MOF-Incorporated Binuclear N-Heterocyclic Carbene-Cobalt Catalyst for Efficient Conversion of CO2 to Formamides

Environmental problem caused by carbon emission is of widespread concern. Involving CO₂ as C₁ resource into chemical synthesis is one of the most attractive ways for carbon recycling. Herein, the first example of host-guest composites featuring metal-organic framework (MOF)-encapsulated binuclear N-heterocyclic carbene (NHC) complex, Co₂ @MIL101, was developed with the molecularly dispersed [Co(IPr)Br]₂ (μ-Br)₂ (Co₂ ) loading in the cage of MIL-101(Cr) via a "ligand-in-dimer-trap" strategy, which was comprehensively investigated through various techniques including synchrotron X-ray absorption, electron microscopy, X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and others. The noble-metal-free double-sites catalyst Co₂ @MIL101 exhibited promising stability, activity, efficiency, reusability, and substrate adaptability for converting CO₂ into various formamides with amines and hydrosilanes and achieved the best performance for one of the most useful formamides, N-methyl-N-phenylformamide (MFA), among the recyclable catalysts at ambient conditions, providing a reliable approach to successfully unify the advantages of both homo- and heterogeneous catalysts. Density functional theory calculations were applied to illustrate the superior activity of the binuclear NHC complex center as double-sites catalyst toward the activation of CO₂ .

A CO2 -Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion

Free N-heterocyclic carbenes (NHCs) are generally prepared by treatment of imidazolium precursors with strong alkali reagents, which usually produces inactive NHC dimers. This treatment would destroy porous supports and thus make supported NHC catalysts difficult to recovery and reuse. Herein, we report the first stable CO₂ -masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline covalent organic framework (COF). The stable NHC-CO₂ moieties in the COF-NHC-CO₂ could be transformed in situ into isolated NHCs by heating, which exhibit superior catalytic performances in hydrosilylation and N-formylation reactions with CO₂ . The NHC sites can reversibly form NHC-CO₂ and thus can be easily recycled and reused while maintaining excellent catalytic activity. Density functional theory calculations revealed that NHC sites can be fully exposed after removal of CO₂ -masks and rapidly react with silanes, which endows COF-NHC with high catalytic activity.

Highly Efficient Conversion of Propargylic Alcohols and Propargylic Amines with CO2 Activated by Noble-Metal-Free Catalyst Cu2 O@ZIF-8

The cyclization reactions of propargylic alcohols and propargylic amines with CO₂ are important in industrial applications, but it was a great challenge that non-noble-metal catalysts catalyzed both reactions under mild conditions. Herein, the catalyst Cu₂ O@ZIF-8 was prepared by encapsulating Cu₂ O nanoparticles into robust ZIF-8, and it can effectively catalyze the cyclization of both propargylic alcohols and propargylic amines with CO₂ into valuable α-alkylidene cyclic carbonates and oxazolidinones with turnover numbers (TONs) of 12.1 and 19.6, which can be recycled at least five times. The mechanisms were further uncovered by NMR, FTIR, ¹³ C isotope-labeling experiments and DFT calculations, in which Cu₂ O and DBU can synergistically activate the C≡C bond and the hydroxy/amino group of substrates. Importantly, it is the first example of a noble-metal-free catalyst that can catalyze both propargylic alcohols and propargylic amines with CO₂ simultaneously.

Crystal Structure Transformation in Hydrogen-bonded Organic Frameworks via Ion Exchange

Hydrogen-bonded organic frameworks (HOFs) have emerged as rapidly growing porous materials while established permanent porosities are very fragile and difficult to stabilize due to weak hydrogen-bonding interactions among building units. Herein, we report a stable hydrogen-bonded metallotecton framework (termed as HOF-ZJU-102) that was constructed through hydrogen-bonding networks between cationic metal-organic complexes [Cu₂ (Hade)₄ (H₂ O)₂ ]⁴⁺ (Hade=adenine) and GeF₆ ^2- anions. The framework not only shows permanent porosity, but also exhibits efficient separation performance of C₂ H₂ /C₂ H₄ at room temperature. More interestingly, its crystal structure could be irreversibly transformed into isostructural counterpart HOF-ZJU-101 by ion exchange in the SiF₆ ^2- containing solution, evidenced by multiple characterization techniques including gas sorption measurements, ¹⁹ F NMR spectra, FTIR and EDS. Utilizing such an ion exchange mechanism, the collapsed HOF-ZJU-102 could be restored into HOF-ZJU-101 by simply soaking in the salt solution.

