Palladium(I)-bridging allyl dimers for the catalytic functionalization of CO2

Article abstract of DOI:10.1021/ja110708k

In general, the chemistry of both η¹-allyl and η³-allyl Pd complexes is extremely well understood; η¹-allyls are nucleophilic and react with electrophiles, whereas η³-allyls are electrophilic and react with nucleophiles. In contrast, relatively little is known about the chemistry of metal complexes with bridging allyl ligands. In this work, we describe a more efficient synthetic methodology for the preparation of Pd^I-bridging allyl dimers and report the first studies of their stoichiometric reactivity. Furthermore, we show that these compounds can activate CO₂ and that an N-heterocyclic carbene-supported dimer is one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO ₂.

Full text of DOI:10.1021/ja110708k

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pubs.acs.org/JACS
Palladium(I)-Bridging Allyl Dimers for the Catalytic Functionalization
of CO2
Damian P. Hruszkewycz,† Jianguo Wu,† Nilay Hazari,* and Christopher D. Incarvito
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
S Supporting Information
b
bridging allyl dimers and report the first studies of their stoichio-
ABSTRACT: In general, the chemistry of both η1-allyl and
metric reactivity. Our preliminary studies suggest that bridging
η3-allyl Pd complexes is extremely well understood; η1-allyls
are nucleophilic and react with electrophiles, whereas η3-
allyls are electrophilic and react with nucleophiles. In
contrast, relatively little is known about the chemistry of
metal complexes with bridging allyl ligands. In this work, we
describe a more efficient synthetic methodology for the
preparation of PdI-bridging allyl dimers and report the first
studies of their stoichiometric reactivity. Furthermore, we
show that these compounds can activate CO2 and that an
N-heterocyclic carbene-supported dimer is one of the most
active and stable catalysts reported to date for the carbox-
ylation of allylstannanes and allylboranes with CO2.
allyl ligands are nucleophilic like η1-allyl ligands rather than
electrophilic like η3-allyl ligands. Furthermore, we show that
these compounds can activate CO2 and that an N-heterocyclic
carbene (NHC)-supported dimer is one of the most active and
stable catalysts reported to date for the carboxylation of allyl-
stannanes and allylboranes with CO2.
Initially, the bridging allyl dimers 1-8 were prepared through
the direct reaction of Pd(allyl)2 or Pd(2-methylallyl)2 with the
appropriate free ligand (eq 1). This was a high-yielding and
efficient route that allowed us to isolate a family of bridging allyl
dimers, including those supported by NHC ligands. Our synthesis
is an improvement on the literature route and avoids the isolation
of compounds of the type (η1-allyl)(η3-allyl)Pd(L). However,
Pd(allyl)2 species are also thermally unstable and difficult to
synthesize. A far superior synthesis of 1-8 involves treatment of
the commercially available compounds [(η3-allyl)Pd(μ-Cl)]2 or
[(η3-2-methylallyl)Pd(μ-Cl)]2 with the free ligand followed by
reaction of the (η3-allyl)Pd(L) or (η3-2-methylallyl)Pd(L) inter-
mediate with the appropriate allyl Grignard reagent (Scheme 1).
The monomeric η3-allyl Pd intermediates can be isolated, or the
reaction can be performed in two sequential steps in one pot. In
general, the yields of 1-8 are excellent, with only 4 and 8 being
formed in less than 50% yield.
n recent years, there has been significant interest in the catalytic
functionalization of CO2 because of the potential for use of this
I
greenhouse gas as a readily available and inexpensive source of
carbon in the synthesis of both commodity chemicals and
complex organic molecules.1 Several transition-metal complexes
have been developed as catalysts for carboxylation reactions, and
the key step is proposed to involve a reaction between the metal
complex and CO2.2 Recently, both Wendt and our group have
reported studies of the mechanism of CO2 insertion into
monomeric η1-allyl Pd complexes and shown that these species
are catalytically active for the carboxylation of allylstannanes and
allylboranes.3 During our studies of η1-allyl Pd systems, we
discovered that Pd(I)-bridging allyl dimers also undergo inser-
tion of CO2 to form a bridging carboxylate, and we report our
findings here.
