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, [Mn V H 3 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 S 4 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 3 B 1 ( C 2v ). Additionally, we provide fingerprint spectra not only for [OMn V O] + , but also for the high-spin hydroxidooxidomanganese(IV) ion [OMn IV OH] + in its 4 A'' ( C s ) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).