Control of stereoselectivity in the ring-opening metathesis polymerization of norbornene by the auxiliary ligands butadiene and o-xylylene in well-defined pentamethylcyclopentadiene tantalum carbene complexes

Article abstract of DOI:10.1021/om9802966

cis-Dialkyl complexes of tantalum, Ta(CH₂Ph)₂Cp*(η⁴-C₄H ₆) (2a) (Cp* = η⁵-C₅Me₅), TaMe(CH₂SiMe₃)Cp*(η⁴-butadiene) (6), and TaMe(CH₂CMe₃)Cp*(η⁴-butadiene) (7), were found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene to give poly(norbornene) with a high cis-vinylene double-bond (97-99%) content, while an o-xylylene complex Ta(CH₂Ph)₂(η⁴-o-(CH₂) ₂C₆H₄)Cp* (9) was also an initiator to give poly(norbornene) with a high trans-vinylene double-bond (92-95%) content. When a Cp-butadiene complex Ta(CH₂Ph)₂Cp(η⁴-C₄H₆) (2b) was used as an initiator, we obtained poly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). The factors controlling these stereoselectivities have been investigated, and we obtained the following results: (1) We isolated benzylidene complexes and found that the proportion of anti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalum center, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe₃ resulted in the formation of a benzylidene complex Ta(=CHPh)Cp*(η⁴-C₄H₆)(PMe ₃) (3a) as an antirotamer, which has been characterized by X-ray crystal structure analysis, while a similar treatment of 2b afforded Ta(=CHPh)Cp(η⁴-C₄H₆)(PMe₃) (3b), also in the anti-rotamer form, as revealed by a comparison of the chemical shift value of the benzylic proton of 3b with that of 3a. In sharp contrast to the anti-geometry, a benzylidene complex Ta(=CHPh)(η⁴-o-(CH₂)₂C₆H ₄)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, was obtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it is a syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes are considered as intermediates during the propagation step. We could not isolate any metallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane Cp*-(η⁴-o-xylylene)Ta[CH(Ph)CH(C₁₀H ₆)CH] (15) was isolated in 18% yield when acenaphthylene, instead of the norbornene, was added to 12. The trans-phenyl geometry of the metallacyclobutane ring system, which indicated the retention of the Ta=C bond stereochemistry, was determined by the X-ray analysis of 15, whose structural features were compared with that of Cp*(η⁴-butadiene)Nb[CH₂CH(C₁₀H ₆)CH] (14). (3) In contrast to high cis/trans stereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the ¹³C NMR spectra of the hydrogenated derivatives was found to be almost atactic. This suggests that the alkylidene species would be a resting state during the propagation, and thereby monomer could be added to both faces of the Ta=C bond, leading to the atactic polymer. Consequently, the high cis and trans stereoselectivity thus demonstrated is ascribed to the huge congestion between the Cp* ligand and auxiliary ligands 1,3-butadiene or o-xylylene.

Full text of DOI:10.1021/om9802966

Organometallics 1998, 17, 4183-4195
4183
Con tr ol of Ster eoselectivity in th e Rin g-Op en in g
Meta th esis P olym er iza tion of Nor bor n en e by th e
Au xilia r y Liga n d s Bu ta d ien e a n d o-Xylylen e in
Well-Defin ed P en ta m eth ylcyclop en ta d ien e Ta n ta lu m
Ca r ben e Com p lexes
Kazushi Mashima*
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560, J apan
Michitaka Kaidzu, Yoshiyuki Tanaka, Yuushou Nakayama, and
Akira Nakamura
Department of Macromolecular Science, Graduate School of Science, Osaka University,
Toyonaka, Osaka 560, J apan
J ames G. Hamilton and J ohn J . Rooney
School of Chemistry, The Queen’s University of Belfast, David Keir Building,
Belfast BT9 5AG, Northern Ireland
Received April 17, 1998
cis-Dialkyl complexes of tantalum, Ta(CH2Ph)2Cp*(η4-C4H6) (2a ) (Cp* ) η5-C5Me5),
TaMe(CH2SiMe3)Cp*(η4-butadiene) (6), and TaMe(CH2CMe3)Cp*(η4-butadiene) (7), were
found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of
norbornene to give poly(norbornene) with a high cis-vinylene double-bond (97-99%) content,
while an o-xylylene complex Ta(CH2Ph)2(η4-o-(CH2)2C6H4)Cp* (9) was also an initiator to
give poly(norbornene) with a high trans-vinylene double-bond (92-95%) content. When a
Cp-butadiene complex Ta(CH2Ph)2Cp(η4-C4H6) (2b) was used as an initiator, we obtained
poly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). The
factors controlling these stereoselectivities have been investigated, and we obtained the
following results: (1) We isolated benzylidene complexes and found that the proportion of
anti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalum
center, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe3 resulted
in the formation of a benzylidene complex Ta(dCHPh)Cp*(η4-C4H6)(PMe3) (3a ) as an anti-
rotamer, which has been characterized by X-ray crystal structure analysis, while a similar
treatment of 2b afforded Ta(dCHPh)Cp(η4-C4H6)(PMe3) (3b), also in the anti-rotamer form,
as revealed by a comparison of the chemical shift value of the benzylic proton of 3b with
that of 3a . In sharp contrast to the anti-geometry, a benzylidene complex Ta(dCHPh)(η4-
o-(CH2)2C6H4)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, was
obtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it is
a syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes are
considered as intermediates during the propagation step. We could not isolate any
metallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane Cp*-
(η4-o-xylylene)Ta[CH(Ph)CH(C10H6)CH] (15) was isolated in 18% yield when acenaphthylene,
instead of the norbornene, was added to 12. The trans-phenyl geometry of the metallacy-
clobutane ring system, which indicated the retention of the TadC bond stereochemistry,
was determined by the X-ray analysis of 15, whose structural features were compared with
that of Cp*(η4-butadiene)Nb[CH2CH(C10H6)CH] (14). (3) In contrast to high cis/trans
stereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly-
(norbornene) obtained by the tantalum complexes estimated from the 13C NMR spectra of
the hydrogenated derivatives was found to be almost atactic. This suggests that the
alkylidene species would be a resting state during the propagation, and thereby monomer
could be added to both faces of the TadC bond, leading to the atactic polymer. Consequently,
the high cis and trans stereoselectivity thus demonstrated is ascribed to the huge congestion
between the Cp* ligand and auxiliary ligands 1,3-butadiene or o-xylylene.
