Limits of activity: Weakly coordinating ligands in arylphosphinesulfonato palladium(II) polymerization catalysts

Article abstract of DOI:10.1021/om3000339

The coordination strength of various phosphine oxides OPR₃ toward the olefin polymerization catalyst (P^λO)PdMe (P ^λO = κ²-P,O-Ar₂PC ₆H₄SO₂O with Ar = 2-MeOC₆H ₄) as compared to that of dmso has been determined. Equilibrium constants K_L for the reaction 1-dmso + L ? 1-L + dmso range from 3.5 for electron-rich OPBu₃ to 10^-3 for electron-poor OP(p-CF₃C₆H₄)₃. Complexes derived from more strongly coordinating phosphine oxides, i.e. [(P^λO) PdMe(L)] (1-L; L = OPBu₃, OPOct₃, OPPh₃) have been isolated and fully characterized. Additionally, 1-OPBu₃ and 1-OPPh₃ were analyzed by X-ray diffraction analyses. Complexes derived from weakly coordinating phosphine oxides have eluded isolation due to loss of phosphine oxide and formation of barely soluble multinuclear palladium complexes 1_n by bridging coordination of the sulfonate group to various Pd centers. Hence, the (P^λO)PdMe fragment 1 exhibits an intrinsic limitation with respect to coordination of weak donors. Species 1 generated in situ in the absence of additional ligand (L) has been identified in homo- and copolymerization experiments as well as NMR insertion studies as the most active possible catalyst. Since 1 is generated from the easily available precursor [{(1-Cl)-ν-Na}₂)], these findings give rapid access to highly active (P^λO)PdMe catalysts.

Full text of DOI:10.1021/om3000339

Article
pubs.acs.org/Organometallics
Limits of Activity: Weakly Coordinating Ligands in
Arylphosphinesulfonato Palladium(II) Polymerization Catalysts
̈
Boris Neuwald, Franz Olscher, Inigo Gottker-Schnetmann, and Stefan Mecking*
̈
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany
S
* Supporting Information
ABSTRACT: The coordination strength of various phosphine
oxides OPR3 toward the olefin polymerization catalyst
(P∧O)PdMe (P∧O = κ2-P,O-Ar2PC6H4SO2O with Ar = 2-
MeOC6H4) as compared to that of dmso has been determined.
Equilibrium constants KL for the reaction 1-dmso + L ⇆ 1-L +
dmso range from 3.5 for electron-rich OPBu3 to 10−3 for
electron-poor OP(p-CF3C6H4)3. Complexes derived from more strongly coordinating phosphine oxides, i.e. [(P∧O)PdMe(L)]
(1-L; L = OPBu3, OPOct3, OPPh3) have been isolated and fully characterized. Additionally, 1-OPBu3 and 1-OPPh3 were analyzed
by X-ray diffraction analyses. Complexes derived from weakly coordinating phosphine oxides have eluded isolation due to loss of
phosphine oxide and formation of barely soluble multinuclear palladium complexes 1n by bridging coordination of the sulfonate
group to various Pd centers. Hence, the (P∧O)PdMe fragment 1 exhibits an intrinsic limitation with respect to coordination of weak
donors. Species 1 generated in situ in the absence of additional ligand (L) has been identified in homo- and copolymerization
experiments as well as NMR insertion studies as the most active possible catalyst. Since 1 is generated from the easily available
precursor [{(1-Cl)-μ-Na}2)], these findings give rapid access to highly active (P∧O)PdMe catalysts.
he catalytic insertion polymerization of ethylene and
propylene is one of the most well-studied chemical reactions.
metal center and is more readily displaced by olefinic
substrates. This enabled homooligomerization of methyl
acrylate (MA) and the isolation of ethylene−methyl acrylate
copolymers with more than 50% MA incorporation.13,17 Here,
the weak coordination strength of dmso permitted polymer-
ization at low ethylene pressures and thus high MA/ethylene
ratios. However, limitations for activities and incorporation in
co- and homopolymerization experiments with acrylates
employing the phosphinesulfonato Pd(II) catalyst system
arise from the six-membered κ2-C,O chelates [(P∧O)PdCH-
(R)CH2CH(C(O)OMe)CH2R′]. These six-membered chelates
are formed by (co)monomer insertion after a MA insertion.
The coordination strength of the carbonyl oxygen in these
chelates is comparable to that of dmso.18 For completeness
it should be mentioned that entirely “base-free” species
of the molecular composition [(P∧O)PdMe] have been
isolated12,18,19 or synthesized in situ by abstraction of L from
(P∧O)PdMe(L).5,16,18,20−22 However, so far no improved
polymerization activities in comparison to those of “base-
coordinated” compounds have been reported. For in situ
activated catalysts this might be due to incomplete activation or
side reactions by activation reagents or catalyst precursors.5,16,20
For isolated material the reported low solubility likely renders
part of the catalyst inactive.18,23
T
In terms of application, it is employed for the production of more
than 70 million tons of polyolefins annually.1 In contrast, the
insertion polymerization of polar-substituted vinyl monomers has
long remained elusive. In the past few years major breakthroughs
in the field of copolymerization of these monomers with ethylene
have been achieved with neutral Pd(II) phosphinesulfonato
catalysts, which were first reported by Drent et al.2−4 Among
the most remarkable examples are copolymerizations of ethylene
with acrylonitrile,5,6 vinyl acetate,7 and acrylic acid.8
The coordination strength of the monodentate ligand L in
catalyst precursors [(P∧O)PdMe(L)] (1-L; P∧O = κ2-P,O-
Ar2PC6H4SO2O with Ar = 2-MeOC6H4) has a major impact on
the catalytic activity in homo- and copolymerizations, given the pre-
equilibration (P∧O)PdR(L) + monomer ⇆ (P∧O)PdR(monomer)
+ L which accompanies the chain growth. Thus, more strongly
coordinating ligands shift the equilibrium toward the dormant
species 1-L.9 So far, monodentate ligands, e.g. PPh3, tmeda,
pyridine, 2,6-lutidine, dmso, and derivatives thereof have been
used.5,10−14 Alternatively, bidentate carbon-based ligands, e.g.
η3-allyl or η1,η2-2-methoxycyclooct-5-enyl, are suitable precursors
to initiate chain growth.15,16
The significantly higher activity observed with dmso- vs
pyridine-coordinated catalyst precursors suggests studies of
further more weakly coordinating ligands. Phosphine oxides
(OPR3) as a ligand class lend themselves to this purpose, as
By comparison to the aforementioned N- and P-based
ligands, dimethyl sulfoxide (dmso) binds less strongly to the
Received: January 16, 2012
Published: March 20, 2012
© 2012 American Chemical Society
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Article
they are easily accessible from the corresponding phosphines
and exhibit a defined coordination site at the oxygen atom.
