Synthesis, structure, and butadiene polymerization behavior of alkylphosphine cobalt(II) complexes

Article abstract of DOI:10.1016/j.molcata.2004.10.022

The synthesis, characterization and behavior in polymerization of 1,3-butadiene of some cobalt(II) phosphine complexes (CoCl₂(PEt ₃)₂, CoCl₂(PⁿPr₃) ₂, CoCl₂(PCy_2<

Full text of DOI:10.1016/j.molcata.2004.10.022

Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
Synthesis, structure, and butadiene polymerization behavior of
alkylphosphine cobalt(II) complexes
Giovanni Riccia,∗, Alessandra Fornib, Aldo Bogliaa, Tiziano Mottaa
a
CNR, Istituto per lo Studio delle Macromolecole (ISMAC), via E. Bassini 15, 20133 Milano, Italy
CNR, Istituto di Scienze e Tecnologie Molecolari (ISTM), via E. Golgi 19, 20133 Milano, Italy
b
Received 10 August 2004; received in revised form 15 October 2004; accepted 16 October 2004
Abstract
The synthesis, characterization and behavior in polymerization of 1,3-butadiene of some cobalt(II) phosphine complexes (CoCl2(PEt3)2,
CoCl2(PnPr3)2, CoCl2(PCy2H)2, CoCl2(PtBu2H)2, CoCl2(PtBu2Me)2, CoCl2(PCyp3)2, CoCl2(PCy3)2; Et = ethyl, nPr = normal-propyl,
Cy = cyclohexyl, tBu = tert-butyl, Cyp = cyclopentyl) are described. The X-ray diffraction studies of CoCl2(PCy3)2 and CoCl2(PCy2H)2 are
reported. Treatment of these complexes with methylaluminoxane (MAO) leads to highly active butadiene polymerization catalysts; polymers
having different microstructure have been obtained depending on the type of phosphine ligand bonded to the cobalt atom. An interpretation
for this particular behavior, based on the 1,3-dienes polymerization mechanism previously proposed, is given.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Cobalt phosphine complexes; X-ray structure; Cobalt catalysts; Butadiene polymerization; Polybutadiene
1. Introduction
units are randomly distributed along the polymer chain [8]. A
highly cis-1,4 polybutadiene was obtained with CoCl2–MAO
[9], while a predominantly 1,2 syndiotactic polymer was ob-
tained with the same system in presence of PPh3 [10]. These
latter results seem to suggest that the presence of a ligand
bonded to the metal strongly affects the catalyst chemose-
lectivity and consequently the polymer microstructure. For a
better understanding of this behavior we have prepared sev-
eral cobalt(II) phosphine complexes; the X-ray structures of
some of these complexes have been determined, after which
they have been used in combination with MAO for poly-
merizing butadiene. The most significant results obtained are
reported in this paper.
Cobalt catalysts play a very important role in the field of
butadiene polymerization since, depending on the catalytic
formulation, they can give cis-1,4 polybutadiene or 1,2 syn-
diotatctic polybutadiene, the only two diene polymers of in-
dustrial interest and industrially produced [1].
For instance, the system Co(acac)3–AlEt2Cl–H2O gives
a highly cis-1,4 polybutadiene (∼97%) [2]; equibinary
cis-1,4/1,2 polybutadiene is obtained with the system
Co(acac)3–AlEt3–H2O [3]; when CS2 is added to this system
1,2 syndiotactic polybutadiene is obtained [4]. Highly syn-
diotactic 1,2 polybutadiene is also prepared with the system
(C4H6)(η5-C5H8)Co–CS2 [5], but also the cobalt complex
alone is able to give highly syndiotactic polybutadiene [6].
Moreover, it has been reported that the system
CoBr2(PPh3)2–MAO gives from butadiene a polymer with
a mixed cis-1,4/1,2 structure [7] in which the cis-1,4 and 1,2
2. Experimental
2.1. Materials and methods
∗
Triethylphosphine (PEt3) (Strem, 99% pure), tri-normal-
Corresponding author. Tel.: +39 02 23699376; fax: +39 02 2362946.
