Mechanism of ethylene oligomerization by a cationic palladium(II) alkyl complex that contains a (3,5-Me2-pyrazolyl)2CHSi(p-tolyl) 3 ligand

Article abstract of DOI:10.1021/om7007698

The reactivity of the ethylene oligomerization catalyst (NN)Pd(Me)(ethylene)⁺ (5), which contains a silyl-capped bis-pyrazolyl methane ligand (NN = (3,5-Me₂-pyrazolyl) ₂CHSi(p-tolyl)₃ (1)) was investigated. The reaction of (NN)PdMeCl (3) with Li[B(C₆F₅)₄] and ethylene in CH₂Cl₂ generates 5. Complex 5 undergoes ethylene insertion at -10 °C with a rate constant of k_insert,Me = 3.3(3) × 10^-3 s^-1. Complex 5 catalytically oligomerizes ethylene to branched C₆-C₂₀ internal olefins (-10 to 20 °C; 2.7-30 atm ethylene). NMR studies show that an equilibrium mixture of base-free β-agostic secondary alkyl (NN)PdR⁺ species (8) and ethylene adducts (NN)Pd(R)(ethylene)⁺ (9) is present under oligomerization conditions. Complex 9 decomposes to Pd⁰ at 20 °C, resulting in catalyst decomposition. (NN)Pd(R)(L) ⁺ species containing R groups that can function as internal donors (either η²-acyl or agostic β-H) undergo a dynamic process that exchanges the two pz* rings and are thermally unstable. It is proposed that catalyst decomposition and pz* exchange both involve initial coordination of the internal donor to generate a configurationally labile five-coordinate intermediate, which isomerizes, resulting in pz* site exchange or displacement of one arm of the NN ligand.

Full text of DOI:10.1021/om7007698

6750
Organometallics 2007, 26, 6750–6759
Mechanism of Ethylene Oligomerization by a Cationic Palladium(II)
Alkyl Complex that Contains a (3,5-Me2-pyrazolyl)2CHSi(p-tolyl)3)
Ligand
Matthew P. Conley, Christopher T. Burns, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed July 29, 2007
The reactivity of the ethylene oligomerization catalyst (N^N)Pd(Me)(ethylene)+ (5), which contains a
silyl-capped bis-pyrazolyl methane ligand (N^N ) (3,5-Me2-pyrazolyl)2CHSi(p-tolyl)3 (1)) was inves-
tigated. The reaction of (N^N)PdMeCl (3) with Li[B(C6F5)4] and ethylene in CH2Cl2 generates 5. Complex
5 undergoes ethylene insertion at -10 °C with a rate constant of kinsert,Me ) 3.3(3) × 10-3 s-1. Complex
5 catalytically oligomerizes ethylene to branched C6–C20 internal olefins (-10 to 20 °C; 2.7–30 atm
ethylene). NMR studies show that an equilibrium mixture of base-free ꢀ-agostic secondary alkyl
(N^N)PdR+ species (8) and ethylene adducts (N^N)Pd(R)(ethylene)+ (9) is present under oligomerization
conditions. Complex 9 decomposes to Pd0 at 20 °C, resulting in catalyst decomposition. (N^N)Pd(R)(L)+
species containing R groups that can function as internal donors (either η2-acyl or agostic ꢀ-H) undergo
a dynamic process that exchanges the two pz* rings and are thermally unstable. It is proposed that catalyst
decomposition and pz* exchange both involve initial coordination of the internal donor to generate a
configurationally labile five-coordinate intermediate, which isomerizes, resulting in pz* site exchange or
displacement of one arm of the N^N ligand.
Scheme 1
Introduction
The ethylene oligomerization and polymerization reactivity
of (N^N)PdR(C2H4)+ species that contain bis(N-heterocycle)-
methane (N^N) donor ligands was explored in previous work.1
(N^N)PdR+ cations that contain sterically small N^N ligands
catalytically dimerize ethylene by an insertion/ꢀ-H elimination
mechanism. The catalyst resting state is the (N^N)Pd(Et)-
(ethylene)+ complex. Incorporation of bulky substituents in the
bridging methylene position of the N^N ligand shifts the product
distribution to higher molecular weight due to inhibition of chain
transfer. Increasing the electrophilic character (N-heterocycle
) imidazole < pyridine < pyrazole) and the steric bulk of the
(N^N)Pd unit leads to moderate (up to ca. 10-fold) increases in
ethylene insertion rates of (N^N)Pd(R)(ethylene)+ species.
During this work, we discovered that the {(p-tolyl)3Si)CH-
(pz*)2}PdMe+ cation (pz* ) 3,5-Me2-pyrazole), which contains
a silyl-capped bis-pyrazolyl-methane ligand, displays unique
reactivity. This species exhibits higher ethylene insertion and
oligomerization rates but is much less thermally stable compared
to other (N^N)PdR+ cations. Here we describe a detailed study
of the {(p-tolyl)3Si)CH(pz*)2PdMe+ catalyst aimed at under-
standing the oligomerization behavior and the low thermal
stability of this system.
Results and Discussion
Synthesis and Structure of (N^N)PdCl2 and (N^N)PdMeCl.
The bis-pyrazolyl methane ligand (p-tolyl)3SiCH(pz*)2 (1) was
prepared by deprotonation of (pz*)2CH2 with nBuLi in THF at -78
°C, followed by reaction with (p-tolyl)3SiOTf. The reaction of 1
with (MeCN)2PdCl2 affords {(p-tolyl)3SiCH(pz*)2}PdCl2 (2, Scheme
1). The solid state structure of 2 was determined by X-ray
diffraction (Figure 1) and is similar to those of other {bis(N-
heterocycle)methane}PdII complexes.1b,c,2 The geometry at Pd is
square planar, the (N^N)Pd chelate ring has a boat conformation,
and the (p-tolyl)3Si group occupies the axial position on the
* Corresponding author. E-mail: rfjordan@uchicago.edu.
(1) (a) Burns, C. T.; Jordan, R. F. Organometallics 2007, 26, 6726. (b)
Burns, C. T.; Jordan, R. F. Organometallics 2007, 26, 6737. (c) Tsuji, S.;
Swenson, D. C.; Jordan, R. F. Organometallics 1999, 18, 4758. For studies
of related L2PdR+species see:. (d) Rix, F. C.; Brookhart, M. J. Am. Chem.
Soc. 1995, 117, 1137. (e) Rix, F. C.; Brookhart, M.; White, P. S. J. Am.
Chem. Soc. 1996, 118, 4746. (f) Ittel, S. D.; Johnson, L. K.; Brookhart, M.
Chem. ReV. 2000, 100, 1169. (g) Johnson, L. K.; Killian, C. M.; Brookhart,
M. J. Am. Chem. Soc. 1995, 117, 6414. (h) Mecking, S.; Johnson, L. K.;
Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888. (i) Temple,
D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 2000, 122, 6686. (j) Ikeda, S.; Ohhata, F.; Miyoshi, M.; Tanaka, R.;
Minami, T.; Ozawa, F.; Yoshifuji, M. Angew. Chem., Int. Ed. 2000, 39,
4512. (k) Daugulis, O.; Brookhart, M. Organometallics 2002, 21, 5926. (l)
Doherty, M. D.; Trudeau, S.; White, P. S.; Morken, J. P.; Brookhart, M.
Organometallics 2007, 26, 1261. (m) Ledford, J.; Shultz, C. S.; Gates, D. P.;
White, P. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20,
5266. (n) Salo, E. V.; Guan, Z. Organometallics 2003, 22, 5033.
1
bridging carbon. The H NMR spectrum of 2 contains one set
of pz* resonances, consistent with Cs symmetry. The spectrum
also contains one set of sharp p-tolyl resonances, indicating that
rotation around the Si-CH and Si-tolyl bonds is fast on the
NMR time scale.
The reaction of 1 with (COD)PdMeCl affords {(p-
tolyl)3SiCH(pz*)2}PdMeCl (3, Scheme 1). The 1H NMR
spectrum of 3 at 25 °C in CD2Cl2 solution contains two sets of
10.1021/om7007698 CCC: $37.00
2007 American Chemical Society
Publication on Web 12/08/2007

Cationic Palladium(II) Alkyl Complex
Organometallics, Vol. 26, No. 27, 2007 6751
Scheme 3
effects of the –Si(p-tolyl)3 group. For comparison, incorporation
of a –C(p-tolyl)3 group at the bridge position in a closely related
system caused
a smaller reduction in νCO (i.e., {(p-
tolyl)3CCH(mim)2}Pd{C(dO)Me}CO+ (2118 cm-1
)
vs
{H2C(mim)2}Pd{C(dO)Me}CO+ (2122 cm-1)).1,4 Complex 4
decomposes to Pd0 in a few hours at 25 °C.
