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Making single-atom nickel sites from MOF/polymer composites

A single-atom catalyst (SAC) is pretty much what it says on the tin; individual metal atoms that act as active sites to speed up a reaction, dispersed on a supporting material. SACs are desirable in heterogeneous catalysis as they make use of every metal atom, leading to greater possible catalytic efficiencies and activities. Metal-organic frameworks (MOFs), porous and crystalline materials that are useful in their own right, have been identified as useful precursors for pyrolysis to form nanostructures and single-atom catalysts. Researchers in Switzerland and China have now demonstrated this strategy for the formation of single-atom nickel species for electrocatalysis, where their MOF-derived material showed excellent activity, efficiency and durability for the electrochemical CO2 reduction reaction (CO2RR).

Whilst MOF-derived nanostructures exist, single-atom sites are typically harder to achieve by pyrolysis. Most organic linkers within MOFs typically contain oxygen coordination sites to bind the metal, but this oxygen content is lost in pyrolysis due to CO2 formation. Without other Lewis-basic coordination sites within the MOF, pyrolysis can often lead to metal aggregation rather than dispersed single-atom sites for catalysis. The researchers overcame this challenge by adding both a polymer and a secondary nitrogen-rich compound to the MOF before pyrolysis, aiding in the formation of dispersed and stable single-atom metal sites.

Scheme showing the preparation of nickel-containing nitrogen-doped carbon catalysts, starting from the MOF on the left, adding the polymer in the middle, followed by pyrolysis to create the material on the right.

Figure 1: The preparation of the nickel-containing nitrogen-doped carbon catalysts

The researchers selected Ni2(NDISA) as the MOF to study, with nitrogen-containing naphthalene diimide salicylic acid (NDISA) linkers. They introduced a polydopamine (PDA) polymer into the porous channels of the MOF structure to create Ni2(NDISA)-PDA, and then further subjected the MOF/polymer composite to melamine as an additional nitrogen source. The researchers then subjected the MOF, the MOF/polymer composite and the MOF/polymer composite with melamine to pyrolysis, followed by etching with acid to remove any unbound nickel particles, to form the nickel-containing nitrogen-doped-carbon catalysts Ni/NC, Ni/NC-D and Ni/N-CNT, respectively (Figure 1). The MOF/polymer composite with melamine went on to form carbon nanotubes (CNTs) after pyrolysis as an effect of decomposition of the melamine.

The researchers used a range of techniques to characterise the materials. They found that the Ni/N-CNT material had the highest nickel loading and therefore the greatest number of dispersed single-atom nickel sites, owing to both the addition of the polymer that prevented aggregation of the metal and the addition of the nitrogen-rich melamine to aid nickel binding to the surface. All three materials were tested for electrochemical CO2 reduction, an important carbon neutral cycle. The three materials all showed selective production of CO and H2, with the Ni/N-CNT material showing the greatest faradaic efficiency and stability owing to the greater amount of nickel active sites. Overall, this simple strategy of combining a MOF precursor with a polymer and a nitrogen-rich source successfully enhanced the performance of the MOF-derived material with single-atom nickel sites, and has future potential in a wider variety of electrochemical applications using a range of MOF and polymer building blocks.

 

To find out more, please read:

A metal–organic framework/polymer derived catalyst containing single-atom nickel species for electrocatalysis

Shuliang Yang, Jie Zhang, Li Peng, Mehrdad Asgari, Dragos Stoian, Ilia Kochetygov, Wen Luo, Emad Oveisi, Olga Trukhina, Adam H. Clark, Daniel T. Sun and Wendy L. Queen

Chem. Sci., 2020, 11, 10991-10997

 

About the blogger:

Photograph of the author, Samantha AppsDr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Ferrying electrons with ferrocene to enhance nickel electrochemistry

Redox-active transition metal complexes, those that can undergo multiple oxidation and reduction events, are ideal candidates for electrochemical energy storage and fuel technologies. A significant caveat to employing these complexes for electrochemical processes is that their solubilities can drastically change across redox states, creating insoluble oxidation or reduction products that precipitate out of solution. Even when the insoluble redox product is still chemically-intact (as in a reversible electrochemical reaction), it can often be difficult to electrochemically convert it back to its original, soluble redox state. Researchers in the US have now come up with a new technique to overcome this, using ferrocene as a redox mediator to assist with electron-transfer between the insoluble materials and electrodes.

