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The Organometallic HyperTextBookIndexSearch
Fluxionality

General Information

Dynamic Exchange Processes

Quantitative Aspects of Dynamic NMR

Spin Saturation Transfer

How to Interpret Dynamic NMR Data

  1. Start at the low-T limit. It is usually easiest to start here because (presumably) there is no fluxionality. Thus, your task is simply finding static structures that are consistent with the available data. Use your knowledge of the system, integrals, multiplicities, coupling constants, decoupling data, and chemical shifts as you would in any other system.

  2. Look at the high-T limit. Having assigned your spectrum at low T, it is now easy to understand which groups are interconverting on the NMR time scale.

  3. Find a chemically reasonable pathway for the interconversion. Possibilities to look for include:

    1. Dissociation and recoordination of a ligand. To probe for such behavior, add some of the free ligand to the solution. If the fluxional process involves dissociation of that species, the chemical shift of the free ligand and bound species will come at the weighted average of the individual species. One can also try adding an isotopically-labeled version of the free ligand and see if the label is incorporated into the complex.

    2. Rotation about a hindered bond. We can typically ignore rotation about simple bonds such as a metal-alkyl, metal-Cp and metal-alkoxide because these are so facile that they are almost impossible to freeze out. However, large groups, or phenyl rings with ortho substituents can display hindered rotation. Alkenes and other pi-bonding ligands sometimes have a preferred orientation for coordination; an fragment MO approach can help us assess this (more on that in a future chapter).

    3. Opening/closing of bridges. In dinuclear systems, it is not uncommon for a carbonyl or alkoxide ligand to switch between a bridging and terminal position.

    4. Monomer-dimer or dimer-tetramer equilibrium. Dimers (or dimers of dimers) held together only by weakly bridging ligands often undergo dissociation. Note: This is unlikely in a case where a metal-metal bond exists. To probe such equilibria, try decreasing the concentration, which should give more of the lower nuclearity species at a given temperature, as well as increasing the concentration, which should favor the higher nuclearity species. Likewise, high T should favor the dissociated form and low T the more associated form.

    5. Structural or skeletal rearrangements. Two examples are cyclohexane and Cp2TiS5 as discussed above. 5-coordinate systems are quite notorious for fluxional behavior as the energy barrier between trigonal bipyramid (TBP) and square pyramidal (SP) geometries is often quite low. Such interconversions can occur through a Berry pseudorotation or turnstile mechanism. Watch for an entry on the these two mechanisms at a later date.

  4. Remember to consider other possibilities. Remember that you can never prove a mechanism, only disprove one. For example, perhaps there are two processes being observed, not just one!

Self-Test

Question 1. On the right are 1H NMR spectra of (tetramethylallene)Fe(CO)4 (1) at -60 oC and 30 oC (tetramethylallene is Me2C=C=CMe2). In the low temperature spectrum, the integrated ratios of the peaks are 1:1:2.

When tetramethylallene is combined with 1, the 30 oC 1H NMR spectrum of the mixture "consists of two singlets" (the spectrum is not shown here).

Explain these spectra. Be sure to draw careful and complete structures in your answer. Assign the peaks at high and low temperature and be certain to explain whatever processes are occurring.

two NMR spectra

Question 2. Consider this pseudo-octahedral ethylene complex of osmium. At +80 oC, the 31P-decoupled 1H NMR of this complex in solution with an equimolar amount of added ethylene shows a single sharp line at 6.0 ppm (ignoring the PMe3 and PPh3 resonances).

Cooling this solution to 0 oC results in the splitting of this resonance into a single line at 4.9 ppm (about where free ethylene is observed) and two doublets at 7.5 (J = 2 Hz) and 6.7 (J = 2 Hz) ppm.

Upon cooling to -80 oC, the two doublets split further into a complex multiplet.

Explain these observations. Careful and complete drawings will help you assign equivalent protons and be a large help in solving this problem.

a structural formula

Question 3. The compound shown on the right exhibits fluxional behavior in the proton-decoupled 31P NMR. As the sample is cooled, the singlet resonance broadens and decoalesces between -80 and -100 oC (the T depends on the nature of group X).

Below the coalescence point a pattern that appears to be a quartet is observed. Addition of excess PEt3 to this reaction mixture has no effect on the observed NMR behavior.

Using clear drawings, explain what is being observed and show how this is consistent with the NMR observations.

a structural formula

Suggested Reading

  1. Kegley, S.E.; Pinhas, A. R. Problems and Solutions in Organometallic Chemistry, University Science Books: Mill Valley, California, 1986, pp 20-26.

  2. Gasparro, F. P.; Kolodny, N. H. J. Chem. Ed. 1977, 4, 258-261.

dividing line

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This page was last updated Sunday, March 18, 2007
This document and associated figures are copyright 1996-2014 by Rob Toreki or the contributing author (if any) noted above. Send comments, kudos and suggestions to us by email. All rights reserved.