Cationic nickel complexes containing bulky phosphine ligands: Catalyst precursors for styrene polymerization

Article abstract of DOI:10.1021/om049883j

The cationic complexes [Ni(η³-CH₂C(R)CH ₂)(P)₂] IBPh₄] (R = Me, H; (P)₂ = (PMeⁱ-Pr₂)₂, (PPhⁱPr ₂)₂, 1,2-bis(diisopropylphosphino)ethane (dippe)) and [Ni(η-2-MeInd)(P)₂] [BPh₄] (2-Melnd = 2-methylindenyl; (P)₂ = (PMeⁱPr₂)₂, (PPh ⁱPr₂)₂, dippe) have been prepared and characterized. These materials are catalyst precursors for the nonliving polymerization of styrene to polystyrene without the need of a cocatalyst such as methylaluminoxane. The polymerization reactions are carried out in refluxing 1,2-dichloroethane, using a 1:1000 catalyst to styrene ratio, yielding essentially atactic polystyrenes with M_n in the range 14 500-155 000 Da and polydispersities between 1.89 and 9.70. The mass properties of the polymers and the overall yield depend much on the nature of the catalyst precursor.

Full text of DOI:10.1021/om049883j

Organometallics 2004, 23, 3139-3146
3139
Ca tion ic Nick el Com p lexes Con ta in in g Bu lk y P h osp h in e
Liga n d s: Ca ta lyst P r ecu r sor s for Styr en e
P olym er iza tion
Manuel J ime´nez-Tenorio,† M. Carmen Puerta,† Isabel Salcedo,† Pedro Valerga,*,†
Sandra I. Costa,‡ Leonel C. Silva,‡ and Pedro T. Gomes‡
Departamento de Ciencia de Materiales e Ingenier´ıa Metalu´rgica y Qu´ımica Inorga´nica,
Facultad de Ciencias, Universidad de Ca´diz, 11510 Puerto Real, Ca´diz, Spain, and Centro de
Quı´mica Estrutural, Departamento de Engenharia Qu´ımica, Instituto Superior Te´cnico,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Received February 13, 2004
The cationic complexes [Ni(η3-CH2C(R)CH2)(P)2][BPh4] (R ) Me, H; (P)2 ) (PMeiPr2)2,
(PPhiPr2)2, 1,2-bis(diisopropylphosphino)ethane (dippe)) and [Ni(η-2-MeInd)(P)2][BPh4] (2-
MeInd ) 2-methylindenyl; (P)2 ) (PMeiPr2)2, (PPhiPr2)2, dippe) have been prepared and
characterized. These materials are catalyst precursors for the nonliving polymerization of
styrene to polystyrene without the need of a cocatalyst such as methylaluminoxane. The
polymerization reactions are carried out in refluxing 1,2-dichloroethane, using a 1:1000
catalyst to styrene ratio, yielding essentially atactic polystyrenes with Mn in the range
14 500-155 000 Da and polydispersities between 1.89 and 9.70. The mass properties of the
polymers and the overall yield depend much on the nature of the catalyst precursor.
In tr od u ction
cationic species. Among several activators, methylalu-
minoxane (MAO) is widely used as cocatalyst.13 How-
ever, there are also cationic or neutral NiII and PdII
systems capable of olefin polymerization without the
need of a cocatalyst.14
The field of catalytic olefin oligomerization and
polymerization mediated by late-transition-metal com-
plexes1 has been renewed following the reports by
Brookhart and co-workers of a family of new cationic
NiII and PdII catalysts containing R-diimine ligands.2-9
These complexes are very effective catalysts for the
polymerization of ethylene and R-olefins, as well as the
copolymerization of ethylene with R-olefins and polar
comonomers.4,6,9-12 In most cases, the term “catalyst”
applies to the catalyst precursor, which, in general, is
a complex that ought to undergo activation by use of a
cocatalyst to generate the actual catalytically active
Concerning the polymerization of styrene, the reac-
tions mediated by the aluminum-free cationic complex
[Ni(η3-CH2C(CH3)CH2)(COD)][PF6] have been studied
in detail.15 This compound is a very active homogeneous
catalyst precursor for the low-molecular-weight (Mn )
1000-3000 Da) polymerization of styrene. The addition
of phosphine ligands16 to this system causes an increase
in the catalytic activity, which is dramatic in some
instances (e.g. upon addition of 1 equiv of PPh3).17 The
activity increase is less pronounced when the incoming
phosphine has larger cone angles. An important in-
crease in the stereoregularity of the resulting polymers
is also observed upon addition of bulky monodentate
phosphine ligands, e.g. PCy3, since the isotactic content
* To whom correspondence should be addressed. E-mail:
pedro.valerga@uca.es. Tel: +34 956 016340. Fax: +34 956 016288.
† Universidad de Ca´diz.
‡ Instituto Superior Te´cnico.
(1) Reviews on late-transition-metal-catalyzed olefin polymeriza-
tion: (a) Ittel, S. D.; J ohnson, L. K.;. Brookhart, M. Chem. Rev. 2000,
100, 1169. (b) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew.
Chem., Int. Ed. 1999, 38, 429. (c) Gibson, V. C.; Spitzmesser, S. K.
Chem. Rev. 2003, 103, 283-315.
(2) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414.
(3) Killian, C. M.; Tempel, D. J .; J ohnson, L. K.; Brookhart, M. J .
Am. Chem. Soc. 1996, 118, 11664.
(4) J ohnson, L. K.; Mecking, S.; Brookhart, M. J . Am. Chem. Soc.
1996, 118, 267.
(5) Killian, C. M.; J ohnson, L. K.; Brookhart, M. Organometallics
1997, 16, 2005.
(6) Mecking, S.; J ohnson, L. K.; Wang, L.; Brookhart, M. J . Am.
Chem. Soc. 1998, 120, 888.
(7) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65.
(8) Gates, D. P.; Svejda, S. A.; On˜ate, E.; Killian, C. M.; J ohnson,
L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320.
(9) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085.
(10) Marques, M. M.; Correia, S. G.; Ascenso, J .; Ribeiro, A. F. G.;
Gomes, P. T.; Dias, A. R.; Foster, P.; Rausch, M. D.; Chien, J . C. W. J .
Polym. Sci., Part A: Polym. Chem. 1999, 37, 2457.
(11) Marques, M. M.; Fernandes, S.; Correia, S. G.; Ascenso, J . R.;
Nunes, T.; Pereira, S. G.; Ribeiro, A. F. G.; Gomes, P. T.; Dias, A. R.;
Rausch, M. D.; Chien, J . C. W. Macromol. Chem. Phys. 2000, 201, 2464.
(12) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479.
