Synthesis, Structure, Theoretical Studies, and Ligand Exchange Reactions of Monomeric, T-Shaped Arylpalladium(II) Halide Complexes with an Additional, Weak Agostic Interaction

Article abstract of DOI:10.1021/ja037928m

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdP ^tBu₂, P^tBu₃, or Ph ₅FcP^tBu₂ (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)₂ (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL₂ (L = 1-AdP^tBu₂, P^tBu ₃, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdP^tBu₂)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdP^tBu₂)₂ or Pd(P^tBu₃)₂. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd...H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.

Full text of DOI:10.1021/ja037928m

Published on Web 01/14/2004
Synthesis, Structure, Theoretical Studies, and Ligand
Exchange Reactions of Monomeric, T-Shaped
Arylpalladium(II) Halide Complexes with an Additional, Weak
Agostic Interaction
James P. Stambuli,† Christopher D. Incarvito,† Michael Bu¨hl,§ and John F. Hartwig*,†
Contribution from the Department of Chemistry, Yale UniVersity, P.O. Box 208107,
New HaVen, Connecticut 06520-8107, and Max-Planck-Institut fu¨r Kohlenforschung,
Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany
Received August 14, 2003; E-mail: john.hartwig@yale.edu
Abstract: A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L ) 1-AdPtBu2, PtBu3, or
Ph5FcPtBu2 (Q-phos); X ) Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative
addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or
to Pd(dba)2 (dba ) dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the
corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride
to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition
of phenyl triflate to PdL2 (L ) 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes.
The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate
in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative
addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by
X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which
has the largest trans influence, is located trans to the open coordination site. The complexes appear to be
stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-
coordination site of the palladium center. The strength of the Pd‚‚‚H bond, as assessed by tools of density
functional theory, depended upon the donating properties of the ancillary ligands on palladium.
Introduction
A vacant coordination site on a transition-metal complex is
required for many classic organometallic reactions.1,2 Therefore,
unsaturated metal complexes are proposed as intermediates in
most metal-catalyzed reactions, such as hydrogenation,3 hydro-
formylation,4,5 olefin polymerization,6,7 and palladium-catalyzed
cross-coupling reactions.8-11 Many of the palladium-catalyzed
cross-couplings are proposed to proceed through unsaturated,
three-coordinate arylpalladium(II) halide complexes (Figure 1).
† Yale University.
§ Max-Planck-Institut fu¨r Kohlenforschung.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; John Wiley & Sons: New York, 2001.
Figure 1. Proposed catalytic cycle for many cross-coupling reactions.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, 1987.
Such intermediates are believed to form directly from oxidative
addition of an aryl halide to a zerovalent palladium source with
a bulky ligand12-14 or by dissociation of a dative ligand from a
four-coordinate arylpalladium halide complex prior to trans-
metalation15 and reductive elimination.16
(3) Halpern, J. Inorg. Chim. Acta 1981, 50, 11.
(4) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.; Gavney,
J. A., Jr.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535.
(5) Breit, B.; Winde, R.; Mackewitz, T.; Paciello, R.; Harms, K. Chem.-Eur.
J. 2001, 7, 3106.
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11539.
(7) Liu, Z. X.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R. J.
Am. Chem. Soc. 2001, 123, 11193.
(8) Culkin, D. A.; Hartwig, J. F. Acc. Chem. Res. 2003, 36, 234.
(9) Hartwig, J. F. Pure Appl. Chem. 1999, 71, 1417.
(10) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513.
(11) Milstein, D.; Stille, J. K. J. Am. Chem. Soc. 1979, 101, 4992.
(12) Stambuli, J. P.; Bu¨hl, M.; Hartwig, J. F. J. Am Chem. Soc. 2002, 124,
9346.
(13) Hartwig, J. F.; Paul, F. J. Am. Chem. Soc. 1995, 117, 5373.
(14) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J. P.; Sadighi, J. P.;
Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 4369.
9
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J. AM. CHEM. SOC. 2004, 126, 1184-1194
10.1021/ja037928m CCC: $27.50 © 2004 American Chemical Society

Monomeric, T-Shaped Arylpalladium(II) Halide Complexes
A R T I C L E S
Scheme 1
The observation and isolation of monomeric three-coordinate
arylpalladium(II) halide complexes are rare. Several arylpalla-
dium halide complexes containing an aryl group, a halide, and
a single phosphine or arsine ligand have been isolated, but these
complexes are typically stabilized by coordination of solvent
or by dimerization. Amatore and Jutand detected by NMR spec-
troscopy an arylpalladium(II)iodide complex with a single arsine
ligand, but the vacant site of this complex was occupied by a
strongly coordinating solvent such as DMF.17 Arylpalladium-
(II) halide complexes ligated by a single hindered P(o-tol3) have
also been isolated, but these complexes are dimeric with bridging
halide ligands.18,19 Evidence for an equilibrium between a
biscarbene arylpalladium halide complex and a species generated
by dissociation of one carbene was recently reported, but the
structure of the complex resulting from ligand dissociation was
not determined.20
halide and triflate complexes in this work were isolated as
monomeric species with a single phosphine ligand and without
coordinated solvent.
Results
1. Synthesis of LPd(Ar)(Br) and LPd(Ar)(I) by Oxidative
Addition. Reaction of the combination of Pd(dba)2 and the
hindered alkyl phosphine ligand PtBu3 or (1-adamantyl)PtBu2
[(1-Ad)P(tBu)2] with a large excess of phenyl bromide generated
monomeric arylpalladium(II) bromide complexes containing a
single phosphine ligand (Scheme 1). However, the presence of
dba as a side product complicated the isolation of these low-
coordinate palladium complexes. In the presence of dba, the
arylpalladium complexes formed the palladium(I) dimer, [1-Adt-
Three-coordinate d8 complexes of other metals have been
isolated. For example, addition of AgPF6 to (PPh3)3RhCl
generated (PPh3)3Rh+,21 and addition of LiN(SiMe3)2 to (PPh3)3-
22
RhCl generated (PPh3)2RhN(SiMe3)2 in studies many years
ago. More recently, Hofmann has prepared a three-coordinate
RhI complex that contains a bulky bidentate phosphine and a
neopentyl group.23 This complex is stabilized by an agostic
interaction between a γ-C-H bond on the neopentyl group and
the RhI center.
25
Bu2PPdBr]2 and 1,1,5-triphenylpenta-1,4-dien-3-one (eq 1).
The organic product most likely was formed by the insertion
of dba into the Pd-C bond of the arylpalladium(II) halide
followed by â-hydride elimination, as would occur in a Heck
reaction,26 and the palladium product was most likely formed
by decomposition of the hydrido halide complex to form the
stable palladium(I) dimer.27 Thus, a more general procedure for
generating these complexes with a variety of aryl halides and
ligands was needed.
A method to prepare arylpalladium halide complexes with a
single phosphine donor would facilitate fundamental studies on
structure and reactivity of a class of compound that serves as a
reactive intermediate in a series of catalytic processes that are
widely used in synthetic organic chemistry.24 We recently
synthesized the first examples of arylpalladium(II) complexes
that are monomeric, possess a single phosphine ligand, and lack
coordinated solvent.12 These complexes were formed by oxida-
tive addition of phenyl bromide or phenyl iodide to a combina-
tion of Pd(dba)2 (dba ) dibenzylidene acetone) and a hindered
trialkylphosphine and by oxidative addition of aryl iodide to
PdL2, with L ) PtBu3. Although four compounds with this
coordination sphere were isolated, a more general method was
needed if a series of these compounds were to be isolated.
Here we describe such improved methods and the use of this
method to prepare a series of new three-coordinate arylpalladium
halide complexes containing trialkyl and ferrocenyl dialkyl-
phosphines and the conversion of these complexes to arylpal-
ladium triflate complexes that have been inaccessible by
oxidative addition. Extensive structural data and computational
studies provide a better understanding of the origin of the low-
coordinate, mononuclear structures. All of the arylpalladium
1.1 Development of a Method To Prepare LPd(Ar)(X) L
) 1-AdPtBu2, PtBu3. Reactions of PdL2 (L ) (1-Ad)PtBu2 1a
or PtBu3 1b) with aryl halides in THF solvents would eliminate
the presence of dba and its complicating side reactions.