In general, the chemistry of both η1-allyl and η3-allyl Pd
complexes is well-understood; η1-allyls are nucleophilic and react
with electrophiles, whereas η3-allyls are electrophilic and react
with nucleophiles.4 In contrast, relatively little is known about the
chemistry of metal complexes with bridging allyl ligands.5 Only a
handful of species of this type have been prepared, and their
reactivity has not been probed in detail.6 A particularly interest-
ing class of bridging allyl Pd species consists of PdI dimers, which
contain a metal-metal bond, two bridging allyl ligands, and a
terminal phosphine ligand.6b,7 Three examples of these complexes
have been described, and generally they are generated by
reductive dimerization from isolated complexes of the type
(η1-allyl)(η3-allyl)Pd(L) with concomitant elimination of
hexadiene.6b,7a,7c Unfortunately, (η1-allyl)(η3-allyl)Pd(L) com-
plexes are air-, moisture-, and thermally sensitive, which makes
working with these compounds difficult. Here we describe a
more efficient synthetic methodology for the preparation of PdI-
1
Compounds 1-8 were characterized by H and 13C NMR
spectroscopy and elemental analysis (1 and 3 have been pre-
viously reported3c,7a). In addition, crystals of 2, 4, and 6 were
grown from saturated solutions at temperatures from -35 °C to
room temperature. Compounds 2, 4, and 6 have similar geome-
tries, and their structures are shown in Figure 1, Figure S1 in the
Supporting Information, and Figure 2, respectively. Each allyl
ligand is bound through the central carbon to both Pd atoms,
Received: December 8, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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Scheme 1
2-methylallyl ligands) are oriented in a syn geometry, so the
central hydrogen atoms (or methyl groups) are eclipsed and on
the same face of the molecule. The phosphine or NHC ligands
are bent away from the central hydrogen atom (or methyl group)
of the allyl ligands, and the P-Pd-Pd or C-Pd-Pd (C of
NHC) bond angles are significantly less than 180°. The short
Pd-Pd distances [e.g., Pd(1)-Pd(2) = 2.7112(4) Å for 2] are
consistent with a single bond between the two metal centers and
an oxidation state of PdI. Compounds 2 and 6 are only the second
and third examples of structurally characterized Pd complexes
with two bridging allyl ligands,7b while 4 is just the second
example of a well-characterized NHC-supported Pd(I) species.8
In order to probe the reactivity of bridging allyl dimers,
compounds 2-4 were used as models. Treatment with 1 equiv
of diphenylacetic acid led to protonation of one of the bridging
allyl ligands and the formation of dimers 9-11 containing both a
bridging carboxylate ligand and a bridging allyl ligand (eq 2).
These products were spectroscopically characterized. Although
the reactions with 2 and 3 gave quantitative conversion, a second
unidentified product was formed in ∼30% yield in the case of the
NHC-supported complex 4. Surprisingly, treatment with further
equivalents of acid did not lead to protonation of the second
bridging allyl ligand. Phenylthiol also reacted with 2-4 to give
species 12-14 containing one bridging thiolate ligand and one
bridging allyl ligand (eq 3), which were characterized spectro-
scopically. These protonation reactions with thiols are similar to
the protonation reactions that PdI species with one bridging allyl
ligand and one bridging cyclopentadienyl ligand have been
shown to undergo.9 Reaction of 3 with 1 equiv of HCl gave
the mixed allyl-chloride species 15 (eq 4). Control experiments
showed that whereas (η1-allyl)(η3-allyl)Pd(PPh3) reacts with
HCl to form (η3-allyl)PdCl(PPh3), no further reaction of the
electrophilic η3-allyl product with excess HCl is observed. Over-
all, these protonation reactions suggest that our bridging allyl is
nucleophilic like an η1-allyl.
Figure 1. X-ray structure of 2 (hydrogen atoms have been omitted for
clarity). Selected bond lengths (Å) and angles (deg): Pd(1)-Pd(2),
2.7112(4); Pd(1)-P(1), 2.2591(12); Pd(2)-P(2), 2.2512(12); Pd-
(1)-C(1), 2.106(5); Pd(1)-C(2), 2.611(2); Pd(1)-C(4), 2.151(5);
Pd(1)-C(5), 2.437(4); Pd(2)-C(3), 2.158(5); Pd(2)-C(2),
2.429(4); Pd(2)-C(6), 2.120(5); Pd(2)-C(5), 2.613(5); P(1)-Pd-
(1)-Pd(2), 157.47(5); P(2)-Pd(2)-Pd(1), 153.66(4).
