Synthesis, structural, and electron topographical analyses of a dialkylbiaryl phosphine/arene-ligated palladium(I) dimer: Enhanced reactivity in Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1021/ja0558995

The treatment of bis(2-(dicyclohexylphosphino)-2′,6′- dimethoxybiphenyl)PdCl₂ with AgBF₄ produces an air-stable phosphine/arene-ligated Pd(I) dimer with two seemingly identical Pd-arene interactions by X-ray crystallography. However, NMR and theoretical electron topographical analyses of this complex distinguish between these two interactions. One interaction is classified as an arenium-like complex, while the other is classified as a π-interaction. Additionally, this complex is a suitable precatalyst for high yielding Suzuki-Miyaura coupling reactions in short reaction times.

Full text of DOI:10.1021/ja0558995

Published on Web 12/31/2005
Synthesis, Structural, and Electron Topographical Analyses of
a Dialkylbiaryl Phosphine/Arene-Ligated Palladium(I) Dimer:
Enhanced Reactivity in Suzuki-Miyaura Coupling Reactions
Timothy E. Barder
Contribution from the Department of Chemistry, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139
Received September 3, 2005; E-mail: tbarder@mit.edu
Abstract: The treatment of bis(2-(dicyclohexylphosphino)-2′,6′-dimethoxybiphenyl)PdCl2 with AgBF4 pro-
duces an air-stable phosphine/arene-ligated Pd(I) dimer with two seemingly identical Pd-arene interactions
by X-ray crystallography. However, NMR and theoretical electron topographical analyses of this complex
distinguish between these two interactions. One interaction is classified as an arenium-like complex, while
the other is classified as a π-interaction. Additionally, this complex is a suitable precatalyst for high yielding
Suzuki-Miyaura coupling reactions in short reaction times.
Phosphines as supporting ligands for metals, particularly
palladium, have become ubiquitous in the field of cross-coupling
chemistry.1 The continuing examination of structural features
of catalyst systems that engender efficacy in coupling processes
has become an essential element in ligand design. Particularly,
creating not only an electron-rich phosphine center but also
modifying other structural features of the phosphine ligand has
been valuable in constructing efficient catalysts. In recent years,
Figure 1. Recently developed 2-dicyclohexylphosphine biaryl ligands.
we have focused on increasing electron density and bulk on
the biaryl backbone of 2-dialkylphosphino biaryls (Figure 1).2
analyze and further understand these types of interactions, we
These modifications have created highly reactive and stable
sought to induce a Pd-arene interaction involving the electron-
catalyst systems for various Pd-catalyzed cross-coupling pro-
rich bottom ring of 1 with a highly electrophilic Pd(II) center.
cesses. Recently, we reported the X-ray crystal structure of 1•Pd-
Herein, we present our findings and analysis of this endeavor.
(dba)2b,2d (where 1 ) 2-(dicyclohexylphosphino)-2′,6′-dimethoxy-
Synthesis of Complex 6. We recently reported the structure
biphenyl and dba ) trans,trans-dibenzylideneacetone) which
of 4,2d which does not possess any Pd-arene interactions, as
the nonphosphine containing ring of the ligand is pointed away
from the Pd center. We envisioned treating this complex with
possesses an η1-Pd interaction with the 1′ ipso carbon. Similar
Pd-arene interactions exist with complexes composed of other
2-dicyclohexylphosphino biaryl ligands, such as 2•Pd(dba) and
a Ag(I) salt to prepare 5 (Figure 2). Complex 5 would possess
3•Pd(dba),3 as well as with MOP•Pd(II) and MAP•Pd(II)
a highly electrophilic Pd center, which could be stabilized by
complexes4 (where MOP ) 2-diphenylphosphino-2′-methoxy-
1,1′-binaphthyl and MAP ) 2-diphenylphosphino-2′-N,N′-
(dimethylamino)-1,1′-binaphthyl). This unique interaction has
been previously suggested to provide stability for the Pd center
in both the Pd(II) and Pd(0) states;2b,4 however, little is known
the nonphosphine containing rings of the ligands. However,
when a mixture of 4 and AgBF4 in CH2Cl2 was stirred under
argon at ambient temperature for 3 h, the desired product, 5,
was not formed, but rather a Pd(I) dimer, 6, was produced. The
deep red complex was isolated from an acetone/ether mixture
at -25 °C and crystallized in C2/c with two tetrafluoroborate
about the physical nature of Pd-arene interactions. In hopes to
(1) (a) Negishi, E. Acc. Chem. Res. 1982, 15, 340-348. (b) Negishi, E.; de
Meijere, A., Eds. Handbook of Organopalladium Chemistry for Organic
Synthesis; Wiley-Interscience: New York, 2002. (c) Tsuji, J. Palladium
Reagents and Catalysts: New PerspectiVes for the 21st Century; John Wiley
& Sons Ltd.: Chichester, West Sussex, U.K., 2004.