S0276-7333(98)00296-9 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/27/1998

4184 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
In tr od u ction
Alkylidene complexes of transition metals have at-
tracted much interest since they have been used as
catalysts as well as reagents for reactions such as alkene
formation from carbonyl compounds, olefin metathesis,
and ring-opening metathesis polymerization (ROMP) of
cyclic olefins.1-6 Well-defined metathesis catalysts such
as titanacyclobutanes,5,7-9 Mo(NR)(CHR′)(OR′′)2,10-18
and RuCl2(CHR)(PR′3)219-24 have recently proven useful
in ROMP, have led to a more precise description of the
whole mechanism, and have also acheived living ROMP
with high selectivity. Generally, the properties of
polymers sensitively depend not only on molecular
weight and polydispersity but also on the microstructure
of the polymer backbone. Poly(norbornene) and its
derivatives are interesting materials as the control of
polymer microstructure in terms of both the cis/trans
double-bond ratio and the relative orientation of the
cyclopentylene rings, i.e., tacticity, Figure 1, has pro-
vided a challenge.25 Although recently a number of
stereocontrolled polymerizations of norbornadiene de-
rivatives have been reported,10-18 there are relatively
few examples of the stereospecific polymerization of
norbornene derivatives, and these have involved the use
of classical catalyst systems such as ReCl525 and OsCl3,26
which yield cis syndiotactic polymer and (mesitylene)-
W(CO)3,25 which gave trans isotactic material.
F igu r e 1. Four possible stereoregularities of poly(nor-
bornene).
metallocene-like TaCp*(diene) (Cp* ) pentamethylcy-
clopentadienyl) fragment, isoelectronic and isolobal to
group 4 metallocene fragments Cp2M, which are 14-
electron species; we had found that TaCp*(diene) sta-
bilized various reactive species such as benzyne27 and
cationic alkyls28 similar to the corresponding metal-
locene complexes of group 4 metals. We prepared and
characterized cis-dialkyl complexes bearing TaCp*(η4-
butadiene), TaCp(η4-butadiene), and TaCp*(η4-o-xy-
lylene) fragments, and all these complexes were catalyst
precursors for the ROMP of norbornene. We have found
that a combination of Cp* and butadiene ligands affords
a polymer having cis-vinylene bonds, while a combina-
tion of Cp* and o-xylylene ligands results in a trans-
polymer; however, the Cp-butadiene system has no
stereoselectivity. We have also found that these poly-
(norbornene)s were almost atactic on the basis of the
13C NMR spectra of the hydrogenated derivatives. The
outline of a mechanism is discussed on the basis of the
isolation of two kinds of benzylidene complexes, an anti-
rotamer for a butadiene complex and a syn-rotamer for
an o-xylylene complex, together with metallacyclobu-
tanes. A part of this work has been the subject of
preliminary communications.29
Grubbs work on a titanocene-carbene system promp-
ted us to study a new catalyst system based on the
(1) Nugent, W. A.; Meyer, J . M. Metal-Ligand Multiple Bonds;
Wiley-Interscience: New York, 1988; p 133.
(2) For comprehensive reviews: Ivin, K. J . Olefin Metathesis;
Academic Press: New York, 1983. Dragutan, V.; Balaban, A. T.;
Dimonie, M. Olefin Metathesis and Ring Opening Polymerization of
Cyclo-Olefins, 2nd ed.; Wiley-Interscience: New York, 1985. Ivin, K.
J .; Mol, J . C. Olefin Metathesis and Metathesis Polymerization;
Academic Press: London, 1997.
(3) Feldman, J .; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1.
(4) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.
(5) Grubbs, R. H.; Tumas, W. Science 1989, 243, 907.
(6) Novak, B. M.; Risse, W.; Grubbs, R. H. Adv. Polym. Sci. 1990,
102, 47.
(7) Gilliom, L. R.; Grubbs, R. H. J . Am. Chem. Soc. 1986, 108, 733.
(8) Petasis, N. A.; Fu, D.-K. J . Am. Chem. Soc. 1993, 115, 7208.
(9) Grubbs, R. H.; Gilliom, L. R. J . Mol. Catal. 1988, 46, 255.
(10) Bazan, G. C.; Khosravi, E.; Schrock, R. R.; Feast, W. J .; Gibson,
V. C.; O’Regan, M. B.; Thomas, J . K.; Davis, W. M. J . Am. Chem. Soc.
1990, 112, 8378.
(11) McConville, D. H.; Hofmeister, G. E.; Schrock, R. R. J . Am.
Chem. Soc. 1993, 115, 4413.
(12) Oskam, J . H.; Schrock, R. R. J . Am. Chem. Soc. 1993, 115,
11831, and references therein.
(13) O’Dell, R.; McConville, D. H.; Hofmeister, G. E.; Schrock, R. R.
J . Am. Chem. Soc. 1994, 116, 3414, and references therein.
(14) Schrock, R. R.; Lee, J .-K.; O’Dell, R.; Oskam, J . H. Macromol-
ecules 1995, 28, 5933.
Resu lts a n d Discu ssion
(15) Totland, K. M.; Boyd, T. J .; Lavoie, G. G.; Davis, W. M.; Schrock,
R. R. Macromolecules 1996, 29, 6114.
(16) Feast, W. J .; Gibson, V. C.; Marshall, E. L. J . Chem. Soc., Chem.
Commun. 1992, 1157.
(17) Feast, W. J .; Gibson, V. C.; Ivin, K. J .; Kenwright, A. M.;
Khosravi, E. J . Chem. Soc., Chem. Commun. 1994, 1399.
(18) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158.
(19) Nguyen, S. T.; J ohnson, L. K.; Grubbs, R. H.; Ziller, J . W. J .
Am. Chem. Soc. 1992, 114, 3974.
(20) Nguyen, S. T.; Grubbs, R. H. J . Am. Chem. Soc. 1993, 115, 9858.
(21) Lynn, D. M.; Kanaoka, S.; Grubbs, R. H. J . Am. Chem. Soc.
1996, 118, 784, and references therein.
Syn t h esis of Ca t a lyst P r ecu r sor s a n d Ben -
zylid en e Com p lexes. F r om Dien e Com p lexes. A
new series of cis-dialkyl complexes bearing 14-electron
“Ta(η5-C5R5)(η4-C4H6)” (R ) H, Me) fragments, which
are isoelectronic to metallocene fragments of group 4
metals, were found to be catalyst precursors for the
ROMP of norbornene. We have already communicated
the synthesis and the crystal structure of the dibenzyl
tantalum complex Ta(CH2Ph)2Cp*(η4-C4H6) (2a ), which
(22) Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc. 1996,
118, 100.
(23) Wilhelm, T. E.; Belderrain, T. R.; Brown, S. N.; Grubbs, R. H.
