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We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na 8 K 8 [Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] ( Co 9 POM ) and nanoporous n -type TiO 2 photoelectrodes (henceforth "TiO 2 - Co 9 POM ") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO 2 - Co 9 POM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO 2 in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound Co 9 POM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that Co 9 POM serves as an efficient WOC, extracting photogenerated holes from TiO 2 on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA 4 H 5 [PMo 6 V 6 O 40 ] ( PV 6 Mo 6 ) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number ( x = 0-4 and 6) in TBA salts of PV x Mo 12- x O 40 (3+ x )- ( PVMo ) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc + under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo /Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV 3 Mo 9 and PV 4 Mo 8 are 8.9 and 48 s -1 , respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo 12 is more positive than that of PVMo 11 (-236 and -405 mV vs Fc/Fc + ); however, the initial reduction rates are 1.06 × 10 -4 s -1 and 0.036 s -1 for PMo 12 and PVMo 11 , respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo 11 and PV 2 Mo 10 , where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕ CS ) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕ abs ), charge separation (ϕ CS ), and photoreduction (ϕ red ), i.e ., IPCE = ϕ abs ϕ CS ϕ red . Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕ CS and interfacial recombination at the junction that decreases ϕ red . Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO 2 , the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA 4 H 5 [PMo 6 V 6 O 40 ] ( PV 6 Mo 6 ) and (b) Cu(ClO 4 ) 2 in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 10 2 in less than one hour with a turnover frequency (TOF) of 6 × 10 -2 s -1 with respect to PV 6 Mo 6 . Multiple electrochemical, spectroscopic, and other methods establish that (1) PV 6 Mo 6 , a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu(II)/Cu(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV 6 Mo 6 and reoxidation of reduced PV 6 Mo 6 by O 2 ; and (3) the analogous polytungstate-based system, TBA 4 H 5 [PW 6 V 6 O 40 ] ( PV 6 W 6 ), has nearly identical cyclic voltammograms (CV) as PV 6 Mo 6 but has almost no catalytic activity: it does not exhibit self-redox buffering.

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the locations and percent occupancies of different 3d metals in these heterogeneous catalysts. Without such information, it is hard to quantify catalysis, stability, and other properties of the WOC as a function of the catalyst active site structure. This study combines the site selective synthesis of a homogeneous WOC with two adjacent 3d metals, [Co 2 Ni 2 (PW 9 O 34 ) 2 ] 10- ( Co 2 Ni 2 P 2 ) as a tractable molecular model for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) that quantifies both the location and percent occupancy of Co (∼97% outer-central-belt positions only) and Ni (∼97% inner-central-belt positions only) in Co 2 Ni 2 P 2 . This mixed-3d-metal complex catalyzes water oxidation an order of magnitude faster than its isostructural analogue, [Co 4 (PW 9 O 34 ) 2 ] 10- ( Co 4 P 2 ). Four independent and complementary lines of evidence confirm that Co 2 Ni 2 P 2 and Co 4 P 2 are the principal WOCs and that Co 2+ (aq) is not. Density functional theory (DFT) studies revealed that Co 4 P 2 and Co 2 Ni 2 P 2 have similar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multistep mechanisms, including likely Co-OOH formation, with the energetics of most steps being lower for Co 2 Ni 2 P 2 than for Co 4 P 2 . Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts.

Materials that both sequester chemical warfare agents (CWAs) and then catalytically decontaminate the entrapped CWAs are highly sought. This article reports such a system for air-based catalytic removal of the sulfur mustard (HD) simulant, 2-chloroethyl ethyl sulfide (CEES). Hypercrosslinked polymers (HCPs) sequester CEES, and an HCP-embedded oxidation system comprising tribromide, nitrate, and acid (NO x Br x H + ) simultaneously catalyzes the aerobic and selective, oxidative conversion of the entrapped CEES to the desired far less-toxic sulfoxide under ambient conditions (air and temperature). (NO x Br x H + ) has been incorporated into three HCPs, a fluorobenzene HCP (HCP-F), a methylated HCP (HCP-M), and an HCP with acidic moieties (HCP-A). HCP-A acts as both an absorbing material and a catalytic component due to its acidic side chains. All three HCP/NO x Br x H + systems work rapidly under these optimally mild conditions. No light or added oxidants are required. The HCP/NO x Br x H + systems are recyclable.

Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO 2 and its effect on charge carriers by transient reflectance spectroscopy. Increasing the TiO 2 layer thickness from 0 to 35 nm increases the field in the GaP depletion region, enhancing the rate and efficiency of interfacial electron transfer from the GaP to TiO 2 on the ps time scale as well as retarding interfacial recombination on the microsecond time scale. This study demonstrates a general method for providing a microscopic view of the photogenerated charge carrier's pathway and loss mechanisms from the bulk of the electrode to the long-lived separated charge at the interface that ultimately drives the photoelectrochemical reactions.

The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs 8 Nb 6 O 19 and (Et 2 NH 2 ) 8 [(α-PW 11 O 39 Zr(μ-OH)(H 2 O)) 2 ]·7H 2 O, (2) probing the effects that common ambient gases, such as CO 2 , SO 2 , and NO 2 , have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.

This Review summarizes the recent progress made in the field of chemical threat reduction by utilizing new in situ analytical techniques and combinations thereof to study multifunctional materials designed for capture and decomposition of nerve gases and their simulants. The emphasis is on the use of in situ experiments that simulate realistic operating conditions (solid-gas interface, ambient pressures and temperatures, time-resolved measurements) and advanced synchrotron methods, such as in situ X-ray absorption and scattering methods, a combination thereof with other complementary measurements (e.g., XPS, Raman, DRIFTS, NMR), and theoretical modeling. The examples presented in this Review range from studies of the adsorption and decomposition of nerve agents and their simulants on Zr-based metal organic frameworks to Nb and Zr-based polyoxometalates and metal (hydro)oxide materials. The approaches employed in these studies ultimately demonstrate how advanced synchrotron-based in situ X-ray absorption spectroscopy and diffraction can be exploited to develop an atomic- level understanding of interfacial binding and reaction of chemical warfare agents, which impacts the development of novel filtration media and other protective materials.

