Synthetic, structural, and dynamic NMR studies of (bisphosphine)palladium(0) complexes of dibenzylideneacetone

Article abstract of DOI:10.1016/S0022-328X(00)00524-6

Two complexes of the type (R₂PCH₂CH₂PR₂)Pd(dba) have been prepared by the reaction of Pd₂(dba)₃·CHCl₃ with R₂PCH₂CH₂PR₂ (R = ⁱPr (1), 74%; Cy (2), 57%; dba = dibenzylideneacetone). X-ray crystallographic studies of 1 and 2 reveal that the coordinated dba ligand adopts an s-trans, s-trans conformation in which the palladium is coordinated to one C=C bond in an η²-fashion. Variable temperature, ¹H- and ³¹P{H}-NMR spectroscopy of 1 show two distinct dynamic processes in solution. In the ¹H-NMR spectra, a rapid intramolecular exchange of coordinated and uncoordinated C=C bonds is observed with the estimated ΔG^?_ex being 14 kcal mol^-1. In the ³¹P{H}-NMR spectra, a facile interconversion of the predominant s-trans, s-trans conformer with the minor s-trans, s-cis, and s-cis, s-cis conformers begins to occur at higher temperatures. Molecular mechanics calculations place the relative energies of the three isomers at 0, 0.9, and 4.7 kcal mol^-1, respectively. An intramolecular mechanism for double bond exchange is proposed to occur via a symmetric transition state involving the s-cis, s-cis conformer. Strong coordination of dba to palladium in 1 is proposed to account for slow reactions with PhX where the relative rates of oxidative addition were found to increase in the order of X = Cl ? Br < I.

Full text of DOI:10.1016/S0022-328X(00)00524-6

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Journal of Organometallic Chemistry 616 (2000) 10–18
Synthetic, structural, and dynamic NMR studies of
(bisphosphine)palladium(0) complexes of dibenzylideneacetone
Steven M. Reid, Joel T. Mague, Mark J. Fink *
Department of Chemistry, Perci6al Stern Hall, Tulane Uni6ersity, New Orleans, LA 70118-5698, USA
Received 25 April 2000; received in revised form 7 July 2000
Abstract
Two complexes of the type (R2PCH2CH2PR2)Pd(dba) have been prepared by the reaction of Pd2(dba)3·CHCl3 with
R2PCH2CH2PR2 (R=iPr (1), 74%; Cy (2), 57%; dba=dibenzylideneacetone). X-ray crystallographic studies of 1 and 2 reveal
that the coordinated dba ligand adopts an s-trans, s-trans conformation in which the palladium is coordinated to one CꢀC bond
1
in an h2-fashion. Variable temperature, H- and 31P{H}-NMR spectroscopy of 1 show two distinct dynamic processes in solution.
In the 1H-NMR spectra, a rapid intramolecular exchange of coordinated and uncoordinated CꢀC bonds is observed with the
estimated DG‡ex being 14 kcal mol−1. In the 31P{H}-NMR spectra, a facile interconversion of the predominant s-trans, s-trans
conformer with the minor s-trans, s-cis, and s-cis, s-cis conformers begins to occur at higher temperatures. Molecular mechanics
calculations place the relative energies of the three isomers at 0, 0.9, and 4.7 kcal mol−1, respectively. An intramolecular
mechanism for double bond exchange is proposed to occur via a symmetric transition state involving the s-cis, s-cis conformer.
Strong coordination of dba to palladium in 1 is proposed to account for slow reactions with PhX where the relative rates of
oxidative addition were found to increase in the order of X=ClꢀBrBI. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; Dibenzylideneacetone; X-ray; Intramolecular exchange
Jutand for a variety of mono- and bidentate-phosphines
(Scheme 1) [4–6]. This mechanism was found to be
operative for reaction mixtures that contain Pd(dba)2
and one equivalent of a bisphosphine PꢁP that bears
aryl substituents on phosphorus to afford complexes of
the type (PꢁP)2Pd. Subsequent redistribution with
Pd(dba) eventually generates (PꢁP)Pd(dba) [7].
The burgeoning utility of palladium sources such as
Pd2(dba)3·CHCl3 in catalytic reactions has sparked an
increasing number of investigations of isolated com-
plexes (PꢁP)Pd(dba) and (P)2Pd(dba). An early report
by Pierpont et al. [8] described the X-ray crystal struc-
ture of (bipy)Pd(dba) which appears to be the first
structurally characterized example of molecules of the
type L2Pd(dba) (bipy=2,2%-bipyridine) [9]. More recent
examples include related complexes that contain chelat-
ing [10–15] and monodentate [16,17] ancillary ligand
motifs.
