Question

Where would you expect to find the 1H1H NMR signal of (CH3)2Mg(CH3)2Mg relative to the TMS signal

Answer 1

Answer:

• In diethyl ether, (CH₃)₂Mg(CH₃)₂Mg, show peaks at -1.46 and -1.74 ppm, at -100 °C
• In tetrahydrofuran (THF), monomer (CH₃)₂Mg show peaks at -1.76 and -1.81 ppm, at -60 °C

Explanation:

The 1H NMR signal of (CH₃)₂Mg can be obtained in diethyl ether and tetrahydrofuran. (CH₃)₂Mg tends to polymerize in diethyl ether but mostly remain monomer in tetrahydrofuran. The negative value of chemical shifts shows that the protons of (CH₃)₂Mg are more shielded than the protons of TMS.

In diethyl ether, the structure of (CH₃)₂Mg is predicted in Figure A (attached). The A group in the structure represents the solvent. The chemical shift at -1.46 ppm is associated with protons of the bridging methyl groups while the signal at -1.74 ppm is for the proton of the terminal methyl group.

In THF, the two possible structures of (CH₃)₂Mg are predicted in Figure B (attached).  Chemical shift at -1.76 ppm is associated with unsolvated monomer of (CH₃)₂Mg while the shift at -1.81 refers to the protons of methyl group bonded to the solvated (CH₃)₂Mg. The solvent is represented as Y in the Figure B.

This study is reported by J. Heard in "NMR of organomagnessium compounds".