Alumoxanes as cocatalysts in the palladium-catalyzed copolymerization of carbon monoxide and ethylene: Genesis of a structure-activity relationship

Article abstract of DOI:10.1021/om9508492

The palladium-catalyzed copolymerization of carbon monoxide and ethylene to give polyketone polymers, [CH₂CH₂C(O)]_n, has been accomplished by the use of either (dppp)-Pd(OAc)₂ or (dppp)Pd[C(O)^tBu]Cl in the presence of a tert-butyl alumoxane, [(^tBu)Al(μ₃-O)]_n (n = 6, 7, 9) or [(^tBu)₇Al₅(μ₃-O) ₃(μ-OH)₂] cocatalyst. The effects on the catalytic activity of the alumoxane and palladium concentrations, the alumoxane structure, and the identity of the phosphine ligands were determined. The function of the alumoxane is shown to depend on the choice of palladium catalyst precursor. With (dppp)Pd[C(O)^tBu]Cl the alumoxane abstracts chloride to give a catalytically active cationic palladium complex directly. In contrast, the alumoxane initially alkylates the palladium in (dppp)Pd(OAc)₂ and subsequently abstracts the remaining acetate anion, yielding the active cationic palladium complex. The catalytic activity is highly dependent on the structure of the alumoxane. A comparative study indicates the cocatalytic activity to be [(^tBu)Al(μ₃-O)]₇ > [(^tBu)Al(μ₃-O)]₆ > [(^tBu)Al(μ₃-O)]₉ ? [(^tBu)₇Al₅(μ₃-O) ₃(μ-OH)₂]. This observed cocatalytic activity correlates with the predicted latent Lewis acidity of the alumoxanes. A discussion of the palladium-alumoxane complex is presented with respect to the model compound [(^tBu)₆Al₆(μ₃-O) ₄(μ-OH)₂(μ-O₂-CCCl₃) ₂], prepared by the reaction of [(^tBu)Al(μ₃-O)]₆ with HO₂CCCl₃. The steric effects of the catalyst active site, as determined by the alkyl bridge length (n) in R₂P(CH₂)_nPR₂ and the alkyl substituents R, were probed for the catalyst precursor compounds [R₂P(CH₂)_n-PR₂]Pd[C(O) ^tBu]Cl (R = Ph, n = 2 (dppe), 3 (dppp), 4 (dppb); R-Me (dmpe), C₆H₁₁ (dcpe), n = 2). The concept of "pocket angle" has been developed to account for the observed steric effects. The detection of vinyl end groups on low-molecular-weight oligomers is indicative of catalyst turnover via a β-hydride-elimination chain termination. A proposed catalyst mechanism and a pathway to catalyst activation are presented. The molecular structures of (dppp)Pd[C(O)^tBu]Cl, (dppe)Pd[C(O)^tBu]Cl, (dmpe)Pd[C(O)^tBu]Cl, (dcpe)Pd[C(O)^tBu]Cl, and [(^tBu)₆Al₆(μ₃-O) ₄(μ-OH)₂(μ-O₂CCCl₃) ₂] have been determined by X-ray crystallography.

Full text of DOI:10.1021/om9508492

Organometallics 1996, 15, 2213-2226
2213
Alu m oxa n es a s Coca ta lysts in th e P a lla d iu m -Ca ta lyzed
Cop olym er iza tion of Ca r bon Mon oxid e a n d Eth ylen e:
Gen esis of a Str u ctu r e-Activity Rela tion sh ip
Yoshihiro Koide,1a Simon G. Bott,1b and Andrew R. Barron*,1a
Departments of Chemistry, Rice University, Houston, Texas 77005, and
University of North Texas, Denton, Texas 76203
Received October 27, 1995X
The palladium-catalyzed copolymerization of carbon monoxide and ethylene to give
polyketone polymers, [CH2CH2C(O)]n, has been accomplished by the use of either (dppp)-
Pd(OAc)2 or (dppp)Pd[C(O)tBu]Cl in the presence of a tert-butyl alumoxane, [(tBu)Al(µ3-O)]n
(n ) 6, 7, 9) or [(tBu)7Al5(µ3-O)3(µ-OH)2] cocatalyst. The effects on the catalytic activity of
the alumoxane and palladium concentrations, the alumoxane structure, and the identity of
the phosphine ligands were determined. The function of the alumoxane is shown to depend
on the choice of palladium catalyst precursor. With (dppp)Pd[C(O)tBu]Cl the alumoxane
abstracts chloride to give a catalytically active cationic palladium complex directly. In
contrast, the alumoxane initially alkylates the palladium in (dppp)Pd(OAc)2 and subsequently
abstracts the remaining acetate anion, yielding the active cationic palladium complex. The
catalytic activity is highly dependent on the structure of the alumoxane. A comparative
study indicates the cocatalytic activity to be [(tBu)Al(µ3-O)]7 > [(tBu)Al(µ3-O)]6 > [(tBu)Al-
(µ3-O)]9 . [(tBu)7Al5(µ3-O)3(µ-OH)2]. This observed cocatalytic activity correlates with the
predicted latent Lewis acidity of the alumoxanes. A discussion of the palladium-alumoxane
complex is presented with respect to the model compound [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2-
CCCl3)2], prepared by the reaction of [(tBu)Al(µ3-O)]6 with HO2CCCl3. The steric effects of
the catalyst active site, as determined by the alkyl bridge length (n) in R2P(CH2)nPR2 and
the alkyl substituents R, were probed for the catalyst precursor compounds [R2P(CH2)n-
PR2]Pd[C(O)tBu]Cl (R ) Ph, n ) 2 (dppe), 3 (dppp), 4 (dppb); R ) Me (dmpe), C6H11 (dcpe),
n ) 2). The concept of “pocket angle” has been developed to account for the observed steric
effects. The detection of vinyl end groups on low-molecular-weight oligomers is indicative
of catalyst turnover via a â-hydride-elimination chain termination. A proposed catalyst
mechanism and a pathway to catalyst activation are presented. The molecular structures
of (dppp)Pd[C(O)tBu]Cl, (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, (dcpe)Pd[C(O)tBu]Cl,
and [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2] have been determined by X-ray crystallography.
In tr od u ction
We have recently reported that polyketone polymers
may be synthesized using a palladium catalyst, (dppp)-
Pd(OAc)2 or (dppp)Pd[C(O)tBu]Cl, with an isolable tert-
butyl alumoxane cocatalyst, [(tBu)Al(µ3-O)]6 (eq 1).10 The
polymers are perfectly alternating ethylene/CO, have
high molecular weights (16 700 < Mn < 58 400), and
show no spectroscopic evidence for furanization or cross-
linking.11
The palladium-catalyzed copolymerization of olefins
and carbon monoxide to form polyketone polymers is
well established. The cationic palladium phosphine
catalysts commonly employed may either be preformed,
e.g., [Pd(PPh3)n(MeCN)4-n][BF4],2,3 or formed in situ
from the reaction of [R2P(CH2)3PR2]Pd(OAc)2 with non-
coordinating acids (e.g., HBF4).4 Despite the diversity
of polyketone catalyst systems reported, all are based
upon a cationic transition-metal center, [LnMX]+, and
a non-coordinating anion.5-9
(1)
* To whom all correspondence should be addressed.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) (a) Rice University. (b) University of North Texas.
(2) Sen, A.; Lai, T. J . Am. Chem. Soc. 1982, 104, 3520.
(3) Lai, T.; Sen, A. Organometallics 1984, 3, 866.
(4) See for example: (a) Drent, E.; Wife, R. L. U.S. Patent 4,970,-
294, 1990. (b) Van Leeuwen, P. W. N.; Roobeek, C. F.; Wong, P. K.
Eur. Pat. Appl. EP 393,790, 1990. (c) Drent, E. Eur. Pat. Appl. EP
390,292, 1990.
(5) See: (a) Brookhart, M.; Grant, B.; Volpe, A. F., J r. Organome-
tallics 1992, 11, 3920. (b) Brookhart, M.; Rix, F. C.; DeSimone, J . M.;
Barborak, J . C. J . Am. Chem. Soc. 1992, 114, 5894.
(6) Drent, E.; Van Broekhoven, J . A. M.; Doyle, M. J . J . Organomet.
Chem. 1991, 417, 235.
The function of the alumoxane cocatalyst was in-
tended to mimic the noncoordinating anion in the more
traditional palladium catalyst systems. However, we
have recently shown that in the case of zirconocene-
catalyzed polymerization of ethylene the alumoxane is
not an innocent noncoordinating anion but has a
(8) Sen, A.; J iang, Z. Macromolecules 1993, 26, 911.
(9) Klaubunde, U.; Tulip, T. H.; Roe, D. C.; Ittel, S. D. J . Organomet.
Chem. 1987, 334, 141.
(7) Chepaikin, E. G.; Bezruchenko, A. P.; Belov, G. P. Izv. Akad.
Nauk SSSR, Ser. Khim. 1990, 9, 2181.
(10) Koide, Y.; Barron, A. R. Macromolecules 1996, 29, 1110.
(11) Koide, Y.; Barron, A. R. Main Group Met. Chem. 1995, 18, 405.
S0276-7333(95)00849-1 CCC: $12.00 © 1996 American Chemical Society