(13) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391.
(14) Recent examples of cationic and neutral NiII and PdII aluminum-
free catalysts: (a) Mecking, S. Coord. Chem. Rev. 2000, 203, 325 and
references therein. (b) Wang, C.; Friederich, S.; Younkin, T. R.; Li, R.
T.; Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998,
17, 3149. (c) Younkin, T. R.; Connor, E. F.; Henderson, J . I.; Friedrich,
S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (d) Liu,
W.; Malinoski, J . M.; Brookhart, M. Organometallics 2002, 21, 2836.
(e) Hicks, F. A.; J enkins, J . C.; Brookhart, M. Organometallics 2003,
22, 3533. (f) Bellabarba, R. M.; Gomes, P. T.; Pascu, S. I. Dalton 2003,
4431. (g) Li, Y.-S.; Li, Y.-R.; Li, X.-F. J . Organomet. Chem. 2003, 667,
185.
(15) Ascenso, J . R.; Dias, A. R.; Gomes, P. T.; Roma˜o, C. C.;
Neibecker, D.; Tkatchenko, I.; Revillon, A. Makromol. Chem. 1989, 190,
2773.
(16) We have recently reported that the addition of SbPh3 also
produces an increase in the catalytic activity, leading mostly to styrene
dimers: J ime´nez-Tenorio, M.; Puerta, M. C.; Salcedo, I.; Valerga, P.;
Costa, S. I.; Gomes. P. T.; Mereiter, K. Chem. Commun. 2003, 1168.
(17) Ascenso, J . R.; Dias, A. R.; Gomes, P. T.; Roma˜o, C. C.;
Tkatchenko, I.; Revillon, A.; Pham, Q.-T. Macromolecules 1996, 29,
4172.
10.1021/om049883j CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/20/2004

3140 Organometallics, Vol. 23, No. 13, 2004
J ime´nez-Tenorio et al.
Sch em e 1
inert toward phosphine substitution and as such cannot
be used in catalysis in the absence of cocatalysts,
although a moderate level of catalytic activity can be
induced at higher temperatures.20 Monophosphine Ni-
(η-indenyl) complexes are therefore more reactive than
the bis(phosphine) derivatives. However, in contrast
with this, it has been reported very recently that the
complexes [Ni(η-1-R-Ind)Cl(PPh3)] (R ) cyclopentyl,
benzyl) are able to catalyze the polymerization of
styrene effectively in the presence of NaBPh4 and PPh3,
at 80 °C, in toluene, to give atactic polystyrenes with
Mn values in the range 4000-7000 Da.27 The cationic
indenyl complexes [Ni(η:σ-1-IndCH2CH2NMe2)(PR3)]+
(R ) Ph, Me, Cy) contain an amine group tethered to
the indenyl ligand, which is hemilabile. These com-
pounds are capable of initiating styrene polymerization
at 80 °C, yielding polystyrenes with Mn values of 24 500
Da (R ) Ph), 35 000 Da (R ) Cy), and 61 000 Da (R )
Me), whereas at 20 °C these values are lower, 6500 Da
(R ) Cy) and 13 000 Da (R ) Me), with narrower
molecular weight distributions.23,25 The complexes [Ni-
(η:σ-1-IndCH2Y)(PPh3)]+ bearing different amino tethers
(Y ) 2-pyridyl, CH2-pyrrolidine, CH2NiPr2) were also
employed as catalysts in the polymerization of styrene,
demonstrating that the nature of the N moiety influ-
ences the catalytic activity.26
We have described the preparation of cationic 2-
methylallyl, allyl, and 2-methylindenyl nickel com-
lexes bearing the bulky phosphine ligand 1,2-bis(diiso-
propylphosphino)ethane (dippe).28 We have now pre-
pared new derivatives containing the bulky monoden-
tate phosphine ligands PRiPr2 (R ) Me, Ph). All these
complexes have been tested as catalysts for the polym-
erization of styrene. These catalytic reactions, together
with the characterization of the new complexes and of
the resulting polystyrenes, are described in the present
work.
Pm may change from 0.65 to 0.90.17,18 However, the
values of Mn for the polymers remain low (below 2000
Da). NMR data have shown the in situ formation of
cationic complexes of the type [Ni(η3-CH2C(CH3)CH2)-
(PR3)2]+ (PR3 ) PPh3, P(OPh3), P(o-Tol)3, PMe3, PnBu3,
PCy3). A mechanism has been proposed involving ad-
dition of styrene to a Ni-H bond, due to either phos-
phine dissociation or exchange with the monomer, to
yield a styryl complex, bound to the nickel center in an
η3-benzylic fashion, which by multiple insertion and
subsequent â-hydride elimination gives rise to regio-
regular head-to-tail polystyrene chains containing meth-
yl and unsaturated end groups (Scheme 1). Strong
evidence in favor of this mechanism was provided by
the NMR detection and isolation of a cationic η3-R-
methylbenzyl nickel species, catalytic studies on struc-
turally characterized cationic η3-benzyl nickel com-
plexes, and microstructural studies of the polymers
obtained.15,17-19
Exp er im en ta l Section
All synthetic operations were performed under a dry dini-
trogen or argon atmosphere following conventional Schlenk
techniques. Tetrahydrofuran, diethyl ether, and petroleum
ether (boiling point range 40-60 °C) were distilled from the
appropriate drying agents. All solvents were deoxygenated
immediately before use. PMeiPr2 and PPhiPr2, as well as the
complexes [Ni(COD)2], [Ni(η-2-MeInd)Br(PPh3)], [Ni(η3-CH2C-
(R)CH2)(dippe)][BPh4] (R ) Me (1c), H (2c)), and [Ni(η-2-
MeInd)(dippe)][BPh4] (4c) were obtained according to the
literature.28 Styrene was dried by stirring over CaH2 and then
purified by trap-to-trap distillation. NMR spectra were taken
on Varian Unity 400 MHz or Varian Gemini 200 MHz
equipment. Microanalysis was performed by the Serveis
Cient´ıfico-Te`cnics, Universitat de Barcelona. Molecular weight
distributions were determined at 35 °C by GPC/SEC using a
Waters 150 CV equipment fitted with three serial-mounted
Ultrastyragel columns: HR1, HR3, HR4 (10 µm; 7.8 × 300
mm). The polymer samples were eluted with THF at a flow
rate of 1 cm3 min-1. The system was calibrated using polysty-
rene standards (TSK Tosoh Co.).