counteranions and one acetone molecule. To our surprise, this
complex is extremely air-stable in the solid state and solution
(dichloromethane); no significant decomposition was observed
within 1 week. We attribute this stability to the Pd-arene
interactions which shield the Pd centers, thereby preventing them
from interacting with oxygen or from self-aggregation.
The synthesis of an analogous dimer was attempted with
2-(2′,4′,6′-tri-isopropyl)-dicyclohexylphosphine as the supporting
ligand. The reaction appeared to proceed smoothly; however,
upon attempted isolation and crystallization, a mixture of Pd
(2) (a) Huang, X.; Anderson, K. W.; Zim, D.; Jiang, L.; Klapars, A.; Buchwald,
S. L. J. Am. Chem. Soc. 2003, 125, 6653-6655. (b) Walker, S. D.; Barder,
T. E.; Martinelli, J. R.; Buchwald, S. L Angew. Chem., Int. Ed. 2004, 43,
1871-1876. (c) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004,
126, 13028-13032. (d) Barder, T. E.; Walker, S. D.; Martinelli, J. R.;
Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 4685-4696.
(3) Barder, T. E.; Buchwald, S. L. Unpublished results.
(4) Kocovsky, P.; Vyskocil, S.; Cisarova, I.; Sejbal, J.; Tislerova, I.; Smrcina,
M.; Lloyd-Jones, G. C.; Stephen, S. C.; Butts, C. P.; Murray, M.; Langer,
V. J. Am. Chem. Soc. 1999, 121, 7714-7715.
9
898
J. AM. CHEM. SOC. 2006, 128, 898-904
10.1021/ja0558995 CCC: $33.50 © 2006 American Chemical Society

Dialkylbiaryl Phosphine/Arene-Ligated Pd(I) Dimer
A R T I C L E S
Figure 2. Attempted (dashed arrow) and actual (solid arrow) reactions between AgBF4 and 4 (ORTEP diagram with hydrogens removed for clarity with
thermal elliposoids at 30% probability) to produce a dicationic bis-phosphino Pd complex.
that of (Ph•Pd)2(AlCl4)2 prepared by refluxing PdCl2, Al, and
AlCl3 in benzene. An X-ray crystal structure was obtained of
this Pd(I) dimer which possessed indistinguishable Pd-C1,1A,6,6A
bond distances of 2.34 Å (Figure 4a). More recently, a trinuclear
Pd “sandwich” was reported by Sharp (Figure 4b)8 and a Pd(I)
dimer “sandwich” with sulfur based ligands by Pfeffer (Figure
Figure 3. Various types of Pd(I) dimer complexes.
4c).9 Other interesting Pd(I) dimers possessing Pd-arene
interactions include a (dppp•Pd)22+ complex from van Leeuwen
black and free ligand was observed. Additionally, ground-state
geometry optimizations were performed on an analogous
structure to 6, but lacking the 2′,6′-dimethoxy groups. A
(Figure 4d)10 and a biarylphosphine Pd(I) dimer complex
possessing a bridging bromide from Vilar (Figure 4e)11 with
similar Pd-arene bond lengths to 6.
stationary point was found possessing a similar arrangement of
the ligands around the Pd centers relative to 6. However, the
electron topographical analyses (namely the bond ellipticity
values for the Pd-arene interactions) are drastically different
for this structure than for 6, suggesting both interactions are
π-interactions possessing some π-symmetry.6 These data may
suggest that electron-donating groups on the 2′,6′ positions of
the ligand are necessary to stabilize the Pd centers.
