Cationic zirconocene hydrides: A new type of highly effective initiators for carbocationic polymerizations

Article abstract of DOI:10.1021/om990304y

The reaction of [CPh₃][B(C₆F₄R)₄] with [Cp′₂-ZrH₂]₂ (Cp′ = C₅H₄SiMe₃) gives the new binuclear hydrido complexes [Cp′₄Zr₂H₃][B(C₆F ₄R)₄J (R = F, SiPr₃ⁱ), which are highly active initiators for the polymerization of isobutene and isobutene - isoprene copolymerizations at near-zero concentrations of ionizing solvents. The structure of the trinuclear hydride [Cp′₅(η¹:η⁵-C₅H ₃-SiMe₃)Zr₃H₄J⁺[B(C ₆F₄SiPrⁱ₃)₄]^- is reported.

Full text of DOI:10.1021/om990304y

Organometallics 1999, 18, 2933-2935
2933
Ca tion ic Zir con ocen e Hyd r id es: A New Typ e of High ly
Effective In itia tor s for Ca r boca tion ic P olym er iza tion s
Andrew G. Carr, David M. Dawson, Mark Thornton-Pett, and
Manfred Bochmann*
School of Chemistry, University of Leeds, Leeds LS2 9J T, U.K.
Received April 26, 1999
Summary: The reaction of [CPh3][B(C6F4R)4] with [Cp′2-
ZrH2]2 (Cp′ ) C5H4SiMe3) gives the new binuclear
hydrido complexes [Cp′4Zr2H3][B(C6F4R)4] (R ) F, SiPri3),
which are highly active initiators for the polymerization
of isobutene and isobutene-isoprene copolymerizations
at near-zero concentrations of ionizing solvents. The
structure of the trinuclear hydride [Cp′5(η1:η5-C5H3-
group organometallic complexes are active in this way,
including Cp*TiMe2(µ-Me)B(C6F5)3,3 [AlCp2]+,4 (C5H4-
SiMe3)2Y(µ-Me)B(C6F5)3,5 [Zr{N(SiMe3)2}3]+,6 Cp2ZrMe2/
[CPh3][B(C6F5)4]/H2O,7 Cp2ZrMe2/B(C6F5)3,8 and [SiMe3]-
[B(C6F5)4],9 which give high-molecular-weight isobutene
polymers at temperatures as high as -20 °C.
SiMe3)Zr3H4]+[B(C6F4SiPri ) ]- is reported.
Although after a few monomer additions to the first
initiation product the course of the polymerization
should be independent from the organometallic species
and at best be influenced by the nature of the counter-
anion, we observed that for a given anion different
organometallic species exhibit very different initiator
efficiencies, with some leading to high polymer yields,
while others give comparably low conversions before
becoming inactive. We now find that cationic zir-
conocene hydrides constitute a new family of unusually
effective initiators, in particular for the polymerization
and copolymerization of isobutene at near-zero concen-
trations of chlorocarbon solvents.
3 4
The copolymerization of isobutene and isoprene to
butyl rubber is a large-scale industrial process based
on a AlCl3/H2O solution as “catalyst” in an ionizing
solvent (chloromethane).1 The polymerization is initi-
ated by protons and follows a carbocationic mechanism,
with tertiary carbocations as the propagating species
(eq 1). Since under such conditions, especially in the
Cationic zirconocene hydrides [Cp2ZrH]+ and [Cp2-
ZrH(L)]+ have previously been made by hydrogenoly-
sis of the corresponding methyl complexes; all are
mononuclear.10,11 In contrast, the reaction of [CPh3]-
[B(C6F4R)4]10d,12 with [Cp′2ZrH2]2 (Cp′ ) C5H4SiMe3)13
gives the binuclear complexes [Cp′4Zr2H3][B(C6F4R)4]
(1)
-
presence of nucleophiles such as H2O, Cl- and AlCl4
,
chain transfer via deprotonation of the -CMe2+ cations
is facile, polymers of adequately high molecular weight
are only obtained at very low (-100 °C) temperatures,
a costly and energy-intensive procedure.
(1: a , R ) F; b, R ) SiPri ) (Scheme 1).14 The reaction
3
(3) Barsan, F.; Baird, M. C. J . Chem. Soc., Chem. Commun. 1995,
1065. Barsan, F.; Karam, A. R.; Parent, M. A.; Baird, M. C. Macro-
molecules 1998, 31, 8439.
