Mechanical Interlocking Enhances the Electrocatalytic Oxygen Reduction Activity and Selectivity of Molecular Copper Complexes

Bimetallic Synergy in Oxygen Reduction: How Tailored Metal-Metal Interactions Amplify Cooperative Catalysis

Bimetallic cooperative catalysis, inspired by cytochrome c oxidase and multicopper oxidase, plays a crucial role in the development of four-electron oxygen reduction catalysts. The distance between metals is a crucial factor affecting the cooperative effect, but its precise influence on bimetallic cooperativity in selective oxygen reduction catalysis still awaits an in-depth understanding. Herein, we employ a series of dicopper complexes with varying linkers to systematically adjust the Cu···Cu distance for electrocatalytic oxygen reduction. Structure-activity relationship analyses reveal that catalysts with a shorter dicopper center exhibited significantly higher four-electron selectivity (approaching 100% for BPMPDCu2 and BPMANCu2) than that with a longer distance (below 80% for 6-HPACu2) in an aqueous solution (pH 7.0). Notably, the catalytic activity of BPMPDCu2 is 11 times and 237 times faster than those of 6-HPACu2 and BPMANCu2, respectively, which does not correlate directly with their Cu···Cu distances. Further investigations into low-valent LCuI2 intermediates, supported by DFT calculations, indicate that the oxygen binding process is the rate-determining step under electrocatalytic conditions and is sensitive to the CuI···CuI distance. The closest BPMANCuI2 characterized by strong CuI-CuI interactions and the more distant 6-HPACuI2 with its separated dicopper sites both hinder effective O2 binding. In contrast, BPMPDCuI2 maintains an optimal Cu···Cu distance that facilitates O2 binding and ensures robust bimetallic cooperativity throughout the catalytic cycle. This work underscores the significance of metal-metal distance regulation in bimetallic cooperatively selective oxygen reduction and provides valuable insights for the rational design of high-performance oxygen reduction catalysts.

Mechanical and Covalent Tailoring of Copper Catenanes for Selective Aqueous Nitrate-to-Ammonia Electrocatalysis

Electrocatalytic nitrate reduction reaction (NO3RR) for the selective generation of ammonia (NH3) enables the removal of deleterious nitrate pollutants while simultaneously upcycling them into a value-added fertilizer. The development of nonprecious metal-derived catalysts such as those featuring copper (Cu) as earth-abundant alternatives for the state-of-the-art precious metal catalysts is of urgent need yet suffering from the activity-selectivity-durability trilemma. Rational design of molecular Cu complexes with well-defined coordination structures permitting systematic structure-activity relationship (SAR) investigations is key to addressing the challenge. Here, a series of molecular Cu(I) complexes with [2]catenane ligands are developed as NO3RR electrocatalysts for the first time. By engineering multiple cationic ammoniums on the catenane backbone, acceptance of the anionic nitrate substrate as well as the release of the cationic ammonium product are promoted, thereby facilitating a higher Faradaic efficiency and product selectivity toward ammonia via an 8e- pathway. Of note, the mutual Coulombic repulsion between the multiply charged ligands is overcome by the mechanical interlocking such that the catalyst integrity can be maintained under practical conditions. This report highlights the promise of employing mechanically interlocked ligands as a platform for customizing metal complexes as catalysts for redox processes involving multiple proton-coupled electron transfer steps.

