Monohapto-allyl Pd(II) complexes with bidentate hybrid P,N ligands

Article abstract of DOI:10.1039/b212393m

Using a new oxazoline-phosphonite P,N ligand, we establish that in contrast with commonly held belief, bidentate ligands can lead to unusually stable η¹-allyl complexes of Pd(II); CO insertion into their Pd-C σ-bond affords the corresponding 3-butenoyl complexes under mild conditions.

Full text of DOI:10.1039/b212393m

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Monohapto-allyl Pd(II) complexes with bidentate hybrid P,N
ligands†
Pierre Braunstein,a Jing Zhanga and Richard Welterb
a Laboratoire de Chimie de Coordination, UMR CNRS 7513, Université Louis Pasteur,
4 rue Blaise Pascal, F-67070 Strasbourg Cédex, France. E-mail: braunst@chimie.u-strasbg.fr
b Laboratoire DECMET, UMR CNRS 7513, Université Louis Pasteur, 4 rue Blaise Pascal,
F-67070 Strasbourg Cédex, France
Received 13th December 2002, Accepted 9th January 2003
First published as an Advance Article on the web 21st January 2003
Using a new oxazoline-phosphonite P,N ligand, we estab-
lish that in contrast with commonly held belief, bidentate
ligands can lead to unusually stable ꢀ1-allyl complexes
of Pd(II); CO insertion into their Pd–C ꢁ-bond affords
the corresponding 3-butenoyl complexes under mild
conditions.
Transition metal allyl complexes display a rich chemistry and
have a considerable importance in homogeneous catalysis
where they often represent key intermediates.1 Whereas η3 bond-
ing mode is the most general for this ligand, η1-allyl complexes
have been isolated mainly with platinum,2 rhodium,3 very
recently with iridium,4 or early transition metals.5 The bonding
features of an allyl fragment to a transition metal affect the
stereochemistry of reactions proceeding via allyl intermediates
and it is well known in Pd-allyl chemistry that an η3–η1–η3
dynamic behaviour has considerable relevance to enantio-
selection.1 It is therefore essential to recognize the exact bond-
ing mode of the allyl ligand in a complex in order to understand
or rationalize its reactivity. In Pd chemistry, the η3-bonding
mode of the allyl ligand is the rule and only very few Pd com-
Fig. 1 ORTEP18 view of the structure of [Pd(η1-C3H5}Cl(NOPMe2
-
N,P)] 1. Thermal ellipsoids shown at 50% probability. Selected bond
lengths (Å) and angles (Њ): Pd(1)–P(1) 2.200(1), Pd(1)–N(1) 2.180(2),
Pd(1)–Cl(1) 2.387(1), Pd(1)–C(21) 2.070(2), C(21)–C(22) 1.475(3),
C(22)–C(23) 1.322(4); C(21)–Pd(1)–P(1) 92.31(7), C(21)–Pd(1)–Cl(1)
88.43(7), N(1)–Pd(1)–P(1) 83.22(5), N(1)–Pd(1)–Cl(1) 96.15(5), Pd(1)–
C(21)–C(22) 101.5(1), C(21)–C(22)–C(23) 126.5(2), Pd(1)–P(1)–O(1)
112.5(6), P(1)–O(1)–C(13) 123.2(1), O(1)–C(13)–C(16) 107.9(1), C(13)–
C(16)–N(1) 125.1(2), C(16)–N(1)–Pd 121.5(1).
plexes containing η1-allyl ligands have been characterized in
solution,6 and even more rarely in the solid-state,7,8 despite their
considerable importance as reactive species or proposed inter-
mediates in C–C coupling reactions.1 Although the idea that a
strong and rigid tridentate ancillary ligand coordinated to a
Pd() centre will trigger the occurrence of the rare η1-bonding
mode of the allyl ligand is completely logical,6,7 we now estab-
lish that this does not represent a prerequisite. Reaction of the
new bidentate ligand 1-(4,4Ј-dimethyl-4,5-dihydrooxazol-2-yl)-
1-methyl)diphenylphosphonite,9 abbreviated NOPMe2
, with
question of the need or not for a tridentate ligand to stabilize
η1-allyl Pd() complexes. With the ligand bis(2-oxazoline-4,4-
dimethyl-2-hydroxydimethyl)phenylphosphonite, abbreviated
NOPONMe2, we recently characterized another η1-allyl chloro
Pd complex, 2, in which NOPONMe2 did not behave as a tri-
dentate, but as a chelating ligand with a dangling oxazoline
moiety.8a
10
[Pd(η3-C3H5)(µ-Cl)]2 afforded in 89% yield the new η1-allyl
chloro Pd() complex 1 which has been fully characterized (see
ESI),† including by X-ray diffraction.‡ There are two different
but almost identical molecules in the asymmetric unit. One of
them is represented in Fig. 1.
