Two pyridine dipyrrolide neptunium(IV) complexes, (MesPDPPh)NpCl2(THF) and Np(MesPDPPh)2, where (MesPDPPh)2- is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-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. Electrochemistry of a series of bis-ligated actinide complexes, An(MesPDPPh)2 (An = Th, U, Np), is presented.

Polyoxotungstates have previously been established to facilitate the hydrogenation of small molecule substrates via hydrogen atom transfer from reactive hydroxyl groups formed at the assembly surface. Understanding structure-function relationships that dictate the thermochemistry and kinetics of proton-coupled electron transfer is key to controlling this chemistry. In this work, we combine comprehensive electrochemical experiments and density functional theory calculations to address how different polyoxotungstate morphologies, specifically W6O19-2, W10O32-4, SiW12O40-4, and P2W18O62-6, affect the bond dissociation free energies of surface hydroxides (BDFE(O-H)) formed upon reduction of the assembly in acidic media. Our results reveal increasing hydroxide bond strengths with increasing cluster size, and that anisotropic cluster geometries result in substantial thermodynamic differentiation of H-binding sites. We demonstrate an excellent agreement between theory and experiments on the reported BDFE(O-H) values and, importantly, we elucidate how cluster size and shape affect electronic properties (local charges and frontier molecular orbitals), giving rise to sites with increased preference for hydrogen binding, demonstrated in higher BDFE(O-H). Overall, this work aids the understanding and design of polyoxometalates exhibiting surface sites with tailored interaction strengths.

Redox flow batteries (RFBs) are emerging as a promising battery technology for grid-scale energy storage. The utilization of non-aqueous solvents expands the repertoire of existing electrolytes toward wider electrochemical windows, which is critical for achieving high energy densities. Successful implementation of non-aqueous RFBs on a large scale necessitates identification of suitable charge carriers through the thorough evaluation of key physicochemical properties, such as redox potential, solubility, solution resistance, transport, and electrokinetic properties. These characteristics further inform the performance metrics of the resulting batteries. To date, there is a lack of systematic guidelines and protocols that direct synthetic chemists with consistent procedures to screen electrolytes for practical applications. This is especially true for researchers interested in studying redox-active inorganic molecules as charge carriers for these applications. In this tutorial-review, we discuss the design criteria, testing methods, and H-cell experimental design for inorganic candidates for emergent non-aqueous redox flow battery technologies. We also present a general framework and recommendations on testing procedures that are suitable in different scenarios based on the relevant chemical information that is desired on a given electrolyte. Finally, we conclude the discussion on our envisioned strategies to enable predictive design strategies for next-generation non-aqueous redox flow batteries.

The synthesis of a niobium-(V) substituted polyoxovanadate-alkoxide (NbPOV-alkoxide; [NbV5O7(OCH3)12]) is reported. Addition of 5,10-dihydrophenazine to [NbV5O7(OCH3)12] results in formation of the 2 H+/e- reduced assembly, [NbV5O6(OH2)-(OCH3)12], via proton-coupled electron transfer. [NbV5O6(OH2)-(OCH3)12] has a bond dissociation free energy (BDFE-(O-H)avg) of 62.3 kcal mol-1, resembling that of its homometallic congener, [V6O6(OH2)-(OCH3)12] (BDFE-(O-H)avg = 62.3 kcal mol-1). Single-crystal X-ray diffraction reveals that [NbV5O6(OH2)-(OCH3)12] exists as a mixture of two structural isomers, with the vanadium-aquo moiety formed in either the trans- or cis- positions relative to the Nb-(V) dopant. The formation of two regioisomers is a departure from prior observations of H-atom uptake at the surface of heterometal-doped polyoxovanadate-alkoxides, and is credited to distortions in intercluster metal oxygen bond lengths. Improved selectivity for the trans- isomer is achieved by decreasing the dielectric constant of the reaction solvent. Computational analysis predicts the preferential formation of trans-[NbV5O6(OH2)-(OCH3)12] in solvents with low dielectric constants as a result of changes to the dispersed charge across the assembly.