Development of technologies for protection against chemical warfare agents (CWAs) is critically important. Recently, polyoxometalates have attracted attention as potential catalysts for nerve-agent decomposition. Improvement of their effectiveness in real operating conditions requires an atomic-level understanding of CWA decomposition at the gas-solid interface. We investigated decomposition of the nerve agent Sarin and its simulant, dimethyl chlorophosphate (DMCP), by zirconium polytungstate. Using a multimodal approach, we showed that upon DMCP and Sarin exposure the dimeric tungstate undergoes monomerization, making coordinatively unsaturated Zr(IV) centers available, which activate nucleophilic hydrolysis. Further, DMCP is shown to be a good model system of reduced toxicity for studies of CWA deactivation at the gas-solid interface.

The generation of five types of Ln(III)-containing tungstotellurates(VI), dimeric (DMAH) 12 Na 2 [H 10 (WO 2 ){Ln(H 2 O) 5 (TeW 18 O 65 )} 2 ]· nH 2 O (abbreviated as {Ln 2 Te 2 W 37 }; Ln = Eu, Gd, or Tb; DMAH = dimethylammonium), tetrameric (DMAH) 21 Na 7 [H 16 {Ln(H 2 O) 5 (TeW 18 O 64 )} 4 ]· nH 2 O (abbreviated as {Ln 4 Te 4 W 72 }, Ln = Eu or Gd), 2:2 dimeric (DMAH) 12 [H 6 {Tb(H 2 O) 3 (TeW 17 O 61 )} 2 ]·25H 2 O (abbreviated as {Tb 2 Te 2 W 34 }), 1:1 monosubstituted (DMAH) 7 Na 2 [H 2 Tb(H 2 O) 4 (TeW 17 O 61 )]·21H 2 O (abbreviated as {TbTeW 17 }), and three-dimensional polymer (DMAH) 2 [HTb(H 2 O) 4 {TeW 6 O 24 }]·14H 2 O (abbreviated as {TbTeW 6 } n ), provides insight into the rich condensation chemistry of lacunary and other Dawson-type polyoxometalates. The pH and the type of Ln 3+ source both dictate which of these new complexes form. To our knowledge, {Ln 4 Te 4 W 72 } is the highest-nuclearity tungstotellurate to date, and {Tb 2 Te 2 W 34 } and {TbTeW 17 } contain the first lacunary {TeW 17 O 61 }. Electrospray ionization mass spectra analyses indicate that the Dawson-like building blocks, {TeW 18 O 65 } and {TeW 17 O 61 }, found in solid structures are also present in solution. The intense photoluminescence (characteristic green emission) of {TbTeW 6 } n , 100× greater than those of {Tb 2 Te 2 W 37 }, {Tb 2 Te 2 W 34 }, and {TbTeW 17 }, is explained by analysis of all 4 X-ray structures and multiple structure-intensity correlations.

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O 2 -based oxidations is reoxidation of the reduced POM by O 2 . We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POM red (α-PW 12 O 40 4- and α-SiVW 11 O 40 6- ) reacting with O 2 , is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POM red by Fe(III) and Cu(I) by O 2 to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PW 12 O 40 3- and α-SiVW 11 O 40 5- , are far better oxidants of organic substrates than the previously studied POMs, α-SiW 12 O 40 4- and α-AlW 12 O 40 5- , this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O 2 /air-based processes.

Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al 2 O 3 ) applied by the atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large family of molecular WOCs when on electrode surfaces. In this study, [{Ru IV 4 (OH) 2 (H 2 O) 4 }(γ-SiW 10 O 34 ) 2 ] 10- (Ru 4 Si 2 ) is supported on hematite photoelectrodes and then protected by ALD Al 2 O 3 ; this ternary system was characterized before and after photoelectrocatalytic water oxidation by Fourier transform infrared, X-ray photoelectron spectroscopy, energy-dispersive X-ray, and voltammetry. All these studies indicate that Ru 4 Si 2 remains intact with Al 2 O 3 ALD protection, but not without. The thickness of the Al 2 O 3 layer significantly affects the catalytic performance of the system: a 4 nm thick Al 2 O 3 layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al 2 O 3 layers thicker than 6.5 nm appear to completely bury the Ru 4 Si 2 catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr 6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.

Two novel polyoxovanadate (POV)-containing fluorosurfactants, each with two hydrophobic fluorinated "tails" and one nanosized, hydrophilic, rigid POV "head group", are synthesized for the first time. They self-assemble into spherical, bilayer vesicles in acetonitrile/water mixed solvents, as evidenced by systemic studies using laser light scattering (LLS) and electron microscopy techniques. The vesicle sizes demonstrate dynamic change over different solvent compositions mainly as a result of the solvent swelling of the fluorocarbon chains, although the charge number on the POVs changes over the solvent polarity as well.

The Co 2p 3/2 X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate K 5 H[CoW 12 O 40 ]·xH 2 O were measured. The low-energy dd transition features at 0.55 eV, unmeasurable with ultraviolet-visible (UV/vis) spectroscopy, were experimentally revealed in 2p3d RIXS spectra. RIXS simulations based on ligand-field multiplet theory were performed to assess the potential cobalt tetragonal symmetry distortion, which is described with the ligand-field parameters 10D q (-0.54 eV), D s (-0.08 eV), and D t (0.005 eV). Because 2p3d RIXS probes not only the optical spin-allowed transitions but also the spin-forbidden transitions, we show that the current 2p3d RIXS simulation enables a series of dd feature assignments with higher accuracy than those from previous optical data. Furthermore, by wave-function decomposition analyses, we demonstrate the more realistic and detailed origins of a few lowest dd transitions using both one-electron-orbital and term-symbol descriptions.