2214 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
contributory role in the polymerization due to its com-
plexation to the zirconium center.12 Furthermore, we
proposed that the cocatalytic activity of each alumoxane,
[(tBu)Al(µ3-O)]n (n ) 6-9), can be related to its structure
as measured by the latent Lewis acidity.13 On the basis
of our results with the Cp2ZrMe2/alumoxane-catalyzed
polymerization of ethylene, it seems reasonable to
expect that the alumoxane function in the palladium-
catalyzed copolymerization of ethylene and CO also be
nonbenign. In this regard we have investigated the
effects on the catalytic activity of the alumoxane and
palladium concentrations, the alumoxane structure, and
the identity of the phosphine ligands. The results of
this study are reported herein.
isomer, [(tBu)Al(µ3-O)]n, (n ) 6 (III),14 7 (IV),15 9 (V)14)
and [Al5(tBu)7(µ3-O)3(µ-OH)2] (VI).15
As a general procedure, a CH2Cl2 solution of alumox-
ane was added to a solution of the palladium catalyst,
previously purged with CO, in a Teflon-lined autoclave
(see Experimental Section). The reaction mixture was
then pressurized to 300 psi16 with an equimolar mixture
of CO and ethylene and heated to 60 °C. The pressure
of the vessel was monitored during the course of the re-
action. The reaction was quenched by release of the CO/
ethylene pressure upon completion of the polymeriza-
tion. The polymer was isolated by filtration and washing.
All of the polyketone polymers isolated are white,
independent of catalyst or reaction pressure.10 Spec-
troscopically the polyketone polymers prepared using
alumoxane cocatalysts are identical with those previ-
ously reported;17 however, all the polymers prepared
using alumoxane cocatalysts are stable high-density
modifications of the R-phase.18 Full spectroscopic and
structural characterization data have been reported
elsewhere.10
Ca ta lytic Dep en d en ce on Alu m oxa n e. We have
previously shown there to be a strong structural depen-
dence for individual alumoxanes on the catalytic activity
in the ring-opening polymerization of â-lactones19 and
the zirconocene-alumoxane polymerization of ethyl-
ene.12 Furthermore, we have attempted to correlate this
structural dependence with the latent Lewis acidity of
the Al-O cages, as well as with the steric strain in the
anionic product.12
Resu lts a n d Discu ssion
In our preliminary experiments we adapted the
literature catalyst system, by reaction of (dppp)Pd(OAc)2
(I) with a mixture of tert-butyl alumoxanes, [(tBu)Al-
(µ3-O)]n, in place of the acid cocatalyst. (dppp)Pd(OAc)2
was replaced with the palladium pivaloyl compound
(dppp)Pd[C(O)tBu]Cl (II) to simplify the catalyst initia-
tion reaction. Previous studies have shown that (Ph3P)2-
Pd[C(O)Me]Cl is a precursor to an active catalyst upon
addition of AgBF4.3 In subsequent experiments the
mixture of alumoxanes was substituted by a single
In order to determine the effect of the alumoxane
structure on the catalytic activity of the palladium-
catalyzed copolymerization of CO and ethylene, we have
investigated the rate of polymerization using four dif-
ferent alumoxanes. The alumoxanes investigated are
[(tBu)Al(µ3-O)]6 (III), [(tBu)Al(µ3-O)]7 (IV), [(tBu)Al(µ3-
O)]9 (V), and [(tBu)7Al5(µ3-O)3(µ-OH)2] (VI).
As can be seen from Figure 1, the relative rates of
activity are [(tBu)Al(µ3-O)]7 > [(tBu)Al(µ3-O)]6 > [(tBu)-
Al(µ3-O)]9 . [(tBu)7Al5(µ3-O)3(µ-OH)2]. The observed
cocatalytic activity correlates well with the predicted
latent Lewis acidity of the alumoxanes.12 Furthermore,
the relative activity of the alumoxanes remains constant
throughout catalysis. In each case a large excess of
alumoxane was employed ([alumoxane] ) 10[Pd]; see
below). The observation of a correlation between alu-
moxane structure and sustained catalytic activity is
important and suggests that the alumoxane is not a
spectator counterion but is actively involved in the
activity of the catalyst site. Further evidence for the
active role of the alumoxane is the concentration depen-
dence of the alumoxane on the rate of polymerization.
(14) Mason, M. R.; Smith, J . M.; Bott, S. G.; Barron, A. R. J . Am.
Chem. Soc. 1993, 115, 4971.
(15) Harlan, C. J .; Mason, M. R.; Barron, A. R. Organometallics
1994, 13, 2957.
(16) 100 psi ) 5170 Torr ) 6.89 bar.
(17) Lommerts, B. J .; Klop, E. A.; Aerts, J . J . Polym. Sci., Polym.
Phys. 1993, 31, 1319.
(18) Polyketone, prepared from carbon monoxide and ethylene, is
known to crystallize in two forms: an R- and a â-phase. There is,
however, some disagreement as to the relationship of the phases.
Several workers have proposed that the â-phase is a high-temperature
form of the R-phase. Alternatively, it has been proposed that the change
in crystal structure is due to defects in the chains introduced upon
heating; see: Chatani, Y.; Takizawa, T.; Murahashi, S.; Sakata, Y.;
Nishimura, Y. J . Polym. Sci. 1961, 55, 811.
(12) Harlan, C. J .; Bott, S. G.; Barron, A. R. J . Am. Chem. Soc. 1995,
117, 6465.
(13) We have defined latent Lewis acidity to be the ability of
electron-precise cage compounds to act as Lewis acids upon cage
opening. The extent of latent Lewis acidity is dependent on the ring
strain within the cage.
(19) Wu, B.; Lenz, R. W.; Harlan, C. J .; Barron, A. R. Can. J .
Microbiol. 1995, 41, 274.

Alumoxanes as Cocatalysts
Organometallics, Vol. 15, No. 9, 1996 2215
F igu r e 1. Relative catalytic activity as a function of the
alumoxanes [(tBu)Al(µ3-O)]6 (hexamer), [(tBu)Al(µ3-O)]7
(heptamer), [(tBu)Al(µ3-O)]9 (nonamer), and [(tBu)7Al5(µ3-
O)3(µ-OH)2] (pentamer). In all reactions (dppp)Pd(OAc)2
was used as the palladium source.
F igu r e 3. Polyketone yield as a function of palladium
concentration (mM), relative to a constant concentration
of [(tBu)Al(µ3-O)]6 (0.25 mM): (9) (dppp)Pd(OAc)2; (0)
(dppp)Pd[C(O)tBu]Cl. The reactions were performed over
2 h at 60 °C and 300 psi (CO, ethylene).
Ca ta lytic Dep en d en ce on P a lla d iu m . The poly-
mer yield increases with increased (dppp)Pd(OAc)2
concentration in the presence of an excess of the
alumoxane [(tBu)Al(µ3-O)]6 (see Figure 3). This is
consistent with each palladium producing a single active
site. Clearly, catalysis is not due to a minor contami-
nant or metallic palladium formed by thermal or reduc-
tive decomposition. A maximum catalytic activity is
observed for a (dppp)Pd(OAc)2/alumoxane molar ratio
of 0.5, which indicates that two molecules of [(tBu)Al-
(µ3-O)]6 per (dppp)Pd(OAc)2 are required for the forma-
tion of the catalytically active species. Such an obser-
vation is consistent with a two-step reaction between
(dppp)Pd(OAc)2 and two molecules of [(tBu)Al(µ3-O)]6;
the first equivalent of alumoxane reacts with the
palladium via metathesis of one acetate group (eq 2),
F igu r e 2. Polyketone yield as a function of [(tBu)Al-
(µ3-O)]6 concentration (mM), relative to a constant concen-
tration of (dppp)Pd(OAc)2 (0.16 mM). All reactions were
performed over 2 h at 60 °C and 300 psi (CO, ethylene).
(dppp)Pd(OAc)2 + [(tBu)Al(µ3-O)]6 f
(dppp)Pd(tBu)(OAc) + [(tBu)5Al6(O)6(OAc)] (2)
(dppp)Pd(tBu)(OAc) + [(tBu)Al(µ3-O)]6 f
[(dppp)Pd(tBu)]+ + [(tBu)6Al6(O)6(OAc)]- (3)
The dependence of the rate of polyketone synthesis
on the alumoxane concentration was determined for
both [(tBu)Al(µ3-O)]6 and [(tBu)Al(µ3-O)]9: T ) 60 °C,
Pi ) 300 psi, [(dppp)Pd(OAc)2] ) 0.16 mM. As can be
seen from Figure 2, the polymerization rate increases
as the alumoxane concentration is increased relative to
that of palladium. The catalytic activity appears to
reaches a maximum with a large excess of alumoxane,
suggesting that all the palladium centers are acti-
vated.20 However, two important points should be
noted. First, even under conditions of large excess,
[(tBu)Al(µ3-O)]6 is more reactive than [(tBu)Al(µ3-O)]9.
Second, the relative alumoxane concentration to reach
maximum activity is lower for [(tBu)Al(µ3-O)]6 than for
[(tBu)Al(µ3-O)]9. Possible explanations for these obser-
vations are discussed below.
and the second alumoxane is required to abstract the
remaining acetate group to form a cationic palladium
complex similar to that previously proposed as the active
catalyst (eq 3). If an excess of (dppp)Pd(OAc)2 is used,
then the catalytic activity is close to zero. This observa-
tion suggests that (dppp)Pd(OAc)2 reacts with the active
catalyst (see below).
We presume that no reaction occurs between the
carboxyl-substituted alumoxane formed from the me-
tathesis of the first acetate, [(tBu)5Al6(O)6(OAc)], and the
palladium alkyl acetate (dppp)Pd(tBu)(OAc). Attempts
to prepare (dppp)Pd(tBu)(OAc), from the reaction of
t
(dppp)Pd(OAc)2 and BuMgCl, to test this hypothesis
While [(tBu)Al(µ3-O)]7 is slightly more active than
[(tBu)Al(µ3-O)]6, the ease of synthesis of the latter as
compared to the former determined that all subsequent
experiments were performed with pure [(tBu)Al(µ3-O)]6.
were unsuccessful (see Experimental Section).
If, of the two alumoxanes required for catalysis, the
first alkylates the palladium and the second abstracts
the acetate to yield a cationic species, then if (dppp)-
Pd[C(O)tBu]Cl (II) is used in place of (dppp)Pd(OAc)2
only one alumoxane should be required to produce an
active catalyst. This is indeed observed, as shown in
Figure 3. As with (dppp)Pd(OAc)2 the polymer yield
(20) Under our conditions, there is sometimes a decrease in the
polymer yield at higher alumoxane concentrations. This observation
is contrary to expectations and is possibly indicative of the instability
of the palladium-alumoxane complex, in the absence of ethylene/CO.

