Generation of cationic indenyl silylamide gadolinium and scandium complexes [(Ind)Ln{N(SiMe3)2}]+[B(C6F 5)4]- and their reactivity for 1,3-butadiene polymerization

Article abstract of DOI:10.1039/b801707g

Highly efficient cis-polymerization of butadiene was achieved by using new bis(indenyl) silylamide rare earth complexes with the cooperation of both a borate salt and i-Bu₃Al; treatment of these complexes with organoboron compounds unexpectedly yielded new cationic mono(indenyl) amido species relevant to polymerization. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801707g

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Generation of cationic indenyl silylamide gadolinium and scandium complexes
[(Ind)Ln{N(SiMe3)2}]+[B(C6F5)4]− and their reactivity for 1,3-butadiene
polymerization†
Olivier Tardif* and Shojiro Kaita
Received 30th January 2008, Accepted 14th March 2008
First published as an Advance Article on the web 28th March 2008
DOI: 10.1039/b801707g
Highly efficient cis-polymerization of butadiene was achieved
by using new bis(indenyl) silylamide rare earth complexes
with the cooperation of both a borate salt and i-Bu3Al;
treatment of these complexes with organoboron compounds
unexpectedly yielded new cationic mono(indenyl) amido
species relevant to polymerization.
(Ln = Gd, Sc) in THF overnight, and treatment of the resulting
chloride derivatives with K{N(SiMe3)2} in toluene for 16 h, after
removal of THF in vacuum (Scheme 1). For the preparation of
(2-Ph-Ind)2Gd{N(SiMe3)2} (3), GdCl3 and (2-Ph-Ind)Li(THF)
were reacted for 7 d at room temperature. These complexes
were characterized by elemental analysis, IR and NMR (for
the scandium diamagnetic derivatives 4) spectroscopy, and X-ray
diffraction analysis for 2–4.
Selective 1,4-cis polymerization of 1,3-butadiene is of increasing
importance, since high 1,4-cis content in the polybutadiene chain
brings excellent elastomeric properties to the resulting rubbers,
which is very useful in tyre manufacturing.1 Industrially, high cis-
polybutadiene is produced with Ziegler–Natta Ti-, Co-, Ni- and
Nd-based catalysts.2,3a In particular, neodymium-based catalysts
provide the highest cis-1,4-content in polybutadiene.3 In 1999,
samarocene (C5Me5)2Sm(thf)2 was shown to polymerize butadiene
in the presence of aluminoxane (MMAO).4 Since then, the high
potential of lanthanocenes such as [(C5Me5)2Ln(AlMe4)]2 and re-
lated cationic lanthanocene [(C5Me5)2Ln]+[B(C6F5)4]− complexes
as precatalysts for well controlled 1,4-cis butadiene polymer-
ization, upon activation with [Ph3C][B(C6F5)4]/i-Bu3Al and i-
Bu3Al, respectively, has been demonstrated.5 With these systems,
gadolinium-based catalysts showed very high activity together
with the highest cis-selectivity. However, polybutadiene with cis-
content greater than 99% was only obtained under extreme
conditions (Tp < −20 ◦C), resulting in reduced activity and limited
molecular weight control.
We report herein that high cis-polybutadiene (> 99%) and high
activity can be obtained under less severe conditions by using
new bis(indenyl) hexamethyldisilylamide rare earth complexes by
cooperative action of both a cationizing reagent and i-Bu3Al. We
also describe the unexpected displacement of one indenyl ligand
in the reaction of these silylamide complexes with cocatalysts such
as ammonium borate and trityl borate salts.
The new bis(indenyl) silylamide complexes (2-R-Ind)2Ln{N-
(SiMe3)2}(Ln = Gd, R = H (1), Me (2), Ph (3); Ln = Sc, R =
Me (4)) were prepared following a two step one-pot procedure,
by reaction of (2-R-Indenyl)Li (R = H, Me, Ph) with LnCl3
Scheme 1 Preparation of bis(indenyl) silylamide rare earth complexes
1–4.
The polymerization of 1,3-butadiene was performed in toluene
at 20 ◦C. As a single component, the neutral bis(indenyl)
silylamide complexes 1–4 do not polymerize butadiene, neither
do the two-component catalytic systems formed from mixtures
of 1–4/[PhNMe2H][B(C6F5)4], 1–4/[Ph3C][B(C6F5)4] and 1–4/i–
Bu3Al, under these conditions. The addition of one equivalent of
triisobutyl aluminium (i-Bu3Al) to 1/[PhNMe2H][B(C6F5)4] did
also not produce an active system for polymerization (Table 1,
run 1). However, when [Ph3C][B(C6F5)4] was used in place
of [PhNMe2H][B(C6F5)4], 1/[Ph3C][B(C6F5)4]/i-Bu3Al (1 equiv.)
induced the polymerization of butadiene with the conversion
of 333 equiv. of monomers reaching 64%, in 15 min (run 2).
