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

Abstract

Recent advances in the study of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) include structural information attained from several X-ray crystallographic (XRD) and spectroscopic (XANES and EXAFS) investigations. The possible structural features gleaned from these studies have enabled synthetic chemists to design more accurate model complexes, which in turn, offer better insight into the possible pathways used by PSII to drive photosynthetic water oxidation catalysis. Mononuclear model compounds have been used to advance the knowledge base regarding the physical properties and reactivity of high-valent (Mn(IV) or Mn(V)) complexes. Such investigations have been especially important in regard to the manganyl (Mn(IV)=O or Mn(V)≡O) species, as there are no reports, to date, of any structural characterized multinuclear model compounds that incorporate such a functionality. Dinuclear and trinuclear model compounds have also been thoroughly studied in attempts to draw further comparison to the physical properties observed in the natural system and to design systems of catalytic relevance. As the reactive center of the OEC has been shown to contain an oxo-Mn(4)Ca cluster, exact structural models necessitate a tetranuclear Mn core. The number of models that make use of Mn(4) clusters has risen substantially in recent years, and these models have provided evidence to support and refute certain mechanistic proposals. Further work is needed to adequately address the rationale for Ca (and Cl) in the OEC and to determine the sequence of events that lead to O(2) evolution.

Binuclear [(cod)(Cl)Ir(bpi)Ir(cod)]+ for Catalytic Water Oxidation

“…the 'blue dimer'. 6 Many (mononuclear, binuclear and multinuclear) variations of the original design have been synthesized since. 7,8 Remarkably, poorly defined, heterogeneous water oxidation catalysts tend to outperform their welldefined homogeneous counterparts, and significant results were recently reported using iridium colloids, [9] a cobalt phosphate system, 10 and recently even a molecular iron complex.…”

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis

“…The complex could not oxidize water but give to us important spectroscopic information of Mn(V) = O group that proposed as an intermediate in biological water oxidation (Miller et al 1998). However, as considered by Pecoraro, these synthetic Mn(V) = O complexes are stabilized by special ligand(s) and the activity of Mn(V) = O could be completely different from Mn(V) = O of the WOC in PSII (Mullins & Pecoraro, 2008). (Miller et al, 1998) (a) and a Mn(IV) complex terminal hydroxo ligands (b) (Yin et al, 2006).…”

“…There are many mono, di, tri and tetra nuclear manganese complexes as structural models for the WOC in PSII (Mullins & Pecoraro, 2008). The WOC in PSII is a tetranuclear manganese complex ( Fig.1) (Umena et al, 2011).…”

“…Regarding these mononuclear manganese complexes, we could obtained many information about spectroscopic properties of Mn(V) compounds as there are no Mn(V) synthetic examples of dinuclear or higher nuclearity structures that have been crystallographically characterized (Fig. 2.) (Mullins & Pecoraro, 2008). Water and terminal hydroxo ligands in Mn(IV) complexes are very important as there are suggested as one of the substrates for oxygen production in the WOC of PS II.…”

“…The relationship between Mn-Mn distance and Mn-O-Mn angle has been considered for such complexes (Mullins & Pecoraro, 2008). In 1988, there was suggestion that WOC had a mononuclear manganes center in close proximity to a trinuclear center and this suggestion was emerged basedon magnetic, spectroscopic, and crystallographic studies in 2000 (Mullins & Pecoraro, 2008).…”