Cationic palladium η3-allyl complexes with hemilabile P,O-ligands: Synthesis and reactivity. Insertion of ethylene into the Pd-allyl function

Article abstract of DOI:10.1021/om960061z

Cationic palladium allyl complexes [(η³-C₃H₅)Pd(κ ²P∧O)]⁺SbF₆- (2[SbF₆], P∧O ≡ Ph₂P(CH₂)₂C-(=O)OEt; 3[SbF₆], o-Ph₂PC₆H₄C(=O)OEt; 4[SbF₆], Ph₂P(CH₂)₂P(=O)Ph₂) have been prepared. In all complexes the oxygen donor can be displaced by other ligands such as carbon monoxide and ethylene. Displacement of an ester donor occurs much more readily than displacement of the phosphine oxide function. Above 0°C, the resulting ethylene complexes [(η³-C₃H₅)-Pd(C₂H ₄)(κ¹P～O)]⁺ react to give (1,2,5-η³)-pent-1-en-5-yl complexes [(H₂C=CH(CH₂)₃Pd-(κ ²P∧O)]⁺. A rate constant of e.g. k(17°C) = (2.27 ± 0.11) × 10^-4 s^-1 was determined for P,O ≡ Ph₂P(CH²)²C(O)OEt by ¹H NMR spectroscopy. Using 2-4 as catalyst precursors for ethylene dimerization, the allyl moiety is ultimately cleaved from the metal center as 1,4-pentadiene.

Full text of DOI:10.1021/om960061z

2650
Organometallics 1996, 15, 2650-2656
Ca tion ic P a lla d iu m η3-Allyl Com p lexes w ith Hem ila bile
P ,O-Liga n d s: Syn th esis a n d Rea ctivity. In ser tion of
Eth ylen e in to th e P d -Allyl F u n ction
Stefan Mecking and Wilhelm Keim*
Institut fu¨r Technische Chemie der RWTH Aachen, Worringer Weg 1, 52074 Aachen, Germany
Received J anuary 31, 1996X
Cationic palladium allyl complexes [(η3-C3H5)Pd(κ2P∧O)]+SbF6- (2[SbF6], P∧O ≡ Ph2P(CH2)2C-
(dO)OEt; 3[SbF6], o-Ph2PC6H4C(dO)OEt; 4[SbF6], Ph2P(CH2)2P(dO)Ph2) have been prepared.
In all complexes the oxygen donor can be displaced by other ligands such as carbon monoxide
and ethylene. Displacement of an ester donor occurs much more readily than displacement
of the phosphine oxide function. Above 0 °C, the resulting ethylene complexes [(η3-C3H5)-
Pd(C2H4)(κ1P∼O)]+ react to give (1,2,5-η3)-pent-1-en-5-yl complexes [(H2CdCH(CH2)3Pd-
(κ2P∧O)]+. A rate constant of e.g. k(17 °C) ) (2.27 ( 0.11) × 10-4 s-1 was determined for
1
P,O ≡ Ph2P(CH2)2C(O)OEt by H NMR spectroscopy. Using 2-4 as catalyst precursors for
ethylene dimerization, the allyl moiety is ultimately cleaved from the metal center as 1,4-
pentadiene.
Sch em e 1
In tr od u ction
Since their initial discovery in the 1950s,1 transition-
metal η3-allyl complexes have been ubiquitous in orga-
nometallic chemistry. One of the key points of interest
is their occurrence as intermediates in a variety of
metal-mediated transformations of organic substrates.
Closely related is the use of η3-allyl complexes as
precursors to homogeneous catalysts, namely for C-C
linkage reactions. The basis for such reactions is the
formation and breakage of metal-carbon bonds, and
thus a catalyst precursor which already contains M-C
bonds can provide a convenient entry into the catalytic
cycle. Prominent examples are the nickel catalysts
developed by Wilke et al. for the dimerization of pro-
pene2 (Scheme 1).
Late-transition-metal catalysts frequently employ
alkyl- or arylphosphines as stabilizing and selectivity
controlling ligands. These soft donor ligands form stable
bonds to the late-metal center. In multidentate func-
tionalized phosphine ligands, which in addition to the
soft donor also contain hard donor sites, the latter
coordinate only weakly to the metal center and can
easily be displaced by other ligands. This property has
been termed hemilabile.3 In catalysis and stoichiomet-
ric reactions, such ligands have the potential of provid-
ing a coordination site for an incoming ligand and of
stabilizing reactive intermediates by reversible coordi-
nation of the hard donor (eq 1).
defined catalyst precursors for the cooligomerization of
carbon monoxide with ethylene5 and the codimerization
of styrene with ethylene.5a,6
Resu lts a n d Discu ssion
P r ep a r a t ion a n d Ch a r a ct er iza t ion of η3-Allyl
Com p lexes. Cationic palladium allyl complexes were
prepared by the general procedure outlined in Scheme
2. Compound 1 was isolated for comparison of its
spectroscopic properties, but generally the reaction
solution of the compounds [(η3-C3H5)Pd(I)(κ1P∼O)] was
allowed to directly react with AgSbF6. When [(η3-C3H5)-
PdI]2 was employed, better results were obtained in the
halide abstraction reaction than with the corresponding
chloro dimer. The analogs of 2 with n ) 1 (R ≡ Me)
(4) (a) Empsall, H. D.; Hyde, E. M.; Pawson, D.; Shaw, B. J . Chem.
Soc., Dalton Trans. 1977, 1292-1298. (b) Keim, W.; Kowaldt, F. H.;
Goddard, R.; Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1978, 17, 466-
467. (c) Braunstein, P.; Matt, D.; Dusausoy, Y.; Fischer, J .; Mitschler,
A.; Ricard, L. J . Am. Chem. Soc. 1981, 103, 5115-5125. (d) Higgins,
S. J .; Taylor, R.; Shaw, B. L. J . Organomet. Chem. 1987, 325, 285-
292. (e) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27-110
and references cited therein.