Figure 2. X-ray structure of 6 (hydrogen atoms have been omitted for
clarity). Selected bond lengths (Å) and angles (deg): Pd(1)-Pd(2),
2.6920(5); Pd(1)-P(1), 2.22457(16); Pd(2)-P(2), 2.2246(15); Pd-
(1)-C(1), 2.118(7); Pd(1)-C(2), 2.510(2); Pd(1)-C(5), 2.112(7);
Pd(1)-C(6), 2.549(4); Pd(2)-C(3), 2.108(6); Pd(2)-C(2),
2.553(4); Pd(2)-C(8), 2.122(7); Pd(2)-C(6), 2.511(5); P(1)-Pd-
(1)-Pd(2), 157.47(5); P(2)-Pd(2)-Pd(1), 153.66(4).
while the terminal carbons bind only to the Pd atom in closest
proximity. Interestingly, although in the 2-methylallyl compound
6 the central carbon of the 2-methylallyl groups is approximately
equidistant from the two Pd atoms, there is a significant distor-
tion in both 2 and 4 (the Pd-central carbon distance varies by
∼0.2 Å in 2 and 4). In all of the compounds, the two allyl (or
Reaction of excess H2 with 2-4 resulted in the generation of
propene and propane and the formation of Pd black. In the case
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COMMUNICATION
of 4, a 50% yield of the Pd0 species Pd(NHC)2 (which was
crystallographically characterized; see Figure S2) was obtained
along with the Pd black (eq 5). In these reactions, H2 presumably
reacts with the bridging allyl to form propene and generate a
species containing a Pd-H bond. The Pd-H bond then
decomposes to form Pd black or, in the case of the NHC
complex, a stable Pd0 species.
The most interesting reactions of the bridging allyl dimers
were observed with CO2. Compounds 1, 2, and 4-8 all react
with CO2 to form the corresponding bridging carboxylates 17-
23 (eq 6); these are the first examples of a reaction between CO2
and a bridging allyl on any metal. All of the reactions gave
quantitative conversion except for insertion into 8, which gave
the expected product 23 and an unidentified product in a 4:1
ratio. As a result, 23 was characterized only spectroscopically and
not studied further. Compounds 17-23 are stable at room
temperature in solution, and further exposure to CO2 does not
lead to the insertion of a second molecule of CO2. The insertion
of CO2 is irreversible, and exposure of 19 to 13CO2 did not lead
to the incorporation of the label into the product. The IR spectra
of 17-23 are consistent with a bridging carboxylate ligand, with
the difference between the CO2 stretches being less than 200
cm-1 in all cases.10 For example, two CO2 stretches were
observed at 1558 and 1392 cm-1 for 22. These bands shifted
to 1517 and 1366 cm-1 when 13CO2 was used to synthesize 22
(calcd 1523 and 1360 cm-1).
Figure 3. X-ray structure of 20 (hydrogen atoms have been omitted for
clarity). Selected bond lengths (Å) and angles (deg): Pd(1)-Pd(2),
2.6386(5); Pd(1)-P(1), 2.3018(10); Pd(2)-P(2), 2.3086(10); Pd-
(1)-O(1), 2.175(3); Pd(2)-O(2), 2.164(3); Pd(1)-C(6), 2.041(5);
Pd(1)-C(7), 2.464(1); Pd(2)-C(7), 2.456(4); Pd(2)-C(8),
2.046(5); P(1)-Pd(1)-Pd(2), 172.92(3); P(2)-Pd(2)-Pd(1),
169.83(2).
Figure 4. X-ray structure of 22 (hydrogen atoms have been omitted for
clarity). Selected bond lengths (Å) and angles (deg): Pd(1)-Pd(2),
2.6267(3); Pd(1)-P(1), 2.3044(9); Pd(2)-P(2), 2.3069(10); Pd-
(1)-O(1), 2.133(3); Pd(2)-O(2), 2.186(3); Pd(1)-C(6),
2.044(6); Pd(1)-C(7), 2.397(3); Pd(2)-C(7), 2.494(3); Pd(2)-
C(8), 2.035(6); P(1)-Pd(1)-Pd(2), 161.16(2); P(2)-Pd(2)-Pd-
(1), 172.98(2).
mechanism of CO2 insertion is unclear, but it is interesting to
note that whereas nucleophilic terminal η1-allyl ligands react
with CO2, electrophilic terminal η3-allyl ligands do not. This
again suggests that the bridging allyl ligand has reactivity similar
to that of an η1-allyl ligand.
