Reactivity of electrophilic palladium alkyl cations stabilized by electron-rich chelating diphosphine ligands. Evidence for dinuclear intermediates and the formation of a dinuclear mixed-valence methyl cation

Article abstract of DOI:10.1039/a800152i

The reactivity of the electron-rich palladium alkyl cation, [Pd(dippe)R]⁺BAr₄^- (dippe = 1,2-bis(diisopropylphosphino)ethane; R = η³-CH₂Ph or CH₃; BAr₄ = {B[3,5-(F₃C)₂C₆H₃]₄}) with a variety of small molecules is reported. Although the benzyl cation is unreactive towards carbon monoxide and dihydrogen, the corresponding methyl cation, [Pd(dippe)Me(s)]⁺ (s = Et₂O, THF or o-dichlorobenzene) reacts rapidly with H₂ to produce the dihydride-bridged dimer {[(dippe)Pd]₂(μ-H)₂}²⁺, and with CO to produce the dinuclear mixed-valence, cationic complex [Pd(dippe)(μ-CO)Pd(dippe)Me][BAr₄]. In addition, the methyl cation can abstract alkyl groups from neutral dialkyl complexes; thus, the addition of [Pd(dippe)Me(s)]⁺ to Pd(dippe)(CH₂Ph)₂ results in the formation of the methyl benzyl derivative Pd(dippe)Me(CH₂Ph) and the cationic benzyl cation [Pd(dippe)(η³-CH₂Ph)]⁺. Methyl group interchange is also observed for the reaction of the methyl cation with the neutral dimethyl; when [Pd(dippe)Me(s)]⁺ is mixed with Pd(dippe)(¹³CH₃)₂, the carbon-13 label is immediately scrambled to the cation. These exchange reactions are suggested to occur via dinuclear intermediates. The mixed-valence dinuclear species mentioned above has been investigated in some detail; mechanistic studies have indicated that the addition of CO to [Pd(dippe)Me(s)]⁺ probably proceeds via simple substitution of the solvent by CO to generate the expected mononuclear methyl carbonyl cation [Pd(dippe)Me(CO)]⁺, followed by migratory insertion to give the acetyl carbonyl derivative [Pd(dippe)(COMe)(CO)]⁺. The final product is the dinuclear mixed-valence species [Pd(dippe)(μ-CO)Pd(dippe)Me][BAr₄], which is accompanied by the formation of acetone (Me₂CO) and the dicarbonyl dication [Pd(dippe)(CO)₂]²⁺. Presumably, methyl transfer occurs at some stage from the methyl cation to generate the methyl-acetyl complex, Pd(dippe)Me(COMe); reductive elimination of acetone under CO from the methyl-acetyl complex produces the Pd⁰ complex Pd(dippe)CO which then reacts with the starting methyl cation to generate the dinuclear mixed-valence species. Addition of CO to the mixed-valence species does not result in formation of mononuclear complexes, rather 1 equivalent of CO adds to form a new dinuclear complex with two bridging COs.

Full text of DOI:10.1039/a800152i

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Reactivity of electrophilic palladium alkyl cations stabilized by
electron-rich chelating diphosphine ligands. Evidence for dinuclear
intermediates and the formation of a dinuclear mixed-valence methyl
cation‡
Michael D. Fryzuk,* Guy K. B. Clentsmith and Steven J. Rettig†
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC,
Canada V6T 1Z1
The reactivity of the electron-rich palladium alkyl cation, [Pd(dippe)R]ϩBAr4Ϫ (dippe = 1,2-bis(diisopropyl-
phosphino)ethane; R = η3-CH2Ph or CH3; BAr4 = {B[3,5-(F3C)2C6H3]4}) with a variety of small molecules is
reported. Although the benzyl cation is unreactive towards carbon monoxide and dihydrogen, the corresponding
methyl cation, [Pd(dippe)Me(s)]ϩ (s = Et2O, THF or o-dichlorobenzene) reacts rapidly with H2 to produce the
dihydride-bridged dimer {[(dippe)Pd]2(µ-H)2}2ϩ, and with CO to produce the dinuclear mixed-valence, cationic
complex [Pd(dippe)(µ-CO)Pd(dippe)Me][BAr4]. In addition, the methyl cation can abstract alkyl groups from
neutral dialkyl complexes; thus, the addition of [Pd(dippe)Me(s)]ϩ to Pd(dippe)(CH2Ph)2 results in the formation
of the methyl benzyl derivative Pd(dippe)Me(CH2Ph) and the cationic benzyl cation [Pd(dippe)(η3-CH2Ph)]ϩ.
Methyl group interchange is also observed for the reaction of the methyl cation with the neutral dimethyl; when
[Pd(dippe)Me(s)]ϩ is mixed with Pd(dippe)(13CH3)2, the carbon-13 label is immediately scrambled to the cation.
These exchange reactions are suggested to occur via dinuclear intermediates. The mixed-valence dinuclear species
mentioned above has been investigated in some detail; mechanistic studies have indicated that the addition of CO
to [Pd(dippe)Me(s)]ϩ probably proceeds via simple substitution of the solvent by CO to generate the expected
mononuclear methyl carbonyl cation [Pd(dippe)Me(CO)]ϩ, followed by migratory insertion to give the acetyl
carbonyl derivative [Pd(dippe)(COMe)(CO)]ϩ. The final product is the dinuclear mixed-valence species
[Pd(dippe)(µ-CO)Pd(dippe)Me][BAr4], which is accompanied by the formation of acetone (Me2CO) and the
dicarbonyl dication [Pd(dippe)(CO)2]2ϩ. Presumably, methyl transfer occurs at some stage from the methyl cation
to generate the methyl–acetyl complex, Pd(dippe)Me(COMe); reductive elimination of acetone under CO from
the methyl–acetyl complex produces the Pd0 complex Pd(dippe)CO which then reacts with the starting methyl
cation to generate the dinuclear mixed-valence species. Addition of CO to the mixed-valence species does not
result in formation of mononuclear complexes, rather 1 equivalent of CO adds to form a new dinuclear complex
with two bridging COs.
Soluble palladium complexes are widely used as catalysts for
many organic transformations.1,2 Recently, cationic PdII deriv-
atives have come under intense scrutiny as catalysts for the
copolymerization of carbon monoxide and olefins, and for
the polymerization of functionalized olefins.3–19 In both of
these processes, one of the propagating species is believed to
consist of a cationic square-planar derivative (A) stabilized by a
bidentate chelating ligand, usually a diphosphine or a diimine,
with the other two sites occupied by monomer and growing
polymer.20
such as H2 and CO and make comparisons with the neutral
dialkyl analogues. What we find is that the reactivity of these
cations is generally enhanced as compared with their neutral
counterparts, and that dinuclear products and intermediates
play an important role in the chemistry of these species.
Results and Discussion
Preparation of cationic alkyls and hydrides of PdII
We have previously reported the preparation of dinuclear
hydrides of palladium()22 and catalytically active pal-
ladium(0)23 complexes that incorporate electron-rich chelating
ligands bearing isopropyl substituents, i.e. 1,2-bis(diisopropyl-
phosphino)ethane (dippe) and 1,3-bis(diisopropylphosphino)-
propane (dippp). To prepare cationic alkyl complexes, we chose
the method developed by Brookhart et al. involving proton-
ation of neutral dialkyl complexes using HBAr4 (HBAr4 =
[H(OEt2)2]ϩ{B[3,5-(F3C)2C6H3]4}Ϫ).7 The starting dialkyl deriv-
atives, Pd(dippe)R2 and Pd(dippp)R2 (where R = Me or CH2Ph)
are prepared via standard preparative routes as described in the
Experimental section. Protonation of the dibenzyl complex
Pd(dippe)(CH2Ph)2 leads to the formation of the η3-benzyl
derivative [Pd(dippe)(η3-CH2C6H5)]ϩBAr4Ϫ [equation (1)].
The 31P-{1H} NMR spectrum of 1 shows the required AX
pattern due to inequivalent phosphorus nuclei, i.e. a pair of
+
D
D
P
diphosphine or diimine
polymer; L co-ordinated monomer
Pd
D
D
L
P
A
From a design point of view, such a configuration would
appear ideal since the chelating ancillary ligand restricts the two
reactive sites to be cis disposed, exactly as required for the
migratory insertion type reaction.21 In addition, the steric and
electronic properties of the ancillary ligand can be varied to
influence the stereochemistry and molecular weight of the
resulting polymers.5
In this paper we report the stoichiometric reactivity of
electrophilic palladium() alkyl cations with small molecules
1
doublets at δ 88.7 and 75.2 (JPPЈ = 32.1 Hz), and the H and
1H-{31P} NMR spectra are representative of an unsymmetrical
complex. Of particular note are the four isopropyl methyl
† Professional Officer: UBC Structural Chemistry Laboratory.
‡ Non-SI units employed: atm = 101 325 Pa; mmHg = 133.322 Pa.
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016
2007

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stronger donor. The 31P-{1H} NMR spectrum of 2 thus consists
+
H
of an AX pattern and in different NMR solvents the chemical
shift of the two doublets change noticeably. From a starting
solution of Pd(dippe)Me2 plus HBAr4 in Et2O–C6D6 the fol-
lowing 31P-{1H} NMR parameters can be generated: an AX
pattern at δ 84.2 and 63.1 (JAX = 17.4 Hz) due to [Pd(dippe)-
Me(OEt2)]ϩ 2a; following addition of a few drops of THF, an
AX pattern at δ 86.0 and 69.0 (JAX = 17.0 Hz), due to [Pd-
(dippe)Me(THF)]ϩ 2b; and following addition of excess PPh3,
an AMX pattern at δ 78.3, 70.0 and 29.8 (JAX = 368.0,
JAM = 21.9, JMX = 29.5 Hz) due to [Pd(dippe)Me(PPh3)]ϩ 2c.