The researchers studied the redox-active nickel complex, [Ni(PPh2NPh2)2(CH3CN)]2+, which is often used as a catalyst for electrochemical hydrogen evolution. The electrochemistry of the nickel complex was explored under non-catalytic conditions, where the absence of a proton source was previously unexplored. Cyclic voltammetry experiments of [Ni(PPh2NPh2)2]2+ in acetonitrile indicated two, one-electron reduction events that correspond to the NiII/Iand NiI/0 redox couples, both of which were electrochemically and chemically reversible at low concentrations (Figure 1). The researchers noted a concentration dependence, where the reversibility is increasingly lost at higher concentrations of the complex (blue scan, Figure 1A). This was attributed to the formation of the two-electron reduced product, [Ni(PPh2NPh2)2], which proved insoluble in acetonitrile, precipitating out of solution upon its electrochemical formation and depositing on the electrode surface.

Cyclic voltammograms of [Ni(PPh2NPh2)2]2+ in acetonitrile

Figure 1. Cyclic voltammograms of [Ni(PPh2NPh2)2]2+, showing the two redox events with the two, separate peaks. A) Concentration dependence, whereby reversibility decreases upon increasing concentration and B) Scan-rate dependence, whereby reversibility is regained at higher scan-rates.

Once the researchers established the electrochemically-driven solubility changes for the nickel complex, they looked at enhancing the overall reversibility of this reaction. Whilst the two-electron reduction to form insoluble [Ni(PPh2NPh2)2] proceeded smoothly, regenerating [Ni(PPh2NPh2)2]2+ by oxidation of this insoluble product was slow and inefficient, due to poor electron transfer between the deposited material and the electrode. The researchers therefore added ferrocene as a freely diffusing redox mediator to the electrochemical reaction, to essentially shuttle electrons from the insoluble reduction product to the electrode. This proved successful, with subsequent electrochemical experiments of [Ni(PPh2NPh2)2]2+ in the presence of ferrocene showing faster and catalytic regeneration of the original nickel complex.

Redox cycle scheme for [Ni(PPh2NPh2)2]2+

Figure 2. A scheme showing the redox cycle of [Ni(PPh2NPh2)2]2+, with annotations to describe the experimental kinetics observed.

In addition to the experimental studies, the researchers also turned to mathematical modelling to gain more understanding of electrochemically-driven solubility cycling in electrochemical reactions. Two models were presented showing the effect of the deposited materials on the electrochemical response, either with or without possible electrode inhibition effects. Overall, the researchers have presented a unique strategy for improving the reversibility of redox reactions that are limited by insoluble redox products, which is beneficial for systems where both materials deposit on electrodes or are suspended in solution.

 

To find out more, please read:

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

Katherine J. Lee, Kunal M. Lodaya, Cole T. Gruninger, Eric S. Rountree and Jillian L. Dempsey

Chem. Sci., 2020, 11, 9836-9851

 

About the blogger:

Dr. Samantha Apps just finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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A radical twist to halogenations using boron tribromide

Organoboranes are extremely useful reagents for chemical synthesis; their Lewis acidic nature makes them reactive towards nucleophilic species, and their ability to participate in free-radical processes widely expands their synthetic use. Trialkylboranes (BR3) are the most widely studied in terms of borane radical chemistry, whereby alkyl radicals (R) can be generated through homolytic substitution at the boron atom under oxygen conditions to then participate in various alkylation reactions, as shown in Scheme 1a. This extremely mild radical-initiation system, using just O2 instead of heat or light for radical generation, is highly desirable in chemical synthesis, particularly for the formation of thermally unstable products.

Scheme showing radical generation from organoboranes

Scheme 1: (A) Previously known radical chemistry with organoboranes and (B) radical reactivity using trihaloboranes.

Researchers in both China and the US have now applied this concept of radical generation using trihaloboranes for halogenation. Halogenation reactions are extremely important in chemical synthesis, since the resulting halogenated products are ideal precursors for installing a wide range of functional groups through substitution chemistry. Typically, halogenation of organic molecules using trihaloboranes has been attributed to their Lewis acidic nature, but the researchers have now shown that these reagents can also act as halogen radical donors (as shown in Scheme 1b).

