b](PF₆ )₃ with Ag₂ O was achieved. The molecular structures of the cages were established by X-ray diffraction studies along with NMR and MS analyses. The existence of a single assembly in solution was supported by diffusion-ordered spectroscopy (DOSY) ¹ H NMR spectra. Further, transmetalation reactions of these self-assembled complexes, [Ag₆ (3 a,b)₄ ](PF₆ )₆ , with Cu^I /Au^I -ions provided various coinage metal-NHC complexes having diverse molecular compositions, which included the first example of a hexanuclear Cu^I -dodecacarbene complex, [Cu₆ (3 b)₄ ](PF₆ )₆ .