These data are in accord with a square-planar palladium cation
whose fourth co-ordination site is filled by a labile solvent
molecule. For this reason, we will refer to 2 as the cation
[Pd(dippe)Me(s)]ϩ where s is the reaction- or NMR-solvent.
Additional information can be obtained by examination of
the 13C-{1H} and 31P-{1H} NMR spectra of the carbon-13
labelled species [Pd(dippe)13Me(s)]ϩ (2-13C1). The 31P-{1H}
NMR spectrum of 2-13C1 is an ABX pattern with two non-
equivalent phosphorus nuclei (JPPЈ = 16.7 Hz) coupled to a
single 13C nucleus (JCP = 86.1, JCPЈ = 2.8 Hz). The two values of
2JCP are again consistent with a trans and cis coupling respect-
ively across a square-planar centre. The corresponding 13C-
{1H} NMR spectrum exhibits an apparent doublet of doublets
for the enriched methyl resonance due to the strong trans coup-
ling and the cis coupling of much smaller magnitude (this split-
ting is not well resolved in the 31P-{1H} NMR spectrum). The
non-ambiguous coupling constants and chemical shifts detailed
above for the [Pd(dippe)Me]ϩ system prove useful in monitor-
ing the reactivity of this complex. As a sidebar, elemental
analyses of the methyl cations 2 and 3 do not show evidence for
the presence of co-ordinated solvent; presumably, in the solid
state, the anion weakly co-ordinates enough so that the solvent
can be removed by pumping under vacuum. Attempts to obtain
X-ray quality crystals of these cations have failed thus far.
In the reactions with small molecules, the electrophilic methyl
cation 2 shows enhanced reactivity as compared to the neutral
precursor. For example, Pd(dippe)Me2, the parent complex of
2, is completely inert to dihydrogen. In contrast, a THF solu-
tion of 2, placed under 1–4 atm of H2, rapidly develops a red
colour, the 31P-{1H} NMR spectroscopy reveals a singlet at
δ 92.0, due to equivalent phosphorus donors. The correspond-
ing 1H NMR spectrum reveals a binomial quintet in the hydride
region at δ Ϫ5.75 (JPH = 55.0 Hz) indicative of rapid fluxional
exchange similar to that found for the related neutral hydride
dimers.22 Solution spectroscopy therefore indicates that a
dinuclear Pd species, {[(dippe)Pd]2(µ-H)2}[BAr4]2 4[BAr4]2,
results from the activation of dihydrogen by 2. As shown by the
corresponding reaction between H2 and [Pd(dippe)13Me(s)]ϩ
(2-13C1) and the observation of 13CH4 in solution, the methyl
residue of 2 is lost as methane as shown in equation (3).
H
P
P
P
P
CH2Ph
CH2Ph
Et2O
(–PhCH3)
–
+ HBAr4
Pd
(1)
Pd
BAr4
1 [BAr4]
signals between δ 1.07 and 0.75 and the two isopropyl methine
resonances at δ 2.37 and 1.98; the H-{31P} NMR spectrum
1
again shows the inequivalence of the methylene backbone of
the ligand with the two triplets required for the A2X2 spin-
system of the CH2CHЈ2 linkage. Significantly, signals due to the
ortho protons of the benzyl group are observed at δ 6.41.24 This
upfield value indicates η3-co-ordination and the assignment is
further evidenced by an NOE experiment: irradiation of the
benzylic protons results in signal enhancement of the peak at
δ 6.41, due to the ortho protons, and a negative NOE with the
meta protons at δ 7.3. The ortho and benzylic protons each
appear as single multiplets in the 1H NMR spectrum and
their equivalence is maintained even at Ϫ90 ЊC, as is the in-
equivalence of the phosphorus nuclei in the 31P-{1H} NMR
spectrum. This dynamic behaviour is consistent with a fast η1–
η3 suprafacial shift of the palladium nucleus, which inter-
changes both syn and anti benzyl protons, and ortho-phenyl
protons but maintains the non-equivalence of the phosphine
donors.25
The preparation of the same benzyl cation 1 could also be
effected by benzyl abstraction from the starting neutral
derivative Pd(dippe)(CH2C6H5)2 by B(C6F5)3, one of the Lewis
acids that has been used in the synthesis of single-site olefin
polymerization catalysts.26 Although a 1:1 mixture of the two
reagents generated an oil, the solution spectra of the product
were equivalent to that of 1, except for the benzylic signals due
to [B(C6F5)3CH2C6H5]Ϫ observed in the H NMR spectrum.
1
The labeled derivative, 1-d7, was also prepared by protonation
with HBAr4 on Pd(dippe)(CD2C6D5)2 [prepared in turn from
Pd(dippe)Cl2 and 2 equivalents of KCD2C6D5], and a doublet
was observed at δ 2.30 (JHD = 2.5 Hz) in the 2H NMR spectrum
of the reaction mixture due to extruded HD2CC6D5.
Although cationic, the reactivity of [Pd(dippe)(η3-CH2-
C6H5)]ϩ is unremarkable. Cation 1 is inert to both CO and H2,
and is in fact air-stable. This is not too surprising as examples
of stable η3-allyl palladium cations are legion, and their
chemistry is mature.27 We therefore chose to examine proton-
ation reactions of Pd(dippe)Me2 and Pd(dippp)Me2. To an
ethereal solution of either palladium dimethyl complex, the
addition of 1 equivalent of HBAr4 causes a rapid effervescence,
undoubtedly of methane, and the methyl cations [Pd(dippe)-
Me(OEt2)]ϩ 2 and [Pd(dippp)Me(OEt2)]ϩ 3 are obtained as
shown for 2[BAr4] in equation (2).
+
2+
+
P
P
4 atm H2
THF
(–MeH)
Me
s
P
P
H
H
P
P
P
P
P
Pd
Me
Me
Me
Pd
Pd
1/2
Et2O
(3)
–
+ HBAr4
BAr4
Pd
Pd
(2)
P
s
(–MeH)
4
2 (s = THF)
(s = Et2O)
2[BAr4]
The orientation of the bridging hydrides of 4, shown to be
planar in equation (3), is speculation at this point and is based
on the propensity for four-co-ordinate, PdII systems to be
square planar; however, other geometries22 are certainly
possible and cannot be ruled out.
With regards to the enhanced reactivity of the methyl cation
2 with H2, it is interesting to note that the cationic nature of 2 is
not a sufficient condition to augment reactivity since, as already
mentioned, the 16-e benzyl cation 1 is also inert to H2. Thus it
Because the remaining methyl group bound to the Pd centre
cannot offer additional π interaction as can an allyl or
benzyl group, the metal nucleus is now highly electrophilic and
is loosely stabilized by whatever solvent molecules are available.
In different NMR solvents, or even an ethereal solution of
the cation to which different solvents are successively added,
this gives rise to a series of solution spectra which indicates
the progressive filling of the vacant co-ordination site by the
2008
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016

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would appear that the reactivity is a result of both co-ordinative
unsaturation at the palladium centre and the positive charge,
although it is difficult to rank the relative merit of either effect
since they are intimately connected in these systems; neutral
PdII systems with labile ligands would be the obvious targets to
try to compare to these cationic systems.
+
C6H5
CH2
P
P
P
Pd
Pd
Me
CH2C6H5
P
The electrophilicity of the palladium nucleus in 2 can also be
demonstrated by the reaction of 2 with Pd(dippe)(CH2C6H5)2.
The stoichiometric solution of methyl cation 2 and Pd(dippe)-
(CH2C6H5)2 in Et2O–C6D6 leads to the formation of two
products, of different solubility, both of which give rise to AX
patterns in the 31P-{1H} NMR spectrum. The identity of the
benzene insoluble but ether soluble product is the η3-benzyl
cation 1, whereas the other product, which is soluble in benzene
B
cation is not generated. It must simply be that the Lewis acidity
of the palladium nucleus of 2 renders the complex exceptionally
kinetically labile. Hydrocarbyl transfer was also observed in the
reaction between [Cp*2ZrCH3]ϩ and Cp*2Zr(13CH3)2, where
the Zr nucleus was similarly Lewis-acidic, and the anion
[B(C6F5)3Me]Ϫ, similarly poorly co-ordinating.26
In both reactions shown in equations (4) and (5) an inter-
mediate cannot be observed in the 31P-{1H} NMR spectrum,
and yet a hydrocarbyl residue has been demonstrably trans-
ferred from one PdII nucleus to another. In order to account for
this process a dinuclear species, as an intermediate or transition
state, must be invoked. One possibility is shown in B, in which
the hydrocarbyl residues bridge both metal centres.
1
and displays both methyl and benzylic resonances in its H
NMR spectrum, was identified as Pd(dippe)(Me)(CH2C6H5) 5.
Complex 5 was unequivocally characterized by independent
synthesis from Pd(dippe)MeCl and benzyl potassium. Evi-
dently, the acidic palladium centre of 2 abstracts a benzyl group
from Pd(dippe)(CH2C6H5)2 to give quantitative yields of the
neutral, mixed hydrocarbyl complex, 5, and the stabilized η3-
benzyl cation 1 [equation (4)].
