Metal‐Carbene‐Templated Photochemistry in Solution: A Universal Route towards Cyclobutane Derivatives

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.

Coordination Polymer-Mediated Molecular Surgery for Precise Interconversion of Dicyclobutane Compounds

A Cd(II)-based coordination polymer {[Cd₂(5-F-1,3-bpeb)₂(FBA)₄]·H₂O}_n (CP₁) was obtained from Cd(II) salt, 5-fluoro-1,3-bis[2-(4-pyridyl)ethenyl]benzene (5-F-1,3-bpeb), and p-fluorobenzoic acid (HFBA). Within the one-dimensional chain structure of CP₁, a pair of 5-F-1,3-bpeb was arranged in a face-to-face style. Upon UV irradiation and heat treatment, multiple cyclobutane isomers, including specific monocyclobutanes (1 with an endo-cyclobutane ring in CP₁-1 and 1' with an exo-cyclobutane ring in CP₁-1') and dicyclobutanes (endo,endo-dicyclobutane 2α in CP₁-2α, exo,endo-dicyclobutane 2β in CP₁-2β, and exo,exo-dicyclobutane 2γ in CP₁-2γ) were stereoselectively produced. These isomers could be interconverted inside the CP via cutting/coupling specific bonds, which may be regarded as a type of molecular surgery. The precision of cutting/coupling relied on the thermal stability of the cyclobutanes and the alignment of the reactive alkene centers. The conversion processes were tracked through nuclear magnetic resonance, in situ powder X-ray diffraction, and IR spectroscopy. This approach can be considered as skeletal editing to construct complex organic compounds directly from one precursor.

Controllable Strategy for Metal-Organic Framework Light-Driven [2 + 2] Cycloaddition Reactions via Solvent-Assisted Linker Exchange

Flexible olefinic trans-1,2-bis(4-pyridyl)ethene linkers were postsynthetically introduced into the metal-organic frameworks (MOFs) containing parallel rigid 4,4'-bipyridine linkers with a spacing of less than 4.2 Å by the linker exchange strategy, and then, the MOF satisfied Schmidt criteria could be obtained. Eventually, MOF products connected by cyclobutane derivatives were formed by the photochemical [2 + 2] cycloaddition reaction under UV irradiation.

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).

Template-driven construction of [8]-imidazolium macrocycles

A controllable and efficient template-driven strategy for the rational construction of polyimidazolium macrocycles has been developed.

Regioselective Synthesis of Chromones via Cyclocarbonylative Sonogashira Coupling Catalyzed by Highly Active Bridged-Bis(N-Heterocyclic Carbene)Palladium(II) Complexes

The one-pot regioselective and catalytic synthesis of bioactive chromones and flavones was achieved via phosphine-free cyclocarbonylative Sonogashira coupling reactions of 2-iodophenols with aryl alkynes, alkyl alkynes, and dialkynes. The reactions are catalyzed by new dibromidobis(NHC)palladium(II) complexes. The new bridged N,N'-substituted benzimidazolium salts (L1, L2, and L3) and their palladium complexes C1, C2, and C3 were designed, prepared, and fully characterized using different physical and spectroscopic techniques. The molecular structures of complexes C1 and C3 were determined by single-crystal X-ray diffraction analysis. They showed a distorted square planar geometry, where the Pd(II) ion is bonded to the carbon atoms of two cis NHC carbene ligands and two cis bromido anions. These complexes displayed a high catalytic activity in cyclocarbonylative Sonogashira coupling reactions with low catalyst loadings. The regioselectivity of these reactions was controlled by using diethylamine as the base and DMF as the solvent.

Ultrastable and Highly Catalytically Active N-Heterocyclic-Carbene-Stabilized Gold Nanoparticles in Confined Spaces

Controlling the size and surface functionalization of nanoparticles (NPs) can lead to improved properties and applicability. Herein, we demonstrate the efficiency of the metal-carbene template approach (MCTA) to synthesize highly robust and soluble three-dimensional polyimidazolium cages (PICs) of different sizes, each bearing numerous imidazolium groups, and use these as templates to synthesize and stabilize catalytically active, cavity-hosted, dispersed poly-N-heterocyclic carbene (NHC)-anchored gold NPs. Owing to the stabilization of the NHC ligands and the effective confinement of the cage cavities, the as-prepared poly-NHC-shell-encapsulated AuNPs displayed promising stability towards heat, pH, and chemical regents. Most notably, all the Au@PCCs (PCC=polycarbene cage) exhibited excellent catalytic activities in various chemical reactions, together with high stability and durability.