Redox Chemistry of a Dimanganese(II,III) Complex with an Unsymmetric Ligand: Water Binding, Deprotonation and Accumulative Light‐Induced Oxidation

Abstract

AbstractA dinuclear manganese complex {[Mn2II,IIIL(μ‐OAc)2]ClO4} has been synthesised, where L is the dianion of 2‐{[bis(pyrid‐2‐ylmethyl)amino]methyl}‐6‐{[(3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)(pyrid‐2‐ylmethyl)amino]methyl}‐4‐methylphenol, an unsymmetric binucleating ligand with two coordinating phenol groups. The two manganese ions, with a Mn–Mn distance of 3.498 Å, are bridged by the two bidentate acetate ligands and the 4‐methylphenolate group of the ligand, resulting in a N3O3 and N2O4 donor set of MnII and MnIII, respectively. Electrochemically [Mn2II,IIIL(μ‐OAc)2]+ is reduced to [Mn2II,IIL(μ‐OAc)2] at E1/2(1) = –0.53 V versusFc+/0 and oxidised to [Mn2III,IIIL(μ‐OAc)2]2+ at E1/2(2) = 0.38 V versus Fc+/0. All three redox states have been characterised by EPR, IR and UV/Vis spectroscopy. Subsequent oxidation of [Mn2II,IIIL(μ‐OAc)2]2+ [E1/2(3) = 0.75 V vs. Fc+/0] in dry acetonitrile results in an unstable primary product with a lifetime of about 100 ms. At high scan rates quasireversible voltammetric behaviour is found for all three electrode processes, with particularly slow electron transfer for the II,III/II,II [k°(1) = 0.002 cm s–1] and III,III/II,III [k°(2) = 0.005 cm s–1] couples, which can be rationalised in terms of major distortions of the MnIII centres. In aqueous media the bridging acetates are replaced by water‐derived ligands. Deprotonation of these stabilises higher valence states, and photo‐induced oxidation of the manganese complex results in a Mn2III,IVLcomplex with oxo or hydroxo bridging ligands, which isfurther oxidised to an EPR‐silent product. These results demonstrate that a larger number of metal‐centred oxidations can be compressed in a narrow potential range if build up of charge is avoided by charge‐compensating reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)