Indenyl complexes, capable of undergoing facile ring
slippage of the indenyl ligand (Ind) involving change
from η5 to a η3 allyl-like coordination (indenyl effect),
have also shown to be active in olefin oligomerization
and polymerization.20-26 Thus, Zargarian and co-work-
ers have reported that the neutral complexes [Ni(η-1-
i
R-indenyl)X(PR3)] (R ) Me, Pr; X ) Cl, Me, CtCPh;
PR3 ) PPh3, PMe3) in combination with MAO are
capable of polymerizing ethylene to fairly high molec-
ular weight polyethylene at a modest level of activity.22
Interestingly, these authors have reported that the
cationic derivatives [Ni(η-indenyl)(PR3)2]+ are quite
(18) Ascenso, J . R.; Dias, A. R.; Gomes, P. T.; Roma˜o, C. C.; Pham,
Q.-T.; Neibecker, D.; Tkatchenko, I. Macromolecules 1989, 22, 998.
(19) Ascenso, J . R.; Dias, A. R.; Gomes, P. T.; Roma˜o, C. C.;
Tkatchenko, I.; Revillon, A. Polyhedron 1989, 8, 2449.
(20) Zargarian, D. Coord. Chem. Rev. 2002, 233, 157.
(21) Vollmerhaus, R.; Be´langer-Garie´py, F.; Zargarian, D. Organo-
metallics 1997, 16, 4762.
[Ni(η3-CH2C(R)CH2)(P MeiP r 2)2][BP h 4] (R ) Me (1a ), H
(2a )). To a slurry of [Ni(COD)2] (0.54 g, 2 mmol) in diethyl
ether (20 mL) cooled to -80 °C using a liquid N2-ethanol bath
(22) Dubois, M.-A.; Wang, R.; Zargarian, D.; Tian, J .; Vollmerhaus,
R.; Li, Z.; Collins. S. Organometallics 2001, 20, 663.
(23) Groux, L. F.; Zargarian, D. Organometallics 2001, 20, 3811.
(24) Groux, L. F.; Zargarian, D.; Simon, L. C.; Soares, J . B. P. J .
Mol. Catal. 2003, 193, 51.
(25) Groux, L. F.; Zargarian, D. Organometallics 2003, 22, 4759.
(26) Groux, L. F.; Zargarian, D. Organometallics 2003, 22, 3124.
(27) Sun, H.; Li, W.; Han, X.; Shen, Q.; Zhang, Y. J . Organomet.
Chem. 2003, 688, 132.
(28) J ime´nez-Tenorio, M.; Puerta, M. C.; Salcedo, I.;, Valerga, P.
Dalton 2001, 653.

Cationic Nickel Complexes with Bulky Phosphines
Organometallics, Vol. 23, No. 13, 2004 3141
was added 3-bromo-2-methylpropene (for 1a ) or allyl bromide
(for 2a ) (2 mL of a stock 1 M solution in diethyl ether, 2 mmol).
The mixture was warmed to room temperature and stirred for
1 h. During this time it changed from yellow to red. At this
stage PMeiPr2 (0.3 mL, 2 mmol) was added. The mixture was
stirred for 15 min, and then the solvent was removed under
vacuum. The residue was extracted with methanol. The
orange-brown solution was filtered through Celite in order to
remove finely divided black metallic nickel, not always but very
often present in the reaction mixture. An excess of solid
NaBPh4 (ca. 0.4 g) was added to the filtered solution. The
yellow or orange microcrystalline precipitate generated was
filtered off, washed with ethanol and petroleum ether, and
dried under vacuum. The complexes were recrystallized from
acetone-ethanol. Data for 1a : yield 0.52 g, 75% based on the
starting amount of PMeiPr2. Anal. Calcd for C42H61BNiP2: C,
PCH3); 17.71, 19.03 (s, P(CH(CH3)2)2); 23.93 (br, P(CH(CH3)2)2).
Data for 3b are as follows. Anal. Calcd for C24H38Br2NiP2: C,
1
47.5; H, 6.31. Found: C, 47.1; H, 6.24. H NMR (CD3COCD3,
298 K): very broad signals. 31P{1H} NMR (CD3COCD3, 298
1
K): featureless. P{1H} NMR (CD3COCD3, 223 K): δ 26.2 s.
13C{1H} NMR: not recorded.
[Ni(η-2-MeIn d )(P RiP r 2)2][BP h 4] (R ) Me (4a ), P h (4b)).
To a solution of [Ni(η-2-MeInd)Br(PPh3)] (0.53 g, 1 mmol) in
methanol (20 mL) was added PMeiPr2 (0.3 mL, 2 mmol) or
PPhiPr2 (0.42 mL, 2 mmol). The mixture was stirred for 12 h
at room temperature. Addition of an excess of solid NaBPh4
(ca. 0.4 g) yielded a brown-orange precipitate, which was
filtered off, washed with ethanol and petroleum ether, and
dried under vacuum. The numbering scheme used for the
identification of the NMR signals for these compounds are
based on the atom labeling
1
72.3; H, 8.82. Found: C, 72.0; H, 8.58. H NMR (CDCl3, 298
K): δ 0.92 (m, 24 H, P(CH(CH3)2)2); 0.97 (m, 6 H, PCH3); 1.63
(s, 3 H, CH2C(CH3)CH2); 1.81 (m, 4 H, P(CH(CH3)2)2); 1.96 (s
br, 2 H, CHantiC(CH3)CHanti); 3.57 (s br, 2 H, CHsynC(CH3)-
CHsyn); 6.76 (t, J HH ) 7.3 Hz, 4 H), 6.91 (t, J HH ) 7.3 Hz, 8
H), 7.33 (br, 8 H), B(C6H5)4. 31P{1H} NMR (CDCl3, 298 K): δ
28.3 s. 13C{1H} NMR (CDCl3, 298 K): δ 5.5 (PCH3); 17.7, 18.2,
19.7, 19.8 (s, P(CH(CH3)2)2); 23.1 (s, CH2C(CH3)CH2); 27.5 (m,
P(CH(CH3)2)2); 63.1 (m, CH2C(CH3)CH2); 127.3 (s, CH2C(CH3)-
CH2). Data for 2a : yield 0.48 g, 70% based on the starting
amount of PMeiPr2. Anal. Calcd for C41H59BNiP2: C, 72.1; H,
8.70. Found: C, 71.9; H, 8.52. 1H NMR (CDCl3, 298 K): δ 1.09
(m, 6 H, PCH3); 1.49 (m, 24 H, P(CH(CH3)2)2); 1.94 (m, 4 H,
3
3
Data for 4a : yield 0.46 g, 60%. Anal. Calcd for C48H63BNiP2:
C, 74.7; H, 8.23. Found: C, 74.6; H, 8.11. 1H NMR (CD3COCD3,
298 K): δ 0.90 (br, 6 H, PCH3); 0.95 (m, 24 H, P(CH(CH3)2)2);
1.80 (m, 4 H, P(CH(CH3)2)2); 2.32 (s, 3 H, HC(10)); 5.33 (s, 2 H,
HC(1)-C(3)); 7.05 (m, 2 H, HC(6)-C(9)); 7.44 (m, 2 H, HC(7)-C(8)); 6.76
3
P(CH(CH3)2)2); 2.14 (d, J HH ) 16 Hz, 2 H, CHantiCHCHanti);
