1,721,076 research outputs found
Photocatalytic Oxidation of Alcohols with Organic Dyes: From Homogeneous Systems to Sensitized Materials
No full textThe oxidation of alcohols retains an enormous importance in
industry and in synthetic organic chemistry. The renaissance
in photocatalysis is offering new routes to conduct this class
of transformations under benign and sustainable conditions. In
this concept article, we highlight the efforts that have been
conducted in light-driven oxidation of alcohols using organic
photocatalysts (PC). After an overview of relevant industrial processes,
we will discuss relevant examples of PC employed in solution or when integrated in photosynthetic schemes within
semiconductor slides or nanoparticles. The combination of the
photochemical systems with electrochemical routes or their
engineering in flow will be considered. Where possible, we highlight
also mechanistic aspects that reveal the key steps involved
and contribute to improve the performance of the process.
We finally present an outlook on the future perspectives and
developments in the field
Hydrogen peroxide activation by fluorophilic polyoxotungstates for fast and selective oxygen transfer catalysis
No full textFluorophilic polyoxotungstates perform the selective epoxidations of internal and terminal double bonds by hydrogen peroxide (H2O2) activation in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), under mild temperature conditions. A hybrid synergy of supramolecular interactions, involving the inorganic cluster and the fluorinated solvent, is envisaged to boost H2O2 activation and the oxygen transfer mechanism. 1,2-Epoxides have been obtained with >99% selectivity and 98% yield at T = 40-70 °C
Unveiling the reactivity of CO2 with carbanions: a theoretical analysis of the carboxylation step
No full textThe synthetic insertion of carbon dioxide into organic scaffolds typically requires the reaction of CO2 with a carbanion (carboxylation step), with the latter being generated through chemical, electrochemical, or photochemical routes. Still, little is known about the energetic and structural requirements of this step. In this work, we unveil the reactivity of CO2 with a selected set of 28 carbanions through DFT calculations and provide linear free-energy relationships that correlate the Delta G(0) and the Delta G double dagger of the carboxylation step. These reveal a Leffler-Hammond parameter alpha = 0.26 +/- 0.02 and an intrinsic barrier Delta G double dagger(0) = 12.7 +/- 0.3 kcal mol(-1) (omega b97XD/aug-cc-pvtz//omega b97XD/def2tzvp level of theory), indicative of smooth reactivity of carbanions with CO2. This reactivity is further associated with the basicity of the carbanions (expressed as the pK(aH) of the conjugate acid), in a linear Br & oslash;nsted plot between calculated Delta G double dagger and experimental pK(aH) (slope beta = 0.40 +/- 0.04 kcal mol(-1)). According to the Mayr-Patz equation, calculations allow the extrapolation of electrophilicity values for CO2 in the range from -15.3 to -18.7, in good agreement with a single reported experimental value of -16.3. Concerning the structural changes occurring in the transition state, the major energy penalty comes from the distortion of CO2. These findings can be useful in designing novel reactivity targeting carbon dioxide fixation
Optically Active Polyoxotungstates Bearing Chiral Organophosphonate Substituents
No full textDivacant Keggin-type polyoxotungstates [γ-XW10O36]8– with X = Si or Ge, were functionalized with chiral phosphoryl groups. The hybrid compounds [(R*PO)2(γ-XW10O36)]4– with R = N-protected aminoalkyl groups or O-protected amino acid derivatives, were isolated. The solution characterization of the products was performed by different techniques: 183W, 31P, 13C, and 1H NMR spectroscopy, electrospray ionization mass spectrometry, UV/Vis spectroscopy, and circular dichroism (CD). The experimental data confirm the covalent grafting of the organic moieties onto the polyanionic surface. A chirality transfer, from the pendant organic arm to the inorganic framework is apparent from CD studies. Multiple Cotton effects were observed in the region of charge-transfer transitions pertaining to W–O bonds. Furthermore, the 183W NMR spectra are consistent with the expected C2 symmetry, resulting from introduction of two organic stereocenters. The title complexes were used in the presence of hydrogen peroxide to perform the oxidation of methyl p-tolyl sulfide. Implications for the design of enantioselective catalysts based on these derivatives are discussed
Tetrametallic Oxygen Evolution Catalysts. Primary Interactions and Photochemical Processes with Ru(bpy)32+ Photosensitizer
No full textWater oxidation is a key step common to most artificial photosynthetic reaction schemes.[1] It is a very complex process, involving the 4-electron oxidation of two water molecules, the formation of a new O-O bond, and the release of four protons. As such, it is considered to be the real bottleneck towards artificial photosynthesis.[2]
In a standard photocatalytic cycle, a strong oxidant is irreversibly generated by reaction of an excited photosensitizer (P) and a sacrificial electron acceptor (S) that decomposes upon reduction. The photocatalytic mechanism leading to water oxidation is generally assumed to involve four sequential hole transfer steps from the photochemically oxidized sensitizer to the catalyst (C), which then evolves to a series of high valent intermediates.
