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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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Controlling chirality at an osmium centre

Chirality is the concept of non-superimposable mirror images. A simple example of chiral objects are our hands, and in chemistry, molecules are chiral if they have a stereogenic (or asymmetric) centre. Like our hands when it comes to writing or dextrous tasks, chiral compounds are ubiquitous in chemistry, where often only one enantiomer (a chiral isomer) is selective for binding or reactivity. Chiral transition-metal complexes are also desirable for asymmetric catalysis, where the majority of systems have chiral ligands to impart chirality in the complex. The alternative ‘chiral-at-metal’ approach imparts chirality at the metal centre by the selective arrangement of asymmetric, achiral ligands, allowing the metal centre to act as both the stereogenic centre and the reactive centre for catalysis.

Chiral-at-metal complexes

Figure 1: General structure for chiral-at-metal complexes using chelating, achiral ligands (above), and the specific structures of the enantiomers for the new osmium system Os1 (below)

Researchers in Germany have adopted this chiral-at-metal approach for asymmetric catalysis, previously reporting Ir(III), Rh(III), Ru(II) and Fe(II) systems, and have now translated this to the first example of a chiral-at-osmium complex (Figure 1). A new octahedral osmium(II) complex was synthesised (Os1), with two bidentate phenanthrolinium ligands (plus carbonyl and bound acetonitrile ligands) that are coordinated in a non-C2-symmetric fashion to create the stereogenic osmium metal centre. Complex Os1 was initially synthesised as a racemic mixture (rac-Os1), in which the researchers were able to resolve to the individual enantiomers using a chiral auxiliary ligand. This method involved replacing the labile acetonitrile ligand with a chiral auxiliary ligand, (S)-2, to form a mixture of diastereomers Δ-(S)-Os2 and Λ-(S)-Os2, which were separable by solubility. Δ-(S)-Os2 precipitated out of the reaction solution, and was isolated and purified by filtration and washing in a >99:1 diastereomeric ratio (d.r.), and the Λ-(S)-Os2 that remained in solution was purified by column chromatography in a >99:1 d.r. The separated diastereomers were then treated with acid in an acetonitrile solution to replace the chiral auxiliary ligand back to the bound acetonitrile, to give the separated enantiomers Δ-Os1 and Λ-Os1, with >99:1 enantiomeric ratios (Scheme 1).

Synthesis of chiral-at-osmium complex

Scheme 1: Synthesis of Os1, starting with formation of the racemic mixture, followed by enantiomeric resolution using a chiral auxiliary ligand (S)-2

The researchers then tested the catalytic activity of the separated enantiomers of Os1 with respect to intramolecular C(sp3)–H aminations that proceed via transition metal nitrenoid intermediates. Δ-Os1 showed catalytic activity for the amination of various nitrene precursor reagents, such as the conversion of sulfonylazides (3) to cyclic sulfonylamides (4) and azidoformates (5) to 2-oxazolidinones (6), tolerating various substrate functional groups (a-c) (Scheme 2). Significantly, the chiral osmium catalyst gave high catalytic yields and enantiomeric purities, particularly in comparison to the previously reported ruthenium analogue (see Scheme 2 for comparative ratios).

Catalysis using chiral-at-osmium complex

Scheme 2: Catalytic C(sp3)-H aminations using Δ-Os1, with comparisons in activity and enantiomeric purity to the previous ruthenium analogue

Overall, this new chiral-at-osmium complex has shown superior catalytic activity with greater enantiomeric selectivity for intramolecular C(sp3)–H aminations, additionally providing the first examples of catalytic enantioselective ring-closing C-H amination of 2-oxazolidinones. This catalytic activity can be attributed to the labile acetonitrile ligand of Os1, which also proved beneficial for allowing enantiomeric resolution of the initial racemic mixture by substitution with a chiral auxiliary ligand.

 

To find out more, please read:

Asymmetric Catalysis with Chiral-at-Osmium Complex

Guanghui Wang, Zijun Zhou, Xiang Shen, Sergei Ivlev and Eric Meggers*

Chem. Commun., 2020, 56, 7714-7717

 

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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Bacteria vs. bacteria: studying small molecules for microbial competition

Antibiotic resistant bacteria, and the subsequent diseases caused by their infection, are of serious global concern. As more and more bacteria develop antibiotic resistance and jeopardise the current treatments for serious infections, there is a strong imperative to both develop new medicines and to understand these bacterial pathogens. Pseudomonas aeruginosa, Staphylococcus aureus and species from the genus Burkholderia are all such antibiotic resistant bacteria that contribute to various human diseases, and they can pose a serious threat to cystic fibrosis patients through chronic lung infections. These are often polymicrobial infections, meaning that the different bacteria interact by association and can alter the impact of the resulting disease.

