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Review
. 2008 Mar 27;363(1494):1293-303; discussion 1303.
doi: 10.1098/rstb.2007.2226.

A ligand field chemistry of oxygen generation by the oxygen-evolving complex and synthetic active sites

Theodore A Betley  1 , Yogesh Surendranath, Montana V Childress, Glen E Alliger, Ross Fu, Christopher C Cummins, Daniel G Nocera
Affiliations
Review

A ligand field chemistry of oxygen generation by the oxygen-evolving complex and synthetic active sites

Theodore A Betley et al. Philos Trans R Soc Lond B Biol Sci. .

Abstract

Oxygen-oxygen bond formation and O2 generation occur from the S4 state of the oxygen-evolving complex (OEC). Several mechanistic possibilities have been proposed for water oxidation, depending on the formal oxidation state of the Mn atoms. All fall under two general classifications: the AB mechanism in which nucleophilic oxygen (base, B) attacks electrophilic oxygen (acid, A) of the Mn4Ca cluster or the RC mechanism in which radical-like oxygen species couple within OEC. The critical intermediate in either mechanism involves a metal oxo, though the nature of this oxo for AB and RC mechanisms is disparate. In the case of the AB mechanism, assembly of an even-electron count, high-valent metal-oxo proximate to a hydroxide is needed whereas, in an RC mechanism, two odd-electron count, high-valent metal oxos are required. Thus the two mechanisms give rise to very different design criteria for functional models of the OEC active site. This discussion presents the electron counts and ligand geometries that support metal oxos for AB and RC O-O bond-forming reactions. The construction of architectures that bring two oxygen functionalities together under the purview of the AB and RC scenarios are described.

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Figures

Figure 1
Figure 1
Current proposals for the various electron counts of the S-states of the Kok cycle.
Figure 2
Figure 2
Qualitative frontier molecular orbital splitting diagrams for (a) an octahedral metal complex and a metal oxo residing in a (b) tetragonal field, (c) trigonal bipyramidal and (d) tetrahedral ligand field. The table shows the d-electron count that supports a multiple metal oxo bond and the preferred mechanism for the O–O bond coupling, where AB is acid/base and RC, radical coupling. Only low-spin configurations are considered. The assignment of AB and RC will change with high-spin configurations.
Figure 3
Figure 3
(a) The water oxidation centre in photosystem II (Ferreira et al. 2004) and (b) the Hangman porphyrin (I) assemble the oxygen of two waters for coupling, (II) activate the water to oxo by PCET and (III) position a high-valent oxo along the reactive metal hydroxide vector. Though the resting state of the Hangman is a FeIII–OH⋯H2O complex, the Hangman porphyrin is prepared by the introduction of an FeII into the porphyrin core and the assembly of two waters. Production of the Compound I intermediate of the Hangman thus results from an overall (4e−,4H+) process.
Figure 4
Figure 4
Bimetallic ruthenium compounds capable of water splitting: Meyer's blue oxo dimer complex [(bpy)2(OH2)RuIII(μ-O)RuIII(OH2)(bpy)2]4+(1) and cofacial diruthenium platforms on the scaffolds of dimethylxanthene (2), dibenzofuran (3) and anthracene (4,5).
Figure 5
Figure 5
The trigonal bipyramidal field offered by the tris(pyrrolide)amine framework. The complex is stabilized for the oxo complexes of VIV and VV formal oxidation states.
Figure 6
Figure 6
Hexaanionic cryptands to support the coupling of two high-valent oxos by an RC mechanism.
Figure 7
Figure 7
Trianionic platforms to support high-valent, late transition metal-oxo cores in a tetrahedral ligand field.
See this image and copyright information in PMC

References

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