Development of novel dinuclear molybdenum complexes with long-lived excited states for the photochemical N2 activation

• Applicant: Dr. Sascha Ossinger
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2021 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 460752300

The overall goal of this proposed project is the rational development of catalysts for the light-driven activation of metal-bound dinitrogen. To this end, the primary goal is the establishment of long-lived MLCT excited states on Mo-N=N-Mo dimers, from which photochemical N2 activation will occur. The growing world population combined with an increase in the demand of food have led to greater importance in the production and use of nitrogenous fertilizers in recent decades. Therefore, industrial ammonia production via the HABER-BOSCH process, which consumes 1 2% of the world’s energy supply, is one of the most important large-scale industrial processes. Besides this, the synthetic nitrogen fixation on well-defined transition metal complexes as a further way to convert N2 to NH3 has developed considerably in the last years. In the field of synthetic nitrogen fixation, only a handful of characterized molecular systems are capable of photochemical or light-driven N2 activation. For some of these molecular compounds the mechanisms of thermal N2 splitting is understood in much detail. Whereas, the operating principles of light-driven N2 activation are still poorly understood. Therefore, if light-driven N2 splitting could ultimately be performed in catalytic fashion, this would have very important socio-economic implications and on a more fundamental level, the development of photoactive excited states on N2-bridged dimers will open up new possibilities in the area of light-driven N2 activation and inorganic photochemistry in general. Recently, CHIRIK and SCHOLES described a Mo-N=N-Mo dimer, and its photophysical properties provided the first direct insight into MLCT states that could be suitable for N2 splitting. This molecular compound exhibits only a very short excited-state lifetime, and therefore, photochemical reactions cannot compete with the rapid nonradiative deactivation of the 3MLCT state. Consequently, this proposed project aims to develop new N2-bridged Mo-N=N-Mo dimers with a coordination environment that combines a more idealized octahedral coordination geometry with a stronger ligand field resulting in long-lived 3MLCT excited states. These new compounds, including a variety of different combinations of equatorial chelate and axial monodentate ligands, will be synthesized and fully characterized. Furthermore, the excited states investigations and the light-driven N2-activation studies of the target complex will be performed using time-resolved UV-Vis and IR spectroscopy. In addition, all investigations in this project will be supported by computational studies. Finally, systems with long-lived enough excited states will be investigated in view of reductive 3MLCT quenching by sacrificial donors, which represents the key step towards the development of a photocatalyst for the light-driven formation of NH3 from N2.

• DFG Programme: WBP Fellowship
• International Connection: Switzerland
• Host: Professor Dr. Oliver S. Wenger