1. Introduction
Coordinatively unsaturated and zero-valent palla-
dium fragments of the types (P)2Pd and (PꢁP)Pd are
generally accepted as the active catalytic species in
many kinds of reactions (P=monodentate phosphine;
PꢁP=chelating bisphosphine) [1]. Convenient sources
of palladium(0) in these reactions are the air-stable
complexes Pd2(dba)3·CHCl3 and Pd(dba)2 (dba=
dibenzylideneacetone) [2,3]. The mechanism of the in
situ
formation
of
dba-stablilized
complexes
(PꢁP)Pd(dba) has been established by Amatore and
Previous studies of zero-valent palladium complexes
in this laboratory led to the discovery of [(dcpe)Pd]2
which furnishes the highly reactive (dcpe)Pd(0) frag-
ment through a rapid dimer-monomer equilibrium
Scheme 1.
* Corresponding author. Fax: +1-504-8655596.
E-mail address: fink@tulane.edu (M.J. Fink).
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00524-6

S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
11
(dcpe=1,2-bis(dicyclohexylphosphino)ethane) [18]. The
limiting small scale synthesis of this complex together
with its instability toward oxygen and water prompted
us to pursue a more straightforward and practical route
to (PꢁP)Pd(0) from related complexes (PꢁP)Pd(dba).
Herein we describe the synthesis and X-ray crystal
structures of two such species containing electron-rich
chelating bisphosphines. Additionally, we report a dy-
namic NMR study on one of these complexes.
(dippe=1,2-bis(diisopropylphosphino)ethane). Large
orange crystals of 1 were obtained in 74% yield from
cold benzene–pentane at −20°C whereas complex 2
was isolated as dark red–orange cubes in 57% yield
from cold CH2Cl2–Et2O.
2. Results
2.1. Synthesis
(1)
Slow addition of dippe or dcpe to a dark purple
solution of Pd2(dba)3·CHCl3 in benzene gradually gave
an orange solution of (dippe)Pd(dba) (1) or an orange
precipitate of (dcpe)Pd(dba) (2) as shown in Eq. (1)
The dippe derivative 1 decomposes in air over several
months and much more rapidly (1–2 days) in hydrocar-
bon solution. Even more rapid decomposition was ob-
served in CDCl3 and CD2Cl2 to give unidentifiable
products (31P{H}-NMR). In contrast, the more steri-
cally congested 2 was found to be more robust and
could be manipulated under air and in chlorinated
solvents for short periods of time without appreciable
decomposition. 1H-NMR spectra of 1 (C6D6) and 2
(toluene-d8) at 20°C each reveal broad resonances in-
cluding those for one coordinated dba ligand while the
31P{H}-NMR spectra each consist of two broad reso-
nances of equal intensity. The origin of these broad
resonances will be discussed in a later section.
2.2. X-ray crystallography
The solid state structures of 1 and 2 were determined
by X-ray crystallography. ORTEP views of 1 and 2 are
given in Figs. 1 and 2, respectively, while experimental
details and relevant metrical data for both complexes
are presented in Tables 1–3. A comparison of bond
angles and distances in 1 and 2 indicates that the core
geometries of their structures are virtually superimpos-
able while minor differences between the two are not
statistically significant. The bond distances of the coor-
Fig. 1. ORTEP view and atom numbering scheme of 1. Most hydrogen
atoms have been omitted for clarity. Thermal ellipsoids are drawn at
the 30% probability level.
,
,
dinated olefins (1.428(8) A for 1 and 1.408(7) A for 2)
are significantly shorter than those observed for the
,
related (dppe)Pd(dba) (1.522(8) A) [10] and
,
[(Cy2PCH2)2NMe]Pd(dba) (1.52(3) A) [12] but com-
parable to those in [2-(2%-dicyclohexylphos-
phinophenyl)-2-methyl-1,3-dioxolane]Pd(dba) (1.423(3)
,
,
A) [11], [P(Ph2Np)3]2Pd(dba) (1.44(3) A) and
,
(PBz3)2Pd(dba) (1.45(4) A) [16], and both molecules of
,
(2,2%-bipyridine)Pd(dba) (1.42(1) and 1.45(1) A) [8]. In
accordance with structural data for the five complexes
that contain symmetrical ancillary ligand arrays men-
tioned above, there is no statistical difference between
both PdꢁColefin bond distances in either 1 or 2. The only
Fig. 2. ORTEP view and atom numbering scheme of 2. A molecule of
CH2Cl2and most hydrogen atoms have been omitted for clarity.
Thermal ellipsoids are drawn at the 30% probability level.

