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(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 <i>k</i>_H/<i>k</i>_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 (<i>J. Am. Chem. Soc.</i> <b>2015</b>, 137, 14481–14487)

Spectroscopic Investigations of Catalase Compound II: Characterization of an Iron(IV) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme

We report on the protonation state of <i>Helicobacter pylori</i> catalase compound II. UV/visible, Mössbauer, and X-ray absorption spectroscopies have been used to examine the intermediate from pH 5 to 14. We have determined that HPC-II exists in an iron­(IV) hydroxide state up to pH 11. Above this pH, the iron­(IV) hydroxide complex transitions to a new species (p<i>K</i>_a = 13.1) with Mössbauer parameters that are indicative of an iron­(IV)-oxo intermediate. Recently, we discussed a role for an elevated compound II p<i>K</i>_a in diminishing the compound I reduction potential. This has the effect of shifting the thermodynamic landscape toward the two-electron chemistry that is critical for catalase function. In catalase, a diminished potential would increase the selectivity for peroxide disproportionation over off-pathway one-electron chemistry, reducing the buildup of the inactive compound II state and reducing the need for energetically expensive electron donor molecules