Arylpalladium phosphonate complexes as reactive intermediates in Phosphorus-Carbon bond forming reactions

Article abstract of DOI:10.1021/om800906m

Phosphorus-carbon bond formation from discrete transition metal complexes have been investigated through a combination of synthetic, spectroscopic, crystallographic, and computational methods. Reactive intermediates of the type (diphosphine)Pd(aryl)(P(0)(OEt)₂) have been prepared, characterized, and studied as possible intermediates in metal-mediated coupling reactions. Several of the reactive intermediates were characterized crystallographically, and a discussion of the solid state structures is presented. In contrast to other carbon-heteroelement bond forming reactions, palladium complexes containing electron-donating substituents on the aromatic fragment exhibited faster rates of reductive elimination. Large bite angle diphosphine ligands induced rapid rates of elimination, while bipyridine and small bite angle diphosphine ligands resulted in much slower rates of elimination. An investigation of the effect of typical impurities on the elimination reaction was carried out. While excess diphosphine, pyridine, and acetonitrile had little effect on the observed rate, the addition of water slowed the phosphorus-carbon bond forming reaction. Coordination of water to the complex was observed speetroscopically and crystallographically. Computational studies were utilized to probe the reaction pathways for P-C bond formation via Pd catalysis.

Full text of DOI:10.1021/om800906m

Organometallics 2009, 28, 1193–1201
1193
Arylpalladium Phosphonate Complexes as Reactive Intermediates in
Phosphorus-Carbon Bond Forming Reactions
Mark C. Kohler,† Thomas V. Grimes,# Xiaoping Wang,‡ Thomas R. Cundari,*,‡ and
Robert A. Stockland, Jr.*,†
Department of Chemistry, Bucknell UniVersity, Lewisburg, PennsylVania 17837, and Department of
Chemistry, Center for AdVanced Scientific Computing and Modeling (CASCaM), UniVersity of North
Texas, Denton, Texas 76203
ReceiVed September 17, 2008
Phosphorus-carbon bond formation from discrete transition metal complexes have been investigated
through a combination of synthetic, spectroscopic, crystallographic, and computational methods. Reactive
intermediates of the type (diphosphine)Pd(aryl)(P(O)(OEt)2) have been prepared, characterized, and studied
as possible intermediates in metal-mediated coupling reactions. Several of the reactive intermediates
were characterized crystallographically, and a discussion of the solid state structures is presented. In
contrast to other carbon-heteroelement bond forming reactions, palladium complexes containing electron-
donating substituents on the aromatic fragment exhibited faster rates of reductive elimination. Large bite
angle diphosphine ligands induced rapid rates of elimination, while bipyridine and small bite angle
diphosphine ligands resulted in much slower rates of elimination. An investigation of the effect of typical
impurities on the elimination reaction was carried out. While excess diphosphine, pyridine, and acetonitrile
had little effect on the observed rate, the addition of water slowed the phosphorus-carbon bond forming
reaction. Coordination of water to the complex was observed spectroscopically and crystallographically.
Computational studies were utilized to probe the reaction pathways for P-C bond formation via Pd
catalysis.
and steric effects of the organic fragments on the individual
steps of the transformation is needed.
We and others have been interested in elucidating the key
mechanistic steps in palladium-catalyzed P-C bond forming
reactions.11-39 To this end, we recently studied arylphosphonate
Introduction
Carbon-heteroelement bond forming reactions are critical
transformations used to construct a range of organic architec-
tures. For the formation of phosphorus-carbon bonds, Arbuzov
and related reactions are some of the most popular approaches
and are used to generate alkylphosphonates in high yields.1
However, aryl halides are typically unreactive under Arbuzov
conditions. To resolve this issue, nickel-,2 copper-,3,4 and
palladium-catalyzed5-9 coupling reactions of hydrogen phos-
phonates and other R2P(O)H substrates with aryl halides have
been developed.10 Despite the harsh conditions of these reac-
tions, the transformations have become powerful assets to the
synthetic chemist due to high conversions and minimization of
unwanted secondary processes. To develop highly active
catalysts for this reaction, an understanding of the electronic
(11) (a) Kohler, M. C.; Stockland, R. A., Jr.; Rath, N. P. Organometallics
2006, 25, 5746. (b) Stockland, R. A., Jr.; Wilson, B. D.; Goodman, C. C.;
Giese, B. J.; Shrimp, F. L., II. J. Chem. Ed. 2007, 84, 694.
(12) (a) Levine, A. M.; Stockland, R. A., Jr.; Clark, R.; Guzei, I.
Organometallics 2002, 21, 3278. (b) Ide, D. M.; Eastlund, M. P.; Jupe,
C. L.; Stockland, R. A., Jr. Curr. Org. Chem. 2008, 12, 1258. (c) Kalek,
M.; Stawinski, J. Organometallics 2007, 26, 5840. (d) Kalek, M.; Stawinski,
J. Organometallics 2008, 27, 5876. (e) Blank, N. F.; Moncarz, J. R.;
Brunker, T. J.; Scriban, C.; Anderson, B. J.; Amir, O.; Glueck, D. S.;
Zakharov, L. N.; Golen, J. A.; Incarvito, C. D.; Rheingold, A. L. J. Am.
Chem. Soc. 2007, 129, 6847. (f) Moncarz, J. R.; Brunker, T. J.; Jewett,
J. C.; Orchowski, M.; Glueck, D. S.; Sommer, R. D.; Lam, K.-C.; Incarvito,
C. D.; Concolino, T. E.; Ceccarelli, C.; Zakharov, L. N.; Rheingold, A. L.
Organometallics 2003, 22, 3205.
* Corresponding author. E-mail: rstockla@bucknell.edu.
# Current address: Fukui Institute for Fundamental Chemistry, Kyoto
University, Kyoto 606-8103, Japan.
(13) Stockland, R. A., Jr.; Levine, A. M.; Giovine, M. T.; Guzei, I. A.;
Cannistra, J. C. Organometallics 2004, 23, 647.
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37, 94.
† Bucknell University.
‡ University of North Texas.
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2006, 71, 5020.
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10.1021/om800906m CCC: $40.75
2009 American Chemical Society
Publication on Web 01/21/2009

1194 Organometallics, Vol. 28, No. 4, 2009
Kohler et al.
