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Modeling the Oxygen-Evolving Complex of Photosystem II. Synthesis, Redox Properties, and Core Interconversion Studies of Dimanganese Complexes Having {MnIII2(μ-O)(μ-OAc)2}2+, {MnIIIMnIV(μ-O)2(μ-OAc)}2+, and {MnIV2(μ-O)2(μ-OAc)}3+ Cores with MeL as a Terminal Ligand:  A New Asymmetric Mixed-Valence Core,1

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journal contribution
posted on 02.05.1998, 00:00 by Tapan Kumar Lal, Rabindranath Mukherjee
The synthesis of [MnIIIMnIV(μ-O)2(μ-OAc)(MeL)2][ClO4]2·H2O (1a), [MnIIIMnIV(μ-O)2(μ-OAc)(MeL)2][BF4]2·2MeCN (1b), [MnIII2(μ-O)(μ-OAc)2(MeL)2][PF6]2·H2O (2a), [MnIII2(μ-O)(μ-OAc)2(MeL)2][ClO4]2·H2O (2b), and [MnIV2(μ-O)2(μ-OAc)(MeL)2][ClO4]3·H2O (3), containing a common tridentate facially capping ligand with chelate ring asymmetry, (2-pyridylmethyl)(2-pyridylethyl)methylamine (MeL), is described. X-ray crystallographic analysis of complex 1b revealed that it has an asymmetric MnIIIMnIV(μ-O)2(μ-OAc)}2+ core, as a result of different binding modes adopted by MeL at the Mn(III) versus the Mn(IV) site. The present group of complexes have both kinds of dimanganese core structure as that in the proposed tetranuclear structural model for the OEC in PSII. Temperature-dependent magnetic properties reveal the following results:  2J = −288, −244, and +2 cm-1 for 1a, 3, and 2a, respectively. Complex 1a has a doublet ground state and its X-band EPR spectrum at 77 K exhibits a 16-line pattern at g = 2. Owing to their substitutional lability, various core interconversion studies have been done on 1a, 2b, and 3, in considerable detail. The redox properties of 1a, 2a, and 3 have been systematically investigated. Complex 1a is reversibly oxidized by a one-electron process at 1.0 V to generate the MnIV2 dimer and is reduced irreversibly to MnIII2 species at −0.1 V vs SCE. Complex 3 exhibits two reductive redox processes at potentials almost identical (within experimental error) to that of 1a. Complex 2a displays a scan-rate dependent oxidative process at 1.2 V; an irreversible reductive process is also observed to generate MnIIIMnII species at 0.0 V vs SCE. On cycling of the scans between the potential limits 0.4 to 1.4 V, at the expense of the higher potential response at 1.0 V the lower potential response at 0.82 V grows. Coulometric oxidation of 2a at 1.2 V reveals that a net two-electron has been released, thereby generating one-electron oxidized form of 1a, due to an ECE mechanism. The successes of the synthetic reactions and the core interconversion studies presented here are the consequences of a series of disproportionation reactions. For 2a the electron-transfer processes are accompanied by protonation/deprotonation phenomena of relevance to OEC in PSII. To pinpoint the role of Cl- during redox transitions in OEC, the reactions of chloride ion with 1a, 2b, and 3 have been investigated by means of absorption spectroscopy and cyclic voltammetry.