Semi‐heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides

Graphitic Carbon Nitride as a Photocatalyst for Decarboxylative C(sp2)-C(sp3) Couplings via Nickel Catalysis

The development of robust and reliable methods for the construction of C(sp2)-C(sp3) bonds is vital for accessing an increased array of structurally diverse scaffolds in drug discovery and development campaigns. While significant advances towards this goal have been achieved using metallaphotoredox chemistry, many of these methods utilise photocatalysts based on precious-metals due to their efficient redox processes and tuneable properties. However, due to the cost, scarcity, and toxicity of these metals, the search for suitable replacements should be a priority. Here, we show the use of commercially available heterogeneous semiconductor graphitic carbon nitride (gCN) as a photocatalyst, combined with nickel catalysis, for the cross-coupling between aryl halide and carboxylic acid coupling partners. gCN has been shown to engage in single-electron-transfer (SET) and energy-transfer (EnT) processes for the formation of C-X bonds, and in this manuscript we overcome previous limitations to furnish C-C over C-O bonds using carboxylic acids. A broad scope of both aryl halides and carboxylic acids is presented, and recycling of the photocatalyst demonstrated. The mechanism of the reaction is also investigated.

TEMPO/O2 Synergistically Mediated BiBrO-Photocatalyzed Decarboxylative Phosphorylation of N-Arylglycines

With both TEMPO and O2 (in air) as the homogeneous redox mediators, BiBrO as the heterogeneous semiconductor photocatalyst, the first example of semi-heterogeneous photocatalytic decarboxylative phosphorylation of N-arylglycines with diarylphosphine oxides was established. A series of α-amino phosphinoxides were efficiently synthesized.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Gold-Catalyzed C-O Cross-Coupling Reactions of Aryl Iodides with Silver Carboxylates

We have developed a C-O cross-coupling reaction of (hetero)aryl iodides with silver carboxylates via a AuI/AuIII catalytic cycle. The transformation featured exclusive chemoselectivity and moisture/air insensitivity. Aromatic and aliphatic (including primary, secondary, and tertiary) silver carboxylates are all suitable substrates. Moreover, this protocol worked well intermolecularly and intramolecularly. Most importantly, good yields were obtained regardless of the substrates' electronic effect and steric hindrance.

Carbon Vacancies Steer the Activity in Dual Ni Carbon Nitride Photocatalysis

The manipulation of carbon nitride (CN) structures is one main avenue to enhance the activity of CN-based photocatalysts. Increasing the efficiency of photocatalytic heterogeneous materials is a critical step toward the realistic implementation of sustainable schemes for organic synthesis. However, limited knowledge of the structure/activity relationship in relation to subtle structural variations prevents a fully rational design of new photocatalytic materials, limiting practical applications. Here, the CN structure is engineered by means of a microwave treatment, and the structure of the material is shaped around its suitable functionality for Ni dual photocatalysis, with a resulting boosting of the reaction efficiency toward many CX (X = N, S, O) couplings. The combination of advanced characterization techniques and first-principle simulations reveals that this enhanced reactivity is due to the formation of carbon vacancies that evolve into triazole and imine N species able to suitably bind Ni complexes and harness highly efficient dual catalysis. The cost-effective microwave treatment proposed here appears as a versatile and sustainable approach to the design of CN-based photocatalysts for a wide range of industrially relevant organic synthetic reactions.

A π-conjugated covalent organic framework enables interlocked nickel/photoredox catalysis for light-harvesting cross-coupling reactions

Covalent organic frameworks (COFs) are an outstanding platform for heterogeneous photocatalysis. Herein, we synthesized a pyrene-based two-dimensional C[double bond, length as m-dash]C linked π-conjugated COF via Knoevenagel condensation and anchored Ni(ii)-centers through bipyridine moieties. Instead of traditional dual metallaphotoredox catalysis, the mono-metal decorated Ni@Bpy-sp2c-COF interlocked the catalysis mediated by light and the transition metal. Under light irradiation, enhanced energy and electron transfer in the COF backbone, as delineated by the photoluminescence, electrochemical, and control experiments, expedited the excitation of Ni centers to efficiently catalyze diverse photocatalytic C-X (X = B, C, N, O, P, S) cross-coupling reactions with efficiencies orders of magnitude higher than the homogeneous controls. The COF catalyst tolerated a diverse range of coupling partners with various steric and electronic properties, delivering the products with up to 99% yields. Some reactions were performed on a gram scale and were applied to diversify pharmaceuticals and complex molecules to demonstrate the synthetic utility.

