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1.
J Am Chem Soc ; 2023 Apr 10.
Artículo en Inglés | MEDLINE | ID: mdl-37036435

RESUMEN

A high-valent manganese(IV)-hydroxo porphyrin π-cation radical complex, [Mn(IV)(OH)(Porp+•)(X)]+, was synthesized and characterized spectroscopically. The Mn porphyrin intermediate was highly reactive in alkane hydroxylation and oxygen atom transfer reactions. More importantly, the Mn porphyrin intermediate reacted with water at a fast rate, resulting in the dioxygen evolution. To the best of our knowledge, we report the first manganese Cpd I model compound bearing a porphyrin π-cation radical ligand with a high reactivity in oxidation reactions, including water oxidation.

2.
J Am Chem Soc ; 144(39): 17824-17831, 2022 10 05.
Artículo en Inglés | MEDLINE | ID: mdl-36154168

RESUMEN

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin (S = 5/2) Fe(III) ions in an FeN4Cl2 coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H+/1e- reduction of surface Fe(III)-OH surface. A catalytic wave is observed in the presence of NO3-, with an onset potential of -1.1 V vs SCE. At pH 6.0, GCC-FeDIM rapidly reduces NO3- to ammonium and nitrite with 88 and 6% Faradaic efficiency, respectively. Mechanistic studies, including in situ X-ray absorption spectroscopy, suggest that electrocatalytic NO3- reduction involves an iron nitrosyl intermediate. The Fe-N bond length (1.65 Å) is similar to that observed in {Fe(NO)}6 complexes, which is supported by the results of DFT calculations.


Asunto(s)
Compuestos de Amonio , Grafito , Cloruros , Compuestos Férricos/química , Hierro/química , Modelos Moleculares , Nitratos , Nitritos , Óxidos de Nitrógeno
3.
J Am Chem Soc ; 143(38): 15556-15561, 2021 09 29.
Artículo en Inglés | MEDLINE | ID: mdl-34529428

RESUMEN

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O2)(13-TMC)]+ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, Mössbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = 5/2 Fe(III) species binding an O2 unit. A notable observation was an unusually high νO-O at ∼1000 cm-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high νO-O value and unprecedented electrophilic reactivity in oxidation reactions.


Asunto(s)
Complejos de Coordinación/química , Compuestos Férricos/química , Benzaldehídos/química , Cinética , Ligandos , Estructura Molecular , Oxidación-Reducción , Oxígeno/química , Sulfuros/química
4.
J Am Chem Soc ; 142(2): 884-893, 2020 01 15.
Artículo en Inglés | MEDLINE | ID: mdl-31865704

RESUMEN

Artificial photosynthesis could promise abundant, carbon-neutral energy, but implementation is currently limited by a lack of control over the multi-electron catalysis of water oxidation. Discoveries of the most active catalysts still rely heavily on serendipity. [Ru(tpy)(bpy)(H2O)]2+ (1; bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine) is representative of the largest known class of water oxidation catalysts. We undertook an extensive spectroscopic analysis of the prototypical 1 water oxidation catalyst and its fastest known analog [Ru(EtO-tpy)(bpy)(H2O)]2+ (2), capable of 10 times faster water oxidation, to investigate the mechanism of action and factors controlling catalytic activity. EPR and resonance Raman spectroscopy did not detect the proposed [RuV═O] intermediate in 1 and 2 but indicated the possible formation of N-oxides. A lag phase was observed prior to O2 evolution, suggesting catalyst modification before the onset of catalysis. The reactive intermediate [Ru(tpy)(bpy-NO)(H2O)]2+ (1-NO; bpy-NO = 2,2'-bipyridine-N-oxide) proposed by combined spectroscopic and DFT analysis was de novo synthesized and demonstrated 100-fold greater catalytic activity than 1. Thus, in situ transient formation of small amounts of the Ru complex with N-oxide ligands can significantly activate single-site Ru-based catalysts. Furthermore, the rate of O2 evolution was found to correlate with the redox potential of the ligand. This observation might assist with rational design of new catalysts.

