Mechanistic studies of palladium(II)-a-diimine-catalyzed polymerizations of cis- and trans-2-butenes

Article abstract of DOI:10.1021/om040077m

Complexes [(N∧N)Pd(CH₃(L)]BAr′₄ (3, 4: L = NCAr′; 5: L = OEt₂) (N∧N ≡ ArN=C(R)-C(R)=NAr, 3, 5: Ar = 2,6-C₆H₃(Me)₂; 4: Ar = 2,6-C ₆H₃(ⁱPr)₂; Ar′ ≡ 3,5-C₆H₃(CF₃)₂) catalyze the polymerization of trans- and cis-2-butene. Both the productivity and the molecular weight of poly(cis-2-butene) are much lower than those of poly(trans-2-butene). The polymers exhibit atactic regioregular microstructure with a methyl group on every third backbone carbon. Low-temperature NMR studies show that migratory insertion in the η²-butene complexes [(N∧N)Pd(CH₃)(CH₃CH=CHCH₃)]⁺ (6 and 11) occurs to give isomerized alkyl olefin complexes [(N∧N)Pd(CH ₂CH₂CH(CH₃)₂)(CH₃CH= CHCH₃)]⁺ (8 and 12). The first insertion barrier of trans-2-butene (19.1 kcal/mol) is slightly lower than that of cis-2-butene (19.3 kcal/mol), while the subsequent insertion barrier of cis-2-butene (20.2 kcal/mol) is 0.6 kcal/mol higher than that for trans-2-butene (19.6 kcal/mol). The isopentyl palladium complexes 14a and 14b, which most closely model the propagating species, exhibit nearly equal binding affinities for cis- and trans-2-butene.

Full text of DOI:10.1021/om040077m

Organometallics 2004, 23, 6099-6107
6099
Articles
Mechanistic Studies of
Palladium(II)-r-Diimine-Catalyzed Polymerizations of
cis- and trans-2-Butenes
Weijun Liu and Maurice Brookhart*
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received June 1, 2004
Complexes [(N∧N)Pd(CH3)(L)]BAr′4 (3, 4: L ) NCAr′; 5: L ) OEt2) (N∧N ≡ ArNdC(R)-
C(R)dNAr, 3, 5: Ar ) 2,6-C6H3(Me)2; 4: Ar ) 2,6-C6H3(iPr)2; Ar′ ≡ 3,5-C6H3(CF3)2) catalyze
the polymerization of trans- and cis-2-butene. Both the productivity and the molecular weight
of poly(cis-2-butene) are much lower than those of poly(trans-2-butene). The polymers exhibit
atactic regioregular microstructure with a methyl group on every third backbone carbon.
Low-temperature NMR studies show that migratory insertion in the η2-butene complexes
[(N∧N)Pd(CH3)(CH3CHdCHCH3)]+ (6 and 11) occurs to give isomerized alkyl olefin
complexes [(N∧N)Pd(CH2CH2CH(CH3)2)(CH3CHdCHCH3)]+ (8 and 12). The first insertion
barrier of trans-2-butene (19.1 kcal/mol) is slightly lower than that of cis-2-butene (19.3
kcal/mol), while the subsequent insertion barrier of cis-2-butene (20.2 kcal/mol) is 0.6 kcal/
mol higher than that for trans-2-butene (19.6 kcal/mol). The isopentyl palladium complexes
14a and 14b, which most closely model the propagating species, exhibit nearly equal binding
affinities for cis- and trans-2-butene.
Introduction
A unique feature of late metal catalyst systems based
on Ni(II) and Pd(II) is the ability to produce polyolefins
with very different microstructures relative to those
produced using early metal catalysts.1-3 In a series of
papers employing R-diimine complexes bearing bulky
aryl groups as illustrated in Figure 1, we,4-6 the DuPont
group,7,8 and subsequently others9-12 have shown that
polymerization of ethylene results in production of
branched polyethylenes, while polymerization of R-ole-
(1) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
Figure 1. (R-diimine)Ni(II) and Pd(II) olefin polymeriza-
1169-1203.
tion catalysts.
(2) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534-540.
(3) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283-
315.
fins results in polymers with many fewer branches than
expected compared to polymers produced via monomer
enchainment through sequential 1,2-additions. Poly-
(cyclopentene) exhibits 1,3-monomer enchainment.13
These unique microstructures are attributed to the
ability of Ni or Pd to migrate along the polymer chain
via a series of â-hydride elimination/reinsertion reac-
tions14 (“chain walking”) in competition with monomer
insertion. For example, in the case of ethylene initial
insertion forms a primary metal alkyl bond; subsequent
chain walking produces secondary metal alkyl species,
(4) Johnson, L. K.; Killian, C. K.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415.
(5) Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart, M. J.
Am. Chem. Soc. 1996, 118, 11664-11665.
(6) Gates, D. P.; Svejda, S. A.; On˜ate, E.; Killian, C. M.; Johnson,
L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320-
2334.
(7) Johnson, L. K.; Killian, C. M.; Arthur, S. D.; Feldman, J.;
McCord, E. F.; McLain, S. J.; Kreutzer, K. A.; Bennett, A. M. A.;
Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J.;
Brookhart, M. S. WO 9623010, 1996.
(8) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999,
283, 2059-2062.
(9) Pellecchia, C.; Zambelli, A.; Oliva, L.; Pappalardo, D. Macro-
molecules 1996, 29, 6990-6993.
(10) Pellecchia, C.; Zambelli, A. Macromol. Rapid Commun. 1996,
17, 333-338.
(13) McLain, S. J.; Feldman, J.; McCord, E. F.; Gardner, K. H.;
Teasley, M. F.; Coughlin, E. B.; Sweetman, K. J.; Johnson, L. K.;
Brookhart, M. Macromolecules 1998, 31, 6705-6707.
(14) Mo¨hring, V. M.; Fink, G. Angew. Chem., Int. Ed. Engl. 1985,
24, 4, 1001-1003.
(11) Pellecchia, C.; Zambelli, A.; Mazzeo, M.; Pappalardo, D. J. Mol.
Catal. A: Chem. 1998, 128, 229-237.
(12) Pappalardo, D.; Mazzeo, M.; Pellecchia, C. Macromol. Rapid
Commun. 1997, 18, 1017-1023.
10.1021/om040077m CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/20/2004

6100 Organometallics, Vol. 23, No. 26, 2004
Liu and Brookhart
Scheme 1. Chain-Straightening Model in r-Diimine Ni(II)- and Pd(II)-Catalyzed Propylene Polymerization
which upon migratory insertion creates a branch in the
polymer chain. Fewer branches than expected in poly-
(R-olefin)s arise due to 2,1-insertion followed by chain
walking to form a primary metal alkyl bond, which then
undergoes insertion. Insertion of an R-olefin into a pri-
mary bond is sterically favored over insertion into a
secondary alkyl bond. In the case of propylene, this
process can result in 1,3-enchainment, as illustrated in
Scheme 1.