Generation of {(p-tolyl)3SiCH(pz*)2}PdMe(C2H4)+. The
methyl ethylene complex [{(p-tolyl)3SiCH(pz*)2}PdMe(C2H4)]-
[B(C6F5)4] (5) was generated quantitatively by the reaction of 3
and [Li(Et2O)2.8][B(C6F5)4] in CD2Cl2 in the presence of
ethylene at -60 °C (Scheme 2). Under these conditions insertion
is not observed. The 1H NMR spectrum of 5 at -60 °C contains
two sharp sets of pz*-H4 resonances (δ 6.06, 5.85), consistent
with C1 symmetry. The Pd-Me resonance appears at δ 0.38,
ca. 0.63 ppm downfield from the Pd-Me resonance of 3. The
1H NMR spectrum contains separate signals for bound (δ 4.15,
4.38) and free (δ 5.38) ethylene at -60 °C. Similarly, the 13C
spectrum contains separate resonances for bound (δ 87.5, br)
and free (δ 130.0) ethylene. These data are consistent with slow
exchange of bound and free ethylene and fast rotation around
the Pd-(C2H4 centroid) axis in 5.5
Figure 1. ORTEP view (50% probability ellipsoids) of {(p-
tolyl)3SiCH(pz*)2}PdCl2 (2). H atoms are omitted. Key bond
distances (Å) and angles (deg): Pd(1)-Cl(1) 2.28(1); Pd(1)-Cl(2)
2.29(1); N(1)-Pd(1) 2.03(3); N(3)-Pd(1) 2.03(3); N(3)-N(4)
1.38(4); N(1)-N(2) 1.37(4); N(4)-C(11) 1.46(5); N(2)-C(11)
1.45(5); C(11)-Si(1) 1.95(1); N(3)-Pd(1)-N(1) 88.2(1);
N(3)-Pd(1)-Cl(1) 91.7(9); N(1)-Pd(1)-Cl(1) 179.2(9); N(3)-
Pd(1)-Cl(2) 177.1(9); N(1)-Pd(1)-Cl(2) 92.8(9); Cl(1)-Pd(1)-
Cl(2) 87.4(4). Angles between intersecting planes N(1)-N(2)-
N(3)-N(4) and N(1)-Pd(1)-N(3) 158.1; N(1)-N(2)-N(3)-N(4)
and N(2)-C(11)-N(4) 133.2; two heterocycle planes 126.8.
Scheme 2
To probe the dynamics of Pd-(C2H4) rotation, 5 was
generated in CDCl2F solution and low-temperature NMR spectra
1
were recorded. At -120 °C, the H NMR spectrum contains
four broad signals (δ 5.21, 4.54, 4.26, 2.86) and the 13C
spectrum contains two broad signals (δ 83.6, 92.1) for the bound
ethylene, indicating that ethylene rotation is slow on the
1
chemical shift time scale at this temperature. The H ethylene
signals at δ 4.54 and 4.25 coalesce at -105 °C, from which
q
the barrier to ethylene rotation was determined to be ∆Grot
)
8.2 kcal mol-1
.
pz* resonances consistent with C1 symmetry. As in 2, fast
rotation around the Si-CH and Si-tolyl bonds is observed.
Generation of a Pd Acyl Carbonyl Complex. The acyl
carbonyl complex {(p-tolyl)3SiCH(pz*)2}Pd{C(dO)Me}(CO)+
(4) was generated quantitatively by the reaction of 3 and
[Li(Et2O)2.8][B(C6F5)4] in CD2Cl2 in the presence of CO
(Scheme 2). The 13C NMR spectrum of 4 at -60 °C in the
presence of excess CO contains sharp resonances for bound CO
(δ 171.3) and free CO (δ 184), indicating that intermolecular
CO exchange is slow on the NMR time scale under these
conditions.3 The νCO value for the carbonyl ligand of 4 in
CD2Cl2 solution is 2122 cm-1 (cf. free CO 2139 cm-1), which
indicates that 4 contains a moderately electrophilic Pd center.
This value is lower than the values for {H2C(pz)2}Pd-
{C(dO)Me}CO+ (2133 cm-1) and {H2C(pz*)2}Pd{C(dO)Me}-
CO+ (2132 cm-1),1a most likely due to the inductive and field
Kinetics of Ethylene Insertion of 5. In the presence of excess
ethylene at -10 °C, 5 undergoes ethylene insertion to yield
higher Pd-alkyl complexes and eventually ethylene oligomers
(Scheme 3). The rate of insertion of 5 is first-order in Pd and
zero-order in ethylene. The insertion rate constant, kinsert,Me
)
3.3(3) × 10-3 s-1, was determined by monitoring the disap-
pearance of the Pd-Me 1H NMR resonance. The insertion
barrier is ∆Gq ) 18.3 kcal mol-1. The kinsert,Me value for 5 is
greater than that for {Me2C(pz)2}PdMe(C2H4)+ (2.2 × 10-4
s-1), {H2C(pz)2}PdMe(C2H4)+ (3.6
×
10-4 s-1), and
{H2C(pz*)2}PdMe(C2H4)+ (1.3 × 10-3 s-1) at the same
temperature.1a,c 5 undergoes ethylene insertion faster than any
of the {bis(N-heterocycle)methane}Pd(R)(ethylene)+ species
studied previously.1
Ethylene Oligomerization Behavior of {(p-tolyl)3SiCH-
(pz*)2}PdMe+. Preparative scale ethylene oligomerizations were
performed using 5 generated in situ by the reaction of 3 and
[Li(Et2O)2.8][B(C6F5)] in CH2Cl2 in the presence of ethylene.
At -10 °C under 2.7 atm of ethylene, 5 produces a distribution
of branched C6–C20 internal olefins (Figure 2). Very little Pd0
formation is observed under these conditions, indicating that
(2) (a) Minghetti, G.; Cinellu, M. A.; Bandini, A. L.; Banditelli, G.;
Demartin, F.; Manassero, M. J. Organomet. Chem. 1986, 315, 387. (b)
Burns, C. T.; Shen, H.; Jordan, R. F. J. Organomet. Chem. 2003, 683, 240.
(c) Done, M. C.; Ruther, T.; Cavell, K. J.; Kilner, M.; Peacock, E. J.;
Braussaud, N.; Skelton, B. W.; White, A. J. Organomet. Chem. 2000, 607,
78. (d) Newkome, G. R.; Gupta, V. K.; Theriot, K. J.; Ewing, J. C.;
Wicelinski, S. P.; Huie, W. R.; Fronczek, F. R.; Watkins, S. F. Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, C40, 1352. (e) Sanchez-
Mendez, A.; de Jesus, E.; Flores, J. C.; Gomez-Sal, P. Inorg. Chem. 2007,
46, 4793.
(4) (a) Increasing the electron density at a remote position of a ligand
lowered νCO in related complexes: Wu, F.; Jordan, R. F. Organometallics
2006, 23, 5631. (b) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2002, 124,
5272. (c) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100.
(5) The four ethylene hydrogens comprise an ABCD system under slow
rotation conditions, and an AA′BB′ system under fast rotation conditions.
(3) (a) For analogous compounds see: Wu, F.; Foley, S. R.; Burns, C. T.;
Jordan, R. F. J. Am. Chem. Soc. 2005, 127, 1841. (b) Rix, F. C.; Brookhart,
M. J. Am. Chem. Soc. 1996, 118, 6414.

6752 Organometallics, Vol. 26, No. 27, 2007
Conley et al.
species.8 These assignments were confirmed by a COSY
spectrum. Sharp singlets are observed for the (p-tolyl)3SiCH
(δ 6.45) and pz*-H4 (δ 6.05, 5.92) resonances.
At 0 °C, the 1H NMR spectrum of 6 contains a broad signal
at δ -2.10 for the isopropyl methyl hydrogens and a singlet at
δ 3.09 for the PdCHMeCH2-µ-H methine hydrogen. The
spectrum also contains two sets of pz* resonances, showing that
the sides of the N^N ligand remain inequivalent at this
temperature. These results imply that in-place rotation around
the Pd-C bond and concomitant exchange of agostic and
nonagostic Me units is fast at 0 °C.9
Reaction of 6 with Ethylene. The reaction of 6 with 1 equiv
of ethylene in CD2Cl2 at -80 °C results in partial reversible
formation of the n-propyl ethylene species [{(p-
tolyl)3SiCH(pz*)2}PdnPr(C2H4)][B(C6F5)4] (7, (eq 2)). Complex
7 is stable in CD2Cl2 solution up to -35 °C, at which
Figure 2. Distribution of olefins produced by 5. Conditions: in situ
generation of
5 from 30 µmol of 3 and 30 µmol of
1
temperature ethylene insertion occurs. The H NMR spectrum
[Li(Et2O)2.8][B(C6F5)] in 20 mL of CH2Cl2. Blue bars: -10 °C,
2.7 atm of ethylene; red bars: 25 °C, 2.7 atm of ethylene; green
bars: 25 °C, 2.7 atm of ethylene.
of 7 in CD2Cl2 at -80 °C contains a broad resonance for the
bound ethylene (δ 4.26)10 and sharp singlets for the (p-
tolyl)3SiCH (δ 6.70) and pz*-H4 (δ 6.13, 5.90) hydrogens.
The n-propyl resonances were identified with the aid of a
COSY spectrum (-80 °C; PdCH2CH2CH3: δ 1.72, 0.77;
PdCH2CH2CH3: δ 1.94; PdCH2CH2CH3: δ 0.77).
catalyst decomposition is minimal. The turnover frequency (13
× 10-3 s-1) is ca. 4 times higher than the ethylene insertion
rate of 5. At 25 °C under 2.7 atm of ethylene, 5 produces a
distribution of branched C6–C12 internal olefins (39 branches/
103 C). The product distribution is very similar at 30 atm of
ethylene pressure. Rapid formation of Pd0 is observed during
the 25 °C reactions, indicating that the catalyst is not stable at
this temperature.6
To probe the nature of the decomposition process, a CH2Cl2
solution of 5 was exposed to ethylene (650 mm on demand) at
25 °C to model the oligomerization conditions. Pd0 was
recovered in 85% mass balance after 2 h. NMR and ESI-MS
analysis of the product mixture showed that [(p-tolyl)3-
SiCH(pz*)2]H+ was a major decomposition product, indicating
that the ligand remains intact upon decomposition of the
catalyst.7
Generation of [{(p-tolyl)3SiCH(pz*)2}PdCHMeCH2-µ-H]-
[B(C6F5)4] (6). The reaction of 5 in CDCl2F at 0 °C in the
absence of ethylene generates the ꢀ-H agostic isopropyl species
[{(p-tolyl)3SiCH(pz*)2}PdCHMeCH2-µ-H][B(C6F5)4] (6) in 95%
yield along with ca. 5% of higher ꢀ-agostic alkyl complexes
(eq 1). Complex 6 is stable in CDCl2F solution below 0 °C.
