Ethylene oligomerization catalyzed by a unique phosphine - Oxazoline palladium(II) complex. Propagation and chain transfer mechanisms

Article abstract of DOI:10.1021/om061025v

Palladium(II)-catalyzed ethylene oligomerization reactions using a new [(P∧N)Pd(CH₃)(NCArF)][SbF₆] complex, 2b (P∧N = phosphine-oxazoline, Ar_F = 3,5-(CF₃)₂C ₆H₃), are described. Analysis of the X-ray crystal structure of the (P∧N)Pd(CH₃)Cl precatalyst, 2a, reveals a unique axial Pd...H interaction (2.33 A) with a ligand C_sp3-H bond. Exposure of this complex to ethylene in methylene chloride produces a Schulz-Flory distribution of α-olefins ranging from C₄ to C₂₄. Effects of varying ethylene pressure and reaction temperature on oligomerizations are reported. The turnover frequency exhibits saturation behavior as ethylene pressure is increased; carrying out a Lineweaver - Burk analysis provides a barrier of 17.5 kcal/mol for migratory insertion of the palladium alkyl ethylene species. The invariance of the SchulzFlory constant with added quantities of nitrile suggests that chain transfer occurs through a process best described as chain transfer to monomer. The catalyst resting state is shown to vary as a function of ethylene pressure and nitrile concentration. Oligomerization experiments performed with a related phosphine-oxazoline complex lacking Pd...H-C_sp3 axial interaction lead only to ethylene dimerization.

Full text of DOI:10.1021/om061025v

Organometallics 2007, 26, 1261-1269
1261
Ethylene Oligomerization Catalyzed by a Unique
Phosphine-Oxazoline Palladium(II) Complex. Propagation and
Chain Transfer Mechanisms
Mark D. Doherty, Ste´phane Trudeau, Peter S. White, James P. Morken, and
Maurice Brookhart*
Department of Chemistry, The UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599
ReceiVed NoVember 7, 2006
Palladium(II)-catalyzed ethylene oligomerization reactions using a new [(P∧N)Pd(CH3)(NCArF)][SbF6]
complex, 2b (P∧N ) phosphine-oxazoline, ArF ) 3,5-(CF3)2C6H3), are described. Analysis of the X-ray
crystal structure of the (P∧N)Pd(CH3)Cl precatalyst, 2a, reveals a unique axial Pd‚‚‚H interaction (2.33
Å) with a ligand Csp3-H bond. Exposure of this complex to ethylene in methylene chloride produces a
Schulz-Flory distribution of R-olefins ranging from C4 to C24. Effects of varying ethylene pressure and
reaction temperature on oligomerizations are reported. The turnover frequency exhibits saturation behavior
as ethylene pressure is increased; carrying out a Lineweaver-Burk analysis provides a barrier of 17.5
kcal/mol for migratory insertion of the palladium alkyl ethylene species. The invariance of the Schulz-
Flory constant with added quantities of nitrile suggests that chain transfer occurs through a process best
described as chain transfer to monomer. The catalyst resting state is shown to vary as a function of
ethylene pressure and nitrile concentration. Oligomerization experiments performed with a related
phosphine-oxazoline complex lacking Pd‚‚‚H-Csp3 axial interaction lead only to ethylene dimerization.
The concept of incorporating steric bulk into the axial sites
of d8 square-planar complexes to inhibit chain transfer has
provided a design feature that has been used to prepare several
other polymerization and oligomerization catalysts. Highly
active neutral Ni(II) catalysts for ethylene polymerization have
been prepared from monoanionic salicylaldiminato ligands
bearing an ortho-disubstituted N-aryl ring.18,19 Similar systems
based on anilinotropone and related ligands also polymerize
ethylene to give high molecular weight polymers with low
molecular weight distributions.20-23 Other related Pd(II) systems
for ethylene polymerization/oligomerization that incorporate one
or two ortho-disubstituted aryl groups into the ligand backbone
include bis-aryl-substituted phosphindene (P∧P) complexes
(polymerization),24 mixed aryl phosphinidene-arylimine (P∧N)
complexes (oligomerization),25 arylimine-phosphine (N∧P)
Introduction
The polymerization of ethylene and R-olefins using late
transition metal complexes has been an active area of research
over the last decade.1-5 Most thoroughly studied have been
cationic Ni(II) and Pd(II) aryl-substituted R-diimine complexes
bearing ortho substituents, which are effective catalysts for the
polymerization of ethylene and R-olefins to high molecular
weight materials through a coordination-insertion mechanism.1-10
The orthogonal orientation of the two N-aryl rings with respect
to the metal square plane positions the ortho substituents above
and below the axial coordination sites of the metal. This feature
has been shown to be responsible for inhibiting chain transfer
relative to chain propagation, thus resulting in polymer formation
from ethylene and other olefinic monomers.3,7,11-17
(13) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001,
123, 11539-11555.
* Corresponding author. E-mail: mbrookhart@unc.edu.
(1) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38, 428-447.
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1100.
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(3) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203.
(15) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am.
Chem. Soc. 1997, 119, 6177-6186.
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B 1997, 101, 7877-7880.
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Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460-462.
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Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149-3151.
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22, 3533-3545.
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1995, 117, 6414-6415.
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Soc. 1998, 120, 888-899.
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1999, 121, 10634-10635.
(11) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L.
K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320-2334.
(12) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,
M. J. Am. Chem. Soc. 2000, 122, 6686-6700.
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21.
10.1021/om061025v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/20/2007

1262 Organometallics, Vol. 26, No. 5, 2007
Doherty et al.
Given that the presence of axial bulk has a significant impact
on catalyst reactivity, the axial Pd‚‚‚H interaction of 1 led us
to wonder if this axial interaction would provide sufficient steric
bulk to effectively retard chain transfer and result in oligomer-
ization or polymerization of ethylene. We report here the
synthesis of new (phosphine-oxazoline)palladium(II) com-
plexes related to 1 and mechanistic studies of their activity as
ethylene oligomerization catalysts.
Results
Figure 1. (phosphine-oxazoline)PdCl2.
