title: In situ characterization of cofacial Co(IV) centers in Co4O4cubane: Modeling the high-valent active site in oxygen-evolving catalysts.
abstract: The Co4O4cubane 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)2state may be captured in a Co(III)2(IV)2cubane. We demonstrate that the Co(III)2(IV)2cubane 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)2cubane, 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)2dimer. The exchange coupling in the cofacial Co(IV)2site allows for parallels to be drawn between the electronic structure of the Co4O4cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)2center on O-O bond formation.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138.
authors.forename: Casey N
authors.initials: CN
authors.lastname: Brodsky
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Ryan G
authors.initials: RG
authors.lastname: Hadt
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Dugan
authors.initials: D
authors.lastname: Hayes
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Reinhart
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138.
authors.forename: Nancy
authors.initials: N
authors.lastname: Li
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439.|Department of Chemistry, Northwestern University, Evanston, IL 60208.
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138; dnocera@fas.harvard.edu.
authors.forename: Daniel G
authors.identifier: 0000-0001-5055-320X
authors.initials: DG
authors.lastname: Nocera
keywords: electrocatalysis; oxygen evolution reaction; renewable energy; solar-to-fuels; water splitting