Chemical Science Blog

The mighty gear is essential in machines. Even when scaling down the size of the machine, like from cars to small wristwatches, gears are necessary to transmit motion and mechanical power across the system. Machines can be decreased in size all the way to the nanoscale with molecular machines, where individual molecules can produce mechanical motion in response to external stimuli. Just as in macroscopic machines (e.g. cars), the addition of gears to nanomachines is needed for creating more complex assemblies with controlled motion, extending the applications of these molecules beyond the fundamental.

A team of researchers from France and Japan have now reported a series of molecular gears, with the aim of achieving correlated motion within trains of gears across a surface (Figure 1). To achieve this correlated motion, the researchers designed desymmetrised organometallic molecular gears based around star-shaped ruthenium piano-stool complexes. These molecular gears incorporated a facially capping hydrotris(indazolyl)borate ligand at one end, which both anchors the complex to the surface and lifts the central metal centre away to enable a rotational axis around the ruthenium. At the other end, the molecular gears have a cyclopentadienyl core to act as the cogwheel, functionalised with bulky groups that mimic the teeth that allow correlated motion between the gears (Figure 2). The researchers set out to make these molecular gears with lower symmetry to allow for detailed on-surface mechanical studies, by changing one of the five teeth (i.e. the functionalised groups around the cyclopentadienyl core) to include a steric or chemical tag– this is shown in Figures 1 and 2 by the green section.

The researchers developed a modular synthetic approach to achieve desymmetrisation of the star-shaped ruthenium molecular gears, based on post-functionalisation of the central cyclopentadienyl core with Ni(II) porphyrins to act as the teeth of the cogwheels. They used an unsymmetrical 1,2,3,4,5-penta(p-halogenophenyl)cyclopentadienyl as the core; the p-halogenophenyl groups are all pre-activated to allow for further functionalisation, but one of the five is a p-iodophenyl group that chemoselectively reacts over the other four p-bromophenyl groups. Scheme 1 shows a sequential synthetic route towards one of the desymmetrised molecular gears: the p-iodophenyl group is first functionalised with a unique porphyrin (shown in green), before subsequent functionalisation of the four other p-bromophenyl groups with the same porphyrins (shown in blue), all using palladium-catalysed cross-coupling reactions.

The researchers varied their approach to changing the unique porphyrinic tooth for the molecular gear, using either steric tagging (with one longer linker between the porphyrin and p-halogenophenyl group) or chemical tagging, using either one distinct electron-deficient porphyrin (achieved by using p-cyanophenyl substituents on the tetrapyrrole core) or one distinct metal porphyrin (Zn(II) instead of Ni(II)). The synthesised desymmetrised molecular gears were characterised using spectroscopic and electrochemical techniques, and the researchers are currently undertaking further mechanical studies to understand the correlated motion of these gears on surfaces.

 

To find out more, please read:

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors

Seifallah Abid, Yohan Gisbert, Mitsuru Kojima, Nathalie Saffon-Merceron, Jérôme Cuny, Claire Kammerer* and  Gwénaël Rapenne*

Chem. Sci., 2021, Advance Article

 

About the blogger:

Dr. 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.

Methylene (-CH₂) is one of the simplest and most important building blocks for chemical synthesis. Methylenation reactions add methylene groups to molecules and often proceed using transition metal methylene complexes. Titanium methylene complexes are excellent for methylenations and have been used in a variety of reactions such as olefin metathesis, polymerisations or olefination of carbonyls. Early examples of such titanium methylenation reagents include Tebbe’s reagent that can generate a terminally bound mononuclear titanium methylidene, Cp₂Ti=CH₂ (Figure 1a), or a methylenation reagent prepared from CH₂Br₂, Zn and TiCl₄ (with catalytic lead), referred to as ‘CH₂X₂-Zn(Pb)-TiCl₄’.

Researchers from Japan have been interested in the ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenation reagent and in particular, deducing the molecular structure of the reactive species. Earlier studies have revealed two key steps in the preparation of this methylenating reagent: the first is that a zinc methylene species, ‘CH₂(ZnX₂)’, is formed by the reaction of CH₂X₂ with Zn and catalytic lead (Figure 1b), and the second is that the Ti(IV) chloride reagent is reduced to Ti(III) chloride by Zn(0) simultaneously (Figure 1c). The researchers hypothesised that a reactive titanium methylidene species (similar to that generated from Tebbe’s reagent in Figure 1a) should form via a transmetallation event between the zinc methylene species and the Ti(III) chloride, and thus be the reactive methylenating species of the ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenation reagent.