Synthesis of [(L∧L′)Pd(CH3)(NCArF)][A] (A ) SbF6,
B(ArF)4; ArF ) 3,5-(CF3)2C6H3) Complexes. Treatment of
(COD)PdMeCl with the appropriate bidentate ligand (2-4) led
to isolation of the corresponding (L∧L′)PdMeCl complexes, 2a-
4a, as light yellow solids, which have been fully characterized
complexes (oligomerization),26 arylphosphinidene-thioaryl (P∧S)
complexes (oligomerization/polymerization),25 and phena-
cylphosphine (P∧O) complexes (oligomerization).27,28
A number of bidentate Pd(II) complexes lacking ligand
substitution patterns that place substituents near axial sites prove
to be simple ethylene dimerization catalysts.29-33 For example
protonation of the (1,10-phenanthroline)PdMe2 complex gener-
ates an ethylene dimerization catalyst in which the resting state
is the ethyl ethylene complex (1,10-phenanthroline)Pd(C2H4)(CH2-
CH3)+. Several simple bidentate phosphine Pd(II) complexes
also exhibit dimerization catalysis.32,33 In cases where substit-
uents on phosphorus are small, the ethyl ethylene complex,
(P∧P)Pd(C2H4)(CH2CH3)+, is the resting state, but in cases
where phosphorus bears larger substituents the agostic ethyl
complex, (P∧P)Pd(CH2CH2-µ-H)+, is the resting state and the
turnover frequency is first-order in ethylene.32
1
by H, 13C, and 31P{1H} NMR spectroscopy as well as X-ray
crystallography (Scheme 1). In complex 2a the 1H NMR
chemical shift of the benzylic hydrogen is shifted downfield
with respect to the free ligand at 8.83 ppm (∆δ ) 2.68 ppm)
and coupled to phosphorus (2JHP ) 4 Hz), analogous to complex
1. The CH3 signals for 2a and 3a appear at 0.62 ppm (3JHP
)
4 Hz) and 0.41 ppm (3JHP ) 3 Hz), respectively, as doublets
due to phosphorus coupling, while the CH3 signal for 4a appears
at 1.19 ppm as a singlet.
Single crystals of 2a and 3a suitable for X-ray diffraction
analysis were obtained by slow evaporation of dichloromethane-
d2 and dichloromethane solutions, respectively. An ORTEP
drawing and key bond distances and angles of 2a are given in
Figure 2, while those for 3a are given in Figure 3 (crystal-
lographic data are summarized in Table 5, see Experimental
Section). Both complexes are four-coordinate with square-planar
geometry, the most notable features being the cis orientation
of the methyl and phosphine ligands in both complexes as well
as the axial interaction with the C-H bond of the ligand
backbone in 2a (Pd(1)-H(11) 2.33 Å, Pd(1)-H(11)-C(11)
144.1°). As with complex 1, this interaction is most likely a
result of steric constraints imparted to the ligand backbone upon
coordination to palladium.
In connection with other research,34 the (phosphine-oxazo-
line)palladium(II) dichloride complex, 1, shown in Figure 1, was
synthesized. Examination of the X-ray crystal structure revealed
an axial interaction between the Csp3-H bond of the ligand back-
bone and the palladium with a Pd‚‚‚H separation of 2.32 Å and
a Pd‚‚‚H-Csp3 angle of 144.3°. Upon coordination to palladium,
1
the H NMR chemical shift of the benzylic proton exhibits a
large downfield shift from 6.15 to 9.42 ppm (∆δ ) 3.27 ppm)
and exhibits phosphorus coupling (2JH,P ) 2.6 Hz). These obser-
vations are consistent with those for Pt(II)35-40 and Pd(II)41,42
complexes possessing similar M‚‚‚H-C interactions, which are
best described as three-center, four-electron interactions.
The cationic nitrile complexes 2b and 3b were prepared by
addition of AgSbF6 and 3,5-bis(trifluoromethyl)benzonitrile to
methylene chloride solutions of 2a and 3a at room temperature
(26) Daugulis, O.; Brookhart, M. Organometallics 2002, 21, 5926-
5934.
(27) Liu, W.; Malinoski, J. M.; Brookhart, M. Organometallics 2002,
21, 2836-2838.
(28) Malinoski, J. M.; Brookhart, M. Organometallics 2003, 22, 5324-
5335.
(29) Andrieu, J.; Braunstein, P.; Naud, F.; Adams, R. D. J. Organomet.
Chem. 2000, 601, 43-50.
(30) An exception to this trend was reported using cationic bis(pyrazolyl)-
pyridine palladium complexes as catalysts for the polymerization of ethylene.
See: Mohlala, M. S.; Guzei, I. A.; Darkwa, J.; Mapolie, S. F. J. Mol. Catal.
A 2005, 241, 93-100.
(31) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-
1138.
(32) Ledford, J.; Shultz, C. S.; Gates, D. P.; White, P. S.; DeSimone, J.
M.; Brookhart, M. Organometallics 2001, 20, 5266-5276.
(33) Shultz, C. S.; Ledford, J.; DeSimone, J. M.; Brookhart, M. J. Am.
Chem. Soc. 2000, 122, 6351-6356.
(34) Trudeau, S.; Morken, J. P. Tetrahedron 2006, 62, 11470-11476.
(35) Albinati, A.; Anklin, C. G.; Pregosin, P. S. Inorg. Chim. Acta 1984,
90, L37-L38.
(36) Anklin, C. G.; Pregosin, P. S. Magn. Reson. Chem. 1985, 23, 671-
675.
1
(Scheme 1). The H NMR spectrum of 2b shows a similar
downfield shift of the benzylic proton with respect to the free
ligand at 8.48 ppm (∆δ ) 2.33 ppm) as well as coupling to
phosphorus (2JHP ) 4 Hz). Both methyl doublets in 2b and 3b
exhibit a slight downfield shift relative to 2a and 3a and appear
at 0.75 ppm (3JHP ) 2 Hz) and 0.49 ppm (3JHP ) 2 Hz),
respectively. Signals for the bound nitrile appear at 8.31 (Hortho
)
and 8.28 (Hpara) ppm for 2b and 8.38 (Hortho) and 8.29 (Hpara
)
ppm for 3b. Complex 4b can be prepared in a similar manner
by treatment of 4a with NaB(ArF)4 and 3,5-bis(trifluoromethyl)-
benzonitrile in methylene chloride at room temperature.
Ethylene Oligomerization. The results of a series of ethylene
oligomerization reactions carried out in methylene chloride using
catalysts 2b-4b are summarized in Table 1. Complex 2b
produces a Schulz-Flory distribution of olefins containing an
even number of carbons. The Schulz-Flory constant, R,
represents the probability of chain propagation and is calculated
from the ratio of Cn+2/Cn oligomers produced as shown in eq
(37) Albinati, A.; Anklin, C. G.; Ganazzoli, F.; Ruegg, H.; Pregosin, P.
S. Inorg. Chem. 1987, 26, 503-508.
(38) Albinati, A.; Arz, C.; Pregosin, P. S. Inorg. Chem. 1987, 26, 508-
513.
(39) Albinati, A.; Arz, C.; Pregosin, P. S. J. Organomet. Chem. 1988,
356, 367-379.
(40) Albinati, A.; Pregosin, P. S.; Wombacher, F. Inorg. Chem. 1990,
29, 1812-1817.
(41) Mann, B. E.; Bailey, P. M.; Maitlis, P. M. J. Am. Chem. Soc. 1975,
97, 1275-1276.
(42) Roe, D. M.; Bailey, P. M.; Moseley, K.; Maitlis, P. M. J. Chem.
Soc., Chem. Commun. 1972, 1273-1274.