Organometallics 1997, 16, 3867.
(24) Belderrain, T. R.; Grubbs, R. H. Organometallics 1997, 16, 4001.
(25) Hamilton, J . G. Polymer, in press.
(26) Al-Samak, B.; Amir-Ebrahimi, V.; Carvill, A. G.; Hamilton, J .
G.; Rooney, J . J . Polymer Int. 1996, 41, 85.
(27) Mashima, K.; Tanaka, Y.; Nakamura, A. Organometallics 1995,
14, 5642.
(28) Mashima, K.; Fujikawa, S.; Tanaka, Y.; Urata, H.; Oshiki, T.;
Tanaka, E.; Nakamura, A. Organometallics 1995, 14, 2633.
(29) Mashima, K.; Tanaka, Y.; Kaidzu, M.; Nakamura, A. Organo-
metallics 1996, 15, 2431.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4185
was prepared in 83% yield by reaction of TaCl2Cp*(η4-
C4H6) (1a ) with 2 equiv of benzyl Grignard reagent in
THF.29 The Cp analogue Ta(CH2Ph)2Cp(η4-C4H6) (2b)
was obtained as brown crystals in 51% yield by a similar
reaction of TaCl2Cp(η4-C4H6) (1b) with 2 equiv of benzyl
Grignard reagent in THF followed by recrystallization
from a mixture of toluene and hexane. The structure
of 2b was characterized on the basis of NMR spectros-
copy and compared with that found for 2a . The 1H
NMR spectrum of 2b showed AB type benzylic signals
at δ 1.27 and 1.86 (J ) 10.9 Hz) which are shifted to
lower field compared to those (δ 0.40 and 1.23, J ) 10.9
Hz) found for 2a . We have already reported the
thermolysis of 2a in the presence of PMe3 to afford a
benzylidene complex Ta(dCHPh)Cp*(η4-C4H6)(PMe3)
(3a ) that has a PMe3 ligand and a benzylidene moiety
(anti-isomer with the phenyl group pointed away from
the Cp* ligand).29 Similarly, thermolysis of 2b in the
presence of PMe3 at 40 °C for 25 h provided a ben-
zylidene complex Ta(dCHPh)Cp(η4-C4H6)(PMe3) (3b),
which decomposed readily during attempted isolation.
Thus, the complex 3b was characterized in solution by
its 1H NMR spectrum, which exhibited a doublet,
characteristic of an R-benzylidene proton, at δ 10.52
(J H-P ) 5.9 Hz), downfield from that of 3a (δ 9.67).
Ch a r t 1
or without the donor ligand PMe3 failed; however, they
were found to be catalyst precursors for the ROMP of
norbornene, indicating that the formation of an alky-
lidene species, via a methane-producing R-hydrogen
abstraction, proceeded under the polymerization condi-
tions.
F r om o-Xylylen e Com p lexes. A dibenzyl complex
Ta(CH2Ph)2(η4-o-(CH2)2C6H4)Cp* (9), which has an o-
xylylene auxiliary ligand instead of the η4-butadiene,
is expected to be a catalyst precursor for the ROMP of
norbornene, so we started by preparing the o-xylylene
complex TaCl2(η4-o-(CH2)2C6H4)Cp* (8). Treatment of
TaCl4Cp* with 1 equiv of o-C6H4(CH2MgCl)2 in THF
resulted in the formation of 8 in 38% yield.
Although three coordination modes (A, B, and C,
Chart 1) for a mononuclear o-xylylene complex are
possible, A and B can be ruled out by NMR spectros-
1
copy.30 Thus the H NMR spectrum of 8 displayed an
ABq-type signal due to the benzylic protons of the
o-xylylene ligand; this together with signals due to
aromatic ring protons at normal chemical shift ruled
out the mode A. In the 13C NMR spectrum we obtained
direct information to distinguish the modes B and C, a
signal due to the benzylic carbons of the o-xylylene
ligand being displayed at δ 69.9 with a normal coupling
constant (142 Hz) for J C-H. The chemical shift values
are comparable to that of the known benzyl complexes
of tantalum, e.g., 2a (δ 70.6, 117 Hz), Ta(CH2Ph)2(d
CHPh)Cp* (10) (δ 71.8, 120 Hz),31 TaCl2(CH2Ph)2Cp*
(δ 97.3, 123 Hz),31 TaCl2(CH2CMe3)(CH2Ph)Cp (cis-
isomer: δ 96, 126 Hz; trans-isomer: δ 98.6, 128 Hz),32
and TaCl3(CH2Ph)2(PMe3)2 (δ 82; 133 Hz),33 except for
the highly shielded complex of Ta(dCHPh)(CH2Ph)Cp2
(11) (δ 28.2, 123 Hz).34 The coupling constant (142 Hz)
of the benzylic carbons in 8 is larger than those (120-
133 Hz) found for other benzyl complexes of tan-
talum,31-34 indicating that the structure of 8 can be best
described as mode C with some contribution of mode
B. This is the case with other o-xylylene complexes of
early transition metals, e.g., Ti, Nb, and W.35-39 Thus,
Reaction of 1a with Me3SiCH2MgCl and Me3CCH2-
MgCl in THF resulted in the formation of monalkylated
complexes TaCl(CH2SiMe3)Cp*(η4-C4H6) (4) and TaCl-
(CH2CMe3)Cp*(η4-C4H6) (5); however, no dialkylated
complex was obtained due to the steric bulkiness of the
Cp* ligand and of these Grignard reagents. A similar
monophenylated complex TaCl(Ph)Cp*(η4-C4H6) has
been prepared by the reaction of TaCl2Cp*(η4-C4H6) with
phenyl Grignard reagent.27 The monoalkylated com-
plexes 4 and 5 reacted readily with 1 equiv of methyl
Grignard reagent, forming oily products TaMe(CH2-
SiMe3)Cp*(η4-C4H6) (6) and TaMe(CH2CMe3)Cp*(η4-
C4H6) (7), respectively. Since complexes 6 and 7 could
not be isolated as pure forms, we determined their
structures on the basis of 1H NMR spectral data: a
singlet at -0.45 ppm being attributed to the methyl
group bound to the tantalum atom along with a set of
signals due to trimethylsilylmethyl or neopentyl groups
in the right integral ratio. The asymmetry of the
tantalum atom in the complexes 6 and 7 resulted in the
six different signals due to the six protons of the
coordinated butadiene moiety. All attempts to prepare
alkylidene complexes by the thermolysis of 6 and 7 with
(30) McGrady, J . E.; Stranger, R.; Brown, M.; Bennett, M. A.
Organometallics 1996, 15, 3109, and references therein.