E-mail address: giovanni.ricci@ismac.cnr.it (G. Ricci).
propylphosphine (PnPr3) (Strem, min. 95% pure), dicy-
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.10.022

236
G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
clohexylphosphine (PCy2H) (Strem, 98% pure), di-tert-
butylphosphine (PtBu2H) (Strem, >95% pure), di-tert-
butylmethylphosphine (PtBu2Me) (Strem, >98% pure), tri-
cyclopentylphosphine (PCyp3) (Strem, >97% pure), tricy-
clohexylphosphine (PCy3) (Strem, 97% pure), anhydrous
cobalt dichloride (Aldrich, 99.9% pure) and MAO (Cromp-
ton, 10 wt.% solution in toluene) were used as received. Ethyl
alcohol (Carlo Erba, 96◦) was degassed under vacuum, then
by bubbling dry dinitrogen and kept over molecular sieves;
pentane (Carlo Erba, >99% pure) was refluxed over Na/K
alloy for ca. 8 h, then distilled and stored over molecular
sieves under dry dinitrogen; toluene (Carlo Erba, 99.5% pure)
was refluxed over Na for ca. 8 h, then distilled and stored
over molecular sieves under dry dinitrogen. 1,3-Butadiene
(Air Liquide, >99.5% pure) was evaporated from the con-
tainer prior to each run, dried by passing through a column
packed with molecular sieves and condensed into the reac-
tor which had been precooled to −20 ◦C. All the phosphine
cobalt complexes were synthesized as indicated below, fol-
lowing a general experimental procedure reported in [11].
The elemental analyses of the cobalt complexes were per-
formed by the analytical laboratories of Polimeri Europa-
Centro Ricerche Novara-“Istituto Guido Donegani”; infrared
spectra were recorded as KBr disks with a Bruker IFS 48 in-
strument.
was extracted in continuous with boiling pentane for about
24 h. At the end of the extraction a crystalline blue prod-
uct was formed; the supernatant solution was removed and
further crops of crystals were obtained by cooling it at
−30 ◦C. Yield: 3.82 g (92% based on CoCl2). Anal. Calcd.
for C18H42CoCl2P2: Co, 13.09; Cl, 15.75; P, 13.76. Found:
Co, 13.3; Cl, 16.1; P, 13.8.
Spectroscopic data: IR (KBr) ν (cm−1) 2961s, 2873s,
1462s, 1417s, 1378m, 1223m, 1088s, 1050m, 905w, 850m,
761s, 724m, 645w, 434w.
2.4. Synthesis of CoCl2(PCy2H)2
To a solution of CoCl2 (0.81 g, 6.2 mmol) in ethyl alco-
hol, dicyclohexylphosphine (3.69 g, 18.6 mmol) was added
under stirring. A blue precipitate is immediately formed; the
suspension was kept under vigorous stirring for 24 h, then
filtered. The blue solid on the filter was washed with ethyl
alcohol (3× 10 mL) and pentane (2× 10 mL), then dried un-
der vacuum. It was then extracted in continuous with boiling
pentane for several days; at the end a crystalline product is
formed on the bottom of the Schlenk flask containing the ex-
tracting solution and further crops of crystals can be isolated
by removing the supernatant pentane solution and cooling it
at −30 ◦C. Yield: 2.74 g (83.9% based on CoCl2).
Anal. Calcd. for C24H46CoCl2P2: Co, 11.20; Cl, 13.47; P,
11.77. Found: Co, 11.1; Cl, 13.4; P, 11.9.
2.2. Synthesis of CoCl2(PEt3)2
Spectroscopic data: IR (KBr) ν (cm−1) 2917s, 2847s,
1450s, 1347w, 1327w, 1302w, 1264m, 1194m, 1182m,
1112m, 1074w, 1002m, 917w, 891m, 867w, 853m, 822m,
806s, 713w, 511w.
Triethylphosphine (2 mL, 13.9 mmol) was added to a solu-
tion of CoCl2 (0.60 g, 5.53 mmol) in ethyl alcohol (50 mL).
The solution immediately became dark blue. The reaction
was kept under stirring for 20 h at room temperature, then
the solvent was removed under vacuum; a large amount of
blue needles were formed while removing the solvent. The
blue crystalline residue was washed with small amounts of
pentane at low temperature and dried again under vacuum;
then it was extracted in continuous with boiling pentane for
about 24 h. At the end of the extraction a crystalline blue
product was formed; the supernatant solution was removed
and further crops of crystals were obtained by cooling it at
−30 ◦C. Yield: 1.72 g (64.2% based on CoCl2).