Phosphine-Oxazoline Pd(II) Complex
Organometallics, Vol. 26, No. 5, 2007 1263
Scheme 1. Synthesis of Cationic Pd(II)-CH3 Complexes
1.43 The primary products are linear R-olefins with the remainder
(up to 25%) being made up of linear 2-alkenes (as determined
by GC analysis). Branched olefins are not observed under any
reaction conditions examined. While complex 2b produces a
distribution of olefins, catalysis employing complexes 3b and
4b produces solely butenes under the reaction conditions;
therefore the remainder of this section will deal with catalyst
2b.
be described using the Michaelis-Menton equation (eq 2),
which can be rearranged into a linear expression (eq 3). A plot
of 1/TOF versus 1/[PC2H4], a Lineweaver-Burk plot, should
yield a straight line with a y-intercept at 1/TOFMAX and a slope
of Km/TOFMAX. TOFMAX represents the turnover frequency
under conditions where the reaction is saturated in ethylene.
The Lineweaver-Burk plot of data from Table 1 (entries 2, 5,
and 6) as shown in Figure 4 provides a TOFMAX of 3600 mol
ethylene per mol Pd per hour and kobs ) 1.0 s-1 at 25 °C, which
corresponds to ∆Gq ) 17.5 kcal/mol.
rate of propagation
rate of propagation + rate of chain transfer
R )
)
moles of Cn+2
(1)
moles of Cn
The oligomerization data presented in Table 1 exhibit several
trends. An increase in the ethylene pressure results in higher
observed turnover frequency (TOF) (entries 2, 5, and 6) while
not significantly altering the R values. However, doubling the
pressure (entry 2 vs 5) does not result in a doubling of the TOF,
while increasing the pressure by a factor of 3.5 (entry 2 vs 6)
does not result in a similar increase in the TOF. This indicates
that at higher ethylene pressures the reaction is approaching
saturation behavior. This approach to saturation behavior can
Figure 3. ORTEP structure of (PHOX)PdMeCl‚CH2Cl2, 3a‚CH2-
Cl2. The CH2Cl2 has been omitted for clarity. Selected bond lengths
(Å) and angles (deg): Pd(1)-C(1) 2.097(3), Pd(1)-Cl(1) 2.369-
(2), Pd(1)-N(2) 2.141(3), N(2)-C(6) 1.276(4), C(6)-C(7) 1.469-
(4), C(7)-C(12) 1.415(4), P(1)-C(12) 1.830(3), Pd(1)-P(1)
2.208(2); C(1)-Pd(1)-Cl(1) 88.69(1), Cl(1)-Pd(1)-N(2) 92.18-
(1), N(2)-Pd(1)-P(1) 84.41(1), P(1)-Pd(1)-C(1) 94.69(1), Pd-
(1)-N(2)-C(6) 130.0(2), N(2)-C(6)-C(7) 127.0(2), C(6)-C(7)-
C(12) 122.4(2), C(7)-C(12)-P(1) 120.5(2), C(12)-P(1)-Pd(1)
108.48(1).
Table 1. Oligomerization of Ethylene in Methylene Chloride
[cat]
P(C2H4)
(psig)
T
(°C)
time
(h)
entry catalyst (×105 M)
Ra
TOFb
Figure 2. ORTEP structure of (StePHOX)PdMeCl, 2a. Selected
bond lengths (Å) and angles (deg): Pd(1)-C(1) 2.046(4), Pd(1)-
Cl(1) 2.364(9), Pd(1)-N(2) 2.173(3), N(2)-C(6) 1.268(5), C(6)-
C(7) 1.500(6), C(7)-C(11) 1.524(5), C(11)-C(12) 1.509(6),
C(12)-C(17) 1.414(5), C(17)-P(1) 1.838(4), P(1)-Pd(1) 2.225-
(2), Pd(1)-H(11) 2.33, H(11)-C(11) 0.98; C(1)-Pd(1)-Cl(1)
90.64(1), Cl(1)-Pd(1)-N(2) 86.90(9), N(2)-Pd(1)-P(1) 90.47-
(9), P(1)-Pd(1)-C(1) 91.97(1), Pd(1)-N(2)-C(6) 132.7(3), N(2)-
C(6)-C(7) 127.3(4), C(6)-C(7)-C(11) 116.6(3), C(7)-C(11)-
C(12) 115.5(4), C(11)-C(12)-C(17) 123.3(4), C(12)-C(17)-
P(1) 125.2(3), C(17)-P(1)-Pd(1) 117.04(1), Pd(1)-H(11)-C(11)
144.1.
1
2
3
4
5
6
7
8
9
2b
2b
2b
2b
2b
2b
2b
3b
4b
9.7
9.7
9.7
9.7
9.7
9.7
9.7
9.7
9.7
200
200
200
200
400
700
200
200
200
25
25
25
25
25
25
50
25
25
0.53
0.53
0.54
0.55
0.49
0.53
0.35
N/Ac
N/Ac
0.5
1
3
8.33
1
1
1
1
1
1310
1560
980
790
2260
2560
15 760
b
a Schulz-Flory R. Turnover frequency ) mol of ethylene consumed/
c
mol of Pd catalyst per hour. Only butenes produced.

1264 Organometallics, Vol. 26, No. 5, 2007
Doherty et al.
V
MAX[S]
V )
(2)
(3)
{Km + [S]}
Km
VMAX[S]
1
V
1
)
+
VMAX
An increase in reaction temperature from 25 °C to 50 °C
results in a 10-fold increase in catalyst activity (entry 2 vs 7).
This increase in activity is also accompanied by an increase in
the rate of chain termination relative to propagation, as can be
seen by the lower value of the Schulz-Flory constant, R. At
longer reaction times, the catalyst activity decreases as a result
of catalyst decay (Table 1, compare entries 1-4). The Schulz-
Flory R values remain constant, indicating that the decomposi-
tion products, although unknown, do not significantly interfere
with the oligomerization process.
Effects of Excess NCArF, Determination of Keq, and
NCArF Self-Exchange Rates. Addition of excess nitrile to the
reaction mixture inhibits the rate of formation of R-olefins, as
can be seen by the data shown in Table 2. This decrease in
activity is not accompanied by a decrease in the Schulz-Flory
R value, indicating the added nitrile has not affected the ratio
of chain transfer to chain propagation rates. The high concentra-
tions of NCArF necessary to effectively inhibit oligomerization
indicate that under normal oligomerization conditions (no added
nitrile) an alkyl nitrile species is present only in low concentra-
tions relative to other species (see below).
To assess the relative binding affinities of ethylene and 3,5-
bis(trifluoromethyl)benzonitrile, the equilibrium constant for the
reaction shown in Scheme 2 was determined. When complex
2b was treated with ethylene (5, 10, and 20 equiv) at -80 °C,
an equilibrium mixture of 2b and 2c was generated and the
ratio of the two complexes determined by integration of the
two methyl signals at 0.600 and 0.391 ppm, respectively.
Measurement at -80 °C was necessary to avoid complications
from insertion chemistry of 2c and rapid (NMR time scale)
interconversion of these species due to associative ligand
exchange. The measured Keq was 0.13, indicating a ca. 1 order
of magnitude difference in binding affinities. Since the ∆S value
for this equilibrium is expected to be close to zero, the ∆G
value of 0.78 kcal/mol can be used to estimate Keq at higher
temperatures.
