Isolation of a Rare Terminal Zr(IV) Peroxide and Insight into Polyoxovanadate Support Impact on Thioether Oxidation.

Substitutional lability of the terminal methoxide ligand on a Zr(IV) substituted polyoxovanadate-alkoxide (POV-alkoxide) via protonolysis is presented. Addition of excess water or stoichiometric 2,2,2-trifluoroethanol results in the exchange of the terminal methoxide ligand for a hydroxide or 2,2,2-trifluoroethoxide ligand, respectively. The lability of the terminal methoxide ligand at zirconium is leveraged to access a relatively stable terminal peroxide bound to a POV-alkoxide supported Zr(IV) center, via addition of hydrogen peroxide adducts compatible with organic solvent.

Photoluminescence of a Uranium(IV) Alkoxide Complex.

In this report, we describe the photoluminescence of a homoleptic uranium(IV) alkoxide complex. Excitation of [Li(THF)][U(O Bu)] leads to the first example of photoluminescence from a well-defined actinide complex originating from an f-f excitation, supported by second order multiconfigurational electronic structure calculations including spin-orbit coupling. These calculations show strong spin-orbit coupling between the excited triplet and singlet states for the 5f-orbital manifold, which leads to a long-lived excited state lifetime of 0.

Neptunium Pyridine Dipyrrolide Complexes.

Two pyridine dipyrrolide neptunium(IV) complexes, (PDP)NpCl(THF) and Np(PDP), where (PDP) is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1-pyrrol-2-yl)pyridine, have been prepared. Characterization of the complexes has been performed through a combination of solid- and solution-state methods, including single-crystal X-ray diffraction and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono- and bis-ligated species.

Engineering mechanisms of proton-coupled electron transfer to a titanium-substituted polyoxovanadate-alkoxide.

Metal oxides are promising catalysts for small molecule hydrogen chemistries, mediated by interfacial proton-coupled electron transfer (PCET) processes. Engineering the mechanism of PCET has been shown to control the selectivity of reduced products, providing an additional route for improving reductive catalysis with metal oxides. In this work, we present kinetic resolution of the rate determining proton-transfer step of PCET to a titanium-doped POV, TiVO(OCH) with 9,10-dihydrophenazine by monitoring the loss of the cationic radical intermediate using stopped-flow analysis.

Implications of Charge and Heteroatom Dopants on the Thermodynamics and Kinetics of Redox Reactions in Keggin-Type Polyoxometalates.

The utilization of polyoxometalate-based materials is largely dictated by their redox properties. Detailed understanding of the thermodynamic and kinetic efficiency of charge transfer is therefore essential to the development of polyoxometalate-based systems for target applications. Toward this end, we report electrochemical studies of a series of heteroatom-doped Keggin-type polyoxotungstate clusters [PWO] ( ), [VWO] ( ), [P(VW)O] ( ), and [V(VW)O] ( ) to elucidate the role of the identity and spatial location of heteroatoms and overall cluster charge on the rate constants of electron transfer and redox reaction entropies.

p of alcohols dictates their reactivity with reduced uranium-substituted thiomolybdate clusters.

The uranium-substituted thiomolybdate cluster, (Cp*3Mo3S4)UCp*, has been demonstrated as a model for water reduction by single uranium atoms supported on a molybdenum sulfide surface (U@MoS). In this study, the scope of O-H bond activation is expanded through the investigation of the reactivity of various alcohols with differing p values for the -OH proton. The reaction of (Cp*3Mo3S4)UCp* with stoichiometric amounts of methanol, phenol, 2,6-dichlorophenol, and nonafluoro--butyl alcohol affords the corresponding mono-alkoxide species, (Cp*3Mo3S4)Cp*U(OR), a uranium-metalloligand cooperative activation of the O-H bond.

Vanadium Substitution Dictates H Atom Uptake at Lindqvist-type Polyoxotungstates.

Understanding how modification of molecular structures changes the thermochemistry of H atom uptake can provide design criteria for the formation of highly active catalysts for reductive transformations. Herein, we describe the effect of doping an atomically precise polyoxotungstate with vanadium on proton-coupled electron transfer (PCET) reactivity. The Lindqvist-type polyoxotungstate [WO] displays reversible redox chemistry, which was found to be unchanged in the presence of acid, indicating an inability to couple reduction with protonation.

Venturing Past Uranium: Synthesis of a Np(IV) Polyoxomolybdate-Alkoxide Sandwich Complex.

