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, [(5C5H5)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 airsensitive anionic species.
The structures of the cis/trans isomers of [(5-C5H5)Fe(CO2)2]2 are shown below:
O
O
C
C
Fe
Fe
OC
C
O
Cis
Fe
Fe
CO
OC
CO
C
O
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(3allyl)dipalladium(II)), the second compound being analyzed, is given below:
H
C
C
Pd
H
H
Cl
Cl
Pd
H
H
C
C
H
H
C
H
C
H
H
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 CC 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 16electron 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:
Hb
Hc
C
Ha
C
Ha
C
Hb
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 dimer in
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
Acetone
Acetonitrile
DMSO
Dichloromethane
Chloroform
Freezing Point (C)
-98
-45
18
-97
-60
Boiling Point (C)
56.5
81
100
40
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.