(31) Messerle, L. W.; J ennische, P.; Schrock, R. R.; Stucky, G. J .
Am. Chem. Soc. 1980, 102, 6744.
(32) Wood, C. D.; McLain, S. J .; Schrock, R. R. J . Am. Chem. Soc.
1979, 101, 3210.
(33) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J . D.; Schrock,
R. R. J . Am. Chem. Soc. 1980, 102, 6236.
(34) Schrock, R. R.; Messerle, L. W.; Wood, C. D.; Guggenberger, L.
J . J . Am. Chem. Soc. 1978, 100, 3793.
(35) Lappert, M. F.; Raston, C. L.; Skelton, B. W.; White, A. H. J .
Chem. Soc., Chem. Commun. 1981, 485.
(36) Lappert, M. F.; Raston, C. L.; Skelton, B. W.; White, A. H. J .
Chem. Soc., Dalton Trans. 1984, 893.

4186 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
(4) and 2.249(4) Å)29 and 11 (2.30(1) Å).34 The bonding
parameters within the o-xylylene and the pentameth-
ylcyclopentadienyl ligand are unexceptional. The Ta-
Câ distances (2.584(8) and 2.578(8) Å) in the o-xylylene
ligand are longer than those in butadiene complexes
such as 1b (2.424(11)-2.410(12) Å),40 indicating the
presence of bonding interaction between Ta and the Câ
carbons as observed for various η4-o-xylylene complexes
(modes B and C) of early transition metals such as Ti,
Nb, and W.35-39 The fold angle (104.3°) between Ta-
C1-C4 and C1-C2-C3-C4 of 8 is larger than those
found for 1b (94.6°)40 and is smaller than those found
for some of the o-xylylene complexes of Ti (109.5° and
129°)36,39 and Nb (126°);36 this is in accord with the NMR
result, where bonding mode C has some contribution
from mode B.
The dichloro complex 8 reacted readily with 2 equiv
of PhCH2MgCl in THF to give the corresponding diben-
zyl complex 9 in 62% yield. Complex 9 is thermally
unstable, and its structure was determined by NMR
F igu r e 2. ORTEP drawing of 8 with numbering scheme.
Hydrogen atoms are omitted for clarity.
1
spectroscopy supported by elemental analysis. The H
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) of 8
NMR spectrum of 9 displayed two sets of signals
assignable to two benzyl ligands and the o-xylylene
ligand. In the 13C NMR spectrum, the benzylic carbons
of the benzyl ligands appeared at δ 79.9 (1J CH ) 119
Hz), while the benzylic carbons of the o-xylylene ligand
occurred at 68.8 (1J CH ) 137 Hz); here a smaller
coupling constant than that found for 8 suggested a
larger contribution from mode C.
Bond Distances (Å)
Ta-Cl(1)
Ta-C(1)
Ta-C(3)
Ta-C(11)
Ta-C(13)
Ta-C(15)
C(2)-C(3)
2.400(2)
2.221(8)
2.578(8)
2.448(8)
2.445(8)
2.419(8)
1.41(1)
Ta-Cl(2)
2.405(2)
2.584(8)
2.220(8)
2.448(8)
2.420(8)
1.51(1)
Ta-C(2)
Ta-C(4)
Ta-C(12)
Ta-C(14)
C(1)-C(2)
C(3)-C(4)
1.49(1)
Monitoring the thermolysis of 9 in C6D6 at 20 °C by
1H NMR spectroscopy revealed a smooth R-hydrogen
abstraction accompanied by the elimination of toluene,
as the benzylidene complex Ta(dCHPh)(η4-o-(CH2)2C6H4)-
Cp* (12) was formed. Thus, the complex 12 was isolated
in 17% yield by thermolysis of 9 at 40 °C for 4 h followed
by recrystallization from a mixture of toluene and
hexane. The benzylidene complex 12 is a dark red and
extremely air-sensitive crystalline solid, but it can be
stored at room temperature under argon for months
without decomposition. It is noteworthy that 12 can be
isolated without the electron-donating phosphine ligand
like the phosphine-free benzylidene complex 10;31 this
is in sharp contrast to the PMe3 adduct 3a obtained by
the thermolysis of 2a in the presence of PMe3.29 It is
initially difficult to rule out a benzylidene-bridged
dinuclear structure, but a crystallographic study finally
confirmed a mononuclear structure for 12 (vide infra).
Bond Angles (deg)
Cl(1)-Ta-Cl(2)
Cl(1)-Ta-C(4)
Cl(2)-Ta-C(4)
Ta-C(1)-C(2)
C(1)-C(2)-C(3)
89.53(9)
Cl(1)-Ta-C(1)
Cl(2)-Ta-C(1)
C(1)-Ta-C(4)
Ta-C(4)-C(3)
C(2)-C(3)-C(4)
83.8(3)
137.9(2)
74.1(3)
85.7(5)
116.5(8)
138.2(2)
84.1(3)
85.5(5)
113.7(8)
the H and 13C NMR evidence, along with the crystal-
lographic study (vide infra), designated bonding mode
C for the structure of 8.
1
1
The H NMR spectrum of 12 showed a singlet at δ 4.99
due to the R-benzylidene proton, indicating the presence
of a single alkylidene rotamer. The chemical shift value
of the alkylidene carbon of 12 is higher shifted than
those (around 10 ppm) found for 3, being a result of
shielding by the aromatic ring of the o-xylylene unit.
In the 13C NMR spectrum, the benzylidene carbon atom
Slow cooling of a toluene solution of 8 provided single
crystals suitable for an X-ray diffraction study. An
ORTEP drawing of 8 is shown in Figure 2, and the
selected bond distances and angles are summarized in
Table 1. The coordination of the tantalum atom can be
described as the four-legged piano stool geometry com-
prised of the Cp* ligand, two CR atoms of the o-xylylene
ligand, and two chloro ligands. The Ta-CR distances
(2.221(8) and 2.220(8) Å) are comparable to those of a
benzyl complex of tantalum 10 (2.188(15) and 2.233-
(14) Å),31 but are slightly shorter than those of 2a (2.277-
appeared at δ 230.8 with a coupling constant J C-H
)
85 Hz. This chemical shift is comparable to that found
for 10 (δ 220, J C-H ) 82 Hz)31 and otherwise is smaller
than those of the corresponding carbon atoms in related
complexes, 3a (δ 237.3, J C-H ) 112 Hz, J C-P ) 19 Hz)
and 11 (δ 246, J C-H ) 127 Hz).34 The small coupling
constant of 12 is consistent with the large Ta-CR-Câ
(37) Bristow, G. S.; Lappert, M. F.; Martin, T. R.; Atwood, J . L.;
Hunter, W. F. J . Chem. Soc., Dalton Trans. 1984, 399.