2.5. Synthesis of CoCl2(PtBu2H)2
Di-tert-butylphosphine(1.62 g, 11.1 mmol) was added to a
solutionofCoCl2 (0.48 g, 3.7 mmol)inethylalcohol(20 mL).
An extremely dense suspension was immediately formed,
which was kept under vigorous stirring for ca. 24 h, then
filtered. A blue residue is obtained, which was washed with
ethyl alcohol (3× 10 mL) and pentane (2× 10 mL), dried
under vacuum and then extracted in continuous with boiling
pentane for a few days. Crystals of the complex are formed
directly during the extraction and further crops of crystals are
obtained by removing the supernatant pentane solution and
cooling it at −30 ◦C. Yield: 1 g (64% based on CoCl2).
Anal. Calcd. for C16H38CoCl2P2: Co, 13.96; Cl, 16.79; P,
14.67. Found: Co, 13.8; Cl, 17.1; P, 14.9.
Anal. Calcd. for C12H30CoCl2P2: Co, 16.10; Cl, 19.36; P,
16.92. Found: Co, 16.2; Cl, 19.5; P, 17.0.
Spectroscopic data: IR (KBr) ν (cm−1) 2970s, 2938m,
2880m, 1459s, 1417s, 1383m, 1259w, 1121w, 1038s, 1013w,
771s, 758s, 736m, 699w, 623w.
2.3. Synthesis of CoCl2(PnPr3)2
Spectroscopic data: IR (KBr) ν (cm−1) 3002w, 2981m,
2955s, 2902m, 2868m, 1471s, 1394w, 1370s, 1208w, 1180s,
1141w, 1110w, 1030s, 962w, 944m, 920w, 885m, 820m,
804m, 616m, 584w, 464m, 409w.
Tri-normal-propylphosphine (4.6 mL, 23.1 mmol) was
added under stirring to a solution of CoCl2 (1.20 g,
9.22 mmol) in ethyl alcohol (50 mL). The solution became
immediately dark blue; after 20 h under stirring the sol-
vent was removed under vacuum at room temperature. The
blue residue was washed with small amounts of pentane
at low temperature and dried again under vacuum; then it
2.6. Synthesis of CoCl2(PtBu2Me)2
To a solution of CoCl2 (0.37 g, 2.8 mmol) in ethyl alcohol
(25 mL) a solution of di-tert-butylmethylphosphine (1.35 g,

G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
237
Table 1
8.4 mmol)inethylalcohol(10 mL)wasadded. Abluesuspen-
◦
˚
Selected bond lengths (A) and angles ( ) for CoCl2(PCy2H)2
sion was immediately formed; it was kept under stirring for
24 h, then filtered. The blue residue on the filter was washed
with small amounts of ethyl alcohol and pentane, then dried
under vacuum. Crystals of CoCl2(PtBu2Me)2 were obtained
by extraction of the blue solid in continuous with boiling pen-
tane for some days; the crystals were formed directly during
the extraction and further crops of crystals were obtained
by removing the pentane solution and cooling it at −30 ◦C.
Yield: 0.88 g (70.3% based on CoCl2).
Co Cl 2.2182(6)
Co
P
P
P
C1
C7
2.3942(6)
1.8370(16)
1.858(3), 1.804(12)
109.81(3)
105.87(3)
Cl Co Clꢀ
P
Co Pꢀ
Cl Co
C1
P
C7
109.69(2)
104.73(10), 118.1(4)
P
Anal. Calcd. for C18H42CoCl2P2: Co, 13.09; Cl, 15.75; P,
13.76. Found: Co, 13.7; Cl, 16.3; P, 13.5.
2.9. X-ray crystallographic studies
Spectroscopic data: IR (KBr) ν (cm−1) 2963s, 2949s,
2905m, 2873m, 1475s, 1396m, 1372s, 1304s, 1182w, 1127s,
1101s, 1047m, 1025m, 943m, 886s, 818s, 759m, 726w,
641m, 485w, 471w, 455m, 414w.
A summary of the experimental details concerning the X-
ray diffraction study of CoCl2(PCy2H)2 and CoCl2(PCy3)2
is reported in Table 4. The crystals used for data collection
were entirely covered with perfluorinated oil to reduce crys-
tal decay. X-ray data were collected on a Bruker Smart Apex
CCD area detector equipped with fine-focus sealed tube oper-
ating at 50 kV and 30 mA. The first 100 frames were collected
also at the end of the data collection to monitor crystal decay.
