A lithium-air battery and gas handling system demonstrator.

The lithium-air (Li-air) battery offers one of the highest practical specific energy densities of any battery system at >400 W h kgsystem-1. The practical cell is expected to operate in air, which is flowed into the positive porous electrode where it forms Li2O2 on discharge and is released as O2 on charge. The presence of CO2 and H2O in the gas stream leads to the formation of oxidatively robust side products, Li2CO3 and LiOH, respectively. Thus, a gas handling system is needed to control the flow and remove CO2 and H2O from the gas supply. Here we present the first example of an integrated Li-air battery with in-line gas handling, that allows control over the flow and composition of the gas supplied to a Li-air cell and simultaneous evaluation of the cell and scrubber performance. Our findings reveal that O2 flow can drastically impact the capacity of cells and confirm the need for redox mediators. However, we show that current air-electrode designs translated from fuel cell technology are not suitable for Li-air cells as they result in the need for higher gas flow rates than required theoretically. This puts the scrubber under a high load and increases the requirements for solvent saturation and recapture. Our results clarify the challenges that must be addressed to realise a practical Li-air system and will provide vital insight for future modelling and cell development.

Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes.

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host-guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By "plugging" the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond.

Silver(I)-Catalyzed Synthesis of Cuneanes from Cubanes and their Investigation as Isosteres.

Bridged or caged polycyclic hydrocarbons have rigid structures that project substituents into precise regions of 3D space, making them attractive as linking groups in materials science and as building blocks for medicinal chemistry. The efficient synthesis of new or underexplored classes of such compounds is, therefore, an important objective. Herein, we describe the silver(I)-catalyzed rearrangement of 1,4-disubstituted cubanes to cuneanes, which are strained hydrocarbons that have not received much attention since they were first described in 1970. The synthesis of 2,6-disubstituted or 1,3-disubstituted cuneanes can be achieved with high regioselectivities, with the regioselectivity being dependent on the electronic character of the cubane substituents. A preliminary assessment of cuneanes as scaffolds for medicinal chemistry suggests cuneanes could serve as isosteric replacements of trans-1,4-disubstituted cyclohexanes and 1,3-disubstituted benzenes. An analogue of the anticancer drug sonidegib was synthesized, in which the 1,2,3-trisubstituted benzene was replaced with a 1,3-disubstituted cuneane.

Host-Guest Chemistry in Boron Nitride Nanotubes: Interactions with Polyoxometalates and Mechanism of Encapsulation.

Boron nitride nanotubes (BNNTs) are an emerging class of molecular container offering new functionalities and possibilities for studying molecules at the nanoscale. Herein, BNNTs are demonstrated as highly effective nanocontainers for polyoxometalate (POM) molecules. The encapsulation of POMs within BNNTs occurs spontaneously at room temperature from an aqueous solution, leading to the self-assembly of a POM@BNNT host-guest system. Analysis of the interactions between the host-nanotube and guest-molecule indicate that Lewis acid-base interactions between W═O groups of the POM (base) and B-atoms of the BNNT lattice (acid) likely play a major role in driving POM encapsulation, with photoactivated electron transfer from BNNTs to POMs in solution also contributing to the process. The transparent nature of the BNNT nanocontainer allows extensive investigation of the guest-molecules by photoluminescence, Raman, UV-vis absorption, and EPR spectroscopies. These studies revealed considerable energy and electron transfer processes between BNNTs and POMs, likely mediated via defect energy states of the BNNTs and resulting in the quenching of BNNT photoluminescence at room temperature, the emergence of new photoluminescence emissions at cryogenic temperatures (<100 K), a photochromic response, and paramagnetic signals from guest-POMs. These phenomena offer a fresh perspective on host-guest interactions at the nanoscale and open pathways for harvesting the functional properties of these hybrid systems.

Application of a Synthetic Ferredoxin-Inspired [4Fe4S]-Peptide Maquette as the Redox Partner for an [FeFe]-Hydrogenase.

