The modularity and synthetic flexibility of metal–organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF–enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Water is known to play an important role for the crystallization and stability of Zr‐based metal‐organic frameworks (MOFs). This work investigates its effect on the vapor‐assisted conversion (VAC) synthesis of UiO‐67 MOF thin films on Au‐coated Si substrates. We demonstrate the equilibration processes taking place during the VAC procedure, confirming the gradual equilibration of all solutions upon heating. The presence of water affects the vapor phase composition but does not significantly impact the acetic acid equilibration rate. However, the preparation of UiO‐67 thin films by VAC is highly sensitive to the water content in the reaction. Some water is required for the formation of the zirconium clusters, but excessive water in the reaction vial yields poorly crystalline materials. Atmospheric water that is taken up by the vapor source can be sufficient to reduce crystallinity dramatically. This complication can be partially overcome by increasing the amount of acetic acid in the vapor source.

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O 2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5 OMe )Cl] + (Py5 OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy) 3 ] 2+ /S 2 O 8 2– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the M III /M II oxidation, but none was able to yield a detectable amount of a M IV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl – or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond M III . On that basis, design features for next-generation molecular WOCs are suggested.

We present the synthesis of the first mixed-ring rhenium fulvene sandwich complex, [Re(η 6 -C 5 H 4 CH 2 )(η 6 -C 6 H 6 )] + , from the respective carbinol precursor [Re(η 5 -C 5 H 4 CH 2 OH)(η 6 -C 6 H 6 )]. The demanding preparation on the basis of the reactive cross-conjugated π system of the fulvene ligand restricts the synthetic accessibility for such fulvene complexes, and the only pathways elaborated originate from the respective carbinols. In contrast to related systems, a suitable rhenium-containing precursor did not exist hitherto. Recently, we described the synthesis of the mixed-aromatic complex [Re(η 5 -C 5 H 4 CHO)(η 6 -C 6 H 6 )] which gave access to the carbinol complex [Re(η 5 -C 5 H 4 CH 2 OH)(η 6 -C 6 H 6 )] and the title compound, both described herein. With [Re(η 6 -C 5 H 4 CH 2 )(η 6 -C 6 H 6 )] + in hand, the susceptibility of the exocyclic methylidene group of the coordinated pentafulvene to nucleophilic attacks was investigated with a variety of Lewis bases (hydride, cyanide, amide, alkoxide, thiolate, and phosphine moieties). The characteristic NMR pattern and Xray crystal structures of [Re(η 6 -C 5 H 4 CH 2 )(η 6 -C 6 H 6 )] + and postfunctionalized [Re(η 5 -C 5 H 4 CH 2 R)(η 6 -C 6 H 6 )] complexes are presented to confirm their authenticities.