Preparation of Nanostructured Ta3N5 Electrodes by Alkaline Hydrothermal Treatment Followed by NH3 Annealing and Their Improved Water Oxidation Performance.

Solar water splitting is a clean and sustainable process for green hydrogen production. It can reduce the fossil fuel consumption. Tantalum nitride (Ta3N5) is one of the limited candidates of semiconductors, which absorb a broad range of visible light and are thermodynamically able to split water without external bias potential. In the present work, we introduce a facile method to prepare a nanostructured Ta3N5 photoanode in a two-step process: hydrothermal deposition of perovskite-type NaTaO3 in a hydrofluoric acid-free NaOH aqueous solution followed by heat treatment in NH3 atmosphere. The resulted bare Ta3N5 electrode was subsequently modified with a Ni-doped CoFeO x (Ni:CoFeO x ) as a water oxidation catalyst. After the cocatalyst loading, the electrode shows a photocurrent of about 5.3 mA cm-2 at 1.23 V vs reversible hydrogen electrode. The electrode maintained about 82% of its initial photocurrent after 7 h irradiation. In addition, a continuous oxygen evolution occurred for 3 h at Faraday efficiency of 96%. This performance is superior to that of the single-layer-modified Ta3N5 photoanodes reported so far. This remarkable improvement on the photochemical performance could be due to the uniform nanostructured surface morphology of the present Ta3N5 photoanode. Other alkaline salt treatments, such as LiOH and KOH, do not give such nanostructured morphology and accordingly exhibit lower performance than the one treated in NaOH.

Efficient Solar-Assisted O2 Reduction Using a Cofacial Iron Porphyrin Dimer Catalyst Integrated into a p-CuBi2 O4 Photocathode.

A cofacial iron porphyrin heterodimer, Fe2 TPFPP-TMP showed high electrocatalytic activity, selectivity, and stability for O2 reduction to H2 O both in homogeneous non-aqueous and heterogeneous neutral aqueous solutions. When it is integrated to FTO/p-CuBi2 O4 (FTO=fluorine doped tin oxide) photocathode prepared by a simple novel method, a remarkable efficient solar-assisted O2 reduction is achieved in neutral potassium phosphate (KPi) or basic NaOH solutions saturated with O2 .

A new preparation of a bifunctional crystalline heterogeneous copper electrocatalyst by electrodeposition using a Robson-type macrocyclic dinuclear copper complex for efficient hydrogen and oxygen evolution from water.

The development of low-cost, stable bifunctional electrocatalysts, which operate in the same electrolyte with a low overpotential for water splitting, including the oxygen evolution reaction and the hydrogen evolution reaction, remains an attractive prospect and a great challenge. In this study, a water soluble Robson-type macrocyclic dicopper(ii) complex has been used for the first time as a catalyst precursor for the generation of a copper-based bifunctional heterogeneous catalyst film, which can be used for both HER and OER at a near neutral pH. In sodium borate buffer at pH 9.20, this complex decomposed to give a Cu(OH)2/Cu2O-based thin film on FTO that catalyzes both hydrogen production and water oxidation. The morphology, nature and composition of the thin film were fully characterized by scanning electron microscopy, powder X-ray diffraction, X-ray photoelectron, and energy dispersive X-ray spectroscopies. The catalyst film showed high stability during the course of electrolysis in either the cathodic or the anodic direction for more than 4 h. Faradaic efficiencies of ∼92% for HER and ∼96% for OER were achieved. The switch between the two half-reactions of catalytic water splitting was fully reversible in nature.

The secondary coordination sphere and axial ligand effects on oxygen reduction reaction by iron porphyrins: a DFT computational study.

Oxygen reduction reaction (ORR) catalyzed by a bio-inspired iron porphyrin bearing a hanging carboxylic acid group over the porphyrin ring, and a tethered axial imidazole ligand was studied by DFT calculations. BP86 free energy calculations of the redox potentials and pK a's of reaction components involved in the proton coupled electron transfer (PCET) reactions of the ferric-hydroxo and -superoxo complexes were performed based on Born-Haber thermodynamic cycle in conjunction with a continuum solvation model. The comparison was made with iron porphyrins that lack either in the hanging acid group or axial ligand, suggesting that H-bond interaction between the carboxylic acid and iron-bound hydroxo, aquo, superoxo, and peroxo ligands (de)stabilizes the Fe-O bonding, resulting in the increase in the reduction potential of the ferric complexes. The axial ligand interaction with the imidazole raises the affinity of the iron-bound superoxo and peroxo ligands for proton. In addition, a low-spin end-on ferric-hydroperoxo intermediate, a key precursor for O-O cleavage, can be stabilized in the presence of axial ligation. Thus, selective and efficient ORR of iron porphyrin can be achieved with the aid of the secondary coordination sphere and axial ligand interactions.

