Electron-rich phosphines, switchable lewis (super)acids and cooperative metal complexes: new prospects for the activation of CO2, N2, H2, N2O and SO2 and their utilization as synthetic building blocks

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The activation of small molecules such as CO2, N2, H2, N2O and SO2 for energy-efficient chemical transformations is a key challenge of current research. The present project is concerned with the development of new methods for the utilization of these molecules as synthetic building blocks and for their transformation into fuels for chemical energy storage. In our previous studies we showed that very electron-rich phosphines PR3 were obtained by attaching strong donor substituents R (R = imidazolin-2-ylidenamino) at the phosphorus atom. The resulting phosphines are powerful ligands in homogeneous catalysis and they form reversible CO2 adducts with low energy barriers. Based on these results, the synthesis of new donor substituents R with optimized properties (photoswitchability, water stability, variable steric and electronic properties) is proposed in the present project. These R substituents constitute the common synthetic building blocks in the following three subprojects: In subproject 1 the activation of CO2 and SO2 with uncharged strong Lewis bases is proposed targeting towards the Lewis base-catalyzed functionalization of CO2 and the controlled release of SO from SO2 under mild conditions. In this respect, phosphines, phosphine oxides and guanidine bases with variable steric and electronic properties will be developed, and their ability to form CO2 and SO2 adducts will be investigated. Subproject 2 is concerned with the activation of H2, N2O, CO2 and N2 using novel Lewis (super)acids with switchable Lewis acidity. In this context, sterically protected, trigonal planar phosphorus and boron cations will be prepared. These Lewis acids comprise the feature that their Lewis acidity can be increased by irradiation with UV light or by protonation. The goal is to activate N2O, CO2 and SO2 by complexation to the Lewis acids and to enables their subsequent reaction with weakly nucleophilic substrates such as alkenes, alkines or aromatic compounds. Liberation of the reaction products from the Lewis acid shall be accomplished by switching of the Lewis acidity. In subproject 3 the cooperative activation of N2 and H2 at Cu, Au, Ru and Fe complexes is proposed. Specific ligand design and the generation of polymetallic structures shall allow for the isolation of N2 complexes with the coinage metals. A second focus lies on the catalytic hydrogenation of N2 to form NH3 using Ru and Fe pincer complexes.