Activation of a water molecule using a mononuclear Mn complex: from Mn-aquo, to Mn-hydroxo, to Mn-oxyl via charge compensation

Abstract

Activation of a water molecule by the electrochemical oxidation of a Mn-aquo complex accompanied by the loss of protons is reported. The sequential (2 × 1 electron/1 proton) and direct (2 electron/2 proton) proton-coupled electrochemical oxidation of a non-porphyrinic six-coordinated Mn(II)OH2 complex into a mononuclear Mn(O) complex is described. The intermediate Mn(III)OH2 and Mn(III)OH complexes are electrochemically prepared and analysed. Complete deprotonation of the coordinated water molecule in the Mn(O) complex is confirmed by electrochemical data while the analysis of EXAFS data reveals a gradual shortening of an Mn–O bond upon oxidation from Mn(II)OH2 to Mn(III)OH and Mn(O). Reactivity experiments, DFT calculations and XANES pre-edge features provide strong evidence that the bonding in Mn(O) is best characterized by a Mn(III)-oxyl description. Such oxyl species could play a crucial role in natural and artificial water splitting reactions. We provide here a synthetic example for such species, obtained by electrochemical activation of a water ligand.

Artificial photosynthesis: understanding water splitting in nature

“…Key properties are (i) that it can stabilize multiple oxidation states to be accessed over a narrow range of redox potentials. This property is derived from protein/cofactor interactions being able to tune the pK a of cofactor bound water (substrate) molecules allowing sequential H þ release upon each Mn oxidation event; (ii) the possibility that the heterobimetallic cubane allows the formation of an energetically accessible Mn V -oxo or Mn IV -oxyl radical in the transition state [85][86][87][88]; and (iii) that the Mn ions represent trapped valence states of local high-spin states, which magnetically couple together in a precise way in each S-state.…”

Experimental and Computational X-ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged MnIV Dimers Relevant to Redox-Active Metalloproteins

“…15 Model compounds assessing the properties and reactivities of Mn IV -OH and Mn IV =O as well as Fe IV -OH and Fe IV =O species have been reported by several groups. 16,17,18,19,20,21 Que and co-workers have examined Fe complexes that are remarkable for their H-atom abstraction capabilities. 22,23 In some systems, it is possible to resolve the protonation state of oxo bridged species through single-crystal X-ray crystallography 6 where unusually long or short M-O and M-OH bonds have been reported.…”

Mono‐/Multinuclear Water Oxidation Catalysts

“…[70] For OÀO bond formation,t wo routes werep roposed:t he concerted path involves the reaction betweenM n IV -oxy and water, whereas the two-stepp ath involves coordination of the Mn ion to water,w hich can lead to OÀOb ond formation from the MnÀOH bond. [73] In 2015, Gupta et al reported that high oxidation state Mn V =Oe xisted and radicalc oupling for OÀO bond formation was al ikely mechanism during the OER. Dey and coworkers found am ononuclearM n-corrole complex that could catalyzet he OER with fast kinetics.…”

“…also reported that the OÀOb ond formation was formed by Mn IV =Oo rM n III -oxy. [73] In 2015, Gupta et al reported that high oxidation state Mn V =Oe xisted and radicalc oupling for OÀO bond formation was al ikely mechanism during the OER. [74] In 2017, Li et al researched the catalytic mechanismo f [Mn{Py 2 N(tBu) 2 (H 2 O) 2 }] 2 + for the OER.…”