Bachelor Project: Daniël Preschel (0592412) Light-induced generation of hydrogen by supramolecular assemblies based on Fe2-Hydrogenase models Synthesis and characterization of new supramolecular linkers. Using hydrogen as a primary energy source seems a promising solution towards our dependence on classical fuels, and climate change as well. However this can only be considered durable if hydrogen is produced in the same manner, for example by solar power, being the most liable candidate supporting future large-scale applications (1). As quite often, nature is the main source of inspiration in chemistry: photosynthesis is carried out by plants; absorbing the energy of sunlight and storing it in a chemical form, as chlorophyll (a chromophore) captures photons and provides excited electrons that are used for the reduction of substrates. Also, researchers discovered the class of [Fe2] hydrogenase (H2-ase) enzymes, capable of catalyzing the reduction of protons into molecular hydrogen. In theory, combining these valuable properties into a single compound creates a photoactive catalyst, capable of generating hydrogen upon irradiation with visible light. Ever since the structure of the highly efficient enzyme has been determined in quite some detail, researchers have tried to produce synthetic H2-ase mimics. Literature has already reported photoactive artificial supramolecular complexes that are capable of generating hydrogen indeed (2). The active site is composed of 2 Fe atoms, coordinated by carbonyl ligands. In these experiments one of them is substituted by pyridyl-functionalized supramolecular linkers, which automatically attach to the chromophore (ZnTPP), see fig. 1. The self-assembly of the photo-sensitizer (PS) is a very convenient feature in this kind of research, as it facilitates the synthesis of many closely related complexes in an easy way. More recently, scientists have also coordinated an isocyanide ligand to the [Fe2] active site, more faithfully resembling the highly efficient H2-ase enzymes found in nature (3). Indeed, this system showed improved stability under actual photocatalysis conditions. This is a very stimulating result, as deactivation of the catalyst remains a huge challenge thus far. Inspired by the promising results found in literature, we present the synthesis and characterization of a small series of new supramolecular linkers, based on pyridyl-functionalized isocyanides. The new linkers are varied in length, and the position of the pyridine. This alters the distance and orientation of the chromophore towards the [Fe2] active centre, determining the spatial organization of the complete assembly. The goal was to relate the structural variations of these supramolecular complexes to their performance and stability upon photoreduction by means of reproducible photocatalysis experiments. We have also synthesized a new supramolecular phosphoramidite ligand, which has already proven to be very successful in producing H2. However, this type of ligand is very soluble in organic solvents, preventing crystallization. Therefore, we modified the ligand with an aromatic backbone, which is expected to improve the crystallization properties, enabling the possibility to gather more information about the structure of this compound. The synthesis of the new supramolecular linkers, and their [Fe2] complexes was successful, but turned out to be challenging as especially isocyanides are very difficult to handle. They were characterized by 1H- and 31P-NMR, and IR measurements, giving detailed information about their structure. Luminescence titration experiments monitored by UV-vis showed the quenching of fluorescent intensity of the chromophore, upon association with the Fe2-complex. These measurements indicate the transport of excited electrons towards the [Fe2] catalytic centre, but were not performed accurately enough to determine the association constant between the [Fe2]-complex and ZnTPP. Also, the electrocatalytic reductions potentials of the new [Fe2] complexes were established by means of cyclic voltammetry. In the presence of acid, the current measured at the negative potential associated with the reduction of the catalytic centre gradually increased. This significant electrocatalytic response proves the [Fe2] phophoramidite complex is an active electrocatalyst indeed. However, no photocatalysis experiments have been carried out yet, due to set-up problems of the GC equipment. So more research is still needed to further investigate the photocatalytic properties of these supramolecular [Fe2] complexes. fig.1. - a. Supramolecular photocatalytic assembly based on an [Fe2]-H2-ase mimic, using a pyridine-substituted linker to connect ZnTPP (the PS) to the Fe2 core. Upon irradiation this assembly forms H2. b. The [Fe2]-H2-ase enzyme found in nature: the Fe2 core is coordinated by carbonyl and cyanide ligands, and electron transfer is facilitated by Ferredoxin. References: 1. N. Amaroli, V. Balzani. The Hydrogen Issue. ChemSusChem, 2011, 4, p.21-36. 2. A.M. Kluwer, R. Kapre, F. Hartl, M Lutz, A.L. Spek, A.M. Brouwer, P.W.N.M. van Leeuwen, J.H.N. Reek. Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for molecular hydrogen evolution. PNAS, 2009, 106. No.26. 3. F. Wang, W.G. Wang, X. Wang, H. Wang, C. Tung, L Wu. A highly Efficient Photocatalytic System for Hydrogen Production by a Robust Hydrogenase Mimic in an Aqueous Solution Angewandte Chemie. 2011, 50, p.1-6.