Complexes 20 and 22 were crystallographically characterized,
and the structures are shown in Figures 3 and 4. The single
bridging 2-methyallyl ligand is bound to each Pd through two
carbons, in an analogous fashion to 2 and 6, while the bridging
carboxylate binds only through the oxygen atoms. The alkyl
group of the carboxylate is on the opposite face of the molecule
from the methyl group of the allyl, and the P-Pd-Pd bond
angles are much closer to 180 °C than in 2 and 6. The Pd-Pd
distances [Pd(1)-Pd(2) = 2.6267(3) Å for 20] are shorter than
those observed in 2 and 6 but still consistent with a single bond.
The rates of reaction with CO2 vary depending on the
substituent on the allyl group and the ancillary ligand, with
reactions observed from room temperature to 70 °C. For the allyl
ligand, the rate is NHC > PEt3 > PMe3, with no reaction observed
for the PPh3-supported ligand, while for the 2-methylallyl ligand,
the rate is NHC > PPh3 > PEt3 > PMe3. The reactions are faster
for 2-methyallyl than for allyl. In all cases, the reactions of the
dimeric species are significantly slower than for the monomeric
species (η1-2-methylallyl)(η3-2-methyallyl)Pd(L) (L = PR3,
NHC), which react with CO2 at approximately -30 °C.3c The
We have recently demonstrated that complexes of the type
(η1-2-methylallyl)(η3-2-methylallyl)Pd(L) (L = PR3, NHC) are
extremely effective catalysts for the carboxylation of allylstan-
nanes and allylboranes.3d The proposed mechanism involves two
steps: insertion of CO2 followed by transmetalation. No reaction
was observed in transmetalation reactions between the phos-
phine-supported carboxylates (17, 18, and 20-22) and tributy-
lallylstannane. However, reaction of 19 with tributylallylstannane
resulted in the generation of 4 and a tin carboxylate, suggesting
that catalysis may be possible. In fact, 4 is a highly active catalyst
for the carboxylation of allylstannanes with CO2 (eq 7 and
Table 1) and is comparable in reactivity with the best systems
that have been reported.3a,d Compound 4 is also able to catalyze
the carboxylation of allylboranes with an efficiency similar to that
of our previously reported system.3d The major advantage of the
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Table 1. Carboxylation of Different Allylstannanes and Al-
lylboranes with CO2 in C6D6 Using Complex 4 as the
Catalysta
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M. S. G.; Roodt, A.; Wendt, O. F. Organometallics 2010, 29, 3521. (c)
Wu, J.; Green, J. C.; Hazari, N.; Hruszkewycz, D. P.; Incarvito, C. D.;
Schmeier, T. J. Organometallics 2010, 29, 6369. (d) Wu, J.; Hazari, N.
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substrate
time (h)
conversion (%)
Me3Sn(2-methylallyl)
nBu3Sn(2-methylallyl)
Me3Sn(allyl)
20
24
26
24
26
55
79
80
70
82
60
81
nBu3Sn(allyl)
(pinacol)B(allyl)
(pinacol)B(2-methylallyl)
a Reaction conditions: substrate (0.118 mmol), catalyst 4 (3.8 mg,
0.0059 mmol), and CO2 (1 atm) in 0.25 mL of C6D6 at room
temperature (unless otherwise stated).
dimeric system is that it is significantly easier to synthesize than
monomeric systems and is more stable; thus, there is less catalyst
decomposition, and the catalyst is capable of higher turnover
numbers.
(7) (a) Henc, B.; Jolly, P. W.; Salz, R.; Stobbe, S.; Wilke, G.; Benn, R.;
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In conclusion, we have developed a facile synthesis for the
preparation of allyl-bridged PdI dimers and performed the first
reactivity studies with these molecules. An NHC-supported
dimer is one of the most active catalysts reported to date for
the catalytic carboxylation of allylstannanes and allylboranes with
CO2. From our results, it appears that bridging allyls react more
like terminal η1-allyls than η3-allyls, and future work will look to
further compare the reactivity of bridging allyls with terminal η1-
allyls and understand the mechanism of CO2 insertion into
bridging allyls.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental protocols, char-
b
acterization data for all new compounds, and X-ray crystal-
lographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
nilay.hazari@yale.edu
Author Contributions
†These authors contributed equally.
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