Table 1. Arylpalladium Phosphonate Precursors
formation from discrete reaction intermediates of the type
bu2bipyPd(Ar)(P(O)(OR)2) (eq 1).11 These complexes repre-
sented the first examples of isolated four-coordinate arylpalla-
dium complexes containing a phosphonate fragment and were
quite resistant to reductive elimination at 25 °C. An investigation
of the conditions necessary to induce reductive elimination from
these intermediates would provide insight into the key
phosphorus-carbon bond forming step in palladium-catalyzed
arylphosphonate synthesis. Furthermore, the chelating ligand
forced the aryl and phosphonate fragments to be bound to the
metal center in a cis fashion; thus, isomerization was not a
prerequisite for reductive elimination from these compounds.
The arylpalladium phosphonate compounds were quite stable
at 25 °C in the solid state and in solution. However, all examples
underwent clean reductive elimination in C6D6 solutions at
elevated temperatures in sealed tubes (90-120 °C). In contrast
with other carbon-heteroelement bond forming reactions from
Pd(II),40 metal-bound aryl fragments bearing electron-donating
groups accelerated the elimination process, while -NO2, -Cl,
or -CN groups resulted in sluggish phosphorus-carbon bond
formation. Although the use of bipyridine-based ligands facili-
tated the isolation and characterization of these potential
intermediates, most palladium-catalyzed coupling reactions
involve monodentate or bidentate phosphine donors. The
efficacy of diphosphine ligands for promoting the formation of
phosphorus-carbon bonds by reductive elimination from dis-
crete reaction intermediates of the type (diphosphine)Pd(Ar)-
(P(O)(OR)2) would be of interest for the design and development
of more efficient catalysts.
R
compound
4-C6H4NO2
4-C6H4CN
C6H5
4-C6H4Me
4-C6H4OMe
2-C6H4Me
2,6-C6H3Me2
1
2
3
4
5
6
7
sors to palladium and gold phosphonate species.11,12a,13 For
example, treatment of (bu2bipy)Pd(R)Cl with 1.0 equiv of
Ag[P(O)(OR)2] generated (bu2bipy)Pd(R)(P(O)(OR)2) com-
pounds in high yields. Simple filtration and removal of the
volatiles typically afforded high yields of the desired complexes.
For this work, diphosphine-ligated compounds were the syn-
thetic targets, and it seemed appropriate that treatment of
(diphosphine)Pd(R)Cl species with the silver phosphonate salts
should result in the formation of the desired compounds.
However, extensive experimentation yielded only decomposition
and the formation of palladium mirrors. Thus an alternate
method for the generation of the diphosphinePd(R)(P(O)(OR)2)
species was needed. Since diphosphines are typically stronger
donors for palladium than bipyridines, addition of diphosphine
to a solution of the bu2bipyPd(Ar)(P(O)(OR)2) species could
result in displacement of the bipyridine ligand and result in the
formation of the desired complexes. Using bu2bipyPd(Ph)-
(P(O)(OEt)2) as a model system, addition of 1.0 equiv of dppe
(in C6D6 solution) resulted in immediate and quantitative
elimination of the bipyridine and generation of dppePd(Ph)-
1
(P(O)(OEt)2). Additionally, analysis of the H NMR spectrum
of this reaction mixture within a few minutes of diphosphine
addition revealed the presence of PhP(O)(OEt)2. This suggested
that elimination from the diphosphine-ligated compounds will
occur at significantly reduced temperatures as compared to the
bipyridine species.
Results and Discussion
Synthesis of Organopalladium Precursors. We have re-
cently reported that silver phosphonates are convenient precur-
With the development of a successful system for the
preparation of the desired intermediates, a series of compounds
were prepared using a range of arylpalladium precursors (Table
1) to provide a comparison of the electronic and steric effects
of the metal-bound aryl fragment. The diphosphines, dppe, dppp,
and dpephos, were selected and -P(O)(OEt)2 was chosen as
the prototypical phosphonate fragment due to its rich history in
the synthesis of organic compounds.41 Using dppe as the
diphosphine, treatment of C6D6 solutions of 1-7 with dppe
generated the arylpalladium phosphonate species by displace-
ment of the bu2bipy ligand. For 1-5, the displacement reaction
was complete within a few minutes at 25 °C. For 6 and 7, the
displacement reaction was slower, with the latter taking several
hours to reach completion. Sharp resonances were observed for
the free bu2bipy ligand and bound diphosphine, suggesting that
bu2bipy does not compete with the diphosphine for the metal
center. For 1-3, 6, and 7, the sluggish rate of reductive
elimination from the diphosphine-ligated species facilitated
isolation of these compounds. Monitoring the displacement
reaction by NMR spectroscopy in tol-d8 revealed that the
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I. P. Russ. J. Org. Chem. 2003, 39, 797.
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Albinato, A. Organometallics 2000, 19, 3062.
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Reactions; New Aspects in Phosphorus Chemistry; 2004; Vol. 232, p 25.
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Kobata, C.; Nune, S. K.; Tanaka, M. Tetrahedron Lett. 2007, 48, 4669.
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(41) dppe ) bis(diphenylphosphino)ethane, dppp ) bis(diphenylphos-
phino)propane, dpephos ) (oxydi-2,1-phenylene)bis(diphenylphosphine).

Arylpalladium Phosphonate Complexes
Organometallics, Vol. 28, No. 4, 2009 1195
displacement of bu2bipy by dppe was quite rapid at 25 °C, but
it was very slow at temperatures below 10 °C and essentially
nonexistent below -10 °C.
complex. Alternatively, the elimination could be occurring from
a mixed system where one end of the dpephos is attached to
the palladium along with one end of the bipyridine ligand. The
broadened signals in the 31P{1H} NMR spectrum for the free
ligand and bu2bipyPd(4-C6H4CN)(P(O)(OEt)2) at low temper-
atures suggested a dynamic process; however, the rapid
arylphosphonate formation from the mixture precluded further
investigation of the process.
Changing the diphosphine to dppp resulted in similar results
for most of the displacement reactions; however, elimination
often occurred rapidly from the generated dpppPd(Ar)(P(O)(O-
Et)2) compounds. This required the dppp ligated compounds to
be generated and monitored in situ. For the precursors containing
metal-bound aryl rings with electron-donating substituents, the
rate of displacement was on the order of the reductive elimina-
tion, and the reactive intermediates were observed only at low
temperature. For example, injecting C6D6 to the mixture of
bu2bipyPd(4-C6H4OMe)(P(O)(OEt)2)/dppp followed by shaking
for 5 s and flash freezing resulted in a mixture of unreacted
starting materials (40%), reactive intermediate (5%), and
products ((4-C6H4OMe)P(O)(OEt)2/dppp2Pd(0), 55%). Similar
to the reactions involving dppe, careful monitoring of this
reaction by 1H and 31P{1H} NMR spectroscopy using tol-d8 as
the solvent revealed that the effective temperature range for the
displacement reaction was 15-25 °C. Only a trace amount of
displacement/elimination was observed below -10 °C.