Reaction with carbon monoxide
+
As part of our interest in the reactivity of the electron-rich PdII
methyl cation 2, we examined its reaction with CO. Based on
many model studies for palladium alkyl phosphine and diimine
complexes,11,28–32 one would predict that the sequence of reac-
tions would involve CO first displacing the solvent molecule,
then migratory insertion would occur to yield a Pd–acyl species,
which would be trapped either by solvent or by CO. These
acyl species are regarded as important intermediates in the
copolymerization of CO and ethylene.5,33 Moreover, this is
simply an example of migratory insertion, which has long been
recognized as a fundamental organometallic reaction path-
way.21 However, when a solution of 2 in [2H8]THF is placed
under 1–4 atm of CO, instead of observing a new doublet of
doublets in the 31P-{1H} NMR spectrum, as would be generated
by the AX spin system of a [Pd(dippe)(COMe)s]ϩ species
(s = solvent molecule or CO), the major feature observed was a
singlet at δ 58.7, indicating equivalence of the phosphorus
donors of the dippe ligand, accompanied by the appearance of
a deep orange colour for the solution. In addition, it was
observed that this product remained stable under added pres-
sure of CO (i.e. ca. 5 atm), and signals attributable to acyl
groups could not be identified in the 1H NMR spectrum. Under
an atmosphere of 13CO the 31P-{1H} NMR signal at δ 58.7
splits into a doublet of doublets (JPC = 28.1; JPCЈ = 25.5 Hz) and
quintet absorptions with the same coupling constants were
observed at low field in the 13C-{1H} NMR spectrum (δ 236.4
and 228.1 respectively). When the corresponding experiment
was performed using 2-13C1 under CO, the 31P-{1H} NMR spec-
trum revealed a doublet at δ 58.7 (JPC = 14.4 Hz), and in the
13C-{1H} NMR spectrum a binomial quintet was observed at
δ 47.3 (JCP = 14.6 Hz). These results indicate the formation
of some dinuclear palladium species that is bridged by two
chemically different carbonyl groups and in which the four
phosphorus donors are equivalent. However, as will be seen in
the next section, the solid-state molecular structure required
modification of this conclusion.
P
P
P
P
CH2C6H5
CH2C6H5
Me
s
Pd
+
Pd
2 (s = Et2O)
(4)
Et2O–C6D6
r.t.
+
H
H
P
P
P
P
CH2C6H5
Me
+
Pd
Pd
5
1
A potentially more interesting result occurs when an ethereal
solution of 2 is mixed with 1 equivalent of its labelled parent
molecule, Pd(dippe)(13CH3)2. Hydrocarbyl transfer is again
observed with the decrease in intensity of the second order
[AX]2 multiplet due to Pd(dippe)(13CH3)2 in the 31P-{1H} or
13C-{1H} NMR spectra and the observation of simpler ABX
patterns due to isotopomeric [Pd(dippe)Me]ϩ/[Pd(dippe)Me*]ϩ
and Pd(dippe)Me2/Pd(dippe)MeMe* [equation (5)].
It is clear that there is no thermodynamic driving force in this
last reaction in that a more stable species such as the η3-benzyl
+
P
P
P
P
Me
s
Me*
Me*
Pd
Pd
+
2
(s = Et2O)
(5)
Et2O–C6D6
r.t.
Solid-state and solution structure of [Pd(dippe)(ì-CO)Pd(dippe)-
Me][BAr4] 6[BAr4]
+
Amber crystals from the reaction of 2 with CO were obtained
from Et2O–toluene. The structure is shown in Fig. 1; crystal
data and selected bond lengths and angles are listed in Tables 1
and 2 respectively. Each palladium nucleus of 6 resides in a
distinct co-ordination environment. The dipalladium core of 6
is monocationic, and the structure depicted is associated with a
BAr4 anion that is isolated from both palladium nuclei though
P
P
Me
s
Pd
P
+
Pd
C
Me*
Me*
P
2–13
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016
2009

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Table 1 Crystallographic data for [(dippe)Pd(µ-CO)(dippe)PdMe]-
{B[3,5-(CF3)2C6H3]4}ؒ0.5C7H8 *
Formula
M
C62H79BF24OP4Pd2ؒ0.5C7H8
1689.84
Colour, habit
Crystal size/mm
Crystal system
Space group
Yellow, plate
0.15 × 0.35 × 0.40
Triclinic
¯
P1 (no. 2)
a/Å
b/Å
c/Å
α/Њ
17.134(2)
18.454(2)
12.932(2)
93.22(1)
94.98(1)
70.366(7)
3835.1(8)
2
1.463
1714
6.49
0.82–1.00
ω–2θ
β/Њ
γ/Њ
U/Å3
Z
ρcalc/g cmϪ3
F(000)
µ(Mo-Kα)/cmϪ1
Transmission factors
Scan type
Scan range/Њ in ω
Scan speed/Њ minϪ1
Data collected
2θmax/Њ
Crystal decay (%)
Total reflections
Unique reflections
Rmerge
Reflections with I у 3σ(I)
No. of variables
R
RЈ
S
1.26 ϩ 0.35 tan θ
16 (up to 8 rescans)
ϩh, k,
50
l
22.5
13 984
13 489
0.054
5769
927
0.044
0.039
2.05
Fig. 1 Molecular structure and numbering scheme for the cation
[Pd(dippe)(µ-CO)Pd(dippe)Me]ϩ
6
Max. ∆/σ (last cycle)
0.14
Ϫ0.51, ϩ0.48
Residual density/e ÅϪ3
closer in space to Pd(1). In the cation, Pd(2) interacts with the
carbonyl group with a Pd᎐C separation of 1.873(10) Å, mean-
while its interaction with C(30) of the methyl group is minimal;
here the separation is 2.94(1) Å. On the other hand, Pd(1) is
separated from C(29) by 2.32(1) Å, yet its distance of 2.172(7)
Å from the methyl carbon, C(30), clearly indicates that the
methyl group is covalently bound to this centre.34 Given this
non-symmetric bridging of the carbonyl group across the
palladium centres, and the disposition of the methyl group, it
is reasonable to formulate 6 as a mixed-valence species of Pd,
that is, as an adduct of a basic Pd0 complex, Pd(dippe)CO, and
a Lewis-acidic PdII species, [Pd(dippe)Me]ϩ. Indeed, 6 could be
reformulated as a conventional square-planar palladium co-
ordination complex, with Pd(1) as the central metal atom and
the Pd(dippe)CO moiety as the fourth ligand, if the relevant
bond angles are considered. The angles P(2)᎐Pd(1)᎐C(30)
and P(1)᎐Pd(1)᎐Pd(2) are tolerably close to 180Њ [177.7(2) and
170.32(6)Њ respectively], and the angles Pd(2)᎐Pd(1)᎐C(30) and
P(1)᎐Pd(1)᎐C(30) [78.5(2) and 91.9(2)Њ] are reasonably close to
90Њ. In this respect, of possible significance is the separation
between the palladium nuclei; its exceptionally short value of
2.6886(8) Å {cf. approx. 2.81 Å for the [(dippp)Pd]2(µ-H)2
series}22 may represent an electrostatic interaction between the
electron-rich Pd0 centre and the cationic PdII, or, more likely a
co-ordinate bond35 from Pd(2) to cationic Pd(1). This bond is
bridged by the CO group, which may be designated as semi-
bridging36 on the basis of its unequal span across the two non-
equivalent palladium centres and the bent Pd(2)᎐C(29)᎐O(1)
angle of 160.1(2)Њ. The P(2)᎐Pd(1)᎐C(30) angle of 177.7(2)Њ
also seems to preclude an agostic interaction between Pd(2) and
the methyl group; if such an agostic interaction were operating
a more acute angle might be expected. Also of interest is the
unusual disposition of the co-ordination planes: the plane
defined by P(1)᎐Pd(1)᎐P(2) is tilted to that defined by
P(3)᎐Pd(2)᎐P(4) by an angle of 92.10Њ, i.e. the co-ordination
planes of the palladium nuclei are normal to each other; as
discussed below, this structural feature complicated the solution
behaviour.
* T 294 K, Rigaku AFC6S diffractometer, Mo-Kα radiation (λ =
0.710 69 Å), graphite monochromator, takeoff angle 6.0Њ, aperture
6.0 × 6.0 mm at a distance of 285 mm from the crystal, stationary
background counts at each end of the scan (scan/background time ratio
2:1), σ2(F2) = [S2(C ϩ 4B)]/Lp2 (S = scan rate, C = scan count, B =
normalized background count), function minimized Σw(|Fo| Ϫ |Fc|)2
where w = 4Fo /σ2(Fo ), R = Σ||Fo| Ϫ |Fc||/Σ|Fo|, RЈ = (Σw(|Fo| Ϫ |Fc|)2/
2
2
¹
¹
Σw|Fo|2)² and S = [Σw(|Fo| Ϫ |Fc|)2/(m Ϫ n)]². Values given for R, RЈ and
S are based on those reflections with I у 3σ(I).
