Main Group Metals in Oxidation State Zero for Bond Activation and Catalysis

forthcoming

Historically, main group metal organometallics are used as stoichiometric reagents in organic synthesis. This holds especially for highly polar s-block metal compounds like BuLi or Grignard’s. Only since the start of this century, it has been recognized that they also make for remarkably versatile catalysts. Parallel to these developments, the low-oxidation-state chemistry of the early main group metals emerged with the seminal isolation of first MgI complexes. Since 2007 numerous examples have been reported. First highly reactive complexes of an s-block metal in the formal zero-oxidation-state (ZOS) were reported by our group (“Strongly reducing magnesium(0) complexes”, Nature 2021, 592, 717). MM-ZERO aims to develop main group metal chemistry in the ZOS. In contrast to transition metals, for which there are many examples of TM(0) complexes, it is a bold challenge to isolate complexes of electron-rich main group metals. This holds especially for the electropositive s-block metals. Devoid of d-orbitals for backbonding, they easily oxidize. Building upon our preliminary work, we propose strategies to advance ZOS main group metal chemistry and push the frontiers of knowledge. Smart ligand design and the introduction of ball-milling are key to access hydrocarbon-soluble complexes with ZOS-metals of groups 1-3. While this first part is of fundamental nature, the second part of the project aims to activate main group metals for hydrogenation catalysis. Preliminary work shows that Ba activated by Metal-Vapour-Synthesis (MVS, i.e. cocondensation in an organic matrix) is an unexpectedly active alkene hydrogenation catalyst. Moreover, in combination with MVS-activated Fe, benzene can be even hydrogenated at 20 °C. Ba/Fe-cooperativity is demonstrated by the fact that both metals alone are not active. Combining abundant main group and 3d-TM’s, it is our ultimate goal to develop sustainable, highly active (de)hydrogenation catalysts for hydrogen storage applications.