Synthesis and reactivity of (PCP) palladium hydroxy carbonyl and related complexes toward CO2 and phenylacetylene

Article abstract of DOI:10.1021/om0611709

{2,6-Bis[(di-tert-butylphosphino)methyl]phenyl}palladium hydroxide, PCPPdOH, undergoes a fast insertion of CO₂ to give the bicarbonate complex (PCP)PdO₂COH. It also reacts with CO to give a mononuclear hydroxy carbonyl complex, (PCP)

Full text of DOI:10.1021/om0611709

2426
Organometallics 2007, 26, 2426-2430
Synthesis and Reactivity of (PCP) Palladium Hydroxy Carbonyl and
Related Complexes toward CO2 and Phenylacetylene
Roger Johansson and Ola F. Wendt*
Organic Chemistry, Department of Chemistry, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden
ReceiVed December 22, 2006
{2,6-Bis[(di-tert-butylphosphino)methyl]phenyl}palladium hydroxide, PCPPdOH, undergoes a fast
insertion of CO2 to give the bicarbonate complex (PCP)PdO2COH. It also reacts with CO to give a
mononuclear hydroxy carbonyl complex, (PCP)PdCOOH. The Pd-C bond of this is unreactive toward
olefins. The complex undergoes a slow conversion to the formate complex, (PCP)PdO2CH, via a
decarboxylation to give the hydride, which undergoes a fast normal insertion of CO2 to give the formate.
The hydride, (PCP)PdH, reacts with phenyl acetylene to give the acetylide complex, (PCP)PdCCPh, in
a C-H exchange reaction. This is similarly obtained from the reaction of (PCP)PdMe or (PCP)PdPh
with phenyl acetylene.
in a similar fashion. Recently, a palladium hydroxy carbonyl
compound based on a {2,6-bis[(diisopropylphosphino)methyl]-
phenyl} ligand framework was reported.7 The complex could
not be isolated and underwent decarbonylation to give a
dinuclear, CO2-bridged unit.
We were interested in tuning the behavior of such (PCP)Pd
systems, and here we report on the synthesis of a stable
mononuclear palladium hydroxy carbonyl compound based on
the {2,6-bis[(di-tert-butylphosphino)methyl]phenyl} ligand that
could be characterized in the solid state. We also report on its
reactivity and that of the related hydride, especially with respect
to C-H bond activation of phenyl acetylene in a σ-bond
metathesis type of reaction.
Introduction
Metallocarboxylic acids, or hydroxy carbonyl compounds,
have been proposed as intermediates in a number of reactions
such as the methoxy carbonylation of 1-alkynes1 and the
reduction of CO2 with H2, as well as in electrochemical
reduction2 and the water gas shift reaction,3 but reports on their
isolation and characterization are relatively few.4 The reported
reactivity of hydroxy carbonyl compounds include decarboxy-
lation reactions to give the corresponding hydride and nucleo-
philic substitution reactions on the coordinated CO unit.4h
Mononuclear palladium hydroxy carbonyl compounds have not
been isolated, as opposed to their platinum analogues.4f,h
However, they have been proposed to be involved in the
hydroxy carbonylation of aryl halides,5 and it has also been
shown that alkoxy carbonyl compounds can undergo insertion
of alkene followed by â-elimination to give unsaturated esters.6
We have an interest in the carboxylation of hydrocarbons, and
one possible way to generate unsaturated carboxylic acids would
be insertion of alkene into a palladium hydroxy carbonyl moiety
Experimental Section
General Procedures and Materials. All experiments were
carried out using standard high-vacuum line or Schlenk techniques
or in a glovebox under nitrogen.8 Solvents were dried over and
distilled from Na/benzophenone prior to use. If nothing else is
stated, all commercially available reagents were used as received
from Aldrich. Complexes 1,9 4,10 7, and 811 were prepared according
to literature. NMR measurements were performed in C6D6 unless
* Corresponding author. E-mail: ola.wendt@organic.lu.se.
(1) (a) Gabriele, B.; Costa, M.; Salerno, G.; Chiusoli, G. P. J. Chem.
Soc., Perkin Trans. 1994, 83-87. (b) Chiusoli, G. P.; Costa, M.; Cucchia,
L.; Gabriele, B.; Salerno, G.; Veltri, L. J. Mol. Catal. A 2003, 204-205,
133-142.