The rate of exchange of bound nitrile with free nitrile was
determined using line-broadening analysis of 1H NMR spectra
taken at low temperature (Scheme 3). The line width at half-
height for the bound nitrile in 2b in the absence of free nitrile
was measured in CD2Cl2 at -70 °C. Line widths at half-height
(ω) were measured (in Hz) in the presence of added nitrile at
-70 °C. The change in line width, ∆ω, was found to be
proportional to the concentration of added nitrile consistent with
associative exchange. The second-order rate constants for
exchange were determined using the equation for the slow
exchange approximation, k ) π(∆ω)/[NCArF], where [NCArF]
is the concentration of free nitrile in solution (Table 3). The
rate data and the free energy of activation for this process are
summarized in Table 3.
Figure 4. Lineweaver-Burk plot of ethylene oligomerization data.
Table 2. Effect of Added NCArF on Ethylene
Oligomerization
entry NCArF equiva [NCArF] (×105 M)
R
time (h) TOFb
1
2
3
4
5
6
7
0
3
10
0
29
97
485
970
2425
4850
0.53
0.51
0.50
0.52
0.52
0.53
0.52
1
1
1
2
2
2
2
1560
1230
1380
840
750
610
50
100
250
500
380
a Reaction conditions: [2b] ) 9.7 × 10-5 M, 200 psig of ethylene, 25 °C,
b
100 mL of CH2Cl2. TOF ) mol of ethylene consumed/mol of Pd catalyst
per hour.
Scheme 4). After workup, 5 was isolated as a light yellow solid
and was characterized by 1H, 13C, and 31P{1H} NMR spectros-
1
copy. The H NMR spectrum of 5 shows the benzylic proton
shifted downfield relative to the free ligand at 9.65 ppm (∆δ )
3.50 ppm) and coupled to phosphorus (2JHP ) 6 Hz) as in
complexes 1, 2a, and 2b. The two CH3 signals appear as two
doublets at 0.95 ppm (3JHP ) 7 Hz) and 0.88 (3JHP ) 9 Hz)
ppm, respectively.
Protonation of a CD2Cl2 solution of 5 with [H(OEt2)2]-
[B(ArF)4] at -78 °C followed by addition of 20 equiv of
ethylene generated the palladium(II) methyl ethylene complex
2c (Scheme 4). The rate of the migratory insertion reaction of
2c was monitored by measuring the disappearance of the
1
palladium methyl resonance in the H NMR spectrum at -21,
-15, and -10 °C. Data are summarized in Table 4. First-order
kinetics is observed with an average free energy of activation,
∆Gq, of 19.8 kcal/mol. The subsequent rates of migratory
insertions of higher homologues, (L∧L)Pd((CH2)nCH3)(C2H4)+,
must be considerably faster since ethylene is consumed at a
much faster rate than the decrease of the methyl signal of 2c.
This considerable difference in rate precluded a quantitative
measurement of the rate of subsequent insertion(s) by NMR
spectroscopy.
Discussion
The ethylene oligomerization results presented in Table 1
illustrate the effect of the axial Csp3-H‚‚‚Pd interaction on the
reactivity of palladium(II) complex 2b. Complexes 3b and 4b,
both of which lack any axial bulk, produce butenes, while
complex 2b produces linear R-olefins up to C24. The proposed
oligomerization mechanism for this reaction is similar to that
of related late metal systems43,44 and can be broken up into three
parts: initiation, propagation, and chain transfer. The first
insertion constitutes the “initiation” step and must involve
migratory insertion of the methyl ethylene complex 2c. Given
Synthesis and Migratory Insertion Reaction of [(StePHOX)-
Pd(CH3)(C2H4)][B(ArF)4] (ArF ) 3,5-(CF3)2C6H3), 2c. The
precursor to complex 2c, the palladium(II) dimethyl complex
5, was prepared by addition of ligand 2 to a diethyl ether solution
of (TMEDA)Pd(CH3)2 at -30 °C followed by warming to 25 °C
overnight (TMEDA ) N,N,N′,N′-tetramethylethylenediamine,
(44) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997,
16, 2005-2007.
(43) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74.

Phosphine-Oxazoline Pd(II) Complex
Organometallics, Vol. 26, No. 5, 2007 1265
Scheme 2. Determination of Keq
Scheme 3. Associative Exchange of 2b with Free NCArF
Scheme 4. Synthesis of the Cationic Pd(II) Methyl Ethylene Complex
Scheme 5. Kinetics of Migratory Insertion in 2c
the relative binding affinities of ethylene and 3,5-bis(trifluo-
equiv is present (Table 1) the nitrile complex is present in very
low concentrations relative to the ethylene complex 2c′, and
thus the primary species present in addition to 2c′ is the agostic
species 2d.
romethyl)benzonitrile Keq ≈ 0.27 (extrapolated from -80 °C),
at 25 °C, 200 psi of ethylene, the methyl ethylene complex 2c
must be the major species in solution (2c/2b ≈ 6800).45
Migratory insertion generates a propyl complex that can undergo
chain transfer to yield propylene or further insertions and chain
transfer to yield odd carbon number oligomers. Once an odd
oligomer is produced, the catalyst enters the primary propagation
cycle, which is described in Scheme 6. Since hundreds of
turnovers are achieved prior to analysis of products, the small
fraction of odd oligomers are not a significant fraction of the
product and are not detected.
Scheme 6 is consistent with the dependence of the TOF on
ethylene pressure and the approach to saturation as established
by the Lineweaver-Burk plot. The ratio of the maximum TOF
(ca. 3600 s-1) to the observed TOF at 200 psi ethylene (ca.
