Grignard reagents (RMgX) and compounds such as methyllithium (MeLi) are extremely useful in organic synthesis. The corresponding species of heavier metals, such as calcium, have been much more challenging to synthesize due to the low reactivity of the metal and the high reactivity of the products. The reactions usually involve stabilization with bulky ligands. The synthesis of simple alkylcalcium species without such ligands has been difficult.

Reiner Anwander and colleagues, University of Tübingen, Germany, have synthesized dimethylcalcium ([CaMe₂]_n). The team used a salt metathesis of calcium bis(trimethylsilyl)amide [Ca{N(SiMe₃)₂}₂]₂ with methyllithium to give the desired product as a white amorphous precipitate. To avoid impurities, the researchers developed a method to reduce the chloride content of commercially available MeLi solutions. They used K[N(SiMe₃)₂] to remove the chloride as KCl.

The dimethylcalcium product could be crystallized from chilled tetrahydrofuran (THF) solutions as the heptametallic adduct [(THF)₁₀Ca₇Me₁₄]. In spite of the fact that [CaMe₂]_n slowly decomposes in solution, the researchers were able to perform reactions with it at low temperatures and with short reaction times.The team converted the compound to iodo complexes and to complexes with the bulky ligand Tp^tBu,Me (Tp^tBu,Me = tris(3-tBu-5-Me-pyrazolyl)borate). Dimethylcalcium could also be used in Grignard-type reactions to transfer methyl groups.

• Dimethylcalcium,
  Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, Reiner Anwander,
  J. Am. Chem. Soc. 2018.
  https://doi.org/10.1021/jacs.7b12984