(2)
The dynamics of ethylene rotation of 7 were probed by low-
1
temperature NMR in CDCl2F solution. At -115 °C, the H
NMR spectrum of 7 contains four resonances (δ 5.41, 4.54,
3.94, 2.83), and the 13C NMR contains two resonances (δ 93.7,
1
86.1) for the bound ethylene, indicating slow rotation. The H
ethylene signals at δ 4.54 and 3.94 coalesce at -90 °C, giving
q
∆Grot ) 8.1 kcal mol-1, similar to the value for 5.
The equilibrium constant for ethylene coordination to 6 (eq
2), Keq ) [7][6]-1[C2H4]-1, was determined over the temper-
ature range -80 to -35 °C by 1H NMR using the characteristic
(p-tolyl)3SiCH resonances of 6 and 7. At -80 °C, Keq ) 430
M-1. Thermodynamic parameters for this equilibrium were
determined by a van’t Hoff analysis and are listed in Table 1.
Extrapolation of the van’t Hoff plot to –10 °C, the optimum
ethylene oligomerization temperature, gives Keq ) 2.8 M-1
.
NMR Monitoring of the Reaction of 5 with Excess
Ethylene. The 5-catalyzed ethylene oligomerization reaction at
-10 °C in CD2Cl2 was monitored by 1H NMR by periodically
cold-quenching the reaction and collecting 1H NMR spectra at
-80 °C. These spectra showed that a mixture of ꢀ-H agostic
higher secondary alkyl complexes (8, R ) distribution of alkyl
groups) and alkyl ethylene complexes (9, R′ ) distribution of
alkyls groups) was present, as shown in Scheme 4. After the
ethylene is consumed, the only Pd species in solution is 8.
(1)
The 1H NMR spectrum of 6 in CDCl2F at -115 °C contains
a PdCHMeCH2-µ-H resonance at δ 3.31, a nonagostic methyl
resonance at δ 0.77, and PdCHMeCH2-µ-H resonances at δ
-0.04 and 0.81. The agostic ꢀ-H appears as a triplet at δ -8.91
(2JHH ) 18 Hz), which is characteristic of ꢀ-H agostic Pd-iPr
(6) The oligomerization distribution produced by 5 at 30 atm of ethylene
fit a Shultz-Flory distribution (R ) 0.35 and ꢀ ) 1.8). However,
oligomerizations at 2.7 atm of ethylene pressure did not produce Shultz-Flory
distributions.
(7) The compound [1H][B(C6F5)4] was generated quantitatively by the
reaction of 1 (10 mg, 20 µmol) and [H(OEt2)2][B(C6F5)4] (16 mg, 20 µmol)
in CD2Cl2 (0.7 mL) for 10 min at 25 °C. 1H NMR (CD2Cl2, 25 °C): δ
12.63 (br s, 1H, H+), 7.24 (m, 12 H, 4-Me-C6H4), 6.50 (s, 1H, CH), 6.06
(s, 2H, pz* H4), 2.39 (s, 9H, 4-Me-C6H4), 2.21 (s, 3H, pz* Me), 2.05 (s,
3H, pz* Me). ESI-MS: [(pz*)2CHSi(p-tolyl)3]H+ calcd m/z 505.2, found
505.2. [1H][B(C6F5)4] decomposes in CD2Cl2 solution at 25 °C (ca. 25%
after 24 h).
(8) Tempel, D. J.; Brookhart, M. Organometallics 1997, 17, 2290.
(9) Two diastereomers are possible for 6, which differ in the relative
configurations of the N^N bridging carbon and the isopropyl methine carbon.
Exchange of the agostic and nonagostic methyl groups by Pd-C bond
rotation interconverts the diastereomers of 6. However, only one set of
resonances is observed for 6 at all temperatures. It is likely that the NMR
spectra of the two diastereomers are very similar due to the remoteness of
the two stereocenters.
(10) The broad resonance at δ 4.26 integrates for 4H and results from
the overlap of the two resonances expected for the bound ethylene, which,
based on the low-temperature limit spectrum, have very similar chemical
shifts (∆δ ) 0.12).

Cationic Palladium(II) Alkyl Complex
Organometallics, Vol. 26, No. 27, 2007 6753
Table 1. Thermodynamic Parameters for Ethylene Coordination to
Scheme 5
{(p-tolyl)3SiCH(pz*)2}PdR+ Speciesa
i
6 (R ) Pr)b,c
8 (R ) alkyl)d,e
∆H (kcal mol-1
∆S (cal mol-1 K-1
∆G293 (kcal mol-1 f
Keq at -10 °C (M-1 g
)
-7.4(5)
-27(2)
0.51
-5.8(3)
-23(2)
0.94
)
)
)
2.8
0.72
a Determined in CD2Cl2 solution. b Coordination of ethylene to 6
gives the n-propyl ethylene complex 7. c Based on NMR data in the
temperature range -80 to -35 °C. d R ) distribution of oligoethylene
chains. e Based on NMR data in the temperature range -80 to -40 °C.
the reaction of 8 with ethylene-d4 at -10 °C in CD2Cl2 was
studied. The 1H NMR spectrum (collected at -80 °C) showed
that after 20 min at –10 °C the characteristic (p-
tolyl)3SiCH(pz*)2 resonances for 8 and 9 were present, but the
agostic ꢀ-H, terminal ꢀ-H, and methine C-H resonances of 8
and the ethylene resonance of 9 were absent. Upon subsequent
reaction with unlabeled ethylene, the ꢀ-H agostic, terminal ꢀ-H,
and methine C-H signals of 8 and the ethylene resonance of 9
appeared within 20 min. These results confirm that 8 and 9 are
active growing species.
f ∆G evaluated at 293 K. g Keq
)
[(N^N)PdR(ethylene)+]-
[[(N^N)PdR+]-1[ethylene]-1; determined by extrapolation of the van’t Hoff
plot.
Scheme 4
Catalyst Stability. The results above show that {(p-
tolyl)3SiCH(pz*)2}PdR+ species exist as an equilibrium mixture
of 8 and 9 under catalytic ethylene oligomerization conditions.
As noted above, fast catalyst decomposition to Pd0 occurs in
ethylene oligomerization at 25 °C. However, CD2Cl2 solutions
of 8 are very stable at 25 °C (<10% decomposition after 2 days).
Therefore, the low catalyst stability must result from instability
of 9.
1
The H NMR data for 8 are similar to those for isopropyl
1
analogue 6. The H NMR chemical shifts (CD2Cl2, -80 °C)
for the (p-tolyl)3SiCH (δ 6.37), pz*-H4 (6.04, 5.93) and agostic
ꢀ-H (δ -9.50, m) units of 8 are very similar to those for 6.11
1
The alkyl region of the H spectrum of 8 was challenging to
Dynamic Properties of (N^N)Pd Complexes. The 1H NMR
spectrum of {(p-tolyl)3SiCH(pz*)2}Pd{C(dO)Me}(CO)+ (4)
at -60 °C contains two sharp sets of pz* signals, consistent
with the expected inequivalence of the pz* units. However,
the pz* resonances broaden and coalesce to one set as the
temperature is raised; for example, the pz*-H4 signals
coalesce at -40 °C as shown in Figure 3. In contrast, the
(p-tolyl)3SiCH and Pd{C(dO)Me} resonances remain sharp
under these conditions. Similarly, the 13C NMR spectrum of
4 exhibits broadening of the pz* resonances but not the (p-
tolyl)3SiCH or Pd{C(dO)Me} signals at -40 °C. The bound
and free CO signals remain sharp at -40 °C, indicating that
exchange of free and coordinated CO is slow.14 These results
show that 4 undergoes a dynamic process that exchanges the
pz* units but does not involve the Pd{C(dO)Me} unit and
does not involve intermolecular CO exchange (eq 3).15
assign because a distribution of alkyl chain lengths is present
(vide infra). However, the -80 °C COSY spectrum contains
correlations between the ꢀ-H agostic, terminal ꢀ-H (δ 0.42, m),
and R-methine signals (δ 3.03, m), and these signals integrate
in a 1/1/1 ratio, confirming the ꢀ-H agostic secondary alkyl
structure.12
The NMR data for 9 are similar to those for the n-propyl
analogue 7. The H NMR chemical shifts (CD2Cl2, -80 °C)
for the (p-tolyl)3SiCH (δ 6.69), pz*-H4 (δ 6.08, 5.82), and bound
ethylene (δ 4.15, br) units of 9 are very similar to those for 7.
1
1
The H NMR spectrum of 9 in CDCl2F solution at -120 °C
contains four ethylene signals (δ 4.45, 3.91, 3.57, 2.75). The δ
q
3.91 and 3.57 resonances coalesce at -90 °C, giving ∆Grot
)
8.3 kcal mol-1, similar to the values for 5 and 7.
The equilibrium constant for ethylene coordination to 8
(Scheme 4), Keq ) [9][8]-1[C2H4]-1, was determined by H
1
NMR from -80 to -40 °C, using the (p-tolyl)3SiCH resonances
for 8 and 9. The thermodynamic parameters for this equilibrium
are similar to the values for 6 and are listed in Table 1. Keq is
estimated to be 0.72 M-1 at -10 °C, slightly lower than the
value for 6.
Characterization of Alkyl Chain Length Distribution in
8. To determine alkyl chain length distribution in 8, this species
was reacted with CO and then NaOMe/MeOH to generate the
corresponding distribution of methyl esters as shown in Scheme
5.13 GC-MS analysis showed that a distribution of C5 to C17
methyl esters formed. Multiple isomers were detected for each
carbon number. These results are consistent with branching in
the ethylene oligomers produced by 5 in Scheme 3.