We have yet to reconcile the solid-state structure of 6 with
that observed in solution, and in fact under 13CO the species in
solution is certainly not 6 but an allied dinuclear palladium
complex to whose core two distinct carbonyl units are co-
ordinated (viz. the 31P-{1H} NMR spectrum exhibits a doublet
of doublets with two values of JPC observed). Nevertheless, an
analytically pure, crystalline sample of 6 dissolved in [2H8]THF
does give a singlet at δ 58.6 in its 31P-{1H} NMR spectrum, and
if 13CO is introduced the doublet of doublet pattern is observed
centred at δ 58.8. It appears that the 31P-{1H} NMR chemical
shifts of the two species are more or less coincident, and that
the species giving rise to the ABX pattern is the dicarbonyl,
[Pd(dippe)(µ-13CO)2Pd(dippe)Me][BAr4] 6a[BAr4], the carbonyl
adduct of 6;37 the structure of the dicarbonyl is unknown, but
must be similar to the isolated monocarbonyl 6 with the added
proviso that the two bridging carbonyls are inequivalent. All
attempts to isolate dicarbonyl 6a simply result in isolation of 6
as the extra CO group is lost upon crystallization. It is worth-
while adding in passing that zerovalent palladium species such
as Pd(dippp)CO (see below) and Pd(dippp)(CO)2 have been
characterized in solution, and the CO ligand is quite labile.38
Admittedly, the low-field carbonyl absorptions in the 13C-{1H}
NMR spectrum observed for 6a, at δ 236.4 and 228.1, could be
consistent with acyl carbons, but since both carbonyls bridge
two palladium centres (as witnessed by their appearance as
binomial quintets), the low-field chemical shifts are quite
appropriate for bridging ligands.39,40
2010
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016

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+
+
Table 2 Selected intramolecular bond distances (Å) and angles (Њ)
observed in [Pd(dippe)(µ-CO)Pd(dippe)Me][BAr4] 6[BAr4]
O
C
O
C
P
P
P
P
P
P
PdA
PdA
PdA
PdB
PdA
PdB
Pd(1)᎐Pd(2)
Pd(1)᎐P(1)
Pd(1)᎐P(2)
Pd(1)᎐C(29)
Pd(1)᎐C(30)
Pd(2)᎐P(3)
Pd(2)᎐P(4)
Pd(2)᎐C(29)
P(1)᎐C(1)
2.6886(8)
2.302(2)
2.332(2)
2.32(1)
2.172(7)
2.325(2)
2.340(2)
1.87(1)
P(2)᎐C(5)
P(2)᎐C(6)
1.829(8)
1.852(7)
1.85(1)
H3C
H3C
P
P
P(3)᎐C(15)
P(3)᎐C(17)
P(3)᎐C(18)
P(4)᎐C(16)
P(4)᎐C(19)
P(4)᎐C(20)
O(1)᎐C(29)
C(2)᎐C(2)
C(15)᎐C(16)
1.82(1)
1.844(9)
1.837(8)
1.835(9)
1.841(8)
1.130(9)
1.51(1)
+
+
O
C
O
C
P
P
P
P
P
PdB
PdA
PdB
1.831(7)
1.822(8)
1.84(1)
P
C
CH3
S
P
H3
P(1)᎐C(3)
P(1)᎐C(4)
1.41(1)
P(2)᎐C(2)
1.849(7)
C
+
Pd(2)᎐Pd(1)᎐P(1)
Pd(2)᎐Pd(1)᎐P(2)
Pd(2)᎐Pd(1)᎐C(29)
Pd(2)᎐Pd(1)᎐C(30)
P(1)᎐Pd(1)᎐P(2)
P(1)᎐Pd(1)᎐C(29)
P(1)᎐Pd(1)᎐C(30)
P(2)᎐Pd(1)᎐C(29)
P(2)᎐Pd(1)᎐C(30)
C(29)᎐Pd(1)᎐C(30)
Pd(1)᎐Pd(2)᎐P(3)
Pd(1)᎐Pd(2)᎐P(4)
Pd(1)᎐Pd(2)᎐C(29)
P(3)᎐Pd(2)᎐P(4)
P(3)᎐Pd(2)᎐C(29)
P(4)᎐Pd(2)᎐C(29)
Pd(1)᎐P(1)᎐C(1)
Pd(1)᎐P(1)᎐C(3)
Pd(1)᎐P(1)᎐C(4)
C(1)᎐P(1)᎐C(3)
170.32(6)
102.94(5)
43.1(2)
Pd(1)᎐P(2)᎐C(2)
Pd(1)᎐P(2)᎐C(5)
Pd(1)᎐P(2)᎐C(6)
C(2)᎐P(2)᎐C(5)
C(2)᎐P(2)᎐C(6)
C(5)᎐P(2)᎐C(6)
Pd(2)᎐P(3)᎐C(15)
Pd(2)᎐P(3)᎐C(17)
Pd(2)᎐P(3)᎐C(18)
C(15)᎐P(3)᎐C(17)
C(15)᎐P(3)᎐C(18)
C(15)᎐P(3)᎐C(18)
Pd(2)᎐P(4)᎐C(16)
Pd(2)᎐P(4)᎐C(19)
Pd(2)᎐P(4)᎐C(20)
C(16)᎐P(4)᎐C(19)
C(16)᎐P(4)᎐C(20)
C(16)᎐P(4)᎐C(20)
Pd(1)᎐C(29)᎐Pd(2)
Pd(1)᎐C(29)᎐O(1)
Pd(2)᎐C(29)᎐O(1)
107.2(2)
117.0(3)
119.0(3)
103.3(3)
103.4(3)
105.1(4)
106.1(3)
120.7(3)
114.6(3)
105.4(6)
104.4(5)
104.1(5)
105.9(3)
120.4(3)
118.5(3)
104.3(4)
102.7(4)
102.8(4)
79.0(4)
O
C
P
P
P
PdB
78.5(2)
P
CH3
86.74(7)
135.4(2)
91.9(2)
100.9(3)
177.7(2)
81.4(3)
170.32(6)
102.94(5)
43.1(2)
86.74(7)
135.4(2)
100.9(3)
106.6(3)
114.7(3)
120.0(3)
103.7(4)
106.7(5)
103.7(5)
Scheme 1
+
+
O
C
P
P
P
P
CH3
P
THF
+ CO
Pd
Pd
Pd
(6)
s
H3C
P
6
2 (s = THF)
31P-{1H} NMR: δ 58.6 (s)
IR νCO (KBr disc): 1830 cm–1
120.8(8)
160.1(2)
C(1)᎐P(1)᎐C(4)
C(3)᎐P(1)᎐C(4)
Me(s)]ϩ 2, formally a PdII species, and carbon monoxide in
THF. The product isolated is a PdII/Pd0 dimer. Thus, some-
where along the reaction pathway a divalent palladium nucleus
undergoes a formal two-electron reduction to Pd0.
As regards the apparent equivalence of the phosphorus
nuclei in the 31P-{1H} NMR spectrum, clearly an exchange pro-
cess must be operating which serves to equate the phosphorus
nuclei. Such a process must not only interchange the phos-
phorus nuclei on each one of the dippe ligands, it must also
equate the donor nuclei on alternate ligands. Lability of the CO
group would partially satisfy the first requirement, however
since 6 is a 30-e dinuclear species, it is likely that the CO remains
tightly bound to the metal core. Alternatively, the equivalence
of the phosphorus donors may be explained by an intermediate
with both a bridging methyl and a bridging carbonyl group, and
with the co-ordination planes coplanar. Transfer of the methyl
or carbonyl group may occur in either a stepwise or concerted
fashion, and rapid equilibration results in the observation of a
singlet in the 31P-{1H} NMR spectrum. A proposal for this
fluxional process is shown in Scheme 1. We suggest that one end
of the dinuclear unit rotates to generate two coplanar units
which then undergo a symmetrization process through a species
such as C with equivalent palladium nuclei; such a rearrange-
ment allows for methyl group and CO transfer between the two
palladium centres and exchange of the phosphorus donors by
virtue of the sense of the rotation to form C.
When the CO reaction was performed in Et2O–C6D6 with
2-13C1, the 31P-{1H} NMR spectrum revealed two sets of doub-
let of doublets at δ 73.7 and 70.7 (JPPЈ = 40.6 Hz), and in the
13C-{1H} NMR spectrum a doublet of doublets at δ 47.3 was
observed (JCP = 38.7, JCPЈ = 17.3 Hz). This species is formulated
as the square-planar, acyl–carbonyl complex, [Pd(dippe)-
(CO13CH3)CO]ϩ 7 with the two phosphorus–carbon coupling
3
constants of the JCP type corresponding to a trans and cis
coupling respectively. When the reaction of 2-13C1 was per-
formed in the poorly co-ordinating solvent o-dichlorobenzene
under a deficiency of CO, the carbonyl adduct of the labelled
methyl cation, [Pd(dippe)(13CH3)CO]ϩ 8, was observed (Scheme
2), as indicated by a new ABX pattern in the 31P-{1H} NMR at
δ 84.2 and 83.8 (JPPЈ = 21.2 Hz), and the corresponding spec-
trum in the 13C-{1H} NMR centred at δ Ϫ4.9 [JCP (trans) = 105,
JCPЈ (cis) = Ϫ8.5 Hz], whose upfield value indicates a Pd᎐Me
group rather than a Pd–acyl.41 These results are summarized in
Scheme 2.
From these latter experiments one can obtain spectroscopic
parameters for acyl–carbonyl 7 and methyl–carbonyl 8 to use in
subsequent experiments described below; the change in solvents
shown in Scheme 2 provided the best conditions to prepare the
putative intermediates 7 and 8.
As mentioned above, at moderate pressures of CO (1–4 atm),
signals due to a putative Pd–acyl species were conspicuously
absent from a solution of 2 in [2H8]THF. However, when an
NMR sample of the labelled precursor 2-13C1 was prepared
under lower pressures of CO (300 mmHg pressure), signals
attributable to both [Pd(dippe)(CO13CH3)CO]ϩ 7 and [Pd-
(dippe)(13CH3)CO]ϩ 8 could be observed in the 31P-{1H} NMR
spectrum, which diminished in intensity relative to the peak due
to 6 on standing. Also a peak at δ 81.0 was observed that grows
At low temperature (<Ϫ70 ЊC), the singlet at δ 58.6 splits into
four broad signals, i.e. four phosphorus environments, and is
therefore consistent with the solid-state structure. Such fluxion-
ality was also observed in the related cation, {[(dippp)Pd]2-
(µ-H)(µ-CO)}ϩ,39,40 but in this case the limiting spectrum was
not reached.
Mechanistic experiments
The mechanism of formation of dinuclear 6 from the mono-
nuclear methyl cation 2 and CO is not obvious. As shown as a
reminder in equation (6), the initial reactants were [Pd(dippe)-
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016
2011

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+
O
C
P
P
P
P
O
P
P
13CH3
(8)
CO
CH3
+
Pd
Pd CO
Pd
C
THF
CH3
H3C
CH3
s
2–13C1
31P-{1H} NMR spectrum. The identity of this species is prob-
ably [Pd(dippe)(CO)2](BAr4)2 9 since under 13CO, the peak at
δ 81.0 appears as a second-order multiplet due to the [AX]2
spin system of the square-planar [Pd(dippe)(13CO)2]2ϩ dication.
