Synthesis and characterization of palladium(ii) complexes with new diphosphonium-diphosphine and diphosphine ligands. Production of low molecular weight alternating polyketones via catalytic CO/ethene copolymerisation

Article abstract of DOI:10.1039/b601154c

The bis-cationic diphosphonium-diphosphine 6,7-di(di-2-methoxyphenyl) phosphinyl-2,2,4,4-tetra(di-2-methoxyphenyl)-2λ⁴, 4λ⁴-diphosphoniumbicyclo[3.1.1]heptane-bis(PF₆) ((o-MeO-PCP)(PF₆)₂) and the diphosphine rac-2,4-bis((di-2-methoxyphenyl)phosphino)pentane (rac-o-MeO-bdpp) have been synthesized. Both ligands have been employed to coordinate PdCl₂ and Pd(OAc)₂ to give [PdCl₂(o-MeO-PCP)](PF₆) ₂ (1a), PdCl₂(rac-o-MeO-bdpp) (1b), [Pd(OAc) ₂(o-MeO-PCP)](PF₆)₂ (2a) and Pd(OAc) ₂(rac-o-MeO-bdpp) (2b). The ligands and complexes have been fully characterized in solution by multinuclear NMR spectroscopy. In addition, 1a and 1b have been authenticated by single crystal X-ray structure analyses. The Pd^II complexes 1a and 1b have been employed as catalyst precursors for the CO/ethene copolymerisation in water-acetic acid mixtures, while 2a and 2b have been tested in methanol in the presence of p-toluenesulfonic acid. Irrespective of the reaction media, perfectly alternating polyketones were obtained in excellent yields and with number-average molecular weights ranging from 7.1-13.9 kg mol^-1 with the diphosphonium-diphosphine catalysts and from 37.2-48.2 kg mol^-1 with the diphosphine catalysts. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b601154c

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www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of palladium(II) complexes with new
diphosphonium-diphosphine and diphosphine ligands. Production of low
molecular weight alternating polyketones via catalytic CO/ethene
copolymerisation
Claudio Bianchini,*a Peter Bru¨ggeller,*b Carmen Claver,c Georg Czermak,b Alexander Dumfort,b
Andrea Meli,a Werner Oberhauser*a and Eduardo J. Garcia Suareza
Received 25th January 2006, Accepted 28th March 2006
First published as an Advance Article on the web 13th April 2006
DOI: 10.1039/b601154c
The bis-cationic diphosphonium-diphosphine 6,7-di(di-2-methoxyphenyl)phosphinyl-2,2,4,4-tetra(di-2-
methoxyphenyl)-2k4,4k4-diphosphoniumbicyclo[3.1.1]heptane-bis(PF6) ((o-MeO-PCP)(PF6)2) and the
diphosphine rac-2,4-bis((di-2-methoxyphenyl)phosphino)pentane (rac-o-MeO-bdpp) have been
synthesized. Both ligands have been employed to coordinate PdCl2 and Pd(OAc)2 to give
[PdCl2(o-MeO-PCP)](PF6)2 (1a), PdCl2(rac-o-MeO-bdpp) (1b), [Pd(OAc)2(o-MeO-PCP)](PF6)2 (2a)
and Pd(OAc)2(rac-o-MeO-bdpp) (2b). The ligands and complexes have been fully characterized in
solution by multinuclear NMR spectroscopy. In addition, 1a and 1b have been authenticated by single
crystal X-ray structure analyses. The PdII complexes 1a and 1b have been employed as catalyst
precursors for the CO/ethene copolymerisation in water–acetic acid mixtures, while 2a and 2b have
been tested in methanol in the presence of p-toluenesulfonic acid. Irrespective of the reaction media,
perfectly alternating polyketones were obtained in excellent yields and with number-average molecular
weights ranging from 7.1–13.9 kg mol−1 with the diphosphonium-diphosphine catalysts and from
37.2–48.2 kg mol−1 with the diphosphine catalysts.
Below each sketch is reported the productivity as kg(polyketone)
Introduction
(g(Pd) × h)−1 exhibited by the corresponding PdII catalysts under
The design of conformationally rigid polyphosphine ligands for
comparable reaction conditions. For comparative purposes, it
also reports the PdII catalyst with the commonly used 1,3-
bis(diphenylphosphino)propane (dppp) ligand.
the coordination of late transition metals is a subject of much
interest in organometallic chemistry and homogeneous catalysis.1
Indeed, decreasing the flexibility of the supporting ligand de-
creases the number of possible conformations of the metal–
ligand assembly, thus leading to improved selectivity. Unlike
selectivity, the effect of the skeletal rigidity on the activity is unpre-
dictable as it mainly depends on the reaction under investigation
rather than on the structural and conformational properties of
the metal–diphosphine precursor. In the particular case of the
copolymerisation of CO with ethene by palladium(II) catalysts,2 a
direct correlation between catalytic productivity and structural
rigidity of the chelating diphosphine has been unambiguously
observed for a number of ligands, including C2-bridged3 and C3-
bridged4 diphosphines, 1,1ꢀ-bis(diorganylphosphino)ferrocenes,5
and bis(phosphino)[3]ferrocenophanes.6 Scheme 1 shows the
molecular sketches of two 1,3-diphosphine ligands bearing back-
bone substituents that decrease the ligand flexibility (rac-bdpp
and meso-bdpp; bdpp = 2,4-bis(diphenylphosphino)pentane).
Scheme 1
In this work, we report the synthesis of the new stereochemically
rigid, C3-bridged, bis-cationic diphosphonium-diphosphine 6,7-
di(di-2-methoxyphenyl)phosphino-2,2,4,4-tetra(di-2-methoxy-
phenyl)-2k4,4k4-diphosphonium bicyclo[3.1.1]heptane that has
−
been isolated as a PF6 salt ((o-MeO-PCP)(PF6)2, Scheme 2).
aIstituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di
Ricerca CNR di Firenze, via Madonna del Piano 10, 50019, Sesto Fioren-
tino, Italy. E-mail: claudio.bianchini@iccom.cnr.it, werner.oberhauser@
iccom.cnr.it; Fax: +39-055-5225203
bInstitut fu¨r Allgemeine, Anorganische und Theoretische Chemie der Uni-
versita¨t Innsbruck, Innrain 52a, Innsbruck 6020, Austria. E-mail: Peter.
