Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published February 26, 2020 | Supplemental Material + Accepted Version
Journal Article Open

S = 3 Ground State for a Tetranuclear Mn^(IV)₄O₄ Complex Mimicking the S₃ State of the Oxygen Evolving Complex

Abstract

The S₃ state is currently the last observable intermediate prior to O–O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn^(IV)₄ core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S₃ state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S₃ state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn^(IV)₄O₄ cuboidal complex as a spectroscopic model of the S₃ state. Results show that this Mn^(IV)₄O₄ complex has an S = 3 ground state with isotropic ⁵⁵Mn hyperfine coupling constants of −75, −88, −91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer–monomer magnetic coupling model of pseudo-octahedral Mn^(IV) centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S₂ state to the S₃ state. This same spin state change is observed following oxidation of the previously reported Mn^(III)Mn^(IV)₃O₄ cuboidal complex to the Mn^(IV)₄O₄ complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.

Additional Information

© 2020 American Chemical Society. Received: September 25, 2019; Published: February 4, 2020. D.A.M. thanks Prof. Troy Stich (Wake Forest University) for valuable discussions and for reading the paper. This research was supported by the NIH (T.A. grant R01-GM102687), the Dreyfus Teacher–Scholar Program (T.A.), Dow Next Generation Educator (instrumentation), NSF-1531940 (Caltech EPR facility), and the Division of Chemical Sciences, Geosciences, and Biosciences (R.D.B. grant DE-SC0007203) of the Office of Basic Energy Sciences of the U.S. Department of Energy. Part of this work (XAS data collection) was carried out at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. XAS studies were performed with support of the Office of Science, OBES, Division of Chemical Sciences, Geosciences, and Biosciences (CSGB) of the DOE under contract no. DE-AC02-05CH11231 (J.Y.). Author Contributions: H.B.L. and D.A.M.: These authors contributed equally to this work. The authors declare no competing financial interest.

Attached Files

Accepted Version - jacs.9b10371.pdf

Accepted Version - nihms-1587212.pdf

Supplemental Material - ja9b10371_si_001.pdf

Files

jacs.9b10371.pdf
Files (3.8 MB)
Name Size Download all
md5:1e1d3e49526f68852c3d79368f750510
1.0 MB Preview Download
md5:06be508d3f5b6a545183a46247ca4e60
1.4 MB Preview Download
md5:cddcdfaee9d06d93d693925ac8a70aca
1.3 MB Preview Download

Additional details

Created:
August 19, 2023
Modified:
October 19, 2023