12
S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
Table 3
exception to this trend appears to be [(Cy2PCH2)2-
NMe]Pd(dba) for which a weak Pd···Ocarbonyl interac-
,
Selected bond lengths (A) and angles (°) for 2
Bond lengths
Table 1
PdꢁP(1)
PdꢁP(2)
2.283(1)
2.307(1)
2.159(5)
2.159(5)
1.852(5)
1.863(5)
1.408(7)
1.406(7)
Crystal data and structure refinement for 1 and 2
PdꢁC(28)
PdꢁC(29)
P(1)ꢁC(1)
P(2)ꢁC(2)
C(28)ꢁC(29)
C(1)ꢁC(2)
Compound
(dippe)Pd(dba)
(1)
(dcpe)Pd(dba)·
CH2Cl2 (2)
Color/shape
Orange/column
C31H46OP2Pd
293(2)
Orthorhombic
Pna21
Orange/block
C44H64Cl2OP2Pd
293(2)
Triclinic
P1
Chemical formula
Temperature (K)
Crystal system
Space group
Bond angles
C(28)ꢁPdꢁC(29)
P(1)ꢁPdꢁP(2)
P(2)ꢁPdꢁC(29)
P(1)ꢁPdꢁC(28)
P(2)ꢁPdꢁC(28)
P(1)ꢁPdꢁC(29)
38.1(2)
88.04(5)
162.0(1)
147.9(2)
124.0(2)
109.9(2)
Unit cell dimensions
,
a (A)
21.470(1)
8.6569(8)
17.193(2)
12.017(1)
19.751(1)
9.5524(9)
95.515(6)
99.598(8)
77.948(6)
2181.7(3)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
tion was proposed to shorten one PdꢁColefin bond
(PdꢁO=3.296 A). The corresponding Pd···Ocarbonyl dis-
,
V (A )
Z
3195.7(5)
4
1.253
0.701
,
Dcalc (g cm−3)
Absorption coefficient
(mm−1)
1.291
0.652
,
,
tance in 1 is 3.293 A and in 2 is 3.177 A but we find no
clear evidence of Pd···Ocarbonyl interactions in either
complex. The C15ꢁC16ꢁP1ꢁP2 torsion angle in 1 (1.5°)
and C28ꢁC29ꢁP1ꢁP2 torsion angle in 2 (2.8°) indicate
only slight twists between each set of ligand manifolds.
The P1ꢁPdꢁP2 bite angles in 1 (88.26(5)°) and 2
(88.04(5)°) are similar to that observed previously for
(dppe)Pd(dba) (87.19(5)°).
Radiation
Mo–Ka
Mo–Ka
(u=0.71073)
Enraf–Nonius
CAD4/q/2q
1.90–25.98
(u=0.71073)
Enraf–Nonius
CAD4/q/2q
1.75–25.24
Diffractometer/scan type
q Range for data
collection (°)
Reflections measured
Independent/observed
reflections
3242
8170
3242/2663
[I\|2(I)]
3242/1/332
7765/4491
[I\|2(I)]
7765/0/459
2.3. Reactions between 1 and halobenzenes
Data/restraints/
parameters
It is generally accepted that a key step in many
palladium catalyzed reactions that involve aryl halides
(or pseudo-halides) is the oxidative addition of ArX to
L2Pd(0) species to furnish intermediates of the type
L2Pd(Ar)X. Although it would be more convenient in
practice to generate such intermediates from mixtures
of Pd2(dba)3 (or Pd(dba)2) and phosphines L, we under-
took a brief survey of reactions between isolated 1 and
PhX in order to assess the relative ease with which
oxidative additions to 1 proceed in the absence of
excess dba (X=Cl, Br, I). As depicted in Eq. (2), 1 was
treated with five equivalents of PhX in toluene-d8 at
Extinction coefficient
Goodness-of-fit on F2
Final R indices
[I\|2(I)]
None
1.045
R1=0.0247,
wR2=0.0604
R1=0.0466,
wR2=0.0652
None
1.007
R1=0.0437,
wR2=0.0952
R1=0.1508,
wR2=0.1165
R indices (all data)
Table 2
,
Selected bond lengths (A) and angles (°) for 1
Bond lengths
PdꢁP(1)
PdꢁP(2)
PdꢁC(15)
PdꢁC(16)
P(1)ꢁC(14)
P(2)ꢁC(13)
C(15)ꢁC(16)
C(13)ꢁC(14)
2.310(1)
2.282(1)
2.151(4)
2.148(4)
1.838(5)
1.853(5)
1.428(8)
1.528(7)
1
65°C and then monitored by H- and 31P{H}-NMR.