1500 s-1) suggests that ca. 40% of the palladium complex under
these conditions is present as alkyl ethylene complex 2c′. The
remainder of the palladium complexes must be an equilibrium
mixture of the nitrile complex 2b′ and the agostic alkyl complex
2d. It is possible that the agostic alkyl complexes are in
equilibrium with olefin hydride complex 2e′, but experimen-
tal12,13,46 and theoretical14,15,47,48 studies on related complexes
suggest the more stable form is the â-agostic species. The
addition of free nitrile retards the rate as the equilibrium shifts
from the ethylene complex to the nitrile complex. The fact that
a large excess of nitrile is necessary to significantly reduce the
TOF (see Table 2) implies that under conditions where only 1
The TOFMAX, 1.0 s-1, is the rate constant of migratory
insertion of 2c′ at 25 °C and corresponds to a ∆Gq ) 17.5 kcal/
mol. The ∆Gq for migratory insertion of the methyl ethylene
complex 2c is ca. 19.8 kcal/mol in the temperature range -10
to -21 °C. This difference is consistent with the observation
(see above) that subsequent insertions occur at faster rates than
the insertion of the methyl complex and with previous observa-
tions of methyl versus ethyl migratory insertion rates.13,46
Insight into the chain transfer mechanism can be gleaned from
the data in Tables 1 and 2. Data in Table 1 indicate there is no
dependence of the Schulz-Flory R value on ethylene concentra-
tion. This is consistent with a chain transfer mechanism
involving either associative displacement of an olefin from an
Table 5. Crystallographic Data for 2a and 3a
2a
3a‚CH2Cl2
formula
fw
C27H29ClNO3PPd
588.33
C23H23Cl3NOPPd
573.14
cryst syst
space group
a (Å)
b (Å)
c (Å)
orthorhombic
P212121
9.6232(4)
11.1381(5)
24.2794(10)
90
monoclinic
P21/c
13.245(18)
10.760(11)
17.492(12)
90
102.08(8)
90
2438(4)
4
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
Table 3. Second-Order Rate Constants for Nitrile Exchange
2602.37(19)
4
1.502
at -70 °C
D
calcd (Mg/m3)
1.562
[2b] (M)
[NCArF] (M)
rate const (M-1 s-1
)
∆Gq (kcal/mol)
λ (Å)
0.71073
0.906
0.71073
1.171
8.09 × 10-3
7.90 × 10-3
8.87 × 102
9.3
9.3
µ (mm-1
)
8.09 × 10-3
3.25 × 10-2
1.17 × 103
cryst dimens (mm3)
T (K)
0.20 × 0.20 × 0.10
0.30 × 0.25 × 0.10
100(1)
298(2)
Table 4. First-Order Rate Constants for Ethylene Insertion
in 2c
2 θ range (deg)
1.68-26.00
26 615
5111
0.0378
0.0867
1.57-28.15
47 387
5921
0.0347
0.0821
no. of rflns
no. of indep rflns
R1
wR2
Rall
GOF
T (°C)
k
ins (×105 s-1
)
∆Gq (kcal/mol)
-10
-15
-21
16.3
10.0
5.3
19.9
19.8
19.6
0.0438
1.063
0.0399
1.139

1266 Organometallics, Vol. 26, No. 5, 2007
Scheme 6. Proposed Mechanism of Ethylene Oligomerization: Propagation
Doherty et al.
Scheme 7. Proposed Mechanism of Ethylene Oligomerization: Chain Transfer
olefin hydride (eq 4) or chain transfer to monomer in which
olefin displacement occurs following intramolecular hydrogen
transfer in the alkyl ethylene complex (eq 5). Clearly in the
chain transfer to monomer mechanism there will be no
dependence of R on ethylene concentration. The ratio of transfer
to propagation rates is simply the ratio of the two first-order
rate constants shown in eq 5. Concerning the associative
displacement mechanism, the ratio of the rates of chain transfer
to propagation is given by eq 6. Since the ethylene complex
2c′ is in equilibrium with the olefin hydride complex 2e′ (eq 4,
[2e′] ) Keq[2c′]/[C2H4]), eq 6 reduces to eq 7, and there is no
dependence of R on ethylene concentration. Thus, in the absence
of added nitrile, the data do not distinguish between the two
chain transfer mechanisms.
The fact that the Schulz-Flory R value is unaffected by even
extremely high concentrations of nitrile (500 equiv, Table 2)
eliminates the associative displacement mechanism. Since the
nitrile has an order of magnitude stronger binding affinity than
ethylene and since we have established that associative self-
exchange in the nitrile complex 2b is extremely facile, olefin
hydride complexes 2e′ should react rapidly with the nitrile to
produce the nitrile hydride complex 2f (eq 4). This provides
another competitive “associative displacement” pathway for
chain transfer (the nitrile hydride would reenter the catalytic
cycle) and the R value would decrease rapidly with an increase
in nitrile concentration, which is inconsistent with data in Table
2. Increase in nitrile concentration as noted above will decrease
the concentration of ethylene complex 2c′ but will not effect
the ratio of kpropagation/ktransfer in eq 5 and thus will not impact R
in the chain transfer to monomer mechanism, consistent with
our observations.
(45) An ethylene concentration of 0.18 M atm-1 at 25 °C in methylene
chloride and the maximum possible concentration of free nitrile under
oligomerization conditions of 9.7 × 10-5 M (see Table 1) were used together
with the equilibrium constant estimated at 25 °C, Keq ) 0.27 (see Scheme
2), to estimate the 2c/2b ratio of ca. 6800 under oligomerization conditions.
The solubility of ethylene in methylene chloride at 25 °C at elevated
pressures was determined previously (see ref 9).
(46) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975-3982.
(47) Musaev, D. G.; Froese, R. D. J.; Svensson, M.; Morokuma, K. J.
Am. Chem. Soc. 1997, 119, 367-374.
(48) Musaev, D. G.; Svensson, M.; Morokuma, K.; Stro¨mberg, S.;
Zetterberg, K.; Siegbahn, P. E. M. Organometallics 1997, 16, 1933-1945.
νtransfer
k
transfer[2e′][C2H4]
kinsertion[2c′]
R )
)
(6)
νpropagation

Phosphine-Oxazoline Pd(II) Complex
ktransferKeq[2c′][C2H4]
Organometallics, Vol. 26, No. 5, 2007 1267
2,34 and 353-55 were prepared according to literature procedures.
Polymer grade ethylene (99.9%) was purchased from Matheson and
used as received. Methylene chloride-d2, toluene-d8, and chloro-
form-d were purchased from Cambridge Isotope Laboratories,
degassed using freeze-pump-thaw techniques, and stored over
4 Å molecular sieves. GC analysis was performed on an Agilent
6850 Series GC System using a J. & W. Scientific HP-1 column
(100% dimethylpolysiloxane, 30 m × 0.32 mm i.d., 0.25 µm film
thickness; temperature progression: 3 min isothermal at 50 °C,
k
transferKeq
)
(7)
kinsertion
kinsertion[2c′][C2H4]
Conclusions
A new phosphine-oxazoline palladium(II) catalyst bearing
an axial Pd‚‚‚H interaction has been developed for ethylene
oligomerization. The oligomers produced are linear olefins of
which the majority (>75%) are R-olefins with a Schulz-Flory
R value of ∼0.5 at 25 °C. The proposed mechanism of
oligomerization beginning with methyl nitrile complex 2b,
[(P∧N)Pd(CH3)(NCArF)][SbF6] (P∧N ) phosphine-oxazoline,
ArF ) 3,5-(CF3)2C6H3), involves insertion of ethylene into a
Pd-CH3+ bond to generate a propyl complex (initiation), which
can undergo chain transfer to yield propylene or further
insertions and chain transfer to yield odd carbon number
oligomers.
Following the first chain transfer event, the catalyst enters
the primary propagation cycle. The major palladium complexes
present under oligomerization conditions are the alkyl agostic
species, 2d, and the alkyl ethylene species, 2c′, and their ratios
depend on ethylene pressure. The turnover frequency varies with
ethylene pressure and follows Michaelis-Menton kinetics.
