Photoactive Metal–Organic Framework and Its Film for Light-Driven Hydrogen Production and Carbon Dioxide Reduction

Stable Zinc-Based Metal-Organic Framework Photocatalyst for Effective Visible-Light-Driven Hydrogen Production

Herein, a new Zn-MOF material, [Zn(L1)(L2)], 1, was built successfully through a one-pot solvothermal method. The 3D MOF structure was determined by Single X-ray diffraction analysis, IR, and elemental analysis. A series of PXRD tests of 1 after being immersed in different solvents and pH solutions demonstrated the good stability of 1. Interestingly, this material displayed high catalytic activity for the visible-light-driven hydrogen generation under the illumination of white LED in pure water or a mixture of DMF and H₂O without additional photosensitizers and cocatalysts. Besides, the studies also showed that the catalytic activity changed constantly as well as the solvent ratio adjustment of DMF and H₂O from 4:6 to 2:8. Additionally, the catalytic activity reached the best value (743 μmol g^-1 h^-1) when the solvent ratio was 4:6. The heterogeneous nature and recyclability of the MOF catalyst, as well as several factors that affect the catalytic activity, were investigated and described in detail. Moreover, the photocatalytic mechanism for the hydrogen generation of 1 was also proposed based on the fluorescence spectra and UV-vis absorption.

Artificial Metalloenzymes: Recent Developments and Innovations in Bioinorganic Catalysis

Cellular life is orchestrated by the biochemical components of cells that include nucleic acids, lipids, carbohydrates, proteins, and cofactors such as metabolites and metals, all of which coalesce and function synchronously within the cell. Metalloenzymes allow for such complex chemical processes, as they catalyze a myriad of biochemical reactions both efficiently and selectively, where the metal cofactor provides additional functionality to promote reactivity not readily achieved in their absence. While the past 60 years have yielded considerable insight on how enzymes catalyze these reactions, a need to engineer and develop artificial metalloenzymes has been driven not only by industrial and therapeutic needs, but also by innate human curiosity. The design of miniature enzymes, both rationally and through serendipity, using both organic and inorganic building blocks has been explored by many scientists over the years and significant progress has been made. Herein, recent developments over the past 5 years in areas that have not been recently reviewed are summarized, and prospects for future research in these areas are addressed.

Harnessing the Untapped Catalytic Potential of a CoFe2O4/Mn-BDC Hybrid MOF Composite for Obtaining a Multitude of 1,4-Disubstituted 1,2,3-Triazole Scaffolds

Metal-organic frameworks derived nanostructures with extraordinary variability, and many unprecedented properties have recently emerged as promising catalytic materials to address the challenges in the field of modern organic synthesis. In this contribution, the present work reports the fabrication of an intricately designed magnetic MOF composite based on Mn-BDC (manganese benzene-1,4-dicarboxylate/manganese terephthalate) microflakes via a facile and benign in situ solvothermal approach. Structural information about the as-synthesized hybrid composite has been obtained with characterization techniques such as TEM, SEM, XRD, FT-IR, AAS, EDX, ED-XRF, and VSM analysis. Upon investigation of catalytic performance, the resulting material unveils remarkable efficacy toward facile access of a diverse array of pharmaceutically active 1,2,3-triazoles from a multicomponent coupling reaction of terminal alkynes, sodium azide, and alkyl or aryl halides as coupling partners. In addition to a wide substrate scope, the catalyst with highly accessible active sites also possesses a stable catalytic metal center along with superb magnetic properties that facilitate rapid and efficient separation. The prominent feature that makes this protocol highly desirable is the ambient and greener reaction conditions in comparison to literature precedents reported to date. Further, a plausible mechanistic pathway is also proposed to rationalize the impressive potential of the developed catalytic system in the concerned reaction. We envision that findings from our study would not only provide new insights into the judicious design of advanced MOF based architectures but also pave the way toward greening of industrial manufacturing processes to tackle critical environmental and economic issues.

Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes

Four new triphenylamine ligands with different substituents in the para position and their corresponding copper(II) complexes are reported. This study includes their structural, spectroscopic, magnetic, and electrochemical properties. The complexes possess a dinuclear copper(II) paddle-wheel core, a building unit that is also common in metal-organic frameworks. Electrochemical measurements demonstrate that the triphenylamine ligands and the corresponding complexes are susceptible to oxidation, resulting in the formation of stable radical cations. The square-wave voltammograms observed for the complexes are similar to those of the ligands, except for a slight shift in potential. Square-wave voltammetry data show that, in the complexes, these oxidations can be described as individual one-electron processes centered on the coordinated ligands. Spectroelectrochemistry reveals that, during the oxidation of the complexes, no difference can be detected for the spectra of successively oxidized species. For the absorption bands of the oxidized species of the ligands and complexes, only a slight shift is observed. ESR spectra for the chemically oxidized complexes indicate ligand-centered radicals. The copper ions of the paddle-wheel core are strongly antiferromagnetic coupled. DFT calculations for the fully oxidized complexes indicate a very weak ferromagnetic coupling between the copper ions and the ligand radicals, whereas a very weak antiferromagnetic coupling is found among the ligand radicals.