Ethylene-d4 Labeling Experiments. To confirm that 8 and
9 are involved in the catalytic ethylene oligomerization cycle,
(11) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 11539.
(12) The acetonitrile complex [{(p-tolyl)3SiCH(pz*)2}Pd(R)MeCN]-
[B(C6F5)4] 8b was generated to probe the alkyl chain distribution. However,
reaction of MeCN with 8 in CD2Cl2 at -10 °C yields an equilibrium mixture
of 8 and 8b (Keq ) [8b][8]-1[MeCN]-1 ) 240 M-1), from which little
information about the alkyl chain distribution could be obtained.
(13) Liu, J.; Heaton, B. T.; Iggo, J. A.; Whyman, R. Chem. Commun.
2004, 1326.
Figure 3. Variable-temperature 1H NMR of 4 in CD2Cl2. The pz*-
H4 region is shown. The first-order rate constants for the pz*
exchange (kexch) were determined by simulation of the observed
spectra.

6754 Organometallics, Vol. 26, No. 27, 2007
Conley et al.
Table 2. Activation Parameters for pz* Site Exchange in
{(p-tolyl)3SiCH(pz*)2}Pd Species Determined by
Variable-Temperature NMR
Scheme 6
a
∆Gq
∆Hq
∆Sq
293
complex
(kcal mol-1
)
(kcal mol-1
)
(cal mol-1 K-1
)
3
4
6
7
9
17.8
11.0
15.4
11.4
11.3
21.7(6)
9.2(3)
13(2)
-6(2)
7.2(4)
8.5(4)
-14(2)
-9(2)
a ∆Gq evaluated at 293 K.
Activation parameters for the exchange were obtained from
1
the variable-temperature H NMR spectra and are listed in
Table 2. Complex 8a (Scheme 5), which contains a distribu-
tion of alkyl chains, also undergoes fast pz* site exchange.16
The alkyl ethylene complexes 7 and 9 also display selective
(3)
broadening of the pz* signals, even at -80 °C, indicating that
pz* exchange is fast in these cases. The activation parameters
for pz* exchange for 7 and 9 are listed in Table 2 and are similar
to those for 4. The rate of pz* exchange in 7 and 9 is
independent of ethylene concentration over a wide range (7:
10–140 mM; 9; 70–1340 mM). Therefore, pz* exchange cannot
be explained by intermolecular ethylene exchange.
up to 0 °C, but these resonances are selectively broadened at
20 °C. The barrier to pz* exchange estimated from the excess
line width is ∆Gq ) 15.4 kcal mol-1 (Table 2).17
298
Mechanism of Pz* Exchange and Relationship to
Catalyst Stability. Interestingly, there is a qualitative correlation
between the thermal stability and the barriers to pz* site
exchange of {(p-tolyl)3SiCH(pz*)2}Pd species. Complexes 3 and
8 are very stable at 25 °C and exhibit very slow pz* site
exchange, while 4 and 9 are unstable at 25 °C and exhibit fast
pz* exchange rates. A common feature of the unstable, fast-
pz*-exchange species is that they contain a possible internal
donor (either η2-acyl or agostic ꢀ-H),18 while the more stable,
slow-pz*-exchange species do not. These considerations suggest
that catalyst decomposition and pz* exchange both involve
initial coordination of the internal donor. A reasonable mech-
anism that explains catalyst decomposition and pz* exchange
is shown in Scheme 6.
In contrast, the other {(p-tolyl)3SiCH(pz*)2}Pd species studied
here undergo much slower (or no) pz* exchange. The 1H NMR
spectrum of 3 (in 1,1,2,2-tetrachloroethane-d2) contains two
sharp sets of pz* signals at 25 °C, but these resonances broaden
as the temperature is raised, showing that slow pz* exchange
occurs. Activation parameters for pz* exchange of 3 were
determined from the coalescence of the pz*-H4 resonances (δ
5.92, 5.58) and are listed in Table 2. The pz* exchange barrier
for 3, ∆Gq298 ) 17.8 kcal mol-1, is much higher than those for
4, 7, and 9. EXSY spectra of 3 at 20 °C confirm that pz*
exchange occurs and show that the barrier is similar in CD2Cl2
(∆Gq ) 17.7 kcal mol-1) and THF-d8 (17.9 kcal mol-1).
293
The absence of a significant solvent dependence of the barrier
suggests that solvent coordination does not play a critical role
in the pz* exchange of 3. Similarly, the H NMR spectra of 8
(at 25 °C) and 5 (up to -40 °C; insertion occurs above this
temperature) contain two sharp sets of pz* signals characteristic
of slow pz* exchange.
The key feature of Scheme 6 is the formation of a configu-
rationally labile five-coordinate intermediate B by coordination
of the internal donor X (ꢀ-H agostic or η2-acyl). B can isomerize
(via trigonal bipyramidal intermediates that are not shown) to
C, which contains a pz* ligand in the axial site from which it
can dissociate (D), ultimately leading to loss of the N^N ligand
and decomposition. B can also isomerize to E and then C′,
leading to decomposition. Alternatively, E can isomerize to F
and then B′, ultimately resulting in pz* exchange (A′). A more
complete mechanism showing the trigonal bipyramidal inter-
mediates is given in the Supporting Information.
1
Complex 6 undergoes pz* exchange at an intermediate rate.
The 1H NMR spectra of 6 exhibit two sharp sets of pz* signals
(14) Even if intermolecular CO exchange were fast on the NMR time
scale, the pz* site exchange would not be explained since associative CO
exchange is expected to be stereospecific.
(15) A similar exchange process was noted for H2C(pz)2Pd{C-
(dO)Me}CO+ and H2C(3,5-Me2-pz)2Pd{C(dO)Me}CO+ but was not studied
in detail. See ref 1a.
(17) For 6 at 20 °C, the excess linewidth of the pz*-H4/H4′ resonance
is ∆ω1/2 ) 6 Hz (relative to the (p-tolyl)3SiCH resonance), and kexch
)
(16) The 1H NMR spectrum of 8a contains one broad resonance for the
two pz*-H4 hydrogens at–40 °C. The 13C NMR pz* resonances are
broadened into the baseline and the bound and free CO resonances are sharp
at–40 °C. The results show that pz* exchange is fast on the NMR time
scale and does not involve intermolecular CO exchange at -40 °C.
π∆ω1/2 ) 19 s-1
.
(18) (a) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem.
Soc. 2001, 123, 9172. (b) Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1996,
118, 7337. (c) Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics
1996, 15, 5568.

Cationic Palladium(II) Alkyl Complex
Organometallics, Vol. 26, No. 27, 2007 6755
Scheme 7
columns of activated alumina and BASF R3-11 oxygen scav-
enger. Diethyl ether and tetrahydrofuran were distilled from Na/
benzophenone ketyl. Dichloromethane was distilled from P2O5.
Cl2DCCDCl2, CDCl3, and CD2Cl2 were distilled from CaH2 or
P2O5. CDCl2F was prepared according to the literature and
distilled from P2O5.19 CO was purchased from Aldrich and used
as received. Ethylene (research grade) was obtained from
Matheson and used as received. [Li(Et2O)2.8][B(C6F5)4] was
obtained from Boulder Scientific and used as received. The Et2O
content of the [Li(Et2O)2.8][B(C6F5)4] salt was determined by
1H NMR with C6Me6 as an internal standard. (MeCN)2PdCl2
was obtained from Strem and used as received. The compounds
(pz*)2CH2 (pz* ) 3,5-Me2-pyrazole),20 (p-tolyl)3SiCl,21 (p-
tolyl)3SiOSO2CF3,22 and (cod)Pd(Me)Cl23 were prepared as
described in the literature. All other chemicals were purchased
from standard suppliers and used without further purification.
Elemental analyses were performed by Midwest Microlabs
(Indianapolis, IN). Infrared spectra were obtained at 25 °C under
an N2 atmosphere using a Nicolet NEXUS 470 FT-IR spec-
trometer. GC-MS analyses were performed on a HP-6890
instrument equipped with a HP-5973 mass selective detector.
NMR spectra were recorded in Teflon-valved NMR tubes on
Bruker DRX-400 or -500 spectrometers at ambient probe
1
temperature unless otherwise indicated. H and 13C chemical
Mechanism of Ethylene Oligomerization. The key observa-
tions pertaining to the mechanism of ethylene oligomerization
by 5 are as follows: (i) 5 produces a distribution of branched
internal olefins, (ii) the oligomer distribution is essentially
independent of ethylene pressure, (iii) an equilibrium mixture
of ꢀ-H agostic species 8 and ethylene adducts 9 is present under
catalytic conditions, (iv) catalyst deactivation occurs rapidly at
25 °C with loss of the N^N ligand, and (v) 8 is stable at 25 °C,
while 9 is unstable at 25 °C. A mechanism that is consistent
with these results is shown in Scheme 7. The alkyl ethylene
complex 9 undergoes insertion to generate the primary alkyl
species 8′. Species 8′ either is trapped by ethylene to yield a
primary alkyl ethylene adduct 9′ or chain walks to generate a
secondary alkyl species (8) and the corresponding ethylene
adduct 9. Species 9 and 9′ insert, resulting in chain growth, or
decompose as described above. Chain transfer occurs by
ethylene displacement of olefin from the (N^N)Pd(H)(olefin)+
chain-walk intermediates, or by ꢀ-H transfer to coordinated
ethylene in 9 or 9′.
shifts are reported versus SiMe4 and were determined by
reference to the residual solvent peaks. 19F and 11B chemical
shifts were referenced to external neat CFCl3 and BF3 · Et2O,
respectively. Coupling constants are reported in Hz. NMR probe
temperatures were calibrated by a MeOH thermometer.24 For
variable-temperature NMR experiments, the probe was main-
tained at a given temperature for 20–30 min to allow the sample
to reach thermal equilibrium.