Furthermore, a great variety of phosphines are commercially
available and allow for electronic and steric fine tuning. While
chelating, hemilabile ligands (X∧O; X  N, P, O; O =
phosphine oxide) and especially the phosphine−phosphine oxide
ligands have attracted much attention in homogeneous
catalysis,24−28 the application of monodentate tertiary phosphine
oxides in homogeneous catalysis is rare,29−33 even though
coordination toward metal centers has been well studied.34
The relative coordination strength of these phosphine oxides
1
in comparison to that of dmso (KOPR ) was determined by H
3
1
NMR spectroscopy. The H resonance of dmso in CD2Cl2 is
shifted downfield from 2.54 ppm (free dmso) to 2.95 ppm by
complexation to the Pd center of (P∧O)PdMe (1).41 Partial
replacement by OPR3 leads to a high-field shift due to a fast
equilibrium between Pd-bound and uncoordinated dmso. From
the shift difference the ratio between 1-dmso and 1-L and
consequently KL at 25 °C was calculated (cf. the Supporting
Information). The results are summarized in Table 1. Whereas
RESULTS AND DISCUSSION
■
Table 1. KL for 1-dmso + L ⇆ 1-L + dmso
Coordination Strength of Phosphine Oxides. Since the
coordination strength is influenced by steric as well as elec-
tronic properties, both parameters should be varied indepen-
dently. Here, the cone angle θ and the electronic parameter χ35
of the corresponding phosphines enable an educated selection
of phosphine oxides.36 For this study we chose OPBu3,
OPOct3, OPPh3, OP(o-Tol)3, and OP(p-CF3C6H4)3,36−38 for
which θ and χ parameters have been reported, as well as the
even more electron deficient OP(3,5-(CF3)2C6H3)3 (Figure 1).
entry
ligand
amt of L (equiv)
δeq
KL
3.5
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
OPBu3
OPOct3
OPPh3
1.0
1.2
2.68
2.66
2.67
2.78
2.77
2.91
2.82
2.54
3.3
0.2
9.2
OP(o-Tol)3
11.3
10.2
9.2
0.03
OP(p-CF3C6H4)3
OP(3,5-(CF3)2C6H3)3
MeOH
0.04
a
a
∼0.001
0.02
≫102
9.4
2.6-lutidine
1.4
a
Δδ is low due to the limited solubility of OPR3; consequently, the
inaccuracy of KL is enhanced.
both alkyl phosphine oxides coordinate slightly more strongly
than dmso (KOPBu = 3.5, KOPOct = 3.3), the more bulky and
3
3
electron-deficient OPPh3 exhibits KOPPh = 0.2 for the
3
equilibrium 1-dmso + OPR3 ⇆ 1-OPR3 + dmso. An even
weaker coordination is evident for the comparison with OP(o-
Tol)3 (KOPTol = 0.03) and OP(p-CF3C6H4)3 (KOP(p‑CF Ar) = 0.04,
3
3
3
Table 1; compare KL values for MeOH and 2,6-lutidine). Hence,
the coordination strength can be controlled by either steric bulk
or electron deficiency over a large range. The introduction of a
second electron-withdrawing group in OP(3,5-(CF3)2C6H3)3
further reduces the coordination strength significantly
(KOP(3,5‑CF Ar) ≈ 0.001).42
Figure 1. Coordination strength of phosphine oxides based on the
parameters θ and χ35 of the corresponding phosphines.36,37
3
3
We note here that only minor changes of KOPPh in the
Phosphine oxides not available commercially were easily
synthesized by phosphine oxidation with H2O2 (cf. the
Supporting Information).
3
temperature range from −25 to 25 °C have been observed and
that ΔH° = 8 kJ mol−1 and ΔS° = 13 J mol−1 K−1 have been
determined by a van’t Hoff analysis (cf. the Supporting
Information). Extrapolation to typical polymerization con-
With regard to the electronic properties, a comparison of the
literature-derived electronic parameters with the observed 31P
NMR shifts of the phosphine oxides show a good correlation.
With increasing electron deficiency χ increases, while δ
decreases. It is important to note that the 31P NMR shift of
phosphine oxides is a reliable measure of the basicity as
opposed to the 31P NMR shift of phosphines, which is also
influenced by sterics.39 As expected, the aryl phosphine oxides
exhibit a weaker basicity than OPBu3. In comparison to OPPh3
the steric bulk is increased by the introduction of a methyl
group in an ortho position in OP(o-Tol)3. It should be noted
that the steric influence is less than is indicated by the cone
angle of the corresponding phosphine, because the distance
between the metal center and the aryl groups is increased in
comparison to that in the phosphine. In contrast, the
introduction of a CF3 group in a para position in OP(p-
CF3C6H4)3 does not change the steric properties but increases
electron deficiency. In OP(3,5-(CF3)2C6H3)3 electron defi-
ciency is further increased while the steric influence is believed
to be similar to that of OP(p-CF3C6H4)3 (θ = 145° vs ∼151°
for P(3,5-Me2C6H3)3).40
ditions, i.e. 90 °C, results in K (90 °C) = 0.4, which com-
OPPh3
pares to KOPPh (25 °C) = 0.2. We assume that a similar small
3
temperature dependence applies to all KOPR values and that
3
KOPR < 1 should result in more active precatalysts 1-L: since
3
during polymerization the monomer can also compete with L
for coordination to (P∧O)PdR (R = growing chain) according
to the equilibrium [(P∧O)PdR(L)] + ethylene ⇆ [(P∧O)PdR-
(ethylene)] + L, complexes 1-L derived from phosphine oxides
with a coordination strength weaker than that of dmso, i.e. from
OPPh3, OP(o-Tol)3, OP(p-CF3C6H4)3, and OP(3,5-(CF3)2C6H3)3,
are expected to exhibit higher turnover frequencies than 1-dmso
as long as saturation kinetics are not reached. Consequently,
such complexes 1-L represent valuable synthetic targets for
highly active single-component catalysts.
