Characterization of Metastable Intermediates Formed in the Reaction between a Mn(II) Complex and Dioxygen, Including a Crystallographic Structure of a Binuclear Mn(III)–Peroxo Species

Abstract

Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O2. Due to their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a binuclear Mn-peroxo, either as a precursor to O2, or derived from O2, in both photosynthetic H2O oxidation and DNA biosynthesis, arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [MnII(SMe2N4(6-MeDPEN))] +(1), and the characterization of intermediates formed en route to a binuclear mono-oxo bridged Mn(III) product {[MnIII(SMe2N4(6-MeDPEN)]2-(μ-O)}2+ (2), the oxo atom of which is derived from 18O2. At low-temperatures, a dioxygen intermediate, [Mn(SMe2N4(6-MeDPEN))(O2)]+ (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k1(−10 °C)= 3780±180M−1s−1, ΔH1‡ = 26.4±1.7 kJ mol−1, ΔS1‡ = − 75.6±6.8 J mol−1K−1), and then convert more slowly (k2(−10 °C)= 417±3.2 M−1s−1, ΔH2‡ = 47.1±1.4 kJ mol−1, ΔS2‡ = − 15.0±5.7 J mol−1K−1) to a species 3 with isotopically sensitive stretches at νo-o (Δ18O) = 819(47) cm−1, kO–O= 3.02 mdyn/Å, and νMn-O(Δ18O) = 611(25) cm−1 consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with the formation of a binuclear Mn-peroxo. This was verified by X-ray crystallography, where the peroxo of {[MnIII(SMe2N4(6-Me-DPEN)]2(trans–μ–1,2–O2)}2+ (3) is shown to be bridging between two Mn(III) ions in an end-on trans-μ-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear μ–oxo bridged product derived from O2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.

Potassium, Cesium, and Ammonium Peroxogermanates with Inorganic Hexanuclear Peroxo Bridged Germanium Anion Isolated from Aqueous Solution

“…The same shift of peroxide 17 O NMR signal to the weak field occurs in the spectrum of 1 M sodium alkali solution in 4% aqueous hydrogen peroxide ( Figure S3c). However, the significant intensity decrease of the signal assigned to the oxo-or hydroxo ligands in germanate anions after hydrogen peroxide addition demonstrates the substitution of initial coordination environment of germanium(IV) atoms and can be caused by peroxide coordination.…”

“…The intensity of this signal (determined by normalization versus water signal) decreased 2 times after addition of 8% hydrogen peroxide solution to the initial germanate solution, and the peak of peroxide appeared at 219.7 ppm ( Figure S3b). Most probably, the shift of peroxide compared to 17 O NMR spectrum of pure hydrogen peroxide (180 ppm) is induced by deprotonation of H 2 O 2 molecules at high pH and does not prove peroxide coordination to germanium.…”

“…All experiments were carried out in the range 40−400°C under nitrogen flow at a heating rate of 5°/min. 17 O NMR Spectrometry. 17 O NMR spectra were collected on a Bruker Avance-500 (11.7483T) spectrometer at resonance frequency of 67.8 MHz.…”

“…17 O NMR Spectrometry. 17 O NMR spectra were collected on a Bruker Avance-500 (11.7483T) spectrometer at resonance frequency of 67.8 MHz. The measurements were performed using a single pulse sequence with rf pulse duration of 8 μs and recycling time of 0.031 s. Experiments were carried out at 5°C.…”

Nucleophilic Reactivity of a Metal-Bound Superoxide Ligand

“…A putative manganese(III)-superoxide could have reacted as a nucleophile with a second manganese(II) center in the formation of a dimanganese(III)-peroxide species, 29 although this reactivity is best accounted for by simple electron transfer. Que and coworkers have mechanistically inferred a nucleophilic iron(III)-superoxide as a transient intermediate in the reaction of an iron(II) complex with O 2 .…”