The complexity and energy demand of the H 2 O oxidation half-reaction represents the bottleneck for the development of sustainable, environmentally friendly H 2 economies using H 2 O as energy feedstock. In Nature, photosynthetic H 2 O oxidation processes occur in photosystem-II (PS-II) and are facilitated by the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn 4 CaO 5 } with cubane-like topology. In recent years, the use of manganese-based H 2 O oxidation catalysts has attracted significant scientific attention, not only to mimic and understand naturally occurring processes, but also due to the low toxicity and high abundancy of Mn in the Earth's crust. Here we report the catalytic H 2 O oxidation activity at pH 7.2 of a high-nuclearity manganese-oxo cluster. The species, which contains multiple cubane motifs and which is stabilized by redox-active aromatic organic ligands, gives rise to an onset overpotential as low as 255 mV when dispersed in a carbon paste matrix, achieving high current densities of 10 mA cm −2 and even 100 mA cm −2 at η = 482 and 654 mV, respectively. The electrodes show good stability under turnover conditions for 7 h. Additionally, direct light-induced O 2 evolution measurements confirm a reaction rate of 0.72 s −1 and turnover number (TON) of up to 55. The outlined experimental concept demonstrates how a synergistic effect between non-innocent, redoxactive organic ligands and bioinspired Mn oxo-clusters resembling the natural {Mn 4 CaO 5 } unit, which are dispersed in a conductive carbon matrix and protected by a Nafion membrane, can facilitate remarkably high catalytic activity under neutral, environmentally friendly pH conditions.

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called ‘rod MOFs’, which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand’s carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

The water oxidation half-reaction is considered the bottleneck in the development of technological advances to replace fossil fuels with sustainable and economically affordable energy sources. In natural photosynthesis, water oxidation occurs in the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn4CaO5} with a cubane-like topology that is embedded within a redox-active protein environment located in photosystem II (PS II). Therefore, the preparation of biomimetic manganese-based compounds is appealing for the development of efficient and inexpensive water oxidation catalysts. Here, we present the water oxidation catalytic activity of a high-nuclearity mixed-metal manganese-strontium cluster, [MnIII12MnII6Sr(μ4-O8)(μ3-Cl)8(HLMe)12(MeCN)6]Cl2∙15MeOH (Mn18Sr) (HLMe = 2,6-bis(hydroxymethyl)-p-cresol), in neutral media. This biomimetic mixed-valence cluster features different cubane-like motifs and it is stabilized by redox-active, quinone-like organic ligands. The complex displays a low onset overpotential of 192 mV and overpotentials of 284 and 550 mV at current densities of 1 mA cm−2 and 10 mA cm−2, respectively. Direct O2 evolution measurements under visible light-driven water oxidation conditions demonstrate the catalytic capabilities of this cluster, which exhibits a turnover frequency of 0.48 s−1 and a turnover number of 21.6. This result allows for a direct comparison to be made with the structurally analogous Mn-oxo cluster [MnIII12MnII7(µ4-O)8(µ3-OCH3)2(µ3-Br)6(HLMe)12(MeOH)5(MeCN)]Br2·9MeCN·MeOH (Mn19), the water oxidation catalytic activity of which was recently reported by us. This work highlights the potential of this series of compounds towards the water oxidation reaction and their amenability to induce structural changes that modify their reactivity.