The 31C NMR spectrum is very diagnostic for a η1-allyl
ligand, with characteristic chemical shifts at δ 29.5, 105.9, and
141.0 for the Pd–CH , ᎐CH and –CH᎐ carbons.8a Consistent
᎐
᎐
2
2
with Pearson’s antisymbiotic effect,11 the η1-allyl ligand is trans
to the nitrogen donor (Fig. 1), a ligand of weaker trans influ-
ence than phosphorus. These results allow us to answer the
This behaviour was not due to an impossibility for NO-
PONMe2 to function as a planar tridentate ligand since
[Pd(NCMe)(NOPONMe2-N,P,N )](BF4)2 3 is perfectly stable.8a
Although the dangling oxazoline arm in 2 was involved in the
dynamic behaviour of the complex, our present results indicate
that it did not play any significant role in promoting the η1-allyl
bonding mode. This establishes that the paradigm for a strong
† Electronic supplementary information (ESI) available: preparations
and selected spectroscopic data for 1 and 4–6. See http://www.rsc.org/
suppdata/dt/b2/b212393m/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Fig. 2 ORTEP18 view of the structure of [Pd(η3-C3H5}(NOPMe2
-
Fig.
3
ORTEP18 view of the structure of [Pd{C(O)C3H5}-
N,P)]PF6 4. Thermal ellipsoids shown at 50% probability. Selected bond
lengths (Å) and angles (Њ): Pd–P 2.244(1), Pd–N 2.112(2), Pd–C(21)
2.102(3), Pd–C(22) 2.144(4), Pd–C(23) 2.268(3), C(21)–C(22) 1.330(5),
C(22)–C(23) 1.245(6); N–Pd–P 91.41(5), N–Pd–C(21) 173.4(1), N–Pd–
C(23) 106.5(1), P–Pd–C(21) 95.17(9), P–Pd–C(22) 130.1(1), P–Pd–
C(23) 162.1 (1), C(21)–Pd–C(23) 66.9(1), P–O(1)–C(13) 123.3(1), O(1)–
C(13)–C(16) 109.0(1), C(13)–C(16)–N 129.0(2).
Cl(NOPONMe2-N,P)] 6. Thermal ellipsoids shown at 50% probability.
Selected bond lengths (Å) and angles (Њ): Pd–P 2.210(1), Pd–N(1)
2.222(1), Pd–Cl 2.372(1), Pd–C(23) 1.983(2), C(23)–O(5) 1.201(2),
C(23)–C(24) 1.535(3), C(24)–C(25) 1.493(3), C(25)–C(26) 1.306(3);
C(23)–Pd–P 88.69(5), C(23)–Pd–Cl 85.29(5), N(1)–Pd–P 88.49(4),
N(1)–Pd–Cl 98.20(4), Pd–P–O(1) 112.07(5), Pd–P–O(3) 112.94(5).
tridentate ligand being required to observe the η1-allyl bonding
mode in Pd() chemistry is no longer valid.
These carbonylation reactions under very mild conditions
contrast with the inactivity of [Pd(η3-2-MeC3H4)Cl(PMe3)]15
and support the view that CO insertion into η3-allyl palladium
cationic complexes occurs via first coordination of the counter
ion to form an η1-allyl intermediate. This hypothesis was pre-
sented Ozawa, Yamamoto and co-workers although they had
not succeeded in isolating such η1-allyl intermediates.15 In
As expected, chloride abstraction from 1 led to the cationic
η3-allyl Pd() complex 4 (Fig. 2).†‡ The longer Pd–C(23) dis-
tance compared to Pd–C(21) reflects the larger trans influence
of the P donor. Its 1H NMR spectrum in CDCl3 at 297 K shows
an AB spin system for the two methylenic protons of the oxazo-
line and the methyl region exhibits two peaks at δ 1.70 and 1.79
for the diastereotopic OC(CH3)2 protons. Unlike the situation
in [Pd(η3-C3H5}(NOPONMe2-N,P)]PF6,8a the five protons of the
allyl fragment of 4 give rise to five peaks at room temperature,
with the terminal protons cis to P exhibiting two doublets at
δ 2.89 (3JHH = 12.3 Hz) and 3.69 (3JHH = 6.9 Hz), respectively.