Complex Synthesis and Characterization. For the
synthesis of phosphine oxide complexes 1-OPR3 standard
procedures were not applicable, since they are either based
on introduction of the ligand with the Pd precursor, as with
L = tmeda from [(tmeda)PdMe2],10,11,15,16 or subsequent ligand
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substitution by a more strongly coordinating ligand.5,10,12,14,19,43
Notably the more weakly coordinating dmso could be
introduced by substitution of tmeda. This substitution occurs
since tmeda is removed from the equilibrium 1/2 (1)2-tmeda +
1
dmso ⇆ 1-dmso + /2 tmeda under vacuum, due to the
considerably higher volatility of tmeda vs dmso.13 However, an
analogous procedure, e.g. solvent evaporation from a mixture of
(1)2-tmeda and phosphine oxide in high-boiling solvents, did
not result in the isolation of clean products. In an alternative
approach, multinuclear “base-free” palladium alkyl complexes
which are accessible, for example, by pyridine or lutidine
abstraction with B(C6F5)312,18,19 may be suitable precursors for
the preparation of phosphine oxide complexes 1-OPR3.
However, a more convenient synthesis starts from [{(1-Cl)-
μ-Na}2)].44 Thus, chloride abstraction from easily accessible
[{(1-Cl)-μ-Na}2)] in the presence of phosphine oxides is
expected to generate 1-OPR3 if the presence of more strongly
coordinating ligands is avoided (Scheme 1).
Figure 2. Molecular structure of 1-OPBu3 with 50% probability
ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Pd(1)−P(1) = 2.203(1); Pd(1)−C(1) =
2.010(3); Pd(1)−O(1) = 2.157(2); Pd(1)−O(6) = 2.129(2); O(6)−
P(2) = 1.514(2); Pd(1)−O(6)−P(2) = 132.1(1); P(1)−Pd(1)−O(6) =
174.4(1).
The viability of this general route was demonstrated by the
synthesis and isolation of 1-dmso. As expected, also the
lower 31P shift (Δδ = 8 ppm for OPPh3 vs 18 ppm for OPBu3/
OPOct3). In the IR spectrum again a decrease of the PO
stretching frequency from 1189 to 1150 cm−1 is observed, which
agrees with ν(OP) 1145 cm−1 reported for the complex
[Pd(NO3)2(OPPh3)(PPh3)].46,48
Scheme 1. Synthesis of 1-L
Suitable crystals for X-ray analysis were obtained from a
CHCl3 solution. As for all reported (P∧O)PdMe structures, the
palladium complex adopts a square-planar geometry and the
methyl group is situated in the position trans to the sulfonate
group (Figure 3). Coordination of the phosphine oxide leads
only to a small elongation of the O−P bond (1.503(3) vs
∼1.49 Å49 in OPPh3),50 which is in accordance with the
aforementioned bis(nitrato)−Pd(II) complex (vide supra; O−
P = 1.501(8) Å)48 and OPPh3 complexes of other metals, for
which in general a length change of <1% is observed.45 In
comparison to 1-OPBu3 the Pd−OP bond length is slightly
elongated (2.138(3) Å for 1-OPPh3 vs 2.129(2) Å for 1-
OPBu3; Figure 2), which is in agreement with the reduced
coordination strength of OPPh3. Phosphine oxides are
capable of showing a rather wide range of M−O−P bond angles
(∼180−140°), while the bonding mode is discussed to be end-on,
in contrast to the case for thio- and seleno-phosphorylic units,
which show a side-on coordination (∠(M−E−P) ≈ 115−98°).45
In comparison the M−O−P bond angles for Pd complexes seem
to be at the lower end of the scale, independent of
the steric bulk at the Pd center created by additional ligands
(∠(Pd−O−P) = 133.5(1)° (1-OPPh3), 132.1(1)° (1-OPBu3),
132.1(4)° [Pd(NO3)2(OPPh3)(PPh3)]).48
complexes 1-OPBu3 and 1-OPOct3 with the slightly more
strongly coordinating alkyl phosphine oxides in comparison to
dmso could be isolated. In contrast to more strongly
coordinating ligands such as pyridine, the Pd−CH3 group
3
1
exhibits no visible JPH coupling in the H NMR spectrum at
25 °C, as was also observed for 1-dmso.18 Coordination of the
phosphine oxide in solution is further evidenced by a downfield
shift of the OPBu3 signal in the 31P NMR spectrum from 48 to
66 ppm.45,46 In the solid state coordination of the phosphine
oxide is evidenced by the shift of the ν(OP) band in the IR
spectrum to lower frequencies from 1154 to 1113 cm−1 (Table 2).
This observed frequency decrease can be related to a lowering of
the O−P bond order due to coordination.47 In addition, the
molecular connectivity of 1-OPBu3 could be established by X-ray
diffraction analysis (Figure 2). The analogous complex 1-OPOct3
exhibits very similar properties (cf. the Supporting Information).
The synthesis of 1-OPPh3 could also be achieved. Here,
coordination is evidenced by a shift in the 31P NMR from 27 to
35 ppm. A weaker coordination in solution by comparison to the
trialkyl phosphine oxide complexes may be reflected in the much
Table 2. 31P NMR Shift and OP IR Band of Pd-Coordinated vs Noncoordinated Phosphine Oxide OPR3
c
entry
R
ν(OP)coord (cm−1
)
ν(OP)free (cm−1
)
Δν(OP) (cm−1
)
δ(31P)coord (ppm)
δ(31P)free (ppm)
Δδ(31P) (ppm)
2-1
2-2
2-3
2-4
2-5
2-6
Bu
1113
1107
1150
1154
1145
1189
1198
1185
1216
41
38
39
b
66
65
35
29
40
48
47
27
25
37
18
18
8
Oct
Ph
a
p-CF3C6H4
o-Tol
n.d.
4
a
n.d.
b
3
a
d
3,5-(CF3)2C6H3
n.d.
b
21
21
0
a
b
Clear identification of the ν(OP)coord band was not possible due to weak intensity and numerous overlapping bands. Disappearance of the
c
d
ν(OP)free band is observable. From isolated raw material directly after dissolving in CD2Cl2. No clear homogeneous reaction mixture was
obtained.
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Scheme 2. Decomposition of 1-L
PdCH2SiMe3}2].12 An extensive IR comparison revealed a
further detail. Removal of the coordinating ligand from 1-L and
transformation to 1n leads to the disappearance of a strong
band at ∼1000 cm−1 assigned to νsym(SO3) and growth of a
very strong band at ∼950 cm−1 (Figure 4, cf. Supporting
Information). Such a frequency shift could be induced after
coordination of the SO3 group to a further palladium center:
e.g., in 1n. In this context it is important to note that for
all other (ligand coordinated) (P∧O)PdMe complexes 1-L
described in this work the νsym(SO3) band can always be found
at around 1000 cm−1 (Figure 4), which is in agreement with
literature data for Pd-coordinated benzenesulfonic acid.54
Hence the absence of a strong band around 950 cm−1 in the
IR spectra can be seen as a reliable indication for the formation
of the discrete ligand-coordinated (P∧O)PdMe species 1-L.
