NMR Investigation of Temperature and Solvent Effects on Molecular Fluxionality:

Using Allylpalladium Chloride Dimer and Carboncyclopentadienylironcarbonyl Dimer

Adapted from:

Szafran, Z.; Pike, R.M.; Singh, M.M. “Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience.”

John Wiley and Sons, Inc.: New York, 1991.

Objective:

In this lab, two compounds will be analyzed using NMR spectroscopy to determine effects of varying temperature and solvent on molecular fluxionality. You will also synthesize the allylpalladium complexes.

Introduction:

The first compound carboncyclopentadienylironcarbonyl dimer, [(5-C5H5)Fe(CO2)2]2, has some unusual structural features, not frequently found in other carbonyl compounds:

  1. It is a dimeric compound.
  2. The molecule possesses both terminal and bridging carbonyl groups.
  3. It is a diamagnetic compound having a short Fe-Fe bond.
  4. The two Fe atoms and the two bridging CO groups are coplanar. One C5H5 ring and one terminal CO group lie above this plane, and the other C5H5 and CO are situated below this plane.
  5. In solution, the compound exists mainly as cis and trans isomers, both being present in rapid equilibrium. Trace amounts of nonbridged species are also present in solution.
  6. Because of the presence of the metal-metal bond, the compound is very sensitive to reduction. It can be easily reduced by sodium to a very air-sensitive anionic species.

The structures of the cis/trans isomers of [(5-C5H5)Fe(CO2)2]2 are shown below:

Cis Trans

All of the properties listed above make this compound an ideal target for detailed investigation by both IR and NMR (both 1H and 13C). While IR and 1H NMR spectra of the compound help in determining the nature of the C5H5 and CO groups, the 13C NMR spectrum, combined with variable temperature studies establishes the dynamic process of the fluxional behavior of this molecule. The terminal CO groups show IR stretches in the range from 1850-2150 cm-1, with the bridging carbonyl group stretches occurring from 1750- 1850 cm-1.

The structure of allylpalladium chloride dimer (di--chlorobis(3-allyl)dipalladium(II)), the second compound being analyzed, is given below:

Allylpalladium chloride dimer

This dimer is synthesized from palladium(II), which forms a variety of square planar organometallic complexes with various olefinic organic groups. In the case of PdCl2 reacted with allyl bromide, the allylpalladium complex shown above may be synthesized. Complexes between a metal salt and an olefin have been known since 1827. In the palladium complexes, the olefin donates electron density from its filled  orbital to an empty palladium  symmetry orbital. The palladium, in turn, donates electron density from a filled  orbital to the empty olefin * orbital. This results in a lowering of the C-C bond order and a consequent lowering of the olefin IR absorption frequency.

There is some difficulty in assigning the number of electrons donated by the allyl group. Viewing the allyl group as a neutral ligand (most convenient in this case), it would function as a 1(monohapto)- or 3(trihapto)- electron donor, depending on whether it were  or  bound. If  bound, the allyl group is bidentate (occupies two coordination sites), while if  bound, it is monodentate. Alternatively, the allyl group can be treated as an anion, where it functions as a 2- or 4-electron donor.

In noncoordinating species, the complex is found in the  form, where it is a 16-electron species, the most stable electronic arrangement for square planar geometry. (The simplest electron count in this case is Pd2+ = 8 electrons, allyl anion = 4, 2 x chloride ion = 4, total = 16 electrons.)

When the allyl group is  bound, the complex is stereochemically rigid. There are three types of non-equivalent hydrogen atom, which are given below:

Non-equivalent protons in the allyl group

Hydrogen C is clearly unique, being part of the only CH group. The B hydrogen atoms are syn to hydrogen C, and the A hydrogen atoms are anti to hydrogen C. The 1H NMR spectrum would therefore show three signals. When the allyl group is  bound, there is free rotation about the C-C single bond, thus rendering the A and B hydrogen atoms equivalent. The 1H NMR spectrum would therefore show only two signals.

Both temperature and solvent can dramatically affect molecular fluxionality. By varying the temperature at which the compound is analyzed, the available thermal energy can be manipulated and therefore the rate at which rotations occur can be manipulated. As the temperature is lowered, the available thermal energy is less than the rotational activation energy, and rotations are slowed sufficiently to detect the presence of rotational isomers in solution. At low temperatures the time spent by each ligand in a given chemical environment is long compared with the time for a nuclear transition. As a result, the measuring technique “sees” separate resonances for each kind. As the temperature is increased, the ligands spend less and less time in each chemical environment, and as the environments become less and less well defined, the peaks begin to broaden. When the exchange processes become sufficiently rapid as to reduce the residence time in any environment far below the time scale of a nuclear transition, the peaks begin to coalesce. Solvent can affect molecular fluxionality through several means. Solvents can provide substituents, which can be substituted on to the molecule being analyzed resulting in a different complex altogether. Polar solvents can also stabilize various conformations of the molecule, resulting in the observation of isomers of different energy levels.

Experimental Procedures:

Carboncyclopentadienylironcarbonyl Dimer Analysis:

Dissolve 25.0 mg of the carboncyclopentadienylironcarbonyl dimerin approximately 500 L of the solvent of your choice. The temperature range for this portion of the experiment is –75C to 25C. The solvents are given in the table below:

Table 1:

Solvent

/ Freezing Point (C) /

Boiling Point (C)

Acetone / -98 / 56.5
Acetonitrile / -45 / 81
DMSO / 18 / 100
Dichloromethane / -97 / 40
Chloroform / -60 / 61

You should collect at least five 13C spectra in the temperature range given above and using the parameters given in Table 2. Repeat the experiment using a different solvent.

Table 2:

Parameter / New Value
X-offset / 150 ppm
X-sweep / 400 ppm

Allylpalladium Chloride Dimer Synthesis and Analysis:

Synthesis:

Add 100 mg of finely divided PdCl2 to a 25-mL round-bottomed flask equipped with a magnetic stirring bar. Add 3.0 mL of glacial acetic acid and 3.0 mL of water. Attach a water condenser and place the apparatus in a hot water bath on a magnetic stirring hot plate. Heat the mixture, with stirring, to 100C for 15 minutes.

Using an automatic delivery pipette, add 500 L of allyl bromide to the reaction solution through the top of the condenser. Heat the solution to 60C, with stirring, for one hour. NOTE: DO NOT HEAT THE SOLUTION OVER 60C OR DECOMPOSITION WILL OCCUR.

Cool the pale yellow mixture to room temperature. Add 3.0 mL of methylene chloride, swirl, and transfer the bottom layer into a clean 25-mL round-bottomed flask using a Pasteur pipette. Repeat this extraction procedure two additional times if any solid remains. Combine the liquid extractions and rotary evaporate the solution overnight to remove any solvent. The allylpalladium chloride dimer product should be a yellow powder.

Analysis:

IR:

Obtain the IR spectrum of the product and compare the spectrum to that of allyl bromide.

NMR:

Dissolve one half of the product (~20 mg) in a minimum amount of DMSO. Dissolve the other one half of the product in a minimum amount of one other solvent listed in Table 1. The temperature range for this portion of the experiment is 0C to 60C. Obtain the 1H NMR spectrum for both solutions at 0, 40, and 60C in addition to room temperature. Use normal proton parameters.