However, the oxidative addition of aryl halides to these
palladium(0) complexes are not favorable thermodynamically
in dilute solutions, and reactions of aryl halides with 1a and 1b
in THF solvent required elevated temperatures (70 °C) and long
reaction times (>16 h for phenyl bromide), even when a 40-
fold excess of aryl halide was used. The long reaction times
led to formation of side products including L2Pd(H)Br12,28 and
[HPR3]2[PdBr4].29 The presence of these species prevented isola-
tion of pure arylpalladium(II) halide complexes by this method.
A high concentration of phenyl bromide could cause the
oxidative addition products to form from the palladium(0)
species in higher conversions and with faster rates without
influencing the rate of decomposition of the arylpalladium halide
(15) Ricci, A.; Angelucci, F.; Bassetti, M.; Lo Sterzo, C. J. Am. Chem. Soc.
2002, 1060.
(16) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn.
1981, 54, 1868.
(17) Amatore, C.; Bahsoun, A. A.; Jutand, A.; Meyer, G.; Ntepe, A. N.; Ricard,
L. J. Am. Chem. Soc. 2003, 125, 4212.
(18) Paul, F.; Patt, J.; Hartwig, J. F. Organometallics 1995, 14, 3030.
(19) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. Organometallics 1996,
15, 2745.
(20) Lewis, A. K. d. K.; Caddick, S.; Cloke, F. G. N.; Billingham, N. C.;
Hitchcock, P. B.; Leonard, J. J. Am. Chem. Soc. 2003, 125, 10066.
(21) Uma, R.; Davies, M. K.; Cre´visy, C.; Gre´e, R. Eur. J. Org. Chem. 2001,
3141.
(25) Stambuli, J. P.; Kuwano, R.; Hartwig, J. F. Angew. Chem., Int. Ed. 2002,
41, 4746.
(22) Cetinkaya, B.; Lappert, M. F.; Torroni, S. J. Chem. Soc., Chem. Commun.
1979, 843.
(26) Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314.
(27) Dura-Vila, V.; Mingos, D. M. P.; Vilar, R.; White, A. J. P.; Williams, D.
J. J. Organomet. Chem. 2000, 600, 198.
(23) Urtel, H.; Meier, C.; Eisentrager, F.; Rominger, F.; Joschek, J. P.; Hofmann,
P. Angew. Chem., Int. Ed. 2001, 40, 781.
(24) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules, 2nd ed.; University Science Books: Sausalito, CA, 1999.
(28) Clark, H. C.; Goel, A. B.; Goel, S. J. Organomet. Chem. 1979, 166, C29.
(29) Goel, R. G.; Montemayor, R. G. Inorg. Chem. 1977, 16, 2183.
9
J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004 1185

A R T I C L E S
Stambuli et al.
Scheme 2
Scheme 3
complexes. Thus, the reactions of PdL2 [L ) 1-AdPtBu2(1a),
PtBu3(1b)] were evaluated in neat aryl bromide.
1.2. Preparation of LPd(Ar)(X), L ) 1-AdPtBu2, P(tBu)3.
Scheme 2 summarizes the oxidative addition reactions that form
T-shaped arylpalladium bromide and iodide complexes ligated
by 1-AdPtBu2 and P(tBu)3. Warming of 1-adamantyl di-tert-
butylphosphine and tri-tert-butylphosphine palladium(0) com-
plexes 1a,b in neat phenyl bromide and phenyl iodide formed
the oxidative addition products 2a,b and 3a,b at 70 °C. The
adamantylphosphine-ligated bromide and iodide complexes 2a
and 3a were isolated in high yields after addition of pentane to
the aryl halide solution, but the tri-tert-butylphosphine com-
plexes 2b and 3b were isolated in somewhat lower yields.
The oxidative addition of phenyl chloride generally occurs
more slowly than the addition of phenyl bromide and iodide,30
but Pd(0) complexes of PtBu3 and 1-AdPtBu2 catalyze the
coupling of aryl chlorides under mild conditions.25,31 Despite
this catalytic activity of PtBu3 and 1-AdPtBu2 palladium
complexes, no arylpalladium(II) chloride complexes were
observed by 31P NMR spectroscopy after heating of Pd(0)
complexes 1a or 1b in neat phenyl chloride at 70 °C for 20 h.
Only the starting palladium(0) complex and free ligand were
observed by 31P NMR spectroscopy. We have previously estab-
lished an equilibrium between the combination of free chloro-
toluene and [Pd(PtBu3)2] and the combination of the arylpalla-
dium chloride complex and free PtBu3. This solution contains
smaller amounts of the arylpalladium chloride complex than
the Pd(0) complex, even in neat chlorotoluene.32 Thus, oxidative
addition of chloroarenes to these Pd(0) complexes cannot gen-
erate high yields of arylpalladium chloride complexes containing
unactivated aryl groups for thermodynamic reasons, and the
inability to observe the phenylpalladium chloride complex is
likely due to the instability of the product over long times at
elevated temperatures.
P(tBu)3 or 1-AdPtBu2 have been reported.34 The reactions of
1a and 1b with phenyl triflate at 70 °C produced several
products, none of which displayed NMR signals consistent with
the arylpalladium triflate complex 5a.
1.3. Preparation of LPd(Ar)(X) Complexes with Related
Hindered Phosphines. 1.3.1. L ) 2-AdPtBu2. The reaction
conditions of neat aryl halide required for the formation of 2a,b
in high yield were not necessary to generate the arylpalladium-
(II) bromide complex (2-AdtBu2P)Pd(Br)Ph (2c) containing the
less hindered 2-adamantyl-di-tert-butylphosphine ligand (2-
AdPtBu2). Reaction of Pd(dba)2, 1 equiv of 2-AdPtBu2, and 40
equiv of PhBr in THF solvent at 25 °C formed the oxidative
addition product 2c in 86% yield after addition of pentane to
the reaction solution (eq 2). Pd(dba)2 was a suitable precursor
because complex 2c was more stable toward dba and less soluble
than complexes 2a,b with more hindered trialkylphosphines.
1.3.2. L ) 1-di-tert-butylphosphino-1′,2′,3′,4′,5′-pentaphen-
ylferrocene (Q-phos). Reactions of the Pd(0) complex ligated
by 1-di-tert-butylphosphino-1′,2′,3′,4′,5′-pentaphenylferrocene
(Q-phos) with aryl halides and triflates are summarized in
Scheme 3. Reaction of (Q-phos)2Pd(0) with phenyl bromide and
iodide occurred in THF to form the arylpalladium halide
complexes 2d and 3d with a single phosphine ligand in 58 and
69% isolated yield, respectively. These reactions occurred at
room temperature instead of at 70 °C, as required for reactions
of 1a,b. Again, treatment of the palladium(0) complex with
phenyl chloride did not produce an oxidative addition product.
Only free Q-phos and starting 1c were detected by 31P NMR
spectroscopy after heating of 1c for 20 h at 60 °C in phenyl
chloride. Free ligand and palladium metal were observed at
higher temperatures.
The addition of phenyl triflate often occurs at rates similar
to those for the addition of phenyl bromide.33 However, few
catalytic couplings of aryl triflates catalyzed by complexes of
(30) Grushin, V. V.; Alper, H. Chem. ReV. 1994, 94, 1047.
(31) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(32) Roy, A. H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 1232.
(33) Jutand, A.; Mosleh, A. Organometallics 1995, 14, 1810.
(34) Littke, A.; Dai, C.; Fu, G. J. Am. Chem. Soc. 2000, 122, 4020.
9
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Monomeric, T-Shaped Arylpalladium(II) Halide Complexes
A R T I C L E S
Scheme 4
strong infrared band at frequencies lower than those of typical
C-H stretches. The H NMR chemical shift of the hydrogen
1
involved in a C-H agostic interaction is typically shifted upfield
from its position when unbound to the metal, and the C-H
coupling constant involving an agostic hydrogen is often lower
than that of the analogous free C-H bond.36,37
All infrared spectra of the P(tBu)3, (1-Ad)P(tBu)2, and Q-phos
complexes showed that any agostic interaction must be weak.
