Probing the Local Environment in Potassium Salts and Potassium-Promoted Catalysts by Potassium Valence-to-Core X-ray Emission Spectroscopy.

Potassium plays an important role in biology as well as a promoter in heterogeneous catalysis. There are, however, limited characterization techniques for potassium available in the literature. This study elucidates the potential of element-selective X-ray emission spectroscopy (XES) for characterizing the coordination environment and the electronic properties of potassium.

Facile electrocatalytic proton reduction by a [Fe-Fe]-hydrogenase bio-inspired synthetic model bearing a terminal CN ligand.

An azadithiolate bridged CN bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe-Fe] Hase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H to H between pH 0-3 at diffusion-controlled rates (10 M s) 10 s at pH 3 with an overpotential of 140 mV.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage.

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N to give a tetra-iron complex with two nitrides (Rodriguez et al., , 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate.

Structural correlations of nitrogenase active sites using nuclear resonance vibrational spectroscopy and QM/MM calculations.

The biological conversion of N to NH is accomplished by the nitrogenase family, which is collectively comprised of three closely related but unique metalloenzymes. In the present study, we have employed a combination of the synchrotron-based technique of Fe nuclear resonance vibrational spectroscopy together with DFT-based quantum mechanics/molecular mechanics (QM/MM) calculations to probe the electronic structure and dynamics of the catalytic components of each of the three unique M Nase enzymes (M = Mo, V, Fe) in both the presence (holo-) and absence (apo-) of the catalytic FeMco clusters (FeMoco, FeVco and FeFeco). The results described herein provide vibrational mode assignments for important fingerprint regions of the FeMco clusters, and demonstrate the sensitivity of the calculated partial vibrational density of states (PVDOS) to the geometric and electronic structures of these clusters.

Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e/4H Oxygen Reduction Rates at Lower Overpotentials.

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e/4H reduction of oxygen to water at rates that are 2-3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins.

Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction.

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose p values vary from ∼9 to 15 in acetonitrile.

Elucidation of Factors That Govern the 2e/2H vs 4e/4H Selectivity of Water Oxidation by a Cobalt Corrole.

Considering the importance of water splitting as the best solution for clean and renewable energy, the worldwide efforts for development of increasingly active molecular water oxidation catalysts must be accompanied by studies that focus on elucidating the mode of actions and catalytic pathways. One crucial challenge remains the elucidation of the factors that determine the selectivity of water oxidation by the desired 4e/4H pathway that leads to O rather than by 2e/2H to HO. We now show that water oxidation with the cobalt-corrole CoBr as electrocatalyst affords HO as the main product in homogeneous solutions, while heterogeneous water oxidation by the same catalyst leads exclusively to oxygen.

Electrocatalytic Water Oxidation by a Phosphorus-Nitrogen O═PN-Pincer Cobalt Complex.

Water oxidation is a primary step in natural as well as artificial photosynthesis to convert renewable solar energy into chemical energy/fuels. Electrocatalytic water oxidation to evolve O, utilizing suitable low-cost catalysts and renewable electricity, is of fundamental importance considering contemporary energy and environmental issues, yet it is kinetically challenging owing to the complex multiproton/electron transfer processes. Herein, we report the first cobalt-based pincer catalyst for catalytic water oxidation at neutral pH with high efficiency under electrochemical conditions.

Homogeneous Electrochemical Reduction of CO to CO by a Cobalt Pyridine Thiolate Complex.

The chemical and electrochemical reduction of CO to value added chemicals entails the development of efficient and selective catalysts. Synthesis, characterization and electrochemical CO reduction activity of a air-stable cobalt(III) diphenylphosphenethano-bis(2-pyridinethiolate)chloride [{Co(dppe)(2-PyS)}Cl, ] complex is divulged. The complex reduces CO under homogeneous electrocatalytic conditions to produce CO with high Faradaic efficiency (FE > 92%) and selectivity in the presence of water.

A designed second-sphere hydrogen-bond interaction that critically influences the O-O bond activation for heterolytic cleavage in ferric iron-porphyrin complexes.

Heme hydroperoxidases catalyze the oxidation of substrates by HO. The catalytic cycle involves the formation of a highly oxidizing species known as Compound , resulting from the two-electron oxidation of the ferric heme in the active site of the resting enzyme. This high-valent intermediate is formed upon facile heterolysis of the O-O bond in the initial Fe-OOH complex.