Data reduction was made using SAINT programs; absorption
corrections based on multiscan were obtained by SADABS
[12]. The structures were solved by SIR92 [13] and refined
on F2 by full-matrix least-squares using SHELX97 [14]. The
program ORTEP-III [15] was used for molecular graphics.
2.7. Synthesis of CoCl2(PCyp3)2
Tricyclopentylphosphine (3.0 g, 12.5 mmol) was added
to a solution of CoCl2 (0.64 g, 4.9 mmol) in ethyl alcohol
(40 mL). A blue suspension was rapidly formed: it was kept
under stirring for 24 h, then filtered. The blue residue was
washed with ethyl alcohol and pentane, dried under vacuum,
then extracted in continuous with boiling pentane for a few
days. A crystalline product is formed directly during the ex-
traction, while further crops of crystals are obtained from the
supernatant by concentrating and cooling at low temperature.
Yield: 2.44 g (82.1% based on CoCl2).
2.10. Polymerization
All operations were carried out under an atmosphere of dry
dinitrogen. A standard procedure is reported. 1,3-Butadiene
was condensed into a 25 mL dried glass reactor kept at
−20 ◦C, then toluene was added and the solution so obtained
was brought to the desired polymerization temperature. MAO
and the cobalt compound were then added, as toluene solu-
Anal. Calcd. for C30H54CoCl2P2: Co, 9.72; Cl, 11.69; P,
10.21. Found: Co, 9.6; Cl, 11.7; P, 10.4.
Spectroscopic data: IR (KBr) ν (cm−1) 2956s, 2911m,
2867s, 1449s, 1372w, 1301m, 1262m, 1238m, 1128s, 910w,
852m, 536m, 523m, 484w.
Table 2
◦
˚
Selected bond lengths (A) and angles ( ) for CoCl2(PCy3)2
2.8. Synthesis of CoCl2(PCy3)2
Co Cl1
Co Cl2
Co P1
Co P2
P1 C1
P1 C7
P1 C13
P2 C19
2.2319(6)
2.2389(6)
2.4375(6)
2.4345(6)
1.848(2)
1.854(2)
1.857(2)
1.881(4), 1.882(4)
1.852(2)
1.851(2)
112.69(3)
125.61(2)
110.76(2)
98.78(2)
Tricyclohexylphosphine (6.24 g, 22.2 mmol) was added
to a solution of CoCl2 (1.16 g, 8.9 mmol) in ethyl alcohol
(50 mL). After a few minutes a blue suspension was formed;
it was kept under stirring for 24 h, then filtered. The residue
on the filter was washed with ethanol (3× 10 mL), then dried
under vacuum. A crystalline product was obtained by ex-
tracting the blue residue in continuous with boiling pentane;
blue crystals were formed directly during the extraction. The
supernatant was removed and further crops of crystals were
obtained by cooling it at −30 ◦C. Yield: 5.51 g (89.6% based
on CoCl2).
P2 C25
P2 C31
Cl1 Co Cl2
P1 Co P2
Cl1 Co P1
Cl1 Co P2
Cl2 Co P1
Cl2 Co P2
C1 P1 C7
C1 P1 C13
C7 P1 C13
C19 P2 C25
C19 P2 C31
C25 P2 C31
99.14(2)
110.31(2)
110.68(10)
103.04(9)
105.60(10)
118.7(2), 95.8(2)
96.7(2), 112.6(2)
104.20(10)
Anal. Calcd. for C36H66CoCl2P2: Co, 8.53; Cl, 10.27; P,
8.79. Found: Co, 8.6; Cl, 10.5; P, 8.9.
Spectroscopic data: IR (KBr) ν (cm−1) 2928s, 2849s,
1447s, 1345w, 1329w, 1301w, 1268m, 1199w, 1176m,
1128w, 1113m, 1004m, 917w, 888m, 850m, 818w, 738w,
515m, 488w.