No reaction was observed for X=Cl, even after five
days. The reaction of 1 with PhBr, however, was judged
to be complete after five days to give (dippe)Pd(Ph)Br
(3a) and that with PhI after 72 h to give (dippe)Pd(Ph)I
(3b), therefore leading us to assign relative reactivities
1
of PhX in the order ClꢀBrBI. The H-NMR spectra
Bond angles
C(15)ꢁPdꢁC(16)
P(1)ꢁPdꢁP(2)
P(1)ꢁPdꢁC(15)
P(2)ꢁPdꢁC(16)
P(1)ꢁPdꢁC(16)
P(2)ꢁPdꢁC(15)
38.8(2)
88.26(5)
160.9(2)
149.5(1)
122.1(1)
110.8(2)
of reaction mixtures containing 3a and 3b are similar
and each shows resonances for one equivalent of free
dba. Additionally, inequivalent methylene protons in
the PCH2CH2P backbone give rise to two multiplets;
similar patterns are observed for the dippe methyne
protons.

S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
13
2.4. Dynamic NMR studies of 1
1
The observation of broad resonances in the H- and
31P{H}-NMR spectra of 1 and 2 at 20°C suggested
dynamic behavior of 1 and 2 in solution. The low
solubility of 2 in hydrocarbon solvents (e.g. C6D6 and
toluene-d8) precluded variable temperature NMR spec-
tra of this complex in solution; however, detailed NMR
studies on the more soluble 1 was possible. The 400.08
1
MHz H-NMR spectrum of 1 in toluene-d8 at −80°C
shows two different sets of olefinic resonances for the
dba ligand (Fig. 3). Two well-resolved doublets at l
7.34 and 8.06 (JHH=15 Hz) correspond to the protons
of one uncoordinated double bond of dba. Two com-
plex resonances at l 5.13 and 5.84 are attributed to the
protons of a double bond coordinated to palladium.
Inequivalent phenyl groups on the dba ligand give rise
to two separate resonances for each set of ortho (l 7.40
and 7.50) and meta (l 7.14 and 7.21) protons while the
para protons give rise to two overlapping resonances at
l 7.01. Each dippe methyl group is evidently rendered
magnetically inequivalent as is evidenced by eight sets
of partially resolved and overlapping doublets of dou-
blets that in turn obscure resonances from the
PCH2CH2P ligand backbone. Additionally, inequiva-
lent CHMe2 protons give rise to two multiplets at l
1.41 and 1.89, respectively, in a 3:1 ratio.
(2)
Four clearly resolved doublet of doublets between l 0.7
and 1.4 indicate four magnetically inequivalent sets of
dippe methyl groups as would be expected for square
planar complexes of Cs point symmetry as shown in Eq.
(2). The 31P{H}-NMR spectra of 3a and 3b are also
similar in that they are each comprised of an AB
pattern of doublets at l 68.58 and 75.48 (JPP=19 Hz)
for 3a and l 68.08 and 72.43 (JPP=18 Hz) for 3b. The
values of these coupling constants JPP are considerably
less than those reported by Amatore et al. for a range
of complexes (P–P)Pd(Ph)I that contain chelating
phosphines less basic than dippe [7].
Fig. 3. 1H-NMR (400.08 MHz) variable temperature spectra of 1 in toluene-d8. aFree dba; bsolvent impurity.

14
S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
Fig. 4. 31P{H}-NMR (161.96 MHz) variable temperature spectra of 1 in toluene-d8. Not shown on the figure is a very weak AB pattern centered
at 62.4 ppm. It is only observed at temperatures below −30°C, and is attributed to the minor (51%) s-cis, s-trans conformer of 1.
The 161.96 MHz 31P{H}-NMR spectrum at −80°C
consists primarily of a well-resolved AB pattern (l=
67.75 and 67.49; JPP=6.43 Hz). Close inspection of the
baseline revealed the presence of a minor component
(ca. 1%) that gave rise to another AB pattern (l 63.82
and 60.86; JPP= ꢁ9 Hz). Both the low temperature
1H- and 31P{H}-NMR data are consistent with coordi-
nation of dba to palladium through one olefinic bond
representative of a primarily static solution structure of
1 similar to that found in the solid state (Fig. 1).
When the temperature was raised, the 1H- and
31P{H}-NMR spectra indicated that 1 exhibits dynamic
behavior in solution. As Fig. 3 indicates, the sharp
olefinic resonances of the dba ligand at −80°C gradu-
ally broadened as the sample was warmed and finally
coalesced at 20°C. Upon further warming to 120°C, a
single set of two broad olefinic resonances gradually
emerged at l 6.59 and 6.46. These chemical shifts are
the approximate arithmetic means of those for the
protons of the uncoordinated and coordinated dba
double bonds, respectively, in the −80°C spectrum.