(38) Mena, M.; Royo, P.; Serrano, R.; Pellinghelli, M. A.; Tiripicchio,
A. Organometallics 1988, 7, 258.
(39) Mena, M.; Royo, P.; Serrano, R.; Pellinghelli, M. A.; Tiripicchio,
A. Organometallics 1989, 8, 476.
(40) Yasuda, H.; Tatsumi, K.; Okamoto, T.; Mashima, K.; Lee, K.;
Nakamura, A.; Kai, Y.; Kanehisa, N.; Kasai, N. J . Am. Chem. Soc.
1985, 107, 2410.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4187
benzylidene complexes of group 5 metals having a half-
metallocene fragment, e.g., 3a 29 and Cp*M(N-2,6-i-
Pr2C6H3)(dCHPh)(PMe3) (13a , M ) V;41 13b, M )
Nb42). The large Ta-C(11)-C(12) angle of 164.8(7)° is
almost the same as that (166.0°) found for 10 and is
1
consistent with the small H-13C coupling constant for
the R-carbon atom.
P olym er iza tion of Nor bor n en e. The isolated ben-
zylidene-phosphine complexes 3a and 3b did not have
any catalytic activity for the ROMP of norbornene,
presumably due to the presence of the phosphine ligand,
which severely prevents norbornene coordination at the
tantalum center. Phosphine-free carbene species, gen-
erated in situ from thermolysis of dialkyl complexes 2,
6, 7, and 9, together with the phosphine-free ben-
zylidene complex 12 can initiate the polymerization of
norbornene, and the results are summarized in Table
3. The activity of these precursors depends crucially
on the temperature since heating is required to generate
the “carbene species” by thermal decomposition, and at
higher temperature the yield of the polymer increased
to some extent. Higher molecular mass polymers were
formed with neat monomer. The stereoselectivity of the
reaction, in terms of the polymers’ cis/ trans ratio, can
be controlled by changing the ancillary ligands (Cp or
Cp*, butadiene, or o-xylylene) on the tantalum center.
F igu r e 3. ORTEP drawing of 12 with numbering scheme.
Hydrogen atoms except one bound to the carbenic carbon
are omitted for clarity.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) of 12
Bond Distances (Å)
Ta-C(1)
2.180(9)
2.59(1)
1.925(9)
2.419(8)
2.44(1)
1.47(1)
1.43(1)
1.49(1)
Ta-C(2)
2.572(8)
2.169(8)
2.445(9)
2.415(8)
2.416(8)
1.42(1)
Ta-C(3)
Ta-C(4)
Ta-C(11)
Ta-C(22)
Ta-C(24)
C(1)-C(2)
C(2)-C(5)
C(11)-C(12)
Ta-C(21)
Ta-C(23)
Ta-C(25)
C(2)-C(3)
C(3)-C(4)
1.50(1)
Several notable features of the polymerization reac-
tion can be pointed out. (1) The catalyst precursors 2a ,
6, and 7, which have both Cp* and butadiene ligands,
gave 97-99% cis-polymers. The catalyst precursor 2a
(Table 3, runs 1-7) was superior to 6 and 7 (runs 8-11)
in the ROMP of norbornene in that the latter two
precursors resulted in a low chemical yield (12-22%
yield after 50 h) and larger polydispersity (Mw/Mn 3.25-
4.67); however the reactions were highly stereoselective
(cis content 97-98%). These findings indicated that the
benzylidene species was generated from 2a much more
readily than the alkylidenes from 6 and 7, which
requires the release of methane. (2) The Cp complexes
2b showed no stereoselectivity in double-bond formation
(runs 12 and 13), indicating that the Cp and butadiene
ligands on the tantalum atom exert similar steric effects.
In this context the isoelectronic and analogous “Cp2Tid
CHR” species has been reported to catalyze the ROMP
of norbornene to give poly(norbornene) without any
stereoselectivity.7,8 (3) In sharp contrast to this high
cis stereoselectivity, the o-xylylene complex 9 gave
polymers with 92-95% trans double bonds (runs 14-
18). The isolated benzylidene complex 12 also initiated
the ROMP of norbornene (run 19) with almost the same
results as when using the complex 9. (4) The different
stereochemistry of the polymers derived from the buta-
diene complex and the o-xylylene complex is attributed
to the significantly larger bulk of the latter ligand and
in nonbonded repulsions which are manifest in the
different geometry of the rotamers of the initiating
alkylidene species, in accordance with the structural
features of 3a 29 and 12.
Bond Angles (deg)
C(1)-Ta-C(4)
C(4)-Ta-C(11)
Ta-C(4)-C(3)
C(1)-C(2)-C(3)
83.0(4)
108.7(4)
87.8(5)
C(1)-Ta-C(11)
114.6(4)
87.3(5)
164.8(7)
119(1)
Ta-C(1)-C(2)
Ta-C(11)-C(12)
C(2)-C(3)-C(4)
120.4(9)
angle (vide infra), as expected by the relationship
between the M-C-C bond angle in alkylidene com-
plexes and the H-13C coupling constant for the R-car-
1
bon atom.1
Figure 3 shows an ORTEP drawing of 12, and selected
bond distances and angles are summarized in Table 2.
Complex 12 has 16 valence electrons and has a three-
legged piano stool geometry comprised of C(1), C(4), and
C(11). The structural features of 12 are closely related
to the Schrock complex 10, which also has a three-legged
piano stool geometry: two benzyl groups and one
benzylidene moiety, the difference is the “up” and
“down” orientations of the two benzyl ligands in 10.31
The Ta-C(11) bond distance (1.925(9) Å) is much
shorter than that of 3a (2.044(8) Å)29 and 11 (2.07(1)
Å),34 but longer than that (1.883(14) Å) of 10.31 The Ta-
CR bond distances (2.169(8) and 2.180(9) Å) of the
o-xylylene ligand are slightly shorter than those (2.188-
(15)-2.233(14) Å) found for the benzyl ligands in 10.31
The o-xylylene ligand is sterically more demanding than
the Cp* ligand, and so the phenyl group of the ben-
zylidene ligand in 12 points up “toward” the Cp* ligand,
i.e., the same, syn, direction as that observed in 10,31
but in sharp contrast to the anti-geometry found for
Hyd r ogen a tion of P oly(n or bor n en e). To deter-
mine the stereoregularity of the various poly(nor-
(41) Buijink, J .-K.; Teuben, J . H.; Kooijman, H.; Spek, A. L.
Organometallics 1994, 13, 2922.