Copper-based complexes have been largely neglected as potential water reduction catalysts. This article reports the synthesis and characterization of a tetra-copper-containing polyoxotungstate, Na3K7[Cu4(H2O)2(B-α-PW9O34)2]·30H2O (Na3K7-Cu4P2). Cu4P2 is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate ([Ir(ppy)2(dtbbpy)][PF6]) as a light absorber and triethanolamine (TEOA) as sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of ∼1270 per Cu4P2 catalyst is obtained after 5 h of irradiation (light-emitting diode; λ = 455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9% is achieved. Both oxidative and reductive quenching pathways are observed by measuring the luminescence intensity of excited state [Ir(ppy)2(dtbbpy)](+*) in the presence of Cu4P2 or TEOA, respectively. Many stability studies (e.g., UV-vis absorption, FT-IR, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy) show that catalyst Cu4P2 undergoes slow decomposition under turnover conditions; however, both the starting Cu4P2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuOx particles. Considering the high abundance and low cost of copper, the present work provides considerations for the design and synthesis of efficient, molecular, water-compatible Cu-based water reduction catalysts.

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-)). Voltammetric studies with dissolved and surface-confined forms of [1(γ-SiW10O36)2](10-) suggest that the oxidized forms of 1 can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of 1 also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO3. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that 1 is one of the most active molecular electrocatalysts for methanol and ethanol oxidation.

The syntheses, structural characterization, and catalytic properties of two different nickel-containing polyoxometalates (POMs) are presented. The dinickel-containing sandwich-type POM [Ni2(P2W15O56)2](20-) (Ni2) exhibits an unusual αααα geometry. The trinickel-containing Wells-Dawson POM [Ni3(OH)3(H2O)3P2W16O59](9-) (Ni3) shows a unique structure where the [α-P2W15O56](12-) ligand is capped by a triangular Ni3O13 unit and a WO6 octahedron. Ni3 shows a high catalytic activity for visible-light-driven hydrogen evolution, while the activity for Ni2 is minimal. An analysis of the structures of multinickel-containing POMs and their hydrogen evolution activity is given.

A novel manganese(IV)-containing symmetrical heteropolyvanadate was prepared by the transformation of K7[MnV13O38]·18H2O (K7MnV13) to K4Li2[MnV14O40]·21H2O (1) at pH 4. The heteropolyanion [MnV14O40](6-) (MnV14) is composed of a MnO6 octahedron surrounded by 14 edge-sharing VO6 octahedra. The simplified representation of MnV14 has a new topology termed jba1 with a total point symbol of {3(10)}2{3(14).4(7)}4{3(18).4(10)}8{3(44).4(46).5}. In the crystal lattice of 1, MnV14 packs with potassium ions, forming a 2D layered K2[MnV14O40](4-) network (ABABAB...). Four K(+) ions cap the four square O4 faces of MnV14, apparently stabilizing the heteropolyanion. Compound 1 catalyzes the t-BuOOH-based oxidation of 2-chloroethyl ethyl sulfide (a mustard gas simulant). The magnetic and catalytic properties of 1 are discussed.

We report a hybrid computational approach to calculate electron transfer between a type-I CdSe/ZnS core/shell quantum dot (QD) with a varying shell thickness and the functionalized anthraquinone (AQ) molecule. This novel approach combines the traditional electron/hole confinement theory in the effective mass approximation for the QD and molecular orbital theory for the AQ molecule. In the present study, the QD's electron and hole envelope wave functions are solutions of the effective-mass Schrödinger equation, and the AQ wave function is obtained at the density functional level. Electron-transfer rate calculations are based on Marcus's theory with the coupling strength computed according to an one-electron orbital perturbation model. We show that in a heptane solution, the LUMO of AQ and the 1Se electron orbital of QD are involved in the charge separation (CS) process. The charge recombination (CR) process, on the other hand, occurs from the singly occupied molecular orbital of the AQ radical (which corresponds to the LUMO in AQ) to a trapped hole state of the QD within the band gap. The calculations support previously reported interpretations of the role of the ZnS shell as a hindrance in the CS and CR process.

Epilepsy is the most common neurological disorder, with over 50 million people worldwide affected. Recent evidence suggests that the transient receptor potential cation channel subfamily V member 1 (TRPV1) may contribute to the onset and progression of some forms of epilepsy. Since the two nonpsychotropic cannabinoids cannabidivarin (CBDV) and cannabidiol (CBD) exert anticonvulsant activity in vivo and produce TRPV1-mediated intracellular calcium elevation in vitro, we evaluated the effects of these two compounds on TRPV1 channel activation and desensitization and in an in vitro model of epileptiform activity. Patch clamp analysis in transfected HEK293 cells demonstrated that CBD and CBDV dose-dependently activate and rapidly desensitize TRPV1, as well as TRP channels of subfamily V type 2 (TRPV2) and subfamily A type 1 (TRPA1). TRPV1 and TRPV2 transcripts were shown to be expressed in rat hippocampal tissue. When tested on epileptiform neuronal spike activity in hippocampal brain slices exposed to a Mg(2+)-free solution using multielectrode arrays (MEAs), CBDV reduced both epileptiform burst amplitude and duration. The prototypical TRPV1 agonist, capsaicin, produced similar, although not identical effects. Capsaicin, but not CBDV, effects on burst amplitude were reversed by IRTX, a selective TRPV1 antagonist. These data suggest that CBDV antiepileptiform effects in the Mg(2+)-free model are not uniquely mediated via activation of TRPV1. However, TRPV1 was strongly phosphorylated (and hence likely sensitized) in Mg(2+)-free solution-treated hippocampal tissue, and both capsaicin and CBDV caused TRPV1 dephosphorylation, consistent with TRPV1 desensitization. We propose that CBDV effects on TRP channels should be studied further in different in vitro and in vivo models of epilepsy.

