Water oxidation by copper‐based complexes to form dioxygen has attracted attention in recent years, with the aim of developing efficient and cheap catalysts for chemical energy storage. In addition, ...high‐valent metal–oxo species produced by the oxidation of metal complexes in the presence of water can be used to achieve substrate oxygenation with the use of H2O as an oxygen source. To date, this strategy has not been reported for copper complexes. Herein, a copper(II) complex, (RPY2)Cu(OTf)2 (RPY2=N‐substituted bis2‐pyridyl(ethylamine) ligands; R=indane; OTf=triflate), is used. This complex, which contains an oxidizable substrate moiety (indane), is used as a tool to monitor an intramolecular oxygen atom transfer reaction. Electrochemical properties were investigated and, upon electrolysis at 1.30 V versus a normal hydrogen electrode (NHE), both dioxygen production and oxygenation of the indane moiety were observed. The ligand was oxidized in a highly diastereoselective manner, which indicated that the observed reactivity was mediated by metal‐centered reactive species. The pH dependence of the reactivity was monitored and correlated with speciation deduced from different techniques, ranging from potentiometric titrations to spectroscopic studies and DFT calculations. Water oxidation for dioxygen production occurs at neutral pH and is probably mediated by the oxidation of a mononuclear copper(II) precursor. It is achieved with a rather low overpotential (280 mV at pH 7), although with limited efficiency. On the other hand, oxygenation is maximum at pH 8–8.5 and is probably mediated by the electrochemical oxidation of an antiferromagnetically coupled dinuclear bis(μ‐hydroxo) copper(II) precursor. This constitutes the first example of copper‐centered oxidative water activation for a selective oxygenation reaction.
Testing water: High‐valent metal–oxo species produced by the oxidation of metal complexes in the presence of water can be used to achieve substrate oxygenation with the use of water as an oxygen source. Intramolecular and stereoselective oxygen atom transfer is achieved on an N‐substituted bis2‐pyridyl(ethylamine) indane ligand upon oxidation of copper(II) precursor in water (see figure).
The synthesis of a dinuclear copper(II) complex, supported by a 1,3‐diamino‐2‐propanol‐based tetraamide ligand, is reported. Structural properties in the solid state and in solution, by means of XRD ...analysis and NMR spectroscopy, respectively, provide evidence of a highly flexible complex that can display several conformations, leading to the image of the wings of a butterfly. The complex was fully characterized and the redox properties were investigated. Room‐temperature spectro‐electrochemistry was used to monitor the formation of a metastable mono‐oxidized product that displayed an absorption band centered at λ=463 nm. EPR investigation of the low‐temperature, chemically generated, mono‐oxidized product reveals the presence of an intermediate described as a mixed‐valent CuIICuIII species, which is a model of the possible highly oxidizing intermediate in particulate methane monooxygenase.
Fleeting species: A flexible dinuclear copper(II) complex based on a tetraamide ligand that resembles a butterfly flapping its wings is characterized, along with its one‐electron oxidized species, which consists of an unstable mixed‐valent copper(II)/copper(III) species (see figure).
The chemistry of borane (BH
3) complexes of tricoordinated phosphorus compounds is reviewed, including the preparation, coordination chemistry, reactivity and synthetic applications.
The stereoselective copper‐mediated hydroxylation of intramolecular C−H bonds from tridentate ligands is reinvestigated using DFT calculations. The computational study aims at deciphering the ...mechanism of C−H hydroxylation obtained after reaction of Cu(I) precursors with dioxygen, using ligands bearing either activated (L1) or non‐activated (L2) C−H bonds. Configurational analysis allows rationalization of the experimentally observed regio‐ and stereoselectivity. The computed mechanism involves the formation of a side‐on peroxide species (P) in equilibrium with the key intermediate bis‐(μ‐oxo) isomer (O) responsible for the C−H activation step. The P/O equilibrium yields the same activation barrier for the two complexes. However, the main difference between the two model complexes is observed during the C−H activation step, where the complex bearing the non‐activated C−H bonds yields a higher energy barrier, accounting for the experimental lack of reactivity of this complex under those conditions.
Computational insights on reactivity: The stereoselective copper‐mediated intramolecular C−H bond hydroxylation is investigated using DFT calculations. The computed mechanism involves a dinuclear side‐on μ‐η2:η2‐peroxo dicopper(II) adduct in equilibrium with the key oxidizing intermediate bis(μ‐oxo) dicopper(III) species. The difference of reactivity towards internal substrates bearing activated or non‐activated C−H bonds is rationalized.
Interfacial modulation of laccase activity
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•Site-directed immobilization of laccase on core-shell magnetic nanoparticles.•Aliphatic aldehyde, aromatic aldehyde or azide functional ...groups provide different chemical environment.•Active site orientation at the surface of nanoparticles modulates oxidation of substrates.
We explored the coupling of laccases to magnetic nanoparticles (MNPs) with different surface chemical coating. Two laccase variants offering two opposite and precise orientations of the substrate oxidation site were immobilised onto core-shell MNPs presenting either aliphatic aldehyde, aromatic aldehyde or azide functional groups at the particles surface. Oxidation capabilities of the six-resulting laccase-MNP hybrids were compared on ABTS and coniferyl alcohol. Herein, we show that the original interfaces created differ substantially in their reactivities with an amplitude from 1 to > 4 folds depending on the nature of the substrate. Taking enzyme orientation into account in the design of surface modification represents a way to introduce selectivity in laccase catalysed reactions.