1
otherwise stated. H, 13C, and 31P NMR spectra were recorded on
a Varian Unity INOVA 500 spectrometer working at 499.77 MHz
(1H). Chemical shifts are given in ppm downfield from TMS using
residual solvent peaks (1H, 13C NMR) or using H3PO4 as an external
reference (31P). Elemental analyses was performed by H. Kolbe
Mikroanalytisches Laboratorium, Mu¨lheim an der Ruhr, Germany.
Coupling constants are given in Hz.
Preparation of PCPtBuPdO2COH (2). In a sealable NMR tube
1 (3.2 mg) was dissolved in C6D6 (0.6 mL). The solution was
pressurized with approximately 4 atm of CO2. Within minutes the
(2) Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L.; Bernatis,
P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois, D. L.
Organometallics 1994, 13, 4844-4855.
(3) Ford, P. C.; Rokicki, A. AdV. Organomet. Chem. 1988, 28, 139-
217.
(4) (a) Elliott, P. I. P.; Haslam, C. E.; Spey, S. E.; Haynes, A. Inorg.
Chem. 2006, 45, 6269-6275. (b) Lee, D. W.; Jensen, C. M.; Morales-
Morales, D. Organometallics 2003, 22, 4744. (c) Gibson, D. H.; Sleadd,
B. A.; Mashuta, M. S.; Richardson, J. F. Acta Crystallogr., Sect. C: Struct.
Commun. 1998, 54, 1584. (d) Gibson, D. H.; Ding, Y.; Andino, J. G.;
Mashuta, M. S.; Richardson, J. F. Organometallics 1998, 17, 5178. (e)
Toyohara, K.; Nagao, H.; Adachi, T.; Yoshida, T.; Tanaka, K. Chem. Lett.
1996, 27. (f) Bennett, M. A.; Jin, H.; Willis, A. C. J. Organomet. Chem.
1993, 451, 249. (g) Mandal, S. K.; Ho, D. M.; Orchin, M. J. Organomet.
Chem. 1992, 439, 53. (h) Bennett, M. A.; Robertson, G. B.; Rokicki, A.;
Wickramasinghe, W. A. J. Am. Chem. Soc. 1988, 110, 7098. (i) Fujita, E.;
Szalda, D. J.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1988, 110, 4870. (j)
Balbach, B. K.; Helus, F.; Oberdorfer, F.; Ziegler, M. L. Angew. Chem.,
Int. Ed. Engl. 1981, 20, 470.
(7) Ca´mpora, J.; Palma, P.; del R´ıo, D.; AÄ lvarez, E. Organometallics
2004, 23, 1652-1655.
(8) Burger, B. J.; Bercaw, J. E. In New DeVelopments in the Synthesis,
Manipulation and Characterization of Organometallic Compounds, Vol.
357; Wayda, A., Darensbourg, M. Y., Eds.; American Chemical Society:
Washington, DC, 1987.
(9) Johansson, R.; O¨ hrstro¨m, L.; Wendt, O. F. Submitted.
(10) Moulton C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020-1024.
(11) Johansson, R.; Jarenmark, M.; Wendt, O. F. Organometallics 2005,
24, 4500-4502.
(5) Grushin, V. V.; Alper, H. J. Am. Chem. Soc. 1995, 117, 4305-4315.
(6) Otsuka, S.; Nakamura, A.; Yoshida, T.; Naruto, M.; Ataka, K. J.
Am. Chem. Soc. 1973, 95, 3180. For a review see: del R´ıo, I.; Claver, C.;
van Leeuwen, P. W. N. M. Eur. J. Inorg. Chem. 2001, 2719.