A tetra-nickel-containing polyoxotungstate, Na6K4[Ni4(H2O)2(PW9O34)2]·32H2O (Na6K4-Ni4P2), has been synthesized in high yield and systematically characterized. The X-ray crystal structure confirms that a tetra-nickel cluster core [Ni4O14] is sandwiched by two trivacant, heptadentate [PW9O34](9-) POM ligands. When coupled with (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate [Ir(ppy)2(dtbbpy)][PF6] as photosensitizer and triethanolamine (TEOA) as sacrificial electron donor, the noble-metal-free complex Ni4P2 works as an efficient and robust molecular catalyst for H2 production upon visible light irradiation. Under minimally optimized conditions, Ni4P2 catalyzes H2 production over 1 week and achieves a turnover number (TON) of as high as 6500 with almost no loss in activity. Mechanistic studies (emission quenching, time-resolved fluorescence decay, and transient absorption spectroscopy) confirm that, under visible light irradiation, the excited state [Ir(ppy)2(dtbbpy)](+)* can be both oxidatively and reductively quenched by Ni4P2 and TEOA, respectively. Extensive stability studies (e.g., UV-vis absorption, FT-IR, mercury-poison test, dynamic light scattering (DLS) and transmission electron microscopy (TEM)) provide very strong evidence that Ni4P2 catalyst remains homogeneous and intact under turnover conditions.

Two MOF-like but all-inorganic polyoxometalate-based networks, [Na7X2W18Sn9Cl5O68·(H2O)m]n (1, X = Si, m = 35; 2, X = Ge, m = 41), and the molecular tetramer Na6[{Na(μ-OH2)(OH2)2}6{Sn6(B-SbW9O33)2}2]·50H2O (3) have been prepared and characterized by X-ray diffraction and spectroscopic methods. All three compounds exhibit unique structural features, and networks 1 and 2 incorporate the highest nuclearity of Sn(II)-containing POMs to date. Tetramer 3 comprises bridging Sn(II) ions with [B-SbW9O33](9-) units and exhibits two highly unusual features, a long-range Sb···Sb interaction and an intramolecular charge-transfer transition involving donation of the lone-pair electron density on both Sb(III) and Sn(II) to the POM. The electronic structure and excited-state dynamics have been studied by transient spectroscopy, spectroelectrochemistry, DFT calculations, and resonance Raman spectroscopy. The synergistic effect of two types of stereoactive lone-pairs on Sb(III) and Sn(II) is critical for the charge-transfer absorption feature in the visible.

We describe an extension of the conventional Fourier grid discrete variable representation (DVR) to the bound state problem of a particle with a position-dependent mass. An infinite order DVR, derived for a variable mass kinetic energy operator, coupled with an efficient grid contraction scheme yields essentially exact eigenvalues for a chosen grid spacing. Implementation of the method is shown to be very practical due to the fact that in a DVR no integral evaluation is necessary and that the resultant kinetic energy matrix is sparse. Numerical calculations are presented for exciton states of spherical, cylindrical, and toric Type I (CdSe/ZnS) core-shell quantum dots. In these examples, electron-hole interaction is treated explicitly by solving a self-consistent Schrödinger-Poisson equation on a contracted DVR grid. Prospective applications of the developed approach to calculating electron transfer rates between adsorbed molecular acceptors and quantum confined nanocrystals of generic shape, dimensionality, and composition are also discussed.

The water insoluble complex Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2], ([Ru(II)bpy]5[Ru(III)4POM]), was synthesized from Rb8K2[{Ru(IV)4O4(OH)2(H2O)4}(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [Ru(II)bpy]5[Ru(III)4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)3](3+)([Ru(III)bpy]) acting as the electron transfer mediator and [Ru(V)4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s(-1) was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [Ru(II)bpy]5[Ru(III)4POM] → [Ru(III)bpy]5[Ru(V)4POM] + 13 e(-) and [Ru(III)bpy]5[Ru(V)4POM] + 2H2O → [Ru(III)bpy]5[Ru(IV)4POM] + O2 + 4H(+). The voltammetry of related water insoluble [Ru(II)bpy]2[S2M18O62] (M = W and Mo) and [Fe(II)Phen]x[Ru(III)4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [Ru(III)bpy] is the electron transfer mediator and [Ru(V)4POM] is the species responsible for oxygen evolution.

An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

A polyoxometalate-supported trirhenium carbonyl cluster, mimicking metal oxide supported interfacial dyadic structures, has been synthesized and characterized. Multiple techniques, including computational and transient absorption spectroscopy, have been applied to characterize the charge-transfer dynamics occurring at the interfaces of this "double cluster". The stepwise kinetics of charge separation and recombination has been thoroughly investigated.