(5) (a) Britovsek, G. J . P.; Keim, W.; Mecking, S.; Sainz, D.; Wagner,
T. J . Chem. Soc., Chem. Commun. 1993, 1632-1634. (b) Keim, W.;
Maas, H.; Mecking, S. Z. Naturforsch. 1995, 50B, 430-438.
(6) The application of analogous nickel complexes as catalyst precur-
sors for the oligomerization of ethylene has been described recently:
Bonnet, M. C.; Dahan, F.; Ecke, A.; Keim, W.; Schulz, R. P.; Tkatch-
enko, I. J . Chem. Soc., Chem. Commun. 1994, 615-616.
We report here the preparation of cationic palladium
η3-allyl complexes with bidentate P,O-ligands4 and their
reactivity toward ethylene. We have previously de-
scribed the application of these complexes as well-
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Smidt, J .; Hafner, W. Angew. Chem. 1959, 71, 284.
(2) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 185-206.
(3) J effrey, J . C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18, 2658-
2666.
S0276-7333(96)00061-1 CCC: $12.00 © 1996 American Chemical Society

Pd η3-Allyl Complexes with P,O-Ligands
Organometallics, Vol. 15, No. 11, 1996 2651
Sch em e 2
and n ) 3 (R ≡ Et) can also be prepared by this
procedure in good yields. Compounds 2-4 were isolated
as light brown or white solids. 2 can be handled at room
temperature under an inert-gas atmosphere for brief
periods of time. 3 and 4 are less thermosensitive, and
they can also be exposed to air without any apparent
decomposition. In the 1H NMR spectra, the protons H1,
H2, and H5 of the allyl ligand give rise to sharp signals,
whereas the resonances of the protons trans to the
oxygen donor, H3 and H4, are broad at room tempera-
ture due to syn-anti exchangesa behavior well-known
for square-planar allyl complexes with a hard and a soft
donor ligand7 (numbering H1(syn)H2(anti)C1C2H5C3H4-
(syn)H3(anti); C1 is trans to P). Coordination of the ester
or phosphine oxide function causes characteristic changes
in the IR and NMR spectra; representative data are
given in Table 1. The wavenumber of the carbonyl band
in 2 and 3 is lowered by ca. 80 cm-1 compared to the
free ligand or 1. The NMR signals of the OCH2 protons
and the carbonyl carbon atom are shifted downfield.
Correspondingly, in complex 4 ν(PdO) is lowered in
comparison to the free ligand, and the 31P NMR signal
of the phosphine oxide function is shifted toward lower
field.
The strength of the palladium-oxygen bond relative
to other ligands is of interest in estimating the coordi-
nation behavior of the P,O-ligand in the active species
in catalysts with these ligands5 (monodentate P or
bidentate P,O), and it is also decisive for the activation
of complexes 2-4 as catalyst precursors (vide infra).
Therefore, the cationic complexes were reacted with tert-
butyl isocyanide and carbon monoxide (eq 2). CO was
F igu r e 1. 31P{1H} NMR spectra of the equilibrium 4 +
C2H4 h 4-E (cf. eq 5; 121 MHz, CD2Cl2, total 5 equiv of
ethylene).
donor and coordinated isonitrile are observed (complex
2-tBu NC, ν(CO) 1732 cm-1, ν(CN) 2209 cm-1; 4-tBu NC,
ν(PdO) 1196 cm-1, ν(CN) 2209 cm-1 (free tBuNC, ν 2139
cm-1)). The ester function is also readily displaced upon
bubbling carbon monoxide through a CH2Cl2 solution
of the complexes (2-CO, ν(CO ester) 1732 cm-1, ν(CO)
2135 cm-1; 3-CO, ν(CO ester) 1697 cm-1, ν(CO) 2132
cm-1). The carbonyl complex 2-CO was also character-
ized by 13C NMR spectroscopy utilizing 13C-enriched
carbon monoxide (δ 180.5 (PdsCO), 171.6 (d, C(O)O),
61.7 (OCH2); for complete data cf. Experimental Sec-
tion). The IR band of the coordinated carbonyl ligand
is very similar to that of gaseous CO (2143 cm-1),
indicating weak binding to the cationic palladium
center.8 Accordingly, when a solution of 2-CO is purged
of special interest, as it is one of the substrates used in
reactions with these complexes as catalyst precursors.5
The reactivity toward ethylene is discussed in the next
section. Addition of 1 equiv of tBuNC resulted in
complete displacement of the ester or phosphine oxide
donor; only the IR bands of the uncoordinated oxygen
(7) (a) Vrieze, K. In Dynamic Nuclear Magnetic Resonance Spec-
troscopy; J ackman, L. M., Cotton, F. A., Eds.; Academic Press: New
York, 1975; pp 441-487. (b) Meyer, H.; Zschunke, A. J . Organomet.
Chem. 1984, 269, 209-216.
(8) In blank experiments, with CO bubbled through neat CH2Cl2,
no free carbon monoxide could be detected by IR or 13C NMR
spectroscopy; obviously the solubility is too low.

2652 Organometallics, Vol. 15, No. 11, 1996
Ta ble 1. Sp ectr oscop ic Da ta for Com p lexes 1-4 a n d F r ee Liga n d sa
Mecking and Keim
IR (CH2Cl2)
ν(CdO) or
13C and 31P NMR
1H NMR
δ(OCH2R)
δ(CdO)
or δ(PdO)
complex or ligand
δ(OCH2R)
δ(Ph2PCH2)
ν(PdO)/cm-1
Ph2P(CH2)2C(O)OEt
o-Ph2PC6H4C(O)OEt
1
2
4.06
4.20
4.03
4.36
4.45
173.0
166.9
172.3
180.9
171.3
33.1
60.5
61.2
61.1
65.8
65.8
-15.0
-3.9
+19.1
+22.6
+22.3
-11.4
+19.8
1730
1709
1731
1655
1641
1190
1127
3
Ph2P(CH2)2P(O)Ph2
4
48.6
a
Room temperature; NMR solvent: CD2Cl2, CH2Cl2-CD2Cl2, or CDCl3 (for a given compound, no significant deviations of the chemical
shifts measured in different solvents were observed).
with argon, the carbonyl complex is completely con-
verted back to 2 again. The phosphine-phosphine oxide
complex 4 shows a markedly different behavior toward
CO: At room temperature, no reaction with carbon
monoxide was detected by IR spectroscopy. Variable-
temperature 31P NMR spectroscopy9 established, how-
ever, that 4 is in a temperature-dependent dynamic
equilibrium with the carbonyl complex 4-CO (eq 3).
considerable interest. Powell and co-workers12 and
others13 were able to isolate 4-enyl insertion products.