5.
Inorg Chem ; 59(19): 13880-13887, 2020 Oct 05.
Artículo en Inglés | MEDLINE | ID: mdl-32924462

RESUMEN

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [RuII(bpy)2(bpyNO)]2+ (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine-N-oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [RuII(bpy)2(bpyNO)]2+ using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic analysis was augmented with the DFT calculations. We concluded that upon 530-650 nm light excitation, 3[RuIII(bpyNO-•)(bpy)2]2+ is formed similarly to the 3[RuIII(bpy-•)(bpy)2]2+ light-induced state of the well-known photosensitizer [RuII(bpy)3]2+. An electron localization on the bpyNO ligand was confirmed by obtaining a unique EPR signal of reduced [RuII(bpy)2(bpyNO-•)]+ (gxx = 2.02, gyy = 1.99, and gzz = 1.87 and 14N hfs Axx = 12 G, Ayy = 34 G, and Azz = 11 G). 3[RuIII(bpyNO-•)(bpy)2]2+ may evolve via breaking of the Ru-O-N fragment at two different positions resulting in [RuIV═O(bpy)2(bpyout)]2+ for breakage at the O-|-N bond and [RuII(H2O)(bpy)2(bpyNOout)]2+ for breakage at the Ru-|-O bond. These pathways were found to have comparable ΔG. A reduction of [RuIV═O(bpy)2(bpyout)]2+ may result in water elimination and formation of [RuII(bpy)3]2+. The expected intermediates, [RuIII(bpy)2(bpyNO)]3+ and [RuIII(bpy)3]3+, were detected by EPR. In addition, a new signal with gxx = 2.38, gyy = 2.10, and gzz = 1.85 was observed and tentatively assigned to a complex with the dissociated ligand, such as [RuIII(H2O)(bpy)2(bpyNOout)]3+. The spectroscopic signatures of [RuIV═O(bpy)2(bpyout)]2+ were not observed, although DFT analysis and [RuII(bpy)3]2+ formation suggest this intermediate. Thus, [RuII(bpy)2(bpyNO)]2+ has potential as a light-induced oxidizer.

6.
Angew Chem Int Ed Engl ; 59(32): 13502-13505, 2020 Aug 03.
Artículo en Inglés | MEDLINE | ID: mdl-32369663

RESUMEN

Hypervalent FeV =O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized FeV species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H2 O2 : reactions that entail breaking of the O-O bond to form a FeV =O fragment. The key FeV =O species proposed to initiate the O-O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [FeV =O,(OH)(Pytacn)]2+ (Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) (2) generated during catalytic water oxidation. X-ray absorption spectroscopy (XAS) confirmed the FeV oxidation state and the presence of a FeV =O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 FeV center. 2 is the first spectroscopically characterized high spin oxo-FeV complex and constitutes a paradigmatic example of the FeV =O(OH) species proposed to be responsible for catalytic water oxidation reactions.

7.
Angew Chem Int Ed Engl ; 58(45): 16124-16129, 2019 11 04.
Artículo en Inglés | MEDLINE | ID: mdl-31489757

RESUMEN

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce4+ ion, [(dpaq)MnIV (O)]+ -Ce4+ (1-Ce4+ ), was synthesized by reacting [(dpaq)MnIII (OH)]+ (2) with cerium ammonium nitrate (CAN). 1-Ce4+ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI-MS, resonance Raman, XANES, and EXAFS, showing an Mn-O bond distance of 1.69 Šwith a resonance Raman band at 675 cm-1 . Electron-transfer and oxygen atom transfer reactivities of 1-Ce4+ were found to be greater than those of MnIV (O) intermediates binding redox-inactive metal ions (1-Mn+ ). This study reports the first example of a redox-active Ce4+ ion-bound MnIV -oxo complex and its spectroscopic characterization and chemical properties.

8.
J Photochem Photobiol A Chem ; 290: 101-108, 2014 Sep 15.
Artículo en Inglés | MEDLINE | ID: mdl-25197204

RESUMEN

A new strategy for encoding polypeptide libraries with photolabile tags is developed. The photoassisted assay, based on conditional release of encoding tags only from bound pairs, can differentiate between peptides which have minor differences in a form of post-translational modifications with epigenetic marks. The encoding strategy is fully compatible with automated peptide synthesis. The encoding pendants are compact and do not perturb potential binding interactions.