While early metal catalysts are highly active for
polymerization of R-olefins, none are known to homopo-
lymerize disubstituted internal olefins, presumably due
to steric constraints. Alternating copolymerization of
ethylene and 2-butenes has been reported using early
metal catalysts, as has random copolymerizations where
the mole fraction of ethylene incorporated exceeds
50%.15-19 Recently, we have reported the homopolym-
erization of trans-2-butene using Ni(II) diimine catalysts
1 and 2 (Figure 2).20 Turnover rates at 25 °C are ca.
polymer is regioregular, and the chain bears a methyl
group on every third carbon. The 13C NMR analysis
established a totally atactic stereochemistry. Curiously,
although trans-2-butene is polymerized with good activ-
ity, we were unable to observe polymer formation from
cis-2-butene. Coates has reported formation of a par-
tially isotactically enriched polymer using R-diimine Ni-
(II) catalysts constructed to possess C2 symmetry.21 In
these cases only trans-2-butene yielded polymer.
In this article we report the homopolymerization of
cis- and trans-2-butenes by Pd(II) diimine complexes
and an in-depth mechanistic study that provides insight
into the differences in behavior of these monomers
toward both Ni(II) and Pd(II) diimine catalysts.
Results and Discussion
Polymerization of trans- and cis-2-Butenes. The
catalysts employed for the copolymerization reactions
were the cationic R-diimine palladium complex 3 and
4 (Scheme 2). The R-diimine ligands22 and the pallad-
ium chloride complexes,4 [(ArNdC(An)-(An)CdNAr)-
Pd(Me)(Cl)], were synthesized following the procedures
in the literature. The cationic complexes 3 and 4 were
prepared in an analogous manner to the procedure pub-
lished for the corresponding compound [(ArNdC(Me)-
(Me)CdNAr)Pd(Me)(NCAr′)].23
The activity of R-diimine palladium complex 3 and 4
for the polymerization of 2-butenes was investigated.
Both catalysts were found to be active for the polym-
erization of both trans- and cis-2-butene without the
need for an activator or cocatalyst. Polymerizations were
initiated by the addition of the palladium catalyst to a
butene-saturated solution in a glass pressure reaction
vessel (Fischer-Porter bottle) and stirring under mono-
mer pressure. The polymerization results are sum-
marized in Table 1.
The results show that with the same catalyst and un-
der similar reaction conditions, the productivity (shown
as turnover number, TON) of cis-2-butene polymeriza-
tion is much lower than that for trans-2-butene. The
reasons for this difference in reactivity are addressed
Figure 2. (R-dimine)Ni(II) complexes.
1700 turnovers/hour for 1 and 1100/hour for 2. The
microstructure of the polymer is illustrated below. The
(15) Natta, G.; Dall’asta, G.; Mazzanti, G.; Pasquon, I.; Valvassori,
A.; Zambelli, A. J. Am. Chem. Soc. 1961, 83, 3343.
(16) Natta, G. Pure Appl. Chem. 1962, 4, 363.
(17) Longo, P.; Grisi, F.; Guerra, G.; Cavallo, L. Macromolecules
2000, 33, 4647-4659.
(18) Ahn, C.-H.; Tahara, M.; Uozumi, T.; Jin, J.; Tsubaki, S.; Sano,
T.; Soga, K. Macromol. Rapid Commun. 2000, 21, 385-389.
(19) It has been reported that Ti and V catalyst systems successfully
catalyze the homopolymerization of internal olefins, but it is believed
that the polymerization occurs because the internal olefins isomerize
to form R-olefins first. This process is termed a monomer-isomerization
polymerization. Addition of group VIII transition metal compounds
such as NiX2 (X ) Br, Cl), which are known to be good isomerization
catalysts, accelerates such polymerization reactions. (a) Endo, K.; Ueda,
R.; Otsu, T. J. Polym. Sci., Polym. Chem. 1991, 29, 807-812. (b) Endo,
K.; Ueda, R.; Otsu, T. Macromolecules 1991, 24, 6849-6852. (c) Endo,
K.; Ueda, R.; Otsu, T. Polym. J. 1991, 23, 1173-1178. (d) Endo, K.;
Ueda, R.; Otsu, T. Makromol. Chem. 1992, 193, 539-547. (e) Endo,
K.; Ueda, R.; Otsu, T. Makromol. Chem. 1993, 194, 2623.
(20) Leatherman, M. D.; Brookhart, M. Macromolecules 2001, 34,
2748-2750.
(21) Cherian, A. E.; Lobkovsky, E. B.; Coates, G. W. Chem. Commun.
2003, 20, 2566-2567.
(22) van Koten, G.; Virieze, K. Adv. Organomet. Chem. 1982, 21,
151-239.
(23) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am.
Chem. Soc. 1998, 120, 888-899.

Pd(II)-R-Diimine-Catalyzed Polymerizations
Organometallics, Vol. 23, No. 26, 2004 6101
Scheme 2. Synthesis of r-Diimine Pd(II) Catalysts 3 and 4
Table 1. trans- and cis-2-Butene Polymerization
the repeat units with ethyl branches, -CH2-CH-
(CH2CH3)-.
Dataa
pressure/
psi
branches/
1000Cc
Migratory Insertion of trans-2-Butene and cis-
2-Butene: Barriers. The precursor employed for gen-
eration of 2-butene complexes is the cationic R-diimine
palladium complex 5 (Scheme 3), which was synthesized
using procedures in the literature.4,24 Addition of ca. 4
equiv of trans-2-butene to a CD2Cl2 solution of 5 at -80
°C leads to clean formation of the η2-olefin complex 6.
The displacement is fast and complete at this temper-
ature. Key 1H NMR resonances for the η2-trans-2-butene
complex 6 include δ 5.16 and 4.52 ppm for the bound
olefinic protons and δ 0.51 ppm for the Pd-CH3 methyl
group. Complex 6 is stable when the temperature is
below -30 °C, and no measurable migratory insertion
was observed at this stage.
c
entry catalyst 2-butene
TONb
1110 25 800
180 2300
390 24 800
120 2200
Mn
1
2
3
4
3
3
4
4
trans
cis
trans
cis
14
13
14
13
264
294
264
281
a Reaction conditions: 10 µmol of catalyst, 15 mL of toluene,
25 °C, 3 h. b TON: turnover number. c Determined by 1H NMR
spectroscopic analysis of the polymer.
by mechanistic studies described below. At the same
time, the molecular weight of poly(cis-2-butene) is ca.
