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(C₆F₅)₃, HBAr^F, TFA) to the high-valent manganese-oxo complex Mn^V(O)(TBP₈Cz) results in the stabilization of a valence tautomer Mn^IV(O-LA)(TBP₈Cz^•+). The Zn^II and B(C₆F₅)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)(TBP₈Cz^•+) 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)(TBP₈Cz^•+) valence tautomer as compared to the Mn^V(O) valence tautomer when LA = Zn^II, B(C₆F₅)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(C₆F₅)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)(TBP₈Cz^•+) 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).