The reaction of carbon dioxide with palladium-allyl bonds

Article abstract of DOI:10.1021/om1007456

A family of palladium allyl complexes of the type (2-methylallyl) ₂Pd(L) (L = PMe₃ (1), PEt₃ (2), PPh₃ (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2- ylidene) have been prepared through the reaction of (2-methylallyl) ₂Pd with the appropriate free ligand. Compounds 1-4 contain one η¹- and one η³-2-methylallyl ligand, and 3 was characterized by X-ray crystallography. These complexes react rapidly with CO₂ at low temperature to form well-defined unidentate palladium carboxylates of the type (η³-2-methylallyl)Pd(OC(O)C ₄H₇)(L) (L = PMe₃ (6), PEt₃ (7), PPh₃ (8), NHC (9)). The structure of 9 was elucidated using X-ray crystallography. The mechanism of the reaction of 1-4 with CO₂ was probed using a combination of experimental and theoretical (density functional theory) studies. The coordination mode of the allyl ligand is crucial, and whereas nucleophilic η¹-allyls react rapidly with CO₂, η³-allyls do not react. We propose that the reaction of palladium η¹-allyls with CO₂ does not proceed via direct insertion of CO₂ into the Pd-C bond but through nucleophilic attack of the terminal olefin on electrophilic CO₂, followed by an associative substitution at palladium.

Full text of DOI:10.1021/om1007456

Organometallics 2010, 29, 6369–6376 6369
DOI: 10.1021/om1007456
The Reaction of Carbon Dioxide with Palladium-Allyl Bonds
Jianguo Wu,† Jennifer C. Green,‡ Nilay Hazari,*,† Damian P. Hruszkewycz,†
Christopher D. Incarvito,† and Timothy J. Schmeier†
†The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut, 06520,
United States, and ‡The Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, U.K.
Received July 29, 2010
A family of palladium allyl complexes of the type (2-methylallyl)2Pd(L) (L = PMe3 (1), PEt3 (2),
PPh3 (3), NHC (4); NHC=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) have
been prepared through the reaction of (2-methylallyl)2Pd with the appropriate free ligand. Com-
pounds 1-4 contain one η1- and one η3-2-methylallyl ligand, and 3 was characterized by X-ray
crystallography. These complexes react rapidly with CO2 at low temperature to form well-defined
unidentate palladium carboxylates of the type (η3-2-methylallyl)Pd(OC(O)C4H7)(L) (L=PMe3 (6),
PEt3 (7), PPh3 (8), NHC (9)). The structure of 9 was elucidated using X-ray crystallography. The
mechanism of the reaction of 1-4 with CO2 was probed using a combination of experimental and
theoretical (density functional theory) studies. The coordination mode of the allyl ligand is crucial,
and whereas nucleophilic η1-allyls react rapidly with CO2, η3-allyls do not react. We propose that the
reaction of palladium η1-allyls with CO2 does not proceed via direct insertion of CO2 into the Pd-C
bond but through nucleophilic attack of the terminal olefin on electrophilic CO2, followed by an
associative substitution at palladium.
Introduction
(70 °C).3a The mechanism of this reaction has not been
thoroughly investigated but is proposed to involve the inser-
tion of CO2 into palladium(II) allyls formed in situ, and recent
work by Johansson and Wendt supports this hypothesis.3b
Despite these intriguing results, there has been little re-
search investigating stoichiometric reactions of well-defined
palladium allyl complexeswith CO2, and the feasibility, scope,
and mechanism of this reaction remain unclear. Preliminary
work almost 30 years ago suggested that palladium allyls react
with CO2, but well-defined and characterized metal complexes
were not isolated.6 In a review article in 1985 Jolly reported
that the reaction of (2-methylallyl)2Pd(PPh3) with CO2 led to
the formation of a crystallographically characterized insertion
product, but no details of the reaction were presented.7 More
recently Wendt and Johansson demonstrated that mixtures of
palladium allyl and palladium hydride species with pincer
ligands react with CO2 to give palladium carboxylate species
and proposed a preliminary mechanism for this transforma-
tion.3b However, analysis of this reaction was complicated by
the presence of two complexes in the starting material.
In this work, we conclusively establish that the reaction of
CO2 with a variety of palladium allyl complexes is facile and
demonstrate that the coordination mode of the allyl ligand is
critical. Furthermore, through a combination of experimental
and computational studies, we propose a mechanism for this
reaction which does not involve direct coordination of CO2 to
the metal center.
In recent years there has been significant interest in the
catalytic functionalization of CO2, due to the potential of
this greenhouse gas as a readily available and inexpensive
source of carbon in the synthesis of both commodity chemi-
cals and complex organic molecules.1 Several transition-
metal complexes have already been developed as catalysts
for carboxylation reactions, and the key step is proposed to
involve a reaction between the metal center and CO2.1a-d In
particular the formation of palladium carboxylates from the
reactions of palladium allyl species with CO2 has been
suggested as a crucial step in several catalytic cycles. These
include systems for the coupling of CO2 with butadiene to
form lactones,2 the carboxylation of allyl stannanes3 and
allenes,4 and the carboxylative coupling of allylstannanes
and allyl halides.5 A pioneering example in this area was the
report by Nicholas and Shi that Pd(PPh3)4 can catalyze the
coupling of allylstannanes with CO2 to form tin carbox-
ylates at elevated pressure (33 atm of CO2) and temperature
*Towhom correspondence should beaddressed. E-mail: nilay.hazari@
yale.edu.
(1) (a) Leitner, W. Coord. Chem. Rev. 1996, 153, 257. (b) Yin, X.; Moss,
J. R. Coord. Chem. Rev. 1999, 181, 27. (c) Aresta, M.; Dibenedetto, A.
Dalton Trans. 2007, 2975. (d) Correa, A.; Martín, R. Angew. Chem., Int.
Ed. 2009, 48, 6201. (e) Sakakura, T.; Kohnoa, K. Chem. Commun. 2009,
1312. (f ) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388.
(2) (a) Braunstein, P.; Matt, D.; Nobel, D. J. Am. Chem. Soc. 1988,
110, 3207. (b) Pitter, S.; Dinjus, E. J. Mol. Catal. A: Chem. 1997, 125, 39.
(c) Aresta, M.; Quaranta, E.; Tommasi, I. New J. Chem. 1994, 18, 133.
(3) (a) Shi, M.; Nicholas, K. M. J. Am. Chem. Soc. 1997, 119, 5057.
(b) Johansson, R.; Wendt, O. F. Dalton Trans. 2007, 488.
(6) (a) Santi, R.; Marchi, M. J. Organomet. Chem. 1979, 182, 117.
(b) Hung, T.; Jolly, P. W.; Wilke, G. J. Organomet. Chem. 1980, 190, C5.
