Abstract
Alcohol to aldehyde conversion is a critical reaction in the industry. Herein, a new electrochemical method is introduced that converts 1 mmol of alcohols to aldehydes and ketones in the ...presence of
N
-hydroxyphthalimide (NHPI, 20 mol%) as a mediator; this conversion is achieved after 8.5 h at room temperature using a piece of Ni foam (1.0 cm
2
) and without adding an extra-base or a need for high temperature. Using this method, 10 mmol (1.08 g) of benzyl alcohol was also successfully oxidized to benzaldehyde (91%) without any by-products. This method was also used to oxidize other alcohols with high yield and selectivity. In the absence of a mediator, the surface of the nickel foam provided oxidation products at the lower yield. After the reaction was complete, nickel foam (anode) was characterized by a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry, which pointed to the formation of nickel oxide on the surface of the electrode. On the other hand, using other electrodes such as Pt, Cu, Fe, and graphite resulted in a low yield for the alcohol to aldehyde conversion.
The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for ...dioxygen reduction, whose selectivity can be changed from a preferential 4e–/4H+ dioxygen-reduction (to water) to a 2e–/2H+ process (to hydrogen peroxide) only by increasing the temperature from −50 to 25 °C. A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV–vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to 1 an entropically unfavorable process. This makes the end-on μ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H2O2. In contrast, at −50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O–O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O2-to-2H2O catalytic conversion in an overall 4e–/4H+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.
The FeFe-hydrogenase (HydA1) from green algae is the minimal enzyme for efficient biological hydrogen (H sub(2)) production. Its active-site six-iron center (H-cluster) consists of a cubane, 4Fe4S ...sub(H), cysteine-linked to a diiron site, 2Fe sub(H). We utilized the spin-polarization of the iron K beta X-ray fluorescence emission to perform site-selective X-ray absorption experiments for spectral discrimination of the two sub-complexes. For the H-cluster in reduced HydA1 protein, XANES and EXAFS spectra, K beta emission lines (3p arrow right 1s transitions), and core-to-valence (pre-edge) absorption (1s arrow right 3d) and valence-to-core (K beta super(2,5)) emission (3d arrow right 1s) spectra were obtained, individually for 4Fe4S sub(H) and 2Fe sub(H). Iron-ligand bond lengths and intermetal distances in 2Fe sub(H) and 4Fe4S sub(H) were resolved, as well as fine structure in the high-spin iron containing cubane. Density functional theory calculations reproduced the X-ray spectral features and assigned the molecular orbital configurations, emphasizing the asymmetric d-level degeneracy of the proximal (Fe sub(p)) and distal (Fe sub(d)) low-spin irons in 2Fe sub(H) in the non-paramagnetic state. This yielded a specific model structure of the H-cluster with a bridging carbon monoxide ligand and an apical open coordination site at Fe sub(d) in 2Fe sub(H). The small HOMO-LUMO gap ( similar to 0.3 eV) enables oxidation and reduction of the active site at similar potentials for reversible H sub(2) turnover by HydA1, the LUMO spread over 4Fe4S sub(H) supports its role as an electron transfer relay, and Fe sub(d) carrying the HOMO is prepared for transient hydride binding. These features and the accessibility of Fe sub(d) from the bulk phase can account for regio-specific redox transitions as well as H sub(2)-formation and O sub(2)-inhibition at the H-cluster. We provide a conceptual and experimental framework for site-selective studies on catalytic mechanisms in inhomogeneous materials.
Understanding the mechanisms of electron transfer (ET) in photosynthetic reaction centers (RCs) may inspire novel catalysts for sunlight-driven fuel production. The electron exit pathway of type II ...RCs comprises two quinone molecules working in series and in between a non-heme iron atom with a carboxyl ligand (bicarbonate in photosystem II (PSII), glutamate in bacterial RCs). For decades, the functional role of the iron has remained enigmatic. We tracked the iron site using microsecond-resolution x-ray absorption spectroscopy after laser-flash excitation of PSII. After formation of the reduced primary quinone, QA−, the x-ray spectral changes revealed a transition (t½ ≈ 150 μs) from a bidentate to a monodentate coordination of the bicarbonate at the Fe(II) (carboxylate shift), which reverted concomitantly with the slower ET to the secondary quinone QB. A redox change of the iron during the ET was excluded. Density-functional theory calculations corroborated the carboxylate shift both in PSII and bacterial RCs and disclosed underlying changes in electronic configuration. We propose that the iron-carboxyl complex facilitates the first interquinone ET by optimizing charge distribution and hydrogen bonding within the QAFeQB triad for high yield QB reduction. Formation of a specific priming intermediate by nuclear rearrangements, setting the stage for subsequent ET, may be a common motif in reactions of biological redox cofactors.