(42) Cockcroft, J . K.; Gibson, V. C.; Howard, J . A. K.; Poole, A. D.;
Siemeling, U.; Wilson, C. J . Chem. Soc., Chem. Commun. 1992, 1668.

4188 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
Ta ble 3. P olym er iza tion of Nor bor n en e Ca ta lyzed by Bu ta d ien e Com p lexes 2a , 2b, 6, a n d 7 a n d o-Xylylen e
Com p lexes 9 a n d 12
c
c
run
compd
temp (°C)
solv
S/Ca
time (h)
yieldb (%)
Mn
Mw/Mn
cisd (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
2a
2a
2a
2a
2a
2a
2a
6
6
7
7
2b
2b
9
9
9
45
45
45
45
65
65
65
60
60
65
65
60
60
45
45
65
50
50
65
toluene
THF
toluene
THF
toluene
THF
none
toluene
THF
toluene
THF
toluene
THF
toluene
THF
100
100
100
100
100
100
1000
100
100
100
100
100
100
100
100
100
500
100
100
50
50
100
100
30
30
87
50
50
50
50
32
32
30
30
6
30
35
54
60
84
94
43
13
12
14
22
42
16
14
5
4000
4100
5600
5200
8900
8600
32 300
2700
3300
2100
2600
3400
1800
2000
5800
3700
22 300
20 300
2800
2.36
2.00
1.93
1.92
1.63
1.88
2.04
3.25
4.05
3.30
4.67
2.46
2.32
1.72
2.00
1.49
1.85
1.91
2.13
97
97
98
98
97
97
99
97
98
98
97
50
50
8
5
7
7
7
THF
none
none
toluene
12
6
73
8
9
9
12
15
30
30
10
a
b
S/C: the ratio of norbornenes/catalyst. Methanol-insoluble fraction. c GPC analysis in THF versus polystyrene standards. Theoretical
Mn value is less than the experimental one.7 Stereochemistry of double bonds in polymer as determined by 1H NMR.7,8,50
d
Ta ble 4. Hyd r ogen a tion of P oly(n or bor n en e)s Der ived fr om P olym er s F or m ed Usin g Com p lexes 2a a n d 9
a s Ca ta lyst P r ecu r sor sa
c
c
run
complex
temp (°C)
solv
yieldb (%)
Mn
Mw/Mn
cisd (%)
mc (%)
1
2
3
2a
2a
9
65
65
50
THF
toluene
none
64
67
23
12 000
12 000
19 000
1.49
1.496
2.34
97
98
5
57
55
57
a
b
The ratio of [norbornenes]/[catalyst] was 100. Each polymerization was quenched after 30 h. Methanol-insoluble fraction. c GPC
analysis in THF versus polystyrene standards. Theoretical Mn value is less than the experimental one.7 Stereochemistry of double
bonds in polymer as determined by 1H NMR.7,8,50 e Overall tacticity of hydrogenated form of the polymer expressed as % isotactic (m)
junctions as determined from its hydrogenated derivative.
d
bornene)s, we carried out a hydrogenation of the double
bonds using diimine generated in situ from p-toluene-
sulfonylhydrazide (see the Experimental Section),26 and
thus we obtained the hydrogenated product in almost
quantitative yield. The fully hydrogenated polymers
were soluble in CDCl3 at 50 °C, and high-quality 13C
NMR spectra for three samples were obtained for
estimation of the relative proportions of dyad and triad
structures.26 The results for these hydrogenated poly-
mers are shown in Table 4. The poly(norbornene)s that
were obtained using the catalyst precursors 2a and 9
were high cis and high trans, respectively, and atactic,
presumably as a result of the monomer adding to the
metastable Ta species without any face selectivity. This
result however was not expected because the high-cis
polymers formed from anti-7-methylnorbornene using
2a were 88% isotactic.29 Although the reasons for this
difference are not clear, this is an important observation
because it provides further evidence of an apparent
tendency for polymers formed from the prototypal
norbornene and norbornadiene, with given catalyst
systems, to be much less tactic than those of their
derivatives.43
However acenaphthylene is a cyclic olefin that is
expected to react with the alkylidene species to form a
metallacyclobutane, and furthermore C-C bond fission
is prevented compared with the metallacyclobutane
derived from the addition of norbornene. We have in
fact successfully isolated and spectroscopically charac-
terized the acenaphthylene adduct of niobium, Cp*(η4-
butadiene)Nb[CH2CH(C10H6)CH] (14), which was found
to have only a low activity in the ROMP of norbornene.44
Thus we investigated reactions of the bis(benzyl) com-
plexes 2, 6, 7, and 9 along with the benzylidene
complexes 3 and 12 with a small excess of acenaphth-
ylene under thermal conditions where a benzylidene
species is generated. In the reactions of 2, 3, 6, 7, and
9, we could not detect any metallacycles at all, and only
decomposed products were detected by NMR spectros-
copy. On the other hand, the reaction of the complex
12 with acenaphthylene gave rise to a deep violet
tantalacyclobutane Cp*(η4-o-xylylene)Ta[CH(Ph)CH-
(C10H6)CH](15) in 18% yield (eq 1), which is air stable
in solid form, but decomposed readily in solution upon
exposure to air. The formation of 15 in the reaction of
9 with acenaphthylene was detected by NMR spectros-
copy, but 15 could not be isolated.
P r ep a r a t ion of a Mod el Ta n t a la cyclob u t a n e
R OMP In t er m ed ia t e. As mentioned above, nor-
bornene efficiently added to the alkylidene species
derived from the tantalum catalyst precursors 3 and 12
to give poly(norbornene), and so it is not surprising that
all attempts to isolate a metallacyclobutane, regarded
as an intermediate in the propagation of ROMP,2 failed.
1
The structure of 15 was determined by H and 13C
NMR spectroscopy as well as X-ray crystallographic
1
analysis (vide infra). In the H NMR spectrum of 15, a
signal at δ 2.14 due to a â-hydrogen atom, which is
(43) Amir-Ebrahimi, V.; Corry, D. A. K.; Hamilton, J . G.; Rooney,
J . J . J . Mol. Catal., in press.
(44) Mashima, K.; Kaidzu, M.; Nakayama, Y.; Nakamura, A. Or-
ganometallics 1997, 16, 1345.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4189
F igu r e 4. ORTEP drawing of 15 with numbering scheme.
Hydrogen atoms are omitted for clarity.
benzylic to the acenaphthylene moiety, was observed.
