Synthesis and characterization of new pacman-type transition metal complexes and their application as nitrogenase mimics in the catalytic dinitrogen activation

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Today, the activation of dinitrogen is one of the most important reactions both for industry relevant syntheses and, in a societal point of view, to warrant food for a growing world population. In industry, the iron-catalyzed Haber-Bosch process is used to produce about 150 million tons of ammonia per year from dinitrogen and dihydrogen. However, this process requires a high amount of energy. The development of homogenous catalysts for converting dinitrogen at ambient conditions is therefore an important target of research today. However, it is also one of the most challenging problems in small molecule activation, which has fascinated many scientists for more than 50 years. Nevertheless, this research field is still rather poorly explored and only two homogeneous systems are known up to date, which are able to reduce dinitrogen to ammonia under catalytic conditions. The primary object of the research plan is the synthesis of novel bimetallic complexes with macrocyclic pacman-type ligands. Afterwards, these complexes are examined towards their application as catalysts for the reduction of dinitrogen. First of all, the calixpyrrole ligands are going to be synthesized and subsequently converted with a variety of transition metal precursors, [MoCl3(thf)3] in thf. Molybdenum and iron should be the priority metals, because of the strong focus to nitrogenase enzymes. Calixpyrroles are a promising class of macrocyclic ligand which combines the coordinative properties with the exceptional design characteristics and synthetic versatility of Schiff-base condensation procedures that can assist in ligand synthesis and engender structural control in the resultant bimetallic complexes. Depending on the steric and electronic demands, there are many possibilities to vary both the size of the arene between the two imins and the nature of the R groups between the pyrrols. Additionally, different metals can be introduced into the pockets of the calixpyrrole ligand. The resultant bimetallic molecular cleft facilitates extensive stoichiometric and catalytic small-molecule chemistry. After a full characterization, the complexes are converted with sodium-amalgam under an atmosphere of dinitrogen. The catalytic potential in the activation of dinitrogen is investigated by slow addition of cobaltocene to reduce the dinitrogen to ammonia. Luthidinium triflate acts as a redox equivalent. The results will be compared to previous literature reports and the catalysts will be modified in terms of electronic and steric properties to improve the catalytic efficiency.