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Zero valent metal centres in metal-organic frameworks

Crystalline MOF structures with high porosity serve well for a range of applications including shape selective catalysis, gas storage or capture, and drug delivery. These entail the prominence of new methodologies for the synthesis of MOFs, either with unusual metal ion oxidation states or already existing molecular counterparts. In this vein, researchers from TU Denmark have used a ligand transferability strategy for the reaction of homoleptic carbonyl complexes of the Group 6 elements, M(CO)6 (M = Cr, Mo, W), with neutral bridging ligands for the formation of heteroleptic M(CO)6-nLn complexes. For the first time, they have presented the synthesis and characterization of a series of MOFs obtained from M(CO)6 (M = Cr, Mo, W) through the substitution of the ditopic pyrazine (pyz) ligand.

Scheme 1: Synthesis of Cr, Mo, and W.

The synthesis follows reaction at an elevated temperature between M(CO)6 (M = Cr, Mo, W) and an excess of the pyz ligand in a sealed ampoule (Scheme1), leading to dark coloured crystalline products. The reaction is highly favourable for the Mo complex, due to the low dissociation energy of the first Mo–CO bond (119 kJ mol-1), and a reaction temperature of 150°C produced a high yield of crystals. The same reaction conditions provided a minute amount of product for M = Cr and no indication of reactivity for M = W. However, a higher temperature of 200°C lead to the production of the W complex due to the high dissociation energy of the W-CO bond (142 kJ mol-1).

The single-crystal X-ray diffraction pattern of the dark shiny crystals suggests that the composition is fac-M(CO)3(pyz)3/2.1/2 pyz (with M = Cr, Mo, W) (Fig. 1). Cr and Mo crystallize in the triclinic P1 space group, whereas W crystallizes with higher symmetry in the monoclinic C2/m space group. The crystal structure predicts that the three remaining carbonyl ligands reach into the voids of the hexagonal tiling. The hexagon is in chair conformation leading to <M–M–M angles approaching 90° with hexagonal pore channels of roughly 5.5–6.5 Å. The hexagon for W is nearly equilateral in nature compared to Mo, which exhibits unequal hexagon edges.

Fig 1: Single crystal structure of Cr, Mo, and W: (a) View of the hexagonal pore channels in W. (b) Stack of coordination layers in W. (c) Chair conformation of the hexagonal arrangement shown for W. The solvent molecules inside the pores and the disorder are not shown for clarity in (a–c). (d–f) Hexagonal fragments of Cr, Mo, and W together with the pyz molecule contained in the pores (not visible for W due to solvent mask, see methods). The positional disorder of the pyz molecules in W is also shown.

The typical IR absorption band related to M-CO stretching is explored for characterizing the complexes. A red shift of ~310 cm-1 in the spectra of the average carbonyl stretching band was obtained, which is approximated as a ~25% decrease of the force constant associated with the C–O bond. The thermogravimetric analysis showed that the Mo complex has the highest thermal stability with an onset of degradation around 180°C, whereas Cr and W underwent significant mass loss from around 150°C and 130°C, respectively. A similar trend is observed for the stability of the complexes in atmospheric air.

These first examples of structurally characterized zero-valent MOFs with metal nodes derived from metal carbonyls could be attractive architectures for the exploration of catalytic applications as they facilitate the possibility for the non-destructive removal of pore-filling molecules.

 

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Zero-valent metals in metal–organic frameworks: fac-M(CO)3(pyrazine)3/2

Laura Voigt, Rene´ Wugt Larsen, Mariusz Kubus and Kasper S. Pedersen*

Chem. Commun., 2021, 57, 3861

About the writer:

Damayanti Bagchi, PhD, is a postdoctoral researcher in Irene Chen’s lab at University of California, Los Angeles, United States. She has 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-bio interface and phage therapy. She is interested in science communication and science policy-diplomacy. She enjoys travelling and experimenting with various cuisines!

You can find her on Twitter @DamayantiBagchi

 

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HOT article: Plug-and-play aqueous electrochemical atom transfer radical polymerization

Paul Wilson and colleagues at University of Warwick, UK, recently published their Communication on a simplified ‘plug-and-play’ approach to aqueous electrochemical atom transfer radical polymerization. In this video Paul gives more detail about the team’s work.


