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Converting triplet dioxygen into a powerful oxidant is fundamentally important to life. The study reported herein quantitatively examines the formation of a well-characterized, reactive, O 2 -derived thiolate ligated Fe III -superoxo using low-temperature stopped-flow kinetics. Comparison of the kinetic barriers to the formation of this species via two routes, involving either the addition of (a) O 2 to [Fe II (S 2 Me2 N 3 (Pr,Pr))] (1) or (b) superoxide to [Fe III (S 2 Me2 N 3 (Pr,Pr))] + (3) is shown to provide insight into the mechanism of O 2 activation. Route (b) was shown to be significantly slower, and the kinetic barrier 14.9 kJ mol -1 higher than route (a), implying that dioxygen activation involves inner-sphere, as opposed to outer sphere, electron transfer from Fe(ii). H-bond donors and ligand constraints are shown to dramatically influence O 2 binding kinetics and reversibility. Dioxygen binds irreversibly to [Fe II (S 2 Me2 N 3 (Pr,Pr))] (1) in tetrahydrofuran, but reversibly in methanol. Hydrogen bonding decreases the ability of the thiolate sulfur to stabilize the transition state and the Fe III -superoxo, as shown by the 10 kJ mol -1 increase in the kinetic barrier to O 2 binding in methanol vs. tetrahydrofuran. Dioxygen release from [Fe III (S 2 Me2 N 3 (Pr,Pr))O 2 ] (2) is shown to be 24 kJ mol -1 higher relative to previously reported [Fe III (S Me2 N 4 (tren))(O 2 )] + (5), the latter of which contains a more flexible ligand. These kinetic results afford an experimentally determined reaction coordinate that illustrates the influence of H-bonding and ligand constraints on the kinetic barrier to dioxygen activation an essential step in biosynthetic pathways critical to life., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Oxoiron(IV) is a common catalytic byproduct observed in the oxidation of alkenes by the combination of H 2 O 2 and nonheme iron catalysts including complex 1 , Fe II PDP* (where PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-( R , R )-2,2'-bipyrrolidine). The oxoiron(IV) species have been proposed to arise by O-O homolysis of the peroxyiron(III) or acylperoxyiron(III) intermediates formed during the presumed Fe III -Fe V catalytic cycle and have generally been regarded as off-pathway. We generated complex 1 IV ═O (λ max = 730 nm, ε = 350 M -1 cm -1 ) directly from 1 and an oxygen atom donor IBX i -Pr (isopropyl 2-iodoxybenzoate) in acetonitrile in the temperature range from -35 to +25 °C under stopped-flow conditions. Species 1 IV ═O is metastable (half-life of 2.0 min at +25 °C), and its decay is accelerated in the presence of organic substrates such as thioanisole, alkenes, benzene, and cyclohexane. The reaction with cyclohexane- d 12 is significantly slower (KIE = 4.9 ± 0.4), suggesting that a hydrogen atom transfer to 1 IV ═O is the rate limiting step. With benzene- d 6 , no significant isotope effect is observed (KIE = 1.0 ± 0.2), but UV-vis spectra show the concomitant formation of an intense 580 nm band likely due to the Fe(III)-phenolate chromophore, suggesting an electrophilic attack of 1 IV ═O on the aromatic system of benzene. Treatment of 1 IV ═O with H 2 O 2 resulted in rapid decay of its 730 nm visible band ( k = 102.6 ± 4.6 M -1 s -1 at -20 °C), most likely occurring by a hydrogen atom transfer from H 2 O 2 . In the presence of excess H 2 O 2 , the oxoiron(IV) is transformed into peroxyiron(III), as seen from the formation of a characteristic 550 nm visible band and g eff = 2.22, 2.16, and 1.96 electron paramagnetic resonance (EPR) spectroscopy signals. Reductively formed 1 III -OOH was able to re-enter the catalytic cycle of alkene epoxidation by H 2 O 2 , albeit with lower yields versus those of oxidatively formed (i.e., 1 + H 2 O 2 ) peroxyiron(III) owing to a loss of ca. 40% active iron. As such, the oxoiron(IV) species can be reintroduced to the catalytic cycle with extra H 2 O 2 , acting as an iron reservoir. Alternatively, peroxycarboxylic acids, which have a stronger O-H bond dissociation energy, do not reduce 1 IV ═O , ensuring that more oxidant is productively employed in substrate oxidation. While this reaction with H 2 O 2 may occur for other nonheme oxoiron(IV) complexes, the only previously reported examples are 3 IV ═O and 4 IV ═O , which are reduced by hydrogen peroxide 130- and 2900-fold more slowy, respectively (as in Angew. Chemie - Int. Ed. 2012 , 51 ( 22 ), 5376 - 5380 , DOI: 10.1002/anie.201200901 ).

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 Mn III Mn IV bimetallic species, and 0.5 equiv after its formation.