3.92 (br, 2 H, CHsynCHCHsyn); 4.85 (m, 1 H, CH2CHCH2); 6.76
3
3
(t, J HH ) 7.3 Hz, 4 H), 6.91 (t, J HH ) 7.3 Hz, 8 H), 7.33 (br,
8 H), B(C6H5)4. 31P{1H} NMR (CD3COCD3, 298 K): δ 28.5 s.
13C{1H} NMR (CD3COCD3, 298 K): δ 6.2 (PCH3); 15.6 (s,
C(10)); 17.5, 18.2, 19.7, 20.0 (s, P(CH(CH3)2)2); 28.1 (m, P(CH-
(CH3)2)2); 80.8 (br, C(1), C(3)); 117.6 (s, C(2)); 119.2 (s, C(6),
C(9)); 121.8 (s, C(4), C(5)); 127.7 (s, C(7), C(8)). Data for 4b:
yield 0.49 g, 55%. Anal. Calcd for C58H67BNiP2: C, 77.8; H,
7.54. Found: C, 77.6; H, 7.39. 1H NMR (CDCl3, 298 K): δ 1.43
(m, 24 H, P(CH(CH3)2)2); 2.39 (m, 4 H, P(CH(CH3)2)2); 2.59 (s,
3 H, HC(10)); 5.87 (s, 2 H, HC(1)-C(3)); 6.95 (m, 2 H, HC(6)-C(9));
7.43 (m, 2 H, HC(7)-C(8)); 6.80-7.30 (m, 10 H, PC6H5); 6.76 (t,
3
3
(t, J HH ) 7.3 Hz, 4 H), 6.91 (t, J HH ) 7.3 Hz, 8 H), 7.33 (br,
8 H), B(C6H5)4. 31P{1H} NMR (CD2Cl2, 298 K): δ 27.5 s.
13C{1H} NMR (CDCl3, 298 K): δ 8.75 (PCH3); 17.9, 18.2, 19.8
(s, P(CH(CH3)2)2); 27.5 (m, P(CH(CH3)2)2); 66.4 (t, J CP ) 3 Hz,
CH2CHCH2); 114.9 (s, CH2CHCH2).
[Ni(η3-CH2C(CH3)CH2)(P P h iP r 2)2][BP h 4] (1b). This com-
pound was prepared by following a procedure identical with
that for 1a , using PPhiPr2 (0.42 mL, 2 mmol) instead of PMei-
Pr2. Yield: 0.49 g, 60% based on the starting amount of
PPhiPr2. Anal. Calcd for C52H65BNiP2: C, 76.0; H, 7.97.
1
3
3J HH ) 7.3 Hz, 4 H), 6.91 (t, J HH ) 7.3 Hz, 8 H), 7.33 (br, 8
Found: C, 75.9; H, 7.69. H NMR (CD2Cl2, 298 K): δ 1.24 (m,
H), B(C6H5)4. 31P{1H} NMR (CDCl3, 298 K): δ 33.6 s. 13C{1H}
NMR (CDCl3, 298 K): δ 14.5 (s, C(10)); 15.0, 18.1, 19.5 (s,
P(CH(CH3)2)2); 23.3 (m, P(CH(CH3)2)2); 87.9 (br, C(1), C(3));
116.9 (s, C(2)); 119.2 (s, C(6), C(9)); 121.8 (s, C(4), C(5)); 125.7
(s, C(7), C(8)), 129-132 (s, P(C6H5)).
24 H, P(CH(CH3)2)2); 2.06 (s, 3 H, CH2C(CH3)CH2); 2.50 (m, 4
H, P(CH(CH3)2)2); 2.89 (s br, 2 H, CHantiC(CH3)CHanti); 4.07 (s
br, 2 H, CHsynC(CH3)CHsyn)); 6.80-7.30 (m, 10 H, PC6H5); 6.76
3
3
(t, J HH ) 7.3 Hz, 4 H), 6.91 (t, J HH ) 7.3 Hz, 8 H), 7.33 (br,
8 H), B(C6H5)4. 31P{1H} NMR (CD2Cl2, 298 K): δ 38.0 s.
13C{1H} NMR (CDCl3, 298 K): δ 14.9, 15.0, 18.4, 19.9 (s, P(CH-
(CH3)2)2); 20.2 (s, CH2C(CH3)CH2); 23.3 (m, P(CH(CH3)2)2); 67.6
(m, CH2C(CH3)CH2); 128.2 (s, CH2C(CH3)CH2); 129-132 (s,
P(C6H5)).
X-r a y St r u ct u r e Det er m in a t ion s. Crystal data and
experimental details are given in Table 1. X-ray data were
collected on a MSC-RIGAKU AFC6S diffractometer (graphite-
monochromated Mo KR radiation, λ ) 0.710 73 Å). Corrections
for Lorentz and polarization effects, for crystal decay, and
for absorption were applied. All structures were solved by
direct methods using the program SHELXS97.29a Struc-
ture refinement on F2 was carried out with the program
SHELXL97.29b ORTEP30 was used for plotting. All non-
hydrogen atoms were anisotropically refined. Hydrogen atoms
were refined in idealized positions riding on the atoms to which
they were bonded.
Gen er a l P r oced u r e for Styr en e P olym er iza tion Rea c-
tion s. A Schlenk tube was loaded with styrene (10 g), 1,2-
dichloroethane (2 mL), and the corresponding catalyst (0.1%
mol) under dinitrogen. The system was heated to the point of
reflux using an oil bath. The temperature of the bath was
maintained in the interval 90-95 °C. The reaction time was
[NiBr 2(P RiP r 2)2] (R ) Me (3a ), P h (3b)). To a slurry of
anhydrous NiBr2 (0.22 g, ca. 1 mmol) in ethanol (20 mL) was
added either PMeiPr2 (0.3 mL, 2 mmol) or PPhiPr2 (0.42 mL,
2 mmol). The color immediately changed to dark purple upon
addition of PMeiPr2, with precipitation of a crystalline mate-
rial, whereas upon addition of PPhiPr2 the mixture became
orange. In the latter case, the mixture was gently heated for
several minutes using a warm water bath. Both reaction
mixtures were stirred at room temperature for 12 h. The
resulting crystalline precipitate, dark purple in the case of 3a
and brown in the case of 3b , was filtered off, washed with
ethanol and petroleum ether, and dried under vacuum.