View the open access article ‘Plug-and-play aqueous electrochemical atom transfer radical polymerization’ here

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A cradle for stable cysteine sulfenic acids

How can you study an unstable biomolecule? One research team’s answer to this is to surround it in a protective cradle. The molecules in question here are cysteine sulfenic acids (Cys-SOH); although they are crucial in various cellular processes and biological functions, they have proven elusive for isolated small-molecule studies due to their instability. Researchers in Japan have now reported the first synthesis and isolation of a stable small molecule cysteine sulfenic acid by employing a protective cradle for the reactive Cys-SOH unit. This bio-inspired design mimics the supramolecular protein environments that can stabilise Cys-SOH residues in situ.

Chemical structure of the small molecule cradled Cys-SOH, with the cradle highlighted in teal

Figure 1. The chemical structure of the protective cradle for cysteine-derived reactive intermediates, including Cys-SOH (framework in blue, cysteine in black).

The researchers utilised the m-phenylene dendrimer framework from their previous studies as the molecular cradle to encapsulate cysteine intermediates for further study (Figure 1). A cysteine unit was first installed into the framework (denoted as Bpsc) via the central benzoyl group to generate the cysteine thiol residue, Cys-SH. The cradled Cys-SH was then converted to the cysteine sulfenic acid by treatment with H2O2, to give the isolable small molecule Cys-SOH (compound 6). The cysteine sulfenic acid 6 proved to be air and thermally stable, and the researchers characterised 6 using NMR and IR spectroscopies, as well as X-ray crystallographic analysis that definitively revealed its structure (Figure 2).

Crystal structure showing the connectivity of the cradled Cys-SOH

Figure 2. The crystal structure of the isolated Cys-SOH small molecule 6, cradled by the Bpsc framework (in green).

Once the structure of the cradled cysteine sulfenic acid 6 was established, the researchers investigated its biologically relevant reactivity. The reaction of Cys-SOH with a thiol is important for redox regulation and protein folding, and the researchers observed that 6 did indeed react with the thiol N-acetylcysteine methyl ester to yield the expected disulfide product. Although this reaction initially proved slow, the researchers were able to show that this was not due to steric hinderance from the Bpsc cradle. Optimisation of this reaction revealed that the addition of triethylamine significantly enhanced the reactivity, and this suggests that the presence of basic residues in the vicinity of cysteine sulfenic acids are important for biological disulfide formations.

In addition to the disulfide formation, the researchers also investigated the reactivity of their cysteine sulfenic acid 6 towards reduction and trapping experiments. The Cys-SOH 6 could be reduced by DTT (dithiothreitol) to regenerate the Cys-SH thiol residue, demonstrating the reversibility of the redox processes between Cys-SH and Cys-SOH within the cradle. The researchers also looked at trapping the Cys-SOH residue using various chemical probes to mimic trapping experiments used for protein-generated cysteine sulfenic acids. They observed the expected formation of the thiol adducts, and again noticed enhanced reactivity in the presence of triethylamine.

Overall, the researchers have demonstrated the first example of an isolable and stable cysteine sulfenic acid that is enabled by a cradling framework. The protective cradle was shown to not impact the biologically-relevant reactivity of the Cys-SOH residue, and ultimately, this framework could serve as a useful biorepresentative model for future studies on Cys-SOH to further understand its behaviour.

 

To find out more, please read:

Isolable small-molecule cysteine sulfenic acid

Tsukasa Sano, Ryosuke Masuda, Shohei Sase and Kei Goto*

Chem. Commun., 2021, 57, 2479-2482

 

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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How sea anemones inspired material design

What do sea anemones and two-dimensional (2D) carbon-based materials have in common? Sea anemones are predatory marine animals and 2D materials are often used for applications in catalysis and energy storage. Most would assume there is little in common between this obscure pairing, however, researchers in China took inspiration from the unique anatomy of sea anemones (particularly their tentacles that can trap and capture prey) for their design of new 2D hybrid porous carbon materials.

