Metastable solutions of Magnesium(I) halides and their manyfold reaction potential

• Applicant: Professor Dr. Hansgeorg Schnöckel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2013 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 238026424

In the past two decades we have investigated a sophisticated experimental method for the synthesis of metastable Al(I)- and Ga(I) halide solutions: the trapping of high-temperature molecules AlX / GaX. The groundbreaking results of these investigations were considered as highlights of fundamental research (c.f. textbooks: e.g. Al4Cp*4, Al4Br4, Al22Cl20) and in the field of nanomaterials (e.g. Al77R20, Ga84R20, superconductivity). In submitting this project we intend to initiate the access to a similar variety of subvalent chemistry with the focus on the neighboring element magnesium; i.e. starting from gaseous, radical MgX high-temperature molecules (X = halogen) - Mg+ is isoelectronic to the Na atom - new molecules with MgMg bonds shall be synthesized. Thus, this project is opening a further field of a novel Mg chemistry, which started about 5 years ago with a, however unprecedented, singular study reported by our Australian colleagues in a highly regarded Science contribution in 2007. However, the then presented classical synthesis of two molecules with one MgMg bond each (RMgMgR) is limited to a particular bond type with chelated, i.e. stabilized magnesium(I). In the last year, we have solved a main problem of this project; extensive preliminary spectroscopic, thermodynamic and quantumchemical investigations resulted in the generation of gaseous radicalic Mg(I) halides at about 1100 K and their quenching in metastable, cooled solutions: MgBr molecules are formed together with a small amount of gaseous MgBr2, if HBr is led over solid MgB2 at about 1100 K. As a further aspect of our project it could be proved experimentally that - as often postulated before - Mg(I) halides are involved in the synthesis of Grignard compounds; thus a problem of long standing in organic chemistry could be solved, as just 100 years ago V. Grignard was awarded the Nobel Prize for discovering the compounds named after him. Despite this essential success - i.e. the synthesis of metastable MgBr solutions - a) the isolation of defined subhalides from such donorstabilized solutions, b) their use as a reducing agent and c) their substitution by appropriate ligands (e. g. Cp*-, N(SiMe3)2-, PR2-) is especially challenging, as metastable MgX solutions will disproportionate already at T>-40°C: 2 MgX = Mg + MgX2. In addition of these main aspects of the project, namely, the synthesis and characterization of small subvalent compounds such as (MgCp*)n molecules which are expected to display unusual bonding, our final aim is to obtain metalloid Mg clusters (MgnRm with n > m) in analogy to the Al50Cp*12 cluster.

• DFG Programme: Research Grants