Analysis of this data using a Lineweaver-Burk plot provides a
maximum TOF of 3600 h-1 at ethylene saturation, which
corresponds to the rate of migratory insertion of the intermediate
palladium alkyl ethylene complex 2c′ (kobs ) 1.0 s-1, ∆Gq )
17.5 kcal/mol at 25 °C). This barrier is consistent with the
insertion barrier of ∆Gq ) 19.8 kcal/mol.
The insensitivity of the Schulz-Flory R value to variations
in ethylene pressure and nitrile concentration together with the
similar binding affinities of nitrile and ethylene and rapid
associative nitrile exchange strongly suggest that chain transfer
occurs via a “chain transfer to monomer” mechanism rather than
by associative displacement of olefin from an olefin hydride
complex.
Ethylene oligomerization performed with complex 3b, which
has similar electronic properties to 2b but lacks the axial
C-H‚‚‚Pd interaction, yields strictly butenes. Other related Pd
complexes lacking substituents that shield the axial site also
show only dimerization activity. These observations highlight
the significance of the axial bulk provided by the ligand Csp3-H
bond in 2b.
1
10 °C/min heat up, 3 min isothermal at 250 °C). All H, 13C, 19F,
and 31P NMR spectra were recorded on Bruker Avance 300, 400,
or 500 MHz spectrometers. Chemical shifts are reported relative
to residual CHCl3 (δ 7.27 for 1H), CHDCl2 (δ 5.32 for 1H), C6D5-
1
CHD2 (quintet centered at δ 2.09, J ) 2.3 Hz for H), CD2Cl2 (δ
54.00 for 13C), C6D5CD3 (δ 20.4 for 13C), and CFCl3 (δ 0.00 19F).
(StePHOX)Pd(CH3)Cl, 2a. A flame-dried Schlenk flask was
charged with (COD)PdMeCl (0.100 g, 0.377 mmol) and methylene
chloride (4 mL). A solution of ligand 2 (0.173 g, 0.399 mmol) in
methylene chloride (6 mL) was added by cannula. The clear solution
was stirred at room temperature for approximately 1 h. The yellow
solution was then concentrated in vacuo to ∼1-2 mL, and hexane
(20 mL) was added to precipitate 2a as a light yellow solid. The
solid was washed with hexane (2 × 5 mL) and dried in vacuo at
room temperature for 2 h (0.194 g, 87%). X-ray quality crystals
were grown via slow evaporation of methylene chloride-d2 from a
concentrated solution of 2a. 1H NMR (CD2Cl2, 400 MHz): δ 8.83
3
2
(1H, dd, JHH ) 8 Hz, JHP ) 4 Hz), 7.97 (1H, dd), 7.63 (1H, t),
7.60 to 7.20 (11H, m), 7.10 (1H, t), 4.23 (2H, overlapping m), 3.98
(1H, dd), 3.74 (1H, m), 2.79 (1H, m), 1.68 (3H, s), 1.67 (3H, s),
0.620 (3H, d, JHP ) 4 Hz). 13C NMR (CD2Cl2, 100.6 MHz): δ
3
165.3, 143.2, 136.9, 133.9, 133.8, 133.2, 132.6, 131.6, 131.4, 131.1,
130.9, 130.3, 129.2, 129.0, 128.7, 128.6, 128.5, 128.3, 127.8, 127.6,
112.1, 77.2, 75.7, 68.9, 26.7, 26.3, 3,68. 31P{1H} NMR (CD2Cl2,
161.9 MHz): δ 23.12 (s). Anal. Calcd for C27H29PNO3ClPd: C,
55.12; H, 4.97; N, 2.38. Found: C, 55.47; H, 4.94; N, 2.33.
(PHOX)PdMeCl, 3a. A flame-dried Schlenk flask was charged
with (COD)PdMeCl (0.076 g, 0.285 mmol) and methylene chloride
(4 mL). A solution of ligand 3 (0.100 g, 0.302 mmol) in methylene
chloride (6 mL) was added by cannula. The clear solution was
stirred at room temperature for approximately 1 h. The yellow
solution was then concentrated in vacuo to ∼1-2 mL, and hexane
(20 mL) was added to precipitate 3a as a light yellow solid. The
solid was washed with hexane (2 × 5 mL) and dried in vacuo at
room temperature for 2 h (0.129 g, 93%). X-ray-quality crystals
were obtained via slow evaporation of methylene chloride from a
1
concentrated solution of 3a. H NMR (CD2Cl2, 300.13 MHz): δ
8.11 (1H, ddd), 7.61 (1H, tt), 7.56 to 7.41 (11H, m), 7.15 (1H,
3
ddd), 4.45 (4H, t), 0.406 (3H, d, JHP ) 3.3 Hz). 31P{1H} NMR
Experimental Section
(CD2Cl2, 121.49 MHz): δ 34.7 (s). Anal. Calcd for C22H21-
NOPClPd‚CH2Cl2‚0.25 C5H12‚0.4 H2O: C, 48.85; H, 4.34; N, 2.35.
Found: C,49.37; H, 4.02; N, 2.55.
(phen)PdMeCl, 4a. This complex was prepared according to
previously reported procedures.56,57 1H NMR (CDCl3, 400.13
MHz): δ 9.46 (1H, bs), 8.98 (1H, bs), 8.50 (1H, d, 3JHH ) 5 Hz),
8.43 (1H, d, 3JHH ) 5 Hz), 7.94 (2H, bs), 7.84 (2H, bs), 1.19 (3H,
s).
General Methods. All manipulations of air- and/or moisture-
sensitive compounds were conducted using standard Schlenk
techniques. Argon was purified by passage through columns of
BASF R3-11 catalyst (Chemalog) and 4 Å molecular sieves.
Toluene, pentane, hexane, methylene chloride, and diethyl ether
were deoxygenated and dried over a column of activated alumina.
Materials. 1,5-Cyclooctadiene, 3,5-bis(trifluoromethyl)benzoni-
trile, 1,10-phenanthroline, and AgSbF6 were used as received from
Aldrich. Na[B(ArF)4] was purchased from Boulder Scientific and
purified according to reported procedures.49 (COD)PdMeCl,50
(TMEDA)Pd(CH3)2,51 H(OEt2)2B(ArF)452 (ArF ) 3,5-(CF3)2C6H3),
[(StePHOX)PdMe(3,5-NCC6H3(CF3)2)][SbF6], 2b. A flame-
dried Schlenk flask was charged with 2a (0.150 g, 0.255 mmol)
and 3,5-NCC6H3(CF3)2 (0.047 mL, 0.280 mmol) in methylene
(53) Dawson, G. J.; Frost, C. G.; Williams, J. M. J.; Coote, S. J.
Tetrahedron Lett. 1993, 34, 3149-3150.
(54) Sprinz, J.; Helmchen, G. Tetrahedron Lett. 1993, 34, 1769-1772.
(55) von Matt, P.; Pfaltz, A. Angew. Chem., Int. Ed. 1993, 32, 566-
568.
(49) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579-
3581.