A Copper(I) Catenane Decorated Metal-Organic Layer as a Heterogenous Catalyst for Dehydrogenative Cross-Coupling

While earth-abundant metals are green and sustainable alternatives to precious metals for catalytic chemical conversions, the fast ligand exchange involving most of the base metals renders their development into robust, reusable catalysts very challenging. Described in this work is a new type of heterogeneous catalyst derived from a 2D metal-organic layer (MOL) grafted with catenane-coordinated Cu(I) complexes. In addition to the good substrate accessibility, easy functionalization, and other favorable features due to the MOL support, the mechanical bond in the anchored catenane ligands also represents a new mechanism to dynamically confine the coordination environment and kinetically stabilize the coordinated Cu(I) to give a well-defined, active yet stable heterogeneous catalyst. Pilot catalytic studies using a model dehydrogenative C─O cross-coupling reaction showed that the Cu(I) catenane-grafted MOL led to exclusive formation of the C─O coupled product, whereas control catalysis using a similar Cu(I) catalyst supported by non-interlocked macrocyclic ligands was found to also give a C─C coupled by-product, whose formation was found to be mediated by the uncontrolled oxidation of the Cu(I) to Cu(II), highlighting the distinctive roles and untapped potential of the catenane coordination in developing base metal-derived catalysts for challenging catalytic conditions.

A Thiourea-Based Rotaxane Catalyst: Nucleophilic Fluorination Phase-Transfer Process Unlocked by the Mechanical Bond

We report a five-component clipping approach using activated isophthaloyl-derived esters to synthesize an amide-based thiourea rotaxane. This method overcomes acyl chloride limitations with nucleophilic thiourea threads. The steric hindrance of the mechanical bond enables, for the first time, an interlocked thiourea as a hydrogen-bonding phase-transfer organocatalyst in nucleophilic fluorinations. This highlights how mechanical bonds expand thiourea catalysis to processes previously incompatible with conventional catalysts.

Ring-in-Ring Assembly Facilitates the Synthesis of a [12]Cycloparaphenylene ABC-Type [3]Catenane

Cycloparaphenylenes (CPPs) represent a significant challenge for the synthesis of mechanically interlocked architectures, because they lack heteroatoms, which precludes traditional active and passive template methods. To circumvent this problem and explore the fundamental and functional properties of CPP rotaxanes and catenanes, researchers have resorted to unusual non-covalent and even to labor-intensive covalent template approaches. Herein, we report a ring-in-ring non-covalent template strategy that makes use of the surprisingly strong non-covalent inclusion of crown ethers into suitably sized CPPs. By threading a secondary ammonium salt through the crown ether and closing the third ring via CuAAC click reaction, we obtained a rare ABC-type hetero-[3]catenane comprising [12]CPP, 24-crown-8 and a dibenzylammonium macrocycle. X-ray crystallography shed light on the ring-in-ring pre-organization and the [3]catenane topology was confirmed by NMR and MS-MS studies. Molecular simulations provided insights into the intriguing ring-vs.-ring-vs.-ring dynamics of the [3]catenane, which are highly dependent on the protonation state of the dibenzylammonium site. This ring-in-ring assembly strategy opens new avenues for the synthesis of complex CPP architectures and their use in functional supramolecular systems.

Functionalization of a [2]Catenane with Donor-Acceptor Chromophores Using a Metal Template and Click Reactions

Synthesizing molecules with significant topological features, such as catenanes, tailored with specific groups to confer desired functionality, is essential for investigating various properties arising from the entanglement due to mechanical bonds. This investigation can pave the way for uncovering novel functional materials employing mechanically interlocked molecules (MIMs). In this direction, we have synthesized a π-donor (D) and π-acceptor (A) functionalized [2]catenane using a non-labile Co(III) metal ion as a template with pyridine-diamide templating center and utilizing click reaction for ring-closing. The donor group is a fluorene derivative, and the acceptor is a benzophenazine derivative, commonly employed in synthesizing conjugated polymers for various optoelectronic devices. Synthetically, the acceptor group was introduced into a macrocycle with a pyridine diamide unit. It was then threaded with a ligand having alkyne terminals to obtain the desired [2]pseudorotaxane utilizing cobalt ion as a template. Ring-closing was then performed with a di-azide functionalized molecule with the donor chromophore. The desired D-A functionalized [2]catenane was obtained after demetalation. All the starting materials, macrocycle, and entangled structures have been characterized by 1H-NMR, 13C-NMR, and mass spectroscopy. Some of these materials were also characterized by single-crystal X-ray analysis. The photophysical properties are studied by UV-visible and fluorescence spectroscopy.

