Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

Systematic investigations on H atom transfer (HAT) thermodynamics
of metal O2 adducts is of fundamental importance for the
design of transition metal catalysts for substrate oxidation and/or
oxygenation directly using O2. Such work should help elucidate
underlying electronic-structure features that govern the OO–H
bond dissociation free energies (BDFEs) of metal-hydroperoxo species,
which can be used to quantitatively appraise the HAT activity of the
corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous
CoIII- and MnIII-hydroperoxo complexes, 3-Co and 3-Mn, were calculated to be 79.3 and
81.5 kcal/mol, respectively, employing the Bordwell relationship based
on experimentally determined pKa values
and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn. To further verify these values, we tested
the HAT capability of their superoxo congeners, 2-Co and 2-Mn, toward three different substrates possessing varying
O–H BDFEs. Specifically, both metal-superoxo species are capable
of activating the O–H bond of 4-oxo-TEMPOH with an O–H
BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract
a H atom from 2,4-di-tert-butylphenol with an O–H
BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol
with an O–H BDFE of 85.6 kcal/mol. Further computational investigations
suggested that it is the high spin state of the MnIII center
in 3-Mn that renders its OO–H BDFE higher than
that of 3-Co, which features a low-spin CoIII center. The present work underscores the role of the metal spin
state being as crucial as the oxidation state in modulating BDFEs.