Chelating behaviour of a phosphinoalcohol leading to a stable alcohol palladium complex

Article abstract of DOI:10.1016/S0022-328X(00)00755-5

Using the enantiopure phosphinoalcohol ligand (P,OH) (1) [(P,OH) = [(1S),(2S),(4R)-1-OH-1-Me-2-PPh₂-4-C(Me)=CH₂-c-C ₆H₇], the stable Pd(II) complexes [(dmba)Pd(P,OH)Cl] (2) (dmba = o -C₆H₄CH₂NMe₂), [(dmba)Pd(P,O H)]PF₆ (3) and [(dmba)Pd(P,O)] (4) have been prepared. Conductivity and NMR measurements showed that complex 2 gives rise to a solvent-dependent equilibrium in solution between the neutral and the cationic species [(dmba)Pd(P,OH)Cl] (2a) and [(dmba)Pd(P,O H)]Cl (2b), respectively. The crystal structures of 2b and [(dmba)Pd(P,O H)]PF₆ (3) have been determined by X-ray diffraction. In the solid state, both cationic complexes display chelating behaviour for the P,OH ligand.

Full text of DOI:10.1016/S0022-328X(00)00755-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 621 (2001) 218–223
Chelating behaviour of a phosphinoalcohol leading to a stable
alcohol palladium complex
Chris Mattheis, Pierre Braunstein *
Laboratoire de Chimie de Coordination, UMR CNRS 7513, Uni6ersite´ Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received 4 September 2000; accepted 13 September 2000
Dedicated to Professor H. Brunner on the occasion of his 65th birthday, with our best wishes and most sincere congratulations
Abstract
Using the enantiopure phosphinoalcohol ligand (P,OH) (1) [(P,OH)=[(1S),(2S),(4R)-1-OH-1-Me-2-PPh2-4-C(Me)ꢀCH2-c-
C6H7], the stable Pd(II) complexes [(dmba)Pd(P,OH)Cl] (2) (dmba=o-C6H4CH2NMe2), [(dmba)Pd(P,OH)]PF6 (3) and
[(dmba)Pd(P,O)] (4) have been prepared. Conductivity and NMR measurements showed that complex 2 gives rise to a
solvent-dependent equilibrium in solution between the neutral and the cationic species [(dmba)Pd(P,OH)Cl] (2a) and
[(dmba)Pd(P,OH)]Cl (2b), respectively. The crystal structures of 2b and [(dmba)Pd(P,OH)]PF6 (3) have been determined by X-ray
diffraction. In the solid state, both cationic complexes display chelating behaviour for the P,OH ligand. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Synthesis; Chiral ligands; Ditopic ligands; Functional phosphines; Palladium; X-ray structures
1. Introduction
phosphinoalcohol ligand 1 was introduced by coordina-
tion of the phosphine moiety to the metal centre, then
the Pd–O bond is created by reaction with a base
[3c–f]. The intermediate phosphinoalcohol palladium
complex 2 could also be isolated. An interesting feature
of this complex is the strong tendency for chelation
displayed by the ligand which leads after dissociation of
the chloride ion to the formation of a cationic complex
in the solid state. This structure is partly retained in
solution.
Palladium and platinum alkoxide complexes have
aroused special interest because they offer a rich chem-
istry with versatile reactivity [1]. They undergo for
instance insertion reactions of carbon monoxide, car-
bon dioxide and olefins into the metal–oxygen bond
and are assumed to be intermediates in several catalytic
and stoichiometric reactions [2]. The difficulties often
encountered in isolating them are mainly due to their
sensitivity towards b-hydride elimination reactions [1].
However, an appropriate choice of ligands has recently
allowed the preparation of several palladium and plat-
inum alkoxide complexes [1]. One possible approach in
this context is the use of chelating P, O ligands [3].
