Are metalloid clusters essential intermediates during the dissolution process of metals in general and, in special, of magnesium during the formation of Grignard Reagents (GR)?

• Applicant: Professor Dr. Hansgeorg Schnöckel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324039962

A prominent example for the dissolution process of a base metal is the formation of the Grignard Reagent (GR): Mg(solid) + RX --> RMgX (X= halide, R= e.g. alkyl). Based on experimental findings MgX/ MgR radicals were postulated in the reaction mixture. This conclusion is reflected in the description within the textbooks up to now: Mg(I) species (e.g. MgCl) are essential intermediates during the formation process of GR. In a recently published article in Chem. Sciences (this result was just presented as a highlight in Nachrichten aus der Chemie) we could show, that not MgX/MgR radicals like postulated so far, but metalloid Mg clusters (e.g. MgnXm, n>m) are essential intermediates during the formation of GR. Because of the very fast reaction of the dissolution process of base metals it was not possible so far to trap an intermediate as a snapshot on this route and to determine the crystal structure. In order to reach this goal, which however will be a dream of future for the direct detection of intermediates during the dissolution process, we decided for an alternative, however experimentally sophisticated, way, which we successfully proceeded for e.g. Al in the last two decades: At about 1000°C MgBr molecules are generated in the gas phase which are subsequently trapped at -196°C and afterwards stored as a metastable solution below -40°C. During the controlled disproportionation and the simultaneous substitution of the halide by the Cp* moiety the metalloid cluster Mg16Cp*8Br4K is formed. This formation process is highly sensitive because the disproportionation reaction of MgBr to solid Mg and MgBr2 is much faster than the substitution reaction. Nevertheless we were now successful via this route to trap the first intermediate between solid Mg and e.g. MgBr2/MgR2: The generated Mg16 cluster has been detected with the help of the ESI procedure in the high resolution FT-ICR spectrum. In the applied project we want to get crystals for the structure determination of the Mg16 and further metalloid clusters for the first time in order to investigate a new territory with the help of synthetic as well as with DFT methods of calculated nanoscaled metalloid Mg clusters as molecular models. The expected results will sustainably modify the description within textbooks of a simply seeming however highly complex but fundamental chemical reaction.

• DFG Programme: Research Grants