P. aeruginosa and Burkholderia generally interact competitively with S. aureus, reducing its viability. This is achieved by the secretion of small molecule respiratory toxins which include 2-alkyl-4(1H)-quinolone N-oxides (AQNOs) by P. aeruginosa or 3-methyl-2-alkyl-4-quinolone N-oxides (MAQNOs) by Burkholderia (see Figure 1). Researchers in Germany and Austria sought to understand the antagonistic interactions of these bacteria and have now reported the synthesis of various representative AQNOs and MAQNOs and investigated their action against S. aureus.

Quinolone derivatives secreted by bacteria

Figure 1: Structures of the quinolone derivatives produced by P. aeruginosa and Burkholderia that act against S. aureus

The researchers approached the synthesis of the AQNOs and MAQNOs by starting with the preparation of the corresponding quinolones, and then converting them to the quinolone N-oxides. They focussed on the C9 nonyl-/nonenyl- derivatives, NQNOs and MNQNOs, as previous studies showed this alkyl chain length proved the most active against S. aureus. Mass spectrometry and fragmentation was primarily used to characterise the synthesised compounds, and the researchers were able to establish a new library of standards to be used for the identification of quinolones and quinolone N-oxides. This therefore allowed the researchers to quantify the specific quinolone derivatives produced by certain strains of P. aeruginosa and Burkholderia using this standard library, as shown in Figure 2.

Quinolone standard library

Figure 2: Quantification of the quinolones (AQs and MAQs) and quinolone N-oxides (AQNOs and MAQNOs) secreted by P. aeruginosa (strains PAO1 and PA14) and Burkholderia thailandesis using calibration against the established standard library

 

The researchers then investigated the possible activity of these quinolone derivatives against S. aureus. The activity of S. aureus was measured using a chromogenic assay, by varying concentrations of the quinolone derivatives until a minimum inhibitory concentration (MIC) was reached, with complete respiratory inhibition of the bacteria. The C9-quinolones (before N-oxidation) showed no inhibition against S. aureus at the highest concentrations tested, but the corresponding quinolone N-oxides (NQNOs and MNQNOs) showed activity against the bacteria. More specifically, unsaturated derivatives were more active, and the MNQNOs, with 3-methylation of the quinolone core, showed the greatest antibiotic activity against S. aureus. These results suggest that the methylated quinolones produced by species of Burkholderia, as well as unsaturared quinolones produced by P. aeruginosa, have an important role in competitive interactions against S. aureus in polymicrobial infections.

 

To find out more, please read:

Profiling structural diversity and activity of 2-alkyl-4(1H)-quinolone N-oxides of Pseudomonas and Burkholderia

Dávid Szamosvári, Michaela Prothiwa, Cora Lisbeth Dieterich and Thomas Böttcher

Chem. Commun., 2020, 56, 6328-6331

 

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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Studying Anticancer Agents with XFM

It turns out that rhenium-based compounds have been showing some promising anticancer activity as they’re stable, allow for real time imaging, structurally diverse, and have low off-site toxicity. The most commonly studied complexes are based around a Re(I) tricarbonyl core with the other three binding sites occupied by ligands of varying complexity. Researchers in the US and Australia developed a tricarbonyl Re isonitrile polypyridyl complex fac-[Re(CO)3(dmphen)(para-tolyl-isonitrile)]+, where dmphen = 2,9-dimethyl-1,10-phenanthro-line, called TRIP for short. TRIP showed promising cytotoxicity and can be imaged using confocal fluorescence microscopy, taking advantage of the emissive metal to ligand charge transfer (MLCT) state. The persistence of the emission indicates that the ligands remain bound to the Re even within cells. The complex’s cytotoxicity stems from its inducement of cells to accumulate misfolded proteins, resulting in apoptosis from the unfolded protein response (UPR). UPR induced cell death is relatively uncommon and led the researchers to find a method to characterize the speciation of TRIP in vitro. They used synchrotron X-ray fluorescence microscopy (XFM) to probe the cellular uptake and distribution of TRIP and an iodo-derivative I-TRIP by looking at elemental signals.