In the course of current studies with phosphinoalco-
hols, we discovered the advantages of the easily accessi-
ble limonene-based ligand 1 [4]. The tertiary character
of its alcohol function prevents b-hydride elimination
and enabled us to obtain the stable palladium alkoxide
4. It was prepared in a stepwise approach: first the
2. Results and discussion
2.1. Synthesis
Using a modified literature method we prepared the
phosphinoalcohol ligand (1S)-trans-(+)-1-hydroxy-1-
methyl-2-diphenylphosphino-4-isopropene-cyclohexane
(1), (abbreviated P,OH in the following), which is
derived from trans-(+)-limonene oxide [4]. Reaction of
1 with [(dmba)Pd(m-Cl)]2 (dmba=o-C6H4CH2NMe2) in
the ratio 2:1 in dichloromethane at room temperature
led to the formation of an air-stable complex 2a in 93%
yield (see Scheme 1).
* Corresponding author. Tel. and fax: +33-388-416030.
E-mail address: braunst@chimie.u-strasbg.fr (P. Braunstein).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00755-5

C. Mattheis, P. Braunstein / Journal of Organometallic Chemistry 621 (2001) 218–223
219
2.2. NMR in6estigations
In order to obtain information about the structure of
2, a comparison of the 31P{1H}-NMR data of 2, 3 and
4 recorded in different solvents was most revealing
(Table 1). The signal for the neutral alkoxide complex 4
appears in CDCl3 at l 58.8 ppm. No significant shift is
observed in C6D6 (l 59.4 ppm). The signal for the
cationic 3 appears at l 48.7 ppm in CDCl3 and at l
48.5 ppm in C6D6. However, the situation changes
dramatically for 2: whereas in the polar solvents CDCl3
or CD3OD the resonance appears at l 49.5 ppm, a
highfield shift of about 6 ppm is observed in C6D6. The
similarity of the chemical shifts between 2 in polar
solvents and 3 indicates that chloride dissociation read-
ily occurs in such a medium. This situation corresponds
to the solid state structure determined for 2b (vide
infra). Taking into account the high field shift of the
signals on going from 4, where the oxygen is strongly
covalently bonded to palladium, to 3 (and 2 in polar
solvents), where the oxygen is less strongly bonded, the
further highfield shift for 2 in apolar C6D6 suggests a
structure with less strong O“Pd or no oxygen bonding
at all. We assume an equilibrium between the neutral
and the cationic form for 2 in solution (Eq. (3)). The
31P{1H}-NMR shifts are consistent with those observed
for the related neutral and cationic ketophosphine com-
Scheme 1.
The related cationic complex [(dmba)Pd(P,OH)]PF6
(3) was readily prepared in 80% yield by reaction of 2a
with TlPF6 (Eq. (1)). This complex forms colourless
needles from dichloromethane–n-hexane.
(1)
plexes [(dmba)PdCl{Ph2PCH2C(O)Ph}] (l 29.5 ppm)
¸¹¹¹¹¹¹¹¹º
and [(dmba)Pd{Ph2PCH2C(O)Ph}]CF3SO3 (l 34.8
ppm), respectively [5]. Clearly, polar solvents lead 1 to
bind in a chelating mode by dissociation of the palla-
dium–chloride bond and form 2b is stabilized whereas
in non-polar solvents, the neutral form 2a of this
complex is preferred, and the phosphinoalcohol ligand
is only bound via the phosphorus atom and the chlo-
ride ligand remains bonded to the palladium.
The neutral complex [(dmba)Pd(P,O)] (4) was ob-
tained in 69% yield by treatment of 2a with potassium
hydride. The deprotonated P,O ligand now acts as a
three-electron donor chelate (Eq. (2)).
(2)
Table 1
(3)
Selected 31P{1H}- and 13C-NMR data of 2, 3 and 4 (l in ppm)
31P{1H}
13C C(1)
C(2)
C(7)
1
2
CDCl3
CDCl3
CDCl3
C6D6
CD3OD 48.7
CDCl3
C6D6
CDCl3
C6D6
11.2
49.5
49.3
43.8
72.9
79.0
42.7 (d, 13.8 Hz)
48.5 (d, 24.2 Hz)
29.4
22.2
a
77.3
80.7
80.1
48.2 (d, 24.4 Hz)
not found
48.3 (d, 24.4 Hz)
22.0
22.8
22.0
With the objective of confirming the equilibrium of
Eq. (3) using 13C- and 1H-NMR spectroscopy, we
observed a shift of the C(1) resonance to lower field
when compared to the free ligand (Tables 1 and 2). But
all signals of 2 and 3 are very close to each other, and
we were unable to detect significant differences between
3
4
48.7
48.5
58.8
59.4
75.5
76.2
54.6 (d, 13.5 Hz)
57.6 (d, 25.5 Hz)
26.9
27.6
a At −60°C.