The final diphosphine studied in this investigation was
dpephos. This diphosphine exhibits a large bite angle (105°)
and has been used in a number of palladium-catalyzed
reactions.42-45 Treatment of C6D6 solutions of 1-7 with this
ligand resulted in immediate generation of the arylphosphonate
For comparison studies, methylpalladium phosphonate com-
plexes were prepared using similar procedures. Treatment of
bu2bipyPdMeCl with 1 equiv of Ag[P(O)(OEt)2] in CH2Cl2
generated bu2bipyPdMe(P(O)(OEt)2) in high yield after filtration
and removal of the volatiles. Treatment of this precursor with
dppe and dppp rapidly generated the diphosphinePdMe(P(O)(O-
Et)2) species. In contrast to the arylpalladium phosphonate
species, these alkyl analogues were quite stable, and no
elimination was observed from the dppe analogue after standing
at 22 °C for several months.
All of the observable intermediates exhibited static structures
in solution with three resonances in the 31P{1H} NMR spectrum.
The phosphonate group gave rise to a signal between 70.8 and
78.8 ppm, with the species containing the more electron-
releasing metal-bound aryl groups giving rise to higher fre-
quency chemical shifts. The trans 2JPP between the phosphonate
and the phosphorus atom is significantly larger than the cis
couplings and ranges between 551.4 and 589.9 Hz. The two
cis couplings ranged between 16.7 and 46.2 Hz.
1
(>97% by NMR). No intermediates were observed in the H
or 31P{1H} NMR spectrum of these reactions. To investigate
this further, the displacement/elimination reaction was monitored
by NMR spectroscopy at low temperatures using the model
system of bu2bipyPd(4-C6H4CN)(P(O)(OEt)2)/dpephos. This
precursor was selected since the complexes incorporating
electron-withdrawing groups on the metal-bound aryl exhibit
the slowest rates of reductive elimination and would provide
the best opportunity to observe the intermediate. At temperatures
below -20 °C (tol-d8), sharp resonances for bu2bipyPd(4-
C6H4CN)(P(O)(OEt)2) and dpephos were observed in the
31P{1H} NMR spectrum. Warming this solution to 0 °C resulted
in broadening of the signal for bu2bipyPd(4-C6H4CN)-
(P(O)(OEt)2) and the dpephos. Lowering the temperature to -50
°C resulted in regeneration of sharp signals for the arylpalladium
complex and free dpephos. Warming this solution to a higher
temperature (10 °C) resulted in broadening of the signals for
both bu2bipyPd(4-C6H4CN)(P(O)(OEt)2) and dpephos along with
generation of the arylphosphonate. No assignable signals for
an intermediate of the type (dpephos)Pd(4-C6H4CN)(P(O)(OEt)2)
were observed. Allowing this solution to stand for a few seconds
at 25 °C and flash freezing in liquid nitrogen followed by
observation at -50 °C also did not reveal an intermediate. In
contrast to all of the reactions involving dppe, where displace-
ment of the bipyridine ligand was relatively rapid compared to
elimination, the rate of elimination is faster than the displace-
ment reaction and no intermediates were observed. This could
be due to complete displacement of the bu2bipy ligand and
The solid state structures of several precursors and aryl
palladium phosphonate species were studied by single-crystal
X-ray diffraction. The molecular structures are presented in
Figure 1, and structural data are summarized in Table 3. Selected
bond distances and angles are presented in Tables 4 and 5.
Single crystals of bu2bipyPd(Ph)(P(O)(OEt)2) and bu2bipyPd(4-
C6H4NO2)(P(O)(OEt)2) were grown by slow diffusion of pentane
into CDCl3 solutions of the complexes at 0 °C. These com-
j
pounds crystallize in the triclinic space group P1 with two
molecules per unit cell. The metal-carbon bond lengths of the
two compounds are similar, and the palladium-nitrogen bonds
are also similar despite being trans to different groups. Although
many aspects of the structures are similar, the P-Pd-C and
P-Pd-N (cis) angles differ by nearly 9°. Additionally, the Pd-
Cipso-Cpara angle deviates significantly from linearity in bu2bipy-
Pd(Ph)(P(O)(OEt)2) (168.0°) and bu2bipyPd(4-C6H4NO2)-
(P(O)(OEt)2) (172.5°). In fact, analyzing the arylpalladium
complexes in the Cambridge Structural Database ligated by
dipyridyl- or ethylene-bridged diphospine ligands revealed that
bu2bipyPd(Ph)(P(O)(OEt)2) has the largest deviation from
linearity of any reported compound. Compounds dppePd(4-
C6H4CN)(P(O)(OEt)2) and dppePdMe(P(O)(OEt)2) crystallized
in the orthorhombic and monoclinic space groups P212121 and
P21/c, respectively. The palladium-carbon and palladium-
phosphonate bonds are elongated in these complexes relative
to the bu2bipy-ligated compounds due to the greater trans
influence of the phosphine donors, and the palladium-alkyl
bond is slightly longer than the palladium-aryl bond. The
Pd-Cipso-Cpara angle in the arylpalladium complex is closer to
linearity (176.0°) than in the related bipyridine palladium
complexes. All of the crystals grown in this investigation contain
a water molecule as a solvent of crystallization. In all structures
the water is hydrogen bonded to the phosphoryl oxygen with
oxygen-oxygen distances between 2.710(3) and 2.8078(15) Å
and O-H-O angles between162(3)° and 175(2)°.
formation of
a
four-coordinate dpephosPd(4-C6H4CN)-
(P(O)(OEt)2) that simply has a very fast rate of reductive
elimination due to the large bite angle or rapid dissociation of
one end of the dpephos to generate a three-coordinate palladium
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Reductive Elimination Studies. To investigate the electronic
and steric effects of the metal-bound aryl fragment on reductive

1196 Organometallics, Vol. 28, No. 4, 2009
Kohler et al.
