Search

Interaction of three free-base meso-tris(p-X-phenyl)corroles H3[TpXPC] (X = H, CH3, OCH3) with Re2(CO)10 at 235 C in the presence of K2CO3 in o-dichlorobenzene has led to putative rhenium biscorrole sandwich compounds with the formula ReH[TpXPC]2. DFT calculations and Re L3 EXAFS measurements suggest a seven-coordinate metal center, with the extra hydrogen located on one of the corrole nitrogens. The complexes can be deprotonated by a base such as DBU, resulting in substantial sharpening of UV-visible spectra and split Soret bands, consistent with the generation of C2-symmetric anions. Both the seven-coordinate neutral and the eight-coordinate anionic forms of the complexes represent a new coordination motif in the field of Re-porphyrinoid interactions.

Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.

Binding of Lewis acidic metal ions and Bronsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)MnIV(O)]-HOTf}+ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., νMn-O at ∼793 cm-1) and 1-Mn+ (Mn+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and Mn+ ions, suggesting that H+ and Mn+ bind at the metal-oxo moiety of [(dpaq)MnIV(O)]+. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)MnIV(OH)](OTf)2 (4), with a Mn-O bond distance of 1.8043(19) A and a Mn-O stretch at 660 cm-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mn+ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mn+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal-Nx (MNx) structure, the replacement of N atoms by some other atoms with relatively weak electronegativity has been regarded as a promising strategy for optimizing the coordination environment of an active metal center and promoting its catalytic performance, which is still a challenge. Herein, we proposed a new synthetic strategy of an in situ phosphatizing of triphenylphosphine encapsulated within metal-organic frameworks for designing atomic Co1-P1N3 interfacial structure, where a cobalt single atom is costabilized by one P atom and three N atoms (denoted as Co-SA/P-in situ). In the acidic media, the Co-SA/P-in situ catalyst with Co1-P1N3 interfacial structure exhibits excellent activity and durability for the hydrogen evolution reaction (HER) with a low overpotential of 98 mV at 10 mA cm-2 and a small Tafel slope of 47 mV dec-1, which are greatly superior to those of catalyst with Co1-N4 interfacial structure. We discover that the bond-length-extended high-valence Co1-P1N3 atomic interface structure plays a crucial role in boosting the HER performance, which is supported by in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculation. We hope this work will promote the development of high performance metal single-atom catalysts.

Electrochemical and chemical studies reveal that the amido complex (PNHxP)Fe(CO)(H)(X) (FeN 1, x = 0, X = 0; Fe(H)(NH) 2, x = 1, X = H; PNHP = bis[2-(diisopropylphosphino)ethyl]amine) is active for the electrocatalytic oxidation of isopropanol. At room temperature, the amido FeN 1 dehydrogenates isopropanol to form acetone. The resulting amino hydride complex Fe(H)(NH) 2 is subsequently oxidized by one electron at a low potential (-0.74 V versus ferrocene/ferrocenium, Fc0/+) in tetrahydrofuran. In the presence of strong base (phosphazene base P2-Et, Et-N = P2(dma)5, P2), this oxidation process becomes a two-electron, two-proton process that regenerates FeN 1. FeN 1 is active for the electrooxidation of isopropanol in the presence of strong base (i.e., P2) with an onset potential near -1 V versus Fc0/+. By cyclic voltammetry, fast turnover frequencies of 1.7 s-1 for isopropanol oxidation are achieved with FeN 1. Controlled potential electrolysis studies confirm that the product of isopropanol electrooxidation is acetone, generated with high Faradaic efficiency (∼100%).

A combination of Pt L3-edge X-ray absorption spectroscopy (EXAFS and XANES) and DFT (TPSS) calculations have been performed on powder samples of the archetypal platinum porphyrinoid complexes PtII[TpCF3PP], PtIV[TpCF3PP]Cl2, and PtIV[TpCF3PC](Ar)(py), where TpCF3PP2- = meso-tetrakis(p-trifluoromethylphenyl)porphyrinato and TpCF3PC3- = meso-tris(p-trifluoromethylphenyl)corrolato. The three complexes yielded Pt L3-edge energies of 11 566.0 eV, 11 567.2 eV, and 11 567.6 eV, respectively. The 1.2 eV blueshift from the Pt(II) to the Pt(IV) porphyrin derivative is smaller than expected for a formal two-electron oxidation of the metal center. A rationale was provided by DFT-based Hirshfeld which showed that the porphyrin ligand in the Pt(IV) complex is actually substantially oxidized relative to that in the Pt(II) complex. The much smaller blueshift of 0.4 eV, going from PtIV[TpCF3PP]Cl2, and PtIV[TpCF3PC](Ar)(py), is ascribable to the significantly stronger ligand field in the latter compound.