The researchers selected boron tribromide (BBr3) as a bromide radical donor (Br), since the B-O bond that forms upon radical generation using O2 is much stronger than the B-Br bond that breaks, making the process thermodynamically favourable. They applied this approach to investigate the hydrobromination of cyclopropanes, for the novel and selective formation of the anti-Markovnikov haloalkane product. The researchers initially optimised the reaction of cyclopropylbenzene (1a) with BBr3/O2 and found that the addition of a proton source (e.g. H2O or alcohol) was sufficient to terminate the radical reaction and give the anti-Markovnikov product (2a) as the major species (Scheme 2). Using these conditions, the substrate scope could be expanded for the hydrobromination of a wide range of cyclopropanes, including typically challenging alcohol or amine-functionalised substrates.

Reaction optimisation scheme and table for hydrobromination of cyclopropane with BBr3

Scheme 2: Initial reaction optimisation of hydrobromination of cyclopropylbenzene (1a) to give the anti-Markovnikov product (2a) as the major species.

To establish that the hydrobromination reactivity was occurring via a radical process rather than a possible acid-mediated pathway, the researchers conducted a series of control experiments. The addition of radical scavengers resulted in only the formation of the Markovnikov product, suggesting the radical process is necessary for the anti-Markovnikov selectivity observed. The absence of oxygen also shut down the reactivity, which further indicates the radical pathway as shown in Scheme 1b. Additional computations modelled a possible pathway analogous to the established radical alkylation using trialkylboranes, showing an energetically favourable radical pathway for the hydrobromination of cyclopropylbenzene using BBr3/O2 (Figure 1).

Energy profile diagram for the radical hydrobromination of cyclopropanes

Figure 1: The calculated energy profile for the hydrobromination reaction

The results in this study demonstrate that trihaloboranes, like trialkylboranes, can act as radical donors for halogenation reactions, allowing for previously unreported anti-Markovnikov selectivity in the hydrobromination of cyclopropanes. This radical reactivity could be applied in the future for the halogenation of many different organic molecules, giving way to new methods to affect selectivity that cannot be achieved using traditional acid-mediated pathways.

 

To find out more, please read:

Boron tribromide as a reagent for anti-Markovnikov addition of HBr to cyclopropanes

Matthew H. Gieuw, Shuming Chen, Zhihai Ke, K. N. Houk and Ying-Yeung Yeung

Chem. Sci., 2020, 11, 9426-9433

 

About the blogger:

Dr. Samantha Apps just finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Aggregation makes fluorescent probes better and brighter

Fluorescence, the phenomenon where a molecule re-emits light upon absorption of electromagnetic radiation, is used in biological imaging to visualise structures, processes and diseases. Emission of these fluorescent molecules, known as fluorophores, in the near-infrared region is particularly advantageous, allowing for enhanced tissue penetration and reduced photodamage. Near-infrared (NIR) fluorophores are therefore attractive probes for bioimaging but are currently limited with problems such as low brightness or quenching of the emission by aggregation.

To overcome this aggregation-caused quenching effect, researchers in China turned to fluorophores that have aggregation-induced emission (AIE) properties. Aggregation-induced emission (AIE) is a concept where molecules only fluoresce upon aggregation in concentrated solutions, and not in dilute solutions where they can freely rotate. The researchers therefore designed their fluorophore to contain the molecular rotor tetraphenylethene, that can induce AIE effects and therefore boost and brighten the fluorescence.

The researchers prepared a suite of fluorophores using a central donor-acceptor-donor core, with methoxy-tetraphenylethene (MTPE) as the donor and thieno[3,4,-b]pyrazine (TP) as the acceptor. Substituents on the TP acceptor were varied, and the effects on aggregation and the fluorescence were investigated. Density functional theory calculations gave the researchers insight into the molecular conformations of the fluorophores, as shown in Figure 1. The 3 variants all showed twisted geometries (top row, Figure 1), indicating high degrees of rotation, which could then be restricted through aggregation and give rise to the desired AIE effects. Additionally, the calculations measured electronic distributions, confirming high degrees of electron conjugation in the molecules (see the HOMO diagrams, Figure 1) that are essential for fluorescence.

DFT results of AIE fluorophores

Figure 1: Results from density functional theory calculations to show molecular geometries and electron conjugation within the suite of fluorophores

The fluorescence characteristics of the variants were measured by absorption and emission/photoluminescence spectra. The absorption spectra in DMSO (Figure 2a) shows absorptions between 518 and 543 nm, with the most red-shifted (longer wavelength) absorption displayed for the most conjugated variant (MTPE-TP3). The effect of aggregation on the fluorescence was measured by adding water (in which the fluorophores showed poor solubility) to the DMSO solutions, and the resulting photoluminescence intensities showed an increase with higher water fractions. This increase in brightness (i.e. intensity) is explained by the water affecting aggregation of the fluorophores and inducing the AIE effect (Figures 2b and c).