Structural data of the oxygen-evolving complex (OEC) in photosystem II (PSII) determined by X-ray crystallography, quantum chemistry (QC), and extended X-ray absorption fine structure (EXAFS) ...analyses are presently inconsistent. Therefore, a detailed study of what information can be gained about the OEC through a comparison of QC and crystallographic structure information combined with the information from range-extended EXAFS spectra was undertaken. An analysis for determining the precision of the atomic coordinates of the OEC by QC is carried out. OEC model structures based on crystallographic data that are obtained by QC from different research groups are compared with one another and with structures obtained by high-resolution crystallography. The theory of EXAFS spectra is summarized, and the application of EXAFS spectra to the experimental determination of the structure of the OEC is detailed. We discriminate three types of parameters entering the formula for the EXAFS spectrum: (1) model-independent, predefined, and fixed; (2) model-dependent that can be computed or adjusted; and (3) model-dependent that must be adjusted. The information content of EXAFS spectra is estimated and is related to the precision of atomic coordinates and resolution power to discriminate different atom-pair distances of the OEC. It is demonstrated how a precise adjustment of atomic coordinates can yield a nearly perfect representation of the experimental OEC EXAFS spectrum, but at the expense of overfitting and losing the knowledge of the initial OEC model structure. Introducing a novel type of penalty function, it is shown that moderate adjustment of atomic coordinates to the EXAFS spectrum limited by constraints avoids overfitting and can be used to validate different OEC model structures. This technique is used to identify the OEC model structures whose computed OEC EXAFS spectra agree best with the measured spectrum. In this way, the most likely S-state and protonation pattern of the OEC for the most recent high-resolution crystal structure of PSII are determined. We find that the X-ray free-electron laser (XFEL) structure is indeed not significantly affected by exposure to XFEL pulses and thus results in a radiation-damage-free model of the OEC.
Water‐oxidizing calcium–manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X‐ray absorption spectroscopy at the manganese ...and calcium K edges. The amorphous, birnesite‐type oxides are obtained through a simple protocol that involves electrodeposition followed by active‐site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all‐MnIV oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of MnIII ions at catalytic potentials is identified as a prerequisite for catalytic activity.
Catalyst Ca‐n do: Water‐oxidizing calcium–manganese oxides mimic the inorganic core of the biological catalyst. The oxides are electrodeposited and modified by active‐site creation through annealing at moderate temperatures. The ability to undergo redox transitions and the presence of a minority fraction of MnIII ions at catalytic potentials facilitates water‐oxidation catalysis.
In addition to oxometal M(n+)double bond, length as m-dashO and imidometal M(n+)double bond, length as m-dashNR units, transient metal-iodosylarene M((n-2)+)-Odouble bond, length as m-dashIPh and ...metal-iminoiodane M((n-2)+)-N(R)double bond, length as m-dashIPh adducts are often invoked as a possible "second oxidant" responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.
Prototypic dinuclear metal cofactors with varying metallation constitute a class of O2-activating catalysts in numerous enzymes such as ribonucleotide reductase. Reliable structures are required to ...unravel the reaction mechanisms. However, protein crystallography data may be compromised by x-ray photoreduction (XRP). We studied XPR of Fe(III)Fe(III) and Mn(III)Fe(III) sites in the R2 subunit of Chlamydia trachomatis ribonucleotide reductase using x-ray absorption spectroscopy. Rapid and biphasic x-ray photoreduction kinetics at 20 and 80 K for both cofactor types suggested sequential formation of (III,II) and (II,II) species and similar redox potentials of iron and manganese sites. Comparing with typical x-ray doses in crystallography implies that (II,II) states are reached in <1 s in such studies. First-sphere metal coordination and metal-metal distances differed after chemical reduction at room temperature and after XPR at cryogenic temperatures, as corroborated by model structures from density functional theory calculations. The inter-metal distances in the XPR-induced (II,II) states, however, are similar to R2 crystal structures. Therefore, crystal data of initially oxidized R2-type proteins mostly contain photoreduced (II,II) cofactors, which deviate from the native structures functional in O2 activation, explaining observed variable metal ligation motifs. This situation may be remedied by novel femtosecond free electron-laser protein crystallography techniques.
Background: Typical FeFe and MnFe cofactors bind to numerous enzymes such as ribonucleotide reductases. Crystallographic data suggest x-ray photoreduction (XPR) effects.
Results: Rapid XPR-induced cofactor changes were monitored using time-resolved x-ray absorption spectroscopy.
Conclusion: The XPR-induced cofactor states differ significantly from the native configurations, but comply with crystallographic structures.
Significance: Structure determination for high-valent dimetal-oxygen cofactors requires free electron-laser protein crystallography combined with x-ray spectroscopy.
The active site for hydrogen production in FeFe hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the ...development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated FeFe model complex, (CO)3FeI1-(bdtCl2)-FeI2(CO)(Ph2P–CH2–NCH3–CH2–PPh2) (1, bdt = benzene-1,2-dithiolate), in comparison to iron–carbonyl references. Kβ emission spectra (Kβ1,3, Kβ′) revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the Kβ1,3 energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (Kβ2,5) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d → 1s emission and 1s → 3d absorption spectra and supported by DFT, the HOMO–LUMO gap of 1 is about 2.8 eV. Kβ-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various Kβ emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on FeFe hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.
The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four ...protons that drive the downstream process of CO
fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn
Ca complex (Mn
CaO
) which is progressively oxidized upon the absorption of light by the chlorophyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to dioxygen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of O
from a new water molecule, which disappears on completion of the reaction, implicating it in the O-O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn
Ca cluster.
PDF