This is at lower field compared with that (δ 1.46) of 1444
and is at lower field than those found for Cp*(η4-
C4H6)Nb[CH2CH(C5H8)CH] (16) (δ -1.69),44 Cp2Ti[CH2-
CH(C5H8)CH] (17) (δ 0.14),7 and (DIPP)3Ta[CH(tBu)-
CH(C5H8)CH] (18) (DIPP ) 2,6-di(isopropyl)phenoxy) (δ
0.88).45 The orientation of the phenyl group on the
cyclobutane ring system could not be probed by 1H NMR
experiments, but its trans orientation with respect to
the acenaphthylene moiety can be determined by the
X-ray analysis (vide infra) and is thus consistent with
the small geminal coupling constants observed for the
R-hydrogen atoms (δ 3.17, J ) 7.9 Hz and 4.12, J ) 7.7
Hz). The assignment of the 13C NMR signals to CR (δ
89.7 and 92.1) and Câ (δ 20.6, J CH ) 139 Hz) in the
tantalacyclobutane was possible by way of a 1H-13C
COSY experiment and comparison with those of 14 (CR,
δ 74.1 and 111.6; Câ, δ 20.7, J CH ) 145 Hz).44 The
chemical shift values (δ 63.6 and 64.0) and coupling
constants (139 and 140 Hz) for benzylic carbons of the
o-xylylene ligand are normal and comparable to those
found for 8, 9, and 12; the coupling constants suggest a
conventional o-xylylene ligand with a large contribution
of the canonical form C in Chart 1.
F igu r e 5. ORTEP drawing of 14 with numbering scheme.
Hydrogen atoms are omitted for clarity.
Figure 4 shows the solid-state structure of 15, and
selected bond distances and angles are shown in Table
5. Complex 15 has a four-legged piano stool geometry
composed of a capping Cp* ligand with four carbon atom
legs, C(1), C(4), C(11), and C(13), and has a puckered
metalacyclobutane structure with a phenyl group at
C(11) trans to the acenaphthylene ring. This confirms
that the addition of acenaphthylene to the TadC bond
of 12 proceeded in the fashion shown to minimize the
steric interaction between the Cp* ligand and the
approaching acenaphthylene; the “syn” orientation of
the “TadCHPh” moiety in 12 has been retained during
addition reaction. The fold angle of 12.5° between Ta-
C(11)-C(13) and C(11)-C(12)-C(13) is smaller than
that of 16 (29.6°) and those of the tantalacyclobutane
complexes 1845 and (DIPP)3Ta[CH(Ph)CH(tBu)CH2] (19)
(DIPP ) 2,6-di(isopropyl)phenoxy), which are 15.5° and
25.2°, respectively.45,46 The rather small fold angle of
15 might be attributed to steric congestion caused by
the Cp* and the o-xylylene ligands together with the
added acenaphthylene moiety. The distance (2.637(6)
(45) Wallace, K. C.; Dewan, J . C.; Schrock, R. R. Organometallics
1986, 5, 2162.
(46) Wallace, K. C.; Liu, A. H.; Dewan, J . C.; Schrock, R. R. J . Am.
Chem. Soc. 1988, 110, 4964.

4190 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) of 14 a n d 15
Sch em e 1
14 (M ) Nb)
15 (M ) Ta)
Bond Distances (Å)
2.18(3)
M-C(1)
2.212(7)
2.582(6)
2.611(6)
2.228(7)
2.224(6)
2.637(6)
2.199(6)
1.470(9)
1.415(9)
1.46(1)
M-C(2)
2.34(2)
2.33(2)
2.28(2)
2.28(2)
2.40(2)
2.18(1)
1.38(3)
1.38(3)
M-C(3)
M-C(4)
M-C(11)
M‚‚‚C(12)
M-C(13)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(11)-C(12)
C(12)-C(13)
C(11)-C(31)
1.40(3)
1.65(3)
1.56(3)
1.577(8)
1.614(9)
1.463(9)
Bond Angles (deg)
71(1)
C(1)-M-C(4)
72.8(3)
73.9(2)
128.4(2)
88.6(3)
89.7(3)
139.4(3)
86.6(4)
87.6(4)
86.0(4)
86.1(3)
136.1(5)
114.1(6)
115.1(6)
112.8(5)
111.2(5)
105.4(5)
128.5(5)
119.7(6)
C(11)-M-C(13)
C(1)-M-C(11)
C(1)-M-C(13)
C(4)-M-C(11)
C(4)-M-C(13)
M-C(1)-C(2)
80.6(9)
134(1)
101(1)
74(1)
133.7(9)
78(2)
74(1)
73(1)
77(1)
131(1)
116(3)
115(2)
127(2)
107(2)
104(2)
Mech a n istic Ou tlin e. To understand the factors
that determine the stereochemistry of the double bonds
in poly(norbornene) and the high cis/ trans stereoselec-
tivity and poor stereoregularity (almost atactic), we
isolated two benzylidene complexes, 3a and 12, which
can be regarded as key intermediates in the ROMP
catalyzed by our tantalum catalyst system. 3a has an
anti-benzylidene ligand, while 12 is syn, and it is
expected that the stability of the rotamer of the alky-
lidene species during the propagation will control the
stereochemistry of the double bond of poly(norbornene)
because syn- and anti-rotamers of molybdenum com-
plexes of the type Mo(NAr)(dCHMe2Ph)(OR)2 (Ar ) 2,6-
Me2C6H3, 2,6-iPr2C6H3; OR ) O-tBu, OCMe(CF3)2, etc.)
have been demonstrated to play a major role in deter-
mining whether certain other polymers contain cis or
trans double bonds.12,17 With the butadiene complex 2a
as a catalyst precursor, the relative geometry of the
alkylidene species, and presumably the propagating
chain end of the polymer, is mostly anti (see 20, 23, and
24 in Scheme 1). On the other hand, the o-xylylene
ligand is much more sterically demanding than the
butadiene ligand and thus induces the formation of a
metastable anti-alkylidene species 27 from 26 (see
Scheme 2). The formation of syn-28 and -29, similar to
12 (as shown in Scheme 2), seems to be taking place,
although this could not be detected. Accordingly, we
can control the orientation of the rotational isomer by
varying an auxiliary ligand, e.g., butadiene or o-xy-
lylene, on the tantalum center.
M-C(4)-C(3)
M-C(11)-C(12)
M-C(13)-C(12)
M-C(13)-C(14)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(11)-C(12)-C(13)
C(11)-C(12)-C(21)
C(12)-C(13)-C(14)
C(12)-C(11)-C(14)
C(12)-C(11)-C(14)
Å) of Ta‚‚‚C(12) in 15 is longer than the corresponding
distances found for 16 (2.44(1) Å) and 18 (2.382(16) Å),
but is shorter than that (2.782(24) Å) of 19. The bond
distance (2.224(6) Å) of Ta-C(11) is longer than that
(2.199(6) Å) of Ta-C(13), although both C(11) and C(13)
are benzylic carbons, presumably due to the presence
of the acenaphthylene ring system. The angles of C(1)-
Ta-C(4) (72.8(3)°) and C(11)-Ta-C(13) (73.9(2)°), which
are of chelating ligands, are smaller than the angles of
C(1)-Ta-C(13) (88.6(3)°) and C(4)-Ta-C(11) (89.7(3)°).