The synthesis of a Np(IV) polyoxomolybdate-alkoxide sandwich complex, (TBA)[Np{MoO(OMe)NO}] (TBA = tetrabutylammonium), is reported. This compound represents a rare example of a neptunium polyoxometalate cluster isolated outside of water, allowing for characterization of its electrochemical properties in nonaqueous solvents. Complexation of An(IV) cations fine-tunes the redox properties of the cluster, with the observed four reversible reductive events varying slightly both in potential and peak separation depending on the actinide present.

Amphiphilic, phosphonic acid-capped cadmium selenide quantum dots sensitize a thiomolybdate catalyst for hydrogen production.

Combining a molecular thiomolybdate cluster, [Mo3S13]2-, with cadmium selenide quantum dots capped with tetraethyleneglycol monomethyl ether phosphonate (TEGPA) ligands results in a highly active photocatalytic system for the production of hydrogen under visible light irradiation. The system reaches turnover numbers exceeding 30 000 and remains active for ∼200 h.

Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(v) sandwich complex.

The synthesis and characterization of a series of (TBA)[M{MoO(OMe)NO}] (M = Zr, Hf, Th, and U) sandwich complexes is reported. A preformed lacunary, Lindqvist-type, polyoxomolybdate-alkoxide cluster provides access to first examples of actinide-polyoxomolybdate sandwich complexes isolated under non-aqueous conditions. Incorporation of metal(iv) cations into this framework was found to "switch on" reversible redox chemistry at the {Mo} ligands, with the Zr and Hf containing complexes accepting up to two electrons, while the Th and U derivates accommodate as many as four additional electrons.

Molecular Models of Atomically Dispersed Uranium at MoS Surfaces Reveal Cooperative Mechanism of Water Reduction.

Single atoms of uranium supported on molybdenum sulfide surfaces (U@MoS) have been recently demonstrated to facilitate the hydrogen evolution reaction (HER) through electrocatalysis. Theoretical calculations have predicted uranium hydroxide moieties bound to edge-sulfur atoms of MoS as a proposed transition state involved in the HER process. However, the isolation of relevant intermediates involved in this process remains a challenge, rendering mechanistic hypotheses unverified.

O reduction proton-coupled electron transfer by a V(III) aquo on a polyoxovanadate-alkoxide cluster.

We report the transfer of H-atoms from a reduced polyoxovanadate alkoxide [OctN][VO(OH)(OMe)] concerted proton-electron transfer. Oxygen reduction is compared between bridging and terminal O-H bonds revealing similar mechanisms, providing new insight to design criteria for metal-oxide electrocatalysts that faciliate oxygen reduction by concerted-proton electron transfer.

Impact of Surface Ligand Identity and Density on the Thermodynamics of H Atom Uptake at Polyoxovanadate-Alkoxide Surfaces.

An understanding of how molecular structure influences the thermodynamics of H atom transfer is critical to designing efficient catalysts for reductive chemistries. Herein, we report experimental and theoretical investigations summarizing structure-function relationships of polyoxovanadate-alkoxides that influence bond dissociation free energies of hydroxide ligands located at the surface of the cluster. We evaluate the thermochemical descriptors of O-H bond strength for a series of clusters, namely [VO(OH)(TRIOL)] ( = 2, 4, 6; R = NO, Me) and [VO(OMe)(OH)(TRIOL)], via computational analysis and open circuit potential measurements.

Elucidation of Design Criteria for V-based Redox Mediators: Structure-Function Relationships that Dictate Rates of Heterogeneous Electron Transfer.

Redox mediators are attractive solutions for addressing the stringent kinetic stipulations required for efficient energy conversion processes. In this work, we compare the electrochemical properties of four vanadium complexes, namely [V(acac)], [VO(OMe)], [BuN][VO(TRIS)], and [BuN][VO(NO)] in non-aqueous solutions on glassy carbon electrodes. The goal of this study is to investigate the electron transfer kinetics and diffusivity of these compounds under identical experimental conditions to develop an understanding of structure-function relationships that dictate the physicochemical properties of vanadium oxide assemblies.

Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes.

A series of pyridine dipyrrolide actinide(IV) complexes, (PDP)AnCl(THF) and An(PDP) (An = U, Th, where (PDP) is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1-pyrrol-2-yl)pyridine), have been prepared. Characterization of all four complexes has been performed through a combination of solid- and solution-state methods, including elemental analysis, single crystal X-ray diffraction, and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono- and bis-ligated species.

Heterometal Dopant Changes the Mechanism of Proton-Coupled Electron Transfer at the Polyoxovanadate-Alkoxide Surface.