Nanostructured Carbon Nitride for Continuous-Flow Trifluoromethylation of (Hetero)arenes

Efficient catalytic methods for the trifluoromethylation of (hetero)arenes are of particular importance in organic and pharmaceutical manufacturing. However, many existing protocols rely on toxic reagents and expensive or sterically hindered homogeneous catalysts. One promising alternative to conduct this transformation involves the use of carbon nitride, a non-toxic photocatalyst prepared from inexpensive precursors. Nonetheless, there is still little understanding regarding the interplay between physicochemical features of this photocatalyst and the corresponding effects on the reaction rate. In this work, we elucidate the role of carbon nitride nanostructuring on the catalytic performance, understanding the effect of surface area and band gap tuning via metal insertion. Our findings provide new insights into the structure-function relationships of the catalyst, which we exploit to design a continuous-flow process that maximizes catalyst-light interaction, facilitates catalyst reusability, and enables intensified reaction scale-up. This is particularly significant given that photocatalyzed batch protocols often face challenges during industrial exploitation. Finally, we extrapolate the rapid and simplified continuous-flow method to the synthesis of a variety of functionalized heteroaromatics, which have numerous applications in the pharmaceutical and fine chemical industries.

Organocatalysis with carbon nitrides

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Dual Nickel Photocatalysis for O-Aryl Carbamate Synthesis from Carbon Dioxide

We report the use of dual nickel photocatalysis in the synthesis of O-aryl carbamates from aryl iodides or bromides, amines, and carbon dioxide. The reaction proceeded in visible light, at ambient carbon dioxide pressure, and without stoichiometric activating reagents. Mechanistic analysis is consistent with a Ni(I-III) cycle, where the active species is generated by the photocatalyst. The rate-limiting steps were the photocatalyst-mediated reduction of Ni(II) to Ni(I) and subsequent oxidative addition of the aryl halide. The physical properties of the photocatalyst were critical for promoting formation of O-aryl carbamates over various byproducts. Nine new phthalonitrile photocatalysts were synthesized, which exhibited properties that were vital to achieve high selectivity and activity.

Poly(heptazine imide) ligand exchange enables remarkable low catalyst loadings in heterogeneous metallaphotocatalysis

The development of heterogeneous metallaphotocatalysis is of great interest for sustainable organic synthesis. The rational design and controllable preparation of well-defined (site-isolated) metal/photo bifunctional solid catalysts to meet such goal remains a critical challenge. Herein, we demonstrate the incorporation of privileged homogeneous bipyridyl-based Ni-catalysts into highly ordered and crystalline potassium poly(heptazine imide) (K-PHI). A variety of PHI-supported cationic bipyridyl-based Ni-catalysts (L_nNi-PHI) have been prepared and fully characterized by various techniques including NMR, ICP-OES, XPS, HAADF-STEM and XAS. The L_nNi-PHI catalysts exhibit exceptional chemical stability and recyclability in diverse C-P, C-S, C-O and C-N cross-coupling reactions. The proximity and cooperativity effects in L_nNi-PHI significantly enhances the photo/Ni dual catalytic activity, thus resulting in low catalyst loadings and high turnover numbers.

Photoinduced Nickel-Catalyzed Carbon-Heteroatom Coupling

Herein, we report visible light-promoted single nickel catalysis for diverse carbon-heteroatom couplings under mild conditions. This mild, general, and robust method to couple diverse nitrogen, oxygen, and sulfur nucleophiles with aryl(heteroaryl)/alkenyl iodides/bromides exhibits a wide functional group tolerance and is applicable to late-stage modification of pharmaceuticals and natural products. On the base of preliminary mechanistic studies, a Ni^I /Ni^III cycle via the generation of active Ni^I complexes that appear from homolysis of Ni^II -I rather than Ni^II -aryl bond was tentatively proposed.