Figure 1. Molecular structures of dppePd(C6H4CN)(P(O)(OEt)2), dppePd(Me)(P(O)(OEt)2), and bu2bipyPd(4-C6H4R)(P(O)(OEt)2) (R )
NO2,H). Thermal ellipsoids are set at 50% probability.
elimination from these compounds, the phosphorus-carbon
PhP(O)(OEt)2, free bu2bipy, and the internal standard. Com-
parison of the integral values for the starting complex and
arylphosphonate revealed a greater than 95% conversion into
Ph(P(O)(OEt)2). Analysis of the reaction mixture by GC and
GCMS confirmed the lack of secondary products. Since the
elimination was typically carried out in the presence of added
diphosphine or PPh3 in order to trap the palladium(0) product
formed in the reaction, elimination reactions were carried out
using 5 and 10 equiv of diphosphine. Addition of excess
diphosphine had a negligible effect on the rate of the elimination,
suggesting the free phosphine simply traps the generated Pd(0).
The reaction is also insensitive to 5 or 10 equiv of bu2bipy.
Ruling out involvement of the former in the rate of the reductive
elimination reaction was critical since many of the arylpalladium
phosphonate complexes eliminated rapidly upon formation and
were generated in situ from the combination of 1-7 and
diphosphine. Doubling the concentration of the arylpalladium
phosphonate had no effect on the rate of the elimination process.
Since palladium-catalyzed arylphosphonate-forming reactions
often employ hygroscopic hydrogen phosphonates, the effect
of water on the reductive elimination was investigated. The
addition of 1 equiv of water resulted in a negligible change in
the rate of the elimination reaction. Notably, the complex was
quite stable to the water, and >95% conversion into Ph-
P(O)(OEt)2 was still observed. No hydrolysis products were
observed in this reaction. Analysis of the 1H NMR spectrum of
1
bond forming reaction was monitored using H and 31P{1H}
NMR spectroscopy. Solutions (C6D6) of the intermediates were
generated and held at 21.6 °C for three half-lives, while NMR
spectra were systematically collected. Comparison of the integral
values for the intermediate complex and aryl phosphonate
relative to an unreactive internal standard revealed the concen-
tration of the intermediate and product. The reported kinetic
data in the following tables represent the average of two
independent experiments.
To provide a baseline for the elimination reactions, initial
studies focused on the aryl palladium phosphonate complex
containing the electron neutral -C6H5 group and dppe as the
diphosphine. The results of the initial screening are summarized
in Table 6. Generally, a dry NMR tube was charged with the
arylpalladium phosphonate precursor, 2 equiv of dppe, (meth-
oxymethyl)diphenylphosphine oxide (internal standard for both
1H and 31P NMR), and dry C6D6. NMR spectra were collected
within minutes and revealed complete displacement of the
bu2bipy ligand and quantitative generation of the dppePdPh-
(P(O)(OEt)2 as well as a small amount of arylphosphonate.
Analysis of the integral data over three half-lives generated a
linear first-order plot with t1/2 ) 342 min. The t1/2 values for
these elimination reactions were used to investigate the effect
of different additives. At the end of the reaction, the only signals
1
in the H and 31P NMR spectrum were due to (dppe)2Pd(0),

Arylpalladium Phosphonate Complexes
Organometallics, Vol. 28, No. 4, 2009 1197
Table 2. 31P{1H} NMR Data for the Reactive Intermediates
with this observation is the presence of 1 equiv of water per Pd
complex in virtually all of the crystallized compounds. In
all of the crystalline samples, the water is hydrogen bonded
to the phosphoryl oxygen. It should be pointed out that
although the compounds are isolated as anhydrous materials,
they are quite hygroscopic and will absorb water upon
exposure to the atmosphere. To further probe the tolerance/
intolerance of the system to the presence of water, analogous
reactions were carried out in the presence of 10 equiv of
water. Curiously, instead of accelerating the elimination reac-
tion, the t1/2 for the complex was significantly increased. Similar
to the reactions using 1 equiv of water, the compound was stable
to the added water and the product distribution remained the
same (>95% conversion into PhP(O)(OEt)2). It is noteworthy
to point out that free palladium was not observed in any of these
transformations. Thus, while the reductive elimination was
tolerant of small amounts of water, increasing the level of water
contamination lead to decreased reaction rates, but no change
in product distributions.
To probe the electronic effects of adding donating and
withdrawing groups to the metal-bound aryl fragments on the
rate of phosphorus carbon bond formation, elimination from
discrete species of the type diphosphinePd(C6H4R)(P(O)(OEt)2
(R ) OMe, Me, CN, NO2) was investigated. The results of this
investigation are presented in Table 7. Using this data, several
observations and conclusions can be made about the elimination
process. First, electron-poor aryl groups exhibited slower rates
of reductive elimination relative to electron-rich aryl groups.
Using the dppePdPh(P(O)(OEt)2) as a baseline, incorporating a
4-CN group cuts the rate of reductive elimination in half, while
adding an electron-donating 4-OMe group significantly increases
the rate. These observations are consistent with a mechanism
involving intramolecular attack of the ipso carbon from the
metal-bound aryl fragment on the phosphorus center to generate
the new phosphorus carbon bond. The presence of electron-
donating groups would accelerate this process, while electron-
withdrawing groups would slow the bond-forming reaction.
While this is an attractive explanation, our kinetic model cannot
rule out the possibility of a concerted reductive elimination.
Changing the supporting ligand from dppe to other diphos-
phine ligands dramatically affected the rate of the phosphorus-
carbon bond forming reaction. Using dppp as the ancillary ligand
resulted in significant increases in the rates of all the elimination
reactions. The complexes dpppPd(4-C6H5R)P(O)(OEt)2 (R )
Me, OMe) eliminated so rapidly at room temperature that an
accurate determination of the rates was not possible. Comparing
the two diphosphines revealed that elimination from the dppp
derivatives was always faster than the dppe analogues. The
observed increase in the rate of the reductive elimination reaction
upon increasing the bite angle of the diphosphine is consistent
with other bond-forming reactions. Changing the diphosphine
to dpephos (bite angle ) 105°) resulted in rapid reductive
elimination for all cases. The large bite angle of dpephos could
promote the reductive elimination reaction from a four-
coordinate species, or dissociation of one end of the diphosphine
could generate a three-coordinate intermediate that undergoes
rapid reductive elimination. It should be pointed out that our
kinetic model cannot distinguish between dissociation of one
end of the diphosphine to generate a three-coordinate intermedi-
ate that rapidly undergoes reductive elimination followed by
rapid addition of the dangling phosphine and simple elimination
from a four-coordinate complex.
a Data recorded in C6D6 at 21.7 °C. b Data recorded in C6D6 at 8.0
°C.
the reaction mixture revealed the water peak at 2.45 ppm. This
is significantly shifted from where water in pure C6D6 appears
(0.5 ppm). The change in the chemical shift of the water is
presumably due to hydrogen bonding between the water and
the phosphoryl oxygen of dppePdPh(P(O)(OEt)2). Consistent
The electronic effect observed in these metal phosphonate
systemsstandsincontrasttovirtuallyallothercarbon-heteroelement

1198 Organometallics, Vol. 28, No. 4, 2009
Kohler et al.
Table 3. Structure Information for Alkyl and Arylpalladium Phosphonate Complexes
dppePd(4-C6H4CN)(P(O)(OEt)2) bu2bipyPd(4-C6H4NO2)(P(O)(OEt)2)
dppePdMe(P(O)(OEt)2)
bu2bipyPdPh(P(O)(OEt)2)
formula
C37H38NO3P3Pd · H2O
C28H38N3O5PPd · H2O
C31H37O3P3Pd · (0.5H2O) ·
C28H39N2O3PPd · H2O
(0.25THF) · (0.25C5H12)
formula wt
cryst syst
space group
a (Å)
b (Å)
c (Å)
762.01
orthorhombic
P212121
18.1086(12)
21.8118(15)
27.2808(18)
90
652.00
701.99
monoclinic
P21/c
12.2388(5)
14.5238(6)
37.2223
90
98.9350(10)
90
607.00
triclinic
triclinic
j
j
P1
P1
10.8000(7)
12.5432(9)
12.6532(9)
95.8280(10)
100.1240(10)
115.2270(10)
100(2)
11.2013(4)
11.6154(4)
13.3073(5)
105.030(1)
113.038(1)
103.874(1)
100(2)
R (deg)
ꢀ (deg)
90
90
γ (deg)
temp (K)
100(2)
10775.4(13)
12
100(2)
6536.1(5)
8
V (Å3)
1496.20(18)
2
1420.82(9)
2
Z
θmax (deg)
27.45
1.409
26.98
1.447
26.21
1.427
27.77
1.419
d(calcd) (Mg m-3
)
no. of rflns collected
no. of indep rflns
R(I > 2σ(I))
134 068
18 415
66 097
17 609
24 468 [R(int) ) 0.0340]
R1 ) 0.0240
wR2 ) 0.0529
1.093
6493 [R(int) ) 0.0226]
R1 ) 0.0270
wR2 ) 0.0668
1.027
13 050 [R(int) ) 0.0257]
R1 ) 0.0378
wR2 ) 0.1042
1.026
6674 [R(int) ) 0.0215]
R1 ) 0.0193
wR2 ) 0.0506
1.093
GOF
absolute struct param
0.00(1)
Table 4. Selected Bond Distances (Å) and Angles (deg) for
bu2bipyPd(4-C6H4R)(P(O)(OEt)2 Complexes
Table 6. Effect of Additives of Elimination Reactiona
R ) H
R ) NO2
Pd(1)-C(1)
Pd(1)-N(1)
Pd(1)-N(2)
Pd(1)-P(1)
1.9965(13)
2.1336(11)
2.1349(11)
2.2266(3)
1.4988(10)
98.82(3)
76.97(4)
93.89(5)
90.47(4)
170.57(5)
173.98(3)
1.994(2)
2.1274(17)
2.1254(16)
2.2381(5)
1.4979(14)
107.53(5)
76.81(6)
94.17(7)
81.32(6)
169.81(17)
175.15(5)
P(1)-O(1)
N(2)-Pd-P(1)
N(2)-Pd-N(1)
N(1)-Pd-C(1)
C(1)-P(1)
C(1)-N(2)
N(1)-Pd-P(1)
#
additive
t1/2 (min)b
1
2
3
4
5
6
7
8
none
10 equiv bu2bipy
5 equiv dppe
10 equiv dppe
10 equiv PPh3
10 equiv PhC’CPh
1 equiv H2O
342
365
348
350
361
376
409
1046
Table 5. Selected Bond Distances (Å) and Angles (deg) for
dppePd(R)(P(O)(OEt)2 Complexes
10 equiv H2O
a Reactions carried out in C6D6 at 22 °C. b The half-life is the time
required for 1/2 of the complex to be consumed and is reported as the
average of duplicate experiments.
amides50 containing Pd(II) complexes was accelerated by the
presence of electron-withdrawing groups on the metal-bound
aryl fragment. Although the obvious difference between the
phosphonate systems presented in this work and the elimination
studies involving -OR, -NR2, and -PR2 is the absence of the
lone pair on the heteroatom, recent studies have suggested that
the electronic effects observed in carbon-heteroelement bond
forming reactions are the result of perturbations of the σ-network
and that π-contributions are minimal.40 It should also be noted
that the rate of reductive elimination of an arylphosphine from
a diphosphine-ligated palladium phosphide was accelerated upon
incorporation of an electron-withdrawing group (-NO2) into
the metal-bound aryl group.12e The Hammett analysis of the
rates of elimination relative to the complex containing the
C ) Me
C ) 4-C6H4CN
Pd(1)-C(1)
2.101(3)
2.3325(7)
2.3215(7)
2.2941(8)
1.500(3)
100.88(3)
84.35(3)
90.06(9)
84.51(9)
174.32(9)
169.82(3)
2.065(2)
2.3575(5)
2.3203(6)
2.2962(6)
1.4961(17)
99.37(2)
84.73(2)
91.33(6)
84.70(6)
175.91(6)
168.63(2)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-P(3)
P(3)-O(1)
P(1)-Pd-P(3)
P(2)-Pd-P(1)
P(2)-Pd-C(1)
C(1)-Pd-P(3)
C(1)-Pd-P(1)
P(2)-Pd-P(3)
bond forming reactions that have been reported from Pd(II).40,46
Elegant work by Hartwig and others has shown that reductive
elimination from arylpalladium alkoxides,47,48 sulfides,49 and
(48) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1997, 119, 6787.
(46) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852.
(47) Mann, G.; Shelby, Q.; Roy, A. H.; Hartwig, J. F. Organometallics
2003, 22, 2775.
(49) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzei,
I. A. J. Am. Chem. Soc. 1998, 120, 9205.
(50) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 8232.