The Equatorial Ligand Effect on the Properties and Reactivity of Iron(V) Oxo Intermediates.

High-valent metal oxo oxidants are common catalytic-cycle intermediates in enzymes and known to be highly reactive. To understand which features of these oxidants affect their reactivity, a series of biomimetic iron(V) oxo oxidants with peripherally substituted biuret-modified tetraamido macrocyclic ligands were synthesized and characterized. Major shifts in the UV/Vis absorption as a result of replacing a group in the equatorial plane of the iron(V) oxo species were found.

Formally Ferric Heme Carbon Monoxide Adduct.

Formally ferric carbonyl adducts are reported in a series of thiolate-bound iron porphyrins. Resonance Raman data indicate the presence of both Fe-S and Fe-CO bonds, and EPR data of this S = 1/2 species indicate a ligand-based electron hole, giving this complex an Fe(II)-thiyl radical electronic ground state. The FTIR data show that the C-O vibrations are substantially higher than in the corresponding ferrous-thiolate CO adducts.

Role of 2 sphere H-bonding residues in tuning the kinetics of CO reduction to CO by iron porphyrin complexes.

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO2. It has been recently established that the reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(ii)-CO22- and a Fe(ii)-COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO2 reduction.

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e/4H O Reduction: Activation of O-O Bond by 2nd Sphere Hydrogen Bonding.

Facile and selective 4e/4H electrochemical reduction of O to HO in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e/4H electrochemical O reduction to HO is possible with rate constants as fast as 10 M s under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate.

Iron(ii) complexes of 2-mercaptopyridine as rubredoxin site analogues.

The synthesis, characterization and reactivity studies of iron(ii) complexes [Fe(PySH)](OTf), 1-(OTf)2, [Fe(PySH)](ClO), 1-(ClO4)2, and [Fe(PyS)], (2), of a 2-mercaptopyridine (PySH) ligand are discussed. The X-ray crystal structures of both 1-(OTf)2 and 1-(ClO4)2 reveal a distorted tetrahedral geometry at the iron(ii) center with identical constituents. All the pyridine nitrogen atoms are protonated and thiolate ions are coordinated to the iron(ii) center.

Ligand-Induced Tuning of the Oxidase Activity of μ-Hydroxidodimanganese(III) Complexes Using 3,5-Di-tert-butylcatechol as the Substrate: Isolation and Characterization of Products Involving an Oxidized Dioxolene Moiety.

Oxidase activities of a μ-hydroxidodimanganese(III) system involving a series of tetradentate capping ligands HL with a pair of phenolate arms have been investigated in the presence of 3,5-di-tert-butylcatechol (HDBC) as a coligand cum-reductant. The reaction follows two distinctly different paths, decided by the substituent combinations (R and R) present in the capping ligand. With the ligands HL and HL, the products obtained are semiquinonato compounds [Mn(L)(DBSQ)]·2CHOH (1) and [Mn(L)(DBSQ)]·CHOH (2), respectively.

Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with Fe═O and Fe═O Units.

In this report we compare the geometric and electronic structures and reactivities of [Fe(O)] and [Fe(O)] species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe═O vibration of the [Fe(O)] complex 2 is at 798 cm, compared to 862 cm for the corresponding [Fe(O)] species 3, a 64 cm frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe═O complexes.

Activating Fe(I) Porphyrins for the Hydrogen Evolution Reaction Using Second-Sphere Proton Transfer Residues.

Iron porphyrin complexes with second-sphere distal triazole residues show a hydrogen evolution reaction (HER) catalyzed by the Fe(I) state in both organic and aqueous media, whereas an analogous iron porphyrin complex without the distal residues catalyzes the HER in the formal Fe(0) state. This activation of the Fe(I) state by the second-sphere residues lowers the overpotential of the HER by these iron porphyrin complexes by 50%. Experimental data and theoretical calculations indicate that the distal triazole residues, once protonated, enhance the proton affinity of the iron center via formation of a dihydrogen bond with an Fe(III)-H intermediate.

Theoretical exploration of the mechanism of formylmethanofuran dehydrogenase: the first reductive step in CO2 fixation by methanogens.