The transfer of two H-atom equivalents to the titanium-doped polyoxovanadate-alkoxide, [TiVO(OCH)], results in the formation of a V(III)-OH site at the surface of the assembly. Incorporation of the group (IV) metal ion results in a weakening of the O-H bonds of [TiVO(OH)(OCH)] in comparison to its homometallic congener, [VO(OH)(OCH)], resembling more closely the thermodynamics reported for the one-electron reduced derivative, [VO(OH)(OCH)]. An analysis of early time points of the reaction of [TiVO(OCH)] and 5,10-dihydrophenazine reveals the formation of an oxidized substrate, suggesting that proton-coupled electron transfer proceeds via initial electron transfer from substrate to cluster prior to proton transfer.

Molybdenum sulphide clusters as redox-active supports for low-valent uranium.

The preparation of an actinide substituted cubane cluster, (Cp*3Mo3S4)Cp*UI2, and its reduced derivatives are reported. Structural and spectroscopic investigations provide insight into the unique interactions between the actinide and its redox-active molybdenum sulphide metalloligand, serving as a model to study atomically-dispersed, low-valent actinide ions on MoS surfaces. To probe the ability of the assembly to facilitate multielectron small molecule activation, the reactivity of the fully-reduced cluster, (Cp*3Mo3S4)Cp*U, with azobenzene was investigated.

Improved solubility of titanium-doped polyoxovanadate charge carriers for symmetric non-aqueous redox flow batteries.

Non-aqueous redox flow batteries constitute a promising solution for grid-scale energy storage due to the ability to achieve larger cell voltages than can be readily accessed in water. However, their widespread application is limited by low solubility of the electroactive species in organic solvents. In this work, we demonstrate that organic functionalization of titanium-substituted polyoxovanadate-alkoxide clusters increases the solubility of these assemblies over that of their homoleptic congeners by a factor of >10 in acetonitrile.

Efficient Hole Transfer from CdSe Quantum Dots Enabled by Oxygen-Deficient Polyoxovanadate-Alkoxide Clusters.

A limitation of the implementation of cadmium chalcogenide quantum dots (QDs) in charge transfer systems is the efficient removal of photogenerated holes. Rapid hole transfer has typically required the functionalization of hole acceptors with groups that can coordinate to the surface of the QD. In addition to being synthetically limiting, this strategy also necessitates a competitive binding equilibrium between the hole acceptor and native, solubilizing ligands on the nanocrystal.

Acidity of Carboxylic Acid Ligands Influences the Formation of VO(A) and VO(B) Nanocrystals under Solvothermal Conditions.

Vanadium dioxide (VO) can adopt many different crystal structures at ambient temperature and pressure, each with different, and often desirable, electronic, optical, and chemical properties. Understanding how to control which crystal phase forms under various reaction conditions is therefore crucial to developing VO for various applications. This paper describes the impact of ligand acidity on the formation of VO nanocrystals from the solvothermal reaction of vanadyl acetylacetonate (VO(acac)) with stoichiometric amounts of water.

Surface ligand length influences kinetics of H-atom uptake in polyoxovanadate-alkoxide clusters.

The uptake of hydrogen atoms (H-atoms) at reducible metal oxide nanocrystal surfaces has implications in catalysis and energy storage. However, it is often difficult to gain insight into the physicochemical factors that dictate the thermodynamics and kinetics of H-atom transfer to the surface of these assemblies. Recently, our research group has demonstrated the formation of oxygen-atom (O-atom) defects in polyoxovanadate-alkoxide (POV-alkoxide) clusters conversion of surface oxido moieties to aquo ligands, which can be accomplished addition of two H-atom equivalents.

Manipulating Ligand Density at the Surface of Polyoxovanadate-Alkoxide Clusters.

We present the post-synthetic modification of a polyoxovanadate-alkoxide (POV-alkoxide) cluster via the reactivity of its cationic form, [VO(OCH)], with water. This result indicates that cluster oxidation increases the lability of bridging methoxide ligands, affording a ligand exchange reaction that serves to compensate for the increased charge of the cluster core. This synthetic advance affords the isolation of a series of POV-alkoxide clusters with varying degrees of μ-O ligands incorporated at the surface, namely, [VO(OCH)], [VO(OCH)], and [VO(OCH)].

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions.

Regioselectivity of concerted proton-electron transfer at the surface of a polyoxovanadate cluster.

Proton-coupled electron transfer (PCET) is an important process in the activation and reactivity of metal oxide surfaces. In this work, we study the electronic structure of a reduced polyoxovanadate-alkoxide cluster bearing a single bridging oxide moiety. The structural and electronic implications of the incorporation of bridging oxide sites are revealed, most notably resulting in the quenching of cluster-wide electron delocalization in the most reduced state of the molecule.