Intraligand Charge Transfer Enables Visible-Light-Mediated Nickel-Catalyzed Cross-Coupling Reactions

We demonstrate that several visible-light-mediated carbon-heteroatom cross-coupling reactions can be carried out using a photoactive Ni^II precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl₂ ). The activation of this precatalyst towards cross-coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metal-to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl₂ with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

Ni(II)-Mediated Photochemical Oxidative Esterification of Aldehydes with Phenols

The photopromoted, Ni-catalyzed acceptorless dehydrogenation esterification of phenols and aromatic aldehydes has been achieved in an oxidant- and external photosensitizer-free manner. This reliable and atom-economical transformation was tolerant to a wide range of functional groups and proceeded efficiently to give various aryl benzoates in moderate to high yields. Additionally, this photocatalytic system displayed high activity for the hydrogen-evolution cross coupling of aliphatic aldehydes and phenols employing dual nickel and aromatic aldehyde catalysis.

Dual Photoredox Ni/Benzophenone Catalysis: A Study of the NiII Precatalyst Photoreduction Step

The combination of Ni^IIX₂ salts with a bipyridine-type ligand and aromatic carbonyl-based chromophores has emerged as a benchmark precatalytic system to efficiently conduct cross-couplings mediated by light. Mechanistic studies have led to two scenarios in which Ni⁰ is proposed as the catalytic species. Nonetheless, in none of these studies has a Ni^II to Ni⁰ photoreduction been evidenced. By exploiting UV-visible, nuclear magnetic resonance, resonance Raman, electron paramagnetic resonance, and dynamic light scattering spectroscopies and also transmission electron microscopy, we report that, when photolyzed by UVA in alcohols, the structurally defined [Ni^II₂(μ-OH₂)(dtbbpy)₂(BPCO₂)₄] complex 1 integrating a benzophenone chromophore is reduced into a diamagnetic Ni^I dimer of the general formula [Ni^I₂(dtbbpy)₂(BPCO₂)₂]. In marked contrast, in THF, photolysis led to the fast formation of Ni⁰, which accumulates in the form of metallic ultrathin Ni nanosheets characterized by a mean size of ∼100 nm and a surface plasmon resonance at 505 nm. Finally, it is shown that 1 combined with UVA irradiation catalyzes cross-couplings, that is, C(sp³)-H arylation of THF and O-arylation of methanol. These results are discussed in light of the mechanisms proposed for these cross-couplings with a focus on the oxidation state of the catalytic species.

An Integrated Carbon Nitride-Nickel Photocatalyst for the Amination of Aryl Halides Using Sodium Azide

The synthesis of primary anilines via sustainable methods remains a challenge in organic synthesis. We report a photocatalytic protocol for the selective synthesis of primary anilines via cross‐coupling of a wide range of aryl/heteroaryl halides with sodium azide using a photocatalyst powder consisting of nickel(II) deposited on mesoporous carbon nitride (Ni‐mpg‐CN_x). This heterogeneous photocatalyst contains a high surface area with a visible light‐absorbing and adaptive “built‐in” solid‐state ligand for the integrated catalytic Ni site. The method displays a high functional group tolerance, requires mild reaction conditions, and benefits from easy recovery and reuse of the photocatalyst powder. Thereby, it overcomes the need of complex ligand scaffolds required in homogeneous catalysis, precious metals and elevated temperatures/pressures in existing protocols of primary anilines synthesis. The reported heterogeneous Ni‐mpg‐CN_x holds potential for applications in the academic and industrial synthesis of anilines and exploration of other photocatalytic transformations.

Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.

Organometallic catalysis under visible light activation: benefits and preliminary rationales

Organometallic catalysis under visible light activation is an emerging field. Activation by photosensitization or by direct light absorption of organometallic complexes can facilitate or trigger elementary steps in a catalytic cycle such as pre-catalyst reduction, oxidative addition, transmetalation and reductive elimination, as well as the ability of generating radical intermediates, widening the structural diversity offered by classical couplings. This perspective aims to highlight key examples of these light-induced or enhanced processes, with an emphasis on the underlying mechanisms involved.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Organic Photocatalyst in the Dual Catalytic Arylation of Alkyl Bis(catecholato)silicates

Mesoporous graphitic carbon nitride (mpg-CN) is introduced as a heterogeneous photocatalyst to perform dual photoredox- and nickel-catalyzed cross-coupling reactions between alkyl bis(catecholato)silicates as radical precursors and aryl or alkenyl bromides. The synergy between this recyclable photocatalyst and the broadly applied homogeneous nickel complex [Ni(dtbbpy)Br₂] gives access to C(sp²)-C(sp³) cross-coupling products in a sustainable fashion. The recycled mpg-CN photocatalyst was analyzed by time-resolved emission spectroscopy and EPR spectroscopy.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Ni Single Atoms on Carbon Nitride for Visible-Light-Promoted Full Heterogeneous Dual Catalysis

