Molecular Co-Catalyst Confined within a Metallacage for Enhanced Photocatalytic CO2 Reduction.

The construction of structurally well-defined supramolecular hosts to accommodate catalytically active species within a cavity is a promising way to address catalyst deactivation. The resulting supramolecular catalysts can significantly improve the utilization of catalytic sites, thereby achieving a highly efficient chemical conversion. In this study, the Co-metalated phthalocyanine (Pc-Co) was successfully confined within a tetragonal prismatic metallacage, leading to the formation of a distinctive type of supramolecular photocatalyst (Pc-Co@Cage). The host-guest architecture of Pc-Co@Cage was unambiguously elucidated by single-crystal X-ray diffraction (SCXRD), NMR, and ESI-TOF-MS, revealing that the single cobalt active site can be thoroughly isolated within the space-restricted microenvironment. In addition, we found that Pc-Co@Cage can serve as a homogeneous supramolecular photocatalyst that displays high CO2 to CO conversion in aqueous media under visible light irradiation. This supramolecular photocatalyst exhibits an obvious improvement in activity (TONCO = 4175) and selectivity (SelCO = 92%) relative to the nonconfined Pc-Co catalyst (TONCO = 500, SelCO = 54%). The present strategy provided a rare example for the construction of a highly active, selective, and stable photocatalyst for CO2 reduction through a cavity-confined molecular catalyst within a discrete metallacage.

Unravelling the Roles of Solvophobic Effects and π⋠⋠⋠π Stacking Interactions in the Formation of [2]Catenanes Featuring Di-(N-Heterocyclic Carbene) Building Blocks.

A series of [2]catenanes has been prepared from di-NHC building blocks by utilizing solvophobic effects and/or π⋅⋅⋅π stacking interactions. The dinickel naphthobiscarbene complex syn-[1] and the kinked biphenyl-bridged bipyridyl ligand L2 yield the [2]catenane [2-IL](OTf)4 by self-assembly. Solvophobic effects are pivotal for the formation of the interlocked species. Substitution of the biphenyl-linker in L2 for a pyromellitic diimide group gave ligand L3 , which yielded in combination with syn-[1] the [2]catenane [3-IL](OTf)4 . This assembly exhibits enhanced stability in diluted solution, aided by additional π⋅⋅⋅π stacking interactions. The π⋅⋅⋅π stacking was augmented by the introduction of a pyrene bridge between two NHC donors in ligand L4 . Di-NHC precursor H2 -L4 (PF6 )2 reacts with Ag2 O to give the [Ag2 L4 2 ]2 [2]catenane [4-IL](PF6 )4 , which shows strong π⋅⋅⋅π stacking interactions between the pyrene groups. This assembly was readily converted into the [Au2 L4 2 ]2 gold species [5-IL](PF6 )4 , which exhibits exceptional stability based on the strong π⋅⋅⋅π stacking interactions and the enhanced stability of the Au-CNHC bonds.

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.

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)-CNHC 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 CNHC-M-CNHC 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.

Construction and Hierarchical Self-Assembly of Multifunctional Coordination Cages with Triangular Metal-Metal-Bonded Units.

Herein, a series of face-capped (Tr2M3)4L4 (Tr = cycloheptatrienyl cationic ring; M = metal; L = organosulfur ligand) tetrahedral cages 1-3 functionalized with 12 appended crown ether moieties were designed and synthesized. The reversible binding of ammonium cations with peripheral crown ether moieties to adjust internal guest-binding was realized. Combination of a bisammonium linker and cage 3 led to the formation of a supramolecular gel SPN1 via host-guest interactions between the crown ether moieties and ammonium salts. The obtained supramolecular gel exhibited multiple-stimuli responsiveness, injectability, and excellent self-healing properties and could be further developed to a SPN1-based drug delivery system. In addition, the storage modulus of SPN1 was 20 times higher than that of the model gel without Pd-Pd bonded blocks, and SPN1 had better self-healing properties compared with the latter, demonstrating the importance of such cages in improving mechanical strength without losing the dynamic properties of the material. The cytotoxicity in vitro of the drug-loaded (doxorubicin or methotrexate) SPN1 was significantly improved compared to that of free drugs.

