Structurally characterized intermediates in the stepwise insertion of CO-ethylene or CO-methyl acrylate into the metal-carbon bond of Pd(II) complexes stabilized by (phosphinomethyl)oxazoline ligands

Article abstract of DOI:10.1039/b613865a

The initial CO-ethylene or CO-methyl acrylate insertion steps into the Pd-Me bond of methylpalladium(ii) complexes with (phosphinomethyl)oxazoline ligands, leading to metallacycles, have been fully characterized, including by X-ray diffraction. The Royal

Full text of DOI:10.1039/b613865a

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Structurally characterized intermediates in the stepwise insertion of
CO–ethylene or CO–methyl acrylate into the metal–carbon bond of
Pd(II) complexes stabilized by (phosphinomethyl)oxazoline ligands{
Magno Agostinho and Pierre Braunstein*
Received (in Cambridge, UK) 25th September 2006, Accepted 9th November 2006
First published as an Advance Article on the web 23rd November 2006
DOI: 10.1039/b613865a
and that of the Pd(II) complexes 3a,b (Scheme 2) in ethylene–CO
copolymerisation.7 Starting from 3a,b, we have now isolated the
initial intermediates in CO–ethylene or CO–methyl acrylate
copolymerisation reactions, without the need to use excess methyl
acrylate. The structures of the new insertion products 4b, 6b and
7a,b have been determined by X-ray diffraction as well as those of
the known 2a,b and 3a,b for comparative purposes.{
Reactions of 2a,b and 3a,b with CO in CH2Cl2 at room
temperature were monitored by 31P{1H} and 1H NMR. CO
insertion into their Pd–Me bond produced within a few minutes
the acyl complexes 4a,b and 5a,b, respectively (Scheme 2), as
evidenced by the large high-field shift of the 31P{1H} NMR
resonance (Dd = 214.7 4a, Dd = 214.8 4b, Dd = 215.4 5a, Dd =
215.1 5b, Table 1).
The initial CO–ethylene or CO–methyl acrylate insertion steps
into the Pd–Me bond of methylpalladium(II) complexes with
(phosphinomethyl)oxazoline ligands, leading to metallacycles,
have been fully characterized, including by X-ray diffraction.
The palladium-catalyzed alternating copolymerisation of olefins
and carbon monoxide, which leads to the formation of
polyketones, has become a major field of research in both
academic and industrial laboratories.1
Although the basic reaction mechanism of CO–olefin copoly-
merisation, which involves mutually cis sites of square-planar
Pd(II) species, has been established,1,2 detailed investigations on the
early stages of the chain-growth process have mostly been carried
out with strained alkenes owing to the difficulties often encoun-
tered in the isolation of intermediates. The first structural reports of
an ethylene–CO coupling product by Green et al., using mono-
dentate PPh3 and a N,O ligand,3a then by us using a P,O ligand3b or
a diphosphine-bridged heterodimetallic Fe–Pd complex3c were
followed by only a few examples with P,N,3d,e N,N3f and P,P3g
chelating ligands. Furthermore, despite the considerable interest in
the copolymerisation of olefins with polar monomers, such as
methyl acrylate,2e,4 only a few CO–methyl acrylate coupling
products have been isolated and characterized.2c,3b,c,5 Using a
bidentatephosphine-imine(P,N)ligand, Reddyetal. havereported
what appears to be the only structure of a CO–methyl acrylate
coupling product.3e These authors used a large excess of olefin (33
to 67 equiv.) in CH2Cl2, with a reaction time between 1–3 h.
Following the synthesis of the ligands 1a,b and of the Pd(II)
methyl complexes 2a,b (Scheme 1), we investigated the catalytic
activity of Ni(II) complexes with 1b in ethylene oligomerisation,6
In order to detect the usually elusive palladium acyl, carbonyl
complexes of the type [Pd{C(O)Me}(CO)(P,N)]OTf, a CD2Cl2
solution of 3b was exposedtoanatmosphereof13CO, and 31P{1H},
13C{1H} and 1H NMR spectra were recorded at different
temperatures. At room temperature, the 31P{1H} NMR resonance
for 5b is a doublet centred at d 21.6 (2JPC = 9.7 Hz). Its acyl carbon
appears in the 13C{1H} spectrum as an intense doublet at d 222.7
(2JPC = 9.7 Hz), and the weak signal observed at d 179.8
corresponds to the coordinated CO of the acyl, carbonyl derivative.
Upon decreasing the temperature to 260 uC, the 31P{1H}
resonance significantly broadened and shifted to d 19.1 while the
13C{1H} resonances for the acyl carbon and the coordinated CO
also became broader and shifted to d 222.6 and 175.5, respectively.
