Magnesium Cyanide or Isocyanide?

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Spectroscopy

Background:

• The binding preference of the cyanide (CN-) ligand to metal centers is crucial for understanding organometallic complexes.
• Investigating magnesium-cyanide interactions provides insights into the reactivity and structure of early transition metal complexes.

Purpose of the Study:

• To determine the preferred binding mode (Mg-CN vs. Mg-NC) of the cyanide ligand to a magnesium center.
• To characterize the resulting magnesium complexes using spectroscopic and computational methods.

Main Methods:

• Synthesis of monomeric magnesium complexes using a dipyrromethene ligand (MesDPM) to prevent dimerization.
• X-ray crystal structure determination of magnesium-isocyanide complexes.
• Density Functional Theory (DFT) calculations to analyze binding energies and isomerization barriers.
• Infrared (IR) spectroscopy and 13C Nuclear Magnetic Resonance (NMR) spectroscopy for structural and dynamic analysis.

Main Results:

• A high preference (≈95:5) for the isocyanide binding mode (Mg-NC) over the cyanide mode (Mg-CN) was observed.
• IR spectroscopy revealed distinct stretching frequencies for Mg-NC (2085 cm-1) and Mg-CN (2162 cm-1), confirmed by 13C labeling.
• DFT calculations indicated a low isomerization barrier (12.0 kcal/mol) for CN rotation, consistent with observed dynamic behavior in NMR studies.

Conclusions:

• Magnesium complexes with the CN- ligand are predominantly isocyanides (Mg-NC).
• The dynamic rotation of the CN ligand in solution and solid states is facilitated by a low isomerization barrier.
• These findings suggest that Mg-CN complexes should be classified as isocyanides rather than cyanides.