Chemical access to the mononuclear Mn(III) [(mL)Mn(OMe)] + complex (mLH = N,N′-bis-(2-pyridylmethyl)- N-(2-hydroxybenzyl)- N′-methyl-ethane-1,2-diamine) and electrochemical oxidation to the Mn(IV) [(mL)Mn(OMe)] 2+ species

Abstract

Chemical oxidation in acetonitrile of the previously reported phenolato-bridged binuclear Mn(II) complex [(mL)MnMn(mL)] 2+ ( 1), where mLH is pentadentate N,N′-bis-(2-pyridylmethyl)- N-(2-hydroxybenzyl)-N′-methyl-ethane-1,2-diamine ligand [C. Hureau, et al., Chem. Eur. J. 2004, 10, 1998–2010] using iodosylbenzene PhIO (dissolved in methanol) is described. The addition of one to four equivalents of PhIO per Mn ion leads to the transient formation of the mono-μ-oxo binuclear Mn 2(III,III) complex [(mL)Mn(μ-O)Mn(mL)] 2+ ( 2), previously studied. After addition of five equivalents of PhIO per Mn ion, the mononuclear Mn(III) species [(mL)Mn(OMe)] + ( 3) is quantitatively generated. The UV–Vis spectrum of 3 displays a broad band at 456 nm ( ε = 1000 L mol −1 cm −1) attributed to phenolato to Mn(III) charge transfer transition. Complex 3 exhibits a reversible oxidation wave at E 1/2 = 0.68 V versus SCE, and the mononuclear Mn(IV) complex [(mL)Mn(OMe)] 2+ ( 3 ox ) can thus be generated by exhaustive electrolysis at 1.0 V versus SCE. The 9.4 GHz EPR spectrum of complex 3 ox shows a strong transition near g = 4 consistent with a rhombically distorted S = 3/2 system with a zero-field splitting dominating the Zeeman effect. UV–Vis spectrum displays a large phenolato to Mn(IV) charge transfer transition at 670 nm ( ε = 2450 L mol −1 cm −1).