In a separate experiment we tried to access these Pd2ϩ complexes
by the addition of 2 equivalents of HBAr4 to a solution of
Pd(dippe)Me2 in THF. The evolution of a gas was indeed
observed, but unfortunately the species so produced, presum-
ably [Pd(dippe)(THF)2][BAr4]2 10b[BAr4]2, rapidly polymerizes
the THF solvent which makes characterization impossible. In
CO
CO
o-dichlorobenzene
Et2O
+
+
O
C
13CH3
P
Pd
P
P
Pd
13CH3
CO
P
CO
7
8
31P-{1H} NMR: δ 73.7 (dd), 70.7 (dd)
13C-{1H} NMR: δ 47.3 (dd)
CD3CN spectroscopic parameters for the dication [Pd(dippe)-
31P-{1H} NMR: δ 84.2 (m), 83.8 (m)
13C-{1H} NMR: δ –4.90 (m)
(N᎐CCD3)2][BAr4]2 10a[BAr4]2 can be obtained and its 31P-{1H}
᎐
᎐
NMR spectrum exhibits a singlet at δ 118.9. However, CO is
too poorly co-ordinating to displace the strongly bound
acetonitrile molecules from the palladium centre, and 9 was
therefore inaccessible by this route. However, when Pd(dippe)-
Me2 is treated with 2 equivalents of HBAr4 in THF, but under
stringent temperature control (i.e. a temperature of Ϫ78 ЊC
was not exceeded) evidence for 9 could be obtained. Thus, after
the reaction mixture was warmed to Ϫ20 ЊC and the volatiles
were removed, a solution of the residue in CD2Cl2 under 12CO
exhibited a singlet at δ 83.8 in the 31P-{1H} NMR spectrum.
When the solution was warmed to room temperature, the
singlet at δ 83.8 diminished in intensity and a new singlet was
observed at δ 115.6. From the original mixture, crystals of the
bis(THF) dication 10b were indeed isolated, after recrystalliz-
ation from methylene chloride at low temperature (<Ϫ20 ЊC).
In CD2Cl2 10a briefly gives a singlet in the 31P-{1H} NMR
spectrum at δ 118.9, to be replaced by a singlet due to the
decomposition product at δ 115.6. Thus, one can conclude that
the singlet at δ 81.9 in the original reaction between 2 and CO
was in fact [Pd(dippe)(CO)2]2ϩ. The slight discrepancy in chem-
ical shift may be explained by the different NMR solvent used
in each case. To summarize, the mononuclear acyl 7 combines
with a Pd᎐Me complex, whose methyl group is exchanged for
the carbonyl of 7 (possibly via the intermediacy of a dinuclear
intermediate). The complex Pd(dippe)(COMe)Me is thereby
formed, which, in the presence of CO, reductively eliminates
acetone to generate the zerovalent palladium complex, Pd-
(dippe)CO [equations (7) and (8)]. Meanwhile, the other
product arising from the reaction of 7 and 8, the dication
[Pd(dippe)(CO)2]2ϩ 9, eventually decomposes by reacting with
the solvent. This decomposition side reaction does not compli-
cate isolation of the dinuclear final product 6.
The extrusion of acetone thus produces one of the elements
required for formation of 6/6a, the zerovalent Pd(dippe)CO
unit. It is proposed that this electron-rich moiety acts as a
donor towards [Pd(dippe)Me]ϩ 2, which is present in the mix-
ture, in a type of Lewis acid–metal base interaction. Therefore
we can finally balance the reaction of CO and 2 to give 6 as
shown in equation (9).
As further confirmation of the mixed-valence nature of
dinuclear 6, we prepared an analogue using independently pre-
pared Pd(dippp)CO, and authentic [Pd(dippe)Me(OEt2)]ϩ 2.
The use of the three-carbon backbone ligand dippp for the
starting Pd0 portion represents a negligible chemical change,
but a dramatic change in magnetic resonance results, and this
provides excellent handles for 31P-{1H} NMR spectroscopy. As
shown in equation (10), if Pd(dippp)CO is generated in situ
{i.e. by placing a THF solution of [Pd(dippp)]2 under CO},
and 1 equivalent of [Pd(dippe)Me(OEt2)]ϩ 2 is added, a pair of
triplets are observed in the 31P-{1H} NMR spectrum of the
solution at δ 53.8 and 23.8 (JPPЈ = 28.5 Hz). This result is precisely
Scheme 2
in with the peak at δ 58.7 but then diminishes in intensity on
standing with several new absorptions above δ 100 observed.
The observation of 7 and 8 seems to indicate that these species
are intermediates in the formation of 6/6a, and not the
converse.
When the volatiles were distilled away from an NMR sample
of 6 prepared using 13CO, a peak at δ 204.0 was observed in the
13C-{1H} NMR spectrum of the distillate, and a doublet reson-
1
ance was observed at δ 2.03 (JHC = 6.0 Hz) in the H NMR
spectrum, i.e. chemical shifts consistent with acetone.42,43 The
identity of this coproduct was confirmed by GC-MS, which,
along with peaks for Et2O and [2H8]THF, gave a molecular
ion at m/z 59, i.e. corresponding to H3C(13CO)CH3. For the
experiment performed with 2-13C1 under 12CO, H313C(CO)-
13CH3 was identified by GC-MS. The origin of acetone is thus
due to the methyl group of the starting material 2. In order to
account for the coupling of two methyl units and the one
carbonyl we must invoke hydrocarbyl transfer, presumably
between 7 and 8 or 2, of a methyl group to a palladium acyl
species (or conversely, an acyl group to a palladium methyl
species), of the kind represented in equation (7). Under CO,
+
+
O
C
P
P
P
P
CH3
s
CH3
Pd
+
Pd
CO
7
s = solvent or CO
CO THF
(7)
2+
O
P
C
P
CO
CO
CH3
Pd
Pd
+
P
P
CH3
9
such a process would produce two new compounds: a neutral
PdII complex, Pd(dippe)(COMe)Me, which reductively elimin-
ates to give acetone and a zerovalent palladium species, pre-
sumably Pd(dippe)CO [equation (8)]; and a PdII species,
[Pd(dippe)]2ϩ or one of its carbonyl adducts. It is this dicationic
species that therefore gives rise to the peak at δ 81.9 in the
2012
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016

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from this work, we have shown that there is a further point to
+
consider and that is the formation of catalytically inactive
species due to the presence of strongly co-ordinating solvents
such as THF. In the case reported here, catalyst poisoning
occurs by formation of an inactive dinuclear complex.
P
P
CH3
s
+
3
Pd
3 CO
In this report we have described some reactivity patterns for
[Pd(dippe)Me(s)]ϩ 2. The fact that this cationic system shows
enhanced reactivity towards small molecules as compared to
the inertness of its parent complex, Pd(dippe)Me2, is apparent
from its reaction with dihydrogen. Late metals can activate
small molecules by oxidative addition pathways, and the reac-
tion of 2 to give {[Pd(dippe)]2(µ-H)2}ϩ 4 could be accounted for
by an oxidative addition of 2 to give a PdIV species, followed by
a reductive elimination of methane to give 4. Such a mechanism
is in marked contrast to the inability of both neutral PdII and
Pd0 to add dihydrogen.45 The rapid reaction of 2 with H2 could
even be said to resemble hydrogenolysis (i.e. σ-bond meta-
thesis46), a process that is normally associated with the alkyls of
the early transition metals. This apparent reversal of the nor-
mal trends of reactivity of the Periodic Table has been noted
in a similar system, viz. {Cp*Ir(PMe3)Me(CH2Cl2)}BAr4,47,48
a cationic complex of IrIII which will induce selective C᎐H
activation in alkanes.
2
O
C
–
(9)
2+
THF
H3C
CH3
+
O
C
P
P
P
CO
CO
P
+
Pd
Pd
Pd
P
H3C
P
6
9
what is expected if adduct formation between Pd(dippp)CO
and2occurs,andtheformationof{[Pd(dippp)(µ-CO)Pd(dippe)-
Me]} 11, the dippp analogue of 6, is indicated.
It should be noted that only one of the two limiting, instant-
aneous structures of 11 is shown in equation (10); as before, an
Experimental
+
The GC-MS analysis was performed by the Mass Spectro-
metry Service of the University of British Columbia upon a
KRATOS MS80 RFA instrument. Other procedures are
identical to that reported previously.22,23
P
P
CH3
s
Pd
+
The salt HBAr4 was prepared by a literature method,7 as was
B(C6F5)3.49 Methyllithium (Aldrich, halide content <0.05 mol
O
C
P
P
+
2
P
THF
(10)
Pd
Pd
L
Ϫ1) was used as supplied as a 1.4 mol LϪ1 solution in Et2O;
H3C
P
PhCH2MgCl (Aldrich) was used as supplied as a 1.0 mol LϪ1
solution in Et2O; Mg(13CH3)2 was prepared by a literature
method and used as either a 0.05 mol LϪ1 solution in Et2O,
or recrystallized from benzene–pentane as Mg(13CH3)2ؒdiox
(diox = 1,4-dioxane).50 The metal precursor Pd(COD)MeCl was
prepared according to the literature,51 as were Pd(dippe)Cl2 and
Pd(dippp)I2.22,23 Methylene chloride was distilled from CaH2
under N2. Acetonitrile was dried over activated 4 Å molecular
sieves, distilled from trap to trap, and stored over molecular
sieves. The solvents CD2Cl2 and CD3CN were distilled from
CaH2 after a prolonged period at reflux under N2; [2H8]THF
was distilled from sodium benzophenone under N2; C6D4Cl2-o
was distilled from CaH2 under N2. All deuteriated solvents were
subjected to four freeze–pump–thaw cycles.