Decomposition of 1-OP(o-Tol)3 and 1-OP(p-CF3C6H4)3 to
Figure 3. Molecular structure of 1-OPPh3 with 50% probability
ellipsoids. All hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−P(1) =
2.199(1); Pd(1)−C(1) = 2.089(3); Pd(1)−O(1) = 2.150(3); Pd(1)−
O(6) = 2.138(3); O(6)−P(2) = 1.503(3); Pd(1)−O(6)−P(2) =
133.5(1); P(1)−Pd(1)−O(6) = 175.2(1).
Limitations for Coordinating Ligands Weaker than
OPPh3. The isolation of 1-OPPh3 already revealed a relatively
weak binding of this phosphine oxide to the Pd center, which
affects the workup procedure: coordinated OPPh3 can be
extracted from the complex by extensive washing with toluene,
leading to partially insoluble material. In contrast, 1-OP(o-
Tol)3 and 1-OP(p-CF3C6H4)3, having phosphine oxides with a
further decreased coordination strength (Table 1), were only
obtained as crude products that are not stable in solution for a
prolonged period of time. For the solid raw materials isolated
by solvent evaporation after filtration a shift of the ν(PO) band
in the IR spectra can be observed (Figure 4),51 evidencing a
Pd−OPR3 interaction. Dissolving the isolated material in
CH2Cl2 yields a clear solution, and the 31P NMR spectrum of
the dissolved raw material shows a further reduced but
significant shift for the OPR3 resonances (Δδ(OP(o-Tol)3) =
3 ppm; Δδ(OP(p-CF3C6H4)3) = 4 ppm). However, within
1−12 h in solution white precipitates form. Detailed analysis of
these precipitates by ATR-IR and NMR spectroscopy in
CD3OD confirmed the decomposition of 1-OP(o-Tol)3 and
1-OP(p-CF3C6H4)3 to [{(P∧O)PdMe}n] (1n) (Scheme 2 and
Figure 4; cf. Supporting Information).12,18,19,52,53
1
free phosphine oxide and 1n can be monitored by H NMR
spectroscopy over the course of several hours, as 1n precipitates
upon formation, which results in diminishing signals for
1-OP(o-Tol)3 and 1-OP(p-CF3C6H4)3, whereas OPR3 remains
in solution (Figure 5).
In the case of the significantly more weakly coordinating
OP(3,5-(CF3)2C6H3)3, crude 1-OP(3,5-(CF3)2C6H3)3 already
It is assumed that 1n is bridged via coordination of multiple
Pd centers to the sulfonate groups in analogy to [{(P∧O)-
1
Figure 5. H NMR spectra (expansion, 400 MHz, CD2Cl2) of the
1
decomposition of 1-OP(o-Tol)3 with time and H NMR spectrum
(400 MHz, MeOD) of the resulting precipitate.55
contains substantial amounts of 1n, as evidenced by the IR
spectrum, the limited solubility, and the intensity ratio of anisyl
methoxy to aromatic 3,5-(CF3)2C6H3 1H resonances (cf.
Supporting Information). The instability of 1-OP(o-Tol)3
and 1-OP(p-CF3C6H4)3 and their decomposition toward 1n
and free phosphine oxide as well as the elusive isolation of
1-OP(3,5-(CF3)2C6H3)3 clearly point to a coordination
Figure 4. ATR-IR spectra of OP(o-Tol)3, ligated complexes 1-L (L =
OP(o-Tol)3, dmso), the “base-free” complex 1n, and isolated precipitate
from a 1-OP(o-Tol)3 solution in CH2Cl2.
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Figure 6. Molecular structure of 1-MeOH with 50% probability
ellipsoids. All solvent molecules and hydrogen atoms, except the
hydroxyl hydrogen H24, are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Pd(1)−P(1) = 2.200(1); Pd(1)−C(1) =
2.032(2); Pd(1)−O(1) = 2.158(1); Pd(1)−O(6) = 2.139(2); O(6)−
C(22) = 1.429(3); Pd(1)−O(6)−C(22) = 118.2(1); P(1)−Pd(1)−
O(6) =176.9(0).
Figure 7. First-order consumption of Pd−Me by insertion of MA
([Pd] = 0.02 mol L−1 in CD2Cl2, 25 °C).
strength of palladium-coordinated sulfonate which effectively
competes with these phosphine oxides.
However, kinetic control may allow for the isolation of
complexes of weakly coordinating ligands such as methanol.
While methanol binds less strongly to (P∧O)PdMe than do
OP(o-Tol)3 or OP(p-CF3C6H4)3 (Table 1, entries 1-4 and 1-5
vs 1-7), 1-MeOH was isolated by crystallization from methanol
solution and analyzed by X-ray diffraction analysis (Figure 6).
However, even solid 1-MeOH in the absence of a methanol
atmosphere loses methanol within 20 min at room temperature
and forms 1n, as evidenced by ATR-IR spectroscopy (cf.
Supporting Information).
NMR Studies of the Influence of Coordinating Ligand
L on Monomer Insertion. To study the influence of the
coordinating ligand on monomer insertion in detail, the
insertions of ethylene and methyl acrylate were investigated
by NMR spectroscopy.
Figure 8. First-order consumption of Pd−CH(C(O)OMe)CH2-Me by
consecutive insertion of MA ([Pd] = 0.02 mol L−1 in CD2Cl2, 25 °C).
dmso-coordinated insertion product κ2-C,O-[(P∧O)PdCH(C-
(O)OMe)CH2Me(dmso)].13
Influence of Ligand L on Polymerization Activities.
Replacement of pyridine or 2,6-lutidine by the significantly
more weakly coordinating dmso in catalyst precursors 1-L has
resulted in a nearly 10-fold activity increase in ethylene
homopolymerizations at a pressure of 5 bar. At this low
ethylene pressure the equilibrium [(P∧O)PdR(L)] + ethylene ⇆
[(P∧O)PdR(ethylene)] + L (R = growing chain) is believed to
be shifted far to the ethylene complex for L = dmso, while
pyridine or 2,6-lutidine compete much more effectively with
ethylene and require higher [ethylene] in order to reach
saturation kinetic conditions. Arguably, the most promising
species to reach saturation kinetic behavior at the lowest
possible [ethylene] is the “base-free” catalyst precursor
(P∧O)PdMe (1). However, the formation of 1n from 1 (e.g.,
after pyridine abstraction from 1-pyridine) and the low
solubility of 1n have so far prevented catalytic activities higher
than those observed for 1-dmso.18 Therefore, 1-OPR3 (R = Ph,
o-Tol, p-CF3C6H4) species described in this work are of interest
for achieving saturation kinetic conditions at the lowest possible
[ethylene]. To this end, polymerizations at variable ethylene
concentrations with the isolable defined precatalysts 1-dmso,
1-OPBu3, and 1-OPPh3 were studied. In addition, the polymer-
ization of ethylene in the presence of 1 prepared by in situ chloride
abstraction from 1/2 [{(1-Cl)-μ-Na}2)] with 1 equiv of AgBF4 was
investigated. Note that, in this context, [{(1-Cl)-μ-Na}2)] contains
up to 1 equiv of a coordinating solvent such as acetone or
diethyl ether per Na depending on the preparative workup.
