A Highly Reactive Chromium(V)–Oxo TAML Cation Radical Complex in Electron Transfer and Oxygen Atom Transfer Reactions

We report the synthesis, characterization,
and electron-transfer
(ET) oxidation reactivity of a chromium­(V)–oxo TAML cation
radical complex binding Sc3+ ion, {[CrV(O)­(TAML•+)]-Sc3+}3+ (2-Sc).
Its precursors, such as [CrV(O)­(TAML)]− (1) and {[CrV(O)­(TAML)]-Sc3+}2+ (1-Sc), were also characterized spectroscopically
and/or structurally. In ET and oxygen atom transfer (OAT) reactions,
while 1 and 1-Sc were sluggish oxidants, 2-Sc was a highly reactive oxidant with an extremely small
reorganization energy. For example, in ET oxidation reactions, nanosecond
laser-induced transient absorption measurements were performed to
examine the fast ET from electron donors (e.g., ferrocene derivatives)
to 2-Sc, affording a small reorganization energy (λ
= 0.26 eV) of ET, which is even much smaller than the λ values
reported in the ET reduction of heme Compound I (Cpd I) models and
non-heme metal–oxo complexes. Such a small reorganization energy
is ascribed to the TAML ligand centered ET reduction of 2-Sc. The λ value of 0.26 eV was also obtained in the electron
self-exchange reaction between 2-Sc and 1-Sc. In OAT reactions, the rate constants of the sulfoxidation of
thioanisole derivatives by 2-Sc at −40 °C
were much greater than those reported in the oxidation of thioanisoles
by heme Cpd I and non-heme metal–oxo complexes. The reactivity
of 2-Sc in hydrogen atom transfer (HAT) reactions is
also discussed briefly. To the best of our knowledge, this Cr­(V)-oxo
TAML cation radical complex binding Sc3+ ion, {[CrV(O)­(TAML•+)]-Sc3+}3+, with an extremely small reorganization energy is one of the most
powerful high-valent metal–oxo oxidants in ET and OAT reactions.