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, Mossbauer, 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.

A combination of Pt L

Precision synthesis of thin films requires an improved mechanistic understanding of the structural evolution of materials at the atomic scale. Atomic layer deposition (ALD) is a critical nanofabric...

Surface lattice oxygen in transition-metal oxides plays a vital role in catalytic processes. Mastering activation of surface lattice oxygen and identifying the activation mechanism are crucial for the development and design of advanced catalysts. A strategy is now developed to create a spinel Co3 O4 /perovskite La0.3 Sr0.7 CoO3 interface by in situ reconstruction of the surface Sr enrichment region in perovskite LSC to activate surface lattice oxygen. XAS and XPS confirm that the regulated chemical interface optimizes the hybridized orbital between Co 3d and O 2p and triggers more electrons in oxygen site of LSC transferred into lattice of Co3 O4 , leading to more inactive O2- transformed into active O2-x . Furthermore, the activated Co3 O4 /LSC exhibits the best catalytic activities for CO oxidation, oxygen evolution, and oxygen reduction. This work would provide a fundamental understanding to explain the activation mechanism of surface oxygen sites.

Cobalt(III) hydrides, formed via protonation of basic cobalt(I) centers, have long been recognized as key intermediates in the electrocatalytic reduction of protons to hydrogen. An understanding of the structural and electronic factors that govern their formation is key to developing more efficient and potent catalysts. A combination of Co K-edge X-ray absorption spectroscopy, extended X-ray absorption fine structure, density functional theory (DFT), and time-dependent DFT methods have been used to investigate several cyclopentadienyl (Cp) Co(III)L (L = ligand) species and their two-electron reduced Co(I) analogues. The results reveal that when L is strongly π-accepting, the reduced species demonstrates strong backbonding between the electron-rich Co(I) center and the ligand L, resulting in a weakly basic Co center that does not protonate to form a Co(III)-H. In contrast, a weakly π-accepting or σ-donating ligand system results in an electron-rich Co(I) center, which is readily protonated to form a Co(III)-H. This study reveals the strength of a combined X-ray spectroscopy/theory method in understanding the role of ligands in tuning the electronic structure and subsequent reactivity of the metal center.

The regulation of catalytic activity in the oxygen reduction reaction (ORR) is significant to the development of metal–air batteries and other oxygen involving energy conversion devices. Herein, we propose an atomic interface strategy to construct a single atom copper catalyst (denoted as Cu-SA/SNC) which exhibits enhanced ORR activity with a half-wave potential of 0.893 V vs. RHE in alkaline media. Moreover, synchrotron-radiation-based X-ray absorption fine structure (XAFS) investigations together with density functional theory (DFT) calculations reveal that the isolated bond-shrinking low-valence Cu (+1)–N4–C8S2 atomic interface moiety serves as an active site during the ORR process, and the synergistic mechanism between the Cu species and the carbon matrix at the atomic interface plays a critical role in boosting the ORR efficiency, by adjusting the reaction free energy of intermediate adsorption. This atomic interface concept may provide an alternative methodology for the rational design of advanced oxygen electrode materials and new probability to improve their catalytic performance.

Methyl-coenzyme M reductase (MCR) catalyzes both synthesis and anaerobic oxidation of methane (AOM). Its catalytic site contains Ni at the core of Cofactor F430. The Ni ion, in its low-valent Ni(I) state lights the fuse leading to homolysis of the C-S bond of methyl-coenzyme M (methyl-SCoM) to generate a methyl radical, which abstracts a hydrogen atom from Coenzyme B (HSCoB) to generate methane and the mixed disulfide CoMSSCoB. Direct reversal of this reaction activates methane to initiate anaerobic methane oxidation. Based on crystal structures, which reveal a Ni-thiol interaction between Ni(II)-MCR and inhibitor CoMSH, a Ni(I)-thioether complex with substrate methyl-SCoM has been transposed to canonical MCR mechanisms. Similarly, a Ni(I)-disulfide with CoMSSCoB is proposed for the reverse reaction. However, this Ni(I)-sulfur interaction poses a conundrum for the proposed hydrogen atom abstraction reaction because the >6 Å distance between the thiol group of SCoB and the thiol of SCoM observed in the structures appears too long for such a reaction. Spectroscopic, kinetic, structural and computational studies described here establish that both methyl-SCoM and CoMSSCoB bind to the active Ni(I) state of MCR through their sulfonate groups, forming a hexacoordinate Ni(I)-N/O complex, not Ni(I)-S. These studies rule out direct Ni(I)-sulfur interactions in both substrate-bound states. As a solution to the mechanistic conundrum, we propose that both forward and reverse MCR reactions emanate through long-range electron transfer from Ni(I)-sulfonate complexes with methyl-SCoM or CoMSSCoB, respectively.