Fluorescence spectra and aggregation effects of AIE fluorophores

Figure 2: a) Absorption spectra of the fluorophore variants; b) photoluminescence spectra of the most conjugated variant, MTPE-TP3 with different water fractions; c) corresponding photoluminescence intensity plotted against water fractions for all three variants. d) to f) additionally indicate the effect of increased viscosity (and aggregation) upon glycerol addition to the fluorophores.

The researchers also formulated nanoparticles for each fluorophore variant to allow for better water solubility and therefore biocompatibility. They found that the absorption and emission of the nanoparticles became both brighter and more red-shifted and were now within the near-infrared range for favourable biological imaging. In vitro and in vivo testing of these nanoparticles in breast cancer cells and tumour-bearing mice verified that the AIE-nanoparticles are suitable for biological imaging, and indicate their potential to assist with tumour diagnosis in future clinical settings.

 

To find out more, please read:

Simultaneously boosting the conjugation, brightness and solubility of organic fluorophores by using AIEgens

Ji Qi, Xingchen Duan, Yuanjing Cai, Shaorui Jia, Chao Chen, Zheng Zhao, Ying Li, Hui-Qing Peng, Ryan T. K. Kwok, Jacky W. Y. Lam, Dan Ding  and  Ben Zhong Tang

Chem. Sci., 2020, 11, 8438-8447

 

About the blogger:

Dr. Samantha Apps is a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

 

 

 

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Biradical bismuth makes its debut

Low-valent compounds are attractive in chemical synthesis and catalysis due to their highly reactive nature. Carbenes are the archetypal example, where the carbon atom is divalent with two valence electrons, but group 15 analogues have also gained recent interest as reactive intermediates in fundamental transformations. These low-valent compounds (E-R), where the group 15 atom (E) has an oxidation state of +1 and is bound to just one additional atom, are extremely reactive and therefore challenging to isolate. The lighter congeners of nitrogen and phosphorus (as nitrenes, N-R, and phosphinidenes, P-R) have been isolated, but the heavier homologues are much more difficult to access and tend to undergo degradation. Stabilisation through adduct formation with Lewis bases had previously allowed for the formation of the heaviest group 15 bismuth homologue, and these stabilised bismuthinidenes showed potential in electrocatalytic and photophysical applications. Researchers in Germany and Switzerland have now reported for the first time the generation of a free and non-stabilised organometallic bismuthinidene compound, methylbismuth (BiMe), in the gas phase (Figure 1).

Low valent group 15 structures

Figure 1: Structures and examples of low-valent group 15 compounds, with their electronic ground state configuration

The researchers targeted the non-stabilised organometallic bismuthinidene using a top-down approach, by breaking the Bi-C bonds of the higher valent and well-defined BiMe3 precursor (Scheme 1). They achieved this by pyrolysis of BiMe3, with subsequent analysis by photoelectron-photoion coincidence spectroscopy (PEPICO), that allows the recording of photoionisation mass spectra to detect ions produced by the pyrolysis. As shown by the photoionisation mass spectra in Figure 2, pyrolysis resulted in methyl loss through Bi-C homolytic cleavage, with higher pyrolysis power (bottom trace) showing full conversion of BiMe3 with by of m/z = 254. Stepwise methyl loss down to atomic bismuth was observed with m/z = 209 for Bi+, but notably BiMe+ was observed at m/z = 224, indicating bismuthinidene formation.

BiMe generation

Scheme 1: Stepwise methyl abstraction from BiMe3 to generate bismuthinidene BiMe in the  gas phase by flash pyrolysis

Photoionisation mass spectra for BiMe

Figure 2: Photoionisation mass spectra showing methyl loss in the conversion of BiMe3 to BiMe by pyrolysis. Top trace = without pyrolysis, middle trace = low pyrolysis power, bottom trace = high pyrolysis power

The researchers further probed the electronic nature of the generated bismuthinidene by additional photoelectron spectroscopy and simulations. An ionisation energy of 7.88 eV was determined, and indicated the triplet (biradical) ground state (structure 3 in Scheme 1) as the lowest energy structure. This contrasts to the lighter N and P congeners, where the methylene species are the most energetically favoured (like 5 in Scheme 1). The researchers also conducted experiments to investigate the stepwise methyl abstraction via BiMe2, determining a bond dissociation energy of 210 kJ mol-1 for the first Bi-C homolytic cleavage and demonstrating that this methyl abstraction could also be achieved under moderate reaction conditions. Overall, this report indicates that non-stabilised bismuthinidenes can be generated, with the potential for future exploitation as reactive intermediates in synthetic chemistry.

