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Title: 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(C 6F 5) 3, HBAr F, TFA) to the high-valent manganese–oxo complex Mn V(O)(TBP 8Cz) results in the stabilization of a valence tautomer Mn IV(O-LA)(TBP 8Cz •+). The Zn II and B(C 6F 5) 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)(TBP 8Cz •+) 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)(TBP 8Cz •+) valence tautomer as compared to the MnV(O) valence tautomer when LA = Zn II, B(Cmore » 6F 5) 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 k H/ k D = 25–27 was observed for LA = B(C 6F 5) 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. Furthermore, the reactivity of Mn IV(O-LA)(TBP 8Cz •+) 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.« less

Authors:
 [1];  [2];  [3];  [4];  [1]
  1. The Johns Hopkins Univ., Baltimore, MD (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  3. The Johns Hopkins Univ., Baltimore, MD (United States); Univ. of Wisconsin, Madison, WI (United States)
  4. The Johns Hopkins Univ., Baltimore, MD (United States); Chulalongkorn Univ., Bangkok (Thailand)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1350592
Grant/Contract Number:
GM101153; AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Inorganic Chemistry
Additional Journal Information:
Journal Volume: 55; Journal Issue: 20; Journal ID: ISSN 0020-1669
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Baglia, Regina A., Krest, Courtney M., Yang, Tzuhsiung, Leeladee, Pannee, and Goldberg, David P.. High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage. United States: N. p., 2016. Web. doi:10.1021/acs.inorgchem.6b02109.
Baglia, Regina A., Krest, Courtney M., Yang, Tzuhsiung, Leeladee, Pannee, & Goldberg, David P.. High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage. United States. doi:10.1021/acs.inorgchem.6b02109.
Baglia, Regina A., Krest, Courtney M., Yang, Tzuhsiung, Leeladee, Pannee, and Goldberg, David P.. 2016. "High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage". United States. doi:10.1021/acs.inorgchem.6b02109. https://www.osti.gov/servlets/purl/1350592.
@article{osti_1350592,
title = {High-valent manganese–oxo valence tautomers and the influence of Lewis/Brönsted acids on C–H bond cleavage},
author = {Baglia, Regina A. and Krest, Courtney M. and Yang, Tzuhsiung and Leeladee, Pannee and Goldberg, David P.},
abstractNote = {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},
journal = {Inorganic Chemistry},
number = 20,
volume = 55,
place = {United States},
year = 2016,
month = 9
}

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  • Equilibrium constants for mono- and bisligation of imidazole (ImH) with (meso-tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) chloride ((Me/sub 8/TPP)Mn/sup III/Cl) have been determined so that the concentrations (dry CH/sub 2/Cl/sub 2/) of the three species (Me/sub 8/TPP)Mn/sup III/Cl, ((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and ((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl may be calculated at different ImH concentrations. The equilibrium constants for ligation of the one and two imidazoles are K/sub 1/ = 245 M/sup -1/ and ..beta../sub 2/ = 1.80 x 10/sup 5/ M/sup -1/. The reaction of p-cyano-N,N-dimethylaniline N-oxide (NO) with the manganese(III)porphyrin (under the pseudo-first-order conditions of (NO)/sub i/ >> ((Me/sub 8/TPP)Mn/sup III/Cl)/sub i/ and in themore » presence and absence of ImH) is first order in both NO and manganese(III) porphyrin, and the rate-controlling step involves oxygen transfer with formation of higher valent manganese-oxo porphyrin species plus p-cyano-N,N-dimethylaniline (DA). From the dependence of the pseudo-first-order rate constants (k/sub obsd/) upon (ImH)/sub i/ and a knowledge of the equilibrium constants for imidazole ligation there has been calculated the second-order rate constants for the kinetic terms k/sub 1/(NO)((Me/sub 8/TPP)Mn/sup III/Cl), k/sub 2/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and k/sub 3/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl). Comparison of the second-order rate constants (k/sub 1/ = 3.3 x 10/sup -2/ M/sup -1/ s/sup -1/, k/sub 2/ = 5.53 M/sup -1/ s/sup -1/, and k/sub 4/ = 7.32 x 10/sup -2/ M/sup -1/ s/sup -1/) establishes that ligation by one imidazole increases the rate of reaction of the manganese(III) porphyrin with NO by approx. 166-fold.« less
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