Figure 1. Chemical structures of TRIP and I-TRIP

I-TRIP is particularly well-suited to this type of study, as the iodine provides an additional spectroscopic handle on the isonitrile ligand absent in TRIP. Of course, the researchers had to confirm that I-TRIP possessed similar cytotoxicity and working mechanism to TRIP. Various assays and biological studies showed evidence of comparable cytotoxicity and mechanism, demonstrating that altering the substitution of the isonitrile ligand doesn’t significantly impact the bioactivity of the complex. With that settled, the experiments could move to the synchrotron to probe elemental distributions.

Figure 2. XFM elemental distribution maps of HeLa cervical cancer cells treated with either DMSO (control), TRIP, or I-TRIP.

Cells treated with both TRIP and I-TRIP show a clear Re signal, confirming that they can enter and persist in cells. Critically, the colocalization of the Re and I maps for I-TRIP samples indicate that the isonitrile ligand remains bound as a part of the Re complex inside the cells. This strongly suggests that the Re complex is intact while it induces cell death, adding to the developing mechanistic understanding of their activity. This work shows the utility of XRM as a technique to study the distribution of organometallic complexes in living cells. Additionally, the tunability and stable bioactivity of the Re complexes shows that they’re amenable to study by a wide range of techniques that will allow for further mechanistic probing.

To find out more, please read:

X-Ray fluorescence microscopy reveals that rhenium(I) tricarbonyl isonitrile complexes remain intact in vitro

Chilaluck C. Konkankit, James Lovett, Hugh H. Harris and Justin J. Wilson

Chem. Commun., 2020, 56, 6515-6518

About the blogger:

Dr. Beth Mundy is a recent PhD in chemistry from the Cossairt lab at the University of Washington in Seattle, Washington. Her research focused on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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A precious pairing

Understanding metal-metal interactions is of fundamental interest to chemists, especially in the design of new materials or catalysts. Heterometallic metal-metal bonding is particularly fascinating, since the unique properties of each metal can be combined or even manipulated, to enhance structural, electronic or even photochemical effects. Gold and platinum are one such pairing of interest and there have already been several applications of AuPt clusters reported for catalysis. However, well-defined, homogeneous AuPt complexes are comparatively under explored, but there is a huge potential for these heterodinuclear complexes to catalyse useful chemical transformations. Research in Germany by Butschke and co-workers now describes the first example of an AuPt complex with a bound olefin, as a valuable, formal Au+IPt0 precursor for further reactivity and chemical transformations (Figure 1).

Figure 1: The new AuPt heterodinuclear complex with Pt-bound olefins (right), and existing examples in the literature with Pt-bound phosphines (left and centre).

The new cationic AuPt complex described in this report differs from the existing literature by the presence of weakly bound olefin ligands (in this case, norbonene/nbe) coordinated to the platinum. The other existing examples have only strongly σ-donating phosphine ligands coordinated to the Pt centre, which increases the overall stability of the complexes and renders them unreactive for further chemistry. In contrast, the nbe ligands are more weakly bound, and have a much lower dissociation energy, creating a more reactive complex which is therefore a valuable precursor to other formal Au+IPt0 complexes. This increased reactivity is reflected in the preparation and subsequent manipulations of the complex, which had to be conducted at low temperatures to prevent decomposition.

Figure 2: The X-ray crystal structure of the new AuPt complex.

The new AuPt heterodinuclear complex prepared in this report was characterised by a range of spectroscopic and structural techniques. Single-crystal X-ray diffraction confirmed the molecular structure of the complex, as shown in Figure 2. A rearrangement of the three axial nbe ligands was observed in the new AuPt complex compared to the [Pt(nbe)3] platinum precursor; the three olefin ligands are arranged in a spoke-wheel geometry with the bridging methylenes of nbe all pointing in the same direction away from the gold (an ‘up-up-up’ configuration, in comparison to an ‘up-up-down’ arrangement as in the Pt precursor). NMR spectroscopic characterisation also helped to elucidate and confirm the structure. The 195Pt-NMR resonance of the AuPt complex was particularly noteworthy, showing a similar chemical shift to that of the Pt precursor, which indicates little to no electronic change at Pt0 in the new AuPt complex. This was also reflected in the 13C NMR resonances for the olefinic carbons, which again, were similar in the AuPt complex and the Pt precursor.

Figure 3: Comparing the new AuPt complex (3) to other systems with fewer bound olefins, in terms of the Au-Pt bond dissociation energy (x-axis) and the overall charge transfer between the Au and Pt (y-axis), according to three different calculations.