220
C. Mattheis, P. Braunstein / Journal of Organometallic Chemistry 621 (2001) 218–223
Table 2
Selected 1H-NMR data of 2, 3 and 4 (l in ppm)
Me
Me
NMe2
NCH2
3.96
C(9)HH C(9)HH OH
1
2
CDCl3 1.09 1.42
CDCl3 1.13 1.54 3.01 (d, 2.2 Hz), 3.11 (d, 2.5
Hz)
4.57
4.66
4.62
4.97
n.f.
9.62
C6D6
C6D6
0.86 1.53 2.60
0.89 1.52 2.60, 2.67
3.04 (dd, 2.5, 13.8 Hz), 3.72 (d, 13.5 Hz)
3.09 (dd, 2.9, 13.7 Hz), 3.68 (d, 13.7 Hz)
4.01 (d, 14.2 Hz), 4.15 (d, 14.1 Hz)
5.02
4.98
4.64
5.16
5.14
4.94
9.49(br)
9.72(br)
n.f.
a
CD3OD 1.17 1.53 2.85 (d, 2.1 Hz), 2.91 (d, 2.0
Hz)
3
4
CDCl3 1.14 1.51 2.92
3.97
3.24
4.61
4.65
4.95
5.12
5.01
5.90
C6D6
0.96 1.46 2.42 (d, 2.3 Hz), 2.61 (d, 2.5
Hz)
CDCl3 1.12 1.62 2.90 (d, 2.5 Hz), 2.94 (d, 1.3
Hz)
3.85 (dd, 3.0, 13.8 Hz), 4.1 (d, 14.1 Hz)
3.44 (d, 15 Hz), 3.61 (d, 12 Hz)
3.55 (d, 1.8 Hz)
4.68
4.84
4.84
4.84
4.96
4.92
C6D6
1.26 1.53 2.73 (d, 2.2 Hz), 2.74 (d, 2.2
Hz)
a
C6D6
1.29 1.52 2.71 (d, 2.2 Hz), 2.75 (d, 1.8
Hz)
a At higher concentration (20 mg dissolved in 1 ml benzene).
polar and non-polar solvents, neither for C(1) nor for
the aromatic ipso carbon which is in trans position to
oxygen when the alcohol moiety coordinates to palla-
dium. An interesting feature was noticed in the 1H-
NMR spectrum in benzene: depending on the
concentration of 2 or 4 we observe differences for the
signals of NMe2 and NCH2 (a dependence on the
concentration of 3 in benzene could not be studied
owing to the poor solubility of this complex). Whereas
at low concentration of 2 the NMe2 signal appears as a
singlet, at higher concentration two singlets were ob-
served. Concentrating a benzene solution of 4 led to a
slightly larger splitting for the NMe2 group, while in
contrast, the NCH2 signal changes from two doublets
to only one doublet (we also found in the 31P-NMR
spectra a slight dependence on the concentration: at
higher concentration the signal shifts up to 1.5 ppm to
higher field). These observations point to the occur-
rence of intermolecular phenomena which have not
been further investigated.
Within experimental error, these values are almost the
same (\m[Pd]Cl=74.1 S cm2 mol−1, \m[(Pd+)+
(Cl−)]=71.4 S cm2 mol−1). This indicates that 2 is
completely dissociated in methanol and is present as 2b.
2.4. Crystal structure of 2b and 3
Crystals of 2b suitable for X-ray determination were
obtained from dichloromethane–n-hexane. They con-
tain one molecule of dichloromethane per formula unit.
In the solid state, a cationic palladium complex is
separated from the chloride anion (Fig. 1). The ligand
arrangement around the metal is square planar. The
palladium is chelated by the dmba moiety and coordi-
nation of the phosphorus occurs in cis position to the
aryl group, in agreement with the antisymbiotic effect.