238
G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
Table 3
Polymerization of butadiene with different cobalt catalysts
Run
Catalyst
Polymerizationa
Polymer
Co-compound
θb (◦)
Time (min)
Conv (%)
Nc (min−1
)
cis-1,4d (%)
1,2d (%)
Mw (g × mol−1
)
Mw/Mn
1
2
3
4
5
6
7
CoCl2(PEt3)2
132
132
143
150
161
30
35
30
35
40
39
32
74.8
85.5
70.8
60.4
59.0
56.4
44.4
129
127
122
89
76
85
39.9
38.2
51.0
59.0
94.0
73.6
77.4
60.1
61.8
49.0
41.0
6.0
CoCl2(PnPr3)2
CoCl2(PCy2H)2
CoCl2(PtBu2H)2
CoCl2(PtBu2Me)2
CoCl2(PCyp3)2
CoCl2(PCy3)2
280000
275000
260000
245000
250000
1.9
2.0
1.8
2.0
2.1
26.4
22.6
170
72
a
b
c
d
Polymerization conditions: butadiene, 2 mL; toluene, 16 mL; Al/Co = 1000; moles of Co, 5 × 10−6; +20 ◦C.
Phosphine cone angle, as reported by Tolman (Ref. [21]).
N: moles of butadiene polymerized per mol of Co per minute.
Determined by 1H NMR, as shown in Fig. 3. Trans-1,4 units are almost negligible, as indicated by the IR spectra of the polymers.
tions, in the order. The polymerization was terminated with
methanol containing a small amount of hydrochloric acid,
the polymer was coagulated and repeatedly washed with
methanol, then dried in vacuum at room temperature.
about 10 wt.%. The infrared spectra of the polymers were per-
formed with a Bruker IFS 48 instrument, using polymer films
on KBr disks. The films were obtained by deposition from
solutions in benzene or hot (ca.100 ◦C) solutions in 1,2,4-
trichlorobenzene.
MW and MWD were determined by GPC analysis using
the universal calibration method. The GPC system was com-
posed by a Agilent 1100 pump, a detector IR Agilent 1100
and PL Mixed-A columns; tetrahydrofurane (1 mL/min) was
used as solvent at 25 ◦C.
2.10.1. Polymer characterization
1
13C and H NMR measurements were performed with a
Bruker AM 270 instrument. The spectra were obtained in
C2D2Cl4 at 103 ◦C (hexamethyldisiloxane, HMDS, as inter-
nal standard). The concentration of polymer solutions was
Table 4
Crystal data, details of data collection and results of the refinement for CoCl2(PCy2H)2 and CoCl2(PCy3)2
Compound
CoCl2(PCy2H)2
CoCl2(PCy3)2
C36H66Cl2CoP2
Formula
C
24H46Cl2CoP2
Mr
526.38
690.66
Crystal system
Space group
Z
Monoclinic
P2/c
2
Monoclinic
P21/n
4
Dcalc (g cm−3
)
1.271
1.206
˚
a (A)
11.941(2)
10.4845(10)
˚
b (A)
5.7615(12)
15.8069(15)
˚
c (A)
20.296(4)
99.96(3)
1375.3(5)
22.973(2)
91.903(10)
3805.1(6)
β (◦)
3
˚
V (A )
Crystal size
0.35 mm × 0.25 mm × 0.10 mm
0.30 mm × 0.20 mm × 0.05 mm
Colour, habit
Light blue, Rectangular prism
Light blue, Rectangular prism
µ (mm−1
)
0.944
0.698
Radiation
T (K)
Mo K␣
293(2)
68.6
Mo K␣
293(2)
58.0
2θmax (◦)
h, k, l ranges
−18 → 18; −9 → 9; −31 → 31
−14 → 14; −21 → 21; −31 → 31
Intensity decay (%)
0.00
0.00
Absorption correction
Multi-scan (Bruker SADABS)
0.876, 1.000
20912
Multi-scan (Bruker SADABS)
0.926, 1.000
79767
T
min, Tmax
Measured reflections
Rint
0.0250
0.0420
Independent reflections
Reflections with I > 2σ(I)
No. of parameters, restraints
R(F2), wR(F2)
R[F2 > 2σ(F2)], wR[F2 > 2σ(F2)]
Goodness-of-fit
5448
4074
187, 0
0.0583, 0.1157
0.0403, 0.1065
1.041
10103
7881
416, 12
0.0608, 0.1251
0.0453, 0.1151
1.050
(ꢀ/σ)max
ꢀρ max, ꢀρmin (eA
0.001
0.645, −0.238
0.002
0.631, −0.291
−3
˚
)

G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
239
Polybutadiene microstructure was determined as reported
in [16].
