High-Valent Metal Tetrapyrroles for Surface-Supported Catalysis

The macrocycle of tetrapyrroles provides one of nature`s most versatile coordination sites for catalytically active metal ions. Metallocorroles and porphyrins are the best known members of this class of naturally abundant catalytic molecules. The facile access, robustness, and easily tunable properties have established metallotetrapyrroles as key components in advanced systems like catalysts, therapeutics, in-vivo biomarkers, or dye-sensitized photovoltaic cells. Their characteristic ability of stabilizing metal ions in high oxidation states such as +III, +IV and +V bestows them selective catalytic activity for reactive pathways prospective for novel applications such as, e. g., tumor elimination via Mn(III), Fe(III), Au(III) or Ga(III) and atherosclerosis prevention. Recent liquid-phase investigations have indicated a high potential of metallocorroles for small-molecule activation (e. g. O2 and CO 2 ). The chemistry of corroles remained largely undeveloped for decades because of severe synthetic obstacles that were overcome not before the late 1990s and, so far, literally all studies have been conducted in liquid phase. Thus, the fundamental knowledge on the electronic properties, conformation and functionality of corroles supported on solid surfaces remains to be explored. Our proposal concerns the investigation, characterization and development of corrole-based agent systems supported by technologically relevant metal and semiconductor surfaces (Au, Si) for improved small-molecule activation. We focus on well- selected metallocorroles that are regarded as prospective catalytic molecules for the selective O2 and CO 2 reduction in the liquid phase. As a reference for comparison, we will simultaneously investigate the related porphyrin complexes. An improved catalytic performance is aspired by employing well-defined surface templates that assist not only the desired stabilization of high-valent central metal ions, but act as prospective technological platforms, as well. We shall comprehensively characterize the elementary properties and processes that determine specific catalytic activities. Our proposal is an interdisciplinary feedback-oriented collaboration between four complementary research groups from Austria and Germany with dedicated expertise in experimental and theoretical physics as well as synthetic and analytic chemistry. Based on already existing collaborations, we perform concerted experimental multi-technique investigations (low temperature scanning tunneling microscopy; photoelectron-, Xray absorption, magnetic-resonance- and optical spectroscopy; electrochemistry) in combination with state-of-the-art model calculations from first principles. Our long term vision is a control of functionalized metallocorroles and -porphyrins supported on technologically relevant surface templates for selective small- molecule activation.

We demonstrate the development of a novel catalyst capable of electrochemically generating dioxygen (O2) from water (H2O) and vice versa two key processes in fuel cell applications. This bi-functional property was studied by our research consortium in detail and the reaction kinetics of this catalysed chemical reaction was determined. Our catalyst is based on a metal-organic molecule named manganese-corrole and works with different technically relevant supporting materials, like e.g. silver or graphite. Our catalyst is highly stable and can be regenerated. The Austrian part of the project covers the synthesis, chemical and physical characterization of the catalyst, with such high sensitivity, that even single catalyst molecules can be investigated. The international project partners determined structural properties of only a few nanometres thin catalyst films on different supporting materials and provided essential theoretical contributions and simulation results for better understanding the catalysts mode of operation.