(50) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13, 303-
306.
(51) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten,
G. Organometallics 1989, 8, 2907-2917.
(52) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920-3922.
(56) Albano, V. G.; Castellari, C.; Cucciolito, M. E.; Panunzi, A.;
Vitagliano, A. Organometallics 1990, 9, 1269-1276.
(57) De Felice, V.; Albano, V. G.; Castellari, C.; Cucciolito, M. E.; De
Renzi, A. J. Organomet. Chem. 1991, 403, 269-277.

1268 Organometallics, Vol. 26, No. 5, 2007
Doherty et al.
chloride (2 mL). A solution of AgSbF6 (0.092 g, 0.268 mmol) in
methylene chloride (3 mL) was added via cannula. The solution
was diluted with methylene chloride (5 mL) and stirred at room
temperature. After ∼30 min, the light brown precipitate (AgCl)
was removed by cannula filtration and washed with methylene
chloride (2 × 5 mL). The washings were combined with the light
yellow filtrate and concentrated in vacuo to ∼1-2 mL, and pentane
(20 mL) was added to precipitate 2b as an off-white solid. The
solid was washed with pentane (2 × 5 mL) and dried in vacuo at
room temperature for 2 h (0.222 g, 85%). 1H NMR (CD2Cl2, 300.13
cannula filtration, washed with pentane (2 × 5 mL), and dried in
vacuo at room temperature for 2 h (2.02 g, 97%). 1H NMR (CD2-
3
4
Cl2, 400.13 MHz): δ 8.94 (1H, dd, JHH ) 5 Hz, JHH ) 1 Hz),
3
4
3
8.87 (1H, dd, JHH ) 5 Hz, JHH ) 1 Hz), 8.67 (1H, dd, JHH ) 8
Hz, 4JHH ) 1 Hz), 8.62 (1H, dd, 3JHH ) 8 Hz, 4JHH ) 1 Hz), 8.44
(2H, bs), 8.39 (1H, bs), 8.08 (1H, d, 8 Hz), 8.05 (1H, d, 8 Hz),
7.96 (1H, dd, JHH ) 8 Hz, JHH ) 5 Hz), 7.93 (1H, dd, JHH ) 8
Hz, JHH ) 5 Hz), 7.72 (8H, s), 7.53 (4H, s), 1.43 (3H, s). Anal.
3
3
3
3
Calcd for C54H26F30N3BPd: C, 46.19; H, 1.87; N, 2.99. Found: C,
46.08; H, 1.92; N, 2.73.
3
2
MHz): δ 8.48 (1H, dd, JHH ) 8 Hz, JHP ) 4 Hz), 8.31 (2H, s,
NCArF o-H), 8.28 (1H, s, NCArF p-H), 8.01 (1H, dd, JHH ) 4.8
and 7.5 Hz), 7.72 (1H, br t, JHH ) 7.5 Hz), 7.67 to 7.25 (10H, m),
7.15 (2H, overlapping dd’s, J ) 1.0 Hz, 1.5, 8.1, and 8.1 Hz,),
4.35 (1H, m), 4.11 (1H, d, 3JHH ) 8.1 Hz), 3.83 (2H, overlapping
(StePHOX)PdMe2, 5. A flame-dried Schlenk flask was charged
with (TMEDA)PdMe2 (0.075 g, 0.297 mmol) and diethyl ether (2
mL) and cooled to -30 °C. A solution of ligand 2 (0.128 g, 0.297
mmol) in diethyl ether (3 mL) was added via cannula. The solution
was diluted with diethyl ether (10 mL) and stirred at -30 °C for
3 h and allowed to warm to room temperature overnight while
stirring. Volatiles were removed in vacuo to give the product 5 as
a yellow solid. The solid was washed with pentane (6 × 10 mL) to
remove remaining starting material and dried in vacuo for 3 h (0.074
m’s), 2.79 (1H, m), 1.73 (3H, s), 1.68 (3H, s), 0.75 (3H, d, 3JHP
)
2.4 Hz). 13C NMR (CD2Cl2, 75.5 MHz): δ 167.9, 141.5, 141.3,
136.9, 133.8, 133.7, 133.3, 133.2, 133.0, 132.0, 131.2, 129.7, 129.6,
129.3, 129.1, 129.0, 128.9, 128.6, 128.5, 127.9, 127.7, 126.3, 125.6,
122.2 (q, 2 CF3,1JCF ) 273.4 Hz), 118.0, 112.3, 111.9, 76.8, 69.4,
26.5, 26.4, 5.8. 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 23.03 (s).
19F NMR (CD2Cl2, 376.5 MHz): δ -64.1 (s, NCArF), -120 to
-140 (br m, SbF6). Anal. Calcd for C36H33F12O3N2PSbPd: C,
42.03; H, 3.23; N, 2.72. Found: C, 41.82; H, 3.03; N, 2.46.
1
3
g, 44%). H NMR (C7D8, 500.13 MHz): δ 9.65 (1H, dd, JHH
)
2
7.0 Hz, JHP ) 6 Hz), 8.02 (1H, dd), 7.35 to 6.75 (17H, m), 4.12
(1H, d), 3.46, (1H, q), 3.21 (1H, q), 2.87 (1H, q), 2.39 (1H, q),
3
1.90 (3H, s), 1.62 (3H, s), 0.95 (3H, d, JHP ) 6.8 Hz), 0.88 (3H,
d, 3JHP ) 8.6 Hz). 13C NMR (C7D8, 125.7 MHz): δ 164.4, 143.9,
137.6, 137.5, 137.2, 135.2, 135.0, 133.5, 133.4, 133.2, 132.8, 131.6,
131.2, 111.4, 77.3, 76.2, 67.3, 58.3, 54.7, 46.0, 34.5, 26.8, 26.1,
22.9, 14.4, 3.7, 2.9, -3.4. 31P{1H} NMR (C7D8, 202.45 MHz): δ
11.78 (s). Anal. Calcd for C28H33O3NPPd: C, 59.11; H, 5.85; N,
2.46. Found: C, 58.94; H, 5.61; N, 2.50.
[(PHOX)PdMe(3,5-NCC6H3(CF3)2)][SbF6], 3b. A flame-dried
Schlenk flask was charged with 3a (0.100 g, 0.205 mmol) and 3,5-
NCC6H3(CF3)2 (0.038 mL, 0.225 mmol) in methylene chloride (2
mL). A solution of AgSbF6 (0.074 g, 0.215 mmol) in methylene
chloride (3 mL) was added via cannula. The solution was diluted
with methylene chloride (5 mL) and stirred at room temperature.
