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 .

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.

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.

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.

Why charging Li-air batteries with current low-voltage mediators is slow and singlet oxygen does not explain degradation.

Although Li-air rechargeable batteries offer higher energy densities than lithium-ion batteries, the insulating Li2O2 formed during discharge hinders rapid, efficient re-charging. Redox mediators are used to facilitate Li2O2 oxidation; however, fast kinetics at a low charging voltage are necessary for practical applications and are yet to be achieved. We investigate the mechanism of Li2O2 oxidation by redox mediators. The rate-limiting step is the outer-sphere one-electron oxidation of Li2O2 to LiO2, which follows Marcus theory. The second step is dominated by LiO2 disproportionation, forming mostly triplet-state O2. The yield of singlet-state O2 depends on the redox potential of the mediator in a way that does not correlate with electrolyte degradation, in contrast to earlier views. Our mechanistic understanding explains why current low-voltage mediators (<+3.3 V) fail to deliver high rates (the maximum rate is at +3.74 V) and suggests important mediator design strategies to deliver sufficiently high rates for fast charging at potentials closer to the thermodynamic potential of Li2O2 oxidation (+2.96 V).

A Cooperative Photoactive Class-I Hybrid Polyoxometalate With Benzothiadiazole-Imidazolium Cations.

An organic-inorganic hybrid species based on the Wells-Dawson polyoxotungstate [P2W18O62]6- and novel fluorescent benzothiadiazole-imidazolium cations, [BTD-4,7-ImH]2+, has been synthesized. X-ray crystallographic analysis shows that the inorganic and organic components form a hydrogen-bonded superstructure and that the cations are revealed to be non-equivalent with varying degrees of rotation between the BTD and imidazolium rings due to competition between weak intra- and intermolecular interactions. The UV-vis diffuse reflectance spectra indicate that the hybrid has a band gap of 3.13 eV, while the solid-state fluorescence properties of the cation are quenched in the hybrid material, suggesting the existence of electron transfer between the inorganic and organic components. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the polyoxometalate (POM) and BTD-4,7-ImH precursors, estimated through UV-vis absorption spectroscopy and cyclic voltammetry, indicate that electron transfer from the BTD cations to the POM may occur in the excited state.