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2013, 49, 5696-5698.
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absorption or MCD spectra in the visible and near-UV regions. Thus, these experiments selectively probe the MnIII complexes.
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53. Despite numerous attempts, we were unable to acceptably fit VTVH MCD data collected for [MnIII(O2)(N4py)]+ at higher energies. We presume this is due to the overlap of electronic
transitions with different polarizations at higher energies. Alternatively, larger MCD B- term contributions could also affect the quality of VTVH MCD data collected at these wavelengths.
54. Nivorozhkin, A. L.; Anxolabéhère-Mallart, E.; Mialane, P.; Davydov, R.; Guilhem, J.; Cesario, M.; Audière, J.-P.; Girerd, J.-J.; Styring, S.; Schussler, L.; Seris, J.-L., Inorg.
Chem. 1997, 36, 846-853.
55. bpmg = 2-[[2-[bis(pyridin-2-ylmethyl)amino]-ethyl](methyl)amino]acetic acid and mcbpen = N-methyl-N'-carboxymethyl-N,N'-bis(pyridylmethyl)ethane-1,2-diamine.
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61. The peroxo frontier orbitals, which are O–O -antibonding, are conventionally classified as in-plane (ip*) or out-of-plane (op*) depending on their orientation with respect to the metal-peroxo bond vectors.
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75
Chapter 3
Reaction Landscape of a Pentadentate N5-Ligated Mn
IIComplex with O
2•-and H
2O
2Includes Conversion of a Peroxomanganese(III) Adduct to a
Bis(-oxo)dimanganese(III,IV) Species
This work has been reproduced from: D. F. Leto, S. Chattopadhyay, V. W. Day and T. A. Jackson, Dalton Trans., 2013, 42, 13014 - 13025, with permission from The Royal Society of Chemistry.
76
3.1. Introduction.
The diverse chemistry of manganese-dependent enzymes commonly features the reaction of a redox-active manganese ion with molecular oxygen or one of its reduced derivatives (i.e., superoxide, hydrogen peroxide, and water).1-3 For example, Mn-dependent dioxygenases4,5 and oxalate oxidase6,7 react with molecular oxygen to perform substrate oxidations. Manganese superoxide dismutase3,8,9 and manganese catalase2 react with superoxide and hydrogen peroxide, respectively, as defense against reactive oxygen species. The tetramanganese cofactor in the oxygen-evolving complex of photosystem II converts water to O2.10,11 Finally, it was recently
established that Mn-ribonucleotide reducatase (Mn-RNR) from Bacillus subtilis requires superoxide for the assembly of an oxo-bridged dimanganese(III,IV) species.12 The MnIIIMnIV form of Mn-RNR rapidly converts to a MnIII2-Y cofactor that itself generates a cysteine radical
to initiate the conversion of nucleotides to deoxynucleotides.13-15 The molecular mechanism by which the MnIIIMnIV form of Mn-RNR is assembled is unknown, although a (hydro)- peroxomanganese(III) species was proposed as an intermediate.12
Examination of the spectroscopic properties and reactivity of Mn-containing model complexes has played a prominent role in advancing our understanding of Mn enzymes.1,2,11,16-18 With respect to Mn-RNR, for example, there are numerous model complexes featuring bis(µ- oxo)dimanganese(III,IV) cores that serve as potential mimics of the MnIIIMnIV form of the enzyme.11,19 These model complexes, which are often observed upon treatment of manganese(II) complexes with H2O2 at room temperature,2,19 are generally very stable thermodynamically.
