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

The addition of Lewis or Bronsted acids (LA = Zn(OTf)(2), B(C6F5)(3), HBArF, 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(center dot+)). The Zn-II and B(C6F5)(3) complexes were characterized by manganese Kedge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (Is 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 angstrom and a fifth N/O ligand at 1.61 angstrom, 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 angstrom). The reactivity of Mn-IV(O-LA)(TBP8Cz(center dot+)) toward C-H substrates was examined, and it was found that H-center dot 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(center dot+)) valence tautomer as compared to the Mn-V(O) valence tautomer when LA = Zn-II, B(C6F5)(3), and HBArF, whereas for LA = TFA, the C-H cleavage rate is slightly slower than when compared to Mnv(0). A large, nonclassical kinetic isotope effect of k(H)/k(D) = 25-27 was observed for LA = B(C6F5)(3) and HBArF, 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(center dot+)) 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(center dot+)) recently reported