Background of Pd(I) Dimers. There have been numerous
Pd(I) complexes prepared and characterized by X-ray crystal-
lography in the past 60 years;5 however, the majority of these
complexes contain a bridging ligand, which are often CO,
hyride, oxygen, halides, isonitriles, and even phosphines (chelat-
ing and nonchelating) between the two Pd centers (Figure 3).
Additionally, olefins have been shown to act as bridging
ligands between the two Pd centers. However, there have been
very few reports of Pd(I) dimer “sandwich” complexes where
both Pd centers are positioned between two arenes. One early
report of a complex described by Allegra in 1965 and 19707 is
Description of the X-ray Crystal Structure of 6. Complex
6 has a Pd-Pd bond length of 2.7037(3) Å, which is consistent
with other Pd(I) dimers (Pd-Pdmin 2.4878(7), Pd-Pdmax 3.1852-
(6) Å).12 There exists not only one but two Pd-arene interac-
tions with the nonphosphine containing ring of the biaryl ligand.
Curiously, the Pd-C(10A) and Pd-C(7) distances are nearly
identical (2.1901(17) and 2.1970(16) Å, respectively), which
are substantially shorter than the Pd-P bond length, 2.3045(4)
Å (Figure 5). The Pd-C(8) and Pd-C(9A) distances are 2.2989-
(16) and 2.3870(17) Å, respectively, which are within the sum
of the van der Waals radii of Pd and carbon atoms. The C(7)-
Pd1 bond length of 2.1970(16) Å is also substantially shorter
(by 0.177 Å) than the C(7)-Pd1 bond length in the previously
reported structure of 1•Pd(dba).2b,d Unlike the 1•Pd(dba) X-ray
crystal structure where little to no bond length deviations were
observed in the nonphosphine containing ring of the ligand
relative to 4, which lacks a Pd-arene interaction, bond length
deviations are observed in 6. The C(7)-C(ortho) distances
lengthen from to 1.397(3) and 1.399(3) Å in 4 to 1.453(2) and
1.435(2) Å in 6, while the O-C(ortho) distances shorten from
1.372(3) and 1.363(3) Å in 4 to 1.345(2) and 1.348(2) Å in 6.
(5) (a) Murahashi, T.; Kurosawa, H. Coord. Chem. ReV. 2002, 231, 207-228.
(b) Kurosawa, H. J. Organomet. Chem. 2004, 689, 4511-4520.
(6) The Cartesian coordinates for this optimized structure are included in the
Supporting Information. Additionally, the bond ellipticity for the Pd1-
C(7) bond critical point is very large (1.042) and for the Pd1-C(10A) bond
critical point is much smaller (0.2128). The Pd1-C(7) value clearly
illustrates that an elliptical-like orbital (residing both on C7 and C8, but
with only one bond critical point to Pd) is interacting with the Pd center,
in direct contrast to 6. For comparison, in the optimized structure of 1•Pd-
(dba),2d where dba ) trans,trans-dibenzylidineacetone, there exists an η2
interaction between Pd and one of the olefins of dba where the Pd-C bond
critical points have ellipticity values of 0.7829 and 0.3077.
(8) Kannan, S.; James, A. J.; Sharp, P. R. J. Am. Chem. Soc. 1998, 120, 215-
216.
(9) Dupont, J.; Pfeffer, M.; Rotteveel, M. C.; De Cian, A.; Fischer, J.
Organometallics 1989, 8, 1116-1118.
(10) Budzelaar, P. H. M.; Van Leeuwen, P. W. N. M.; Roobeek, C. F.; Orpen,
A. G. Organometallics 1992, 11, 23-25.
(11) Christmann, U.; Vilar, R.; White, A. J. P.; Williams, D. J. Chem. Commun.
2004, 1294-1295.
