How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O–O Bond Cleavage, and Reactivity

Herein we quantitatively investigate
how metal ion Lewis acidity and steric properties influence the kinetics
and thermodynamics of dioxygen binding versus release from structurally
analogous Mn–O2 complexes, as well as the barrier
to Mn peroxo O–O bond cleavage, and the reactivity of Mn oxo
intermediates. Previously we demonstrated that the steric and electronic
properties of MnIII–OOR complexes containing N-heterocyclic
(NAr) ligand scaffolds can have a dramatic influence on
alkylperoxo O–O bond lengths and the barrier to alkylperoxo
O–O bond cleavage. Herein, we examine the dioxygen reactivity
of a new MnII complex containing a more electron-rich,
less sterically demanding NAr ligand scaffold, and compare
it with previously reported MnII complexes. Dioxygen binding
is shown to be reversible with complexes containing the more electron-rich
metal ions. The kinetic barrier to O2 binding and peroxo
O–O bond cleavage is shown to correlate with redox potentials,
as well as the steric properties of the supporting NAr ligands.
The reaction landscape for the dioxygen chemistry of the more electron-rich
complexes is shown to be relatively flat. A total of four intermediates,
including a superoxo and peroxo species, are observed with the most
electron-rich complex. Two new intermediates are shown to form following
the peroxo, which are capable of cleaving strong X–H bonds.
In the absence of a sacrificial H atom donor, solvent, or ligand,
serves as a source of H atoms. With TEMPOH as sacrificial H atom donor,
a deuterium isotope effect is observed (kH/kD = 3.5), implicating a hydrogen atom
transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene
is produced prior to the formation of an EPR detected MnIIIMnIV bimetallic species, and 0.5 equiv after its formation.