Hybrid 2D carbon-based materials combine the desirable properties of 2D porous carbon materials (large surface area and good electronic conductivity) with additional functional components (e.g. metal ions or inorganic acids), which can enhance and control the properties of the material. However, the construction of hybrid materials is often limited by poor dispersion of the functional component on the carbon structure, as the functional precursors tend to aggregate on the surface, and thus diminish the desired properties for the material. To overcome this undesirable aggregation, the researchers turned to their sea-anemone, bio-inspired strategy, where they used carbon-based molecular brushes as building blocks to imitate tentacles, allowing functional precursors to be trapped within the brush network make the hybrid materials (Figure 1).

Figure 1. (a) A conventional strategy for the construction of 2D hybrid materials, where the functional components are shown to aggregate on the surface. (b) The sea-anemone inspired approach by the researchers, starting from a carbon molecular brush network that traps the functional component (Lewis acids), leading to 2D hybrid materials after carbonization.

The researchers used poly(4-vinylpyridine)-grafted-graphene oxide (GO-g-P4VP) as the molecular brush substrate and carbon source for the creation of their 2D hybrid materials. They created the substrate by covalently grafting a thin layer of poly(4-vinylpyridine) (P4VP) from a single-layer graphene oxide (GO) surface and used microscopy techniques to characterise the resulting materials, where various protuberances were observed on the GO sheets to signify the brushes. Next, the GO-g-P4VP brushes were subjected to various Lewis acids (Co2+ ions, B or P; using either cobalt nitrate, boric acids or phytic acids, respectively). The researchers observed spontaneous chemisorption and immobilisation of the Lewis acids within the brush structure, due to the strong interaction with the Lewis basic pyridine groups of the P4VP chains. Lastly, the material was then carbonized at 800 °C to produce the 2D hybrid porous carbon materials (2DHPCs), functionalised with Co (2DHPC-Co), B (2DHPC-B) or P species (2DHPC-P).

The new 2DHPCs were characterised using microscopy techniques and X-ray diffraction, which confirmed 2D morphologies of the materials, uniform distribution of the Lewis acid species and porous structures. The researchers then demonstrated the potential of these high-porosity and functionalised materials by testing them in applications as oxygen evolution reaction (OER) electrocatalysts or as sulfur hosts within Li-S batteries, where in both cases, the 2DHPCs excelled compared to other graphene-based materials. Ultimately, the researchers have demonstrated that their sea-anemone inspired strategy allows for the successful construction of high performance 2D hybrid materials, and could be translated for the design of promising new materials for future energy conversion and storage applications.

 

To find out more, please read:

A versatile sea anemone-inspired strategy toward 2D hybrid porous carbons from functional molecular brushes

Xidong Lin, Zelin Wang, Ruliang Liu, Shaohong Liu,* Kunyi Leng, He Lou, Yang Du, Bingna Zheng, Ruowen Fu and Dingcai Wu*

Chem. Commun., 2021, 57, 1446–1449

 

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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Toroids or fibres? The different assemblies for scissor-shaped azobenzene dyads

Imagine if a pile of laundry could spontaneously organise itself into a neat, folded pile of clothes (and how useful this would be!). This principle can actually happen for some molecules in a process known as self-assembly, whereby a molecule or molecules will organise themselves spontaneously to form a supramolecular structure. Molecules can be specifically designed to self-assemble, by relying on the inclusion of functional groups and motifs that create repulsions and interactions to form unique nanostructures and functional materials.

Researchers in Japan have been studying scissor-shaped azobenzene dyads that can self-assemble by folding, using π-π stacking and hydrogen-bonding (Figure 1a and 1b). They previously observed two key self-assembly processes for two example dyads with formation of either toroidal (doughnut-shaped) structures through intramolecular folding, which can subsequently stack onto each other to create tubular motifs; or fibre/ribbon-type structures through one-dimensional helical stacking. The researchers hypothesised that these different self-assembly pathways could be attributed to differences in the degree of intermolecular aggregation of the dyads and set about verifying this using alkylated or perfluoroalkylated azobenzene dyads (2 and 3, Figure 1a) that have different aggregation properties.

Figure 1. a) Structure of azobenzene dyads 1-3. b) Structure of the folded dyad. (c and d) Self-assembly pathways for dyads 2 (c) and 3 (d).

