A Balancing Act: Stability versus Reactivity of Mn(O) Complexes

Heather M. Neu, Regina A. Baglia, and David P. Goldberg*

Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States

Acc. Chem. Res., Article ASAP

DOI: 10.1021/acs.accounts.5b00273

Publication Date (Web): September 9, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/pdf/10.1021/acs.accounts.5b00273

*E-mail: dpg@jhu.edu.

 Special Issue

Published as part of the Accounts of Chemical Research special issue “Synthesis in Biological Inorganic Chemistry”.

Biography

Heather Neu received her B.S. degree in Chemistry from the University of Wisconsin-Superior in 2007, where she conducted independent undergraduate research through the McNair Scholars program under the direction of Professor Troy S. Bergstedt. In 2010, she earned her M.S. from the University of Minnesota Duluth, with Professor Victor N. Nemykin. She is currently a Ph.D. candidate at Johns Hopkins University working with Professor David P. Goldberg.

Biography

Regina Baglia received her B.S. degree in Biochemistry from Temple University in 2011, where she conducted undergraduate research through the Diamond Research Scholars program under the direction of Professor Michael J. Zdilla. She is currently a Ph.D. candidate at Johns Hopkins University working with Professor David P. Goldberg.

Biography

David Goldberg received his B.A. degree from Williams College (1989) and a Ph.D. degree from M.I.T (1995). After completing a postdoctoral fellowship at Northwestern University, he moved to Johns Hopkins University in 1998, where he is currently a Professor of Chemistry.

Abstract

Conspectus

A large class of heme and non-heme metalloenzymes utilize O2 or its derivatives (e.g., H2O2) to generate high-valent metal–oxo intermediates for performing challenging and selective oxidations. Due to their reactive nature, these intermediates are often short-lived and very difficult to characterize. Synthetic chemists have sought to prepare analogous metal–oxo complexes with ligands that impart enough stability to allow for their characterization and an examination of their inherent reactivity. The challenge in designing these molecules is to achieve a balance between their stability, which should allow for their in situ characterization or isolation, and their reactivity, in which they can still participate in interesting chemical transformations. This Account focuses on our recent efforts to generate and stabilize high-valent manganese–oxo porphyrinoid complexes and tune their reactivity in the oxidation of organic substrates.

Dioxygen can be used to generate a high-valent MnV(O) corrolazine (MnV(O)(TBP8Cz)) by irradiation of MnIII(TBP8Cz) with visible light in the presence of a C–H substrate. Quantitative formation of the MnV(O) complex occurs with concomitant selective hydroxylation of the benzylic substrate hexamethylbenzene. Addition of a strong H+ donor converted this light/O2/substrate reaction from a stoichiometric to a catalytic process with modest turnovers. The addition of H+ likely activates a transient MnV(O) complex to achieve turnover, whereas in the absence of H+, the MnV(O) complex is an unreactive “dead-end” complex. Addition of anionic donors to the MnV(O) complex also leads to enhanced reactivity, with a large increase in the rate of two-electron oxygen atom transfer (OAT) to thioether substrates. Spectroscopic characterization (Mn K-edge X-ray absorption and resonance Raman spectroscopies) revealed that the anionic donors (X–) bind to the MnV ion to form six-coordinate [MnV(O)(X)]− complexes. An unusual “V-shaped” Hammett plot for the oxidation of para-substituted thioanisole derivatives suggested that six-coordinate [MnV(O)(X)]− complexes can act as both electrophiles and nucleophiles, depending on the nature of the substrate. Oxidation of the MnV(O) corrolazine resulted in the in situ generation of a MnV(O) π-radical cation complex, [MnV(O)(TBP8Cz•+)]+, which exhibited more than a 100-fold rate increase in the oxidation of thioethers. The addition of Lewis acids (LA; ZnII, B(C6F5)3) to the closed-shell, diamagnetic MnV(O)(TBP8Cz) stabilized a paramagnetic valence tautomer MnIV(O)(TBP8Cz•+)(LA), which was characterized as a second π-radical cation complex by NMR, EPR, UV-vis, and high resolution cold spray ionization MS. The MnIV(O)(TBP8Cz•+)(LA) complexes are able to abstract H• from phenols and exhibit a rate enhancement of up to ∼100-fold over the parent MnV(O) valence tautomer. In contrast, a large decrease in rate is observed for OAT for the MnIV(O)(TBP8Cz•+)(LA) complexes. The rate enhancement for hydrogen atom transfer (HAT) may derive from the higher redox potential for the π-radical cation complex, while the large rate decrease seen for OAT may come from a decrease in electrophilicity for an MnIV(O) versus MnV(O) complex.