Bio-inspired cofacial Fe porphyrin dimers for efficient electrocatalytic CO2 to CO conversion: Overpotential tuning by substituents at the porphyrin rings.

Efficient reduction of CO2 into useful carbon resources particularly CO is an essential reaction for developing alternate sources of fuels and for reducing the greenhouse effect of CO2. The binuclear Ni, Fe-containing carbon monoxide dehydrogenase (CODHs) efficiently catalyzes the reduction of CO2 to CO. The location of Ni and Fe at proper positions allows their cooperation for CO2 to CO conversion through a push-pull mechanism. Bio-inspired from CODHs, we used several cofacial porphyrin dimers with different substituents as suitable ligands for holding two Fe ions with suitable Fe-Fe separation distance to efficiently and selectively promote CO2 to CO conversion with high turnover frequencies, TOFs. The substituents on the porphyrin rings greatly affect the catalysis process. By introducing electron-withdrawing/-donating groups, e.g. electron-withdrawing perfluorophenyl, at all meso positions of the porphyrin rings, the catalysis overpotential, η was minimized by ≈0.3 V compared to that obtained by introducing electron-donating mesityl groups. The Fe porphyrin dimers among reported catalysts are the most efficient ones for CO2 to CO conversion. Control experiments indicate that the high performance of the current CO2 to CO conversion catalysts is due to the presence of binuclear Fe centers at suitable Fe-Fe separation distance.

The secondary coordination sphere controlled reactivity of a ferric-superoxo heme: unexpected conversion to a ferric hydroperoxo intermediate by reaction with a high-spin ferrous heme.

A bio-inspired heme complex involving both a proton donor and an axial imidazole ligand reduces the activation energy for the formation of a ferric hydroperoxo intermediate. A high-spin ferrous heme is shown to be capable of reducing its superoxy species to generate a ferric hydroperoxo intermediate for the first time.

Efficient electrocatalytic CO2 reduction with a molecular cofacial iron porphyrin dimer.

A cofacial iron tetraphenyl porphyrin dimer, , bio-inspired by the Ni-Fe containing metalloenzyme, carbon monoxide dehydrogenase (CODH), efficiently and selectively catalyses the electrochemical reduction of CO2 to CO in DMF/10% H2O solution at the electro-generated Fe(0)(por) species with high Faradic efficiency (95%) and TOF (4300 s(-1)) at a moderate overpotential, η = 0.66 V.

A cryo-generated ferrous-superoxo porphyrin: EPR, resonance Raman and DFT studies.

The resonance Raman analysis of cryo-generated ferrous-superoxy heme has been performed for the first time, and its structure and the reaction mechanism are rationalized by DFT calculations. The presence of another electronic tautomer of ferrous-superoxy heme is predicted computationally.

Covalent grafting of carbon nanotubes with a biomimetic heme model compound to enhance oxygen reduction reactions.

The oxygen reduction reaction (ORR) is one of the most important reactions in both life processes and energy conversion systems. The replacement of noble-metal Pt-based ORR electrocatalysts by nonprecious-metal catalysts is crucial for the large-scale commercialization of automotive fuel cells. Inspired by the mechanisms of dioxygen activation by metalloenzymes, herein we report a structurally well-defined, bio-inspired ORR catalyst that consists of a biomimetic model compound-an axial imidazole-coordinated porphyrin-covalently attached to multiwalled carbon nanotubes. Without pyrolysis, this bio-inspired electrocatalyst demonstrates superior ORR activity and stability compared to those of the state-of-the-art Pt/C catalyst in both acidic and alkaline solutions, thus making it a promising alternative as an ORR electrocatalyst for application in fuel-cell technology.

Isolation of a Mn(IV) acylperoxo complex and its monooxidation ability.