3. Results and discussion
3.1. Synthesis and characterization of the cobalt
phosphine complexes
The cobalt phosphine complexes were prepared fol-
lowing the experimental procedure already described in
the literature for this type of compounds [11]. The fol-
lowing phosphines were used: triethylphosphine (PEt3),
tri-normal-propylphosphine (PnPr3), dicyclohexylphos-
phine (PCy2H), di-tert-butylphosphine (PtBu2H), di-tert-
butylmethylphosphine (PtBu2Me), tricyclopentylphosphine
(PCyp3) and tricyclohexylphosphine (PCy3).
Anhydrous cobalt dichloride was dissolved in ethanol,
then the phosphine (P/Co molar ratio = 2.5–3) was added; de-
pending on the type of phosphine, completely homogeneous
blue solutions or heterogeneous blue suspensions were ob-
tained. In case of homogeneous solution, the solvent was re-
moved under vacuum and a crystalline product was obtained
by continuous extraction of the residue with boiling pentane;
in case of heterogeneous suspension, the precipitate was fil-
tered off and then again a crystalline product was obtained by
continuous extraction of the residue with boiling pentane. A
more detailed description of the cobalt complexes synthesis
is reported in Section 2.
Single crystals suitable for X-ray structure determina-
tion were obtained for the complexes CoCl2(PCy2H)2 and
CoCl2(PCy3)2 by recrystallization from pentane solution at
low temperature (−30 ◦C). In the case of CoCl2(PCyp3)2,
several attempts of recrystallization (from pentane, toluene
or pentane/toluene mixed solutions) yielded seemingly good
prismatic crystals, which however X-ray analysis proved to
be twinned. The large disorder probably affecting the cy-
clopentyl groups prevented in all cases to solve structure for
the different twin components. For the other complexes, only
microcrystalline compounds have been obtained. Attempts
to precipitate suitable monocrystals are in progress.
Fig. 1. ORTEP plot [15] of CoCl2(PCy2H)2 with atom numbering scheme.
Displacement ellipsoids are drawn at the 20% probability level.
to structural disorder which affects one cyclohexyl ring in
both structures, the C7–C12 one in CoCl2(PCy2H)2 and the
C19–C24 ring in CoCl2(PCy3)2 (in Figs. 1 and 2 only one
of the two alternative conformations is shown). However,
by comparing the weighted mean value of all the P–C dis-
˚
tances in CoCl2(PCy2H)2, 1.838(3) A, and in CoCl2(PCy3)2,
˚
1.855(4) A, we observe also in this case a small lengthening
associated to the increased bulkiness of the substituents. In
all cases the cyclohexyl groups are in the chair conformation,
with bond lengths and angles having values similar to those
normally observed for this kind of aliphatic ring.
The metal–phosphorous (M P) distances in both
CoCl2(PCy2H)2 and CoCl2(PCy3)2 complexes appear to be
particularly long if compared with those observed in the
same complexes of other transition metals whose struc-
ture is reported in literature. In NiCl2(PCy2H)2 [17], the
˚
two M P distances measure 2.1392 and 2.1618 A and in
NiCl2(PCy3)2 [18], PdCl2(PCy3)2 [19] and PtCl2(PCy3)2
[20] complexes, the unique M P bond lengths are 2.2779,
˚
2.3628 and 2.3372 A, respectively. On the other hand, the
Co Cl distances are fairly similar to the Ni Cl distances
˚
in the complexes with PCy2H [17] (2.189 and 2.203 A) and
˚
PCy3 [18] (2.1880 A) ligands, and shorter than the Pd Cl
The molecular structures of CoCl2(PCy2H)2 and
CoCl2(PCy3)2 are shown in Figs. 1 and 2, respectively,
and selected bond lengths and angles are reported in
Tables 1 and 2. The effect of the increased dimensions of
the phosphine groups on going from CoCl2(PCy2H)2 to
CoCl2(PCy3)2 is clearly visible by comparing correspond-
ing Co Cl and especially Co P bond lengths. In both cases
˚
they undergo an increase, by 0.0172(6) and 0.0418(6) A on
average, respectively, on going from the former to the lat-
ter complex, indicating an electronic rearrangement within
the core of the two complexes. The increased sterical de-
mands are also responsible of an enlargement by 19.74(3)◦
in the P Co P bond angle. The comparison between corre-
sponding P C bond lengths in the two complexes appears
to be less significant in terms of experimental error, owing
Fig. 2. ORTEP plot [15] of CoCl2(PCy3)2 with atom numbering scheme.