Yields: 0.36 g, 80% for 3a ; 0.46 g; 75% for 3b. Data for 3a are
as follows. Anal. Calcd for C14H34Br2NiP2: C, 34.8; H, 7.10.
1
Found: C, 35.1; H, 6.99. H NMR (CD3COCD3, 298 K): very
1
broad signals. H NMR (CD3COCD3, 193 K): δ 1.05 (br, 6 H,
(29) (a) SHELXS97, Program for Crystal Structure Solution; Uni-
versity of Go¨ttingen, Go¨ttingen, Germany, 1990. (b) SHELXL97,
Program for Crystal Structure Refinement; University of Go¨ttingen,
Go¨ttingen, Germany, 1997.
(30) Farruggia, L. J . ORTEP-3 for Windows, version 1.076. J . Appl.
Crystallogr. 1997, 30, 565.
PCH3); 1.23 (d br, 12 H, P(CH(CH3)2)2); 1.41 (d, br, 12 H,
P(CH(CH3)2)2); 2.24 (br, 4 H, P(CH(CH3)2)2). 31P{1H} NMR
(CD3COCD3, 298 K): featureless. 31P{1H} NMR (CD3COCD3,
188 K): δ 13.9 s. 13C{1H} NMR (CD2Cl2, 228 K): δ 2.13 (s br,

3142 Organometallics, Vol. 23, No. 13, 2004
J ime´nez-Tenorio et al.
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for 1b
a n d 3a
1b
3a
formula
fw
T (K)
C
52H65BNiP2
C
14H34Br2NiP2
821.54
293(2)
482.88
293(2)
cryst size (mm)
cryst syst
space group
cell params
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
0.40 × 0.18 × 0.12 0.32 × 0.25 × 0.20
triclinic
orthorhombic
P1h (No. 2)
Pna21 (No. 33)
13.233(3)
14.501(7)
12.416(3)
99.22(3)
94.25(2)
101.47(3)
2290(1)
2
18.871(5)
7.516(2)
14.330(5)
2032(2)
4
Z
Fcalcd (g cm-3
)
1.221
5.40
1.578
5.03
µ(Mo KR) (cm-1
F(000)
)
F igu r e 1. ORTEP drawing (30% thermal ellipsoids) of the
cation [Ni(η3-CH2C(CH3)CH2)(PPhiPr2)2]+ in complex 1b.
Hydrogen atoms have been omitted. Selected bond lengths
(Å) and angles (deg) with estimated standard deviations
in parentheses: Ni(1)-C(1), 2.035(5); Ni(1)-C(2), 2.079(5);
Ni(1)-C(3), 2.137(6); Ni(1)-P(1), 2.245(2); Ni(1)-P(2),
2.334(2); C(1)-C(2), 1.417(9); C(2)-C(3), 1.361(9); C(2)-
C(4), 1.426(9); P(1)-Ni(1)-P(2), 111.29(6); C(1)-C(2)-C(3),
119.9(6).
880
984
max and min transmissn
1-0.82
1.44-0.79
factors
θ range for data collecn (deg) 2.57 < θ < 26.82
no. of rflns collected
no. of unique rflns
2.58 < θ < 25.05
1642
1642 (Rint
0.000)
1024
6667
6667 (Rint
0.110)
4013
)
)
no. of obsd rflns (I > 3σI)
no.of parameters
final R1, wR2 values
(I > 3σI)
final R1, wR2 values
(all data)
514
182
0.063, 0.154
0.116, 0.182
+1.361, -0.616
0.065, 0.156
0.113, 0.179
+1.002, -0.963
with MeOH and addition of NaBPh4.28
residual electron density
peaks (e Å -3
)
measured from the moment in which the reflux was initiated.
Two milliliter aliquots of the reaction mixture were taken
every 1 h, for an extended period of 6 h. The reaction was
quenched by addition of methanol to every aliquot. The
volatiles were pumped off, and the residue was treated with
methanol to yield a white precipitate of polystyrene, which was
filtered off and dried under vacuum. The polystyrene was
dissolved in dichloromethane (10 mL) and filtered through a
silica gel plug. The silica gel was washed with several small
portions of dichloromethane in order to remove all traces of
polystyrene. The solvent was removed from the combined
filtrate. The resulting residue was crushed with methanol, and
the polystyrene was filtered off, washed with petroleum ether,
and dried under vacuum to constant weight.
This method did not work for the preparation of the
allyl complex [Ni(η3-CH2CHCH2)(PPhiPr2)2][BPh4]. In
this case, the only isolated product was the allyl-phos-
phonium salt [CH2dCHCH2PPhiPr2][BPh4]. The 31P-
{1H} NMR spectra of 1a ,b and 2a consist of one singlet,
In some instances, the reaction was left running for a period
of 19 h. At the end of this period, the reaction mixture was
treated also as described above.
1
whereas the H NMR spectra display separated reso-
In all cases, the polymerization reactions were contrasted
against a blank consisting of a reaction mixture identical with
the system under study, but with no catalyst added. The small
amounts of polystyrene obtained in the blank reaction (only
significant in a few instances, usually after long reaction times)
were discounted from the amounts of polystyrene obtained in
the catalyzed reactions, and the resulting corrected figures
were used for the calculation of the yields.