The NMR spectra of cationic Pd compounds contain signals
of the free B(C6F5)4- anion. 13C{1H} NMR (CD2Cl2, -60 °C):
δ 147.5 (dm, J ) 241, C2), 137.8 (dm, J ) 238, C4), 135.8
(dm, J ) 249, C3), 123.6 (br, C1). 11B NMR (CD2Cl2, -60
°C): δ -16.9 (s). 19F NMR (CD2Cl2, -60 °C): δ -133.7 (br s,
2F, o-F), -163.0 (t, J ) 23, 1F, p-F), -167.0 (t, J ) 19, 2F,
m-F). Solutions of cationic species generated in situ from the
reaction of 2 and [Li(Et2O)2.8][B(C6F5)4] contain LiCl and free
Et2O. In some experiments the Et2O was used as an internal
standard in NMR experiments.
(pz*)2CHSi(p-tolyl)3 (1). A solution of (3,5-Me2-pz)2CH2
(0.727 g, 3.56 mmol) in THF (75 mL) was cooled to -78 °C, and
nBuLi (2.66 M in hexanes, 1.40 mL, 3.74 mmol) was added over
5 min while the mixture was stirred. A white precipitate formed.
The mixture was stirred at -78 °C for 1 h. A solution of (p-
tolyl)3SiOTf (1.68 g, 3.74 mmol) in THF (30 mL) cooled to
-78 °C was added by cannula. The resulting yellow solution
was stirred at -78 °C for 2 h, allowed to warm to 25 °C, and
stirred for 12 h. The yellow mixture was quenched with saturated
aqueous NH4Cl (150 mL) and extracted with CH2Cl2 (3 × 75
mL). The clear yellow CH2Cl2 extract was washed with aqueous
10% Na2CO3 (200 mL), saturated aqueous NaHCO3 (200 mL),
and H2O (200 mL) and finally dried over MgSO4. The solvent
Conclusion
The {(p-tolyl)3SiCH(pz*)2}Pd(Me)(ethylene)+ cation (5)
catalytically oligomerizes ethylene to branched C6–C20 internal
olefins. NMR studies show that an equilibrium mixture of base-
free ꢀ-agostic secondary alkyl (N^N)PdR+ species (8) and
ethylene adducts (N^N)Pd(R)(ethylene)+ (9) is present under
oligomerization conditions. At 20 °C, 9 undergoes fast site
exchange of the pz* rings and decomposes readily to Pd0, while
8 does not undergo pz* site exchange and is stable. It is proposed
that the decomposition and pz* exchange of 9 both involve
initial coordination of a ꢀ-H to generate a configurationally labile
five-coordinate intermediate, which can isomerize, resulting in
pz* site exchange or displacement of one arm of the N^N ligand.
(19) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629.
(20) Joshi, V. S.; Sarkar, A.; Rajamohanan, P. R. J. Organomet. Chem.
1991, 409, 341.
Experimental Section
(21) Bassindale, A. R.; Ellis, R. J.; Taylor, P. G. J. Chem. Res., Synop.
1996, 34.
General Considerations. All manipulations were performed
using glovebox or Schlenk techniques under purified N2.
Nitrogen was purified by passage through columns containing
activated molecular sieves and Q-5 oxygen scavenger. Pentane,
hexanes, toluene, and benzene were purified by passage through
(22) Asadi, A.; Avent, A. G.; Eaborn, C.; Hill, M. S.; Hitchcock, P. B.;
Meehan, M. M.; Smith, J. D. Organometallics 2002, 21, 2183.
(23) Rülke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(24) Van Geet, A. L. Anal. Chem. 1970, 42, 679.

6756 Organometallics, Vol. 26, No. 27, 2007
Conley et al.
was removed under vacuum. The resulting white solid was
purified by column chromatography using neutral alumina,
eluting with pentane (150 mL) and then pentane/CH2Cl2 (1:1
v/v, 150 mL). The second fraction was dried over MgSO4 and
taken to dryness under vacuum to afford a white solid (1.38 g,
77%). 1H NMR (CDCl3): δ 7.38 (m, 6H, 4-Me-C6H4), 7.07 (m,
6H, 4-Me-C6H4), 6.64 (s, 1H, CH), 5.68 (s, 2H, pz* H4), 2.31
(s, 9H, 4-Me-C6H4) 2.08 (s, 6H, pz*-Me), 1.78 (s, 6H, pz*-
Me). 13C{1H} NMR (CDCl3): δ 146.3, 140.1, 139.2, 136.8,
128.9, 128.2, 106.4, 67.8, 21.5, 13.4, 10.9. Anal. Calcd for
C32H36N4Si: C, 76.15; H, 7.19; N, 11.10. Found: C, 75.76; H,
7.27; N, 10.57.
108.2 (4/4′-pz*), 107.7 (4/4′-pz*), 63.1 (CH), 43.7 (COMe), 21.5
(4-Me-C6H4), 15.3, 14.4, 11.8, 11.2; the free CO resonance
appears at δ 184.3. IR (CD2Cl2, cm-1): 2122 (VCO), 1746 (Vacyl).
Generation of [{(p-tolyl)3Si)CH(pz*)2}PdMe(H2CdCH2)]-
[B(C6F5)4] (5). A valved NMR tube containing a CD2Cl2 (0.7
mL) solution of
2
(0.013 g, 0.020 mmol) and
[Li(Et2O)2.8][B(C6F5)4] (0.018 g, 0.020 mmol) was cooled to
-196 °C, and ethylene (0.18 mmol) was added by vacuum
transfer. The tube was sealed, warmed to -78 °C, and
transferred to a precooled (-60 °C) NMR probe, and NMR
spectra were recorded at -60 °C. The 1H NMR spectrum
established that 5 had formed (100% vs Et2O after 20 min).
Under these conditions (-60 °C), exchange of coordinated and
free ethylene is slow, but rotation of the bound ethylene is fast
on the NMR chemical shift time scale. 1H NMR (CD2Cl2, -60
°C): δ 7.21 (m, 6H, 4-Me-C6H4), 7.06 (m, 6H, 4-Me-C6H4),
6.59 (s, 1H, CH), 6.06 (s, 1H, pz* H4/H4′), 5.85 (s, 1H, pz*
H4/H4′), 4.38 (m, 2H, H2CdCH2), 4.15 (br m, 2H, H2CdCH2),
2.34 (s, 9H, 4-Me-C6H4), 2.32 (s, 3H, pz* Me), 2.07 (s, 3H,
pz* Me), 2.04 (s, 3H, pz* Me), 1.82 (s, 3H, pz* Me), 0.38 (s,
3H, PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ 151.5 (3/3′-
pz*), 150.6 (3/3′-pz*), 142.0 (5/5′-pz*), 141.9, 141.1 (5/5′-pz*),
136.2, 129.3, 124.1, 108.2 (4/4′-pz*), 108.1 (4/4′-pz*), 87.5 (br,
H2CdCH2), 63.3 (CH), 21.3 (4-Me-C6H4), 14.9, 12.6, 11.7, 11.3,
7.0 (PdMe). A solution of 5 in CDCl2F was generated as
described above and analyzed by variable-temperature (-60 °C
{(p-tolyl)3SiCH(pz*)2}PdCl2 (2). A solution of (MeCN)2PdCl2
(0.259 g, 1.00 mmol) and (3,5-Me2-pz)2CHSi(p-tolyl)3 (0.530 g,
1.05 mmol) in CH2Cl2 (25 mL) was stirred for 2 h at 25 °C.
The orange solution was taken to dryness under vacuum. The
resulting orange solid was suspended in Et2O (25 mL), stirred
for 30 min, and collected by filtration. The solid was washed
with pentane (20 mL) and dried under vacuum to yield 2 as an
1
orange powder (0.587 g, 86%). H NMR (CDCl3): δ 7.45 (m,
6H, 4-Me-C6H4), 7.18 (m, 6H, 4-Me-C6H4), 6.58 (s, 1H, CH),
5.77 (s, 2H, pz* H4), 2.54 (s, 6H, pz* 5-Me), 2.35 (s, 9H, 4-Me-
C6H4), 1.96 (s, 6H, pz* 3-Me). 13C{1H} NMR (CDCl3): δ 153.9,
141.3, 140.4, 137.0, 129.5, 125.0, 108.0, 65.4, 21.7, 15.6, 11.7.
Anal. Calcd for C32H36Cl2N4PdSi: C, 56.35; H, 5.32; N, 8.21.
Found: C, 56.40; H, 5.31; N, 8.10.