Displacement ellipsoids are drawn at the 20% probability level.

240
G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
˚
[19] and Pt Cl [20] bond lengths (2.3011 and 2.3167 A, re-
spectively), as expected.
3.2. Polymerization of 1,3-butadiene
The results obtained in the polymerization of butadiene
with the systems prepared by combining MAO with the dif-
ferent cobalt phosphine complexes are reported in Table 3
and can be summarized as follows.
Catalysts based on cobalt alkyl-phosphine complexes give
from butadiene polymers having different structure (cis-1,4;
mixed cis-1,4/1,2; predominantly 1,2) depending on the type
of ligand. The cis-1,4 content seems to increase with increas-
ing the ligand steric hindrance (catalysts using cyclic phos-
phines, PCy3 and PCyp3, make an exception to this rule,
even if they too give a predominantly cis-1,4 polybutadi-
ene). In general, a predominantly cis-1,4 polymer is obtained
with catalysts using more hindered ligands (θ ≥ 160◦; θ is the
phosphine cone angle as reported by Tolman [21]) (Table 3,
runs 5–7), while polybutadienes with a predominantly 1,2
structure or mixed cis-1,4/1,2 structure are obtained in the
case of less hindered phosphines (Table 3, runs 1–4). When
polybutadienes with a mixed structure are obtained, the cis-
1,4 and 1,2 units are randomly distributed along the polymer
chain, as indicated by the fact that polymers fractionation
with different solvents always give polymeric fractions hav-
ing almost the same structure as the crude polymer; it was
never possible to isolate polymer fractions having a predom-
inantly cis-1,4 or 1,2 structure. This result confirms what al-
ready reported in the literature: a random distribution of the
different butadiene units (cis-1,4 and 1,2) was also observed
by Soga in the polymerization of butadiene with the system
CoBr2(PPh3)2-MAO [8]. The 1H NMR spectra of some of the
polymers of Table 3, from which the cis-1,4/1,2 content (trans
units are almost negligible as indicated by the IR spectra of
the polymers in which the band at 967 cm−1 is extremely
low) can be determined as reported, are shown in Fig. 3.
The type of phosphine has also some influence on the
catalyst activity; the data reported in Table 3 seem to indicate
that it is higher for catalysts based on cobalt complexes with
less hindered phosphines. This fact could be interpreted by
admitting an easier alkylation of the cobalt complexes with
less hindered phosphines, with consequent formation of a
larger number of active sites. Nevertheless it is also true that,
in general, 1,3-diene polymerizations giving 1,2 polymers
are much faster (larger kinetic constant Kp) than those giving
cis-1,4 polymers.
Fig. 3. 1H NMR spectra of polybutadienes obtained with (a) CoCl2(PnPr3)2-
MAO (Table 3, run 2); (b) CoCl2(PtBu2H)2-MAO (Table 3, run 4);
(c) CoCl2(PCyp3)2-MAO (Table 3, run 6); (d) CoCl2(PtBu2Me)2-MAO
(Table 3, run 5).
unit is obtained. The type of ligand (only one is supposed to
remain bonded to the metal in the active site since otherwise,
in case of two ligands, we would have an electron in excess
with respect to the rare gas (18e) rule [23]) can influence the
reciprocal orientation of the allylic unit and the monomer;
depending on the nature of the phosphine, the C1 or the C3
insertion can be favored with formation of cis-1,4 or 1,2 units,
respectively. When the new incoming monomer has the same
probability to insert at C1 (Fig. 4a) or C3 (Fig. 4b), an equib-
The different chemoselectivity exhibited, in the polymer-
ization of butadiene, by the catalyst systems reported above
can be interpreted on the basis of the diene polymerization
mechanism proposed by Porri et al. [22]. Fig. 4 shows the well
knownschemealreadyreportedinpreviouspapers[22b]. Cis-
1,4 units and 1,2 units can originate from the same allylic unit:
if the new entering monomer inserts at C1 of the allylic unit,
a cis 1,4 unit is formed; if the monomer inserts at C3, a 1,2
Fig. 4. Scheme of formation of cis-1,4 units vs. 1,2 units.

G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 226 (2005) 235–241
241
inary cis-1,4/1,2 (50:50) polymer can form, as happened for
example in the polymerization of 1,3-butadiene with the sys-
tem Fe(acac)3-Al(iBu)3-(phen) [24].