2216 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
increases with increased (dppp)Pd[C(O)tBu]Cl concen-
tration until the Pd/alumoxane ratio is ca. 1, at which
point catalysis reaches a maximum. The fact that the
catalysis reaches a maximum at one alumoxane per
palladium suggests that each alumoxane is only able
to react with 1 equiv of (dppp)Pd[C(O)tBu]Cl. However,
at higher palladium concentrations catalysis remains
constant. This is unlike the Cp2ZrMe2/[(tBu)Al(µ3-O)]6-
catalyzed polymerization of ethylene, where we have
evidence that each [(tBu)Al(µ3-O)]6 can react with two
Cp2ZrMe2 molecules.12
Ta ble 1. Rela tive Ca ta lytic Activity a n d Estim a ted
In ter ior Con e An gle a s a F u n ction of th e
P h osp h in e Liga n d in
[R2P (CH2)n P R2]P d [C(O)tBu ]Cl
e
initial rate
total polymer
yieldc (g)
θ|d
θ
phosphinea nb dP/dt (psi‚min-1
)
(deg) (deg)
dppe
dppp
dppb
dmpe
dcpe
2
3
4
2
2
0.19
0.75
0.12
0.216
1.42
0.025
none
1.25
132
108
93
141
106
129
146
102
174
115
0.83
a
Abbreviations: dppe ) 1,2-bis(diphenylphosphino)ethane,
It is interesting to note that the absolute yield of
polyketone appears to be greater using (dppp)Pd[C(O)t-
Bu]Cl than (dppp)Pd(OAc)2, even under optimum condi-
tions for each palladium catalyst precursor. This is
presumably related to either the magnitude of the
equilibrium constant between the neutral and “cationic”
palladium complexes (eq 4; X ) Cl, OAc) or the
coordinating ability of the resulting alumoxane complex
[(tBu)6Al6(O)6(X)]- (X ) Cl versus OAc). The reversible
nature of this type of reaction has been observed pre-
viously.12
dppp ) 1,3-bis(diphenylphosphino)propane, dppb ) 1,4-bis(diphen-
ylphosphino)butane, dmpe ) 1,2-bis(dimethylphosphino)ethane,
dcpe ) 1,2-bis(dicyclohexylphosphino)ethane. b R2P(CH2)nPR2. c Af-
d
ter 10 h. Parallel to the PdP2 plane. e Perpendicular to the PdP2
plane.
1,2-bis(dimethylphosphino)ethane) and (dcpe)Pd[C(O)t-
Bu]Cl (dppe ) 1,2-bis(dicyclohexylphosphino)ethane)
allows for the effects of alkyl substitution to be probed.
(dppp)Pd(P)X + [(tBu)Al(µ3-O)]6 h
[(dppp)Pd(P)]+ + [(tBu)6Al6(O)6(X)]- (4)
P ) polymer
Ca ta lytic Dep en d en ce on th e P h osp h in e Liga n d .
Several groups have reported a significant effect on
catalytic activity of the chelate ring size (as deter-
mined by the alkyl bridge length n) for a number of
catalytic systems containing bidentate phosphines,
R2P(CH2)nPR2: e.g., the rhodium-catalyzed hydrogena-
tion of styrene.21 In the case of palladium-catalyzed
polyketone synthesis, Drent and co-workers6 observed
that the maximum activity and highest molecular
weights were obtained by the use of a propyl-bridged
diphosphine; i.e., n ) 3. These workers suggested that
the control exerted by the phosphine was due to the
ability of the diphosphine to stabilize both square-planar
and trigonal-bipyramidal geometries, the maximum
activity being observed for phosphines in which the bite
angle (P-Pd-P) allows complexation in both geom-
etries. Subsequently, Chien and co-workers observed
the same trend in catalytic activity but proposed an
alternative rationale based upon the ability of the
various phosphines to undergo chelate opening.22
To determine how the steric and electronic properties
of the phosphine ligand effect the catalytic activity in
the alumoxane cocatalyzed system, we systematic-
ally varied the chelate ring size (n), and the alkyl
substituents (R), for the catalyst precursor compounds
[R2P(CH2)nPR2]Pd[C(O)tBu]Cl. A comparison of the
relative catalytic activity of (dppp)Pd[C(O)tBu]Cl (II)
with (dppe)Pd[C(O)tBu]Cl (VII; dppe ) 1,2-bis(diphen-
ylphosphino)ethane) and (dppb)Pd[C(O)tBu]Cl (VIII;
dppe ) 1,2-bis(diphenylphosphino)butane) will deter-
mine the effect of phosphine chelate ring size. Similarly,
a comparison of the catalytic activity of (dppe)Pd-
[C(O)tBu]Cl with that of (dmpe)Pd[C(O)tBu]Cl (dmpe )
The relative catalytic activity of each compound was
determined with [(tBu)AlO]6 as the cocatalyst (see Table
1). In addition, the molecular structures of (dppp)Pd-
[C(O)tBu]Cl, (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]-
Cl, and (dcpe)Pd[C(O)tBu]Cl were determined by X-ray
crystallography (see below).
The relative rate of catalysis and total polymer yield
as a function of the phosphine in [Ph2P(CH2)nPPh2]Pd-
[C(O)tBu]Cl follow the order previously observed for the
acid cocatalyzed catalysts; i.e., n ) 3 > 2 > 4.6 However,
(dmpe)Pd[C(O)tBu]Cl shows almost no activity, even for
oligomer formation, and (dcpe)Pd[C(O)tBu]Cl exhibits
an activity comparable to that of (dppp)Pd[C(O)tBu]Cl.
These latter observations are contrary to both of the
proposed structure-activity relationships.6,22 Clearly,
if chelate ring size is the most important factor in
controlling catalyst activity, then (dmpe)Pd[C(O)tBu]Cl,
(dppe)Pd[C(O)tBu]Cl, and (dcpe)Pd[C(O)tBu]Cl should
be comparable catalysts, which they are not. Similarly,
if chelate opening is the controlling effect, then the more
basic the phosphine, the lower its activity should be;
i.e., dppe > dmpe > dcpe. However, this trend is not
observed. Thus, an alternative explanation must be
sought.
It is clear that for the ethylene-bridged phosphines
(dmpe, dppe, and dcpe) catalytic activity increases with
the increased steric bulk of the phosphine’s alkyl
substituents, as mirrored by the increasing alkyl cone
angles: Me in dmpe (90°), Ph in dppe (105°), and C6H11
in dcpe (117°).23 Furthermore, increasing the length of
the phosphine backbone not only increases the bite
angle of the phosphine but also forces the phenyl
substituents toward the palladium, as indicated by a
decrease in the average Pd-P-Ph angle measured from
(21) Poulin, J .-C.; Dang, T.-P.; Kagan, H. B. J . Organomet. Chem.
1975, 84, 87.
(22) Xu, F. Y.; Zhao, A. X.; Chien, J . C. W. Makromol. Chem. 1993,
194, 2579.
(23) Tolman, C. A. Chem. Rev. 1977, 77, 313.

Alumoxanes as Cocatalysts
Organometallics, Vol. 15, No. 9, 1996 2217
the solid-state structures (see below): 117° in (dppe)-
Pd[C(O)tBu], 114° in (dppp)Pd[C(O)tBu]Cl, and 113° in
(dppb)PdX2 (estimated24). Thus, increasing the length
of the phosphine’s backbone results in increased steric
crowding at the palladium. Given the foregoing, it
would seem reasonable that some form of steric control
is in operation.
formed as an intermediate prior to olefin insertion. In
order for olefin insertion/acyl migration to occur, the
ethylene must be coplanar with the migrating acyl
group, shown in XI.27 Thus, it is in the ligand plane
While the steric bulk of monodentate phosphines is
readily estimated by Tolman’s cone angle,23 there has
been no simple method to estimate the steric effects of
bidentate phosphines. Using the space-filling models
derived from crystallographic data, it is difficult to
compare the steric hindrance of the “pocket” in the dppe,
dppp, and dppb complexes. It is desirable, therefore,
to derive a semiquantitative measure of the size of the
active site at palladium.
that steric hindrance becomes a controlling factor in
ethylene coordination/insertion.
If we assume that the P-C bonds in [Ph2P(CH2)n-
PPh2]PdX2 compounds are free to rotate, then if solid-
state structures of [Ph2P(CH2)nPPh2]Pd[C(O)tBu]Cl are
used as models (see below), a measure may be made of
the pocket angle at palladium:25 both parallel (θ|, IX)
and perpendicular (θ , X) to the PdP2 plane. The
It is reasonable to propose that if the internal cone
angle of the [Ph2P(CH2)nPPh2]Pd moiety is sufficiently
small (i.e., n ) 4), then ethylene is precluded from com-
plexation: the equilibrium shown in eq 5 is shifted
predominantly to the left. However, it is not imme-
diately obvious why little or no catalytic activity is
observed for large phosphine pocket angles at pal-
ladium, such as in the case of (dppe)Pd[C(O)tBu]Cl and
(dmpe)Pd[C(O)tBu]Cl. The following explanations are
consistent with observation and literature precedent. (1)
The lack of steric hindrance about palladium provides
a facile pathway for the decomposition of the active
catalyst. This is consistent with the observation of
palladium metal during reactions using (dmpe)Pd-
[C(O)tBu]Cl; however, little palladium metal is observed
during reactions using (dppe)Pd[C(O)tBu]Cl. (2) The
formation of stable metallocycles of the type shown in
Scheme 1 have been reported for the insertion of
norbornadiene.28 Such chelate formation will oppose the
insertion of the next monomer unit (carbon monoxide)
by blocking the required coordination site on palladium
and thus inhibiting polymerization (Scheme 1). The
magnitude of this effect will depend on the coordination
strength of the carbonyl group â to the palladium center.
The relative stability of such a chelate ligand will be
dependent on the effectiveness of the carbonyl oxygen-
palladium electrostatic interaction, which will be de-
termined by the chelate ring size (in the present case
independent of the phosphine ligand) and the steric
hindrance of the phosphine ligand. For complexes in
which the phosphine has a small pocket angle (i.e.,
dppp, dcpe) the phosphine’s substituents may inhibit,
or even preclude, chelation as a consequence of the steric
parallel pocket angle (θ|) is defined as the angle sub-
tended between two planes perpendicular to the PdP2
plane, and bisecting at the metal, that can exclude the
van der Waals surface of all the ligands over all
rotational orientations about the P-C bonds. Similarly,
the perpendicular pocket angle (θ ) is defined as the
angle subtended between two planes parallel with the
P2 vector, and bisecting at the metal, that can exclude
the van der Waals surface of all the ligands over all
rotational orientations about the P-C bonds. Calcu-
lated values for θ| and θ are given in Table 1.26
From Table 1 it can be clearly seen that there is an
optimum size for the active site. Furthermore, catalysis
is primarily dependent on the size of the interior cone
angle in the plane of the ligands around palladium, i.e.,
θ|. This is not surprising, since the most sterically
crowded species during the catalytic cycle is the ethyl-
ene complex [{Ph2P(CH2)nPPh2}Pd(H2CdCH2)C(O)R]+,
(24) We have been unable to obtain crystals of (dppb)Pd[C(O)tBu]-
Cl suitable for X-ray crystallography. Thus, the Pd-P-Ph angles and
pocket angles were determined from related structures; see: (a)
Haggerty, B. S.; Housecroft, C. E.; Rheingold, A. L.; Shaykh, B. A. M.
J . Chem. Soc., Dalton Trans. 1991, 2175. (b) Werner, H.; Ebner, M.;
Bertleff, W.; Schubert, U. Organometallics 1983, 2, 891. (c) Ganter,
C.; Orpen, A. G.; Bergamini, P.; Costa, E. Acta Crystallogr., Sect. C
1994, C50, 507.
(25) We have defined the “pocket angle” to be the interior cone angle
of a chelating phosphine.
(26) The pocket angles given in Table 1 are calculated from X-ray
crystal structures of the palladium complexes; however, similar values
are obtained by assuming a generic M-P bond distance of 2.28 Å, as
is done for cone angle measurements; cf. ref 23.
(27) Hoffmann et al. have shown that the thermodynamically stable
conformation of a palladium olefin complex is with the olefin bound
perpendicular to the P2PdL2 plane; see: Albright, T. A.; Hoffmann,
R.; Thibeault, J . C.; Thorn, D. L. J . Am. Chem. Soc. 1979, 101, 3801.
However, the ethylene must be coplanar with the acyl group for
migration to occur: Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 2079.
(28) (a) van Asselt, R.; Gielens, E. C. G.; Ru¨lke, R. E.; Vrieze. K.;
Elsevier, C. J . J . Am. Chem. Soc. 1994, 116, 977. (b) Brumbaugh, J .
S.; Whittle, R. R.; Parvez, M. A.; Sen, A. Organometallics 1990, 9, 1735.