Photochemical Properties of Host-Guest Supramolecular Systems with Structurally Confined Metal-Organic Capsules

Inspired by natural photosynthesis, researchers have designed symmetric metal-organic hosts with large inner pockets that are spontaneously generated through preorganized ligands and functionalized metallocorners to construct dye-containing host-guest systems. The abundant noncovalent interaction sites in the pockets of the hosts facilitated substrate-catalyst interactions for possible enrichment, fixation, and activation of substrates/reagents, providing special electron transfer pathways for regio- or stereoselectively photocatalytic chemical transformations. In this Account, we focus our attention on metal-organic hosts that contain photoactive or redox-active units to evaluate electron transfer and charge separation between host and guest units in these supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transfer processes within the structurally confined pockets of these interesting metal-organic hosts. We have been engaged in developing methods to isolate a series of chromophores for charge separation in supramolecular systems, incorporating organic dyes as photosensitizers in metal-organic hosts with electron acceptor/donor guests is a promising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related to these inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host-guest supramolecular systems to fix and isolate the donor-acceptor pair with a short through-space distance provided a new PET pathway to stabilize the charge-separated ion pair. Highly efficient photosynthetic systems can be obtained when charge transfer to electron donors/acceptors occurs faster than the charge recombination. This Account starts with a brief summary of the potential approaches for constructing photoactive metal-organic hosts through the incorporation of dye molecules within ligand backbones or as a part of the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing the photoinduced charge separation and electron transfer pathways within the dye-incorporated metal-organic hosts. We also searched for strategies for constructing photoactive supramolecular systems through encapsulating dye molecules within the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages due to the structural confinement: avoiding excited state quenching caused by other chemical species, including aggregated dyes, stabilizing the radical intermediate and tuning the absorption or emission of the guest through electron/energy transfer pathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energy metabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects of promoting and controlling light-induced chemical transformations.

Expanding the Horizon of Multicomponent Oxidative Coupling Reaction via the Design of a Unique, 3D Copper Isophthalate MOF-Based Catalyst Decorated with Mixed Spinel CoFe2O4 Nanoparticles

This work discloses the first ever magnetically retrievable copper isophthalate-based metal-organic framework (MOF) decorated with surface-modified cobalt ferrite (CoFe2O4) nanoparticles that have been utilized as catalytic reactors for obtaining a relatively large number of biologically active benzimidazole scaffolds. A facile one-pot solvothermal approach was employed for obtaining spherical and monodisperse CoFe2O4 nanoparticles, which were subsequently modified using suitable protecting and functionalizing agents. Finally, these functionalized magnetic nanoparticles were anchored onto the three-dimensional copper isophthalate MOF via a covalent immobilization methodology. The exploitation of advanced microscopic tools such as transmission electron microscopy and scanning electron microscopy provided valuable insights into the morphology of the immobilized MOF. These results indicated that the surface-modified magnetic nanoparticles had grown onto the surface of copper-5-nitroisophthalic acid MOF. A greener C-H functionalization strategy that involves the multicomponent oxidative cross-coupling between two different set of amines (sp2-hybridized nitrogen-containing anilines and sp3-hybridized nitrogen-containing alkyl/aryl amine derivatives) and sodium azide has been incorporated to provide access to a broad spectrum of the value-added target benzimidazole moieties. It is interesting to note that this magnetic MOF-catalyzed protocol not only replaces toxic solvents with water, which is a green solvent, but also enhances the economic competitiveness since the magnetic catalyst can be readily recovered and recycled for eight consecutive runs.

Metal-Organic Frameworks for CO2 Chemical Transformations

Carbon dioxide (CO₂ ), as the primary greenhouse gas in the atmosphere, triggers a series of environmental and energy related problems in the world. Therefore, there is an urgent need to develop multiple methods to capture and convert CO₂ into useful chemical products, which can significantly improve the environment and promote sustainable development. Over the past several decades, metal-organic frameworks (MOFs) have shown outstanding heterogeneous catalytic activity due in part to their high internal surface area and chemical functionalities. These properties and the ability to synthesize MOF platforms allow experiments to test structure-function relationships for transforming CO₂ into useful chemicals. Herein, recent developments are highlighted for MOFs participating as catalysts for the chemical fixation and photochemical reduction of CO₂ . Finally, opportunities and challenges facing MOF catalysts are discussed in this ongoing research area.