The insertion of ethylene into the Pd−Me bond of 1-OPPh3
and of species 1 generated by in situ chloride abstraction from
[{(1-Cl)-μ-Na}2)] with AgBF4 in the absence of additional
ligands was monitored at −15 °C (likely, in 1 the [(P∧O)-
PdMe] fragment is weakly coordinated by the methylene
chloride solvent).56 The determined first-order rate constants
(22 equiv of ethylene as compared to palladium) show that
at −15 °C 1 is consumed slightly more quickly than 1-OPPh3
(k1‑OPPh ,ethylene = 5.7 × 10−4 s−1, k1,ethylene = 7.0 × 10−4 s−1; cf.
3
Supporting Information). A similar trend with increasing
coordination strength was also observed for the reaction of
[(P∧O)PdMe(L)] and 1 with MA. Rate constants for the first
insertion 1stk1‑L range from 0.6 × 10−3 s−1 for 1-OPBu3 to 3.2 ×
10−3 s−1 for 1 at 0.02 mol L−1 palladium and 0.3 mol L−1 MA
(Figure 7). Insertion of MA at 25 °C into [(P∧O)PdCH(C-
(O)OMe)CH2Me] also proceeds the fastest for 1 (secondk1 =
9.2 × 10−5 s−1) as compared to 1-OPBu3 (secondk1‑OPBu = 8.2 ×
3
10−6 s−1) (Figure 8).
Since the rate constants for the second MA insertion
are significantly affected by the applied ligands, it can be
concluded that the coordination strength of dmso, OPPh3,
and OPBu3 exceeds that of the carbonyl group in the possible
four-membered chelate κ2-C,O-[(P∧O)PdCH(C(O)OMe)-
CH2Me]. This is in accordance with the isolation of the
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Polymerization results (Figure 9) show that in situ generated 1
as well as 1-OPPh3, 1-OPBu3, and 1-dmso exhibit essentially
similar activities at a given ethylene pressure (2, 3.5, 5, and 10 bar,
90 °C). As far as slight differences beyond experimental error are
influence on the activity also in ethylene−MA copolymeriza-
tions at low [ethylene], which correlates to the presence and
the nature of the coordinating ligand and its equilibrium
constant KL vs dmso: at 3.5 bar of ethylene and 0.5 mol L−1
MA in situ generated 1 produces 50% more copolymer than
1-OPBu3, while the copolymer composition remains essentially
identical (Table 3 entries 3-1 vs 3-4). The increase in activity is
steady in the order 1-OPBu3 < 1-dmso < 1-OPPh3 < 1, which
reflects the coordination strengths of the present OPR3 and
dmso ligands (vide supra, Table 1). Also note that with decreas-
ing coordination strength a slight increase in molecular weight
is evident (Table 3). This increase in molecular weight is
tentatively explained by a ligand-induced opening of the six-
membered chelates [(P∧O)PdCH(R)CH2CH(C(O)OMe)-
a
Table 3. Ethylene−Methyl Acrylate Copolymerization
cat.
yield
(g)
XMA
TOF
TOF
d
M /
wf
b
c
e
entry precursor
(%)
C2H4
MA
Mn
Mn
3-1 1-OPBu3
3-2 1-dmso
3-3 1-OPPh3
0.8
0.9
1.0
1.2
14.2
14.8
13.7
13.2
1927
2047
2404
2888
319
356
380
439
2.0
2.3
2.5
2.6
1.7
1.7
1.8
1.8
Figure 9. Dependence of average activity in ethylene homopolyme-
rization on pressure for 1 and 1-L (L = dmso, OPBu3, OPPh3).
Reaction conditions: t = 30 min; T = 90 °C; V(toluene) = 100 mL;
[Pd] = 40 μmol L−1. Dashed lines are merely a guide to the eye.
3-4 1 (in
situ)
a
Reaction conditions: total volume of toluene + MA, 50 mL; [MA] =
observed, these tend to reflect the coordination strength of the
ligand L or the absence of L ([{(1-Cl)-μ-Na}2)]/AgBF4).
These findings qualitatively agree with previous studies of
the effect of dmso on ethylene polymerization activities with
1-dmso, which showed that the equilibrium [(P∧O)PdR-
(ethylene)] + dmso ⇆ [(P∧O)PdR(dmso)] + ethylene (R =
growing chain) does render a portion of the metal centers
inactive by coordination of the 1 equiv of dmso introduced with
the catalyst precursor. However, the ethylene complex is
strongly favored in this equilibrium already at low ethylene
pressure, such that the inactive portion is rather small, and
saturation kinetic conditions are met.18
0.5 mol L−1; 3.5 bar ethylene pressure; 93 °C; 20 μmol Pd(II); 1 h
b
c
1
reaction time. Determined by H NMR in CDCl3. In units of (mol
d
of C2H4) (mol of Pd)−1 h−1. In units of (mol of MA) (mol of Pd)−1
e
f
h−1. In units of 103 g mol−1, determined by 1H NMR. Determined by
GPC.
CH2R′] and concomitant β-H elimination, whereas opening
by the monomer leads to the following insertion.
While the increase in activity is limited, in situ generated 1, i.e.
[{(1-Cl)-μ-Na}2] plus AgBF4, proves to be the most active
catalyst for ethylene−MA copolymerizations. This is quite
remarkable, since from a synthetic point of view [{(1-Cl)-μ-
Na}2] is easily available in a one-step reaction from Na(P∧O) and
[(cod)PdMeCl] without further time-consuming transformations.