The first example of monooxygenation by a high-valent Mn(IV) complex with a peroxide is described. A key Mn(IV) acylperoxo intermediate, which uses m-chloroperoxybenzoic acid as the oxygen donor, is directly observed by electro-spray ionization mass spectrometry and resonance Raman spectroscopy.

Crystal structure of aldoxime dehydratase and its catalytic mechanism involved in carbon-nitrogen triple-bond synthesis.

Aldoxime dehydratase (OxdA), which is a unique heme protein, catalyzes the dehydration of an aldoxime to a nitrile even in the presence of water in the reaction mixture. Unlike the utilization of H(2)O(2) or O(2) as a mediator of catalysis by other heme-containing enzymes (e.g., P450), OxdA is notable for the direct binding of a substrate to the heme iron. Here, we determined the crystal structure of OxdA. We then constructed OxdA mutants in which each of the polar amino acids lying within ∼6 Šof the iron atom of the heme was converted to alanine. Among the purified mutant OxdAs, S219A had completely lost and R178A exhibited a reduction in the activity. Together with this finding, the crystal structural analysis of OxdA and spectroscopic and electrostatic potential analyses of the wild-type and mutant OxdAs suggest that S219 plays a key role in the catalysis, forming a hydrogen bond with the substrate. Based on the spatial arrangement of the OxdA active site and the results of a series of mutagenesis experiments, we propose the detailed catalytic mechanism of general aldoxime dehydratases: (i) S219 stabilizes the hydroxy group of the substrate to increase its basicity; (ii) H320 acts as an acid-base catalyst; and (iii) R178 stabilizes the heme, and would donate a proton to and accept one from H320.

Axial ligand effects on vibrational dynamics of iron in heme carbonyl studied by nuclear resonance vibrational spectroscopy.

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory calculation (DFT) have been applied to illuminate the effect of axial ligation on the vibrational dynamics of iron in heme carbonyl. The analyses of the NRVS data of five- (5c) and six-coordinate (6c) heme-CO complexes indicate that the prominent feature of (57)Fe partial vibrational density of state ((57)FePVDOS) at the 250-300 cm(-1) region is significantly affected by the association of the axial ligand. The DFT calculations predict that the prominent (57)FePVDOS is composed of iron in-plane motions which are coupled with porphyrin pyrrole in-plane (ν(49), ν(50), and ν(53)), an out-of-plane (γ(8)) (two of four pyrrole rings include the in-plane modes, while the rest of pyrrole rings vibrate along the out-of-plane coordinate), and out-of-phase carbonyl C and O atom displacement perpendicular to the Fe-C-O axis. Thus, in the case of the 5c CO-heme the prominent (57)FePVDOS shows sharp and intense feature because of the degeneracy of the e symmetry mode within the framework of C(4v) symmetry molecule, whereas the association of the axial imidazole ligand in the 6c complex with the lowered symmetry results in split of the degenerate vibrational energy as indicated by broader and lower intensity features of the corresponding NRVS peak compared to the 5c structure. The vibrational energy of the iron in-plane motion in the 6c complex is higher than that in 5c, implying that the iron in the 6c complex includes stronger in-plane interaction with the porphyrin compared to 5c. The iron in-plane mode above 500 cm(-1), which is predominantly coupled with the out-of-phase carbonyl C and O atom motion perpendicular to Fe-C-O, called as Fe-C-O bending mode (δ(Fe-C-O)), also suggests that the 6c structure involves a larger force constant for the e symmetry mode than 5c. The DFT calculations along with the NRVS data suggest that the stiffened iron in-plane motion in the 6c complex can be ascribed to diminished pseudo-Jahn-Teller instability along the e symmetry displacement due to an increased a(1)-e orbital energy gap caused by σ* interaction between the iron d(z(2)) orbital and the nitrogen p orbital from the axial imidazole ligand. Thus, the present study implicates a fundamental molecular mechanism of axial ligation of heme in association with a diatomic gas molecule, which is a key primary step toward versatile biological functions.

Photophysical analysis of 1,10-phenanthroline-embedded porphyrin analogues and their magnesium(II) complexes.