11 times lower than that of poly(trans-2-butene). This
is thought to be mainly due to the slow propagation rate
of cis-2-butene as well as the relative chain transfer
rates. For example, with catalyst 3, each palladium
center undergoes, on average, ca. two chain transfers
during trans-2-butene polymerization (entry 1), while
about four chain transfers occur on average during cis-
2-butene polymerization (entry 2). This difference in
relative chain transfer rates together with the difference
in propagation rates account for the lower molecular
weight of poly(cis-2-butene).
Migratory insertion of the trans-2-butene occurs at
-30 °C and higher temperatures, yielding exclusively
the η2-olefin palladium alkyl complex 8, which is as-
sumed to be formed from the direct 2,3-insertion product
7 via a chain-straightening isomerization mechanism
as proposed in Scheme 4. The chain-straightening
process involves â-hydride elimination, olefin rotation,
and reinsertion. Neither complex 7 nor other intermedi-
ates were observed under these conditions, presumably
due to the fast rate of the isomerization and trapping
processes. No identifiable ethyl-branched alkyl product
was observed, which indicates that pathway A is favored
over pathway B (Scheme 4) probably due to less favored
trapping of the â-substituted primary alkyl species 10
Increasing the steric bulk of the ortho aryl substituent
from methyl to isopropyl results in a decrease in
productivity, while the molecular weights of polymer
produced remain basically the same.
Analysis by 1H NMR spectroscopy indicates that the
polymers contain slightly more than half (260-290) the
number of branches per 1000 carbon atoms expected
from standard 2-butene enchainment (500 branches/
1000C). As shown in Figure 3, the 13C NMR spectrum
of the polymer is similar to that for the poly(trans-2-
butene) produced by nickel catalysts20 and shows dis-
tinct methyl (20.1 ppm), methylene (34.8 ppm), and
methine (33.6 ppm) carbon resonances, which is con-
sistent with a regioregular polymer microstructure
carrying a methyl branch on every third carbon. The
ca. 1:2:1 pattern of methyl resonance indicates an atactic
microstructure with a 1:2:1 ratio of rr:mr:mm dyads.
The minor resonances at 11.6 and 30.5 ppm belong to
the methyl and methylene carbons, respectively, of
1
relative to 9.25 Key H NMR features of the insertion
product 8 include δ 0.59 and 0.40 ppm resonances for
Figure 3. 13C NMR spectrum of poly(trans-2-butene).

6102 Organometallics, Vol. 23, No. 26, 2004
Scheme 3. Synthesis of r-Diimine Pd(II) Catalysts 3 and 4
Liu and Brookhart
Scheme 4. Proposed Pd(II)-Catalyzed 2-Butene Polymerization Mechanism
the diastereotopic methyl groups at the terminus of the
butene in CD2Cl2 at -80 °C results in clean formation
of the η2-olefin complex 11 (Scheme 5). Similar to the
case of trans-2-butene, the displacement is fast and
complete at this temperature. Key 1H NMR resonances
for the η2-cis-2-butene complex 11 include δ 5.12 ppm
for the bound olefinic protons and δ 0.33 ppm for the
Pd-CH3 methyl group. Complex 11 is also stable when
the temperature is below -30 °C, and no measurable
migratory insertion was observed at this stage.
Similar migratory insertion behavior as seen in the
trans-2-butene case was observed at -30 °C and higher
temperatures, yielding exclusively the η2-olefin pal-
ladium alkyl complex 12, the product formed from the
direct 2,3-insertion to yield 7 followed by isomerization
via the mechanism proposed in Scheme 5. The 2,3-
insertion product 7 is identical to the one for trans-2-
butene insertion (Scheme 3) and was not observed.
Again, no identifiable ethyl branched alkyl product was
observed. Key 1H NMR resonances of the insertion
alkyl chain, and δ 4.83 and 4.68 ppm vinylic signals of
the bound trans-2-butene. At -30 °C, no significant
amount of subsequent insertion product was observed.
The energy barrier (∆Gq) of trans-2-butene migratory
insertion was determined to be 19.1 ( 0.1 kcal/mol at
-20 °C by following the disappearance of the Pd-CH3
signal (complex 6) by 1H NMR spectroscopy. The barrier
is about 0.7 kcal/mol higher than that for ethylene
migratory insertion to the Pd-Me bond in the analogous
system (∆Gq ) 18.4 kcal/mol at -20 °C),24 which means
that the insertion rate of ethylene is ca. 4 times faster
than that of trans-2-butene at this temperature.
The insertion chemistry of cis-2-butene was also
investigated. Treatment of 5 with ca. 4 equiv of cis-2-
(24) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686-6700.
(25) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 11539-11555.

Pd(II)-R-Diimine-Catalyzed Polymerizations
Organometallics, Vol. 23, No. 26, 2004 6103
Scheme 5. Migratory Insertion of cis-2-Butene into the Pd-Me Bond of Complex 5
product 12 appear at δ 0.46 ppm for the two equivalent
methyl groups at the end of the alkyl chain, δ 5.24 ppm
for the olefinic protons of bound trans-2-butene, and δ
1.19 ppm for the methylene protons of Pd-CH2. At -30
°C, no significant amount of subsequent insertion
product was observed.
kcal/mol higher than that for the first insertion of cis-
2-butene (∆Gq ) 19.3 kcal/mol at -10 °C). Compared
with the insertion barrier for trans-2-butene, this
activation barrier is 0.6 kcal/mol higher, which corre-
sponds to a ca. 3-fold difference in insertion rates at this
temperature.
Relative Binding Affinities. For bulky internal
olefins such as trans- and cis-2-butenes, their binding
affinities to the metal center could possibly affect the
productivities of the transition metal catalyzed polym-
erization reactions, given the fact that there are often
some stronger ligands such as acetonitrile and benzoni-
trile in the reaction mixture.
The energy barrier (∆Gq ) 19.3 ( 0.1 kcal/mol) of cis-
2-butene migratory insertion was determined in a
1
manner similar to that for trans-2-butene by H NMR
spectroscopy at -20 °C. The barrier is about 0.2 kcal/
mol higher than that for trans-2-butene migratory
insertion (∆Gq ) 19.1 kcal/mol at -20 °C), which shows
that the first insertion rate of trans-2-butene is only
slightly faster (ca. 1.6-fold) than that of cis-2-butene at
this temperature. This difference is less than that in
the real polymerization reactions (Table 1, entries 1 and
2), where the trans-2-butene propagation rate is ca. 6
times faster than that for cis-2-butene under similar
reaction conditions.
Rates of Subsequent Insertion Reactions. Sub-
sequent insertions of trans- and cis-2-butenes into the
growing polymer chain, which mimic the real chain
propagation during the polymerization reactions, were
investigated by following the decreasing signal for free
trans- and cis-2-butenes in the solution in the presence
of complex 8 or 12.
