Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+:: Chemical and electrochemical access to a new mono-μ-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)n(II)(mL)](ClO4)(2) (1(ClO4)(2)) has been obtained with the new [N4O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)- N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0800 mT at the X band and over 100-1450 mT at the Q-band, features that are usually observed for dinuclear Mn-II complexes. Cyclic voltammetry of I exhibits two irreversible oxidation waves at E-1(p) = 0.89 V and E-2(p) = 1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E-1(p), = 0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20degreesC, 1 is transformed into the mono-mu-oxo species [(mL)Mn-III-(mu-O)-Mn-III(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn-III-(mu-O)(2)-Mn-IV(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn-III charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the (MnMnIII)-Mn-III/(MnMnIV)-Mn-III and (MnMnIV)-Mn-III/(MnMnIV)-Mn-IV oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixedvalent species [(mL)Mn-III-(mu-O)Mn-IV(mL)](3+) (2ox). The UV/Vis spectrum of 2ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn-IV charge-transfer transition. 2ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2ox are proposed.