Brueggeller@uibk.ac.at; Fax: +43-512-5072934
cDept. de Qu´ımica F´ısica i Inorganica, Facultat de Qu´ımica, Universitat
Rovira i Virgili, c/Marcel.li Domingo s/n, 43007, Tarragona, Spain
Scheme 2
2964 | Dalton Trans., 2006, 2964–2973
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1
or ketone/ester end groups. 13C{ H} NMR (HFIP–C6H6-d6,
100.62 MHz, ppm): d 6.80 (CH2CH3), 27.90 (CH3OC(O)CH2),
52.50 (C(O)OCH3), 176.10 (CH3O(O)C), 212 (CH2CH2C(O)),
217 (CH3CH2C(O)).2 The number-average molecular weight (Mn)
of the copolymers was determined by 1H NMR spectroscopy.
Bis-cationic complexes of (o-MeO-PCP)(PF6)2 with PdCl2 and
Pd(OAc)2 (OAc = acetate), namely [PdCl2(o-MeO-PCP)](PF6)2
(1a) and [Pd(OAc)2(o-MeO-PCP)](PF6)2 (2b), have been synthe-
sized and fully characterized. As a comparison to a neutral and less
strained C3-bridged diphosphine, we have also prepared the new
diphosphine rac-2,4-bis(di(2-methoxyphenyl)phosphino)pentane
(rac-o-MeO-bdpp) as well as its PdCl2 and Pd(OAc)2 com-
plexes PdCl2(rac-o-MeO-bdpp) (1b) and Pd(OAc)2(rac-o-MeO-
bdpp) (2b). All these PdII complexes have been tested as catalyst
precursors for the copolymerisation of CO and ethene in different
solvents, showing high activity and a remarkable control on the
molecular weight of the alternating polyketone products.
Preparations
(o-MeO-PCP)(PF6)2. The tetraphosphine o-MeO-dppcb
(0.62 g, 0.60 mmol) was stirred in CH2Cl2 (30 mL) for 7 d at room
temperature. The resultant yellow solution was concentrated
to 5 mL and diethyl ether (150 mL) was added to complete
the precipitation of a yellow product, which was then filtered
off, dried under vacuum and dissolved in water (120 mL). The
water solution was filtered through a column of Celite to remove
any trace of suspended material. The addition of solid NaPF6
(2.00 g, 12.00 mmol) to the resultant clear solution caused
the precipitation of an off-white powder, which was filtered
off, washed three times with water (20 mL) and then dried
under vacuum. Yield 90.5% (0.72 g, 0.54 mmol). C61H62F12O8P6
Experimental
Materials and physical measurements
All reactions and manipulations were carried out under a nitrogen
atmosphere by using Schlenk-type techniques. The solvents
were generally distilled over dehydrating reagents and were
deoxygenated before use. The reagents were used as purchased
from Aldrich or Fluka, unless stated otherwise. PdCl2(COD)
(COD = 1,5-cyclooctadiene),7 bis(o-methoxyphenyl)phosphine8
and meso/rac-2,4-pentanediol dimesylate9 were prepared
according to literature methods. The synthesis of trans,trans,trans-
tetrakis((di-2-methoxyphenyl)phosphino)cyclobutane (o-MeO-
dppcb) will be described elsewhere.10 All the isolated solid
samples were collected on sintered glass frits and washed with
appropriate solvents before being dried under a stream of
nitrogen. Copolymerisation reactions were performed with a
250 mL stainless steel autoclave, constructed at the ICCOM-CNR
(Florence, Italy), equipped with a magnetic drive stirrer and a
Parr 4842 temperature and pressure controller. The autoclave was
connected to a gas reservoir to maintain a constant pressure over
the catalytic reactions. GC/MS analyses of the solutions were
performed on a Shimadzu QP 5000 apparatus equipped with a
SPB-1 Supelco fused silica capillary column (30 m, 0.25 mm id,
0.25 lm film thickness). Deuterated solvents for routine NMR
1
(1336.29): calc. C 54.83, H 4.64; found C 54.91, H 4.71%. H
NMR (CD2Cl2, 400.13 MHz, ppm): d 3.38 (s, 12H, OCH3), 3.52
3
(s, 12H, OCH3), 4.25 (t, J(HP) 18.0 Hz, 2H, P–CH), 4.37 (t,
3J(HP) 9.6 Hz, 2H, P–CH), 4.59 (t, 2J(HP) 15.3 Hz, 2H, P–CH2),
6.87–7.70 (m, 32H, Ar-H). 13C{ H} NMR (CD2Cl2, 100.62 MHz,
1
ppm): d 8.02 (t, 1J(CP) 50.0 Hz, P-CH2), 32.00 (m, P-CH), 32.31
(m, P-CH), 55.78 (s, O-CH3), 56.51 (s, O-CH3), 104.70 (d, 1J(CP)
90.6 Hz, P-C(ipso)), 112.83 (s, Ar-C), 113.50 (s, Ar-C), 121.20 (s,
Ar-C), 121.85 (s, Ar-C), 122.80 (s, Ar-C), 132.72 (s, Ar-C), 133.25
(s, Ar-C), 135.08 (s, Ar-C), 138.53(s, Ar-C), 161.64 (s, O-C(Ar)),
162.02 (s, O-C(Ar)). 31P{ H} NMR (CD2Cl2, 161.98 MHz, ppm)
1
d 25.08 (t, 3J(PP) 16.0 Hz), −17.16 (br. t).
meso/rac-2,4-Dibromopentane. This mixture of stereoisomers
was synthesized using a modified version of the synthetic protocol
reported by Wiberg and Lowry.13 LiBr (7.33 g, 84.4 mmol)
was added to an acetone solution of meso/rac-2,4-pentanediol
dimesylate (5.50 g, 21.1 mmol). The mixture was refluxed for
20 h, then it was cooled to room temperature. The solvent was
removed by distillation and the crude residue was suspended in
a solvent mixture of water–Et2O–n-pentane (60 mL) (30 : 15 :
15, v/v/v). The organic layer was separated and the solvent was
removed under reduced pressure to give a yellow liquid residue.
Yield 65.7% (26 : 74) (meso : rac) (3.19 g, 13.862 mmol). C5H10Br2
(229.85): calc. C 26.13, H 4.35; found C 26.40, H 4.55%. 1H NMR
(CDCl3, 200.13 MHz, ppm) d 1.73 (d, 3J(HH) 6.72 Hz, 6H, CH3),
1.77 (d, 3J(HH) 6.60 Hz 6H, CH3), 2.15 (m, 3H, meso-CHH, rac-
CH2), 2.47 (dt, 2J(HH) 14.2 Hz, 3J(HH) 8.0 Hz, 1H, meso-CHH),
4.21 (m, 2H, Br–CH), 4.38(m, 2H, Br–CH).
