In the worldwide search for sustainable energy technologies, water oxidation by abundant low-cost materials is of key importance. In nature, this process is efficiently catalyzed by an intricate manganese–calcium (Mn₄Ca) complex bound to the proteins of photosystem II (PSII). Recently synthetic manganese–calcium oxides were found to be active catalysts of water oxidation but at the atomic level their structure has remained elusive. To investigate these amorphous catalysts, extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both manganese and calcium was performed. The XAS results reveal striking similarities between the synthetic material and the natural Mn₄Ca complex. The oxidation state of manganese in the active oxides was found to be close to +4, but Mn^III ions are present as well at a level of about 20%. Neighboring Mn ions are extensively interconnected by two bridging oxygens, a characteristic feature of layered manganese oxides. However, the oxides do not exhibit long-range order, as opposed to canonical, but catalytically inactive Mn^III- or Mn^IV-oxides. Two different Ca-containing motifs were identified. One of them results in the formation of Mn₃CaO₄ cubes, as also proposed for the natural paragon in PSII. Other calcium ions likely interconnect oxide-layer fragments. We conclude that these readily synthesized manganese–calcium oxides are the closest structural and functional analogs to the native PSII catalyst found so far. Evolutionary implications are considered. From the differences to inactive manganese oxides, we infer structural features facilitating the catalysis of water oxidation in both the protein-bound Mn₄Ca complex of PSII and in the synthetic oxides.

In reaction sequences for light driven water-splitting into H₂ and O₂, water-oxidation is a crucial reaction step. In vivo, the process is catalysed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn₄ cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca²⁺ is removed from its structure. Activity can be restored not only by the addition of Ca²⁺ but also Sr²⁺ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compounds for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, preparing a series of K-, Ca-, Sr- and Mg-containing birnessites. Structural motifs within these materials were determined using X-ray absorption spectroscopy (XAS) showing that all materials have similar atomic structures despite their different elemental compositions. Water-oxidation experiments were carried out to elucidate structure-reactivity relations. These experiments demonstrated that the oxides—like the OEC—require the presence of calcium in their structures to reach maximum catalytic activity. As another similarity to the OEC, Sr²⁺ is the “second best choice” for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca²⁺ in biological water-oxidation. Additionally, they offer valuable hints for the development of synthetic, manganese-based water-oxidation catalysts for artificial photosynthesis.

One of the most fundamental processes of the natural photosynthetic reaction sequence is the light-driven oxidation of water to molecular oxygen. In vivo, this reaction takes place in the large protein ensemble Photosystem II, where a μ-oxido-Mn₄Ca- cluster, the oxygen-evolving-complex (OEC), has been identified as the catalytic site for the four-electron/four-proton redox reaction of water oxidation. This Perspective presents recent progress for three strategies which have been followed to prepare functional synthetic analogues of the OEC: (1) the synthesis of dinuclear manganese complexes designed to act as water-oxidation catalysts in homogeneous solution, (2) heterogeneous catalysts in the form of clay hybrids of such Mn₂-complexes and (3) the preparation of manganese oxide particles of different compositions and morphologies. We discuss the key observations from the studies of such synthetic manganese systems in order to shed light upon the catalytic mechanism of natural water oxidation. Additionally, it is shown how research in this field has recently been motivated more and more by the prospect of finding efficient, robust and affordable catalysts for light-driven water oxidation, a key reaction of artificial photosynthesis. As manganese is an abundant and non-toxic element, manganese compounds are very promising candidates for the extraction of reduction equivalents from water. These electrons could consecutively be fed into the synthesis of “solar fuels” such as hydrogen or methanol.

A set of six multinuclear manganese complexes was screened for the ability to catalyse reactions yielding O₂ under coherent experimental conditions; we identify a much larger number of manganese compounds than previously known that catalyse oxygen formation.