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1. Concerted proton–electron transfer reactions of manganese–hydroxo and manganese–oxo complexes

   https://doi.org/10.1039/d0cc01201g  

   Mayfield, Jaycee R.; Grotemeyer, Elizabeth N.; Jackson, Timothy A. (January 2020, Chemical Communications)

   The enzymes manganese superoxide dismutase and manganese lipoxygenase use Mn III –hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C–H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn–oxo motifs in attacking substrate C–H bonds, presumably by a concerted proton–electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn–hydroxo and Mn–oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of Mn III –hydroxo and Mn IV –oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around Mn III –hydroxo and Mn IV –oxo motifs permit elucidation of structure–activity relationships. For Mn III –hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the Mn III/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the p K a of the Mn II –aqua product, which represents the proton-transfer component for CPET. For Mn IV –oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the Mn IV –oxo unit results in large rate enhancements for C–H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of Mn III –hydroxo and Mn IV –oxo adducts. 

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2. Manganese-based permanent magnet materials

   https://doi.org/10.1016/j.pmatsci.2021.100872  

   Keller, Thomas; Baker, Ian (February 2022, Progress in Materials Science)
3. Manganese Cycling in the Oceans

   https://doi.org/10.1002/9781119300762.wsts0065  

   Tebo, Bradley M. Luther (December 2019, Encyclopedia of Water: Science, Technology, and Society)

   Manganese (Mn) is an essential element for life. Although its concentration is at (sub)nanomolar levels throughout the ocean, it affects the oxygen concentration of the ocean because it is central to the photosynthetic formation of dioxygen, O2, in photosystem center II. Mn inputs into the ocean are from atmospheric transport of particles and their dissolution to form dissolved Mn, and from the flux of dissolved Mn from rivers, sediments and hydrothermal vents. The main removal mechanism is transport of particulate Mn from dust and organic matter to the sediments. The environmental chemistry of manganese centers on its +2, +3 and +4 oxidation states. Most recent data show that Mn(II) is dissolved, that Mn(IV) is particulate MnO2, and that Mn(III) can be particulate or dissolved when bound to organic complexes [denoted as Mn(III)-L]. Mn(II) is oxidized primarily by microbial processes whereas MnO2 is reduced by abiotic and biotic processes. Photochemical processing aids redox cycling in surface waters. In suboxic zones, which are defined as areas with dissolved O2 concentrations below 3 M, both oxidation and reduction processes can occur but usually at different depths. In suboxic zones, dissolved Mn is also released from organic matter during its decomposition and from MnO2 reduction. 

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4. Light-driven formation of manganese oxide by today’s photosystem II supports evolutionarily ancient manganese-oxidizing photosynthesis

   https://doi.org/10.1038/s41467-020-19852-0  

   Chernev, Petko; Fischer, Sophie; Hoffmann, Jutta; Oliver, Nicholas; Assunção, Ricardo; Yu, Boram; Burnap, Robert L.; Zaharieva, Ivelina; Nürnberg, Dennis J.; Haumann, Michael; et al (December 2020, Nature Communications)

   Abstract Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III,IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today’s photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by down-sizing of protein-bound manganese-oxide nanoparticles to finally yield today’s catalyst of photosynthetic water oxidation. 

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5. Manganese Oxide Catalyzed Hydrolysis of Polyphosphates

   https://doi.org/10.1021/acsearthspacechem.9b00220  

   Wan, Biao; Huang, Rixiang; Diaz, Julia M.; Tang, Yuanzhi (September 2019, ACS Earth and Space Chemistry)