Insertion of carbon dioxide into (PCP)PdII-Me bonds

Article abstract of DOI:10.1021/om0505561

A (PCP)PdMe complex has been synthesized and structurally characterized, displaying a very long Pd-Me bond. It is reactive toward CO₂ insertion, giving the corresponding acetate in quantitative yield. The methyl complex can be regenerated using ZnMe₂, and catalytic carboxylation is possible in benzene.

Full text of DOI:10.1021/om0505561

4500
Organometallics 2005, 24, 4500-4502
Insertion of Carbon Dioxide into (PCP)PdII-Me Bonds
Roger Johansson, Martin Jarenmark, and Ola F. Wendt*
Organic Chemistry, Department of Chemistry, Lund University, P.O. Box 124,
S-221 00 Lund, Sweden
Received July 4, 2005
Summary: A (PCP)PdMe complex has been synthesized
and structurally characterized, displaying a very long
Pd-Me bond. It is reactive toward CO2 insertion, giving
the corresponding acetate in quantitative yield. The
methyl complex can be regenerated using ZnMe2, and
catalytic carboxylation is possible in benzene.
There is no information about a stoichiometric insertion
step, but such a step seems likely.6 Our earlier inves-
tigations of Pd-Me complexes with the methyl group
trans to nitrogen or phosphorus show that they are
either inert or, in the presence of small amounts of
water, give carbonate complexes.7 We therefore decided
to try to labilize the Pd-C bond, and our main focus
has been on PCP-type complexes which combine high
thermal stability with a reactive site trans to the trans-
labilizing in-plane aryl ring. PCP complexes of pal-
ladium have been shown to catalyze a large number of
C-C bond forming reactions.8 Here we report the
synthesis of (PCP)Pd complexes with hydrocarbyl ligands
in the fourth position and their reactivity toward CO2.
We also report the structural characterizations of
(PCP)Pd methyl and acetate complexes.
With the increasing awareness of the world’s finite
petroleum reserves, carbon dioxide has become interest-
ing as a feedstock for the synthesis of useful organic
molecules.1 The formation of C-C bonds would be of
particular value, but of course this poses difficulties,
since CO2 is an inert molecule that does not react readily
under most conditions. There are examples of a number
of transition-metal carbon bonds that undergo inser-
tion,2 and there are also examples of catalytic reactions
that form new C-C bonds.3
To develop new reactions for catalysis, we decided to
investigate the insertion into Pd-C σ-bonds. Palladium
is known to activate C-H bonds in hydrocarbons,4
which are interesting substrates to react with CO2 in a
C-C bond-forming reaction. Also, palladium-allyl com-
plexes are known to undergo insertion reactions with
CO2;5 however, there are no known examples of inser-
tions into Pd-C σ-bonds. Furthermore, Fujiwara and
co-workers have reported palladium-catalyzed direct
carboxylations of arenes, albeit with low conversions.
The free (PCP)H (1) ligands were synthesized accord-
ing to literature methods9 and reacted with Pd(TFA)2
to give the TFA complexes 2 and 4 (cf. Scheme 1). These
Scheme 1
* To whom correspondence should be addressed. E-mail: ola.wendt@
organic.lu.se.
(1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman,
E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.;
Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard,
W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons,
J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.;
Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt,
L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W.
Chem. Rev. 2001, 101, 953 and references therein.
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Pandey, K. K. Coord. Chem. Rev. 1995, 140, 37. (c) Darensbourg, D.
J.; Kudaroski Hankel, R.; Bauch, C. G.; Pala, M.; Simmons, D.; White,
J. N. J. Am. Chem. Soc. 1985, 107, 7463. (d) Darensbourg, D. J.;
Gro¨tsch, G.; Wiegreffe, P.; Rheingold, A. L. Inorg. Chem. 1987, 26,
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Tekavec, T. N. J. Am. Chem. Soc. 2002, 124, 15188.
undergo reaction with methyllithium in diethyl ether
to give the corresponding palladium methyl compounds,
which display a characteristic high-field virtual triplet
around 0.4 ppm in C6D6 and a single 31P{1H} resonance,
in line with a trans geometry. Reactions were sluggish
and 5 could not be isolated, since the reaction mixture
(4) (a) Boele, M. D. K.; von Strijdonk, G. P. F.; de Vries, A. H. M.;
Kamer, P. C. J.; de Vries, J. G.; van Leeuven, P. W. N. M. J. Am. Chem.
