Water Oxidation Catalysis by Molecular Metal-Oxides

Abstract Water oxidation catalysis is recognized as the bottleneck for the development of efficient devices based on artificial photosynthesis, that is the light driven water splitting into hydrogen and oxygen. A recent breakthrough in this field, is the development of a molecular, fast and robust water oxidation catalyst namely a fully inorganic tetranuclear ruthenium complex with polyoxometalate ligands. The crystal structure of [Ru4(μ-O)4(μ-OH)2(H2O)4(SiW10O36)2]10-, 1, evidences the entrapment of an adamantane like, tetranuclear ruthenium(IV)-oxo core, by two decatungtosilicate units. Several spectroscopic techniques confirm the maintenance of the structure in aqueous solution. In the presence of Ce(IV) as sacrificial electron acceptor, 1 catalyzes water oxidation to oxygen, showing up to 500 turnovers and a turnover frequency of 0.125 s-1. The synergistic effect of the four ruthenium centres has a fundamental effect on such astounding performance, as confirmed by spectroscopic and computational characterization of five competent intermediates involved in the catalytic cycle, in strict analogy with the natural paradigm of the oxygen evolving centre in Photosystem II. Interestingly, 1 efficiently catalyzes water oxidation in the presence of photogenerated oxidants, as well; this fundamental feature is probably related to very fast hole scavenging of anionic 1 from cationic photogenerated oxidants, such as Ru(bpy)33+. Thus, 1 is an ideal candidate for the assembly of high efficient oxygenevolving anodes into nanostructured devices for artificial photosynthesis

Supramolecular organic???inorganic domains integrating fullerene-based acceptors with polyoxometalate-bis-pyrene tweezers for organic photovoltaic applications

A strategy to improve organic photovoltaics, and to enhance the device efficiency, builds on the design of interfacial layered (IFL) materials implementing the performance of the photoactive acceptor/donor system. A novel IFL blend has been engineered by a supramolecular organic-inorganic heterojunction integrating polyoxometalate-bis-pyrene (pyrPOM) receptors that can selectively bind fullerene-based acceptors through π-π interactions and in particular the most used phenyl-C61-butyric acid methyl ester (PCBM) PCBM. The resulting pyrPOM@PCBM IFL, assembled by means of the Langmuir-Blodgett approach, has been fully characterized both in solution and on solid supports by means of the Langmuir-Schaefer method, featuring a high dielectric function, good polarizability and piezo-responsive behavior, which suggest ferroelectric properties. An organic solar cell is realized interposing the IFL between poly(3-hexylthiophene) (P3HT) as polymer donor and the PCBM acceptor layers, thus enhancing the open circuit voltage of the solar device by about 34% under an applied bias of ±5 V. © 2021 The Royal Society of Chemistry

Selective targeting of proteins by hybrid polyoxometalates: Interaction between a bis-biotinylated hybrid conjugate and avidin

The Keggin-type polyoxometalate [\u3b3-SiW10O36]8 12 was covalently modified to obtain a bis-biotinylated conjugate able to bind avidin. Spectroscopic studies such as UV-vis, fluorimetry, circular dichroism, coupled to surface plasmon resonance technique were used to highlight the unique interplay of supramolecular interactions between the homotetrameric protein and the bis-functionalized polyanion. In particular, the dual recognition mechanism of the avidin encompasses (i) a complementary electrostatic association between the anionic surface of the polyoxotungstate and each positively charged avidin subunit and (ii) specific host-guest interactions between each biotinylated arm and a corresponding pocket on the tetramer subunits. The assembly exhibits peroxidase-like reactivity and it was used in aqueous solution for L-methionine methyl ester oxidation by H2O2. The recognition phenomenon was then exploited for the preparation of layer-by-layer films, whose structural evolution was monitored in situ by ATR-FTIR spectroscopy. Finally, cell tracking studies were performed by exploiting the specific interactions with a labeled streptavidin

Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes. Efficient hydrogen evolution from water is one promising route towards a renewable energy economy and sustainable development. Here we show a tridimensional electrocatalytic interface, featuring a hierarchical, co-axial arrangement of a palladium/titanium dioxide layer on functionalized multi-walled carbon nanotubes. The resulting morphology leads to a merging of the conductive nanocarbon core with the active inorganic phase. A mechanistic synergy is envisioned by a cascade of catalytic events promoting water dissociation, hydride formation and hydrogen evolution. The nanohybrid exhibits a performance exceeding that of stateof- the-art electrocatalysts (turnover frequency of 15000 H2 per hour at 50mVoverpotential). The Tafel slope of B130mV per decade points to a rate-determining step comprised of water dissociation and formation of hydride. Comparative activities of the isolated components or their physical mixtures demonstrate that the good performance evolves from the synergistic hierarchical structure

Vanadium-catalyzed, microwave-assisted oxidations with H 2 O 2 in ionic liquids*

Abstract: The application of vanadium(V) catalysts in selective oxidation with peroxides offers an efficient procedure that is compatible with different functional groups and leads to good yields and selectivities. However, the search for more efficient and sustainable procedures that employ H 2 O 2 as oxidant remains an important topic. In the last few years, striking results have been obtained by applying microwave (MW) activation in metal-catalyzed reactions carried out in ionic liquids (ILs). In the present study, results achieved with vanadiumbased catalysts in oxidations of various substrates with H 2 O 2 are presented; in particular, epoxidation of alkenes and sulfoxidation of thioethers have been investigated. Notably, in the latter oxidation, a significant improvement in the rate of reaction and an increase in selectivity have been observed in the case of hydrophobic ILs in combination with MW activation

DFT calculations of Ru-99 chemical shifts with all-electron and effective core potential basis sets

The nuclear shielding of Ru-99 in several complexes has been investigated computationally by DFT methods with effective core potential and all-electron basis sets. Shieldings calculated with ECP bases correlate very satisfactorily with available experimental data, although they are ca. one order of magnitude lower than the experimental values. The influence of molecular geometry (semiempirical vs. DFT) on the nuclear shielding is also examined and discussed, particularly in the case of species containing Ru-S bonds {[RuCl2(DMSO)(4)] and alpha-[PW11RuO39(DMSO)](5-)}. It is shown that such calculations may help the assignment of signals in Ru-99 NMR spectra of mono- and polynuclear complexes

Relativistic DFT calculation of Ru-99 NMR parameters: Chemical shifts and spin-spin coupling constants

The nuclear shielding of Ru-99 in a wide variety of complexes was investigated computationally by DFT methods, including relativistic effects (by means of the Pauli and ZORA approximations), up to spin-orbit coupling and Slater all-electron or frozen-core basis sets. Mononuclear complexes, a trinuclear cluster and a ruthenium-substituted polyoxometalate were included. Chemical shifts calculated in this way correlated very satisfactorily with experimental values, with fit lines having slopes close to unity. In the few cases where a comparison was possible, spin-spin coupling constants involving Ru-99 were also successfully calculated

Effective core potential DFT calculations of nuclear shielding as a tool for the prediction and assignment of the tungsten chemical shift in mono- and polynuclear complexes

The shielding of the W-183 nucleus in mononuclear tungsten complexes and in the Keggin heteropolyoxotungstate PW12O403- has been investigated by a density functional theory (DFT) method with effective core potentials. Calculated [PW12O40]3- shieldings correlate with experimental data, although they are one order of magnitude lower than the experimental values, which is reflected in low slopes (< 0.1) of the correlation lines. The influence of molecular geometry (semi-empirical vs. DFT) on the nuclear shielding is examined. There is a fair correlation between atomic charges at W and nuclear shielding only for species sharing the same formal oxidation state

Vicinal tungsten-tungsten coupling constants in polyoxotungstates: DFT calculations challenge an empirical rule

The empirical rule that edge and corner junctions give rise to small and large vicinal NMR tungsten\u2013tungsten coupling constants, respectively, in polyoxometalates has led to the assumption that the W-O-W angle is the sole factor involved. Why then is the corner 2JWW coupling in \u3b3-[SiW10O36]8 12 (see picture) so small? Relativistic DFT calculations show why this is indeed expected