Resonances from the dba phenyl groups also coalesced
to give one set of sharp ortho, meta, and para proton
signals at 120°C. Furthermore, the 120°C spectrum
shows that the dippe ligand resonances consist of two
sets of overlapping methyl doublets of doublets while
the CHMe2 protons give rise to two well-resolved mul-
tiplets at l 1.62 and 1.77. A single multiplet for the
PCH2CH2P backbone appeared at l 1.23.
Results from the corresponding variable temperature
31P{H}-NMR experiment are presented in Fig. 4. The
sharp AB pattern of the major species in the −80°C
31P{H}-NMR spectrum of 1 was preserved up to
−30°C above which it gradually collapsed to give two
increasingly broad resonances of equal intensity.
Analogous behavior (not shown in Fig. 4) was also
observed for the AB pattern of the minor species. One
AB pattern then reemerged at 60°C and remained
relatively unchanged up to 120°C, whereas no minor
component was observed at all above −30°C. As
shown in Fig. 4, the chemical shifts of the major species
at −80°C simultaneously diverged and shifted slightly
upfield with increasing temperature.
3. Discussion
A correct interpretation of the dynamic NMR behav-
ior of 1 in solution must take into account two distinct
processes: the facile interconversion of the rotational

S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
15
isomers of the complex, and the exchange between
coordinated and free olefins of the dba ligand. The
variable temperature H-NMR data are consistent with
an intramolecular exchange of coordinated and uncoor-
dinated dba double bonds as shown in Scheme 2. We
have ruled out any intermolecular mechanism that in-
volved dba since a trace impurity of free dba gave rise
to sharp 1H signals that did not coalesce over the
temperature range of this experiment.
The ground state s-trans, s-trans conformer I is that
observed in the solid state. Successive rotations by 180°
about each CcarbonylꢁC(H)C(Ph) single bond would give
s-cis, s-trans conformer II followed by the s-cis, s-cis
conformer III. Early structural work on dba complexes
of palladium demonstrated that the dba ligands may
adopt both s-cis, s-trans and s-cis, s-cis conformers in
the solid state [3,19–21], a fact that was confirmed by a
more recent and accurate determination of the structure
of Pd2(dba)3·CH2Cl2 [14]. The exchange of coordinated
and uncoordinated dba double bonds is proposed to
occur from the s-cis, s-cis conformer via the transition
state IV in which the geometry about palladium is
pseudo-tetrahedral. This is similar to the low energy
configuration observed in the X-ray crystal structure of
(dcpe)Ni(h4-C4H6) [22]. This mechanism nicely accom-
modates the fact that the two phosphorus nuclei remain
inequivalent over the entire temperature range of the
variable temperature NMR experiments.
The mechanism in Scheme 2 is analogous to that
proposed by Jolly et al. for trigonal planar complexes
of the type (R2PCH2CH2PR2)Pd(h2-C4H6) in which
intramolecular exchange of coordinated and uncoordi-
nated butadiene double bonds was observed both in
1
i
t
solution [24] and in the solid state (R=Cy, Pr, Bu)
[22]. The same mechanism was invoked to explain
similar double bond exchange phenomena in
(tBu2PCH2CH2PtBu2)Ni(C4H6) [23] and (R2PCH2CH2-
PR2)Pd(1,5-C6H10) [25] although in these examples ex-
change of the phosphorus nuclei was also observed at
t
higher temperatures (R=iPr, Bu).
The changes in lineshape in the variable temperature
31P{H}-NMR reflect a second process — quite dis-
tinct from the double bond exchange discussed
above — involving rapid interconversion of the con-
formers shown in Scheme 2. Molecular mechanics cal-
culations on 1 confirmed the presence of I, II, and III
as local minima on the conformational potential energy
surface with relative energies of 0, 0.9, and 4.7 kcal
mol−1, respectively. The rotational barrier for I“II
was estimated to be 5 kcal mol−1 and that for II“III
was estimated to be 7.4 kcal mol−1. These values are
consistent with the tentative assignment to II as the
minor component in the −80°C 31P{H}-NMR spec-
trum. At higher temperatures, however, the population
of intermediates II and III would be expected to in-
crease, but so would the rates of exchange between
those species, therefore precluding any spectral assign-
ments to intermediates other than II. For this reason it
was impossible to accurately analyze conformer equi-
libria from 31P{H}-NMR spectra by computer simula-
tion. We note, however, that the observed change in
lineshapes and chemical shifts with increasing tempera-
ture are qualitatively consistent with increasing popula-
tions of conformers II and III.