To confirm their hypothesis, the researchers studied the reactivity of multiple combinations of a zinc methylene species (1) and titanium(III or IV) chloride reagents, with and without additional ligands (such as phosphines, amines or ethers). The researchers found that most combinations of reagents resulted in methylene loss via the generation of methane or ethylene, but the combination of TMEDA adducts of the zinc methylene (1a) and Ti(III) chloride (2) gave clean conversion to a new titanium methylene species 3 (Scheme 1). Although the researchers originally hypothesised the formation of a mononuclear titanium methylidene via methylene transmetallation from zinc to titanium, the new species 3 was revealed to be a dinuclear, bridging methylene complex. The dinuclear species was characterised using NMR spectroscopy and single-crystal X-ray diffraction techniques, and the connectivity of the bridging methylene was conclusively established by the X-ray crystal structure.

After elucidating the structure of the dinuclear titanium methylene complex, the researchers tested 3 as a methylenating reagent and observed successful methylene transfer reactions from 3 to esters, terminal olefins and 1,3-dienes. A further computational mechanistic study for the reactivity of 3 and a 1,3-diene was performed, where the DFT calculations indicated a mononuclear titanium methylidene as the reactive species, generated from the dinuclear titanium methylene complex. These calculations corroborate the researchers’ initial hypothesis and correlate with Tebbe’s reagent, where the reactive methylenating agent is also a mononuclear titanium methylidene that is generated from a dinuclear bridging methylene complex.

 

To find out more, please read:

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(III) methylene complex

Takashi Kurogi,* Kaito Kuroki, Shunsuke Moritani and Kazuhiko Takai*

Chem. Sci., 2021, Advance Article

 

About the blogger:

Dr. 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.

Activating a bond is the first step towards bond breaking processes for synthesis and catalysis. Despite the major role of transition metals in a variety of bond activation processes, C–X bond activation of halogenated benzenes (PhX, X = F, Cl, Br, I) is still challenging; there are very few examples of metal···XPh complexes, even though they are crucial for C–X bond activation and catalysis. There are even fewer examples of main group metal···XPh complexes, with no examples of main group complexes of bromobenzene or iodobenzene.

Researchers in Germany have been studying cationic magnesium complexes with the β-diketiminate ligand (^RBDI, R = methyl or t-butyl), where their extreme Lewis acidity makes them ideal candidates for halobenzene complex formation for C–X activation. Building upon their previous findings of the formation of a Mg-chlorobenzene complex, the researchers have now demonstrated the preparation of the full series of halobenzene complexes, including the first examples of coordination of bromobenzene and iodobenzene to a main group metal, as shown in Scheme 1.

The researchers found that both the smaller methyl-substituted complex, (^MeBDI)Mg⁺, and the bulkier t-butyl substituted complex, (^tBuBDI)Mg⁺, were able to bind fluorobenzene to form Mg···FPh complexes (1–3 in, Scheme 1), owing to the high polarity of PhF that can compete with the Mg···B(C₆F₅)₄^–  (the magnesium–anion) interaction. The other halobenzene complexes (Mg···XPh for X = Cl, Br, I; VI, 4, 5) could only be accessed with the use of the bulkier ^tBuBDI ligand. This was attributed to the fact that the bulky t-butyl substituents essentially turn off the Mg···B(C₆F₅)₄^– interaction, which in turn allows the less polar PhX halobenzenes to bind to the magnesium centre.

The researchers isolated and fully characterised the Mg-halobenzene complexes, and used X-ray crystallography and DFT calculations to further understand their properties. The interaction of the strongly Lewis acidic (BDI)Mg⁺ cation with the halobenzene resulted in C–X activation as shown by elongation of the C–X bonds in the crystal structures. Additionally, the solid-state structures showed that the Mg···X–Ph angle is the most linear for PhF and decreases in size (i.e. bends more) for the larger halogens. This increased bending for the larger halogens is explained by the halogen σ-hole, which is a region of positive electrostatic potential on the surface of the halogen opposite to the C–X bond, that increases with halogen size. As shown by the schematic in Figure 1, the presence of a larger halogen hole forces a more acute Mg···X interaction relative to the C–X bond.