(7) (a) Allegra, G.; Immirzi, A.; Porri, L. J. Am. Chem. Soc. 1965, 87, 1394-
1395. (b) Allegra, G.; Cassagrande, G. T.; Immirzi, A.; Porri, L.; Vitulli,
G. J. Am. Chem. Soc. 1970, 92, 289-293.
(12) Murahashi, T.; Otani, T.; Mochizuki, E.; Kai, Y.; Kurosawa, H. J. Am.
Chem. Soc. 1998, 120, 4536-4537.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 899

A R T I C L E S
Barder
Figure 4. Isolated and Characterized Pd(I) complexes.
Figure 5. ORTEP diagram of 6 with hydrogens, acetone, and tetrafluoroborate anions removed for clarity. Thermal ellipsoids at 50% probability.
Suzuki-Miyaura Reactions Utilizing 6. As 6 is composed
of a 1:1 ratio of ligand:Pd, which is believed to be the ratio of
the active catalyst in cross-coupling reactions with 2-phosphi-
nobiaryl based ligands,2d,14 Suzuki-Miyaura coupling reactions
were attempted with 6 as the precatalyst. Heating the reaction
mixture composed of an aryl boronic acid, aryl chloride, base,
and 6 to temperatures g 90 °C caused rapid formation of Pd
black, which is most likely due to the lack of excess ligand to
stabilize the Pd after reduction to the zero oxidation state.
However, decreasing the reaction temperature to 70 °C in
toluene prevented the formation of Pd black, and reactions of
even trisubstituted biaryls proceeded smoothly and very rapidly
(Table 1, entry 1). This is further evidence that the catalytic
cycle is composed of a monoligated-Pd rather than a bis-ligated-
Pd center as with other smaller ligands, i.e., triphenylphosphine
or even tri-tert-butylphosphine. Additionally, arenes possessing
sensitive functional groups, such as 3-chlorobenzaldehyde,
reacted quickly with 2-methoxyphenylboronic acid in 92%
isolated yield using only 0.2% 6 in 1 h at 70 °C (Table 1, entry
Selected Bond Lengths (Å) and Angles (deg)
Pd(1)-Pd(1A) 2.7037(3)
Pd(1)-P(1) 2.3045(4)
Pd(1)-C(8) 2.2989(16)
Pd(1)-C(7) 2.1970(16)
C(7)-C(12) 1.453(2)
Pd(1)-C(10A) 2.1901(17)
Pd(1)-C(9A) 2.3870(17)
O(1)-C(12) 1.348(2)
C(7)-C(8) 1.435(2)
P(1)-Pd(1)-Pd(1A) 169.596(12)
C(6)-C(7)-Pd(1) 114.32(11)
O(2)-C(8) 1.345(2)
C(7)-Pd(1)-C(10A) 170.26(7)
C(10)-C(7)-Pd(1) 88.41
These bond length deviations suggest an arenium ion13 (also
commonly referred to as a σ complex or Wheland intermediate)
has formed, where the bottom ring of the ligand has undergone
electrophilic addition to the Pd center and is stabilized by an
ortho methoxy group. The 172 ppm (broad singlet) 13C NMR
chemical shift of C8 suggests substantial double bond character
in the O(2)-C(8) bond, which is consistent with the formation
of a stabilized arenium ion.4 Further evidence from electron
topography analyses for this complex existing as a stabilized
arenium ion is provided below.
(14) Strieter, E. R.; Blackmond, D. G.; Buchwald, S. L. J. Am. Chem. Soc.
2003, 125, 13978-13980 and references therein.
(13) Olah, G. A. J. Am. Chem. Soc. 1971, 94, 808-820.
9
900 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Dialkylbiaryl Phosphine/Arene-Ligated Pd(I) Dimer
A R T I C L E S
Scheme 1. Proposed Reduction of 6 to 1•Pd(0) by Phenylboronic Acid
Table 1. Suzuki-Miyaura Couplings Using 3 as the Precatalysta
∼0.1% of the highly reactive monoligated-Pd(0) species present
even if the reduction of the monoligated-Pd(II) species was
difficult at this temperature. These results initially suggest that
higher temperatures are required for the disproportionation of
6. However, as the reduction of biaryl phosphine Pd(II) species
(a key step in the mechanism proposed in Scheme 1) is not yet
understood, further studies on the activation of 6 will be
conducted after more data are amassed regarding the reduction
of biaryl phosphine Pd(II) complexes.
