Chemical Science Blog

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.

We are pleased to share a selection of our referee-recommended HOT articles for December. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

Efficacy analysis of compartmentalization for ambient CH₄ activation mediated by a Rh^II metalloradical in a nanowire array electrode
Benjamin S. Natinsky, Brandon J. Jolly, David M. Dumas and Chong Liu
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC05700B, Edge Article

Supertetrahedral polyanionic network in the first lithium phosphidoindate Li₃InP₂ – structural similarity to Li₂SiP₂ and Li₂GeP₂ and dissimilarity to Li₃AlP₂ and Li₃GaP₂
Tassilo M. F. Restle, Volker L. Deringer, Jan Meyer, Gabriele Raudaschl-Sieber and Thomas F. Fässler
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC05851C, Edge Article

One class classification as a practical approach for accelerating π–π co-crystal discovery
Aikaterini Vriza, Angelos B. Canaj, Rebecca Vismara, Laurence J. Kershaw Cook, Troy D. Manning, Michael W. Gaultois, Peter A. Wood, Vitaliy Kurlin, Neil Berry, Matthew S. Dyer and Matthew J. Rosseinsky
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC04263C, Edge Article

Fast reversible isomerization of merocyanine as a tool to quantify stress history in elastomers
Yinjun Chen, C. Joshua Yeh, Qiang Guo, Yuan Qi, Rong Long and Costantino Creton
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC06157C, Edge Article

Peptide sequence mediated self-assembly of molybdenum blue nanowheel superstructures
Shan She, Weimin Xuan, Nicola L. Bell, Robert Pow, Eduard Garrido Ribo, Zoe Sinclair, De-Liang Long and Leroy Cronin
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC06098D, Edge Article

Enhanced voltammetric anion sensing at halogen and hydrogen bonding ferrocenyl SAMs
Robert Hein, Xiaoxiong Li, Paul D. Beer and Jason J. Davis
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC06210C, Edge Article

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
DOI: 10.1039/D0SC06321E, Edge Article

Submit to Chemical Science today! Check out our author guidelines for information on our article types or find out more about the advantages of publishing in a Royal Society of Chemistry journal.

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We are pleased to share a selection of our referee-recommended HOT articles for November. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

Single-nucleotide resolution of N⁶-adenine methylation sites in DNA and RNA by nitrite sequencing
Yasaman Mahdavi-Amiri, Kimberley Chung Kim Chung and Ryan Hili
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC03509B, Edge Article

Biomimetic hydrogen-bonding cascade for chemical activation: telling a nucleophile from a base
Hyunchang Park and Dongwhan Lee
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC05067A, Edge Article

Is there a photochemical Hammond postulate?
Christian G. Bochet and Freya M. Harvey
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC04370B, Edge Article

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character
Zhe Wang, Rikuo Akisaka, Sohshi Yabumoto, Tatsuo Nakagawa, Sayaka Hatano and Manabu Abe
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC05311B, Edge Article

Room temperature conductance switching in a molecular iron(III) spin crossover junction
Senthil Kumar Karuppannan, Alejandro Martín-Rodríguez, Eliseo Ruiz, Phimphaka Harding, David J. Harding, Xiaojiang Yu, Anton Tadich, Bruce Cowie, Dongchen Qi and Christian A. Nijhuis
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC04555A, Edge Article

On-surface isostructural transformation from a hydrogen-bonded network to a coordination network for tuning the pore size and guest recognition
Dong-Dong Zhou, Jun Wang, Pin Chen, Yangyong He, Jun-Xi Wu, Sen Gao, Zhihao Zhong, Yunfei Du, Dingyong Zhong and Jie-Peng Zhang
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC05147K, Edge Article

Born–Oppenheimer approximation in optical cavities: from success to breakdown
Csaba Fábri, Gábor J. Halász, Lorenz S. Cederbaum and Ágnes Vibók
Chem. Sci., 2021, Advance Article
DOI: 10.1039/D0SC05164K, Edge Article

 

Submit to Chemical Science today! Check out our author guidelines for information on our article types or find out more about the advantages of publishing in a Royal Society of Chemistry journal.

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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.

By Damayanti Bagchi, Guest Web Writer

Hypoxia, an event of inadequate oxygen supply to solid tumor, is related to aggressive propagation of malignancy by altering cancer cell metabolism. It also induces resistance to standard cancer chemotherapeutics and requires ‘out of the box’ thinking to deal with, often termed as alternative strategies.

Photodynamic therapy (PDT) uses a light activated chemical, termed photosensitizer (PS), which can behave like a drug only in the presence of specific light. The photon energy could excite the PS molecule that can produce reactive oxygen species (ROS) which in turn destroys cancer cells. PDT requires an oxygen-filled environment to be effective, which restricts its application in hypoxic tumors. Alternatively, photochemotherapy (PCT) follows a similar mechanism to PDT, and differs only by exploiting an oxygen-independent reaction pathway and hence could be used under hypoxic conditions.

The search for new effective PCT agents needs optimization in a lot of parameters given the stability of the compound, and sensitivity to different physical (light, heat) and chemical (pH, tumor micro-environment) conditions. The replication of hypoxic tumor conditions in a lab-based cell culture set up is quite challenging. Despite all of these tricks, scientists from the US, Canada and Italy came up with a new class of heavy-metal based photo-chemo agents, in a cross-country collaborative scientific investigation.

The researchers synthesized a range of Os(II)-oligothienyl-appended metal-ligand complexes that can produce highly effective cytotoxicity in hypoxia, through light- induced electron transfer reactions via triplet intra-ligand charge transfer excited states. The novel complex is defined as [Os(phen)2(IP-nT)]Cl2, where phen =1,10- phenanthroline, IP = imidazo[4,5-f][1,10]phenanthroline, and nT = thiophenes of varying chain lengths n (n = 0–4). Scheme 1 shows details of the structural variety.