1
to -120 °C) NMR. H NMR (CDCl2F, -120 °C): δ 5.21 (br
{(p-tolyl)3SiCH(pz*)2}PdMeCl (3). A flask was charged with
(cod)PdMeCl (0.265 g, 1.00 mmol) and (3,5-Me2-pz)2CHSi(p-
tolyl)3 (0.510 g, 1.01 mmol), and Et2O (15 mL) was added by
syringe at 25 °C. A white precipitate formed rapidly. The
mixture was stirred at 25 °C for 4 h, and the solid was collected
by filtration. The solid was washed with Et2O (2 × 10 mL),
washed with pentane (10 mL), and dried under vacuum to yield
a white solid (0.499 g, 75%). The isolated solid contained 0.12
m, H2CdCH2), 4.54 (br m, H2CdCH2), 4.26 (br m, H2CdCH2),
2.86 (br m, H2CdCH2). 13C{1H} (CDCl2F, -120 °C) δ 92.1
(br, H2CdCH2), 83.6 (br, H2CdCH2).25 The rotational barrier
∆Gqrot ) 8.2 kcal mol-1 was obtained from the coalescence of
the δ 4.54 and 4.26 resonances, using the coalescence ap-
proximation (k ) π(νa - νb)/21/2; krot ) 310 s-1 at Tcoalescence
) -105 °C).26
Kinetics of Insertion of [{(p-tolyl)3Si)CH(pz*)2}PdMe-
(H2CdCH2)][B(C6F5)4] (5). A CD2Cl2 solution of 5 containing
free ethylene (280 mM) was generated in a valved NMR tube as
described above and maintained at -10 °C. Periodically the tube
was cold-quenched in a -78 °C bath for 5 min and transferred
to a precooled (-60 °C) NMR probe, where a 1H NMR
spectrum was recorded. Values of I0,PdMe, IPdMe, and IEt2O where
I0,PdMe ) the integral of the Pd-Me resonance of 5 at the start
of the experiment, IPdMe ) the integral of the Pd-Me resonance
at time point, and IEt2O ) the integral of the Et2O internal
standard, were obtained. A plot of ln(APdMe/A0,PdMe), where APdMe
) IPdMe/IEt2O and A0,PdMe ) I0,PdMe/IEt2O, versus time was linear
(r2 ) 0.996). The slope of the line equals –kinsert,Me. For 5,
kinsert,Me ) 3.3(3) × 10-3 s-1 at -10 °C (∼3 half-lives). An
identical result was obtained for [free H2CdCH2] ) 110 mM,
which shows that the insertion rate is zero-order in ethylene.
Low-Pressure Oligomerization of Ethylene by [{(p-
tolyl)3SiCH(pz*)2}PdMe(H2CdCH2)][B(C6F5)4] (5). A 200 mL
Fischer-Porter bottle equipped with a magnetic stir bar was charged
with 2 (0.020 g, 0.030 mmol) and [Li(Et2O)2.8][B(C6F5)4] (0.027
g, 0.030 mmol). The bottle was attached to a stainless steel
double-manifold vacuum/ethylene line, placed under vacuum
to remove the nitrogen atmosphere, and then charged with
ethylene (3 atm). The bottle was cooled (-78 °C) and vented
to decrease the ethylene pressure to 1 atm, and CH2Cl2 (20 mL)
was added by syringe. The ethylene pressure was immediately
1
1
equiv of Et2O, which was quantified by H NMR. H NMR
(CD2Cl2): δ 7.34 (m, 6H, 4-Me-C6H4), 7.14 (m, 6H, 4-Me-
C6H4), 6.58 (s, 1H, CH), 5.92 (s, 1H, pz* H4/H4′), 5.63 (s, 1H,
pz* H4/H4′), 2.38 (s, 3H, pz* Me), 2.30 (s, 9H, 4-Me-C6H4),
2.15 (s, 3H, pz* Me), 2.15 (s, 3H, pz* Me), 1.92 (s, 3H, pz*
Me), -0.25 (s, 3H, PdMe). 13C{1H} NMR (CD2Cl2): δ 151.0
(3/3′-pz*), 150.4 (3/3′-pz*), 140.5, 140.2 (5/5′-pz*), 138.6 (5/
5′-pz*), 136.5, 128.7, 126.0, 107.0 (4/4′-pz*), 106.0 (4/4′-pz*),
63.3 (CH), 21.2 (4-Me-C6H4), 14.9 (pz* Me), 13.6, (pz* Me),
11.7 (pz* Me), 11.6 (pz* Me), -6.5 (PdMe). Anal. Calcd for
C33H39ClN4PdSi · 0.12Et2O: C, 59.97; H, 5.99; N, 8.36. Found:
C, 59.70; H, 6.05; N, 8.35.
Generation of [{(p-tolyl)3SiCH(pz*)2}Pd{C(dO)Me}-
(CO)][B(C6F5)4] (4). A valved NMR tube containing a CD2Cl2
(0.7 mL) solution of
2 (0.020 g, 0.030 mmol) and
[Li(Et2O)2.8][B(C6F5)4] (0.027 g, 0.030 mmol) was cooled to
-196 °C and exposed to CO (1 atm) for 5 min The tube was
sealed and warmed to -78 °C. The tube was briefly warmed to
23 °C and vigorously shaken. A slurry of a white solid in a
colorless supernatant was obtained. The tube was kept at -78
°C and transferred to a precooled (-60 °C) NMR probe, and
NMR spectra were recorded. The 1H NMR spectrum established
1
that 4 had formed (100% vs Et2O). H NMR (CD2Cl2, -60
°C): δ 7.22 (m, 6H, 4-Me-C6H4), 7.17 (m, 6H, 4-Me-C6H4),
6.50 (s, 1H, CH), 6.00 (s, 1H, pz* H4/H4′), 5.92 (s, 1H, pz*
H4/H4′), 2.60 (s, 3H, COMe), 2.34 (s, 9H, 4-Me-C6H4), 2.18
(s, 3H, pz* Me), 2.12 (s, 3H, pz* Me), 2.09 (s, 3H, pz* Me),
1.77 (s, 3H, pz* Me). 13C{1H} NMR (CD2Cl2, -60 °C): δ 210.7
(C(O)Me), 171.3 (PdCO), 151.8 (3/3′-pz*), 150.6 (3/3′-pz*),
142.9 (5/5′-pz*), 142.3, 141.9 (5/5′-pz*), 137.0, 129.3, 123.5,
(25) The pz* and (p-tolyl)3SiCH resonances do not shift or broaden
significantly due to temperature or solvent.
(26) The other two ethylene 1H NMR resonances (δ 5.42, 2.86) were
broadened into the baseline at -105 °C.

Cationic Palladium(II) Alkyl Complex
Organometallics, Vol. 26, No. 27, 2007 6757
increased 3 atm, and the solution was warmed to the desired
temperature and stirred for 18 h, during which time the total
pressure was kept constant (3 atm) by feeding ethylene on
demand. The ethylene feed was turned off and the reactor was
vented. In the -10 °C experiments, the product mixture was a
slurry of a white solid in a pale yellow supernatant. In the room-
temperature experiments, the product mixture was a slurry of a
black solid in a yellow supernatant. Toluene (50 µL, for use as
an internal standard) was added when the pressure reached 1
atm. An aliquot (ca. 200 µL) of the mixture was filtered through
a small pad of silica with 2-methylbutane (3 mL) as the eluent.
The filtrate was analyzed by quantitative GC-MS to determine
the oligomer distribution. A solution of HCl in MeOH (1 M, 5
mL) was added to the remainder of the mixture, and the mixture
was taken to dryness under vacuum. The residue was taken up
in hexanes (20 mL) and filtered through a plug of silica gel.
The solvent was removed under vacuum to afford a mixture of
(4/4′-pz*), 106.7 (4/4′-pz*), 63.3 (CH), 43.9 (PdCHMeCH2-µ-
H), 21.3 (4-Me-C6H4), 20.7 (PdCHMeCH2-µ-H), 14.7, 14.3, 14.2
(br s, PdCHMeCH2-µ-H), 11.7, 10.8. Key 1H–1H COSY
correlations (CDCl2F, -115 °C): δ 3.31 (PdCHMeCH2-µ-H)
coupled to δ 0.85 (PdCHMeCH2-µ-H), 0.77 (PdCHMeCH2-µ-
H), and 0.04 (PdCHMeCH2-µ-H); δ 0.85 (PdCHMeCH2-µ-H)
coupled to δ 0.04 (PdCHMeCH2-µ-H); δ 0.85 and 0.04
(PdCHMeCH2-µ-H) coupled to δ 0.77 (PdCHMeCH2-µ-H) and
δ -8.90 (PdCHMeCH2-µ-H). The solution of 6 in CDCl2F was
warmed to 0 °C, and broadening of the PdCHMeCH2-µ-H
resonances was observed. 1H NMR (CDCl2F, 0 °C): δ 7.18 (m,
6H, 4-Me-C6H4), 7.07 (m, 6H, 4-Me-C6H4), 6.63 (s, 1H, CH),
6.03 (s, 1H, pz* H4/H4′), 5.92 (s, 1H, pz* H4/H4′), 3.09 (s,
1H, PdCHMeCH2-µ-H), 2.36 (s, 9H, 4-Me-C6H4), 2.32 (s, 3H,
pz* Me), 2.18 (s, 3H, pz* Me), 2.09 (s, 3H, pz* Me), 1.80 (s,
3H, pz* Me), -2.10 (br, ca. 5H,29 PdCHMeCH2-µ-H). 13C NMR
(CDCl2F, 0 °C) δ 152.3 (3/3′-pz*), 142.7 (3/3′-pz*), 141.8 (5/
5′-pz*), 141.5 (5/5′-pz*), 136.5, 129.9, 124.3, 107.9 (4/4′-pz*),
106.7 (4/4′-pz*), 63.5 (CH), 43.7 (PdCHMeCH2-µ-H), 20.7 (4-
Me-C6H4), 14.1, 13.9, 11.0, 10.1; the PdCHMeCH2-µ-H and
PdCHMeCH2-µ-H were not observed.