Higher conversion and better molecular weight control was
reached with 4 equiv. of i-Bu3Al (run 4). Under these conditions,
the ternary systems composed of 1–4, a borate salt, and i-
Bu3Al, successfully promoted the polymerization of butadiene,
reaching nearly full monomer conversion within 15 min (runs
4–10). The MWD of the polymers are unimodal and narrow
(Mw/Mn = 1.10–1.47), consistent with single site behavior of
these catalytic systems. Remarkably, polybutadienes produced by
the gadolinium based catalysts (1–3), possess a very high 1,4-
cis microstructure, and negligible or nil 1,2-vinyl content. In
contrast, the scandium complex 4 afforded polybutadiene with
a cis-content of 87.6% (run 10), showing that stereoregularity is
strongly dependent on the metal center.5d The three-component
catalytic systems composed of 3/[PhNMe2H][B(C6F5)4]/i-Bu3Al,
or 3/[Ph3C][B(C6F5)4]/i-Bu3Al were the most active, and the most
cis-1,4-stereospecific, yielding polybutadienes with very narrow
Elastomer Precision Polymerization Laboratory, RIKEN (The Institute of
Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama, 351-0198,
Japan. E-mail: olivier@riken.jp; Fax: +81 48 467 2790; Tel: +81 48 467 5908
† Electronic supplementary information (ESI) available: Detailed exper-
imental procedures, NMR spectra of the in situ generation of 6-[d8],
1H NMR spectra of the neutral gadolinium complexes 1–3, 13C NMR
spectrum, GPC and IR charts of representative polymer products. CCDC
reference numbers 670476 and 670547–670551. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b801707g
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Table 1 1,3-Butadiene polymerization using 1–4/[B]N/i-Bu3Al, 1–4/[B]C/i-Bu3Al, and 5/i-Bu3Ala
Microstructure (%)c
d
d
Run
[Ln]0
[Borate]b
Yield (%)
1,4-cis
1,2
—
0
0
<0.1
0
<0.2
<0.2
0
Mn (× 10−4
)
Mw/Mn
1e
2e
3f
4
5
6
7
8
9
10
11
1
1
1
1
1
2
2
3
3
4
5
[B]N
[B]C
[B]C
[B]C
[B]N
[B]C
[B]N
[B]C
[B]N
[B]C
—
0
64
93
93
84
95
90
100
100
100
100
—
—
—
>99
98.9
98.6
>99
98.7
99
>99
>99
87.6
88.2
39.7
23.8
10.8
10.8
10.2
12.4
13.3
12.0
18.3
35.3
1.41
1.22
1.15
1.47
1.17
1.24
1.10
1.13
1.15
1.22
0
12.1
11.8
a Performed in toluene (total volume = 20 mL); Tp = 20 ◦C; polymerization time = 15 min; [Ln]0 = 3 × 10−5 mol, [butadiene]0/[Ln]0 = 333; [i-
Bu3Al]0/[Ln]0 = 4. b [Borate]: [B]N = [PhNMe2H][B(C6F5)4], [B]C = [Ph3C][B(C6F5)4], [B]N/[Ln]0 = [B]C/[Ln]0 = 1. c Determined by IR spectroscopy in
CS2. d Determined by gel permeation chromatography vs. polystyrene standards. e [i-Bu3Al]0/[Ln]0 = 1. f [i-Bu3Al]0/[Ln]0 = 2.
MWDs (runs 8, 9). For comparison, polybutadiene obtained by
using the aluminate gadolinium complex [(C5Me5)2Gd(AlMe4)]2
under the same conditions as those described in runs 4–10,
possesses a significantly lower 1,4-cis content (96.4%).6
gave single crystals of 5. An X-ray diffraction study,‡ unambigu-
ously, revealed the displacement of one indenyl ligand, and showed
that 5 exists as separated [(2-Me-Ind)Sc{N(SiMe3)2}(PhNMe2)]+
cations and [B(C6F5)4]− anions (Fig. 1).