P
P
Pd CO
11
exchange process serves to equilibrate the phosphorus nuclei of
each ligand (cf. Scheme 1).
Conclusion
From this work, one major conclusion emerges and that is that
dinuclearity figures prominently in these cationic palladium
methyl complexes stabilized with electron-rich diphosphine
ligands. It is useful to review the occurrences of dinuclear sys-
tems in this study. The reaction of the methyl cation 2 with H2
generates a dinuclear palladium hydride species; the interchange
of alkyl groups between neutral dialkyl derivatives and cationic
alkyl complexes is proposed to involve a dinuclear intermediate;
and the formation of a dinuclear mixed-valence complex is
observed upon carbonylation of the methyl cation 2 in THF.
This is hardly coincidence but not easily rationalized. If one
examines the literature on Pd chemistry, dinuclearity is not
particularly common;15 it would appear that electron-rich
diphosphine ancillary ligands tend to promote this kind of
behaviour. Nitrogen-based ligands such as diimines, bipyridine
or phenanthroline apparently do not promote formation of
dinuclear Pd complexes; nevertheless, it should be emphasized
that beyond these observations we have no explanation.
The copolymerization of carbon monoxide with olefins is
typically carried out in MeOH or CH2Cl2.4,5,8,11,16,32 To our
knowledge no one has reported the use of THF for this reac-
tion although there is one brief report44 that mentions that no
polymerization was observed in THF. It stands to reason that
any strongly co-ordinating solvent will compete for substrate
binding (i.e. co-ordination of ethylene or carbon monoxide)
and reduce the tendency for the propagation step. However,
Preparations
Pd(dippe)Me2. To a slurry of Pd(dippe)Cl2 (1.54 g; 3.50
mmol) in THF (30 mL) at Ϫ78 ЊC was added a solution of
MeLi (5.0 mL; 7.0 mmol) dropwise over 5 min. The cooling
bath was then removed, and as the mixture attained room
temperature a reaction ensued evidenced by the disappearance
of the Pd(dippe)Cl2 and the formation of a clear, dark brown
solution. The solution was stirred for 1 h after which time the
solvent was removed in vacuo. The brown residue was extracted
with toluene (25 mL), passed through a frit lined with Celite to
remove LiCl, and concentrated to a small volume (ca. 3 mL).
The brown solution was layered with pentane (10 mL) and
cooled to Ϫ40 ЊC to give colourless crystals of Pd(dippe)Me2
1
after 12 h (1.25 g; 89% yield). H NMR ([2H8]toluene, 500.13
MHz): δ 1.88 (spt, 4 H, CHMeMeЈ, JHMe = 7.1), 1.14 (m, 4 H,
PCH2CH2P), 1.06 (dd, 12 H, CHMeMeЈ, JHMe = 7.1, JMeP
=
15.4), 0.84 (dd, 12 H, CHMeMeЈ, JHMe = 7.1, JMeP = 12.4), 0.72
[dd, 6 H, PdMe2, JMeP (trans) = 6.6, JMeP (cis) = 1.0 Hz].
31P-{1H} NMR ([2H8]toluene): δ 61.0 (Found: C, 47.69; H, 9.42.
Calc. for C16H38P2Pd: C, 48.19; H, 9.60%).
Following the preparation of Pd(dippe)Me2, Pd(dippe)-
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016
2013

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(13CH3)2 was similarly prepared, with Pd(dippe)Cl2 (0.509 g;
1.16 × 10Ϫ3 mol) and Mg(13CH3)2(C4H8O)2 (0.165 g; 1.16 × 10Ϫ3
mol). Recrystallization from toluene (1 mL) layered with
hexanes gave colourless crystals (0.371 g; 80% yield). 1H NMR
(C6D6, 500.13 MHz): δ 1.88 (spt, 4 H, CHMeMeЈ, JHMe = 7.1),
1.14 (m, 4 H, PCH2CH2P), 1.09 (dd, 12 H, CHMeMeЈ,
JHMe = 7.1, JMeP = 15.4), 0.83 (dd, 12 H, CHMeMeЈ, JHMe = 7.1,
JMeP = 12.4), 0.87 [ddd, 6 H, Pd*Me2, JHC = 124.0, JMeP
(trans) = 6.6, JMeP (cis) = 1.0 Hz]. 31P-{1H} (C6D6, 81.015 MHz):
δ 65.4 [m, JPC (trans) = 108.5, JPC (cis) = Ϫ10.5, JPPЈ = 8.1 Hz].
13C-{1H} (C6D6, 50.32 MHz): δ 0.0 [m, JCP (trans) = 108.5, JCP
(cis) = Ϫ10.5, JCCЈ = 0.5 Hz]. Microanalysis was not obtained.
[Pd(dippe)(s)Me][BAr4] 2[BAr4]. To a solution of Pd(dippe)-
Me2 (0.126 g; 3.16 × 10Ϫ4 mol) in Et2O (5 mL) was added a
solution of HBAr4 (0.320 g; 3.16 × 10Ϫ4 mol) in Et2O (5 mL) at
Ϫ10 ЊC. The solution vigorously effervesced and lightened in
intensity. The volatiles were removed in vacuo and the residue
was recrystallized from Et2O (3 mL) and placed in the freezer.
Colourless crystals appeared after 24 h (0.323 g; 82% yield). 1H
NMR ([2H8]THF, 500.13 MHz): δ 7.79 (br s, 8 H, o-H of Ar4),
7.58 (br s, 4 H, p-H of Ar4), 3.35 (q, 4 H, free OCH2CH3,
JHHЈ = 7.0), 2.43 and 2.31 (spt, 4 H, CHMeMeЈ, JHMe = 7.2),
2.15 and 2.11 (dt, 4 H, PCH2CHЈ2P, JHP = 13.3, JHHЈ = 6.1),
1.31, 1.26, 1.24 and 1.21 (dd, 24 H, CHMeMeЈ, JMeP = 16.0,
JMeH = 7.2), 1.07 (t, 6 H, free OCH2CH3, JHHЈ = 7.0), 0.63 [dd,
3 H, Pd᎐Me, JMeP (trans) = 6.3, JMeP (cis) = 0.9 Hz]. 31P-{1H}
NMR ([2H8]THF, 202.42 MHz): δ 86.0 (d, 1 P, JPPЈ = 17.0), 69.0
(d, 1 P, JPPЈ = 17.0 Hz) (Found: C, 45.37; H, 4.00. Calc. for
C47H47BF24P2Pd: C, 45.27; H, 3.80%).
Pd(dippp)Me2. The complex Pd(dippp)Me2 was synthesized
as for Pd(dippe)Me2, with Pd(dippp)I2 (0.565 g; 8.88 × 10Ϫ4
mol) and MeLi (1.3 mL; 1.80 × 10Ϫ3 mol). Successive recrystal-
lizations from toluene (3 mL) layered with hexanes (6 mL) gave
1
Pd(dippp)Me2 as colourless crystals (0.179 g; 49% yield). H
Labelled 2-13C1 was prepared likewise from Pd(dippe)(13CH3)2
and HBAr4. 31P-{1H} NMR (C6D6–Et2O, 81.015 MHz): δ 87.9
[dd, 1 P, JPPЈ = 16.7, JPC (cis) = 2.8], 67.3 [dd, 1 P, JPЈP = 16.7, JPC
(trans) = 86.1 Hz]. 13C-{1H} NMR (C6D6–Et2O, 50.32 MHz):
δ 4.49 [dd, JCP (trans) = 86.1, JCP (cis) = 2.8 Hz].
NMR (C6D6, 299.99 MHz): δ 2.45 (m, 2 H, PCH2CH2CH2P),
1.92 (m, 4 H, CHMeMeЈ, JHMe = 7.2), 1.58 (m, 4 H, PCH2CH2-
CH2P), 1.14 and 0.91 (dd, 24 H, CHMeMeЈ, JMeH = 7.2,
JMeP = 15.0), 0.59 [dd, 6 H, PdMe2, JMeP (trans) = 5.0, JMeP
(cis) = 1.0 Hz]. 31P-{1H} NMR (C6D6, 121.41 MHz): δ 16.4
(Found: C, 49.15; H, 9.95. Calc. for C17H40P2Pd: C, 49.46; H,
9.77%).
[Pd(dippp)(s)Me][BAr4] 3[BAr4]. As for 2 with Pd(dippp)Me2
(0.086 g; 2.08 × 10Ϫ4 mol) and HBAr4 (0.211 g; 2.08 × 10Ϫ4
mol). Recrystallization from Et2O (2 mL) gave clear crystals
1
Pd(dippe)(CH2Ph)2. As for Pd(dippe)Me2 with Pd(dippe)Cl2
(1.09 g; 2.48 × 10Ϫ3 mol) and PhCH2MgCl (5 mL; 5.0 × 10Ϫ3
mol). Recrystallization from toluene (2 mL) layered with
hexanes (10 mL) afforded Pd(dippe)(CH2Ph)2 as clear brown
which darkened on standing (0.100 g; 38% yield). H NMR
(CD2Cl2, 200.13 MHz): δ 7.63 (br s, 8 H, m-H of Ar4), 7.50 (br
s, 4 H, p-H of Ar4), 3.67 (q, 4 H, OCH2CH3, JHHЈ = 7.0), 2.39
(m, 2 H, PCH2CH2CHЈ2P), 2.10 (m, 4 H, CHMeMeЈ), 1.59 and
1.43 (m, 4 H, PCH2CH2CHЈ2P), 1.15 (t, 6 H, OCH2CH3,
JHHЈ = 7.0), 1.10 (m, 24 H, CHMeMeЈ), 0.60 [dd, 3 H, Pd᎐Me,
JMeP (trans) = 5.0, JMeP (cis) = 1.0 Hz]. 31P-{1H} NMR (CD2Cl2,
81.015 MHz): δ 49.6 (d, 1 P, JPPЈ = 39.2), 16.2 (d, 1 P, JPPЈ = 39.2
Hz) (Found: C, 46.01; H, 3.59. Calc. for C48H49BF24P2Pd: C,
45.72; H, 3.92%).