The proton-coupled electron transfer to the surface of a Keggin-type polyoxomolybdate, [nBu4N]3[PMo12O40], is described. The thermochemistry of surface O-H bonds generated upon reduction/protonation of the assembly is determined (∼67 kcal mol-1). Kinetic investigations provide evidence that proton-electron pairs are added to the polyoxomolybdate surface through a concerted mechanism.

Proton-coupled electron transfer (PCET) is an important mechanism that defines the reactivity of H atom equivalents at reducible metal oxide (MOx) surfaces. To better understand structure-function properties that dictate the thermochemistry and kinetics of PCET at MOx surfaces, our group has employed polyoxovanadate-alkoxide (POV-alkoxide) complexes as molecular models of extended materials. In this work, we investigate the influence of anionic dopants on PCET reactivity in POV-alkoxides. We present the synthesis and characterization of two anion-substituted POV-ethoxides, [V6O6X(OC2H5)12]- (X = Cl or SCN). Reactivity of these assemblies with a potent H atom transfer reagent, 9,10-dihydrophenazine, in acetonitrile (MeCN) affords formation of the 2e-/2H+ reduced species, [V6O6X(MeCN)(OC2H5)12]-. The identity of the (pseudo)halide dopant influences the rate of the reaction, wherein the thiocyanate-substituted species exhibits H atom uptake at rates 2× faster than its chloride congener, and 5× faster than the fully oxygenated assembly, [V6O7(OC2H5)12]-. Collectively, these results provide insight into the role the identify of the dopant plays in controlling the kinetics of H atom uptake/transfer at the surfaces of MOx.

The new Ceiii centered sandwich-type complex (TBA)3[Ce{W4O13(OMe)4MoNO}2] is reported. The redox properties of this molecule, and its all-molybdenum analogue, (TBA)3[Ce{Mo5O13(OMe)4NO}2], were investigated using cyclic voltammetry. The data reveals the presence of reversible Ceiv/Ceiii redox couples at modest potentials. One electron oxidation of the complexes provides facile access to the corresponding Ceiv derivatives, which were fully characterized. 17O NMR spectroscopy reveals that the chemical shifts of the oxygen nuclei directly bound to Ceiv are much higher than the corresponding signals in isostructural, diamagnetic, Zriv, Hfiv, or Thiv centered complexes. Density functional theory (DFT) calculations indicate that the increase in chemical shift correlates with an increase in the covalency of the Miv-O bonds, illustrating that 17O NMR spectroscopy is a powerful experimental tool for interrogating the nature of metal oxygen bonding in diamagnetic complexes.

The structure and density of surface capping ligands in cadmium chalcogenide quantum dots (QDs) are important considerations for controlling the efficiency of charge separation via the transfer of electrons or holes to molecular acceptors. Here we show how the manipulation of the surface ligand density of oleic acid-capped cadmium selenide (CdSe) QDs impacts the efficiency of hole transfer (HT) to polyoxovanadate alkoxides. Meerwein's salt is used as a ligand-stripping agent, providing opportunities to quantitatively manipulate the ligand density at the surface of the nanocrystal, as evidenced by 1H NMR spectroscopy. Time-resolved photoluminescence and transient absorption spectroscopies reveal that the extent of HT is quantitatively related to increased surface accessibility. Collectively, these results show that the reduction of surface ligand density can be used to tune the extent of interactions of molecular acceptors with QDs, providing a route to control charge-transfer processes relevant to improving the efficiency of QDs as photosensitizers.

An externally exposed V dopant imposes H-atom uptake at the polyoxotungstate surface and dictates the proton–coupled electron transfer mechanism.

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. Isolation of the terminal peroxide complex allows for investigation into the impact of the sterically protected, electron-rich POV-alkoxide support on the activation of hydrogen peroxide at Zr(IV). While the isolated peroxide complex is inactive towards the oxidation of thioethers, the methoxy terminated Zr(IV) functions as a precatalyst for the reaction. Mechanistic analysis reveals electrophilic oxidation conditions with hydrogen peroxide substrates, with a nucleophilic parameter ( χ N u ${{{{\bf\chi}}}_{{\bf N u}}}$ ) of 0.09±0.02. In thioether oxidation reactions, selectivity for sulfoxide products (95-99 %) in acetonitrile is observed, suggesting the use of a reduced POV-alkoxide prevents over-oxidation of substrate.