While rate constants for the MA insertion from NMR studies
vary by factors of 5−10 (Figures 7 and 8), the polymer yields
only vary by a factor of 1.5 (Table 3). In addition to the
aforementioned effect of different concentrations in NMR
studies vs polymerization experiments, the stability of six-
membered chelates C formed in copolymerization by coor-
dination of a penultimate incorporated MA-derived repeating
unit can contribute to reducing the effect of other ligands L on
activity at a given amount of catalyst precursor added per
reaction volume (Scheme 3).18 Equilibration of these chelates
with incoming olefin is strongly shifted to C as compared to the
olefin-coordinated species D. Even if olefin insertion from
species A and B were sensitive to the coordination strength of L
due to the equilibration according to KA and KB, the following
rate-determining insertion from D is mainly affected by KC2 and
thus is mostly insensitive to L, as long as L is not significantly
more strongly coordinating than the κ-O carbonyl oxygen in C.
As noted previously, olefin insertion from D represents a
bottleneck in the MA−ethylene copolymerization which is
intrinsically linked to the coordination strength of the carbonyl
oxygen in C.57
As outlined, the equilibria [(P∧O)PdR(L)] + monomer ⇆
[(P∧O)R(monomer)] + L are relevant in polymerization
studies (activation of catalyst precursors, reversible deactivation
of active species) as well as NMR investigations of insertion
rates (preequilibria to insertion). It is worth noting that the
effect of the coordination strength of L will be much more
pronounced in NMR studies ([Pd] ≈ 10−2 mol L−1) vs
polymerization studies ([Pd] ≈ 10−4−10−6 mol L−1), as here
concentrations of metal species and consequently of free L
liberated from the metal precursor are typically much higher,
while monomer concentrations are usually roughly similar.
While all the above experimental observations fit into this
conclusive picture, they also suggest that the strong depend-
ence of average polymerization rates in the regime of up to
5 bar of ethylene pressure (Figure 9) is not simply related to
competitive relative binding of monomer vs L (or sulfonate).
For the low-pressure polymerizations at 2, 3.5, and 5 bar of
ethylene a pronounced drop in catalytic activity over time is
observed by mass-flow monitoring. Thus, at 2 bar the ethylene
uptake into the reactor decreases to 65−40% of its initial value
after 30 min polymerization time. At 5 bar the decrease of
ethylene uptake is not as pronounced but still drops to 80−70%
of the initial values (cf. Supporting Information). Possibly, a
dependence of catalyst stability on ethylene concentration
affects the polymerization behavior.
The choice of catalyst precursor, i.e. [{(1-Cl)-μ-Na}2] plus
AgBF4, 1-OPBu3, 1-OPPh3, or 1-dmso, has a measurable
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400, a Bruker Avance DRX 600, or a Bruker Avance III 600 spectro-
Scheme 3. Rate-Determining Equilibria in the
Copolymerization of Ethylene with MA
1
meter, equipped with a cryoprobe head. H and 13C NMR chemical
shifts were referenced to the solvent signal. 19F and 31P NMR chemical
shifts were referenced to CFCl3 and 85% H3PO4, respectively.
Multiplicities are given as follows (or combinations thereof): s, singlet,
d, doublet, t, triplet, vt, virtual triplet, m, multiplet. The identity and
1
purity of metal complexes was established by H, 13C, and 31P NMR
1
1
and elemental analysis. NMR assignments were confirmed by H, H
gCOSY, 1H,13C gHSQC, and 1H,13C gHMBC experiments. For
copolymers molecular weights were determined by 1H NMR and
polydispersity indexes were determined by GPC on a Polymer
Laboratories PL-GPC 50 instrument with two PLgel 5 μm MIXED-C
columns and an RI detector in THF against a polystyrene standard.
Elemental analysis and FAB mass spectra were obtained by the
Analytical Services at the Department of Chemistry, University of
Konstanz. Elemental analyses were performed on an Elementar Vario
MICRO cube instrument. FAB mass spectra were obtained with a
double-focusing Finnagan MAT 8200 mass spectrometer equipped
with a Ion Tech (Teddington, U.K) FAB ion source. ESI mass spectra
were recorded on a Bruker Esquire 3000+ instrument.
SUMMARY AND CONCLUSION
■
Phosphine oxides are easily available monodentate ligands with
a defined coordination site and allow wide manipulation of
coordination strength due to manifold possible steric and
electronic modifications. Determination of the coordination
strength KL toward the Pd center of (P∧O)PdMe for several
1
phosphine oxides OPR3 in comparison to dmso by H NMR
spectroscopy revealed that the coordination strength decreases
with increasing steric bulk and electron deficiency. The
investigated OPR3 ligands cover a range of KL from ∼3 to
0.001 vs dmso, decreasing in the order R = Bu ≈ Oct > Ph >
oTol ≈ p-CF3-C6H4 > 3,5-(CF3)2C6H3. Preparative synthesis
afforded the new complexes 1-OPBu3, 1-OPOct3, and
1-OPPh3, as shown by complete characterization by NMR and
IR spectroscopy, mass spectrometry, and elemental analysis. In
addition, X-ray diffraction analysis of 1-OPBu3 and 1-OPPh3
confirm the κ-O coordination of the phosphine oxides to
[(P∧O)PdMe(OPBu3)] (1-OPBu3; P∧O = κ2-P,O-2-(2-MeOC6H4)2-
PC6H4SO3)). A suspension of 99 mg (0.08 mmol, 0.5 equiv) of [{(1-Cl)-
μ-Na}2], 33 mg (0.17 mmol, 1.1 equiv) of AgBF4, and 35 mg (0.16 mmol,
1.0 equiv) of OPBu3 in 5 mL of CH2Cl2 was stirred for 30 min in the
dark. The resulting precipitate was filtered off to afford a yellow
solution. The filtrate was evaporated and the resulting residue was
washed with pentane and dried under vacuum to yield 1-OPBu3 as a
beige solid (100 mg, 0.14 mmol, 88%). Crystals suitable for X-ray
diffraction analysis were obtained from a saturated toluene solution
at −20 °C.
palladium. So far, 1-OPPh3 with KOPPh = 0.2 is the most
3
weakly coordinated stable (P∧O)PdMe complex synthesized.
Studies with weaker coordinating ligands disclosed that for
[(P∧O)PdMe(L)] the minimum of coordination strength for L
is limited, due to the ability of the (P∧O)PdMe fragment to
form ill-defined multinuclear palladium complexes 1n. More
weakly coordinating ligands still can temporally stabilize the
(P∧O)PdMe fragment, but ultimately 1n forms, as evidenced by
NMR and IR spectroscopic studies.