Visible-light-driven organic transformations are of great interest in synthesizing valuable fine chemicals under mild conditions. The merger of heterogeneous photocatalysts and transition metal catalysts has recently drawn much attention due to its versatility for organic transformations. However, these semi-heterogenous systems suffered several drawbacks, such as transition metal agglomeration on the heterogeneous surface, hindering further applications. Here, we introduce heterogeneous single Ni atoms supported on carbon nitride (NiSAC/CN) for visible-light-driven C–N functionalization with a broad substrate scope. Compared to a semi-heterogeneous system, high activity and stability were observed due to metal–support interactions. Furthermore, through systematic experimental mechanistic studies, we demonstrate that the stabilized single Ni atoms on CN effectively change their redox states, leading to a complete photoredox cycle for C–N coupling.

In this work, the first demonstration of heterogeneous photoredox C–N coupling is reported using Ni atoms on C₃N₄. Due to metal–support interactions, high activity and stability were observed during visible-light-driven C–N functionalization.

Radical defects modulate the photocatalytic response in 2D-graphitic carbon nitride

Graphitic carbon nitride (gCN) is an important heterogeneous metal-free catalytic material. Thermally induced post-synthetic modifications, such as amorphization and/or reduction, were recently used to enhance the photocatalytic response of these materials for certain classes of organic transformations, with structural defects possibly playing an important role. The knowledge of how these surface modifications modulate the photocatalytic response of gCN is therefore not only interesting from a fundamental point of view, but also necessary for the development and/or tuning of metal-free gCN systems with superior photo-catalytic properties. Herein, employing density functional theory calculations and combining both the periodic and molecular approaches, in conjunction with experimental EPR measurements, we demonstrate that different structural defects on the gCN surface generate distinctive radical defect states localized within the electronic bandgap, with only those correlated with amorphous and reduced gCN structures being photo-active. To this end, we (i) model defective gCN surfaces containing radical defect states; (ii) assess the interactions of these defects with the radical precursors involved in the photo-driven alkylation of electron-rich aromatic compounds (namely perfluoroalkyl iodides); and (iii) describe the photo-chemical processes triggering the initial step of that reaction at the gCN surface. We provide a coherent structure/photo-catalytic property relationship on defective gCN surfaces, elaborating how only specific defect types act as binding sites for the perfluoroalkyl iodide reagent and can favor a photo-induced charge transfer from the gCN surface to the molecule, thus triggering the perfluoroalkylation reaction.

The nature of radical defects governs the photocatalytic activity of graphitic carbon nitride.

Photocatalytic C-H Azolation of Arenes Using Heterogeneous Carbon Nitride in Batch and Flow

The functionalization of aryl C(sp² )-H bonds is a useful strategy for the late-stage modification of biologically active molecules, especially for the regioselective introduction of azole heterocycles to prepare medicinally-relevant compounds. Herein, we describe a practical photocatalytic transformation using a mesoporous carbon nitride (mpg-CN_x ) photocatalyst, which enables the efficient azolation of various arenes through direct oxidation. The method exhibits a broad substrate scope and is amenable to the late-stage functionalization of several pharmaceuticals. Due to the heterogeneous nature and high photocatalytic stability of mpg-CN_x , the catalyst can be easily recovered and reused leading to greener and more sustainable routes, using either batch or flow processing, to prepare these important compounds of interest in pharmaceutical and agrochemical research.

Nickel-Catalyzed C-Heteroatom Cross-Coupling Reactions under Mild Conditions via Facilitated Reductive Elimination

The formation of C-heteroatom bonds represents an important type of bond-forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd- or Cu-catalyzed C-heteroatom cross-couplings under high-temperature conditions, recent advances in homo- and heterogeneous Ni-catalyzed C-heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of Ni^III and excited Ni^II intermediates facilitate the reductive elimination step to achieve mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg-CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Ligand-Metal Charge Transfer Induced via Adjustment of Textural Properties Controls the Performance of Single-Atom Catalysts during Photocatalytic Degradation