The infrared spectra of free ligands 1-AdPtBu2, PtBu3, and
Reaction of phenyl triflate with complex 1c at 60 °C for 2 h
produced the arylphosphonium salt of Q-phos, [FcP(Ph)tBu2-
(Ph)5]+[OTf]-. The same phosphonium salt was also generated
by the addition of silver triflate to a solution of the Q-phos-
ligated bromide complex 2d. No reaction occurred between free
Q-phos and the aryl triflate at 60 °C over the same time frame,
which implies that the P-C bond is formed at the metal center.
Most likely, the arylpalladium triflate complex is unstable to
P-C bond-forming reductive elimination of the phosphonium
salt.
Q-phos contained C-H bands between 3000 and 2800 cm-1
.
The infrared spectra of 2a-5a, 2b-3b, and 2d-3d did not
contain any medium-strong C-H stretching bands below 2800
cm-1 that would indicate a weakened C-H bond resulting from
an agostic interaction.
In contrast, the infrared spectrum of complex 2c containing
the 2-adamantyl ligand contained a weakened C-H stretching
band that clearly indicates the presence of an agostic interac-
tion. The infrared spectrum of free 2-AdPtBu2 contained C-H
bands from 3000 to 2850 cm-1. The infrared spectrum of
2. Independent Route to LPd(Ph)(OTf) (L ) (1-Ad)P-
(tBu)2). A route to the arylpalladium triflate complex ligated
by (1-Ad)P(tBu)2 was developed starting from the phenylpal-
ladium bromide complex, as shown in Scheme 4. Treatment of
bromide complex 2a with silver triflate in toluene at room
temperature produced palladium triflate 5a in good yield. Com-
plex 5a decomposed to many products when heated at 70 °C
for 2 h, and this thermal instability explains the difficulty in
isolating this complex from an oxidative addition at this tem-
perature. The infrared spectrum of triflate 5a in THF solution
showed that the triflate was coordinated to palladium. Free
triflate typically vibrates at 1280 cm-1, and coordinated triflates
complex 2c contained a medium-strong band at 2700 cm-1
.
Thus, the agostic interaction in complex 2c with the secondary
carbon attached to phosphorus appears to be stronger than that
in the complexes with the larger ligands. This stronger agostic
interaction and the increased rigidity of the ligand that would
result from this interaction may account for some of the
increased stability and lower solubility of this complex, relative
to those of the arylpalladium halides with the larger 1-AdPtBu2
and PtBu3 ligands.
vibrate closer to 1380 cm-1 35
.
The infrared spectrum of 5a in
None of the H or 13C NMR spectra of the arylpalladium
1
solution contained a band at 1395 cm-1. Moreover, the 31P NMR
chemical shift in the relatively nonpolar noncoordinating solvent
benzene was similar to that in THF. The infrared spectrum of
5a as a KBr pellet in the solid state was less conclusive. This
spectrum contained a strong band at 1317 cm-1, which falls
between the typical frequencies for free and bound triflates.
X-ray crystallography, however, showed that the triflate was
bound to palladium in the solid state as well (vide infra).
3. Spectroscopic Characteristics. All of the arylpalladium
halide and triflate complexes were fully characterized by
common spectroscopic techniques; all but one complex were
amenable to microanalysis. Several complexes were character-
ized by X-ray diffraction, as described below. The 1:1 ratio of
phosphine to palladium in each of the isolated complexes was
determined by integration of the resonances of the phosphine
versus those of the palladium-bound aryl group. One might
expect that the 31P NMR chemical shifts of the complexes in
this work would differ from those of phosphine ligands in more
conventional coordination spheres. However, no clear trends
in chemical shifts could be gleaned from comparisons of the
shifts of the three-coordinate palladium(II) relative to those of
dimeric arylpalladium halide complexes, bisphosphine arylpal-
ladium halide complexes, or the respective palladium(0) precur-
sors. The mononuclearity of these complexes was determined
by X-ray diffraction.
complexes in this work provided evidence for an agostic
interaction. 1H NMR spectra of complexes 2b, 2d, and 3d were
obtained from room temperature to -100 °C in CD2Cl2, and
1H NMR spectra of (1-Ad)P(tBu)2 complex 2a, and (2-Ad)P-
(tBu)2 complex 2c were obtained between 20 °C and -115 °C
in CDCl2F.38 The 1H NMR spectra of Q-phos-ligated complexes
2d and 3d broadened at low temperatures because of confor-
mational changes of the pentaphenylferrocenyl group on the
NMR time scale, but no resonance emerged at high field at -90
°C in CD2Cl2. The 1H NMR spectrum of (2-Ad)P(tBu)2 complex
2c in CDCl2F at -115 °C was complex due to its low symmetry,
and it was also broadened at this temperature. However, no
upfield resonance that would signify an agostic interaction was
observed at this low temperature. In addition, we measured the
C-H coupling constants of the ligand hydrogens by two-
dimensional J-resolved 13C NMR spectroscopic methods, and
no evidence for a reduced value of these couplings in the
complexes was obtained. Even the methine J1 values of the
CH
2-adamantyl complex 2c, which showed reduced C-H stretch-
ing frequencies, were within 1.5 Hz of those of the free ligand.
The lack of an upfield shift and reduced C-H coupling constant
in these complexes could result from the absence of an agostic
interaction in solution, a small upfield shift and change in
coupling constant from the agostic interaction, or a small change
in value and an equilibration of the agostic and free hydrogens
at a rate faster than the NMR time scale. Computational studies
described in section 4 imply that there is a small difference in
chemical shift between the agostic and free hydrogens.
3.1. Evaluation of Potential Agostic Interactions by 1H and
13C NMR and IR Spectroscopic Studies. We probed for the
presence of an agostic interaction in each complex by infrared
and low-temperature 1H NMR and 13C NMR spectroscopy. The
infrared spectra of many agostic compounds contain a medium-
(36) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988,
36, 1.
(37) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395.
(38) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629.
(35) Lawrance, G. A. Chem. ReV. 1986, 86, 17.
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A R T I C L E S
Stambuli et al.
Figure 2. ORTEP diagram of (1-AdPtBu2)Pd(Ph)Br (2a) at 30% ellipsoids.
Figure 4. ORTEP diagram of P(tBu)3Pd(Ph)Br (2b) at 30% ellipsoids.
Figure 5. ORTEP diagram of PtBu3Pd(m-xylyl)I (3b) at 30% ellipsoids.
Figure 3. ORTEP diagram of (1-AdPtBu2)Pd(Ph)OTF (5a) at 30%
ellipsoids.
3.2. X-ray Diffraction Studies. Several of the arylpalladium
halide and triflate complexes were characterized by X-ray
diffraction. The ORTEP diagrams of 2a, 5a, 2b, 3b, and 2d
are pictured sequentially in Figures 2-6. Selected bond angles
and distances are presented in Tables 2-4. Each complex
adopted a T-shaped geometry with the phenyl group located
trans to the open coordination site. Each complex also contained
a hydrogen atom of a phosphine ligand located within 2.4 Å of
the metal center at the coordination site that lacks a heavy atom.
The hydrogen atom nearest the metal in complex 2a was located
and refined. In the other cases, the M-H distances were
estimated by placing the hydrogen atoms in idealized positions.
The M-H distance depended on the electronic properties of
the ancillary ligands in a fashion that could be rationalized by
considering the electrophilicity of the metal. The more electro-
philic the metal center, the shorter the M-H distance. Therefore,
the metal coordination sphere of these complexes contains three
conventional metal-ligand bonds and an additional weak agostic
interaction together in a square-planar arrangement.
Figure 6. ORTEP diagram of (Q-phos)Pd(Ph)Br (2d) at 30% ellipsoids.