1
measurements were dried over molecular sieves. 1H, 13C{ H} and
1
31P{ H} NMR spectra were obtained on either a Bruker ACP
200 (200.13, 50.32 and 81.01 MHz, respectively) or a Bruker
Avance DRX-400 spectrometer (400.13, 100.62 and 161.98 MHz),
respectively. Chemical shifts are reported in ppm (d) with reference
to either TMS as an internal standard (1H and 13C NMR) or 85%
H3PO4 as an external standard (31P NMR). High pressure NMR
(HPNMR) experiments were carried out on the Bruker ACP 200
spectrometer, using a 10 mm HPNMR tube (Saphikon sapphire
tube; titanium high pressure charging head constructed at the
ICCOM-CNR).11 Elemental analyses were performed using a
Carlo Erba Model 1106 elemental analyser. Infrared spectra were
recorded on a FT-IR Spectrum GX instrument (Perkin Elmer).
The conductivity of ionic compounds was measured with an
Orion model 990101 conductance cell connected to a model 101
conductivity meter. The conductivity data were obtained at a
sample concentration of ca. 10−3 M in nitroethane solutions.12
rac-o-MeO-bdpp. n-BuLi (6.80 mL, 10.8 mmol) was slowly
added to bis(o-methoxyphenyl)phosphine (2.00 g, 8.13 mmol)
dissolved in anhydrous and degassed THF (150 mL) at 0◦ C. After
the yellow Li salt precipitated, the suspension was stirred for
1 h at room temperature. A 26 : 74 mixture of meso/rac-2,4-
dibromopentane (0.92 g, 4.02 mmol) was added dropwise to
this suspension at room temperature, then the yellow solution
was stirred overnight. Afterwards, the reaction was quenched
with water (3 mL), concentrated to dryness, and washed with
water and ethanol. Yield of meso/rac-o-MeO-bdpp 71.0% (1.62 g,
1
Polyketone products were analysed by IR, 1H and 13C{ H} NMR
spectroscopy. The NMR measurements were performed in a
solvent mixture of 1,1,1,3,3,3-hexafluoroisopropanol-d2–C6H6-d6
(5 : 1, v/v) showing a perfectly alternating structure with ketone
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2.89 mmol). Recrystallisation of the mixture from CH2Cl2/EtOH
led to the precipitation of a pure sample of rac-o-MeO-bdpp as
an off-white powder. Yield 36.0% (0.82 g, 1.46 mmol). C33H38O4P2
(560.26): calc. C 70.70, H 6.83; found C 70.65, H 6.85%. 1H NMR
Pd(OAc)2(rac-o-MeO-bdpp) (2b). Compound 1b (0.26 g,
0.35 mmol) was dissolved in degassed CH2Cl2 (30 mL) under
nitrogen. To this solution was added Ag(OAc) (0.13 g, 0.77 mmol)
and the resultant suspension was stirred for 30 min. The precipi-
tated AgCl was removed by filtration through a column of Celite
and the clear solution was concentrated to ca. 5 mL. The addition
of a 1 : 1 (v/v) mixture of n-hexane and diethyl ether (20 mL)
completed the precipitation of 2b as yellow microcrystals which
were filtered off, and dried in a stream of nitrogen. Yield 60%
(0.16 g, 0.21 mmol). C37H44O8P2Pd (784.69): calc. C 56.60, H 5.65;
found C 56.63, H 5.66%. 1H NMR (CD2Cl2, 400.13 MHz, ppm) d
1.01 (dd, 3J(HP) 16.0 Hz, 3J(HH) 6.9 Hz, 6H, CH3), 1.10 (s, 6H,
OAc–CH3), 1.88 (m, 2H, CH2), 3.12 (m, 2H, P–CH), 3.68 (s, 6H,
3
3
(CDCl3, 200.13 MHz, ppm) d 1.06 (dd, J(HP) 15.8 Hz, J(HH)
6.8 Hz, 6H, CH3), 1.53 (m, 2H, CH2), 2.75 (m, 2H, P–CH), 3.63
(s, 6H, OCH3), 3.69 (s, 6H, OCH3), 6.72–7.83 (m, 16H, Ar-H).
1
31P{ H} NMR (CDCl3, 81.01 MHz, ppm) d −23.8 (s). All attempts
to isolate the meso ligand in a pure form were unsuccessful.
[PdCl2(o-MeO-PCP)](PF6)2 (1a). PdCl2(COD) (0.03 g,
0.10 mmol) was added to a degassed solution of (o-MeO-
PCP)(PF6)2 (0.13 g, 0.10 mmol) in CH2Cl2 (10 mL). The resultant
mixture was stirred under nitrogen for 2 h, then n-hexane (50 mL)
was added slowly to complete the precipitation of 1a as an
off-white solid, which was filtered off, washed with n-hexane
(5 mL) and dried under vacuum. Yield 85.2% (0.13 g, 0.08 mmol).
C61H62Cl2F12O8P6Pd (1513.61): calc. C 48.40, H 4.09; found C
48.55, H 4.21%. 1H NMR (DMSO-d6, 400.13 MHz, ppm) d 3.64
(s, 12H, OCH3), 3.70 (s, 12H, OCH3), 3.90 (m, 2H, P–CH), 4.09
(td, 3J(HP) 15.6 Hz, 2J(HP) 8.8 Hz, 2H, P–CH), 4.79 (br. t,
1
OCH3), 3.93 (s, 6H, OCH3), 6.95–7.68 (m, 16H, Ar-H). 31P{ H}
NMR (CD2Cl2, 161.98 MHz, ppm) d 31.78 (s).
Catalytic copolymerisation of CO and ethene
HPNMR experiment in water–AcOH with 1b as catalyst pre-
cursor. A 10 mm sapphire HPNMR tube was charged with a
suspension of 1b (0.01 g, 0.02 mmol) in a mixture of D2O and
AcOH (0.4 mL and 1.6 mL, respectively) under nitrogen and then
1
2J(HP) 15.0 Hz, 2H, P–CH2), 6.35–8.85 (m, 32H, Ar-H). 31P{ H}
NMR (DMSO-d6, 161.98 MHz, ppm) d 30.60 (t, 3J(PP) 24.9 Hz),
24.20 (br. t).
◦
1
placed into the NMR probe maintained at 20 C. After a 31P{ H}
NMR spectrum was acquired, the sapphire tube was removed from
the NMR probe and pressurized with a 1 : 1 mixture of CO–C2H4
Crystals of 1a·2CH3COCH3 suitable for X-ray analysis were
to a total pressure of 40 bar at room temperature. A 31P{ H} NMR
1
obtained by slow evaporation of an acetone solution of 1a.
spectrum was recorded at 20 ◦C and then the probe-head was
heated to 85 ◦C. This temperature was maintained for 1 h, during
[Pd(OAc)2(o-MeO-PCP)](PF6)2 (2a). Pd(OAc)2 (0.02 g,
0.10 mmol) was added to a degassed solution of (o-MeO-
PCP)(PF6)2 (0.13 g, 0.10 mmol) in CH2Cl2 (10 mL). The resultant
mixture was stirred at room temperature for 12 h, then it was
concentrated to a smaller volume (3 mL). The addition of n-hexane
(50 mL) caused the precipitation of 2a as an orange red powder,
which was filtered off, washed with n-hexane (5 mL) and dried
under vacuum. Yield 82.6% (0.13 g, 0.08 mmol). C65H68F12O12P6Pd
(1560.71): calc. C 50.02, H 4.36; found C 50.22, H 4.44%. 1H NMR
(DMSO-d6, 400.13 MHz, ppm) d 1.15 (s, 6H, OAc–CH3), 3.20–
which time 31P{ H} NMR spectra were acquired every 20 min.
