Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)2(η2-O2)]

Exploring the influence of H-bonding and ligand constraints on thiolate ligated non-heme iron mediated dioxygen activation

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, O2-derived thiolate ligated FeIII-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) O2 to [FeII(S2 Me2N3(Pr,Pr))] (1) or (b) superoxide to [FeIII(S2 Me2N3(Pr,Pr))]+ (3) is shown to provide insight into the mechanism of O2 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 O2 binding kinetics and reversibility. Dioxygen binds irreversibly to [FeII(S2 Me2N3(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 FeIII-superoxo, as shown by the 10 kJ mol-1 increase in the kinetic barrier to O2 binding in methanol vs. tetrahydrofuran. Dioxygen release from [FeIII(S2 Me2N3(Pr,Pr))O2] (2) is shown to be 24 kJ mol-1 higher relative to previously reported [FeIII(SMe2N4(tren))(O2)]+ (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.

Reversible Single-Electron-Transfer to Oxygen in a Stable N-Heterocyclic Carbene Palladium(I) Metalloradical

The chemical and electrochemical one-electron oxidation of [Pd(IPr)2] (1) leads to the formation of mononuclear palladium(I) complex [Pd(IPr)2][PF6] (2). This bench-stable metalloradical has been fully characterized and its structure confirmed by X-ray diffraction analysis. EPR and DFT studies confirm the localization of the unpaired electron onto the metal center. Low temperature NMR and EPR measurements reveal the ability of complex 2 to reversibly coordinate and reduce the dioxygen molecule, leading to the formation of a three-coordinate complex, [PdII(IPr)2(η1-O2)]+ (4), in which the unpaired electron has been transferred to the superoxido ligand.

Mechanistic Studies for Pd(II)(O2) Reduction Generating Pd(0) and H2O: Formation of Pd(OH)2 as a Key Intermediate

Molecular oxygen (O₂) remains to be an ideal yet underutilized feedstock for the oxidative transformation of organic substrates and renewable energy systems such as fuel cells. Palladium (Pd) has shown particular promise in enabling these applications. The present study describes a Pd-mediated O₂ reduction to water via C-H activation of 9,10-dihydroanthracene (DHA) by a Pd(II) η²-peroxo complex 1O₂. The reaction yields stoichiometric anthracene and Pd(0) product 1 and is notable in two respects. First, plots of concentrations of the reaction participants over time have distinctly sigmoidal shapes, indicating that conversion accelerates over time and implying autocatalysis. Second, the reaction proceeds via a genuine monometallic Pd(II) dihydroxide 1(OH)₂ directly observed to grow and decay as an intermediate. Confirming its role as an intermediate, the dihydroxide 1(OH)₂ was found to mediate C-H oxidation of DHA on par in activity with the peroxo compound 1O₂. Mechanistic studies with density functional theory (DFT) calculations suggested that both 1O₂ and 1(OH)₂ react with DHA by hydrogen atom transfer (HAT) and that autocatalysis in the 1O₂ reaction results from oxidative addition of the initial Pd(II) complex 1O₂ to the Pd(0) product 1. This reaction forms a transient bis(μ-oxo) Pd(II) dimer 1O₂1 that is more active in the HAT oxidation of DHA than the initial 1O₂.