Atoms in Molecules and ELF Analyses. To further explore
the electronic nature of this complex, we turned to electron
topography techniques. Although specialized high resolution
X-ray diffraction techniques exist that locate bonding electrons,
standard X-ray diffraction techniques only locate electron
density surrounding but not between atoms (i.e., bonds). The
theory of Atoms in Molecules developed by Bader16 allows for
the topographical analysis of electron density, F(r), and location
and analysis of extrema, i.e., critical points, within the electron
density. In particular, the existence of a bond path containing a
bond critical point (3, -1), where 3 is the rank, ω, i.e., number
of nonzero eigenvalues (curvatures) in standard Cartesian
coordinates, and -1 is the signature, σ, i.e., the sum of the signs
of the three nonzero eigenvalues (curvatures), confirms the
presence of a bonding interaction. However, the rank and
signature do not allow for further analyses of the nature of the
bond, and more in depth topographical values were required.
Additionally, the electron localization function (ELF) of Becke
and Edgecombe17 which was further developed by Silvi and
Savin18 provides insight into the classification of chemical bonds
by visualization of valence shell regions in molecules.
a Reaction conditions: 1 mmol of ArCl, 1.5 mmol of ArB(OH)2, 2 mmol
of K3PO4, 0.2 mol % 3, 0.5 M PhMe, 70 °C.
3). Bis(phosphine)-µ-bromide Pd(I) dimers have also been used
in the amination of aryl halides, Suzuki-Miyaura coupling
reactions, and R-arylation of ester enolates.10,15
In all examples in Table 1, the reaction mixture containing
the red complex, 6, immediately changed color to yellow or
beige upon heating. This color change is likely due to the rapid
reduction of 6 to 1•Pd(0) by the aryl boronic acid with
concurrent generation of the respective biaryl (Scheme 1). Most
likely, disproportionation occurs to form a monoligated-Pd(0)
and a monoligated-Pd(II) species. Reduction of the Pd(II) species
can then occur by the aryl boronic acid. To initially probe the
activation of 6, the reaction of o-tolyl boronic acid and 2-chloro-
m-xylene with 0.2% 6 was conducted at 40 °C for 1 h. This
reaction is identical to the reaction in Table 1, entry 1 (for which
a 96% yield was obtained) except for the lower temperature.
However, only ∼25% conversion of the aryl chloride was
observed when the reaction temperature was 40 °C. Surprisingly,
the much easier reaction of o-tolyl boronic acid with 4-n-
butylchlorobenzene only proceeded to ∼50% conversion of aryl
chloride using 0.2% 6 in 1 h at 40 °C. Since disproportionation
of 6 yields both a monoligated Pd(II) and Pd(0) species (the
presumed active catalyst), the reaction of o-tolyl boronic acid
with 4-n-butylchlorobenzene should proceed very rapidly if
disproportionation readily occurs at 40 °C as there would be
All calculations were conducted on a home-built Linux cluster
consisting of 24 Xeon processors. The ground-state optimization
of (1)2Pd2 was completed using Gaussian 0319 with the
2+
B3LYP hybrid functional.20 For C, H, O, and P atoms, the
6-31G(d) basis set was used, and for the Pd center, LANL2DZ+
ECP21 was employed. The X-ray structure coordinates were used
as a starting point for the optimization which was constrained
to C2 symmetry. Due to the large size of this calculation, a
(16) Bader, R. F. W. Atoms in Molecules; Oxford Unversity Press: New York,
2003.
(17) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397-5403.
(18) Silvi, B.; Savin, A. Nature 1994, 371, 683-686.
