title: Oxygen-atom transfer reactivity of axially ligated Mn(V)-oxo complexes: evidence for enhanced electrophilic and nucleophilic pathways.
abstract: Addition of anionic donors to the manganese(V)-oxo corrolazine complex Mn(V)(O)(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn(V)(O)(TBP8Cz)(X)](-) complexes (X = F(-), N3(-), OCN(-)) exhibit a ∼5 cm(-1) downshift of the Mn-O vibrational mode relative to the parent Mn(V)(O)(TBP8Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate < cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent Mn(V)(O)(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [Mn(V)(O)(TBP8Cz)(X)](-) (X = CN(-) or F(-)) as the oxidant, and unusual "V-shaped" Hammett plots were obtained. These results are rationalized based upon a change in mechanism that hinges on the ability of the [Mn(V)(O)(TBP8Cz)(X)](-) complexes to function as either an electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the one-electron-oxidized cationic Mn(V)(O)(TBP8Cz(•+)) complex yielded a linear Hammett relationship for all substrates (ρ = -1.40), consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence of axial donors on high-valent Mn(V)(O) porphyrinoid complexes.
authors.affiliation: Department of Chemistry, The Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Heather M
authors.initials: HM
authors.lastname: Neu
authors.forename: Tzuhsiung
authors.initials: T
authors.lastname: Yang
authors.forename: Regina A
authors.initials: RA
authors.lastname: Baglia
authors.forename: Timothy H
authors.initials: TH
authors.lastname: Yosca
authors.forename: Michael T
authors.initials: MT
authors.lastname: Green
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Quesne
authors.forename: Sam P
authors.initials: SP
authors.lastname: de Visser
authors.forename: David P
authors.initials: DP
authors.lastname: Goldberg
mesh_terms: D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D011166:Porphyrins; D008345:Manganese; D010100:Oxygen