(4) Dawson, D. M.; Bochmann, M. Angew. Chem. 1996, 108, 2371;
Angew. Chem., Int. Ed. Engl. 1996, 35, 2226.
(5) Song, X.; Thornton-Pett, M.; Bochmann, M. Organometallics
1998, 17, 1004.
(6) Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromol. Rapid
Commun. 1998, 19, 205.
(7) Shaffer, T. D.; Ashborough, J . R. J . Polym. Sci., Part A: Polym.
Chem. 1997, 35, 329.
(8) Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromolecules 1998,
31, 2035.
(9) J acobs, S.; Kennedy, J . P. Polymer Bull. 1998, 41, 503. Kennedy,
J . P.; Pi, Z.; J acobs, S. Abstr. Pap.-Am. Chem. Soc. 1999, 217, 173-
PMSE.
(10) (a) Yang, X.; Stern, C. L.; Marks, T. J . Angew. Chem., Int. Ed.
Engl. 1992, 31, 1375. (b) Yang, X.; Stern, C. L.; Marks, T. J . J . Am.
Chem. Soc. 1994, 116, 10015. (c) J ia, L.; Yang, X.; Ishihara, A.; Marks,
T. J . Organometallics 1995, 14, 3135. (d) J ia, L.; Yang, X.; Stern, C.
L.; Marks, T. J . Organometallics 1997, 16, 842. (e) Chen, Y. X.; Marks,
T. J . Organometallics 1997, 16, 3649.
Cationic group 4 metallocenes stabilized by very
weakly coordinating anions are well-known as Ziegler-
Natta catalysts for the polymerization of 1-alkenes.2 The
feature common to all these compounds is the highly
electrophilic metal center and the essentially nonnu-
cleophilic character of the anion. Such systems should
therefore (a) be capable of acting as cationic initiators
and (b) minimize nucleophile-assisted chain transfer
and, hence, allow cationic polymerizations to be con-
ducted at significantly higher temperatures. Indeed, as
we and others have shown, several types of cationic or
zwitterionic early-transition-metal, lanthanide, and main-
(1) Kenndy, J . P.; Mare´chal, E. Carbocationic Polymerization;
Wiley: New York, 1982. Kenndy, J . P.; Iva´n, B. Designed Polymers by
Carbocationic Macromolecular Engineering: Theory and Practice;
Hanser: Munich, 1991. Nuyken, O.; Pask, S. D. In Comprehensive
Polymer Science; Allen, G., Bevington, J . C., Eds.; Pergamon: Oxford,
U.K., 1989; Vol. 3, p 619, and references therein.
(2) Kaminsky, W. J . Chem. Soc., Dalton Trans. 1998, 1413. Boch-
mann, M. J . Chem. Soc., Dalton Trans. 1996, 255. Grubbs, R. H.;
Coates, G. W. Acc. Chem. Res. 1996, 29, 85. Brintzinger, H. H.; Fischer.
D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1143.
(11) (a) J ordan, R. F.; Bradley, P. K.; Baenziger, N. C.; LaPointe,
R. E. J . Am. Chem. Soc. 1990, 112, 1289. (b) Guo, Z.; Bradley, P. K.;
J ordan, R. F. Organometallics 1992, 11, 2690. (c) Guo, Z.; Swenson,
D. C.; J ordan, R. F. Organometallics 1994, 13, 1424.
(12) Chien, J . C. W.; Tsai, W. M.; Rausch, M. D. J . Am. Chem. Soc.
1991, 113, 8570. Bochmann, M.; Lancaster, S. L. J . Organomet. Chem.
1992, 434, C1.
(13) Larsonneur, A.; Choukroun, R. Organometallics 1993, 12, 3216.
10.1021/om990304y CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/10/1999

2934 Organometallics, Vol. 18, No. 16, 1999
Communications
Sch em e 1
F igu r e 1. Structure of the cation in 2. Selected bond
lengths (Å) and angles (deg): Zr(1)-C(19) ) 2.304(6), Zr-
(1)-H(1) ) 2.04(5), Zr(1)-H(2) ) 2.24(6), Zr(3)-H(1) )
2.11(5), Zr(3)-H(2) ) 2.00(6), Zr(3)-H(3) ) 2.02(6), Zr(2)-
H(2) ) 2.18(6), Zr(2)-H(3) ) 2.17(6), Zr(2)-H(4) ) 1.73-
(5); C(19)-Zr(1)-H(1) ) 118(2), C(19)-Zr(1)-Zr(3) )
88.40(14), H(1)-Zr(1)-H(2) ) 59(2), H(2)-Zr(2)-H(3) )
62(2), H(2)-Zr(2)-H(4) ) 117(3), H(3)-Zr(2)-H(4) ) 66-
(3), H(1)-Zr(3)-H(2) ) 62(2), H(2)-Zr(3)-H(3) ) 67(2),
H(1)-Zr(3)-H(3) ) 129(2).