10.1021/om0611709 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/24/2007

Synthesis of (PCP) Palladium Hydroxy Carbonyl Complexes
Organometallics, Vol. 26, No. 9, 2007 2427
Table 1. Crystal Data for 2, 3, and 4
Scheme 1
2
3
4
formula
fw
space group
a/Å
b/Å
c/Å
C25H44O3P2Pd
560.93
Pbca
15.7850(3)
16.3154(4)
21.8306(3)
90
C25H44O2P2Pd
544.94
C2/c
30.3983(11)
12.8608(7)
15.4378(9)
90
111.773(4)
90
5604.8(5)
8
1.292
0.79
2.68-32.66
24 782
C24H44P2Pd
500.93
I41cd
16.36170(10)
16.36170(10)
19.3825 (3)
90
R/deg
â/deg
γ/deg
V/Å3
Z
was left stirring for 2 days. After evaporation of all volatile
90
90
90
90
components and recrystallization in acetone, 6 was isolated (44 mg,
1
0.085 mmol, 85%). H NMR (C6D6): δ 7.63 (d, JHH ) 7.6, 2H),
5622.22(19)
8
1.323
0.80
2.27-32.73
35 760
5188.80(9)
8
1.282
0.85
2.49-33.08
51 183
7.18 (t, JHH ) 7.5, 2H), 7.12 (m, 3H) 7.00 (t, JHH ) 7.1, 1H), 3.23
(vt, JPH ) 8.0, 4H), 1.34 (vt, JPH ) 13.7, 36H). 31P{1H} NMR: δ
80.3; 13C{1H}: δ 152.1 (t, JPC ) 10.2), 130.9 (s), 128.3 (s), 128.1
(s), 127.9 (s), 125.0 (s), 124.6 (s), 121.4 (vt, JPC ) 21.8), 118.5
D
calcd/g cm-3
µ/mm-1
θ range/deg
no. of reflns
collected
(s), 118.3 (t, JPC ) 2.9), 37.8 (vt, JPC ) 20.4), 34.9 (vt, JPC
17.2), 29.5 (vt, JPC ) 6.6).
)
no. of unique
reflns
9186
9536
9643
Crystallography. Intensity data were collected with an Oxford
Diffraction Xcalibur 3 system using ω-scans and Mo KR (λ )
0.71073 Å) radiation.12 Intensity data were extracted and integrated
using Crysalis RED.13 The structures were solved by direct methods
and refined by full-matrix least-squares calculations on F2 using
SHELXTL 5.1.14 Non-H atoms were refined with anisotropic
displacement. All hydrogen atoms were constrained to parent sites,
using a riding model. Crystal data and details about data collection
are given in Table 1. Selected bond distances and angles are given
in Table 2. All crystallographic data are available in CIF format
and are given in the Supporting Information.
Reaction of 7 and 8 with PhCCH. In a sealable NMR tube 7
or 8 was dissolved in C6D6 (0.6 mL). PhCCH (3.5 µL) was added,
and the tube was heated to 80 °C. The reaction with 7 to yield 6
was finished after roughly 12 h. The sample of 8 was heated at
100 °C for 14 days until all of the PhCCH was consumed.
R(F) (I > 2σ(I))a
wR(F2) (all data)b
Sc
0.038
0.093
0.91
0.052
0.186
1.55
0.033
0.095
1.02
Rint
0.061
0.027
0.053
aR ) ∑(|Fo| - |Fc|)/∑|Fo|. wR ) [∑w(|Fo| - |Fc|)2/∑|Fo| ]1/2. S )
b
2
c
[∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
Table 2. Selected Angles and Distances for the Complexes
Studied
2
3
4
Pd-C
2.005
2.305
2.319
2.121
164.46
2.070
2.297
2.300
2.056
165.6
2.072
2.258
Pd-P1
Pd-P2
Pd-Xa
P-Pd-P
167.89
a X denotes the binding atom of the ligand in the fourth position.
Results and Discussion
corresponding carbonate complex was formed. After slow evapora-
tion of the solvent, crystals suitable for X-ray diffraction studies
formed as colorless prisms. Anal. Found: C, 53.6; H, 8.0.
C25H44O3P2Pd requires: C, 53.5; H, 7.9. 1H NMR (C6D6): δ 6.99
(t, JHH ) 7.5, 1H), 6.88 (d, JHH ) 7.8, 2H), 2.91 (vt, JPH ) 8, 4H),
1.31 (vt, JPH ) 13, 36H). 31P{1H} NMR: δ 73.1. 13C{1H} NMR:
δ 162.5 (s), 154.0 (s), 151.9 (t, JPC ) 10), 124.9 (s), 122.5 (vt, JPC
) 20), 34.8 (vt, JPC )14.5), 33.5 (vt, JPC ) 20), 29.3 (vt, JPC ) 7).
