An official website of the United States government
In order to shed light on metal-dependent mechanisms for O–O bond cleavage, and its microscopic reverse, we compare herein the electronic and geometric structures of O2-derived binuclear Co(III)– and Mn(III)–peroxo compounds. Binuclear metal peroxo complexes are proposed to form as intermediates during Mn-promoted photosynthetic H2O oxidation, as well as a Co-containing artificial leaf inspired by nature’s photosynthetic H2O oxidation catalyst. Crystallographic characterization of an extremely activated peroxo is made possible by working with substitution-inert, low-spin Co(III). Density functional theory (DFT) calculations show that the frontier orbitals of the Co(III)–peroxo compound differ noticeably from the analogous Mn(III)–peroxo compound. The highest occupied molecular orbital (HOMO) associated with the Co(III)–peroxo is more localized on the peroxo in an antibonding π*(O–O) orbital, whereas the HOMO of the structurally analogous Mn(III)–peroxo is delocalized over both the metal d-orbitals and peroxo π*(O–O) orbital. With low-spin d6 Co(III), filled t2g orbitals prevent π-back-donation from the doubly occupied antibonding π*(O–O) orbital onto the metal ion. This is not the case with high-spin d4 Mn(III), since these orbitals are half-filled. This weakens the peroxo O–O bond of the former relative to the latter.
more »
« less
Synthesis of chemically active models of the oxygen-evolving complex
Photosynthesis, one of Nature’s most essential processes, is known to be reliant on the oxidation of water to dioxygen. However, this process has yet to be synthetically replicated, or its mechanism fully elucidated. Fundamental understanding of nature’s water oxidation reaction could offer clues to the design of superior catalysts for solar water splitting as a source of alternative energy. In order to work towards this, various structural mimics of nature’s CaMn4 water oxidation catalyst (the Oxygen Evolving Complex) have been synthesized and characterized. These clusters were prepared from Manganese and Cobalt, precursors and complexed to 2-pyridinemethanol. The resulting biomimetic models consisted of Mn-O and Co-O bridges, which are similar to the nature of the OEC itself. Electrocatalytic reactivity of these systems will be presented.
more »
« less
- Award ID(s):
- 1800105
- PAR ID:
- 10187687
- Date Published:
- Journal Name:
- ACS Spring 2020 National Meeting and Expo
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The oxygen evolving complex (OEC) of photosystem II is responsible for the four-electron oxidation of water to dioxygen in oxygenic photosynthetic organisms. These organisms use light to drive this thermodynamically uphill process to harvest high-energy electrons from water for cellular energy, generating O2 as a byproduct. While numerous details of the operation of the OEC are well understood, the molecular mechanism of O=O formation remains under debate. Model complex chemistry from our group will be presented wherein we have prepared cluster systems with little or no terminal multidentate ligation in order to generate geometrically flexible and reactive clusters. Two systems will be presented: a series of Ca-Mn-O hemicubane clusters with variable calcium content, and their electrocatalytic activity in activation of water in oxidation reactions, and a Mn cubane cluster with a pendant Mn=O moiety that evolves dioxygen from an oxidation state analagous to the turnover state of the natural systemmore » « less
-
Abstract Mechanisms for the cold atmospheric plasma (CAP) treatment of cells in solution are needed for more optimum design of plasma devices for wound healing, cancer treatment, and bacterial inactivation. However, the complexity of organic molecules on cell membranes makes understanding mechanisms that result in modifications (i.e. oxidation) of such compounds difficult. As a surrogate to these systems, a reaction mechanism for the oxidation of cysteine in CAP activated water was developed and implemented in a 0-dimensional (plug-flow) global plasma chemistry model with the capability of addressing plasma-liquid interactions. Reaction rate coefficients for organic reactions in water were estimated based on available data in the literature or by analogy to gas-phase reactions. The mechanism was validated by comparison to experimental mass-spectrometry data for COST-jets sustained in He/O2, He/H2O and He/N2/O2mixtures treating cysteine in water. Results from the model were used to determine the consequences of changing COST-jet operating parameters, such as distance from the substrate and inlet gas composition, on cysteine oxidation product formation. Results indicate that operating parameters can be adjusted to select for desired cysteine oxidation products, including nitrosylated products.more » « less
-
Abstract Marine microbes produce extracellular reactive oxygen species (ROS) such as superoxide and hydrogen peroxide (H2O2) as a result of regulated and nonregulated physiological and metabolic reactions. ROS production can be a sink and cryptic recycling flux of dissolved oxygen that may rival other key fluxes in the global oxygen cycle; however, the low abundance and high turnover rate of ROS makes this figure difficult to constrain. One key step in determining the disparity between the gross production of ROS and the net sink of dissolved oxygen lies in understanding the degradation pathways of H2O2in the marine water column. In this study, we use isotope‐labeling techniques to determine the redox fate of H2O2in a range of marine environments off the West Coast of California. We find that H2O2reduction is greater than or equal to H2O2oxidation at most sampled depths, with notable exceptions in some surface and intermediate water depths. The observation that H2O2oxidation can exceed reduction in the dark ocean indicates the presence of an oxidizing decay pathway that is not among the known suite of microbially mediated enzymatic pathways (i.e., catalase and peroxidase), pointing to an abiotic and/or a nonenzymatic decay pathway at intermediate water depths. These results highlight the complexity and heterogeneity of ROS decay pathways in natural waters and their unconstrained regulation of oxygen levels within the ocean.more » « less
-
Tellurorhodamine, 9-mesityl-3,6-bis(dimethylamino)telluroxanthylium hexafluorophosphate ( 1 ), photocatalytically oxidizes aromatic and aliphatic silanes and triphenyl phosphine under mild aerobic conditions. Under irradiation with visible light, 1 can react with self-sensitized 1 O 2 to generate the active telluroxide oxidant ( 2 ). Silanes are oxidized to silanols and triphenyl phosphine is oxidized to triphenyl phoshine oxide either using 2 , or 1 with aerobic irradiation. Kinetic experiments coupled with a computational study elucidate possible mechanisms of oxidation for both silane and phosphine substrates. First-order rates were observed in the oxidation of triphenyl phosphine and methyldiphenyl silane, indicating a substitution like mechanism for substrate binding to the oxidized tellurium( iv ). Additionally, these reactions exhibited a rate-dependence on water. Oxidations were typically run in 50 : 50 water/methanol, however, the absence of water decreased the rates of silane oxidation to a greater degree than triphenyl phosphine oxidation. Parallel results were observed in solvent kinetic isotope experiments using D 2 O in the solvent mixture. The rates of oxidation were slowed to a greater degree in silane oxidation by 2 ( k H / k D = 17.30) than for phosphine ( k H / k D = 6.20). Various silanes and triphenyl phosphine were photocatalytically oxidized with 1 (5%) under irradiation with warm white LEDs using atmospheric oxygen as the terminal oxidant.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1021/scimeetings.0c06076