The researchers prepared the supramolecular assemblies of the new dyads by cooling hot solutions (at 90 °C) of 2 or 3 in methylcyclohexane. Both absorption spectroscopy and dynamic light scattering (DLS) measurements indicated self-assembly of the dyads after cooling to 20 °C, and the resulting assemblies precipitated from the cooled solutions with minutes. The structures of these assemblies were then revealed by atomic force microscopy (AFM), showing different nanostructures for 2 and 3. The alkylated dyad 2 assembled into both toroidal and cylindrically extended nanostructures (Figure 2a and b), where the cylinders were composed of stacked toroidal components (as in Figure 1c). On the other hand, the perfluoroalkylated dyad 3 assembled into entangled fibres after cooling (Figure 2c and d), with no well-defined nanostructures observed even with moderate heating.

Figure 2. AFM images of the dyad assemblies after cooling to 20 °C. a) toroids of 2 and b) nanotubes of 2, with insets showing cross-sectional analyses along the orange lines. (c and d) supramolecular fibres of 3 [c) cooling to 20 °C and d) cooling to 30 °C], with inset showing cross-section analysis along the green line.

The researchers investigated the two different self-assembly pathways of 2 and 3 using multiple characterisation techniques. They found that the perfluoroalkylated dyad 3 self-assembles directly into extended fibres as a result of the enhanced intermolecular interactions provided by fluorophilic interactions, whereas the unique toroidal assembly for the alkylated dyad 2 is a result of weak molecular interactions. The extended fibres of self-assembled dyad 3 were found to form organogels at higher concentrations, in which the researchers also studied the photoresponsive properties. Overall, the results confirm the original hypothesis of different self-assembly pathways for dyads with different aggregation properties, and this study will guide future work into creating targets that can switch their self-assembly pathways under external stimuli.

 

To find out more, please read:

Self-assembly of alkylated and perfluoroalkylated scissor-shaped azobenzene dyads into distinct structures

Hironari Arima, Takuho Saito, Takashi Kajitani and Shiki Yagai *

Chem. Commun., 2020, 56, 15619-15622

 

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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One-dimensional carbon ladders

Nanomaterials are small and mighty. Scaling down to the molecular level imparts desirable properties to materials, for example high conductivity that is useful for electronic applications. Conjugated polymers (CPs) are one-dimensional nanomaterials with promising potential in optoelectronics and spintronics, due to their enhanced conductivity as a result of the delocalised π-electrons along the backbone of the polymer. Doubly-linked CPs, referred to as π-conjugated ladder polymers, have more unique features compared to their singly-linked counterparts, but such nanomaterials are often difficult to synthesise.

Polycyclic aromatic molecules such as acenes have shown potential as organic semiconductors and are therefore ideal materials for new CPs, but polymer formation by coupling the acenes is limited by repulsions between hydrogen atoms of adjacent acene motifs. These constraints can be overcome by installing wider ethynylene or cumulene π-conjugated bridges between the acenes, as demonstrated by researchers from Spain and the Czech Republic in their design and synthesis of a new one-dimensional π-conjugated ladder polymer (Figure 1). This ladder polymer is based on doubly-linked pentacenes (5 linearly-fused benzene rings).

(a) shows the chemical structures in a scheme of the precursor (brominated pentacene) and the polymer structure (ladder pentacene polymer), where the pentacene is 5 linearly fused benzene molecules. (b) and (c) show brighter spots on a dark surface that correspond to the chains of new polymer, where in (c), the individual pentacenes can be differentiated (d) and (e) show even more detail where the fine structure of the pentacenes and linkages of the polymer can be made out.

Figure 1. (a) The synthesis of the ladder polymer, starting from the 8BrPN precursor. (b) Wide STM image showing chains of the new polymer on the Au(111) surface. (c) Close-up high-resolution STM image of one polymer chain corresponding to the green rectangle in (b). (d) nc-AFM image of the molecular structure of the ladder polymer. (e) Simulated nc-AFM image of (d).