The coordination environment around the palladium
centre is completed by the oxygen atom of the P,OH
ligand. Although the proton of the hydroxy group
could not be located, the Pd–O bond distance (2.174(3)
,
A) clearly indicates a dative interaction. In the related
complexes with a negatively charged oxygen donor
[(dmba)P¸d¹{P¹h¹2P¹C¹H¹2¹C¹(O¹º)O}],
[(dmba)-
[(dmba)-
2.3. Conducti6ity in6estigations
atom
¸¹¹¹¹¹¹¹¹¹¹º
Pd{Ph2PCH···C(···O)OEt}]
and
Conductivity measurements were carried out in
methanol at room temperature. Complex 2 behaves like
a 1:1 electrolyte with a molar conductivity of \m=74.1
S cm2 mol−1, which is similar to the value found
by Pringle et al. for the palladium complex
P¸d¹[P¹h¹2P¹C¹{¹C¹(O¹)¹N¹H¹P¹h¹}C¹(¹P¹hº)O], the Pd–O bond dis-
tance is shorter than in 2b (2.105(3), 2.117(5)
[PdCl2(PPh2CH2CMe2OH)2] (\m=78 S cm2 mol−1
)
[3f]. Since we were not only interested in the value of
the molar conductivity \m but also in the degree of
dissociation of 2 in methanol, we compared its value
\m[Pd]Cl with the sum of \m[Pd+] and \mCl−. The
latter were determined by subtracting the values \m of
complexes fully dissociated into ions (see Scheme 2).
Scheme 2. All values \m in S cm2 mol−1
.

C. Mattheis, P. Braunstein / Journal of Organometallic Chemistry 621 (2001) 218–223
221
,
bond length of 0.9 A, the H–Cl bond distance is
,
calculated to be ca. 2.2–2.4 A, which is less than the
H–Cl bond distances typically considered as short 5
,
2.52 A) [9]. Interactions with the dichloromethane
molecule are not observed. Several solid-state structures
of palladium complexes with phosphino alcohols have
been reported. Two examples contain the 2-
diphenylphosphino cyclohexanol, a ligand very similar
to 1. However, in all these structures, the ligand coordi-
nates only in a monodentate fashion via the phospho-
rus atom and no dissociation of the chloride coligand
occurs [10]. This indicates that very slight differences in
electronic and steric properties may be sufficient to
control the mono- or bidentate coordination mode of
the phosphino alcohols.
In the crystal structure of 3, determined for compara-
tive purposes, we observe the expected ligand arrange-
ment with the chelating P,OH ligand (Fig. 2). Like in
the crystal structure of 2b there is a molecule of
dichloromethane of solvation in the unit cell. The pro-
ton of the hydroxy group could not be located. All
Fig. 1. ORTEP view of the molecular structure of [(dmba)Pd(P,OH)]Cl
(2b). The second phenyl substituent at P is not shown for clarity.
Thermal ellipsoids are drawn at the 30% probability level.
Table 3
,
Selected bond lengths (A) and angles (°) for 2b with estimated
standard deviations in parentheses
Pd–P
Pd–O
Pd–N
Pd–C(27)
Pd–Cl
O–Cl
2.241(1)
2.174(3)
2.115(5)
1.984(5)
3.853
P–Pd–O
P–Pd–C(27)
O–Pd–N
N–Pd–C(27)
Pd–O–Cl
P–O–Cl
84.5(1)
99.7(2)
93.3(2)
82.6(2)
97.4
2.913
92.7
,
and 2.096(3) A, respectively) [6]. The same holds true
for the cationic palladium alcohol complex
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Me2NCH2CH2NMe2)Pd(o-C6H4CH2CH2OH)]NO3
,
(Pd–O 2.076(4) A) [7], and even for the
neutral palladium alcohol compounds
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Pd{OC(NMe2)CH2(Me2)OH}Cl(NO2)],
¸¹¹¹¹¹¹º
[PdNCH2CH2NH2)
»¹¹¹¹¹¹¼
Fig. 2.