Rigorous computational investigation using quantum mechanical calculations indicates all the Os(II) complexes have the requisite triplet state energies to sensitize ROS production by Type II energy transfer pathway but the actual effect in cells would likely depend on the local concentrations of these complexes and the redox environment. The detailed experimental photophysical studies reveal more on the structure-activity relationship of the complexes. All these complexes show broad absorption in the visible light region (400-550 nm) and weaker absorption in the near-infra red (NIR 700 nm) range. The excited state absorption spectra clearly show the difference in the long-lived triplet state with increasing conjugation with the addition of ligands (n=1-4). Figure 1 depicts the different photophysical pathways present in the complexes. The extended conjugation in Os-4T provides the longest triplet excited state lifetimes (3–4ms and 14–16ms), which suggests it’s superior ROS generation capability over the other complexes.

The complex Os-4T provides remarkable photocytotoxicity in human melanoma cells and the activity is dependent on the wavelength of the light source used. Authors reported an exceedingly high phototherapeutic index (PI) of 6500 (a value of more than 100 is highly active) under red light irradiation. The significant activity attained using NIR light expands the potential activity of the complex Os-4T via implication of the NIR biological window I. However, the light dependency of cytotoxicity changes in hypoxic conditions in which PI is found to be 90. The potential toxicity of the heavy metal-based complex is also investigated, and the complex is found to be safe up to 200 mg/kg doses in mice, which is pretty high compare to other PCT agents. The study gives the first insight into Os-based photo-chemo agents with remarkably high activity in both oxygen-rich normoxic and oxygen-deficient hypoxic conditions. Researchers are currently expanding this work and validating the effects of Os-4T in mice models and hope to proceed with clinical human trials for the eradication of the long-standing invincible problem of cancer.

To find out more, please read:

Breaking the barrier: an osmium photosensitizer with unprecedented hypoxic phototoxicity for real world photodynamic therapy

John A. Roque, III, Patrick C. Barrett, Houston D. Cole, Liubov M. Lifshits, Ge Shi, Susan Monro, David von Dohlen, Susy Kim, Nino Russo, Gagan Deep, Colin G. Cameron,* Marta E. Alberto* and Sherri A. McFarland*

Chem. Sci., 2020, 11, 9784-9806

About the blogger:

Dr Damayanti Bagchi is a postdoctoral researcher in Irene Chen’s lab at University of California, Los Angeles, United States. She obtained her PhD in Physical Chemistry from Satyendra Nath Bose National Centre for Basic Sciences, India. Her research is focused on spectroscopic studies of nano-biomaterials. She is interested in exploring light enabled therapeutics. She enjoys food and experimenting with various cuisines, which she found resembles products/ side products of chemical reactions!

We are pleased to share a selection of our referee-recommended HOT articles for October. We hope you enjoy reading these articles and congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download. You can find our full 2020 HOT article collection here.

 

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
DOI: 10.1039/D0SC04512H, Edge Article

Effect of curvature and placement of donor and acceptor units in cycloparaphenylenes: a computational study
Terri C. Lovell, Kaylin G. Fosnacht, Curtis E. Colwell and Ramesh Jasti
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03923C, Edge Article

Total syntheses of spiroviolene and spirograterpene A: a structural reassignment with biosynthetic implications
Hyung Min Chi, Charles J. F. Cole, Pengfei Hu, Cooper A. Taylor and Scott A. Snyder
Chem. Sci., 2020, 11, 10939-10944
DOI: 10.1039/D0SC04686H, Edge Article

Coligand role in the NHC nickel catalyzed C–F bond activation: investigations on the insertion of bis(NHC) nickel into the C–F bond of hexafluorobenzene
Maximilian W. Kuntze-Fechner, Hendrik Verplancke, Lukas Tendera, Martin Diefenbach, Ivo Krummenacher, Holger Braunschweig, Todd B. Marder, Max C. Holthausen and Udo Radius
Chem. Sci., 2020, 11, 11009-11023
DOI: 10.1039/D0SC04237D, Edge Article

Multimerized self-assembled caged two-in-one siRNA nanoparticles for photomodulation of RNAi-induced gene silencing
Changmai Chen, Nannan Jing, Zhongyu Wang, Yu Zhang, Wei Chen and Xinjing Tang
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03562A, Edge Article

Selective inhibition of the K⁺ efflux sensitive NLRP3 pathway by Cl⁻ channel modulation
Tessa Swanton, James A. Beswick, Halah Hammadi, Lucy Morris, Daniel Williams, Stephane de Cesco, Lina El-Sharkawy, Shi Yu, Jack Green, John B. Davis, Catherine B. Lawrence, David Brough and Sally Freeman
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC03828H, Edge Article

Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state
Zhen Xu, Clàudia Climent, Christopher M. Brown, Duane Hean, Christopher J. Bardeen, David Casanova and Michael O. Wolf
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04715E, Edge Article

Cation recognition on a fullerene-based macrocycle
Yoshifumi Hashikawa and Yasujiro Murata
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC05280A, Edge Article

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets
Grazia Rovelli, Michael I. Jacobs, Megan D. Willis, Rebecca J. Rapf, Alexander M. Prophet and Kevin R. Wilson
Chem. Sci., 2020, Advance Article
DOI: 10.1039/D0SC04611F, Edge Article

 

Submit to Chemical Science today! Check out our author guidelines for information on our article types or find out more about the advantages of publishing in a Royal Society of Chemistry journal.

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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.