A Manganese(V)–Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion

A mononuclear non-heme manganese­(V)–oxo
complex, [MnV(O)­(TAML)]− (1), was synthesized
by activating dioxygen in the presence of olefins with weak allylic
C–H bonds and characterized structurally and spectroscopically.
In mechanistic studies, the formation rate of 1 was found
to depend on the allylic C–H bond dissociation energies (BDEs)
of olefins, and a kinetic isotope effect (KIE) value of 16 was obtained
in the reactions of cyclohexene and cyclohexene-d10. These results suggest that a hydrogen atom abstraction
from the allylic C–H bonds of olefins by a putative MnIV–superoxo species, which is formed by binding O2 by a high-spin (S = 2) [MnIII(TAML)]− complex, is the rate-determining step.
A Mn­(V)–oxo complex binding Sc3+ ion, [MnV(O)­(TAML)]−–(Sc3+) (2), was also synthesized in the reaction of 1 with Sc3+ ion and then characterized using various spectroscopic techniques.
The binding site of the Sc3+ ion was proposed to be the
TAML ligand, not the Mn–O moiety, probably due to the low basicity
of the oxo group compared to the basicity of the amide carbonyl group
in the TAML ligand. Reactivity studies of the Mn­(V)–oxo intermediates, 1 and 2, in oxygen atom transfer and electron-transfer
reactions revealed that the binding of Sc3+ ion at the
TAML ligand of Mn­(V)–oxo enhanced its oxidizing power with
a positively shifted one-electron reduction potential (ΔEred = 0.70 V). This study reports the first
example of tuning the second coordination sphere of high-valent metal–oxo
species by binding a redox-inactive metal ion at the supporting ligand
site, thereby modulating their electron-transfer properties as well
as their reactivities in oxidation reactions.