1
C12–C20 oligomers, which was characterized by GC-MS. H
NMR (CDCl3): δ 5.36 (m, RHCdCHR′), 5.08 (br s,
RR′CdCHR′′), 1.98 (m, allylic H), 1.60 (H3CRCdCR′R′′), 1.25
(CH2, CH), 0.83 (m, CH3). The branch number (39 branches/
1
103 C) carbons was determined by H NMR.27,28
Generation of [{(p-tolyl)3SiCH(pz*)2}Pd(CH2CH2CH3)-
(H2CdCH2)][B(C6F5)4] by Reversible Ethylene Coordina-
tion (7). A valved NMR tube containing a CD2Cl2 (0.6 mL)
solution of 6 (0.020 mmol) was cooled to -196 °C, and ethylene
(0.020 mmol) was added by vacuum transfer. The sample was
warmed to -60 °C for 1 h and placed in a precooled (-80 °C)
NMR probe, and NMR spectra were recorded. The spectra
showed that an equilibrium mixture of 6, 7, and free ethylene
High-Pressure Oligomerization of Ethylene by [{(p-
tolyl)3SiCH(pz*)2}PdMe(H2CdCH2)][B(C6F5)4] (5). An au-
toclave was charged with 2 (0.020 g, 0.030 mmol) and
[Li(Et2O)2.8][B(C6F5)4] (0.027 g, 0.030 mmol), CH2Cl2 (20 mL)
was added by syringe, and the autoclave was pressurized with
ethylene (30 atm) that was fed on demand. The mixture was
stirred for 18 h at 25 °C. The autoclave was vented and toluene
(50 µL, for use as an internal standard) was added when the
pressure reached 1 atm. The product mixture was a slurry of a
black solid in a yellow supernatant. An aliquot (ca. 200 µL) of
the mixture was filtered through a small pad of silica with
2-methylbutane (3 mL) as the eluent. This solution was analyzed
by quantitative GC-MS to determine the oligomer distribution.
A solution of HCl in MeOH (1 M, 5 mL) was added to the
remainder of the mixture, and the mixture was taken to dryness
under vacuum. The residue was taken up in hexanes (20 mL)
and filtered through a plug of silica gel. The solvent was
removed under vacuum to afford a mixture of C12–C20 oligomers
(0.045 g), which was characterized by GC-MS.
had formed. The equilibrium constant, Keq
)
[7][6]-1
[H2C)CH2]-1, was determined by 1H NMR from -80 to -35
°C using the integrals of the (p-tolyl)SiCH resonances for 6
and 7 and the free ethylene resonance, referenced to the Et2O
internal standard. Keq ) 430 M-1 at -80 °C; ∆H ) -7.4(5)
kcal mol-1 and ∆S ) -27(2) eu. The equilibrium expression
was confirmed by two experiments in which [Pd] and
1
[H2CdCH2] were varied. H NMR (CD2Cl2, -80 °C): δ 7.26
(m, 6H, 4-Me-C6H4), 7.09 (m, 6H, 4-Me-C6H4), 6.70 (s, 1H,
CH), 6.13 (s, 1H, pz* H4/H4′), 5.90 (s, 1H, pz* H4/H4′), 4.26
(br m, 4H, H2CdCH2), 2.39 (s, 12H, 4-Me-C6H4 and pz* Me),
2.04 (pz* Me), 1.94 (m, 2H, PdCH2CH2CH3), 1.72 (s, 4H, pz*
Me and PdCH2CH2CH3), 0.77 (m, 4H, PdCH2CH2CH3 and
PdCH2CH2CH3). 13C{1H} NMR (CD2Cl2, -80 °C): δ 150.5
(3/3′-pz*), 148.6 (3/3′-pz*), 142.2 (5/5′-pz*), 142.1, 140.9 (5/
5′-pz*), 136.7, 129.5, 124.8, 108.3 (4/4′-pz*), 107.8 (4/4′-pz*),
64.0 (CH), 28.9 (PdCH2CH2CH3), 23.7 (PdCH2CH2CH3), 21.6
(4-Me-C6H4), 15.1, 14.6, 12.7 (PdCH2CH2CH3), 12.3, 11.2. Key
1H – 1H COSY interactions (CD2Cl2, -80 °C): δ 1.72
(PdCH2CH2CH3) coupled to δ 0.77 (PdCH2CH2CH3) and 1.94
(PdCH2CH2CH3); δ 1.94 (PdCH2CH2CH3) coupled to δ 0.77
(PdCH2CH2CH3). A solution of 7 in CDCl2F was generated as
described above, and variable-temperature (-80 to -115 °C)
Generation of [{(p-tolyl)3SiCH(pz*)2}PdCHMeCH2-µ-
H][B(C6F5)4] (6). A valved NMR tube containing a CDCl2F (0.6
mL) solution of 2 (13 mg, 0.020 mmol) and [Li(Et2O)2.8][B(C6F5)4]
(18 mg, 0.020 mmol) was cooled to -196 °C, and ethylene
(0.020 mmol) was added by vacuum transfer. The solution was
thawed at -78 °C, warmed to 0 °C for 30 min, cooled to -78
°C, and transferred to a precooled (-115 °C) NMR probe, and
NMR spectra were recorded. The NMR spectra established that
1
6 had formed (100% vs Et2O). H NMR (CDCl2F, -115 °C):
δ 7.27–7.15 (br m, 12H, 4-Me-C6H4), 6.45 (s, 1H, CH), 6.05
(s, 1H, pz* H4/H4′), 5.92 (s, 1H, pz* H4/H4′), 3.31 (m, 1H,
PdCHMeCH2-µ-H), 2.35 (s, 9H, 4-Me-C6H4), 2.31 (s, 3H, pz*
Me), 2.19 (s, 3H, pz* Me), 2.11 (s, 3H, pz* Me), 1.71 (s, 3H,
pz* Me), 0.85 (m, 1H, PdCHMeCH2-µ-H), 0.77 (d, J ) 6 Hz,
3H, PdCHMeCH2-µ-H), -0.04 (m, 1H, PdCHMeCH2-µ-H),
-8.91 (t, J ) 18 Hz, 1H, PdCHMeCH2-µ-H). 13C{1H} NMR
(CDCl2F, -115 °C): δ 151.8 (3/3′-pz*), 149.5 (3/3′-pz*), 142.6
(5/5′-pz*), 141.9 (5/5′-pz*), 140.9, 136.9, 129.3, 124.3, 108.0
1
NMR experiments were conducted. H NMR (CDCl2F, -115
°C): δ 5.41 (br m, H2CdCH2), 4.54 (br m, H2CdCH2), 3.94
(br m, H2CdCH2), 2.83 (br m, H2CdCH2). 13C{1H} (CDCl2F,
-115 °C): δ 93.7 (br, H2CdCH2), 86.1 (br, H2CdCH2).8 The
q
rotational barrier ∆Grot ) 8.1 kcal mol-1 was obtained from
the coalescence of the δ 4.54 and 3.94 resonances, using the
coalescence approximation (krot ) 670 s-1 at Tcoalescence ) -90
°C).30
(27) Daugulis, O.; Brookhart, M.; White, P. S. Organometallics 2002,
21, 5935.
(29) The integral is lower than expected due to peak broadness; ∆ν1/2
) 250 Hz.
(28) The oligomer mixtures were hydrogenated (MeOH, 1 atm of H2,
10% Pd/C). GC-MS analysis of the product showed that a mixture of
branched alkanes was formed.
(30) The other two bound ethylene 1H NMR resonances (δ 5.41, 2.83)
were broadened into the baseline at -90 °C.

6758 Organometallics, Vol. 26, No. 27, 2007
Conley et al.
Table 3. Summary of X-Ray Diffraction Data for 2 · (3CHCl3)
Generation of [{(p-tolyl)3SiCH(pz*)2}PdCHRCHR-µ-
H][B(C6F5)4] (8b, R ) alkyl). A valved NMR tube containing
a CH2Cl2 (0.6 mL) solution of 2 (13 mg, 0.020 mmol) and
[Li(Et2O)2.8][B(C6F5)4] (18 mg, 0.020 mmol) was cooled to
-196 °C, and ethylene (1.1 mmol) was added by vacuum
transfer. The solution was warmed to -10 °C for 6 h. The
volatiles were removed under vacuum to yield a brown oily
residue, which was washed with pentane (3 × 2 mL). The tube
was evacuated and CD2Cl2 (0.6 mL) was added by vacuum
transfer at -78 °C. The tube was transferred to a precooled
(-80 °C) probe and spectra were recorded. The spectra
established that 8 had formed in essentially quantitative yield.