9.
J Catal ; 4292024 Jan.
Artículo en Inglés | MEDLINE | ID: mdl-38187083

RESUMEN

Photoelectrochemical water splitting can produce green hydrogen for industrial use and CO2-neutral transportation, ensuring the transition from fossil fuels to green, renewable energy sources. The iron-based electrocatalyst [FeII4FeIII(µ-3-O)(µ-L)6]3+ (LH = 3,5-bis(2-pyridyl)pyrazole) (1), discovered in 2016, is one of the fastest molecular water oxidation catalysts (WOC) based on earth-abundant elements. However, its water oxidation reaction mechanism has not been yet fully elucidated. Here, we present in situ X-ray spectroscopy and electron paramagnetic resonance (EPR) analysis of electrochemical water oxidation reaction (WOR) promoted by (1) in water-acetonitrile solution. We observed transient reactive intermediates during the in situ electrochemical WOR, consistent with a coordination sphere expansion prior to the onset of catalytic current. At a pre-catalytic (~+1.1 V vs. Ag/AgCl) potential, the distinct g~2.0 EPR signal assigned to FeIII/FeIV interaction was observed. Prolonged bulk electrolysis at catalytic (~+1.6 V vs. Ag/AgCl) potential leads to the further oxidation of Fe centers in (1). At the steady state achieved with such electrolysis, the formation of hypervalent FeV=O and FeIV=O catalytic intermediates was inferred with XANES and EXAFS fitting, detecting a short Fe=O bond at ~1.6 Å. (1) was embedded into MIL-126 MOF with the formation of (1)-MIL-126 composite. The latter was tested in photoelectrochemical WOR and demonstrated an improvement of electrocatalytic current upon visible light irradiation in acidic (pH=2) water solution. The presented spectroscopic analysis gives further insight into the catalytic pathways of multinuclear systems and should help the subsequent development of more energy- and cost-effective catalysts of water splitting based on earth-abundant metals. Photoelectrocatalytic activity of (1)-MIL-126 confirms the possibility of creating an assembly of (1) inside a solid support and boosting it with solar irradiation towards industrial applications of the catalyst.

10.
ChemSusChem ; 16(5): e202202124, 2023 Mar 08.
Artículo en Inglés | MEDLINE | ID: mdl-36479638

RESUMEN

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.23 V potential which is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe2 O3 , featuring the highly oxidizing holes, is not stable at low pH. Here, the key examples of Fe-based metal-organic framework (MOF) water oxidation photoelectrocatalysts active at pH=1 are presented. Fe-MIL-126 and Fe MOF-dcbpy structures were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) linkers and their mixtures. Presence of dcbpy linkers allows integration of metal-based catalysts via coordination to 2,2'-bipyridine fragments. Fe-based MOFs were doped with Ru-based precursors to achieve highly active MOFs bearing [Ru(bpy)(dcbpy)(H2 O)2 ]2+ WOC. Materials were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, resonance Raman, X-ray absorption spectroscopy, fs optical pump-probe, electron paramagnetic resonance (EPR), diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. It is shown that under reaction conditions, FeIII and RuIII oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe3 O nodes emerge as photosensitizers able to drive prolonged O2 evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-WOC modification for faster WOC catalysis, or Ru-WOC substitution to 3d metal-based systems. The findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

11.
ChemSusChem ; 15(4): e202101657, 2022 Feb 18.
Artículo en Inglés | MEDLINE | ID: mdl-34905663

RESUMEN

Catalytic water oxidation is an important process for the development of clean energy solutions and energy storage. Despite the significant number of reports on active catalysts, systematic control of the catalytic activity remains elusive. In this study, descriptors are explored that can be correlated with catalytic activity. [Ru(tpy)(pic)2 (H2 O)](NO3 )2 and [Ru(EtO-tpy)(pic)2 (H2 O)](NO3 )2 (where tpy=2,2' : 6',2"-terpyridine, EtO-tpy=4'-(ethoxy)-2,2':6',2"-terpyridine, pic=4-picoline) are synthesized and characterized by NMR, UV/Vis, EPR, resonance Raman, and X-ray absorption spectroscopy, and electrochemical analysis. Addition of the ethoxy group increases the catalytic activity in chemically driven and photocatalytic water oxidation. Thus, the effect of the electron-donating group known for the [Ru(tpy)(bpy)(H2 O)]2+ family is transferable to architectures with a tpy ligand trans to the Ru-oxo unit. Under catalytic conditions, [Ru(EtO-tpy)(pic)2 (H2 O)](NO3 )2 displays new spectroscopic signals tentatively assigned to a peroxo intermediate. Reaction pathways were analyzed by using DFT calculations. [Ru(EtO-tpy)(pic)2 (H2 O)](NO3 )2 is found to be one of the most active catalysts functioning by a water nucleophilic attack mechanism.