The palladium hydroxide 1 was prepared as previously
described from the corresponding chloride via the nitrate.9
Terminal palladium hydroxides are rare and include the iso-
propyl analogue of 1. Within minutes of pressurizing a sample
of 1 in C6D6 with 4 atm of CO2 the corresponding bicarbonate,
2, was formed, cf. Scheme 1. This reactivity is about the same
as that found for other palladium hydroxide complexes,15 but it
is orders of magnitude higher than the corresponding (with the
same ligand system) insertion into a Pd-Me bond, which
requires heating for 48 h. Compound 2 was isolated and fully
characterized by NMR spectroscopy, elemental analysis, and
X-ray crystallography. The molecular structure shows some
unusual arrangement and is given in Figure 1. The phenyl group
of the ligand backbone is tilted in comparison to what is
commonly found for pincer complexes. Also, the benzylic
carbons are not in the plane of the phenyl ring and neither is
the palladium. The phenyl ring is forced out of position by one
of the t-Bu groups of a neighboring complex. The carbonate
groups of two neighboring units exhibit hydrogen bonding with
an O-H‚‚‚O distance comparable to other carbonate hydrogen-
bonding distances (2.607(4) Å). We were unable to find any
signal in the 1H NMR corresponding to the bicarbonate proton,
Preparation of PCPtBuPdCOOH (3). In a Strauss flask PCPtBu
-
PdOH (1) (0.068 g; 0.13 mmol) was dissolved in C6H6, and this
solution was stirred in an atmosphere of CO for 2 h. The solvent
was removed, and the solid was washed with pentane. This left a
gray, amorphous solid that was used without further purification
(0.032 g; 0.059 mmol; 45%). Slow diffusion of pentane into a
benzene solution gave crystals suitable for X-ray diffraction studies.
1H NMR (C6D6): δ 7.13 (t, 3JHH ) 5, 1H), 7.12 (d, 3JHH ) 5, 2H),
3.27 (vt, 2JPH ) 7, 4H), 1.27 (vt, 3JPH ) 13, 36H). 31P{1H} NMR:
δ 79.3. 13C{1H} NMR: δ 213.4 (s), 172.4 (s), 151.2 (t), 125.2 (s),
122.2 (s), 121.1 (t, JPC ) 9), 38.7 (vt, JPC ) 21), 35.0 (vt, JPC
15), 29.5 (s).
)
Reaction of 4 with CO2. In a sealable NMR tube 4 was dissolved
in C6D6 (0.6 mL). The tube was pressurized with approximately 4
atm of CO2. The reaction was complete in less than 1 h, giving
1
PCPtBuPdO2CH, 5, as the sole product. H NMR (C6D6): δ 9.08
(12) Crysalis CCD; Oxford Diffraction Ltd.: Abingdon, Oxfordshire,
UK, 2005.
(13) Crysalis RED; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK,
2005.
(14) Sheldrick, G. M. SHELXTL5.1, Program for Structure Solution and
Least Squares Refinement; University of Go¨ttingen: Go¨ttingen, Germany,
1998.
(15) Ganguly, S.; Mague, J. T.; Roundhill, D. M. Inorg. Chem. 1992,
31, 3831-3835. Ruiz, J.; Mart´ınez, M. T.; Florenciano, F.; Rodr´ıguez, V.;
Lo´pez, G. Inorg. Chem. 2003, 42, 3650-3661.
(t, JPH ) 1.7, 1H), 7.00 (t, JHH ) 7.5, 1H), 6.89 (d, JHH ) 7.5, 2H),
2.92 (vt, JPH ) 7.9, 4H), 1.26 (vt, JPH ) 13.7, 36H). 31P{1H}
NMR: δ 73.9. 13C{1H} NMR: δ 167.6 (s), 154.9 (s), 151,7 (t, JPC
) 10.3), 125.0 (s), 122.5 (vt, JPC ) 22), 34.8 (vt, JPC ) 14), 33.7
(vt, JPC ) 21), 29.3 (vt, JPC ) 6).
Preparation of PCPtBuPdCCPh (6). In a round bottomed flask
4 (50 mg; 0.0998 mmol) was dissolved in C6H6 (15 mL).