Voltammetric studies of the Ru-containing polyoxometalate water oxidation molecular catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-) where 1 represents the {Ru4O4(OH)2(H2O)4} core and 1(0) stands for its initial form with all ruthenium centers in the oxidation state IV) have been carried out in aqueous media over a wide range of pH (2-12 using Britton-Robinson buffer) and ionic strength. Well-defined voltammograms in buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO3, NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, MgSO4, or Na2SO4). A combination of data derived from dc cyclic, rotating disk electrode, and Fourier transformed large amplitude ac voltammetry allow the assignment of two processes related to reduction of the framework and the complete series of Ru(III/IV) and Ru(IV/V) redox processes and also provide their reversible potentials. Analysis of these data reveals that K(+) has a significantly stronger interaction with 1(1) (the number inside bracket stands for the number of electrons removed from 1(0)) than found for the other cations investigated, and hence its presence significantly alters the pH dependence of the 1(0)/1(1) reversible potential. Comparison of experimental data with theory developed in terms of equilibrium constants for process 1(0)/1(1) reveals that both H(+) and K(+) interact competitively with both 1(0) and 1(1). Importantly, reversible potential data reveal that (i) proton transfer does not necessarily need to be coupled to all electron transfer steps to achieve catalytic oxidation of water, (ii) the four-electron oxidized form, 1(4), is capable of oxidizing water under all conditions studied, and (iii) under some conditions, the three-electron oxidized form, 1(3), also exhibits considerable catalytic activity.

Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) [Co4(H2O)2(PW9O34)2](10-) (Co4POM), initial reports of an efficient, molecular catalyst have been challenged by studies suggesting that formation of cobalt oxide (CoOx) or other byproducts are responsible for the catalytic activity. Thus, we describe a series of experiments for thorough examination of active species under catalytic conditions and apply them to Co4POM. These provide strong evidence that under the conditions initially reported for water oxidation using Co4POM (Yin et al. Science, 2010, 328, 342), this POM anion functions as a molecular catalyst, not a precursor for CoOx. Specifically, we quantify the amount of Co(2+)(aq) released from Co4POM by two methods (cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry) and show that this amount of cobalt, whatever speciation state it may exist in, cannot account for the observed water oxidation. We document that catalytic O2 evolution by Co4POM, Co(2+)(aq), and CoOx have different dependences on buffers, pH, and WOC concentration. Extraction of Co4POM, but not Co(2+)(aq) or CoOx into toluene from water, and other experiments further confirm that Co4POM is the dominant WOC. Recent studies showing that Co4POM decomposes to a CoOx WOC under electrochemical bias (Stracke and Finke, J. Am. Chem. Soc., 2011, 133, 14872), or displays an increased ability to reduce [Ru(bpy)3](3+) upon aging (Scandola, et al., Chem. Commun., 2012, 48, 8808) help complete the picture of Co4POM behavior under various conditions but do not affect our central conclusions.

We present results on parameterization of reactive force field [van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396-9409] for investigating the properties of the [Nb6O19Hx]((8-x)-) Lindqvist polyoxoanion, x = 0-8, in water. Force-field parameters were fitted to an extensive data set consisting of structures and energetics obtained at the Perdew-Burke-Ernzerhof density functional level of theory. These parameters can reasonably describe pure water structure as well as water with an excess of H(+) and OH(-) ions. Molecular dynamics simulations were performed on [Nb6O19Hx]((8-x)-), x = 0-8, submerged in bulk water at 298 K. Analysis of the MD trajectories showed facile H atom transfer between the protonated polyoxoanion core and bulk water. The number of oxygen sites labeled with an H atom was found to vary depending on the pH of the solution. Detailed analysis shows that the total number of protons at bridging (terminal), η-O (μ2-O), sites ranges from 3(1) at pH 7, to 2(0) at pH 11, to 1(0) at pH 15. These findings closely reflect available experimental measurements.

Di- and tricobalt silicotungstate complexes, K(5)Na(4)H(4)[{Na(3)(μ-OH(2))(2)Co(2)(μ-OH)(4)} (Si(2)W(18)O(66))]·37H(2)O (1) and K(6)Na(3)[Na(H(2)O){Co(H(2)O)(3)}(2){Co(H(2)O)(2)}(Si(2)W(18)O(66))]·22H(2)O (2), have been synthesized through reaction of cobalt chloride and [A-α-SiW(9)O(34)](10-) in acidic buffer solution. They have been characterized by X-ray crystallography, elemental analysis, cyclic voltammetry, infrared, and UV-vis spectroscopy. In 1, two cobalt atoms as well as three sodium atoms are incorporated in the central pocket of the [Si(2)W(18)O(66)](16-) polyanion. In 2, one cobalt atom and one sodium atom are incorporated in the pocket of [Si(2)W(18)O(66)](16-); two other cobalt atoms in this complex protrude outside the pocket and connect with WO(6) units of other [Si(2)W(18)O(66)](16-) polyanions to form a one-dimensional polymeric structure. The crucial parameters in the synthesis of these two compounds are discussed, and their stability in different buffer solutions is studied. The decomposition of 1 or 2 in heated potassium acetate buffer (pH 4.8, 1 M) yields K(11)[{Co(2)(H(2)O)(8)}K(Si(2)W(18)O(66))]·17H(2)O (3) based on spectroscopic studies and an X-ray crystal structure.

A new series of complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These two-component polyoxometalate (POM) compounds have been made by reaction of solvated {M(CO)(3)}(+) ions (M = Re or Mn) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb or Bi) POM multidentate ligands in aqueous solution. These syntheses reveal that the fac-{WO(OH)(2)}(2+) groups in the terminal positions of these two POM ligands are easily replaced by the topologically equivalent units fac-{M(CO)(3)}(+). Four compounds, [X(2)W(20)O(70){M(CO)(3)}(2)](12-) (1a: X = Sb, M = Re; 1b: X = Bi, M = Re; 2a: X = Sb, M = Mn; 2b: X = Bi, M = Mn) have been isolated and characterized of X-ray crystallography, spectroscopic, and computational methods. The charge transfer dynamics, investigated by femtosecond transient absorption (TA) spectroscopy of 1a and 1b combined with the density functional theory (DFT) calculations indicate that both complexes exhibit metal-to-polyoxometalate charge-transfer (MPCT) from the Re centers to the POM ligands, while MPCT from the Mn centers to the POM ligands in 2a and 2b leads to decomposition of starting compounds. The studies suggest a general synthetic route to a potentially very large class of POM-based hybrid compounds.

