Designing molecular catalysts for proton reduction and water oxidation
Abstract
Chemical energy conversion has become an important research field in times of increasing energy demand and the need for sustainable alternatives to fossil fuels. This project focuses therefore on a) biomimetic proton reduction catalysts with tethering ligands promoting restricted rotation and b) development of water oxidation catalysts for both chemical and photo-induced reduction.
a) The chemistry of low-valent Fe compounds as models of the H2ase active sites has reached a remarkable degree of maturity over the last decade. Ligands to catalytic metal centres in enzymes are held in place through hydrogen bonding that keeps the system in a non-relaxed, higher energy state. Ligand re-organizations are prevented, which enables turnover frequencies exceeding 10.000 s-1. In contrast, synthetic catalysts that are in solution without any additional degree of organization will always relax to a thermodynamic minimum configuration. The most prominent example for this behavior in the context of [FeFe] H2ase mimics is the rearrangement of a terminal hydride which is the kinetic protonation product, to a bridging position. From a catalysis perspective, this rotation is highly unfavorable as the terminal hydride is more nucleophilic and reduced at milder potential than the bridging hydride. At present, no system has ever been presented that addresses this issue in a catalysis context. This project focuses therefore on the design of compounds holding rigid ligands which might prevent the free rotation of the ligands and thereby stabilize the terminal hydride. Preliminary DFT calculations (BP86) showed indeed a trend of increased rotation barriers when bulky tethered ligands are installed to the catalyst. Based on these initial results, we want to compare possible structures of further model compounds and calculate transition states of the ligand rotation.
b) Only few examples for functional water oxidation catalysis have been presented so far in the literature. This project focuses therefore on the identification of molecular catalyst motifs based on non-precious metals such as iron and cobalt which catalyze water oxidation both chemically and photo-induced. DFT (B3LYP) will be employed to optimize the geometry of small molecules in order to predict possible binding sites and thereby postulate a probable reaction mechanism.