ORTEP
view of the molecular structure of
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
{cyclo-CHCH2C(O)HC(Me)HCH2N(CH2CH2OH)}]
[(dmba)Pd(P,OH)]PF6 (3). The second phenyl substituent at P is not
shown for clarity. Thermal ellipsoids are drawn at the 30% probabil-
ity level.
»¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¼
and
[Pd{PPh2CH2S(O)Me}Cl2(MeOH)]
(Pd–O
,
2.059(4), 2.045(6), 2.148(2) A) [8]. The bond distances
and angles around the dmba moiety and around the
phosphorus atom are all in the expected range (Table
3). The chloride ion is located below the palladium
coordination plane, rather close and almost at the
vertical of the oxygen atom (bond angles Pd–O–Cl and
P–O–Cl of 97.4 and 92.7°, respectively). Whereas the
Table 4
,
Selected bond lengths (A) and angles (°) for 3 with estimated stan-
dard deviations in parentheses
Pd–P
Pd–O
Pd–N
Pd–C(13)
Pd–F(1)
Pd–F(2)
2.251(2)
2.206(5)
2.119(7)
1.968(7)
3.406
O–F(1)
O–F(2)
P–Pd–O
P–Pd–C(27)
O–Pd–N
N–Pd–C(27)
3.060
2.977
84.6(1)
97.6(2)
95.7(2)
82.1(3)
,
value of the Pd–Cl distance (3.853 A) excludes any
interaction, the O–Cl distance of 2.913 A is indicative
of a short hydrogen bridge (with an estimated O–H
,
4.52

222
C. Mattheis, P. Braunstein / Journal of Organometallic Chemistry 621 (2001) 218–223
bond distances and angles are similar to those in 2b
(Table 4), except for the longer Pd–O bond distance
4.3. Synthesis of complex 2
To solution of 1 (0.95 g, 2.8 mmol) in
dichloromethane (20 ml) [(dmba)Pd(m-Cl)]2 (0.78 g, 1.4
mmol) was added at room temperature. After the solu-
tion was stirred for 30 min, it was filtered and concen-
trated to 8 ml and then n-hexane (50 ml) was added. At
−30°C 1.60 g (2.6 mmol, 93%) of the off-white product
crystallized. Anal. Calc. for C31H39ClNOPPd: C, 60.59;
H, 6.40. Found: C, 60.96; H, 6.55%.
,
(2.206(5) A). This lengthening may be due to the
a
presence of stronger hydrogen bonding: a bifurcated
H-bonding occurs between the OH proton and two
,
fluor atoms of the PF6 anion (O–F: 2.977 and 3.060 A).
3. Conclusion
We have synthesized the very stable, chiral palladium
phosphinoalcohol complex 2 whose solid-state structure
shows that the ligand coordinates in a chelating mode
by displacement of a chloride ligand. The bonding
behaviour of phosphinoalcohols depends critically on
small steric and electronic factors, as evidenced by the
fact that ligands similar to 1 gave palladium complexes
with only monodentate coordination via phosphorus.
In solution, complex 2 gives rise to an equilibrium
between the cationic (polar solvents) and the neutral
species (apolar solvents). Its deprotonation afforded the
neutral complex [(dmba)Pd(P,O)] (4) with a chiral,
chelating phenolate ligand. Nickel phosphino pheno-
lates have been recently shown to catalyse the oligomer-
ization of ethylene into linear a-olefins [11] and early
transition metal complexes with chelating phenolate
ligand continue to attract attention as catalyst precur-
sors in a-olefin polymerization [12].
4.4. Synthesis of complex 3
Solid TlPF6 (0.41 g, 1.2 mmol) was added to a
solution of 2 (0.36 g, 0.59 mmol) in dichloromethane
(15 ml). After the mixture was stirred overnight, it was
filtered and concentrated to 3 ml and then n-hexane (20
ml) was added. At −30°C, 0.34 g (0.47 mmol, 80%) of
the white product crystallized. Anal. Calc. for
C31H39F6NOP2Pd: C, 51.43; H, 5.43. Found: C, 51.03;
H, 5.90%.