The mechanism depicted in eq 4 was proposed. How-
ever, most of these studies employed strained olefins,
and direct observation of insertion of a simple, unacti-
vated olefin such as ethylene has not been reported.14
The spectroscopic observations resemble those ob-
tained with ethylene, depicted in Figure 1 and described
in more detail in the following section. Above -30 °C,
interconversion between 4 and 4-CO is fast on the NMR
time scale, resulting in one averaged signal each for the
phosphine and the phosphine oxide functions. At room
temperature, the chemical shifts and the coupling
constants of these signals are nearly identical with the
spectrum of 4, in accordance with the IR spectroscopic
results. Below -30 °C, two sets of signals emerge (4, δ
49.5 and 18.1, 3J (P,P) ) 6 Hz; 4-CO, δ 32 and 21.0,
3J (P,P) ) 52 Hz (CD2Cl2, -87 °C)). Complex 4-CO is
the major species at temperatures below ca. -50 °C (1
atm of CO). In conclusion, the P,O-ligands in all the
cationic allyl complexes display hemilabile behavior;
however, the phosphine oxide donor binds significantly
more strongly to the palladium center than an ester
donor.
Upon saturation of a solution of 2 with ethylene at
room temperature, the carbonyl band of 2 disappears;
instead, a strong band at ν(CO) 1732 cm-1 and a weaker
band at ν(CO) 1664 cm-1 are observed. In the further
course of the experiment, the intensity of the latter band
increases, while the band at 1732 cm-1 decreases in
intensity. Within 1 h, the band at ν 1664 cm-1 becomes
the most intense. If the solution is purged with argon
during the initial stages of the experiment, the band at
ν(CO) 1732 cm-1 disappears, and 2 is recovered (ν(CO)
1655 cm-1). By multinuclear NMR spectroscopy and
1D- and 2D-decoupling experiments, the species could
be unambigiously identified as 2-E and 5 (Scheme 3).
When ethylene is added to a CD2Cl2 solution of 2 at
-30 °C, clean conversion to 2-E occurs. Due to rapid
exchange of coordinated with excess free ethylene, only
one broad averaged signal is observed (δ 5.1, total 3
equiv of ethylene; free ethylene in blank experiment δ
5.4). When the solution is warmed, allyl-ethylene
coupling occurs to yield the pent-4-en-1-yl complex 5.15
This complex is stabilized by recoordination of the
oxygen donor of the hemilabile ligand; obviously, the
ester function is not displaced by the excess ethylene,
Rea ction s w ith Eth ylen e. The reaction of (η3-allyl)-
metal intermediates with olefins, resulting in net inser-
tion of the olefin into the allyl-metal moiety, is assumed
to be a key step in various economically important
processes, e.g. the oligomerization2,10a,b or polymeri-
zation10b-d of dienes and the codimerization of dienes
with ethylene.11 Therefore, this reaction has received
(9) The 1H NMR spectra display broad, averaged signals of less
diagnostic value.
(10) (a) Keim, W.; Behr, A.; Ro¨per, M. In Comprehensive Organo-
metallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: Oxford, U.K., 1982; Vol. 8; pp 371-462. (b) J olly, P.
W. In ref 10a, pp 671-711. (c) Taube, R.; Wache, S.; Sieler, J . J .
Organomet. Chem. 1993, 459, 335-347 and references cited therein.
(d) Durand, J . P.; Dawans, F.; Teyssie, P. J . Polym. Sci., Part A: Polym.
Chem. 1970, 8, 979-990.
(11) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley-Interscience: New York, 1992; pp 76-78. (b) Tolman, C. A. J .
Am. Chem. Soc. 1970, 92, 6777-6784, 6785-6790.
(12) Hughes, R. P.; Powell, J . J . Organomet. Chem. 1973, 60, 387-
407.
(13) (a) Gallazzi, M. C.; Hanlon, T. L.; Vittuli, G.; Porri, L. J .
Organomet. Chem. 1971, 33, C45-C46. (b) Sadownick, J . A.; Lippard,
S. L. Inorg. Chem. 1973, 12, 2659-2665. (c) Taube, R.; Bo¨hme, P.;
Gehrke, J .-P. Z. Anorg. Allg. Chem. 1989, 578, 89-98.
(14) Very recently, the reaction of similar palladium allyl complexes
with various olefins was directly observed by NMR spectroscopy:
DiRenzo, G. M.; Brookhart, M. J . Am. Chem. Soc., in press.

Pd η3-Allyl Complexes with P,O-Ligands
Organometallics, Vol. 15, No. 11, 1996 2653
Sch em e 3
tempted crystallizations were hampered by the inevi-
table formation of oils.
When the reaction 2-E f 5 was monitored by 1H
NMR spectroscopy, the first-order rate constants were
obtained for different temperatures: k(10 °C) ) (9.6 (
0.5) × 10-5 s-1, k(17 °C) ) (2.27 ( 0.11) × 10-4 s-1 and
k(24 °C) ) (4.6 ( 0.8) × 10-4 s-1 (solvent CD2Cl2). At
40 °C the reaction was to fast to be followed accurately,
a rate of ca. 2.3 × 10-3 s-1 was estimated (CDCl3). The
exchange of H3 and H4 in the NMR spectra of 2-4 (vide
supra) occurs via a η1-allyl species.7 In contrast, no
analogous evidence for dynamic behavior was observed
for 2-E. Though it is by no means ruled out, no evidence
for intermediacy of a η1-allyl species in the conversion
to 5 was found, and thus our data would also be in
agreement with C-C linkage occurring between the
ethylene and the η3-bound allyl ligand. From the kinetic
data a free enthalpy of activation ∆Gq(17 °C) ) (91.2 (
0.8) kJ mol-1 results.
