6.1 Determination of energetics of fluxional molecules by nmr

Introduction to fluxionality

It does not take an extensive knowledge of chemistry to understand that as-drawn chemical structures do not give an entirely correct picture of molecules. Unlike drawings, molecules are not stationary objects in solution, the gas phase, or even in the solid state. Bonds can rotate, bend, and stretch, and the molecule can even undergo conformational changes. Rotation, bending, and stretching do not typically interfere with characterization techniques, but conformational changes occasionally complicate analyses, especially nuclear magnetic resonance (NMR).

For the present discussion, a fluxional molecule can be defined as one that undergoes an intramolecular reversible interchange between two or more conformations. Fluxionality is specified as intramolecular to differentiate from ligand exchange and complexation mechanisms, intermolecular processes. An irreversible interchange is more of a chemical reaction than a form of fluxionality. Most of the following examples alternate between two conformations, but more complex fluxionality is possible. Additionally, this module will focus on inorganic compounds. In this module, examples of fluxional molecules, NMR procedures, calculations of energetics of fluxional molecules, and the limitations of the approach will be covered.

Examples of fluxionality

Bailar twist

Octahedral trischelate complexes are susceptible to Bailar twists, in which the complex distorts into a trigonal prismatic intermediate before reverting to its original octahedral geometry. If the chelates are not symmetric, a Δ enantiomer will be inverted to a Λ enantiomer. For example not how in [link] with the GaL ₃ complex of 2,3-dihydroxy-N,N‘-diisopropylterephthalamide ( [link] ) the end product has the chelate ligands spiraling the opposite direction around the metal center.

Berry pseudorotation

D _3h compounds can also experience fluxionality in the form of a Berry pseudorotation (depicted in [link] ), in which the complex distorts into a C _4v intermediate and returns to trigonal bipyrimidal geometry, exchanging two equatorial and axial groups . Phosphorous pentafluoride is one of the simplest examples of this effect. In its ¹⁹ FNMR, only one peak representing five fluorines is present at 266 ppm, even at low temperatures. This is due to interconversion faster than the NMR timescale.

Sandwich and half-sandwich complexes

Perhaps one of the best examples of fluxional metal complexes is (π ⁵ -C ₅ H ₅ )Fe(CO) ₂ (π ¹ -C ₅ H ₅ ) ( [link] ). Not only does it have a rotating η ⁵ cyclopentadienyl ring, it also has an alternating η ¹ cyclopentadienyl ring (Cp). This can be seen in its NMR spectra in [link] . The signal for five protons corresponds to the metallocene Cp ring (5.6 ppm). Notice how the peak remains a sharp singlet despite the large temperature sampling range of the spectra. Another noteworthy aspect is how the multiplets corresponding to the other Cp ring broaden and eventually condense into one sharp singlet.