The synthesis, characterization, photophysical properties, and theoretical analysis of a series of tetraaza porphyrin analogues (H-Pn: n=1-4) containing a dipyrrin subunit and an embedded 1,10-phenanthroline subunit are described. The meso-phenyl-substituted derivative (H-P1) interacts with a Mg(2+) salt (e.g., MgCl(2), MgBr(2), MgI(2), Mg(ClO(4))(2), and Mg(OAc)(2)) in MeCN solution, thereby giving rise to a cation-dependent red-shift in both the absorbance- and emission maxima. In this system, as well as in the other H-Pn porphyrin analogues used in this study, the four nitrogen atoms of the ligand interact with the bound magnesium cation to form Mg(2+)-dipyrrin-phenanthroline complexes of the general structure MgX-Pn (X=counteranion). Both single-crystal X-ray diffraction analysis of the corresponding zinc-chloride derivative (ZnCl-P1) and fluorescence spectroscopy of the Mg-adducts that are formed from various metal salts provide support for the conclusion that, in complexes such as MgCl-P1, a distorted square-pyramidal geometry persists about the metal cation wherein a chloride anion acts as an axial counteranion. Several analogues (HPn) that contain electron-donating and/or electron-withdrawing dipyrrin moieties were prepared in an effort to understand the structure-property relationships and the photophysical attributes of these Mg-dipyrrin complexes. Analysis of various MgX-Pn (X=anion) systems revealed significant substitution effects on their chemical, electrochemical, and photophysical properties, as well as on the Mg(2+)-cation affinities. The fluorescence properties of MgCl-Pn reflected the effect of donor-excited photoinduced electron transfer (d-PET) processes from the dipyrrin subunit (as a donor site) to the 1,10-phenanthroline acceptor subunit. The proposed d-PET process was analyzed by electron paramagnetic resonance (EPR) spectroscopy and by femtosecond transient absorption (TA) spectroscopy, as well as by theoretical DFT calculations. Taken together, these studies provide support for the suggestion that a radical species is produced as the result of an intramolecular charge-transfer process, following photoexcitation. These photophysical effects, combined with a mixed dipyrrin-phenanthroline structure that is capable of effective Mg(2+)-cation complexation, lead us to suggest that porphyrin-inspired systems, such as HPn, have a role to play as magnesium-cation sensors.

Cytochrome c(552) from Thermus thermophilus engineered for facile substitution of prosthetic group.

The facile replacement of heme c in cytochromes c with non-natural prosthetic groups has been difficult to achieve due to two thioether linkages between cysteine residues and the heme. Fee et al. demonstrated that cytochrome c(552) from Thermus thermophilus, overproduced in the cytosol of E. coli, has a covalent linkage cleavable by heat between the heme and Cys11, as well as possessing the thioether linkage with Cys14 [Fee, J. A. (2004) Biochemistry 43, 12162-12176]. Prompted by this result, we prepared a C14A mutant, anticipating that the heme species in the mutant was bound to the polypeptide solely through the thermally cleavable linkage; therefore, the removal of the heme would be feasible after heating the protein. Contrary to this expectation, C14A immediately after purification (as-purified C14A) possessed no covalent linkage. An attempt to extract the heme using a conventional acid-butanone method was unsuccessful due to rapid linkage formation between the heme and polypeptide. Spectroscopic analyses suggested that the as-purified C14A possessed a heme b derivative where one of two peripheral vinyl groups had been replaced with a group containing a reactive carbonyl. A reaction of the as-purified C14A with [BH(3)CN](-) blocked the linkage formation on the carbonyl group, allowing a quantitative yield of heme-free apo-C14A. Reconstitution of apo-C14A was achieved with ferric and ferrous heme b and zinc protoporphyrin. All reconstituted C14As showed spontaneous covalent linkage formation. We propose that C14A is a potential source for the facile production of an artificial cytochrome c, containing a non-natural prosthetic group.

Oxoferryl porphyrin/hydrogen peroxide system whose behavior is equivalent to hydroperoxoferric porphyrin.

The reaction between H(2)O(2) and a pyridine-coordinated ferric porphyrin encapsulated by a cyclodextrin dimer yielded a hydroperoxoferric porphyrin intermediate, PFe(III)-OOH, which rapidly decomposed to oxoferryl porphyrin (PFe(IV)═O). Upon reaction with H(2)O(2), PFe(IV)═O reverted to PFe(III)-OOH, which was converted to carbon monoxide-coordinated ferrous porphyrin under a CO atmosphere. PFe(IV)═O in the presence of excess H(2)O(2) behaves as PFe(III)-OOH.

