Many Mg-Mg bonds form the core of the Mg16Cp*8Br4K cluster anion: the key to a reassessment of the Grignard reagent (GR) formation process?

T Kruczyński; F Henke; M Neumaier; K H Bowen; H Schnöckel

doi:10.1039/c5sc03914b

Many Mg-Mg bonds form the core of the Mg₁₆Cp*8Br₄K cluster anion: the key to a reassessment of the Grignard reagent (GR) formation process?

Chem Sci. 2016 Feb 1;7(2):1543-1547. doi: 10.1039/c5sc03914b. Epub 2015 Nov 26.

Authors

T Kruczyński¹, F Henke¹, M Neumaier², K H Bowen³, H Schnöckel¹

Affiliations

¹ Karlsruhe Institute of Technology (KIT) , Institute of Inorganic Chemistry , Engesserstr. 15 , D-76131 Karlsruhe , Germany . Email: hansgeorg.schnoeckel@kit.edu.
² Karlsruhe Institute of Technology (KIT) , Institute of Meteorology and Climate Research - Atmospheric Trace Gases and Remote Sensing , H.-v.-Helmholtz-Platz 1 , 76344 Leopoldshafen , Germany.
³ Johns Hopkins University , Department of Chemistry , 3400 North Charles St. , Baltimore , MD 21218 , USA.

Abstract

It caused a sensation eight years ago, when the first room temperature stable molecular compound with a Mg-Mg bond (LMgMgL, L = chelating ligand) containing magnesium in the oxidation state +1 was prepared. Here, we report the preparation of a [Mg₁₆Cp*8Br₄K]^- cluster anion (Cp* = pentamethylcyclopentadiene) with 27 Mg-Mg bonds. It has been obtained through the reaction of KCp* with a metastable solution of MgBr in toluene. A highly-resolved Fourier transform mass spectrum (FT-MS) of this cluster anion, brought into vacuum by electrospraying its solution in THF, provides the title cluster's stoichiometry. This Mg₁₆ cluster together with experiments on the metastable solution of MgBr show that: during the formation process of GRs (Grignard reagents) which are involved in most of sophisticated syntheses of organic products, not the highly reactive MgBr radical as often presumed, but instead the metalloid Mg₁₆Cp*8Br₄ cluster anion and its related cousins that are the operative intermediates along the pathway from Mg metal to GRs (e.g. Cp*MgBr).