'Bacterial-type' ferredoxins host a cubane [4Fe4S]2+/+ cluster that enables these proteins to mediate electron transfer and facilitate a broad range of biological processes. Peptide maquettes based on the conserved cluster-forming motif have previously been reported and used to model the ferredoxins. Herein we explore the integration of a [4Fe4S]-peptide maquette into a H2 -powered electron transport chain. While routinely formed under anaerobic conditions, we illustrate by electron paramagnetic resonance (EPR) analysis that these maquettes can be reconstituted under aerobic conditions by using photoactivated NADH to reduce the cluster at 240 K. Attempts to tune the redox properties of the iron-sulfur cluster by introducing an Fe-coordinating selenocysteine residue were also explored. To demonstrate the integration of these artificial metalloproteins into a semi-synthetic electron transport chain, we utilize a ferredoxin-inspired [4Fe4S]-peptide maquette as the redox partner in the hydrogenase-mediated oxidation of H2 .

Four Redox Isomers of a [3 × 3] Copper-Iron Heterometal Grid.

A mixed-valence heterometallic nonanuclear [3 × 3] grid complex, [CuI2CuII6FeIII(L)6](BF4)5·MeOH·9H2O (1; MeOH = methanol), was synthesized by a one-pot reaction of copper and iron ions with multidentate ligand 2,6-bis[5-(2-pyridinyl)-1H-pyrazol-3-yl]pyridine (H2L). 1 showed five quasi-reversible one-electron redox processes centered at +0.74, +0.60, +0.39, +0.27, and -0.13 V versus SCE, assignable to four CuI/CuII processes and one FeII/FeIII couple, respectively. The two-electron-oxidized species [CuII8FeIII(L)6](PF6)7·4MeOH·7H2O (12eOx), the two-electron-reduced species [CuI4CuII4FeIII(L)6](PF6)3·2H2O (12eRed), and the three-electron-reduced species [CuI4CuII4FeII(L)6](PF6)2·5MeOH·H2O (13eRed) were isolated electrochemically. The four redox isomers were characterized by single-crystal X-ray analysis, SQUID magnetometry, and Mössbauer spectroscopy.

Electronic Structure and Photoactivity of Organoarsenic Hybrid Polyoxometalates.

Organofunctionalization of polyoxometalates (POMs) allows the preparation of hybrid molecular systems with tunable electronic properties. Currently, there are only a handful of approaches that allow for the fine-tuning of POM frontier molecular orbitals in a predictable manner. Herein, we demonstrate a new functionalization method for the Wells-Dawson polyoxotungstate [P2W18O62]6- using arylarsonic acids which enables modulation of the redox and photochemical properties. Arylarsonic groups facilitate orbital mixing between the organic and inorganic moieties, and the nature of the organic substituents significantly impacts the redox potentials of the POM core. The photochemical response of the hybrid POMs correlates with their computed and experimentally estimated lowest unoccupied molecular orbital energies, and the arylarsonic hybrids are found to exhibit increased visible light photosensitivity comparable with that of arylphosphonic analogues. Arylarsonic hybridization offers a route to stable and tunable organic-inorganic hybrid systems for a range of redox and photochemical applications.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials.

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Self-Assembled Surfactant-Polyoxovanadate Soft Materials as Tuneable Vanadium Oxide Cathode Precursors for Lithium-Ion Batteries.

The mixing of [V10 O28 ]6- decavanadate anions with a dicationic gemini surfactant (gem) leads to the spontaneous self-assembly of surfactant-templated nanostructured arrays of decavanadate clusters. Calcination of the material under air yields highly crystalline, sponge-like V2 O5 (gem-V2 O5 ). In contrast, calcination of the amorphous tetrabutylammonium decavanadate allows isolation of a more agglomerated V2 O5 consisting of very small crystallites (TBA-V2 O5 ). Electrochemical analysis of the materials' performance as lithium-ion intercalation electrodes highlights the role of morphology in cathode performance. The large crystallites and long-range microstructure of the gem-V2 O5 cathode deliver higher initial capacity and superior capacity retention than TBA-V2 O5 . The smaller crystallite size and higher surface area of TBA-V2 O5 allow faster lithium insertion and superior rate performance to gem-V2 O5 .

Diphosphoryl-functionalized Polyoxometalates: Structurally and Electronically Tunable Hybrid Molecular Materials.

Herein, we report the synthesis and characterization of a new class of hybrid Wells-Dawson polyoxometalate (POM) containing a diphosphoryl group (P2 O6 X) of the general formula [P2 W17 O57 (P2 O6 X)]6- (X=O, NH, or CR1 R2 ). Modifying the bridging unit X was found to impact the redox potentials of the POM. The ease with which a range of α-functionalized diphosphonic acids (X=CR1 R2 ) can be prepared provides possibilities to access diverse functionalized hybrid POMs. Compared to existing phosphonate hybrid Wells-Dawson POMs, diphosphoryl-substituted POMs offer a wider tunable redox window and enhanced hydrolytic stability. This study provides a basis for the rational design and synthesis of next-generation hybrid Wells-Dawson POMs.

