Functional Light-Responsive Metal Carbonyl Systems

The formation of metal carbonyls represents some of the earliest observations of organometallic chemistry. Metal interactions with carbon monoxide play a central role in the fields of catalysis and bioorganometallic research including medicinal inorganic chemistry. In the present project, we will focus on the photoreactivity of novel metal carbonyl systems with improved light- responsive properties. One major goal of the current research proposal is the synthesis of water soluble derivatives of light-controlled organometallic compounds. Furthermore, systematic studies will be carried out to provide photoresponsive carbonyl systems that can be controlled by red-light or near infrared light (NIR) excitation. Potential applications of these novel compounds for the design of different photocatalytic processes will be explored in detail. These efforts will include the characterization of NIR-sensitive systems for light-controlled CO- release (photo-CORMs), novel organometallic multielectron transfer catalysts for the investigation of photoinduced CO 2 -reduction in homogeneous solution, and polynuclear compounds for bio-inspired H 2 -photogeneration based on water soluble metal carbonyl complexes.

Coordination compounds containing at least one directly connected metal-carbon bond are classified as metal-organic systems (organometallics). Among the oldest representatives of this class of compounds are carbonyl complexes of the transition metals, where C-bound carbon monoxide (CO) is present as a ligand. The CO-molecule is characterized by prominent s-donor and p-acceptor properties and therefore can act as a strongly coordinating ligand. It imposes a large ligand-field splitting effect and thus can serve for stabilizing metal centers in low oxidation states, and also for inducing low-spin electronic structures. Due to these effects, the properties of metal-carbonyl complexes are frequently playing a central role in certain branches of chemistry and homogeneous catalysis. More recently, it has also been discovered that the unique features of organometallics as catalysts including certain carbonyl complexes of first-row transition metals (eg. Fe, Co, Ni carbonyl compounds) are systematically employed in natural enzymatic systems. This branch of bio-organometallic chemistry today represents a vibrant and rapidly growing research field. Within the framework of the FWF-project Functional Light-Responsive Metal Carbonyl Systems it was attempted to mimic the reactivity properties of different naturally occuring CO-complexes by means of photochemistry and photocatalysis using synthetic organometallic compounds. For this purpose, first of all very stable, intensely-coloured and in some cases also highly water-soluble and biocompatible carbonyl-complexes had to be prepared. Structures, spectroscopic properties and light-dependent reactivity of the new compounds were characterized in detail. In the further course of these investigations, several novel photochemically and catalytically active metal-organic systems could be introduced. Their potential applications in three different research fields were examined. In summary, new systems for the biomimetic activation of small molecules, the controlled release of physiologically relevant compounds and for the photochemical conversion and storage of solar energy were described. For example, in order to reach the goal of photocatalytic hydrogen production driven by sunlight, a completely new class of iron-carbonyl-based hydrogenase enzyme model compounds could be developed. Moreover, several new catalysts for the reduction of CO have been reported. Finally, an unprecedented biocompatible model system with light-dependent heme-oxygenase function suitable for phototherapeutic purposes could be developed, which can be utilized for the controlled release of small amounts of the gaseous signalling molecule carbon monoxide (CO) wihtin life cells under physiologically relevant conditions.