Complex 3 seemed a more promising starting com-
pound for formation of a crystalline pentenyl complex.
Due to its aromatic backbone, 3 can readily be obtained
as a solid by precipitation or crystallization, in contrast
to 2. Analogous to 2, 3 reacts with excess ethylene at
-30 °C to yield 3-E. When the temperature is raised,
insertion yielding the pentenyl complex occurs with
rates similar to those for 2. However, accompanying
conversion of ethylene to butenes occurs much faster
than with 2/5. In accordance with this behavior, 3 is a
precursor to much more active ethylene dimerization
catalysts than 2.19 Thus, although mixtures of 3 and
the pentenyl complex could be precipitated as solids
from solution as anticipated, rapid depletion in ethylene
prevented reasonable conversions to the pentenyl com-
plex.
in contrast to the behavior of the corresponding allyl
complex 2.16 Considering the NMR spectroscopic prop-
erties of the pentenyl ligand, the signals of the olefinic
protons are shifted to lower field in comparison to a free
R-olefin by ∆δ ) 0.5-1.4. Coupling to the phosphine
donor and the diastereotopy of the allylic protons Ha and
Ha′ at low temperatures17 also clearly indicate coordina-
tion to the metal center. The magnitudes of the PsC
coupling constants of the PdCH2- (no coupling ob-
served) and the vinyl group of the pentenyl ligand
(dCH2, 10 Hz; dCH-, 5 Hz) support the stereochem-
istry at the square-planar palladium center depicted in
Scheme 3.18 In the NMR experiments, formation of 5
is accompanied by the conversion of ethylene, present
in excess, to butenes. The cationic allyl complexes 2-4
are precursors to very active catalysts for the dimer-
ization of ethylene,5 and it is assumed that during the
NMR experiment a small, and thus undetectable,
amount of a catalytically active species is formed (vide
infra). At higher temperatures, further conversion of
5 can occur. Following the reaction of 2-E to give 5 at
+40 °C (CDCl3), after 1 h minor amounts of two new
species with 31P NMR resonances at δ 39.4 and δ 21.0,
respectively, were observed. On the basis of H,H-COSY
and 13C NMR data, the species with δ 21.0 was
tentatively assigned as the syn,syn-1,3-dimethylallyl
complex analogous to 2, which may form by rearrange-
ment of 5. Similar isomerizations of a pentenyl to an
allyl complex have been described.15b,c Complex 5 was
isolated as an off-white solid. Due to impurities of 2
(formed from unreacted 2-E upon workup in vacuo) or
by compounds formed by further reaction of 5, an
analytically pure sample could not be obtained. At-
The phosphine-phosphine oxide complex 4 displays
different properties toward ethylene than the phos-
phino-ester complexes. When ethylene is added at -30
°C, no substitution of the phosphine oxide donor is
1
observed by H and 31P NMR spectroscopy. However,
the signals are slightly broadened and shifted compared
to the spectrum of 4 in the absence of ethylene. Vari-
able-temperature 31P NMR spectroscopy9 (Figure 1)
showed that 4 is in a dynamic equilibrium with the
ethylene complex 4-E (eq 5).20 At temperatures higher
(15) For similar complexes, prepared by different routes, see: (a)
Lehmkuhl, H.; Rufinska, A.; Benn, R.; Schroth, G.; Mynott, R. Liebigs
Ann. Chem. 1981, 317-332. (b) Parra-Hake, M.; Rettig, M. F.;
Williams, J . L.; Wing, R. M. Organometallics 1986, 5, 1032-1040. (c)
Flood, T. C.; Statler, J . A. Organometallics 1984, 3, 1795-1803. (d)
Flood, T. C.; Bittler, S. P. J . Am. Chem. Soc. 1984, 106, 6076-6077.
(e) Ermer, S. P.; Struck, G. E.; Bitler, S. P.; Richards, R.; Bau, R.; Flood,
T. C. Organometallics 1993, 12, 2634-2643. (f) Pedone, C.; Benedetti,
E. J . Organomet. Chem. 1971, 31, 403-414.
(16) This observation must be kept in mind, when estimating the
coordination behavior of the P,O-ligand in an assumed intermediate
in catalysis from the relative coordination strength of the oxygen donor
in model compounds, such as 2-4, as the other ligands coordinated to
the metal center obviously have a strong impact.
(17) Tc ≈ 17 °C (300 MHz). It is assumed that this exchange occurs
between two species with opposite enantiotopic faces of the olefinic
moiety binding to the metal center. This requires intermediate
decoordination of the olefinic function.
than -30 °C, only traces of 4-E are present, whereas at
temperatures lower than ca. -80 °C 4-E is the major
species (1 atm of ethylene).
Correspondingly, when the temperature is raised to
room temperature, no formation of a pentenyl complex
is observed. However, after the NMR sample is stored
(18) For comparative 13C NMR data cf. ref 15e. The J (H,P) values
provide no clear picture considering the stereochemistry in comparison
to literature data: (a) de Graaf, W.; Boersma, J .; Smeets, W. J . J .;
Spek, A. L.; van Koten, G. Organometallics 1989, 8, 2907-2917. (b)
Lindner, E.; Dettinger, J .; Fawzi, R.; Steimann, M. Chem. Ber. 1993,
126, 1347-1353.
(19) Mecking, S. Dissertation, RWTH Aachen, 1994.
(20) For a theoretical treatment of exchange between two nonequally
populated sites cf.: Sandstro¨m, J . Dynamic NMR Spectroscopy;
Academic Press: London, 1982; pp 25-29, 80-84.

2654 Organometallics, Vol. 15, No. 11, 1996
Mecking and Keim
for 1 day at room temperature, in addition to the 1H
NMR signals of 4, weak signals (integrating for ca. 10%
relative to 4) analogous to the pentenyl ligand in 5 are
observed. All ethylene had been converted to butenes.
