The addition of Lewis or Brönsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese–oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII 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–Nave = 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 MnV(O) starting material (Mn–O = 1.55 Å). The reactivity of MnIV(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 MnIV complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, 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 kH/kD = 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. Furthermore, the reactivity of MnIV(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 MnIV(O–H)(tpfc•+) recently reported.
Baglia, Regina A., et al. "High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage." Inorganic Chemistry, vol. 55, no. 20, Sep. 2016. https://doi.org/10.1021/acs.inorgchem.6b02109
Baglia, Regina A., Krest, Courtney M., Yang, Tzuhsiung, Leeladee, Pannee, & Goldberg, David P. (2016). High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage. Inorganic Chemistry, 55(20). https://doi.org/10.1021/acs.inorgchem.6b02109
Baglia, Regina A., Krest, Courtney M., Yang, Tzuhsiung, et al., "High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage," Inorganic Chemistry 55, no. 20 (2016), https://doi.org/10.1021/acs.inorgchem.6b02109
@article{osti_1350592,
author = {Baglia, Regina A. and Krest, Courtney M. and Yang, Tzuhsiung and Leeladee, Pannee and Goldberg, David P.},
title = {High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage},
annote = {The addition of Lewis or Brönsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese–oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII 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–Nave = 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 MnV(O) starting material (Mn–O = 1.55 Å). The reactivity of MnIV(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 MnIV complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, 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 kH/kD = 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. Furthermore, the reactivity of MnIV(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 MnIV(O–H)(tpfc•+) recently reported.},
doi = {10.1021/acs.inorgchem.6b02109},
url = {https://www.osti.gov/biblio/1350592},
journal = {Inorganic Chemistry},
issn = {ISSN 0020-1669},
number = {20},
volume = {55},
place = {United States},
publisher = {American Chemical Society (ACS)},
year = {2016},
month = {09}}