Magnesium Cyanide or Isocyanide?

Abstract

Preference for the binding mode of the CN− ligand to Mg (Mg−CN vs. Mg−NC) is investigated. A monomeric Mg complex with a terminal CN ligand was prepared using the dipyrromethene ligand MesDPM which successfully blocks dimerization. While reaction of (MesDPM)MgN(SiMe3)2 with Me3SiCN gave the coordination complex (MesDPM)MgN(SiMe3)2⋅NCSiMe3, reaction with (MesDPM)Mg(nBu) led to (MesDPM)MgNC⋅(THF)2. A Mg−NC/Mg−CN ratio of ≈95:5 was established by crystal‐structure determination and DFT calculations. IR studies show absorbances for CN stretching at 2085 cm−1 (Mg−NC) and 2162 cm−1 (Mg−CN) as confirmed by 13C labeling. In solution and in the solid state, the CN ligand rotates within the pocket. The calculated isomerization barrier is only 12.0 kcal mol−1 and the 13C NMR signal for CN decoalesces at −85 °C (Mg−NC: 175.9 ppm, Mg−CN: 144.3 ppm). Experiment and theory both indicate that Mg complexes with the CN− ligand should not be named cyanides but are more properly defined as isocyanides.