Aerobic Oxidation of Formaldehyde Catalyzed by Polyvanadotungstates

Three tetra-<i>n</i>-butylammonium (TBA) salts of polyvanadotungstates, [<i>n</i>-Bu₄N]₆[PW₉V₃O₄₀] (<b>PW</b>_<b>9</b><b>V</b>_<b>3</b>), [<i>n</i>-Bu₄N]₅H₂PW₈V₄O₄₀ (<b>PW</b>_<b>8</b><b>V</b>_<b>4</b>), and [<i>n</i>-Bu₄N]₄H₅PW₆V₆O₄₀·20H₂O (<b>PW</b>_<b>6</b><b>V</b>_<b>6</b>), have been synthesized and shown to be effective catalysts for the aerobic oxidation of formaldehyde to formic acid under ambient conditions. These complexes, characterized by elemental analysis, Fourier transform infrared spectroscopy, UV–vis spectroscopy, and thermogravimetric analysis, exhibit a catalytic activity for this reaction comparable to those of other polyoxometalates. Importantly, they are more effective in the presence of water than the metal oxide-supported Pt and/or Au nanoparticles traditionally used as catalysts for formaldehyde oxidation in the gas phase. The polyvanadotungstate-catalyzed oxidation reactions are first-order in formaldehyde, parabolic-order (slow, fast, and slow again) in catalyst, and zero-order in O₂. Under optimized conditions, a turnover number of ∼57 has been obtained. These catalysts can be recycled and reused without a significant loss of catalytic activity

Near Unity Quantum Yield of Light-Driven Redox Mediator Reduction and Efficient H₂ Generation Using Colloidal Nanorod Heterostructures

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 (MV²⁺), 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 H₂, 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 MV²⁺ 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

Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN₂S₂) charge transfer complex were synthesized and used as photosensitizers for the generation of H₂ from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b>. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π–π* absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN₂S₂ complexes. The shift of the rhodamine π–π* absorption maxima in going from <b>Pt-RDM1</b> to <b>Pt-RDM3</b> correlates well with the HOMO–LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H₂ generation when attached to platinized TiO₂ nanoparticles (Pt-TiO₂). After 40 h of irradiation, systems containing <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b> exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine ¹π–π* state to the singlet charge transfer (¹CT) state of the PtN₂S₂ chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine ¹π–π* state to a charge separated (CS) RDM^(0•)-Pt^(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN₂S₂ dyads for H₂ generation correlate well with the relative energies of the CS state and the PtN₂S₂ ³CT state used for H₂ production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H₂ from aqueous protons

Hole Removal Rate Limits Photodriven H₂ Generation Efficiency in CdS-Pt and CdSe/CdS-Pt Semiconductor Nanorod–Metal Tip Heterostructures

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

Syntheses, Structural Characterization, and Catalytic Properties of Di- and Trinickel Polyoxometalates

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– (<b>Ni</b>_<b>2</b>) exhibits an unusual αααα geometry. The trinickel-containing Wells–Dawson POM [Ni₃(OH)₃(H₂O)₃P₂W₁₆O₅₉]^9– (<b>Ni</b>_<b>3</b>) shows a unique structure where the [α-P₂W₁₅O₅₆]^12– ligand is capped by a triangular Ni₃O₁₃ unit and a WO₆ octahedron. <b>Ni</b>_<b>3</b> shows a high catalytic activity for visible-light-driven hydrogen evolution, while the activity for <b>Ni</b>_<b>2</b> is minimal. An analysis of the structures of multinickel-containing POMs and their hydrogen evolution activity is given

Catalytic Light-Driven Generation of Hydrogen from Water by Iron Dithiolene Complexes

The development of active, robust systems for light-driven hydrogen production from aqueous protons based on catalysts and light absorbers composed solely of earth abundant elements remains a challenge in the development of an artificial photosynthetic system for water splitting. Herein, we report the synthesis and characterization of four closely related Fe bis­(benzenedithiolate) complexes that exhibit catalytic activity for hydrogen evolution when employed in systems with water-soluble CdSe QDs as photosensitizer and ascorbic acid as a sacrificial electron source under visible light irradiation (520 nm). The complex with the most electron-donating dithiolene ligand exhibits the highest activity, the overall order of activity correlating with the reduction potential of the formally Fe­(III) dimeric dianions. Detailed studies of the effect of different capping agents and the extent of surface coverage of these capping agents on the CdSe QD surfaces reveal that they affect system activity and provide insight into the continued development of such systems containing QD light absorbers and molecular catalysts for H₂ formation

Catalytic Light-Driven Generation of Hydrogen from Water by Iron Dithiolene Complexes