2218 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
Sch em e 1. Su ggested Ch ela te F or m a tion for
P a lla d iu m Com p lexes w ith a Su fficien tly La r ge
P h osp h in e P ock et An gle
F igu r e 4. Molecular structure of (dppp)Pd[C(O)tBu]Cl.
Thermal ellipsoids are shown at the 30% level, and
hydrogen atoms are omitted for clarity.
repulsion between the alkyl substituents on phosphorus
and the carbonyl. In contrast, complexes containing
phosphines with a large pocket angle (i.e., dmpe) will
have significantly less steric hindrance, and chelation
may become so strong that the insertion of the next
monomer is inhibited completely; therefore, chain propa-
gation cannot occur. This rationalization would also
provide a plausible explanation for the formation of
oligomer with the dppe ligand.
While our results agree with these explanations, other
possibilities may exist. What is clear, however, is the
importance of the size (steric hindrance) and possibly
shape of the active catalytic site in these palladium
complexes.
Molecu lar Str u ctu r es of [R2P (CH2)n P R2]P d[C(O)-
tBu ]Cl. The molecular structures of (dppp)Pd[C(O)tBu]-
Cl, (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, and
(dcpe)Pd[C(O)tBu]Cl are shown in Figures 4-7, respec-
tively; selected bond lengths and angles are given in
Table 2.
F igu r e 5. Molecular structure of (dppe)Pd[C(O)tBu]Cl.
Thermal ellipsoids are shown at the 30% level, and
hydrogen atoms are omitted for clarity.
The palladium centers are, as expected, essentially
square planar; i.e., ∑(X-Pd-Y) ) 359.1(2)-360.0(1)°.
As can be seen from Table 2, the chelate phosphine’s
bite angle, P(1)-Pd(1)-P(2), is only dependent on the
magnitude of the Pd-P-Cn-P cycle. A similar trend
has been reported for [R2P(CH2)nPR2]PdCl2 compounds.29
In contrast, the Cl(1)-Pd(1)-C(1) angle is independent
of the phosphine bite angle and appears to depend on
the conformation of the phosphine’s alkyl substituents.
Thus, while the Cl(1)-Pd(1)-C(1) angles in (dppp)Pd-
[C(O)tBu]Cl and (dppe)Pd[C(O)tBu]Cl are similar, those
in (dmpe)Pd[C(O)tBu]Cl and (dcpe)Pd[C(O)tBu]Cl are
significantly larger. It is clear from Figure 7 that in
the solid state two of the cyclohexyl groups in (dcpe)-
Pd[C(O)tBu]Cl are positioned to minimize steric interac-
tion with the pivaloyl and chloride ligands,30 resulting
in a Cl(1)-Pd(1)-C(1) angle similar to that in (dmpe)-
Pd[C(O)tBu]Cl. This is in contrast to the observed
solution steric hindrance of the dcpe ligand in catalysis
(see above).
The Pd-P bond distances fall into two distinct
groups: those trans to the chloride (2.247(2)-2.254(1)
Å) and those trans to the pivaloyl group (2.366(1)-2.409-
(2) Å), consistent with the greater trans influence
exerted by the latter. It has previously been reported31
that the M-P bond distance in metallacyclic compounds
is related to the number of atoms in the metallacycle.
A similar trend is observed for (dppp)Pd[C(O)tBu]Cl
versus (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, and
(dcpe)Pd[C(O)tBu]Cl (see Table 2). The Pd(1)-C(1)
distances (2.038(5)-2.071(4) Å) are slightly shorter than
previously reported values for Pd-alkyl (2.08-2.09 Å),32
concordant with the σ-donor ability of an acyl being
stronger than that of an alkyl substituent. The Pd(1)-
Cl(1) distances are within the range expected; however,
an interesting correlation is observed: the Pd-Cl bond
distance is inversely proportional to the trans-P-Pd-
(31) Ros, R.; Lenarda, M.; Pahor, N. B.; Calligaris, M.; Delise, P.;
Randaccio, L.; Graziani, M. J . Chem. Soc., Dalton Trans. 1976, 1937.
(32) See for example: (a) Wisner, J . M.; Bartczark, T. J .; Ibers, J .
A. Organometallics 1986, 5, 2044. (b) de Graaf, W.; Boersma, J .;
Smeets, W. J . J .; Spek, A. L.; van Koten, G. Organometallics 1989, 8,
2907.
(29) Steffen, W. L.; Palenik, G. J . Inorg. Chem. 1976, 15, 2432.
(30) A similar confirmation was previously observed for (dcpe)PdCl2;
see: Ganguly, S.; Mague, J . T.; Roundhill, D. M. Acta Crystallogr.,
Sect. C 1994, C50, 217.

Alumoxanes as Cocatalysts
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for [R2P (CH2)n P R2]P d [C(O)tBu ]Cl
phosphine
Organometallics, Vol. 15, No. 9, 1996 2219
dppp
dppe
dmpe
dcpe
Pd(1)-Cl(1)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-C(1)
P(1)-C(11)
P(1)-C(111)
P(1)-C(117)
P(2)-C(21)
P(2)-C(211)
P(2)-C(217)
O(1)-C(1)
2.386(2)
2.362(3)
2.247(2)
2.377(2)
2.042(8)
1.838(8)
1.812(9)
1.824(9)
1.815(8)
1.816(9)
1.825(9)
1.18(1)
2.382(2)
2.340(2)
2.229(2)
2.041(7)
1.81(1)
1.81(1)
1.784(8) [C(112)]
1.82(1)
1.81(1)
2.380(1)
2.254(1)
2.366(1)
2.071(4)
1.845(5)
1.831(4)
1.843(5)
1.836(5)
1.834(4)
1.848(4)
1.193(5)
1.563(6)
2.409(2)
2.254(1)
2.038(5)
1.815(6)
1.819(5)
1.834(6)
1.828(6) [C(13)]
1.828(5)
1.818(6)
1.194(6)
1.552(8)
1.786(9) [C(212)]
1.204(9)
1.55(1)
C(1)-C(2)
1.53(1)
Cl(1)-Pd(1)-P(1)
Cl(1)-Pd(1)-P(2)
Cl(1)-Pd(1)-C(1)
P(1)-Pd(1)-P(2)
P(1)-Pd(1)-C(1)
P(2)-Pd(1)-C(1)
Pd(1)-P(1)-C(11)
Pd(1)-P(1)-C(111)
Pd(1)-P(1)-C(117)
Pd(1)-P(2)-C(21)
Pd(1)-P(2)-C(211)
Pd(1)-P(2)-C(217)
Pd(1)-C(1)-O(1)
Pd(1)-C(1)-C(2)
O(1)-C(1)-C(2)
92.51(5)
173.08(5)
85.7(2)
91.58(5)
170.8(1)
89.4(2)
116.7(2)
107.9(2)
121.1(2)
115.4(2) [C(13)]
109.4(2)
120.9(2)
118.4(4)
121.5(3)
119.9(5)
179.69(9)
94.74(9)
86.7(2)
85.57(7)
93.0(2)
173.9(3)
106.7(2)
120.3(3)
114.4(2)
104.6(3)
120.1(3)
115.6(3)
118.4(7)
121.5(6)
119.9(8)
91.42(7)
177.07(7)
91.7(2)
85.66(7)
174.2(2)
91.2(2)
106.3(3)
118.1(3)
117.8(3) [C(112)]
108.0(3)
120.7(4)
115.9(3) [C(212)]
118.4(6)
122.4(5)
119.1(6)
173.99(5)
89.06(5)
90.8(1)
86.63(4)
92.9(1)
171.7(1)
109.0(2)
111.8(1)
121.7(2)
106.5(1)
119.1(1)
115.1(1)
118.9(3)
122.5(3)
118.1(4)
F igu r e 6. Molecular structure of (dmpe)Pd[C(O)tBu]Cl.
Thermal ellipsoids are shown at the 30% level, and
hydrogen atoms are omitted for clarity.
F igu r e 7. Molecular structure of (dcpe)Pd[C(O)tBu]Cl.
Thermal ellipsoids are shown at the 30% level, and
hydrogen atoms are omitted for clarity.
Cl angle. Such a correlation is the obverse of that
expected from trans-influence arguments: i.e., the closer
a ligand is to being trans, the longer the metal-ligand
bond distance.33 In the present case the correlation is
consistent with an orbital overlap argument: the closer
to linear the P-Pd-Cl vector, the greater the σ-bonding
overlap between the ligand donor orbital and the
are all puckered due to the preference of the PCH2CH2P
moiety for a staggered conformation.35 In all the
structures the pivaloyl group is oriented out of the
palladium coordination plane to reduce steric conges-
tion.
P olyk eton e P olym er En d -Gr ou p An a lysis. While
the formation of the R-phase of polyketone clearly
indicates that the polymerization reaction involves the
alternating insertion of carbon monoxide and ethylene,
2
2
palladium dx -y
.
The six-membered PdP2C3 ring in (dppp)Pd[C(O)tBu]-
Cl is in a chair conformation, resulting in the axial-
equatorial arrangement of the phenyl groups.34 The
five-membered Pd-P-C-C-P rings in (dppe)Pd[C(O)-
tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, and (dcpe)Pd[C(O)tBu]Cl
the high molecular weights obtained (16 700 < Mn
<
58 400) precluded spectroscopic identification of the end
(35) See for example: (a) Salt, J . E.; Girolami, G. S.; Wilkinson, G.;
Motevalli, M.; Thornton-Pett, M.; Hursthouse, M. B. J . Chem. Soc.,
Dalton Trans. 1985, 685. (b) Girolami, G. S.; Wilkinson, G.; Galas, A.
M. R.; Thornton-Pett, M.; Hursthouse, M. B. J . Chem. Soc., Dalton
Trans. 1985, 1339. (c) Salt, J . E.; Wilkinson, G.; Motevalli, M.;
Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1986, 1141. (d) Barron,
A. R.; Salt, J . E.; Girolami, G. S.; Wilkinson, G.; Motevalli, M.;
Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1987, 2947.
(33) See for example: (a) Lyons, D.; Wilkinson, G.; Thornton-Pett,
M.; Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1984, 695. (b)
Barron, A. R.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J . Chem.
Soc., Dalton Trans. 1987, 837.
(34) Hermann, W. A.; Brossmer, C.; Priermeier, T.; O¨ fele, K. J .
Organomet. Chem. 1994, 481, 97.