This is indicative of an equilibrium between CO and OTf
coordination which, at 2100 uC, is completely shifted towards
the acyl, carbonyl species and the 31P{1H} resonance becomes a
doublet of doublets centred at d 18.3 (2JPC(cis) = 4.1, 2JPC(trans)
=
83.8 Hz) while the acyl carbon and coordinated CO 13C{1H}
resonances appear as doublets at d 223.1 and 175.3, respectively
(see ESI for details).{ These results are in agreement with those for
related complexes stabilized by P,P ligands,8 but at variance with
those with a P,N ligand in which no significant amount of
palladium acyl, carbonyl species was detected at 270 uC.3d
Complexes 2a,b, 3a,b and 4b have slightly distorted square
planar coordination geometries with the methyl (2a,b and 3a,b)
and acyl (4b) ligands cis to the phosphorus atom (ESI),{ in
agreement with the donor groups with the largest trans influence
avoiding a mutually trans position, as observed in other complexes
of the type [Pd(Me)Cl(P,N)].3e,9 The Pd–N bond distance in 2a
Scheme 1
Laboratoire de Chimie de Coordination (UMR 7177 CNRS), Institut de
Chimie, Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070
Strasbourg Ce´dex, France. E-mail: braunst@chimie.u-strasbg.fr;
Fax: +33 390 241 322
{ Electronic supplementary information (ESI) available: Complete sets of
crystallographic parameters for 2a,b, 3a,b, 4b, 6b, 7a,b; experimental
procedures and spectroscopic characterizations. See DOI: 10.1039/
b613865a
(Table 2) is longer than that reported for the analogous PdCl2
7
complex [2.058(2) A], which reflects the larger trans influence of
˚
the methyl group compared with chloride. The Pd–C distance of
58 | Chem. Commun., 2007, 58–60
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Scheme 2 All reactions were performed at room temperature in CH2Cl2.
Ethylene or methyl acrylate insertion into the Pd–acyl bond of
5a and 5b was completed in less than 1 h at room temperature
under atmospheric pressure (31P NMR monitoring) and afforded
6a,b or 7a,b, respectively (Scheme 2). In all four complexes,
coordination of the ketonic oxygen atom to Pd (see nCO, Table 1)
results in a stabilizing chelate which makes b-hydrogen elimination
less likely.11 These complexes are stable at room temperature for
several hours in solution and weeks in the solid state, which
illustrates the beneficial role of the P–N chelates. The 31P{1H}
NMR signals of 6a,b and 7a,b are shifted to low field relative
Table 1 Selected IR and NMR data of the ligands and complexes
IR
NMRd
1H
nCN
nCO
31P
1a 1660a (s)
1b 1660a (s)
2a 1647a (s)
2b 1628a (s)
3a 1633b (s)
3b 1632b (s)
215.8
215.8
33.1
32.8
37.4
37.4
18.4
18.0
22.0
22.3
3
0.55 Pd–CH3 (d, JPH = 2.7)
3
0.65 Pd–CH3 (d, JPH = 3.3)
0.60 Pd–CH3 (s)
0.70 Pd–CH3 (s)
4
4a 1642b (s) 1684b (s) 2.20 PdC(O)CH3 (d, JPH = 1.1)
4b 1632b (s) 1685b (s) 2.17 PdC(O)CH3 (d, JPH = 1.6)
4
1
to those of 5a,b (Table 1). In the H NMR spectrum of 6a,b the
5a 1644a (s) 1704a (s)
5b 1655a (s) 1707a (s)
Pd–CH2 protons give rise to a triplet of doublets (d 1.65, 6a and
d 1.67, 6b) whereas the CH2CLO protons appear as a broad triplet
nCN/CO
6a 1634b (s)
6b 1629b (s)
nC(O)OMe
4+5
2.45 C(O)CH3 (s)
2.49 C(O)CH3 (s)
34.4
34.7
32.8
34.3
(d 3.08, 6a and d 3.12, 6b), indicating a smaller
J
HP
coupling.{
The CH and CH2 protons Ha, Hb and Hc of 7a,b were
unambiguously identified and resonate at d 2.46, 2.90 and 3.26 (7a)
and d 2.46, 2.87 and 3.27 (7b) respectively (vicinal and geminal JHH
and JHP coupling constants are given in the ESI).{
7a 1633b (s) 1683b (s) 2.52 C(O)CH3 (s)
7b 1629b (s) 1684b (s) 2.55 C(O)CH3 (s)
a
b
d
In CH2Cl2. In KBr, cm21
.
In CDCl3, ppm, J in Hz.