The development of active, robust systems for light-driven hydrogen production from aqueous protons based on catalysts and light absorbers composed solely of earth abundant elements remains a challenge in the development of an artificial photosynthetic system for water splitting. Herein, we report the synthesis and characterization of four closely related Fe bis­(benzenedithiolate) complexes that exhibit catalytic activity for hydrogen evolution when employed in systems with water-soluble CdSe QDs as photosensitizer and ascorbic acid as a sacrificial electron source under visible light irradiation (520 nm). The complex with the most electron-donating dithiolene ligand exhibits the highest activity, the overall order of activity correlating with the reduction potential of the formally Fe­(III) dimeric dianions. Detailed studies of the effect of different capping agents and the extent of surface coverage of these capping agents on the CdSe QD surfaces reveal that they affect system activity and provide insight into the continued development of such systems containing QD light absorbers and molecular catalysts for H₂ formation

Differentiating Homogeneous and Heterogeneous Water Oxidation Catalysis: Confirmation that [Co₄(H₂O)₂(α-PW₉O₃₄)₂]^10– Is a Molecular Water Oxidation Catalyst

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 (CoO_<i>x</i>) 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. <i>Science</i>, <b>2010</b>, <i>328</i>, 342), this POM anion functions as a molecular catalyst, not a precursor for CoO_<i>x</i>. Specifically, we quantify the amount of Co²⁺(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, Co²⁺(aq), and CoO_<i>x</i> have different dependences on buffers, pH, and WOC concentration. Extraction of Co₄POM, but not Co²⁺(aq) or CoO_<i>x</i> into toluene from water, and other experiments further confirm that Co₄POM is the dominant WOC. Recent studies showing that Co₄POM decomposes to a CoO_<i>x</i> WOC under electrochemical bias (Stracke and Finke, <i>J. Am. Chem. Soc.</i>, <b>2011</b>, <i>133</i>, 14872), or displays an increased ability to reduce [Ru­(bpy)₃]³⁺ upon aging (Scandola, et al., <i>Chem. Commun</i>., <b>2012</b>, <i>48</i>, 8808) help complete the picture of Co₄POM behavior under various conditions but do not affect our central conclusions

Three Candesartan Salts with Enhanced Oral Bioavailability

Three new salts, [H₃N­(CH₂)₂NH₃]­[can]·2H₂O (<b>1</b>), [H₃N­(CH₂)₃NH₃]­[can]·2H₂O (<b>2</b>), and [NH₄]­[Hcan] (<b>3</b>), of the minimally soluble antihypertensive drug, Candesartan (H₂can), have been prepared by solvent-assisted grinding. Salts <b>1–3</b> also have been thoroughly characterized by single-crystal X-ray diffraction, powder X-ray diffraction, Fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance, thermogravimetry, and differential scanning calorimetry. In the case of <b>1</b> and <b>2</b>, two protons of carboxyl and tetrazole groups of Candesartan transfer to the diamine, resulting in salts where both hydrogen bonding and electrostatic interactions that link the Candesartan and diamine (diammonium) units into a one-dimensional supramolecular ribbon. However, unlike the case in <b>1</b> and <b>2</b>, only one proton from the carboxyl group of Candesartan transfers to ammonia in <b>3</b> and ionic components now assemble into a three-dimensional supramolecular network. Dissolution studies indicate that both the apparent solubility and dissolution rate of salts <b>2</b> and <b>3</b> in phosphate buffer are dramatically improved compared to those of the original active pharmaceutical ingredient (API). Furthermore, to evaluate the absorption effect of salts <b>1–3</b> <i>in vivo</i>, pharmacokinetic studies were performed in rats. It is notable that the oral bioavailability of salts <b>1–3</b> is enhanced by 1.3, 2.5, and 3.1 times, respectively, compared to that of the API

Cu-based Polyoxometalate Catalyst for Efficient Catalytic Hydrogen Evolution

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(<i>B</i>-α-PW₉O₃₄)₂]·30H₂O (Na₃K₇-<b>Cu</b>_<b>4</b><b>P</b>_<b>2</b>). <b>Cu</b>_<b>4</b><b>P</b>_<b>2</b> is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4′-di-<i>tert</i>-butyl-2,2′-dipyridyl)-bis­(2-phenylpyridine­(1<i>H</i>))-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 <b>Cu</b>_<b>4</b><b>P</b>_<b>2</b> 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 <b>Cu</b>_<b>4</b><b>P</b>_<b>2</b> 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 <b>Cu</b>_<b>4</b><b>P</b>_<b>2</b> undergoes slow decomposition under turnover conditions; however, both the starting <b>Cu</b>_<b>4</b><b>P</b>_<b>2</b> as well as its molecular decomposition products are the dominant catalytically active species for H₂ evolution not Cu or CuO_<i>x</i> 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