Exploring Cation-Anion Redox Processes in One-Dimensional Linear Chain Vanadium Tetrasulfide Rechargeable Magnesium Ion Cathodes.

For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulfide anions [S2]2- forming one-dimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption near-edge spectroscopy (XANES), and 51V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V4+ to [S2]2-, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2- to S2-. We report the formation of a previously unknown intermediate in the Mg-V-S compositional space, Mg3V2S8, comprising [VS4]3- tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V5+ and S2- containing intermediate on charging instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic-anionic redox processes as a potential way of achieving higher capacities for MIBs.

A Versatile Approach to Dynamic Amide Bond Formation with Imine Nucleophiles.

Dynamic covalent chemistry has rapidly become an important approach to access supramolecular structures. While the products generated in these reactions are held together by covalent bonds, the reversible nature of the transformations can limit the utility of many these systems in creating robust materials. We describe herein a method to form stable and commonly employed amide bonds by exploiting the reversible coupling of imines and acyl chlorides. The reaction employs easily accessible reagents, is dynamic under ambient conditions, without catalysts, and can be trapped with simple hydrolysis. This offers an approach to create broad families of amide products under thermodynamic control, including the selective formation of amide macrocycles or polymers.

Strengthening the Magnetic Interactions in Pseudobinary First-Row Transition Metal Thiocyanates, M(NCS)2.

Understanding the effect of chemical composition on the strength of magnetic interactions is key to the design of magnets with high operating temperatures. The magnetic divalent first-row transition metal (TM) thiocyanates are a class of chemically simple layered molecular frameworks. Here, we report two new members of the family, manganese(II) thiocyanate, Mn(NCS)2, and iron(II) thiocyanate, Fe(NCS)2. Using magnetic susceptibility measurements on these materials and on cobalt(II) thiocyanate and nickel(II) thiocyanate, Co(NCS)2 and Ni(NCS)2, respectively, we identify significantly stronger net antiferromagnetic interactions between the earlier TM ions-a decrease in the Weiss constant, θ, from 29 K for Ni(NCS)2 to -115 K for Mn(NCS)2-a consequence of more diffuse 3d orbitals, increased orbital overlap, and increasing numbers of unpaired t2g electrons. We elucidate the magnetic structures of these materials: Mn(NCS)2, Fe(NCS)2, and Co(NCS)2 order into the same antiferromagnetic commensurate ground state, while Ni(NCS)2 adopts a ground state structure consisting of ferromagnetically ordered layers stacked antiferromagnetically. We show that significantly stronger exchange interactions can be realized in these thiocyanate frameworks by using earlier TMs.

Post-Assembly Reactivity of N-Aryl Iminoboronates: Reversible Radical Coupling and Unusual B-N Dynamic Covalent Chemistry.

Post-assembly reaction of a dynamic covalent iminoboronate system following addition of Cp2 Co resulted in the formation of a series of new reductively coupled dianionic dimers via C-C bond formation. The dimers formed as a mixture of BN-containing isomeric products: diastereomers rac5 and meso5, with coupled five-membered rings, and enantiomeric rac6, with a fused six-membered ring bicyclic system from C-C bond formation and rearrangement of the B-N bonds. Each isomer was identified using 1 H NMR spectroscopy in combination with single crystal X-ray structure determination. Interestingly, interconversion between the coupled five-membered rings (rac5 ) and fused bicyclic systems (rac6 ) was found to occur through an unprecedented breaking and reforming of the B-N covalent bond. Further, the coupled products could be converted quantitatively back to their iminoboronate precursors with addition of the electron abstractor Ph3 C+ .

Development and Cycloaddition Reactivity of a New Class of Pyridine-Based Mesoionic 1,3-Dipole.

We describe here the development and structural characterization of a new type of mesoionic 1,3-dipole, which can be generated in the one-step reaction of imines with pyridine- or quinoline-based acid chlorides. Coupling the formation of these dipoles with alkyne cycloaddition can open a general and modular route to synthesize indolizines from combinations of available and diversifiable building blocks.

Mg(PF6)2-Based Electrolyte Systems: Understanding Electrolyte-Electrode Interactions for the Development of Mg-Ion Batteries.

