Ca K-Edge XAS as a Probe of Calcium Centers in Complex Systems

Martin‐Diaconescu, Vlad; Gennari, Marcello; Gerey, Bertrand; Tsui, Emily Y.; Kanady, Jacob S.; Tran, Rosalie; Pécaut, Jacques; Maganas, Dimitrios; Krewald, Vera; Gouré, Eric; Duboc, Carole; Yano, Junko; Agapie, Theodor; Collomb, Marie‐Noëlle; DeBeer, Serena

doi:10.1021/ic501991e

Cited by 44 publications

(49 citation statements)

References 94 publications

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“…The XANES spectra in Ref. display strong similarities to the spectra described herein. It is conceivable that XANES changes caused by annealing relate to the formation of the Mn 3 CaO 4 motif.…”

Section: Resultssupporting

confidence: 54%

“…These changes are clearly less pronounced in the 0.31 Ca/Mn sample. In a series of Ca model compounds with differing coordination environments, the Ca pre‐edge was highly sensitive to coordination number and ligand geometry of the Ca ion . Seven‐coordinated calcium compounds in cubane‐type structures showed higher pre‐edge intensity when compared with non‐cubane eight‐ and seven‐coordinated compounds.…”

Section: Resultsmentioning

confidence: 99%

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Electrosynthesis of Biomimetic Manganese–Calcium Oxides for Water Oxidation Catalysis—Atomic Structure and Functionality

et al. 2015

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Water-oxidizing calcium-manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.

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Section: Resultssupporting

confidence: 54%

Section: Resultsmentioning

confidence: 99%

Electrosynthesis of Biomimetic Manganese–Calcium Oxides for Water Oxidation Catalysis—Atomic Structure and Functionality

et al. 2015

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“…-SOC complexes under different environmental conditions. As an example, Martin-Diaconescu et al (2015) have recently successfully probed the coordination environment of synthetic Ca complexes. While powerful, these direct spectroscopic techniques require the use of synchrotron light source and can only be applied to small amounts of samples with limited compositional complexity.…”

Section: Building Empirical Evidence For Ca-soc Interactionsmentioning

confidence: 99%

Calcium-mediated stabilisation of soil organic carbon

2017

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Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (Ca Exch ) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca 2? can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner-and outer-sphere bridging by Ca 2? can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO 3 ) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al-or Fe-to Ca, and their respective effects on SOC dynamics.

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“…46,70,71 Spin unrestricted, time-dependent calculations for XAS spectra were ) using acetonitrile as solvent.…”

Section: Computational Detailsmentioning

confidence: 99%

Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel–Oxygen Species

et al. 2016

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Abstract.Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed towards the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This is in part related to the "Oxo Wall" concept, which predicts that late transition elements cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO -at low temperature by UV/Vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is +4, experimental and computational analysis indicate that 3 is best formulated as a Ni III complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 1/ClO -

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Ca K-Edge XAS as a Probe of Calcium Centers in Complex Systems

Cited by 44 publications

References 94 publications

Electrosynthesis of Biomimetic Manganese–Calcium Oxides for Water Oxidation Catalysis—Atomic Structure and Functionality

Electrosynthesis of Biomimetic Manganese–Calcium Oxides for Water Oxidation Catalysis—Atomic Structure and Functionality

Calcium-mediated stabilisation of soil organic carbon

Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel–Oxygen Species

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