1. Abstract
The long-term research goal of the Shelby Group is to design
palladium compounds bonded to negatively charged diphosphorus
compounds and to test them for catalysis. We have explored two
ways to synthesize these compounds. In one route, we
successfully deprotonated the diphosphine ligand, however, its
coordination to palladium proved difficult. The other route,
coordination of the neutral diphosphine to palladium followed by
deprotonation seems more promising. We have narrowed our
focus on unsymmetric diphosphine ligands that contain only
aromatic substituents with different electronic and/or spatial
character to further examine conditions that favor the diphosphine
to become negatively charged. Currently, we are working to
synthesize two unsymmetric ligands, (2-MeO-C6H4)2PCH2PPh2 and
(3,5-dimethylphenyl)2PCH2PPh2. We have two general methods to
make the Ar2PCH2PPh2 ligands. One method forms the
unsymmetric phosphine sulfide percursor, Ph2PSCH2PAr2, which is
then reduced with sodium. The (2-MeO-C6H4)2PCH2PPh2 ligand
was made by this sulfide route. Although this method does not
appear to be a good option for making the methoxy (2-MeO-
C6H4)2PCH2PPh2 ligand because the oxygen might react with
sodium, it may be a promising method for the (3,5-
dimethylphenyl)2PCH2PPh2 ligand, which does not contain oxygen.
Another method for making the Ar2PCH2PPh2 ligands is to treat
Ar2PCH2Li with Ph2PCl to make the desired ligand directly. The
direct method appears to be favorable for the synthesis of both
target ligands, and we are at the stage of making minor changes to
improve product yields.
Background
Symmetrical and unsymmetrical diphosphinomethane ligands (R2P
– CH2 – PR2) have widespread use in transition metal chemistry and
catalysis. The ligand can act as a bridging ligand (where the two
phosphorus atoms bond to two different metals) or as a chelating
ligand (where the two phosphorus atoms bond to the same metal).
When the methylene ( – CH – ) backbone of the ligand is
deprotonated to form , isolated complexes adopt bonding modes
in which the ligand (that has become negatively-charged R2P – CH –
PR2 ) is a mono-, bi-, or tri-dentate ligand by bonding to one, two,
or three metal centers using its methanide carbon and phosphorus
atoms. Our research focuses on the synthesis of palladium
complexes containing unsymmetrical diphosphine ligands and the
steric and/or electronic effects of the phosphorus substituents on
the reactivity at the methylene carbon. Our results suggest that
sterically bulky phosphorus substituents favor the formation of
binuclear Pd Complexes in which the ligands bridge between the
metal centers, and that the methylene proton is more acidic when
the phosphine substituents are aromatic.
Results
Throughout reactions 1-5, two different substituents were used. These two aromatic
substituents will be represented by Ar2. The following ligands are shown below :
3,5-dimethlyphenyl 2-MeO-C6H4
Reaction 1:
• Methyl lithium reacted with Ar2PCl to give Ar2PCH3 .
• The percent yield of Ar2PCH3 was not calculated due to its rapid need in reaction 2.
Scheme 1: Synthesis of Ar2PCH3
Reaction 2:
 LinBu reacted with Ar2PCH3 to give Ar2PCH2Li.
 Reflux was done promptly after addition on LitBu.
 The percent yield of Ar2PCH2Li was not calculated due to its rapid need in reaction 3.
Scheme 2 : Synthesis of Ar2PCH2Li
Reaction 3:
 Ph2PCl reacted with Ar2PCH2Li to give rise to Ar2PCH2PPh2.
Scheme 3 : Synthesis of Ar2PCH2PPh2
Phosphorus NMR Data Results:
Future Research
Reaction Four:
-Similar to analogue in scheme 4, Ph2PCH2P(3,5-dimethylphenyl)2 and Ph2PCH2P(2-MeO-
C6H4)2 will each be coordinated to Pd to give PdCl2[Ph2PCH2P(3,5-dimethylpheny)2] and
PdCl2[Ph2PCH2P(2-MeO-C6H4)2], respectively.
Scheme 4 : Synthesis of PdCl2[Ph2PCH2P(otolyl)2]
Reaction Five:
-Similar to analogue in scheme 5, PdCl2[Ph2PCH2P(3,5-dimethlyphenyl)2] and
PdCl2[Ph2PCH2P(2-MeO-C6H4)2] will each be reduced by LinBu to give the zero valent
palladium diphosphine dimer.
Scheme 5 : Synthesis of the zero valent palladium diphosphine dimer
Figure 4: Structure of the zero valent palladium diphosphine dimer
Acknowledgements
We thank the Chemistry Department of DePaul University and Louis Stokes Alliance for
Minority Participation for the equipment and funding needed to supply our research
project.
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Studies of Steric and Electronic Behaviors of Palladium
Compounds Bonded to Diphosphorus
Darcy Velazquez, Khrystyna Hlukhenka, Stephanie Pacheco, & Dr. Quinetta Shelby
Figure 1. (3,5-dimethylpheny)2PCH2PPh2 Figure 2. (2-MeO-C6H4)2PCH2PPh2
Figure 3. (3,5-dimethylpheny)2PCH2PPh2 via sulfur route