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Published October 1, 2013 | Published
Journal Article Open

Role of oxido incorporation and ligand lability in expanding redox accessibility of structurally related Mn4 clusters

Abstract

Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O_2 formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the six oxidation states from Mn^(II)_(3)Mn^III to Mn^(III)_(2)Mn^(IV)_2 with the same ligand framework (L) by incorporating four oxido ligands. A 1,3,5-triarylbenzene framework appended with six pyridyl and three alkoxy groups was utilized along with three acetate anions to access tetramanganese complexes, Mn_(4)O_x, with x = 1, 2, 3, and 4. Alongside two previously reported complexes, four new clusters in various states were isolated and characterized by crystallography, and four were observed electrochemically, thus accessing the eight oxidation states from Mn^(II)_4 to Mn^(III)Mn^(IV)_3. This structurally related series of compounds was characterized by EXAFS, XANES, EPR, magnetism, and cyclic voltammetry. Similar to the ligands in the active site of the protein, the ancillary ligand (L) is preserved throughout the series and changes its binding mode between the low and high oxido-content clusters. Implications for the rational assembly and properties of high oxidation state metal–oxido clusters are presented.

Additional Information

© 2013 Royal Society of Chemistry. Received 20th May 2013; Accepted 18th July 2013. We are grateful to California Institute of Technology, the Searle Scholars Program, an NSF CAREER (CHE-1151918) award (TA), the Rose Hill Foundation, and the NSF GRFP (J.S.K.) for funding. We thank L. M. Henling for assistance with crystallography. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF CRIF:MU award to Caltech (CHE-0639094). We acknowledge the Gordon and Betty Moore Foundation, the Beckman Institute, and the Sano-Aventis BRP for their support of the Molecular Observatory at Caltech. The work carried out by RDB was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-11ER16282). SSRL is operated by Stanford University for the DOE and supported by its Office of Biological and Environmental Research, and by the NIH, NIGMS (including P41GM103393) and the NCRR (P41RR001209). X-ray spectroscopy studies were supported by the NIH (F32GM100595 to R.T.) and by the Director of the Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences, DOE (DE-AC02-05CH11231 to J.Y.).

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August 19, 2023
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