Arylpalladium Phosphonate Complexes
Organometallics, Vol. 28, No. 4, 2009 1199
Table 7. Electronic and Steric Effect on Alkyl and Arylphosphonate
Elimination from Model Compoundsa
Figure 2. Hammett analysis for elimination from dppePd(4-
C6H4X)(P(O)(OEt)2) species.
electron-neutral phenyl group is shown in Figure 2. A nearly
linear relationship between σp and the rate of elimination was
obtained (FΙ ) -2.7; R2 ) 0.99). Comparisons between these
FΙ values and other carbon-heteroelement bond forming reac-
tions would be of interest; however, the inductive σ parameter
provided poor correlations for elimination of aryl sulfides and
aryl amines.40,46,50 Linear correlations were observed using a
parameter that accounted for the resonance effects of the
substituents (σR). The FΙ value for carbon-carbon bond forming
reactions from Pd(II) would also be a useful comparison for
this chemistry; however, the formation of biaryls from palla-
dium(II) is often too rapid for accurate measurement. The
formation of biaryls by reductive elimination from diarylplati-
num complexes of the type (DPPF)Pt(C6H4R)2 and (DPPF)-
Pt(C6H4CF3)(C6H4R) has been studied, and FI values of -0.9
and -1.5 have been determined.51
The steric effect of the metal-bound aryl group was inves-
tigated by adding one or two methyl groups to the ortho
positions of the metal-bound aryl fragment. Adding a single
methyl group resulted in an elimination rate that was roughly
equivalent to having a phenyl group attached to the metal.
However, incorporating two methyl groups drastically reduced
the rate of the reaction. This is consistent with π-type coordina-
tion of the eliminated arylphosphonate species to the palladium
complex.49 Only having one group does not strongly inhibit this
coordination; however, two ortho substituents significantly
affects the coordination and slows the reaction. The sluggish
rate could also be due to the bulky groups hindering close
approach of phosphorus and carbon.
A comparison of the electronic and steric properties of the
metal-bound aryl group revealed that changing the identity of
the diphosphine reversed the importance of these properties.
Comparing the rates of elimination for dppePdR(P(O)(OEt)2)
complexes bearing 2,6-C6H3Me2 group and a 4-C6H4CN group
revealed that the electronic component is more important for
the rate of elimination in these species, and slower rates of
elimination were observed for the sterically hindered species.
Curiously, increasing the chain length of the diphosphine ligand
reverses this observation. For example, the steric component
overpowers the electronic and the rate of elimination for
dpppPd(4-C6H4CN)(P(O)(OEt)2) is faster than from dpppPd(2,6-
C6H3Me2)(P(O)(OEt)2).
a Reactions carried out in C6D6 at 22 °C. b Rates (kobs) are reported in
s-1 (× 100 000) and are (10%. c The elimination was too fast for
accurate rate determination. d Less than 1% elimination was observed
after standing at 22 °C for 43 200 min. 5% elimination product was
observed after standing at 22 °C for 45 120 min. The relative rate is by
comparison to the electron neutral phenyl derivative.
(51) Shekhar, S.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 13016.

1200 Organometallics, Vol. 28, No. 4, 2009
Kohler et al.
Table 8. Summary of Calculated Gas Phase Kinetic Data
substituent
σp
k (1/s)
log(k/kH)
NMe2
NH2
OMe
Me
-0.83
-0.66
-0.27
-0.17
0
0.23
0.42
0.54
0.66
0.78
1.99 × 104
9.96 × 103
5.57 × 103
6.05 × 102
7.54 × 102
1.76 × 102
6.56 × 10
6.64
1.42
1.12
0.87
-0.10
0
-0.63
-1.06
-2.05
-1.28
-1.43
H
C2H
CHO
CF3
CN
3.95 × 10
NO2
2.82 × 10
Table 9. Summary of Calculated Solvent Phase Kinetic Data
substituent
σp
k (solv, 1/s)
log(k/kh) (solv)
NMe2
NH2
OMe
Me
-0.83
-0.66
-0.27
-0.17
0
0.23
0.42
0.54
0.66
0.78
3.85 × 104
8.50 × 103
5.06 × 103
2.99 × 102
4.40 × 102
1.43 × 102
2.67 × 10
3.40
1.94
1.29
1.06
-0.17
0
-0.49
-1.22
-2.11
-1.59
-1.77
H
Figure 3. Hammett plot for phosphorus-carbon bond forming
reaction (gas phase).
C2H
CHO
CF3
CN
1.12 × 10
NO2
7.55
Computational Investigation of the Reductive Elimina-
tion Reaction. A Hammett analysis utilizing density functional
theory was carried out using the substituents and associated
kinetic data listed in Table 8 (gas phase simulations) and Table
9 (simulations in benzene solvent) and plotted in Figures 3 and
4. For this computational analysis, a reductive elimination
mechanism was assumed for the P-C bond forming reaction.
Both the gas phase and solvated (benzene) values produce a
negative FI of moderate magnitude (-2.0 (gas phase) or -2.4
(benzene)). Electron-withdrawing groups in the para position
retard the reaction, so it is assumed that the Pd-C(ipso) bond
donates electrons to the Pd center in the reductive elimination.
These DFT results, which are consistent in both magnitude and
direction with the experimental Hammett analysis (Vide supra),
support the contention that the mechanism of P-C bond
formation is a reductive elimination for which electron-releasing
substituents accelerate the process. Furthermore, the small
difference between the benzene and gas phase calculated FI
argues against mechanisms in which there is significant charge
buildup in the transition state.
Figure 4. Hammett plot for phosphorus-carbon bond forming
reaction (solvated: benzene).
procedures.11a All yields are based upon isolated material unless
specified. 1H and 13C chemical shifts were determined by reference
to residual protonated solvent resonances or Me4Si. All coupling
constants are given in hertz. 31P{1H} NMR spectra were referenced
to external H3PO4 (0 ppm). Integratable 31P{1H} NMR spectra were
recorded using an inverse-gated decoupling sequence (decoupling
only during acquisition) using a 30 s recycle delay. Arylphospho-
nates were identified by comparison of the spectroscopic data to
authentic samples or literature values.2,11a,52,53
Summary
A series of arylpalladium phosphonate complexes have been
prepared, characterized, and studied as key intermediates in
palladium-catalyzed coupling reactions. All of the palladium
complexes eliminated the arylphosphonate species in high yields
under mild conditions. In contrast to virtually all other
carbon-heteroatom bond forming reactions from palladium(II),
the presence of electron-withdrawing groups on the metal-bound
aryl group induced slower rates of reductive elimination. The
Hammett analysis revealed a linear dependence on sigma with
a FΙ ) -2.7. A computational study corroborated this experi-
mental investigation and also found a linear dependence on σp
with F ) -2.4 for the solvated (benzene) model system.