Phenylacetylene (0.13 mL; 1.18 mmol) was added, and the solution

2428 Organometallics, Vol. 26, No. 9, 2007
Johansson and Wendt
Scheme 2
complexes, it exhibits the typical bent-back P-Pd-P angle
(165.5°) because of the limitations from the rigid ligand
backbone. When heating a benzene solution of 3, it decomposes
by loss of CO2, forming 4. This is seen as a new signal in the
31P NMR spectrum at 95 ppm. All spectral features fit well with
those reported for 4 in the literature and were also compared
with an independently synthesized sample (vide infra). The
elimination of CO2 from 3 is not reversible since the reaction
of 4 with CO2 does not produce the hydroxycarbonyl but rather
the formate complex, 5, in a normal insertion that parallels our
previously studied reaction of PCPtBuPdMe.11 Thus, heating 3
in a closed vessel slowly converts it into 5 through loss of CO2
followed by normal insertion into the palladium hydride bond.
These reactions are summarized in Scheme 2. The reactivity of
3 is in stark contrast to that reported for the corresponding
isopropyl complex PCPi-PrPdCOOH. In that case the hydroxy-
carbonyl could not be isolated but only detected in solution
under carbon monoxide pressure. It is reported that when the
pressure of CO is removed, the complex loses carbon monoxide
to re-form the palladium hydroxide, which reacts with a
remaining hydroxy carbonyl under loss of water, forming a CO2-
bridged dimer, which could be isolated and characterized.7 For
3, there is no evidence of a monomer-dimer equilibrium in
solution, and attempts to make the dimeric analogue by reacting
1 with 3 were unsuccessful. The difference in behavior is most
likely due to the higher steric hindrance of the tert-butyl groups
in 3, making it thermodynamically favorable to lose CO2 and
obtain the small hydride in the fourth position as opposed to
the larger hydroxide group. The unwillingness of 3 to dimerize
can also be explained by steric factors. Many metal hydroxy
carbonyls with open coordination sites decompose by loss of
CO2,4a but for palladium and platinum complexes with two trans-
bound phosphines and a phenyl group trans to the hydroxy
carbonyl the preferred decomposition route is normally through
loss of carbon monoxide.4f,h,7 To investigate the reactivity of
the Pd-C bond of 3, it was reacted with methyl acrylate, phenyl
acetylene, and ethylene, but no insertion products were detected
in any of these reactions. This seems to rule out a reactivity
pattern where a new C-C bond could be formed from a hydroxy
carbonyl in a similar fashion as reported for an alkoxy carbonyl
as mentioned in the Introduction. The olefins gave a mixture
of unidentified decomposition products, but the reaction with
phenyl acetylene gave a main product at 80 ppm in the 31P NMR
spectrum in addition to minor (20%) decomposition products.
The main product displays two characteristic peaks at around
118 ppm corresponding to the sp-hybridized carbons, and it was
shown to be the acetylide complex, 6. The peak at 118.3 displays
a typical triplet pattern due to coupling with the phosphorus
atoms and can hence be assigned to the sp carbon bonded to
the palladium. An HMBC 2D-NMR showed that the peak at
118.5 corresponds to the sp carbon next to the phenyl ring.
Compound 6 could never be obtained in an analytically pure
state; it always contained small amounts of an insoluble
hydrocarbon, presumably the phenyl acetylene polymer, thus
giving a higher carbon analysis than expected.
Figure 1. DIAMOND drawing of 2. Hydrogen atoms are omitted
for clarity.
Figure 2. DIAMOND drawing of 3. Hydrogen atoms are omitted
for clarity.
possibly due to exchange of that proton with adventitious water
in the solvent or a fast monomer-dimer equilibrium in solution,
but the 13C NMR signal for the bicarbonate was located at 162.5
ppm. The insertion of CO2 into a palladium oxygen bond is a
well-known reaction,16 but there are few reports on any
structurally characterized mononuclear palladium bicarbonate
complexes.17 We have earlier reported on the isolation of a
bicarbonate complex from the reaction between carbonic acid
and palladium methyl complexes.18
Carbon monoxide reacts in the same way as was previously
reported for making hydroxycarbonyls of the platinum group
metals,4f,h resulting in a gray solid that was identified as the
hydroxycarbonyl compound 3, cf. Scheme 1. This is a slightly
thermally sensitive compound that was characterized by multi-
nuclear NMR and X-ray crystallography. It failed to give a
satisfactory elemental analysis due to thermal decomposition.