1
◦
The tube was then allowed to cool to 20 C and a final 31P{ H}
NMR spectrum was acquired. The formation of copolymer was
observed.
1
Autoclave experiments in water–AcOH with 1a and 1b as catalyst
precursors. A deoxygenated mixture of distilled water and AcOH
(100 mL) was introduced by suction into an autoclave (250 mL),
previously evacuated by a vacuum pump, containing the catalyst
precursor (0.005 mmol). The autoclave was charged with a 1 : 1
CO–C2H4 mixture to 30 bar at room temperature. The temperature
was increased to 85 ◦C while the pressure inside the autoclave was
maintained at 40 bar. The autoclave was stirred (1100 rpm) for
the desired time, then it was cooled with an ice-water bath. The
unreacted gases were released and the insoluble copolymer was
filtered off, washed with water, and dried under vacuum at 60◦ C
to constant weight.
3
3.50 (m, 24H, OCH3), 3.88 (br. t, J(HP) 15.0 Hz, 2H, P–CH),
3.99 (td, 3J(HP) 15.3 Hz, 2J(HP) 8.2 Hz, 2H, P–CH), 4.78 (br. t,
1
2J(HP) 15.0 Hz, 2H, P–CH2), 6.60–8.00 (m, 32H, Ar-H). 31P{ H}
NMR (DMSO-d6, 161.98 MHz, ppm) d 32.50 (br. s), 24.45 (br. s).
PdCl2(rac-o-MeO-bdpp) (1b). rac-o-MeO-bdpp (0.11 g,
0.20 mmol) was dissolved in degassed CH2Cl2 (20 mL) under
nitrogen. To this colourless solution was added PdCl2(COD)
(0.06 g, 0.20 mmol). The resultant suspension was stirred for 1 h,
then it was concentrated to half the volume. The addition of a 1 :
1 (v/v) mixture of n-pentane and diethyl ether (20 mL) completed
the precipitation of 1b as an off-white microcrystalline solid, which
was filtered off and dried in a stream of nitrogen. Yield 69.8%
(0.10 g, 0.14 mmol). C33H38Cl2O4P2Pd (737.58): calc. C 53.71, H
Autoclave experiments in MeOH with 2a and 2b as catalyst
precursors. MeOH (reagent grade from Aldrich, 100 mL) was
introduced under nitrogen into an autoclave (250 mL) containing
the catalytic precursor (0.005 mmol) and the desired amount of
p-toluenesulfonic acid monohydrate (TsOH). The autoclave was
pressurized with a 1 : 1 CO–C2H4 mixture to 30 bar at room
temperature. The temperature was increased to 85 ◦C while the
pressure inside the autoclave was maintained at 40 bar. The
autoclave was stirred (1100 rpm) for the desired time, then it was
cooled with an ice-water bath. The unreacted gases were released
and the insoluble copolymer was filtered off, washed with water,
and dried under vacuum at 60 ◦C to constant weight. The filtered
solutions were analysed by GC/MS.
1
5.19; found C 53.69, H 5.20%. H NMR (CD2Cl2, 400.13 MHz,
ppm) d 0.86 (dd, 3J(HP) 15.9 Hz, 3J(HH) 6.7 Hz, 6H, CH3), 1.82
(m, 2H, CH2), 3.24 (m, 2H, P–CH), 3.72 (s, 6H, OCH3), 3.90 (s,
1
6H, OCH3), 6.80–7.70 (m, 16H, Ar-H). 31P{ H} NMR (CD2Cl2,
161.98 MHz, ppm) d 34.50(s).
Crystals of 1b suitable for X-ray analysis were obtained by slow
diffusion of toluene into a saturated CH2Cl2 solution at room
temperature.
2966 | Dalton Trans., 2006, 2964–2973
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Table 1 Crystallographic data for 1a·2CH3COCH3 and (S,S)-1b
1a·2CH3COCH3
(S,S)-1b
Empirical formula
Formula weight
T/K
C67H74Cl2F12O10P6Pd
1630.41
243(2)
0.71073
C33H38Cl2O4P2Pd
737.87
293(2)
0.71073
˚
k/A
Crystal system, space group
Monoclinic, P21/c
14.3205(2)
20.4675(3)
24.5985(3)
90.0
91.8346(9)
90.0
7206.2(2)
Monoclinic, P21
10.6259(7)
13.6492(11)
11.9265(9)
90.0
107.937(7)
90.0
1645.7(2)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
4
2
Dc/Mg m−3
1.503
0.552
1.489
0.859
l/mm−1
F(000)
3336
756
Crystal size/mm
Absorption correction
Refinement method
Data/restraints/parameter
Goodness-of-fit on F2
Final R indices
0.25 × 0.15 × 0.10
0.40 × 0.20 × 0.10
MULTI-SCAN
SADABS
Full-matrix least-squares on F2
13017/0/895
Full-matrix least-squares on F2
10051/1/391
1.017
0.855
[I > 3r(I)] R1 = 0.0477, wR2 = 0.1497
R1 = 0.1017, wR2 = 0.1685
0.630, −0.451
[I > 2r(I)] R1 = 0.0451, wR2 = 0.0808
R1 = 0.1015, wR2 = 0.0949
1.122, −0.375
R indices (all data)
Largest diff. peak, hole/e A
3
˚
X-Ray crystallographic data collection and refinement of the
structures
The crystallographic data for 1a·2CH3COCH3 and (S,S)-1b
are summarized in Table 1. Graphite-monochromated Mo-Ka
˚
radiation (k = 0.71073 A) was used. The data collection was
performed on a Nonius Kappa CCD diffractometer using ␾-
x-scans and on an Oxford Diffraction CCD diffractometer
using x-scans for 1a·2CH3COCH3 and (S,S)-1b, respectively. Cell
refinement, data reduction and empirical absorption correction
were carried out with the Denzo and Scalepack programs14a
for 1a·2CH3COCH3 and with the Oxford diffraction software
and SADABS14b for (S,S)-1b. All structure determination cal-
culations for 1a·2CH3COCH3 were performed with SHELXTL
NT 5.10 including SHELXS-97 and SHELXL-97,14c while the
WINGX package14d with SIR-97,14e SHELXL-97 and ORTEP-
3 programs14f were used for (S,S)-1b. Final refinements on F2
were carried out with anisotropic thermal parameters for all non-
hydrogen atoms. All hydrogen atoms were included using a riding
model with isotropic U values depending on the Ueq of the adjacent
carbon atoms.
