Characterizations of chloro and aqua Mn(II) mononuclear complexes with amino-pyridine ligands. Comparison of their electrochemical properties with those of Fe(II) counterparts.

The solution behavior of mononuclear Mn(II) complexes, namely, (L(5)(2))MnCl (1), (L(5)(3))MnCl (2), (L(5)(2))Mn(OH(2)) (3), (L(5)(3))Mn(OH(2)) (4), and (L(6)(2))Mn(OH(2)) (6), with L(5)(2/3) and L(6)(2) being penta- and hexadentate amino-pyridine ligands, is investigated in MeCN using EPR, UV-vis spectroscopies, and electrochemistry. The addition of one chloride ion onto species 6 leads to the formation of the complex (L(6)(2))MnCl (5) that is X-ray characterized. EPR and UV-vis spectra indicate that structure and redox states of complexes 1-6 are maintained in MeCN solution. Chloro complexes 1, 2, and 5 show reversible Mn(II)/Mn(III) process at 0.95, 1.02, and 1.05 V vs SCE, respectively, whereas solvated complexes 3, 4, and 6 show an irreversible anodic peak around 1.5 V vs SCE. Electrochemical oxidations of 1 and 5 leading to the Mn(III) complexes (L(5)(2))MnCl (7) and (L(6)(2))MnCl (8) are successful. The UV-vis signatures of 7 and 8 show features associated with chloro to Mn(III) LMCT and d-d transitions. The X-ray characterization of the heptacoordinated Mn(III) species 8 is also reported. The analogous electrochemical generation of the corresponding Mn(III) complex was not possible when starting from 2. The new mixed-valence di-mu-oxo (L(5)(2))Mn(muO)(2)Mn(L(5)(2)) species (9) can be obtained from 3, whereas the sister (L(5)(3))Mn(muO)(2)Mn(L(5)(3)) species can not be generated from 4. Such different responses upon oxidations are commented on with the help of comparison with related Mn/Fe complexes and are discussed in relation with the size of the metallacycle formed between the diamino bridge and the metal center (5- vs 6-membered). Lastly, a comparison between redox potentials of the studied Mn(II) complexes with those of Fe(II) analogues is drawn and completed with previously reported data on Mn/Fe isostructural systems. This gives us the opportunity to get some indirect insights into the metal specificity encountered in enzymes among which superoxide dismutase is the archetypal model.