The rate of the subsequent insertion of trans-2-butene
(eq 1) was measured in CD2Cl2 at -10 °C, and the
corresponding activation barrier is 19.6 ( 0.1 kcal/mol,
which is ca. 0.5 kcal/mol higher than the barrier of first
insertion (∆Gq ) 19.1 kcal/mol at -10 °C). This follows
the same trend as ethylene homopolymerization chem-
istry, where the barrier for subsequent insertion (∆Gq
) 18.6 kcal/mol at -20 °C) is about 0.2 kcal/mol higher
than that of the first insertion (∆Gq ) 18.4 kcal/mol at
-20 °C).24
The relative binding affinities of trans- and cis-2-
butenes were investigated using 1H NMR spectroscopy.
The equilibrium constants for the reactions depicted in
eq 3 were determined by low-temperature 1H NMR
spectroscopy and are summarized in Table 2. The
equilibrium constant could not be precisely measured
in a single competition experiment because the reso-
nances of trans- and cis-2-butenes overlap with each
other. Trimethoxyvinylsilane was chosen as a compari-
son ligand because its spectrum is clearly separated
from those of trans- and cis-2-butenes. The relative
binding affinities of trans-2-butene versus trimethox-
yvinylsilane and cis-2-butene versus trimethoxyvinyl-
Table 2. Equilibrium Constants for Eq 3
T
∆G
entrya (°C)
L
L′
Keq (kcal/mol)
1
2
3
4
5
6
7
8
9
-70 trimethoxyvinylsilane cis-2-butene 3.4
-70 trimethoxyvinylsilane trans-2-butene 0.66
-0.49
0.17
-70 trans-2-butene
cis-2-butene 5.1b
-0.66
-0.46
0.017
-0.48
-0.47
-0.15
-0.32
-60 trimethoxyvinylsilane cis-2-butene 3.0
-60 trimethoxyvinylsilane trans-2-butene 0.96
-60 trans-2-butene
cis-2-butene 3.1b
-50 trimethoxyvinylsilane cis-2-butene 2.9
-50 trimethoxyvinylsilane trans-2-butene 1.4
Similarly, the subsequent insertion (eq 2) rate for cis-
2-butene was measured at -10 °C. The corresponding
energy barrier is 20.2 ( 0.1 kcal/mol, which is ca. 0.9
-50 trans-2-butene
cis-2-butene 2.0b
a Solvent: CD2Cl2. b Calculated.

6104 Organometallics, Vol. 23, No. 26, 2004
Liu and Brookhart
Scheme 6. Synthesis of Pd(II) Dialkyl Complex 13
silane were determined and used to calculate the
relative binding affinities of trans-2-butene versus cis-
2-butene as given by Keq for the equation in eq 3, where
L ) trans-2-butene, L′ ) cis-2-butene.
Scheme 7. Trapping of Agostic Cations 14a,b with
2-Butenes
Results show that cis-2-butene binds slightly more
strongly than trans-2-butene at low temperatures. At
-70 °C, the equilibrium constant is ca. 5.1 (corre-
sponding ∆G ) -0.66 kcal/mol), while it is 3.1 (corre-
sponding ∆G ) -0.48 kcal/mol) at -60 °C and 2.0
(corresponding ∆G ) -0.32 kcal/mol) at -50 °C. A rough
Van’t Hoff analysis yields approximate enthalpy and
entropy values for the equilibrium (∆H ) -4.1 kcal/mol,
∆S ) -17 eu). From these data the equilibrium con-
stant at 25 °C was calculated to be 0.2, which means
the palladium center slightly favors trans-2-butene
over cis-2-butene under polymerization reaction condi-
tions.
Considering that the palladium methyl complexes 6
and 11 are not good models for the real polymerization
system, dialkyl palladium complex 13 (Scheme 6) was
employed for further investigation of the binding affini-
ties of the 2-butenes. The palladium dichloride com-
plexes [(ArNdC(An)-(An)CdNAr)Pd(Cl)2] were syn-
thesized following the procedure in the literature.26
Complex 13 was synthesized in a manner analogous to
the procedure published for the corresponding com-
pound [(ArNdC(An)-(An)CdNAr)Pd(n-Pr)2].25
Addition of 1 equiv of HBAr′(OEt2)2 to the solution of
13 in CD2Cl2 at -78 °C results in formation of 1 equiv
of 2-methylpentane and two â-agostic complexes, 14a
and 14b, in a ratio of ca. 3:1, as evidenced by two broad
agostic hydrogen signals at -7.35 and -7.25 ppm. The
downfield signals, which partially overlap the reso-
nances of 2-methylpentane, are broadened due to rapid
isomerization, and structures of the isomers present
cannot be definitively assigned. However, treatment of
these agostic species with either trans-2-butene or cis-
2-butene results in clean formation of 8 or 12, respec-
tively (Scheme 7). These trapping experiments, which
yield the most stable primary alkyl complexes, are
consistent with our earlier reports of similar trapping
experiments.25
Table 3. Equilibrium Constants for Eq 4
T
∆G
entrya (°C)
L1
L2
Keq (kcal/mol)
1
2
3
4
5
6
7
8
9
-80 trimethoxyvinylsilane cis-2-butene
1.6
-0.18
-0.20
0.02
-0.28
-0.28
0.00
-0.35
-0.31
-0.04
-80 trimethoxyvinylsilane trans-2-butene 1.7
-80 trans-2-butene
cis-2-butene
0.94b
2.0
-70 trimethoxyvinylsilane cis-2-butene
-70 trimethoxyvinylsilane trans-2-butene 2.0
-70 trans-2-butene
cis-2-butene
1.0b
2.3
-60 trimethoxyvinylsilane cis-2-butene
-60 trimethoxyvinylsilane trans-2-butene 2.1
-60 trans-2-butene cis-2-butene
1.10b
a Solvent: CD2Cl2. b Calculated.
methoxyvinylsilane was again used as a comparison
ligand for the same reason as above. The equilibrium
constants of the trans-2-butene/trimethoxyvinylsilane
pair and trimethoxyvinylsilane/cis-2-butene pair were
used to calculate the relative binding affinities of trans-
2-butene versus cis-2-butene as given by Keq for the
equation shown in eq 4, where L ) trans-2-butene and
L′ ) cis-2-butene.
Results show that there is almost no difference
between the binding strength of trans-2-butene and that
of cis-2-butene. At -80 °C, the equilibrium constant is
ca. 0.94 (corresponding ∆G ) 0.02 kcal/mol), while it is
1.0 (corresponding ∆G ) 0.00 kcal/mol) at -70 °C and
1.1 (corresponding ∆G ) -0.04 kcal/mol) at -60 °C.
Earlier studies showed that in the polymerization of
R-olefins using nitrile complexes analogous to 3 and 4
the catalyst resting state was an equilibrium mixture
of the R-olefin complex and the nitrile complex.27 In the
case of the bulkier 2-butenes, this equilibrium clearly
will be shifted more to the side of the nitrile complex.
