Sensitized Photochemical CO2 Reduction by Hetero-Pacman Compounds Linking a ReI Tricarbonyl with a Porphyrin Unit

The hetero-Pacman architecture places two different metal coordination sites in close proximity, which can support efficient energy and/or electron transfer and allow for cooperative activation of small molecules. Here, the synthesis of dyads consisting of a porphyrin unit as photosensitizer and a rhenium unit as catalytically active site, which are held together by the rigid xanthene backbone, is presented. Mononuclear [(NN)Re(CO)₃(Cl)] complexes for CO₂ reduction in which NN represents a bidentate diimine ligand (e.g., bipyridine or phenanthroline) lack light absorption in the visible region, resulting in poor photocatalysis upon illumination with visible light. To improve their visible-light absorption, we have focused on the incorporation of a strongly absorbing free base or zinc porphyrin unit. Resulting photocatalytic experiments showed a strong dependence of the catalytic performance on both the type of photosensitizer and the excitation wavelengths. Most notably, the intramolecular hetero-Pacman system containing a zinc porphyrin unit showed much better catalytic activity in the visible region (excitation wavelengths >450 nm) than the free base porphyrin version or the corresponding mononuclear rhenium compound or an intermolecular system comprised of a 1:1 mixture of the mononuclear analogues.     

Improved Stability and Tunable Functionalization of Parallel β-Sheets via Multicomponent N-Alkylation of the Turn Moiety

In contrast to the myriad of methods available to produce α-helices and antiparallel β-sheets in synthetic peptides, just a few are known for the construction of stable, non-cyclic parallel β-sheets. Herein, we report an efficient on-resin approach for the assembly of parallel β-sheet peptides in which the N-alkylated turn moiety enhances the stability and gives access to a variety of functionalizations without modifying the parallel strands. The key synthetic step of this strategy is the multicomponent construction of an N-alkylated turn using the Ugi reaction on varied isocyano-resins. This four-component process assembles the orthogonally protected turn fragment and incorporates handles serving for labeling/conjugation purposes or for reducing peptide aggregation. NMR and circular dichroism analyses confirm the better-structured and more stable parallel β-sheets in the N-alkylated peptides compared to the non-functionalized variants.