 

To find out more, please read:

Methylbismuth: an organometallic bismuthinidene biradical

Deb Pratim Mukhopadhyay, Domenik Schleier, Sara Wirsing, Jacqueline Ramler, Dustin Kaiser, Engelbert Reusch, Patrick Hemberger, Tobias Preitschopf, Ivo Krummenacher, Bernd Engels,* Ingo Fischer* and Crispin Lichtenberg*

Chem. Sci., 2020, Advance Article

 

About the blogger:

Dr. Samantha Apps is a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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Porous supports for hemes to mimic enzymatic transformations

Nature is the ultimate molecular designer. The complex structures of proteins are crucial for allowing the many processes that support life, including the many chemical transformations that occur. A diverse array of biological reactions are catalysed by iron porphyrin active sites (also known as hemes), and rely on the local protein environments to envelop and stabilise the reactive intermediates that can form in the process. Whilst chemists can easily synthesise iron porphyrins to imitate the reactive centre, mimicking the surrounding protein superstructure is less trivial.

Metal-organic frameworks (MOFs) represent one strategy (as an alternative to proteins) to support iron porphyrins for chemical catalysis. These frameworks can precisely separate each heme unit, thereby sequestering each active site in a similar fashion to a protein, and the porous nature allows for diffusion of reagents into the catalyst. Importantly, the pore environment can be precisely controlled and modified, allowing for enhancements to the catalytic activity of the supported heme.

Researchers in the US have now reported a new method to modify a heme-containing metal-organic framework to enhance the catalytic activity towards C-H bond activation. They studied the porphyrinic Zr-based framework, PCN-224, where the porphyrin is suspended between Zr6 nodes (Figure 1).  Exchange of the formate and benzoate ligands around the Zr6 node in PCN-224 was achieved by initial treatment with acetic anhydride to give acetate ligands in the new material (PCN-224’, 1), and further reactivity with methanol resulted in Zr-hydroxy ligands (2) – see insert to Figure 1. Additionally, iron was installed into the modified framework 1 by reaction with FeCl3 and base, to give 1FeCl. Here, an FeCl was installed in each porphyrin unit in the MOF, and further hydroxylation reactivity (similar to the 1 to 2 transformation) resulted in the formation of 2FeCl.

PCN-224 framework

Figure 1. The structure of the PCN-224 framework. Insert (below) shows modifications to PCN-224 to give 1 and 2, with varying ligands around the Zr6 node (in green).

The modified PCN-224 frameworks were characterised by various techniques. Powder X-ray diffraction showed the retention of bulk crystallinity of the material upon ligand substitution. UV-Vis and 57Fe Mössbauer spectroscopy confirmed iron coordination within the porphyrin units of the framework. Importantly, the modification of the Zr6 ligands was structrually confirmed by single-crystal X-ray analysis, and DRIFTS spectra showed the expected O-H stretches for the hydroxy ligands in 2 and 2FeCl. The porosity of the framework was maintained upon the modifications, as shown by surface area measurements, making the new materials ideal candidates for catalytic testing.

PCN-224 catalytic activity

Figure 2. A chart showing the catalytic activity of the heme-frameworks (1FeCl and 2FeCl) compared to the molecular iron porphyrin complex ((TPP)FeCl) for cyclohexane oxidation.

The researchers studied the effects of the framework modifications using the catalytic oxidation of cyclohexane as a model reaction. The researchers compared the oxidation of cyclohexane with iodosylbenzene in CH2Cl2 using either the molecular iron porphyrin complex, or the metallated porphyrin frameworks 1FeCl or 2FeCl. Whilst low yields of oxidation products (cyclohexanol, cyclohexanone and chlorocyclohexane) were observed for the molecular iron porphyrin complex, higher yields of 68% and 26% were noted for the frameworks 1FeCl and 2FeCl, with corresponding turnovers of 14 and 5, respectively (Figure 2). The higher catalytic activity for the acetylated framework (1FeCl) was attributed to the lack of acidic protons within the framework that would impair any oxidation reactivity at the heme centre. Ultimately, the results show an enhancement to the catalysis when the heme is supported and protected, demonstrating that MOFs are ideal supports for modelling enzymatic reactions.