+
is quantitative. If an excess of CPh3 over [Cp′2ZrH2]2
is employed, 1 is again formed together with unreacted
CPh3+; there is no further reaction to mononuclear [Cp′2-
ZrH]+.
The compounds are obtained as colorless crystals from
toluene. Two of the hydride ligands are bridging (1a : δ
-2.02 and -2.66), the third is terminal (δ 4.55).15 The
compounds are fluxional. Below -60 °C all three hy-
dride resonances are resolved; at -50 °C the signal for
Ha and Hc (-2.02 and 4.55) coalesce, while Hb (-2.66)
begins to broaden. At -30 °C interchange of all three
hydrides is fast on the NMR time scale. The compounds
are stable in dichloromethane solution up to -30 °C for
a period of hours but decompose at higher temperatures.
The reaction of [Cp′2ZrH2]2 with either 1 or 2 equiv
of B(C6F5)3 proceeds in an analogous manner to give
[Cp′4Zr2H3][HB(C6F5)3] (1c); again, there is no evidence
for a mononuclear cationic hydride. In addition to the
signals for [Cp′4Zr2H3]+ seen for 1a and 1b, the H-B
signal of 1c is found at δ 3.49 (-90 °C). Compound 1c
is less stable in CD2Cl2 and begins to decompose at -30
°C.
The synthesis of 1 may be accompanied by the
formation of cationic zirconocene hydrides of higher
nuclearity, as shown by the isolation of a second hydrido
complex during the recrystallization of 1b, [Cp′5(η1:η5-
C5H3SiMe3)Zr3H4]+[B(C6F4SiPri ) ]-‚3.5(toluene) (2), as
3 4
pale yellow-green crystals, evidently the product of
metalation of one of the Cp′ ligands. The compound was
identified by X-ray diffraction. The cation (Figure 1) is
trinuclear, with one Zr-C σ-bond to a neighboring Cp′
ligand and three roughly coplanar bridging hydrides and
one terminal hydride. This large cation is stabilized by
the bulky anion [B(C6F4SiPri ) ]- 10d and contains 3.5
3 4
(disordered) toluene molecules of crystallization.16
For isobutene polymerizations equimolar amounts of
[Cp′2ZrH2]2 and [CPh3][B(C6F5)4] were mixed in cold
(-78 °C) CH2Cl2. Aliquots of this solution were injected
into liquid isobutene equilibrated at the given temper-
atures (Table 1).17 Rapid polymerization ensues. Whereas
under these conditions polymerizations initiated with
Cp2ZrMe2/B(C6F5)3 or [CPh3][B(C6F5)4]7,8 are typically
isothermal or lead only to small temperature increases,
initiation with the hydride system produced a signifi-
cant polymerization exotherm. The conversion rates and
polymer yields are dramatically higher than in the case
of Cp2ZrMe2 under otherwise identical conditions, while
the polymer molecular weights are comparable. Al-
though isoprene is known as a powerful retardant of
both polymer yields and polymer molecular weights,1
(14) Preparation of 1a : A toluene solution (20 mL) of [CPh3]-
[B(C6F5)4] (0.5 g, 0.54 mmol) was added to a stirred toluene solution
(5 mL) of [ZrCp′2H2]2 (0.4 g, 0.55 mmol) cooled to 0 °C. The reaction
mixture was stirred at this temperature for 30 min before it was
warmed to room temperature and all volatiles were removed in vacuo.
The white solid residue was washed with light petroleum to remove
HCPh3. The compound proved insufficiently soluble in toluene for
recrystallization. Anal. Calcd for C56H55BF20Si4Zr2: C, 47.6; H, 3.92.