The authors then further probed the binding of the gold centre to the platinum, and why there was no apparent significant change in the electronics between the new AuPt complex and the Pt precursor. A comparison to the existing AuPt complexes reported revealed that these are often assigned formally as Au-IPt+II, where there is a dative interaction between the Lewis base (Pt) and the Lewis Acid (Au). In contrast, the new AuPt complex in this report is formally assigned as Au+IPt0, where there is considerably less charge transfer in the metal-metal bonding, as shown by DFT calculations (see Figure 3). This formal Au+IPt0 assignment ultimately results in the coordinated nbe olefin ligands having a low dissociation energy (i.e. they are highly labile and susceptible to ligand substitution), which is further supported by DFT calculations and is reflected in the lack of an identifiable electrospray-ionisation mass spectrometry peak for the [M]+ ion. Therefore, this new AuPt complex is a desirable precursor for the preparation of other formal Au+IPt0 complexes, which will allow for future reactivity studies on these unusual heterodinuclear systems.

To find out more, please read:

A heterodinuclear, formal Au+IPt0 complex with weakly bound alkene ligands

Lukas D. Ernst, Konstantin Koessler, Andreas Peter, Daniel Kratzert, Harald Scherer and Burkhard Butschke

Chem. Commun., 2020, 56, 5350-5353

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Generating Electricity from Urea Using NiCoMo/Graphene Catalysts

Electro-oxidizing urea in water has the dual benefit of generating electricity and treating wastewater. In alkaline media, the sluggish kinetics of the urea oxidation reaction, CO(NH2)2 + 6OH → N2↑ + CO2↑ + 5H2O + 6e, due to its transfer of six electrons demand efficient catalysts to speed up this process.

A research team led by Xiujuan Sun and Rui Ding, both at Xiangtan University, China, used a co-precipitation method to synthesize urea-oxidation catalysts. These catalysts comprised of graphene-anchored nanoparticles of metallic Ni, Co, and Mo, as well as their alloys and hydro/oxides (NCM/G) (Figure 1). NCM/G with the optimal composition displayed a mass activity of 140.9 mA cm-2 mgcat-1 and an onset potential of 1.32 V vs. RHE (current density = 10 mA/cm2). Their results are published in Chemical Communications (DOI: 10.1039/D0CC02132F).

Figure 1. Representative (a) transmission electron microscopy image and (b-f) elemental mappings of the synthesized catalyst.

The catalysts exhibited different catalytic activities dependent on their chemical compositions, which were tunable by varying the moles of the Ni, Co, and Mo precursors. Cyclic voltammograms of various NCM/G all showed markedly increased current density at potentials beyond 0.3 V vs. Ag/AgCl in urea-containing aqueous solutions (Figure 2a), marking their catalytic activity for urea oxidation. Among all the tested catalysts, NCM/G with Ni:Co:Mo = 80:10:10 achieved the largest current density at 0.6 V vs. Ag/AgCl, indicating its highest catalytic activity. Additionally, chronoamperometry demonstrated that increasing the Mo content was beneficial for maintaining catalyst stability, as the current density of NCM/G with the lowest amount of Mo (the black curve in Figure 2b) decayed the fastest. Combining the results of cyclic voltammetry and chronoamperometry, the authors deduced that the optimal molar ratio of Ni:Co:Mo was 80:10:10.

Figure 2. (a) Cyclic voltammograms (scan rate: 1 mV/s) and (b) chronoamperometry (potential: 0.5 V vs. Ag/AgCl) profiles of NCM/G with different Ni, Co, and Mo contents. Electrolyte: 1.0 M KOH + 0.33 M urea in water. The molar ratios of Ni:Co:Mo of NCM/G 90505, 811, and 71515 are 90:5:5. 80:10:10, and 70:15:15, respectively.

The optimization of the chemical composition demonstrated in this work can rationalize the development of high-performance, metallic electrocatalysts for urea oxidation.

For expanded understanding, please read:

Trimetallic NiCoMo/Graphene Multifunctional Electrocatalysts with Moderate Structural/Electronic Effects for Highly Efficient Alkaline Urea Oxidation Reaction

Wei Shi, Xiujuan Sun, Rui Ding, Danfeng Ying, Yongfa Huang, Yuxi Huang, Caini Tan, Ziyang Jia, and Enhui Liu

Chem. Commun., 2020, DOI: 10.1039/D0CC02132F

 

Tianyu Liu acknowledges Zacary Croft at Virginia Tech, U.S., for his careful proofreading of this post.

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from the University of California, Santa Cruz, in the United States. He is passionate about the communication of scientific endeavors to both the general public and other scientists with diverse research expertise to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Comm. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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