As seen in Table 1, the Pd-P and Pd-Cipso bond distances
were similar among all the compounds. Complex 5a has the
shortest Pd-P bond distance, which may be a result of the weak
9
1188 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004

Monomeric, T-Shaped Arylpalladium(II) Halide Complexes
A R T I C L E S
Table 1. Bond Distances (in angstroms) of Arylpalladium(II) Halide Complexes Determined by X-ray Diffraction and Values Determined
from DFT (BP86/ECP1 level, in parentheses)
a
a
complex
Pd−P
Pd−C
Pd−X
Pd‚‚‚H
Pd‚‚‚C
2a, L ) 1-AdPtBu2, X ) Br
2.296(1)
(2.331)
1.977(4)
(1.989)
nd
2.4711(5)
(2.444)
nd
2.26(3)
(2.284)
nd
2.799(4)
(3.012)
nd
2c, L ) 2-AdPtBu2, X ) Br
5a, L ) 1-AdPtBu2, X ) OTf
2b, L ) PtBu3, X ) Br
ndb
(2.319)
2.2523(16)
(2.320)
2.2854(13)
(2.332)
2.294(1)
(2.347)
(1.998)
1.967(3)
(1.979)
1.977(5)
(1.987)
1.982(4)
(1.989)
1.969(3)
(2.451)
(2.154)
(3.155)
2.159(2)
(2.221/2.348)
2.4537(8)
(2.438)
2.6126(4)
(2.620)
2.06c
2.74c
(2.730)
2.18c
(3.426)
2.81c
(2.421)
(3.087)
3b (3b′),a L ) PtBu3, X ) I
2d, L ) Q-phos, X ) Br
2.33c
2.94c
(2.461)
2.13c
(3.142)
2.78c
2.2567(10)
2.4292(6)
a
b
c
Experiment for 3b with Ar ) 2,4-m-xylyl; DFT results for 3b′ with Ar ) Ph. Not determined. Determined from idealized positions.
Table 2. Bond Angles (in degrees) of Arylpalladium(II) Halide
Complexes Determined by X-ray Diffraction and Values
Determined by DFT (BP86/ECP1 level, in parentheses)
to occupy the open coordination site if this substituent can
donate a hydrogen atom to the metal. One might expect an aryl
group of the pentaphenylferrocenyl unit to bind to the open site,
but these groups are located further from the metal than the
nearest hydrogen of the tert-butyl group and in an inappropriate
geometry to donate to the metal.
complex
P−Pd−C
X−Pd−C
X−Pd−P
2a, L ) 1-AdPtBu2, X ) Br
100.8(1)
(102.9)
(98.8)
100.58(9)
(99)
99.92(15)
(103.0)
100.9(1)
(102.6)
100.71(11)
91.4(1)
(92.8)
(92.0)
90.29(10)
(88)
93.14(5)
(93.9)
94.0(1)
(94.6)
91.94(11)
162.61(3)
(164.2)
(169.2)
169.12(5)
(171)
166.88(4)
(163.0)
164.57(3)
(162.4)
166.16(3)
2c, L ) 2AdPtBu2, X ) Br
5a, L ) 1-AdPtBu2, X ) OTf
Distortions in bond angles at phosphorus are revealed by the
angles summarized in Table 3. The phosphine ligands in all
the complexes pivot at the phosphorus toward the open
coordination site. The Pd-P-C angle that includes the alkyl
group nearest the metal was found to be 12-24° smaller than
the next smallest of the Pd-P-C angles. Moreover, the P-C-C
angle that includes the carbon bearing the hydrogen closest to
the metal was smaller than the other two P-C-C angles. This
difference in P-C-C angles was smaller than the difference
in Pd-P-C angles but was statistically significant. The P-C-C
angle that includes the carbon bearing the hydrogen nearest the
metal was found to be 1.2-6.6° smaller than the next smallest
of the other two P-C-C angles. The P-Pd-C bond angle was
found to be smaller in the complexes that contain the smaller
Pd‚‚‚H distance. The triflate complex 5a, which has the shortest
Pd‚‚‚H distance of 2.06 Å, has the smallest P-Pd-C bond angle
of 92.31(9)°, while complex 3b, which has the largest Pd‚‚‚H
bond distance of 2.33 Å, has the largest P-Pd-C bond angle
of 96.5(1)°. These distortions are consistent with the presence
of an agostic interaction that would require a distortion of the
angles at phosphorus and at the tert-butyl or adamantyl carbon
bound to phosphorus, but they may also simply originate from
a drive to relieve steric congestion. For example, Caulton and
co-workers reported that the one P-Ru-C bond angle in
RuCl2(CO)(PiPr3)2 was 10° smaller than the smaller of the other
two P-Ru-C angles,39 but the Ru‚‚‚H distance of this complex
was longer than would be characteristic of an agostic interaction.
Likewise, one Pd-P-C angle of the tri-o-tolylphosphine ligand
in {Pd[P(o-tol)3](p-C6H4-n-Bu)Br}2 was found to be 5° smaller
than the smaller of the other two angles, and this complex
contains square-planar coordination geometries with no agostic
interactions.18
2b, L ) PtBu3, X ) Br
3b(3b′),a L ) PtBu3, X ) I
2d, L ) Q-phos, X ) Br
a
Experiment for 3b with Ar ) 2,4-m-xylyl; DFT results for 3b′ with
Ar ) Ph.
electron-donating ability of the triflate ligand bound to pal-
ladium. The Pd-O bond length in triflate 5a is shorter than the
palladium-halogen bond lengths because of the smaller size
of the oxygen atom. The palladium-bromide bond distances
did not vary with changes in the phosphine ligand.
The bond angles of the coordination sphere of the metal are
shown in Table 2. All of the complexes that were examined by
X-ray analysis have angles about the palladium that total
approximately 360°, but the angles deviated significantly from
90 or 180°. The P-Pd-Cipso bond angles are all about 10° larger
than the idealized 90° angle of a T-shaped geometry. In contrast,
the X-Pd-Cipso and X-Pd-P bond angles varied signifi-
cantly between complexes of the different ligands. For instance,
PtBu3-ligated 2b and 3b have X-Pd-Cipso angles greater than
93°, while the 1-AdP(tBu)2- and Q-phos-ligated complexes 2a
and 2d have X-Pd-Cipso angles less than 92°. The magnitudes
of the X-Pd-P bond angles were found to vary with the
identity of the halide. Bromide complexes 2b and 2d contain
X-Pd-P angles around 166°, while the triflate complex 5a has
a larger angle of 169° and the iodide 3b has a smaller angle of
165°. The 1-adamantyl-ligated bromide complex 2a has an
X-Pd-P angle of 163°, which is much different from the same
angle in bromides 2b and 3b and is the smallest X-Pd-P angle
of the complexes we studied.
The overall conformation and detailed metrical parameters
of the phosphine ligand provide information on how the
complexes accommodate their steric bulk. The adamantyl cages
of the phosphines in complexes 2a and 5a are positioned nearest
to the open coordination site. In contrast, the largest group on
the Q-phos ligand, the pentaphenylcyclopentadienyl group, is
positioned distal to the open coordination site in 2d. Thus, the
substituent that occupies the largest volume of the ligand appears
4. Computational Studies of 2a, 2b, 3b′, and 2c. To
complement the experimental studies and to provide a further
assessment of the presence or absence of an agostic bonding
interaction, computational studies were conducted. Several
complexes, including those prepared recently40 and some
(39) Huang, D.; Streib, W. E.; Bollinger, J. C.; Caulton, K. G.; Winter, R. F.;
Scheiring, T. J. Am. Chem. Soc. 1999, 121, 8087.
(40) Gerisch, M.; Krumper, J. R.; Bergman, R. G.; Tilley, T. D. Organometallics
2003, 22, 47.
9
J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004 1189

A R T I C L E S
Stambuli et al.
Table 3. Bond Angles Containing the Phosphorus Atom of Complexes 2a, 5a, 2b, and 3b Determined by X-ray Diffraction
complex
atoms
angle (deg)
atoms
angle (deg)
2a
Pd(1)-P(1)-C(7)
Pd(1)-P(1)-C(21)
Pd(1)-P(1)-C(17)
Pd(1)-P(1)-C(7)
Pd(1)-P(1)-C(21)
Pd(1)-P(1)-C(17)
Pd(1)-P(1)-C(9)
Pd(1)-P(1)-C(1)
Pd(1)-P(1)-C(5)
Pd(1)-P(1)-C(13)
Pd(1)-P(1)-C(17)
Pd(1)-P(1)-C(9)
Pd(1)-P(1)-C(45)
Pd(1)-P(1)-C(1)
Pd(1)-P(1)-C(45)
95.5(1)
109.0(1)
122.7(1)
92.31(9)
113.99(9)
118.60(8)
93.79(19)
115.0(19)
116.9(2)
96.5(1)
108.2(1)
123.0(1)
94.52(12)
118.68(11)
119.57(12)
P(1)-C(7)-C(15)
P(1)-C(7)-C(8)
P(1)-C(7)-C(16)
P(1)-C(7)-C(12)
P(1)-C(7)-C(13)
P(1)-C(7)-C(8)
P(1)-C(9)-C(12)
P(1)-C(9)-C(11)
P(1)-C(9)-C(10)
P(1)-C(13)-C(16)
P(1)-C(13)-C(14)
P(1)-C(13)-C(15)
P(1)-C(45)-C(46)
P(1)-C(45)-C(47)
P(1)-C(45)-C(48)
106.0(3)
110.9(3)
116.9(3)
105.83(16)
110.52(14)
116.48(17)
106.6(5)
107.8(6)
118.1(5)
107.9(3)
109.7(3)
117.0(3)
104.0(2)
110.6(3)
117.2(3)
L ) 1-AdPtBu2, X ) Br
5a
L ) 1-AdPtBu2, X ) OTf
2b
L ) PtBu3, X ) Br
3b
L ) PtBu3, X ) I
2d
L ) Q-phos, X ) Br
3b′, near 2810 cm-1 for 2a, and near 2750 cm-1 for 2c.50 As
predicted by these calculations, the experimental infrared
spectrum of 2c contained a C-H stretch at 2710 cm-1, which
is sufficiently far from the standard frequency of C-H stretches
to assign it to an agostic interaction.