The significant number of exchanging spins observed
1
in the H-NMR spectra rendered full lineshape analysis
too computationally complex. Consequently, the free
energy of activation for this exchange process DG‡ex was
estimated from the coalescence temperature (ꢁ20°C;
Fig. 3) for the aromatic ortho protons that yielded a
value of ꢁ14 kcal mol−1. This is greater than the
estimated values for double bond exchange in
(tBu2PCH2CH2PtBu2)Pd(C4H6) (DGe‡x=12 kcal mol−1
;
solid state) [22] and (tBu2PCH2CH2PtBu2)Ni(C4H6)
The relatively slow reactions of 1 with PhI and
especially PhBr in toluene at elevated temperatures are
(DG‡ex=7–8 kcal mol−1; solution) [23].
Scheme 2. Proposed intermolecular exchange of coordinated and uncoordinated double bonds in 1 (phenyl groups have been omitted for clarity).

16
S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
cur more rapidly with increasing basicity of the bispho-
sphine, our studies suggest that this trend can be
substantially attenuated when dba is present as a coor-
dinating ligand.
Scheme 3.
5. Experimental
the likely consequences of dba being tightly bound to
palladium. Dissociation of dba from L2Pd(dba) to give
a highly reactive 14 electron species L2Pd(0) is generally
accepted to occur before the oxidative addition of PhX
to palladium [6]. A facile equilibrium involving free dba
and (dippe)Pd(0) is clearly not observed for 1 on the
NMR time scale, although we cannot rule out the
possibility that it occurs on the chemical time scale.
Amatore et al. have studied the kinetics of oxidative
addition of PhI to (PꢁP)Pd(dba) (PꢁP=DIOP, dppf, or
BINAP). The overall reaction rates were dependent
both on the position of the dissociative equilibria and
the relative rates of reaction of PhI with (PꢁP)Pd(dba)
and (PꢁP)Pd(0) (Scheme 3). As expected, (PꢁP)Pd(0)
was found to be the most reactive species towards PhI;
however, reaction with (PꢁP)Pd(dba) does occur but
with rates several orders of magnitude less than those
for (PꢁP)Pd(0) [7]. In contrast, Pregosin et al. demon-
strated that other complexes of the type (PꢁP)Pd(dba)
do not undergo oxidative addition with PhI at all, even
after prolonged reaction times. The authors concluded
that the presence of dba could be detrimental to cata-
lytic reactions in which bidentate ligands are employed
[26]. Our qualitative results are consistent with the
mechanism presented in Scheme 3. The overall reactiv-
ity of (dippe)Pd(0) should be high towards PhI and
PhBr [27]; therefore, the low reactivity of 1 towards
these halobenzenes is the likely consequence of the
extremely low availability of (dippe)Pd(0) from the
dissociation equilibrium.
5.1. General procedures and instrumentation
All air-sensitive manipulations were conducted under
an atmosphere of dry argon in oven-dried glassware
using standard Schlenk techniques. Benzene, Et2O, and
pentane were freshly distilled from purple sodium ben-
zophenone ketyl just prior to use. Other solvents were
of reagent grade and used without further purification.
C6D6 and toluene-d8 were dried over sodium benzophe-
none ketyl and then transferred via vacuum transfer
,
into either storage flasks containing 4 A molecular
sieves or directly into NMR tubes. Chlorobenzene,
bromobenzene, and iodobenzene were distilled from
,
CaH2 under Ar and stored over 4 A molecular sieves.
1,2-bis(dicyclohexylphosphino)ethane (dcpe) was pur-
chased from Strem Chemicals, Inc. 1,2-bis(diisopropy-
lphosphino)ethane (dippe) [28] and Pd2(dba)3·CHCl3
[2,3] were prepared according to published procedures.
NMR spectra were acquired on a GE Omega 400
MHz spectrometer with the following operating fre-
quencies: 1H- (400.08 MHz), 31P{H}-(161.96 MHz).
Chemical shifts are reported in ppm and coupling con-
stants in hertz. All spectra were acquired at ca. 20°C
unless otherwise noted. Elemental analyses were per-
formed by Schwarzkopf Micoanalytical Laboratory,
Inc. (Woodside, NY).