(c) Behr, A.; Von Ilsemann, G. J. Organomet. Chem. 1984, 276, C77.
(7) Jolly, P. W. Angew. Chem., Int. Ed. 1985, 24, 283.
(4) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254.
(5) Franks, R. J.; Nicholas, K. M. Organometallics 2000, 19, 1458.
r
2010 American Chemical Society
Published on Web 11/02/2010
pubs.acs.org/Organometallics

6370 Organometallics, Vol. 29, No. 23, 2010
Wu et al.
Figure 1. X-ray structure of 3(hydrogen atoms omitted for clarity).
˚
Selected bond lengths (A) and angles (deg): Pd(1)-P(1) =
2.2822(9), Pd(1)-C(7) = 2.240(3), Pd(1)-C(5) = 2.154(4), Pd-
(1)-C(6)=2.186(3), Pd(1)-C(1)=2.111(2), C(1)-C(2)=1.480(5),
C(2)-C(3) = 1.343(6), C(2)-C(4) = 1.465(5), C(5)-C(6) =
1.409(5), C(6)-C(7) = 1.384(6), C(6)-C(8) = 1.500(6); Pd(1)-
C(1)-C(2)=115.6(2), Pd(1)-C(5)-C(6)=72.3(2), Pd(1)-C(6)-
C(7) = 73.9(2).
Figure 2. X-ray structure of 9 (hydrogen atoms, methyl groups
of the NHC ligand, and solvent molecules omitted for clarity). Se-
lected bond lengths (A) and angles (deg): Pd(1)-C(1) = 2.036(3),
˚
Pd(1)-C(33)=2.205(4), Pd(1)-C(34)=2.147(4), Pd(1)-C(35)=
2.069(6), Pd(1)-O(1)=2.112(3), C(33)-C(34)=1.378(7), C(34)-
C(35)=1.405(7), C(34)-C(36)=1.505(7), O(1)-C(28)=1.265(5),
C(28)-O(2)=1.222(5), C(28)-C(29)=1.531(6), C(29)-C(30)=
1.495(8), C(30)-C(31) = 1.487(9), C(30)-C(32) = 1.311(9);
Pd(1)-C(33)-C(34)=69.2(2), Pd(1)-C(34)-C(35)=67.6(2),
Pd(1)-O(1)-C(28)=122.0(2), C(1)-Pd(1)-C(35)=100.20(18),
C(33)-Pd(1)-O(1)=103.61(18), O(1)-Pd(1)-C(1)=89.02(13).
Results and Discussion
Synthesis of Palladium Allyls and Reaction with CO2.
A family of palladium allyl complexes of the type (2-methyl-
allyl)2Pd(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4);
NHC=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imid-
azol-2-ylidene) were prepared through the reaction of
(2-methylallyl)2Pd with 1 equiv of ligand, L, at low tempera-
ture (eq 1). The 1H NMR spectra of 1-4 are all highly
fluxional and indicate rapid exchange between the 2-methyl-
allyl ligands. At low temperature (-60 °C) the exchange is
sufficiently slow on the NMR time scale to suggest the com-
plexes contain one η1- and one η3-2-methylallyl ligand and
have a structure analogous to that proposed for (allyl)2Pd(L)
(L = PR3).8 For example at room temperature the 1H NMR
spectrum of (2-methylallyl)2Pd(PEt3) (2) contains only two
resonances for the 2-methylallyl ligands in the ratio 8:6 (for
the four CH2 groups and two CH3 groups, respectively). In
contrast, at low temperature 10 resonances are observed for
the 2-methylallyl ligands, as each of the 8 protons associated
with CH2 groups gives a unique resonance and the two methyl
groups give distinct signals. Compounds 1, 3, and 4 display
similar properties.
Scheme 1
two different coordination modes for the allyl ligands and is
a rare example of a crystallographically characterized palla-
dium η1-allyl (especially without pincer ligands).9 A similar
η1:η3 Pt species has previously been reported, although bond
angles and distances are not available for comparison.7 In
contrast, the related Ni complex (PMe3)Ni(allyl)2 is five-
coordinate,10 with two η3-allyl groups, which presumably
reflects the preference of Ni(II) to form five-coordinate
formally 18e complexes compared with Pd or Pt.
Compounds 1-4 are thermally unstable and decompose in
solution at room temperature in the case of 2 and at tempera-
tures above -20 °C for 1, 3, and 4. In most cases the products
of decomposition are not clear; however, 3 decomposes rela-
tively cleanly to give the dimeric bridging 2-methylallyl PdI
species 5 and 1 equiv of 2,5-dimethylhexa-1,5-diene (eq 2).
A similar process has previously been observed for complexes
(9) (a) Ramdeehul, S.; Barloy, L.; Osborn, J. A.; Cian, A. D.; Fischer, J.
Organometallics 1996, 15, 5442. (b) Barloy, L.; Ramdeehul, S.; Osborn, J. A.;
Carlotti, C.; Taulelle, F.; Cian, A. D.; Fischer, J. Eur. J. Inorg. Chem. 2000,
2523. (c) Braunstein, P.; Naud, F.; Dedieu, A.; Rohmer, M.-M.; DeCian, A.;
Rettig, S. Organometallics 2001, 20, 2966. (d) Kollmar, M.; Helmchen, G.
Organometallics 2002, 21, 4771. (e) Goldfuss, B.; Loschmann, T.; Rominger,
F. Chem. Eur. J. 2004, 10, 5422. (f ) Braunstein, P.; Zhang, J.; Welter, R. Dalton
Trans. 2003, 507. (g) Zhang, J.; Braunstein, P.; Welter, R. Inorg. Chem. 2004,
43, 4172. (h) Guo, R.; Portscheller, J. L.; Day, V. W.; Malinakova, H. C.
Organometallics 2007, 26, 3874.
Crystals of 3 were grown from a toluene/pentane solution
at -35 °C, and the structure is shown in Figure 1. The bond
lengths and angles are consistent with one η1- and one
η3-2-methylallyl ligand, and the coordination geometry
around Pd is square planar. To the best of our knowledge, 3
is the first structurally characterized palladium complex with
(8) (a) Benn, R.; Jolly, P. W.; Mynott, R.; Raspel, B.; Schenker, G.;
Schick, K. P.; Schroth, G. Organometallics 1985, 4, 1945. (b) Krause, J.;
(10) Henc, B.; Jolly, P. W.; Salz, R.; Stobbe, S.; Wilke, G.; Benn, R.;
€
Mynott, R.; Seevogel, K.; Goddard, R.; Kruger, C. J. Organomet. Chem.
€
Goddard, R.; Mynott, R.; Porschke, K.-R. Organometallics 2001, 20, 1992.
1980, 191, 449.