Table 4. Computational Analysis of the Pd‚‚‚H Interaction in
Arylpalladium(II) Halides at the BP86/ECP1 Level (except where
otherwise noted)
νC-H‚‚‚Pd
cm-1
δ (1H)
(ppm)a
F
(au)
∇F
complex
WBI
(au)
2a, L ) 1-AdPtBu2, X ) Br
2c, L ) 2-AdPtBu2, X ) Br
2b, L ) PtBu3, X ) Br
3b′, L ) PtBu3, X ) I
2812
2748
2910
2919
0.6 0.023 0.023 0.075
-0.4 0.041 0.029 0.083
1.1 0.016 0.018 0.058
1.3 0.015 0.017 0.053
1
Calculations of the H NMR spectra of 2a-c and 3b′ were
1
also performed. These calculations showed that the static H
NMR spectrum would show only small differences in chem-
ical shift between the hydrogen located nearest the metal center
and the hydrogens distal from the metal in the tert-butyl or
a
B3LYP/II′′ level.
1
adamantyl groups. H NMR chemical shifts for the agostic
prepared many years ago,41 have been termed agostic without
1H NMR or IR spectroscopic evidence.42-47 Thus, we computed
geometries and harmonic vibrational frequencies for several of
the complexes and conducted a topological analysis of the total
electron density as developed by Bader and termed the atoms-
in-molecules (AIM) theory.48
Geometries and harmonic vibrational frequencies were com-
puted at the BP86/ECP1 level49 for complexes 2a (1-AdPtBu2-
Pd(Ph)Br), 2b (PtBu3Pd(Ph)Br), the phenyl analogue of 3b
(PtBu3Pd(Ph)I, 3b′), and 2c (2-AdPtBu2Pd(Ph)Br). The com-
puted geometrical parameters of 2a, 2b, and 3b′ agree well with
those determined in the solid state by X-ray diffraction, except
for the nonbonded Pd‚‚‚C distances, which are noticeably
overestimated (see Tables 2 and 3). A prime indicator for the
presence of an agostic Pd‚‚‚H interaction is the distance between
the palladium and nearest hydrogen atom. The experimental
Pd‚‚‚H bond distance of 2.26 Å for 2a is close to the optimized
value of 2.28 Å (Table 1). The Pd‚‚‚H distances for PtBu3-ligated
iodide 3b′ and 2-AdPtBu2-ligated 2c were computed to be
somewhat longer and shorter, respectively, than that in 2a (2.46
Å and 2.15 Å, Table 1).
hydrogen in the static structures of 2a and 2c were calculated
to lie near +0.6 ppm and -0.4 ppm, respectively (Table 4).
Exchange between the agostic hydrogen and the large number
of free tert-butyl hydrogens or the appropriate adamantyl
hydrogens was fast on the NMR time scale and, therefore, led
to averaged chemical shift values that are within the typical
range for aliphatic hydrogens. These calculations explain the
absence of 1H NMR spectroscopic evidence for metal-hydrogen
interactions in 2a and 2c.
Population analyses were also used to assess the extent of
bonding interactions. The Wiberg bond indices (WBIs)51 of both
PtBu3 complexes were calculated to be less than 0.02 (Table
4). The WBI of complex 2c, which appears by experimental
data to contain the strongest agostic interaction of the complexes
in this work, was 0.04. A yttrium alkyl complex with a hydrogen
that appeared by X-ray crystallography to participate in an
agostic interaction was computed to have a WBI of 0.013 for
its M-H agostic interaction.42
Topological analyses of the total electron densities (Bader
analyses) were also performed to provide a more precise analysis
of the presence or absence of a Pd‚‚‚H bonding interaction. A
bonding interaction between two atoms creates a so-called bond
critical point (bcp), which is the saddle point of the electron
density between the two nuclei. The magnitude (F) of the
electron density and its Laplacian(∇F) at the bcp provide
information about the strength and type of bond between the
two atoms. The F value between the two atoms is typically larger
for covalent bonds than for ionic and hydrogen bonds and is
very small for repulsive interactions. The ∇F is positive for ionic,
Additional computational data providing evidence for a weak
agostic interaction are collected in Table 4. Frequency calcula-
tions predicted that the C-H vibration involving the hydrogen
atom closest to Pd would occur near 2900 cm-1 for 2b and
(41) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. Chem. Commun.
1982, 802.
(42) Niemeyer, M. Z. Anorg. Allg. Chem. 2002, 628, 647.
(43) Chen, W.-C.; Hung, C.-H. Inorg. Chem. 2001, 40, 5070.
(44) Leung, W.-H.; Zheng, H.; Chim, J. L. C.; Chan, J.; Wong, W.-T.; Williams,
I. D. J. Chem. Soc., Dalton Trans. 2000, 423.
(45) Pupi, R. M.; Coalter, J. N.; Petersen, J. L. J. Organomet. Chem. 1995,
497, 17.
(46) Cuenca, T.; Flores, J. C.; Royo, P. Organometallics 1992, 11, 777.
(47) Bennett, M. A.; Chiraratvatana, C.; Robertson, G. B.; Tooptakong, U.
Organometallics 1988, 7, 1403.
(48) Bader, R. F. W. Atoms in Molecules; Oxford Press: New York, 1990.
(49) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474.
(50) Visual inspection of the corresponding vibrational modes (i.e., eigenvectors)
showed that they are clearly localized at the agostic CH bond for 2a and
2c, whereas in the case of 2b and 3b′ some mixing with the other CH
stretching modes from the same methyl group was apparent.
(51) Wiberg, K. B. Tetrahedron 1968, 24, 1083.
9
1190 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004

Monomeric, T-Shaped Arylpalladium(II) Halide Complexes
A R T I C L E S
Scheme 5
hydrogen, or agostic bonds and negative for covalent bonds.
Therefore, the presence of a bcp, a small F value, and a positive
∇F value would indicate the presence of an agostic interaction
between the palladium and nearest hydrogen of the ligand.
For all complexes 2a-c and 3b′, a bond path between Pd
and the nearest H atom was found and the bcp’s showed small
values of F and small, positive values of ∇F. These data indicate
that a weak agostic interaction is present in 2a-c and 3b′. For
comparison, calculations of the Pd-H bonding in Pd(PH3)2-
(H)2 showed a WBI of 0.52 and a F value of 0.127 au (∇F )
-0.118 au). This comparison suggests that the agostic Pd‚‚‚H
interactions in 2a,c are about 1 order of magnitude weaker than
a typical Pd-H single bond. All computational criteria in Tables
2 and 4 indicate that the strength of the Pd‚‚‚H interaction
increases in the sequence 3b′ e 2b < 2a < 2c. Thus, com-
putational studies and IR spectroscopic measurements suggest
that complexes 2b and 3b′ contain weak agostic interactions,
while complex 2c contains a stronger agostic interaction.
The triflate complex 5a appeared to be a special case for the
application of theory to understanding the M‚‚‚H interaction.
In the solid-state structure determined by X-ray diffraction, the
triflate was coordinated to palladium in a η1-fashion, and the
adamantyl group was oriented to create a close Pd‚‚‚H contact
(around 2 Å when H atoms are placed at standard positions).