The electrochemistry of the water oxidation catalyst, Rb(8)K(2)[{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)] (Rb(8)K(2)-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A'/A to H/H') that are readily detected under dc voltammetric conditions. The six most positive processes (A'/A to F/F'), each involve a one electron ruthenium based charge transfer step (A'/A, B'/B are Ru(IV/V) oxidation and C/C' to F/F' are Ru(IV/III) reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B'/B, E/E', and F/F' processes are quasi-reversible, while the others are close to reversible. A third Ru(IV/V) oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb(8)K(2)-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G') assigned to Ru(III/II) reduction and the multiple electron transfer reduction process (H/H') that arise from the tungstate polyoxometalate framework. A significant shift of E°' to a more positive potential value for process H/H' was observed on removal of K(+) (~100 mV in H(2)SO(4) and ~50 mV in HCl).

The advancement of direct solar-to-fuel conversion technologies requires the development of efficient catalysts as well as efficient materials and novel approaches for light harvesting and charge separation. We report a novel system for unprecedentedly efficient (with near-unity quantum yield) light-driven reduction of methylviologen (MV(2+)), a common redox mediator, using colloidal quasi-type II CdSe/CdS dot-in-rod nanorods as a light absorber and charge separator and mercaptopropionic acid as a sacrificial electron donor. In the presence of Pt nanoparticles, this system can efficiently convert sunlight into H(2), providing a versatile redox mediator-based approach for solar-to-fuel conversion. Compared to related CdSe seed and CdSe/CdS core/shell quantum dots and CdS nanorods, the quantum yields are significantly higher in the CdSe/CdS dot-in-rod structures. Comparison of charge separation, recombination and hole filling rates in these complexes showed that the dot-in-rod structure enables ultrafast electron transfer to methylviologen, fast hole removal by sacrificial electron donor and slow charge recombination, leading to the high quantum yield for MV(2+) photoreduction. Our finding demonstrates that by controlling the composition, size and shape of quantum-confined nanoheterostructures, the electron and hole wave functions can be tailored to produce efficient light harvesting and charge separation materials.

Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.

[P(4)W(35)O(124){Re(CO)(3)}(2)](16-) (1), a Wells-Dawson [α(2)-P(2)W(17)O(61)](10-) polyoxometalate (POM)-supported [Re(CO)(3)](+) complex containing covalent W(VI)-O-Re(I) bonds has been synthesized and characterized by several methods, including X-ray crystallography. This complex shows a high visible absorptivity (ε(470 nm) = 4000 M(-1) cm(-1) in water) due to the formation of a Re(I)-to-POM charge transfer (MPCT) band. The complex was investigated by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the Re(I) center to the POM ligand., (© 2011 American Chemical Society)

A combination of polyanion size and charge allows the Keggin-type polyoxometalate (POM), [CuPW(11)O(39)](5-), a catalyst for some air-based organic oxidations, to fit snuggly in the pores of MOF-199 (HKUST-1), a metal-organic framework (MOF) with the POM countercations residing in alternative pores. This close matching of POM diameter and MOF pore size in this POM-MOF material, [Cu(3)(C(9)H(3)O(6))(2)](4)[{(CH(3))(4)N}(4)CuPW(11)O(39)H] (1), results in a substantial synergistic stabilization of both the MOF and the POM. In addition, this heretofore undocumented POM-MOF interaction results in a dramatic increase in the catalytic turnover rate of the POM for air-based oxidations. While 1 catalyzes the rapid chemo- and shape-selective oxidation of thiols to disulfides and, more significantly, the rapid and sustained removal of toxic H(2)S via H(2)S + 1/2 O(2) → 1/8 S(8) + H(2)O (4000 turnovers in <20 h), the POM or the MOF alone is catalytically slow or inactive. Three arguments are consistent with the catalytic reactions taking place inside the pores. POM activation by encapsulation in the MOF likely involves electrostatic interactions between the two components resulting in a higher reduction potential of the POM.

Two inorganic-organic hybrid clusters with one or two covalently linked pyrene fluorescent probes, [(n-C(4)H(9))(4)N](2)[V(6)O(13){(OCH(2))(3)C(NH(CO)CH(2)CH(2)CH(2)C(16)H(9))}{(OCH(2))(3)C-(NH(2))}] ((TBA(+))(2)1) and [(n-C(4)H(9))(4)N](2)[V(6)O(13){(OCH(2))(3)C(NH(CO)CH(2)CH(2)CH(2)C(16)H(9))}(2)] ((TBA(+))(2)2), respectively, are synthesized from Lindqvist type polyoxometalates (POMs). The incorporation of pyrene into POMs results in amphiphilic hybrid molecules and simultaneously offers a great opportunity to study the interaction between hybrid clusters and their counterions. 2D-NOESY NMR and fluorescence techniques have been used to study the role of counterions such as tetrabutyl ammonium (TBA) in the vesicle formation of the hybrid clusters. The TBA(+) ions not only screen the electrostatic repulsions between the POM head groups but also are involved in the hydrophobic region of the vesicular structure where they interrupt the formation of pyrene excimers that greatly perturbs the luminescence signal from the vesicle solution. By replacing the TBA(+) counterions with protons, the new vesicles demonstrate interesting pH-dependent fluorescence properties.