3
4
1H NMR (400 MHz, CD2Cl2): δ 8.02 (dd, JHH = 7.1, JPH = 4.9,
1H, 6-H), 7.66 (br, 2H, 12-H), 7.52 (vt, J = 7.8, 2H, 10-H), 7.43 (vt,
J = 7.4, 1H, 5-H), 7.32−7.25 (m, 2H, 4-H and 3-H), 7.01 (vt, J = 7.4,
2H, 11-H), 6.93 (dd, 3JHH = 7.8, 4JPH = 4.8, 2H, 9-H), 3.60 (s, 6H, Ar-
OCH3), 1.95−1.85 (m, 6H, 13-H), 1.66−1.58 (m, 6H, 14-H), 1.51−
3
1.43 (m, 6H, 15-H), 0.94 (t, JHH = 7.3, 9H, 16-H), 0.14 (s, 3H, Pd-
CH3). 31P{1H} NMR (162 MHz, CD2Cl2): δ 66.4 (br, OPBu3), 27.2
(PAr). 13C{1H} NMR (101 MHz, CD2Cl2): δ 160.9 (d, 2JPC = 1.4, C8),
Detailed insertion studies by NMR techniques showed that 1
exhibits the highest rate constants for ethylene insertion and for
the first and second MA insertions.
Homo- and copolymerization studies revealed that the most
active catalyst, 1, is available by in situ chloride abstraction from
[{(1-Cl)-μ-Na}2]. So far, 1 generated by in situ chloride
abstraction from [{(1-Cl)-μ-Na}2] represents the best model
substance for a species free of significantly coordinating
monodentate ligand L with the highest possible activity in
ethylene−MA copolymerizations.
2
148.9 (d, 2JPC = 15.7, C1), 138.4 (br, C12), 135.0 (d, JPC = 2.5, C3),
133.7 (C10), 130.5 (C5), 128.6 (br d, 3JPC = 6.0, C4), 128.1 (d, 1JPC
=
52.9, C2), 127.9 (d, 3JPC = 8.0, C6), 120.8 (d, 3JPC = 12.4, C11), 117.1
(d, 1JPC = 59.4, C7), 111.9 (d, 3JPC = 4.5, C9), 55.7 (Ar−OCH3), 27.5
1
3
2
(d, JPC = 64.8, C13), 24.7 (d, JPC = 14.7, C15), 24.3 (d, JPC = 3.8,
C14), 14.0 (C16), 0.9 (br, Pd−CH3). Anal. Calcd for C33H48O6P2PdS:
C, 53.48; H, 6.53. Found: C, 53.53; H, 6.53. MS (FAB): m/z 740
[M]+, 725 [M − Me]+, 522 [M − OPBu3]+, 507 [M − OPBu3 −
Me]+, 216 [OPBu3 + H]+. ATR-IR: 1/λ (cm−1) 2956 (m), 2869 (m),
EXPERIMENTAL DETAILS
■
Unless noted otherwise, all manipulations of air-sensitive compounds
were carried out under an inert atmosphere using standard glovebox or
Schlenk techniques. THF, toluene, CH2Cl2, and MeOH were dried
using standard protocols.58 Pentane and Et2O were dried by passing
through columns equipped with aluminum oxide/molecular sieves 3 Å.
Ethylene (3.5 grade) supplied by Praxair and methyl acrylate (99%)
supplied by Aldrich were used as received. [(tmeda)PdMe2],59
[(cod)PdMeCl],60 2-[bis(2-methoxyphenyl)phosphino]benzenesulfonic
acid,61 1-dmso,13 and [{(1-Cl)-μ-Na}2]44 were prepared by known
procedures. NMR spectra were recorded on a Varian Unity INOVA
1588 (m), 1575 (m), 1476 (m), 1427 (m), 1248 (s), 1159 (s,
νasym(SO3)), 1113 (ss, ν(PO)), 988 (s, νsym(SO3)), 747 (s), 669 (m).
[(P∧O)PdMe(OPOct3)] (1-OPOct3; P∧O = κ2-P,O-2-(2-
MeOC6H4)2PC6H4SO3)). A suspension of 92 mg (0.08 mmol, 0.5 equiv)
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of [{(1-Cl)-μ-Na}2], 32 mg (0.16 mmol, 1.0 equiv) of AgBF4, and
64 mg (0.16 mmol, 1.0 equiv) of OPOct3 in 5 mL of CH2Cl2 was
stirred for 30 min in the dark. The resulting precipitate was filtered off
to afford a yellow solution. The filtrate was evaporated and the
resulting residue was washed with pentane and dried under vacuum to
yield 1-OPOct3 as a beige solid (122 mg, 0.13 mmol, 81%).
radiation source (Mo Kα, λ = 0.710 73 Å) and an image plate
detection system. The structures were solved by Patterson and direct
methods (SHELXS-97),62 completed with difference Fourier
syntheses, and refined with full-matrix least squares using SHELXL-
9763 minimizing w(Fo − Fc2)2.
2
3
4
1H NMR (400 MHz, CD2Cl2): δ 8.03 (dd, JHH = 7.6, JPH = 4.7,
1H, 6-H), 7.67 (br, 2H, 12-H), 7.51 (vt, J = 7.8, 2H, 10-H), 7.42 (vt,
J = 7.2, 1H, 5-H), 7.35−7.21 (m, 2H, 4-H and 3-H), 7.01 (vt, J = 7.5,
ASSOCIATED CONTENT
■
S
* Supporting Information
3
4
Text, tables, figures, and CIF files giving detailed experimental
procedures and analytical data, detailed polymerization
procedures, structural diagrams, selected bond lengths and
angles, and crystallographic data/processing parameters for all
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
2H, 11-H), 6.93 (dd, JHH = 8.2, JPH = 4.7, 2H, 9-H), 3.60 (s, 6H, Ar-
OCH3), 1.96−1.84 (m, 6H, 13-H), 1.69−1.56 (m, 6H, 14-H), 1.49−1.40
(m, 6H, 15-H), 1.38−1.21 (m, 24H, 16-H and 17-H & 18-H and 19-H),
3
0.88 (t, JHH = 6.7, 9H, 20-H), 0.14 (s, 3H, Pd-CH3). 31P{1H} NMR
(162 MHz, CD2Cl2): δ 65.3 (br, OPOct3), 26.1 (PAr). 13C{1H} NMR
2
(101 MHz, CD2Cl2): δ 160.9 (d, JPC = 1.7, C8), 149.2 (br, C1), 138.4
(br, C12), 135.0 (d, 2JPC = 2.7, C3), 133.6 (C10), 130.5 (C5), 128.5 (br
3
1
3
d, JPC = 8.5, C4), 128.2 (d, JPC = 52.8, C2), 128.0 (d, JPC = 8.9, C6),
AUTHOR INFORMATION
3
1
3
■
120.8 (d, JPC = 12.5, C11), 117.2 (d, JPC = 59.7, C7), 111.9 (d, JPC
=
4.6, C9), 55.7 (Ar−OCH3), 32.4 (C18), 31.7 (d, 3JCP = 14.3, C15), 29.7
(C16 & C17), 27.8 (d, 1JPC = 64.5, C13), 23.2 (C19), 22.3 (d, 2JPC = 3.8,
C14), 14.4 (C20), 0.9 (br, Pd−CH3). Anal. Calcd for C45H72O6P2PdS: C,
59.43; H, 7.98. Found: C, 58.18; H, 7.87. MS (FAB): m/z 908 [M]+, 893
[M − Me]+, 773 [H(OPOct3)2]+, 522 [M − OPOct3]+, 507 [M −
OPOct3 − Me]+, 387 [OPOct3 + H]+. ATR-IR: 1/λ (cm−1) 2923(m),
2853 (m), 1588 (w), 1574 (w), 1476 (m), 1464 (m), 1430 (m), 1262 (s),
1251 (s), 1159 (s, νasym(SO3)), 1107 (ss, ν(PO)), 1001 (s, νsym(SO3)),
755 (s), 669 (m).