Methanotrophic bacteria catalyze the aerobic oxidation of methane to methanol using Cu-containing enzymes, thereby exerting a modulating influence on the global methane cycle. To facilitate the acquisition of Cu ions, some methanotrophic bacteria secrete small modified peptides known as “methanobactins,” which strongly bind Cu and function as an extracellular Cu recruitment relay, analogous to siderophores and Fe. In addition to Cu, methanobactins form complexes with other late transition metals, including the Group 12 transition metals Zn, Cd, and Hg, although the interplay among solution-phase configurations, metal interactions, and the spectroscopic signatures of methanobactin-metal complexes remains ambiguous. In this study, the complexation of Zn, Cd, and Hg by methanobactin from Methylocystis sp. strain SB2 was studied using a combination of absorbance, fluorescence, extended x-ray absorption fine structure (EXAFS) spectroscopy, and time-dependent density functional theory (TD-DFT) calculations. We report changes in sample absorbance and fluorescence spectral dynamics, which occur on a wide range of experimental timescales and characterize a clear stoichiometric complexation dependence. Mercury L3-edge EXAFS and TD-DFT calculations suggest a linear model for Hg S coordination, and TD-DFT suggests a tetrahedral model for Zn2+ and Cd2+. We observed an enhancement in the fluorescence of methanobactin upon interaction with transition metals and propose a mechanism of complexation-hindered isomerization drawing inspiration from the wild-type Green Fluorescent Protein active site. Collectively, our results represent the first combined computational and experimental spectroscopy study of methanobactins and shed new light on molecular interactions and dynamics that characterize complexes of methanobactins with Group 12 transition metals.

Acid effects on the chemical properties of metal-oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal(IV)-oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron(V)-oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe(V)-oxo TAML complex (1) and its protonated iron(V)-oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe(V)-oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 108 M-2 and 10(1) M-1, respectively. The reactivities of the proton-bound iron(V)-oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 107 times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (Ered vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (Ered vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the Ered value was observed with a slope of 47 mV/log([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.

Atomic interface regulation is thought to be an efficient method to adjust the performance of single atom catalysts. Herein, a practical strategy was reported to rationally design single copper atoms coordinated with both sulfur and nitrogen atoms in metal-organic framework derived hierarchically porous carbon (S-Cu-ISA/SNC). The atomic interface configuration of the copper site in S-Cu-ISA/SNC is detected to be an unsymmetrically arranged Cu-S1N3 moiety. The catalyst exhibits excellent oxygen reduction reaction activity with a half-wave potential of 0.918 V vs. RHE. Additionally, through in situ X-ray absorption fine structure tests, we discover that the low-valent Cuprous-S1N3 moiety acts as an active center during the oxygen reduction process. Our discovery provides a universal scheme for the controllable synthesis and performance regulation of single metal atom catalysts toward energy applications., Engineering the coordination environment of single atom catalysts offers to opportunity to optimize electrocatalytic activity. In this work, the authors prepare an unsymmetrical Cu-S1N3 single atom site on porous carbon with high performance in the oxygen reduction reaction.

The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal-N

A combination of Cr K-edge XAS and DFT calculations have been performed on archetypal chromium porphyrinoid complexes CrIV[TMP]O (2) and CrV[TPC]O (3), and the results have been compared to the reference compound CrIII[TPP]Cl (1), where TPP2−, TMP2−, and TPC3− are the anions of meso-tetraphenylporphyrin, meso-tetramesitylporphyrin, and meso-triphenylcorrole, respectively. The intensity-weighted average energy position in 1 (5990.9 eV), 2 (5992.0 eV) and 3 (5992.6 eV) are consistent with increasing the metal oxidation state along the series. EXAFS and theoretical analysis of 2 and 3 reveal that the Cr–O bond is longer and weaker in 3 relative to 2, despite the structural similarity and increase in oxidation state in 3. This is also reflected in a comparison of the Cr K-pre-edge transitions. The roughly twenty-fold increase in intensity in the two oxo complexes is unsurprising and well-precedented for other first-row transition metals. However, although 3 had greater overall intensity, the intensity of transitions along the Cr–O bond is greater. EXAFS, DFT and TDDFT analyses show that the more contracted N4 core of the corrole results in a greater out-of-plane displacement of the Cr in 3 relative to 2, which in turn accentuates the lower local symmetry of the Cr in 3 relative to 2. This difference helps us to appreciate the TDDFT result that whereas two 1s→ 3dz2 transitions make up the majority of the intensity in the pre-edge of 2, the pre-edge of 3 also includes fairly intense transitions to molecular orbitals with 3dxz, 3dyz and 3dz2 character, thus accounting for the somewhat higher overall intensity for 3.