The new conjugated ladder polymer was synthesised by vacuum deposition of the precursor molecule (5,7,12,14-tetrakis(dibromomethylene)-5,7,12,14-tetrahydropentacene, 8BrPN) onto a gold surface, Au(111). Subsequent thermal treatment up to 360 °C resulted in the formation of ethynylene linkages between the pentacenes, forming a ladder polymer on the gold surface (Figure 1a). The researchers determined the ladder polymer structure using scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) techniques, as shown in Figure 1b-e. The STM images showed mostly one-dimensional chains with some disordered segments (Figure 1b), and the close-up image in Figure 1c confirms the double-linkages between each pentacene. The nc-AFM images show even more detail, confirming that the backbone of the polymer contains the pentacenes, doubly-linked to adjacent moieties in the 5,7,12 and 14 positions (Figure 1d/e). The nc-AFM characterisation also confirmed ethynylene (–C≡C–) rather than cumulene (=C=C=) linkages, shown by the brighter regions in the middle of the connections that correspond to the ethynylene triple bonds.

The researchers also probed the electronic structure of the new polymer on the gold surface using scanning tunnelling spectroscopy (STS). They determined an electronic bandgap that is 0.22 eV larger than that of a singly cumulene-linked pentacene polymer, thus demonstrating the advantages of the ladder-type structure in this instance. The technique used to create the pentacene ladder polymer in this report could be adopted for the creation of other π-conjugated ladder polymers with varying acenes, which could ultimately be useful for new nanomaterials in optoelectronics and spintronics.

 

To find out more, please read:

On-surface synthesis of doubly-linked one-dimensional pentacene ladder polymers

Kalyan Biswas, José I. Urgel, Ana Sánchez-Grande, Shayan Edalatmanesh, José Santos, Borja Cirera, Pingo Mutombo, Koen Lauwaet, Rodolfo Miranda, Pavel Jelínek,* Nazario Martín* and David Écija*

Chem. Commun., 2020, 56, 15309–15312

 

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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Breaking the rules of valency for silicon and germanium

As every chemist knows, carbon likes to play by the rules and is almost always tetrahedral and tetracoordinate. There are exceptions to this geometry though; examples of planar tetra-, penta- and hexacoordinate carbons have been experimentally detected or theoretically predicted. The other group 14 elements also behave similarly to carbon, but again, planar tetracoordinate (pt) or planar hexacoordinate (ph) silicon or germanium atoms have been reported as exceptions to the tetrahedral, tetracoordinate standard.

Planar pentacoordinate (pp) silicon (ppSi) atoms are desirable in 2D materials, and researchers in China and Mexico have now reported the first examples of planar pentacoordinate silicon and germanium atoms (ppX, where X = Si, Ge). The researchers used a ‘π-localisation’ approach in their design, where the formation of multiple bonds between the surrounding ‘ligand’ atoms and the Si or Ge centre led to viable ppX atoms in XMg4Y (X = Si, Ge; Y = In, Tl) or SiMg3In2 structures (Figure 1).

Structures of the two planar pentacoordinate Si/Ge species investigated. Pentagonal shapes, with 5 surrounding atoms coloured green for Mg or purple for In/Tl, with a central brown atom for Si/Ge, connected to each periphery atom.

Figure 1: Structures for the global minima of the two planar pentacoordinate Si/Ge structures

Based on the precedence of B or Al atoms as ligands for stabilising planar hypercoordinate carbon atoms, the researchers first investigated alkaline earth atoms to stabilise the group 14 centres, looking at XM52- structures (X = Si, Ge; M = Mg, Ca, Be). A local energy minimum and a planar, pentacoordinate D5h geometry was only calculated for the triplet-state magnesium species (Figure 2), indicating that analogues of XMg52- were suitable for further investigation.

The three possible lowest energy structures for XM5(2-). The left structure is pentagonal, with a central Si/Ge connected to each periphery atom. The middle structure shows the Si/Ge connected to 4 M atoms in a square shape, with the fifth M atom bridging above the top two M atoms in the square. The third structure on the right shows the Si/Ge atom connected to the 5 M atoms around it, but in an irregular, non-pentagonal shape, where the M atoms do not all connect outside the central Si/Ge atom.

