Main Group Metal Mediated Hydrogenation Reactions and Catalysis

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The catalytic hydrogenation of unsaturated compounds using molecular dihydrogen (H2) as a clean reducing agent is one of the most important reactions in petroleum and chemical industry as well as organic synthesis. Most hydrogenation catalysts are derived from precious transition-metals that are often toxic and bio-incompatible. Recently, there is an increasing interest in more environment-friendly and cheaper alkaline and alkaline earth metal hydride catalysts that are usually found as dimers and even higher oligomers. Remarkable main-group-metal catalysts were reported for the hydrogenation of activated conjugated alkenes and even unactivated 1-alkenes as well as aldimines (PhHC=NR, R = tBu, iPr, Ph) under mild conditions. However, the detailed catalytic mechanism and especially the potential (metal) cooperative and (ligand) steric effects for understanding such catalytic systems often remain unclear. Very recently, together with Prof. Douglas W. Stephan's experimental group, we have demonstrated that dimeric alkaline metal complexes M2L2 (M = Li, Na, K; L = tBu2P, NCy2), K2L2 (L = N(SiMe3)2) and (C6H5CH2K)2 can facilitate reversible H2-activation and even reactions with a CO/H2 mixture to form new C-C chain and C-H bonds. Because of the complicated electronic structure of large metal hydride catalysts, additional interactions with unsaturated substrate and solvent molecules as well as potentially extensive exploration of reaction paths is required, detailed mechanistic studies of such catalytic hydrogenation reactions are computationally challenging. In this project, based on the efficient GFN2-xTB method (conformational and reaction paths exploration), dispersion-corrected low-cost DFT (fast structure screening), and high-level dispersion-corrected DFT methods (accurate structures and energies, highly correlated DLPNO-CCSD(T) as benchmark when necessary), a multi-level theoretical approach is adopted to efficiently explore various reaction paths, in order to identify the catalytic mechanisms in great detail. Through close interplay with experiment, this project aims at a deep mechanistic understanding of hydrogenation reactions catalyzed by main group metal catalysts as well as a rational design of environment-friendly and cheap new-generation hydrogenation catalysts.