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

Abstract

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 (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 MnIIIMnIV bimetallic species, and 0.5 equiv after its formation.

Advances in the Molecular Catalysis of Dioxygen Reduction

“…These recent studies suggest that there is a continuing benefit to the study of the molecular electrocatalytic reduction of O 2 . The mechanistic fundamentals of homogeneous O 2 activation have relevance to the understanding of biological systems, as well as the development of better ORR and aerobic oxidation catalysts. ,− There is sustained interest in running the latter electrochemically, − given the obvious parallels to the function of Cytochrome P450, − which catalyzes the oxygenation of endogenous and exogenous molecules to produce important metabolites. Mechanistic and thermodynamic understanding of molecular catalysis can be the basis of intriguing systems with careful experimental design: a recent study combined Rh porphyrins with methane and O 2 to assemble an abiotic system capable of activity analogous to methane monooxygenase by exploiting electrochemically generated reaction gradients created by inorganic nanomaterials .…”

Homogeneous catalysis of dioxygen reduction by molecular Mn complexes

“…We note that the stoichiometric chemistry of Mn complexes with thiolate containing ligands and O 2 has been explored extensively by Kovacs and coworkers. [71][72][73][74][75] Upon exposure to dry air, both centers were oxidized by a single electron and a bridging hydroxide species was recovered. Mechanistic studies established that the dinuclear monothiol species united two separate reaction cycles, where either H 2 O 2 or H 2 O production was favored.…”

“…13D and F). 74,122 6. Critical analysis of homogeneous ORR by molecular Mn/Fe/Co Extensive studies on the ORR mediated by metalloporphyrins allows for a critical comparison of activity and mechanism for structurally analogous systems containing Fe, Co and Mn active sites.…”

Electronic Structure and Reactivity of Dioxygen-Derived Aliphatic Thiolate-Ligated Fe-Peroxo and Fe(IV) Oxo Compounds

“…Much of our understanding of the properties of heme and nonheme iron enzyme dioxygen intermediates such as Fe­(III)–superoxos, Fe­(III)–peroxos, and high-valent iron oxos comes from small-molecule chemistry. − Only a small number of these are derived from O 2 , ,− and despite the key role of thiolates in a wide variety of nonheme iron enzymes, only a small number of these contain thiolates in the coordination sphere. ,,, Key properties unique to aliphatic thiolate (RS – or Cys-S – ) ligands include an increase in the potency of high-valent iron oxo intermediates − and, as shown by our group, the potency of iron-superoxo compounds with respect to their ability to cleave strong C–H bonds. , We also recently showed that aliphatic thiolates help to promote peroxo O–O bond cleavage. , Very few well-characterized iron-superoxo compounds have been reported, ,− and of those reported, only two incorporate a thiolate in the coordination sphere , and only one cleaves strong C–H bonds . We recently showed that aliphatic thiolate-ligated [Fe II (S 2 Me2 N 3 (Pr,Pr))] ( 1 ) reacts with O 2 (Scheme ) to form an unprecedented example of an aliphatic thiolate-ligated iron superoxo intermediate, [Fe III (S 2 Me2 N 3 (Pr,Pr))­(O 2 )] ( 2 ), which is capable of cleaving strong C–H bonds (e.g., tetrahydrofuran (THF) (BDE­(C–H) = 92 kcal mol –1 )) at −73 °C with a k H / k D = 4.8 .…”