Mononuclear MnIII and MnIV Bis-terpyridine Complexes: Electrochemical Formation and Spectroscopic Characterizations

The electrochemical behavior of two mononuclear Mn-II bis-terpyridine complexes, [Mn-II(L)(2)](2+) (L = terpy (2,2': 6',2 ''-terpyridine) and Bu-t(3)-terpy (4,4',4 ''-tritert-butyl-2,2':6',2 ''-terpyridine)), has been investigated in dry CH3CN. Under these conditions, the cyclic voltammograms of these complexes exhibit not only the well-known Mn-II/Mn-III oxidation system but also a second metal-based oxidation one, corresponding to the Mn-III/Mn-IV redox couple. These oxidative processes are located at E-1/2 = +0.96 and +1.77 V vs Ag/Ag+ (+1.26 and +2.07 V vs SCE) for the terpy complex and E-1/2 = +0.85 and +1.56 V vs Ag/Ag+ (+1.15 and +1.86 V vs SCE) for the Bu-t(3)-terpy derivative. The one-electron oxidized form of these complexes, [Mn-III((L)(2)](3+), has been quantitatively generated by exhaustive electrolyses at E = 1.30 V, as previously observed in the case of the oxidation of [Mn-II(tolyl-terpy)(2)](2+) (tolyl-terpy = 4'-(4-Methylphenyl)-2,2':6',2 ''-terpyridine) (Romain, S.; Duboc, C.; Neese, F; Riviere, E.; Hanton, L. R.; Blackman, A. G.; Lepretre, J.-C.; Deronzier, A.; Collomb, M.-N. Chem.-Eur. J. 2009, 15, 980-988). Further electrolyses at E = 1.65-1.80 V of [Mn-III(L)(2)](3+) solutions have shown that the [Mn-IV(L)(2)](4+) species is only stable for L = Bu-t(3)-terpy because of the strong electron-donating properties of the tert-butyl substituents. These electrogenerated high-valent complexes are rare examples of mononuclear Mn-III and Mn-IV complexes stabilized solely by neutral N ligands. They have been fully characterized in solution by UV-visible and electron paramagnetic resonance (EPR) spectroscopies. A detailed investigation of the EPR spectra of the [Mn-II(Bu-t(3)-terpy)(2)](2+) and [Mn-IV(Bu-t(3)-terpy)(2)](4+) has allowed the determination of the spin Hamiltonian parameters for both systems (for Mn: IDI = 0.059 cm(-1); vertical bar E vertical bar = 0.014 cm(-1); E/D = 0.259; g(x) = g(y) = g(z) = 2.00 and for Mn-IV: vertical bar D vertical bar = 1.33(6) cm(-1); vertical bar E vertical bar = 0.36(4) cm(-1); E/D = 0.27; g(x) = 1.96(4); g(y) = 1.97(4); g(z) = 1.98(4)).