Acidic open-cage solution containing basic cage-confined nanospaces for multipurpose catalysis

Solvents regulate the packing porosity of a bilayer metal-organic cage

Metal-organic cages (MOCs) are an emerging class of porous materials with promising applications. However, controlling the configuration of the cage packing, which can influence the overall porosity of the materials, remains a difficulty, as many factors can influence the cage assembly and stacking. Herein, we report a solvent strategy to fine-tune the packing configuration of a bilayer MOC, a small triangular prism cage (six Cu ions act as vertices, three nitrate ions act as pillars, and six nitrate ions act as caps) incorporated into a large triangular prism cage (another six Cu ions act as vertices, a couple of oxygen atoms act as pillars and six ligands (L1: 3,5-bis(pyridine-3-yl)-4H-1,2,4-triazole) act as a jointed cap) by the coordination between the triazole nitrogen from L1 and the inner vertex Cu ions. The involved solvents water, acetonitrile (MeCN) and N,N'-dimethylformamide (DMF) form hydrogen bonds with this bilayer MOC, resulting in three different types of packing associated with systemically tuned porosity (NTU-93: 12.2%, NTU-94: 19.3%, and NTU-95: 42.1%). Gas adsorption and breakthrough tests demonstrate that NTU-95 has potential ability for C2H2/C2H4 separation. This work not only shows a case of finely tuned packing of coordination cages, but also provides a powerful tool that may be extended to other cage families.

Photocatalysis Meets Confinement: An Emerging Opportunity for Photoinduced Organic Transformations

The self-assembled metal-organic cages (MOCs) have been evolved as a paradigm of enzyme-mimic catalysts since they are able to synergize multifunctionalities inherent in metal and organic components and constitute microenvironments characteristic of enzymatic spatial confinement and versatile host-guest interactions, thus facilitating unconventional organic transformations via unique driving-forces such as weak noncovalent binding and electron/energy transfer. Recently, MOC-based photoreactors emerged as a burgeoning platform of supramolecular photocatalysis, displaying anomalous reactivities and selectivities distinct from bulk solution. This perspective recaps two decades journey of the photoinduced radical reactions by using photoactive metal-organic cages (PMOCs) as artificial reactors, outlining how the cage-confined photocatalysis was evolved from stoichiometric photoreactions to photocatalytic turnover, from high-energy UV-irradiation to sustainable visible-light photoactivation, and from simple radical reactions to multi-level chemo- and stereoselectivities. We will focus on PMOCs that merge structural and functional biomimicry into a single-cage to behave as multi-role photoreactors, emphasizing their potentials in tackling current challenges in organic transformations through single-electron transfer (SET) or energy transfer (EnT) pathways in a simple, green while feasible manner.

Chiral BINOL-phosphate assembled single hexagonal nanotube in aqueous solution for confined rearrangement acceleration

Creating microenvironments that mimic an enzyme's active site is a critical aspect of supramolecular confined catalysis. In this study, we employ the commonly used chiral 1,1'-bi-2-naphthol (BINOL) phosphates as subcomponents to construct supramolecular hollow nanotube in an aqueous medium through non-covalent intermolecular recognition and arrangement. The hexagonal nanotubular structure is characterized by various techniques, including X-ray, NMR, ESI-MS, AFM, and TEM, and is confirmed to exist in a homogeneous aqueous solution stably. The nanotube's length in solution depends on the concentration of chiral BINOL-phosphate as a monomer. Additionally, the assembled nanotube can accelerate the rate of the 3-aza-Cope rearrangement reaction by up to 85-fold due to the interior confinement effect. Based on the detailed kinetic and thermodynamic analyses, we propose that the chain-like substrates are constrained and pre-organized into a reactive chair-like conformation, which stabilizes the transition state of the reaction in the confined nanospace of the nanotube. Notably, due to the restricted conformer with less degrees of freedom, the entropic barrier is significantly reduced compared to the enthalpic barrier, resulting in a more pronounced acceleration effect.