Highly Efficient Circularly Polarized Luminescence from Chiral Au13 Clusters Stabilized by Enantiopure Monodentate NHC Ligands.

Chiral Au nanoclusters have promising application prospects in chiral sensing, asymmetric catalysis, and chiroptics. However, enantiopure superatomic homogold clusters with crystallographic structures emitting bright circularly polarized luminescence (CPL) remain challenging. In this study, we designed chiral N-heterocyclic carbenes (NHCs), and for the first time enantioselectively synthesized a pair of monovalent cationic superatomic Au13 clusters. This new enantiomeric pair of clusters has a quasi-C2 symmetric core and exhibited CPL with an unprecedent solution-state quantum yield (QY) of 61 % among those of the atomically precise Au nanoclusters. DFT calculations provided insights into the circular dichroism behavior, and revealed the origin of CPL from superatomic Au clusters. This work opens a new avenue for developing novel homochiral nanoclusters using chiral NHC ligands and provides fundamental understanding of the origin of the chiroptics of metal clusters.

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.

Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids.

Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λmax : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λmax : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.

Formation of Metallosupramolecular Helicates and Mesocates from Poly-N-Heterocyclic Carbene Ligands.

In this work, a series of poly-NHC-based tetranuclear silver helicates and mesocates were synthesized from the silver-mediated self-assembly of the ligands involving multiple tridentate CNC-type pincer units and NHC coordination sites. The silver helicate was found to be transferred to a gold mesocate upon metal exchange reaction. The metallosupramolecular helicates and mesocates have been fully characterized by single-crystal X-ray crystallography, mass spectrometry, and multinuclear nuclear magnetic resonance spectroscopies. This study provides an example of the selective preparation of poly-NHC-based helicates or mesocates depending on the size of metal ions and the steric effect of ligands.

A platform for blue-luminescent carbon-centered radicals.

Organic radicals, which have unique doublet spin-configuration, provide an alternative method to overcome the efficiency limitation of organic light-emitting diodes (OLEDs) based on conventional fluorescent organic molecules. Further, they have made great breakthroughs in deep-red and near-infrared OLEDs. However, it is difficult to extend their fluorescence into a short-wavelength region because of the natural narrow bandgap of the organic radicals. Herein, we significantly expand the scope of luminescent radicals by showing a new platform of carbon-centered radicals derived from N-heterocyclic carbenes that produce blue to green emissions (444-529 nm). Time-dependent density functional theory calculations and experimental investigations disclose that the fluorescence originates from the high-energy excited states to the ground state, demonstrating an anti-Kasha behavior. The present work provides an efficient and modular approach toward a library of carbon-centered radicals that feature anti-Kasha's rule emission, rendering them as potential new emitters in the short-wavelength region.

Water-Soluble Self-Assembled Cage with Triangular Metal-Metal-Bonded Units Enabling the Sequential Selective Separation of Alkanes and Isomeric Molecules.

The selective separation of structurally similar aliphatic/aromatic hydrocarbons is an essential goal in industrial processes. In this study, we report the synthesis of a water-soluble (Tr2M3)4L4 (Tr = cycloheptatrienyl ring; M = metal; L = organosulfur ligand) molecular cage (1) via self-assembly of the water-soluble acceptor tripalladium sandwich species [(Tr2Pd3)(CH3CN)][NO3]2 and the attachment onto L of solubilizing methoxyethoxy appendants to be utilized in an energy-friendly alternative approach to the separation of structurally similar molecules under ambient conditions. Cage 1, comprising a hydrophobic inner cavity, exhibited good solubility and stability in aqueous media. It also demonstrated excellent performance in the sequential separation of alkanes (C6-C9), xylene, and other disubstituted benzene isomers and cis/trans-decalin.

A Molecular "A-Type" Tangled Metallocube.