Highly Efficient Self-Assembly of Heterometallic [2]Catenanes and Cyclic Bis[2]catenanes via Orthogonal Metal-Coordination Interactions

Although catenated cages have been widely constructed due to their unique and elegant topological structures, cyclic catenanes formed by the connection of multiple catenane units have been rarely reported. Herein, based on the orthogonal metal-coordination-driven self-assembly, we prepare a series of heterometallic [2]catenanes and cyclic bis[2]catenanes, whose structures are clearly evidenced by single-crystal X-ray analysis. Owing to the multiple positively charged nature, as well as the potential synergistic effect of the Cu(I) and Pt(II) metal ions, the cyclic bis[2]catenanes display broad-spectrum antibacterial activity. This work not only provides an efficient strategy for the construction of heterometallic [2]catenanes and cyclic bis[2]catenanes but also explores their applications as superior antibacterial agents, which will promote the construction of advanced supramolecular structures for biomedical applications.

Water and Air Stable Copper(I) Complexes of Tetracationic Catenane Ligands for Oxidative C-C Cross-Coupling

Aqueous soluble and stable Cu(I) molecular catalysts featuring a catenane ligand composed of two dicationic, mutually repelling but mechanically interlocked macrocycles are reported. The ligand interlocking not only fine-tunes the coordination sphere and kinetically stabilizes the Cu(I) against air oxidation and disproportionation, but also buries the hydrophobic portions of the ligands and prevents their dissociation which are necessary for their good water solubility and a sustained activity. These catenane Cu(I) complexes can catalyze the oxidative C-C coupling of indoles and tetrahydroisoquinolines in water, using H2O2 as a green oxidant with a good substrate scope. The successful use of catenane ligands in exploiting aqueous Cu(I) catalysis thus highlights the many unexplored potential of mechanical bond as a design element for exploring transition metal catalysis under challenging conditions.

Template Assisted Synthesis of Linear [5]Catenane by Post-Functionalization of Templated [2]Catenane and Using Click Reaction

Polymers with all mechanically interlocked rings, such as linear [n]catenanes, have great potential as functional materials due to possible higher degrees of freedom that may contribute to their flexibility but remain elusive. All the synthetic methods used to prepare such a polymer yield mixtures of products. In the absence of higher molecular weight linear [n]catenanes, emphasis on synthesizing low molecular weight oligomers is being pursued. Here, we have described the synthesis of a linear [5]catenane by post-functionalizing a Co(III) templated [2]catenane having a pyridine-diamide unit free for further metal ion coordination. Two molecules were synthesized with suitable threading groups: one, two terminal azide groups, and two, with two terminal alkyne groups to form two [3]pseudorotaxane utilizing Co(III) coordination. These units were then joined, forming a macrocycle, using click reaction, giving the desired metalated linear [5]catenane in 40 % yield. Removal of metal ions leads to linear [5]catenane. In addition, the formation of linear [3] and [2]catenane are also observed. All synthesized structures have been isolated by column chromatographic technique and characterized by 1H-NMR, 13C-NMR, and mass spectroscopy.

Lattice Strained Induced Spin Regulation in Co-N/S Coordination-Framework Enhanced Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is the bottleneck of metal-air batteries and fuel cells. Strain regulation can change the geometry and adjust the surface charge distribution of catalysts, which is a powerful strategy to optimize the ORR activity. The introduction of controlled strain to the material is still difficult to achieve. Herein, we present a temperature-pressure-induced strategy to achieve the controlled lattice strain for metal coordination polymers. Through the systematic study of the strain effect on ORR performance, the relationship between geometric and electronic effects is further understood and confirmed. The strained Co-DABDT (DABDT=2,5-diaminobenzene-1,4-dithiol) with 2 % lattice compression exhibits a superior half-wave potential of 0.81 V. Theoretical analysis reveals that the lattice strain changes spin-charge densities around S atoms for Co-DABDT, and then regulates the hydrogen bond interaction with intermediates to promote the ORR catalytic process. This work helps to understand the catalytic mechanism from the atomic level.