Improved activity and stability Pt-based catalysts for the oxygen reduction reaction (ORR) are needed to perpetuate the deployment of polymer electrolyte fuel cells (PEFCs) in the transportation sector. Here, we use atomic layer deposition of TiO2 and Pt coupled with thermal reductive annealing to prepare Pt3Ti electrocatalysts. The atomic level synthetic control resulted in Pt3Ti nanoparticles with high ORR performance, including a mass activity of 1.84 A/mgPt and excellent electrochemical stability. The Pt3Ti nanoparticles show excellent specific activity — 5.3-fold higher than commercial Pt/C and 3-fold higher than polycrystalline Pt, exceeding the performance of any PtTi catalysts reported to date. Combined experimental and computational efforts indicate that Pt enrichment on the Pt3Ti enhances the activity, and the intrinsic stability of the Pt3Ti phase provides durability. This knowledge, along with the facile fabrication of alloys by atomic layer deposition, can be leveraged to designed improved performance catalysts.

An X-ray absorption spectroscopy (XAS) data-reporting format is developed and its inclusion as standard supporting information in biological XAS publications is proposed. The aim of this text-based data-reporting XIF (XAFS information file) file format is to develop a consistent protocol for XAS data collection, analysis and interpretation, and, optionally, for sharing reference and reported data., The complex structural landscape of biological samples and their sensitivity to X-ray exposure leads to specific challenges in biological X-ray absorption spectroscopy (bio-XAS) research, which in turn has necessitated standardization of various aspects of bio-XAS data measurement, analysis and interpretation. The bio-XAS community is therefore well suited for the development of a data-reporting standard with the specific aim of creating a feedback loop for improving/standardizing data analysis protocols and optionally to make published data available to collaborators/researchers in a meaningful and quantitative format. The XIF (XAFS information file) reporting format presented here contains key experimental and analysis parameters, useful in developing a consistent platform for bio-XAS research worldwide. Such a reporting standard, enforced by the user community and publishing groups alike, can be an important step towards the standardization of data measurement and analysis techniques in bio-XAS.

© 2017 American Chemical Society. A reinvestigation of cobalt-corrole-triphenylphosphine complexes has yielded an unexpectedly subtle picture of their electronic structures. UV-vis absorption spectroscopy, skeletal bond length alternations observed in X-ray structures, and broken-symmetry DFT (B3LYP) calculations suggest partial CoII-corrole•2-character for these complexes. The same probes applied to the analogous rhodium corroles evince no evidence of a noninnocent corrole. X-ray absorption spectroscopic studies showed that the Co K rising edge of Co[TPC](PPh3) (TPC = triphenylcorrole) is red-shifted by ∼1.8 eV relative to the bona fide Co(III) complexes Co[TPC](py)2and Co[TPP](py)Cl (TPP = tetraphenylporphyrin, py = pyridine), consistent with a partial CoII-corrole•2-description for Co[TPC](PPh3). Electrochemical measurements have shown that both the Co and Rh complexes undergo two reversible oxidations and one to two irreversible reductions. In particular, the first reduction of the Rh corroles occurs at significantly more negative potentials than that of the Co corroles, reflecting significantly higher stability of the Rh(III) state relative to Co(III). Together, the results presented herein suggest that cobalt-corrole-triphenylphosphine complexes are significantly noninnocent with moderate CoII-corrole•2-character, underscoring - yet again - the ubiquity of ligand noninnocence among first-row transition metal corroles.