Because of their peculiar nitrogen-rich structure, carbon nitrides are convenient polydentate ligands for designing single atom-dispersed photocatalysts. However, the relation between catalysts' textural properties and their photophysical-photocatalytic properties is rarely elaborated. Herein, we report the preparation and characterization of a series of single-atom heterogeneous catalysts featuring highly dispersed Ag and Cu species on mesoporous graphitic C₃N₄. We show that adjustment of materials textural properties and therefore metal single-atom coordination mode enables ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-ligand charge transfer (LMLCT), properties that were long speculated in single-atom catalysis but never observed. We employ the developed materials in the degradation of organic pollutants under irradiation with visible light. Kinetic investigations under flow conditions show that single atoms of Ag and Cu decrease the number of toxic organic fragmentation products while leading to a higher selectivity toward full degradation. The results correlate with the selected mode of charge transfer in the designed photocatalysts and provide a new understanding of how the local environment of a single-atom catalyst affects the surface structure and reactivity. The concepts can be exploited further to rationally design and optimize other single-atom materials.

Heterogeneous photocatalytic cyanomethylarylation of alkenes with acetonitrile: synthesis of diverse nitrogenous heterocyclic compounds

A visible light-mediated heterogeneous photocatalytic cyanomethylarylation of alkenes with acetonitrile has been established using K-modified carbon nitride (CN-K) as a recyclable semiconductor photocatalyst. This protocol, employing readily accessible alkyl N-hydroxyphthalimide (NHPI) ester as a radical initiator, allows the efficient construction of a broad array of structural diverse nitrogenous heterocyclic compounds including indolines, oxindoles, isoquinolinones, and isoquinolinediones.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx ) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Long-Lived Triplet Excited State in a Heterogeneous Modified Carbon Nitride Photocatalyst

Heterogeneous carbon nitrides have numerous advantages as photocatalysts, including strong light absorption, tunable band edges, and scalability, but their performance and continued development are limited by fast charge recombination and an under-developed mechanistic understanding of photodriven interfacial electron transfer. These shortcomings are a result of complex photophysics, leading to rate asynchrony between oxidation and reduction, as well as redox processes driven out of electronic trap states rather than excited states. We show that a well-defined triplet excited state in cyanamide-modified carbon nitride is realized with appropriately sized particles. The utility of this long-lived excited state is demonstrated by its ability to drive a hydroamidation photoredox cycle. By the tuning of the particle size of CN_x, the oxidation-reduction photochemistry of carbon nitride may be balanced to achieve a redox-neutral closed photocatalytic cycle. These results uncover a triplet excited state chemistry for appropriately sized CN_x particles that preludes a rich energy and electron transfer photochemistry for these materials.

Esterification of Carboxylic Acids with Aryl Halides via the Merger of Paired Electrolysis and Nickel Catalysis

Electrochemistry has been successfully applied in metal catalysis to avoid the usage of chemical redox agents. This strategy proved to be a powerful approach to construct carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. However, most of the developed methods are based on either anodic oxidation or cathodic reduction, in which a sacrificial reaction occurs at the counter electrode. Paired electrolysis merging with metal catalysis is underdeveloped, wherein both anodic and cathodic processes are taking place simultaneously. Herein, we demonstrated that by using esterification of carboxylic acids with aryl halides via paired electrolysis using nickel as the catalyst the respective aryl esters were obtained in good to excellent yields at room temperature in an undivided electrochemical cell.

Metal-Free Photocatalysis: Two-Dimensional Nanomaterial Connection toward Advanced Organic Synthesis

Two-dimensional (2D) nanostructures are a frontier in materials chemistry as a result of their extraordinary properties. Metal-free 2D nanomaterials possess extra appeal due to their improved cost-effectiveness and lower toxicity with respect to many inorganic structures. The outstanding electronic characteristics of some metal-free 2D semiconductors have projected them into the world of organic synthesis, where they can function as high-performance photocatalysts to drive the sustainable synthesis of high-value organic molecules. Recent reports on this topic have inspired a stream of research and opened up a theme that we believe will become one of the most dominant trends in the forthcoming years.

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways*

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Visible-light-promoted thiocyanation of sp2 C–H bonds over heterogeneous graphitic carbon nitrides

Mesoporous graphitic carbon nitride (mpg-C₃N₄) is developed as a practical heterogeneous photocatalyst for C–S bond formation.