Tangled cubes feature the topology of typical Platonic cubes, with their "faces" traversed by edges in different ways. This study generates an "A-type" tangled metallocube from the reaction of binuclear gold-NHC complex and H2 S. The tangled cube topology was validated by multinuclear nuclear magnetic resonance (NMR) spectroscopy, high-resolution electrospray-ionization (HR-ESI) mass spectrometry, and single-crystal X-ray diffraction analysis. This study offers a simple and effective approach to designing and fabricating new, topologically unique molecular structures.

N-heterocyclic carbene-stabilized metal nanoparticles within porous organic cages for catalytic application.

Tuning the surface-embellishing ligands of metal nanoparticles (NPs) is a powerful strategy to modulate their morphology and surface electronic and functional features, impacting their catalytic activity and selectivity. In this work, we report the design and synthesis of a polytriazolium organic cage PIC-T, capable of stabilizing PdNPs within its discrete cavity. The obtained material (denoted Pd@PCC-T) is highly durable and monodispersed with narrow particle-size distribution of 2.06 ± 0.02 nm, exhibiting excellent catalytic performance and recyclability in the Sonogashira coupling and tandem reaction to synthesize benzofuran derivatives. Further investigation indicates that the modulation of N-heterocyclic carbene sites embedded in the organic cage has an impact on NPs' catalytic efficiency, thus providing a novel methodology to design superior NP catalysts.

Supramolecular-controlled regioselective photochemical [4 + 4] cycloaddition within Cp*Rh-based metallarectangles.

Photochemical reactions are vital synthetic means for the synthesis of natural products and highly strained molecules. However, it remains an immense challenge to control the chemo- and regioselectivity in the photoreactions of anthracene derivatives while maintaining high reactivity. Herein, we report the synthesis of two photoactive metallarectangles 1a and 1b by coordination-driven self-assembly of 2,6- and 2,7-bifunctionalized anthracenes with a half-sandwich rhodium template. By taking advantage of the rhodium template, the anthracene groups within the metallarectangles can be preorganized in a parallel fashion and exclusively undergo a photochemical [4 + 4] cycloaddition. As a result, the syn-photodimers were obtained in quantitative yields under irradiation at 365 nm. The photocycloaddition of metallarectangles 1a and 1b was found to be reversible via photo- and thermal cleavage reactions, even after repeating three times. Furthermore, pure organic photodimers 3a and 3b, which are difficult to synthesize by conventional organic methods, can be readily dissociated from the metalloassemblies in high yields.

N-Heterocyclic carbenes and their precursors in functionalised porous materials.

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.

Air-/Heat-Stable Crystalline Carbon-Centered Radicals Derived from an Annelated N-Heterocyclic Carbene.

Organic radicals are open-shell species and have been extensively applied to functional materials due to their unique physicochemical properties with unpaired electrons; however, most of them are highly reactive and short-lived. Herein, a series of stable radicals were readily accessed in two steps from a bis(imino)acenaphthene-supported N-heterocyclic carbene (IPr(BIAN)) through enhancing the delocalization of spin density. The IPr(BIAN)-based radicals 3a-c, obtained by reduction of the corresponding iminium salts 2a-c with KC8, have been spectroscopically and crystallographically (3a,c) characterized. DFT calculations indicate that increasing the electron-withdrawing properties of the para substituent on the carbene carbon atom results in the spin density evolving from the acenaphthene ring to the phenyl ring. The IPr(BIAN)-based radicals 3a-c show excellent stability: they have half-lives of 1 week in well-aerated solutions and feature a high thermal decomposition temperature up to 200 °C.

A metal-carbene template approach enables efficient synthesis of a functionalized cage-annulated crown ether.

A facile synthetic method to form cage-annulated crown ether with anchored imidazolium units was developed. The present work verified the potential application of the metal-carbene template approach (MCTA) in the preparation of novel flexible polyimidazolium cages by photochemical [2+2] cycloaddition reactions and may provide a new method for the preparation of flexible pure organic cages with incorporation of a variety of functional sites.

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 L1Au , H2 L2Au ) 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.

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).