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Published April 11, 2017 | Published + Supplemental Material
Journal Article Open

In situ characterization of cofacial Co(IV) centers in Co_4O_4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

Abstract

The Co_4O_4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)_2 state may be captured in a Co(III)_2(IV)_2 cubane. We demonstrate that the Co(III)_2(IV)_2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)_2(IV)_2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)_2 dimer. The exchange coupling in the cofacial Co(IV)_2 site allows for parallels to be drawn between the electronic structure of the Co_4O_4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)_2 center on O-O bond formation.

Additional Information

© 2017 National Academy of Sciences. Contributed by Daniel G. Nocera, February 7, 2017 (sent for review November 23, 2016; reviewed by Kyle M. Lancaster and James K. McCusker) This material is based upon work supported under the Solar Photochemistry Program of the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences of the US Department of Energy (D.G.N.) and the Solar Energy Conversion program in Chemical Sciences and Engineering Division at Argonne National Laboratory (ANL) under Contract DE-AC02-06CH11357. R.G.H. is an Enrico Fermi Fellow at ANL and D.H. is a Joseph J. Katz Postdoctoral Fellow at ANL. C.N.B. is a National Science Foundation Graduate Research Fellow at Harvard University. Use of beamline 12BM at the Advanced Photon Source at ANL was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. C.N.B. and R.G.H. contributed equally to this work. Author contributions: C.N.B., R.G.H., and D.G.N. designed research; C.N.B., R.G.H., D.H., and N.L. performed research; C.N.B., R.G.H., D.H., B.J.R., N.L., and L.X.C. contributed new reagents/analytic tools; C.N.B., R.G.H., D.H., N.L., and D.G.N. analyzed data; and C.N.B., R.G.H., and D.G.N. wrote the paper. Reviewers: K.M.L., Cornell University; and J.K.M., Michigan State University. The authors declare no conflict of interest. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1701816114/-/DCSupplemental.

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Created:
August 21, 2023
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October 18, 2023