Superoxide. In the currently proposed mechanism for cofactor assembly in Mn-RNR, a
dinuclear MnII center reacts with O2- to generate a bis(µ-oxo)dimanganese(III,IV) species.12,14,15
Our observation that 1 converts to 3 upon reaction with a limiting amount of O2- mimics this
chemistry. In this process, the MnIII-O2 complex serves as an intermediate. We propose that the
nucleophilic peroxo ligand on 2 reacts with the MnII center in 1 to form 3. This proposal is consistent with the following observations: i) 2 does not decay to 3 in significant quantities when the former is generated in higher yields, ii) the addition of 1 to 2 leads to the formation of 3in ~90% yield relative to the amount of 2 generated, and iii) equal amounts of 3are produced when either 0.5 or 1 equiv. 1 is added to 2 (i.e., the formation of 3 is limited by the amount of 2).
The reactivity of 1 with superoxide is highly reminiscent of previous work done independently by Dismukes70 and Perrée-Fauvet72 using MnII(TPP) (TPP = tetraphenylporphyrin). In both cases, a [MnIIIMnIV(-O)2(TPP)]- species, formulated on the basis
of EPR data, was proposed to form upon reaction of [MnII(TPP)] with O2-. Notably, the
formation of the MnIIIMnIV dimer was only observed when the ratio of O2- to [MnII(TPP)(py)]
was 0.5:1.72 When a 1:1 ratio was used, the [Mn(O2)(TPP)]- species was presumed to form in
high yield and the MnIIIMnIV dimer was not observed. Our current work thus extends this prior base of knowledge to a non-porphyrinoid manganese system and provides further details on these transformations.
It is tempting to speculate that the reaction between 1 and 2 to produce 3 proceeds through a dinuclear peroxo-bridged intermediate [MnIIMnIII(µ-O2)(4-N4py)2]3+ (Scheme
100 3.2).70,72 Two electrons are required to convert the peroxo ligand to the two oxo ligands in [MnIIIMnIV(µ-O)2(4-N4py)2]3+. The MnII and MnIII centers of the peroxo-bridged intermediate
could each provide an electron to cleave the peroxo bond, resulting in the formation of 3. Labelling experiments with K18O2 could, in principle, be used to establish the origin of the
bridging oxo ligands in 3. However, in practice, these experiments were complicated due to the limited solubility of superoxide without excess 18-crown-6 ether present in solution. When 18- crown-6 ether was added to increase the solubility of K18O2, the ESI-MS data were dominated by
the 18-crown-6 ether signal.
Groni et al. proposed the formation of a homovalent dinuclear peroxo-bridged intermediate, [MnIIIMnIII(µ-O2)(mL52)2]4+, en route to the formation of bis(µ-
oxo)dimanganese(III,IV) product, [MnIIIMnIV(µ-O2)(mL52)2]3+.38 They suggested that this
peroxo-bridged dimanganese(III,III) intermediate is formed by the reaction of [MnIII(O2)(mL52)]+
with [MnIII(OH2)(mL52)]3+ in a basic aqueous medium.38 Unsupported peroxo-bridged
dimanganese complexes are rather rare. To the best of our knowledge, the only well- characterized example of an unsupported peroxo-bridged dimanganese complex was reported earlier this year by Coggins et al.40 The {[MnIII(SMe2N4(6-Me-DPEN)]2(trans-µ-1,2-O2)}2+ (6-
Me-DPEN = N,N-Bis(6-methyl-2-pyridilmethyl)ethane-1,2-diamine) complex features a peroxo ligand bridged between two MnIII ions in an end-on trans-µ-1,2-fashion.40 A peroxo-bridged Mn cluster, the trinuclear Mn2III µ-peroxo complex [Mn3(dien)2(OAc)2(µ-O2)Cl](ClO4)2 (dien =
diethylenetriamine),73 and a bis(-oxo)dimanganese(IV,IV) complex with a cis--1,2-peroxo bridge, [L2Mn2(µ-O)2(µ-O2)](ClO4)2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane),74 have also
101
3.5. Summary.
The reactivity of [MnII(N4py)(OTf)]+ with superoxide and hydrogen peroxide is unique among synthetic mononuclear MnII complexes as a peroxomanganese(III) intermediate was formed from the reaction of 1 with superoxide, whereas a heterovalent bis(µ- oxo)dimanganese(III,IV) complex was formed in high yields from the reaction of 1 with hydrogen peroxide and base. The MnIII-O2 intermediate was converted to the heterovalent oxo-
bridged manganese(III,IV) dimer by a subsequent reaction with [MnII(N4py)(OTf)]+. This conversion of two MnII species to an oxo-bridged MnIIIMnIV compound using O2- as oxidant
mimics the formation of the MnIIIMnIV form of Mn-RNR. The oxo-bridged dimer was characterized structurally and spectroscopically, which revealed a change in denticity of the N4py ligand from pentadentate in the MnII complex to tetradentate in the oxo-bridged dimer.
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