Also, a new pair of 31P resonances was observed (δ 43.6
and 26.2, J (P,P) ) 4 Hz). Thus, it may be assumed that
4-E displays the same reactivity as 2-E and 3-E.
Sch em e 4
The reactivity of the cationic allyl complexes toward
ethylene/CO mixtures is of interest, as 2-4 have also
been applied as well-defined catalyst precursors for the
cooligomerization of ethylene and CO.5 When a 1:1
ethylene/CO mixture is bubbled through a methylene
chloride solution of 2, despite the higher solubility of
ethylene, the carbonyl complex 2-CO (ν(CO ester) 1732
cm-1, ν(CO) 2135 cm-1) is the only species observed
associative process;25 this would result in an enhance-
ment of the liberation of pentadiene by higher ethylene
concentrations. On basis of the above assumption,
experiments aimed at clarifying the final fate of the allyl
moiety during activation were carried out under ethyl-
ene pressure.
initially; small amounts of 2-E (ν(CO ester) 1732 cm-1
)
A THF solution of 2 (ca. 0.5 mmol in 10 mL) was
stirred under 5 atm of ethylene, and samples were
drawn periodically and analyzed by GC. In samples
taken within the first 3 h, 1,4-pentadiene was identified
by gas chromatography by enrichment with a genuine
sample, and by GC-MS. Small peaks with retention
times corresponding to cis- and trans-1,3-pentadiene
were also observed. After 24 h, no more 1,4-pentadiene
was observed. The use of the weakly coordinating
solvent THF is necessary to generally slow down the
dimerization and isomerization reactions occurring.
Using CH2Cl2, no pentadienes could be detected, pre-
sumably due to rapid isomerization of the initially
formed 1,4-pentadiene to the reactive 1,3-isomers and
subsequent cooligomerization with ethylene, resulting
in distribution of the allyl group over a large number
of different products.
may have been obscured. Eventually, however, the
pentenyl complex 5 (ν(CO ester) 1664 cm-1) is formed,
as in the absence of CO. No insertion of CO into the
allyl moiety was observed with 2-CO. Such reactions
most often require harsh conditions, and to the best of
our knowledge, only one example of formation of an
isolable acyl complex has been reported.21 Thus, it can
be assumed that the first step in the activation of allyl
catalyst precursors in the cooligomerization of ethylene
and CO occurs by insertion of ethylene.
Activa tion of Allyl Com p lexes in Olefin Oligo-
m er iza tion Rea ction s. A generally accepted mecha-
nism for the oligomerization of olefins by late transition
metals involves subsequent insertions of the substrate
into a metal hydride complex as the active species.22
Considering the activation of allyl catalyst precursors
to the active species, from oligomerization reactions of
olefins 1:1 coupling products of the allyl moiety with the
olefinic substrate have been isolated, which provide
indirect evidence for formation of a hydride complex.23
Other mechanisms for oligomerization of olefins with
allyl complexes as catalysts have been proposed24 but
have received little support.
Con clu sion
Cationic palladium allyl complexes with a scope of
bidentate P,O-ligands can be prepared conveniently.
These compounds are well-defined precursors to very
active catalysts for C-C linkage reactions of olefins. The
P,O-ligands in these complexes are hemilabile; the
oxygen donor can be displaced by other ligands such as
carbon monoxide and ethylene. The O-donor of a
phosphino-ester ligand is displaced much more readily
than with a phosphine-phosphine oxide, and the results
imply that the former behaves as a monodentate P-
coordinated ligand under the conditions of catalysis. The
hemilability of the P,O-ligands allowed the direct ob-
servation of coupling of ethylene with an allyl ligand to
yield a σ,π-η3-pent-4-en-1-yl complex. In ethylene dimer-
ization experiments using an allyl complex as a catalyst
precursor, 1,4-pentadiene was identified. These obser-
vations provide support for a palladium hydride as the
catalytically active species, formed by activation of the
allyl precursor through insertion of ethylene and sub-
sequent â-hydride elimination.
As mentioned in the previous section, NMR experi-
ments were accompanied by dimerization of ethylene,
without observation of an active species, presumably
due to its low concentration. Although this dimerization
can cause depletion in ethylene, it occurs at a much
lower rate than in a typical catalytic oligomerization
experiment.5b,19 For instance, exposing a dilute solution
of 3 (0.05 mmol in 20 mL of CH2Cl2) to 30 atm of
ethylene at room temperature results in dimerization
at a rate of 6000 mol/(mol h).19 This dramatic increase
in activity at higher ethylene concentrations may in part
be due to more rapid activation of the catalyst precursor.
A possible mechanism for the formation of a hydride
species is shown in Scheme 4. In this context it is
interesting that very recent results by Brookhart et al.
support that olefin exchange of such complexes is an
Exp er im en ta l Section
(21) Ozawa, F.; Son, T.; Osakada, K.; Yamamoto, A. J . Chem. Soc.,
Chem. Commun. 1989, 1067-1068.
All manipulations were carried out under a dry argon
atmosphere by standard Schlenk techniques. For reactions
above atmospheric pressure a heavy-walled 100 mL round-
bottom glass tube with two necks equipped with glass threads
was used. NMR spectra were acquired on a Bruker CXP-200,
(22) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry, 2nd
ed.; University Science Books: Mill Valley, CA, 1987; pp 577-617.
(23) (a) Bogdanovic, B.; Henc, B.; Karmann, H.-G.; Nu¨ssel, H.-G.;
Walter, D.; Wilke, G. Ind. Eng. Chem. 1970, 62, 34-44. (b) Bogdanovic,
B. Adv. Organomet. Chem. 1979, 17, 105-140. (c) Grenouillet, P.;
Neibecker, D.; Tkatchenko, I. Organometallics 1984, 3, 1130-1132.
(24) Sen, A.; Lai, T.-W. Organometallics 1983, 2, 1059-1060.
(25) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414-6415.