nances corresponding to the anti and syn protons of the
2-methylallyl or allyl ligands. A reexamination of the
1H NMR spectrum of compound 2c showed that the
assignments previously reported for this derivative were
in error.28 Separated resonances for the anti and syn
protons of the allyl ligand in the [Ni(η3-CH2CHCH2)-
3
(dippe)]+ cation at 2.72 ppm (Hanti, d, J HH ) 10.2 Hz)
3
and 4.61 ppm (Hsyn, d, J HH ) 7.2 Hz) are present in
the spectrum, together with one multiplet at 5.31 ppm
corresponding to the proton attached to the central allyl
carbon atom. No coalescence is observed for the reso-
nances of the anti and syn protons when the tempera-
ture is raised to 60 °C for any of the derivatives 1a -c
and 2a ,c. This suggests a relatively high energy barrier
for the Hsyn/Hanti exchange process in all these com-
pounds, and also in the case of compound 2c, in contrast
with the earlier report.28 The crystal structure of 1b was
determined. An ORTEP view of the complex cation
[Ni(η3-CH2C(CH3)CH2)(PPhiPr2)2]+ is shown in Figure
1, together with the most relevant bond distances and
Resu lts a n d Discu ssion
P r ep a r a tion of th e Com p lexes. The 2-methylallyl
derivatives [Ni(η3-CH2C(CH3)CH2)(PMeiPr2)2][BPh4] (1a)
and [Ni(η3-CH2C(CH3)CH2)(PPhiPr2)2][BPh4] (1b), as
well as the allyl complex [Ni(η3-CH2CHCH2)(PMeiPr2)2]-
[BPh4] (2a ), were prepared as described for the related
dippe derivatives [Ni(η3-CH2C(CH3)CH2)(dippe)][BPh4]
(1c) and [Ni(η3-CH2CHCH2)(dippe)][BPh4] (2c): by re-
action of [Ni(COD)2] with either 3-bromo-2-methyl-
propene or allyl bromide and the corresponding
phosphine in diethyl ether, followed by extraction

Cationic Nickel Complexes with Bulky Phosphines
Organometallics, Vol. 23, No. 13, 2004 3143
angles. This complex displays a pseudo-square-planar
“two-legged piano stool” structure, very similar to that
of compound 1c.28 The angle formed by the least-squares
plane defined by the 2-methylallyl carbon atoms and
the plane defined by the NiP2 moiety has a value of
64.6°. The bond lengths C(1)-C(2) and C(2)-C(3) are
not equal and are intermediate between single and
double bonds, as expected. The variation sequence
observed for the Ni-C separations suggests an unsym-
metric bonding mode for the methylallyl ligand. The
P(1)-Ni(1)-P(2) angle of 111.29(6)° is significantly
larger than the value of 90.6(2)° found in the dippe
derivative 1c, due to the absence of the backbone carbon
chain between the two phosphorus atoms. In addition,
the Ni-P bonds are ca. 0.1 Å longer than in 1c,
suggesting an increased steric hindrance for PPhiPr2
compared to dippe. The torsion angle C(23)-P(2)-P(1)-
C(11) has a value of 38.7°, which indicates a staggered
conformation of the PPhiPr2 ligands with respect to each
other.
The preparation of the neutral 2-methylindenyl de-
rivatives [Ni(η-2-MeInd)Br(PR3)] (PR3 ) PMeiPr2,
PPhiPr2) was attempted, since these materials are
precursors for the synthesis of cationic complexes by
halide abstraction. The intended synthetic route to these
neutral compounds was the metathetical exchange of
bromide in [NiBr2(PR3)2] using Li(2-MeInd).
F igu r e 2. VT 31P{1H} NMR spectra of compound 3a in
CD3COCD3.
The complexes [NiBr2(PR3)2] (PR3 ) PMeiPr2 (3a ),
PPhiPr2 (3b)) were obtained by reaction of anhydrous
NiBr2 with the corresponding phosphine in ethanol.
These compounds are crystalline materials and are
diamagnetic in solution (Evans method31) that, however,
display broad resonances in their 1H NMR spectra,
whereas their 31P{1H} NMR spectra are featureless at
room temperature. At low temperature, a broad reso-
nance appears and sharpens as the temperature gets
lower. Thus, at 188 K in CD3COCD3, the 31P{1H} NMR
of 3a consists of one relatively sharp singlet at 13.5 ppm
(Figure 2), or at 26.5 ppm in the case of 3b. This
behavior can be interpreted in terms of a rapid phos-
phine dissociation equilibrium which occurs at a lower
rate and to a lesser extent as the temperature is
lowered. The fact that both PMeiPr2 and PPhiPr2 in
these complexes are easily exchanged by other phos-
phines, e.g. PEt3, is consistent with this hypothesis. The
crystal structure of 3a was determined. An ORTEP view
of the molecular structure of [NiBr2(PMeiPr2)2] is shown
in Figure 3.
The structure consists of a packing of discrete
[NiBr2(PMeiPr2)2] molecules interacting through van der
Waals forces. Each of the molecules is square planar,
with a trans arrangement of bromide and phosphine
ligands. This structure is very similar to that adopted
by the related compounds trans-[NiBr2(PiPr3)2]32 and
trans-[NiBr2(PPhMe2)2].33 All distances and angles in
this complex are within the expected ranges, being
unexceptional.
F igu r e 3. ORTEP drawing (30% thermal ellipsoids) of the
complex [NiBr2(PMeiPr2)2] (3a ). Hydrogen atoms have been
omitted. Selected bond lengths (Å) and angles (deg) with
estimated standard deviations in parentheses: Ni(1)-P(1),
2.254(6); Ni(1)-P(2), 2.251(6); Ni(1)-Br(1), 2.300(3); Ni(1)-
Br(2), 2.301(3); P(1)-Ni(1)-P(2), 179.1(2); Br(1)-Ni(1)-
Br(2), 179.6(2); Br(1)-Ni(1)-P(1), 89.4(2); Br(2)-Ni(1)-
P(2), 88.6(2).
explanation for this behavior might be that the com-
plexes [Ni(η-2-MeInd)Br(PR3)], once generated, undergo
a disproportionation reaction, yielding [NiBr2(PR3)2]
(isolated) plus [Ni(η-2-MeInd)2] (not detected). In any
case, since the desired products were not obtained, these
reactions were not investigated any further.
The complexes [Ni(η-2-MeInd)(PR3)2][BPh4] (PR3
)
PMeiPr2 (4a ), PPhiPr2 (4b)) were prepared in a fashion
analogous to that for the dippe derivative [Ni(η-2-
MeInd)(dippe)][BPh4] (4c).28 The procedure is an
adaptation of the method used by Zargarian and co-
workers for the synthesis of indenyl and 1-methylinde-
nyl nickel phosphine derivatives,34 which consists of the
reaction of [Ni(η-2-MeInd)Br(PPh3)] with 2 equiv of the
corresponding phosphine in MeOH, followed by addition
of NaBPh4.
The reactions of 3a ,b with Li(2-MeInd) failed to give
the desired complexes. The starting materials are
recovered back from these reactions. One possible
(31) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Crawford, T. H.;
Swanson, J . J . Chem. Educ. 1971, 48, 382.
(32) Wunderlich, H. Z. Kristallogr. 1997, 212, 381.
(33) Godfrey, S. M.; McAuliffe, C. A.; Pritchard, R. G. J . Chem. Soc.,
Dalton Trans. 1993, 2875.
(34) Huber, T. A.; Be´langer-Garie´py, F.; Zargarian, D. Organome-
tallics 1995, 14, 4997.

3144 Organometallics, Vol. 23, No. 13, 2004
J ime´nez-Tenorio et al.