Scheme 3
The contemporaneous alkylation of two phosphorus atoms of
o-MeO-dppcb is quite surprising as dichloromethane is a weak
alkylating agent. A driving force to the observed reaction may
be the formation of a six-membered diphosphonium ring which
stabilizes the all-axial positions of the phosphorus donor atoms
in o-MeO-dppcb. The latter ligand can in fact adopt three distinct
conformations in solutions (Scheme 4).10
CCDC reference numbers: 293490 and 293489 for
1a·2CH3COCH3 and (S,S)-1b, respectively. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b601154c
Results and discussion
Scheme 4
Synthesis of the ligands
The bis-cationic diphosphonium-diphosphine (o-MeO-
PCP)(PF6)2 is stable in both protic and aprotic solvents and no
trace of ylide formation was observed even for long residence times
in solution.
The diphosphonium-diphosphine ligand (o-MeO-PCP)(PF6)2 has
been obtained by regioselective bis-alkylation of the tetraphos-
phine o-MeO-dppcb10 with neat CH2Cl2, followed by exchange
of the chloride counteranions with PF6 in water. This procedure
gave (o-MeO-PCP)(PF6)2 in analytically pure form and 90% yield
The 31P{ H} NMR spectrum of (o-MeO-PCP)(PF6)2 consists
−
1
of a broad triplet in the phosphine region at −17.16 ppm and
(Scheme 3).
of a narrow triplet in the phosphonium region at 25.08 ppm.15a
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The 3J(PP) coupling constant of 16 Hz is rather small compared
to other cyclic phosphonium-phosphine ligands.15 This finding
may be due to the conformational flexibility of the six-membered
diphosphonium heterocycle, which, affecting the dihedral angle
between the phosphine and phosphonium units, would average the
PP coupling constant. The presence of a CH2 bridge in between the
phosphonium units is put in evidence by a triplet at 4.59 ppm with
a 2J(HP) value of 15.3 Hz in the 1H NMR spectrum and by a triplet
The presence of a strained 6–4–6 ring system in 1a was
confirmed by a single crystal X-ray structure analysis. A selected
ORTEP drawing of the cation [PdCl2(o-MeO-PCP)]2+ (1a2+) in
1a·2CH3COCH3 is presented in Fig. 1.
1
1
in the 13C{ H}NMR spectrum at 8.02 ppm with a J(CP) value
of 50.0 Hz. The coupling constant values are comparable to those
reported for similar Ar2P+CH2P+Ar2 fragments in six-membered
diphosphonium heterocycles.16
The less strained diphosphine rac-o-MeO-bdpp was synthesized
by the reaction of LiPAr2 (Ar = 2-methoxyphenyl)9 with a 26 : 74
meso : rac mixture of 2,4-dibromopentane in THF (Scheme 5).
Scheme 5
Fig. 1 ORTEP diagram of 1a·2CH3COCH3 with 30% probability ellip-
soids. The two PF6− counterions and the two acetone molecules are omitted
for clarity. Only the ipso carbon atoms of the 2-methoxyphenyl units are
presented on P(2) and P(4).
Only the rac isomer of o-MeO-bdpp could be isolated in a pure
form (36% yield) as an off-white solid by recrystallisation of the
stereoisomeric mixture from CH2Cl2–ethanol.
Besides 1a2+ cations and PF6− anions, the crystal structure shows
the presence of two molecules of acetone in the asymmetric unit.
Crystallographic data and selected bond distances and angles for
the complex cation 1a2+ are reported in Tables 1 and 2, respectively.
The Pd atom shows a square-planar coordination with a deviation
from the coordination plane, defined by the atoms P(1), P(3), Cl(1)
Synthesis and characterization of the catalytic precursors
The reaction of (o-MeO-PCP)(PF6)2 with PdCl2(COD) or
Pd(OAc)2 in CH2Cl2 gave the bis-cationic PdII complexes
[Pd(Cl)2(o-MeO-PCP)](PF6)2 (1a) and [Pd(OAc)2(o-MeO-PCP)]-
(PF6)2 (2a), respectively, as air stable compounds in 85 and 83%
yield, respectively (Scheme 6).
˚
and Cl(2), of 0.1867(8) A in the direction of C(11). The rather
short Pd(1)–P(1) and Pd(1)–P(3) bond distances of 2.2546(13)
˚
and 2.2403(13) A, respectively, lead to the formation of a P(1)–
Pd(1)–P(3) bite angle of 95.05(4)◦, which is ca. 5◦ higher than that
reported for other six-membered PdII complexes with chelating
diphosphines.17 The six-membered ring system containing the
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for 1a·2CH3COCH3
and (S,S)-1b
1a·2CH3COCH3
(S,S)-1b
Pd(1)–P(1)
Pd(1)–P(2)
2.2546(13)
2.2671(11)
2.2757(11)
Scheme 6
Pd(1)–P(3)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
2.2403(13)
2.3288(14)
2.3436(14)
2.3620(11)
2.3608(10)
90.11(4)
Compounds 1a and 2a are slightly soluble in water at room
temperature and very soluble in DMF and DMSO. They have been
characterized in solution by multinuclear NMR spectroscopy.
P(1)–Pd(1)–P(2)
P(1)–Pd(1)–P(3)
Cl(1)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–P(2)
Cl(1)–Pd(1)–P(3)
Cl(2)–Pd(1)–P(1)
Intramolecular distances/A
Pd(1)–O(1)
Pd(1)–O(2)
Pd(1)–O(3)
Pd(1)–O(4)
Pd(1)–O(5)
Pd(1)–O(6)
95.05(4)
91.42(5)
91.53(4)
177.98(4)
1
Both compounds exhibit a 31P{ H} NMR spectrum containing
two signals in a 1 : 1 ratio that are assigned to the phosphonium
atoms (ca. 24.3 ppm) and the coordinated phosphine atoms
(30.60 ppm for 1a and 32.50 ppm for 2a, respectively). In the
177.76(5)
175.82(5)
179.62(5)
˚
3.2949(33)
5.2054(37)
3.5077(36)
5.1814(31)
5.1989(31)
3.4846(35)
case of 1a the two 31P{ H} NMR signals appear as triplets with
1
3
a J(PP) value of 24.9 Hz, while 2a exhibits two broad singlets.