Crystal structures of these complexes reveal typical Mn---Mn separations of ~2.7 Å.11,19-23 Some of these bis(µ-oxo)dimanganese(III,IV) complexes have been shown to oxidize substrates by a H-atom transfer mechanism, thus mimicking tyrosine oxidation by the MnIIIMnIV form of Mn-
77 RNR.24,25
Mononculear side-on peroxomanganese(III)26-38 and end-on alkylperoxomanganese(III)39-
41
intermediates have also been reported and characterized. These intermediates can be generated from their corresponding manganese(II) precursors using a variety of oxidants, including superoxide, hydrogen (or alkyl) peroxide, and molecular oxygen. They often, but not always,30-32 are thermally unstable and must be formed and characterized at low temperatures. Reactivity studies of peroxomanganese(III) adducts have shown that the peroxo group is nucleophilic and attacks aldehydes to initiate a deformylation reaction.27,30,31,34,42 Notably, although several examples of oxo-bridged dimanganese(III,IV) and (alkyl)peroxomanganese(III) species exist, there is, to the best of our knowledge, limited evidence that (alkyl)peroxomanganese(III) adducts can serve as intermediates in the generation oxo-bridged dimanganese(III,IV) complexes.39-41 To better understand the relationship of peroxomanganese(III) and bis(µ- oxo)dimanganese(III,IV) species, we have explored the chemical reactivity of the mononuclear [MnII(N4py)(OTf)](OTf) complex with superoxide and hydrogen peroxide. In a previous, collaborative study with Dorlet and Anxolabéhère-Mallart, we established that the [MnII(N4py)(OTf)](OTf) complex converts to a MnIII-O2 adduct upon treatment with excess
superoxide (Scheme 1).28 On the basis of detailed spectroscopic and computational studies of [MnIII(O2)(N4py)]+, as well as the closely related [MnIII(O2)(mL52)]+ and [MnIII(O2)(imL52)]+
complexes (mL52 = N-methyl-N,N,N,-tris(2-pyridylmethyl)ethane-1,2-diamine and imL52 = N-
methyl-N,N,N,-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine), we concluded that the N4py ligand in [MnIII(O2)(N4py)]+ is bound in an unusual tetradentate fashion (termed herein 4-
N4py), whereas the peroxo is coordinated in a side-on (i.e., 2-O2) mode. Because tetradentate
78 dipyridinylmethane moiety, two isomers of [MnIII(O2)(4-N4py)]+ are possible. Molecular
structures of these isomers, which were previously developed using density functional theory (DFT) computations, are shown in Scheme 3.1.28 While the DFT-optimized structures are very similar, the [MnIII(O2)(4-N4py)]+ isomer with a dissociated methylenepyridine arm was lower in
total energy by ~6 kcal/mol, and TD-DFT-computed spectroscopic properties for this isomer were more consistent with the available spectroscopic data. Thus, we favored this isomer, but were unable to make a definitive conclusion on the basis of the available data.43
Scheme 3.1.
In this chapter, we extend our understanding of the reactivity of [MnII(N4py)(OTf)](OTf) by showing that this complex converts to [MnIIIMnIV(µ-O)2(4-N4py)2]3+ upon treatment with
H2O2 and base even at -40 °C. Intriguingly, the [MnIIIMnIV(µ-O)2(4-N4py)2]3+ complex can be
79 with [MnII(N4py)(OTf)](OTf). As this reaction features the conversion of two MnII species to a MnIIIMnIV compound using O2- as oxidant, it mimics formation of the MnIIIMnIV form of Mn-
RNR.
3.2. Experimental Methods.
3.2.1. Materials and Methods. All chemicals and solvents were obtained from
commercial vendors and were ACS reagent-grade or better and used as received. Electronic absorption spectra were obtained on either a Varian Cary 50 Bio or an Agilent 8453 spectrophotometer, both of which were interfaced with a Unisoku cryostat (USP-203-A) capable of maintaining temperatures between 150 and 373 K. Mass spectrometry experiments were performed using an LCT Primers MicroMass electrospray time-of-flight instrument.
3.2.2. Synthesis. The synthesis of N4py ligand was performed according to a previously
described procedure.44,45 1 was synthesized by reacting N4py ligand with MnII(OTf)2 in an
acetonitrile (MeCN) solution in a 1:1 molar ratio as previously reported.28 3 was synthesized as follows. Five equiv. H2O2 (30% aqueous) and 0.5 equiv. triethylamine (Et3N) were slowly added
to a 36 mM aqueous solution of [MnII(N4py)(OTf)](OTf) (1) at 5 oC while stirring. The solution instantly changed from colorless to dark forest green and gas evolution was observed. After stirring for one hour, the water was removed in vacuo. The solid was redissolved in cold MeCN and a precipitate was formed from the solution by layering with cold ether. The solvent was decanted and the precipitate was washed with cold ether and dried (yield 95%). ESI-MS data were as follows: {[MnIIIMnIV(µ-O)2(N4py)2](OTf)2}+ m/z = 1174.1285 (calc 1174.1305). To
grow X-ray diffraction quality crystals, an aqueous solution of 3 (25 mM) was treated with a