Question

The ¹H NMR spectrum of the given iridium complex at room temperature gave a single signal at 2.6 ppm, and its ³¹P NMR spectrum gave a single signal at 23.0 ppm. When the spectra were recorded at lower temperatures, both these signals split into a complex pattern. The intra-molecular dynamic processes shown by this molecule are: 

 

• A. Berry pseudo-rotation and rotation of the ethylene units along the C=C axis
• B. Berry pseudo-rotation and propeller type rotation of the ethylene units along the Ir-alkene axis
• C. Ray-Dutt twist and rotation of the ethylene units along the C=C axis
• D. Ray-Dutt twist and propeller type rotation of the ethylene units along the Ir-alkene axis

Hint:

Fluxional behavior in organometallic complexes often averages NMR signals at room temperature. Berry pseudo-rotation is characteristic of TBP geometries, while alkene ligands may rotate along their bonding axis with the metal.

Answer 1

The Correct Option is B

The iridium complex depicted exhibits fluxional behavior, as observed from the NMR data. At room temperature:

• ¹H NMR shows a single peak at 2.6 ppm.
• ³¹P NMR shows a single peak at 23.0 ppm.

At lower temperatures, these signals resolve into more complex multiplets, indicating slowed intramolecular processes.

This behavior suggests:

• Berry pseudo-rotation: Typical for a trigonal bipyramidal (TBP) environment, where ligands can interchange between axial and equatorial positions rapidly at room temperature.
• Propeller-type rotation: The coordinated ethylene (C₂H₄) ligands can undergo a propeller-like motion around the Ir–alkene coordination axis.

These dynamic motions average the chemical environments, causing single resonances in NMR at higher temperatures. On cooling, the slower motions lead to distinct environments and signal splitting.

Therefore, the dynamic processes involved are Berry pseudo-rotation and propeller-type rotation of the ethylene units along the Ir–alkene axis.

\[ \boxed{\text{Correct answer is (B)}} \]