Preparation of bu2bipyPd(Me)(P(O)(OEt)2). A 100 mL round-
bottom flask was charged with bu2bipyPdMeCl (0.100 g, 0.235
mmol), Ag[P(O)(OEt)2] (0.058 g, 0.236 mmol), and a magnetic
stirring bar. The flask was capped, evacuated, and filled with
nitrogen. Dichloromethane (50 mL) was added by syringe, and the
reaction mixture was stirred for 2 h in the dark. The reaction mixture
was passed through a short plug of Celite followed by removal of
the volatiles to afford the title compound (0.11 g, 88.7%) as a white
solid. Anal. Calcd for C23H37N2O3PPd: C, 52.42; H, 7.08. Found:
1
C, 52.34; H, 7.01. H NMR (CDCl3, 25 °C): δ 10.11 (d, 1H, J )
6.0, Ar-H), 8.59 (dd, 1H, J ) 5.8, 3.3, Ar-H), 8.01 (s, 1H, Ar-H),
7.94 (s, 1H, Ar-H), 7.51 (m, 2H, Ar-H), 4.08 (m, 4H, -OCH2CH3),
1.44 (s, 9H, -CMe3), 1.41 (s, 9H, -CMe3), 1.31 (t, 6H, J ) 7.0,
-OCH2C H3), 0.79 (d, 3H, J ) 0.8, PdMe). 13C{1H} NMR (CDCl3,
25 °C): δ 163.7 (s, quat), 162.4 (s, quat), 156.1 (d, J ) 1.7, quat),
153.8 (d, J ) 1.5, quat), 153.6 (s, Ar-H), 147.2 (s, Ar-H), 123.6
Experimental Section
General Considerations. Diethyl ether, dichloromethane, and
hexane were dried using a Grubbs-style solvent purification system.
C6D6 and tol-d8 were dried over molecular sieves. The diphosphine
ligands, (methoxymethyl)diphenylphosphine oxide, and bu2bipy
were obtained from Aldrich and used as received. The bu2bipyPd-
(Ar)(P(O)(OEt)2) precursors were prepared following literature
(52) Kagayama, T.; Nakano, A.; Sakaguchi, S.; Ishii, Y. Org. Lett. 2006,
8, 407.
(53) Kottmann, H.; Skarzewski, J.; Effenberger, F. Synthesis 1987, 797.

Arylpalladium Phosphonate Complexes
Organometallics, Vol. 28, No. 4, 2009 1201
(s, Ar-H), 122.8 (d, J ) 2.9, Ar-H), 118.0 (d, J ) 3.0, Ar-H), 117.5
(s, Ar-H), 57.7 (d, J ) 2.9, -OCH2CH3), 35.4 (s, -CMe3), 35.2
(s, -CMe3), 30.34 (s, -CMe3), 30.31 (s, -CMe3), 16.8 (d, J )
7.5, -OCH2CH3), -1.9 (d, J ) 5.2, -PdMe). 31P{1H} NMR
(CDCl3, 25 °C): δ 72.6.
General Procedure for Isolation of DiphosphinePd(R)(P(O)-
(OEt)2) Species. A 10 mL reaction vial was charged with the
appropriate bu2bipyPd(R)(P(O)(OEt)2) precursor and 1 equiv of the
diphosphine. After evacuating and refilling with nitrogen, anhydrous
ether (5 mL) was injected by syringe. After stirring for 3 h, the
solid was separated using a centrifuge, and the resulting solid was
washed with diethyl ether (2 × 3 mL) and dried under vacuum.
31P{1H} NMR spectroscopic data for the isolated and observed
compounds are summarized in Table 2.
0.038 mmol) and dppp (0.016 g, 0.038 mmol) to afford 0.012 g
(47%) as a white solid. Anal. Calcd for C32H39O3P3Pd: C, 57.28;
H, 5.86. Found: C, 56.97; H, 5.49. H NMR (CDCl3, 25 °C): δ
7.89 (m, 4H, Ar-H), 7.41 (m, 3H, Ar-H), 6.91-7.20 (m, 13H, Ar-
H), 4.18 (m, 4H, -OCH2CH3), 1.99 (m, 2H, -PCH2CH2C H2P-),
1.95 (m, 4H, -PCH2CH2CH2P-), 1.37 (m, 3H, PdMe), 1.09
(t, 6H, -OCH2CH3).
1
General Procedure for Observation of DiphosphinePd-
(R)(P(O)(OEt)2) Species. For compounds that underwent rapid
reductive elimination, the following method was used for spectro-
scopic characterization at reduced temperatures. An oven-dried
NMR tube was charged with bu2bipyPd(R)(P(O)(OEt)2), 1 equiv
of diphosphine, and 0.5 mL of C6D6. After shaking for 2 min the
samples were placed into a cooled NMR spectrometer (8 °C) and
1H and 31P{1H} NMR spectra were recorded.
Preparation of dppePd(4-C6H4CN)(P(O)(OEt)2). The general
procedure was followed using bu2bipyPd(4-C6H4CN)(P(O)(OEt)2)
(0.020 g, 0.033 mmol) and dppe (0.013 g, 0.033 mmol) to afford
0.018 g (75%) as a white solid. Anal. Calcd for C37H38NO3P3Pd:
Reductive Elimination of Arylphosphonates. Method A. An
oven-dried NMR tube was charged with isolated diphosphineP-
d(R)(P(O)(OEt)2), 1 equiv of diphosphine, (methoxymethyl)diphe-
1
C, 59.73; H, 5.15. Found: C, 60.05; H, 4.85. H NMR (C6D6, 25
1
nylphosphine oxide (internal standard), and 0.5 mL of C6D6. H
°C): δ 7.93 (m, 4H, Ar-H), 7.62 (m, 2H, Ar-H), 6.88-7.18 (m,
18H, Ar-H), 3.88 (m, 2H, -OCH2CH3), 3.61 (m, 2H, -OCH2CH3),
1.72 (m, 4H, -PCH2CH2P-), 0.80 (t, 6H, J ) 7.2, -OCH2CH3).
Preparation of dppePd(4-C6H4NO2)(P(O)(OEt)2). The general
procedure was followed using bu2bipyPd(4-C6H4NO2)(P(O)-
(OEt)2) (0.020 g, 0.032 mmol) and dppe (0.013 g, 0.032 mmol) to
afford 0.015 g (63%) as a white solid. Anal. Calcd for
NMR spectra were collected over three half-lives. Comparison of
the integral values of the palladium complex and arylphosphonate
relative to the internal standard provided the concentrations of the
complexes at specified times.