Found: C, 48.0; H, 4.3. Preparation of 1b: A toluene solution (20 mL)
of [CPh3][B(C6F4SiPri3)4] (0.8 g, 0.54 mmol) was added to a stirred
toluene solution (20 mL) of [ZrCp′2H2]2 (0.4 g, 0.54 mmol) cooled to 0
°C. The reaction mixture was stirred at this temperature for 30 min.
The solution was filtered and placed in a refrigerator (5 °C) to give a
small yield of 1b as colorless crystals. Crystalline 1b is only sparingly
soluble in toluene. Anal. Calcd for C92H139BF16Si8Zr2: C, 56.18; H, 7.12.
Found: C, 56.2; H, 7.2. Preparation of 2: When the toluene filtrate
from the preparation of 1b was stored at 5 °C for several days, small
pale yellow-green crystals of 2 separated (ca. 0.4 g). The crystals readily
lose solvent on evacuation but proved suitable for X-ray diffraction.
The hydride signals could not be located by NMR.
(16) Crystal data: C108H165BF16Si10Zr3‚3.5C7H8; space group P21/n;
monoclinic; a ) 20.3249(11) Å; b ) 29.797(2) Å; c ) 24.0124(11) Å; â
) 97.026(2)° at 160 K; V ) 14433.3(13) Å3; Z ) 4; final R indices (I >
2σ(I)) R1 ) 0.0761, wR2 ) 0.1890 for 24 803 absorption-corrected
reflections (R(int) ) 0.1100).
(15) In contrast, a related binuclear tungsten hydride has a sym-
metrical structure, [(Cp2WH)2(µ-H)]+: Klingler, R. J .; Huffman, J . C.;
Kochi, J . K. J . Am. Chem. Soc. 1980, 102, 208.

Communications
Organometallics, Vol. 18, No. 16, 1999 2935
Ta ble 1. Isobu ten e Hom o- a n d Cop olym er iza tion s w ith Ca tion ic Zir con ocen e Meth yl a n d Zir con ocen e
Hyd r id e P r ecu r sor sa
amt of
initiator
solvent
(amt, mL)
amt of
IB
isoprene
incorp
run
no.
amt of Zr temp isobutene
isoprene time polymer conversn Mw
(mL)
×
Zr complex
(µmol)
(°C)
(mL)
(min) yield (g)
(%)
10-3 Mw/Mn (mol %)
1
2
3
4
5
6
7
8
9
Cp2ZrMe2
Cp2ZrMe2
Cp2ZrMe2
Cp′2ZrMe2
Cp′2ZrMe2
[Cp′2ZrH2]2
[Cp′2ZrH2]2
[Cp′2ZrH2]2
[Cp′2ZrH2]2
50
50
50
50
50
33
50
33
15
15
-78
-30
-78
-70
-30
-78
-35
-30
-30
-30
100
100
100
100
100
100
100
100
60
CH2Cl2 (1.5)
CH2Cl2 (1.5)
CH2Cl2 (1.5)
CH2Cl2 (1.5)
CH2Cl2 (1.5)
CH2Cl2 (1.0)
CH2Cl2 (3.0)
CH2Cl2 (1.0)
C6H4F2/tol (0.5)c
CH2Cl2 (0.5)
20
20
20
30
30
15
30
20
3
7.3
0.3
1.5
3.3
4.1
13
2,260
554
458
396
85
465
214
142
225
136
1.7
3.5
2.1
2.7
4.1
8.9
2.7
2.7
3.2
7.2
0.5
2.6
5.7
7.1
70
56
42
22
26
2.0
1.5
1.5
1.6
1.6
1.3
40.0b
32.0b
23.8
7.5
1.5
1.3
10 Cp2Zr(allyl)2
60
30
8.9
a
Conditions: 60 or 100 mL of isobutene; initiation by appropriate amounts of premixed solutions of 0.05 M zirconocene complex in
CH2Cl2 and 0.1 M [CPh3][B(C6F5)4] in CH2Cl2 (except for entry 9); stirring rate 1000 rpm. The reactions were terminated by injection of
b
2 mL of methanol. Exothermic reaction. c 1,2-Difluorobenzene/toluene 60:40 (v/v).
Sch em e 2
the conversion rates and molecular weights of the
isobutene-isoprene copolymers obtained with the zir-
conocene hydride initiators are remarkably similar to
the values for isobutene homopolymers made under
comparable conditions, especially at reaction tempera-
tures of -50 °C and above. Of particular significance is
the high conversion in the isobutene-isoprene copo-
lymerization at -30 °C (Table 1, entry 8), i.e., under
conditions where the Cp2ZrMe2/[CPh3][B(C6F5)4] system
produces only trace amounts of copolymer.