Thus, relative binding affinities of the two olefins, cis-
and trans-2-butene, could play a significant role in their
relative polymer propagation rates. However, the fact
that the binding affinities of the two olefins are nearly
Clean generation of either 8 or 12 allows study of the
relative binding affinities as generally illustrated by
reactions in eq 4. The equilibrium constants of these
reactions were measured by low-temperature 1H NMR
spectroscopy and are summarized in Table 3. Tri-
(26) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975-
3982.
(27) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085-
3100.

Pd(II)-R-Diimine-Catalyzed Polymerizations
Organometallics, Vol. 23, No. 26, 2004 6105
et al.30 High-purity (99.0+ %) trans-2-butene and cis-2-butene
were purchased from Aldrich Chemical Co. and used without
further purification. The R-diimine ligands (ArNdC(An)-
C(An)dNAr, An ) acenaphthyl ) 1,8-naphth-diyl)22 and corre-
sponding (R-diimine)palladium(II) chloride complexes24 were
prepared according to literature procedures. Palladium methyl
ether adduct 3,4,24 (cod)Pd(Me)(Cl),31 and [H(OEt2)2]+[BAr′4]-
identical suggests that the main factor in dictating the
higher propagation rate of trans- versus cis-2-butene is
the lower insertion barrier of the trans-2-butene relative
to the cis-2-butene.
Summary
-
- 32
(BAr′4 ) B(3,5-C6H3(CF3)2)4
)
were prepared according to
The R-diimine Pd(II) catalysts described here allow
for the polymerization of both trans- and cis-2-butenes,
although the productivities are much lower than those
for R-diimine Ni(II)-catalyzed trans-2-butene polymer-
izations. The polymers exhibit atactic regioregular mi-
crostructure with a methyl branch on every third
backbone carbon due to the chain-straightening mech-
anism. Both the productivity and the molecular weight
of poly(cis-2-butene) are much lower than those of poly-
(trans-2-butene).
Mechanistic aspects of the polymerization reactions
were investigated by low-temperature NMR spectros-
copy. The isopentyl palladium complexes 14a and 14b,
which most closely model the propagating species,
exhibit nearly equal binding affinities for cis- and trans-
2-butene. The barriers to migratory insertion of the cis-
and trans-2-butene adducts 6 and 11 of the palladium
methyl complex and the adducts 8 and 12 of the
palladium isopentyl complex were measured. For the
latter two complexes, which best model the propagating
species, the insertion barrier for the trans-2-butene
complex (19.6 kcal/mol) is ca. 0.6kcal/mol less than the
barrier for migratory insertion of the cis-2-butene
complex (20.2 kcal/mol). From these data we conclude
that the primary factor in determining the lower
propagation rate of the cis-2-butene relative to the trans-
2-butene is the lower barrier of migratory insertion of
the trans olefin.
In contrast to the Pd system, the nickel diimine
systems show a much greater difference in activities:
trans-2-butene is quite reactive and yields high polymer
with a turnover frequency of ca. 1700/h at 25 °C using
the complex analogous to 3. In the case of the cis isomer
no polymer (or oligomer) is observed. On the basis of
earlier results for R-olefins28 it is clear the catalyst
resting state is Ni agostic species. Thus, the relative
activities are determined by both the difference in
binding affinities and insertion barriers. On the basis
of the results for the Pd systems, it is likely that the
major determining factor is the difference in insertion
barriers.
literature procedures. Dichloromethane-d2 was dried over P2O5
or CaH2, vacuum transferred, degassed by three freeze-
pump-thaw cycles, and stored over activated 4 Å molecular
sieves. Acenaphthenequinone, 2,6-dimethylaniline, and 2,6-
diisopropylaniline were purchased from Aldrich Chemical Co.
and used without further purification. NaBAr4′ was purchased
from Boulder Scientific and dried in vacuo over P2O5 for 3 days.
(Cyclooctadiene)PdCl2 was purchased from Strem Chemicals
and used as received. 1-Bromo-3-methylbutane was purchased
from Aldrich Chemical Co. and used without further purifica-
tion. BrMg(CH2)2(CH)(CH3)2 was prepared using a standard
literature procedure33 and titrated with 2-butanol and 1,10-
phenathroline.
Synthesis of Catalysts. Spectral Data for the BAr′4
1
Counterion. The following H and 13C spectroscopic assign-
ments of the BAr′4 counterion in CD2Cl2 were invariant for
different complexes and temperatures and are not reported
in the spectroscopic data for each of the cationic complexes.
B[3,5-C6H3(CF3)2]4- (BAr′4). 1H NMR (CD2Cl2): δ 7.74 (s,
8H, Ho), 7.57 (s, 4H, Hp). 13C{1H} NMR (CD2Cl2): δ 162.2 (q,
JCB ) 37.4 Hz, Cipso), 135.2 (Co), 129.3 (q, JCF ) 31.3 Hz, Cm),
125.0 (q, JCF ) 272.5 Hz, CF3), 117.9 (Cp).
Synthesis
of
[(ArNdC(An)-C(An)dNAr)Pd(Me)-
(NCAr′)]+BAr′4- (Ar ) 2,6-Me2C6H3) (3). This complex was
synthesized on the basis of literature procedures for the
analogous complex with the methyl-substituted backbone.23
A
flame-dried Schlenk flask was charged with NaBAr′4 (0.1624
g, 1.83 × 10-4 mol) and [(ArNdC(An)-C(An)dNAr)Pd(Me)-
(Cl)] (0.100 g, 1.83 × 10-4 mol) in a drybox under an argon
atmosphere. The Schlenk flask was cooled to -78 °C, and
CH2Cl2 (10 mL) was added via syringe. 3,5-Bis-trifluorometh-
ylbenzonitrile (0.61 mL, ca. 20 equiv) was added, followed by
another portion of CH2Cl2 (6 mL). The resulting bright maroon
solution was allowed to warm to room temperature and stirred
overnight. The reaction mixture was filtered, leaving behind
a white precipitate of NaCl. The solvent was removed in vacuo,
leaving a dark orange oil. Pentane (15 mL) was added to the
oil, and the mixture was vigorously stirred for 1.5 h, during
which time a yellow solid precipitated from the light orange
solution. The solid was filtered and dried in vacuo for 3 h,
yielding 0.276 g of yellow powder (94%). 1H NMR (CD2Cl2, 300
MHz, 25 °C): δ 8.26 (s, 1H, Ar′) 8.22 (d, 1H, J ) 6.3 Hz, An:
Hp), 8.19 (d, 1H, J ) 6.3 Hz, An′: Hp), 8.16 (s, 2H, Ar′), 7.55
(m, 2H, An: Hm, An′: Hm), 7.37 (m, 6H, Haryl), 6.99 (d, 1H, J
) 7.2 Hz, An:Ho), 6.63 (d, 1H, J ) 7.2 Hz, An′:Ho), 2.42 and
2.31 (s, 6H each; C6H3Me2 and C′6H3Me2), 0.86 (s, 3H; PdMe).