2220 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
groups. Such information would allow an understand-
ing of the initiation and chain-termination steps. Fur-
thermore, the presence of a chain-termination step not
associated with the hydrolytic workup of the polymer
should indicate that catalyst turnover is occurring.
While the concentration dependence on the alumox-
ane and palladium in the (dppp)Pd(OAc)2 system indi-
cates that the alumoxane performs two functions to
initiate the catalyst, the use of the pivaloyl substituent
in the (dppp)Pd[C(O)tBu]Cl catalyst precursor should
preform the growth chain. It may be presumed that
upon chloride abstraction the initial reaction would
involve ethylene insertion (eq 6). If this is indeed true,
alumoxane, abstraction of acetate, â-hydride elimination
of the tert-butyl group to yield a hydride, and ethylene
insertion. The active catalyst is therefore [(dppp)PdH]+.
In this manner the alumoxane cocatalyzed polymeriza-
tion is analogous to that observed for the (dppp)Pd-
(OAc)2/HX system.4 The oligomers observed for reac-
tions using (dppp)Pd[C(O)Et]Cl as the catalyst precursor
are identical with those observed using (dppp)Pd(OAc)2
(see Experimental Section).
In all of the oligomers, termination is observed to be
via an alkyl or olefinic group. The alkyl is formed upon
alcoholysis of the living polymer species upon ethanol
workup (see Experimental Section) (eq 7), while the
olefinic group is formed as a result of â-hydride elimina-
tion (eq 8). The observation of oligomeric â-hydride
then the resulting polymers will be terminated by a
C(O)tBu group. We were unsuccessful in the observa-
1
tion of such pivaloyl end groups by H NMR.10 How-
ever, oligomers were isolated by limiting the reaction
times to 15 min and removing insoluble polymers prior
to analysis (see Experimental Section).
The soluble oligomeric species formed using (dppp)-
Pd[C(O)tBu]Cl and [(tBu)Al(µ3-O)]6 were characterized
by 1H and 13C NMR spectroscopy. The 13C NMR
spectrum, by comparison with that of bona fide spectra
t
of BuC(O)R groups, is consistent with the presence of
a C(O)tBu-terminated polymer (see Experimental Sec-
tion). Furthermore, the GC-mass spectrum of the
volatile oligomers shows the predominant species to be
the polyketone XII. Phenyl end groups are observed
for oligomeric species formed using (dppp)Pd[C(O)Ph]-
Cl (see Experimental Section).
elimination products suggests that catalysis turnover
occurs even after only a few CO/ethylene insertion steps.
The result of a single catalyst turnover is, thus, the
formation of the cationic palladium hydride [(dppp)-
PdH]+, similar to that proposed for the (dppp)Pd(OAc)2/
HX system.
It is important to note that there is no evidence for
aldehyde termination upon hydrolysis, consistent with
the reversible CO insertion:5b,28 releasing the CO pres-
sure after polymerization leads to decarbonylation of
Pd-acyl to result in Pd-alkyl, which by either solvolysis
or â-hydride elimination gives the ethyl ketone or vinyl
ketone end group, respectively.
No oligomers with tert-butyl groups were observed
when (dppp)Pd(OAc)2 was used as the palladium pre-
cursor, as either tBuC(O)- or tBuCH2CH2C(O)- ter-
mini. The 13C NMR and mass spectra of the products
formed during short reaction times show the presence
of two types of oligomers: saturated oligo-ketones EtC-
(O)[(C2H4)C(O)]nEt (XIII), and oligomers with ethyl
terminal groups (XIV). The formation of ethyl-termi-
Ch a in In itia tion , P r op a ga tion , a n d Ter m in a tion
Mech a n ism s. On the basis of the foregoing experi-
mental data the palladium/alumoxane-catalyzed co-
polymerization of ethylene and carbon monoxide is
proposed to occur in the manner summarized in Scheme
2.
nated oligomers indicates that the polymers are not
formed via a palladium pivaloyl complex. Instead,
initiation of polymerization must occur in the following
sequence: metathetical alkylation of palladium by the
Catalyst initiation via the formation of a palladium
hydride has been previously observed for polyketone

Alumoxanes as Cocatalysts
Organometallics, Vol. 15, No. 9, 1996 2221
Sch em e 2. P r op osed Ca ta lyst F or m a tion a n d
Ca ta lytic Cycle for P a lla d iu m -Alu m oxa n e
Cop olym er iza tion of Olefin s a n d Ca r bon Mon oxid e
catalyst precursor, then only a single equivalent of alu-
moxane is required to activate the complex, via chloride
abstraction (cf. eq 4). A similar palladium acyl catalyst
has been reported.3 Once catalysis is initiated, both pal-
ladium precursors result in similar products, the only
difference being that (dppp)Pd[C(O)tBu]Cl yields 1 mo-
lar equiv of a pivaloyl-terminated polymer per palladium.
There have been several detailed studies of the chain
propagation mechanism of metal-catalyzed copolymer-
ization of olefins with CO.37 The accepted mechanism,
especially in aprotic solvents,38 involves the alternate
insertion of CO and the olefin into a preformed Pd-
alkyl bond.3,5b,39 By analogy with these previous studies
we propose that a similar mechanism is applicable for
the alumoxane cocatalyzed system (see Scheme 2).
Given the similarity of chain propagation and catalyst
initiation, it would be expected that chain termination
occurs in a manner similar to that for previous aprotic
catalyst systems (i.e., via â-hydride elimination and sol-
volysis upon reaction workup); this is indeed observed.
Ca ta lyst Sp ecia tion . On the basis of the similarity
in chain initiation, propagation, and termination steps
with those of the traditional palladium cationic polym-
erization catalysts, we may expect that the alumoxane
cocatalyst represents an alternative route to the [(dppp)-
Pd(R)]+ complexes. The function of the alumoxane
would therefore be simply as a source of a noncoordi-
nating anion (e.g., [(tBu)6Al6(O)6(X)]-; X ) Cl, OAc) and,
thus, be a direct replacement for the traditional coun-
terions (e.g., [BF4]-, [O3STol]-, [O3SCF3]-, etc.) em-
ployed in polyketone synthesis.2-4 Given our previous
results which demonstrate that the cage structure of
the alumoxane opens to accept an anionic ligand, a
structure such as XV may be reasonably proposed for
the alumoxane counterion formed from the hexamer.
synthesis using cationic palladium catalysts in both
protic3 and aprotic solvents.36 In the alumoxane co-
catalyzed system, use of (dppp)Pd(OAc)2 as the catalyst
precursor also involves hydride formation. As is shown
in Scheme 2, reaction of (dppp)Pd(OAc)2 with 2 molar
equiv of [(tBu)Al(µ3-O)]n results in the following: (1)
metathetical exchange of acetate for tert-butyl (eq 2),
(2) abstraction of the remaining acetate anion (eq 3),
and (3) â-hydride elimination. The alumoxane products
formed in the first and second reaction steps are, in the
case of the hexamer, [(tBu)5Al6(O)6(OAc)] and [(tBu)6Al6-
(O)6(OAc)]-, respectively. The requirement of two alu-
moxanes per palladium precludes [(tBu)5Al6O6(OAc)]
from abstracting the second acetate; i.e., the reaction
shown in eq 9 does not occur. The lack of evidence for
[(tBu)5Al6(O)6(OAc)] + (dppp)Pd(tBu)(OAc) N
[(tBu)5Al6(O)6(OAc)2]- + [(dppp)Pd(tBu)]+ (9)
any tert-butyl end groups for polymers or oligomers
formed using (dppp)Pd(OAc)2 suggests that either CO
insertion into the Pd-C(CH3)3 group is not favored or
the â-hydride elimination reaction is faster than car-
bonyl insertion. If (dppp)Pd[C(O)tBu]Cl is used as the
(37) For a recent review, see: Sen, A. Acc. Chem. Res. 1993, 26,
303.
(38) A mechanism involving cationic Pd-carbene species has been
proposed for the copolymerization of propylene with CO; see: Batistini,
A.; Consiglio, G. Organometallics 1992, 11, 1766.
(39) With regard to the alkyl to acyl reaction step (Scheme 1), recent
studies have demonstrated that alkyl migration rather than carbonyl
insertion is the preferred mechanism for cationic palladium com-
plexes: van Leeuwen, P. W. N. M.; Roobeek, C. F.; van der Heijden,
H. J . Am. Chem. Soc. 1994, 116, 12117.
(36) (a) J iang, Z.; Dahlen, G. M.; Houseknecht, K.; Sen, A. Polym.
Prepr., Am. Chem. Soc., Div. Polym. Chem. 1992, 33, 1233. (b) J iang,
Z.; Dahlen, G. M.; Houseknecht, K.; Sen, A. Macromolecules 1992, 25,
2999.