DFT calculations were also performed and were in good agreement with the solid-state experimental parameters. The researchers calculated complexation enthalpies between 11 and 13 kcal mol^-1, which are weak but still indicate a Mg···X–Ph interaction. This interaction ultimately indicates C–X bond activation, signifying that these main group complexes show potential for C–X bond breaking processes in future catalytic applications.

 

To find out more, please read:

Magnesium–halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex

Alexander Friedrich, Jürgen Pahl, Jonathan Eyselein, Jens Langer, Nico van Eikema Hommes, Andreas Görling and Sjoerd Harder*

Chem. Sci., 2021, Advance Article

 

About the blogger:

Dr. 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.

Hydrogen bonding is all around us. The intermolecular force of attraction between a hydrogen atom bound to an electronegative centre (the hydrogen bond donor, HBD) and another nearby electronegative atom with a lone pair of electrons (the hydrogen bond acceptor, HBA) is present in many chemical structures and can be seen in many biological motifs such as in enzymes or proteins. Beyond a simple HBD-HBA pair (Figure 1a), hydrogen bonding can cascade between multiple HBD-HBA pairs (Figure 1b). In these paired units, the presence of a central proton mediator (e.g. imidazole) can induce polarisation to make the resulting hydrogen bonds stronger, promoting further reactivity and selectivity (Figure 1c).

Some enzymes cleverly make use of cascade hydrogen bonding to control the strength of the hydrogen bonds that form between the limited number of available amino acids. One example is a class of enzymes with a ‘catalytic triad’, whereby a hydrogen bonding array exists between the hydroxyl group of serine, the imidazole group of histidine and the carboxylate group of aspartate residues (Figure 2a). Researchers from South Korea took inspiration from such catalytic triads to create a ‘synthetic triad’ with a biomimetic hydrogen bonding network (Figure 2b, compound 1). The researchers employed a central benzimidazole to their synthetic triad to act as a platform to align the HBD-HBA pairs, instead of the precise three-dimensional structure that would anchor these pairs in enzymes.

The researchers envisioned that their biomimetic small molecule could be used as a fluorescent probe owing to the photophysical properties of the chosen benzimidazole motif. They designed the probe for cyanide detection, where capture of a toxic cyanide ion turns on fluorescence in the probe (Figure 3c). The design of the probe was therefore influenced with the target application in mind, so the researchers systematically added each HBD-HBA pair around the benzimidazole, as shown in Figure 3b.

An aldehyde functional group was selected for the first HBD-HBA pair, due to its ability to form hydrogen bonds that can quench the fluorescence in the absence of cyanide (compound 2). The researchers tested compound 2 for cyanide detection and indeed observed fluorescence, but found that the fluorescence also occurred in the presence of a simple Brønsted base. The design of the probe was then iteratively modified until fluorescence was selective for cyanide addition, with a total of four HBD-HBA pairs around the benzimidazole centre that mutually reinforced one another. This strategy shows promise for the design of other fluorescent probes and could also be utilised for other biological targeting applications.

 

To find out more, please read:

Biomimetic hydrogen-bonding cascade for chemical activation: telling a nucleophile from a base

Hyunchang Park and Dongwhan Lee*

Chem. Sci., 2021, Advance Article

 

About the blogger:

Dr. 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.

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.

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:

Dr. 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.

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(P^Ph₂N^Ph₂)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(P^Ph₂N^Ph₂)₂]²⁺ in acetonitrile indicated two, one-electron reduction events that correspond to the Ni^II/Iand Ni^I/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(P^Ph₂N^Ph₂)₂], which proved insoluble in acetonitrile, precipitating out of solution upon its electrochemical formation and depositing on the electrode surface.

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(P^Ph₂N^Ph₂)₂] proceeded smoothly, regenerating [Ni(P^Ph₂N^Ph₂)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(P^Ph₂N^Ph₂)₂]²⁺ in the presence of ferrocene showing faster and catalytic regeneration of the original nickel complex.

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.

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.

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.

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).

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.

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.

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).

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.

 

 

 

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).

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.

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.

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.

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.

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.