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.

Photocatalytic Terminal C-C Coupling Reaction Inside Water Soluble Nanocages

Developing methods that activate C-H bonds directly with high selectivity for C-C bond formation in complex organic synthesis has been a major chemistry challenge. Recently it has been shown that photoactivation of weakly polarized C-H bonds can be carried out inside a cationic water-soluble nanocage with visible light-mediated host-guest charge transfer (CT) chemistry. Using this novel photoredox activation paradigm, here we demonstrate C-C bond formation to photo-generate 1,3-diynes at room temperature in water from terminal aromatic alkynes for the first time. The formation of cavity-confined alkyne radical cation and the proton-removed neutral radical species highlight the unique C-C coupling step driven by supramolecular preorganization.

Radio- and Photosensitizing Os(II)-Based Nanocage for Combined Radio-/Chemo-/X-ray-Induced Photodynamic Therapies, NIR Imaging, and Drug Delivery

Integration of clinical imaging and collaborative multimodal therapies into a single nanomaterial for multipurpose diagnosis and treatment is of great interest to theranostic nanomedicine. Here, we report a rational design of a discrete Os-based metal-organic nanocage Pd6(OsL3)828+ (MOC-43) as a versatile theranostic nanoplatform to meet the following demands simultaneously: (1) synergistic treatments of radio-, chemo-, and X-ray-induced photodynamic therapies (X-PDT) for breast cancer, (2) NIR imaging for cancer cell tracking and tumor-targeting, and (3) anticancer drug transport through a host-guest strategy. The nanoscale MOC-43 incorporates high-Z Os-element to interact with X-ray irradiation for dual radiosensitization and photosensitization, showing efficient energy transfer to endogenous oxygen in cancer cells to enhance X-PDT efficacy. It also features intrinsic NIR emission originating from metal-to-ligand charge transfer (MLCT) as an excellent imaging probe. Meanwhile, its 12 pockets can capture and concentrate low-water-soluble molecules for anticancer drug delivery. These multifunctions are implemented and demonstrated by micellization of coumarin-loaded cages with DSPE-PEG2000 into coumarin ⊂ MOC-43 nanoparticles (CMNPs) for efficient subcellular endocytosis and uptake. The cancer treatments in vitro/in vivo show promising antitumor performance, providing a conceptual protocol to combine cage-cargo drug transport with diagnosis and treatment for collaborative cancer theranostics by virtue of multifunction synergism on a single-nanomaterial platform.

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

Constructing Ultrastable Metallo-Cages via In Situ Deprotonation/Oxidation of Dynamic Supramolecular Assemblies

Coordination-driven self-assembly enables the spontaneous construction of metallo-supramolecules with high precision, facilitated by dynamic and reversible metal-ligand interactions. The dynamic nature of coordination, however, results in structural lability in many metallo-supramolecular assembly systems. Consequently, it remains a formidable challenge to achieve self-assembly reversibility and structural stability simultaneously in metallo-supramolecular systems. To tackle this issue, herein, we incorporate an acid-/base-responsive tridentate ligand into multitopic building blocks to precisely construct a series of metallo-supramolecular cages through coordination-driven self-assembly. These dynamic cagelike assemblies can be transformed to their static states through mild in situ deprotonation/oxidation, leading to ultrastable skeletons that can withstand high temperatures, metal ion chelators, and strong acid/base conditions. This in situ transformation provides a reliable and powerful approach to manipulate the kinetic features and stability of metallo-supramolecules and allows for modulation of encapsulation and release behaviors of metallo-cages when utilizing nanoscale quantum dots (QDs) as guest molecules.