13C NMR (CD2Cl2, 125 MHz, 25 °C): δ 176.9, (NdC(H)), 169.9
(NdC′(H)), 146.6, 143.2, 142.3, 133.8, 133.5, 133.1, 131.7,
130.0, 129.9, 129.6, 129.1, 128.9, 128.4, 128.3, 128.2, 125.5,
125.4, 125.2, 123.9, 123.3, 121.7, 121.1, 119.7, 111.5) (An: CH
and quaternary C and Ar: Cipso, Co, Cp, Cm and Ar′: Co, Cp),
Experimental Section
General Methods. All manipulations of air- and/or water-
sensitive compounds were performed using standard high-
vacuum or Schlenk techniques. Argon was purified by passage
through columns of BASF R3-11 catalyst (Chemalog) and 4 Å
molecular sieves. Solid organometallic compounds were trans-
ferred in argon-filled Vacuum Atmospheres or MBraun dry-
boxes and were stored in the drybox. 1H and 13C NMR spectra
were recorded on Bruker Avance 300 or 500 MHz spectrom-
eters. Chemical shifts are reported in ppm downfield of TMS
(28) Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc.
1999, 121, 10634-10635.
(29) Binsch, G. Dynamic Nuclear Magnetic Resonance Spectroscopy;
Jackman, L., Cotton, F. A., Eds.; Academic Press: New York, 1975; p
77.
and are referenced to residual 1H NMR signals and to the 13
NMR signals of the deuterated solvents, respectively.
C
(30) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(31) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769-5778.
(32) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
Error analysis of ∆Gq was based on Binsch’s derivation of
σ∆Gq and incorporated an estimate of 10% error in k and (1
°C error in temperature.29
Materials. Diethyl ether, pentane, and methylene chloride
were purified using procedures recently reported by Pangborn
(33) Handbook of Grignard Reagents; Silverman, G. S., Ed.; Dek-
ker: New York, 1996.

6106 Organometallics, Vol. 23, No. 26, 2004
Liu and Brookhart
18.2 and 17.9 (Ar(CH3)2 and Ar(C′H3)2), 7.39 (PdCH3). Anal.
Calcd for (C70H42BF30N3Pd): C, 52.15; H, 2.62; N 2.61.
Found: C, 52.43; H, 2.64; N, 2.63.
reaction solution was added dropwise to acidified methanol
(150 mL), and the polymer was allowed to precipitate. The
polymer was separated as a viscous oil and washed twice with
methanol, then dried in a vacuum oven overnight at 50 °C.
In Situ Generation and NMR Observation of η2-Olefin
Pd(II) Complexes. In a drybox under an argon atmos-
phere, an NMR tube was charged with ca. 0.01 mmol of
[(ArNdC(An)-C(An)dNAr)Pd(Me)(OEt2)]BAr′4. The tube was
capped with a rubber septum and removed from the drybox.
After securing the septum with Teflon tape and Parafilm, the
tube was cooled to -78 °C. CD2Cl2 was added to the NMR tube
via syringe (600 µL), and the septum was rewrapped with
Parafilm. The tube was shaken and warmed slightly to
facilitate dissolution of the complex. After acquiring a spec-
trum at -80 °C, olefin was added via syringe to the solution
cooled to -78 °C, and the NMR tube was briefly shaken to
completely dissolve the additive. The tube was then trans-
ferred to the precooled NMR probe for acquisition of spectra.
The concentrations of the active species and free olefin were
calculated using the BAr′4 or para-acenaphthyl peaks as an
internal standard.
Synthesis
of
[(ArNdC(An)-C(An)dNAr)Pd(Me)-
(NCAr′)]+BAr′4- (Ar ) 2,6-iPr2C6H3) (4). This compound was
synthesized using a procedure identical to that described above
for 3. Reagents used: NaBAr′4 (0.1624 g, 1.83 × 10-4 mol),
[(ArNdC(An)-C(An)dNAr)Pd(Me)(Cl)] (0.100 g, 1.83 × 10-4
mol), 3,5-bis-trifluoromethylbenzonitrile (0.61 mL, ca. 20
equiv). Following isolation of the crude product as an orange
oil, pentane (15 mL) was added and the mixture was cooled
to -78 °C overnight, during which time an orange solid
precipitated from the solution. The solid was filtered and dried
1
in vacuo for 3 h, yielding 0.186 g of orange powder (71%). H
NMR (CD2Cl2, 300 MHz, 25 °C): δ 8.26 (s, 1H, Ar′), 8.19 (d,
1H, J ) 5.1 Hz, An: Hp), 8.17 (d, 1H, J ) 3.0 Hz, An′: Hp),
8.16 (s, 2H, Ar′), 7.60 (m, 2H, An: Hm, An′: Hm), 7.49 (m, 6H,
Haryl), 6.98 (d, 1H, J ) 7.2 Hz, An:Ho), 6.59 (d, 1H, J ) 7.5 Hz,
An′:Ho), 3.35 and 3.24 (m, 2H each; C6H3CH and C′6H3CH),
1,41 (d, 12H, C6H3CH(Me)2), 1.09 and 1.01 (d, 6H each;
C′6H3CH(Me)2), 0.98 (s, 3H; PdMe). 13C NMR (CD2Cl2, 125
MHz, 25 °C): δ 177.0 (NdC(H)), 170.2 (NdC′(H)), 146.5,
141.1, 140.2, 139.6, 139.0, 133.9, 133.2, 131.9, 130.0, 129.7,
129.2, 128.8, 128.2, 126.9, 126.0, 125.9, 125.5, 125.2,
125.0, 123.9, 123.2, 121.7, 121.1, 119.6, 115.2, 111.3) (An: CH
[(ArNdC(An)-(An)CdNAr)Pd(CH3)(η2-trans-CH3-CHd
CH-CH3)]+[BAr′4]- (Ar ) 2,6-C6H3(CH3)2), 6. 1H NMR
(CD2Cl2, 500 MHz, -80 °C): δ 8.12 (d, 1H, J ) 8.0 Hz, An:
Hp), 8.09 (d, 1H, J ) 8.5 Hz, An′: Hp), 7.47 (m, 2H, An: Hm,
An′: Hm), 7.31 (m, 6H, Haryl), 6.55 (d, 1H, J ) 7.0 Hz, An: Ho),
6.46 (d, 1H, J ) 7.0 Hz, An′: Ho), 5.16 and 4.52 (m, 1H each;
Pd(η2-trans-CH3-CHdCH-CH3)), 2.37, 2.20, 2.08, 1.98 (s, 3H
each; Ar-CH3), 1.76 and 1.64 (d, 3H each, J ) 6.0; Pd(η2-trans-
CH3-CHdCH-CH3)), 0.51 (s, 3H; Pd(CH3)).
and quaternary C and Ar:
Cipso, Co, Cp, Cm and Ar′: Co,
Cp), 29.8 and 29.6 (Ar(CH)(CH3)2 and Ar(C′H)(CH3)2), 24.1,
23.9, 23.5, and 23.2 (Ar(CH)(CH3)2 and Ar(CH)(C′H3)2 and
Ar(CH)(C′′H3)2 and Ar(CH)(C′′′H3)2), 9.27 (PdCH3). Anal. Calcd
for (C78H58BF30N3Pd): C, 54.32; H, 3.39; N 2.44. Found: C,
54.05; H, 3.31; N, 2.57.
