Synthesis and crystal structure of a dinuclear palladium complex containing C,O-bridging ester-enolato moieties

Article abstract of DOI:10.1021/om010968j

Palladium ester enolates have been prepared and characterized using IR, NMR, and single-crystal X-ray diffraction techniques. The X-ray diffraction established a dimeric crystal structure for complex {[N∧N]PdCH₂C(O)OCH₃}₂ (N∧N = 1-[1-(5-methylpyrrole-2-yl)ethylidne]amino-2,6-diisopropylbenzene), 2, which contains C,O-bridging enolato groups. Upon reaction with donor molecules (acetonitrile, phosphines), the dimeric 2 cleaves to form monometallic C-bound enolate complexes, 3′ and 4.

Full text of DOI:10.1021/om010968j

1462
Organometallics 2002, 21, 1462-1465
Syn th esis a n d Cr ysta l Str u ctu r e of a Din u clea r
P a lla d iu m Com p lex Con ta in in g C,O-Br id gin g
Ester -En ola to Moieties
Gonglu Tian, Paul D. Boyle, and Bruce M. Novak*
Department of Chemistry, North Carolina State University,
Raleigh, North Carolina 27695-8204
Received November 8, 2001
Palladium ester enolates have been prepared and characterized using IR, NMR, and single-
crystal X-ray diffraction techniques. The X-ray diffraction established a dimeric crystal
structure for complex {[N∧N]PdCH2C(O)OCH3}2 (N∧N ) 1-[1-(5-methylpyrrole-2-yl)ethylidne]-
amino-2,6-diisopropylbenzene), 2, which contains C,O-bridging enolato groups. Upon reaction
with donor molecules (acetonitrile, phosphines), the dimeric 2 cleaves to form monometallic
C-bound enolate complexes, 3′ and 4.
Ch a r t 1
The formation of palladium enolates is assumed to
be a crucial step in numerous organic transformations.
Examples include the direct arylation of esters and
ketones,1 aldol-like and Mannich-type reactions,2,3 and
the arylation of acrylates (Heck reactions).4 Palladium
ester enolates have also been proposed as key interme-
diates in the copolymerzation of ethylene and acrylates.5
Evidence for the presence of palladium enolates in these
organic reactions and polymerizations rests mainly on
spectroscopic data.3a,6 Some palladium enolates have
been isolated and characterized, and various bonding
modes are found for the enolato groups (Chart 1). Unlike
early transition metal and rare earth enolates, carbon-
bound enolates are the most common binding mode
found in palladium complexes due largely to the low
oxophility of palladium.7 When a second coordination
site is available, the C,O-bridging7a,c,8 or η3-oxoallyl
enolate9 can also be seen. The O-bound enolates, al-
though very scarce in late metal complexes, can also be
found in palladium complexes.10
Among the isolated and well-characterized palladium
enolates, most of them are ketonyl complexes. Only a
few examples are known for palladium ester enolates,
and no crystal structure is available to date.7f,11 Herein,
we report the synthesis and the first crystal structure
of palladium ester enolates.
During our studies on group VIII complexes for olefin
copolymerizations with polar monomers, we discovered
that palladium-based neutral complexes possessing
2-iminopyrrole ligands were highly active initiators for
methylacrylate homopolymerization and copolymeriza-
tion with olefins (e.g., 1-hexene, norbornene).12 Realizing
that palladium ester enolates could be reasonable
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C. M.; Arthur, S. D.; Feldman, J .; McCord, E. F.; Mclain, S. J .;
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arethy, A.; Tempel, D. J . Pat. Appl. WO 96/23010, 1996.
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35, 657-659. (b) Sodeoka, M.; Ohrai, K.; Shibasaki, M. J . Org. Chem.
1995, 60, 2648-2649.
(8) (a) Vicente, J .; Arcas, A.; Fernandez-Hernandex, J . M.; Bautista,
D. Organometallics 2001, 20, 2767. (b) Ruiz, J .; Rodriguez, V.; Cutillas,
N.; Pardo, M.; Perez, J .; Lopez, G. Organometallics 2001, 20, 1973-
1982.
(7) (a) Albeniz, A. C.; Catalina, N. M.; Espinet, P.; Redon, R.
Organometallics 1999, 18, 5571-5576. (b) Vicente, J .; Abad, J . A.;
Chicote, M.-T.; Abrisqueta, M.-D.; Lorca, J .-A.; de Arellano, M. C. R.
Organometallics 1998, 17, 1564-1568. (c) Veya, P.; Floriani, C.
Organometallics 1993, 12, 4899-4907. (d) Suzuki, K.; Yamamoto, H.
Inorg. Chim. Acta 1993, 208, 225-229. (e) Byers, P. K.; Canty, A. J .;
Skelton, B. W.; Traill, P. R.; Watson, A. A.; White, A. H. Organome-
tallics 1992, 11, 3085-3088. (f) Burkhardt, E. R.; Bergman, R. G.;
Heathcock, C. H. Organometallics 1990, 9, 30-44. (g) Wanat, R. A.;
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391-400. (b) Yanase, N.; Nakamura, Y.; Kawaguchi, S. Inorg. Chem.
1980, 19, 1575-1581. (c) Yanase, N.; Nakamura, Y.; Kawaguchi, S.
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10.1021/om010968j CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/22/2002

Dinuclear Palladium Complex
Sch em e 1
Organometallics, Vol. 21, No. 7, 2002 1463
The IR spectrum of 2 exhibits a strong ν(CdO)
absorption shifted down to 1594 cm-1 (KBr), suggesting
the coordination of the carbonyl group with a palladium
center. In the 1H NMR spectrum (CDCl3) the methylene
protons of complex 2 appear as two doublets at δ 3.10
and 3.00 ppm (J H-H ) 6.0 Hz). The resonance of the
methoxy protons is observed at δ 2.54 ppm, which is
significantly shifted with respect to organic methyl
esters. This shift is due to shielding by the aromatic
groups that are held in close proximity through the
coordination of the carbonyl group by the second pal-
ladium center. The signals for the isopropyl groups
consist of two septets for the CH and four doublets for
the CH3 groups. The observed NMR pattern is a
consequence of the absence of a molecular symmetry
plane in each fragment of the dimer 2, which has now
been confirmed by the X-ray diffraction (vide infra). In
the 13C NMR spectrum, the resonances at δ 193.1 and
9.1 ppm can be ascribed to the carbonyl carbon and the
CH2 bound to the Pd, the former being shifted downfield
owing to the coordination with palladium. All of these
spectroscopic data are consistent with either a chelated
monomeric η3-oxoallyl complex or a dimer complex held
together by bridging enolates.
Lopez and co-workers have recently reported that the
dimeric complex {(AsPh3)(C6F5)Pd}2{µ-CH2C(O)CH3}2,
A, undergoes dissociation to give two η3-oxoallyl mon-
omeric complexes in chloroform solution.8b The dimer:
monomer ratio increases as the temperature decreases.
In contrast, there is no direct evidence indicating the
dissociation of dimer 2 in noncoordinating solvents (e.g.,
chloroform and benzene). No change was found in the
variable-temperature NMR spectra of complex 2 in
chloroform solution (up to 50 °C). In the IR spectrum,
the value of ν(CdO) absorption measured in chloroform
solution (1595 cm-1) is essentially the same as that
recorded in the solid state. These studies lead us to
conclude that the dimeric structure of 2 is very robust
in noncoordinating solvents. We were, however, able to
observe dissociation in the presence of a coordinating
solvent. The IR spectrum of complex 2 in CHCl3/CH3-
CN (ca. 10:1) displays the complete disappearance of the
absorbance at 1595 cm-1 and the appearance of a strong
carbonyl absorption at 1680 cm-1, indicating that the
coordinated carbonyl group is dissociated, forming the
solvated monomeric palladium complex, 3′. This inter-
intermediates in a propagation mechanism involving
acrylates, we sought to independently synthesize and
study their behavior. Our first attempts to prepare ester
enolates by the insertion of methyl acrylate into com-
plexes possessing Pd-CH3 bonds (e.g., (N∧N)Pd(CH3)L,
where N∧N ) 1-[1-(5-methylpyrrole-2-yl)ethylidne]-
amino-2,6-diisopropylbenzene, and L ) triphenylphos-
phine (PPh3), trimethylphosphine (PMe3), or pyridine)
failed, with nonspecific decomposition and reductive
elimination occurring in all attempts. We later found
out that the Pd-CH3 bond in these neutral palladium
complexes is relatively inert toward the insertion of
methyl acrylate. Therefore, we used the reaction of
lithium enolates with palladium chloride to prepare our
palladium ester enolates.
The palladium chloride dimer 1, obtained from the
reaction of Na2PdCl4 with the N∧N bidentate ligand in
methanol, proved to be a convenient starting material
to prepare the palladium ester enolates. When complex
1 was allowed to react with the lithium enolate of
methyl acetate in THF, a new complex, 2, is formed
(Scheme 1). Complex 2 was later shown to be a pal-
ladium dimer that has the ester-enolato ligands bridging
the palladium centers. The THF used in the reaction
does not compete with the enolate carbonyl group for
coordination as far as the isolated solid product is
concerned. However, the reaction of complex 1 and the
lithium enolate of tert-butyl acetate gave the monomeric
C-bound compound 3 rather than a dimeric complex.
Complex 3 was isolated as a yellow solid that displays
low thermal stability. Substituting the bound THF for
triphenylphosphine stabilizes the complex (vide infra).
Again, the preferred reaction pathway is away from an
η3-oxoallyl enolate and toward the C-bond species. We
attribute the inhibition of dimer formation in this case
to the steric effects introduced by the tert-butyl group.
1
pretation is further supported by the H NMR spectrum
of 2 in CDCl3/CD3CN, which consists of a singlet (δ 2.54
ppm) for Pd-CH2, a septet (δ 3.30 ppm) for the isopropyl
CH groups, and a singlet at δ 3.50 ppm for CH3O group.
No resonances are observed in the vinylic region of
the spectrum, indicating that no O-bound species is
formed.
The dissociation of the dimer 2 can also be achieved
by adding a strong coordination ligand like PPh3. The
resulting air- and moisture-stable monomeric product
has been isolated as complex 4. The C-bound structure
is assigned on the basis of NMR and infrared data. The
phosphorus-proton coupling (J P-H ) 5.1 Hz) and phos-
phorus-carbon coupling (J P-C ) 11.4 Hz) are observed
for the methylene groups in the 1H and 13C NMR
spectra, respectively. The ν(CdO) absorption in the IR
spectrum (1695 cm-1, KBr) is also consistent with the
C-bound enolate structure. Similarly, addition of PPh3

1464 Organometallics, Vol. 21, No. 7, 2002
Tian et al.
result, the two protons on each enolato methylene group
are in a different chemical environment, which is
1
displayed in the H NMR spectrum.
It is worthwhile to point out that the NMR and IR
data are insufficient to determine the structure of
complex 2, since both η3-oxoallyl and C,O-bridging
binding mode would be consistent with these data. On
the other hand, the former binding mode for enolato,
though often invoked in palladium chemistry, has never
been proven structurally. The characterization of the
dimeric structure of complex 2 implies that a dinuclear
process might be a plausible pathway for some pal-
ladium-catalyzed reactions and polymerizations, which
have been assumed to proceed through a η3-oxoallyl
intermediate.
In conclusion we have prepared and characterized
palladium ester enolates. The results indicate that of
the four possible binding modes, the ester-enolato group
in this study prefers to coordinate with palladium
through either a C-bound or a C,O-bridging binding
mode. The C,O-bridging structure of complex 2 has been
confirmed by X-ray diffraction. As far as we know, this
is the first crystal structure of palladium ester enolates.
Since the relevant reactions and polymerizations have
been widely used in organic and polymer chemistry, this
study should contribute to a better understanding of the
reaction mechanism and provide valuable guidelines to
improve catalyst design and reaction yield.
F igu r e 1. X-ray crystal structure of complex 2. Selected
bond lengths (Å) and angles (deg): Pd1-O1 2.053(3), Pd1-
N1 2.003(4), Pd1-N2 2.083(4), Pd1-C24 2.075(4), C23-
C24 1.448(7), C23-O3 1.252(6), C23-O4 1.337(5), N1-
Pd1-O1 171.46(13), N2-Pd1-C24 174.26(16), C20-O1-
Pd1 122.0(3), O1-Pd1-C24 91.85(15), Pd1-C24-C23
107.8(3), C24-C23-O3 127.3 (4). The bond lengths and
angles in the other Pd unit are essentially the same as
above.
into the solution of 3 resulted in the replacement of THF
by PPh3 and produced C-bound enolate 5 as a air- and
moisture-stable solid.
An X-ray crystallographic study ultimately estab-
lished the crystal structure of 2 as the dimer.13 A view
of the molecule is given in Figure 1, and selected bond
lengths and angels are listed in the caption. The ester-
enolato groups connect two palladium atoms with a
head-to-tail arrangement, giving rise to an eight-
membered ring. Each palladium atom is in a square
planar coordination geometry with a slight tetrahedral
distortion. The two palladium coordination planes are
mutually perpendicular, forming a dihedral angle of
93.27(10)°. The two nonsymmetric N∧N ligands are
related by a pseudo C2 symmetry axis. Finally, the
Pd-C bond occupies the position trans to the imino
nitrogen atom. The dimeric structure of complex 2 is
comparable with the recently reported ketonyl-bridged
palladium enolates, A and [{Pd(PPh3)2}2(µ-{CH2C(O)-
Ph})2][CF3SO3]2, B.7c,8b However, the following points
must be stressed: (i) The Pd-C distances (2.074(5) and
2.075(4) Å) are shorter in 2 than in other dimeric
palladium complexes A (2.106(10) and 2.130(11) Å) and
B (2.162(10) and 2.134(10) Å), thus suggesting that the
trans-influence of nitrogen is much smaller than that
of phosphorus or arsenic. The Pd-O distances, 2.053-
(3) and 2.051(3) Å, are also relatively shorter in complex
2. (ii) The corresponding bond distances in the two
independent units of complex 2 are essential identical.
But somewhat different bond distances, especially Pd-C
and Pd-O, are found in the two units of complexes A
and B, respectively. (iii) The enolato groups are twisted
out of the palladium coordination plane with a torsion
angle N1-Pd1-C24-C23 of 160.1(6)°. The free rotation
of the enolato group around the Pd-C bond is arrested
by the formation of the eight-membered ring. As a
Exp er im en ta l Section
All manipulations of air- and/or water-sensitive compounds
were performed using standard Schlenk techniques. The 1H,
13C, and 31P NMR spectra were recorded on a GE-300 or Varian
Gemini-300 spectrometer (300 MHz for 1H) unless otherwise
specified. Chemical shifts for 1H and 13C NMR spectra were
referenced using internal solvent resonances and are reported
relative to tetramethylsilane. The 31P NMR spectra were
referenced to external H3PO4. Infrared spectra were recorded
on
a J asco FT/IR-410 Series Fourier transform infrared
spectrometer. Elemental analysis was performed by Atlantic
Microlab, Inc.
Anhydrous solvents were passed through columns packed
with Q5 catalysts and molecular sieves prior to use. Benzene-
d6 was dried over CaH2, vacuum-transferred, degassed by
repeated freeze-pump-thaw cycles, and stored over 4 Å
molecular sieves. Chloroform-d was dried over 4 Å molecular
sieves. The N∧N bidentate ligand (1-[1-(5-methylpyrrole-2-yl)-
ethylidne]amino-2,6-diisopropylbenzene) was prepared accord-
ing to the procedure reported before.12 Unless otherwise noted,
all compounds were purchased from Aldrich Chemical Co. and
used as received.
P r ep a r a tion of {[N∧N]P d Cl}2 (1). Na2[PdCl4] (2.0 mmol),
prepared in situ from PdCl2 and NaCl, was dissolved in 10
mL of methanol and cooled to -40 °C. The N∧N bidentate
ligand (0.564 g, 2.0 mmol) and sodium acetate (0.164 g, 2.0
mmol) in methanol (10 mL) were transferred to the above
solution via cannula. The resulting mixture was warmed and
stirred at room temperature for 12 h. The red solid formed
during this period was collected by filtration, washed twice
with methanol (ca. 20 mL), and air-dried. Recrystallization
from CH2Cl2/methanol afforded 0.770 g (91% yield) of 1. The
product consisted of two isomers (cis and trans) in the ratio of
77:23. It was used in the following preparation without further
separation. Major isomer: 1H NMR (CDCl3, 400 MHz) δ 7.26-
7.24 (m, 2 H, ph-H4), 7.08 (d, 4 H, ph-H3,5), 6.56 (d, 2 H, pyrrole-
H3), 5.74 (d, 2 H, pyrrole-H4), 3.41 (m, 4 H, iPr-CH), 1.89 (s,
6 H, pyrrole-CH3), 1.80 (s, 6 H, NdC-CH3), 1.47 (d, 12 H, J H-H
(13) Crystal data for 2: 0.34 × 0.32 × 0.30 mm, monoclinic, P21/c,
a ) 12.0009(13) Å, b ) 14.6436(9) Å, c ) 28.262(2) Å, â ) 100.087-
(11)°, V ) 4889.9(7) Å3, Z ) 4, Dc ) 1.350 g‚cm-3, µ ) 0.78 mm-1, λ )
0.71073 Å, 2θ ) 49.8°, F(000) ) 2065.01, T ) 148 K, reflections
measured 8536, of which 8536 were independent, residuals Rf ) 0.049,
Rw ) 0.063.

Dinuclear Palladium Complex
Organometallics, Vol. 21, No. 7, 2002 1465
) 6.8, iPr-CH3), 1.18 (d, 12 H, J H-H ) 6.8, iPr-CH3); 13C NMR
(CDCl3, 400 MHz) δ 169.6 (CdN), 149.5, 143.2, 141.3, 140.3,
127.9, 123,7, 117.9, 110.5 (pyrrole- and ph-C), 28.6 (iPr-CH),
24.2, 24.1 (iPr-CH3), 15.7, 15.6 (pyrrole- and imine-CH3). Minor
isomer: 1H NMR (CDCl3, 400 MHz) δ 7.26-7.24 (m, 2 H, ph-
H4), 6.90 (d, 4 H, ph-H3,5), 6.56 (d, 2 H, pyrrole-H3), 5.81 (d, 2
H, pyrrole-H4), 3.16 (m, 4 H, iPr-CH), 2.24 (s, 6 H, pyrrole-
CH3), 1.85 (s, 6 H, NdC-CH3), 1.21 (d, 12 H, J HH ) 6.8, iPr-
CH3), 1.04 (d, 12 H, J HH ) 6.8, iPr-CH3); 13C NMR (CDCl3,
400 MHz) δ 169.6 (CdN), 149.1, 143.2, 141.3, 140.3, 127.7,
123,5, 118.0, 110.7 (pyrrole- and ph-C), 28.5 (iPr-CH), 24.2,
24.1(iPr-CH3), 16.2, 15.8 (pyrrole- and imine-CH3). Anal. Calcd
for C38H50Cl2N4Pd2: C, 53.91; H, 5.95; N, 6.62. Found: C,
53.74; H, 5.96; N, 6.58. MS(FAB): m/e 844 (M+).
167.0 (CdN), 148.4, 144.5, 143.7, 142.7, 126.0, 123.3, 117.4,
112.1 (pyrrole- and ph-C), 80.2 (OC(CH3)3), 68.8 (thf-CH2O),
28.8 (iPr-CH), 28.5 (-C(CH3)3), 25.7 (thf-CH2), 24.5, 24.4 (iPr-
CH3), 18.2, 16.7 (pyrrole- and imine-CH3), 6.2 (Pd-CH2).
P r ep a r a tion of [N∧N](P h 3P )P d CH2CO2CH3 (4). A mix-
ture of complex 1 (0.423 g, 0.50 mmol), lithium enolate of
methyl acetate (1.0 mmol), PPh3 (0.262 g, 1.0 mmol), and THF
(10.0 mL) was stirred at -78 °C for 2 h and then at room
temperature for 12 h. The solvent was removed under vacuum,
and the residue was taken in 10 mL of benzene. After
filtration, the solvent of the clear filtrate was evaporated to
afford a yellow solid. Recrystallization from ether/pentane
afforded 0.448 g (62% yield) of 4. The complex can also be
prepared in quantitative yield by adding PPh3 into the benzene
1
P r ep a r a tion of {[N∧N]P d CH2CO2CH3}2 (2). A solution
of complex 1 (0.423 g, 0.50 mmol) in 5.0 mL of THF was added
to the solution of lithium enolate generated in situ from the
reaction of LDA (1.0 mmol) and methyl acetate (1.0 mmol) in
THF (5.0 mL) at -78 °C. The resulting mixture was stirred
at -78 °C for 2 h and then allowed to warm to room
temperature over a period of 12 h. The solvent was removed
under vacuum, and the residue was taken in 10 mL of benzene.
After filtration, the solvent of the clear filtrate was evaporated
to afford a yellow solid. Recrystallization from benzene/pentane
afforded 0.202 g (44% yield) of 2. 1H NMR (C6D6): δ 7.15-
6.90 (m, 6 H, ph-H), 6.80 (d, 2 H, pyrrole-H3), 6.16 (d, 2 H,
pyrrole-H4), 3.58 (m, 2 H, iPr-CH), 3.52 (d, 2 H, J H-H ) 6.0
Hz, Pd-CH2), 3.38 (d, 2 H, J H-H ) 6.0 Hz, Pd-CH2), 3.36 (m, 2
H, iPr-CH), 2.57 (s, 6 H, CH3O), 2.31 (s, 6 H, pyrrole-CH3),
1.69 (s, 6 H, NdC-CH3), 1.25 (d, 6 H, iPr-CH3), 1.16 (d, 6 H,
iPr-CH3), 1.03 (d, 6 H, iPr-CH3), 0.87 (d, 6 H, iPr-CH3). 1H
NMR (CDCl3): δ 7.20-7.00 (m, 6 H, ph-H), 6.64 (d, 2 H,
pyrrole-H3), 5.91 (d, 2 H, pyrrole-H4), 3.53 (m, 2 H, iPr-CH),
3.26 (m, 2 H, iPr-CH), 3.10 (d, 2 H, J H-H ) 6.0 Hz, Pd-CH2),
3.01 (d, 2 H, J H-H ) 6.0 Hz, Pd-CH2), 2.54 (s, 6 H, CH3O),
2.18 (s, 6 H, pyrrole-CH3), 1.93 (s, 6 H, NdC-CH3), 1.22 (d, 6
H, iPr-CH3), 1.18 (d, 6 H, iPr-CH3), 1.17 (d, 6 H, iPr-CH3), 1.03
(d, 6 H, iPr-CH3). 13C NMR (C6D6): δ 193.1 (CdO), 167.5 (Cd
N), 148.3, 143.2, 143.1, 143.0, 142.9, 126.3, 123.6, 118.0, 112.3
(pyrrole- and ph-C), 52.9 (CH3O), 28.8, 28.5 (iPr-CH), 24.8,
24.3, 23.7, 23.4 (iPr-CH3), 17.5, 16.1 (pyrrole- and imine-CH3),
9.1 (Pd-CH2). IR (KBr): 1595 (s, CdO), and 1559 cm-1 (s, Cd
N). IR (CHCl3): 1596 (s, CdO), and 1564 cm-1 (s, CdN). IR
(CHCl3/CH3CN ) 10:1): 1680 (s, CdO), and 1565 cm-1 (s, Cd
N). Anal. Calcd for C44H60N4O4Pd2: C, 57.33; H, 6.56; N, 6.08.
Found: C, 57.87; H, 6.61; N, 5.95.
solution of 2. H NMR (C6D6): δ 8.00-7.85 (m, 6 H, PPh3-H),
7.15-7.10 (m, 3 H, ph-H), 7.02 (d, 1 H, pyrrole-H3), 7.00-6.90
(m, 9 H, PPh3-H), 6.17 (d, 1 H, pyrrole-H4), 3.75 (m, 2 H, iPr-
CH), 3.05 (s, 3 H, CH3O), 1.99 (s, 3 H, pyrrole-CH3), 1.50 (d, 2
H, J P-H ) 5.1 Hz, Pd-CH2), 1.34 (s, 3 H, NdC-CH3), 1.28 (d, 6
H, iPr-CH3), 1.14 (d, 6 H, iPr-CH3). 13C NMR (C6D6, 400
MHz): δ 177.7 (CdO), 169.8 (CdN), 148.2, 144.0, 143.3, 142.0,
136.0 (d, J P-C ) 11.4 Hz), 130.9, 128.9, 128.5, 126.9, 124.2,
119.8, 112.7 (pyrrole- and ph-C), 50.4 (CH3O), 28.8, (iPr-CH),
24.7, 24.0 (iPr-CH3), 24.6 (d, J P-C ) 9.1 Hz, Pd-CH2), 18.4, 18.3
(pyrrole- and imine-CH3). 31P NMR (C6D6): δ 35.2 (s). IR
(KBr): 1695 (s, CdO), and 1553 cm-1 (s, CdN). Anal. Calcd
for C40H45N2O2PPd: C, 66.43; H, 6.27; N, 3.87. Found: C,
66.43; H, 6.36; N, 3.78.
P r ep a r a tion of [N∧N](P h 3P )P d CH2CO2Bu t (5). A mix-
ture of complex 1 (0.423 g, 0.50 mmol), lithium enolate of tert-
butyl acetate (1.0 mmol), PPh3 (0.262 g, 1.0 mmol), and THF
(10.0 mL) was stirred at -78 °C for 2 h and then at room
temperature for 12 h. The solvent was removed under vacuum,
the residue was taken in 10 mL of hexane, and, after filtration,
the solvent was evaporated to afford a yellow solid. Recrys-
tallization from hexane at -20 °C afforded 0.590 g (76% yield)
of 5. The complex can also be prepared in high yield by adding
1
PPh3 into the benzene solution of 3. H NMR (C6D6): δ 8.00-
7.80 (m, 6 H, PPh3-H), 7.15-7.10 (m, 3H, ph-H), 7.03 (d, 1 H,
pyrrole-H3), 7.00-6.90 (m, 9 H, PPh3-H), 6.20 (d, 1 H, pyrrole-
H4), 3.78 (m, 2 H, iPr-CH), 2.00 (s, 3 H, pyrrole-CH3), 1.51
(d, 2 H, J P-H ) 4.1 Hz, Pd-CH2), 1.37 (s, 3 H, NdC-CH3), 1.31
(d, 6 H, iPr-CH3), 1.13 (d, 6 H, iPr-CH3), 1.09 (s, 9 H, tBu). 13
C
NMR (C6D6): δ 177.1 (CdO), 169.6 (CdN), 148.2, 144.1, 143.4,
142.1, 136.1 (d, J P-C ) 12.2 Hz), 130.9, 128.9, 128.6, 126.8,
124.2, 119.7, 112.8 (pyrrole- and ph-C), 78.0 (OC(CH3)3), 28.8,
(iPr-CH), 28.6 (-C(CH3)3), 26.6 (d, J P-C ) 9.8 Hz, Pd-CH2),
24.6, 24.3 (iPr-CH3), 18.5, 18.3 (pyrrole- and imine-CH3). 31P
NMR (C6D6): δ 34.0 (s). IR (KBr): 1689 (s, CdO), and 1554
cm-1 (s, CdN). Anal. Calcd for C43H51N2O2PPd: C, 67.49; H,
6.72; N, 3.66. Found: C, 68.14; H, 6.94; N, 3.50.
P r ep a r a tion of [N∧N](THF )P d CH2CO2Bu t (3). A mixture
of complex 1 (0.423 g, 0.50 mmol), lithium enolate of tert-butyl
acetate (1.0 mmol), and THF (10.0 mL) was stirred at -78 °C
for 2 h and then at room temperature for 12 h. The solvent
was removed under vacuum, and the residue was taken in 10
mL of ether. After filtration, the solvent of the clear filtrate
was evaporated to afford a yellow oil, which solidified upon
addition of pentane. The obtained yellow solid, complex 3,
showed low thermal stability and gradually decomposed in
solution even under nitrogen atmosphere. Yield: 0.489 g (85%).
1H NMR (C6D6, 400 MHz): δ 7.10-7.08 (m, 3 H, ph-H), 6.81
(d, 1 H, pyrrole-H3), 6.25 (d, 1 H, pyrrole-H4), 3.56 (m, 2 H,
iPr-CH), 3.48 (m, 4 H, thf-CH2O), 2.88 (s, 2 H, Pd-CH2), 2.81
(s, 3 H, pyrrole-CH3), 1.76 (s, 1 H, NdC-CH3), 1.48 (s, 9 H,
C(CH3)3), 1.44 (d, 6 H, iPr-CH3), 1.38 (m, 4 H, thf-CH2), 1.13
(d, 6 H, iPr-CH3). 13C NMR (C6D6, 400 MHz): δ 186.2 (CdO),
Ack n ow led gm en t. The authors acknowledge the
Dow Chemical Company for support of this work. We
would also like to thank Drs. Michael Mullins, Harold
Boone, and Peter Nickias for many helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: X-ray structure
information for compound 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM010968J