5.2. Preparations
5.2.1. (dippe)Pd(dba) (1)
A stirred dark purple solution of Pd2(dba)3·CHCl3 in
30 ml C6H6 was treated dropwise with neat dippe (760
mg, 2.90 mmol). The solution gradually changed to
orange and was stirred for a total of 15 min, then
concentrated to ꢁ10 ml. Et2O (15 ml) and pentane (15
ml) were added to precipitate free dba which was
allowed to settle and the mother liquor was siphoned
into another flask and stripped to dryness. The result-
ing crude product was crystallized from C6H6–pentane
at 0°C as orange crystals that were washed once with
4. Conclusion
We have prepared two examples of air-stable com-
plexes of the type (PꢁP)Pd(dba). The X-ray crystal
structures of both are in general agreement with the
limited number of other structurally characterized pal-
ladium(0) complexes of dba. However, dynamic 1H-
and 31P{H}-NMR studies revealed that 1 is the first
example in which a dba ligand has been shown to
undergo temperature dependent intramolecular double
bond and conformer exchanges, respectively, particu-
larly to the exclusion of any observable intermolecular
exchange processes. For this latter reason, the reaction
of 1 with halobenzenes is prohibitively slow to be of
practical significance. While oxidative additions of PhX
to (PꢁP)Pd(0) fragments are generally conceded to oc-
1
pentane and dried in vacuo; yield 1.289 g (74%). H-
NMR (C6D6): l 0.3–2.0 (br, 36, dippe), 5.16 (br, 1,
HCꢀCH), 5.73 (br, 1, HCꢀCH), 6.9–7.6 (br, 11, Ph and
HCꢀCH), 8.02 (br, 1, HCꢀCH). 31P{H}-NMR (C6D6):
l 65.83 (br, 1), 66.67 (br, 1). Anal. Calc. for
C31H46P2PdO: C, 61.74; H, 7.69. Found: C, 61.61; H,
7.79%.

S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
17
5.2.2. (dcpe)Pd(dba) (2)
5.3. Molecular mechanics calculations
A solution of dcpe (817 mg, 1.93 mmol) in 10 ml
C6H6 was added during 10 min to a stirred dark purple
solution of Pd2(dba)3·CHCl3 in 15 ml C6H6. Within 5
min after the end of addition, a bright orange solid
precipitated. After 1.25 h, the mixture was poured into
100 ml Et2O and the crude orange product was col-
lected by filtration and then crystallized from CH2Cl2–
Et2O at −20°C as dark orange–red cubes which were
washed with pentane and dried in vacuo at 40°C for 16
Molecular mechanics calculations were performed on
(dippe)Pd(dba) using the MM+ force field of HYPER-
CHEM 5.11. The geometry of the immediate coordina-
tion sphere of the complex (P2PdC2) was taken from
the X-ray structure of 1 and kept frozen during all
energy minimizations. A conformational energy surface
was generated by systematically varying the torsional
angles of the two independent CꢁC bonds to the car-
bonyl carbon, holding those angles fixed, and then
performing an energy minimization. Both sets of angles
were varied systematically from −180 to 180° in 30°
increments. The resultant symmetric energy contour
showed local energy minima corresponding to conform-
ers I, II, and III, with the global minimum belonging to
the s-trans, s-trans conformer I. The relative energies of
I, II, and III were further refined by full geometry
optimization (except for the frozen core) starting from
the local minima of the conformational surface. The
rotational barriers were estimated from the original
conformational map and were not optimized.
1
h; yield 841 mg (57%). H-NMR (toluene-d8): l 0.40–
2.00 (m, 48, dcpe), 5.06 (br, 1, HCꢀCH), 5.55 (br, 1,
HCꢀCH), 6.80–7.50 (m, 11, Ph and HCꢀCH), 7.70 (br,
1, HCꢀCH). 31P{H}-NMR (toluene-d8): l 56.50 (br, 1),
58.37 (br, 1). Anal. Calc. for C43H62OP2Pd: C, 67.66;
H, 8.19. Found: C, 67.28; H, 8.38%.
5.2.3. (dippe)Pd(Ph)Br (3a)
An NMR tube was charged with 1 (38 mg, 0.06
mmol) and PhBr (49 mg, 0.32 mmol, five equivalents).
0.8 ml toluene-d8 was condensed via vacuum transfer
onto the mixture and the NMR tube was then flame-
sealed. The resulting orange solution was kept at 65°C
in an oil bath and monitored by 1H- and 31P{H}-NMR.
After 5 days, the reaction was judged to be complete.
1H-NMR (toluene-d8): l 0.72 (dd, 6, CH(CH3)2,
JHPꢀ17, JHH=7), 0.80 (dd, 6, CH(CH3)2, JHP=15,
5.4. X-ray crystallography
5.4.1. Structure solution and refinement for
(dippe)Pd(dba) (1)
JHH=7), 0.94 (dd, 6, CH(CH3)2, JHP=13, JHH=7),
An orange column-like crystal of 1 of dimensions
0.66×0.40×0.36 mm, obtained by slow diffusion of
pentane into a benzene solution of the complex under
Ar, was affixed to a thin glass fiber with epoxy cement.