The abundant-metal-based polyoxometalate complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is a hydrolytically and oxidatively stable, homogeneous, and efficient molecular catalyst for the visible-light-driven catalytic oxidation of water. Using a sacrificial electron acceptor and photosensitizer, it exhibits a high (30%) photon-to-O(2) yield and a large turnover number (>220, limited solely by depletion of the sacrificial electron acceptor) at pH 8. The photocatalytic performance of this catalyst is superior to that of the previously reported precious-metal-based polyoxometalate water oxidation catalyst [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)](10-).

To probe photoinduced water oxidation catalyzed by the Mn₄O₄L₆ cubane clusters, we have computationally studied the mechanism and controlling factors of the O₂ formation from the [Mn₄O₄L₆] catalyst, 6. It was demonstrated that dissociation of an L = H₂PO₂⁻ ligand from 6 facilitates the direct O-O bond formation that proceeds with a 28.3 (33.4) kcal/mol rate-determining energy barrier at the transition state TS1. This step (the O-O single bond formation) of the reaction is a two-electron oxidation/reduction process, during which two oxo ligands are transformed into to μ²:η²-O₂²⁻ unit, and two ("distal") Mn centers are reduced from the 4+ to the 3+ oxidation state. Next two-electron oxidation/reduction occurs by "dancing" of the resulted O₂²⁻ fragment between the Mn¹ and Mn²/Mn(2')-centers, keeping its strong coordination to the Mn(1')-center. As a result of this four-electron oxidation/reduction process Mn centers of the Mn₄-core of I transform from {Mn¹(III)-Mn(1')(III)-Mn²(IV)-Mn(2')(IV)} to {Mn¹(II)-Mn(1')(II)-Mn²(III)-Mn(2')(III)} in IV. In other words, upon O₂ formation in cationic complex [Mn₄O₄L₅](+), I, all four Mn-centers are reduced by one electron each. The overall reaction I → TS1 → II → III → TS2 → IV → TS3 → V → VI + O₂ is found to be exothermic by 15.4 (10.5) kcal/mol. We analyze the lowest spin states and geometries of all reactants, intermediates, transition states, and products of the targeted reaction.

The mechanism and electron transfer dynamics of the reaction [Ru(II)(mptpy)(2)](4+) + hnu + [S(2)O(8)](2-) --> [Ru(III)(mptpy)(2)](5+) + SO(4)(2-) + SO(4)(-*) were studied using various computational (density functional and exciton interaction theories) and experimental (transient absorption, static and time-resolved fluorescence spectroscopy, and other) techniques. The results were compared with those recently reported for [Ru(bpy)(3)](2+) dye [ref 18]. It was found that the excitation energy of [Ru(mptpy)(2)](4+) is about 0.4-0.5 eV smaller than that of [Ru(bpy)(3)](2+), which is consistent with the measured absorption maxima of 445 and 507 nm, for [Ru(bpy)(3)](2+) and [Ru(mptpy)(2)](4+), respectively. The smaller excitation energy in [Ru(mptpy)(2)](4+) correlates with much slower electron transfer rates to persulfate compared to [Ru(bpy)(3)](2+). The quenching of the photoexcited [Ru(mptpy)(2)](4+) by [S(2)O(8)](2-) occurs via a unimolecular mechanism with formation of a weak ion-pair complex {[Ru(mptpy)(2)](4+)...([S(2)O(8)](2-))(n)}, where n = 1 and 2. The initial photon is absorbed by the [Ru(mptpy)(2)](4+) fragment forming an MLCT state, e.g., the bright singlet state S1. This S1 state undergoes a fast spin-orbit coupling induced intersystem crossing to a lower-lying triplet and rapid subsequent relaxation down to the lowest triplet T1 via internal conversion and collisions with solvent molecules. At this stage, the electron transfer from [Ru(mptpy)(2)](4+) to a loosely attached [S(2)O(8)](2-) occurs in a dark reaction via elongation of the O-O peroxo bond of the oxidant [S(2)O(8)](2-). The electron transfer lifetimes in water are calculated to be 1/kappa(1) = 199.4 ns and 1/kappa(2) = 108.4 ns, for the 1:1 and 1:2 complexes, respectively. The computed electron transfer lifetimes (1/kappa(1)) are in reasonable agreement with their experimental values of 298 and 149 ns for the 1:1 and 1:2 complexes, respectively. The effect of solvent polarity on electron transfer rates is found to be significant: the less polar acetonitrile slows the rate by an order of magnitude compared to water.

A new family of sandwich-type polytungstophosphates containing two different types of metals in the central belt, M'(2)M(2)(PW(9)O(34))(2)(12-) (M' = Na or Li, M = Mn(2+), Co(2+), Ni(2+), and Zn(2+)), have been synthesized and characterized by infrared spectroscopy, (31)P solution NMR spectroscopy, and elemental analysis. Compounds Na(2)Co(2)(PW(9)O(34))(2)(12-) (Na2Co2), Na(2)Ni(2)(PW(9)O(34))(2)(12-) (Na2Ni2), Li(2)Ni(2)(PW(9)O(34))(2)(12-) (Li2Ni2), Na(2)Mn(2)(PW(9)O(34))(2)(12-) (Na2Mn2), and Li(2)Zn(2)(PW(9)O(34))(2)(12-) (Li2Zn2) were characterized by X-ray crystallography. All these compounds have similar structures, in which two transition-metal ions and two alkali metal ions (sodium or lithium) are sandwiched between two [B-alpha-PW(9)O(34)](9-) units; the transition and alkali metals reside in the internal and external (solvent exposed) positions of the central belt, respectively. By adding LiCl to aqueous solutions of compounds Na2M2, lithium-sodium exchanges in the external belt positions are observed by (31)P solution NMR spectroscopy and X-ray crystallography. Magnetic measurements indicate ferromagnetic exchange interactions between the two Ni(2+) ions in Na2Ni2 at 10-300 K and the two Co(2+) ions in Na2Co2 at 6-30 K. In contrast, Na2Mn2 exhibits an antiferromagnetic interaction between the Mn(2+) ions at 2-50 K.