The protonolysis of amido complexes using ammonium bo-
rate is well established in organolanthanide chemistry,7,8 and
cationization by abstraction of the amido ligand using trityl
borate has also been mentioned in the research literature.7c,9
Therefore it was postulated that the complexes 1–4 would react
with [PhNMe2H][B(C6F5)4] or [Ph3C][B(C6F5)4] to yield the cor-
responding cationic species “[(Ind)2Ln]+[B(C6F5)4]−”. However,
the reactions turned out to be very different from our initial
expectations.
The addition of [PhNMe2H][B(C6F5)4] to a toluene solu-
tion of 4 gave the mono(indenyl) silylamide cation [(2-Me-
Ind)Sc{N(SiMe3)2}(PhNMe2)]+[B(C6F5)4]−, 5 (Scheme 2). The
amine complex 5 separates as an oil from the solution mixture, but
can be isolated as a powder in good yield after washing the oil with
hexane. Layering hexane above the oily residue prior to washing
Fig. 1 ORTEP drawings of 4 and 5 with 30% thermal ellipsoids.
−
The hydrogen atoms and the B(C6F5)4 anion are omitted for clar-
ity. Selected bond lengths (A) and angles (◦): 4: Sc(1)–Ind(av.)
˚
2.530(2) and 2.550(2), Sc(1)–N(1) 2.071(2), Sc(1) · · · C(24) 2.850(2),
Sc(1) · · · Si(2) 3.005(1), Sc(1)–N(1)–Si(2) 104.97(7), Sc(1)–N(1)–Si(1)
131.37(8); for 5: Sc(1)–Ind(av.) 2.466(2), Sc(1)–N(1) 2.005(2), Sc(1)–N(2)
2.317(2), Sc(1) · · · C(16) 2.614(2), Sc(1) · · · Si(2) 2.893(1), Sc(1)–N(1)–Si(2)
101.29(7), Sc(1)–N(1)–Si(1) 135.33(9).
˚
As expected, the Sc–Ind (av. 2.466(2) A) and Sc–amido
˚
(2.005(2) A) bond distances in 5 are significantly shorter than those
˚
˚
in the neutral complex 4 (av. 2.550(2), 2.530(2) A and 2.071(2) A,
˚
respectively). The Sc–aniline (2.317(2) A) bond distance is com-
˚
parable to the Sc–amino bond distance of 2.300(3) A in the
cation (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)(j2F-C6F5)B(C6F5)3.10
As in 4, the silylamide ligand is asymmetrically bound to the
Sc metal center, but the distortion appears accentuated in the
cation (Sc–N–Si: 4: 104.97(7) and 131.37(8)◦); 5: 101.29(7) and
135.33(9)◦), so that one SiMe group in 5 appears in close proximity
to the Sc metal center. For example, the C(16) and Si(2) in 5
Scheme 2 Reactions of bis(indenyl) silylamide scandium complex 4 with
anilinium borate and trityl borate ([BArF] = [B(C6F5)4]).
2532 | Dalton Trans., 2008, 2531–2533
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0.324 mm−1, reflections collected: 26223, independent reflections: 14514,
(Rint = 0.0300), Final R indices [I > 2rI]: R1 = 0.0456, wR2 = 0.1214, R
indices (all data): R1 = 0.0649, wR2 = 0.1371. 6: C48H43F20B1Sc1O2Si2N1,
M = 1157.78, T = 90(2) K, monoclinic, space group P21/c (No. 14),
11
˚
˚
are now, 2.614(2) A and 2.893(1) A away from the Sc metal,
˚
˚
respectively, as opposed to 2.850(2) A and 3.005(1) A in 4,
suggesting that electron deficiency at the Sc3+ center is further
relieved by a stronger Sc · · · SiMe intramolecular interaction.12
This interaction is also substantiated by the Si(2)–C(16) (1.919
(2) A) bond distance, which is significantly longer than any of
the other Si–C bonds which range from 1.862(2) to 1.874(2) A.
However, evidence for an intramolecular interaction in solution
was not observed at room temperature, but the 1H and 13C NMR
spectra of 5 in C6D5Cl were consistent with its X-ray structure.