Corresponding Author
*E-mail: stefan.mecking@uni-konstanz.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.N. acknowledges support by the state of Baden-Wurttemberg
̈
by a Landesgraduiertenforderung-Stipend. This work is financi-
̈
ally supported by the DFG (Me 1388/10). S.M. is indebted to
the Fonds der Chemischen Industrie. We thank D. Guironnet
for providing crystals of 1-MeOH.
REFERENCES
■
(1) Mülhaupt, R. Macromol. Chem. Phys. 2003, 204, 289−327.
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PC6H4SO3)). A suspension of 131 mg (0.10 mmol, 0.5 equiv) of [{(1-Cl)-
μ-Na}2], 57 mg (0.20 mmol, 1.0 equiv) of OPPh3, and 40 mg
(0.21 mmol, 1.0 equiv) of AgBF4 in 20 mL of CH2Cl2 was stirred for
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was suspended in 20 mL of pentane and stirred for 3 h. The precipitate
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as a slightly yellow solid (125 mg, 0.16 mmol, 80%). Crystals suitable
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after layering with pentane.
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1H NMR (600 MHz, CD2Cl2): δ 8.04−8.00 (m, 1H, 6-H), 7.82−
7.75 (m, 6H, 14-H), 7.63−7.48 (m, 13H, 10-H and 12-H & 15-H and
16-H), 7.44−7.40 (m, 1H, 5-H), 7.31−7.24 (m, 2H, 3-H and 4-H),
6.98 (vt, J = 7.2, 2H, 11-H), 6.92 (dd, 3JHH = 7.5, 4JPH = 4.7, 2H, 9-H),
3.57 (s, 6H, Ar−OCH3), 0.13 (br s, 3H, Pd-CH3). Note that the Pd-
Me shift is very sensitive to small amounts of impurities, e.g. H2O.
31P{1H} NMR (162 MHz, CD2Cl2): δ 34.9 (br, OPPh3), 27.0 (PAr).
Cavallo, L.; Mecking, S.; Gottker-Schnetmann, I. Proc. Natl. Acad. Sci.
̈
U.S.A. 2011, 108, 8955−8959. (d) Zhou, X.; Jordan, R. F.
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M. J.; Frohlich, R.; Rieger, B.; Kehr, G.; Erker, G. Organometallics
̈
2011, 30, 5248−5257. (f) Anselment, T. M. J.; Wichmann, C.;
Anderson, C. E.; Herdtweck, E.; Rieger, B. Organometallics 2011, 30,
6602−6611. (g) Anselment, T. M. J.; Anderson, C. E.; Rieger, B.;
Boeddinghaus, M. B.; Fassler, T. F. Dalton Trans. 2011, 40, 8304−
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Chem. 2010, 4595−4601. (i) Nagai, Y.; Kochi, T.; Nozaki, K.
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13C{1H} NMR (151 MHz, CD2Cl2): δ 160.9 (C8), 148.9 (br, C1),
138.4 (br, C12), 135.0 (C3), 133.7 (C10), 133.0 (d, 2JPC = 10.2, C14),
1
132.9 (C16), 131.8 (d, JPC = 110.9, C13), 130.5 (C5), 129.1 (d,
3JCP = 12.6, C15), 128.7 (br, C4), 128.1 (br, C6), 128.0 (d, 1JCP = 53.7,
3
1
C2), 120.9 (d, JPC = 12.3, C11), 117.0 (d, JPC = 59.3, C7), 111.9
(d, 3JPC = 4.3, C9), 55.7 (Ar-OCH3), 1.1 (br, Pd-CH3). Anal. Calcd for
C39H36O6P2PdS: C, 58.47; H, 4.53. Found: C, 58.47; H, 4.81. MS
(FAB): m/z 802 [M]+, 785 [M − Me]+, 522 [M − OPPh3]+, 507
[M − OPPh3 − Me]+, 278 [OPPh3 + H]+. ATR-IR: 1/λ (cm−1) 3062
(w), 1587 (m), 1574 (m), 1476 (m), 1434 (m), 1253 (s), 1162
(s, νasym(SO3)), 1150 (s, ν(PO)), 1115 (s), 998 (s, νsym(SO3)), 755
(s), 723 (ss), 694 (s), 670 (s).
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2007, 129, 8948−8949.
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2009, 131, 14606−14607.
(8) Runzi, T.; Frohlich, D.; Mecking, S. J. Am. Chem. Soc. 2010, 132,
̈
̈
17690−17691.
X-ray Crystallography. Crystals of 1-OPBu3, 1-OPPh3, and
1-MeOH were grown as described in the corresponding experimental
section. X-ray diffraction analyses were performed at 100 K on a STOE
IPDS-II diffractometer equipped with a graphite-monochromated
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polymerization with 1-dmso by addition of saturated compounds with
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(52) An analogous but stepwise transformation could be observed by
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10−3, KC1,dmso,80 °C = 1.9, and KC2,dmso,80 °C (=KAKC1) = 2 × 10−3.18 For
L = OPPh3, with the binding strength of OPPh3 vs dmso at 80 °C
being 3.5 × 10−1, equilibrium constants are calculated to be
KA,OPPh ,80 °C ( ≈KB ≈ KD) = 3.1 × 10−3, KC1,OPPh ,80 °C = 6.7 × 10−1,
3
3
and KC2,OPPh ,80 °C (=KAKC1) = 2 × 10−3.
3
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