Characterizations of chloro and aqua Mn(II) mononuclear complexes with amino-pyridine ligands. Comparison of their electrochemical properties with those of Fe(II) counterparts.
Résumé
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))](2+) (3), [(L(5)(3))Mn(OH(2))](2+) (4), and [(L(6)(2))Mn(OH(2))](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](2+) (7) and [(L(6)(2))MnCl](2+) (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))](3+) species (9) can be obtained from 3, whereas the sister [(L(5)(3))Mn(muO)(2)Mn(L(5)(3))](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.