[(ArNdC(An)-(An)CdNAr)Pd(CH3)(η2-cis-CH3-CHd
CH-CH3)]+[BAr′4]- (Ar ) 2,6-C6H3(CH3)2), 11. 1H NMR
(CD2Cl2, 500 MHz, -80 °C): δ 8.12 (d, 1H, J ) 8.0 Hz, An:
Hp), 8.09 (d, 1H, J ) 8.0 Hz, An′: Hp), 7.47 (m, 2H, An: Hm,
An′: Hm), 7.31 (m, 6H, Haryl), 6.57 (d, 1H, J ) 7.5 Hz, An: Ho),
6.51 (d, 1H, J ) 7.5 Hz, An′: Ho), 5.12 (br m, 2H; Pd(η2-cis-
CH3-CHdCH-CH3)), 2.18, 2.16 (s, 6H each; Ar-CH3), 1.68
(br d, 6H; Pd(η2-trans-CH3-CHdCH-CH3)), 0.33 (s, 3H;
Pd(CH3)).
Synthesis of Dialkyl Complex. ((2,6-(CH3)2C6H3)NdC-
(An)-(An)CdN(2,6-(CH3)2C6H3))Pd(CH2CH2CH(Me)2)2 (13).
A flame-dried Schlenk flask was charged with ((2,6-(CH3)2C6H3)-
NdC(An)-(An)-CdN(2,6-(CH3)2C6H3))PdCl2 (0.491 g, 0.87 mmol)
in an argon-filled drybox. The flask was placed under argon
and cooled to -78 °C (dry ice/2-propanol), and Et2O (20 mL)
was added via syringe. BrMg(CH2CH2CH(Me)2) was added as
a solution in Et2O (0.3 M, 6 mL, 1.8 mmol), and the mixture
was stirred at -78 °C for 2 h. MeOH (0.1 mL) was added to
quench any excess Grignard reagent, and the dark mixture
was flash-filtered through Florisil into a clean, flame-dried
Schlenk at 0 °C. The Florisil was washed with Et2O (10
mL) and pentane (10 mL), and the filtrate was reduced in
vacuo to give a red-brown solid, which was dried under re-
duced pressure for 1 h at room temperature and stored at
Determination of Rates of trans- and cis-2-Butene
Migratory Insertion by NMR Spectroscopy. In a drybox
under an argon atmosphere, an NMR tube was charged with
ca. 0.01 mmol of [(ArNdC(An)-C(An)dNAr) Pd(Me)(OEt2)]-
BAr′4. The tube was capped with a rubber septum and removed
from the drybox. After securing the septum with Teflon tape
and Parafilm, the tube was cooled to -78 °C. CD2Cl2 was added
to the NMR tube via syringe (600 µL), and the septum was
rewrapped with Parafilm. The tube was shaken and warmed
slightly to facilitate dissolution of the complex. Then trans-
or cis-2-butene was added via syringe to the solution cooled
to -78 °C, and the NMR tube was briefly shaken to completely
dissolve the additive. The tube was then transferred to the
precooled NMR probe for acquisition of spectra. After acquiring
a spectrum at -80 °C, the probe was warmed to proper
temperature for the migratory insertion studies. The concen-
trations of the active species and free olefin were calculated
using the BAr′4 or para-acenaphthyl peaks as an internal
standard. Rates of migratory insertion were determined by
monitoring the loss of the PdCH3 resonance. The natural
logarithm of the starting alkyl olefin complex resonance was
plotted versus time (first-order treatment) to obtain kinetic
plots. NMR probe temperatures were calibrated using an
Omega type T thermocouple immersed in anhydrous methanol
in a 5 mm NMMR tube.
1
-30 °C in the drybox freezer. Yield: 0.330 g (60%). H NMR
(CD2Cl2, 500 MHz, -80 °C): δ 8.03 (d, 2H, J ) 8.0 Hz, An:
Hp), 7.42 (m, 2H, An: Hm, An′: Hm), 7.22 (m, 6H, Haryl), 6.71
(d, 2H, J ) 7.0 Hz, An:Ho), 2.27 (s, 12H C6H3Me2), 1.09 (m,
4H, Pd(CH2CH2CHMe2)2), 1.02 (m, 2H, Pd(CH2CH2CHMe2)2),
0.87 (m, 4H, Pd(CH2CH2CHMe2)2), 0.58 (d, 12H, J ) 6.5,
Pd(CH2CH2CHMe2)2),. 13C NMR (CD2Cl2, 125 MHz, -80 °C):
δ 167.0, 145.7, 143.0, 130.1, 129.3, 128.7, 128.6, 128.2,
126.1, 123.2, 40.9 (PdCH2CH2CH(CH3)2), 32.5 (PdCH2CH2CH-
(CH3)2), 22.5 (PdCH2CH2CH(CH3)2), 18.1 (ArCH3), 16.9
(PdCH2CH2CH(CH3)2). This compound was not sufficiently
stable for elemental analysis.
General Procedure for Polymerization Reactions of
trans- and cis-2-Butenes. An oven-dried Fischer-Porter
bottle rated to 100 psi was equipped with a magnetic stir bar,
attached to the pressure head unit, and repeatedly evacuated
and backfilled with argon three times. Toluene (10 mL) was
added via syringe and trans- or cis-2-butene pressure was
added and released twice. The pressure was vented and a
solution of the appropriate Pd(II) catalyst (0.010 mmol) in
toluene (5 mL) was added quickly via cannula, and the reactor
was sealed and brought to the appropriate monomer pressure.
