A New Type of Mn–O Cluster with a Mn₄ Chain Unit—Relevance to the Photosynthetic Oxygen Evolving Center

Abstract

Two Mn2(μ‐O)2 units linked by another two oxo bridges make up the cluster core of 1 (see structure on the right; two bipyridine ligands have been omitted from both Mn1 and Mn4). The MnMn distances and magnetic properties of 1 indicate that this cluster may be a model of the reaction center for oxygen evolution in plants. The question whether the Mn4 unit has a chainlike arrangement or a cubane structure in the naturally occurring enzyme is nevertheless still unanswered.

Electro‐ and Photoinduced Formation and Transformation of Oxido‐Bridged Multinuclear Mn Complexes

“…The tetranuclear complex [Mn 4 IV O 6 (bpy) 6 ] 4+ can be electrochemically prepared on a large scale and precipitated by addition of an excess of NaBF 4 to the oxidized solution 41. After filtration the resulting brown powder exhibits the same characteristics as a previous sample prepared chemically by the disproportionation of [Mn III (bpy)Cl 3 (H 2 O)] in aqueous nitric acid solution48,49 or by addition of K 2 CrO 7 to a solution of dinuclear [Mn 2 III,IV O 2 (bpy) 4 ] 3+ 50. It should be noted that [Mn 4 O 6 (bpy) 6 ] 4+ is EPR inactive at helium temperature owing to the S = 0 ground spin state while the one‐electron reduced, mixed‐valent Mn 3 IV Mn III form produced (transiently) by cryogenic radiolytic reduction of [Mn 4 IV O 6 (bpy) 6 ] 4+ exhibits a multiline EPR signal with a spectral width and intensity pattern comparable to that of the PSII S 2 state signal 51.…”

Reflections on small molecule manganese models that seek to mimic photosynthetic water oxidation chemistry

“…Although the dimer-of-dimers model has fallen out of favor with many researchers, there Scheme remains a genuine need to study models that mimic such architectures. In the early 1990s, Girerd and coworkers reported a model complex, [Mn 4 O 6 (bpy) 6 ] 4+ , that is composed of two di--oxo-dimanganese units linked by a di--oxo bridge [219]. Recently, Brudvig and coworkers have offered two alternative linear topologies using Mn 4 IV (terpy) complexes with different combinations of a mono-2 -oxo and bis-2 -oxo-bridged Mn units [91,92].…”

Modeling the Oxygen-Evolving Complex in Photosystem II

“…A good example of this can be found in the chemistry of [(bpy)2Mn(0) 2 Mn(bpy)2] 3+ , which in aqueous solution exists in dynamic equilibrium with at least six other complexes of varying nuclearity. [103][104][105][106] As a result of this, although there has been some success in oxidizing water with manganese complexes, it has been difficult to characterize the active species. One parallel approach, avoiding this problem, has been to try and design a catalyst.…”