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, [(L52)MnCl]+ (1), [(L53)MnCl]+ (2), [(L52)Mn(OH2)]2+ (3), [(L53)Mn(OH2)]2+ (4), and [(L62)Mn(OH2)]2+ (6), with L52/3 and L62 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 [(L62)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 [(L52)MnCl]2+ (7) and [(L62)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-μ-oxo [(L52)Mn(μO)2Mn(L52)]3+ species (9) can be obtained from 3, whereas the sister [(L53)Mn(μO)2Mn(L53)]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.