Chiral Molecular Cage with Tunable Stereoinversion Barriers

The manipulation of chirality in molecular entities that rapidly interconvert between enantiomeric forms is challenging, particularly at the supramolecular level. Advances in controlling such dynamic stereochemical systems offer opportunities to understand chiral symmetry breaking and homochirality. Herein, we report the synthesis of a face-rotating tetrahedron (FRT), an organic molecular cage composed of tridurylborane facial units that undergo stereomutations between enantiomeric trefoil propeller-like conformations. After resolution, we show that the racemization barrier of the enantiopure FRT can be regulated in situ through the reversible binding of fluoride anions onto the tridurylborane moieties. Furthermore, the addition of an enantiopure phenylethanol to the FRT can effectively induce chirality of the molecular cage by preferentially binding to one of its enantiomeric conformers. This study presents a new paradigm for controlling dynamic chirality in supramolecular systems, which may have implications for asymmetric synthesis and dynamic stereochemistry.

A Diverse Array of Large Capsules Transform in Response to Stimuli

The allosteric regulation of biomolecules, such as enzymes, enables them to adapt and alter their conformation to fit specific substrates, expressing different functionalities in response to stimuli. Different stimuli can also trigger synthetic coordination cages to change their shape, size, and nuclearity by reconfiguring the dynamic metal-ligand bonds that hold them together. Here we demonstrate an abiological system consisting of different organic subcomponents and Zn^II metal ions, which can respond to simple stimuli in complex ways. A Zn^II₂₀L₁₂ dodecahedron transforms to give a larger Zn^II₃₀L₁₂ icosidodecahedron through subcomponent exchange, as an aldehyde that forms bidentate ligands is displaced in favor of one that forms tridentate ligands together with a penta-amine subcomponent. In the presence of a chiral template guest, the same system that produced the icosidodecahedron instead gives a Zn^II₁₅L₆ truncated rhombohedral architecture through enantioselective self-assembly. Under specific crystallization conditions, a guest induces a further reconfiguration of either the Zn^II₃₀L₁₂ or Zn^II₁₅L₆ cages to yield an unprecedented Zn^II₂₀L₈ pseudo-truncated octahedral structure. The transformation network of these cages shows how large synthetic hosts can undergo structural adaptation through the application of chemical stimuli, opening pathways to broader applications.

In situ detection of miRNA-21 in MCF-7 cell-derived extracellular vesicles using the red blood cell membrane vesicle strategy

In this work, we constructed a novel membrane fusion strategy for extracellular vesicles (EVs) and red blood cell membrane vesicles (RVs). A nanoscale space is formed, which can improve the efficiency of the probe reaction with miRNA-21, which allows the in situ fluorescence detection of miRNA-21 in EVs.

A Redox-Active Supramolecular Fe4L6 Cage Based on Organic Vertices with Acid-Base-Dependent Charge Tunability for Dehydrogenation Catalysis

Supramolecular cage chemistry is of lasting interest because, as artificial blueprints of natural enzymes, the self-assembled cage structures not only provide substrate-hosting biomimetic environments but also can integrate active sites in the confined nanospaces for function synergism. Herein, we demonstrate a vertex-directed organic-clip chelation assembly strategy to construct a metal-organic cage Fe₄L₆⁸⁺ (MOC-63) incorporating 12 imidazole proton donor-acceptor motifs and four redox-active Fe centers in an octahedral coordination nanospace. Different from regular supramolecular cages assembled with coordination metal vertices, MOC-63 comprises six ditopic organic-clip ligands as vertices and four tris-chelating Fe(N∩N)₃ moieties as faces, thus improving its acid, base, and redox robustness by virtue of cage-stabilized dynamics in solution. Improved dehydrogenation catalysis of 1,2,3,4-tetrahydroquinoline derivatives is accomplished by MOC-63 owing to a supramolecular cage effect that synergizes multiple Fe centers and radical species to expedite intermediate conversion of the multistep reactions in a cage-confined nanospace. The acid-base buffering imidazole motifs play a vital role in modulating the total charge state to resist pH variation and tune the solubility among varied solvents, thereby enhancing reaction acceleration in acidic conditions and rendering a facile recycling catalytic process.