Mg(PF6)2-based electrolytes for Mg-ion batteries have not received the same attention as the analogous LiPF6-based electrolytes used in most Li-ion cells owing to the perception that the PF6(-) anion decomposes on and passivates Mg electrodes. No synthesis of the Mg(PF6)2 salt has been reported, nor have its solutions been studied electrochemically. Here, we report the synthesis of the complex Mg(PF6)2(CH3CN)6 and its solution-state electrochemistry. Solutions of Mg(PF6)2(CH3CN)6 in CH3CN and CH3CN/THF mixtures exhibit high conductivities (up to 28 mS·cm(-1)) and electrochemical stability up to at least 4 V vs Mg on Al electrodes. Contrary to established perceptions, Mg electrodes are observed to remain electrochemically active when cycled in the presence of these Mg(PF6)2-based electrolytes, with no fluoride (i.e., MgF2) formed on the Mg surface. Stainless steel electrodes are found to corrode when cycled in the presence of Mg(PF6)2 solutions, but Al electrodes are passivated. The electrolytes have been used in a prototype Mg battery with a Mg anode and Chevrel (Mo3S4)-phase cathode.

The structures of ordered defects in thiocyanate analogues of Prussian Blue.

We report the structures of six new divalent transition metal hexathiocyanatobismuthate frameworks with the generic formula , M = Mn, Co, Ni and Zn. These frameworks are defective analogues of the perovskite-derived trivalent transition metal hexathiocyanatobismuthates MIII[Bi(SCN)6]. The defects in these new thiocyanate frameworks order and produce complex superstructures due to the low symmetry of the parent structure, in contrast to the related and more well-studied cyanide Prussian Blue analogues. Despite the close similarities in the chemistries of these four transition metal cations, we find that each framework contains a different mechanism for accommodating the lowered transition metal charge, making use of some combination of Bi(SCN)6 3- vacancies, MBi antisite defects, water substitution for thiocyanate, adventitious extra-framework cations and reduced metal coordination number. These materials provide an unusually clear view of defects in molecular framework materials and their variety suggests that similar richness may be waiting to be uncovered in other hybrid perovskite frameworks.

Strongly coloured thiocyanate frameworks with perovskite-analogue structures.

We report the first examples of thiocyanate-based analogues of the cyanide Prussian blue compounds, MIII[Bi(SCN)6], M = Fe, Cr, Sc. These compounds adopt the primitive cubic pcu topology and show strict cation order. Optical absorption measurements show these compounds have band gaps within the visible and near IR region, suggesting that they may be useful for applications where light harvesting is key, such as photocatalysis. We also show that Cr[Bi(SCN)6] can reversibly uptake water into its framework structure pointing towards the possibility of using these frameworks for host/guest chemistry.

Correction: Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium.

Correction for 'Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium' by Evan N. Keyzer et al., Chem. Commun., 2017, 53, 4573-4576.

Regioselective 1,4-hydroboration of pyridines catalyzed by an acid-initiated boronium cation.

The reaction of the commercially available ammonium salt NH4BPh4 with a pyridine-activated pinacolborane species generates a boronium cation that facilitates the 1,4-selective hydroboration of pyridines in polar solvents. This catalytic reaction is amenable to a host of reactive functional groups and provides access to sterically bulky hydroboration products, previously inaccessible by metal-free routes. Further, the regioselectivity of this reaction can be altered by reducing the polarity of the reaction solvent, resulting in greater proportions of the 1,2-hydroboration product.

Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium.

The development of rechargeable Ca-ion batteries as an alternative to Li systems has been limited by the availability of suitable electrolyte salts. We present the synthesis of complexes of Ca(PF6)2 (a key potential Ca battery electrolyte salt) via the treatment of Ca metal with NOPF6, and explore their conversion to species containing PO2F2- under the reaction conditions.

Insights into the electrochemical performances of Bi anodes for Mg ion batteries using 25Mg solid state NMR spectroscopy.

Bi nanowires as anode materials for Mg ion batteries exhibit excellent electrochemical behaviour, forming Mg3Bi2; this is in part ascribed to the rapid Mg mobility between the two Mg sites of Mg3Bi2, as revealed by the 25Mg NMR spectra of Mg3Bi2 formed electrochemically and via ball-milling. A mechanism involving hops into vacant Mg sites is proposed.

A palladium-catalysed multicomponent coupling approach to conjugated poly(1,3-dipoles) and polyheterocycles.

Conjugated polymers have emerged over the past several decades as key components for a range of applications, including semiconductors, molecular wires, sensors, light switchable transistors and OLEDs. Nevertheless, the construction of many such polymers, especially highly substituted variants, typically involves a multistep synthesis. This can limit the ability to both access and tune polymer structures for desired properties. Here we show an alternative approach to synthesize conjugated materials: a metal-catalysed multicomponent polymerization. This reaction assembles multiple monomer units into a new polymer containing reactive 1,3-dipoles, which can be modified using cycloaddition reactions. In addition to the synthetic ease of this approach, its modularity allows easy adaptation to incorporate a range of desired substituents, all via one-pot reactions.