Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential‐determining step. Subsequently, novel cobalt oxide‐based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water‐oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non‐ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox‐potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

Click chemistry has been utilized to access 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridines (BTPs) as versatile extended heteroaromatic building blocks for their exploitation in supramolecular chemistry, in particular foldamer and ligand design. In addition to their high-yielding synthesis using Cu(I)-catalyzed Huisgen-type 1,3-dipolar cycloaddition reactions the formed triazole moieties constitute an integral part of the BTP framework and encode both its pronounced conformational preferences as well as its chelating ability. A diverse set of symmetrical and non-symmetrical BTPs carrying electron-donating and -withdrawing substituents at both terminal aryl and the central pyridine moieties has efficiently been synthesized and could furthermore readily be postfunctionalized with amphiphilic side chains and porphyrin chromophores. In both solution and solid state, the BTP scaffold adopts a highly conserved horseshoe-like anti-anti conformation. Upon protonation or metal coordination, the BTP scaffold switches to the chelating syn-syn conformation. Iron and europium complexes have been prepared, successfully characterized by single-crystal X-ray diffraction analysis, and investigated with regard to their spin state and luminescent properties. The extended heteroaromatic BTP scaffold should prove useful for the design of responsive foldamer backbones and the preparation of new magnetic and emissive materials.

Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).

Going ferryl: FeII complexes with two isomeric bispidine ligands catalyze the oxidation of cyclooctene with H2O2 in MeCN under aerobic conditions with high selectivity for the epoxide (see scheme; L=bispidine ligand). An {FeIVO} complex is postulated to be the oxidant, formed by the homolytic cleavage of the OO bond of an {FeIII‐OOH} intermediate. Performing the reaction under argon gives a mixture of cis and trans diols as well as the epoxide.

[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probed the molecular and electronic configurations of the H-cluster in functional oxidized, reduced, and super-reduced or CO-inhibited HydA1 protein, in particular searching for intermediates with iron-hydride bonds. Site-selective X-ray absorption and emission spectroscopy were used to distinguish between low- and high-spin iron sites in the two subcomplexes of the H-cluster. The experimental methods and spectral simulations were calibrated using synthetic model complexes with ligand variations and bound hydride species. Distinct X-ray spectroscopic signatures of electronic excitation or decay transitions in [4Fe4S]H and [2Fe]H were obtained, which were quantitatively reproduced by density functional theory calculations, thereby leading to specific H-cluster model structures. We show that iron-hydride bonds are absent in the reduced state, whereas only in the super-reduced state, ligand rotation facilitates hydride binding presumably to the Fe-Fe bridging position at [2Fe]H. These results are in agreement with a catalytic cycle involving three main intermediates and at least two protonation and electron transfer steps prior to the H2 formation chemistry in [FeFe]-hydrogenases.