Binuclear Manganese Compounds of Potential Biological Significance. Part 2. Mechanistic Study of Hydrogen Peroxide Disproportionation by Dimanganese Complexes:  The Two Oxygen Atoms of the Peroxide End up in a Dioxo Intermediate

Abstract

The dimanganese(II,II) complexes 1a [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)) and 1b [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g approximately 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) ([Mn(III)Mn(IV)(L)(O)(2)(OBz)](+) for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g approximately 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [Mn(II)Mn(III)(L)(O)(OAc)](+) ([Mn(II)Mn(III)(L)(O)(OBz)](+) for 3b). In the presence of H(2)(18)O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively. When H(2)(18)O(2) is used, 2a and 3a are labeled showing that the oxo ligands come from H(2)O(2). Interestingly, when an equimolar mixture of H(2)O(2) and H(2)(18)O(2) is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H(2)O(2) molecule. All these experiments lead to attribute the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) to 2a and to 3a the formula [Mn(II)Mn(III)(L)(O)(OAc)](+). Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of 1a with tert-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments. Its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 A and three or two N/O atoms at 2.14 A. In addition one N atom was found at a longer distance (2.3 A) and one Mn at 2.63 A. 2a can be one-electron oxidized at E(1/2) = 0.91 V(NHE) (DeltaE(1/2) = 0.08 V) leading to its Mn(IV)Mn(IV) analogue. The formation of 2a from 1a was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate Mn(II)Mn(III) species, resembling 4a [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))(2), the one-electron-oxidized form of 1a, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction.

Catalase vs Peroxidase Activity of a Manganese(II) Compound: Identification of a Mn(III)−(μ-O)2−Mn(IV) Reaction Intermediate by Electrospray Ionization Mass Spectrometry and Electron Paramagnetic Resonance Spectroscopy

“…33 Similar absorption bands (three absorption bands between 400-700 nm) were observed when the dimanga- 21 reported by Latour, were allowed to react with H 2 O 2 in acetonitrile. Mn(III)Mn(IV)-di-µ-oxo complexes containing the tripodal ligands bpia, 17 tpa, bpg, and pda 18 also exhibit similar absorption bands.…”

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

“…15,19,20 Examples include ligands employed to generate PAP, 19,21,23,24,28,[96][97][98][99][100] phosphoesterase, 101 urease, 102,103 catechol oxidase 104 and manganese catalase biomimetics. 105,106 Some ligands generate a hard and a soft coordination site for the generation of heterodinuclear model complexes for PAP metalloenzymes. 24,96 In some cases the ligands have one structural variation in one arm, 23,24,96 in others one donor arm has been omitted.…”

“…24,96 In some cases the ligands have one structural variation in one arm, 23,24,96 in others one donor arm has been omitted. 19,[104][105][106] Often the vacant coordination site is found to be occupied by water or solvent molecules in the complex. 105,106 …”

“…19,[104][105][106] Often the vacant coordination site is found to be occupied by water or solvent molecules in the complex. 105,106 …”

Asymmetric zinc(ii) complexes as functional and structural models for phosphoesterases

“…It has been proposed that these asymmetric complexes are not only more appropriate functional models for the active site of phosphoesterase enzymes, but also that they exhibit enhanced catalytic rates compared with their symmetric counterparts. [1][2][3] Ligands used to generate purple acid phosphatase, 1,4-10 phosphoesterase, 11 urease, 12,13 catechol oxidase 14 and manganese catalase biomimetics 15,16 have been reported. Some ligands engender both a hard and a soft coordination site resulting in heterodinuclear complexes as, for example, models for purple acid phosphatase metalloenzymes.…”