(19) Frisch, M. J., et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(15) (a) Hooper, M. W.; Utsunomiya, M.; Hartwig, J. F. J. Org. Chem. 2003,
68, 2861-2873. (b) Jørgensen, M.; Lee, S.; Liu, X.-X.; Wolkowski, J. P.;
Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 12557-12565. (c) Stambuli,
J. P.; Kuwano, R.; Hartwig, J. F. Angew. Chem., Int. Ed. 2002, 41, 4746-
4748.
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 901

A R T I C L E S
Barder
2
Figure 6. Contour diagrams of F(r) [a, b, c, d] and ∇ F(r) [e, f] (black contours ) positive values, green contours ) negative values) for various planes
in 6. The red circles are (3, -3) critical points, the blue triangles are (3, -1) critical points, and the pink curve in [d] is the bond path connecting Pd1-
C(10A). Contour values for [a-d]: 0.001, 0.002, 0.004, 0.008, 0.02, 0.04, 0.08, 0.2, 0.4, 0.8. Those for [e, f]: -40, -20, -10...10, 20, 40.
Table 2. Selected Topographical Values for 6a
frequency calculation to ensure all positive eigenvalues could
not be conducted; however, (1)2Pd2 was optimized to con-
2
Ai−Aj
ri
rj
F(rc)
∇ F(rc)
λ1
λ2
λ3
ꢀ
2+
Pd-C(7)
2.289 1.972 0.0740 0.1592 -0.0757 -0.0718 0.3066 0.0540
vergence criteria as follows: RMS force ) 0.000 003 hartrees/
bohr and RMS atom displacement ) 0.000 343 hartrees/radian,
with ∂E/∂Xn ) 0.0 (n ) 980). Subsequently, a single-point
energy calculation with concurrent generation of a wave function
file was conducted at 6-311++G(2d,2p) for C, H, O, and P
and LANL2DZ+ECP21 for Pd. The wave function file was
analyzed by AIM2000.22 For the ELF analysis, a formatted
checkpoint file was used employing the program DGrid 3.0.23
The files generated by these programs were visualized using
VMD24 and MOLEKEL.25
Pd-C(10A) 2.358 2.014 0.0618 0.1502 -0.0614 -0.0522 0.2637 0.1754
C(7)-C(8) 1.410 1.348 0.2620 -0.5186 -0.4965 -0.4158 0.3938 0.1942
O-C(8)
1.673 0.907 0.2643 -0.3381 -0.4701 -0.4640 0.5961 0.0132
a All values in atomic units.
(3, -1) critical point between Pd1-C(n), n ) 7, 10A. However,
in Figure 6d, the contours to which the arrows point widen
relative to those in Figure 6a-c. This is suggestive of the
presence of an interaction between Pd1-C(9A), although neither
a (3, -1) nor a (3, +1) critical point (i.e., a ring critical point
(3, +1) which would be present within the triangle formed by
Pd1-C(10A)-C(9A)) could not be located despite numerous
attempts. Additionally, the (3, -1) critical point between Pd1-
C(10A) in Figure 6c and 6d does not lay directly on the Pd1-
C(10A) bond, as it does in Figure 6a and 6b, but is slightly
shifted to the right, i.e, toward C(9A); the pink line in Figure
6d depicts the actual bond path between Pd1-C(10A). This
slight shift is noteworthy as is described in the following section.
It is important to note that the optimized structure of 6 is not
identical to that of the X-ray crystal structure.26 However, the
structure is very similar with key bond distance differences of
Pd1-C(7) 0.06 Å, Pd1-C(10A) 0.11 Å, and Pd1-Pd1A 0.15
Å. In the optimized structure, from which further discussion is
based, the bond length difference between the Pd1-C(7) and
Pd1-C(10A) is only 0.06 Å.