1
The H NMR spectra of 1a and 2a show a broad triplet at ca.
4.8 ppm with a J(HP) coupling constant of 15.0 Hz due to the
phosphonium bridging CH2 group.
3.5361(39)
5.1555(40)
2
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metal centre is connected to a folded cyclobutane ring and to a
further six-membered heterocyclic ring, which contains two phos-
phonium units in the 1,3 positions with respect to the cyclobutane
ring, which exhibits a folding angle of 27.94(37)◦. The two six-
membered ring systems are almost perpendicular to each other,
which is shown by an interplanar angle of 86.43(10)◦ between the
two planes defined by P(1), C(1), C(3), P(3) and P(2), C(2), C(4),
P(4). A folding along the coordinating phosphorus atoms P(1)
and P(3) is apparently hindered by the steric crowding created by
the eight ortho-methoxyphenyl groups.4b Evidence for ring strain
in the six-membered palladacycle is provided by the bond angles
Pd(1)–P(1)–C(1), Pd(1)–P(3)–C(3), P(3)–C(3)–C(2) of 114.97(14),
113.45(14) and 118.5(3)◦, respectively. These deviate significantly
from known unstrained cases that show values of ca. 109◦. The
ortho-methoxyphenyl rings on the phosphorus donor atoms P(1)
and P(3) deviate significantly from an ideal diagonal disposition
with respect to the coordination plane of palladium. This is shown
by the deviation of the four ipso-carbon atoms C(11), C(21), C(51)
Fig. 2 ORTEP diagram of (S,S)-1b with 30% probability ellipsoids.
centre is square-planarly coordinated by two phosphorus and
two chloride atoms. The deviation of the metal centre from the
coordination plane, defined by the atoms P(1), P(2), Cl(1) and
˚
and C(61) of 1.199(5), −1.521(5), −0.711(6) and 2.117(5) A from
the coordination plane defined by the atoms P(1), P(3), Cl(1)
and Cl(2), due to the interplanar angle of 11.47(24)◦ between
the planes defined by Pd(1), P(1), C(1) and Pd(1), P(3), C(3). The
two H atoms attached to the bridging carbon atom C(5) have been
located by difference Fourier maps. Both the P(4)–C(5) and P(2)–
2+
˚
Cl(2), is 0.0192(7) A in the direction of C(1). Unlike 1a , (S,S)-1b
exhibits a typical P(1)–Pd(1)–P(2) bite angle of 90.11(4)◦, which
is comparable to that of many other six-membered palladium
diphosphine complexes.18 The Pd(1)–P(1) and Pd(1)–P(2) bond
˚
C(5) bond lengths of 1.806(4) and 1.803(4) A, respectively, and
˚
lengths of 2.2671(11) and 2.2757(11) A, respectively, are signifi-
the bond angle P(4)–C(5)–P(2) of 117.9(2)◦ match well the values
reported for cyclic CH2 bridged diphosphonium units.16a,18 The
intramolecular distances between Pd(1) and the four o-methoxy
oxygen atoms O(1), O(2), O(5) and O(6), in the range from
cantly longer in (S,S)-1b than in 1a2+ (2.2546(13) and 2.2403(13)
˚
A, respectively). This effect may be accounted for the higher ring
strain of the palladacycle in 1a2+ as compared to (S,S)-1b, which
would trigger a higher s-character of the phosphorus orbitals in
the former complex.21
˚
3.2949(33)–5.2054(37) A, are not bonding interactions. Like those
observed in NiII(o-MeO-dppp) complexes (o-MeO-dppp = 1,3-
bis((di-2-o-methoxyphenyl)phosphino)propane), the diagonally
disposed oxygen atoms O(1) and O(5) are close to pseudo-apical
positions of a distorted octahedron.19 The two acetone molecules
in the crystal lattice build up a web of short intermolecular
distances. Within these short contacts the most important are:
O(11) · · · H(1A) 2.421, O(11) · · · H(5A) 2.301, O(11) · · · H(27A)
Like in other S,S-skewphos metal complexes, the carbon
backbone of the ligand in (S,S)-1b exhibits a diagonal disposition
with respect to the coordination plane, which is the most common
conformation for skewphos metal complexes.20 Strictly related to
the diagonal disposition of the C5 carbon backbone, the four 2-
methoxyphenyl groups are disposed diagonally with respect to
the Pd coordination plane, which is shown by the deviation of
the four ipso carbon atoms from the coordination plane (C(1)
0.9327(51), C(8) −1,7077(45), C(15) 1.6957(43) and C(22) −0.8764
˚
2.698, O(10) · · · H(16A) 2.590 and H(6A) · · · C(25) 2.745 A.
The reaction of rac-o-MeO-bdpp with PdCl2(COD) in CH2Cl2
gave PdCl2(rac-o-MeO-bdpp) (1b), while the analogous reaction
with Pd(OAc)2 yielded Pd(OAc)2(rac-o-MeO-bdpp (2b) as an
impure product. However, the reaction of 1b with Ag(OAc) in
CH2Cl2 gave 2b in a pure form. 1b and 2b were obtained in 70 and
60% yield, respectively (Scheme 7). The diphosphine rac-o-MeO-
bdpp is just the o-methoxy derivative of the well-known ligand
skewphos ligand.20
˚
A). Furthermore, two o-methoxy oxygen atoms, namely O(1) and
O(4), show shorter intramolecular distances to Pd(1) (3.5077(36)
˚
and 3.4846(35) A, respectively), while O(2) and O(3) show by
far a higher intramolecular distance to Pd(1) of 5.1814(31) and
˚
5.1989(31) A, respectively.
Catalytic copolymerisation of CO and ethene
The solvent of choice of most CO/C2H4 copolymerisation
reactions by PdX2(diphosphine) catalysis (Scheme 8) is either
MeOH or water. This implies the use of weakly coordinating co-
ligands (X = TsO−, CF3CO2−) to allow for the activation of either
solvent to give Pd–H, Pd–OMe or Pd–OH initiators.2 Recently,
however, Toniolo et al. have shown that the bis-chloride and bis-
acetate complexes PdX2(diphosphine) (diphosphine = dppe, dppp,
Scheme 7
The crystal structure of 1b shows the S,S stereoisomer (S,S-1b),
which crystallizes in the chiral space group P21. A selected ORTEP
plot of S,S-1b is reported in Fig. 2.
