Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy

High-Spin Manganese(V) in an Active Center Analogue of the Oxygen-Evolving Complex

In a comprehensive investigation of the dinuclear [Mn2O3]+ cluster, the smallest dimanganese entity with two μ-oxo bridges and a terminal oxo ligand, and a simplified structural model of the active center in the oxygen-evolving complex, we identify antiferromagnetically coupled high-spin manganese centers in very different oxidation states of +2 and +5, but rule out the presence of a manganese(IV)-oxyl species by experimental X-ray absorption and X-ray magnetic circular dichroism spectroscopy combined with multireference calculations. This first identification of a high-spin manganese(V) center in any polynuclear oxidomanganese complex underscores the need for multireference computational methods to describe high-valent oxidomanganese species.

Direct spectroscopic evidence for the high-spin state of dioxidomanganese(V)

The spin state of metal centers in many catalytic reactions has been demonstrated to be a rate limiting factor when high-valent metal centers such as manganese are involved. Although numerous manganese(V) complexes, including a few manganese(V) oxo complexes, have been identified, thus far only one of these, [MnVH3 buea(O)], has been directly confirmed to exist in a high spin state. Such a high-spin manganese(V) center may play a crucial role in the dioxygen formation process in the elusive S4 state of the Kok cycle in photosystem II. In this study, we provide direct experimental evidence, using X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS), of gas phase [OMnO]+ as the second known high-spin manganese(V) oxo complex. We conclusively assign the ground state as 3B1 (C2v). Additionally, we provide fingerprint spectra not only for [OMnV O]+, but also for the high-spin hydroxidooxidomanganese(IV) ion [OMnIV OH]+ in its 4A'' (Cs) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).