Two new series of iron meso-tris(para-X-phenyl)corrole (TpXPC) complexes, Fe[TpXPC]Ph and Fe[TpXPC]Tol, in which X=CF3 , H, Me, and OMe, and Tol=p-methylphenyl (p-tolyl), have been synthesized, allowing a multitechnique electronic-structural comparison with the corresponding FeCl, FeNO, and Fe2 (μ-O) TpXPC derivatives. Optical spectroscopy revealed that the Soret maxima of the FePh and FeTol series are insensitive to the phenyl para substituent, consistent with the presumed innocence of the corrole ligand in these compounds. Accordingly, we may be increasingly confident in the ability of the substituent effect criterion to serve as a probe of corrole noninnocence. Furthermore, four complexes-Fe[TPC]Cl, Fe[TPC](NO), {Fe[TPC]}2 O, and Fe[TPC]Ph-were selected for a detailed XANES investigation of the question of ligand noninnocence. The intensity-weighted average energy (IWAE) positions were found to exhibit rather modest variations (0.8 eV over the series of corroles). The integrated Fe-K pre-edge intensities, on the other hand, vary considerably, with a 2.5 fold increase for Fe[TPC]Ph relative to Fe[TPC]Cl and Fe[TPC](NO). Given the approximately C4v local symmetry of the Fe in all the complexes, the large increase in intensity for Fe[TPC]Ph may be attributed to a higher number of 3d holes, consistent with an expected FeIV -like description, in contrast to Fe[TPC]Cl and Fe[TPC](NO), in which the Fe is thought to be FeIII -like. These results afford strong validation of XANES as a probe of ligand noninnocence in metallocorroles. Electrochemical redox potentials, on the other hand, were found not to afford a simple probe of ligand noninnocence in Fe corroles.

Acetyl-CoA synthase (ACS) is a key enzyme in the Wood–Ljungdahl pathway of anaerobic CO2 fixation, which has long been proposed to operate by a novel mechanism involving a series of protein-bound organometallic (Ni–CO, methyl–Ni, and acetyl–Ni) intermediates. Here we report the first direct structural evidence of the proposed metal–carbon bond. We describe the preparation of the highly active metal-replete enzyme and near-quantitative generation of the kinetically competent carbonylated intermediate. This advance has allowed a combination of Ni and Fe K-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure experiments along with density functional theory calculations. The data reveal that CO binds to the proximal Ni of the six-metal metallocenter at the active site and undergoes dramatic structural and electronic perturbation in forming this organometallic Ni–CO intermediate. This direct identification of a Ni–carbon bond in the catalytically competent CO-bound form of the A cluster of ACS provides definitive experimental structural evidence supporting the proposed organometallic mechanism of anaerobic acetyl-CoA synthesis.

The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO)3(Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO)3(Br) 1 (bipy = 2,2'-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (-0.93 V vs Fc+/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at -1.68 V and -1.89 V vs Fc+/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn-K-edge X-ray absorption spectroscopy (XAS), Mn-Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO)3(CH3CN)][CF3SO3] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO)3][K(18-crown-6)] 3 and [Mn(azpy)(CO)3][Cp2Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(-1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO)3 type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer-dimer equilibrium in the singly reduced Mn(0) state. This study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.

International audience; A mononuclear nonheme iron(IV)–amido complex bearing a tetraamido macrocyclic ligand, [(TAML)FeIV(NHTs)]− (1), was synthesized via a hydrogen atom (H atom) abstraction reaction of an iron(V)–imido complex, [(TAML)FeV(NTs)]− (2), and fully characterized using various spectroscopies. We then investigated (1) the pKa of 1, (2) the reaction of 1 with a carbon-centered radical, and (3) the H atom abstraction reaction of 1. To the best of our knowledge, the present study reports for the first time the synthesis and chemical properties/reactions of a high-valent iron(IV)–amido complex.

For ABO3 perovskite oxides, one of the key issues limiting their utilization in heterogeneous catalysis is the dominant presence of catalytically inactive A‐site cations at the surface. The engineering of B‐site terminated perovskites is considered as an effective method to address this issue, especially when dealing with Mn/Co‐based perovskite catalysts. However, to date, such a strategy has not been fully successful and remains a major challenge in the field. Herein, a Mn‐terminated La0.45Sr0.45MnO3 (B‐LSM) is successfully synthesized via a one‐pot hydrothermal method, in which low‐valence Mn ions partially occupy the A site to form the active Mn‐excess phase. Experimental results and theoretical calculations reveal that the presence of the surface Mn termination in B‐LSM optimizes the hybrid orbitals of Mn 3d‐O 2p and promotes the activation of surface lattice oxygen, where the pristine inert lattice O2− is evolved into active and stable lattice O2−x. Such structural optimization significantly reduces the activation energy barriers on going from O2− species to important intermediate O− species during O2 activation. Moreover, this results in good stability and Pt‐like activity for the B‐LSM during CO oxidation. This work offers a new chemical route for the design of advanced perovskite‐type oxides possessing novel functions.