When a full geometry optimization was started from this
experimental structure, the triflate rotated to place a second
oxygen atom near the vacant coordination site and to create an
η2-triflate complex. This coordination in the calculated structure
released the agostic hydrogen and created a Pd‚‚‚H distance of
2.74 Å (Table 1). These computational data do not imply that
the complex adopts a structure in the solution phase that is
different from that in the solid, but they do suggest that the
equilibrium between both coordination modes is finely balanced.
If so, the magnitude of the agostic interaction is comparable to
the magnitude of the coordination of the second oxygen of a
triflate group.
P(o-tol)3 to 2a produced a 1:1 ratio of 1-AdPtBu2-ligated 2a
and the P(o-tol)3-ligated dimer, {[(o-tol)3P]Pd(Ph)(Br)}2. In
contrast, reaction of Pd[P(o-tol)3]2 with PtBu3 generates
Pd[PtBu3]2 and free P(o-tol)3 with no Pd[P(o-tol)3]2 remaining.
2-Adamantyl complex 2c was more stable than complexes
2a, 2b, 2d, and the tri-o-tolylphosphine-ligated arylpalladium
halide complexes. Reaction of 2a with 1 equiv of 2-AdPtBu2
converted all of 2a to 2c. The reaction energy computed for
this transformation, -2.1 kcal/mol at the BP86/ECP1 + ZPE
level, is consistent with this observation. Similarly, the addition
of 1 equiv of tricyclohexylphosphine to 2a produced 0.5 equiv
of [(PCy3)2Pd(Ph)Br] and left 0.5 equiv of 2a unreacted. A trace
amount of Pd[PCy3]2 was formed. Addition of 2 equiv of PCy3
to 2a generated [(PCy3)2Pd(Ph)Br] and free 1-AdPtBu2 as the
only products observed by 31P NMR spectroscopy.
Discussion
Rates of Oxidative Addition and Catalytic Couplings. The
qualitative trends in rates of oxidative addition to the isolated
PdL2 complexes (L ) tri-tertiaryalkylphosphine) correlated with
the rates of reactions catalyzed by these palladium(0) complexes.
For example, the oxidative addition of aryl bromides and
chlorides to PdL2 with L ) PtBu3 or 1-AdPtBu2 required heating,
and no coupling reactions at room temperature have been
described with these PdL2 complexes as catalyst. Workers at
Tosoh reported the coupling of aryl halides and diarylamines
at 100-120 °C using Pd(PtBu3)2 generated in situ as catalyst.52
Fu and Dai have reported Negishi cross-couplings of aryl and
vinyl chlorides catalyzed by Pd(PtBu3)2 at 100 °C,53 and
mechanistic studies on aryl halide aminations with Pd(PtBu3)2
as catalyst were conducted at 95 °C.54 Reactions catalyzed by
palladium with PtBu3 as ligand at room temperature have been
conducted with the combination of Pd(dba)2 and 1 equiv of
31,34,55
PtBu3
or with precursors that contain one PtBu3 ligand
per palladium.25
Geometry of Arylpalladium(II) Complexes. The ability and
means by which the various complexes can adopt a fully square-
planar geometry dictate whether they become monomeric with
two phosphine ligands, dimeric with one phosphine per metal,
or monomeric with one phosphine ligand and a weak agostic
interaction. The oxidative addition of aryl bromide to Pd(PPh3)4
produces the four-coordinate trans-(PPh3)2Pd(Ar)(Br).56 Addi-
tion of aryl bromide to Pd[(P(o-tol)3]2 produces the dimeric,
5. Phosphine Ligand Exchange Processes. Ligand substitu-
tion reactions of 1-AdPtBu2-ligated 2a were conducted with
P(tBu)3, Q-phos, 2-AdP(tBu)2, and several common monoden-
tate phosphines. These reactions are summarized in Scheme 5.
The addition of 1 equiv of PtBu3 to 2a produced a 2.5:1.0
equilibrium ratio of 2a and PtBu3-ligated 2b. The addition of 1
equiv of Q-phos to 2a produced a 5:1 mixture of 2a and Q-phos-
ligated 2d. Thus, the order of bond strengths of these three
ligands to the arylpalladium(II) halide complexes is 1-AdPtBu2
> PtBu3 > Q-phos.
(52) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39, 2367.
(53) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719.
(54) Alcazar-Roman, L. M.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 12905.
(55) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-
Roman, L. M. J. Org. Chem. 1999, 64, 5575.
(56) Moser, W. R.; Wang, A. W.; Kjeldahl, N. K. J. Am. Chem. Soc. 1988,
110, 2816.
Addition of the classic hindered aromatic phosphine ligand
P(o-tol)3 revealed similar binding affinities of P(o-tol)3 and the
hindered alkylphosphines to Pd(II). The addition of 1 equiv of
9
J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004 1191

A R T I C L E S
Stambuli et al.
Table 5. A Comparison of the Bond Angles that Surround the Pd Atoms in {[(o-tol3)P]Pd(Ar)(Br)} and 2a
complex
Br−Pd−Cipso (deg)
Br−Pd−P (deg)
P−Pd−Cipso (deg)
smallest Pd−P−C angle
{[(o-tol3)P]Pd(Ar)(Br)}2
87.1(3)
91.4(1)
174.7(9)
162.61(3)
89.8(3)
100.8(1)
109.5
106.0
(1-AdPtBu2)Pd(Ph)(Br) (2a)
four-coordinate complex {[(o-tol3)P]Pd(Ar)(Br)}2 with one
phosphine per palladium,18,19 while addition of aryl bromides
to Pd(PtBu3)2 generates the monomeric complexes (PtBu3)Pd-
(Ar)(Br). Both the monomeric PPh3 complexes and the dimeric
P(o-tol3) complexes are square planar about the palladium atom.
In contrast, the PtBu3 complex contains a T-shaped orientation
of the three heavy atoms bound to palladium. This complex
could be considered square planar if an agostic hydrogen is
included as the fourth ligand on palladium, but the geometry is
distinct from complexes that adopt a four-coordinate geometry
by coordinating the halide of a second arylpalladium halide
fragment or a second phosphine ligand.
A comparison of the steric properties of P(o-tol)3 and PtBu3
and the geometries of the respective arylpalladium halide
complexes reveals the factors that control the geometry and
nuclearity of arylpalladium halide complexes. Since the time
when Tolman reported that the cone angle of P(o-tol)3 is 12°
larger than the cone angle of PtBu3,57 alternative methods to
measure these cone angles have been reported.58 By solid angle
measurements, the cone angle of P(o-tol)3 is closer to 144° than
to the 194° angle in Tolman’s original report, but the cone angle
of PtBu3 was unchanged.
a conformation that avoids steric interactions with the other
ligands of a planar, four-coordinate geometry.
Summary
This work has revealed a simple method to access a variety
of arylpalladium(II) halide complexes with a novel coordination
sphere by oxidative addition of aryl bromides and iodides to
bis-phosphine palladium(0) complexes. This coordination sphere
contains an aryl group, a halide, and only one phosphine ligand,
along with a weak agostic interaction. In all cases, the aryl
chloride and aryl triflate complexes could not be isolated by
direct oxidative addition to the palladium(0) species but were
prepared from the bromide by anion exchange. Structural studies
of these compounds revealed the geometric distortions that occur
to accommodate the bulky ligands and the origin of the pref-
erence for mononuclear versus dinuclear structures with a single
ligand. Moreover, extensive computational studies have con-
firmed the presence of a weak agostic interaction between the
metal and a ligand hydrogen that appeared in the solid-state
structure but was revealed by spectroscopy in only one case.
Because arylpalladium(II) halide complexes are proposed as
intermediates in many cross-coupling reactions, studies on the
rates of formation of these complexes and studies on the
reactivity of these isolated species should provide insight into
the mechanisms of metal-catalyzed cross-coupling processes.
The weakness of the C-H agostic interaction in compounds
2 and 3 revealed by spectroscopic and computational studies
and the potential for P(o-tol)3 to adopt a smaller cone angle
lead to the difference in structures of the complexes of the
tertiary alkylphosphine ligands in this study and those of the
hindered triarylphosphines. If the agostic interaction were
stronger than the interaction with a bridging halide ligand, then
complexes of P(o-tol)3 could adopt a monomeric structure with
an ortho-tolyl hydrogen occupying the open site. Although
dimeric in their most stable form, the P(o-tol)3 complexes often
react through their monomeric form, and the monomer is likely
to be stabilized by such a weak agostic interaction.