The COOH group gives a characteristic 13C NMR signal at 213
ppm. In the solid state 3 consists of hydrogen-bonded dimers
formed from symmetry equivalent complexes. The molecular
structure is given in Figure 2. The O-O distance is 2.684(9)
Å, indicating a fairly strong hydrogen bond well within the range
commonly seen for hydroxy carbonyl compounds and only
slightly longer than what is expected for carboxylic acids. It is
also markedly shorter than the corresponding platinum analogue.
The dimeric structure is slightly bent (5.4°), and the distances
between the two closest carbons of the tert-butyl groups differ
noticeably between the two sides of the dimer (3.666 vs 4.479
Å for the closest distance). The complex has a distorted square
planar arrangement, and as is almost always seen in PCP
(16) Ruiz, J.; Mart´ınez, M. T.; Florenciano, F.; Rodr´ıguez, V.; Lo´pez,
G. Inorg. Chem. 2003, 42, 3650-3661.
(17) Crutchley, R. J.; Powell, J.; Faggiani, R.; Lock, C. J. L. Inorg. Chim.
Acta 1977, 24, L15.
(18) Pushkar, J.; Wendt, O. F. Inorg. Chim. Acta 2004, 357, 1295.

Synthesis of (PCP) Palladium Hydroxy Carbonyl Complexes
Organometallics, Vol. 26, No. 9, 2007 2429
Scheme 3
Figure 3. DIAMOND drawing of 4. Hydrogen atoms, except for
the hydride, are omitted for clarity.
We suspected the involvement of the palladium hydride in
this reaction and decided to investigate the reactivity of 4 in
some detail. Thus, 4 was synthesized according to a reported
procedure.10 In addition, we were able to obtain an X-ray crystal
structure of 4, and the molecular structure is given in Figure 3.
It forms a monomeric species packed by London forces and
the same distorted square planar arrangement as seen earlier.
The hydride ligand could not be located in the Fourier map,
but its presence was substantiated by the characteristic virtual
Scheme 4
1
triplet in the H NMR spectrum at -3.76 ppm. The similar
there is a parallel C-H exchange reaction directly with the
palladium hydride. The small amount of dihydrogen thus formed
could possibly escape detection by 1H NMR spectroscopy. The
formation of 6 from 3 is therefore proposed to take place via 4.
It can be noted that we observed no formation of styrene in the
reaction of 3 to give 6, suggesting that in this case the reaction
does not go via the vinyl compound.
PCPPhPdH has been reported to be dinuclear in the solid state,19
but again the bulky tert-butyl groups seem to prevent dimer-
ization. The steric bulk is probably also what renders complex
4 thermally stable as compared to many other palladium
hydrides. As mentioned above, 4 reacts with CO2 within minutes
to give 5 as the sole product. The reactivity of the Pd-H bond
is many orders of magnitude higher than that of the correspond-
ing Pd-Me bond. This is in line with the normal reactivity
pattern for M-R (R ) H, alkyl) bonds in 1,2-insertion reactions
and is explained by the stabilization of a concerted transition
state that the spherically symmetrical hydrogen s-orbital gives.20
Compound 5 was identified by comparison with a second sample
synthesized through a different route. Thus, PCPtBuPdOTf was
reacted with NaO2CH, giving 5, which could be characterized
by multinuclear NMR spectroscopy, showing a typical formate
1H NMR shift as a triplet at 9.08 ppm. Unfortunately, 5 failed
to give X-ray quality crystals and decomposed upon standing,
giving reduction to Pd(0).21 Reaction of 4 with phenyl acetylene
gave no insertion but rather the acetylide complex 6 after
approximately 24 h reaction at room temperature. This some-
what surprising outcome could be explained by a C-H exchange
reaction giving 6 and H2 as a byproduct. However, no
dihydrogen was detected, but the product mixture contained
approximately 0.7 equiv of styrene. Also, monitoring the
reaction by NMR spectroscopy provided no evidence of any
intermediate species. A possible reaction sequence to explain
these facts is an initial slow insertion step giving a â-phenylvinyl
species that can undergo a fast C-H exchange reaction with a
second phenyl acetylene, giving 6 and styrene, as outlined in
Scheme 3. Such reaction sequences have been proposed earlier
for a rhenium hydride complex.22 The fact that styrene was