General procedures for crystal orientation, unit cell
determination and refinement, and data collection on
the Enraf–Nonius CAD4 diffractometer have been
published [29]. Those specific to the present structure
are presented in Table 1. The initial orthorhombic cell
indicated by the CAD4 software was confirmed by the
observation of mmm diffraction symmetry and the
space group was uniquely determined by the systematic
absences observed in the final data set. The intensity
data was corrected for a linear 4.5% decay in the
intensity monitors, for Lorentz and polarization effects,
and an empirical absorption coefficient was applied by
using „ scans for several reflections with  near 90°.
1.07 (m, 3, CH(CH3)2), 1.30 (m, 1, CH(CH3)2), 1.34
(dd, 6, CH(CH3)2, JHP=16, JHH=7), 1.81 (m, 2,
PCH2CH2P), 2.19 (m, 2, PCH2CH2P), 6.87 (d, 2, free
dba HCꢀCH, JHH=16), 7.26 (m, 4, free dba Ph), 6.98
(m, 3), 7.41 (m, 6, free dba Ph), 7.62 (m, 2), 7.72 (d, 2,
free dba HCꢀCH, JHH=16), plus resonances for excess
PhBr. 31P{H}-NMR (toluene-d8): l 68.58 (d, 1, JPP
=
19), 75.48 (d, 1).
5.2.4. (dippe)Pd(Ph)I (3b)
An NMR tube was charged with 1 (37 mg, 0.06
mmol) and PhI (63 mg, 0.31 mmol, five equivalents).
0.8 ml toluene-d8 was condensed via vacuum transfer
onto the mixture and the NMR tube was then flame-
sealed. The resulting orange solution was kept at 65°C
in an oil bath and monitored by 1H- and 31P{H}-NMR.
1
After 72 h, 1 had been consumed completely. H-NMR
The SHELXTL-PLUS v.5.1 software package (Bruker
(toluene-d8): l 0.70 (dd, 6, CH(CH3)2, JHP=16, JHH
7), 0.78 (dd, 6, CH(CH3)2, JHP=15, JHH=7), 0.92 (dd,
6, CH(CH3)2, HP=13, HH=7), 1.25 (m, 1,
=
AXS; Madison, WI) was used to carry out the structure
solution and refinement. The position of the Pd atom
was determined from a sharpened Patterson function
and the remainder of the structure was developed by
successive cycles of full-matrix, least-squares refinement
followed by calculation of a difference Fourier map.
Hydrogen atoms were placed in calculated positions
with riding contributions except for those bonded to
C(15) and C(16) (coordinated double bond of dba)
which were refined with isotropic displacement
parameters.
J
J
CH(CH3)2), 1.10 (m, 3, CH(CH3)2), 1.32 (dd, 6,
CH(CH3)2, JHP=16, JHH=7), 1.81 (m, 2, PCH2CH2P),
2.25 (m, 2, PCH2CH2P), 6.87 (d, 2, free dba HCꢀCH,
JHH=16), 6.89 (m, 3), 7.26 (m, 4, free dba Ph), 7.41 (m,
6, free dba Ph), 7.62 (m, 2), 7.72 (d, 2, free dba
HCꢀCH, JHH=16), plus resonances for excess PhI.
31P{H}-NMR (toluene-d8): l 68.08 (d, 1, JPP=18),
72.43 (d, 1).

18
S.M. Reid et al. / Journal of Organometallic Chemistry 616 (2000) 10–18
5.4.2. Structure solution and refinement for
(dcpe)Pd(dba) (2)
fully acknowledged for financial support of this
work.
An orange block-like crystal of 2 of dimensions
0.33×0.30×0.26 mm, obtained by cooling to −20°C
a solution of the complex in CH2Cl2–Et2O, was
mounted into a thin-walled glass capillary which was
then flame-sealed. General procedures for crystal orien-
tation, unit cell determination and refinement, and data
collection on the Enraf–Nonius CAD4 diffractometer
are similar to those reported for 1. Those specific to the
present structure are presented in Table 1. The CAD4
software indicated a triclinic cell initially and no higher
symmetry was found. The intensity statistics indicated a
centrosymmetric space group and therefore P1 was
selected and confirmed as the correct space group by
the successful refinement of the structure. The intensity
data was corrected for a linear 4.3% decay in the
intensity monitors, for Lorentz and polarization effects,
and an empirical absorption coefficient was applied by
using „ scans for several reflections with  near 90°.
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6. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 143662 for compound 1
143661 for compound 2. Copies of this information
may be obtained free of charge from: The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
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Acknowledgements
Dow Corning corporation and the Tulane Center
for Bioenvironmental Research (DSWA) are grate-
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