Experimental Section
1
General Methods. H and 13C NMR spectra were recorded on a
Bruker DPX 400 MHz spectrometer, Bruker DPX 500 MHz spectrom-
eter, or a General Electric Omega 500 spectrometer with tetramethyl-
silane or residual protiated solvent used as a reference. Elemental
analyses were performed by Robertson Microlabs, Inc., Madison, NJ.
All 31P and 13C NMR spectra were proton decoupled. Toluene, tetra-
hydrofuran, ether, and pentane were distilled from sodium-benzophe-
none. Phenyl bromide was purified by distillation; however, the results
were similar to those using phenyl bromide as received. Phenyl iodide,
silver triflate, and tetraoctylammonium chloride were used without
further purification. The syntheses of Pd[1-AdPtBu2]2,12 Pd[PtBu3]2,59
Pd[Q-phos]2,60 1-AdPtBu2,61 2-AdPtBu2,61 2-AdPtBu2Pd(Ph)Br (2c),12
The steric factors that determine monomeric or dimeric
structures are more subtle. Dimerization of the monomeric
complexes in the current work would require substantial
reorganizations and increased steric interactions. Selected angles
from the structures of monomeric 1-AdPtBu2Pd(Ph)(Br) 2a and
the dimeric complex {[(o-tol3)P]Pd(Ar)(Br)}2 determined by
X-ray diffraction are shown in Table 5. The largest differ-
ence between the selected angles of the two complexes is the
Br-Pd-P bond angle. The association of two monomeric 2a
species to form one dimeric complex would require the
Br-Pd-P angle to increase by 12°, the P-Pd-C angle to
decrease by 11°, and the phosphine to pivot back to a more
conventional conformation with relatively equivalent Pd-P-C
angles. Moreover, the open coordination site adopted by one
of the tert-butylmethyl groups would be occupied by the
bridging halogen. With three large substituents in roughly 3-fold
symmetry, a bulky tertiaryalkyl phosphine ligand cannot adopt
12
and PtBu3Pd(m-xylyl)I (3b) have been published previously.
1-AdPtBu2Pd(Ph)Br (2a). In a glovebox, 540 mg (0.810 mmol) of
Pd[1-AdPtBu2]2 (1a), 4.0 mL (38.0 mmol) of phenyl bromide, and a
stir bar were added to a screw-capped vial. The vial was removed from
the glovebox and heated at 70 °C for 2.5 h or until 31P NMR
spectroscopy showed complete consumption of 1. The vial was returned
to the drybox and added to a stirring flask filled with 50 mL of pentane.
An orange precipitate formed immediately. The flask was stirred for 5
min. At this time, the orange precipitate was washed 5 × 10 mL with
pentane. The orange solid was dried under vacuum to yield 70.2% (309
mg, 0.569 mmol) of the desired product. Spectroscopic data for this
complex were published previously.
(59) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem. Soc.
1976, 98, 5850.
(60) Shelby, Q.; Kataoka, N.; Mann, G.; Hartwig, J. F. J. Am. Chem. Soc. 2000,
122, 10718.
(57) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(58) Brown, T. L.; Lee, K. J. Coord. Chem. ReV. 1993, 128, 89.
(61) Stambuli, J. P.; Stauffer, S. R.; Shaughnessy, K. H.; Hartwig, J. F. J. Am.
Chem. Soc. 2001, 123, 2677.
9
1192 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004

Monomeric, T-Shaped Arylpalladium(II) Halide Complexes
A R T I C L E S
1-AdPtBu2Pd(Ph)I (3a). In a glovebox, an NMR tube was charged
with 102 mg (0.153 mmol) of Pd[1-AdPtBu2]2 (1) and 1.0 mL (8.9
mmol) of phenyl iodide. The tube was heated at 70 °C for 7 min or
until 31P NMR spectroscopy showed complete consumption of 1a. The
tube was returned to the drybox and added to a vial filled with 20 mL
of pentane. An orange precipitate was immediately formed. The vial
was stirred for 5 min. At this time, the orange precipitate was washed
5 × 5 mL with pentane. The orange solid was dried under vacuum to
yield 87% (79 mg, 0.13 mmol) of the desired product. The product
decomposed in solution after 1 h. 1H NMR (CD2Cl2, 500 MHz, 0 °C):
δ 1.44 (d, J ) 12.5 Hz, 18H), 1.74-1.80 (br m, 6H), 2.04 (br s, 3H),
2.21 (br s, 6H), 6.79-6.83 (m, 1H), 6.85-6.88 (m, 2H), 7.24 (d, J )
6.5 Hz, 2H). 13C NMR{1H} (CD2Cl2, 500 MHz, 0 °C): δ 29.0 (d, J )
7.8 Hz), 32.4 (d, J ) 2.1 Hz), 41.1 (d, J ) 7.6 Hz), 41.4, 49.2 (d, J )
7.1 Hz), 123.8, 127.0 (d, J ) 2.5 Hz), 132.9 (d, J ) 6.4 Hz), 136.6 (d,
J ) 3.9 Hz). 31P NMR (C6D6, 202 MHz): δ 50.6.
1-AdPtBu2Pd(Ph)CF3SO3 (5a). In a glovebox, 98 mg (0.18 mmol)
of 1-AdPtBu2Pd(Ph)Br (2a) was dissolved in 2 mL of toluene and
stirred. In a separate vial, silver triflate (57 mg, 0.22 mmol) was
dissolved in 2 mL of toluene. The silver triflate solution was added
dropwise to the stirring vial, whose appearance changed from a clear
orange solution to a cloudy dark green mixture. The reaction was stirred
for an additional 2 min. At this time, the reaction mixture was filtered
through a plug of Celite and concentrated to approximately 1 mL. The
resulting bright yellow solution was layered with pentane and cooled
to -35 °C. After 12 h, bright yellow crystals formed that were washed
with 5 mL of pentane and dried under vacuum to yield 78% (87 mg,
0.14 mmol) of the desired product. 1H NMR (C6D6, 500 MHz): δ 1.03
(d, J ) 13.0 Hz, 18H), 1.41-1.55 (br m, 6H), 1.74 (br s, 3H), 1.98 (br
s, 6H), 6.79-6.81 (m, 3H), 7.28-7.31 (m, 2H). 13C NMR{1H} (C6D6,
125 MHz): δ 29.0 (d, J ) 7.3 Hz), 32.1 (d, J ) 1.9 Hz), 36.1, 40.7 (d,
J ) 12.4 Hz), 41.4, 48.8 (d, J ) 10.8 Hz), 125.7, 128.2 (br s), 135.3
(d, J ) 5.8 Hz), 135.4 (d, J ) 2.1 Hz). 31P NMR (C6D6, 202 MHz):
δ 72.1. Anal. Calcd for C25H38F3O3PPdS: C, 48.98; H, 6.25. Found:
C, 48.59; H, 5.87.
6.93 (br m, 3H), 7.20-7.22 (br m, 17H), 7.26-7.28 (br m, 10H). 13C
NMR{1H} (CD2Cl2, 100 MHz): δ 30.5 (d, J ) 3.6 Hz), 39.0 (d, J )
17.2 Hz), 76.7 (25.9 Hz), 78.4 (d, J ) 7.0 Hz), 80.1 (d, J ) 7.7 Hz),
88.8, 124.6, 126.7, 127.4, 128.2, 132.9, 133.3, 135.4, 135.8 (d, J )
2.6 Hz). 31P NMR (C6D6, 202 MHz): δ 49.2. Anal. Calcd for C54H52-
PPdBrFe: C, 66.58; H, 5.38. Found: C, 66.18; H, 5.32.
Q-phos Pd(Ph)I (3d). Complex 3c was prepared in a manner similar
to 2c to give 75.4% (0.0492 mmol, 50.2 mg) of a red powder. 1H NMR
(CD2Cl2, 400 MHz): δ 1.03 (d, J ) 14.0 Hz, 18H), 4.54 (s, 2H), 4.74
(s, 2H), 6.84-6.89 (br m, 3H), 7.12-7.25 (br m, 27H), 7.26-7.28.