2222 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
Any difference in activity between the alumoxanes,
[(tBu)Al(µ3-O)]n, would have to be based upon any
variation in the coordinating ability of each of the
resulting alumoxane anions, [(tBu)nAln(O)nX]-.
It has been previously assumed that the function of
alumoxanes as cocatalysts in the Ziegler-Natta type
polymerization of olefins also involves the formation of
a noncoordinating anion.40-43 However, we have re-
cently demonstrated that the alumoxanes do not form
simple noncoordinating anions but act as ligands to the
metal center to form a discrete complex, e.g., XVI.13
Given our previous results, it is possible that a similar
situation could apply for the palladium-alumoxane
system, i.e., XVII.
Figu r e 8. Molecular structure of [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-
O2CCCl3)2]. Thermal ellipsoids are shown at the 30% level,
and organic hydrogen atoms are omitted for clarity.
While, unlike the case for the zirconocene system, we
have no direct evidence for a palladium-alumoxane
complex, experimental data and model compounds
provide the basis for such a species.
1. The catalytic activity of the palladium-alumoxane
system is highly dependent on the polarity (dielectric
constant) of the solvent: higher activity is observed in
more polar (high dielectric constant) solvents, suggest-
ing that the active catalyst is either highly polar or ionic.
However, little or no activity is observed in coordinating
solvents. We have shown that the tert-butyl alumox-
anes are not affected by THF or MeCN,44 and previous
palladium catalysts for polyketone synthesis have been
carried out in alcohols and water. Thus, the cessation
of catalysis in THF and MeCN cannot be due to
decomposition of the alumoxane or inhibition of CO or
ethylene complexation but must be due to competitive
binding of the solvent in place of the alumoxane.
2. We have not yet been able to obtain structural
information on the palladium-alumoxane complexes;
however, the reaction of [(tBu)Al(µ3-O)]6 with 2 molar
equiv of Cl3CCO2H yields the cage-opened alumoxane
[(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2], whose structure
(Figure 8) has been determined by X-ray crystallography
(see below).45 The structural relationship between the
Al6O6 core of [(tBu)6Al6(µ3-O)4(µ-OH)2(O2CCCl3)2] and
that previously reported for [(Et2O)Li]2[(tBu)6Al6(µ3-O)6-
(40) (a) Sishta, C.; Hathorn, R. M.; Marks, T. J . J . Am. Chem. Soc.
1992, 114, 1112. (b) Resconi, L.; Bossi, S.; Abis, L. Macromolecules
1990, 23, 4489.
(41) (a) J olly, C. A.; Marynick, D. S. J . Am. Chem. Soc. 1989, 111,
7968. (b) Lauher, J . W.; Hoffmann, R. J . Am. Chem. Soc. 1976, 98,
1729.
(42) Dahmen, K. H.; Hedden, D.; Burwell, R. L., J r.; Marks, T. J .
Langmuir 1988, 4, 1212.
(43) Gassman, P. G.; Callstrom, M. R. J . Am. Chem. Soc. 1987, 109,
7875.
(44) The high-yield synthesis of [(tBu)Al(µ3-O)]n is carried out in
MeCN solution.
(45) The formation of [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2] from
[(tBu)Al(µ3-O)]6 by the addition of the carboxylic acid to an Al-O bond
is similar to the reaction during the formation of carboxylate alumox-
anes from the aluminum oxides; see: Landry, C. C.; Pappe´, N.; Mason,
M. R.; Apblett, A. W.; Tyler, A. N.; MacInnes, A. N.; Barron, A. R. J .
Mater. Chem. 1995, 5, 331.
F igu r e 9. Core structures of [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-
O2CCCl3)2] (top) and [(Et2O)Li]2[(tBu)6Al6(µ3-O)6Me2] (bot-
tom). The solid lines represent the common structural
fragments present in each.

Alumoxanes as Cocatalysts
Organometallics, Vol. 15, No. 9, 1996 2223
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
Me2] can readily be seen from Figure 9. On the basis
of this comparison, and the analogy with [Cp2ZrMe]-
[(tBu)6Al6(µ3-O)6Me] (XVI), it is clear that [(tBu)6Al6(µ3-
O)4(µ-OH)2(µ-O2CCCl3)2] represents a suitable model for
the interaction of [(tBu)Al(µ3-O)]6 with (dppp)Pd(OAc)2.
From the forgoing we propose that the alumoxane
forms a complex with the palladium center or a tight
ion pair within a solvent cage. This picture is similar
to the one we have demonstrated for alumoxanes in
zirconocene-catalyzed olefin polymerization. This view
of the alumoxane as a coordinating rather than nonco-
ordinating anion is also consistent with the decreased
rates of polymerization observed for alumoxane cocata-
lyzed versus noncoordinating anion cocatalyzed pro-
cesses.
If the palladium-alumoxane catalyst system is a
complex and not discrete ions, then the relative activity
of each of the alumoxanes cannot be explained by the
relative noncoordinating ability. However, the relative
activity of the alumoxanes may be readily understood
by a consideration of the alumoxane’s latent Lewis
acidity and its effects on catalyst activation.
(d eg) for [(tBu )6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2]
Al(1)-O(123)
Al(1)-C(11)
Al(2)-O(123)
Al(2)-C(21)
Al(2)-O(13a)
Al(3)-O(23)
Al(3)-O(12a)
O(132)-Al(2a)
O(2)-C(1)
1.799(3)
1.952(5)
1.829(3)
1.973(4)
1.999(3)
1.815(3)
1.799(3)
1.999(3)
1.233(6)
Al(1)-O(132)
Al(1)-O(1)
1.765(3)
1.855(3)
1.838(4)
2.057(3)
1.778(3)
1.945(6)
1.799(3)
1.248(6)
Al(2)-O(23)
Al(2)-O(2)
Al(3)-O(132)
Al(3)-C(31)
O(123)-Al(3a)
O(1)-C(1)
O(123)-Al(1)-O(132) 103.8(1) O(123)-Al(1)-C(11) 117.7(2)
O(132)-Al(1)-C(11) 124.0(2) O(123)-Al(1)-O(1)
O(132)-Al(1)-O(1) 102.8(2) C(11)-Al(1)-O(1)
100.3(1)
104.7(2)
O(123)-Al(2)-O(23) 109.7(1) O(123)-Al(2)-C(21) 127.6(2)
O(23)-Al(2)-C(21)
O(23)-Al(2)-O(2)
122.3(2) O(123)-Al(2)-O(2)
83.6(1) C(21)-Al(2)-O(2)
88.5(1)
91.1(2)
88.7(1)
163.7(1)
O(123)-Al(2)-O(13a) 80.6(1) O(23)-Al(2)-O(13a)
C(21)-Al(2)-O(13a) 105.2(1) O(2)-Al(2)-O(13a)
O(132)-Al(3)-O(23) 106.4(2) O(132)-Al(3)-C(31) 122.3(2)
O(23)-Al(3)-C(31)
111.2(2) O(132)-Al(3)-O(12a) 87.8(1)
O(23)-Al(3)-O(12a) 101.0(1) C(31)-Al(3)-O(12a) 124.4(2)
Al(1)-O(123)-Al(2)
Al(2)-O(123)-Al(3a)
118.3(2) Al(1)-O(123)-Al(3a) 115.6(1)
96.8(1) Al(1)-O(132)-Al(3)
126.1(2)
91.6(1)
Al(1)-O(132)-Al(2a) 121.8(2) Al(3)-O(132)-Al(2a)
Al(2)-O(23)-Al(3)
Al(2)-O(2)-C(1)
O(1)-C(1)-C(2)
126.1(2) Al(1)-O(1)-C(1)
129.4(3) O(1)-C(1)-O(2)
116.7(4) O(2)-C(1)-C(2)
128.4(3)
126.4(4)
117.0(4)
The latent Lewis acidity of a cage alumoxane is its
ability to cage-open in the presence of a substrate to
provide a Lewis acidic aluminum site. The latent Lewis
acidity is dependent on the steric strain imposed upon
an Al-O bond by the structure of the cage, ΓAl-O. With
a specific substrate, e.g., (dppp)Pd[C(O)R]Cl, there
exists an equilibrium between the closed and open forms
of the alumoxane, e.g., [(tBu)Al(µ3-O)]6 (eq 10). The
to those we have previously reported for other tert-butyl
alumoxane compounds. One unusual feature of the
structure, however, is the presence of the two five-
coordinate aluminum centers, Al(2) and Al(2a). As is
common for five-coordinate aluminum, the geometry
about Al(2) is that of a distorted trigonal bipyramid:
O(2)-Al(2)-O(132a) ) 163.7(1)° and ∑(Xeq-Al-Xeq) )
359.6(2)°. The two trichloroacetate ligands each bridge
two of the aluminums across opposing six-membered
Al3O3 faces. The bond lengths and angles within the
carboxylate unit are typical of such moieties.46-48 The
ligand bite distance (Al(1)‚‚‚Al(2) ) 3.03 Å) is, however,
significantly smaller than those previously observed for
aluminum carboxylates (3.26-4.18 Å), with a concur-
rent decrease in the Al-O-C angles.
Con clu sion
alumoxane/Pd ratio required to activate all the pal-
ladium centers to catalysis determines the position of
this equilibrium. Thus, in the case of the nonamer and
hexamer (Figure 2) the latent Lewis acidity is greater
in the latter. The position of this equilibrium is also
controlled by the identity of the ligand transferred from
the palladium to the aluminum, i.e., the relative bonding
energies for the Pd-X and Al-X bonds. This is seen
in the observed catalytic activity for (dppp)Pd[C(O)tBu]-
Cl being greater than that for (dppp)Pd(OAc)2 (Figure
1). The identity of X may also control the interaction
between the alumoxane and palladium fragments; e.g.,
acetate may possibly act as a bridging ligand between
palladium and aluminum.
The palladium-catalyzed copolymerization of carbon
monoxide and ethylene to give polyketone polymers has
been accomplished by the use of either (dppp)Pd(OAc)2
or (dppp)Pd[C(O)tBu]Cl in the presence of tert-butyl-
alumoxane cocatalysts.10 The effects on the catalytic
activity of the alumoxane and palladium concentrations
suggest that the active catalytic species is a palladium-
alumoxane complex. The function of the alumoxane in
the catalyst initiation is shown to depend on the choice
of palladium catalyst precursor. With (dppp)Pd[C(O)-
tBu]Cl the alumoxane abstracts chloride, while with
(dppp)Pd(OAc)2 the alumoxane initially alkylates the
palladium and subsequently abstracts the remaining
acetate anion. The catalytic activity is highly dependent
on the structure of the alumoxane employed as the
cocatalyst. A comparative study indicates the cocata-
lytic activity to be [(tBu)Al(µ3-O)]7 > [(tBu)Al(µ3-O)]6 >
Molecu la r Str u ctu r e of [(tBu )6Al6(µ3-O)4(µ-OH)2-
(µ-O2CCCl3)2]. The molecular structure of [(tBu)6Al6(µ3-
O)4(µ-OH)2(µ-O2CCCl3)2] is shown in Figure 8; selected
bond lengths and angles are given in Table 3. The Al6O6
core structure consists of two fused boat conformation
Al3O3 rings and can be described as being derived from
the opening of two opposing edges of the hexagonal-
prismatic core of [(tBu)Al(µ3-O)]6. The geometries and
bond distances around the Al and O atoms are similar
(46) Koide, Y.; Barron, A. R. Organometallics 1995, 14, 4026.
(47) Sobota, P.; Mustafa, M. O.; Utko, J .; Lis, T. J . Chem. Soc.,
Dalton Trans. 1990, 1809.
(48) The molecular structure of [(tBu)2Al(µ-O2CPh)]2 has been
determined by X-ray crystallography: C-O ) 1.254(4) and 1.259(5)
Å, O-C-O ) 122.7(3)°: Barron, A. R. Unpublished results.