Soc. 2002, 124, 1586. (b) Kalyani, D.; Deprez, N. R.; Desai, L. V.;
Sanford, M. S. J. Am. Chem. Soc. 2005, 127, 7330. (c) Zaitsev, V. G.;
Daugulis, O. J. Am. Chem. Soc. 2005, 127, 4156. (d) Orito, K.; Horibata,
A.; Nakamura, T.; Ushito, H.; Nagasaki, H.; Yuguchi, M.; Yamashita,
S.; Tokuda, M. J. Am. Chem. Soc. 2004, 126, 14342. (e) Yokota, T.;
Tani, M.; Sakagushi, S.; Ishii, Y. J. Am. Chem. Soc. 2003, 125, 1476.
(f) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J.
Science 2003, 301, 814. (g) Sen, A. Acc. Chem. Res. 1998, 31, 550.
(5) (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.
(6) (a) Sugimoto, H.; Kawata, I.; Taniguchi, H.; Fujiwara, Y. J.
Organomet. Chem. 1984, 266, C44. (b) Fujiwara, Y.; Tabaki, K.;
Taniguchi, Y. Synlett 1996, 591. (c) Fuchita, Y.; Hiraki, K.; Kamogawa,
Y.; Suenaga, M.; Tohgoh, K.; Fujiwara, Y. Bull. Chem. Soc. Jpn. 1989,
62, 1081.
(7) Pushkar, J.; Wendt, O. F. Inorg. Chim. Acta 2004, 357, 1295.
(8) (a) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am.
Chem. Soc. 1997, 119, 11687. (b) Cotter, W. D.; Barbour, L.; Mc-
Namara, K. L.; Hechter, R.; Lachcotte, R. L. J. Am. Chem. Soc. 1998,
120, 11016.
(9) Gusev, D. G.; Madott, M.; Dolgushin, F. M.; Lyssenko, K. A.;
Antipin, M. Y. Organometallics 2000, 19, 1734.
10.1021/om0505561 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/19/2005

Communications
Organometallics, Vol. 24, No. 19, 2005 4501
gave no reaction at room temperature. The mixture was
then heated to 80 °C and the reaction followed by NMR.
Only 3 was reactive: within 2 days all of the starting
material was consumed and the sole product was the
normally inserted complex 6 (cf. Scheme 2). Isolation
Scheme 2
of this product was unsuccessful, but it could be
independently synthesized according to Scheme 2. The
insertion product was characterized by comparison with
the synthesized sample of PCPPdOAc (6), which was
fully characterized. We also obtained crystals of 6
suitable for X-ray diffraction.13 The molecular structure
is shown in Figure 2. The acetate is η1 coordinated, as
Figure 1. DIAMOND19 drawing of 3. For clarity hydrogen
atoms have been omitted. Selected bond distances (Å) and
angles (deg) with estimated standard deviations: Pd1-P1
) 2.2927(8); Pd1-P2 ) 2.2885(7); Pd1-C1 ) 2.076(2);
Pd1-C9 ) 2.199(3); P1-Pd1-P2 ) 165.48(3); P1-Pd1-
C1 ) 83.24(7); P1-Pd1-C9 ) 97.16(8); P2-Pd1-C1 )
82.74(7); P2-Pd1-C9 ) 97.01(8).
also contained starting material. Compound 3 was fully
characterized and gave crystals suitable for X-ray
crystallography.10 The molecular structure, together
with selected distances and angles, is given in Figure
1. It consists of discrete molecules packed by dispersive
forces. The coordination geometry is distorted square-
planar with the typical “bent-back” P-Pd-P angle of
165.48(3)°. It displays a very long Pd-Me bond of 2.199-
(3) Å, the longest reported Pd-C σ-bond in a mono-
nuclear complex. This is in line with the high trans
influence of the aryl ring and confirms our assumption
that the methyl group should have an enhanced reactiv-
ity. The methyl group of course also exerts a high trans
influence, giving rise to a rather long palladium-aryl
bond of 2.076(2) Å, in the same range as has been
observed trans to a σ(C)-bonded furyl group.11
Figure 2. DIAMOND19 drawing of 6. For clarity hydrogen
atoms have been omitted. Selected bond distances (Å) and
angles (deg) with estimated standard deviations: Pd1-P1
) 2.3058(8); Pd1-P2 ) 2.3118(8); Pd1-C1 ) 2.014(2);
Pd1-O1 ) 2.1279(19); O1-C10 ) 1.252(3); O2-C10 )
1.227(3); P1-Pd1-P2 ) 165.94(3); P1-Pd1-C1 ) 83.17-
(7); P1-Pd1-O1 ) 96.24(6); P2-Pd1-C1 ) 82.77(7); P2-
Pd1-O1 ) 97.71(6), O2-C10-O1 ) 126.0(3).