Ta ble 2. P olym er iza tion of Styr en e Ca ta lyzed by Com p lexes 1, 2, a n d 4
polymerc
10-3Mn
47.6e
b
entrya
cat.
time (h)
yield (%)
Nt (h-1
)
type of distribn
bimodale
10-3Mw
Mw/Mn
distribn, wt %d
e
1
2
3
4
5
6
7
1a
1a
1b
1b
1c
1c
2a
6
19
6
19
6
59
67
45
50
35
81
42
102
35
78
27
60
43
84
111.5
2.34e
unimodal
unimodal
bimodal
152.8
262.1
33.8
4.52
1.95
134.4
19
5
43.4
226.2
31.7
175.3
1.37
1.29
39
61
8
9
10
11
2a
2c
2c
4a
19
5
19
6
88
66
89
60
46
132
48
unimodal
bimodal
100.5
47.0
2.14
102
4.2
39.0
3.0
22.4
1.40
1.74
40
60
12
13
14
15
4a
4b
4b
4c
19
6
19
19
92
25
43
94
48
42
23
48
unimodal
unimodal
140.7
293.3
14.5
9.70
155.2
1.89
a
b
Experimental conditions: styrene (10 g), solvent 1,2-dichloroethane (2 mL), [sty]/[Ni] ) 1000, T ) 90-95 °C. See ref 36. c Determined
by GPC/SEC. Estimated by area integration of both distributions in the GPC/SEC chromatogram. e A unimodal distribution is apparently
d
observed, but a shift from the typical Shulz-Flory shape on the low-molecular-weight side of the GPC/SEC chromatogram denotes the
presence of a superimposed small fraction of low-molecular-weight polystyrene of difficult quantification; the values presented correspond
to the integration of the whole sample.
(dippe)][BPh4], also reported by our research group,28
is not catalytically active under these conditions. The
amount of polystyrene increases steadily with the
reaction time, although the overall yield depends much
on the particular catalyst used. Most often, after a
period of 5-6 h the increase in the viscosity of the
reaction mixture led us to terminate the reaction at this
point by quenching with MeOH and exposure to air. In
general, the rate of polystyrene formation decreases
after this period, although in some cases a significant
increase in the overall polystyrene yield was observed
when the mixture was allowed to react for an extended
period of time of 19 h under reflux. Thus, for instance,
when 4c was used as catalyst, extremely low amounts
The 31P{1H} NMR spectra of 4a ,b consist of one
of polystyrene were obtained after 6 h. However, after
singlet, whereas the 1H NMR spectra show signals
19 h the conversion into polystyrene was over 90%. The
characteristic of the 2-MeInd ligand. The hapticity in
individual yields of polystyrene obtained using each of
solution was estimated by means of 13C{1H} NMR
the catalysts after the appropriate reaction times and
spectroscopy.35 The parameter ∆δ, which represents the
the resulting turnover frequency (Nt)36 are listed in
difference in chemical shift between the 13C{1H} NMR
Table 2. For reaction times of 5-6 h, the yields are
resonances of the ring-junction carbon atoms C(4)-C(5)
between 25% and 66%. Depending on the catalyst, the
in the indenyl complexes 4a ,b and in “free” indenyl (i.e.
yields increase up to 43%, to 94% after 19 h. We have
sodium indenyl, δ 130.7 ppm), has a value at 298 K of
measured the individual polystyrene yields as a function
-8.9 ppm for 4a and -9.0 ppm for 4b. These values
of time for each catalyst. This has allowed us to evaluate
indicate an intermediate η3/η5 hapticity in solution for
the variation of the cumulated turnover number (TON)
both compounds, as has also been previously found for
with the reaction time, as shown in Figure 4, as well as
4c, suggesting that slip-fold distortions are also present
the variation of the turnover frequency. The variation
in the 2-MeInd ligand in these complexes.20
of the cumulated TON with the reaction time shows
P olym er iza t ion R ea ct ion s. The cationic methyl-
clearly a sigmoid profile in all cases. After 5-6 h, the
allyl, allyl, and indenyl derivatives 1a -c, 2a ,c and 4a -c
increase in TON is very smooth, approaching a plateau.
are catalytic precursors for the polymerization of styrene
With respect to the variation of the turnover frequency,
to polystyrene in the absence of a cocatalyst such as
Nt increases to a maximum in the first 1 h, after which
MAO. A range of reaction conditions (solvent, temper-
the activity decreases. Hence, all of the systems behave
ature, catalyst/styrene ratio, reaction time) were evalu-
similarly in terms of polystyrene yields, 4b being the
ated. The best results were obtained using 1,2-dichlo-
least efficient. It is possible that in this case the
roethane as solvent at the reflux temperature (T ) 90-
catalytically active species do not survive in solution for
95 °C), with a catalyst to styrene molar ratio of 1:1000
a prolonged period of time.
under a dinitrogen atmosphere. No catalytic effect was
The polystyrenes obtained in these catalytic reactions
observed for the molar ratio 1:10 000. Interestingly, the
were characterized by GPC/SEC, and their mass fea-
18-electron pentamethylcyclopentadienyl complex [Cp*Ni-
(35) Huber, T. A.; Bayrakdarian, M.; Dion, S.; Dubuc, I.; Be´langer-
Garie´py, F.; Zargarian, D. Organometallics 1997, 16, 5811.
(36) The turnover frequency is defined as Nt ) (moles of converted
styrene)/[(moles of catalyst)(time)].

Cationic Nickel Complexes with Bulky Phosphines
Organometallics, Vol. 23, No. 13, 2004 3145
F igu r e 5. GPC chromatograms of polystyrenes obtained
using compound 4a as catalyst after the reaction times
shown.
F igu r e 4. Variation of the cumulated turnover number
(TON) with time for the polymerization of styrene catalyzed
by complexes 1, 2, and 4.
recently published work on the polymerization of sty-
rene catalyzed by related cationic indenyl nickel com-
plexes.26
tures, at the end of the polymerization reaction, are
listed in Table 2. The GPC traces indicate rather broad
molecular weight distributions, which apparently can
be treated as unimodal with three exceptions (see
below). The values of Mn range from 14 500 to 155 200
Da in the case of the unimodal distributions, with
polydispersity indexes (Mw/Mn) varying inversely. The
lower polydispersity values (Mw/Mn ≈ 2) are observed
for the polystyrenes obtained using as catalysts com-
pounds containing the bidentate phosphine dippe (1c,
2c, and 4c), which also led to polymers having the
highest values of Mn and unimodal distributions. The
largest polydispersity values correspond to the polysty-
renes obtained using complexes containing PPhiPr2 as
coligand (1b and 4b). However, the polystyrenes result-
ing from the reactions catalyzed by compounds contain-
ing the monodentate ligand PMeiPr2 (1a , 2a , and 4a )
are clearly bimodal at the end of 6 h.