[2] P. Racanelli, L. Porri, Eur. Polym. J. 6 (1970) 751.
[3] J. Furukawa, K. Haga, E. Kobayashi, Y. Iseda, T. Yoshimoto, K.
Sakamoto, Polym. J. 2 (1971) 371.
[4] H. Ashitaka, H. Ishikawa, H. Ueno, A. Nagasaka, J. Polym. Sci.:
Polym. Chem. Ed. 21 (1983) 1853.
[5] H. Ashitaka, K. Jinda, H. Ueno, J. Polym. Sci.: Polym. Chem. Ed.
21 (1983) 1951.
4. Conclusions
[6] G. Ricci, S. Italia, L. Porri, Polym. Commun. 29 (1988) 305.
[7] M. Takeuchi, T. Shiono, K. Soga, Polym. Int. 29 (1992) 209.
[8] M. Takeuchi, T. Shiono, K. Soga, Macromol. Rapid Commun. 16
(1995) 373.
[9] D.C.D. Nath, T. Shiono, T. Ikeda, Macromol. Chem. Phys. 203
(2002) 756.
[10] D.C.D. Nath, T. Shiono, T. Ikeda, J. Polym. Sci. A: Polym. Chem.
40 (2002) 3086.
[11] J. Chatt, B.L. Shaw, J. Chem. Soc. (1961) 285.
[12] SMART, SAINT, and SADABS, Bruker Analytical X-ray Systems,
Bruker AXS Inc., Madison, WI, 1997.
We have synthesized and characterized several phosphine
cobalt(II) complexes; some of them are new and in some
cases their crystal structures have been determined.
By polymerizing butadiene with these cobalt complexes
associated with MAO, we have been able to obtain polymers
having a microstructure ranging from predominantly 1,2 to
essentially cis-1,4; these polymers, as it is well known, are
the only two diene polymers of industrial interest.
Finally, the results obtained have also some mechanistic
implications and have been interpreted on the basis of the
diene polymerization mechanism previously proposed [22].
[13] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.C.
Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27 (1994)
435.
[14] G.M. Sheldrick, SHELX-97. Program for the Refinement of Crystal
Structures, University of Go¨ttingen: Go¨ttingen, Germany, 1997.
[15] M.N. Burnett, C.K. Johnson, ORTEP-III: Oak Ridge Thermal El-
lipsoid Plot Program for Crystal Structure Illustrations, Oak Ridge
National Laboratory Report ORNL-6895, 1996.
[16] (a) D. Morero, A. Santambrogio, L. Porri, F. Ciampelli, Chim. Ind.
(Milan) 41 (1959) 758;
Acknowledgements
We thank Mr. Giulio Zannoni for his assistance in NMR
analysis of the polymers. This work has been carried out with
the support of Polimeri Europa.
(b) V.D. Mochel, J. Polym. Sci. A-1 10 (1972) 1009;
(c) J. Zymonas, E.R. Santee, H.J. Harwood, Macromolecules 6
(1973) 129.
[17] R.A. Palmer, H.F. Jun Giles, D.R. Whitcomb, J. Chem. Soc., Dalton
Trans. (1978) 1671.
[18] P.L. Bellon, V. Albano, V.D. Bianco, F. Pompa, V. Scatturin, Ric.
Sci. 3 (1963) 1213.
[19] V.V. Grushin, C. Bensimon, H. Alper, Inorg. Chem. 33 (1994) 4804.
[20] A. Del Pra, G. Zanotti, Inorg. Chim. Acta 39 (1980) 137.
[21] C.A. Tolman, Chem. Rev. 77 (1977) 313.
[22] (a) L. Porri, in: F. Ciardelli, P. Giusti (Eds.), Structural Order in
Polymers, Pergamon Press, Oxford, 1981, p. 51;
(b) L. Porri, A. Giarrusso, G. Ricci, Prog. Polym. Sci. 16 (1991)
405.
Appendix A. Supplementary data
Tables of atomic coordinates and equivalent isotropic dis-
placement coefficients, anisotropic thermal parameters, bond
distances and angles for CoCl2(PCy2H)2 and CoCl2(PCy3)2.
This material is available free of charge via the Internet at
http://sciencedirect.com.
[23] (a) G. Ricci, M. Battistella, L. Porri, Macromolecules 34 (2001)
5766;
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