2
Contour diagrams of F(r) and ∇ F(r) of four different planes
containing the Pd center and carbon atoms nearest the metal
center are depicted in Figure 6. The red circles represent (3,
-3) critical points which are local maxima in F(r), i.e., nuclei,
while the blue triangles represent (3, -1) critical points which
are saddle points in F(r). In Figure 6a-d there exists only one
2
There are two striking differences in the plots of ∇ F(r) for the
Pd1-C(7)-C(8) plane compared to the Pd1-C(10A)-C(9A)
plane. The area of charge concentration between C(7) and Pd1
(Figure 6e, leftmost arrow) is much greater than that compared
to Figure 6f. Additionally, the contour to which the rightmost
arrows point in Figure 6e and 6f encompasses the C(7) and
C(8) atoms, while the corresponding contour in Figure 6f
surrounds the Pd center. This is likely due to the (3, -1) critical
point between Pd1-C(10A) slightly shifting toward C(9A), such
that the bond is becoming more centered between C(10A) and
C(9A) relative to the Pd1-C(7) bond.
(22) Biegler-Ko¨nig, F.; Scho¨nbohm, J.; Bayles, D. J. Comput. Chem. 2001, 22,
545-559.
(23) Kohout, M. DGRID, edition 3.0; Max Planck Institute of Chemical Physics
of Solids: Dresden, 2005.
(24) Humphrey, W.; Dalke, A.; Schulten, K. VMD - Visual Molecular Dynamics.
J. Mol. Graphics 1996, 14, 33-38.
(25) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.0; Swiss
Center for Scientific Computing: Manno, Switzerland, 2000.
(26) See the Supporting Information for the Cartesian coordinates for the
2+
optimized structure of (1•Pd)2
.
9
902 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Dialkylbiaryl Phosphine/Arene-Ligated Pd(I) Dimer
A R T I C L E S
Figure 7. Molecular orbital diagrams of 6, green ) carbon, red ) oxygen, pink ) phosphorus. The Pd centers are located within the surface of the MOs.
(a) HOMO (-0.392 eV), (b) HOMO-1 (-0.399 eV).
Selected electron topographical values for 6 are presented in
Table 2. The most striking values for 6 are the vastly different
bond ellipticity values, ꢀ, defined as (λ1/λ2 - 1). The bond
ellipticity is the ratio of the smallest eigenvalue to the other
negative eigenvalue (only two negative eigenvalues exist for a
bond critical point). By comparing λ1 and λ2, i.e., the amount
of depletion of electron density from the saddle point, in the
direction of the two negative eigenvalues, the bond symmetry
can be analyzed. If λ1 ) λ2, then ꢀ ) 0, which is the case for
a pure sigma bond. For a pure sigma bond, the depletion of
electron density from the bond critical point in the direction of
the two negative eigenvalues is equal, which creates a cylindrical
shape of electron density. However, if |λ1| > |λ2|, then ꢀ > 0
is obtained. In this case, the bond possesses varying amounts
of π character; the greater the absolute value of the ratio of λ1
to λ2, the greater the amount of π character. Hence, the
cylindrical shape of electron density, observed when ꢀ ) 0,
stretches to an ellipse. As a standard reference, in ethane,
benzene, and ethylene, the C-C bond critical points have values
Figure 8. Electron localization function (ELF) plots of the nonphosphine
containing ring of the ligand in 6, torquoise ) carbon-carbon bonds and
red ) oxygen-carbon bonds. ELF isosurface value (a) 0.67, (b) 0.70, (c)
0.74, (d) 0.78.
of ꢀ ) 0.0, 0.2, 0.33.27
The values of ꢀ for the (3, -1) critical point of Pd1-C(7)
and Pd1-C(10A) of the ground-state optimized structure of 6
provide insight into the symmetry of the bond that is present.