4.5. Synthesis of complex 4
To a solution of 2 (0.57 g, 0.93 mmol) in THF (20
ml) potassium hydride (0.2 g, 5 mmol) was added at
0°C. After the mixture was stirred overnight at r.t, it
was filtered and concentrated to 5 ml and then n-hex-
ane (30 ml) was added. At −30°C, 0.37 g (0.64 mmol,
69%) of the beige product crystallized. Anal. Calc. for
C31H38NOPPd: C, 64.41; H, 6.63; N, 2.42. Found: C,
64.42; H, 6.73; N, 2.46%.
4. Experimental
4.1. General procedures
4.6. X-ray crystal structure determinations of 2b and 3
All manipulations were carried out under an atmo-
sphere of dry oxygen-free nitrogen with standard
Schlenk techniques. Solvents were dried and degassed
by refluxing over suitable reagents before use.
The X-ray data for both structures were collected on
a Kappa CCD diffractometer. The structures were
solved using direct methods and refined against ꢀFꢀ.
Absorption corrections were computed from the psi
scans of four reflections. For all computations the
Nonius MOLEN package was used [13]. The hydrogen
4.2. Synthesis of the ligand P,OH (1)
The literature method [4] was modified. To a mixture
of (+) cis/trans limonene oxides (8.6 ml, 26.2 mmol
trans oxide) in 30 ml THF at −80°C was slowly added
a 0.5 M solution of KPPh2 in THF (52.4 ml, 26.2
mmol). After the mixture was stirred for 2 h, it was
slowly warmed up to room temperature (r.t.). Addition
of 60 ml of a degassed NH4Cl solution (10%) in water
was followed by separation of the layers and extraction
of the water phase with THF (2×20 ml). The com-
bined THF extracts were dried over sodium sulfate.
The filtered colourless solution was concentrated to 25
ml and n-pentane (150 ml) was added. At −80°C, 6.8
g (20.1 mmol, 77%) of the white product crystallized.
Anal. Calc. for C22H27OP: C, 78.08; H, 8.04. Found: C,
77.6; H, 8.3%.
atoms were introduced as fixed contributors ((dC–H=
2
,
,
0.95 A, BH=1.3Bequiv(C) A ).
Crystal data for 2b. Colourless crystals from
dichloromethane–n-hexane. Crystal data: C31H38-
ClNOPPd·CH2Cl2, M=698.41; orthorhombic; space
group P212121; a=9.3780(3), b=16.4500(6), c=
3
,
,
21.4210(4) A, U=3304.6(3) A , Z=4, Dcalc=1.40 g
cm−3, T=294 K, v(u(Mo–Ka)=0.71073 A)=0.877
,
mm−1. Number of data with I\3|(I) 2986; weighting
scheme 4F2o/(|2(Fo2)+0.0004F4o); number of variables
352; R=0.030, Rw=0.034.
Crystal data for 3. Colourless crystals from
dichloromethane–n-hexane. Crystal data: C31H38-
F6NOP2Pd·CH2Cl2, M=807.93; orthorhombic; space
group P212121; a=9.3752(4), b=18.5773(8), c=

C. Mattheis, P. Braunstein / Journal of Organometallic Chemistry 621 (2001) 218–223
223
3
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,
mm−1. Number of data with I\3|(I) 4194; weighting
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5. Supplementary material
Crystallographic data for the structures of 2b and 3
have been deposited in cif format with the Cambridge
Crystallographic Data Centre, CCDC Nos 148987 and
148988, respectively. Copies of the data can be obtained
free of charge on application to The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
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We are grateful to the A. von Humboldt Foundation
for a Feodor–Lynen fellowship to C.M. and to the
Ministe`re de l’Education Nationale, de la Recherche et
de la Technologie (Paris) and the CNRS for financial
support and an associate research position to C.M. We
thank A. DeCian (Strasbourg) for solving the crystal
structures and A.A.H. van der Zeijden and M. Mayor
for helpful discussions.
[11] J. Pietsch, P. Braunstein, Y. Chauvin, New J. Chem. (1998) 467,
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