Article
Organometallics, Vol. 29, No. 23, 2010 6371
Figure 3. Model compounds used for computational calculations.
for the NHC complex 9 and the smallest for the PPh3 com-
plex 8. When 2 was reacted with 13C-labeled CO2, 7 with a
13C-labeled carboxylate was formed. The CO2 stretching
frequencies shifted from 1601 and 1338 cm-1 in the un-
labeled complex to 1562 and 1315 cm-1 in the labeled complex
(calculated 1565 and 1309 cm-1). Further confirmation for
the proposed structures of 6-9 was obtained through the
solid-state structure of 9 (Figure 2), which clearly shows
a unidentate carboxylate and an η3-2-methylallyl ligand.
The overall coordination geometry around Pd is approxi-
mately square planar, and the carboxylate group is oriented
away from the allyl ligand. Compound 9 is the first structur-
ally characterized product from the reaction of CO2 with a
palladium allyl.
Figure 4. HOMO of (η1-C3H4Me)(η3-C3H4Me)Pd(PH3) (ii)
and labeling scheme for terminal C atoms.
Table 1. SCF Energies of the Possible Transition States, TS1,
Representing Attack at the Four Terminal Allyl Carbons for
i and iia
allyl
Ca
Cb (TS1)
Cc
Cd
C3H5, i
C4H8, ii
171
141
52
47
76
103
108
134
a Energies (kJ mol-1) are given relative to (η1-allyl)(η3-allyl)PdPH3.
of the type (allyl)2Pd(L) (L = PR3),8 and [(η3-allyl)Pd(PPh3)]2
has been crystallographically characterized.11
For comparison the unsubstituted allyl compounds (allyl)2-
Pd(PMe3),10 (allyl)2Pd(PEt3) and (allyl)2Pd(PPh3)10 were pre-
pared using literature methods. Interestingly, no reaction was
observed with CO2 before decomposition of the starting
material (predominantly to dimeric PdI species)8 occurred at
approximately -10 °C. This suggests that the barrier for reac-
tion with CO2 is higher for the unsubstituted allyls compared
with the 2-methylallyl compounds.13 Furthermore, no varia-
tion in the rate of CO2 insertion into 1-4 was observed
when excess ligand was added to the reaction mixture. These
observations strongly suggest that the reaction does not
follow a straightforward insertion mechanism involving ini-
tial coordination of CO2 to the Pd center, followed by allyl
migration, and the formation of a new C-C bond between the
metal-coordinated carbon of the allyl and CO2 (Scheme 1).
This type of insertion mechanism has been proposed in the
reaction of CO2 with octadienyl species, which is proposed
as a key step in the coupling of butadiene with CO2 to form
lactones.2
Reaction of excess CO2 with 1-4 led to rapid and quanti-
tative formation of the unidentate carboxylate complexes
6-9 (eq 3). Compounds 1-3 react in approximately 1 h
at -20 °C, while compound 4reacts in less than 5 min at -40 °C.
As the ancillary ligand was changed, the rate of reaction
increased in the following order: PPh3 < PEt3 ≈ PMe3 <
NHC. This demonstrates that more electron rich ligands lead
to faster reactions. The differences between the symmetric
and asymmetric CO2 stretches in the IR spectra of 6-9 were
significantly greater than 200 cm-1, which is characteristic of
the presence of a unidentate carboxylate ligand.12 Further-
more, the magnitude of the difference between a symmetric
and asymmetric CO2 stretch was related to the electron
richness of the ligand, with the largest difference observed
Mechanism of Reaction between Palladium Allyls and CO2.
In order to probe the mechanism of reaction between CO2
and palladium allyl species, a density functional theory (DFT)
study was performed. Calculations were carried out on model
(11) Jolly, P. W.; Krueger, C.; Schick, K. P.; Wilke, G. Z. Naturforsch.,
B: Anorg. Chem., Org. Chem. 1980, 35B, 926.
(12) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227.
(13) We believe that if decomposition via dimerization can be suppressed,
species of the type (allyl)2Pd(L) react with CO2 at room temperature. See:
Wu, J.; Hazari, N. Chem. Commun., DOI:10.1039/C0CC03191G.

6372 Organometallics, Vol. 29, No. 23, 2010
Wu et al.
Figure 5. Energy profile for formation of (η3-C3H4Me)(η1-O2CC3H4Me)Pd(PH3) from (η3-C3H4Me)Pd2 (Cf is the CO2 carbon). C-C
distances in the intermediates and transition states are given at the bottom of the figure.
compounds with both allyl and 2-methylallyl groups on Pd
and PH3, PMe3, and a simplified NHC as the supporting
ligands (Figure 3). The ADF program suite was used to opti-
mize geometries and search for transition states.14
Initially the key features of the reaction of ii with CO2 will
be discussed, and then the effect of changing the supporting
ligand and the 2-methylallyl group to an allyl group will be
described. Binding of PH3 to (η3-C3H4Me)2Pd is energetically
favorable, and the isomer of (C3H4Me)2Pd(PH3) with one
a number of stepsas illustratedinFigure 5 andScheme 2 andis
energetically favored. The initial electrophilic attack of CO2
leads to a zwitterionic transition state containing a carboca-
tion on the 2-position of the terminal allyl and a negative
charge on the carboxylate. This transition state collapses to a
zwitterionic intermediate with a formal negative charge on
the carboxylate group and a positive charge on the Pd. The
C3H4MeCO2 group is coordinated in a slipped manner to Pd
by both Ca and Ce, and the Ca-Ce bond acquires multiple-
bond character. This step has the highest activation energy of
the sequence. The zwitterionic intermediate undergoes ligand
substitution of Ca-Ce by a carboxylate oxygen. The barrier
for this reaction is small and leads to a higher energy con-
former of the product, which can undergo ligand rotation to
give the final product. Szabo has proposed a similar mechan-
ism for the reaction of electrophilic aldehydes with η1-allyl Pd
complexes with pincer ligands.16
The four modes of attack at the four terminal allyl carbons
were also compared for i (Table 1). Attack at the uncoordi-
nated terminal carbon of the σ-allyl group was again sig-
nificantly favored. In comparing the effect of the ligand L
only this pathway was explored. The energies of the identi-
fied stationary points for the reaction of i-vi are given in
Table 2. In all cases TS1 is the highest point on the energy
surface and all complexes show a similar intermediate, INT1.
The methylation of the allyl group lowers the activation
energy by approximately 10%, because the resultant tertiary
carbocation is more stable than a secondary carbocation.
These energies are consistent with the experimental observa-
tion that the barrier for reaction of 2-methylallyl-substituted
complexes with CO2 is lower than the barrier for unsubsti-
tuted species. When L = PMe3, NHC0, the activation ener-
gies are similar and significantly lower than for the model
η1- and one η3-allyl ligand is lower in energy (22 kJ mol-1
)
than the isomer with two η3-allyl ligands. This is consistent
with our experimental results and suggests that a complex
of the type (η3-C3H4Me)2Pd(L) could be an easily accessible
intermediatefor exchangebetweenthe 2-methylallylligandsin
(η1-C3H4Me)(η3-C3H4Me)Pd(L) (1-4).