2224 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
[(tBu)Al(µ3-O)]9 . [(tBu)7Al5(µ3-O)3(µ-OH)2]. This ob-
served cocatalytic activity correlates with the predicted
latent Lewis acidity of the alumoxanes, providing
further evidence for the intimate association of the
alumoxane and the palladium site during catalysis. The
steric effects of the catalyst active site, as determined
by the alkyl bridge length (n) in R2P(CH2)nPR2 and the
alkyl substituents R, were probed for the catalyst
precursor compounds [R2P(CH2)nPR2]Pd[C(O)tBu]Cl.
The concept of pocket angle (or interior cone angles) has
been developed as an aid to understanding the steric
effects of chelating phosphines.
This study has allowed for an understanding of the
identity of the active site in the palladium-alumoxane-
catalyzed copolymerization of ethylene and carbon
monoxide. Our results indicate that consideration of the
transition-metal center is not enough. The alumoxane’s
structure and ability to accept anionic ligands affect
activity. The alumoxane thus has a moderating effect
on the active site, possibly by complexation to the
palladium. A similar effect has been observed for the
zirconocene-alumoxane-catalyzed polymerization of eth-
ylene.12
compounds to study bioinorganic systems.52 Our future
studies will be aimed at further understanding this
enzyme-like picture of inorganic transition-metal-alu-
moxane catalyst systems.
Exp er im en ta l Section
Copolymerizations were carried out in an autoclave (Berg-
hof, Germany) equipped with a Teflon liner (105 mL). Mass
spectra were obtained on a J EOL AX-505 H mass spectrometer
operating with an electron beam energy of 70 eV for EI mass
spectra. Ammonia was used as the reagent gas for CI
experiments unless otherwise mentioned. Infrared spectra
(4000-400 cm-1) were obtained using Nicolet 5ZDX-FTIR and
Perkin-Elmer 1600 series spectrometers; samples were pre-
pared as CH2Cl2 solutions or Nujol mulls. NMR spectra were
obtained on Bruker AM-500, Bruker AM-400, and Bruker AM-
300 spectrometers using (unless otherwise noted) dichlo-
romethane-d2.
All synthetic procedures were performed under purified
nitrogen using standard Schlenk techniques or in an argon
atmosphere VAC glovebox unless otherwise mentioned. Sol-
vents were distilled and degassed prior to use. [(tBu)7Al5(µ3-
O)3(µ-OH)2], [(tBu)Al(µ3-O)]6, [(tBu)Al(µ3-O)]7, and [(tBu)Al(µ3-
O)]9 were prepared as previously reported.14,15 Carbon
monoxide, ethylene, and carbon monoxide/ethylene mixture
(48.7% CO) gases were obtained from Matheson Gas. (dppp)-
Henderson49 has recently suggested that many met-
alloenzymatic reactions may be conceptualized from
prior knowledge in organometallic chemistry. In con-
trast, we believe that the cocatalytic function of the
alumoxane, and the description of the phosphine “pocket”
around the palladium center on which polymerization
occurs, requires a view of transition-metal-alumoxane
catalyst systems that is closer to that of a metalloen-
zyme than to traditional transition-metal catalyst sys-
tems.
t
Pd(OAc)2, dppe, dppb, dmpe, dcpe, Pd(PPh3)4 (Strem), BuC-
t
(O)Cl, EtC(O)Cl, PhC(O)Cl, and BuMgCl (Aldrich) were used
as received. Bis(diphenylphosphino)propane (dppp) was re-
crystallized from CH2Cl2.
(P h 3P )2P d [C(O)tBu ]Cl. To a solution of Pd(PPh3)4 (1.55
t
g, 1.34 mmol) in toluene (80 mL) was added BuC(O)Cl (0.16
mL, 1.40 mmol). The resulting solution was stirred at ambient
temperature for 1 h under a dimmed light. Adding Et2O (80
mL) and cooling to -20 °C yielded a light yellow precipitate,
which was filtered and dried under vacuum. Yield: 0.66 g,
66%. 1H NMR: δ 7.75 (12H, s, o-CH), 7.39-7.44 (18H, m,
m-,p-CH), 0.57 [9H, s, C(CH3)3].
(d p p p )P d [C(O)tBu ]Cl. (Ph3P)2Pd[C(O)tBu]Cl (0.66 g, 0.88
mmol) was suspended in toluene (50 mL), and dppp (0.36 g,
0.88 mmol) was added. The solution was stirred overnight.
Addition of Et2O (50 mL) gave a pale yellow precipitate, which
was recrystallized from CH2Cl2. Yield: 0.31 g, 55%. 1H
NMR: δ 7.9-7.5 (8H, br, o-CH), 7.38 (8H, br, m-CH), 7.29 (4H,
br, p-CH), 2.58 [2H, q, J (H-H) ) 5.0 Hz, P-CH2CH2], 2.08
and 2.39 (4H, m, P-CH2), 0.83 [9H, s, C(CH3)3].
The Lewis acidic abstraction of an anionic ligand from
a transition metal (cf. eq 4) is well understood, and we
have shown that the ability of any alumoxane to do so
is related to its latent Lewis acidity (cf. eq 10). In
contrast to previous proposals,40-43 we propose that
instead of acting as a simple noncoordinating anion, the
alumoxane is complexed to the “cationic” palladium
center during some or all of the steps in the catalytic
cycle. Thus, the alumoxane may be thought of as having
a stabilizing/cooperating effect on the “cationic” pal-
ladium center, in a similar manner to the histidine
ligands present in hemoglobin and cytochrome C oxi-
dase.50,51 The diagrammatic representation of the pal-
ladium-alumoxane complex shown in XVII may be
thought of as a model for a catalyst resting state, with
the active state involving dissociation (either partial or
complete to form an ion pair) of the alumoxane anion.
It is particularly attractive to think of the alumoxane
stabilizing the palladium catalyst at the points around
the catalytic cycle shown in Scheme 2, where the
palladium is shown as being formally three-coordinate.
The effective binding of the alumoxane as well as its
ability to abstract the ancillary ligand (OAc- or Cl-)
thus has a major control over the activity of the catalyst
system. The size (and possibly shape) of the phosphine’s
pocket appears to have an effect analogous to that of
the amino acid residues in the peptide chain surround-
ing a metal center in a metalloenzyme. The reverse
analogy is commonly used to justify the use of model
(d p p e)P d [C(O)tBu ]Cl. This compound was prepared us-
ing a procedure analogous to that described for (dppp)Pd-
[C(O)tBu]Cl. Yield: 40%. 1H NMR: δ 7.84 (12H, m, o-CH),
7.74 (6H, m, p-CH), 7.53 (12H, m, m-CH), 2.50 (2H, br,
P-CH2), 0.88 [9H, s, C(CH3)3].
(d p p b)P d [C(O)tBu ]Cl. This compound was prepared us-
ing a procedure analogous to that described for (dppp)Pd-
[C(O)tBu]Cl. Yield: 31%. 1H NMR: δ 7.67 (12H, m, o-CH),
7.34 (6H, m, p-CH), 7.28 (12H, m, m-CH), 2.4-1.8 (8H, m,
CH2), 0.80 [9H, s, C(CH3)3].
(d m p e)P d [C(O)tBu ]Cl. (Ph3P)2Pd[C(O)tBu]Cl (1; 0.08 g,
0.11 mmol) was suspended in toluene (70 mL), and dmpe (17.8
µL, 0.11 mmol) was added. The solution was stirred for 3 h,
and then Et2O (50 mL) was added to yield a pale yellow
precipitate. The precipitate was collected by filtration and
(52) See for example: (a) Tang, S. C.; Koch, S.; Papaefthymiou, G.
C.; Foner, S.; Frankel, R. B.; Ibers, J . A.; Holm, R. H. J . Am. Chem.
Soc. 1976, 98, 2414. (b) Lane, R. W.; Ibers, J . A.; Frankel, R. B.;
Papaefthymiou, G. C.; Holm, R. H. J . Am. Chem. Soc. 1977, 99, 84. (c)
Collman, J . P. Acc. Chem. Res. 1977, 10, 265. (d) Groves, J . T. Adv.
Inorg. Biochem. 1979, 1, 119. (e) Armstrong, W. H.; Spool, A.;
Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S. J . J . Am. Chem. Soc.
1984, 106, 3653. (f) Spool, A.; Williams, I. D.; Lippard, S. J . Inorg.
Chem. 1985, 24, 2156. (g) Caradonna, J . P.; Reddy, P. R.; Holm, R. H.
J . Am. Chem. Soc. 1988, 110, 2139. (h) Craig, J . A.; Holm, R. H. J .
Am. Chem. Soc. 1989, 111, 2111.
(49) Henderson, R. A. J . Chem. Soc., Dalton Trans. 1995, 177.
(50) Dickerson, R. E.; Geis, I. Hemoglobin; Benjamin/Cummings:
Menlo Park, CA, 1983.
(51) Meyer, T. E.; Kamen, M. D. Adv. Protein Chem. 1982, 35, 105.