Template Assisted One-Pot Synthesis of [2], Linear [3], and Radial [4]Catenane via Click Reaction

Design and synthesis of higher order catenane are unexpectedly complex and involve precise cooperation among the precursors overcoming competing and opposing interactions. We achieved synthesis of [2], linear [3], radial [4] in a one-pot reaction by consecutive ring closing through click reactions. This synthesis gave three isolable products due to two, four, and six-click reactions between suitable coupling partners. Yields of the isolate templated-catenane decrease from lower to higher-ordered catenane (40 %, 12 %, and 4 %), probably due to the bite angle as well as the flexibility of the reacting partners. Removal of templating cobalt(III) ion leads to the formation of fully organic [2], linear [3], and radial [4]catenane. These synthesized catenanes were purified by column chromatography and characterized by 1H-NMR, 13C-NMR, and ESI-MS spectroscopy. The synthesized catenanes have free binding sites suitable for post-functionalization and may be used for the synthesis of higher-ordered catenane.

An Ultrasmall Ordered High-Entropy Intermetallic with Multiple Active Sites for the Oxygen Reduction Reaction

Controlling multimetallic ensembles at the atomic level is significantly challenging, particularly for high-entropy alloys with more than five elements. Herein, we report an innovative ultrasmall (∼2 nm) PtFeCoNiCuZn high-entropy intermetallic (PFCNCZ-HEI) with a well-ordered structure synthesized by using the space-confined strategy. By exploiting these combined metals, the PFCNCZ-HEI nanoparticles achieve an ultrahigh mass activity of 2.403 A mgPt-1 at 0.90 V vs reversible hydrogen electrode for the oxygen reduction reaction, which is up to 19-fold higher than that of state-of-the-art commercial Pt/C. A proton exchange membrane fuel cell assembled with PFCNCZ-HEI as the cathode (0.03 mgPt cm-2) exhibits a power density of 1.4 W cm-2 and a high mass-normalized rated power of 45 W mgPt-1. Furthermore, theoretical calculations reveal that the outer electrons of the non-noble-metal atoms on the surface of the PFCNCZ-HEI nanoparticle are modulated to show characteristics of multiple active centers. This work offers a promising catalyst design direction for developing highly ordered HEI nanoparticles for electrocatalysis.

Stabilizing iron single atoms with electrospun hollow carbon nanofibers as self-standing air-electrodes for long-time Zn - air batteries

Developing iron-based single-atom catalysts (Fe SACs) with low cost, high activity and stability is vital for commercialising sustainable energy technologies. However, accurately controlling and identifying structure-activity relationships of Fe SACs remains a significant challenge. Herein, we report Fe/N co-doped carbon nanofiber membranes with highly exposed Fe-N4 sites (Fe/NCNFs), synthesized by electrospinning and pyrolysis. The three-dimensional (3D) hierarchical structure and atomically dispersed pyrrole-type Fe (III)-N4 active sites provide the as-prepared catalyst with a positive half-wave potential of 0.87 V and an ultralow Tafel slope of 53 mV dec-1. As an air cathode catalyst for liquid Zn - air batteries, it delivers a high open-circuit voltage (1.474 V), a large peak power density (190 mW cm-2) and a high durability of 2000 cycles at 5 mA cm-2. As a self-standing air cathode, the as-assembled solid-state Zn - air batteries also show stable cycling with a small discharge/charge voltage gap of 0.65 V, indicating great prospects for developing portable zinc - air batteries.