Coordination of aniline is maintained in such a non-polar solvent,
as suggested by its resonances which are significantly shifted from
those of free amine. In contrast, addition of THF to the amine
cation 5 causes the release of N,N-dimethylaniline and its clean
conversion into the corresponding THF adduct complex [(2-Me-
Ind)Sc{N(SiMe3)2}(THF)2]+[B(C6F5)4]− (6), as shown by NMR
and X-ray diffraction analysis. Alternatively, complex 6 can be
prepared by reacting 4 with [PhNMe2H][B(C6F5)4] in THF, and
isolated in good yield after recrystallization from a mixture of THF
and hexane.13 Thus the reaction of 4 with [PhNMe2H][B(C6F5)4]
◦
˚
a = 15.206(3), b = 19.410(3), c = 16.909(3) A, b = 91.299(2) , V =
3
3
4989.4(15) A , Z = 4, Dc = 1.541 g cm , l = 0.311 mm−1, reflections
collected: 25331, independent reflections: 11383, (Rint = 0.0377), Final R
indices [I > 2rI]: R1 = 0.0403, wR2 = 0.0991, R indices (all data): R1 =
0.0577, wR2 = 0.1096. Crystallographic data for 2 (C26H36Gd1Si2N1), 3
(C36H40Gd1Si2N1), and 7 (C52H71B1Gd1O3Si2N1) can be found in the CIF
file. CCDC reference numbers 670476 (2), 670551(3), 670550 (4), 670548
(5), 670549 (6), 670547 (7).
˚
˚
˚
1 L. Porri, A. Giarrusso, in Comprehensive Polymer Science, ed.
G. C. Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Pergamon,
Oxford, 1989, vol. 4, pp. 53–108.
2 R. Taube, G. Sylvester, in Applied Homogeneous Catalysis with
Organometallic Compounds, ed. B. Cornils and W. A. Herrmann, VCH,
Weinheim, 2000, pp. 285–318.
3 (a) L. Friebe, O. Nuyken and W. Obrecht, Adv. Polym. Sci., 2006, 204,
1; (b) A. Fischbach and R. Anwander, Adv. Polym. Sci., 2006, 204, 155.
4 (a) S. Kaita, Z. Hou and Y. Wakatsuki, Macromolecules, 1999, 32,
9078; (b) S. Kaita, Y. Takeguchi, Z. Hou, M. Nishiura, Y. Doi and Y.
Wakatsuki, Macromolecules, 2003, 36, 7923.
5 (a) S. Kaita, Z. Hou and Y. Wakatsuki, Macromolecules, 2001, 34,
1539; (b) S. Kaita, Z. Hou, M. Nishiura, Y. Doi, J. Kurazumi,
A. C. Horiuchi and Y. Wakatsuki, Macromol. Rapid Commun., 2003,
24, 179; (c) S. Kaita, Y. Doi, K. Kaneko, A. C. Horiuchi and Y.
Wakatsuki, Macromolecules, 2004, 37, 5860; (d) S. Kaita, M. Ya-
manaka, A. C. Horiuchi and Y. Wakatsuki, Macromolecules, 2006, 39,
1359.
6 Butadiene polymerization by using the ternary system
[(C5Me5)2Gd(AlMe4)]2/[Ph3C][B(C6F5)4]/i-Bu3Al (4 equiv.) at 20 ◦C;
100% yield; Mn = 128 100; Mw/Mn = 1.57; 1,4-cis/1,4-trans/1,2 =
96.4/2.6/1.0; at −40 ◦C, [(C5Me5)2Gd(AlMe4)] gave a nearly perfect
cis-polybutadiene (1,4-cis > 99.9%), see ref. 5(d).
7 For examples of silylamide lanthanide complex protonolysis using
ammonium borate, see: (a) G. B. Deacon and C. M. Forsyth, Chem.
Commun., 2002, 2522; (b) W. J. Evans, M. A. Johnston, M. A. Greci,
T. S. Gummersheimer and J. W. Ziller, Polyhedron, 2003, 22, 119; (c) V.
Monteil, R. Spitz and C. Boisson, Polym. Int., 2004, 53, 576.
8 For examples of amido actinide complex protonolysis using ammonium
borate, see: (a) J. C. Berthet, C. Boisson, M. Lance, J. Vigner, M.
Nierlich and M. Ephritikhine, J. Chem. Soc., Dalton Trans., 1995, 3019;
(b) J. C. Berthet, C. Boisson, M. Lance, J. Vigner, M. Nierlich and M.
Ephritikhine, J. Chem. Soc., Dalton Trans., 1995, 3027; (c) C. Boisson,
J. C. Berthet, M. Ephritikhine, M. Lance and M. Nierlich, J. Organomet.
Chem., 1997, 533, 7.