After ∼30 min, the light brown precipitate (AgCl) was removed
by cannula filtration and washed with methylene chloride (2 × 5
mL). The washings were combined with the light yellow filtrate
and concentrated in vacuo to ∼1-2 mL, and pentane (20 mL) was
added to precipitate 3b as an off-white solid. The solid was washed
with pentane (2 × 5 mL) and dried in vacuo at room temperature
General Procedure for Ethylene Oligomerizations. A 300 mL
Parr autoclave was heated under vacuum up to 120 °C and then
was cooled to the desired reaction temperature and backfilled with
argon. In the glovebox, a flame-dried Schlenk flask was charged
with the catalyst, 2b (0.010 g, 0.0097 mmol). The catalyst was
dissolved in methylene chloride (15 mL), undecane was added as
an internal standard (0.011 mL, 0.050 mmol), and the mixture was
cannula transferred into the argon-pressurized autoclave. Fresh
methylene chloride (15 mL) was added to the flask and cannula
transferred into the autoclave. The reaction mixture was diluted
with additional methylene chloride (70 mL), the autoclave was
sealed and pressurized with ethylene to the desired level, and the
stirring motor was engaged. After the predetermined reaction time,
the stirring motor was stopped and the reactor was cooled to 0 °C
to retain as much of the low molecular weight oligomers as possible
and then vented. An aliquot was removed for GC analysis, and the
reaction mixture was quenched by pouring into 200 mL of stirring
methanol. Volatiles were removed on the Rotovap, and the
remaining residue was analyzed by 1H NMR. This procedure was
used for variations in temperature, ethylene pressure, and time. For
the reactions with excess 3,5-bis(trifluoromethyl)benzonitrile, the
desired amount of nitrile was added to the flask prior to cannula
transfer into the autoclave.
1
for 2 h (0.153 g, 81%). H NMR (CD2Cl2, 300.13 MHz): δ 8.38
(2H, s, NCArF o-H), 8.29 (1H, s, NCArF p-H), 8.22 (1H, ddd, J )
1.5 Hz, 4.2 Hz, 8.1 Hz), 7.72 (1H, tt, J ) 1.5 Hz, 7.8 Hz), 7.65 to
7.39 (11H, m), 7.19 (1H, ddd, J ) 1.5 Hz, 7.8 Hz, 11.1 Hz), 4.62
3
3
(2H, t, JHH ) 9.7 Hz), 4.30 (2H, t, JHH ) 9.7 Hz), 0.49 (3H, d,
3JHP ) 2.1 Hz). 13C{1H} NMR (CD2Cl2, 125.76 MHz): δ 164.8
(1C, d, JCP ) 5.2 Hz), 135.3 (1C, d, JCP ) 2.5 Hz), 134.4 (4C, d,
2
JCP ) 12.3 Hz), 133.9 (2C, br m), 133.7 (2C, q, JCF ) 35.2 Hz),
133.4 (1C, d, JCP ) 7.5 Hz), 132.7 (1C, d, JCP ) 5.7 Hz), 132.6
(1C, d, JCP ) 21 Hz), 129.7 (6C, d, JCP ) 11.8 Hz), 129.2 (1C, d,
JCP ) 11.8 Hz), 129.0 (1C, br m), 128.0 (1C, d, JCP ) 47.6 Hz),
1
127.5 (1C, d, JCP ) 58.5 Hz), 122.4 (2C, q, JCF ) 273.4 Hz),
118.0 (1C, s), 112.3 (1C, s), 69.23 (1C, s), 56.7 (1C, s), 2.29 (1C,
s). 31P{1H} NMR (CD2Cl2, 121.49 MHz): δ 36.69 (s). 19F NMR
(CD2Cl2, 376.5 MHz): δ -64.2 (s, NCArF), -120 to -140 (br m,
SbF6). Anal. Calcd for C31H24F12N2OPSbPd: C, 40.14; H, 2.61;
N, 3.02. Found: C, 38.83; H, 2.43; N, 2.85.
General Procedure for Nitrile Self-Exchange. An NMR tube
was charged with complex 2b (5 mg, 0.0049 mmol) and CD2Cl2
[(phen)PdMe(3,5-NCC6H3(CF3)2)][B(ArF)4], 4b. A flame-dried
Schlenk flask was charged with 4a (0.500 g, 1.48 mmol) and 3,5-
NCC6H3(CF3)2 (0.300 mL, 1.78 mmol) in diethyl ether (20 mL).
A solution of Na[B(ArF)4] (1.45 g, 1.63 mmol) in diethyl ether (20
mL) was added via cannula and stirred at room temperature. After
∼30 min, the volatiles were removed in vacuo and the remaining
residue was taken up in methylene chloride (10 mL), resulting in
a light yellow solution and a light brown precipitate (NaCl). The
product was isolated via cannula filtration, and the precipitate was
washed with methylene chloride (2 × 5 mL). The washings were
combined with the light yellow filtrate and concentrated in vacuo
to ∼4-5 mL, and pentane (50 mL) was added to precipitate 4b as
a light yellow oil. The mixture was stirred overnight, resulting in
formation of a light yellow precipitate, which was isolated by
1
(600 µL) under argon and sealed with a septum. An H NMR
spectrum was recorded at RT. The tube was cooled to -80 °C,
and the desired amount of 3,5-bis(trifluoromethyl)benzonitrile was
added via syringe. The reaction was followed by VT-NMR over a
range of 190-238 K. The line width at half-height of the signal
assigned to the ortho-H of bound nitrile in 2b (δ 8.280) was
monitored as a function of temperature, and the rate of nitrile
exchange was determined using the slow exchange approximation.
General Procedure for Ethylene Insertion Reactions. A screw
cap NMR tube was charged with 5 (5 mg, 0.0088 mmol) and cooled
to -78 °C. A solution of [H(OEt2)2][B(ArF)4] (8.9 mg, 0.0088mmol)
in CD2Cl2 (512 µL) was cannula transferred to the NMR tube. 1,3,5-
Trimethoxybenzene (88 µL, 0.1 M in CD2Cl2) was added to the

Phosphine-Oxazoline Pd(II) Complex
Organometallics, Vol. 26, No. 5, 2007 1269
NMR tube via syringe as an internal standard. The tube was warmed
briefly to obtain a homogeneous solution and was then placed back
into the -78 °C bath. The tube was placed in the NMR probe at
-80 °C, and an 1H NMR spectrum was recorded of the [(StePHOX)-
PdMe(OEt2)][B(ArF)4] complex. The tube was then removed from
the probe and placed back into the -78 °C bath, and ethylene was
added via syringe (4.26 mL, 0.176 mmol (20 equiv)). The sample
was then warmed to the desired temperature inside the NMR probe,
standard. A rate of insertion was calculated by plotting the natural
log of the concentration of starting material versus time.
Acknowledgment. We thank the National Science Founda-
tion (grant CHE-0615704) and NIGMS (GM 64451) for support
of this work.
Supporting Information Available: A sample plot of the kinet-
ic analysis of ethylene insertion in 2c and CIF files giving complete
crystallographic data for complexes 2a and 3a. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
and insertion was monitored by H NMR. Disappearance of the
[(StePHOX)PdMe(C2H4)][B(ArF)4] complex was monitored by
integration of the signal corresponding to the Pd-CH3 (δ 0.481)
versus the integration of 1,3,5-trimethoxybenzene as an internal
OM061025V