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

Transition-metal peroxos have been
implicated as key intermediates
in a variety of critical biological processes involving O2. Because of 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 Mn–peroxo,
either as a precursor to, or derived from, O2, in both
photosynthetic H2O oxidation and DNA biosynthesis. These
are 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-Me-DPEN))] + (1), and the characterization of intermediates formed en route to a
binuclear mono-oxo-bridged Mn­(III) product {[MnIII(SMe2N4(6-Me-DPEN)]2(μ-O)}2+ (2), the oxo atom of which is derived from 18O2. At low-temperatures, a dioxygen intermediate, [Mn­(SMe2N4(6-Me-DPEN))­(O2)]+ (4), is observed (by stopped-flow) to rapidly and irreversibly
form in this reaction (k1(−10 °C)
= 3780 ± 180 M–1 s–1, ΔH1⧧ = 26.4 ± 1.7 kJ mol–1, ΔS1⧧ = −75.6 ± 6.8 J mol–1 K–1) and then convert more slowly (k2(−10
°C) = 417 ± 3.2 M–1 s–1, ΔH2⧧ = 47.1
± 1.4 kJ mol–1, ΔS2⧧ = −15.0 ± 5.7 J mol–1 K–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 a binuclear Mn­(O22–) 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.