Crystallographic data and selected bond distances and angles
for (S,S)-1b are reported in Tables 1 and 2, respectively. The Pd
Scheme 8
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Table 3 Alternating copolymerisation of CO with ethene catalysed by 1a and 1b in water–AcOHa
Entry
Precatalyst
Water/mol%
Time/h
Polymer/g
Productivityb
Mn/kg mol−1
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
55
65
65
75
85
55
65
75
75
85
1
1
2
1
1
1
1
1
2
1
3.10
3.70
6.46
3.18
2.19
4.12
4.75
5.28
7.84
3.91
5.83
6.95
6.07
5.98
4.11
7.74
8.93
9.92
7.37
7.35
7.3
7.1
37.4
37.2
10
a Catalytic conditions: catalyst precursor, 0.005 mmol; p(CO) and p(C2H4), 20 and 20 bar; water–AcOH, 100 mL; temperature, 85 ◦C; stirring rate, 1100
rpm. b Expressed as kg(polyketone) (g(Pd) × h)−1
.
dppb) (X = Cl−, OAc−) can generate very active catalysts in water–
AcOH mixtures.22 Therefore, the bis-chloride complexes 1a and 1b
were tested in different mixtures of water–AcOH under standard
experimental conditions (40 bar 1 : 1 CO : ethene, 85 ◦C). The
results obtained are reported in Table 3. Irrespective of catalyst and
solvent composition, perfectly alternating polyketone products
were obtained with exclusively ketone end-groups.
A maximum catalytic activity for 1a was observed with the use
of a 65 and 35 mol% mixture of water–AcOH (entry 2), while a
mixture containing 75 mol% water was the optimum value for 1b
(entry 8). In reactions lasting 1 and 2 h, 1b was slightly more active
than 1a but appreciably less stable with time (entries 2 and 3 vs 8
and 9). However, the most striking catalytic difference between 1a
and 1b was the Mn of the polyketone products: around 7 kg mol−1
with 1a and 37 kg mol−1 with 1b (Table 3). These results indicate
that the chain-transfer rate is significantly faster in the reactions
catalysed by 1a than for the reactions catalysed by 1b.
et al.22 and Zudin et al.23 have also demonstrated that increasing
the water proportion in the water–AcOH mixture increases the
concentration of the cationic PdII–H species by speeding up the
solvolysis process. On the other hand, too large a proportion of
water has been found to have a detrimental effect on the solubility
22
of CO and C2H4. This may account for the significant decrease
in productivity (from 6.95 to 4.11 kg (g(Pd) × h)−1) by increasing
the water proportion from 65 to 85 mol% (Table 3, entries 2,4,5).
The chain-transfer reaction of CO/ethene copolymerisations
performed in acidic aqueous media has been demonstrated to
occur exclusively via protonolysis by water with formation of a
Pd–OH unit that regenerates a Pd–H initiator by WGS.24 In these
conditions, the intimate mechanism of chain transfer involves a b-
hydride elimination in a b-keto alkyl chelate, followed by hydride
migration to give an enolate whose regioselective protonation
terminates the chain growth (Scheme 10).25 The rate of the b-
H elimination determines the overall chain transfer rate, hence it
controls the molecular weight of the polyketone produced.2
b-H elimination reactions in organometallics are steered by
both electronic and steric factors.26 In particular, it is agreed that
the agostic interaction between the metal and b-hydrogen (the
precursor to hydrogen transfer to the metal) is favoured by a low
electron density at the metal centre. In complexes of the same metal
ion, the electron density at the metal depends on the electronic and
steric characteristics of the supporting ligand(s) as well as on the
overall charge of the complex. In the present case, a lower electron-
donating ability of (o-MeO-PCP)(PF6)2 as compared to rac-o-
MeO-bdpp is indicated by the significantly shorter Pd–P bond
The mechanism of the CO/C2H4 copolymerisation by PdII–
diphosphine catalysis has been intensely studied over recent years
and most of the elementary steps involved, especially chain-
transfer and propagation, have been clarified.2 Scheme 9 provides
a brief summary of the mechanism proposed for CO/ethene
copolymerisations catalysed by PdCl2(diphosphine) complexes in
acidic aqueous media.23
˚
distances in 1a as compared to 1b (2.2546(13) and 2.2403(13) A
˚
vs. 2.2671(11) and 2.2757(11) A). As mentioned previously, due to
the ring strain of the six-membered palladacycle, the phosphorus
donor atoms in 1a exhibit a higher s-character, and hence have
shorter van der Waals radii, than in 1b.27 As a further, indirect
proof of the lower metal basicity of complexes with (o-MeO-
PCP)(PF6)2 vs. complexes with rac-o-MeO-bdpp, we have found
that the IR spectrum of the bis-carbonyl complex cis-[Rh(CO)2(o-
MeO-PCP)](PF6)3 shows, in CH2Cl2, m(CO) bands at 2097 and
2052 cm−1, while cis-[Rh(CO)2(rac-o-MeO-bdpp)](PF6) exhibits
red shifted bands at 2086 and 2036 cm−1.10
Scheme 9
It is agreed that the neutral PdII–H complexes are generated
from the bis-chloride precursors by a water gas shift reaction
(WGS),23 and then converted into the catalytically active cationic
PdII–H species by a water-controlled solvolysis process.23a Toniolo
Besides metal basicity, the b-H elimination rate in the propagat-
ing Pd–alkyl (Scheme 10), might be influenced by the interaction
of the metal centre with the oxygen atoms from the ortho-methoxy
2970 | Dalton Trans., 2006, 2964–2973
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Scheme 10
substituents. Indeed, evidence has been reported in the literature
for the coordinative interaction of these oxygen to metal centres.28
In the eventuality of an analogous interaction in the propagating
alkyls generated by 1a and 1b, the oxygen atoms would compete
with the b-hydrogens for coordination, with a retarding effect
on the b-H elimination rate. In this mechanistic picture, the
stereochemical rigidity of (o-MeO-PCP)(PF6)2 would disfavour
the interaction of the o-methoxy units with the palladium centre,
whereas a simple twisting of the C5 backbone of rac-o-MeO-bdpp
would allow for an axial interaction between the o-methoxy groups
and the metal centre.
NMR spectrum acquired at 20 ◦C showed a broad resonance
centred at 28.00 ppm (trace a).29 The HPNMR tube was then
pressurized with a 1 : 1 mixture of CO and C2H4 to a total
1
pressure of 40 bar and a 31P{ H} NMR spectrum was acquired
at 20 ◦C (trace b), which showed no substantial change. The
probe-head was heated to 85 ◦C for 1 h during which time
1
31P{ H} NMR spectra were acquired every 20 min. Since the
first spectrum, a quite sharp singlet appeared at 25.5 ppm. The
31P{ H} spectrum acquired after 1 h at 85 ◦C is reported in
1
Fig. 3 as trace c. When the probe-head was cooled to 20 ◦C,
1
the 31P{ H} NMR spectrum showed the same broad hump as in
In an attempt to gain further information on the present
copolymerisation reactions in water–AcOH, a catalytic run was
performed in a HPNMR tube containing D2O–AcOH (45 mol%
the initial spectrum (trace d). The tube was removed from the
NMR probe showing formation of the copolymer as an off-white
1
powder. A very similar sequence of 31P{ H} HPNMR spectra has
1
D2O) and catalyst precursor 1b. A selected sequence of 31P{ H}
been observed for the CO/ethene copolymerisation catalysed by
Pd(CF3CO2)2(NaDPPPDS) in water.24b It was suggested, and the
same can be said for the present case, that most of the palladium
remains incorporated into the precursor from which a small and
undetectable aliquot is delivered into the catalysis cycle.