To find out more, please read:

Enhancing catalytic alkane hydroxylation by tuning the outer coordination sphere in a heme-containing metal–organic framework

David Z. Zee and T. David Harris

Chem. Sci., 2020, 11, 5447-5452

 

About the blogger:

Dr. Samantha Apps is a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

 

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Capturing nitrenes with iron and light for C-N bond formation

Finding new ways to make chemical bonds is not only a fascination for chemists, but also important for developing greener routes in chemical synthesis. Carbon-nitrogen bonds are one such motif of interest; they are ubiquitous in chemical synthesis as part of amine functional groups, which are present in biologically relevant molecules and pharmaceutical compounds. A powerful method for the formation of C-N bonds involves using nitrenes (R-N:) as the nitrogen source. The use of azides (RN3) to generate the nitrene is particularly attractive, as the only by-product is nitrogen gas, and no additional oxidants are required.

Typical drawbacks associated with the use of nitrenes in amination reactions, i.e. forming C-N bonds for amine functionalities, include the necessity for elevated temperatures (to liberate nitrogen gas from the azide to form the nitrene) and competitive side reactivity (since nitrenes are highly reactive intermediates). Research by Che and co-workers in Hong Kong and China now describes a way to circumvent these disadvantages, using visible light in combination with an iron porphyrin complex to catalyse C-N bond formation by either C-H bond amination or alkene aziridination using organic azide substrates.

Iron porphyrin nitrene capture from organic azide

Figure 1: Trapping of the nitrene (NR) generated from an azide (N3R) by the iron porphyrin (bottom), compared to previous work (top) where a free, reactive nitrene is generated.

The iron porphyrin complex, as described by the researchers, has a dual role in the light-driven C-N bond formation reactivity. Firstly, the iron porphyrin acts as a photosensitiser in this reaction, assisting with the nitrene formation from the organic azide by irradiation. More importantly, the porphyrin complex then acts as a trap for the reactive nitrene and forms a resulting metal-nitrene (or imido) intermediate that can then react with carbon-based substrates for C-N bond formation. Capturing the nitrene at a metal centre, as opposed to generating a free nitrene for subsequent reactivity (as shown in Figure 1), allows for greater selectivity, which is reflected in the extensive substrate scope described by Che and co-workers.

The researchers started their investigation by screening a panel of iron porphyrins to serve as a catalyst in the light-driven C-H amination of indane (as a hydrocarbon substrate) with an electron-deficient aryl azide (Scheme 1). They observed the successful conversion to the C-H amination product, 1-aminoindane, in various yields, where the Fe(TF4DMAP)Cl porphyrin complex (as shown in Scheme 1) was the most effective catalyst, resulting in a 99% yield after 24 hours. Control studies confirmed both light irradiation and the iron porphyrin were required for conversion, and further mechanistic experiments supported the formation of an iron-nitrene intermediate, that could subsequently react with a C-H bond (in this case, within indane) via H-atom abstraction for amination. Translating the same reaction conditions to sp2 carbon substrates, by using styrene instead of indane, resulted in olefin aziridination, showing the applicability of this method to other substrates.

Fe-porphyrin catalysed C-H amination of indane scheme

Scheme 1: C-H amination of indane (1a) with the electron-deficient aryl azide (2a) to form 1-aminoindane (3a), using the optimal iron porphyrin catalyst Fe(TF4DMAP)Cl

After determining the optimal conditions in the above example reactions, Che and co-workers demonstrated an impressive substrate scope for this reactivity, varying either the hydrocarbon substrate or the organic azide for C-H amination. Additionally, they also demonstrated this reactivity could be applied to intramolecular C-H amination, for the formation of imidazolidines or α-azidoketones. Ultimately, this reactivity could be translated to natural product synthesis, and preliminary results in this report showed that late-stage C-H amination using azides could be achieved in complex substrates.

This work elegantly demonstrates a new method for C-N bond formation using organic azides and hydrocarbon or olefin substrates, as the first example of a light driven, iron-porphyrin catalysed, C-H amination or alkene aziridination reaction.

To find out more, please read:

Iron porphyrin catalysed light driven C–H bond amination and alkene aziridination with organic azides

Yi-Dan Du, Cong-Ying Zhou, Wai-Pong To, Hai-Xu Wang and Chi-Ming Che

Chem. Sci., 2020,11, 4680-4686

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