MOF Encapsulating N-Heterocyclic Carbene-Ligated Copper Single-Atom Site Catalyst towards Efficient Methane Electrosynthesis

The exploitation of highly efficient carbon dioxide reduction (CO₂ RR) electrocatalyst for methane (CH₄ ) electrosynthesis has attracted great attention for the intermittent renewable electricity storage but remains challenging. Here, N-heterocyclic carbene (NHC)-ligated copper single atom site (Cu SAS) embedded in metal-organic framework is reported (2Bn-Cu@UiO-67), which can achieve an outstanding Faradaic efficiency (FE) of 81 % for the CO₂ reduction to CH₄ at -1.5 V vs. RHE with a current density of 420 mA cm^-2 . The CH₄ FE of our catalyst remains above 70 % within a wide potential range and achieves an unprecedented turnover frequency (TOF) of 16.3 s^-1 . The σ donation of NHC enriches the surface electron density of Cu SAS and promotes the preferential adsorption of CHO* intermediates. The porosity of the catalyst facilitates the diffusion of CO₂ to 2Bn-Cu, significantly increasing the availability of each catalytic center.

N-Heterocyclic Carbene-Stabilized Ultrasmall Gold Nanoclusters in a Metal-Organic Framework for Photocatalytic CO2 Reduction

Ultrafine gold nanoclusters (Au-NCs) are susceptible to migrate and aggregate, even in the porosity of many crystalline solids. N-heterocyclic carbenes (NHCs) are a class of structurally diverse ligands for the stabilization of Au-NCs in homogeneous chemistry, showing catalytic reactivity in CO₂ activation. Herein, for the first time, we demonstrate a heterogeneous nucleation approach to stabilize ultrasmall and highly dispersed gold nanoclusters in an NHC-functionalized porous matrix. The sizes of gold nanoclusters are tunable from 1.3 nm to 1.8 nm based on the interpenetration of the metal-organic framework (MOF) topology. Control experiments using amine or imidazolium-functionalized MOFs afforded the aggregation of Au species. The resultant Au-NC@MOF composite exhibits a steady and excellent activity in photocatalytic CO₂ reduction, superior to control mixtures without NHC-ligand stabilization. Mechanistic studies reveal the synergistic catalytic effect of MOFs and Au-NCs through the MOF-NHC-Au covalent-bonding bridges.

Heterogeneous Organocatalysts for the Reduction of Carbon Dioxide with Silanes

The utilization of carbon dioxide (CO₂ ) as feedstock for chemical industries is gaining interest as a sustainable alternative to nonrenewable fossil resources. However, CO₂ reduction is necessary to increase its energy content. Hydrosilane is a potential reducing agent that exhibits excellent reactivity under ambient conditions. CO₂ hydrosilylation yields versatile products such as silylformate and methoxysilane, whereas formamides and N-methylated products are obtained in the presence of amines. In these transformations, organocatalysts are considered as the more sustainable choice of catalyst. In particular, heterogeneous organocatalysts featuring precisely designed active sites offer higher efficiency due to their recyclability. Herein, an overview is presented of the current development of basic organocatalysts immobilized on various supports for application in the chemical reduction of CO₂ with hydrosilanes, and the potential active species parameters that might affect the catalytic activity are identified.

A Resin-Supported Formate Catalyst for the Transformative Reduction of Carbon Dioxide with Hydrosilanes

A heterogeneous formate anion catalyst for the transformative reduction of carbon dioxide (CO₂ ) based on a polystyrene and divinylbenzene copolymer modified with alkylammonium formate was prepared from a widely available anion exchange resin. The catalyst preparation was easy and the characterization was carried out by using elemental analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state ¹³ C cross-polarization/magic-angle spinning nuclear magnetic resonance (¹³ C CP/MAS NMR) spectroscopy. The catalyst displayed good catalytic activity for the direct reduction of CO₂ with hydrosilanes, tunably yielding silylformate or methoxysilane products depending on the hydrosilanes used. The catalyst was also active for the reductive insertion of CO₂ into both primary and secondary amines. The catalytic activity of the resin-supported formate can be predicted from the FTIR spectra of the catalyst, probably because of the difference in the ionic interaction strength between the supported alkylammonium cations and formate anions. The ion pair density is thought to influence the catalytic activity, as shown by the elemental and solid-state ¹³ C NMR analyses.

In situ transformation of a tridentate to a tetradentate unsymmetric Schiff base ligand via deaminative coupling in Ni(ii) complexes: crystal structures, magnetic properties and catecholase activity study

An N₂O donor reduced Schiff base in presence of Ni(ClO₄)₂·6H₂O and SCN⁻ transforms into N₂O₂ donor ligand via deaminative coupling. Metal complexes 1 and 3 exhibit catecholase like activity and antiferromagnetic coupling between the Ni(ii) ions.

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.