Development of a simple and scalable synthesis of (TBA)3[W5O18MoNO] provides for the formation of the mixed-metal lacunary polyoxoalkoxide, (TBA)2[W4O13(OMe)4MoNO][Na(MeOH)]. This complex was used to synthesize a series of polyoxoalkoxide sandwich-type complexes with the general formula (TBA)2[M{W4O13(OMe)4MoNO}2], where M = Zr(IV), Hf(IV), Th(IV), U(IV), and Np(IV). Compared to the analogous all-molybdenum complexes, the series have drastically different optical and redox properties. The results indicate that framework metal substitution acts as a tool for "orbital engineering", with Density Functional Theory (DFT) calculations revealing that the major consequence of incorporation of tungsten into the complexes is localization of LUMO and LUMO+1 on the molybdenum centers remaining in the molecule. The change in the distribution of the frontier orbitals translates to discrepancies in the electronic properties of the series. Given the rarity of polyoxometalate complexes featuring a U(V) ion, one electron oxidation of (TBA)2[U(IV){W4O13(OMe)4MoNO}2] was pursued. Isolation of the corresponding U(V) centered sandwich-type complex is reported, only the second example of U(V)-polyoxometalate complex described to date.

A heterometallic thiomolybdate cluster, Cp*3CoMo2S4 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) has been synthesized and identified as a molecular electrocatalyst for proton reduction in dimethylformamide. Compared with its homometallic congener (Cp*3Mo3S4), cobalt incorporation improves activity by lowering the overpotential for proton reduction, consistent with the contrasting catalytic performance of MoS2 and its Co-doped derivative. Isolation of the reduced form of Cp*3CoMo2S4 and subsequent reactivity studies provide insight into the reaction pathway. These findings establish Cp*3CoMo2S4 as a molecular model for extended sulfide materials.

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@MoS2). In this study, the scope of O-H bond activation is expanded through the investigation of the reactivity of various alcohols with differing pKa values for the -OH proton. The reaction of (Cp*3Mo3S4)UCp* with stoichiometric amounts of methanol, phenol, 2,6-dichlorophenol, and nonafluoro-tert-butyl alcohol affords the corresponding mono-alkoxide species, (Cp*3Mo3S4)Cp*U(OR), via a uranium-metalloligand cooperative activation of the O-H bond. This observed reactivity is analogous to the O-H bond activation reported by (Cp*3Mo3S4)UCp* in the presence of water. However, addition of tert-butanol induces protonolysis of the Cp* ligand on uranium, resulting in the formation of a uranium tris-tert-butoxide cluster, (Cp*3Mo3S4)U(OtBu)3. Independent synthesis of (Cp*3Mo3S4)Cp*U(OtBu) was possible via an alternative pathway, eliminating sterics as a justification for the observed discrepancy in reactivity. These results offer insight into the role the -OH proton pKa plays in dictating the mechanism of O-H bond activation of alcohols by the uranium-substituted thiomolybdate cluster.

The mechanism of proton-coupled electron transfer at the surface of titanium-substituted polyoxovanadate-alkoxide clusters can be tuned by judicious selection of substrate.

We investigated proton insertion-coupled electron transfer (PICET) into tungsten oxide films using non-aqueous organic acid electrolytes. Operando UV-vis-NIR spectroelectrochemistry confirms PICET, showing respective broadband and dual-band transmittance changes for WO3 and WO3·H2O in the visible and NIR regions. WO3·H2O retains facile PICET kinetics in the absence of water.

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 [W6O19]2- displays reversible redox chemistry, which was found to be unchanged in the presence of acid, indicating an inability to couple reduction with protonation. However, the incorporation of a single vanadium center into the structure significantly changes the reactivity, and the potential required for one-electron reduction of [VW5O19]3- was shown to vary with the strength of the acid added. Construction of a potential-pK a diagram allowed assessment of the thermodynamics of H atom uptake, indicating BDFE(O-H) approximate to 64 kcal/mol, while chemical synthesis of the reduced/protonated derivative (TBA)3[VW5O19H] was used to probe the position of protonation.