Correction for ‘X-ray absorption spectroscopy of exemplary platinum porphyrin and corrole derivatives: metal- versus ligand-centered oxidation’ by Benjamin D. Matson et al., RSC Adv., 2021, 11, 32269–32274. DOI: 10.1039/D1RA06151H

The Stanford Synchrotron Radiation Lightsource (SSRL) is a national scientific user facility at the SLAC National Accelerator Laboratory that provides high-brightness X-ray beams, innovative experimental facilities, and expert scientific support as a resource to study our world at the atomic and molecular level. Operating within this context and being closely associated with a major research university (Stanford), SSRL is strongly committed to providing unique educational experiences, and serves as a vital training ground for future generations of scientists and engineers. As part of this program, SSRL oversees a series of schools and workshops each year which deliver theoretical, experimental, and hands-on training by leading experts in their respective fields. Several of the courses held this year, attended by graduate students, postdoctoral fellows, educators and junior/senior investigators, are described in this report.

Stabilising waterlogged archaeological wooden artefacts for display presents a challenge for conservators and scientists. Sulfur compounds, incorporated into the wood prior to excavation, can lead to acid formation when exposed to oxygen, and in the presence of iron ions. Strontium carbonate nanoparticles have recently been shown to reduce the production of acid formation at the root by reacting with inorganic sulfur-containing compounds. Here, we show the feasibility of using this treatment on small samples where consolidating treatments have already been performed. It is found that PEG 200 does not prevent the reactivity of the nanoparticles with the sulfur compounds present in the artefacts. A surface brushing application method was found to be successful whilst retaining the visual integrity. In addition, it was found that this technique results in the leaching of iron from the surface layers, preventing future build up of acid catalysed by iron compounds.

Wooden archeological artifacts excavated from marine sources contain problematic sulfur which can result in degradation. A treatment to safely neutralize sulfur using alkaline earth carbonates is tested on samples from the warship the Mary Rose. S K-edge X-ray Absorption Spectroscopy confirms the neutralization of acid already in the wood, by forming insoluble sulfate salts. The treatment also catalyzes the conversion of iron sulfides to soluble iron sulfates, thus removing the future source of this acid. Calcium carbonate is favored as it is non-toxic and economical. This treatment is relevant to all organic artifacts that are threatened by sulfur compounds.

A mononuclear manganese(V)–oxo complex with tetraamido macrocyclic ligand (TAML), [Mn(V)(O)(TAML)](−) (1), is a sluggish oxidant in oxidation reactions. Herein, a mononuclear manganese(V)–oxo TAML cation radical complex, [Mn(V)(O)(TAML(+•))] (2), is reported. It was synthesized by reacting [Mn(III)(TAML)](−) with 3.0 equivalents of [Ru(III)(bpy)(3)](3+) or upon addition of one-electron oxidant to 1 and then characterized thoroughly with various spectroscopic techniques along with DFT calculations. Although 1 is a sluggish oxidant, 2 is a strong oxidant capable of activating C–H bonds of hydrocarbons (i.e., hydrogen atom transfer reaction) and transferring its oxygen atom to thioanisoles and olefins (i.e., oxygen atom transfer reaction).

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface.

Heme oxygenase (HO) catalyzes a key step in heme homeostasis: the O2- and NADPH-cytochrome P450 reductase-dependent conversion of heme to biliverdin, Fe, and CO through a process in which the heme participates both as a prosthetic group and as a substrate. Mammals contain two isoforms of this enzyme, HO2 and HO1, which share the same α-helical fold forming the catalytic core and heme binding site, as well as a membrane spanning helix at their C-termini. However, unlike HO1, HO2 has an additional 30-residue N-terminus as well as two cysteine-proline sequences near the C-terminus that reside in heme regulatory motifs (HRMs). While the role of the additional N-terminal residues of HO2 is not yet understood, the HRMs have been proposed to reversibly form a thiol/disulfide redox switch that modulates the affinity of HO2 for ferric heme as a function of cellular redox poise. To further define the roles of the N- and C-terminal regions unique to HO2, we used multiple spectroscopic techniques to characterize these regions of the human HO2. Nuclear magnetic resonance spectroscopic experiments with HO2 demonstrate that, when the HRMs are in the oxidized state (HO2(O)), both the extra N-terminal and the C-terminal HRM-containing regions are disordered. However, protein NMR experiments illustrate that, under reducing conditions, the C-terminal region gains some structure as the Cys residues in the HRMs undergo reduction (HO2(R)) and, in experiments employing a diamagnetic protoporphyrin, suggest a redox-dependent interaction between the core and the HRM domains. Further, electron nuclear double resonance and X-ray absorption spectroscopic studies demonstrate that, upon reduction of the HRMs to the sulfhydryl form, a cysteine residue from the HRM region ligates to a ferric heme. Taken together with EPR measurements, which show the appearance of a new low-spin heme signal in reduced HO2, it appears that a cysteine residue(s) in the HRMs directly interacts with a second bound heme.

