A molecular proton reduction catalyst [FeFe](dcbdt)(CO)6 (1, dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) with structural similarities to [FeFe]-hydrogenase active sites has been incorporated into a highly robust Zr(IV)-based metal–organic framework (MOF) by postsynthetic exchange (PSE). The PSE protocol is crucial as direct solvothermal synthesis fails to produce the functionalized MOF. The molecular integrity of the organometallic site within the MOF is demonstrated by a variety of techniques, including X-ray absorption spectroscopy. In conjunction with [Ru(bpy)3]2+ as a photosensitizer and ascorbate as an electron donor, MOF-[FeFe](dcbdt)(CO)6 catalyzes photochemical hydrogen evolution in water at pH 5. The immobilized catalyst shows substantially improved initial rates and overall hydrogen production when compared to a reference system of complex 1 in solution. Improved catalytic performance is ascribed to structural stabilization of the complex when incorporated in the MOF as well as the protection of reduced catalysts 1– and 12– from undesirable charge recombination with oxidized ascorbate.

Photosynthesis is performed by a multitude of organisms, but in nearly all cases, it is variations on a common theme: absorption of light followed by energy transfer to a reaction center where charge separation takes place. This initial form of chemical energy is stabilized by the biosynthesis of carbohydrates. To produce these energy-rich products, a substrate is needed that feeds in reductive equivalents. When photosynthetic microorganisms learned to use water as a substrate some 2 billion years ago, a fundamental barrier against unlimited use of solar energy was overcome. The possibility of solar energy use has inspired researchers to construct artificial photosynthetic systems that show analogy to parts of the intricate molecular machinery of photosynthesis. Recent years have seen a reorientation of efforts toward creating integrated light-to-fuel systems that can use solar energy for direct synthesis of energy-rich compounds, so-called solar fuels. Sustainable production of solar fuels is a long awaited development that promises extensive solar energy use combined with long-term storage. The stoichiometry of water splitting into molecular oxygen, protons, and electrons is deceptively simple; achieving it by chemical catalysis has proven remarkably difficult. The reaction center Photosystem II couples light-induced charge separation to an efficient molecular water-splitting catalyst, a Mn(4)Ca complex, and is thus an important template for biomimetic chemistry. In our aims to design biomimetic manganese complexes for light-driven water oxidation, we link photosensitizers and charge-separation motifs to potential catalysts in supramolecular assemblies. In photosynthesis, production of carbohydrates demands the delivery of multiple reducing equivalents to CO(2). In contrast, the two-electron reduction of protons to molecular hydrogen is much less demanding. Virtually all microorganisms have enzymes called hydrogenases that convert protons to hydrogen, many of them with good catalytic efficiency. The catalytic sites of hydrogenases are now the center of attention of biomimetic efforts, providing prospects for catalytic hydrogen production with inexpensive metals. Thus, we might complete the water-to-fuel conversion: light + 2H(2)O --> 2H(2) + O(2). This reaction formula is to some extent already elegantly fulfilled by cyanobacteria and green algae, water-splitting photosynthetic microorganisms that under certain conditions also can produce hydrogen. An alternative route to hydrogen from solar energy is therefore to engineer these organisms to produce hydrogen more efficiently. This Account describes our original approach to combine research in these two fields: mimicking structural and functional principles of both Photosystem II and hydrogenases by synthetic chemistry and engineering cyanobacteria to become better hydrogen producers and ultimately developing new routes toward synthetic biology.

Good potential: A biomimetic model of the iron hydrogenase active site functions as a catalyst for electrochemical proton reduction at relatively moderate negative potentials. The catalytic cycle (see scheme) involves the protonation of the azadithiolate nitrogen atom as its initial step. This system may eventually be used to achieve light‐activated hydrogen evolution.

The first model of the iron hydrogenase active site has been prepared which concomitantly carries a proton and a hydride; the title species was characterized by IR and NMR spectroscopy and is reduced at more positive potential than any other mimic of this kind.

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal–organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.