1
needles (1.00 g; 73% yield). H NMR (C6D6, 500.13 MHz):
δ 7.43 (m, 4 H, o-H of Ph), 7.21 (m, 4 H, m-H of Ph), 6.96 (m,
2 H, p-H of Ph), 3.11 (dd, 4 H, CH2Ph, JHP = 9.0, JHPЈ = 7.5),
1.75 (spt, 4 H, CHMeMeЈ, JHMe = 7.0), 1.00 (m, 4 H, PCH2-
CH2P), 0.91 and 0.82 (dd, 24 H, CHMeMeЈ, JMeP = 16.0,
JMeH = 7.0 Hz). 31P-{1H} NMR (C6D6, 202.47 MHz): δ 62.0
(Found: C, 60.75; H, 7.94. Calc. for C28H46P2Pd: C, 61.03; H,
8.01%).
{[Pd(dippe)]2(ì-H)2}[BAr4]2 4[BAr4]. Complex cation 2 (0.250
g; 1.89 × 10Ϫ4 mol) was dissolved in THF (10 mL), and the
solution was subjected to four freeze–pump–thaw cycles. The
solution was cooled to Ϫ196 ЊC and placed under 4 atm of
dihydrogen. Upon warming to Ϫ78 ЊC the solution developed a
deep red colour. The solution was warmed with stirring to 0 ЊC
and the THF was removed in vacuo. The red residue remaining
was dissolved in THF (2 mL) and cooled to Ϫ40 ЊC. Deep red
needles deposited after 12 h (0.120 g; 51%). 1H NMR
([2H8]THF, 500.13 MHz): δ 7.79 (br s, 16 H, o-H of Ar4), 7.56
(br s, 8 H, p-H of Ar4), 2.20 (spt, 8 H, CHMeMeЈ, JHMe = 7.0),
2.08 (m, 8 H, PCH2CHЈ2P), 1.20 and 1.14 (dd, 48 H, CH-
MeMeЈ, JMeP = 16.5, JMeH = 7.0), Ϫ5.75 [qnt, 2 H, (µ-H)2,
JPH = 55.0 Hz]. 31P-{1H} NMR ([2H8]THF, 202.42 MHz): δ 92.0
(Found: C, 45.01; H, 3.71. Calc. for C92H90B2F48P4Pd2: C, 44.81;
H, 3.68%).
[Pd(dippe)(ç3-CH2Ph)]{B(C6F5)3CH2Ph}. To a solution of
Pd(dippe)(CH2Ph)2 (0.100 g; 1.82 × 10Ϫ4 mol) in Et2O (2 mL)
was added solid B(C6F5)3 (0.093 g; 1.82 × 10Ϫ4 mol) with stir-
ring. The initial brown colour discharged to give a colourless
solution. The solvent was removed in vacuo to give a slimy
brown oil which resisted crystallization. The product was char-
acterized in solution. 1H NMR (CD3CN, 200.12 MHz): δ 7.75,
7.70, 7.58, 7.10 and 6.59 (m, 10 H, CH2Ph and BCH2Ph), 3.09
(d, 2 H, CH2Ph, JHP = 9.4 Hz), 2.54 (s, 2 H, PhCH2B), 2.40 and
2.10 (m, 4 H, CHMeMeЈ), 2.04 and 1.81 (m, 4 H, PCH2CH2ЈP),
1.17, 1.06, 0.95 and 0.92 (m, 24 H, CHMeMeЈ). 31P-{1H} NMR
(CD2Cl2, 121.42 MHz): δ 89.3 (d, 1 P, JPPЈ = 30.6), 76.9 (d, 1 P,
JPPЈ = 30.6 Hz).
[Pd(dippe)(ç3-CH2C6H5)][BAr4] 1[BAr4]. To a solution of
Pd(dippe)(CH2Ph2)2 (0.072 g; 1.28 × 10Ϫ4 mol) in THF (10 mL)
at Ϫ10 ЊC was added a solution of HBAr4 (0.130 g; 1.28 × 10Ϫ4
mol) in THF (1 mL). The original dark colour became clear.
After the solvent was removed, the residue was dissolved in
Et2O (2.5 mL), and the solution placed in the freezer. Colour-
less crystals appeared after 24 h (0.127 g; 76%). 1H NMR
(CD2Cl2, 500.13 MHz): δ 7.75 (br s, 8 H, o-H of Ar4), 7.68 (m,
1 H, p-H of η3-CH2Ph), 7.55 (br s, 4 H, p-H of Ar4), 7.30 (m,
2 H, m-H of η3-CH2Ph), 6.41 (m, 2 H, o-H of η3-CH2Ph),
3.06 (d, 2 H, η3-CH2Ph, JHP = 7.8), 2.37 and 1.98 (dspt, 4 H,
CHMeMeЈ, JHP = 1.8, JHMe = 7.2), 2.04 and 1.76 (dt, 4 H,
PCH2CH2ЈP, JHP = 19.7, JHHЈ = 7.0), 1.07, 1.10, 0.92 and 0.75
(dd, 24 H, CHMeMeЈ, JMeP = 14.7, JMeH = 7.2 Hz). 31P-{1H}
NMR (CD2Cl2, 121.42 MHz): δ 88.7 (d, 1 P, JPPЈ = 32.1), 75.2
(d, 1 P, JPPЈ = 32.1 Hz) (Found: C, 48.16; H, 3.96. Calc. for
C53H51BF24P2Pd: C, 48.11; H, 3.89%).
Methyl transfer between cation 2 and Pd(dippe)(13CH3)2. To a
solution of 2 (0.035 g; 2.81 × 10Ϫ5 mol) in Et2O (0.20 mL) was
added a solution of Pd(dippe)(13CH3)2 (0.12 g; 2.82 × 10Ϫ5 mol)
in C6D6 (0.20 mL). The products were characterized in solution.
31P-{1H} NMR (C6D6–Et2O, 81.015 MHz): δ 71.0 (br s). 13C-
{1H} NMR (C6D6–Et2O, 50.32 MHz): δ Ϫ3.8 (br s).
Pd(dippe)(Me)CH2Ph 5. To a solution of Pd(COD)MeCl
(0.344 g; 1.26 × 10Ϫ3 mol) in THF (20 mL) was added a solu-
tion of dippe (0.330 g; 1.26 × 10Ϫ3 mol) in toluene (2 mL) to
produce a white precipitate. The reaction vessel was then cooled
to Ϫ78 ЊC in a dry-ice–acetone bath and a solution of KCH2Ph
(0.166 g; 1.26 × 10Ϫ3 mol) in THF (5 mL) was added by can-
nulation. The reaction mixture was allowed to warm to room
temperature with stirring, and the solvent was removed in vacuo
to give an orange residue. The residue was taken up in toluene
2014
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016

View Article Online
(20 mL), and this solution was passed through Celite. The fil-
trate was concentrated to 2 mL and then layered with hexanes
(5 mL). After 24 h at Ϫ40 ЊC orange crystals deposited (0.453 g;
76% yield). 1H NMR (C6D6, 500.13 MHz): δ 7.51 (m, 2 H, m-H
of Ph), 7.26 (m, 2 H, o-H of Ph), 6.96 (m, 1 H, p-H of Ph), 3.18
[dd, 2 H, CH2Ph, JHP (trans) = 10.0, JHP (cis) = 8.0], 1.81 and
1.83 (spt, 4 H, CHMeMeЈ, JHMe = 7.5), 1.06 and 1.04 (m, 4 H,
PCH2CH2ЈP), 1.00, 0.96, 0.82 and 0.75 (dd, 24 H, CHMeMeЈ,
JHP = 12.0, JMeH = 7.5), 0.67 [dd, 3 H, Pd᎐Me, JMeP (trans) = 7.0,
JMeP (cis) = 6.0 Hz]. 31P-{1H} NMR (C6D6, 202.42 MHz): δ 68.6
(d, 1 P, JPPЈ = 10.0), 60.4 (d, 1 P, JPPЈ = 10.0 Hz) (Found: C,
55.62; H, 8.88. Calc. for C22H42P2Pd: C, 55.64; H, 8.91%).
vacuo at Ϫ20 ЊC and [Pd(dippe)(CO)2][BAr4]2 9[BAr4]2 was
characterized in solution. 31P-{1H} NMR (CD2Cl2, 121.42
MHz): δ 83.8. The residue of the reaction mixture was
recrystallized from methylene chloride (2 mL) and cooled to
Ϫ40 ЊC. Yellow crystals of [Pd(dippe)(THF)2][BAr4]2 10b
1
appeared after 12 h (0.220 g; 25%). H NMR (CD2Cl2, 500.13
MHz): δ 7.64 (br s, 16 H, o-H of Ar4), 7.49 (br s, 8 H, p-H of
Ar4), 3.86 (m, 8 H, OCH2CH2), 2.32 (spt, 4 H, CHMeMeЈ,
JHMe = 7.3), 2.17 (m, 4 H, PCH2CHЈ2P), 1.97 (m, 8 H,
OCH2CH2), 1.36 and 1.28 (dd, 24 H, CHMeMeЈ, JMeP = 18.5,
JMeH = 7.3 Hz). 31P-{1H} NMR (CD2Cl2, 202.42 MHz): δ 118.9
{Found: C, 45.48; H, 2.97. Calc. for C82H64B2F48OP2Pd i.e.