The growing interest in zinc oxysulfide (Zn(O,S)) thin films as buffer layers has been motivated by higher efficiencies achieved in solar cells. In this work we present insights into the electronic-geometric structure relationship of varying compositions of Zn(O,S) grown by atomic layer deposition (ALD). The X-ray absorption near edge structure (XANES), a local bonding-sensitive spectroscopic tool, with quantum simulations helps link the atomic structure to the unoccupied density of states (DOS) of the films. The infiltration of sulfur into a ZnO matrix results in the formation of S 3p–Zn 4sp–O 2p hybridized orbitals in the near edge X-ray absorption fine structure (NEXAFS) region of both the O and S K-edges. The extent of sulfur incorporation affects the ionicity of Zn, which in turn alters the bond lengths of Zn–O within the structure and its resulting bandgap. Knowing Zn(O,S)'s electronic-geometric structure interplay allows one to predict, tailor, and optimize its buffer layer performance.

Activation of dioxygen (O2) in enzymatic and biomimetic reactions has been intensively investigated over the past several decades. More recently, O–O bond formation, which is the reverse of the O2-activation reaction, has been the focus of current research. Herein, we report the O2-activation and O–O bond formation reactions by manganese corrole complexes. In the O2-activation reaction, Mn(V)-oxo and Mn(IV)-peroxo intermediates were formed when Mn(III) corroles were exposed to O2 in the presence of base (e.g., OH−) and hydrogen atom (H atom) donor (e.g., THF or cyclic olefins); the O2-activation reaction did not occur in the absence of base and H atom donor. Moreover, formation of the Mn(V)-oxo and Mn(IV)-peroxo species was dependent on the amounts of base present in the reaction solution. The role of the base was proposed to lower the oxidation potential of the Mn(III) corroles, thereby facilitating the binding of O2 and forming a Mn(IV)-superoxo species. The putative Mn(IV)-superoxo species was then converted to the corresponding Mn(IV)-hydroperoxo species by abstracting a H atom from H atom donor, followed by the O–O bond cleavage of the putative Mn(IV)-hydroperoxo species to form a Mn(V)-oxo species. We have also shown that addition of hydroxide ion to the Mn(V)-oxo species afforded the Mn(IV)-peroxo species via O–O bond formation and the resulting Mn(IV)-peroxo species reverted to the Mn(V)-oxo species upon addition of proton, indicating that the O–O bond formation and cleavage reactions between the Mn(V)-oxo and Mn(IV)-peroxo complexes are reversible. The present study reports the first example of using the same manganese complex in both O2-activation and O–O bond formation reactions.

International audience; A mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), [FeV(NTs)(TAML)]- (1), was oxidized by one-electron oxidants, affording formation of an iron(V)-imido TAML cation radical species, [FeV(NTs)(TAML+•)] (2); 2 is a diamagnetic (S = 0) complex, resulting from the antiferromagnetic coupling of the low-spin iron(V) ion (S = 1/2) with the one-electron oxidized ligand (TAML+•). 2 is a competent oxidant in C-H bond functionalization and nitrene transfer reaction, showing that the reactivity of 2 is greater than that of 1.

