title: Water-soluble Fe(II)-H2O complex with a weak O-H bond transfers a hydrogen atom via an observable monomeric Fe(III)-OH.
abstract: Understanding the metal ion properties that favor O-H bond formation versus cleavage should facilitate the development of catalysts tailored to promote a specific reaction, e.g., C-H activation or H2O oxidation. The first step in H2O oxidation involves the endothermic cleavage of a strong O-H bond (BDFE = 122.7 kcal/mol), promoted by binding the H2O to a metal ion, and by coupling electron transfer to proton transfer (PCET). This study focuses on details regarding how a metal ion's electronic structure and ligand environment can tune the energetics of M(HO-H) bond cleavage. The synthesis and characterization of an Fe(II)-H2O complex, 1, that undergoes PCET in H2O to afford a rare example of a monomeric Fe(III)-OH, 7, is described. High-spin 7 is also reproducibly generated via the addition of H2O to {[Fe(III)(O(Me2)N4(tren))]2-(μ-O)}(2+) (8). The O-H bond BDFE of Fe(II)-H2O (1) (68.6 kcal/mol) is calculated using linear fits to its Pourbaix diagram and shown to be 54.1 kcal/mol less than that of H2O and 10.9 kcal/mol less than that of [Fe(II)(H2O)6](2+). The O-H bond of 1 is noticeably weaker than the majority of reported M(n+)(HxO-H) (M = Mn, Fe; n+ = 2+, 3+; x = 0, 1) complexes. Consistent with their relative BDFEs, Fe(II)-H2O (1) is found to donate a H atom to TEMPO(•), whereas the majority of previously reported M(n+)-O(H) complexes, including [Mn(III)(S(Me2)N4(tren))(OH)](+) (2), have been shown to abstract H atoms from TEMPOH. Factors responsible for the weaker O-H bond of 1, such as differences in the electron-donating properties of the ligand, metal ion Lewis acidity, and electronic structure, are discussed.
authors.affiliation: Department of Chemistry, University of Washington , Campus Box 351700, Seattle, Washington 98195-1700, United States.
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Brines
authors.forename: Michael K
authors.initials: MK
authors.lastname: Coggins
authors.forename: Penny Chaau Yan
authors.initials: PC
authors.lastname: Poon
authors.forename: Santiago
authors.initials: S
authors.lastname: Toledo
authors.forename: Werner
authors.initials: W
authors.lastname: Kaminsky
authors.forename: Martin L
authors.initials: ML
authors.lastname: Kirk
authors.forename: Julie A
authors.initials: JA
authors.lastname: Kovacs
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D014867:Water / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D014867:Water; D006859:Hydrogen; D010100:Oxygen