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Title: High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage

The addition of Lewis or Brönsted acids (LA = Zn(OTf) 2, B(C 6F 5) 3, HBAr F, TFA) to the high-valent manganese–oxo complex Mn V(O)(TBP 8Cz) results in the stabilization of a valence tautomer Mn IV(O-LA)(TBP 8Cz •+). The Zn II and B(C 6F 5) 3 complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn–N ave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn–O bond length is elongated compared to the Mn V(O) starting material (Mn–O = 1.55 Å). The reactivity of Mn IV(O-LA)(TBP 8Cz •+) toward C–H substrates was examined, and it was found that H abstraction from C–H bonds occurs in a 1:1 stoichiometry, giving a Mn IV complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the Mn IV(O-LA)(TBP 8Cz •+) valence tautomer as compared to the MnV(O) valence tautomer when LA = Zn II, B(Cmore » 6F 5) 3, and HBArF, whereas for LA = TFA, the C–H cleavage rate is slightly slower than when compared to MnV(O). A large, nonclassical kinetic isotope effect of k H/ k D = 25–27 was observed for LA = B(C 6F 5) 3 and HBAr F, indicating that H-atom transfer (HAT) is the rate-limiting step in the C–H cleavage reaction and implicating a potential tunneling mechanism for HAT. Furthermore, the reactivity of Mn IV(O-LA)(TBP 8Cz •+) toward C–H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex Mn IV(O–H)(tpfc •+) recently reported.« less
 [1] ;  [2] ;  [3] ;  [4] ;  [1]
  1. The Johns Hopkins Univ., Baltimore, MD (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  3. The Johns Hopkins Univ., Baltimore, MD (United States); Univ. of Wisconsin, Madison, WI (United States)
  4. The Johns Hopkins Univ., Baltimore, MD (United States); Chulalongkorn Univ., Bangkok (Thailand)
Publication Date:
Grant/Contract Number:
GM101153; AC02-76SF00515
Accepted Manuscript
Journal Name:
Inorganic Chemistry
Additional Journal Information:
Journal Volume: 55; Journal Issue: 20; Journal ID: ISSN 0020-1669
American Chemical Society (ACS)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
Country of Publication:
United States
OSTI Identifier: