Syntheses and characterization of palladium complexes with a hemilabile "pCO" pincer ligand

Article abstract of DOI:10.1021/om101150y

The synthesis of a new pincer ligand (^tBuPCO = 2-(CH ₂P^tBu₂)-6-(CH₂OCH₃)C ₆H₃) is reported. This ligand has been observed to coordinate in three different modes to pal

Full text of DOI:10.1021/om101150y

ARTICLE
pubs.acs.org/Organometallics
Syntheses and Characterization of Palladium Complexes with a
Hemilabile “PCO” Pincer Ligand
Gregory R. Fulmer,† Werner Kaminsky,† Richard A. Kemp,*,‡ and Karen I. Goldberg*,†
†Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
‡Department of Chemistry & Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States, and
Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, United States
S Supporting Information
b
ABSTRACT: The synthesis of a new pincer ligand (tBuPCO =
2-(CH2PtBu2)-6-(CH2OCH3)C6H3) is reported. This ligand
has been observed to coordinate in three different modes to
palladium. The tBuPCO ligand coordinates in a monodentate
fashion through the phosphine moiety in the dimeric
[(tBuPCO)Pd(Cl)(μ-Cl)]2. Bidentate coordination is observed
through the phosphine and the aryl ring in the binuclear
[(tBuPCO)Pd(μ-OH)]2. The traditional tridentate coordina-
tion mode of a pincer is observed in the monomeric complex
(
tBuPCO)PdCl, wherein the ether oxygen provides the third
point of attachment. Each of these novel palladium(II) com-
plexes was characterized by NMR spectroscopy, elemental
analyses, and single-crystal X-ray crystallography. A variety of
other palladium(II) complexes of tBuPCO have also been
prepared and characterized, including the hydroxide complex (tBuPCO)PdOH. The reactivity of the hydroxide complex with
CO2, CO, and H2 is reported.
’ INTRODUCTION
The new monoanionic pincer complex 2-(CH2PtBu2)-
6-(CH2OCH3)C6H3 (tBuPCO, Figure 1, complex 1) has been
synthesized and used to prepare a variety of palladium(II)
complexes. Similar to the chemistry reported for a PCO analogue
(1,3,5-tris-(CH3)-2-(CH2PtBu2)-6-(CH2OCH3)C6H, Figure 1,
complex 2) by Milstein and co-workers,5f the hemilabitity of the
ether arm as compared to the phosphine was expected to allow
the unymmetrical ligand to function as both a bidentate “PC”
chelate and a tridentate “PCO” system. Indeed, the palladium
complexes prepared with the new tBuPCO ligand show a range of
coordination from monodentate to bidentate and tridentate
ligation. The reactivities of (tBuPCO)Pd-type complexes are
described herein and compared to that of the symmetric PCP
analogue.
The seminal work of Moulton and Shaw’s first monoanionic
tridentate pincer ligands opened the door to an extremely useful
class of organometallic complexes.1 Due to the ability to readily
tune the electronics and sterics of these ligands, as well as the
robust nature of pincer metal complexes, this family of com-
pounds has been extensively studied and used in numerous late
transition metal-catalyzed reactions.2,3 The traditional coordina-
tion mode of a monoanionic pincer ligand is tridentate, and the
high stability of many pincer metal species under a variety of
reaction conditions is typically attributed to this strong and rigid
tridentate coordination. While the first monoanionic pincer
ligands were largely symmetric with a central anionic donor
atom flanked by two identical neutral donor atom arms (e.g., so-
called PCP- or NCN-type ligands), recently new, unsymmetric
pincer complexes wherein the neutral donor arms are different
from one another or in which the anionic donor species is located
on one of the arms rather than the central position of the pincer
have been prepared.4 The unique electronic, steric, and coordi-
nation environments provided by these unsymmetric pincer
ligands have enabled new modes of reactivity.4a,5 The inclusion
of a donor moiety on one arm of the pincer that does not bind
tightly to the metal provides potential access to a open site in the
coordination sphere. This hemilabile attribute of the ligand
allows for the ligand to coordinate in a tridentate configuration
in some situations and in a bidentate configuration in others.
’ RESULTS AND DISCUSSION
Synthesis of tBuPCO Complexes. The tBuPCO ligand was
synthesized according to a literature preparation for a similar
compound from R,R0-dibromo-m-xylene by successive nucleo-
philic displacements with sodium methoxide and di-tert-butyl-
phosphine (Scheme 1).5f
Received: December 7, 2010
Published: March 03, 2011
r
2011 American Chemical Society
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Figure 1. tBuPCO ligand, 2-(CH2PtBu2)-6-(CH2OCH3)C6H3 (1), and
Milstein’s PCO ligand, 1,3,5-tris-(CH3)-2-(CH2PtBu2)-6-(CH2O-
CH3)C6H (2).5f
Scheme 1
Figure 2. ORTEP of complex [(tBuPCO)Pd(Cl)(μ-Cl)]2 (3). Ellip-
soids are shown at 50% probability, and hydrogen atoms and a toluene
solvent molecule are omitted for clarity.
Scheme 2
Table 1. Selected Bond Lengths and Angles for Complex 3
bond
length (Å)
bond
angle (deg)
Pd1-P1
Pd1-Cl1
Pd1-Cl2
Pd1-Cl10
2.2765(7)
2.3192(7)
2.2849(7)
2.4442(6)
3.567(9)
P1-Pd1-Cl1
P1-Pd1-Cl2
Cl1-Pd1-Cl2
P1-Pd1-Cl10
Cl1-Pd1-Cl10
94.27(2)
94.95(2)
170.62(2)
175.24(3)
83.04(3)
Reaction of the tBuPCO ligand with bis(benzonitrile)palla-
dium(II) chloride resulted in the formation of a new phosphorus-
containing compound with a single 31P{1H} NMR resonance at
70.5 ppm. Additionally, the presence of four aromatic protons as
established by integration of the 1H NMR signals argues against
coordination of the aryl ring to the palladium center through the
ipso-carbon. This product was isolated by crystallization from a
toluene-pentane solution and analyzed by X-ray crystallogra-
phy. The orange-colored product of this reaction was determined
to be the dimeric species [(tBuPCO)Pd(Cl)(μ-Cl)]2 (3), as
illustrated in Scheme 2. An ORTEP of 3 is shown in Figure 2.
Of note is that the potentially multidentate ligand is coordinated
only in a monodentate fashion. κ1-Coordination to palladium
through the phosphine arm is observed, and the other three sites
of the typical square-planar local geometry about palladium are
occupied by two bridging and one terminal chloride. For a list of
selected bond distances and angles for 3, see Table 1.
When a toluene solution of 3 was heated to 100 °C for an hour,
the solution color changed from orange to bright yellow. Analysis
by 31P{1H} NMR spectroscopy revealed complete conversion of
complex 3 (signal at 70.5 ppm) to a new complex, 4, with a
resonance at 106.1 ppm. However, when the solution was
allowed to cool to room temperature, the signal at 106.1 ppm was
observed to decrease over the course of 12 h as the 70.5 ppm signal
for 3 returned to its former intensity. Complex 4 was observed by
1H NMR spectroscopy to contain only three aromatic proton
resonances, suggesting that C-H bond activation and coordina-
tion of the aryl ring to the metal center had occurred. Further-
more, a downfield shift for the -OCH3 protons (from 3.21 to
3.53 ppm) was observed, suggesting coordination of the ether
group to the palladium. On the basis of the NMR data, it was
suspected that the reaction depicted in Scheme 3 was occurring.
The equilibrium between 3 and 4 shown in Scheme 3 implies
that if a base were to be added to consume the HCl produced in
Pd1 Pd10
3 3 3
Scheme 3
the conversion of 3 to 4, the reaction could be driven to the right.
Indeed, heating a toluene solution of 3 in the presence of
potassium carbonate allowed for the isolation of the yellow-colored
4, which was characterized by NMR spectroscopy, elemental
analysis, and X-ray diffraction. An ORTEP of the monomeric,
tridentate pincer complex (tBuPCO)PdCl (4) is shown in Figure 3.
Selected bond lengths and angles of 4 are given in Table 2.
Additionally, the reaction shown in Scheme 3 can also be pushed
to the left. The addition of HCl to a benzene-d6 (C6D6) solution
of 4 results in the formation of 3.
In an attempt to remove the chloride from complex 4, sodium
tetrakis[3,5-bis(trifluoromethyl)phenyl]boron (NaB(ArF)4) was
added to solutions of 4 in both the noncoordinating solvent
C6D6 and the more-coordinating solvent tetrahydrofuran-d8
(THF-d8). In both cases, however, only the palladium product
[(tBuPCO)Pd]2(μ-Cl)[B(ArF)4] (5), a binuclear species with a
bridging chloride group, was observed to form (Scheme 4). After
filtering the excess NaB(ArF)4 from the solution, integration
1
of the H NMR spectrum revealed one equivalent of B(ArF)4
resonances for every two equivalents of ligand backbone resonances.
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Figure 4. ORTEP of complex [(tBuPCO)Pd]2(μ-OH)[B(ArF)4] (6).
Ellipsoids are shown at 50% probability, and the [B(ArF)4]- counterion,
toluene solvent molecule, and hydrogen atoms are omitted for clarity.
Figure 3. ORTEP of complex (tBuPCO)PdCl (4). Ellipsoids are shown
at 50% probability, and hydrogen atoms are omitted for clarity.
Table 3. Selected Bond Lengths and Angles for Complex 6
Table 2. Selected Bond Lengths and Angles for Complex 4
bond
length (Å)
bond
angle (deg)
bond
length (Å)
bond
angle (deg)
Pd1-P1
Pd1-C1
Pd1-O1
Pd1-O2
Pd2-O2
2.211(3)
1.948(11)
2.180(7)
2.149(7)
2.113(6)
P1-Pd1-O1
C1-Pd1-O2
C1-Pd1-O1
C1-Pd1-P1
Pd1-O2-Pd2
160.70(2)
173.30(4)
79.2(4)
Pd1-P1
Pd1-Cl1
Pd1-C1
Pd1-O1
2.2055(12)
2.4047(11)
1.978(4)
P1-Pd1-O1
C1-Pd1-Cl1
C1-Pd1-O1
C1-Pd1-P1
162.08(8)
172.85(14)
80.07(16)
82.86(14)
81.6(3)
2.146(3)
142.2(3)
Scheme 4
Scheme 5
in situ formation of (tBuPCO)PdOTf was observed by 1H and 31
P
It would appear that once a chloride anion is removed from one
molecule of 4, the resulting three-coordinate species scavenges a
second molecule of 4 to form the Cl-bridged binuclear palladium
species, 5. Complex 5 is quite resistant to removal of a second
chloride by NaB(ArF)4. Heating a C6D6 solution of 5 to 150 °C
with excess NaB(ArF)4 present did not result in abstraction of the
chloride.
When KOH was added to a solution of 5 in THF-d8, con-
version to a new species was observed by NMR spectroscopy.
The spectroscopic data were consistent with assignment of the
product as the hydroxide-bridged binuclear palladium species
[(tBuPCO)Pd]2(μ-OH)[B(ArF)4] (6). As with 5, integration of
the 1H NMR spectrum for complex 6 revealed two equivalents of
ligand backbone resonances and only one equivalent of B(ArF)4
resonances. However, the 1H NMR spectrum of 6 also contained
a resonance for one hydroxide proton (-1.97 ppm). Complex 6
was further characterized by X-ray diffraction, and the ORTEP is
shown in Figure 4. Bond distances and angles of 6 are listed in
Table 3. Further heating of solutions of 6, even in the presence of
excess KOH, produced no changes in either the 1H or 31P NMR
spectra.
NMR spectroscopies. Potassium hydroxide was then added to
form a new hydroxide species, (tBuPCO)PdOH (7), which exhibits
a hydroxide resonance in the 1H NMR spectrum at 1.00 ppm in
C6D6. Additionally, a small set of peaks, including a hydroxide
resonance at -3.09 ppm, was also detected. This minor species
(<5%) was assigned as a (tBuPCO)palladium(II) hydroxide
dimer (70) on the basis of the 1H NMR data. Complex 70 has a
diagnostic upfield chemical shift for the -OCH3 protons,
suggesting the ether arm is not coordinated to the metal center
(Scheme 5).
When a solution of 7 and 70 was heated to 50 °C, the two sets
of resonances coalesced into single, broader sets of peaks, sug-
gestive of a monomer-dimer equilibrium as shown in Scheme 5.
From a benzene-pentane solution of 7 and 70, a single crystal
was grown, and its structure was determined by X-ray diffraction.
The ORTEP of the dimeric species 70 is shown in Figure 5.
Consistent with the NMR data, the structure of 70 contains an
“opened arm” bidentate coordination of the PCO ligand with
two bridging hydroxides to form [(tBuPCO)Pd(μ-OH)]2 (70).
Selected bond lengths and angles of complex 70 are listed in
Table 4.
When silver trifluoromethanesulfonate (AgOTf) was used to
abstract the chloride from 4, a monomer product was observed.
Starting with a THF solution of 4, AgOTf was added, and the
While in the solid state the (tBuPCO)palladium(II) hydroxide
complex exists as a dimer, this configuration does not appear to
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Figure 6. ORTEP of complex (tBuPCO)PdO(CO)OH (8). Ellipsoids
are shown at 50% probability, and hydrogen atoms and benzene solvent
molecule are omitted for clarity.
Figure 5. ORTEP of complex [(tBuPCO)Pd(μ-OH)]2 (70). Ellipsoids
are shown at 50% probability, and hydrogen atoms are omitted for clarity.
Table 4. Selected Bond Lengths and Angles for Complex 70
bond
length (Å)
bond
angle (deg)
Pd1-P1
Pd1-C1
Pd1-O2
Pd1-O20
2.2023(3)
2.0208(10)
2.1101(8)
2.1009(8)
C1-Pd1-O2
P1-Pd1-O20
P1-Pd1-O2
Pd1-O2-Pd10
178.36(4)
172.56(2)
99.06(2)
102.01(3)
Figure 7. ORTEP exhibiting hydrogen bonding between molecules of
8. Ellipsoids are shown at 50% probability, and hydrogen atoms and
benzene solvent molecules are omitted for clarity.
be maintained in solution. Strong evidence that it is primarily the
monomer 7 that exists in solution at room temperature was obtained
by determining the molecular weight (M) of the hydroxide com-
plex. Using a Signer apparatus,6 the molecular weight for the
Table 5. Selected Bond Lengths and Angles for Complex 8
(
tBuPCO)palladium(II) hydroxide complex in a benzene solu-
bond
length (Å)
bond
angle (deg)
tion was found to be M = 4.00 ꢀ 102 g mol-1, in good agreement
with the expected monomeric M7 = 4.03 ꢀ 102 g mol-1 and not
Pd1-P1
Pd1-C1
Pd1-O1
Pd1-O2
2.2038(3)
1.9617(10)
2.1665(8)
2.1148(8)
2.584(12)
2.584(12)
P1-Pd1-O1
C1-Pd1-O2
C1-Pd1-O1
C1-Pd1-P1
O2-C18-03
O2-C18-04
162.03(2)
174.83(4)
80.69(4)
2
dimeric M7 = 8.06 ꢀ 10 g mol-1. The monomer form is also
0
1
consistent with the downfield chemical shift of the H NMR
resonance for the -OCH3 protons (3.34 ppm) of the major
solution species, indicating a coordinated methoxy group. In
contrast the -OCH3 protons of the minor species appear farther
upfield (3.24 ppm), as expected for a methoxy group that is not
coordinated to the metal center, as would be the case for the
dimeric structure 70.
82.49(3)
O4 O30
123.07(10)
116.22(9)
3 3 3
O3 O40
3 3 3
concentration. Cooling the CO2-concentrated solution enabled
crystallization of the complex with the 13C signal at 164.2 ppm.
X-ray crystallography verified the assignment of this complex as
Reactivity of (tBuPCO)PdOH with H2, CO2, and CO. The
reactivity of the palladium hydroxide complex 7 with a variety of
small molecules was investigated. When benzene-d6 solutions of
7 were degassed and placed under an atmosphere of carbon
the η1-bicarbonate species
(
tBuPCO)PdO(CO)OH (8,
Figure 6).
Like its tBuPCP analogue,8a 8 displays intermolecular hydro-
gen bonding in the solid state between the bicarbonate -OH and
a neighboring bicarbonate CdO (see oxygen atoms O4 and O30
in Figure 7), which bridges two independent units of 8 to form a
dimer. The distances O4 O30 and O3 O40 of 2.584(12) Å
1
dioxide, signals for 7 were completely absent in the H NMR
spectrum. Quantitative conversion to two new palladium species,
8 and 9, was observed. Notably, 13C{1H} NMR spectrum in
C6D6 contained new signals, which included free CO2 at 124.8
ppm,7 and singlets at 164.2 and 173.7 ppm corresponding to
complexes 8 and 9, respectively. CO2 insertion into palladium-
(II) hydroxides is precedented,8 and in the cases where η1-
bicarbonato complexes are formed,8a,b a diagnostic 13C NMR
resonance at δ 161-164 ppm (-OCOOH) is observed. Thus, the
signal at 164.2 ppm would be consistent with a palladium(II) η1-
bicarbonate species. The resonances for both complexes were
observed to be quite broad, suggesting that the species were in
equilibrium. The ratio of the species (as determined by changes
in the integrals of the 1H NMR signals for each) changed with
temperature, and at 25 °C complex 8 was present in greater
3 3 3
3 3 3
are characterized as “moderate” hydrogen bonds according to the
classification of Jeffrey.9 Additional bond lengths and angles for
complex 8 are given in Table 5.
Higher temperatures favored the product with the 13C NMR
singlet at 174 ppm, which was designated as complex 9, and at
50 °C, all of complex 8 had been converted to 9. Complex 9 was
also isolated from the CO2 reaction by removing the volatiles
from the reaction mixture. After redissolving the resulting residue
1
in fresh C6D6, the H and 31P NMR resonances for 9 are
observed to be much sharper, suggesting that when CO2 is
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Scheme 6
when dissolved, accounting for the single set of NMR signals
observed. When variable-temperature NMR spectroscopy was
attempted at -90 °C in toluene-d8 to further investigate
potential fluxionality, line broadening was noted for the ether
arm resonance, but only one set of signals corresponding to the
protons in 9 was observed.
With complexes 8 and 9 characterized, the reaction of 7 with
CO2 can be depicted as shown in Scheme 7. Initially, as
precedented for other late metal hydroxide complexes,8 carbon
dioxide inserts into the hydroxide bond to form 8. The formation
of 9 can be achieved through the reaction of two units of 8, where
an equivalent of carbonic acid is released. The carbonic acid side-
product can convert to carbon dioxide and water and can be
eliminated if the mixture is subjected to vacuum to remove the
volatiles. Alternatively, the binuclear carbonato complex 9 could
also be formed via an acid-base reaction between 8 and
unreacted hydroxo complex 7 with the release of water. Notably,
when substoichiometric amounts of CO2 were added to solu-
tions of 7, only the product 9 was observed by NMR spectros-
copy, along with starting material.
Figure 8. ORTEP of binuclear complex [(tBuPCO)Pd]2(μ-CO3) (9).
Ellipsoids are shown at 50% probability, and non-bicarbonate hydrogen
atoms and benzene solvent molecule are omitted for clarity.
Table 6. Selected Bond Lengths and Angles for Complex 9
bond
length (Å)
bond
angle (deg)
Pd1-P1
Pd1-C1
Pd1-O1
Pd1-O2
Pd2-C19
Pd2-O3
Pd2-O4
2.1970(17)
1.966(6)
2.155(5)
2.127(5)
2.018(7)
2.135(5)
2.143(5)
P1-Pd1-O1
C1-Pd1-O2
C1-Pd1-O1
C1-Pd1-P1
Pd1-O2-C18
C19-Pd2-O3
C19-Pd2-O4
162.93(14)
175.2(2)
79.8(2)
83.23(19)
114.7(4)
109.5(2)
171.3(2)
Reactions of the related tBuPCP palladium(II) hydroxide com-
plex with hydrogen gas leads to clean conversion to the corre-
sponding palladium(II) hydride and water.12 In contrast, reactions
of 7 with H2 resulted in some demetalation of the ligand and the
formation of a black precipitate, presumably palladium(0). The
reaction was monitored by NMR spectroscopy, but no hydride-
containing products were detected. Instead, a new species was
evident with virtual triplets resonating for the tert-butyl and
phosphine methylene arm protons. These spectral data would be
consistent with a product containing two tBuPCO ligands with
trans-coordinated phosphines. A resonance in the 1H NMR
spectrum corresponding to a proton bound to the ipso-carbon of
the benzene backbone was also observed. The new species was
identified as (tBuPCO)2Pd0 (10), which was confirmed by the
independent synthesis of the palladium(0) product. When bis-
(dibenzylideneacetone)palladium(0) was added to twoequivalents
of tBuPCO ligand, the resulting product exhibited identical NMR
resonances to the H2 reaction product. Of note, in a similar
palladium system reported by Hartwig and co-workers, the β-
hydride elimination (BHE) of an alkylamine ligand results in the
proposed formation of a palladium hydride species (PC)Pd-H
(PC = 2-(CH2PtBu2)C6H4), which then decomposes presumable
through reductive elimination (RE) to a product analogous to 10.13
As illustrated in Scheme 8, several mechanisms for the
formation of complex 10 can be considered. One mechanism
(Scheme 8, path a) is the generation of a palladium hydride
complex similar to the tBuPCP system,12 followed by RE of the
C-H bond of the ligand backbone. Notably, the hemilabile ether
arm could dissociate, which would allow the hydride ligand to
rearrange to a position cis to the Pd-C bond, facilitating the
C-H reductive elimination. The (tBuPCO)Pd0 could scavenge
free tBuPCO ligand and form 10. The presence of Pd(0) indicates
that phosphine is released from some Pd centers. The other two
absent, there is no equilibrium in solution between 8 and 9.
Crystals of the new species were obtained though the slow
diffusion of pentane into a concentrated sample in benzene,
and the solid-state structure was determined by X-ray diffraction
as the μ-bridged binuclear palladium carbonato species
[(tBuPCO)Pd]2(μ-CO3) (9). As illustrated in Figure 8, the
CO3 moiety in complex 9 is η2- and η1-bound to an “opened
arm” and a “closed arm” (tBuPCO)Pd unit, respectively. Only one
other example of this type of binuclear palladium complex is
reported,10 though similar examples for other late transition
metals are known.11 Additional bond distances and angles for
complex 9 are listed in Table 6.
With knowledge of the structures of complexes 8 and 9, the
equilibrium between these two species in the presence of CO2
can now be better understood. In particular, the exclusive for-
mation of 9 upon removal of the volatiles from the product
mixture containing 8 and 9 is consistent with the reaction shown
in Scheme 6. Evaporation of the C6D6 solution would be accom-
panied by the removal of CO2 and H2O, which are required for
the conversion of 9 to 8.
Of note, only one set of NMR resonances was observed for the
PCO ligand when analyzing solutions of 9, which could suggest a
rapid equilibrium between the binding of O3 to either Pd1 or
Pd2 (see Figure 8). However, this change in the κ1 or κ2 binding
of the CO3 moiety between the two Pd centers would also have
to be accompanied by the “opening” and “closing” of the respec-
tive tBuPCO ether arms. Another option is that the species in
solution is an η1, η1, μ-CO3 binuclear palladium complex where
both tBuPCO ligands are in the “closed arm” configuration.8c
Similar to how the hydroxide complex 7 is a dimer in the solid
state with a κ2-PCO ligand and a monomer in solution with a κ3-
PCO ligand, complex 9 could exist as the symmetric species
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Scheme 7. Possible Reaction Pathways for the Production of 9
Scheme 8. Possible Mechanisms for the Reaction of 7 with H2, Formation of (tBuPCO)2Pd0 (10)
mechanisms proceed via H2 displacement of the ether arm and
formation of a Pd(II) dihydrogen species.14 The hydrogen could
then add across the Pd-O bond through a four-center internal
electrophilic substitution (IES)15 pathway (Scheme 8, path b) to
form a palladium(II) hydrido aquo complex or across the Pd-C
bond through a four-center σ-bond metathesis (SBM)16 pathway
(Scheme 8, path c) to form a three-coordinate palladium(II)
hydrido hydroxo species. It is notable that the reaction of 7 with
H2 proceeds to completion in a matter of hours compared to the
tBuPCP system, which takes over three days. It is possible that the
sterics of the ligand or even the electronics of the complex allow
for a faster reaction with hydrogen. However, the decrease in
reaction time may also be explained by the operation of mech-
anisms b or c, which involve the ability of the hemilabile ligand in
complex 7 to open a coordination site and allow for efficient
binding of the hydrogen.
source of the C-H (or D). Additionally, when a C6D6 solution
1
of 7 was treated with D2, HD gas was detected by H NMR
1
spectroscopy as a 1:1:1 triplet (4.43 ppm, JHD = 42.8 Hz), as
deuterium was observed to scramble into the PdO-H bond. The
two products 7-d1 and 10-d2 and the starting material 7 were also
detected by 31P{1H} NMR spectroscopy at 94.6, 59.5, and
94.5 ppm, respectively. The scrambling of deuterium into the
PdO-H bond of 7 and the production of HD gas argue against
path c, as the scrambling by this mechanism would require
multiple successive steps to be reversible. The relevant scram-
bling would not occur until the last H-OH reductive elimina-
tion, which would also have to be reversible. Scrambling could
occur via path a, if the C-H RE of the ligand backbone were the
slow step of the reaction. Notably no palladium hydride complex
was detected by NMR spectroscopy. Path b, wherein the
scrambling occurs in a reversible IES step, would be the simplest
explanation.
The reactivity of 7 with carbon monoxide was also investi-
gated. Within minutes of CO exposure to 7, the colorless pal-
ladium(II) hydroxide solution changed to an intense orange color.
Quantitative conversion of compound 7 to a thermally unstable
species 11 was observed by NMR spectroscopy. It was not
possible to definitively establish the identity of 11. The initial
1H and 31P{1H} NMR spectra contain resonances that would
be consistent with a palladium(II) hydroxycarbonyl species
To verify that the proton source for complex 10's ipso-carbon
was indeed from H2, a separate experiment was performed in
C6H6 using deuterium gas. Following the reaction of 7 and D2 by
2H NMR spectroscopy, it was discovered that a broad singlet first
appeared at 1.09 ppm, corresponding to the palladium deuter-
oxide signal of (tBuPCO)PdOD (7-d1). Later, the appearance of
2
another H signal was noted at 7.90 ppm, corresponding to a
deuterium bound to the ipso-carbon of the ligand backbone for
(
tBuPCO-d1)2Pd0 (10-d2), confirming that H2 (or D2) is the
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(
tBuPCO)PdCOOH, as precedented in the tBuPCP system,8a and
like the tBuPCP analogue, no proton resonance for the hydroxy
group was observed. A less likely assignment for 11 would be an
asymmetrically bridged binuclear compound, [(tBuPCO)Pd]2-
(μ-CO2), analogous to the iPrPCP system,17 since only one 31
P
NMR resonance is seen. Distinct from the (tBuPCP)PdCOOH
chemistry, where CO2 loss and palladium hydride formation
occurred after heating the hydroxycarbonyl complex,8a when 11
is heated or placed under vacuum, decomposition to 10 is ob-
served. A potential pathway for the formation of 10 involves
carbon dioxide evolution from 11 to form a palladium(II)
hydride species, (tBuPCO)PdH. Similar to the reactions shown
in Scheme 8 for the later steps of the hydrogenolysis of 7,
reductive elimination of the C-H bond of the ligand backbone
and association of additional ligand would result in 10. Attempts
to isolate and further characterize 11 were unsuccessful.
Figure 9. Aromatic carbon-numbering scheme for ligand (X = PtBu2)
and its precursor (X = Br).
2
NMR, H NMR, 13C NMR, 31P NMR, 1D nuclear Overhauser effect
(NOE) difference, and 1H-13C gradient-selected heteronuclear multi-
ple-quantum coherence (gsHMQC) NMR spectra were recorded at
room temperature using a Bruker Avance 500 MHz spectrometer, and
all were referenced to the residual solvent signal (reported in ppm), with
the exception of 31P NMR spectra, which were referenced to an external
standard of 85% H3PO4 set to 0 ppm. All 13C NMR and 31P NMR data
were collected proton-decoupled (13C{1H} and 31P{1H}). Multiplicity
is reported as s, singlet; d, doublet; t, triplet; vt, virtual triplet; q, quartet;
br, broad. Electrospray ionization (ESI) mass spectrometry was per-
formed using a Bruker Esquire liquid chromatograph-ion trap.
Elemental analyses were carried out by Atlantic Microlab, Inc. of
Norcross, GA. The complex bis(benzonitrile)palladium(II)chloride
(PdCl2(C6H5CN)2) was prepared according to a published procedure.18
2-(CH2Br)-6-(CH2OCH3)C6H3. R,R0-Dibromo-m-xylene (5.122
g, 19.6 mmol) and sodium methoxide (1.060 g, 19.6 mmol) were added
to a round-bottom flask and dissolved in THF. The solution was cooled
to 0 °C, whereupon methanol (1.00 mL, 24.7 mmol) was added. The
reaction was stirred for 12 h, resulting in precipitation of a white solid.
The volatiles were removed under reduced pressure, and the product
was extracted with pentane and filtered through a Teflon filter. The
volatiles were removed from the filtrate under vacuum, revealing a col-
orless oil, which was purified by column chromatography (SiO2, CH2Cl2).
Yield: 2.048 g (49.1%). 1H NMR (C6D6, 500 MHz):19 7.15 (s, 1H, H1),
7.07 (d, 1H, JHH =7.4Hz,H5), 6.99 (t, 1H, 3JHH =7.5Hz,H4), 6.96 (d, 1H,
3JHH = 7.6 Hz, H3), 4.11 (s, 2H, CH2O), 4.00 (s, 2H, CH2Br), 3.08 (s, 3H,
OCH3). 13C{1H} NMR (C6D6, 126 MHz): 139.7 (s, C4), 138.3 (s, C2),
128.8 (s, C6), 128.4 (s, C1), 128.3 (s, C3), 127.5 (s, C5), 74.1 (s, CH2O),
57.8 (s, OCH3), 33.4 (s, CH2Br).
’ SUMMARY
In summary, we have prepared and characterized a new series
of unsymmetric palladium pincer complexes. Among the various
new compounds synthesized, examples of mono-, bi-, and
tridentate coordination modes have been demonstrated for the
tBuPCO ligand. Monodentate binding is observed through the
phosphine in the dimeric [(tBuPCO)Pd(Cl)(μ-Cl)]2 complex 3.
The tBuPCO ligand can also function as a bidentate chelate
through the phosphine and the aryl ring in the dimeric
[(tBuPCO)Pd(μ-OH)]2 complex 70. Additionally, the traditional
tridentate pincer coordination is observed in the monomeric
(
tBuPCO)PdCl complex 4, wherein the ether oxygen provides
the third point of attachment. In some cases, equilibrium
mixtures are observed that confirm the ether arm as a hemilabile
donor. The (tBuPCO)PdOH complex 7 was observed to react
with CO2 to form both the η1-bicarbonate species
(
tBuPCO)PdO(CO)OH, 8, and the μ-bridged binuclear palla-
dium carbonato complex [(tBuPCO)Pd]2(μ-CO3), 9. Hydro-
genolysis reactions involving 7 resulted in the Pd(0) decom-
position product (tBuPCO)2Pd0, 10, presumably via the reduc-
tive elimination of the C-H bond of the ligand backbone,
palladium(0) precipitation, and incorporation of released
tBuPCO ligand.
tBuPCO = 2-(CH2PtBu2)-6-(CH2OCH3)C6H3 (1). A round-bot-
tom flask was charged with 2-(CH2Br)-6-(CH2OCH3)C6H3 (1.262 g,
5.92 mmol) and methanol (20 mL). The solution was cooled to 0 °C,
and di-tert-butylphosphine (0.866 g, 5.92 mmol) was added dropwise to
the reaction. The reaction was allowed to warm to room temperature
and stirred for 12 h. A slight excess of triethylamine (0.65 g, 6.4 mmol)
was added to the colorless liquid. The reaction was stirred for an
additional 12 h. The volatiles were removed under reduced pressure,
revealing an oily, white solid residue. The product was extracted with
pentane and filtered through a Teflon filter. The solvent was removed
from the filtrate under vacuum, and the tBuPCO complex was isolated as
a highly viscous, colorless oil. Yield: 1.253 g (75.9%). 1H NMR (C6D6,
500 MHz):19 7.53 (s, 1H, H1), 7.35 (d, 1H, 3JHH = 7.3 Hz, H3), 7.17
(t, 1H, 3JHH = 7.5 Hz, H4), 7.13 (d, 1H, 3JHH = 7.5 Hz, H5), 4.28 (s, 2H,
CH2O), 3.15 (s, 3H, OCH3), 2.75 (d, 2H, 2JHP = 2.2 Hz, CH2P), 1.06
(d, 18H, 3JHP = 10.6 Hz, C(CH3)3). 13C{1H} NMR (C6D6, 126 MHz):
142.2 (d, 2JCP = 12.7 Hz, C2), 139.1 (s, C6), 128.5 (s, C5), 129.3 (d, 3JCP
= 8.8 Hz, C1), 129.2 (d, 3JCP = 8.8 Hz, C3), 128.47 (s, C4), 125.0 (s, C5),
It is notable that although the κ3-(tBuPCO) palladium chloride
and hydroxide complexes could be synthesized, isolated, and
characterized, we were unable to prepare the corresponding
hydride. It appears that C-H reductive elimination involving the
central C position of the aryl ring of the tBuPCO ligand is rapid
enough to prevent any observation of a Pd(II) hydride in this
system. This facile C-H reductive elimination may be related to
the hemilabile nature of the ether functionality, which can
provide a low-energy pathway to a three-coordinate geometry
and a cis configuration of the M-C and M-H bonds. Under-
standing reactivity trends for hemilabile ligands will aid in future
design of new ligands for catalytic reactions.
’ EXPERIMENTAL SECTION
1
74.7 (s, CH2O), 57.7 (s, OCH3), 31.8 (d, JCP1 = 24.2 Hz, C(CH3)3),
2
Unless specified otherwise, all manipulations were carried out under
nitrogen using conventional vacuum line techniques and a glovebox
equipped with a -35 °C freezer. Solvents were purified before use. THF,
benzene, and pentane were purified by passage through columns of
activated alumina and molecular sieves. C6D6, toluene-d8, and THF-d8
were dried over sodium metal-benzophenone. All other reagents were
used as obtained from commercial suppliers, unless otherwise noted. 1H
30.0 (d, JCP = 13.6 Hz, C(CH3)3), 29.0 (d, JCP = 25.5 Hz, CH2P).
31P{1H} NMR (C6D6, 203 MHz): 33.6.
[(tBuPCO)Pd(Cl)(μ-Cl)]2 (3). The tBuPCO ligand 1 (1.20 g, 4.3
mmol) and PdCl2(C6H5CN)2 (1.65 g, 4.3 mmol) were placed into a
round-bottom flask, and toluene (30 mL) was added. The resulting
solution was heated to 100 °C and stirred for 4 h, upon which the
solution turned deep red in color. The reaction was allowed to cool to
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room temperature over the course of 12 h. A large amount of an orange,
powdery solid precipitated out of solution. The solid was isolated on a glass
frit, rinsed with cold pentane, and dried on a high-vacuum line for 12 h.
Yield: 1.53 g (77.7%). A small amount of the product (20 mg, 22 μmol) was
redissolved in a minimum of toluene and layered with pentane, which
resulted in the growth of red-orange crystals suitable for X-ray crystal-
lography. 1H NMR (C6D6, 500 MHz):19 8.28 (m, 2H, H4), 7.83 (s, 2H,
H1), 7.07 (m, 4H, H3,5), 4.35 (br s, 4H, CH2O), 3.25 (br s, 4H, CH2P),
3.21 (br s, 6H, OCH3), 1.28 (d, 36H, 3JHP = 14.6 Hz, C(CH3)3). 31P{1H}
NMR (C6D6, 203 MHz): 70.5 (br s).
150.8 (s, C1), 147.3 (d, JCP = 12.2 Hz, C2), 144.0 (s, C6), 135.5 (s, ArF-
C2,6), 129.9 (q, 3JCF = 28.7 Hz, ArF-C3,5), 125.8 (s, C4), 125.3 (q, 1JCF
=
272.5 Hz, ArF-CF3), 123.0 (d, 3JCP = 22.4 Hz, C3), 119.1 (s, ArF-C4),
118.1 (s, C5), 83.8 (s, CH2O), 61.6 (s, OCH3), 35.3 (d, 1JCP = 19.0 Hz,
1
2
C(CH3)3), 34.5 (d, JCP = 32.0 Hz, CH2P), 28.5 (d, JCP = 4.1 Hz,
C(CH3)3). 31P{1H} NMR (C6D6, 203 MHz): 103.2.
Preparation of (tBuPCO)PdOH (7). The Pd(II) triflate complex
(
tBuPCO)PdOTf was formed in situ from the reaction of 4 and silver
trifluoromethanesulfonate (AgOTf). Into a round-bottom flask were
placed 4 (727.0 mg, 1.73 mmol) and AgOTf (450.0 mg, 1.75 mmol)
followed by benzene (25 mL). The yellow solution became cloudy white
in color. The flask was covered in aluminum foil, and the reaction was
stirred in the dark for 12 h. The solution was filtered through a Teflon
filter to reveal a champagne-colored solution. A portion of the solid
product (2.2 mg) was added to an NMR tube and dissolved in C6D6.
The generation of the palladium triflate complex was confirmed by 1H
(
tBuPCO)PdCl (4). Complex 3 (1.53 g, 1.67 mmol) and an excess of
potassium carbonate (1.31 g, 9.48 mmol) were added to a round-bottom
flask, and THF (30 mL) was added. The solution was heated at 100 °C
and stirred for 12 h. The solution was filtered through a Teflon filter to
reveal a bright, yellow-orange-colored solution. The volatiles were
removed under reduced pressure, and the resulting yellow-orange solid
was dissolved in a minimum of benzene. A pentane-vapor diffusion
chamber was used to crystallize the desired product. The resulting
yellow-colored crystals, which were suitable for X-ray crystallography,
were isolated. Yield: 1.05 g (74.5%). 1H NMR (C6D6, 500 MHz): 6.98
(t, 1H, 3JHH = 7.5 Hz, H4), 6.85 (d, 1H, 3JHH = 7.4 Hz, H3), 6.52 (d, 1H,
3JHH = 7.5 Hz, H5), 4.41 (s, 2H, CH2O), 3.53 (s, 3H, OCH3), 2.87
(d, 2H, 2JHP = 9.7 Hz, CH2P), 1.26 (d, 18H, 3JHP = 14.5 Hz, C(CH3)3).
13C{1H} NMR (C6D6, 126 MHz): 158.0 (s, C1), 147.7 (d, 2JCP = 12.8
1
and 31P NMR spectroscopies. H NMR (C6D6, 500 MHz):19 6.87 (t,
1H, 3JHH = 7.5 Hz, H4), 6.66 (d, 1H, 3JHH = 7.5 Hz, H3), 6.32, (d, 1H,
3JHH = 7.5 Hz, H5), 4.11 (s, 2H, CH2O), 3.53, (s, 3H, OCH3), 2.59 (d,
2
3
2H, JHP = 9.9 Hz, CH2P), 1.10 (d, 18H, JHP = 14.9 Hz, C(CH3)3).
31P{1H} NMR (C6D6, 203 MHz): 103.4. To the benzene solution of the
palladium triflate complex (tBuPCO)PdOTf (1.73 mmol) was added two
equivalents of KOH (194.0 mg, 3.46 mmol). The reaction was stirred at
ambient temperature for 12 h, upon which the solution was filtered
through a Teflon filter. The faintly yellow-colored filtrate was stripped of
its volatiles, resulting in the formation of a white solid. Yield: 520.4 mg
(74.7%). X-ray quality crystals were obtained from a benzene solution as
colorless plates by pentane-vapor diffusion. Data for monomer (“closed
arm” 7): 1H NMR (C6D6, 500 MHz): 7.22 (d, 1H, 3JHH = 7.2 Hz, H5),
7.05 (t, 1H, 3JHH = 7.2 Hz, H4), 6.96 (d, 1H, 3JHH = 7.2 Hz, H3), 5.06 (s,
2H, OCH2), 3.34 (s, 3H, OCH3), 2.91 (d, 2H, 2JHP = 9.9 Hz, CH2P),
1.13 (d, 18H, 3JHP = 13.3 Hz, C(CH3)3), 1.00 (br s, 1H, OH). 13C{1H}
NMR (C6D6, 126 MHz): 153.2 (s, C1), 149.4 (d, 2JCP = 16.5 Hz, C2),
144.8 (s, C6), 128.7 (s, C5), 123.8 (s, C4), 122.8 (d, 3JCP = 19.3 Hz, C3),
77.8 (s, OCH2), 56.9 (s, OCH3), 35.1 (d, 1JCP = 26.4 Hz, CH2P), 34.4 (d,
1JCP = 17.3 Hz, C(CH3)3), 29.3 (s, C(CH3)3). 31P{1H} NMR (C6D6,
203 MHz): 94.5. Anal. Calcd for C34H58O4P2Pd2: C, 50.69; H, 7.25.
Found: C, 50.62; H, 7.17. [(tBuPCO)Pd(μ-OH)]2 (70). Data for dimer
(“opened arm”): 1H NMR (C6D6, 500 MHz): 5.20 (br s, 2H, OCH2),
3.24 (br s, 3H, OCH3), 2.91 (buried under 5, 2H, CH2P), 1.13 (buried
under 5, 18H, C(CH3)3), -3.09 (br s, 1H, OH). 31P{1H} NMR
(C6D6, 203 MHz): 93.4.
3
Hz, C2), 145.2 (s, C6), 124.8 (s, C4), 122.1 (d, JCP = 22.0 Hz, C3),
118.6 (s, C5), 84.2 (s, CH2O), 60.3 (s, OCH3), 35.8 (d, 1JCP = 29.9 Hz,
1
2
CH2P), 35.5 (d, JCP = 19.4 Hz, C(CH3)3), 29.0 (d, JCP = 4.2 Hz,
C(CH3)3). 31P{1H} NMR (C6D6, 203 MHz): 106.1. Anal. Calcd for
C17H28OPClPd: C, 48.48; H, 6.70. Found: C, 48.49; H, 6.61.
Reaction of 4 with NaB(ArF)4 Generating [(tBuPCO)Pd]2(μ-
Cl)[B(ArF)4] (5). Complex 4 (2.0 mg, 4.7 μmol) and sodium tetrakis-
[3,5-bis(trifluoromethyl)phenyl]boron (NaB(ArF)4, 4.2 mg, 4.7 μmol)
were added to a medium-walled NMR tube fitted with a resealable
Teflon valve. The solids were dissolved in C6D6 (0.40 mL), and the
solution was shaken vigorously. After 5 min, the solution was filtered
through a Teflon filter. The filtrate was examined by NMR spectroscopy,
which revealed the formation of the binuclear species 5. 1H NMR
(C6D6, 500 MHz):19 8.40 (s, 8H, ArF-H2,6), 7.66 (s, 4H, ArF-H4), 6.90
(t, 2H, 3JHH = 7.5 Hz, H4), 6.73 (d, 2H, 3JHH = 7.5 Hz, H3), 6.41 (d, 2H,
3JHH = 7.5 Hz, H5), 4.32 (s, 4H, CH2O), 3.42 (s, 6H, OCH3), 2.74 (d,
2
3
4H, JHP = 9.9 Hz, CH2P), 1.04 (d, 36H, JHP = 14.8 Hz, C(CH3)3).
13C{1H} NMR (C6D6, 126 MHz): 162.8 (q, 1JCB = 49.9 Hz, ArF-C1),
152.2 (s, C1), 147.3 (d, JCP = 11.8 Hz, C2), 143.9 (s, C6), 135.5 (s, ArF-
Molecular Weight Determination of 7 Using a Signer
Apparatus. Applying Raoult’s law, the molecular weight of 7 can be
calculated from the following formula (eq 1):
C2,6), 129.9 (q, 3JCF = 28.7 Hz, ArF-C3,5), 126.4 (s, C4), 125.3 (q, 1JCF
=
272.5 Hz, ArF-CF3), 123.3 (d, 3JCP = 23.0 Hz, C3), 119.4 (s, ArF-C4),
118.1 (s, C5), 83.9 (s, CH2O), 61.5 (s, OCH3), 35.8 (d, 1JCP = 19.0 Hz,
GxMstdVstd
1
2
Mx ¼
ð1Þ
C(CH3)3), 34.2 (d, JCP = 31.6 Hz, CH2P), 28.5 (d, JCP = 3.8 Hz,
C(CH3)3). 31P{1H} NMR (C6D6, 203 MHz): 106.1. ESI-MS (methanol
solution): Mþ found 807.3, calcd for C34H56O2P2ClPd2 807.1 (no solvent
molecules or B(ArF)4 are present).
GstdVx
The variables Mstd, Vstd, and Gstd are the molecular weight, volume of
solution, and mass of a ferrocene standard, respectively, and Mx, Vx, and
Gx are the corresponding values for complex 7. Using a Signer apparatus,
one bulb was filled with a benzene solution of freshly sublimed ferrocene
(Mstd = 1.86 ꢀ 102 g mol-1, Vstd = 1.04 mL, Gstd = 1.76 mg). The other
bulb was charged with crystals of complex 7, which were dissolved in
benzene (Vx = 0.86 mL, Gx = 5.20 mg). The solutions were degassed
with three freeze-pump-thaw cycles, and the apparatus was placed in a
dark, draft-free cabinet for two weeks under isothermal conditions.
Volumes were recorded every 1-3 days until they became constant,
whereupon the molecular weight was calculated to be 4.00 ꢀ 102 g mol-1
(Vstd = 1.10 mL, Vx = 0.80 mL), which is consistent with the monomeric
palladium(II) hydroxide structure (M7 = 4.03 ꢀ 102 g mol-1).
Reaction of 5 with KOH Generating [(tBuPCO)Pd]2(μ-OH)-
[B(ArF)4] (6). Complex 5 (12.5 mg, 7.5 μmol) and an excess of
potassium hydroxide (3.5 mg, 62 μmol) were added to an NMR fitted
with a resealable Teflon valve. C6D6 (0.40 mL) was added to the tube,
and the vessel was place in a sonic bath for 5 h. The solution was filtered
through a Teflon filter, and the filtrate was examined by NMR spec-
troscopy, which revealed the formation of the dimeric hydroxide species
6. The solution was concentrated and layered with pentane, resulting in
1
the formation of X-ray quality crystals as colorless plates. H NMR
(C6D6, 500 MHz):19 8.41 (s, 8H, ArF-H2,6), 7.68 (s, 4H, ArF-H4), 6.91
(t, 2H, 3J = 7.4 Hz, H4), 6.75 (d, 2H, 3J = 7.4 Hz, H3), 6.44 (d, 2H, 3J =
7.4 Hz, H5), 4.36 (s, 4H, CH2O), 3.53 (s, 6H, OCH3), 2.74 (d, 4H, 2J =
9.8 Hz, CH2P), 0.98 (d, 36H, 3J = 14.6 Hz, C(CH3)3), -1.97 (s, 1H, OH).
Reaction of 7 with CO2 Generating tBuPCO)PdO(CO)OH (8)
and [(tBuPCO)Pd]2(μ-CO3) (9). An NMR tube fitted with a resealable
Teflon valve was charged with complex 7 (12.2 mg, 30.3 μmol) and
1
13C{1H} NMR (C6D6, 126 MHz): 162.8 (q, JCB = 49.9 Hz, ArF-C1),
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dissolved in C6D6 (0.40 mL). The solution was degassed and then
placed under an atmosphere of carbon dioxide. The quantitative
conversion of 7 to 8 and 9 was observed within minutes by H NMR
The solution was degassed and then pressurized with deuterium gas
(3.5 atm). Over the course of 3 h, the reaction was monitored by 2H and
31P{1H} NMR spectroscopies, confirming the formations of 10-d2 and
7-d1. A separate experiment was run with 7 (4.2 mg, 10 μmol) dissolved
in C6D6 (0.40 mL), which was pressurized with D2 (3.5 atm). The
reaction was followed by 1H and 31P{1H} NMR spectroscopies, and the
resonances for 10-d2 (the H1 peak was absent) and 7-d1 (the OH peak
had greatly decreased in intensity) were detected, as well as HD gas
(δ 4.43, 1:1:1 t, 1JHD = 42.8 Hz). (tBuPCO-d1)2Pd0 (10-d2). 2H NMR
(C6H6, 76.7 MHz): 7.90 (s, D1). 31P{1H} NMR (C6D6, 203 MHz):
59.5. (tBuPCO)PdOD (7-d1). 2H NMR (C6H6, 76.7 MHz): 1.09
(s, OD). 31P{1H} NMR (C6D6, 203 MHz): 94.6.