As described by Hughes and co-workers12 for the
corresponding (PCP)PtRF complex, (PCP)tBuH can also
be reacted with Pd(tmeda)Me2 to give compound 3. On
an NMR scale this reaction was quantitative, generating
methane as the only byproduct, but it was not amenable
to upscaling (cf. Scheme 1). The corresponding phenyl
complexes 8 and 9 could be obtained similarly from the
reaction of 2 and 4 with PhLi. Also in this case the
reactions were sluggish and none of the phenyl com-
plexes could be isolated, but they were characterized in
expected in a pincer complex. The P-Pd-P angle is
again substantially lower than 180°. The Pd-C distance
is 2.014 Å, in line with a lower trans influence of the
acetate compared to the methyl, and the Pd-O distance
is similar to those found in other examples of acetates
trans to aryl groups.14
1
situ using H and 31P NMR spectroscopy. Using Grig-
Chart 1
nard or organotin reagents did not improve this reac-
tion.
Treating the methyl complexes 3 and 5 in C6D6 with
approximately 30 equiv of CO2 in a sealable NMR tube
(10) Crystal data:
C25H46P2Pd, Mr ) 514.96, orthorhombic, a )
11.156(2) Å, b ) 15.780(3) Å, c ) 30.671(6) Å, V ) 5399.7(19) Å3, space
group Pbca, Z ) 8. A total of 53 129 reflections were measured, 8802
of which were unique (Rint )0.067) and were used in all calculations.
The final Rw(F2) value was 0.1262 (all data), and the R(F) value was
0.0424 (I > 2σ(I)) using 254 parameters. CCDC reference number:
277322.
(11) Cotter, W. D.; Barbour, L.; McNamara, K. L.; Hechter, R.;
Lachicotte, R. J. J. Am. Chem. Soc. 1998, 120, 11016.
(12) Hughes, R. P.; Williamson, A.; Incarvito, C. D.; Rheingold, A.
L. Organometallics 2001, 20, 4741.

4502 Organometallics, Vol. 24, No. 19, 2005
Communications
Table 1. Catalytic Carboxylation of ZnMe2 in C6D6
with Various Metal Complexes as Catalysts
Compound 3 was the only palladium hydrocarbyl
complex that displayed any reactivity toward CO2. Thus,
all phenyl complexes were unreactive, presumably due
to the stronger σ bond that sp2-hybridized carbon atoms
form with transition metals, as seen for example in the
bond distances in Figure 1. It is also clear that the
ancillary ligands play a crucial role. Substituting bulky
and electron-releasing tert-butyl groups on the phos-
phorus atoms for phenyl groups results in loss of
reactivity of the Pd-Me bond. It seems clear that the
combination of a fairly weak Pd-C bond and a nucleo-
philic metal center is crucial for the reaction.
We were interested in incorporating the insertion
reaction in some catalytic context and investigated the
possibility of re-forming 3 from 6 using various nucleo-
philes. Obviously MeLi is not interesting, because it
undergoes an uncatalyzed reaction with CO2.15 Car-
boxylation of allyl tin reagents has been shown to be
catalyzed by palladium complexes,16 presumably due to
the easy insertion into palladium allyl complexes. We
therefore investigated the reactivity of 6 (and 2 and 4)
versus various aryl, alkyl, and vinyl tin reagents, but
no transmetalation activity was observed. This is in line
with the results from our investigation of compounds
such as 4 as catalysts in Stille cross-coupling.17 We then
turned our attention to dimethylzinc, which reacts with
6 to form 3 within 3 h at room temperature.
