Synthetic cluster models of biological and heterogeneous manganese catalysts for O2 evolution

Abstract

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. A mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This Award Article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex of photosystem II. These structural motifs are also related to heterogeneous mixed-metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low-oxidation-state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure-reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa value of the redox-inactive metal-aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.

Figure 1

Structure of OEC from single crystal X-ray diffraction studies.

Figure 2

Proposed structure of the OEC from single crystal XRD studies (top left)^- and more open structures proposed from XAS and computational studies (top middle, right).^- Proposed structure of catalytic sites in calcium manganese oxide water oxidation catalysts (bottom left)^- and of cobalt oxide water oxidation catalysts (bottom right).

Figure 3

Recent proposed mechanisms of O–O bond formation in the OEC,^,- with sites of substrate incorporation highlighted in red.

Figure 4

Selected examples of di- and tetramanganese oxido clusters.^,,-,

Figure 5

Structurally characterized heterometallic calcium-manganese clusters with bridging oxide or hydroxide moieties.^,,,

Figure 6

Solid-state structure of 1.

Figure 7

Solid-state structures of tetramanganese complexes 2-6.

Figure 8

Ligand flexibility as function of cluster oxido content and oxidation state: binding modes of dipyridylalkoxide arms.

Figure 9

Truncated solid-state structures of [9-Ca(DME)(OTf)]²⁺ and [9-Ca(OH₂)₃]³⁺.

Figure 10

Comparison of [CaMn₃O₄] structures. (a) Truncated view of the solid-state structure of 11-Ca. (b) CaMn₄O₅ cluster of the OEC as found in the 1.9 Å-resolution structure, with the [CaMn₃O₄] subsite emphasized by thicker bonds. (c) [Ca₂Mn₃O₄] cluster supported by pivalate ligands, with the [CaMn₃O₄] subsite emphasized by bold bonds. (d) Schematic drawing of one proposed structure of calcium-doped manganese oxide materials.

Figure 11

Redox potentials of heterometallic trimanganese tetraoxido (red squares) and dioxido (blue diamonds) clusters plotted against the pK_a of the corresponding metal(aqua)ⁿ⁺ ion, used as a measure of Lewis acidity. Figure reproduced from ref. 136.

Figure 12

Cyclic voltammograms of the [MMn^IV₃O₄]/[MMn^IIIMn^IV₂O₄] couples of N,N-dimethylacetamide solutions of MMn₃O₄ complexes at a scan rate of 100 mV/s. Potentials are referenced to the ferrocene/ferrocenium couple. Figure reproduced from ref. 136.

Scheme 1

Retrosynthetic analysis for the synthesis of an OEC model.

Scheme 2

Metal incorporation into subsite-differentiated Fe₃MS₄ clusters.

Scheme 3

Synthesis of trimetallic complexes LM^II₃(OAc)₃ (1; M = Mn).

Scheme 4

Synthesis and interconversion of tetramanganese complexes 2-7.

Scheme 5

Synthesis of CaMn₃O_x clusters.

Scheme 6

Synthesis of heterometallic dioxido complexes.

Scheme 7

Synthesis of heterometallic tetraoxido cubane complexes.

Scheme 8

Proposed μ-oxido migration equilibria between OEC substates of S₁ and S₂ (top),^- and proposed mechanisms for oxygen-atom transfer and oxidative water incorporation that depend on μ-oxido migration in model clusters (bottom).