Alumoxanes as Cocatalysts
Organometallics, Vol. 15, No. 9, 1996 2225
recrystallized from CH2Cl2/hexane. Yield: 73%. 1H NMR: δ
1.91 (4H, m, PCH2), 1.62 (12H, s, PCH3), 1.15 [9H, s, C(CH3)3].
(d cp e)P d [C(O)tBu ]Cl. This compound was prepared using
a procedure analogous to that described for (dmpe)Pd[C(O)tBu]-
Cl. Yield: 55%. 1H NMR: δ 2.00 (4H, m, PCH2), 1.82 (4H,
m, â-CH2), 1.72 (2H, m, δ-CH2), 1.55 (1H, m, R-CH2), 1.32 (4H,
m, γ-CH2), 1.23 [9H, s, C(CH3)3].
(P h 3P )2P d [C(O)P h ]Cl. This compound was prepared us-
ing a procedure analogous to that described for (Ph3P)2Pd-
[C(O)tBu]Cl. Yield: 90%. 1H NMR: δ 7.66 (12H, s, o-CH),
7.55 [2H, m, o-CH, C(O)Ph], 7.38 (6H, s, p-CH), 7.29 (12H, m,
m-CH), 7.18 [1H, m, p-CH, C(O)Ph], 7.00 [2H, m, p-CH, C(O)-
Ph].
(d p p p )P d [C(O)P h ]Cl. This compound was prepared using
a procedure analogous to that described for (dppp)Pd[C(O)tBu]-
Cl. Yield: 70%. 1H NMR: δ 7.9-7.5 [10H, br, o-CH, dppp
and C(O)Ph], 7.38 (8H, br, m-CH), 7.29 (4H, br, p-CH), 7.10
[1H, m, p-CH, C(O)Ph], 7.00 [2H, m, p-CH, C(O)Ph], 2.58 (2H,
m, P-CH2CH2), 2.0 and 1.8 (4H, m, P-CH2).
(P h 3P )2P d [C(O)Et]Cl. This compound was prepared using
a procedure analogous to that described for (Ph3P)2Pd[C(O)tBu]-
Cl. Yield: 95%. 1H NMR: δ 7.75 [12H, dd, J (P-H) ) 12.0
Hz, J (H-H) ) 8.0 Hz, o-CH], 7.46 [6H, t, J (H-H) ) 8 Hz,
p-CH], 7.42 (12H, dd, J (H-H) ) 8.0 Hz, m-CH), 1.95 [2H, q,
J (H-H) ) 7.0 Hz, C(O)CH2CH3], -0.51 [3H, t, J (H-H) ) 7.0
Hz, C(O)CH2CH3].
(d p p p )P d [C(O)Et]Cl. This compound was prepared using
a procedure analogous to that described for (dppp)Pd[C(O)tBu]-
Cl. Yield: 90%. 1H NMR: δ 7.77 (8H, m, o-CH), 7.47 (12H,
m, m-,p-CH), 2.41 [2H, m, P-CH2CH2], 1.96 [2H, q, J (H-H)
) 8.0 Hz, C(O)CH2CH3], 1.53 (4H, m, P-CH2), -0.05 [3H, t,
J (H-H) ) 8.0 Hz, C(O)CH2CH3].
Attem p ted Syn th esis of (d p p p )P d (tBu )(OAc). To a
solution of (dppp)Pd(OAc)2 (0.30 g, 0.47 mg) in CH2Cl2 (50 mL)
t
was added BuMgCl (0.29 mL as 2.0 M Et2O, 0.47 mmol). The
resultant berry red solution was added dropwise to hexane (30
mL). A white precipitate was removed by filtration, and then
the volume of the filtrate was reduced approximately by half
in vacuo. The filtrate was kept at -20 °C to provide ruby red
crystals of (dppp)Pd(tBu)Cl. Yield: <5%. 1H NMR: δ 7.79
(12H, m, o-CH), 7.53 (6H, m, p-CH), 7.45 (12H, m, m-CH), 2.00
(2H, br, P-CH2CH2), 2.43 (4H, m, P-CH2), 0.08 [9H, s,
C(CH3)3].
[(tBu )6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2]. [(tBu)Al(µ3-O)]6 (300
mg, 0.50 mmol) was dissolved in hexane (5 mL) and the
resulting solution added to a solution of HO2CCCl3 (82 mg,
0.50 mmol) in hexane (5 mL). After the mixture was stirred
for 30 min, a colorless precipitate was formed. Further solid
was precipitated by cooling (-24 °C) for 2 h. All of the solid
was collected by filtration and then recrystallized from hexane.
Yield: ca. 40%. IR (cm-1): 2922 (m), 2869 (w), 2869 (m), 2348
(m), 1467 (s), 1285 (m), 1256 (w), 1200 (w), 1121 (m), 1074
(m), 990 (m), 703 (w), 617 (w). 1H NMR (C6D6): δ 2.49 (2H, s,
OH), 1.19 [36H, s, C(CH3)3], 1.02 [18H, s, C(CH3)3].
Cop olym er iza tion of Ca r bon Mon oxid e w ith Eth ylen e.
In a typical reaction run, alumoxane (0.025 mmol) was placed
in the autoclave under an N2 atmosphere. The autoclave was
then purged with dry carbon monoxide gas for 5 min. A stock
solution (0.33 mM in dichloromethane) of (dppp)Pd(OAc)2 (15
mL) was transferred into the autoclave under carbon monoxide
atmosphere using an appropriate syringe. The autoclave was
then pressurized with the reactant gases at room temperature.
When carbon monoxide and ethylene were applied separately,
carbon monoxide was applied first. The autoclave was placed
in a heating unit, and the reaction was carried out at 60 °C
with vigorous stirring. At the end of a set period of time,
heating was discontinued and the autoclave was cooled to
ambient temperature for 10 min before it was disassembled.
Following filtration, the product was allowed to dry in air to
provide a white powder. Where necessary, the product was
washed with hexane (10 mL) and subsequently with CH2Cl2
(10 mL).

2226 Organometallics, Vol. 15, No. 9, 1996
Koide et al.
En d -Gr ou p An a lysis for P olyk eton es P r ep a r ed Usin g
(d p p p )P d [C(O)R]Cl. A copolymerization reaction was car-
ried out with (dppp)Pd[C(O)R]Cl and [(tBu)Al(µ3-O)]6, under
typical reaction conditions (see above), for ca. 15 min. The
product was filtered and dissolved in HOC(H)(CF3)2/benzene-
d6. Any impurities and insoluble materials were removed by
filtration before being submitted to mass spectral analysis.
Individual volatile oligomers were characterized by GC-MS.
crystal lattice of each structure; however, in the case of (dppp)-
Pd[C(O)tBu]Cl the solvent was disorded, such that the two
chlorine atoms were spread over three sites in a 0.85:0.85:0.3
ratio. All non-hydrogen atoms were refined anisotropically,
except the low-occupancy chlorine atoms in the dppp com-
pound. Hydrogen atoms were included with fixed thermal
parameters and constrained to “ride” upon the appropriate
atoms (d(C-H) ) 0.95 Å, U(H) ) 1.3Beq(C)). All computations
were performed using MolEN.53 A summary of cell param-
eters, data collection, and structure solution is given in Table
4. Scattering factors were taken from ref 54.
t
R ) Bu , Et[C(O)C2H4]2C(O)tBu . MS (EI, %): m/z 226
(M+, 100). 13C NMR: δ 35.6 [C(O)CH2CH2C(O)], 44.3 [C(CH3)3],
25.5 [C(CH3)3], 33.3 [C(O)CH2CH3], 6.7 [C(O)CH2CH3].
R ) P h , Et[C(O)C2H4]n C(O)P h . 13C NMR: δ 133.9 (R-C,
Ph), 131.8 (p-CH, Ph), 128.9 (br, o-,m-CH, Ph), 35.6 [C(O)-
CH2CH2C(O)], 29.6 [C(O)CH2CH3], 6.7 [C(O)CH2CH3].
En d -Gr ou p An a lysis for P olyk eton es P r ep a r ed Usin g
(d p p p )P d (OAc)2. This was carried out in a manner analo-
gous to that described above, except (dppp)Pd(OAc)2 was used
as the palladium catalyst. The presence of terminal ethyl and
vinyl groups was confirmed by 13C NMR: δ 134.0 [-C(O)-
CHdCH2], 129.1 [-C(O)CHdCH2], 29.3 [C(O)CH2CH3], 1.2
[C(O)CH2CH3]. Individual volatile oligomers were character-
ized by GC-MS.
CH3CH2[C(O)C2H4]4C(O)CH2CH3. MS (EI, %): m/z 310
(M+, 10), 281 (M+ - Et, 20), 253 [M+ - C(O)Et, 13], 225
[{C(O)C2H4}3C(O)Et, 80], 197 [{C2H4C(O)}3Et, 65], 141
[{C2H4C(O)}2Et, 55], 113 [C(O)C2H4C(O)Et, 80], 85 [C2H4C-
(O)Et, 25].
CH3CH2[C(O)C2H4]3C(O)CHdCH2. MS (EI, %): m/z 251
(M+ - H, 60), 223 (M+ - Et, 75), 195 [M+ - C(O)Et, 40], 169
[{C(O)C2H4}2C(O)Et, 30], 141 [{C2H4C(O)}2Et, 63], 113 [C(O)-
C2H4C(O)Et, 100], 85 [C2H4C(O)Et, 35].
A crystal of [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2CCCl3)2] was
mounted in a glass capillary attached to the goniometer head
of a Nicolet R3m/V four-circle diffractometer. Data collection
and unit cell and space group determination were all carried
out in a manner previously described in detail.55 The struc-
tures were solved using the direct methods program XS,56
which readily revealed the positions of the Al, O, and some of
the C atoms. Subsequent difference Fourier maps revealed
the position of all of the non-hydrogen atoms. Subsequently,
full refinement was successful. All the hydrogen atoms were
placed in calculated positions (Uiso ) 0.08; d(C-H) ) 0.96 Å)
for refinement. Neutral-atom scattering factors were taken
from the usual source.54 Refinement of positional and aniso-
tropic thermal parameters led to convergence (see Table 4).
Ack n ow led gm en t. Financial support of this work
was provided by the Office of Naval Research (A.R.B.)
and the Robert E. Welch Foundation (S.G.B.). We
gratefully acknowledge one of the reviewers for insight
into the effect of large pocket angles on catalytic activity.
CH3CH2[C(O)C2H4]4C(O)CHdCH2. MS (EI, %): m/z 308
(M+, 10), 279 (M+ - Et, 15), 251 [M+ - C(O)Et, 31], 223 [{C-
(O)C2H4}3C(O)CHdCH2, 62], 141 [{C2H4C(O)}2Et, 66], 113
[C(O)C2H4C(O)Et, 100], 85 [C2H4C(O)Et, 29].
Su p p or tin g In for m a tion Ava ila ble: Full tables of bond
lengths and angles and positional and thermal parameters for
(dppp)Pd[C(O)tBu]Cl,(dppe)Pd[C(O)tBu]Cl,(dmpe)Pd[C(O)tBu]-
Cl, (dcpe)Pd[C(O)tBu]Cl, and [(tBu)6Al6(µ3-O)4(µ-OH)2(µ-O2-
CCCl3)2] (31 pages). Ordering information is given on any
current masthead page.
Cr ysta llogr a p h ic Stu d ies. Crystals of (dppp)Pd[C(O)tBu]-
Cl, (dppe)Pd[C(O)tBu]Cl, (dmpe)Pd[C(O)tBu]Cl, and (dcpe)Pd-
[C(O)tBu]Cl were sealed in glass capillaries under argon and
mounted on the goniometer of the University of North Texas,
Department of Chemistry’s Enraf-Nonius CAD-4 automated
diffractometer. Data collection and cell determinations were
performed in a manner previously described,14 using either the
ω or the θ-2θ scan technique. Pertinent details are given in
Table 4. The structures were solved by Patterson and Fourier
methods and the models refined using full-matrix least-
squares techniques. A molecule of CH2Cl2 was found in the
OM9508492
(53) MolEN, An Interactive Structure Solution Program; Enraf-
Nonius: Delft, The Netherlands, 1990.
(54) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K., 1974; Vol. 4.
(55) Healy, M. D.; Wierda, D. A.; Barron, A. R. Organometallics
1988, 7, 2543.
(56) Nicolet Instruments Corp., Madison, WI, 1988.