The catalytic activity of the neutral bis-acetate complexes 2a
and 2b was tested in MeOH in the presence of 2 or 20 equiv. of
TsOH, which serves both as a scavenger of the acetate ion and as an
oxidizing agent for Pd0 species eventually formed by PdII reduction
(Scheme 9).2 The results of this study are reported in Table 4.
With either catalyst precursor, the copolymer contained keto
and ester end-groups in a 1 : 1 ratio. As observed in the water–
AcOH solvent, the (o-MeO-PCP)(PF6)2 precursor, 2a, gave lower
molecular weight products (7.7–13.9 kg mol−1) compared to
the rac-o-MeO-bdpp precursor (47.0–48.2 kg mol−1). However,
with either catalyst the copolymers exhibited larger Mn values
compared to the products obtained in water–AcOH, which is
consistent with a slower chain transfer rate in MeOH than in
water–AcOH.
HPNMR spectra is presented in Fig. 3.
Increasing the concentration of TsOH had a beneficial effect on
the productivity of 2a (entries 1 vs. 3 and 2 vs. 4) and a detrimental
effect on that of 2b (entries 5 vs. 7 and 6 vs. 8).
The lower activity in the presence of 2 equiv. of TsOH and the
much higher Mn of the polyketones obtained with 20 equiv. of
acid in the reactions with 2a suggest that the acid concentration
is important not only for decreasing the extent of PdII reduction
but also for stabilising the overall metal–ligand assembly, likely by
inhibiting the deprotonation of the diphosphonium-diphosphine
ligand with formation of ylide species.
1
Fig. 3 Selected 31P{ H} HPNMR spectra (sapphire tube, D2O–AcOH,
20–85 ◦C, 81.01 MHz) recorded during a CO/C2H4 copolymerisation
reaction catalysed by 1b: (a) 1b suspended in D2O–AcOH (45 mol% D2O)
at 20 ◦C; (b) after pressurisation of the tube with a 1 : 1 mixture of CO
and C2H4 (40 bar total pressure) at 20 ◦C; (c) spectrum taken after 1 h at
85◦ C); (d) after cooling to 20 ◦C.
A 10 mm sapphire tube was charged with 1b (0.02 mmol) in a
solvent mixture D2O–AcOH (45 mol% D2O) (2 mL). A 31P{ H}
1
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Table 4 Alternating copolymerisation of CO with ethene catalysed by 2a and 2b in MeOH in the presence of TsOHa
Entry
Precatalyst
TsOH (equiv.)
Time/h
Polymer/g
Productivityb
Mn/kg mol−1
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2b
2b
2b
2b
2
2
20
20
2
2
20
20
1
3
1
3
1
3
1
3
4.00
4.50
5.21
13.48
3.94
11.54
2.90
7.52
2.82
9.79
8.45
7.41
7.23
5.45
4.04
7.7
13.9
47.0
48.2
6.45
a Catalytic conditions: catalyst precursor, 0.005 mmol; p(CO) : p(C2H4), 20 : 20 bar; MeOH, 100 mL, temperature, 85 ◦C; stirring rate, 1100 rpm.
b Expressed as kg(polyketone) (g(Pd) × h)−1
.
The reverse dependence of the productivity on the acid con-
centration for the reactions catalysed by the rac-o-MeO-bdpp
precursor 2b (entries 5–8) is quite surprising as the opposite trend
is generally observed for CO/ethene copolymerisations in MeOH
by PdII–diphosphine catalysis.2 In our opinion, this effect is a direct
consequence of the presence of the o-methoxy substituents on the
phenyl rings. Indeed, a large excess of TsOH, containing one water
molecule of crystallization, would promote the creation of a web of
hydrogen bonding interactions around the metal centre, involving
not only the coordinated solvent molecules but also the four
oxygen atoms of the o-methoxyphenyl groups. As a result, the PdII
centrewouldbecome less accessible to the incoming monomers,30,31
and contemporaneously the beneficial effect of the o-methoxy
groups on the propagation rate would be diminished and even
lost.2d,28,32 Support to the hydrogen bonding interactions around
the palladium centre in rac-o-MeO-bdpp catalysis intermediates is
provided by several molecular structures of PdII–aquo complexes
with TsO− anions, which show the presence of hydrogen bonds
between the coordinated water molecules and the oxygen atoms
of both tosylate ligands and counteranions.5c,30 Moreover, DFT
calculations performed on cationic [L3Pd(H2O)]+ compounds have
confirmed that the stability of such compounds is due to an
efficient interaction of the counteranions with the coordinated
water molecule.33
Acknowledgements
Thanks are due to the European Commission (Contract HPRN-
CT-2002-00196, PALLADIUM project). G. C. and A. D. thank
the University of Innsbruck for PhD grants. This research was
also financially supported by the Fonds zur Fo¨rderung der
wissenschaftlichen Forschung, Vienna, Austria and the Tiroler
Wissenschaftsfonds, Innsbruck, Austria.
References
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P. Meunier and P. Kalck, Coord. Chem. Rev., 2003, 236, 143; (b) J. P.
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Conclusions
The pool of available chelating diphosphine ligands has been
enriched by two new members. The bis-cationic diphosphonium-
diphosphine (o-MeO-PCP)(PF6)2 has been synthesized by a ki-
netically controlled regioselective bis-alkylation of o-MeO-dppcb,
using CH2Cl2 as an alkylating agent, while a standard procedure
has been employed to prepare rac-o-MeO-bdpp. Both ligands have
been used to coordinate PdCl2 and Pd(OAc)2 yielding square-
planar complexes that generate active catalysts for the copoly-
merisation of CO and ethene in either water–AcOH mixtures or
MeOH–TsOH. The perfectly alternating polyketones bear either
exclusively ketone or 1 : 1 ketone ester end groups depending
on the reaction solvent. The polyketone products produced with
the diphosphonium-diphosphine catalysts show number-average
molecular weights up to five times smaller than those obtained
with the phosphine catalysts. These results have been interpreted
in terms of faster chain-transfer rate due to the electronic and
steric properties of the diphosphonium-diphosphine ligand.
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