title: Intramolecular Oxyl Radical Coupling Promotes O-O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation.
abstract: The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn(Py2NtBu2)(H2O)2]2+is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a MnV-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin MnIIIion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calculated reaction barrier of only 14.7 kcal mol-1. This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.
authors.affiliation: Department of Chemistry, Indiana University , 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Douglas W
authors.initials: DW
authors.lastname: Crandell
authors.affiliation: Department of Chemistry, Indiana University , 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Song
authors.initials: S
authors.lastname: Xu
authors.affiliation: Department of Chemistry, Indiana University , 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Jeremy M
authors.identifier: http://orcid.org/0000-0002-3206-4725
authors.initials: JM
authors.lastname: Smith
authors.affiliation: Institute for Basic Science (IBS) , Center for Catalytic Hydrocarbon Functionalizations, Daejeon, 34141, South Korea.|Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon, 34141, South Korea.
authors.forename: Mu-Hyun
authors.identifier: http://orcid.org/0000-0002-8832-8187
authors.initials: MH
authors.lastname: Baik