Pd η3-Allyl Complexes with P,O-Ligands
Organometallics, Vol. 15, No. 11, 1996 2655
Bruker AC300, or Varian Unity 500 spectrometer. Chemical
shifts are referenced to internal or external TMS (1H, 13C),
respectively, or to external 85% H3PO4 (31P). NMR probe
temperatures were measured using an external anhydrous
methanol sample. In experiments using non-deuterated sol-
vents, a coaxial tube with the deuterated compound was
inserted as a lock, or a small amount was added directly to
the sample solution. Assignments were confirmed by H,H-
COSY and selective 1H and 31P decoupling of 1H NMR spectra.
Numbering of the allyl ligand in NMR data: H1(syn)H2(anti)-
C1C2H5C3H4(syn)H3(anti); C1 is trans to P. Gas chromato-
graphic analyses were performed on a Siemens Sichromat with
a 50 m Pona HP column. For GC-MS analyses a Varian 3700
gas chromatograph combined with a Varian MAT 112S mass
spectrometer was used. IR spectra were recorded on a Nicolet
510 P FT spectrometer. Solvents were distilled from a drying
agent under argon (methylene chloride, CaH2; pentane and
THF, sodium benzophenone ketyl). Ethylene (purity >99.5%)
and carbon monoxide (purity >99.99%) were used as received.
Genuine samples of 1,4-pentadiene, trans-1,3-pentadiene, and
cis-1,3-pentadiene were purchased from Fluka. [(C3H5)PdI]2
in vacuo at -10 °C yielded 2.31 g (94%) of yellow, microcrys-
talline powder. 1H NMR (500 MHz, CD2Cl2): δ 7.2-7.6 (m,
10 H; Harom), 5.88-5.98 (m, 1H; H5), 5.15 (t, 3J (H1,H5) )
3J (H1,P) ) 7 Hz, 1H; H1), 4.36 (q, 3J (H,H) ) 7 Hz, 2H; OCH2),
4.28 (dd, 3J (H2,H5) ) 14 Hz, 3J (H2,P) ) 9 Hz, 1H; H2), 3.66 (br
s, 1H; H4), 2.90 (br s, 1H; H3), 2.57-2.83 (br m, 4H; CH2CH2),
3
1.34 (t, J (H,H) ) 7 Hz, 3H; CH3). 13C NMR (50 MHz, CH2-
Cl2): 180.9 (CO), 130.1-133.1 (Carom), 121.8 (d, 2J (C,P) ) 5
2
Hz; C2allyl), 88.2 (d, J (C,P) ) 26 Hz; C1allyl), 65.8 (OCH2), 50.5
(C3allyl), 29.3 (CR), 20.8 (d, 1J (C,P) ) 25 Hz; Câ), 13.9 (CH3). 31
P
NMR (81 MHz, CD2Cl2): δ +22.6 ppm. IR (CH2Cl2): ν 1655
cm-1 (CO), 660 cm-1 (ν3 SbF6-). Anal. Calcd for
C
20H24F6O2PPdSb: C, 35.88; H, 3.61. Found: C, 35.81; H,
3.55.
(η3-Allyl)[eth yl o-(d ip h en ylp h osp h in o)ben zoa te-K2P ,-
O]p a lla d iu m (II) Hexa flu or oa n tim on a te (3[SbF 6]). 0.9646
g [(C3H5)PdI]2 (1.758 mmol), 1.1750 g of o-Ph2PC6H4C(O)OEt
(3.514 mmol), and 1.2145 g of AgSbF6 (3.534 mmol) were
reacted by the general procedure given above. The filtrate was
concentrated in vacuo, and the product was then precipitated
as a powder by slow addition of 40 mL of pentane. The
supernatant was decanted, and the solid was washed with
pentane. Drying in vacuo at -10 °C yielded 2.05 g (81%) of 3.
1H NMR (500 MHz, CDCl3): δ 8.29, 7.69, 7.64, and 7.09 (m,
1H each; PC6H4C(O)O), 7.49-7.60 (m, 10 H; HPhenyl), 5.95-
26
was obtained from [(C3H5)PdCl]2 by halide exchange with
NaI.27
F u n ction a lized P h osp h in e Liga n d s. Phosphino car-
boxylic acid esters Ph2P(CH2)nC(O)OR were prepared by the
procedure reported by Meijboom and van Doorn for the acids.28
After evaporation of the ammonia solvent, the reaction mixture
was worked up with methylene chloride/water, and the
products were distilled through a 5 cm column at 0.01 mbar
(bp 120-130 °C). Ethyl o-(diphenylphosphino)benzoate was
prepared by esterification of the corresponding acid,29 using
DCC.30 (2-(Diphenylphosphino)ethyl)diphenylphosphine oxide
was prepared similarly to reported procedures.31 NMR and
IR data of the ligands have previously been reported.31,32
Gen er a l P r oced u r e for P r ep a r a tion of Ca tion ic P ∧O-
Coor d in a ted Allyl Com p lexes. A solution of 1.00 equiv of
the P,O-ligand in methylene chloride (ca. 10 mL) was added
to a methylene chloride solution (ca. 10 mL) of 1-2 mmol of
[(C3H5)PdI]2. After it was stirred for 30 min, the solution was
transferred to a round-bottom flask containing 1.01 equiv of
AgSbF6. After it was stirred vigorously for 5 min, the mixture
was filtered through celite on a frit into a flask kept at -20
°C. The succeeding workups are given below for the individual
compounds; alternately, all compounds could be obtained in
satisfying purity by removing the solvent from the filtrate in
vacuo. The complexes were stored at -20 °C.
3
3
6.04 (m, 1H; H5), 5.11 (t, J (H,H) ) J (H,P) ) 7 Hz, 1H; H1),
3
4.45 (q, J (H,H) ) 7 Hz, additional 1 Hz coupling or diaster-
eomeric conformations, 2H; OCH2), 4.27 (dd, 3J (H,H) ) 14 Hz,
3J (H,P) ) 9 Hz, 1H; H2), 3.51 (br s, 1H; H4), 3.11 (br d, 3J (H,H)
) 10 Hz, 1H; H3), 1.33 (t, 3J (H,H) ) 7 Hz, 3H; CH3). 13C NMR
(126 MHz, CDCl3): δ 171.3 (CO), 129.5-134.4 (Carom), 121.7
(d, 2J (C,P) ) 5 Hz; C2allyl), 86.3 (d, 2J (C,P) ) 27 Hz; C1allyl),
65.8 (OCH2), 52.5 (d, J (C,P) ) 2 Hz; C3allyl), 13.6 (CH3). 31P
2
NMR (202 MHz, CDCl3): δ +22.3 ppm. IR (CH2Cl2): ν 1641
(CdO), 660 cm-1 (ν3 SbF6-). Anal. Calcd for C24H24F6O2-
PPdSb: C, 40.17; H, 3.37. Found: C, 40.09; H, 3.51.