The bis-allyl compounds of group 10 are unusual among
transition-metal organometallic complexes in possessing a
HOMO that is largely ligand based.15 This is also the case
with ii, (η1-C3H4Me)(η3-C3H4Me)Pd(PH3) (Figure 4). The
electrondensityon the terminalcarbonatomsofthe 2-methyl-
allyl groups suggests that they could be sites for electrophilic
attack by the C of CO2. Transition states were identified for
CO2 binding to all four terminal C atoms (Table 1). The
electronic energy of activation was greatest for site a (141 kJ
mol-1) and least for site b (47 kJ mol-1). Values for sites c and
d were 103 and 134 kJ mol-1, respectively. Thus, the uncoor-
dinated terminal carbon appears to be the most vulnerable to
electrophilic attack. No transition states were identified in
which CO2 initially binds to the metal center. The calculations
suggest that, after initial nucleophilic attack by the terminal
carbon of the η1-allyl on CO2, the overall reaction proceeds via
(14) See the Supporting Information for more details.
(15) (a) Boehm, M. C.; Gleiter, R.; Batich, C. D. Helv. Chim. Acta
1980, 63, 990. (b) Li, X.; Bancroft, G. M.; Puddephatt, R. J.; Liu, Z. F.; Hu,
Y. F.; Tan, K. H. J. Am. Chem. Soc. 1994, 116, 9543.
ꢀ
(16) Szabo, K. J. Chem. Eur. J. 2004, 10, 5268.

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Organometallics, Vol. 29, No. 23, 2010 6373
Scheme 2
a
Table 2. SCF Energies of the Identified Stationary Points for the Reaction of i-vi with CO2
compd
Pd(allyl)2
(η1-allyl)(η3-allyl)Pd(L)
TS1
INT1
TS2
INT2
Pd(allyl)(allylCO2)L
i
15
15
41
39
85
79
0
0
0
0
0
0
52
47
36
33
40
35
42
41
18
23
25
25
44
45
20
23
25
25
16
16
-38
-40
0
-34
-35
-40
-41
-38
-43
ii
iii
iv
v
vi
-9
a Energies (kJ mol-1) are given relative to (η1-allyl)(η3-allyl)PdL.
PH3 system, which agrees with the experimental observation
that electron-donating ligands increase the rate of reaction.
The lowering of the barrier presumably occurs because the
more electron-donating ligands allow Pd to assist in stabiliz-
ing the carbocation in TS1 through inductive effects. The
calculations suggest that the activation energy for NHC0 is
slightly higher than for PMe3, whereas experimentally the
NHC complex 4 reacts faster than the PMe3 species 1. How-
ever, there is one significant difference between the two sys-
tems. The formation of v and vi from the bis(allyl)palladium
precursors is energetically much more favorable than iii
and iv. Given that binding of a ligand is not favored entropi-
cally, the pre-equilibrium may well have a significant effect on
the relative rates of the reactions, a larger ligand binding
constant being expected for v and vi than for iii and iv.
Only minor differences were observed between the com-
plexes for the conversion of INT1 into the final products. In
cases iii-v one carboxylate oxygen lay closer to Pd in INT1
than was found for i, ii, and vi.17 In all cases the transition
states found for the associative substitution of Ca and Ce by
the carboxylate O (TS2) were barely distinguished in energy
from INT1, although they are significantly different in struc-
ture.17 In the case of L = PMe3 the substitution proceeded to
the stable product isomer, whereas with NHC0, as with PH3,
a less stable conformer was detected on the reaction path-
way. The conformers formed on the NHC and PH3 pathways
could undergo ligand rotation to give the final product.
Overall, the results of the calculations suggest that only
nucleophilic η1-allyls will react with CO2 and electrophilic
η3-allyls will be unreactive. We prepared the η3-allyls (allyl)-
PdCl(PMe3),18 (allyl)PdCl(NHC),19 (2-methylallyl)PdCl-
(PPh3),20 (2-methylallyl)PdCl(PMe3),21 and (2-methylallyl)-
PdCl(NHC)22 using literature methods (the binding mode
of the allyl ligand has either previously been confirmed
using X-ray diffraction or was confirmed in this work).14
None of these species reacted with CO2, even at elevated
temperatures, providing further support for our proposed
mechanism.
Conclusions
In this work we have conclusively demonstrated that reac-
tions between palladium η1-allyls and CO2 are facile and
structurally characterized the products. Mechanistic inves-
tigations strongly suggest that the reactions of palladium
η1-allyls with CO2 do not proceed via insertion of CO2 into
the Pd-C bond but through nucleophilic attack of the ter-
minal olefin on electrophilic CO2 followed by an associative
ligand substitution. Our results suggests that in current
systems for the catalytic carboxylation of allylstannanes
the difficult steps are either formation of the active catalyst
or transmetalation, as once a palladium allyl is formed
reaction with CO2 will be facile.3 More importantly, they
suggest that the proposed mechanism for the conversion of
CO2 and butadiene into lactones, which involves the direct
insertion of CO2 into a palladium octadienyl species and
an unusual reductive elimination, may not be correct.2
Instead, this reaction probably involves nucleophilic attack
by the octadienyl ligand on CO2. In further work, we will
utilize our results to both improve current catalysts for the
direct incorporation of CO2 into organic compounds and
design new catalysts.
Note Added after Submission
After the submission of this work, a paper by Wendt and
co-workers describing the mechanism of CO2 insertion into
(PCP)Pd(η1-allyl) species (PCP=2,6-C6H3(CH2PtBu2)2) was
published.23 In this elegant study the substituted allyl com-
plex (PCP)Pd(1-methylallyl) was prepared. Reaction of this
species with CO2 resulted in the formation of the new C-C
bond exclusively between the terminal carbon of the allyl
and CO2 (eq 4), which is consistent with our mechanism.
(17) See the Supporting Information for more details, including
calculated bond lengths.
(18) Alberti, D.; Goddard, R.; Poerschke, K.-R. Organometallics
2005, 24, 3907.
(19) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A., III;
Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2004, 23, 1629.
(20) Powell, J.; Robinson, S. D.; Shaw, B. L. Chem. Commun. 1965, 78.
(21) Ozawa, F.; Park, J. W.; Mackenzie, P. B.; Schaefer, W. P.;
Henling, L. M.; Grubbs, R. H. J. Am. Chem. Soc. 1989, 111, 1319.