Figure 2: Structures of XM52- (X = Si, Ge; M = Mg, Ca, Be), where Nimag refers to the numbers of imaginary frequencies [3A1 refers to the triplet state, 1A1 refers to the singlet state]

The researchers calculated the potential energy surfaces of both the singlet and triplet states of XMg52-, optimising the lowest-lying structures. They found that the most energetically favourable isomers of both SiMg52- and GeMg52- were planar tetracoordinate in the singlet state, but there was only a very small energy range (0.4-1.5 kcal mol-1) between these singlet ptX structures and the desirable ppX structure in the triplet state. The researchers therefore turned to substitution of one of the surrounding ‘ligand’ atoms for heavier group 13 elements, to build a suitable-sized cavity for planar pentacoordinate silicon or germanium. They investigated XMg4Y structures (Y = Al-Tl) and found that a planar pentacoordinate structure was the lowest in energy for Y = In or Tl, with a more substantial jump to the next highest energy level structure (2.4-3.9 kcal mol-1). They also investigated disubstituted systems, XMg3Y2, noting that only SiMg3In2 had a ppX structure at the global energy minimum. They also found that the ppXs investigated all had 16 valence electrons, defying the 18-electron rule standard.

Overall, the researchers have theoretically predicted the first examples of thermodynamically and kinetically stable planar pentacoordinate silicon or germanium atoms. These structures combine both stabilising magnesium and larger group 13 element ‘ligands’, and are suitable candidates for future experimental detection, proving that silicon and germanium can too, break the rules of valency and tetrahedral coordination.

 

To find out more, please read:

Planar pentacoordinate silicon and germanium atoms

Meng-hui Wang, Xue Dong, Zhong-hua Cui, Mesías Orozco-Ic, Yi-hong Ding, Jorge Barroso* and Gabriel Merino

Chem. Commun., 2020, 56, 13772-13775

 

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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Excited-state aromaticity for metal systems

Chemists use many rules to describe and predict molecular properties and behaviours. Hückel’s rule is a well-known example, and is used to determine the aromatic character of planar, cyclic molecules containing π-bonds. Aromaticity is the property that increases the stabilisation of a planar molecule by the delocalisation of electrons in π-bonds, and according to Hückel’s rule, a molecule is aromatic if it contains 4n + 2 π-electrons and antiaromatic if it contains 4n π-electrons. Whilst Hückel’s rule applies to the singlet ground state of a molecule (i.e. when all electrons are paired), Baird’s rule is used to establish aromaticity in the triplet state (i.e. two unpaired electrons), and is essentially the opposite; aromatic for 4n and antiaromatic for 4n + 2 π-electrons (Figure 1). Baird’s rule is useful for understanding the excited (triplet) state properties of molecules, but has so far been limited to describing organic species only.

Huckel and Baird aromaticity for metal species

Figure 1. An example of a Baird aromatic all-metal species (left), with orbitals that exhibit Baird aromaticity in the triplet state (right), and Hückel antiaromaticity in the singlet state (centre)

A collaborative effort by researchers in China, Spain and Poland have now demonstrated that Baird aromaticity can be translated to all-metal systems using a series of DFT experiments. The researchers selected derivatives of Al44-­, a planar moiety with 4 π-electrons that is antiaromatic in the ground state (consistent with Hückel’s rule). They determined that in the triplet (T1) state, the two unpaired electrons in Al44-­ occupied σ molecular orbitals (MOs) instead of π MOs; these are the singly-occupied molecular orbitals (SOMOs) shown in Figure 2 that show anti-bonding character for the σ-radial system. The researchers could then infer a triple aromaticity for the triplet state of Al44- from the formal electron count: Hückel aromaticity of the 2 electrons in the σ-tangential system (HOMO); Hückel aromaticity of the 2 electrons in the π-system; and Baird aromaticity of the 4 electrons in the σ-radial system (SOMO, SOMO’ and HOMO-2).