(η3 -Al l y l )[(2 -(d i p h e n y l p h o s p h i n o )e t h y l )d i p h e -
n ylp h osp h in e oxid e-K2P ,O]p a lla d iu m (II) Hexa flu or oa n -
tim on a te (4[SbF 6]). A 0.5266 g amount of [(C3H5)PdI]2 (0.960
mmol), 0.7953 g of Ph2P(CH2)2P(dO)Ph2 (1.919 mmol), and
0.6633 g of AgSbF6 (1.930 mmol) were reacted by the general
procedure given above. The filtrate was concentrated in vacuo
and layered with 20 mL of pentane. Storage at -78 °C yielded
a microcrystalline solid. The supernatant was decanted, and
the solid was washed with pentane. Drying in vacuo at -10
°C yielded 1.27 g (83%) of 4. 1H NMR (500 MHz, CDCl3): δ
7.44-7.82 (m, 20 H; Harom), 5.80-5.89 (m, 1H; H5), 5.06 (t,
3J (H1,H5) ) 3J (H1,P) ) 7 Hz, 1H; H1), 4.16 (dd, 3J (H2,H5) ) 14
Hz, 3J (H2,P) ) 9 Hz, 1H; H2), 3.25 (br d, 3J (H4,H5) ) 5 Hz,
1H; H4), 2.66-2.82 (m, 5H; CH2CH2 and H3). 13C NMR (126
MHz, CDCl3): δ 127.6-133.5 (Carom), 120.4 (d, 2J (C,P) ) 5 Hz;
C2allyl), 85.5 (d, 2J (C,P) ) 27 Hz; C1allyl), 51.4 (d, 2J (C,P) ) 2
(η3-Allyl)[eth yl 3-(d ip h en ylp h osp h in o)p r op ion a te-K2P ,-
O]p a lla d iu m (II) H exa flu or oa n t im on a t e (2[Sb F 6]).
A
1.0116 g amount of [(C3H5)PdI]2 (1.843 mmol), 1.0546 g of
Ph2P(CH2)2C(O)OEt (3.683 mmol), and 1.2776 g of AgSbF6
(3.718 mmol) were reacted by the general procedure given
above. The filtrate was concentrated in vacuo and layered
with 20 mL of pentane. Upon storage at -78 °C, a yellow oil
formed, which slowly crystallized. The supernatant was
decanted, and the crystals were washed with pentane. Drying
1
2
Hz; C3allyl), 23.3 (dd, J (C,P) ) 70 Hz, J (C,P) ) 2 Hz; CH2P-
1
2
(dO)), 19.8 (dd, J (C,P) ) 25 Hz, J (C,P) ) 6 Hz; CH2PPh2).
31P NMR (121 MHz, CD2Cl2): δ 48.6 (d, 3J (P,P) ) 7 Hz; PdO),
19.8 (d, 3J (P,P) ) 7 Hz; CH2PPh2). IR (CH2Cl2): ν 1127 (PdO),
660 cm-1 (ν3 SbF6-). Anal. Calcd for C29H29F6OP2PdSb: C,
43.67; H, 3.66. Found: C, 43.90; H, 3.96.
(26) Tatsuno, Y.; Yoshida, T.; Seiotsuka Inorg. Synth. 1979, 19, 220-
221.
(27) Tibbetts, D. L.; Brown, T. L. J . Am. Chem. Soc. 1969, 91, 1108-
1112.
IR Sp ectr oscop ic Exp er im en ts. The starting complex
was placed in a Schlenk tube and dissolved in methylene
chloride (1-3 mL) to yield a 0.03-0.06 M solution. After an
IR spectrum was obtained, the reagent was added. Liquid
substrates were dosed with an Eppendorf pipet. Gaseous
reagents were bubbled through the solution; slow bubbling was
continued during the entire experiment to maintain a suf-
ficient concentration. During the experiment small samples
were withdrawn and an IR spectrum was obtained im-
mediately (using a normal cell with KBr windows).
(28) van Doorn, J . A.; Meijboom, N. Phosphorus, Sulfur Silicon
Relat. Elem. 1989, 42, 211-222.
(29) Hoots, J . E.; Rauchfuss, T. B.; Wrobleski, D. A. Inorg. Synth.
1982, 21, 175-179.
(30) (a) Neises, B.; Steglich, W. Angew. Chem., Int. Ed. Engl. 1978,
17, 522-524. (b) Trost, B. M.; van Vranken, D. L.; Bingel, C. J . Am.
Chem. Soc. 1992, 114, 9327-9343.
(31) (a) Abatjoglou, A. G.; Kapicak, L. A. Eur. Pat. Spec. 72560, 1982.
(b) Higgins, S. J .; Taylor, R.; Shaw, B. L. J . Organomet. Chem. 1987,
325, 285-292.
(32) Spectroscopic data for Ph2P(CH2)nC(O)OR: (a) Podlaha, J .;
J egorov, A.; Budesinsky, M.; Hanus, V. Phosphorus Sulfur Silicon
Relat. Elem. 1988, 37, 87-93. (b) Blinn, D. A.; Button, R. S.; Farazi,
V.; Neeb, M. K.; Tapley, C. L.; Trehearne, T. E.; West, S. D.; Kruger,
T. L.; Storhoff, B. N. J . Organomet. Chem. 1990, 393, 143-152. For
complete 1H, 13C, and 31P NMR data for all ligands, cf. ref 19.
