Département de chimie moléculaire

The electrochem. behavior of 2 mononuclear MnII bis-terpyridine complexes, [MnII(L)2]2+ (L = terpy (2,2':6',2''-terpyridine) and (Me3C)3-terpy (4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine)), was studied in dry MeCN. Under these conditions, the cyclic voltammograms of these complexes exhibit not only the known MnII/MnIII oxidn. system but also a 2nd metal-based oxidn. one, corresponding to the MnIII/MnIV redox couple. These oxidative processes are located at E1/2 = +0.96 and +1.77 V vs. Ag/Ag+ (+1.26 and +2.07 V vs. SCE) for the terpy complex and E1/2 = +0.85 and +1.56 V vs. Ag/Ag+ (+1.15 and +1.86 V vs. SCE) for the (Me3C)3-terpy deriv. The 1-electron oxidized form of these complexes, [MnIII(L)2]3+, was quant. generated by exhaustive electrolyzes at E = 1.30 V, as previously obsd. (S. Romain et al., 2009) in the case of the oxidn. of [MnII(tolyl-terpy)2]2+ (tolyl-terpy = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine). Further electrolyzes at E = 1.65-1.80 V of [MnIII(L)2]3+ solns. showed that the [MnIV(L)2]4+ species is only stable for L = (Me3C)3-terpy because of the strong electron-donating properties of the tert-Bu substituents. These electrogenerated high-valent complexes are rare examples of mononuclear MnIII and MnIV complexes stabilized solely by neutral N ligands. They were fully characterized in soln. by UV-visible and ESR spectroscopies. A detailed study of the EPR spectra of the [MnII((CMe3)3-terpy)2]2+ and [MnIV((CMe3)3-terpy)2]4+ has allowed the detn. of the spin Hamiltonian parameters for both systems (for MnII: |D| = 0.059 cm-1; |E| = 0.014 cm-1; E/D = 0.259; gx = gy = gz = 2.00 and for MnIV: |D| = 1.33(6) cm-1; |E| = 0.36(4) cm-1; E/D = 0.27; gx = 1.96(4); gy = 1.97(4); gz = 1.98(4)). [on SciFinder(R)]