Molecular orbitals for Al4(4-)

Figure 2. The key molecular orbitals and optimised structure for the triplet state

To further confirm the Baird aromaticity of the triplet Al44-­, the researchers calculated the singlet-triplet energy gaps of both the naked anion and the cation stabilised species, Li3Al4. They noted small energy gaps between the singlet and triplet states, which most likely results from the extra stabilisation of the Baird aromaticity in the triplet state. The researchers also performed an electron density of delocalised bonds (EDDB) analysis to quantify the extent of electron delocalisation (and therefore aromatic character) of Al44-­, Li3Al4 and other metallic systems, in comparison to a classical Baird aromatic organic molecule, cyclobutadiene (CBD). They noted comparable values for π-electron delocalisation in the metallic systems compared to CBD, indicating triplet state aromaticity. Additional calculations carried out by the researchers further proved the Baird aromatic character of the all-metal systems, demonstrating how the concept of Baird aromaticity can be extended beyond organic systems and paving the way for future understanding of aromatic, excited state metallic systems.

 

To find out more, please read:

All-metal Baird aromaticity

Dandan Chen, Dariusz W. Szczepanik, Jun Zhu and Miquel Solà

Chem. Commun., 2020, 56, 12522-12525

 

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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Suppor(tin)g iron for catalytic ammonia formation

Dinitrogen fixation, i.e. converting abundant yet inert nitrogen gas into useable forms, has tantalised chemists for decades. One goal is the transformation of dinitrogen (N2) to ammonia using transition-metal catalysts under acidic and reducing conditions. Just a handful of complexes have been reported so far that can catalyse ammonia formation, and only a few of these are iron based, despite the prevalence of iron in the enzymes that can perform this process biologically. Researchers in the US have now discovered a new iron-based bimetallic system that can catalytically transform N2 to ammonia, providing additional studies that help understand this mechanism (Figure 1).

Iron complexes that catalyse ammonia formation from dinitrogen

Figure 1: Existing iron systems that can transform N2 to ammonia, and the tin-iron system in this study (bottom right)

The bimetallic system reported by the researchers is a tin-supported iron complex, analogous to other bimetallics previously studied in the group. Dinitrogen complexes were targeted since N2 coordination to a metal centre activates it towards further reactivity. The researchers prepared two tin-iron dinitrogen complexes, by either one or two electron reductions of a metal-bromide precursor (LSnFeBr) under a nitrogen atmosphere, to form the neutral LSnFe(N2) (1) or the anionic [LSnFe(N2)] (2), respectively. These complexes were characterised using a range of spectroscopic, electronic and structural techniques, which confirmed both coordination of N2 as well as a direct tin-iron interaction (Figure 2).

Characterisation of tin-iron dinitrogen complexes

Figure 2. A) X-ray crystal structures of tin-iron dinitrogen complexes 1 and 2. B) Characterisation of the N2 functionalisation product 3 with the crystal structure (left) and NMR spectra (right).

The further-reduced anionic species, 2, showed a greater extent of dinitrogen activation compared to the neutral species (1). This was determined by a lower infrared N-N stretching frequency and a longer N-N bond in the solid-state structure of 2, both of which indicate a weaker and more activated N-N bond. The researchers therefore selected 2 for further functionalisation reactivity, and found that the reaction with Me3SiCl (an electrophile) resulted in successful N2 functionalisation, forming the diazenido complex LSnFeN2SiMe3 (3). The diazenido complex was also characterised using a range of techniques (Figure 2b), with the solid-state structure clearly showing the silyl electrophile adding to the N2 ligand. Isolated diazenido complexes, like 3, are extremely rare in the literature, and the researchers attribute this apparent stability of 3 to the presence of the supporting tin centre within the bimetallic system. The importance of the tin-support in this system was further demonstrated by computational analyses of complexes 13, whereby the charge of the distal (and reactive) nitrogen correlated with the charge on the tin centre.

Scheme and table showing catalytic ammonia formation using tin iron complexes and other iron systems

Figure 3: Catalytic ammonia formation from N2 using the tin-iron complexes, with comparison to previously reported iron systems.

The researchers tested all three tin-iron complexes for catalytic ammonia formation, using [Ph2NH2]OTf as an acid and CoCp2* as a reducing agent (Figure 3). Both the neutral dinitrogen complex (1) and the diazenido complex (3) proved successful, with generating up to 5.9 turnovers of ammonia. Additionally, the comparable activity of 3 lends further support to the likely presence of a diazenido intermediate in the mechanism. A comparison with existing iron systems shows that the presence of an iron-support greatly enhances the catalysis compared to unsupported iron (see entry 8, Figure 3), indicating the significance of this metal-support and paving the way towards future catalyst design.

