Ablyasova, Olesya: Oxidation states in manganese oxide clusters: insights from oxygen-evolving complex analogs. , Dissertation, Universität Freiburg, 2024, 2025
10.6094/UNIFR/263506
Open Access Version
Abstract:
The Photosystem II complex is essential for the first stage of photosynthesis, facilitating dioxygen formation through the CaMn4O5 cluster, a key component of the oxygen-evolving complex (OEC). However, studying the S4 state, which is responsible for oxygen evolution, is challenging due to the transient nature and millisecond timescale of the S3-S4-S0 transition. Two competing models for the S4 state propose distinct oxidation states: the oxo-oxyl radical mechanism favors a high-spin Mn(IV)-oxyl species, while the oxo-oxo coupling mechanism requires high-spin Mn(V)-oxo species. Neither Mn(V)-oxo nor Mn(IV)-oxyl species have been experimentally observed in polymanganese oxide clusters, including CaMn4O5. This thesis investigates whether high-spin Mn(V)-oxo or Mn(IV)-oxyl species can be observed in cold gas-phase cationic manganese oxide complexes, serving as subunits of the CaMn4O5 cluster. We explore both mononuclear and polymanganese oxide systems, using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism to probe the oxidation states and spin properties. Computational methods, including multireference configuration interaction, density functional theory, and multiplet ligand-field theory, were employed to support the experimental findings. Additionally, the influence of ligands, such as acetylacetone, on the oxidation states of manganese was studied. XAS results demonstrate that ligand coordination significantly affects the oxidation state, underscoring the importance of local symmetry in determining electronic properties. Overall, this work provides new insights into the electronic structure of manganese oxide clusters and their relevance to the OEC. The discovery of high-spin Mn(V) species in polymanganese oxide complexes contributes to the understanding of the S4 state and may inform future studies on oxygen evolution in photosynthetic water splitting.