To test this reaction in a catalytic fashion, we reacted
an excess of CO2 with ZnMe2 in dry benzene with a
catalyst load of 3-23% (cf. Table 1). The reactivity of
dimethylzinc toward CO2 can be enhanced by stoichio-
metric amounts of N-methylimidazole and other organic
nitrogen donors,18 but in benzene there is no reaction
in the absence of catalyst. Only 2, 3, and 6 are active
catalysts for this transformation, as expected if an
insertion-transmetalation cycle is operating. In this
case it would be expected that 3 and 6 would display
similar reactivities and that these would not be higher
than those of any of the stoichiometric reactions. This
is also the case. The unexpectedly high activity of
compound 2 is somewhat puzzling. Both 2 and 6
undergo transmetalation with ZnMe2 to form 3, and a
possible explanation would be a faster transmetalation
of 2 compared to 6 and an even more rapid equilibrium
between these two. However, the stoichiometric reac-
amt of
cat. / ZnMe2/
conversnc/
%
complex
t/hb
%
mmol
T/°C
TON
8
2
3
3
6
3
12
23
19
11
8
6
3
11
7
9
16
0.08
0.05
0.06
0.08
0.15
0.15
0.20
0.08
0.18
0.09
0.05
room temp
room temp
100
100
no reacn
100
2
4
5
7
100
80
dppePdCl2
90
no reacn
no reacn
no reacn
no reacn
no reacn
no reacn
no reacn
5
90
(PPh3)3RhCl
90
(PCP)NiCla
110
4
7
110
110
Pd(PPh3)4
100
a Refers to [1,3-bis((di-tert-butylphosphino)methyl)benzene]n-
ickel chloride. b Reactions left overnight unless stated otherwise.
Reaction times not optimized. c Determined by 1H NMR.
tions indicate that the insertion reaction is rate deter-
mining and so the reactivity of 3 must somehow be
altered. To probe this, we studied the reactivity of 3
toward CO2 in the presence of 10% 2, but in the absence
of organozinc reagent. Indeed, the methyl peak disap-
pears much faster than in the absence of 2, but we do
not see a corresponding increase of the acetate peak.
We do not fully understand this, but it seems to indicate
that the fast catalysis by 2 does lie in the insertion step
and does not involve any activation of or by the zinc
reagents, as observed by Inoue in the activation of Et2Zn
by base.18 Perhaps the weakly coordinated triflouro-
acetate is substituted by CO2, which becomes activated
by this and in the presence of a weak Pd-Me bond gives
rise to a fast reaction. It seems unlikely, though, that
the enhanced reactivity in the catalysis is due to general
Lewis acid catalysis by the transition metal, since our
results clearly indicate that the activity is very specific;
numerous other metal complexes have been tested (cf.
Table 1) and none were active in this catalysis. Attempts
to fully understand the mechanism for these transfor-
mations are underway in our laboratory.
In conclusion, we have synthesized a (PCP)PdMe
complex that undergoes insertion of CO2 into the Pd-
Me bond. A catalytic reaction has been devised which
converts ZnMe2 to Zn(OAc)2 with a (PCP)Pd complex
as catalyst.
Acknowledgment. Financial support from the Swed-
ish Research Council, the Crafoord Foundation, the
Knut and Alice Wallenberg Foundation, and the Royal
Physiographic Society in Lund is gratefully acknowl-
edged. We thank Prof. Janis Louie for helpful discus-
sions.
(13) Crystal data:
C26H46O2P2Pd, Mr ) 558.97, monoclinic, a )
21.0355(14) Å, b ) 12.5322(7) Å, c ) 10.9438(6) Å, V ) 2822.5(3) Å3,
space group P21/c, Z ) 4. A total of 25 559 reflections were measured,
9530 of which were unique (Rint ) 0.0477) and were used in all
calculations. The final Rw(F2) value was 0.0727 (all data), and the R(F)
value was 0.0423 (I > 2σ(I)) using 280 parameters. CCDC reference
number: 277323.
(14) Canty, A. J.; Minchin, N. J.; Skelton, B. W.; White, A. H. J.
Chem. Soc., Dalton Trans. 1987, 1477.
(15) Volpin, M. E.; Kolomnikov, I. S. Organomet. React. 1975, 5, 313.
(16) (a) Franks, R. J.; Nicholas, K. M, J. Am. Chem. Soc. 1997, 119,
5057. (b) Shi, M.; Nicholas, K. M. Organometallics 2000, 19, 1458.
(17) Olsson, D.; Nilsson, P.; El Masnaouy, M.; Wendt, O. F. Dalton
Trans. 2005, 1924.
(18) Inoue, S.; Yokoo, Y. J. Organomet. Chem. 1972, 39, 11.
(19) Brandenburg, K. DIAMOND, Program for Molecular Graphics;
Crystal Impact, Bonn, Germany, 2000.
Supporting Information Available: Text and a table
giving full experimental procedures and details of the X-ray
crystal structure determination and a CIF file giving crystal
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM0505561