[Pd(dippe)(THF)][BAr4]2: C, 45.44; H, 2.98%}. [Pd(dippe)-
᎐
[Pd(dippe)(ì-CO)Pd(dippe)Me][BAr4] 6[BAr4] and [Pd-
(N᎐CCH ) ][BAr ] 10a[BAr ] was prepared by adding CD CN
᎐
3
2
4 2
4 2
3
(dippe)(ì-13CO)2Pd(dippe)Me][BAr4] 6a[BAr4]. In
a
thick-
(1 mL) to a mixture of HBAr4 (0.102 g; 5.01 × 10Ϫ5 mol) and
walled reactor vessel a solution of 2 (0.480 g; 3.62 × 10Ϫ4 mol)
in THF (10 mL) was subjected to several freeze–pump–thaw
cycles. At room temperature, the reactor was charged with 4
atm CO gas and stirred for 1 h. The solution developed a red–
amber colour during this time. The volatiles were removed in
vacuo and the orange residue was recrystallized from Et2O (2
mL) under CO and a few drops of toluene to give deep amber
Pd(dippe)Me2 (0.020 g; 2.50 × 10Ϫ5 mol), and characterized in
1
solution. H NMR (CD3CN, 200.13 MHz): δ 7.71 (br s, 16 H,
o-H of Ar4), 7.66 (br s, 8 H, p-H of Ar4), 2.57 (spt, 4 H,
CHMeMeЈ, JHMe = 7.3), 2.31 (m, 4 H, PCH2CHЈ2P), 1.38
and 1.30 (dd, 24 H, CHMeMeЈ, JMeP = 15.0, JMeH = 7.3 Hz).
31P-{1H} NMR (CD3CN, 81.015): δ 118.9.
1
plates of 6[BAr4] (0.200 g; 33% yield). H NMR ([2H8]THF,
[Pd(dippp)(ì-CO)Pd(dippe)Me][BAr4] 11[BAr4]. To a solu-
tion of Pd(dippp)CO {prepared from [Pd(dippp)]2 (0.015 g;
2.00 × 10Ϫ5 mol) under CO in 5 mL Et2O} was added 2 (0.050
g; 4.01 × 10Ϫ5 mol). The product was characterized in solution.
31P-{1H} NMR (C6D6, 81.015 MHz): δ 53.8 (t, 1 P, JPPЈ = 28.5),
23.8 (t, 1 P, JPЈP = 28.5 Hz).
500.13 MHz): δ 7.79 (br s, 8 H, o-H of Ar4), 7.51 (br s, 4 H, p-H
of Ar4), 2.38–2.25 (m, 8 H, CHMeMeЈ), 1.97 (m, 8 H, PCH2-
CH2P), 1.49, 1.39, 1.25 and 1.19 (m, 48 H, CHMeMeЈ), 0.55
(m, 3 H, Pd᎐Me). 31P-{1H} NMR ([2H8]THF, 202.42 MHz):
δ 58.6; at Ϫ70 ЊC, 68.5, 64.5, 59.0, 58.0 (Found: C, 48.05;
H, 4.95. Calc. for C69H86BF24OP4Pd2 i.e. {[Pd(dippe)CO][Pd-
(dippe)Me]}[BAr4]ؒC7H8: C, 47.74; H, 5.05%).
X-Ray crystallographic analysis of [(dippe)Pd(ì-CO)Pd(dippe)-
Me]{B[3,5-(CF3)2C6H3]4}ؒ0.5C7H8 6
Similarly was prepared 6-13C1 with 2-13C1 (0.050 g; 4.01 ×
10Ϫ5 mol) in [2H8]THF (0.4 mL). The product was characterized
in solution. 31P-{1H} NMR ([2H8]THF, 81.015 MHz): δ 58.6 (d,
JPC = 14.6 Hz). 13C-{1H} NMR ([2H8]THF, 50.32 MHz): δ 47.3
(qnt, Pd᎐Me, JCP = 14.6 Hz); 13C NMR (q of qnt, Pd᎐Me,
JHC = 127.6, JCP = 14.6 Hz). Similarly was prepared 6a with 2
(0.050 g; 4.01 × 10Ϫ5 mol) in [2H8]THF (0.4 mL). The product
was characterized in solution under 13CO. 31P-{1H} NMR
([2H8]THF, 202.42 MHz): δ 58.7 (dd, JPC = 28.2, JPCЈ = 25.6 Hz).
13C-{1H} NMR ([2H8]THF, 50.32 MHz): δ 236.4 (qnt, CO,
JCP = 28.2), 228.1 (qnt, CO, JCЈP = 25.6 Hz).
Crystallographic data appear in Table 1. The final unit-cell
parameters were obtained by least-squares on the setting
angles for 25 reflections with 2θ = 20.0–22.9Њ. The intensities of
three standard reflections, measured every 200 reflections
throughout the data collections, decayed linearly by 22.5%. The
data were processed52 and corrected for Lorentz and polariz-
ation effects, decay and absorption (empirical, based on azi-
muthal scans).
The structure was solved by direct methods. The structure
analysis was initiated in the centrosymmetric space group P1,
¯
[Pd(dippe)(CO13CH3)CO][BAr4] 7[BAr4]. In a 5 mm NMR
tube, 2-13C1 (0.030 g; 2.27 × 10Ϫ5 mol) was dissolved in Et2O
(0.4 mL) and a few drops of C6D6 were added. The solution was
freeze–pump–thawed, sealed under ca. 2 equivalents of carbon
monoxide, and its 31P-{1H} and 13C-{1H} NMR spectra were
recorded. 31P-{1H} NMR (C6D6–Et2O, 81.015 MHz): δ 73.7
this choice being confirmed by subsequent calculations. The
C(53) trifluoromethyl group was modelled as 1:1 disordered.
Several more CF3 groups and the Pd(2) chelate ring show signs
of minor disordering, but the quality of the data set was not
sufficient to permit full modelling of this disorder. The toluene
solvent is disordered in a complex fashion about a centre of
symmetry; no models having reasonable geometry could be
refined successfully. The seven largest peaks in the toluene
region were refined as carbon atoms with occupancy factors
constrained to total 3.5 and thermal parameters constrained to
be approximately equal. All non-hydrogen atoms except C(63)
and C(69) (which had non-positive definite thermal parameters
when refined anisotropically) were refined with anisotropic
thermal parameters. The hydrogen atoms associated with the
cation and the anion were fixed in calculated positions with
(dd, 1 P, JPPЈ = 40.6, JCP = 38.7), 70.7 (dd, 1 P, JPPЈ = 40.6, JCP
=
17.3 Hz). 13C-{1H} NMR (C6D6–Et2O, 50.32 MHz): δ 47.3 [dd,
CO13CH3, JCP (trans) = 38.7 Hz, JCPЈ (cis) = 17.3 Hz].
[Pd(dippe)(13CH3)CO][BAr4] 8[BAr4]. In a 5 mm NMR tube,
2 (0.030 g; 2.27 × 10Ϫ5 mol) was dissolved in C6D4Cl2-o (0.35
mL). The solution was freeze–pump–thawed, sealed under ca.
2 equivalents of carbon monoxide, and its 31P-{1H} and 13C-
{1H} NMR spectra were recorded. 31P-{1H} NMR (C6D4Cl2-o,
81.015 MHz): δ 84.2 (dd, 1 P, JPPЈ = 21.2, JCP = 105), 83.8 (dd, 1
P, JPPЈ = 21.2, JCP = Ϫ8.5 Hz). 13C-{1H} NMR (C6D4Cl2-o, 50.32
MHz): δ Ϫ4.9 [dd, Pd᎐CH3, JCP (trans) = 105, JCPЈ (cis) = Ϫ8.5
Hz].
C᎐H = 0.98 Å and BH = 1.2 Bbonded
while those associated
atom
with the toluene molecule were not included in the model. No
secondary extinction correction was necessary. Neutral atom
scattering factors and anomalous dispersion corrections were
taken from ref. 53.
᎐
CCDC reference number 186/968.
[Pd(dippe)(CO)2][BAr4]2 9[BAr4]2, [Pd(dippe)(N᎐CCH ) ]-
᎐
3
2
[BAr4]2 10a[BAr4]2 and [Pd(dippe)(THF)2][BAr4]2 10b[BAr4]2.
To a solid mixture of HBAr4 (0.416 g; 4.11 × 10Ϫ4 mol) and
Pd(dippe)Me2 (0.133 g; 2.06 × 10Ϫ4 mol) under vacuum at
Ϫ196 ЊC, was added THF (2.0 mL) by trap to trap distillation.
The reaction vessel was backfilled with CO (ca. 20 equivalents)
and allowed to warm to Ϫ78 ЊC, at which temperature a clear
yellow solution was observed. The solvent was removed in
Acknowledgements
Financial support for this research was generously provided
by the Donors of the Petroleum Research Fund, administered
by the American Chemical Society. We also thank Johnson
Matthey for the loan of palladium salts.
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016
2015

View Article Online
30 G. P. C. M. Dekker, C. J. Elsevier, K. Vrieze, P. W. N. M. van
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37 An alternative formulation of 6a, as [Pd(dippe)(µ-13CO)Pd(dippe)-
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an acetyl group, is untenable due to the observed 13C-{1H} NMR
spectrum of 2-13C1 under 13CO: JCCЈ values of 4–10 Hz are
1
measured; therefore the scalar coupling is not of the JCCЈ type [cf.
1JCC = 40.6 Hz for the AX2 pattern of 13CH3(13CO)CH3, which is
observed in the same spectrum] and a formulation of [Pd(dippe)-
(µ-13CO)2Pd(dippe)Me]ϩ is justified.
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Received 29th December 1997; Paper 8/00152I
2016
J. Chem. Soc., Dalton Trans., 1998, Pages 2007–2016