present in less than equimolar amounts seems to indicate that
We propose that the C-H exchange reaction takes place via
a σ-bond metathesis. σ-Bond metathesis reactions have mainly
been reported for early transition metals in their highest
oxidation state, e.g., Sc(III).23 However, examples from late
transition metals do exist, especially in cases where the
alternative oxidative addition-reductive elimination pathway
requires access to unstable or unusual oxidation states.24 In the
present case, Pd(IV) is obviously unusual especially in the
context of phosphine ligands. To further investigate the pos-
sibility of a C-H exchange, (PCP)PdMe, 7, and (PCP)PdPh,
8, were reacted with phenyl acetylene, cf. Scheme 4. The
reaction with 7 required heating at 80 °C overnight to give 6
and methane. Surprisingly, the phenyl complex reacted even
slower. After 14 days at 100 °C there was still approximately
50% of unreacted (PCP)PdPh present. This could be due to an
unfavorable equilibrium, but this does not explain the low rate
observed. It was difficult to continue the reaction to determine
the equilibrium position since phenyl acetylene was consumed
by a slow concurrent reaction, which presumably is the radical
polymerization. The lower reactivity of 7 as compared to 4 is
of course in line with what is expected for a σ-bond metathesis,
i.e., the higher the s-character of the bond, the higher the
reactivity. One possible explanation for the low reactivity of 8
could be a ground state stabilization by the strong Pd-Ph bond.
If the reaction goes via oxidative addition of the C-H bond,
this is also expected to be a concerted reaction and should follow
the same reactivity trend as a σ-bond metathesis, so this
alternative mechanism does not explain the reactivity observed.
(19) (a) Rimml, H., Venanzi, L. J. Organomet. Chem. 1984, 260 (2),
C52-C54. (b) Ryu, S. Y.; Kim, H.; Kim, H. S.; Park, S. J. Organomet.
Chem. 1999, 592, 194-197.
(20) Brookhart, M.; Hauptman, E.; Lincoln, D. M. J. Am. Chem. Soc.
1992, 114, 10394. Brookhart, M.; Volpe, A. F.; Lincoln, D. M. J. Am. Chem.
Soc. 1990, 112, 5634. Burger, B. J.; Thompson, M. E.; Cotter, W. D.;
Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 1566.
(21) (a) Darensbourg, D. J.; Darensbourg, M. Y.; Goh, L. Y.; Ludvig,
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2430 Organometallics, Vol. 26, No. 9, 2007
Johansson and Wendt
Compound 6 was unreactive toward CO2 also at elevated
temperatures, which is not surprising considering the lack of
reactivity previously seen for palladium carbon complexes with
sp2-hybridized carbons.
There seems to be a general reactivity trend for these (PCP)
palladium complexes when it comes to insertions. With only
one open coordination site, they preferably react with substrates
that can undergo direct electrophilic attack without precoordi-
nation, such as CO2, whereas both 4 and 7 are unreactive toward
migratory insertion substrates that presumably require a preco-
ordination, such as olefins and CO.25
In conclusion we have synthesized the first mononuclear
palladium hydroxy carbonyl and tested its reactivity in reactions
where it has been suggested to participate. Complex 3 shows
no tendency to dimerize and slowly isomerizes to the corre-
sponding formate via CO2 elimination to give the hydride, which
undergoes a normal CO2 insertion. The Pd-C bond of 3 is
unreactive toward unsaturated compounds, ruling out its in-
volvement in a possible route for hydrocarbon carboxylation.
The palladium hydride reacts with phenyl acetylene in a C-H
exchange reaction to give a palladium acetylide. This is proposed
to take place via a σ-bond metathesis. Similar reactions occur
between phenyl acetylene and palladium methyl and phenyl
complexes.
Acknowledgment. Financial support from the Swedish
Research Council, the Crafoord Foundation, the Knut and Alice
Wallenberg Foundation, and the Royal Physiographic Society
in Lund is gratefully acknowledged.
Supporting Information Available: All crystallographic mate-
rial in CIF format. This is available free of charge via the Internet
at http://pubs.acs.org.
(25) Johansson, R; Wendt, O. F. Unpublished results.
OM0611709