In situ inventory of sulfurous products from the sulfur K-edge synchrotron X-radiolysis of l-cysteine in solid-phase and anaerobic (pH 5) and air-saturated (pH 5, 7, and 9) solutions without and with 40% glycerol is reported. Sequential K-edge X-ray Absorption Spectroscopic (XAS) spectra were acquired. l-cysteine degraded systematically in the X-ray beam. Radiolytic products were inventoried by fits using the XAS spectra of sulfur model compounds. Solid l-cysteine declined to 92% fraction after a single K-edge XAS scan. After six scans, 60% remained, accompanied by 14% cystine, 16% thioether, 5.4% elemental sulfur, and smaller fractions of more highly oxidized products. In air-saturated pH 5 solution, 73% of l-cysteine remained after ten scans, with 2% cystine and 19% elemental sulfur. Oxidation increased with 40% glycerol, yielding 67%, 5%, and 23% fractions, respectively, after ten scans. Higher pH solutions exhibited less radiolytic chemistry. All the reactivity followed first-order kinetics. The anaerobic experiment displayed two reaction phases, with sharp changes in kinetics and radiolytic chemistry. Unexpectedly, the radiolytic oxidation of l-cysteine was increased in anaerobic solution. After ten scans, only 60% of the l-cysteine remained, along with 17% cystine, 22% elemental sulfur, and traces of more highly oxidized products. A new aerobic reaction cycle is hypothesized, wherein dissolved dioxygen captures radiolytic H(•) or [Formula: see text] , enters HO(2)(•)/O(2)(•−), reductively quenches cysteine thiyl radicals, and cycles back to O(2). This cycle is suggested to suppress the radiolytic production of cystine in aerobic solution.

The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-μ-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)2(DBED)2(O)2](2+) (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the [Cu(II)2(NO2-XYL)(O2)](2+) complex, only a P species was spectroscopically observed. However, it was not clear whether this O-O bond cleaves to proceed through an O-type structure along the reaction coordinate for hydroxylation of the aromatic xylyl linker. Accurate evaluation of these reaction coordinates requires reasonable quantitative descriptions of the electronic structures of the P and O species. We have performed Cu L-edge XAS on two well-characterized P and O species to experimentally quantify the Cu 3d character in their ground state wave functions. The lower per-hole Cu character (40 ± 6%) corresponding to higher covalency in the O species compared to the P species (52 ± 4%) reflects a stronger bonding interaction of the bis-μ-oxo core with the Cu(III) centers. DFT calculations show that 10-20% Hartree-Fock (HF) mixing for P and ~38% for O species are required to reproduce the Cu-O bonding; for the P species this HF mixing is also required for an antiferromagnetically coupled description of the two Cu(II) centers. B3LYP (with 20% HF) was, therefore, used to calculate the hydroxylation reaction coordinate of P in [Cu(II)2(NO2-XYL)(O2)](2+). These experimentally calibrated calculations indicate that the electrophilic attack on the aromatic ring does not involve formation of a Cu(III)2(O(2-))2 species. Rather, there is direct electron donation from the aromatic ring into the peroxo σ* orbital of the Cu(II)2(O2(2-)) species, leading to concerted C-O bond formation with O-O bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.

Mononuclear metal-dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side-on cobalt(III)-peroxo complex bearing a macrocyclic N-tetramethylated cyclam (TMC) ligand, [Co(III) (15-TMC)(O2 )](+) , was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)-peroxo complex was cleanly converted into an end-on cobalt(III)-hydroperoxo complex, [Co(III) (15-TMC)(OOH)](2+) . The cobalt(III)-hydroperoxo complex was further converted to [Co(III) (15-TMC-CH2 -O)](2+) by hydroxylation of a methyl group of the 15-TMC ligand. Kinetic studies and (18) O-labeling experiments proposed that the aliphatic hydroxylation occurred via a Co(IV) -oxo (or Co(III) -oxyl) species, which was formed by OO bond homolysis of the cobalt(III)-hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.

One and two scandium ions (Sc(3+)) are bound strongly to nonheme manganese(IV)-oxo complexes, [(N4Py)Mn(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)Mn(IV)(O)](2+) (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), to form Mn(IV)(O)-(Sc(3+))1 and Mn(IV)(O)-(Sc(3+))2 complexes, respectively. The binding of Sc(3+) ions to the Mn(IV)(O) complexes was examined by spectroscopic methods as well as by DFT calculations. The one-electron reduction potentials of the Mn(IV)(O) complexes were markedly shifted to a positive direction by binding of Sc(3+) ions. Accordingly, rates of the electron transfer reactions of the Mn(IV)(O) complexes were enhanced as much as 10(7)-fold by binding of two Sc(3+) ions. The driving force dependence of electron transfer from various electron donors to the Mn(IV)(O) and Mn(IV)(O)-(Sc(3+))2 complexes was examined and analyzed in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The smaller reorganization energies and much more positive reduction potentials of the Mn(IV)(O)-(Sc(3+))2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the Mn(IV)(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the Mn(IV)(O) complexes by binding of Sc(3+) ions resulted in the change of mechanism in the sulfoxidation of thioanisoles by Mn(IV)(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc(3+) ions.