Method B. An oven-dried NMR tube was charged with
bu2bipyPd(R)(P(O)(OEt)2), 2 equiv of diphosphine, (methoxym-
ethyl)diphenylphosphine oxide (internal standard), and 0.5 mL of
C6D6. A 1H NMR spectrum was recorded immediately to determine
the extent of bu2bipy displacement. Once the bipyridine ligand was
1
C36H38NO5P3Pd: C, 56.59; H, 5.01. Found: C, 56.88; H, 4.22. H
NMR (C6D6, 25 °C): δ 7.92 (m, 4H, Ar-H), 7.61 (m, 2H, Ar-H),
6.90-7.14 (m, 18H, Ar-H), 3.65 (m, 2H, -OCH2CH3), 3.61 (m,
2H, -OCH2CH3), 1.68 (m, 4H, -PCH2CH2P-), 0.83 (t, 6H, J )
7.2, -OCH2CH3).
Preparation of dppePd(2-C6H4Me)(P(O)(OEt)2). The general
procedure was followed using bu2bipyPd(2-C6H4Me)(P(O)(OEt)2)
(0.020 g, 0.033 mmol) and dppe (0.013 g, 0.033 mmol) to afford
0.010 g (42%) as a white solid. Anal. Calcd for C37H41O3P3Pd: C,
60.62; H, 5.64. Found: C, 60.69; H, 5.31. 1H NMR (C6D6, 25 °C):
δ 8.10 (m, 4H, Ar-H), 6.89-7.35 (m, 20H, Ar-H), 3.70 (m, 2H,
-OCH2CH3), 3.45 (m, 2H, -OCH2CH3), 2.50 (s, 3H, -Me), 1.80
(m, 4H, -PCH2CH2P-), 0.89 (t, 6H, -OCH2CH3).
Preparation of dppePd(C6H5)(P(O)(OEt)2). The general pro-
cedure was followed using bu2bipyPd(C6H5)(P(O)(OEt)2) (0.020
g, 0.034 mmol) and dppe (0.014 g, 0.034 mmol) to afford 0.015 g
(61%) as a white solid. Anal. Calcd for C36H39O3P3Pd: C, 60.13;
H, 5.47. Found: C, 60.27; H, 5.30. 1H NMR (C6D6, 25 °C): δ 7.95
(m, 4H, Ar-H), 7.75 (m, 3H, Ar-H), 6.82-7.13 (m, 18H, Ar-H),
3.95 (m, 2H, -OCH2CH3), 3.62 (m, 2H, -OCH2CH3), 1.72 (m,
4H, -PCH2CH2P-), 0.81 (t, 6H, J ) 7.4, -OCH2CH3).
Preparation of dppePd(2,6-C6H3Me2)(P(O)(OEt)2). The gen-
eral procedure was followed using bu2bipyPd(2,6-C6H3Me2)(P(O)-
(OEt)2) (0.020 g, 0.032 mmol) and dppe (0.013 g, 0.032 mmol) to
afford 0.019 g (79%) as a white solid. Due to the slow rate of
ligand displacement, this reaction was stirred for 24 h before
isolation. Anal. Calcd for C38H43O3P3Pd: C, 61.09; H, 5.80. Found:
C, 61.36; H, 6.37. 1H NMR (C6D6, 25 °C): δ 8.14 (m, 4H, Ar-H),
6.82-7.24 (m, 19H, Ar-H), 3.96 (m, 2H, -OCH2CH3), 3.60 (m,
2H, -OCH2CH3), 2.67 (s, 6H, -Me), 1.82 (m, 4H, -PCH2CH2P-),
0.85 (t, 6H, -OCH2CH3).
Preparation of dppePd(Me)(P(O)(OEt)2). The general proce-
dure was followed using bu2bipyPd(Me)(P(O)(OEt)2) (0.020 g,
0.038 mmol) and dppe (0.015 g, 0.038 mmol) to afford 0.024 g
(96%) as a white solid. Anal. Calcd for C31H37O3P3Pd: C, 56.67;
H, 5.68. Found: C, 56.32; H, 5.86. 1H NMR (C6D6, 25 °C): δ 7.91
(m, 4H, Ar-H), 7.41 (m, 4H, Ar-H), 6.91-7.20 (m, 12H, Ar-H),
4.18 (m, 4H, -OCH2CH3), 1.80 (m, 4H, -PCH2CH2P-), 1.42 (m,
3H, PdMe), 1.08 (t, 6H, -OCH2CH3).
1
fully displaced (within minutes in most cases), H NMR spectra
were systematically collected over three half-lives. Comparison of
the integral values of the palladium complex and arylphosphonate
relative to the internal standard provided the concentrations of the
species.
Computational Studies. All calculations were performed with the
Jaguar 5.5 code using the B3LYP/CSDZ* level of theory. Unless
otherwise noted, stationary points have been confirmed by vibrational
analysis using an analytic Hessian. Additionally, the transition mode
for transition states has been visually inspected to correspond to the
expected mode. All unimolecular rate constants were computed using
RRKM theory at 300 K. For reactions computed in solvent, the
Poisson-Boltzmann equations were solved numerically using the
parameters for benzene. In these cases, the barrier height was adjusted
by adding the difference of the solvation corrections, retaining the same
vibrational analysis of reactant and transition state for the prefactor
computation. The diphosphine ligand is typically dppe in experimental
systems, but the phenyls have been replaced with hydrogens for the
sake of modeling simplicity.
Acknowledgment. The authors thank the National Science
Foundation for the funds to purchase the NMR spectrometer
(CHE-0521108) and acknowledge the U.S. Department of
Education for their support of CASCaM. Calculations employed
the UNT computational chemistry resource, the purchase of
which was supported by a CRIF grant from the U.S. National
Science Foundation (CHE-0342824). T.R.C. acknowledges NSF
(CHE-0701247) for supporting his contribution. The authors
thank Professor John Hartwig, Daniela Ide, and Marco Rod-
riguez-Lopez for helpful discussions.
Supporting Information Available: CIF files for the arylpal-
ladium phosphonate complexes as well as details of the computa-
tional investigation are available free of charge via the Internet at
http://pubs.acs.org.
Preparation of dpppPd(Me)(P(O)(OEt)2). The general proce-
dure was followed using bu2bipyPd(Me)(P(O)(OEt)2) (0.020 g,
OM800906M