13C NMR{1H} (CD2Cl2, 100 MHz): δ 30.5 (d, J ) 4.2 Hz), 39.1 (d,
J ) 16.4 Hz), 77.1 (d, J ) 22.9 Hz), 78.1 (d, J ) 7.4 Hz), 79.9 (d, J
) 5.8 Hz), 88.8, 124.4, 127.3, 128.2, 132.1 (d, J ) 5.5 Hz), 132.9,
133.3, 135.4, 136.0 (d, J ) 4.1 Hz). 31P NMR (C6D6, 202 MHz): δ
45.7.
Synthesis of the Phosphonium Salt [Q-phos(Ph)]+[OTf]-. In a
glovebox, 102 mg (0.066 mmol) of Pd[Q-phos]2 (1c), 0.40 mL of phenyl
triflate (2.5 mmol), 9.0 mL of THF, and a stir bar were added to a
screw-capped vial. The vial was stirred at 60 °C for approximately 2
h or until 31P NMR spectroscopy showed complete consumption of
1c. The dark brown solution was evaporated until all of the THF was
removed. The vial was removed from the glovebox, and 20 mL of
benzene was added. The benzene solution was filtered through a glass-
fritted funnel, and the filtrate was allowed to slowly evaporate at
23-25 °C. After 16 h, the filtrate was decanted to reveal purple crystals,
which were washed with cold benzene and dried under vacuum to yield
1
75% (47 mg, 0.050 mmol) of the desired product. H NMR (CD2Cl2,
500 MHz): δ 1.32 (d, J ) 16.0 Hz, 18H), 4.99 (br s, 4H), 7.08-7.19
(m, 22H), 7.24-7.26 (m, 5H), 7.66-7.68 (m, 1H), 8.17-8.20 (m, 2H).
13C NMR{1H} (CD2Cl2, 125 MHz): δ 29.0, 38.5 (d, J ) 36.6 Hz),
79.7 (d, J ) 8.6 Hz), 83.2 (d, J ) 9.1 Hz), 88.5, 128.0, 128.2, 128.3,
130.1 (d, J ) 11.7 Hz), 132.9, 134.2, 135.3, 135.4. 31P NMR (CH2Cl2,
202 MHz): δ 53.5. Anal. Calcd for C55H52F3FeO3PS: C, 70.51; H,
5.59. Found: C, 70.47; H, 5.47.
Independent Synthesis of [(PCy3)2Pd(Ph)(Br)]. In a drybox, Pd-
(PCy3)2 (232 mg, 0.348 mmol), phenyl bromide (55 uL, 0.52 mmol),
and 2 mL of benzene were stirred at 25 °C for 24 h. At this time, the
white solid that precipitated from the reaction was collected by fil-
tration. The white solid was crystallized from cold ether to yield 58%
PtBu3Pd(Ph)Br (2b). In a glovebox, 293 mg (0.574 mmol) of Pd-
[PtBu3]2 (7), 2.8 mL of phenyl bromide (26 mmol), and a stir bar were
added to a screw-capped vial. The vial was removed from the glovebox
and heated at 70 °C for 2.5 h or until 31P NMR spectroscopy showed
complete consumption of 1b. The vial was returned to the drybox and
added to a stirring flask filled with 30 mL of pentane. An orange
precipitate was immediately formed. The flask was stirred for 5 min.
At this time, the orange precipitate was washed 5 × 10 mL with
pentane. The orange solid was dried under vacuum to yield 64% (170
1
(165 mg, 0.200 mmol) of the title compound. H NMR (C6D6, 500
MHz): δ 1.11-1.22 (br s, 18H), 1.59-1.76 (m, 30H), 2.07-2.09 (m,
12H), 2.29 (br s, 6H), 6.87 (t, J ) 7.0 Hz, 1H), 6.87 (t, J ) 7.0 Hz,
2H), 7.59 (d, J ) 7.0 Hz, 2H). 13C NMR{1H} (C6D6, 125 MHz): δ
27.0, 28.0 (t, J ) 4.0 Hz), 30.6, 34.7 (t, J ) 9.4 Hz), 122.2, 127.5,
138.9 (t, J ) 3.0 Hz), 155.2 (br s). 31P NMR (C6D6, 202 MHz): δ
20.6 Anal. Calcd for C42H71P2BrPd: C, 61.20; H, 8.68. Found: C,
60.97; H, 8.64.
Computational Details. Molecular structures have been fully
optimized at the BP86/ECP1 level, i.e., employing the exchange and
correlation functionals of Becke and Perdew,62-64 respectively, together
with a fine integration grid (75 radial shells with 302 angular points
per shell), relativistic MEFIT effective core potentials with the
corresponding valence basis sets for Pd65 (contraction schemes [6s5p3d]),
and standard 6-31G* basis set66,67 for all other elements. The nature of
each stationary point was characterized by analytical calculation of the
harmonic vibrational frequencies at that level. Nuclear magnetic
shieldings σ have been evaluated for the BP86/ECP1 geometries using
a recent implementation of the gauge-including atomic orbitals (GIAO)-
DFT method,68 involving the functional combinations according to
Becke (hybrid) and Lee, Yang, and Parr69,70 (B3LYP), together with
1
mg, 0.365 mmol) of the desired product. H NMR (C6D6, 400 MHz):
δ 1.00 (d, J ) 12.8 Hz, 27H), 6.74 (t, J ) 7.2 Hz, 1H), 6.82 (t, J ) 7.2
Hz, 2H), 7.43 (m, 2H). 13C NMR{1H} (C6D6, 125 MHz): δ 32.1, 40.2
(d, J ) 8.4 Hz), 124.0, 127.2, 129.0, 137.0 (d, J ) 3.0 Hz). 31P NMR
(C6D6, 202 MHz): δ 63.0. Anal. Calcd for C18H32BrPPd: C, 46.42;
H, 6.93. Found: C, 46.20; H, 6.84.
Q-phos Pd(Ph)Br (2d). In a glovebox, 60 mg (0.039 mmol) of Pd-
[Q-phos]2 (1c), 0.17 mL of phenyl bromide (1.6 mmol), 2.7 mL of
THF, and a stir bar were added to a screw-capped vial. The vial was
stirred in the glovebox for approximately 2.5 h or until 31P NMR
spectroscopy showed complete consumption of 1c. The red solution
was evaporated until all of the THF was removed. At this time,
approximately 15 mL of pentane was added, and the vial was shaken
and cooled at -35 °C. The vial was removed from the freezer, and the
red precipitate that formed was washed repeatedly with ether until all
of the free ligand was removed (as judged by 1H NMR spectroscopy).
The solid was dissolved in a minimal amount of THF. Ether was added
until the clear red solution became cloudy. After cooling to -35 °C
for 26 h, red crystals appeared in the vial. The red crystals were washed
with pentane (1 × 5 mL) and dried under vacuum to yield 29% (11
mg, 0.0.011 mmol) of the desired product. 1H NMR (C6D6, 500
MHz): δ 1.03 (d, J ) 14.0 Hz, 18H), 4.55 (s, 2H), 4.77 (s, 2H), 6.88-
(62) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
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Acta 1990, 77, 123.
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9
J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004 1193

A R T I C L E S
Stambuli et al.
basis II′′, i.e., a [16s10p9d] all-electron basis for Pd, contracted from
the well-tempered 22s14p12d set of Huzinaga and Klobukowski71 and
augmented with two d-shells of the well-tempered series, the recom-
mended IGLO-basis II72,73 on the three donor atoms bonded to Pd, the
nearest CH moiety (without f-functions on the halogens), and a double-ú
basis72 on all other atoms.74 Chemical shifts δ have been calculated
relative to TMS computed at the same level (σ(1H) ) 31.7). These
computations were performed using the Gaussian 98 program package.75
Topological analysis of the charge density (Bader analysis)48 has been
performed at the BP86/ECP1 level using the Morphy program.76
a gift of PdCl2. M.B. thanks Prof. W. Thiel and the Deutsche
Forschungsgemeinschaft for support.
Supporting Information Available: Crystallographic data for
5a, 2b, and 2d as well as NMR spectra for 3a (PDF and CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA037928M
Acknowledgment. We are grateful to the NIH-NIGMS
(GM58108) for support of this work. We thank Merck Research
Laboratories for an unrestricted gift and Johnson-Matthey for
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Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
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1194 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004