 

To find out more, please read:

Bimetallic iron–tin catalyst for N2 to NH3 and a silyldiazenido model intermediate

Michael J. Dorantes, James T. Moore, Eckhard Bill, Bernd Mienert and Connie C. Lu

Chem. Commun., 2020, 56, 11030-11033

 

Samantha Apps acknowledges Michael Dorantes, for proofreading this post, as well as Prof. Connie Lu and the Lu Lab as her colleagues over the last year.

 

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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Capturing toxic gases with a recyclable porous material

Toxic gas removal is essential for preserving the safety of the public and the environment. Examples include gases such as ammonia (NH3) or carbon monoxide (CO) that can be lethal in just parts-per-million concentrations. Typically, removal of these gases is achieved using porous materials with high surface areas such as zeolites, and more recently covalent- or metal-organic frameworks (COFs or MOFs), which soak up the gases by adsorption to sites within the pores of the structure. Ideally such structures would also be recyclable, whereby the gas can be captured and then safely released without damage to the molecular framework of the material. A collaborative effort by researchers across the world has now demonstrated a recyclable strategy for ammonia adsorption by a MOF, using frustrated Lewis pair chemistry to achieve at least 5 cycles of gas capture and release.

SION105-Eu MOF structure

Figure 1. Left: the structure of SION105-Eu, showing its porous nature; right: the chemical structure of the anionic tctb linker ligand (tctb3-)

The researchers selected SION105-Eu as a highly stable MOF, which uses the triply anionic tctb linker that contains a sterically hindered Lewis acidic boron centre (Figure 1). Normally, ammonia (a Lewis base) will interact with a Lewis acidic boron centre to form a Lewis acid-base adduct. However, the steric demands around the Lewis-acidic boron in the tctb linker ligand prevent the formation of a true adduct with ammonia, instead forming a “frustrated Lewis pair” where the ammonia-boron interaction is considered reversible. This notion therefore allows for ammonia capture by adsorption in the MOF through interaction with the boron centre of the ligand, followed by subsequent ammonia release, since a permanent ammonia-boron interaction is prevented by the steric demands of the linker. Additionally, prevention of permanent adsorption of ammonia within the pores of the MOF prevents any possible collapse of the structure/decomposition, which further enhances the recyclability of the material.

The researchers measured the adsorption of ammonia by SION105-Eu by suspending the MOF in an aqueous solution of NH3 at ambient pressures (i.e. with a loose cap on the vial) or high pressures (with a closed vial cap). They observed up to 10 wt% adsorption in the ambient pressure system, and up to 36 wt% adsorption in the closed system (Figure 2, top). The researchers noted retention of the MOF crystallinity upon ammonia adsorption by powder X-ray diffraction measurements (PXRD), where the same pattern was observed before and after (Figure 2, bottom). The PXRD patterns in Figure 2 (bottom) also confirmed the weak, “frustrated Lewis pair” type interactions between the MOF and NH3, by the appearance of only two new peaks as shown in the squared regions of Figure 2 (bottom).

NH3 adsorption by SION105-Eu MOF

Figure 2. Top: Gravimetrically determined ammonia adsorption over time; bottom: powder XRD patterns before (blue) and after (red) ammonia adsorption

 

The recyclability of the MOF was tested by first allowing ammonia adsorption by suspension of the MOF in an ammonia solution for 6 hours, followed by ammonia release through heating the saturated MOF at 75 °C for 30 minutes. The researchers demonstrated this process could be achieved 5 times without change to the ammonia adsorption capacity or structure of the MOF. Overall, this study shows a new frustrated Lewis pair strategy for gas capture and release to render a MOF material recyclable, which has the potential to be applied beyond ammonia removal to other toxic gases.

 

To find out more, please read:

A recyclable metal–organic framework for ammonia vapour adsorption

Tu N. Nguyen, Ian M. Harreschou, Jung-Hoon Lee, Kyriakos C. Stylianou and Douglas W. Stephan

Chem. Commun., 2020, 56, 9600-9603

 

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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