NMR Sp ectr oscop ic Exp er im en ts. The allyl complex
was dissolved in CD2Cl2 (0.8 mL) in an NMR tube capped with
a septum to yield a 0.075 M solution. After a spectrum was
acquired, ethylene or carbon monoxide was bubbled briefly

2656 Organometallics, Vol. 15, No. 11, 1996
Mecking and Keim
through the solution at -30 °C via a cannula, and the sample
was transferred to the precooled probe. For 13C NMR, 0.1-
0.2 M solutions in CH2Cl2 (4 mL, 10 mm NMR tube) were used.
To confirm the identity of species observed in NMR- and IR-
experiments, IR spectra of NMR reaction solutions were
obtained.
The reaction 2-E f 5 was monitored by IR spectroscopy by
the general procedure given above. When the solvent was
removed in vacuo or by precipitation with pentane, 5 was
obtained as an off-white solid. The compound was dried in
vacuo and stored at -20 °C. For attempted crystallization and
purification cf. Results and Discussion.
(η3-Allyl)(η2-et h ylen e)[et h yl 3-(d ip h en ylp h osp h in o)-
p r op ion a te-K1P ]p a lla d iu m (II) Hexa flu or oa n tim on a te (2-
E[SbF 6]). 1H NMR (500 MHz, CD2Cl2, -30 °C): δ 7.44-7.63
(m, 10H; Harom), 5.57-5.66 (m, 2H; H1 and H5), 5.1 (br s;
average of bound and free ethylene (total 3 equiv)), 4.60 (m,
1H; H4), 4.07 (q, 3J (H,H) ) 7 Hz, 2H; OCH2), 3.68 (dd, 3J (H2,H5)
) 13 Hz, 3J (H2,P) ) 9 Hz, 1H; H2), 3.56 (d, 3J (H3,H5) ) 12 Hz,
(η3-Allyl)(ca r bon yl)[eth yl 3-(d ip h en ylp h osp h in o)p r o-
p ion a te-K1P ]p a lla d iu m (II) Hexa flu or oa n tim on a te (2-CO-
[SbF 6]). Solutions of the complex were prepared by the above
general procedures for spectroscopic experiments at room
temperature, using 10% 13CO-enriched carbon monoxide. 13C
3
NMR (50 MHz, CH2Cl2): δ 180.5 (PdsCO), 171.6 (d, J (C,P)
2
) 14 Hz; C(O)OEt), 129.4-132.9 (Carom), 125.9 (d, J (C,P) ) 5
3
1H; H3), 2.96 (q ≡ dt, 3J (H,P) ) J (H,H) ) 8 Hz, 2H; CRH2),
2
Hz; C2allyl), 82.5 (d, J (C,P) ) 22 Hz; C1allyl), 74.3 (C3allyl), 61.7
2.50 (q ≡ dt, 2J (H,P) ) 3J (H,H) ) 8 Hz, 2H; CâH2), 1.21 (t,
3J (H,H) ) 7 Hz, 3H; CH3). 31P NMR (202 MHz, CD2Cl2, -30
°C): δ +16.8. IR (CH2Cl2): ν 1732 cm-1 (CO).
(OCH2), 29.4 (CR), 23.9 (d, 1J (C,P) ) 27 Hz; Câ), 14.2 (CH3). IR
(CH2Cl2): ν 2135 (12CO), 2087 (13CO), 1732 cm-1 (CO of C(O)-
OEt).
((1,2,5-η3)-P en t-1-en -5-yl)[eth yl 3-(d ip h en ylp h osp h in o)-
p r op ion a te-K2P ,O]p a lla d iu m (II) Hexa flu or oa n tim on a te
(5[SbF 6]). 1H NMR (500 MHz, CD2Cl2): δ 7.55-7.66 (m, 10H;
Ack n ow led gm en t. We are grateful to BP Chemicals
for financial support and to Degussa AG for a loan of
palladium chloride. We thank Wolfgang Goertz, Ste-
fanie Vonderbank, and Beate Hofmann for their par-
ticipation in this research as part of their undergraduate
studies.
H
arom), 7.06 (ddtd, 3J (H,Hcis) ) 17 Hz, 3J (H,Htrans) ) 9 Hz,
3J (H,Hvic) ) 6 Hz, 3J (H,P) ) 1.5 Hz, 1H; dCH), 5.61 (br d,
3J (H,HdCH) ) 17 Hz, 1H; CdCHHcis), 5.42 (ddd, J (H,HdCH) )
3
3
2
9 Hz, J (H,P) ) 3 Hz, J (H,H) ) 2 Hz, 1H; CdCHHtrans), 4.26
(q, 3J (H,H) ) 7 Hz, 2H; OCH2), 2.54-2.64 (m, 4H; PCH2CH2),
2.2 (very br s, 2H; CH2CHd), 1.86 (q ≡ dt, J (H,H) ) 3J (H,P)
3
) 6 Hz, 2H; PdsCH2), 1.74 (br, 2H; PdsCH2CH2), 1.31 (t,
3J (H,H) ) 7 Hz, 3H; CH3). At -10 °C, two seperate broad
signals at δ 2.41 and 1.96 are observed for the diasterotopic
allylic protons. Coalescence occurs at ca. +17 °C (300 MHz).
13C NMR (50 MHz, CH2Cl2-CD2Cl2): δ 178.9 (CO), 140.8 (d,
2J (C,P) ) 5 Hz; -CHd), 130.0-134.4 (Carom), 105.8 (d, 2J (C,P)
) 10 Hz; dCH2), 65.0 (OCH2), 36.0, 35.1, and 32.8 (CH2CH2-
Su p p or tin g In for m a tion Ava ila ble: A plot showing data
for kinetic experiments and the H,H-COSY spectrum of 5 (2
pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, can be ordered from the ACS, and can
be downloaded from the Internet; see any current masthead
page for ordering information and Internet access instructions.
CH2), 28.8 (CR), 22.0 (d, J (C,P) ) 28 Hz; Câ), 13.9 (CH3). 31P
1
NMR (202 MHz, CD2Cl2): δ +26.6. IR (CH2Cl2): ν 1664 cm-1
(CO).
OM960061Z