We report the synthesis and the characterization of a trinuclear nickel complex. Solid state and solution studies using X-ray diffraction, NMR and UV-vis spectroscopy highlight the square planar ...geometries around the metal centers and an all-sulfur coordination sphere. It exhibits significant electrocatalytic activity for hydrogen evolution in DMF using Et
3
NHCl as the proton source. DFT studies suggest that sulfur atoms act as proton relay, as proposed in NiFe hydrogenases.
A trinuclear nickel complex with
S
-based ligands is reported as a bio-inspired model of the NiFe hydrogenases' active site. DFT calculations indicate that thiolate and thioether functions are involved as proton relays in the H
2
evolution mechanism.
A detailed kinetic study based on steady-state and pre-steady-state measurements is described for the highly enantioselective epoxide hydrolase Kau2. The enzyme, which is a member of the ...α/β-hydrolase fold family, preferentially reacts with the (S,S)-enantiomer of trans-stilbene oxide (TSO) with an E value of ∼200. The enzyme follows a classical two-step catalytic mechanism with formation of an alkyl-enzyme intermediate in the first step and hydrolysis of this intermediate in a rate-limiting second step. Tryptophan fluorescence quenching during TSO conversion appears to correlate with alkylation of the enzyme. The steady-state data are consistent with (S,S) and (R,R)-TSO being two competing substrates with marked differences in kcat and KM values. The high enantiopreference of the epoxide hydrolase is best explained by pronounced differences in the second-order alkylation rate constant (k2/KS) and the alkyl-enzyme hydrolysis rate k3 between the (S,S) and (R,R)-enantiomers of TSO. Our data suggest that during conversion of (S,S)-TSO the two active site tyrosines, Tyr157 and Tyr259, serve mainly as electrophilic catalysts in the alkylation half-reaction, polarizing the oxirane oxygen of the bound epoxide through hydrogen bond formation, however, without fully donating their hydrogens to the forming alkyl-enzyme intermediate.
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•E value of the enzyme: ∼200. Preferred substrate: (S,S)-trans-stilbene oxide (TSO).•Protein fluorescence quenching correlates with alkylation of the enzyme.•Enzyme follows a two-step catalytic mechanism with a rate-limiting hydrolysis step.•Data suggest profound differences in k2/KS and k3 between both enantiomers of TSO.•Data suggest that active site tyrosines act as electrophilic catalysts.
We have investigated spectrally, crystallographically as well as computationally the charge transfer complexes involving newly synthesized N -aryl- N ′-methyl non-symmetric viologens (AMVs) and ...hexacyanoferrate( ii ) (HCF) anions. The supramolecular binding of AMVs and HCF was studied in solution and in the crystal state for one of the obtained complexes. Substituent effects on the electron affinities of the dicationic AMVs, determined using Mulliken's theory R. S. Mulliken, J. Am. Chem. Soc. , 1952, 74 , 811–824 were quantified. The structure of one of the AMV//Fe II (CN) 6 pairs solved through Single-Crystal X-ray Diffraction (SCXRD), provided insights for the supramolecular binding of the anionic and cationic counterparts and the role of lattice water molecules. Supramolecular binding in solution, studied with the use of NMR spectroscopy, is in agreement with the results obtained in the solid state.
We describe the synthesis and characterization of a new mononuclear palladium complex (PdL) based on a thiosemicarbazone ligand. X‐ray diffraction data collected on single crystals reveal that the ...complex is a neutral mononuclear entity with symmetric square‐planar geometry around the metal center. The complex exhibits an electrocatalytic behavior for proton reduction in DMF solvent using trifluoroacetic acid (TFA) as a proton source. Gas analysis under controlled electrocatalytic conditions during 4 h at a pool mercury working electrode showed a moderate production of dihydrogen. By combining experimental techniques to theoretical calculations, the results are analyzed and compared to those obtained for the previously reported NiL complex in an effort to rationalize the influence of the metal center on the catalytic performance of PdL to mediate hydrogen evolution.
How does the metal center influence the reaction mechanism and the catalytic properties for hydrogen production of thiosemicarbazone‐based complexes?
Polysaccharide oxidative depolymerization is highly desirable to achieve recalcitrant biomass valorization. Inspired by recently discovered Lytic Polysaccharide Monooxygenases, mononuclear copper ...complexes have been prepared and studied in the literature. However, the activities were evaluated on different substrates and under various conditions. In this work we intended to establish a robust and reproducible activity assay, in aqueous solution at a pH close from neutrality and under mild conditions. We have evaluated several complexes on substrates of increasing complexity: the model substrate para‐nitrophenyl‐β‐D‐glucopyranoside (p‐NPG), cellobiose (glucose dimer), as well as on extended substrates (chitin, cellulose and bagasse from agave). The different assays were compared and proof‐of‐concept that bioinspired complexes can oxidatively promote polysaccharide depolymerization was obtained. Finally, we measured level of hydroxyl radicals released by the complexes under comparable experimental conditions and mechanistic pathways are discussed.
The catalytic activity of several LPMO‐bioinspired copper complexes was assayed on different substrates of increasing complexity (soluble to extended insoluble polysaccharides). The different assays are compared and proof‐of‐concept that bioinspired complexes can oxidatively promote polysaccharide depolymerization was obtained. Mechanistic pathways and catalysts stability are discussed.
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