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

Abstract

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^VH₃ 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 (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 ⁴A′′ (C_s) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).