Synthesis of New Morphinan Opioids by TBADT-Catalyzed Photochemical Functionalization at the Carbon Skeleton.

The synthesis of new morphinan opioids by the addition of photochemically generated carbon-centered radicals to substrates containing an enone in the morphinan C-ring, is described. Using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer photocatalyst, diverse radical donors can be used to prepare a variety of C8-functionalized morphinan opioids. This work demonstrates the late-stage modification of complex, highly functionalized substrates.

Insight into the Structural Variation and Sodium Storage Behavior of Polyoxometalates Encapsulated within Single-Walled Carbon Nanotubes.

The host-guest interaction can remarkably alter the physiochemical properties of composite materials. It is crucial to clarify the mechanism by revealing the influence of the host on the electronic structure of the guest molecules. Herein, we study the structural variation of polyoxometalates (POMs) after being confined in single-walled carbon nanotubes (SWNT). What we found is that in addition to the reported charge transfer from SWNT to POM, an intramolecular electron transfer within a single POM cluster can be observed in the POM@SWNT composites. Moreover, the charge density on the bridged oxygen of POMs is prominently enhanced. The structural change and electron reconfiguration of POMs upon encapsulation in SWNT significantly speed up electron and ion transport, leading to the improved electrochemical performance for sodium ions storage.

Molecular redox species for next-generation batteries.

This Tutorial Review describes how the development of dissolved redox-active molecules is beginning to unlock the potential of three of the most promising 'next-generation' battery technologies - lithium-air, lithium-sulfur and redox-flow batteries. Redox-active molecules act as mediators in lithium-air and lithium-sulfur batteries, shuttling charge between electrodes and substrate systems and improving cell performance. In contrast, they act as the charge-storing components in flow batteries. However, in each case the performance of the molecular species is strongly linked to their solubility, electrochemical and chemical stability, and redox potentials. Herein we describe key examples of the use of redox-active molecules in each of these battery technologies and discuss the challenges and opportunities presented by the development and use of redox-active molecules in these applications. We conclude by issuing a "call to arms" to our colleagues within the wider chemical community, whose synthetic, computational, and analytical skills can potentially make invaluable contributions to the development of next-generation batteries and help to unlock of world of potential energy-storage applications.

Electrochemistry of redox-active molecules confined within narrow carbon nanotubes.

Confinement of molecules within nanocontainers can be a powerful tool for controlling the states of guest-molecules, tuning properties of host-nanocontainers and triggering the emergence of synergistic properties within the host-guest systems. Among nanocontainers, single-walled carbon nanotubes - atomically thin cylinders of carbon, with typical diameters below 2 nm and lengths reaching macroscopic dimensions - are ideal hosts for a variety of materials, including inorganic crystals, and organic, inorganic and organometallic molecules. The extremely high aspect ratio of carbon nanotubes is complemented by their functional properties, such as exceptionally high electrical conductivity and thermal, chemical and electrochemical stability, making carbon nanotubes ideal connectors between guest-molecules and macroscopic electrodes. The idea of harnessing nanotubes both as nanocontainers and nanoelectrodes has led to the incorporation of redox-active species entrapped within nanotube cavities where the host-nanotubes may serve as conduits of electrons to/from the guest-molecules, whilst restricting the molecular positions, orientations, and local environment around the redox centres. This review gives a contemporary overview of the status of molecular redox chemistry within ultra-narrow carbon nanotubes (nanotubes with diameters approaching molecular dimensions) highlighting the opportunities, pitfalls, and gaps in understanding of electrochemistry in confinement, including the role of nanotube diameter, size and shape of guest-molecules, type of electrolyte, solvent and other experimental conditions.

Stabilization of Polyoxometalate Charge Carriers via Redox-Driven Nanoconfinement in Single-Walled Carbon Nanotubes.

