Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of ...these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)
). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.
Two crystallized FeFe hydrogenase model complexes, 1 = (μ-pdt)Fe(CO)(2)(PMe(3))(2) (pdt = SC1H2C2H2C3H2S), and their bridging-hydride (Hy) derivative, 1Hy(+) = (μ-H)(μ-pdt)Fe(CO)(2) (PMe(3))(2)(+) ...(BF(4)(−)), were studied by Fe K-edge X-ray absorption and emission spectroscopy, supported by density functional theory. Structural changes in 1Hy(+) compared to 1 involved small bond elongations (<0.03 Å) and more octahedral Fe geometries; the Fe–H bond at Fe1 (closer to pdt-C2) was ~0.03 Å longer than that at Fe2. Analyses of (1) pre-edge absorption spectra (core-to-valence transitions), (2) Kβ(1,3), Kβ', and Kβ(2,5) emission spectra (valence-to-core transitions), and (3) resonant inelastic X-ray scattering data (valence-to-valence transitions) for resonant and non-resonant excitation and respective spectral simulations indicated the following: (1) the mean Fe oxidation state was similar in both complexes, due to electron density transfer from the ligands to Hy in 1Hy(+). Fe 1s→3d transitions remained at similar energies whereas delocalization of carbonyl AOs onto Fe and significant Hy-contributions to MOs caused an ~0.7 eV up-shift of Fe1s→(CO)s,p transitions in 1Hy(+). Fed-levels were delocalized over Fe1 and Fe2 and degeneracies biased to O(h)–Fe1 and C(4v)–Fe2 states for 1, but to O(h)–Fe1,2 states for 1Hy(+). (2) Electron-pairing of formal Fe(d(7)) ions in low-spin states in both complexes and a higher effective spin count for 1Hy(+) were suggested by comparison with iron reference compounds. Electronic decays from Fe d and ligand s,p MOs and spectral contributions from Hys,p→1s transitions even revealed limited site-selectivity for detection of Fe1 or Fe2 in 1Hy(+). The HOMO/LUMO energy gap for 1 was estimated as 3.0 ± 0.5 eV. (3) For 1Hy(+) compared to 1, increased Fed (x(2) − y(2)) − (z(2)) energy differences (~0.5 eV to ~0.9 eV) and Fed→d transition energies (~2.9 eV to ~3.7 eV) were assigned. These results reveal the specific impact of Hy-binding on the electronic structure of diiron compounds and provide guidelines for a directed search of hydride species in hydrogenases.
Water oxidation by photosystem II (PSII) sustains most life on Earth, but the molecular mechanism of this unique process remains controversial. The ongoing identification of the binding sites and ...modes of the two water-derived substrate oxygens ('substrate waters') in the various intermediates (S
states, i = 0, 1, 2, 3, 4) that the water-splitting tetra-manganese calcium penta-oxygen (Mn
CaO
) cluster attains during the reaction cycle provides central information towards resolving the unique chemistry of biological water oxidation. Mass spectrometric measurements of single- and double-labeled dioxygen species after various incubation times of PSII with H
O provide insight into the substrate binding modes and sites via determination of exchange rates. Such experiments have revealed that the two substrate waters exchange with different rates that vary independently with the S
state and are hence referred to as the fast (W
) and the slow (W
) substrate waters. New insight for the molecular interpretation of these rates arises from our recent finding that in the S
state, under special experimental conditions, two different rates of W
exchange are observed that appear to correlate with the high spin and low spin conformations of the Mn
CaO
cluster. Here, we reexamine and unite various proposed methods for extracting and assigning rate constants from this recent data set. The analysis results in a molecular model for substrate-water binding and exchange that reconciles the expected non-exchangeability of the central oxo bridge O5 when located between two Mn(IV) ions with the experimental and theoretical assignment of O5 as W
in all S states. The analysis also excludes other published proposals for explaining the water exchange kinetics.
In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state ...cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn K beta(1,3) line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The K beta(1,3) line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)(3)Mn(IV) in S-0, Mn(III)(2)Mn(IV)(2) in S-1, Mn(III)Mn(IV)(3) in S-2, and Mn(IV)(4) in S-3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S-4 state unlikely. The difference spectra of both manganese K beta(1,3) emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S-2 -> S-3 transition when compared to Mn(III) oxidation in the S-1 -> S-2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S-2 -> S-3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal ligand bonding patterns at the Mn4Ca complex in the S-2 -> S-3 transition are discussed.
Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. During cyclic ...voltammetry and after sudden potential jumps between noncatalytic and catalytic potentials, Ni oxidation/reduction was tracked with millisecond time resolution by a UV/Vis reflectance signal. Optimal catalysis at intermediate Ni/Fe ratios is explained by two opposing trends for increasing Fe content: a) pronounced slowdown of the Ni2+/Ni3+ oxidation step and b) increased reactivity of the most oxidized catalyst state detectable at catalytic potentials. This state may involve an equilibrium between Ni4+ ions and Ni2+ ions with neighboring ligand holes, possibly in the form of bound peroxides.
Redox in slow motion: Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. Increasing Fe content slows down Ni2+/Ni3+ oxidation state changes and enhances oxygen evolution reactivity involving an equilibrium between Ni4+ and Ni2+ states with ligand holes.
Photosynthesis stores solar light as chemical energy and efficiency of this process is highly important. The electrons required for CO
2
reduction are extracted from water in a reaction driven by ...light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn
4
O
5
-cluster. This cyclic process involves five redox intermediates known as the S
0
-S
4
states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between −10 °C and +20 °C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 °C and below, the highest efficiency (
i.e.
lowest miss parameter) was found for the S
1
→ S
2
transition, while the S
2
→ S
3
transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn
4
O
5
-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible "molecular errors" connected to the
e
−
transfer, H
+
transfer, H
2
O binding and O
2
release are identified.
Temperature dependence of the transition inefficiencies (misses) for the water oxidation process in photosystem II were studied by EPR spectroscopy and are explained on the basis of S state dependent structural changes at the CaMn
4
O
5
-cluster.
Irreversible inhibition by molecular oxygen (O2) complicates the use of FeFe-hydrogenases (HydA) for biotechnological hydrogen (H2) production. Modification by O2 of the active site six-iron complex ...denoted as the H-cluster (4Fe4S-2FeH) of HydA1 from the green alga Chlamydomonas reinhardtii was characterized by x-ray absorption spectroscopy at the iron K-edge. In a time-resolved approach, HydA1 protein samples were prepared after increasing O2 exposure periods at 0 °C. A kinetic analysis of changes in their x-ray absorption near edge structure and extended X-ray absorption fine structure spectra revealed three phases of O2 reactions. The first phase (τ1 ≤ 4 s) is characterized by the formation of an increased number of Fe–O,C bonds, elongation of the Fe–Fe distance in the binuclear unit (2FeH), and oxidation of one iron ion. The second phase (τ2 ≈ 15 s) causes a ∼50% decrease of the number of ∼2.7-Å Fe–Fe distances in the 4Fe4S subcluster and the oxidation of one more iron ion. The final phase (τ3 ≤ 1000 s) leads to the disappearance of most Fe–Fe and Fe–S interactions and further iron oxidation. These results favor a reaction sequence, which involves 1) oxygenation at 2FeH+ leading to the formation of a reactive oxygen species-like superoxide (O2−), followed by 2) H-cluster inactivation and destabilization due to ROS attack on the 4Fe4S cluster to convert it into an apparent 3Fe4S+ unit, leading to 3) complete O2-induced degradation of the remainders of the H-cluster. This mechanism suggests that blocking of ROS diffusion paths and/or altering the redox potential of the 4Fe4S cubane by genetic engineering may yield improved O2 tolerance in FeFe-hydrogenase.
Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron ...transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen–oxygen bond formation and compare it with models proposed in the literature.
The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn
CaO
cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow ...exchanging substrate water (W
)', while, in the S
state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W
)'. A critical point for the assignment of W
is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn
CaO
cluster. In this study, we measured the rates of H
O/H
O substrate water exchange in the S
and S
states of PSII core complexes from wild-type (WT)
sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access
the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of W
and W
were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn
CaO
cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that W
exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S
state, namely the indifference of the rate of W
exchange towards Ca/Sr substitution.
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