(22) Navarro, O.; Oonishi, Y.; Kelly, R. A.; Stevens, E. D.; Briel, O.;
Nolan, S. P. J. Organomet. Chem. 2004, 689, 3722.
(23) Johnson, M. T.; Johansson, R.; Kondrashov, M. V.; Steyl, G.;
Ahlquist, M. S. G.; Roodt, A.; Wendt, O. F. Organometallics 2010, 29,
3521.

6374 Organometallics, Vol. 29, No. 23, 2010
Wu et al.
Scheme 3
assignments is given below. All assignments are based on two-
dimensional 1H,13C-HMQC and HMBC experiments. IR spec-
tra were measured using diamond smart orbit ATR on a Nicolet
6700 FT-IR instrument. Robertson Microlit Laboratories, Inc.,
performed the elemental analyses (inert atmosphere). Literature
procedures were followed to prepare the following compounds:
(2-methylallyl)2Pd,24 (allyl)2Pd,24 (allyl)2Pd(PMe3),10 (allyl)2-
Pd(PEt3), (allyl)2Pd(PPh3),10 (allyl)PdCl(PMe3),18 (allyl)PdCl-
(NHC),19 (2-methylallyl)PdCl(PPh3),20 (2-methylallyl)PdCl-
(PMe3),21 and (2-methylallyl)PdCl(NHC).22
DFT Calculations. Quantum chemical calculations were per-
formed using density functional methods of the Amsterdam Density
Functional (Version ADF2007.01) package.9b,25 TZP basis sets were
used with triple-ξ accuracy sets of Slater-type orbitals, with polariza-
tion functions added to all atoms. Relativistic corrections were made
using the ZORA (zero-order relativistic approximation) forma-
lism,26 and the core electrons were frozen up to 1s for C and O, 2p
for P, and 3d for Pd. The local density approximation of Vosko, Wilk,
and Nusair27 was utilized together with the nonlocal exchange cor-
rection by Becke28 and nonlocal correlation corrections by Perdew.29
All quoted electronic structure data from optimized structures use an
integration grid of 6.0 for the minima and 5.0 for the transition states
and were verified as minima using frequency calculations. The transi-
tions states all showed one imaginary frequency with a motion
connecting the reactant and product. The coordinates for the opti-
mized structures are given in the Supporting Information.
Furthermore, DFT studies performed on the reaction be-
tween (PCP)Pd(allyl) and CO2 proposed a two-step mecha-
nism (Scheme 3). In the first step the nucleophilic terminal
olefin of the η1-allyl attacks CO2 to generate a zwitterionic
transition state, with a positive charge on the allyl and a
negative charge on the carboxylate. This transition state
collapses to a zwitterionic intermediate with a formal nega-
tive charge on the carboxylate group and a positive charge on
the Pd. In the second step ligand substitution generates the
observed final product. This mechanism is identical with our
proposed mechanism, although in Wendt’s system the tran-
sition state for ligand substitution is the highest point on the
surface. This is presumably because the conformationally
rigid PCP ligand makes it energetically more costly for the Pd
species to adopt a pseudo-five-coordinate geometry. The
results of both Wendt’s and our studies strongly suggest
that, regardless of the supporting ligand framework, nucleo-
philic attack on CO2 is the preferred pathway by which
η1-allyl Pd species react with CO2.
X-ray Crystallography. The diffraction experiments were
carried out on a Bruker AXS SMART CCD three-circle dif-
fractometer with a sealed tube at 23 °C using graphite-mono-
˚
chromated Mo KR radiation (λ = 0.710 73 A). The software
used was SMART for collecting frames of data, indexing
reflections, and determination of lattice parameters, SAINT
for integration of intensity of reflections and scaling, SADABS
for empirical absorption correction, and SHELXTL for space
group determination, structure solution, and least-squares re-
finements on |F|2. The crystals were mounted at the end of glass
fibers and used for the diffraction experiments. Anisotropic ther-
mal parameters were refined for the rest of the non-hydrogen
atoms. The hydrogen atoms were placed in their ideal posi-
tions. Details of the crystal and refinement data for complexes
3 and 9 are given in Table 3.
Synthesis and Characterization of New Compounds. The following
numbering scheme is used for the purposes of assigning NMR data:
Experimental Section
General Methods. Experiments were performed under a di-
nitrogen atmosphere in an M-Braun drybox or using standard
Schlenk techniques. (Under standard glovebox conditions purg-
ing was not performed between uses of petroleum ether, diethyl
ether, benzene, toluene, and tetrahydrofuran; thus, when any
of these solvents were used, traces of all these solvents were in
the atmosphere and could be found intermixed in the solvent
bottles.) Moisture- and air-sensitive liquids were transferred
by stainless steel cannula on a Schlenk line or in a drybox. The
solvents for air- and moisture-sensitive reactions were dried by
passage through a column of activated alumina followed by
storage under dinitrogen. All commercial chemicals were used
as received except where noted. Palladium chloride was pur-
chased from Pressure Chemical Co., while 2-methylallyl chlo-
ride, PMe3, PEt3, PPh3 and 1,3-bis(2,6-diisopropylphenyl)-1,
3-dihydro-2H-imidazol-2-ylidene were purchased from Aldrich or
Strem. Anhydrous CO2 was obtained from Airgas, Inc., and was
not dried prior to use. Deuterated solvents and 13C-labeled CO2
were obtained from Cambridge Isotope Laboratories. C6D6 and
toluene-d8 were dried over sodium metal. NMR spectra were
recorded on Bruker AMX-400 and -500 spectrometers at ambi-
ent probe temperatures unless noted. Chemical shifts are
reported with respect to residual internal protio solvent for 1H
(η1-2-MeC3H4)(η3-2-MeC3H4)Pd(PMe3) (1). PMe3 (7.8 μL,
0.08 mmol) was added to a stirred solution of bis(2-methyl-
allyl)palladium (16.3 mg, 0.08 mmol) in 1 mL of toluene at -40 °C.
(24) Henc, B.; Jolly, P. W.; Salz, R.; Wilke, G.; Benn, R.; Hoffmann,
€
E. G.; Mynott, R.; Schroth, G.; Seevogel, K.; Sekutowski, J. C.; Kruger,
C. J. Organomet. Chem. 1980, 191, 425.
(25) (a) Fonseca Guerra, C.; Snijder, J. G.; Te Velde, G.; Baerends,
E. J. Theor. Chem. Acc. 1998, 99, 391. (b) Te Velde, G.; Bickelhaupt, F. M.;
Baerends, E. J.; Fonseca Guerra, C.; Van Gisbergen, S. J. A.; Snijders, J. G.;
Ziegler, T. J. Comput. Chem. 2001, 22, 931.
(26) (a) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1993, 99, 4597. (b) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem.
Phys. 1994, 101, 9783. (c) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G.