1
spectroscopy. X-ray quality crystals of 8 were obtained as colorless
prisms by vacuum transferring hexane over the mixture and then filling
the tube with CO2 (1 atm). Elemental analysis of 8 was impractical since
9 cocrystallized with 8. By NMR spectroscopy, both sets of resonances
for 8 and 9 were broad; however, when the sample was cooled to 10 °C,
the peaks for 8 grew in intensity and sharpened. (tBuPCO)PdO(CO)OH
(8). 1H NMR (C6D6, 500 MHz): 6.97 (t, 1H, 3JHH = 7.3 Hz, H4), 6.80
(d, 1H, 3JHH = 7.3 Hz, H3), 6.44 (d, 1H, 3J = 7.3 Hz, H5), 4.28 (s, 2H,
2
CH2O), 3.43 (s, 3H, OCH3), 2.73 (d, 2H, JHP = 9.5 Hz, CH2P), 1.18
(d, 18H, 3JHP = 14.5 Hz, C(CH3)3). 13C{1H} NMR (C6D6, 126 MHz):
164.2 (s, OCOOH), 152.9 (s, C1), 148.4 (d, 2JCP = 13.4 Hz, C2), 145.6
(s, C6), 124.7 (s, C4), 122.2 (d, 3JCP = 22.1 Hz, C3), 118.6 (s, C5), 83.2
Reaction of 7 with CO. Formation of 11. A J. Young NMR
tube was charged with 7 and C6D6, and the subsequent colorless
solution was degassed. Using a gas manifold, 1 atm of CO was added
to the tube. Upon shaking the tube, within minutes the solution began to
change in color to a bright orange. By NMR spectroscopy, the major
species in solution was characterized as the CO insertion product 11;
however, upon heating the sample or placing the solution under vacuum,
decomposition of 11 to 10 was observed. 1H NMR (C6D6, 500 MHz):
6.97 (t, 1H, 3JHH = 7.5 Hz, H4), 6.81 (d, 1H, 3JHH = 7.6 Hz, H3), 6.50,
(d, 1H, 3JHH = 7.5 Hz, H5), 5.09 (s, 2H, CH2O), 3.35, (s, 3H, OCH3),
1
(s, CH2O), 61.7 (s, OCH3), 35.4 (d, JCP = 19.1 Hz, C(CH3)3), 34.3
(d, 1JCP = 30.7 Hz, CH2P), 28.7 (d, JCP =3.8Hz, C(CH3)3). 31P{1H} NMR
(C6D6, 203 MHz): 100.5.
[(tBuPCO)Pd]2(μ-CO3) (9). A C6D6 solution of 8 and 9, obtained
through the reaction of 7 (14.0 mg, 34.8 μmol) and CO2 (1 atm), was
stripped of its volatiles using a vacuum line. The resulting residue was
redissolved in C6D6 and analyzed by NMR spectroscopy, confirming the
exclusive, quantitative formation of 9. The solution was layered with
pentane and kept under vacuum. Over the course of two days, X-ray
quality crystals formed as colorless prisms. Yield: 13.2 mg (91.4%). 1H
NMR (C6D6, 500 MHz): 7.07 (d, 2H, 3JHH = 3.6 Hz, H3,5), 6.89 (t, 1H,
3JHH = 4.4 Hz, H4), 4.94 (s, 2H, CH2O), 3.56 (s, 3H, OCH3), 2.82
(d, 2H, 2JHP = 9.7 Hz, CH2P), 1.21 (d, 18H, 3JHP = 14.1 Hz, C(CH3)3).
13C{1H} NMR (C6D6, 126 MHz): 173.7 (s, CO3), 151.2 (d, 3JCP = 2.8
Hz, C1), 148.8 (d, 2JCP = 14.6 Hz, C2), 146.7 (d, 4JCP = 2.0 Hz, C6),
2
3
2.79 (d, 2H, JHP = 9.8 Hz, CH2P), 1.19 (d, 18H, JHP = 14.5 Hz,
C(CH3)3);. 31P{1H} NMR (C6D6, 203 MHz): 101.2.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data, in-
b
cluding tables of bond lengths and angles, as well as files in CIF
format for 3, 4, 6, 70, 8, and 9 and NMR spectra of non-isolated
complexes are available free of charge via the Internet at http://
pubs.acs.org.
3
124.7 (s, C4), 122.3 (d, JCP = 21.1 Hz, C3), 121.5 (s, C5), 79.5
1
(s, CH2O), 59.9 (s, OCH3), 35.2 (d, JCP = 18.8 Hz, C(CH3)3), 33.8
(d, 1JCP = 29.5 Hz, CH2P), 29.0 (d, 2JCP = 4.3 Hz, C(CH3)3). 31P{1H}
NMR (C6D6, 203 MHz): 102.4. Anal. Calcd for C35H56O5P2Pd2: C,
50.55; H, 6.79. Found: C, 50.59; H, 6.83.
’ AUTHOR INFORMATION
Reaction of 7 with H2 Yielding (tBuPCO)2Pd0 (10). Complex
7 (5.1 mg, 13 μmol) and C6D6 (0.40 mL) were added to an NMR tube
fitted with a resealable Teflon valve. The solution was degassed and the
tube was pressurized with hydrogen gas (7.0 atm). Over the course of an
hour, the reaction mixture changed from colorless to a dark brown-
colored solution with a black precipitate. Resonances for the tert-butyl
and phosphine methylene protons appeared as virtual triplets in the 1H
NMR spectrum. This coupling pattern is indicative of trans-phosphine
ligands coordinated to a metal center, consistent with the assignment of
Corresponding Author
*E-mail: goldberg@chem.washington.edu; rakemp@unm.edu.
’ ACKNOWLEDGMENT
We thank the Department of Energy (DE-FG02-06ER15765)
for support.
the complex as 10.19 1H NMR (C6D6, 500 MHz): 8.34 (d, 2H, 3JHH
=
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=
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(
in C6H6 (0.40 mL) in an NMR tube fitted with a resealable Teflon valve.
1635
dx.doi.org/10.1021/om101150y |Organometallics 2011, 30, 1627–1636

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