We examine the mechanism of electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) in aqueous solutions using time-dependent density functional and transition charge density model levels of theory. The calculations support the existence of a short-lived, optically bright S1 state that decays to lower-lying triplet states T1 and T2. The T1 state leads to the charge transfer products, that is, [Ru(bpy)(3)](3+) + SO(4)(-*) + SO(4)(2-). It was shown that photoinduced (in the 420-520 nm wavelength range) electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) may proceed via both the "unimolecular" and "bimolecular" pathways, in agreement with the previous experimental findings. This distinction arises on the basis of a weak interaction between the two reactants. Analysis of excited electronic states and their spin-orbit mixing suggests that a photon excites a MLCT S1 state of [Ru(bpy)(3)](2+)*/{[Ru(bpy)(3)](2+)...[S(2)O(8)](2-)}* that converts to the lower-lying T2 state via spin-orbit interaction, which is an intermediate in the S1 --> T2 --> T1 chain. Once the system has converted to T1, it can evolve toward charge transfer from [Ru(bpy)(3)](2+)* to [S(2)O(8)](2-) via elongation of the peroxo O-O bond that brings the "zero order" states of the [Ru(bpy)(3)](2+)* and [S(2)O(8)](2-) fragments into (near) resonance. An exciton interaction model in combination with the transition charge density model provides excellent agreement with TD-DFT and allows us to obtain insight into the electron transfer process. The steady-state luminescence studies of the quenching of [Ru(bpy)(3)](2+)* in aqueous solutions by [S(2)O(8)](2-) at pH 7.2 and 20 mM sodium phosphate buffer, the exact conditions used in our previous study of water oxidation [Geletii, Y. V.; Huang, Z. Q.; Hou, Y.; Musaev, D. G.; Lian, T. Q.; Hill, C. L. J. Am. Chem. Soc. 2009, 131, 7522-7523], show that 61% of the [Ru(bpy)(3)](2+) forms a ground state ion-pair complex [Ru(bpy)(3)](2+)...[S(2)O(8)](2-), and the unimolecular ET rate is 2.5 times faster than for the bimolecular process. The concentration of the ion-pair complexes decreases at higher sodium phosphate buffer.

Geometry and electronic structure of five species [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](10-) (1), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](9-) (2), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](8-) (3), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](7-) (4), and [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](6-) (5) with different oxidation states of Ru centers were studied at the density functional and COSMO levels of theory. These species are expected to be among the possible intermediates of the recently reported 1-catalyzed water oxidation (Geletii, Y. V.; Botar, B.; Kogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem. Int. Ed. 2008, 47, 3896-3899 and Sartorel, A.; Carraro, M.; Scorrano, G.; Zorzi, R. D.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. J. Am. Chem. Soc. 2008, 130, 5006-5007). It was shown that RI-BP86 correctly describes the geometry and energy of the low-lying electronic states of compound 1, whereas the widely used B3LYP approach overestimates the energy of its high-spin states. Including the solvent and/or countercation effects into calculations improves the agreement between the calculated and experimental data. It was found that the several HOMOs and LUMOs of the studied complexes are bonding and antibonding orbitals of the [Ru(4)O(4)(OH)(2)(H(2)O)(4)](6+) core, and four subsequent one-electron oxidations of 1, leading to formation of 2, 3, 4, and 5, respectively, involve only {Ru(4)} core orbitals. In other words, catalyst instability due to ligand oxidation in the widely studied Ru-blue dimer, [(bpy)(2)(O)Ru(V)-(mu-O)-Ru(V)(O)(bpy)(2)](4+), is not operable for 1: the latter all-inorganic catalyst is predicted to be stable under water oxidation turnover conditions. The calculated HOMOs and LUMOs of all the studied species are very close in energy and exhibit a "quasi-continuum" or "nanoparticle-type" electronic structure similar to that of nanosized transition metal clusters. This conclusion closely correlates with the experimentally reported oxidation and reduction features of 1 and explains the unusual linear dependence of oxidation potential versus charges for these compounds. The decrease in total negative charge of the system via 1 > 2 > 3 > 4 > 5, on average, decreases the {Ru(4)}-{SiW(10)} distance. It is predicted that at higher pH compound 1 will, initially, release protons from the mu-O(Ru) oxygen centers.

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

The nitrido derivative (n-Bu(4)N)(4)[PW(11)O(39)Ru(VI)N] transfers its nitrogen atom to triphenylphosphine to give quantitatively the bis(triphenylphosphane)iminium cation [Ph(3)PNPPh(3)](+). An intermediate can be prepared by the reaction of a single molecule of triphenylphosphine with the polyoxometalate, the iminophosphorane derivative (n-Bu(4)N)(3)[PW(11)O(39)Ru(V){NPPh(3)}]. The reactivity of the latter species has been investigated to complete the general scheme of the nitrogen-transfer reaction. By the addition of 1 equiv of hydroxide, [PW(11)O(39)Ru(III){N(OH)PPh(3)}](4-) is obtained. The reaction can be reversed by the addition of one proton. The phosphinoxime derivative [PW(11)O(39)Ru(III){N(OH)PPh(3)}](4-) can be prepared in solution but is unstable. It decomposes to yield quantitatively (n-Bu(4)N)(4)[PW(11)O(39)Ru(III){OPPh(3)}]. All of those species have been thoroughly characterized by mass spectrometry, paramagnetic (31)P NMR, IR, Raman, UV-visible, XANES, and EXAFS spectroscopies.