1H and 1H–1H COSY NMR experiments established that 8
comprises a mixture of species with a distribution of ꢀ-H agostic
secondary alkyl complexes. The distribution of alkyl chains was
established by methanolysis of the acyl complex 8a described
below. Key data for 8: 1H NMR (CD2Cl2; -80 °C): δ 7.16 (m,
6H, 4-Me-C6H4), 6.97 (m, 6H, 4-Me-C6H4), 6.37 (s, 1H, CH),
6.04 (s, 1H, pz* H4/H4′), 5.93 (s, 1H, pz* H4/H4′), 3.03 (m,
1H, PdCHRCHR-µ-H), 2.33 (s, 9H, 4-Me of 4-Me-C6H4), 2.29
(s, 3H, pz* Me), 2.13 (s, 3H, pz* Me), 2.06 (s, 3H, pz* Me),
1.69 (s, 3H, pz* Me), 0.42 (m, 1H, PdCHRCHR-µ-H), -9.50
(m, 1H, PdCHRCHR-µ-H). 1H–1H COSY correlations: Pd(CH-
CH(R)-µ-H)(R) to Pd(CHCH(R)-µ-H)(R) and Pd(CHCH(R)-
µ-H)(R) to Pd(CHCH(R)-µ-H)(R). 13C{1H} NMR CD2Cl2; -80
°C): δ 151.9 (3/3′-pz*), 149.5 (3/3′-pz*), 142.8 (5/5′-pz*), 141.9,
141.1 (5/5′-pz*), 136.7, 129.2, 124.1, 108.0 (4/4′-pz*), 106.7
(4/4′-pz*), 63.5 (CH), 22.85 (4-Me-C6H4), 14.6 (pz* Me), 14.4
(pz* Me), 12.1 (pz* Me), 11.1 (pz* Me); the alky region is
complex. The solution of 8 was warmed to 0 °C, and broadening
of the PdCHRCHR-µ-H signals was observed. 1H NMR
(CD2Cl2, 0 °C): δ 7.15 (m, 6H, 4-Me-C6H4), 7.02 (m, 6H, 4-Me-
C6H4), 6.47 (CH), 5.98 (s, 1H, pz* H4/H4′), 5.88 (s, 1H, pz*
H4/H4′), 3.09 (br, 0.5H, trans-PdCHRCHR-µ-H, 2.72 (s, 0.5H
cis-PdCHRCHR-µ-H), 2.32 (s, 9H, 4-Me-C6H4), 2.28 (s, 3H,
pz* Me), 2.14 (s, 3H, pz* Me), 2.05 (s, 3H, pz* Me), 1.71 (s,
3H, pz* Me), -8.75 (br, 0.1H, PdCHRCHR-µ-H), -9.22 (br,
0.4H, PdCHRCHR-µ-H).31
formula
C64H72Cl4N8Pd2Si2 + 6(CHCl3)
fw
2080.29 (including solvent)
0.25 × 0.25 × 0.25
1.543
cryst size (mm)
Dcalc (Mg/m3)
cryst syst
space group
a (Å)
triclinic
j
P1
13.379(2)
b (Å)
18.531(3)
c (Å)
20.735(4)
R (deg)
114.939(3)
103.056(3)
92.893(3)
4478(1)
ꢀ (deg)
γ (deg)
V (Å3)
Z
2
temperature (K)
cryst color, habit
goodness-of-fit on F2
final R indices (I > 2σ(I))a
R indices (all data)a
100
red-orange, fragment
1.082
R1 ) 0.0479, wR2 ) 0.1148
R1 ) 0.0538, wR2 ) 0.1186
2
2 2
a R1 ) ∑Fo| - |Fc/∑|Fo|; wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo ) ]]1/2
,
2
where w ) q[σ2(Fo ) + (aP)2 + bP]-1
.
to pyrazole ring exchange; the alkyl region is complex. A
solution of NaOMe in MeOH (0.050 mL, 0.4 M) was added to
the solution of 8a, resulting in the immediate formation of Pd0.
The mixture was exposed to air and diluted with hexanes (2
mL). The resulting black slurry was filtered through a pad of
silica with hexanes/CH2Cl2 (10/1 by volume). The filtrate was
analyzed by GC/MS, which showed that a mixture of C5–C17
alkyl methyl esters had formed.
Generation of [{((p-tolyl)3Si)HC(pz*)2}PdR(H2CdCH2)]-
[B(C6F5)4] (9). A valved NMR tube containing a solution of 8
(0.020 mmol) in CD2Cl2 (0.5 mL) containing Et2O (0.060 mmol,
as an internal standard) was cooled to -196 °C, and ethylene
(0.82 mmol) was added by vacuum transfer. The tube was
thawed at -78 °C and transferred to a precooled (-80 °C) NMR
probe. NMR spectra showed that an equilibrium mixture of 8,
9, and free ethylene had formed. The equilibrium constant, Keq
Identification of the Distribution of Alkyl Chain Lengths
in 8 by Carbonylation to Generate [{(p-tolyl)3SiCH-
(pz*)2}Pd((CdO)(R)) CO][B(C6F5)4] (8a) and Methanoly-
sis to Give a Mixture of Esters. A valved NMR tube containing
a CD2Cl2 (0.6 mL) solution of 8 (0.020 mmol) was cooled to
-196 °C and exposed to CO (1 atm) for 5 min. The tube was
sealed, warmed to -78 °C, and briefly warmed to 23 °C with
vigorous shaking. A slurry of white solid in a colorless
supernatant formed. The tube was kept at -78 °C and
transferred to a precooled (-40 °C) NMR probe, and NMR
spectra were recorded. The NMR spectra showed that 8a had
1
) [9][8]-1[H2CdCH2]-1, was determined by H NMR from
-80 to -55 °C using the integrals of the (p-tolyl)3SiCH
resonances for 8 and 9 and the free ethylene resonance,
referenced to the Et2O internal standard. Keq ) 35 M-1 at -80
°C. The resonances for the alkyl group (distribution of chain
lengths) could not be assigned because they overlap with those
1
of the N^N ligand and 8. Key data for 9: H NMR (CD2Cl2,
-80 °C): δ 7.20 (m, 6H, 4-Me-C6H4), 7.02 (m, 6H, 4-Me-C6H4),
6.69 (CH), 6.08 (s, 1H, pz* H4/H4′), 5.82 (s, 1H, pz* H4/H4′),
4.15 (br m, 4H, Pd(H2CdCH2)), 2.33 (s, 9H, 4-Me-C6H4), 2.28
(s, 3H, pz* Me), 1.97 (br s, 3H, pz* Me), 1.63 (s, 3H, pz* Me);
the alky region is complex. 13C{1H} NMR (CDCl2F, -100 °C):
δ 151.2 (3/3′-pz*), 150.3 (3/3′-pz*), 142.6, 142.0 (5/5′-pz*),
140.9 (5/5′-pz*), 136.9, 129.4, 124.8. 108.5 (4/4′-pz*), 108.3
(4/4′-pz*), 93.1 (br, Pd(H2CdCH2)), 85.3 (br, Pd(H2CdCH2),
63.8 (CH), 21.2 (4-Me-C6H4), 14.6 (pz* Me), 12.2 (pz* Me),
11.8 (pz* Me), 10.7 (pz* Me); the alkyl region is complex. A
solution of 9 in CDCl2F was generated as described above, and
variable-temperature (-80 to -120 °C) NMR spectra were
1
formed. Key NMR data: H NMR (CD2Cl2, -40 °C; fast pz*
exchange): δ 7.23 (m, 6H, 4-Me-C6H4), 7.17 (m, 6H, 4-Me-
C6H4), 6.54 (s, 1H, CH), 6.03 (br s, 2H, pz* H4/H4′), 2.36 (s,
9H, 4-Me-C6H4), 2.16 (s, 6H, pz* Me), 1.76 (s, 6H, pz* Me);
the alkyl region is complex. 13C{1H} NMR (CD2Cl2, -40 °C,
for 8a generated using 13CO): δ 217.9 (C(O)R), 216.3 (C(O)R),
215.6 (C(O)Me), 215.5 (C(O)R), 215.3 (C(O)R), 215.3 (C(O)R),
212.4 (C(O)R), 212.2 (C(O)R), 172.9 (PdCO), 172.8 (PdCO),
172.7 (PdCO), 171.8 (PdCO), 142.4, 136.9, 129.5, 124.6, 63.4
(CH), 63.3 (CH), 21.5 (4-Me-C6H4); free CO was observed at
δ 184.3; the pz* signals were broadened into the baseline due
1
measured. H NMR (CDCl2F, -120 °C): δ 4.45 (br m, 1H,
(31) The integrals for the agostic-H signals are low due to exchange
between agostic and nonagostic ꢀ-hydrogens. For dynamics of cationic ꢀ-H-
agostic Pd alkyl complexes see: Shultz, L. H.; Tempel, D. J.; Brookhart,
M. J. Am. Chem. Soc. 2001, 123, 11539.
Pd(H2CdCH2)), 3.91 (br m, 1H, Pd(H2CdCH2)), 3.57 (br m,
1H, Pd(H2CdCH2)), 2.75 (br m, 1H, Pd(H2CdCH2)). The
rotational barrier ∆Gq ) 8.3 kcal mol-1 was obtained from the

Cationic Palladium(II) Alkyl Complex
Organometallics, Vol. 26, No. 27, 2007 6759
coalescence of the δ 3.91 and 3.57 resonances, using the
coalescence approximation (krot ) 380 s-1 at Tcoalescence ) -90
°C).32
Mo KR radiation (0.71073 Å). The space group was determined
as P1 based on systematic absences and intensity statistics.
j
Direct methods were used to locate the Pd, Cl, Si, and most C
atoms from the E-map. Repeated difference Fourier maps
allowed recognition of all expect C and N atoms. Following
anisotropic refinement of all non-H atoms, ideal H atom
positions were calculated. Final refinement was anisotropic for
non-H atoms and isotropic-riding for H atoms. The asymmetric
unit contains two molecules of 2, which differ only in the
orientation of the apical Si(p-tolyl)3 group due to rotation around
the (p-tolyl)3Si-CH(3,5-Me2-pz)2 bond, and six CHCl3.
Measurement of pz* Exchange Rates. pz* exchange rates
1
for 3, 4, 7, and 9 were determined by variable-temperature H
NMR spectroscopy. Rate constants (kexch) were determined by
simulation of the pz* H4/H4′ line shapes using gNMR.33 The
line width of the (p-tolyl)3SiCH resonance was used as the line
width in the absence of exchange. The measured kexch values
were independent of [Pd], and of the [free CO] for 4 and [free
ethylene] for 7. Activation parameters were determined from
Eyring plots for each run.
X-Ray Crystallographic Analysis of {(p-tolyl)3SiCH(3,5-
Me2-pz)2}PdCl2 (2). Single crystals of 2 were obtained by slow
diffusion of pentane into a concentrated CHCl3 solution at
23 °C. Crystallographic data are summarized in Table 3. Data
were collected on a Bruker Smart Apex diffractometer using
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG-02-00ER15036).
Supporting Information Available: NMR data, synthetic
procedures, determination of ethylene binding constants and pz*
site exchange rates, and complete mechanism of pz* exchange. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
(32) The other two ethylene 1H NMR resonances (δ 4.45, 2.75) were
broadened into the baseline at -90 °C.
(33) gNMR, V. 4.1.2b; Adept Scientific: Letchworth, UK, 2000.
OM7007698