A theoretical exploration of the possible active site models of methanofuran dehydrogenase reveals that the free energy of the reduction of the carbamate group is substantially negative and is driven by the electron withdrawing amide group next to the carbonyl carbon. Comparison of the computed transition state energies with the experimental energy barrier indicates that the active site is likely to have an axial oxo and equatorial hydrosulfide ligand, the substrate is likely to be protonated and a second-sphere hydrogen-bonding interaction with the axial ligand can, substantially, lower the barrier of this reaction which involves reduction of the carbonyl center of the a carbamate to form an N-formyl group via a hydride shift from a Mo(IV) center.

Second sphere control of spin state: Differential tuning of axial ligand bonds in ferric porphyrin complexes by hydrogen bonding.

An iron porphyrin with a pre-organized hydrogen bonding (H-Bonding) distal architecture is utilized to avoid the inherent loss of entropy associated with H-Bonding from solvent (water) and mimic the behavior of metallo-enzyme active sites attributed to H-Bonding interactions of active site with the 2nd sphere residues. Resonance Raman (rR) data on these iron porphyrin complexes indicate that H-Bonding to an axial ligand like hydroxide can result in both stronger or weaker Fe(III)-OH bond relative to iron porphyrin complexes. The 6-coordinate (6C) complexes bearing water derived axial ligands, trans to imidazole or thiolate axial ligand with H-Bonding stabilize a low spin (LS) ground state (GS) when a complex without H-Bonding stabilizes a high spin (HS) ground state.

Density functional theory calculations on the active site of biotin synthase: mechanism of S transfer from the Fe(2)S(2) cluster and the role of 1st and 2nd sphere residues.

Density functional theory (DFT) calculations are performed on the active site of biotin synthase (BS) to investigate the sulfur transfer from the Fe(2)S(2) cluster to dethiobiotin (DTB). The active site is modeled to include both the 1st and 2nd sphere residues. Molecular orbital theory considerations and calculation on smaller models indicate that only an S atom (not S²⁻) transfer from an oxidized Fe(2)S(2) cluster leads to the formation of biotin from the DTB using two adenosyl radicals generated from S-adenosyl-L-methionine.

Intermediates Involved in the 2e(-)/2H(+) Reduction of CO2 to CO by Iron(0) Porphyrin.

The reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates. The resonance Raman data of intermediate I, which could only be stabilized at -95 °C, indicates that it is a Fe(II)-CO2(2-) adduct and is followed by an another intermediate II at -80 °C where the bound CO2 in intermediate I is protonated to form a Fe(II)-COOH species. While the initial protonation can be achieved using weak proton sources like MeOH and PhOH, the facile heterolytic cleavage of the C-OH bond in intermediate II requires strong acids.

Effect of axial ligand, spin state, and hydrogen bonding on the inner-sphere reorganization energies of functional models of cytochrome P450.

Using a combination of self-assembly and synthesis, bioinspired electrodes having dilute iron porphyrin active sites bound to axial thiolate and imidazole axial ligands are created atop self-assembled monolayers (SAMs). Resonance Raman data indicate that a picket fence architecture results in a high-spin (HS) ground state (GS) in these complexes and a hydrogen-bonding triazole architecture results in a low-spin (LS) ground state. The reorganization energies (λ) of these thiolate- and imidazole-bound iron porphyrin sites for both HS and LS states are experimentally determined.

Electrocatalytic O₂ reduction by [Fe-Fe]-hydrogenase active site models.

The instability of [Fe-Fe]-hydrogenase and its synthetic models under aerobic conditions is an inherent challenge in their development as practical H2 producing electrodes. The electrochemical oxygen reduction reaction of a series of synthetic model complexes of the [Fe-Fe] hydrogenase is investigated, and a dominant role of the bridgehead nitrogen in reducing the amount of partially reduced oxygen species (PROS), which is detrimental to the stability of these complexes, is discovered.

The cobalt corrole catalyzed hydrogen evolution reaction: surprising electronic effects and characterization of key reaction intermediates.

A surprising effect of halide substituents on reduction potentials and catalytic activity of halogenated cobalt corroles has been deduced by experimental and computational methods; the proton-activating cobalt(I) and the cobalt(II) corrole that is formed in the step during which hydrogen is formed are characterized by NMR spectroscopy.