Redox Chemistry of a Dimanganese(II,III) Complex with an Unsymmetric Ligand: Water Binding, Deprotonation and Accumulative Light-Induced Oxidation
✍ Scribed by Magnus F. Anderlund; Joakim Högblom; Wei Shi; Ping Huang; Lars Eriksson; Högni Weihe; Stenbjörn Styring; Björn Åkermark; Reiner Lomoth; Ann Magnuson
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 318 KB
- Volume
- 2006
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Abstract
A dinuclear manganese complex {[Mn~2~^II,III^L(μ‐OAc)~2~]ClO~4~} 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 N~3~O~3~ and N~2~O~4~ donor set of Mn^II^ and Mn^III^, respectively. Electrochemically [Mn~2~^II,III^L(μ‐OAc)~2~]^+^ is reduced to [Mn~2~^II,II^L(μ‐OAc)~2~] at E~1/2~(1) = –0.53 V versusFc^+/0^ and oxidised to [Mn~2~^III,III^L(μ‐OAc)~2~]^2+^ at E~1/2~(2) = 0.38 V versus Fc^+/0^. All three redox states have been characterised by EPR, IR and UV/Vis spectroscopy. Subsequent oxidation of [Mn~2~^II,III^L(μ‐OAc)~2~]^2+^ [E~1/2~(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 Mn^III^ 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 Mn~2~^III,IV^Lcomplex 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)