Whereas in conventional polymerizations chloro-
methane is present in high concentration to ensure
adequate ionization of the initiator system, the reactions
reported here proceed rapidly with very low concentra-
3
tions of chlorocarbons. As entry 9 shows, chlorinated
solvents are not in fact required; very similar results
that of zirconocene methyls. Since insertion of isobutene
into the Zr-H bonds was suspected, attempts were
are obtained using a 60:40 mixture of 1,2-difluoroben-
made to generate the resulting isobutyl cation, [Cp2Zr-
zene and toluene as the solvent for the initiator mixture.
(CH2CHMe2)]+, by reacting Cp2Zr(CH2CHMe2)2 with
Remarkably, the high initiating efficiency is specific to
[CPh3][B(C6F5)4]. However, the reaction proceeds ac-
zirconocene hydrides, whereas analogous hafnocene
hydrides [Cp′2HfH2]2 give significantly lower polymer
yields.
There is evidence that complexes 1 do not act as
initiators themselves but are converted to give as as yet
cording to Scheme 2 to give the allyl complex [Cp2Zr-
(η3-methallyl)]+ (3). The reaction is fast even at -78 °C;
there was no evidence for a cationic isobutyl intermedi-
ate. Complex 3 is also a powerful polymerization initia-
tor (Table 1, entry 10).
unidentified species, possibly via C-H activation pro-
cesses;18 notably, the polymerization of isobutene is
accompanied by the hydrogenation of one molecule of
isobutene per zirconium. The initiation mechanism with
cationic hydrides differs therefore fundamentally from
Cationic allyl complexes of type 3 have been discussed
as deactivation products of 1-alkene polymerization
catalysts.19 Recent theoretical studies have shown that
the reaction [Cp2Zr(alkyl)]+ f [Cp2Zr(allyl)]+ + H2 is
indeed favorable and explains the occurrence of internal
unsaturation in polyethylene.20 Scheme 2 suggests that
the related reaction [Cp2Zr(alkyl)]+ + olefin f [Cp2Zr-
(allyl)]+ + alkane is similarly facile and likely to be
another contributory factor in the formation of allylic
complexes during Ziegler-Natta polymerizations.
(17) The initiator stock solution was prepared by adding a cold (-78
°C) dichloromethane solution of [CPh3][B(C6F5)4] to an equimolar
amount of [Cp′2ZrH2]2 kept at -78 °C. Isobutene polymerizations were
carried out in a rigorously dried double-jacketed all-glass reactor fitted
with an internal thermocouple. Isobutene was dried by passing through
CaCl2, P2O5, and 4A molecular sieves and stored over Et2AlOHex;
isoprene was distilled from sodium and stored over 4A molecular sieves.
During the polymerization runs the reactor was open to a paraffin
bubbler to prevent pressure buildup. The required quantity of isobutene
was condensed into the reactor at 1 bar and equilibrated at the required
temperature for 30 min at a stirring rate of 800 rpm. An aliquot of the
initiator solution was added via syringe. The polymerizations were
terminated by injecting 2 mL of methanol. Reaction times for less active
initiators was 30 min; in other cases the polymerization was terminated
when a significant increase in viscosity was apparent, which cor-
responded to about 50-60% conversion. The polymer was precipitated
with methanol, washed, and dried at 60 °C for 48 h.
Ack n ow led gm en t. This research was supported by
the British Engineering and Physical Sciences Research
Council and Bayer AG, Leverkusen, Germany. We
thank Drs. G. Langstein, W. Obrecht, and R. Com-
mander for helpful discussions.
(18) Carr, A. G.; Dawson, D. M.; Bochmann, M. Manuscript in
preparation.
(19) Eshuis, J . J . W.; Tan, Y. Y.; Meetsma, A.; Teuben, J . H.;
Renkema, J .; Evens, G. G. Organometallics 1992, 11, 362. Christ, C.
S.; Eyler, J . R.; Richardson, D. E. J . Am. Chem. Soc. 1990, 112, 96.
(20) Margl, P. M.; Woo, T. K.; Ziegler, T. Organometallics 1998, 17,
4997.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving spectroscopic data, X-ray structure determination, and
crystal data for 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM990304Y