These data indicate either a static situation or slow rotation on
the NMR time scale of the allyl ligand about the (η3-C3H5)–Pd
axis and no η3–η1–η3 interconversion, the terminal syn- and
anti-protons remaining non-equivalent.12
contrast to e.g. trans-[Pd{C(O)CH CH᎐CH }Br(PMePh ) ],15
᎐
2
2
2 2
complexes 5 and 6 exhibit remarkable stability towards de-
carbonylation or decomposition, probably because of the
energetically favourable trans-P–Pd–Cl and trans-N–Pd–C
arrangements.8a Attempts to isolate further insertion products
with ethylene were unsuccessful, consistent with the lower
reactivity of neutral complexes,16 and prolonged reaction times
(>1 day) led to progressive decarbonylation and formation
of the corresponding η3-allyl complexes (in situ 1H NMR
monitoring).
Bubbling of CO through a solution of 1 in toluene led to the
In conclusion, we have clearly established that appropriate
bidentate chelating ligands are suitable to stabilize η1-allyl Pd
complexes and this still rare bonding situation may occur more
often than expected in numerous stoichiometric or catalytic
transformations involving Pd() allyl complexes. The import-
ance of halide effects on stoichiometric and catalytic reaction
involving transition metal complexes has been recently
reviewed.17 In particular, it is noteworthy that the syn-anti
isomerization of cationic, π-allyl Pd() complexes is consider-
ably enhanced in the presence of added chloride and that
catalytic amounts of halide were found to beneficially influence
both the regio- and enantio-selectivity of asymmetric allylic
alkylations.17
formation of the insertion product 5.†
Its ν(C᎐O) band at 1688 cmϪ1 is indicative of the formation of
᎐
an acyl ligand and a resonance at δ = 226.7 ppm in the 13C
NMR spectrum with a small 2J(P,C) coupling of 10.0 Hz shows
that the acyl group resides in a cis position relative to the phos-
phorus.13 For comparison, the related insertion product 6 was
prepared from 2 (Fig. 3).†‡ The large high-field shift (∆δ =
Ϫ20 ppm) of the 31P NMR resonance (δ = 118.4), compared to
2 (δ = 138.1), also indicates the cis arrangement of the acyl
group relative to the P atom.13a,14 The geometry around pal-
ladium approximates square-planar, with slight deviations aris-
ing from the angles P–Pd–C(23) and N(1)–Pd–Cl of 85.29(5)
and 98.20(4)Њ, respectively. The 1H NMR spectrum of 6 at room
temperature shows only one AB spin system assigned to the
four OCH2 protons.† The methyl region contains four lines
including a broad singlet for the diastereotopic NC(CH3)2,
Acknowledgements
We are grateful to the Ministère de la Recherche (Paris) for
a post-doctoral grant to J. Z., the Centre National de la
Recherche Scientifique (Paris) for financial support and to
R. Graff and J. D. Sauer for NMR experiments.
Notes and references
which is similar to the situation in cis-[PdCl2(NOPONMe2
-
‡ X-Ray structural analyses: the diffraction intensities were collected on
a Kappa CCD diffractometer using Mo-Kα graphite monochromated
radiation (λ = 0.71069 Å). The structures were solved by heavy-atom
Patterson methods and expanded Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
fixed in calculated positions with C–H = 0.98 Å.
N,P)],8a and indicates that the ligand behaves as a fluxional,
hemilabile P,N chelate. In both 5 and 6, the preference for
the soft carbon ligand to avoid a position trans to P is again
consistent with the antisymbiotic effect.11
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Crystal data for 1: C23H29ClNO2PPd, Mr = 524.29, triclinic, space
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¯
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β = 89.447(5), γ = 86.007(5)Њ, V = 2350.2(16) Å3, F(000) = 1072, Z = 4,
Dc = 1.482 g cmϪ3, µ = 0.990 mmϪ1, T = 173(2) K, R1 = 0.0335,
wR2 = 0.0853, GOF = 1.032 for 523 parameters.
Crystal data for 4: C23H29F6NO2P2Pd, Mr = 633.81, orthorhombic,
space group Pbca, a = 15.136(5), b = 17.392(5), c = 20.248(5) Å,
V = 5330(3) Å3, F(000) = 2560, Z = 8, Dc = 1.580 g cmϪ3, µ = 0.879 mmϪ1
R1 = 0.0393, wR2 = 0.1058, GOF = 0.942 for 316 parameters.
,
Crystal data for 6: C26H38ClN2O5PPd, Mr = 631.4, monoclinic, space
group P21/c, a = 14.643(2), b = 10.970(2), c = 18.216(3) Å, β = 97.017(5)Њ,
V = 2904.2(8) Å3, F(000) = 1304, Z = 4, Dc = 1.444 g cmϪ3, µ = 0.823
mmϪ1, T = 173(2) K, R1 = 0.0467, wR2 = 0.1215, GOF = 1.013 for 325
parameters. CCDC reference numbers 193036–193038. See http://
www.rsc.org/suppdata/dt/b2/b212393m/ for crystallographic data in
CIF or other electronic format.
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