Namely, for the Pd1-C(7) bond, ꢀ ) 0.0540, while for the
Pd1-C(10A) bond, a value nearly 4 times greater is observed,
ꢀ ) 0.1754. This difference is substantial as both the Pd-C(7)
and Pd1-C(10A) bonds are identical in length in the X-ray
crystal structure and very similar in the optimized structure
(Pd1-C(7) 2.25 Å, Pd-C(10A) 2.31 Å). Additionally, the value
of F(rc) for the Pd1-C(7) bond critical point is slightly greater
(0.0128), suggesting a stronger bond than Pd1-C(10A). The
large difference in bond ellipticity, difference in F(rc) for the
(3,-1) critical points of Pd1-C(7) and Pd1-C(10A), and shift
of the (3,-1) bond critical point toward C(9A) are strong
indications that the Pd1-C(10A) bond is composed of some π
character while the Pd1-C(7) bond is composed of mostly σ
character. This concept is consistent with the 13C NMR spectrum
obtained of 6, as described above. The upper half of the aromatic
ring (C7-C8-C12) of the ligand exists as an arenium ion
(hence the mainly σ character in the Pd-C(7) bond) with the
Pd center stabilized by an ortho methoxy group. However, the
lower half of the aromatic ring (C9-C10-C11) of the ligand
interacts with the Pd centers through both a cylindrical C10(pz)
(the arrow pointing to the blue surface from C10 in Figure 7a)
orbital and an elliptical-like orbital (the arrow pointing to the
white surface from C10 in Figure 7b) residing on C10 and C9.
The contributions from both the HOMO and HOMO-1, which
are very similar in energy (0.007 eV difference), may produce
an elliptical-like orbital, residing mainly on C10 and slightly
on C9. This orbital may interact with the Pd center, causing
the value of ꢀ at the (3, -1) critical point between Pd1-C(10A)
to be nearly 4 times greater than that at the (3, -1) critical
point between Pd1-C(7). Additionally, the presence of this
elliptical-like orbital helps explains the shifting of the (3, -1)
critical point toward C(9A) thereby creating a bond possessing
some π symmetry.
Finally, we plotted the electron localization function (ELF)
to assist in visualizing the valence shell regions of the
nonphosphine containing ring of the ligand in 6 (Figure 8).
Figure 8a-d clearly depict ELF isosurfaces encapsulating C(7),
while no such encapsulation exists around C(10). The lack of
encapsulation around C(10) is likely due to the C10(pz) orbital
being utilized in the Pd1-C(10A) interaction, i.e., the valence
shell electrons are occupied in the Pd1-C(10A) interaction.
However, as the Pd1-(C7) bond is mainly a σ interaction, the
valence shell electrons of C(7) are accessible and are able to
be visualized.
In conclusion, we have synthesized and characterized a novel
phosphine/arene ligated Pd(I) dimer in the solid and solution
state. This dimer is a suitable precatalyst for efficient and rapid
Suzuki-Miyaura cross-coupling reactions. Additionally, electron
topography studies were conducted that shed light on the true
(27) Koritsanszky, T.; Buschmann, J.; Luger, P. J. Phys. Chem. 1996, 100,
10547-10553.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 903

A R T I C L E S
Barder
nature of the seemingly identical two arene-Pd interactions.
The solid and solution state experimental data agree well with
the theoretical data: although two bonds may be nearly identical
in bond length and type of atoms involved, the electronic
properties of the bond are ever so important in determining the
nature of the bond. Namely, we classify the Pd-arene interac-
tion with proximal methoxy groups as an arenium ion (σ
complex), while the Pd-arene interaction distal from the
methoxy groups is mainly a π interaction possessing some π
symmetry due the indirect bond path linking Pd1-C(10A).
Further analyses to determine the importance of Pd-arene
interactions of dialkylbiaryl phosphine ligated Pd complexes
that lay within the catalytic cycles of amination and Suzuki-
Miyaura coupling reactions are in progress.
PdCl2 used in this work. T.E.B. is grateful to Professor Steve
Buchwald for the freedom that made this work possible as well
as his support and encouragement. T.E.B. also thanks Professor
Kit Cummins for providing insightful advice, Professor Ged
Parkin for stimulating discussions, and Drs. Bill Davis and Peter
Mu¨ller for assistance with the solvent disorders in the X-ray
crystal structures. Additionally, T.E.B. thanks one of the
reviewers for suggesting important and insightful experiments
regarding the activation of 6.
Supporting Information Available: Experimental procedures,
spectral data, coordinates for the calculated structures, X-ray
crystal structure data (PDF), and the complete list of authors
for ref 18. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Institutes of Health
(GM 46059) for support for this work. We are grateful to Merck
for additional support. We thank Engelhard for supplying the
JA0558995
9
904 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006