Selective Four-Electron Reduction of Oxygen by a Nonheme Heterobimetallic CuFe Complex

We report herein the first nonheme CuFe oxygen reduction catalyst ([CuII (bpbp)(μ-OAc)2 FeIII ]2+ , CuFe-OAc), which serves as a functional model of cytochrome c oxidase and can catalyze oxygen reduction to water with a turnover frequency of 2.4×103  s-1 and selectivity of 96.0 % in the presence of Et3 NH+ . This performance significantly outcompetes its homobimetallic analogues (2.7 s-1 of CuCu-OAc with %H2 O2 selectivity of 98.9 %, and inactive of FeFe-OAc) under the same conditions. Structure-activity relationship studies, in combination with density functional theory calculation, show that the CuFe center efficiently mediates O-O bond cleavage via a CuII (μ-η1  : η2 -O2 )FeIII peroxo intermediate in which the peroxo ligand possesses distinctive coordinating and electronic character. Our work sheds light on the nature of Cu/Fe heterobimetallic cooperation in oxygen reduction catalysis and demonstrates the potential of this synergistic effect in the design of nonheme oxygen reduction catalysts.

Catenane and Rotaxane Synthesis from Cucurbit[6]uril-Mediated Azide-Alkyne Cycloaddition

The chemistry of mechanically interlocked molecules (MIMs) such as catenane and rotaxane is full of new opportunities for the presence of a mechanical bond, and the efficient synthesis of these molecules is therefore of fundamental importance in realizing their unique properties and functions. While many different types of preorganizing interactions and covalent bond formation strategies have been exploited in MIMs synthesis, the use of cucurbit[6]uril (CB[6]) in simultaneously templating macrocycle interlocking and catalyzing the covalent formation of the interlocked components is particularly advantageous in accessing high-order catenanes and rotaxanes. In this review, catenane and rotaxane obtained from CB[6]-catalyzed azide-alkyne cycloaddition will be discussed, with special emphasis on the synthetic strategies employed for obtaining complex [n]rotaxanes and [n]catenanes, as well as their properties and functions.

High-Valence Oxides for High Performance Oxygen Evolution Electrocatalysis

Valence tuning of transition metal oxides is an effective approach to design high-performance catalysts, particularly for the oxygen evolution reaction (OER) that underpins solar/electric water splitting and metal-air batteries. Recently, high-valence oxides (HVOs) are reported to show superior OER performance, in association with the fundamental dynamics of charge transfer and the evolution of the intermediates. Particularly considered are the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM). High-valence states enhance the OER performance mainly by optimizing the eg -orbital filling, promoting the charge transfer between the metal d band and oxygen p band. Moreover, HVOs usually show an elevated O 2p band, which triggers the lattice oxygen as the redox center and enacts the efficient LOM pathway to break the "scaling" limitation of AEM. In addition, oxygen vacancies, induced by the overall charge-neutrality, also promote the direct oxygen coupling in LOM. However, the synthesis of HVOs suffers from relatively large thermodynamic barrier, which makes their preparation difficult. Hence, the synthesis strategies of the HVOs are discussed to guide further design of the HVO electrocatalysts. Finally, further challenges and perspectives are outlined for potential applications in energy conversion and storage.

OPA, TPA and ECD spectra of π-conjugated interlocked chiral nanocarbons

This paper presents a theoretical investigation of the optical absorption and molecular chirality of π-conjugated mechanically interlocked nanocarbons, using one photon absorption (OPA) and two photon absorption (TPA) as well as electronic circular dichroism (ECD) spectra. Our findings reveal the optical excitation properties of mechanically interlocked molecules (MIMs) and chirality resulting from interlocked mechanical bonds. While OPA spectra are unable to distinguish interlocked molecules from non-interlocked molecules, we demonstrate that TPA and ECD can effectively discriminate between them, and can also differentiate [2]catenanes from [3]catenanes. Thus, we propose new methods to identify interlocked mechanical bonds. Our results provide physical insight into the optical properties and absolute configuration of π-conjugated interlocked chiral nanocarbons.