The reactor was stirred for the stated amount of time, then
the excess pressure was vented, and the polymerization
quenched with acetone (5 mL) and 6 M HCl (5 mL). The
[(ArNdC(An)-(An)CdNAr)Pd(CH2CH2CH(CH3)2)(η2-
trans-CH3-CHdCH-CH3)]+[BAr′4]- (Ar ) 2,6-C6H3(CH3)2),
8. 1H NMR (CD2Cl2, 500 MHz, -20 °C): δ 8.16 (d, 1H, J )
8.0 Hz, An: Hp), 8.12 (d, 1H, J ) 8.5 Hz, An′: Hp), 7.50 (m,
2H, An: Hm, An′: Hm), 7.35 (m, 6H, Haryl), 6.57 (d, 1H, J ) 7.5
Hz, An: Ho), 6.51 (d, 1H, J ) 7.5 Hz, An′: Ho), 4.83 and 4.68
(m, 1H each; Pd(η2-trans-CH3-CHdCH-CH3)), 2.35, 2.23,

Pd(II)-R-Diimine-Catalyzed Polymerizations
Organometallics, Vol. 23, No. 26, 2004 6107
2.21, 2.06 (s, 3H each; Ar-CH3), 1.88 and 1.71 (d, 3H each, J
) 6.0; Pd(η2-trans-CH3-CHdCH-CH3)), 1.26 (br m, 3H;
PdCH2CH2CH(CH3)2 and PdCH2CH2CH(CH3)2), 0.81 (m, 2H;
PdCH2CH2CH(CH3)2), 0.59 and 0.40 (d, 3H each, J ) 6.0;
PdCH2CH2CH(CH3)2).
via syringe (600 µL), and the septum was rewrapped with
Parafilm. The tube was shaken briefly to facilitate dissolution
of the complex. The tube was then transferred to the precooled
(-80 °C) NMR probe for acquisition of spectra.
[(ArNdC(An)-(An)CdNAr)Pd(CH2(CH-µ-H)CH(CH3)2]+-
[BAr′4]- (Ar ) 2,6-C6H3(CH3)2), 14a,b. 1H NMR (CD2Cl2, 500
MHz, -80 °C): δ 8.12 (d, 1H, J ) 8.5 Hz, An: Hp), 8.09 (d,
1H, J ) 8.5 Hz, An′: Hp), 7.49 (m, 2H, An: Hm, An′: Hm),
7.26 (m, 6H, Haryl), 6.76 (d, 1H, J ) 6.5 Hz, An: Ho), 6.63
(apparent t, 1H, J ) 6.5 Hz, An′: Ho), 2.18 (br m, 6H;
C6H3Me2), 2.10 (m, 6H; C′6H3Me2), 1.45, 0.88, and 0.45 (all br)
for protons on the Pd alkyl chain, -7.25 and -7.35 (both br)
for agostic protons. All Pd-alkyl resonances broadened due to
exchange at this temperature. Isopentane is also present in
the sample (δ 0.75 for methyl protons, 1.33 for the methine
proton, methylene signals overlap with those of diethyl ether).
Relative Binding Affinity Studies. In a drybox under an
argon atmosphere, an NMR tube was charged with ca. 0.01
mmol of [(ArNdC(An)-C(An)dNAr)Pd(Me)(OEt2)]BAr′4. The
tube was capped with a rubber septum and removed from the
drybox. After securing the septum with Teflon tape and
Parafilm, the tube was cooled to -78 °C. CD2Cl2 was added to
the NMR tube via syringe (600 µL), and the septum was
rewrapped with Parafilm. The tube was shaken and warmed
slightly to facilitate dissolution of the complex. Then appropri-
ate amounts of trans- or cis-2-butene and trimethoxyvinylsi-
lane were added via syringe to the solution at -78 °C, and
again the NMR tube was briefly shaken to completely dissolve
the additives. The tube was then transferred to the precooled
(-80 °C) NMR probe for acquisition of spectra. The concentra-
tions of the two complexes and the free olefins were calculated
using the BAr′4 or para-acenaphthyl peaks as an internal
standard.
[(ArNdC(An)-(An)CdNAr)Pd(CH2CH2CH(CH3)2)(η2-
cis-CH3-CHdCH-CH3)]+[BAr′4]- (Ar ) 2,6-C6H3(CH3)2),
1
12. H NMR (CD2Cl2, 500 MHz, -20 °C): δ 8.17 (d, 1H, J )
8.5 Hz, An: Hp), 8.13 (d, 1H, J ) 8.0 Hz, An′: Hp), 7.51 (m,
2H, An: Hm, An′: Hm), 7.35 (m, 6H, Haryl), 6.62 (d, 1H, J ) 7.0
Hz, An: Ho), 6.59 (d, 1H, J ) 7.0 Hz, An′: Ho), 5.24 (m, 2H;
Pd(η2-cis-CH3-CHdCH-CH3)), 2.24, 2.23 (s, 6H each;
Ar-CH3), 1.72 (d, 6H each, J ) 5.0; Pd(η2-cis-CH3-CHd
CH-CH3)), 1.19 (br m, 3H; PdCH2CH2CH(CH3)2 and PdCH2-
CH2CH(CH3)2), 0.79 (m, 2H; PdCH2CH2CH(CH3)2), 0.46 (d, 6H,
J ) 6.5; PdCH2CH2CH(CH3)2).
Determination of Subsequent Insertion Rates by
NMR Spectroscopy. In a drybox under an argon atmos-
phere, an NMR tube was charged with ca. 0.01 mmol of
[(ArNdC(An)-C(An)dNAr)Pd(Me)(OEt2)]BAr′4. The tube was
capped with a rubber septum and removed from the drybox.
After securing the septum with Teflon tape and Parafilm, the
tube was cooled to -78 °C. CD2Cl2 was added to the NMR tube
via syringe (600 µL), and the septum was rewrapped with
Parafilm. The tube was shaken and warmed slightly to
facilitate dissolution of the complex. Then trans- or cis-2-
butene was added via syringe to the solution at -78 °C, and
the NMR tube was briefly shaken to completely dissolve the
additive. The tube was then transferred to the precooled NMR
probe for acquisition of spectra. The concentrations of the
active species and free olefin were calculated using the BAr′4
or para-acenaphthyl peaks as an internal standard. After the
first insertion was complete, the probe was warmed to -10
°C and the zero-order disappearance of the free 2-butene
integral was monitored.
In Situ Formation and NMR Observation of the
â-Agostic Isopentyl Complexes. In a drybox under an argon
atmosphere, an NMR tube was charged with ca. 0.01 mmol of
[(ArNdC(An)-C(An)dNAr)Pd(CH2CH2CH(CH3)2)] (Ar ) 2,6-
C6H3(CH3)2) and 0.01 mmol of HBAr′(OEt2)2. The tube was
capped with a rubber septum and removed from the drybox.
After securing the septum with Teflon tape and Parafilm, the
tube was cooled to -78 °C. CD2Cl2 was added to the NMR tube
Acknowledgment. We thank DuPont and the Na-
tional Science Foundation (CHE-0107810) for financial
support of this work.
Supporting Information Available: Graphs for the
determination of rates of migratory insertion. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM040077M