We describe the preparation of hybrid redox materials based on polyoxomolybdates encapsulated within single-walled carbon nanotubes (SWNTs). Polyoxomolybdates readily oxidize SWNTs under ambient conditions in solution, and here we study their charge-transfer interactions with SWNTs to provide detailed mechanistic insights into the redox-driven encapsulation of these and similar nanoclusters. We are able to correlate the relative redox potentials of the encapsulated clusters with the level of SWNT oxidation in the resultant hybrid materials and use this to show that precise redox tuning is a necessary requirement for successful encapsulation. The host-guest redox materials described here exhibit exceptional electrochemical stability, retaining up to 86 % of their charge capacity over 1000 oxidation/reduction cycles, despite the typical lability and solution-phase electrochemical instability of the polyoxomolybdates we have explored. Our findings illustrate the broad applicability of the redox-driven encapsulation approach to the design and fabrication of tunable, highly conductive, ultra-stable nanoconfined energy materials.

Redox-Active Hybrid Polyoxometalate-Stabilised Gold Nanoparticles.

We report the design and preparation of multifunctional hybrid nanomaterials through the stabilization of gold nanoparticles with thiol-functionalised hybrid organic-inorganic polyoxometalates (POMs). The covalent attachment of the hybrid POM forms new nanocomposites that are stable at temperatures and pH values which destroy analogous electrostatically functionalised nanocomposites. Photoelectrochemical analysis revealed the unique photochemical and redox properties of these systems.

Asymmetric Hybrid Polyoxometalates: A Platform for Multifunctional Redox-Active Nanomaterials.

Access to asymmetrically functionalized polyoxometalates is a grand challenge as it could lead to new molecular nanomaterials with multiple or modular functionality. Now, a simple one-pot synthetic approach to the isolation of an asymmetrically functionalized organic-inorganic hybrid Wells-Dawson polyoxometalate in good yield is presented. The cluster bears two organophosphonate moieties with contrasting physical properties: a chelating metal-binding group, and a long aliphatic chain that facilitates solvent-dependent self-assembly into soft nanostructures. The orthogonal properties of the modular system are effectively demonstrated by controlled assembly of POM-based redox-active nanoparticles. This simple, high-yielding synthetic method is a promising new approach to the preparation of multi-functional hybrid metal oxide clusters, supermolecular systems, and soft-nanomaterials.

A Brønsted-Ligand-Based Iron Complex as a Molecular Switch with Five Accessible States.

A mononuclear FeII complex, prepared with a Brønsted diacid ligand, H2 L (H2 L=2-[5-phenyl-1H-pyrazole-3-yl] 6-benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [FeII (H2 L)2 ](BF4 )2 (1A ), exhibits abrupt spin transition at T1/2 =258 K, and treatment with base yields a deprotonated analogue [FeII (HL)2 ] (1B ), which shows gradual SCO above 350 K. A range of FeIII analogues were also characterized. [FeIII (HL)(H2 L)](BF4 )Cl (1C ) has an S=5/2 spin state, while the deprotonated complexes [FeIII (L)(HL)], (1D ), and (TEA)[FeIII (L)2 ], (1E ) exist in the low-spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid-state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.

Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for Tunable Switching and Hybrid Electrode Fabrication.

Two carboxyl-substituted iron(II) grids, one protonated, [Fe4(HL)4](BF4)4·4MeCN·AcOEt (1), and the other deprotonated, [Fe4(L)4]·DMSO·EtOH (2), where H2L = 4-{4,5-bis[6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl]-1 H-imidazol-2-yl}benzoic acid, were synthesized. Single-crystal X-ray structure analyses reveal that both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by π-π-stacking interactions. 1 showed spin transition between the 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin-crossover between the 4LS and 2LS-2HS states above 300 K. A modified indium-tin oxide (ITO) electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to the original redox processes of iron(II)/iron(III).

A Multi-Redox Responsive Cyanometalate-Based Metallogel.

A TTF-based (TTF=tetrathiafulvalene) tridentate ligand (α-(4'-methyl-4,5-di-n-dodecylthylthiotetrathiafulvalene-5'-ylthio)- α'-[2,2,2-tris(1-pyrazolyl)ethoxy]-p-xylene) (L) with long-chain alkyl moieties was prepared in order to obtain a new multi-redox active gelator based on a mixed-metal octanuclear complex [FeIII4 NiII4 (CN)12 (tp)4 (L)4 ](BF4 )4 (1). The magnetism, electrochemistry, and gelation behavior of 1 were studied and 1,2-dichlorobenzene solutions of 1 are shown to display thermoreversible gelation behavior at room temperature. Furthermore, the gel phase of 1 was shown to undergo room-temperature gel-to-sol transformations induced by both the oxidation and reduction of the gelator complex by F4 TCNQ or [FeII (Cp*)2 ], respectively.