12.
Chem Catal ; 1(2): 407-422, 2021 Jul 15.
Artículo en Inglés | MEDLINE | ID: mdl-37378353

RESUMEN

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co4O4]n+ ([Co4O4Py4Ac4]0, Py = pyridine and Ac = CH3COO-), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, CoIV=O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two CoIV centers and a CoIV=O unit of strong radicaloid character sufficient to support a CoIV=O + H2O = Co-OOH + H+ transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co4O44+ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

13.
ACS Cent Sci ; 6(7): 1189-1198, 2020 Jul 22.
Artículo en Inglés | MEDLINE | ID: mdl-32724853

RESUMEN

Heterogeneous catalysts in the form of atomically dispersed metals on a support provide the most efficient utilization of the active component, which is especially important for scarce and expensive late transition metals. These catalysts also enable unique opportunities to understand reaction pathways through detailed spectroscopic and computational studies. Here, we demonstrate that atomically dispersed iridium sites on indium tin oxide prepared via surface organometallic chemistry display exemplary catalytic activity in one of the most challenging electrochemical processes, the oxygen evolution reaction (OER). In situ X-ray absorption studies revealed the formation of IrV=O intermediate under OER conditions with an Ir-O distance of 1.83 Å. Modeling of the reaction mechanism indicates that IrV=O is likely a catalyst resting state, which is subsequently oxidized to IrVI enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo)electrochemical applications.

14.
Commun Biol ; 3(1): 13, 2020 01 08.
Artículo en Inglés | MEDLINE | ID: mdl-31925322

RESUMEN

Photosynthetic efficiency depends on equal light energy conversion by two spectrally distinct, serially-connected photosystems. The redox state of the plastoquinone pool, located between the two photosystems, is a key regulatory signal that initiates acclimatory changes in the relative abundance of photosystems. The Chloroplast Sensor Kinase (CSK) links the plastoquinone redox signal with photosystem gene expression but the mechanism by which it monitors the plastoquinone redox state is unclear. Here we show that the purified Arabidopsis and Phaeodactylum CSK and the cyanobacterial CSK homologue, Histidine kinase 2 (Hik2), are iron-sulfur proteins. The Fe-S cluster of CSK is further revealed to be a high potential redox-responsive [3Fe-4S] center. CSK responds to redox agents with reduced plastoquinone suppressing its autokinase activity. Redox changes within the CSK iron-sulfur cluster translate into conformational changes in the protein fold. These results provide key insights into redox signal perception and propagation by the CSK-based chloroplast two-component system.


Asunto(s)
Cloroplastos/metabolismo , Histidina Quinasa/metabolismo , Hierro/metabolismo , Oxidación-Reducción , Azufre/metabolismo , Proteínas de Arabidopsis/química , Proteínas de Arabidopsis/metabolismo , Activación Enzimática , Histidina Quinasa/química , Hierro/química , Fotosíntesis , Conformación Proteica , Proteínas Recombinantes , Análisis Espectral , Relación Estructura-Actividad , Azufre/química
15.
J Sulphur Chem ; 29(3-4): 389-400, 2008.
Artículo en Inglés | MEDLINE | ID: mdl-24436724

RESUMEN

Novel photolabile amphiphiles containing thioxanthone-based fluorogenic caging groups are developed. Photoinduced fragmentation in dithiane-thioxanthone adducts was demonstrated to occur with 100% quantum efficiency at λ ~ 320 nm and more than 50% at λ ~ 360 nm. A plausible mechanism involves homolytic fission of a carbon-carbon single bond in the excited thioxanthone followed by disproportionation via hydrogen transfer. The critical feature of the system is that fluorescence of a substituted thioxanthone is recovered as a result of photofragmentation, making dithiane-thioxanthone adducts efficient fluorogenic caging groups. Photolabile amphiphiles containing these fluorogens are synthesized and their photoinduced disassembly is probed while following the fluorescence recovery. This methodology allows for destabilizing supramolecular assemblies of amphiphiles and at the same time offers a feedback mechanism for monitoring the process by fluorescence.

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