Chemical Communications Blog

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).

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 XM₅^2- structures (X = Si, Ge; M = Mg, Ca, Be). A local energy minimum and a planar, pentacoordinate D_5h geometry was only calculated for the triplet-state magnesium species (Figure 2), indicating that analogues of XMg₅^2- were suitable for further investigation.

The researchers calculated the potential energy surfaces of both the singlet and triplet states of XMg₅^2-, optimising the lowest-lying structures. They found that the most energetically favourable isomers of both SiMg₅^2- and GeMg₅^2- 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:

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.

 

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.

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 Al₄^4-_­, 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 Al₄^4-_­ 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 Al₄^4- 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).

To further confirm the Baird aromaticity of the triplet Al₄^4-_­, 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 Al₄^4-_­, 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.

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).

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).

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 1–3, whereby the charge of the distal (and reactive) nitrogen correlated with the charge on the tin centre.

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 1 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.

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.

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).

 

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.

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.

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).

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).

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.

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.

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.

 

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.

 

 

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).

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.

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.

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