Activation of small molecules by P–N(imido)–P pincer complexes of transition metals
Final Report Abstract
The catalytic activation of small molecules and element–H bonds is one important challenge in chemistry. The current project was aiming at improvements in this field. To extend the boundaries of homogeneous catalysis in bond breaking and forming processes, various strategies, based on organometallic chemistry, can be applied as precious tools to extend the boundaries beyond classic systems. Initially, the development of a new ligand class was proposed to improve catalytic bond activating and forming processes by metal ligand cooperativity, MLC. Due to the lack of knowledge about the behavior of P–Nimido–P ligand systems in such processes, especially involving the cleavage and transfer of dihydrogen, we decided to focus the research first on a deep understanding of fundamental steps – the formation of weak N–H bonds during homogeneous catalyzed ammonia formation. The applied thermochemical approach finally leads to improvements in homogeneous catalyzed ammonia synthesis. As one of the largest industrial transformations, the Haber-Bosch reaction, interconverts dinitrogen and dihydrogen to ammonia, sustaining approximately 50 % of the global population and accounting for over 60 % of the N-atoms in the human body. While current Haber-Bosch technology is 75 % efficient, far surpassing nitrogenase enzymes and molecular catalysts, the carbon footprint is substantial – approximately 600 kg of CO2 is produced per 1000 kg of NH3. For ammonia synthesis compatible with renewable, CO2-free hydrogen, conducted in batch, new catalytic methods are required. Here we describe the development of a new approach to ammonia synthesis using proton coupled electron transfer (PCET), enabled by visible light, that can essentially be considered as free energy source. Our approach is founded in thermochemistry; by understanding the strengths of N–H bonds of various nitrogen-containing ligands, ammonia producing systems can be rationally designed. Because N–H bond dissociation free energies (BDFEs) of many intermediates on the way to ammonia are well below the thermodynamic threshold associated with H2 activation, excited-state PCET is an extremely valuable concept where the additional energy is provided by visible light.
Publications
- “Evaluation of Excited State Bond Weakening for Ammonia Synthesis from a Manganese Nitride: Stepwise Proton Coupled Electron Transfer is Preferred over Hydrogen Atom Transfer” Chem. Commun. 2019, 55, 5595-5598
Loose, F.; Wang, D.; Tian, L.; Scholes, G. D.; Knowles, R. R.; Chirik, P. J.
(See online at https://doi.org/10.1039/c9cc01046g) - “N–H Bond Formation in a Manganese(V) Nitride Yields Ammonia by Light-Driven Proton-Coupled Electron Transfer” J. Am. Chem. Soc. 2019, 141, 4795-4799
Wang, D.; Loose, F.; Chirik, P. J.; Knowles, R. R.
(See online at https://doi.org/10.1021/jacs.8b12957)