In this communication we present a study addressing the kinetics of hole transfer processes involved in sacrificial systems as described above, where the sensitizer P is Ru(bpy)32+, the sacrificial acceptor S is S2O82-, the decomposition products of S- are SO42- and SO4- and several ruthenium- and cobalt-based tetrametallic molecular clusters are used as the catalyst C. In these type of processes the oxidized sensitizer is irreversibly produced so that, in principle, very fast hole transfer is not a strict requirement. In practice, however, the sensitizers, in their oxidized form, are often unstable under the reaction conditions used and fast hole scavenging is determinant to minimize their decomposition (usually the main limiting factor in terms of turnover performance). On the other hand, fast hole-transfer rates will become absolutely crucial in regenerative systems where the catalyst must be able to scavenge the hole on the photogenerated oxidant in competition with charge recombination.[3] In addition, given the different structural and chemical nature of each catalyst, special attention has been paid to the interactions with the sensitizer in water solution and its photophysical and kinetic consequences
Tetrametallic Molecular Catalysts for Photochemical Water Oxidation
No full textPhotochemical water splitting is the ideal target reaction in artificial photosynthesis [1,2], potentially able to provide long-term solutions to solar energy conversion and storage. While the reduction of water to hydrogen is of obvious interest as the fuel generating reaction, the kinetic bottleneck towards water splitting is represented by the oxidation of water to molecular oxygen, a complex process involving four-electron abstraction from two water molecules, with formation of a new O–O bond, and release of four protons [3,4]. This is why most of the attention in the area is focusing on the development of efficient water oxidation catalysts (WOCs). Among molecular WOCs, those featuring a reactive set of four transition metals can leverage an extraordinary interplay of electronic and structural properties [5]. These are of particular interest, owing to their close structural, and possibly functional, relationship to the oxygen evolving complex of natural photosynthesis. In the presentation, special attention will be given to two classes of tetrametallic molecular WOCs: (i) M4O4 cubane-type structures stabilized by simple organic ligands [6], and (ii) systems in which a tetranuclear metal core is stabilized by coordination of fully inorganic polyoxometalate (POM) ligands [7,8]
Water Oxidation Catalysis by Molecular Metal-Oxides
No full textWater 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)3 3+. Thus, 1 is an ideal candidate for the assembly of high efficient oxygenevolving anodes into nanostructured devices for artificial photosynthesis
Relativistic DFT Calculations of Polyoxotungstate 183W NMR Spectra: Insight into their Solution Structure
No full textThe good correlation between experimental and calculated 183W NMR chemical shifts of polyoxotungstates allows to predict and assign the spectra of unstable or unknown species, and to address the counterion effect
Bio-inspired oxidations with polyoxometalate catalysts
No full textTransition metal substituted polyoxometalates (TMS-POM) provide a redox-active metal center, (Fe, Ru, Mn or else), with a totally inorganic ligand system, featuring rigid polydentate binding sites, high electron-acceptor character, extreme robustness and interesting structural and coordination properties, in some cases, mimicking the coordination geometry of natural oxygenase enzymes. Their synthesis and reactivity can be promoted by microwave (MW) induced dielectric heating. In particular, MW irradiation induces a very efficient and selective hydrothermal synthesis of the diamagnetic, air-stable [RuII(DMSO)PW11O39]5−. Its catalytic activity has been screened with shunt oxidants like NaIO4 and KHSO5 and PyCl2NO. Under dioxygen and MW irradiation, [RuII(DMSO)PW11O39]5− catalyzes the oxidation of DMSO to DMSO2 in water. FeIII-POMs with nuclearities 1–4, namely [α-Fe(H2O)SiW11O39]5− (FeSiW11), [γ-Fe2(H2O)2SiW10O38]6− (Fe2SiW10), [α-Fe3(H2O)3SiW9O37]7− (Fe3SiW9) and [β-Fe4(H2O)10(XW9O33)2]n− (Fe4X2W18, X = SeIV, TeIV; AsIII, SbIII; n = 4, 6) were also found to catalyze the MW assisted cyclohexane oxygenation with high turnover frequencies. Preliminary results indicate that the Krebs-type Fe polyoxotungstates can also promote the oxidative cleavage of 3,5-ditert-butylcatechol with molecular oxygen
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