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-O-2 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 Mn-III-OOR complexes containing N-heterocyclic (N-Ar) 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 Mn-II complex containing a more electron-rich, less sterically demanding N-Ar ligand scaffold, and compare it with previously reported Mn-II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O-2 binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N-Ar 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 (k(H)/k(D) = 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 (MnMnIV)-Mn-III bimetallic species, and 0.5 equiv after its formation.