1
could be monitored by H NMR spectroscopy in THF-d8. The
clean and quantitative consumption of 4 is completed within
45 min. The resulting NMR spectrum contains resonances for
6-[d8], identical to those of the isolated cation 6, along with
resonances corresponding to an equimolar amount of free 2-
methylindene, and free N,N-dimethylaniline, showing that the
protonolysis reaction is highly selective. The conversion of the
neutral scandium complex 4 into 6-[d8] by using [Ph3C][B(C6F5)4]
in place of [PhNMe2H][B(C6F5)4] was also observed by 1H NMR
spectroscopy in THF-d8. The reaction proceeds via the clean
and selective capture of one (2-Me-Ind)− anion by the Ph3C+
to give a mixture of the corresponding substituted indene 2-Me-
((Ph3C)Ind) and 6-[d8].14
In summary, we have shown that high activity and extremely
high yields of 1,4-cis polybutadiene could be achieved under
relatively smooth conditions, by using the new bis(indenyl)
silylamide rare earth complexes in cooperation with a borate
salt, and i-Bu3Al. The cationization of these complexes, using
[PhNMe2H][B(C6F5)4], and [Ph3C][B(C6F5)4], occurs by selective
displacement of one indenyl ligand, affording new cationic
mono(indenyl) amido rare earth compounds. Further studies in
progress show that this activation process can be extended to other
lanthanocene complexes for the generation of catalysts relevant to
polymerization.
9 (a) I. Kim and J. M. Zhou, J. Polym. Sci., Part A: Polym. Chem., 1999,
37, 1071; (b) I. Kim and C. S. Choi, J. Polym. Sci., Part A: Polym.
Chem., 1999, 37, 1523.
10 X. Li, M. Nishiura, K. Mori, T. Mashiko and Z. Hou, Chem. Commun.,
2007, 4137.
11 The Sc(1) · · · Si(2) distance in 5 is only slightly longer than the
˚
Sc–Si r-bond distances of 2.797(1), and 2.863(2) A reported for
(C5Me5)2Sc(SiH2SiPh3), and (C5H5)2Sc[Si(SiMe3)3](THF), respectively,
see: (a) A. D. Sadow and T. D. Tilley, J. Am. Chem. Soc., 2004, 127, 643;
(b) B. K. Campion, H. H. Richard and T. D. Tilley, Organometallics,
1993, 12, 2584.
Acknowledgements
12 For examples of similar distortion in structurally characterized hex-
amethyldisilylamide lanthanide complexes, see: (a) T. D. Tilley, R. A.
Andersen and A. Zalkin, J. Am. Chem. Soc., 1982, 104, 3725; (b) K. H.
Den Haan, J. L. De Boer, J. H. Teuben, A. L. Spek, B. Kojic-Prodic,
G. R. Hays and R. Huis, Organometallics, 1986, 5, 1726; (c) H. J. Heeres,
A. Meetsma, J. H. Teuben and R. D. Rogers, Organometallics, 1989, 8,
2637; (d) R. Anwander, Top. Curr. Chem., 1996, 179, 32.
13 Protonolysis of (2-Me-Ind)2Gd{N(SiMe3)2} (2) with [HNEt3][BPh4]
in THF gave the mono(indenyl) silylamide cation [(2-Me-
Ind)Gd{N(SiMe3)2}(THF)3]+[BPh4]− (7) as shown by X-ray diffraction
analysis (see ESI†).
14 The reaction of [Ph3C][B(C6F5)4] with (C5Me4)3Al which gives the
aluminocenium cation [(C5Me4)2Al]+[B(C6F5)4]−, is to our knowledge,
the only other similar example of such a cyclopentadienide abstraction
reported in the research literature: S-J. Lee, P. J. Shapiro and B.
Twamley, Organometallics, 2006, 25, 5582.
The authors wish to thank the Integrated Collaborative Research
Program with Industry in RIKEN for generous financial support
of this work.
Notes and references
‡ Crystallographic data for 4: C26H36Sc1Si2N1, M = 463.70, T = 90(2) K,
monoclinic, space group P21/n (No. 14), a = 10.9022(17), b = 15.712(2),
◦
3
3
˚
˚
c = 14.570(2) A, b = 90.884(3) , V = 2495.5(7) A , Z = 4, Dc = 1.234 g cm ,
l = 0.404 mm−1, reflections collected: 13713, independent reflections: 6624,
(Rint = 0.0482), Final R indices [I > 2rI]: R1 = 0.0396, wR2 = 0.1037, R
indices (all data): R1 = 0.0523, wR2 = 0.1071. 5: C48H38F20B1Sc1Si2N2,
M = 1134.75, T = 90(2) K, triclinic, space group P1¯ (No. 2), a =
˚
12.125(3), b = 13.378(5), c = 14.834(4) A, a = 96.813(7), b = 92.136(4),
◦
3
3
˚
c = 97.660(11) , V = 2364.4(12) A , Z = 2, Dc = 1.594 g cm , l =
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Dalton Trans., 2008, 2531–2533 | 2533
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