The crystal structures of 6b and 7a,b (see Fig. 1) were
determined by X-ray diffraction and the latter two established
the regioselective 2,1 insertion of methyl acrylate, which leads to an
a-methoxycarbonyl complex. Deviations from idealized square
planar geometries are small (Table 2). The similar Pd–C distances
in 6b, 7a and 7b are in agreement with that in the only other
˚
1.9701(13) A in 4b, although slightly shorter than in the analogous
Pd–Me complex (2b), is normal for an acetyl–palladium bond. The
acyl group adopts an orientation approximately perpendicular to
the metal coordination plane, as observed in other complexes of
the type [Pd{C(O)Me}Cl(P,N)].3e,10
˚
Table 2 Selected bond lengths [A] and angles [u] in complexes 2a,b, 3a,b, 4b, 6b and 7a,b
2a
2b
3a
3b
4b
6b
7a
7b
Pd1–N1
Pd1–P1
Pd1–C17
Pd1–O2
2.103 (12)
2.212 (4)
2.057 (13)
2.171 (3)
2.1879 (10)
2.049 (4)
2.131 (4)
2.1699 (12)
2.026 (5)
2.156 (3)
2.1550 (18)
2.1771 (12)
2.031 (2)
2.1896 (11)
2.2549 (13)
1.9701 (13)
2.104 (4)
2.1878 (14)
2.030 (5)
2.125 (4)
2.082 (3)
2.2051 (10)
2.046 (4)
2.112 (2)
2.114 (3)
2.2091 (14)
2.052 (4)
2.138 (3)
2.1631 (17)
Pd1–Cl1
N1–C3
C3–C4
C4–P1
C17–C18
C18–C19
C19–O2
2.383 (4)
1.291 (18)
1.468 (19)
1.847 (13)
2.378 (1)
1.279 (5)
1.485 (5)
1.835 (4)
2.3734 (13)
1.2757 (15)
1.4897 (15)
1.8468 (12)
1.506 (2)
1.281 (6)
1.476 (6)
1.839 (5)
1.275 (2)
1.493 (3)
1.837 (2)
1.270 (6)
1.480 (7)
1.841 (5)
1.527 (8)
1.485 (8)
1.248 (6)
1.285 (5)
1.496 (5)
1.837 (4)
1.545 (6)
1.472 (6)
1.220(5)
1.273 (6)
1.498 (6)
1.835 (4)
1.528 (7)
1.486 (7)
1.239 (6)
N1–Pd1–P1
N1–Pd1–C17
N1–Pd1–O2
N1–Pd1–Cl1
P1–Pd1–O2
P1–Pd1–C17
P1–Pd1–Cl1
C17–Pd1–O2
C17–Pd1–Cl1
82.2 (3)
176.4 (5)
82.29 (9)
171.94 (14)
84.85 (11)
174.97 (17)
95.14 (14)
83.03 (6)
174.30 (8)
94.79 (7)
80.66 (3)
173.83 (4)
83.91 (12)
177.1 (2)
100.03 (14)
82.13 (9)
174.17 (15)
96.10 (12)
81.99 (10)
175.94 (15)
100.26 (13)
93.2 (3)
97.94 (9)
97.09 (3)
172.64 (9)
90.68 (15)
177.32 (4)
91.69 (8)
174.72 (10)
93.30 (16)
175.29 (8)
99.24 (11)
177.54 (9)
97.25 (14)
94.5 (4)
174.66 (15)
89.70 (13)
179.21 (4)
96.35 (4)
173.211 (11)
89.00 (18)
90.42 (8)
82.80 (18)
82.10 (14)
80.57 (16)
90.1 (5)
90.09 (13)
86.48 (4)
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Fig. 1 Molecular structure of: a) complex 3b, b) the cation in 6b, c) the cation in 7b (CH groups of phenyl rings and H atoms omitted, except inserted
olefin). Displacement ellipsoids are drawn at the 50% probability level.
reported structure of a CO–methyl acrylate complex.3e The Pd–O
bond distances in 6b, 7a and 7b are similar and compare with those
in related complexes stabilized by P,N ligands.3d,e,12 At least in the
solid state, there is no interaction between the CO2Me group of
7a,b and the metal centre, which would have reduced its
electrophilicity and increased its steric shielding. It is also
interesting to note that related Pd(II) complexes with the P,O
chelating ligand Ph2PNHC(O)Me were generally found to be less
reactive than 3a,b or 5a,b, longer reaction times being required.3b
In addition to the structural characterization of CO–ethylene
and CO–methyl acrylate insertion products, we have spectro-
scopically observed a temperature-dependent equilibrium between
a triflate, acyl complex, 5b, and a cationic carbonyl, acyl
Pd(II) complex. Previous studies have shown that 3a,b catalyse
the CO–ethylene copolymerisation at 60–90 uC.6 Further investi-
gations are in progress to determine the influence of the ligand bite
angle on the reactivity of the chelate ring in 7a,b towards further
insertion of small molecules.
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We thank Luc Brissieux for preliminary results and the CNRS,
the Ministe`re de la Recherche (Paris) and the Europeen
Commission (Palladium Network HPRN-CT-2002-00196 and
COST program) for support. We are grateful to Prof. R. Welter
and Dr A. DeCian (ULP Strasbourg) for the crystal structure
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60 | Chem. Commun., 2007, 58–60
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