Département de chimie moléculaire

To mimic the carboxylate-rich active site of the Mn catalases more closely the authors introduced carboxylate groups into dimanganese complexes in place of N ligands. Dimanganese(III,IV) complexes of tripodal ligands [Mn2(L)2(O)2]3+/+/-/3- were extended from those of tpa (1) and H(bpg) (2) to those of H2(pda) (3) and H3(nta) (4) (tpa = tris(2-picolyl)amine, H(bpg) = bis(2-picolyl)glycylamine, H2(pda) = 2-picolyldiglycylamine, H3(nta) = nitrilotriacetic acid). While [Mn2(pda)2(O)2][Na(H2O)3] (3) could be synthesized at -20° and characterized in the solid state, [Mn2(nta)2(O)2]3- (4) could be obtained and studied only in soln. at -60°. A new synthetic procedure for the dimanganese(III,III) complexes was devised, using stoichiometric redn. of the dimanganese(III,IV) precursor by the benzil radical with EPR monitoring. This enabled the prepn. of the parent dimanganese(III,III) complex [Mn2(tpa)2(O)2](ClO4)2 (5), which was structurally characterized. The UV/visible, IR, EPR, magnetic, and electrochem. properties of complexes 1-3 and 5 were analyzed to assess the electronic changes brought about by the carboxylate replacement of pyridine ligands. The kinetics of the oxo ligand exchanges with labeled H218O was examd. in MeCN soln. A dramatic effect of the no. of carboxylates was evidenced. The influence of the second carboxylate substitution differs from that of the first one probably because this substitution occurs on an out-of-plane coordination while the former occurs in the plane of the [Mn2O2] core. Indeed, on going from 1 to 3 the exchange rate was increased by a factor of 50. Addn. of NEt3 caused a rate increase for 1, but not for 3. The abilities of 1-3 to disproportionate H2O2 were assessed volumetrically. The disproportionation exhibited a sensitivity corresponding to the carboxylate substitution. Probably the carboxylate ligands in 2 and 3 act as internal bases. [on SciFinder(R)]