Synthetic [FeFe] Hydrogenase Active Site Model Complexes

Abstract: [FeFe]-Hydrogenases (H2ases) are metalloenzymes that can catalyze the reversible reduction of protons to molecular hydrogen as part of the metabolism of certain cyanobacteria and green algae. Due to the low availability of the enzyme, synthetic complexes that mimic the natural active site in structure, function and activity are highly sought after. In this thesis, a number of [FeFe]-H2ases active site model complexes were synthesized to answer open questions of the active site and to develop unprecedented bio-inspired proton reduction catalysts.The first part describes the synthesis and the protonation properties of a [Fe2(μ-adt)(CO)4(PMe3)2] (adt = azadithiolate) complex which contains two basic sites that are similar to those found in the enzyme active site. Unusual kinetic factors give rise to four discrete protonation states. The twofold protonated state is the first model complex that simultaneously carries a proton at the azadithiolate nitrogen and a bridging hydride at the Fe-Fe bond.In the second part, a model complex with an unprecedented amine ligand was synthesized and studied. In analogy to the enzyme active site, the labile amine ligand is expelled after electrochemical reduction.The third part describes a series of model complexes with electronically different aromatic dithiolate ligands. It is demonstrated in one case that the tuning of the ligand by electron-withdrawing substituents results in proton reduction catalysis at an overpotential that is lower than that required by the non-substituted parent compound.The design and the synthetic work towards a new ruthenium-diiron dyad for light-driven hydrogen production are presented in the fourth part.In the final part, differently isotope-labelled mixed valent Fe(I)-Fe(II) model complexes were synthesized, in particular the unprecedented 15N labelled analogue, with the aim to provide EPR-spectroscopic references that will allow the elucidation of the nature of the central atom in the dithiolate bridge of the [FeFe] hydrogenase active site.