Isolation of the first ferromagnetically coupled Mn(III/IV) complex

Area of Science:

• Bioinorganic Chemistry
• Photochemistry
• Photosynthesis Research

Background:

• The oxygen-evolving complex (OEC) in Photosystem II (PS II) plays a crucial role in photosynthesis.
• Understanding the structure and function of high-valent, ferromagnetically coupled manganese sites in the OEC is essential.
• Imidazole bridges are implicated in the electronic interactions within the OEC's manganese cluster.

Purpose of the Study:

• To synthesize and characterize binuclear manganese complexes that model imidazole-bridged, ferromagnetically coupled Mn sites.
• To investigate the magnetic and spectroscopic properties of these model complexes.
• To gain insights into the high oxidation states relevant to the OEC in PS II.

Main Methods:

• Synthesis of binuclear manganese complexes: Mn2(III/IV)(dtsalpn)2DCBI (1) and Mn2(III/III)(dtsalpn)2HDCBI (2).
• Temperature-dependent magnetic susceptibility studies.
• Variable temperature Electron Paramagnetic Resonance (EPR) spectroscopy.

Main Results:

• Complex 1 exhibited ferromagnetic coupling between the Mn(III)/Mn(IV) ions with a coupling constant J = +1.4 cm(-1).
• Variable temperature EPR spectra of complex 1 showed a g = 2 multiline signal attributed to an excited state with S = 1/2.
• The synthesized complexes successfully incorporated the dicarboxyimidazole (DCBI) ligand, facilitating imidazole bridging.

Conclusions:

• The synthesized binuclear manganese complexes serve as effective models for studying ferromagnetically coupled Mn sites in the OEC.
• The observed ferromagnetic coupling in complex 1 provides valuable information about the electronic interactions in high-valent manganese clusters.
• These findings contribute to a deeper understanding of the mechanism of water oxidation in photosynthesis.