J. Chem. Phys. 1996, 105, 6505. (d) Vanlenthe, E.; Ehlers, A.; Baerends, E. J.
J. Chem. Phys. 1999, 110, 8943. (e) Vanlenthe, E.; VanLeeuwen, R.;
Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281.
(27) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(28) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38,
3098. (b) Becke, A. D. J. Chem. Phys. 1988, 88, 1053.
and 13C{1H} NMR spectra and to an external standard for 31
spectra (85% H3PO4 at 0.0 ppm). Atom numbering for the peak
P
(29) Perdew, J. P. Phys. Rev. B 1986, 33, 8800.

Article
Organometallics, Vol. 29, No. 23, 2010 6375
Table 3. Crystal and Refinement Data for Complexes 3 and 9
128.9 (Ph), 112.5 (s, C6), 111.7 (s, C2), 100.0 (s, C7), 69.3 (s, C3),
63.0 (s, C1), 26.7 (s, C8), 24.6 (s, C4), 24.4 (s, C5). 31P{1H} NMR
(135 MHz, toluene-d8, -75 °C): δ 34.6 (s).
3
9
(η1-2-MeC3H4)(η3-2-MeC3H4)Pd(NHC) (4). NHC (23.2 mg,
0.06 mmol) was added to a stirred solution of bis(2-methyl-
allyl)palladium (12.1 mg, 0.06 mmol) in 0.5 mL of toluene-d8
at -78 °C in a J. Young NMR tube. The mixture was warmed
to -50 °C, and the NMR data were collected at -50 °C. No
attempts were made to isolate this extremely unstable com-
pound, and it was only characterized spectroscopically.
1H NMR (500 MHz, toluene-d8, -50 °C): δ 7.14 (m, 2H, H14,
overlapping with solvent peak), 6.97 (m, 4H, H13 and H15,
overlapping with solvent peak), 6.44 (s, 2H, H10), 4.83 (s, 1H, H7),
4.64 (s, 1H, H70), 3.36 (br s, 2H, H21), 2.95 (br s, 2H, H17),
2.82 (s, 1H, H1), 2.76 (s, 1H, H3), 2.54 (s, 1H, H10), 2.19 (d, J =
10 Hz, 1H, H5), 2.10 (d, J = 10 Hz, 1H, H50, overlapping with
solvent peak), 1.88 (s, 1H, H30), 1.66 (s, 3H, H8), 1.38 (s, 6H,
H20), 1.34 (s, 6H, H22), 1.29 (s, 3H, H4), 1.04 (s, 6H, H18), 1.01
(s, 6H, H19). 13C{1H} NMR (125.8 MHz, toluene-d8, -50 °C): δ
194.1 (s, C23), 158.7 (s, C6), 145.6 (s, C12), 145.2 (s, C16), 137.0
(s, C11), 130.0 (s, C12), 129.5 (s, C13 and C15), 123.9 (s, C14),
123.2 (s, C10), 100.1 (s, C7), 65.4 (s, C3), 53.2 (s, C3), 50.3 (s, C1),
28.6 (s, C17), 28.4 (s, C21), 26.2 (s. C4), 25.9 (s, C20 or C22), 25.6
(s, C8), 24.8 (s, C22 or C20), 22.8 (s, C18), 22.5 (s, C19).
{(η3-2-MeC3H4)Pd(PPh3)}2 (5). PPh3 (121.8 mg, 0.46 mmol)
was added to a stirred solution of bis(2-methylallyl)palladium
(100.4 mg, 0.46 mmol) in 8 mL of toluene at -30 °C. The mixture
was slowly warmed to room temperature and stirred for 1 h. The
volatiles were then removed under reduced pressure. The result-
ing residue was washed with cold pentane and dried in vacuo to
give 5 as an orange-yellow powder. Yield: 165.2 mg (84%). Anal.
Calcd (found) for C44H44P2Pd2: C, 64.55 (62.50); H, 5.71 (5.10).
1H NMR (500 MHz, C6D6): δ 7.73 (br s), 7.62 (d), 7.28 (s),
7.08 (m) and 7.01 (m) (30H, Ph), 2.96 (m, 4H, allyl), 2.32 (br s,
4H, allyl), 1.58 (m, 6H, CH3). 13C{1H} NMR (125.8 MHz):
140.2 (s), 136.6 (t), 134.7 (s), 134.6 (s), 134.5 (s) and 129.7 (Ph),
115.4 (s, allyl-center C), 38.9 (s, allyl-end C), 38.9 (s, CH3).
31P{1H} NMR (135 MHz, C6D6): δ 29.3 (s).
empirical formula
formula wt
temp (K)
PdPC26H29
478.89
223
PdO2N2C38H54Cl4
819.07
223
18.6604(18)
12.5634(12)
17.8121(17)
90
98.961(2)
90
4124.9(7)
4
˚
a (A)
9.3455(4)
9.9853(5)
13.9430(7)
72.6291(12)
83.6782(13)
70.4556(12)
1170.17(10)
2
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
cryst syst
space group
dcalcd (Mg/m3)
θ range (deg)
triclinic
P1 (No. 2)
1.359
monoclinic
P21/c (No. 14)
1.319
3.1-27.5
0.870
3.1-27.5
0.741
semiempirical from equivalents
μ (mm-1
abs cor
GOF
)
1.051
0.0412, 0.0869
1.043
0.0648, 0.1435
R1,a wR2b (I > 2σ(I))
P
P
P
P
aR1= |Fo| - |Fc|/ |Fo|. b wR2=[ [w(Fo2 - Fc2)2]/ [w(Fo2)2]1/2
.
The mixture was stirred for 0.5 h at -40 °C and was then
warmed to room temperature. The volatiles were removed by
vacuum to give 1 as a yellow oil. Yield: 20.2 mg (92%). This
compound is thermally unstable and was stored at -30 °C (no
elemental analysis was obtained due to the thermal instability).
1H NMR (500 MHz, toluene-d8, -86 °C): δ 5.09 (s, 1H, H7),
4.84 (s, 1H, H70), 3.38 (br s, 1H, H3), 3.00 (s, 2H, H1 and H5),
2.89 (s, 1H, H30), 2.68 (m, 1H, H10), 2.26 (s, 1H, H50), 2.21 (s, 3H,
H8), 1.59 (s, 3H, H4), 0.73 (m, 9H, CH3P). 13C{1H} NMR (125.8
MHz, toluene-d8, -90 °C): δ 157.2 (s, C6), 131.9 (s, C2), 100.5
(s, C7), 63.1 (d, C3, JP-C = 36.5 Hz), 58.1 (s, C5), 28.0 (s, C8),
25.4 (4, C4), 17.7 (s, C1), 16.0 (d, CH3P, JP-C = 25.2 Hz).
31P{1H} NMR (135 MHz, C6D6): δ -17.3 (s).
