High-Valent Manganese–Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C–H Bond Cleavage

Abstract

The addition of Lewis or Brönsted acids (LA = Zn(OTf) 2, B(C 6 F 5 ) 3, HBAr F, TFA) to the high-valent manganese–oxo complex Mn V (O)(TBP 8 Cz) results in the stabilization of a valence tautomer Mn IV (O-LA)(TBP 8 Cz •+ ). The Zn II and B(C 6 F 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 8 Cz •+ ) 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 8 Cz •+ ) valence tautomer as compared to the Mn V (O) valence tautomer when LA = Zn II, B(C 6 F 5 ) 3, and HBAr F, whereas for LA = TFA, the C–H cleavage rate is slightly slower than when compared to Mn V (O). A large, nonclassical kinetic isotope effect of k H / k D = 25–27 was observed for LA = B(C 6 F 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. The reactivity of Mn IV (O-LA)(TBP 8 Cz •+ ) 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 ( J. Am. Chem. Soc. 2015, 137, 14481–14487).