Supramolecular structures and photoelectronic properties of the inclusion complex of a cyclic free-base porphyrin dimer and C60.

A cyclic free-base porphyrin dimer H4-CPD(Py) (CPD = cyclic porphyrin dimer) linked by butadiyne moieties bearing 4-pyridyl groups self-assembles to form a novel porphyrin nanotube in the crystalline state. The cyclic molecules link together through nonclassical C-H⋅⋅⋅N hydrogen bonds and π­π interactions of the pyridyl groups along the crystallographic a axis. H4-CPD(Py) includes a C60 molecule in its cavity in solution. In the crystal structure of the inclusion complex (C60⊂H4-CPD(Py)), the dimer "bites" a C60 molecule by tilting the porphyrin rings with respect to each other, and there are strong π­π interactions between the porphyrin rings and C60. The included C60 molecules form a zigzag chain along the crystallographic b axis through van der Waals contacts with each other. Femtosecond laser flash photolysis of C60⊂H4-CPD(Py) in the solid state with photoexcitation at 420 nm shows the formation of a completely charge-separated state {H4-CPD(Py)·+ + C60·−}, which decays with a lifetime of 470 ps to the ground state. The charge-carrier mobility of the single crystal of C60⊂H4-CPD(Py) was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. C60⊂H4-CPD(Py) has an anisotropic charge mobility (Σµ = 0.16 and 0.13 cm2 V(−1) s(−1)) along the zigzag chain of C60 (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C60⊂H4-CPD(Py) was deposited onto nanostructured SnO2 films on a transparent electrode. The solar cell exhibited photovoltaic activity with an incident photon to current conversion efficiency of 17%.

Formation of an end-on ferric peroxo intermediate upon one-electron reduction of a ferric superoxo heme.

The low-spin end-on ferric peroxo heme intermediate has been proposed as an alternative reactive intermediate involved in the catalytic cycles of enzymes such as nitric oxide synthase and cytochrome P450. This transient heme intermediate has never been captured using synthetic heme models. We demonstrate herein our success in the solution preparation of such an end-on ferric peroxo intermediate derived from a heme model, which features both a group hanging over the porphyrin macrocycle and a covalently appended axial imidazole ligand, through one-electron reduction of its ferric superoxo precursor. The obtained ferric peroxo intermediate was further transformed into the corresponding ferric hydroperoxo species upon protonation. This heme model compound provides a convenient system for sequential preparation of the important and biologically relevant superoxo/peroxo/hydroperoxo heme intermediates through an oxygenation/one-electron reduction/protonation process similar to the mechanisms used by enzyme systems.

Copper(II) and nickel(II) hexafluorophosphate complexes derived from a monoanionic porphyrin analogue: solvato- and thermochromism of the Ni complexes by spin-interconversion.

Phenanporphodimethene (1) is a porphyrin analogue which has a dipyrromethene unit replaced by a 1,10-phenanthroline moiety. This modification effectively coverts a dianionic porphyrin to a monoanionic porphyrin analogue with a porphyrin-like N4 coordination sphere. The ligand 1 forms copper and nickel hexafluorophosphate complexes, Cu-1 and Ni-1, respectively. X-ray crystallographic analysis of Cu-1 indicates that the Cu(II) complex is tetracoordinated by two pyrrolic nitrogen atoms and two phenanthrolic nitrogen atoms and includes a non-bonding PF(6) counter-anion. The Ni-1 complex has similar geometry with a tetracoordinate square-planar structure in non-coordinating solvents such as CHCl(3). In coordinating solvents such as MeOH, the coordination structure adopts an octahedral geometry. These results indicate that Ni-1 can be converted from a low-spin to high-spin configuration by the coordination of two solvent molecules to the nickel center. This solvatochromic conversion of Ni-1 is accompanied by thermochromic behavior resulting from the transformation between square planar and octahedral configurations in THF solution. The redox peak responsible for the nickel-centered redox reaction of Ni-1 is observed at -0.66 V (vs. Fc/Fc(+)) in CH(2)Cl(2) solution, which indicates generation of low valent Ni(I) species. Thus, Ni-1 may be useful for future investigations as a novel structural model of the active site of cofactor F(430) in methyl-coenzyme M reductase.