(η1-2-MeC3H4)(η3-2-MeC3H4)Pd(PEt3) (2). PEt3 (65 μL,
0.44 mmol) was added to a stirred solution of bis(2-methyl-
allyl)palladium (96 mg, 0.44 mmol) in 1 mL of toluene at -40 °C.
The mixture was stirred for 0.5 h at -40 °C and was then
warmed to room temperature. The volatiles were removed by
vacuum to give 2 as a yellow oil. Yield: 141 mg (95%). This
compound is thermally unstable and was stored at -30 °C (no
elemental analysis was obtained due to the thermal instability).
1H NMR (400 MHz, toluene-d8, -70 °C): δ 5.06 (s, 1H, H7),
4.80 (s, 1H, H70), 3.36 (br s, 1H, H3), 3.06 (m, 1H, H1), 3.00
(s, 1H, H5), 2.97 (s, 1H, H30), 2.54 (m, 1H, H10), 2.29 (s, 1H, H50),
2.17 (s, 3H, H8), 1.58 (s, 3H, H4), 1.26 (m, 6H, PCH2), 0.71 (m,
9H, CH3CH2P). 13C{1H} NMR (101 MHz, toluene-d8, -70 °C):
(η3-2-MeC3H4)(η1-CO2C4H7)Pd(PMe3) (6). To a solution of
bis(2-methylallyl)palladium (13.3 mg, 0.06 mmol) in 0.5 mL of
toluene-d8 at -78 °C was added PMe3 (6.4 μL, 0.06 mmol). The
mixture was warmed to room temperature with stirring. Excess
1 atm of CO2 was added via a dual manifold Schlenk line to the
above solution at room temperature. After 0.5 h the mixture was
evaporated to dryness. The residue was dissolved in a minimum
amount of pentane and was then cooled to -35 °C for 6 h. A
white precipitate formed, which was isolated by filtration. The
solid was dried in vacuo to give 6 as a white powder. Yield: 19.1
mg (92%). Anal. Calcd (found) for C12H23O2PPd: C, 42.81
(42.53); H, 6.89 (6.78).
δ 157.3 (s, C6), 131.4 (s, C2), 100.4 (s, C7), 65.0 (d, C3, JP-C
=
35.2 Hz), 57.5 (s, C1), 27.9 (s, C8), 25.2 (s, C4), 17.7 (d, C5,
JP-C = 10.1 Hz), 16.5 (d, CH2P, JP-C = 22.1 Hz), 8.8
(s, CH3CH2P). 31P{1H} NMR (135 MHz, C6D6): δ 21.9 (s).
(η1-2-MeC3H4)(η3-2-MeC3H4)Pd(PPh3) (3). A solution of
PPh3 (141 mg, 0.54 mmol) in 4 mL of a toluene and pentane
mixture (1/7 v/v) at -40 °C was added to a stirred solution
of bis(2-methylallyl)palladium (117 mg, 0.54 mmol) in 2 mL of
pentane at -78 °C. The mixture was warmed to -40 °C and
stirred for another 2 h. The resulting precipitate was collected by
filtration, washed with pentane, and dried in vacuo to give 3 as
a yellow solid. Yellow single crystals for X-ray analysis were
grown from toluene/pentane at -35 °C. Yield: 214.1 mg (83%).
Anal. Calcd (found) for C26H29PPd: C, 65.21 (65.31); H, 6.10
(6.00). This compound is thermally unstable in solution but was
sufficiently stable as a solid to gather satisfactory elemental data.
1H NMR (500 MHz, toluene-d8, -95 °C): δ 7.45 (br s), 7.22 (s)
and 6.94 (br s) (15H, Ph), 4.84 (br s, 2H, H7), 2.43 (br s, 1H, H1),
3.36 (br s, 2H, H3), 2.87 (br s, 1H, H5), 2.70 (br s, 1H, H50), 2.57
(br s, 1H, H10), 2.11 (br s, 3H, H8), 1.67 (br s, 3H, H4). 13C{1H}
NMR (125.8 MHz, toluene-d8, -70 °C): δ 134.1, 134.0, 133.6,
IR: 1602 (νasym(CO2)), 1336 (νsym(CO)) cm-1 1H NMR
.
(500 MHz, C6D6): δ 5.11 (m, 1H, H8), 4.99 (m, 1H, H80), 4.28
(br s, 1H, H3), 3.50 (s, 2H, H6), 3.42 (br s, 1H, H30), 2.16 (s, 3H,
H9), 2.11 (br s, 2H, H1), 1.57 (s, 3H, H4), 0.96 (d, J = 10 Hz, 6H,
PMe3). 13C{1H} NMR (101 MHz, C6D6): δ 176.0 (s, CO2), 144.4
(s, C7), 131.6 (s, C2), 112.1 (s, C8), 75.5 (d, JC-P = 35.4 Hz, C3),
48.0 (s, C6), 46.5 (s, C1), 23.9 (s, C9), 23.8 (s, C4), 15.9 (d,
JC-P = 25.3 Hz, CH3P). 31P{1H} NMR (135 MHz, C6D6):
δ -13.3 (s).
(η3-2-MeC3H4)(η1-CO2C4H7)Pd(PEt3) (7). Excess 1 atm of
CO2 was added via a dual-manifold Schlenk line to an agitated
solution of 2 (20.7 mg, 0.06 mmol) in 0.5 mL of toluene-d8 at
room temperature. After 0.5 h the mixture was evaporated to
dryness to give 7 as a pale yellow solid. Yield: 21.1 mg (90%).
Anal. Calcd (found) for C15H29O2PPd: C, 47.56 (47.29); H, 7.72
(7.57). Pd(PEt3)(η3-2-MeC3H4)(η1-13CO2C4H7) (7-13CO2) was
prepared by performing the same procedure under an atmo-
sphere of 13C-labeled CO2.
IR: 1601 (νasym(CO2)), 1338 (νsym(CO2)) cm-1
.
1H NMR
(400 MHz, C6D6): δ 5.09 (s, 1H, H8), 4.98 (s, 1H, H80), 4.44

6376 Organometallics, Vol. 29, No. 23, 2010
Wu et al.
(d, J =4 Hz, 1H, H3), 3.55 (d, J = 4 Hz, 1H, H30), 3.48 (s, 2H, H6),
2.18 (s, 2H, H1), 2.16 (s, 3H, H9), 1.63 (s, 3H, H4), 1.36
(m, 6H, PCH2), 0.86 (m, 9H, CH3CH2P). 13C{1H} NMR (101
MHz, C6D6): δ 175.9 (s, CO2), 144.5 (s, C8), 131.0 (s, C2), 112.0
(s, C7), 77.0 (d, JC-P = 30.3 Hz, C3), 48.1 (s, C6), 45.3 (s, C1),
23.9 (s, C9), 23.6 (s, C4). 17.4 (d, CH2P, JC-P = 22.2 Hz), 8.8
(s, CH3CH2P). 31P{1H} NMR (135 MHz, C6D6): δ 24.1 (s).
