Activation of small molecules by P–N(imido)–P pincer complexes of transition metals

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The catalytic activation of small molecules in combination with the activation and cleavage of element–H bonds is one important challenge in chemistry. Principles of organometallics and coordination chemistry are therefor particularly claimed. In focus of this project is the development of transition metal complexes with novel chelating pincer ligands that generally confer kinetic and thermal stability to the corresponding complexes. A productive metal ligand cooperation and therefor a high reactivity of pincer complexes in element–H activations can be achieved by the incorporation of functionalities that are capable of accepting, storing and donating protons or hydrogen atoms to the position where they are needed. To combine a productive metal ligand cooperation with the trans-labilizing effect of strong donors, P–C(carbene)–P complexes were developed. However, the application of these complexes in catalysis is limited by mg-scale synthesis as well as deactivation processes. This project presents a novel approach to circumvent these limitations by the combination of phosphorous donors and an imido functionality. The application of an imido bond between the ligand and the transition metal center will be achieved by the development of a :N–CH(~PR2)2 ligand system. The changed structure of the chelating backbone will allow an accurate adjustment of the donor properties of the N- and P-functions. These well adjusted ligand systems will be coordinated to early as well as late transition metal centers in this project. The synthetic concept for P–N(imido)–P ligands allows versatile substitution patterns and synthetic pathways. The synthetic approach to such P–N(imido)–P complexes by N–H activation is estimated to allow moderate conditions for the synthesis. In addition to the synthesis of P–N(imido)–P transition metal complexes the characterization of those complexes is in focus of this project. Therefore IR, NMR and EPR spectroscopic methods as well as mass spectrometry and X-ray diffraction will be applied. The insights gained by the spectroscopic and structural characterization of P–N(imido)–P transition metal complexes will be supplemented by reactivity studies. Therefor reactions of P–N(imido)–P transition metal complexes with CO provide a detailed investigation of the nature of the M=N bond by using the CO as trans-arranged indicator ligand. The application of P–N(imido)–P complexes is another central aspect of this project. At first the capability of P–N(imido)–P complexes for bond activations will be investigated by test reactions with amines, terminal alkynes and dihydrogen. These investigations will be used later on for the development of a catalytic application of P–N(imido)–P complexes.