Département de chimie moléculaire

The dimanganese(II,II) complexes [Mn2(L)(OAc)2(CH3OH)](ClO4) (1a) and [Mn2(L)(OBz)2(H2O)](ClO4) (1b), where HL is the unsym. phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile soln. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies showed that a species (2) exhibiting a 16-line spectrum at g ∼ 2 persists during catalysis. ESI-MS expts. conducted similarly during catalysis assoc. 2a with a peak at 729 (791 for 2b) corresponding to [MnIIIMnIV(L)(O)2(OAc)]+ ([MnIIIMnIV(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 ∼ 2. ESI-MS assocs. 3a with a peak at 713 (775 for 3b) corresponding to [MnIIMnIII(L)(O)(OAc)]+ ([MnIIMnIII(L)(O)(OBz)]+ for 3b). In the presence of H218O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, resp. When H218O2 was used, 2a and 3a are labeled showing that the oxo ligands come from H2O2. Interestingly, when an equimolar mixt. of H2O2 and H218O2 was used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H2O2 mol. All these expts. lead to attribute the formula [MnIIIMnIV(L)(O)2(OAc)]+ to 2a and to 3a the formula [MnIIMnIII(L)(O)(OAc)]+. Freeze-quench/EPR expts. revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ∼100 ms. 2A was prepd. by reaction of 1a with tert-Bu hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS expts. Its structure was studied by x-ray absorption expts. which revealed two or three O atoms at 1.87 {\AA} and three or two N/O atoms at 2.14 {\AA}. In addn. one N atom was found at a longer distance (2.3 {\AA}) and one Mn at 2.63 {\AA}. 2A can be 1-electron oxidized at E1/2 = 0.91 VNHE ($Δ$E1/2 = 0.08 V) leading to its MnIVMnIV analog. The formation of 2a from 1a was monitored by UV-visible and x-ray absorption spectroscopies. Both concur to show that an intermediate MnIIMnIII species, resembling (4a) [Mn2(L)(OAc)2(H2O)](ClO4)2, the 1-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 biol. reaction. [on SciFinder(R)]