High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage [electronic resource].
- Washington, D.C. : United States. Dept. of Energy, 2016. and Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy
- Physical Description:
- pages 10,800-10,809 : digital, PDF file
- Additional Creators:
- SLAC National Accelerator Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information
- Restrictions on Access:
- Free-to-read Unrestricted online access
- The addition of Lewis or Brönsted acids (LA = Zn(OTf)<sub>2</sub>, B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, HBAr<sup>F</sup>, TFA) to the high-valent manganese–oxo complex Mn<sup>V</sup>(O)(TBP<sub>8</sub>Cz) results in the stabilization of a valence tautomer Mn<sup>IV</sup>(O-LA)(TBP<sub>8</sub>Cz<sup>•+</sup>). The Zn<sup>II</sup> and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> 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<sub>ave</sub> = 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<sup>V</sup>(O) starting material (Mn–O = 1.55 Å). The reactivity of Mn<sup>IV</sup>(O-LA)(TBP<sub>8</sub>Cz<sup>•+</sup>) toward C–H substrates was examined, and it was found that H<sup>•</sup> abstraction from C–H bonds occurs in a 1:1 stoichiometry, giving a Mn<sup>IV</sup> complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the Mn<sup>IV</sup>(O-LA)(TBP<sub>8</sub>Cz<sup>•+</sup>) valence tautomer as compared to the MnV(O) valence tautomer when LA = Zn<sup>II</sup>, B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, 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 <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 25–27 was observed for LA = B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> and HBAr<sup>F</sup>, 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<sup>IV</sup>(O-LA)(TBP<sub>8</sub>Cz<sup>•+</sup>) toward C–H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex Mn<sup>IV</sup>(O–H)(tpfc<sup>•+</sup>) recently reported.
- Published through SciTech Connect., 09/30/2016., Inorganic Chemistry 55 20 ISSN 0020-1669 AM, and Regina A. Baglia; Courtney M. Krest; Tzuhsiung Yang; Pannee Leeladee; David P. Goldberg.
- Funding Information:
- GM101153 and AC02-76SF00515
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