NMP makes the difference - facilitated synthesis of [FeFe] hydrogenase mimics.

To overcome the limitations of high reaction temperatures and long reaction times of conventional synthesis routes towards [FeFe] hydrogenase (H2ase) mimicking complexes, we introduced a more efficient synthesis route in the presence of aprotic polar co-solvents such as N-methyl-2-pyrrolidone (NMP). Versatile (di)thiol or disulfide ligands as well as selenium and tellurium analogues were converted to their corresponding complexes. While both reaction times and temperatures were reduced significantly, yields could be increased. Intensive kinetic monitoring of the formation of two [FeFe] H2ase mimics via UV-vis spectroscopy was performed, revealing an increase of the rate constant by one order of magnitude compared to that obtained in the same reaction without NMP. IR spectroscopic examination of the formation of the 1,3-propandithiole analogue (2a) revealed the appearance of a side product, analyzed by IR and UV-vis spectroscopy and mass spectrometry, which was proposed to be a NMP monosubstituted triirondodecacarbonyl (Fe3(CO)11NMP) cluster. Reacting triirondodecacarbonyl (Fe3(CO)12) with NMP in the absence of any additional ligand yielded this species as well. Quantum chemical simulations of Fe3(CO)11NMP indicated structural rearrangements including the omission of bridging carbonyls (µ-CO). Similar observations were made on utilizing other aprotic polar co-solvents.

Covalent Functionalization of CdSe Quantum Dot Films with Molecular [FeFe] Hydrogenase Mimics for Light-Driven Hydrogen Evolution.

CdSe quantum dots (QDs) combined with [FeFe] hydrogenase mimics as molecular catalytic reaction centers based on earth-abundant elements have demonstrated promising activity for photocatalytic hydrogen generation. Direct linking of the [FeFe] hydrogenase mimics to the QD surface is expected to establish a close contact between the [FeFe] hydrogenase mimics and the light-harvesting QDs, supporting the transfer and accumulation of several electrons needed to drive hydrogen evolution. In this work, we report on the functionalization of QDs immobilized in a thin-film architecture on a substrate with [FeFe] hydrogenase mimics by covalent linking via carboxylate groups as the anchoring functionality. The functionalization was monitored via UV/vis, photoluminescence, IR, and X-ray photoelectron spectroscopy and quantified via micro-X-ray fluorescence spectrometry. The activity of the functionalized thin film was demonstrated, and turn-over numbers in the range of 360-580 (short linkers) and 130-160 (long linkers) were achieved. This work presents a proof-of-concept study, showing the potential of thin-film architectures of immobilized QDs as a platform for light-driven hydrogen evolution without the need for intricate surface modifications to ensure colloidal stability in aqueous environments.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic.

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 104-106. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer is observed in the absence of a sacrificial agent or protons which, under real catalytic conditions, would quench remaining holes or could stabilize the charge separation, respectively.

Synthesis of xylan carbonates - An approach towards reactive polysaccharide derivatives showing self-assembling into nanoparticles.

Xylan phenyl carbonate (XPC) derivatives were prepared and characterized comprehensively. By conversion of xylan with phenyl chloroformate either in dipolar aprotic solvents with LiCl or in an ionic liquid, XPC with degrees of substitution (DS) of up to 2.0, i.e., fully functionalized derivatives, could be obtained. The synthesis was studied with respect to the influence of different reaction parameters. It was found that the reaction medium as well as the type of starting xylan strongly affected the efficiency of the derivatization. The derivatives obtained were characterized by FT-IR- and NMR spectroscopy. Surprisingly, it was found that C-3 is the most reactive position in this particular reaction while substitution in position C-2 only occurred if the neighboring position C-3 already carried a phenyl carbonate group. XPC were found to form spherical nanoparticles (NP) of well-defined shape with diameters around 158 nm. These materials possess unique potential as activated NP for advanced applications.