The polystyrenes obtained in the catalytic runs were
1
also characterized by means of H and 13C{1H} NMR
spectroscopy (1,1,2,2-tetrachloroethane-d2, 120 °C). Ac-
cording to the literature, the analysis of the intensity
distribution of the 13C{1H} NMR resonances corre-
sponding to the ipso-carbon atoms of the phenyl groups
of polystyrene gives information relevant to the tacticity
of polystyrenes.15,17,18 The isotactic contents, Pm, calcu-
lated from the 13C{1H} NMR data for all of the polymers
obtained in this work fall in the range 0.57-0.63. Since
pure atactic polystyrene should have Pm ) 0.50, and for
isotactic polystyrene Pm ) 1.00, it can be concluded that
the polystyrenes resulting from the catalytic reactions
under study are essentially atactic.
In summary, the compounds 1a -c, 2a ,c and 4a -c
exhibit a moderate catalytic activity for the nonliving
polymerization of styrene in refluxing 1,2-dichloroet-
hane. The resulting polystyrenes are essentially atactic
and have rather high molecular weights, with the sole
exception of the polymers obtained using 4a as catalyst.
Either bimodal molecular weight distributions or larger
polydispersities are generally obtained when the cata-
lytic precursors contain monodentate phosphine as
coligands. The variation with time of the cumulated
TON and turnover frequencies, as well as the variation
of the values of Mn, seem to indicate the occurrence of
more than one single pathway for the formation of the
polymers.
Comparing our results with the data in the literature
for the polymerization of styrene mediated by related
allyl complexes,15,17,18 working at much lower temper-
atures, we can see that in general there are important
differences in the polymerization products, despite the
existing similarities between the catalytic systems. The
proposed insertion mechanism leads to styrene oligo-
mers ranging from dimers to low-molecular-weight
unimodal polymers, usually containing a certain degree
of stereoregularity, both features depending on the
basicity and bulk of the phoshine ligand used, whereas
in the present work, essentially atactic, rather high
molecular weight bimodal polystyrenes are obtained. It
seems reasonable to think of at least two different
polymerization pathways, as has been mentioned ear-
lier. Initially, the reaction would take place by an
insertion mechanism essentially analogous to that
In fact, we carried out a study of the variation of the
molecular weight distributions with the reaction time
and found out that only the catalysts containing the
bidentate phosphine dippe (1c, 2c, and 4c) give rise to
polystyrenes having unimodal distributions along the
course of the reaction. For all the other cases, the
presence of low-molecular-weight polystyrene at short
reaction times is always visible. As the reaction goes
on, a new fraction of much higher Mn gradually appears.
This is shown in Figure 5, as an example corresponding
to the polymerization of styrene catalyzed by 4a . The
variation of Mn with the conversion into polystyrene for
each of the catalysts indicates that the polymerization
processes hereby considered are not living. The general
trend is that Mn increases initially with conversion to
a value which, after that, remains more or less stable
when the conversion increases. All these facts might
indicate the simultaneous occurrence of two different
polymerization pathways, one of them leading rapidly
(i.e. less than 1 h) to polymers of low molecular weight
and another one which leads to polymers of higher
molecular weight. The latter apparently requires an
induction time to be effective, but it would continue to
be operative for longer reaction times. This would be
also consistent with the observed variation of the
cumulated turnover number with the reaction time
(Figure 4). A similar behavior was also reported in a

3146 Organometallics, Vol. 23, No. 13, 2004
J ime´nez-Tenorio et al.
proposed for the oligomerization/polymerization of sty-
rene catalyzed by cationic allyl nickel phosphine deriva-
tives at lower temperatures.17-19 In this fashion, low-
molecular-weight polystyrenes would be generated.
After approximately 1 h, a second mechanism is active,
leading to atactic, high-molecular-weight polystyrenes.
This could be just a simple cationic polymerization
process or a free-radical mechanism operating at higher
temperatures.26 For instance, the reaction of the metal
complex with 1,2-dichloroethane used as solvent at high
temperature, or even its mere decomposition, might
generate free radicals which initiate a radical polym-
erization of styrene, leading to atactic high-molecular-
weight polystyrenes. In this sense, we have monitored
by 31P{1H} NMR the initial stages of the polymerization
reaction using 2a as catalyst in 1,2-dichloroethane/
tetrachloroethane-d2. When the temperature is raised
to 80 °C, the resonance at ca. 27.5 ppm corresponding
to 2a disappears, and the spectrum becomes featureless.
The 1H NMR spectrum showed formation of polystyrene.
Cooling to room temperature did not restore the original
31P{1H} NMR spectrum. The 31P{1H} NMR spectrum
of a blank sample of 2a , identical with the previous one
but without styrene, showed an identical behavior.
These observations are consistent with the postulated
formation of radical species at high temperature. Thus,
the final product of the polymerization reaction should
be a mixture containing one fraction of low-molecular-
weight polystyrenes of higher isotactic content plus
another fraction of high-molecular-weight atactic poly-
styrenes. The second fraction becomes more important
with longer reaction times, whereas the lighter fraction
would be the main component at the beginning of the
reaction. The insertion mechanism does not necessarily
require phosphine dissociation, since all of the precata-
lysts are formally 16-electron species. However, phos-
phine dissociation might in fact occur for complexes
containing monodentate phosphine ligands, leading to
multiple active sites. This is less likely to occur in the
case of complexes containing bidentate dippe as coli-
gand, and hence the polymerization takes place mainly
through the alternative pathway. This might also ac-
count for the lack of catalytic activity observed for the
18-electron complex [Cp*Ni(dippe)][BPh4]. In any case,
we must remark that this is just a hypothesis, based
upon experimental observations and the comparison of
our results with catalytic polymerization reactions using
closely related systems which have been mechanistically
studied more in detail.
Ack n ow led gm en t. We thank the Ministerio de
Ciencia y Tecnolog´ıa (DGICYT, Project BQU2001-4026,
Accion Integrada HP2001-0064) and J unta de Andaluc´ıa
(PAI FQM188) for financial support. Thanks are also
due to the CRUP (Acc¸a˜o Integrada E-30/02) and to the
Fundac¸a˜o para a Cieˆncia e Tecnologia for financial
support (Project POCTI/QUI/42015/2001, cofinanced by
FEDER) and for fellowships PRAXIS XXI/BD/19638/99
(to S.I.C.) and SSRH/BD/9127/2002 (to L.C.S.).
Su p p or tin g In for m a tion Ava ila ble: Figures giving rel-
evant GPC/SEC chromatograms and CIF files giving X-ray
structural data, including data collection parameters, posi-
tional and thermal parameters, and bond distances and angles
for complexes 1b and 3a . This material is available free of
charge via the Internet at http://pubs.acs.org.
OM049883J