Spectroscopic data for 7-13CO2 are as follows. IR: 1562
washed with 2 ꢀ 10 mL of pentane, revealing 9 as a white
powder. Colorless single crystals for X-ray analysis were grown
from CH2Cl2/pentane at -70 °C. Yield: 355 mg (40%). Anal.
Calcd (found) for C36H50N2O2Pd: C, 66.60 (66.38); H, 7.76
(7.74); N, 4.32 (4.24).
IR: 1611 (νasym(CO2)), 1330 (νsym(CO2)) cm-1. 1H NMR (400
MHz, CD2Cl2): δ 7.47 (t, J = 8 Hz, 2H, H14), 7.31 (d, J = 8 Hz,
4H, H13 and H15), 7.17 (s, 2H, H10), 4.61 (s, 1H, H8), 4.55
(s, 1H, H80), 3.90 (s, 1H, H3), 2.90 (br m, 4H, H10), 2.79 (s, 1H,
H30), 2.65 (s, 2H, H6), 2.03 (br, 2H, H1), 1.66 (s, 3H, H9), 1.35
(s, 6H, H20), 1.33 (s, 6H, H22), 1.27 (s, 3H, H4), 1.13 (s, 6H, H18
orH19), 1.11 (s, 6H, H19 or H18). 13CNMR(500 MHz, CD2Cl2):
δ 186.8 (s, C23), 175.5 (s, C5), 146.5 (s, C11), 144.8 (s, C7), 136.9
(s, C16), 130.2 (s, C14), 129.1 (s, C2), 124.8 (s, C10), 124.4 (s, C12
and C16), 111.1 (s, C8), 71.2 (s, C3), 47.0 (s, C6), 44.9 (s, C1), 29.0
(s, C21), 26.1 (s, C20 and C22), 23.2 (s, C9), 23.1 (s, C18 and C19),
22.5 (s, C4).
(νasym(CO2)), 1315 (νsym(CO2)) cm-1 1H NMR (400 MHz,
.
toluene-d8): δ 4.99 (s, 1H, H8), 4.90 (s, 1H, H80), 4.36 (d, J =
12 Hz, 1H, H3), 3.46 (d, J = 12 Hz, 1H, H30), 3.34 (d, J = 8 Hz,
2H, H6), 2.19 (s, 2H, H1), 2.09 (s, 3H, H9), 1.64 (s, 3H, H4), 1.37
(m, 6H, PCH2), 0.87 (m, 9H, CH3CH2P). 13C{1H} NMR (101
MHz, toluene-d8): δ 175.5 (CO2). 31P{1H} NMR (135 MHz,
toluene-d8): δ 24.2 (s).
(η3-2-MeC3H4)(η1-CO2C4H7)Pd(PPh3) (8). Excess 1 atm of
CO2 was added via a dual-manifold Schlenk line to a stirred
solution of 3 (60 mg, 0.13 mmol) in 0.6 mL of toluene at -40 °C.
The reaction mixture was stirred at -78 °C overnight, and excess
CO2 was removed under vacuum. Cold pentane (12 mL) was
then added. The resulting precipitate was collected by filtration
at -78 °C, washed with cold pentane, and dried in vacuo to give
8 as a pale yellow solid. Yield: 45.2 (69%). Anal. Calcd (found)
for C27H29O2PPd: C, 62.02 (63.73); H, 5.59 (4.31).
Reactions between CO2 and (allyl)2Pd Complexes. In a typical
reaction 10 mg of (allyl)2Pd(PMe3), (allyl)2Pd(PEt3), or
(allyl)2Pd(PPh3) was dissolved in 0.4 mL of toluene-d8 in a J.
Young NMR tube at -78 °C. The mixture was then degassed
using three freeze-pump-thaw cycles and excess 1 atm of CO2
was added via a dual-manifold Schlenk line at -78 °C. No
reaction was observed with CO2 before decomposition of the
starting material occurred at approximately -10 °C.
IR: 1600 (νasym(CO2)), 1339 (νsym(CO2)) cm-1. 1H NMR (400
MHz, toluene-d8, -70 °C): δ 7.67 (m), 7.47 (br s) and 7.17 (br s)
(15H, Ph), 5.02 (s, 2H, H8), 4.92 (s, 1H, H30), 3.51 (s, 1H, H3),
3.50 (s, 2H, H6), 2.24 (s, 1H, H1), 1.98(1H, H10), 1.85(s, 3H, H9),
1.66 (s, 3H, H4). 13C obtained from 1H-13C HMBC & HMQC
(toluene-d8, -70 °C): d 134.3, 134.1, and 128.2 (Ph), 175.9 (s,
CO2), 143.9 (s, C7), 133.6 (s, C2), 112.4 (s, C8), 75.4
(s, C3), 54.7 (s, C1), 47.2 (s, C6), 22.9 (s, C9 & C4). 31P{1H}
NMR (135 MHz, toluene-d8, -70 °C): 23.2 (s).
Reactions between CO2 and Pd η3-Allyl and η3-2-Methylallyl
Complexes. In an typical reaction 10 mg of (allyl)PdCl(NHC),
(2-methylallyl)PdCl(PPh3), (2-methylallyl)PdCl(PMe3), or
(2-methylallyl)PdCl(NHC) was dissolved in 0.4 mL of C6D6 in
a J. YoungNMRtube. The mixturewasthendegassedusingthree
freeze-pump-thaw cycles, and excess 1 atm of CO2 was added
via a dual-manifold Schlenk line. The reaction was monitored
using 1H NMR spectroscopy and heated for extended periods of
time at 80 °C. No reaction was observed.
(η3-2-MeC3H4)(η1-CO2MeC3H4)Pd(NHC) (9). To a stirred
solution of bis(2-methylallyl)palladium (300 mg, 1.4 mmol)
in 20 mL of toluene at -78 °C was added a solution of NHC
(NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imid-
azol-2-ylidene) (539 mg, 1.4 mmol) in 20 mL of toluene
at -78 °C via cannula. The mixture was stirred for 2 h and
was then degassed using three freeze-pump-thaw cycles and
warmed to -40 °C. Excess 1 atm of CO2 was added via a dual-
manifold Schlenk line at -40 °C. After it was stirred for 1 h, the
mixture was evaporated to dryness. The yellow-brown solid was
Acknowledgment. We thank Dr. Nathan West for in-
sightful discussions and the NIH for a Chemical Biology
Training Grant (T.J.S.).
Supporting Information Available: Tables and CIF files giving
X-ray information for 3, 9, and (η3-allyl)Pd(PMe3)Cl and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.