The o-xylylene ligand of 15 is bending away to minimize
the congestion between the metallacyclic moiety and the
o-xylylene ligand; the fold angle (98.7°) between the
plane of Cp ring carbons and the plane of C(1)-C(2)-
C(3)-C(4) is larger than that of 8 (80.7°) and 12 (81.9°).
The tantallacyclic ring system of 15 is essentially the
same as the structure of the niobacyclic ring system of
14 except for the absence of the phenyl group in the ring
system of 14. The structure of 14 is of poor quality, and
thus the accuracy of bond parameters of 14 is limited.
Selected bond distances and angles of 14 are listed in
Table 5, and a drawing of 14 is provided in Figure 5.
14 may be described as having a four-legged piano stool
geometry composed of a capping Cp* ligand and four
carbon atom legs, C(1), C(4), C(11), and C(13), with a
metalacyclobutane geometry that is almost flat (the fold
angle of Ta-C(11)-C(13) and C(11)-C(12)-C(13) )
2.8°). The Nb‚‚‚C(12) distance of 2.40(2) Å was shorter
than that of 15. The tendency of a longer Nb-C(11)
(2.28(2) Å) and a shorter Nb-C(13) (2.18(1) Å) is
comparable to that found for 15.
With regard to half-metallocene complexes of group
5 metals 13a 41 and 13b,42 vide supra, which are iso-
electronic to the complex 3a including the PMe3 ligand,
an “anti”-alkylidene ligation is observed. It therefore
seems curious that complex 12, which also has a 14-
valence TaCp*(η4-o-xylylene) fragment isoelectronic to
the TaCp*(η4-diene) fragment, adopts the syn-isomer
just like the Schrock complex 10, which has the syn-
benzylidene ligand. The syn-geometry found for 12
might be attributed to the CH-π interaction as shown
in Scheme 2,47 and this emphasizes the unique ability
(47) Steiner, T.; Starikov, E. B.; Amado, A. M.; Teixeira-Dias, J . J .
C. J . Chem. Soc., Perkin Trans. 2 1995, 1321.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4191
Sch em e 2
kinds of functionalized norbornadiene monomers have
been polymerized in high stereoselectivity and stereo-
regularity using metal salt-based catalyst systems,40
and recently all-trans highly tactic10,13 and all-cis highly
tactic polymers16 have been obtained by using well-
defined molybdenum alkylidene complexes. Complexes
167 and 1845 are less stereoselective and catalyze the
ROMP of norbornene to give 55 and 62% trans-poly-
(norbornene)s, respectively. The successful stereocon-
trol described in the present study is attributed to the
incorporation of the bulky Cp* ligand with auxiliary
ligands such as butadiene and o-xylylene, and the
benefits of these steric effects are emphasized when one
considers polymerizations using 2b, a Cp-butadiene
complex, as a catalyst precursor; the resulting poly-
(norbornene) had a 1:1 mixture of cis and trans, indicat-
ing no stereoselectivity. Although a phosphine complex
3b was found to be the same anti-rotamer as 3a , two
rotamers, 31 and 32, are likely to be generated during
the propagation process. Norbornene may be added to
31 in two different ways to form metallacyclobutanes
33 and 34, while the addition of norbornene to 32 yields
35 and 36; subsequent metathesis reaction of these four
possible metallacyclobutanes regenerate 31 and 32,
resulting in a nonselective CdC bond formation.
of the o-xylylene ligand to control the chain end isomer
as opposed to that of the butadiene complex.
Metallacyclobutanes have been demonstrated as in-
termediates in ROMP. We assume that norbornene
approaches the TadC bond to minimize the steric
repulsion of the Cp* ligand, and this is confirmed by
the structural study for niobacyclobutane 14, where C(7)
is pointing away from the Cp* ligand; thereby 21 in
Scheme 1 and 25 in Scheme 2 are less favorable
products. We can postulate that, in the case of the
butadiene complexes (Scheme 1), an “all-cis”-metalla-
cycle 22 is likely to be formed, and subsequent ring-
opening would then afford a cis-double bond and an
inserted product 23 along with 24. For the o-xylylene
catalysts (Scheme 2), the trans-metallacycle 26 is as-
sumed to be the product upon addition of monomer to
12; the metathesis reaction resulted in the formation
of a trans-double bond and a new anti-alkylidene
complex 27 that is expected to isomerize to the syn-
species such as 28 and 29. This interconversion seems
to be much faster than the addition of monomer to 27,
which would lead to the formation of the “all-cis”-
metallacycle 30, which then would give a cis-double
bond, as shown in Scheme 1. The formation of 30 is
unfavorable because there is steric repulsion between
the polymer chain end and the o-xylylene ligand, but
this may explain some contamination of the cis-double
bonds in the polymer obtained using the o-xylylene
complexes. Furthermore, it is of interest that the rate
of polymer formation catalyzed by the o-xylylene pre-
cursors is much slower than that of the butadiene
catalysts.
Four possible stereochemical representations of poly-
(norbornene) are shown in Figure 1; alternating addition
of norbornene to each face of the TadC bond (single
rotamer) affords syndiotactic polymer, while the addi-
tion to the same face of the TadC bond results in
isotactic polymer. There are only a few known examples
of tactic poly(norbornene) in contrast to many examples
of tactic polymers made from norbornadiene derivatives;
cis syndiotactic poly(norbornene) has been obtained
using the ReCl5-based heterogeneous catalyst,26 and a
trans, isotactic-rich example was also reported.26 Al-
though quite high stereoselectivity for the double bond
in the polymer was accomplished with the present
tantalum catalyst precursors, the polymers were almost
atactic as determined by 13C NMR spectroscopy of their
hydrogenated derivatives. This result suggests that the
alkylidene species would be in a resting state during
It is rather difficult to control the stereochemistry of
the CdC bond in poly(norbornene), although many

4192 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
1
The H (500, 400, and 270 MHz) and 13C (125, 100, and 68
MHz) NMR spectra were measured on a J EOL J NM-GX500,
a J EOL J NM-GSX400, or a J EOL J NM-EX270 spectrometer.
When C6D6 was used as the solvent, the spectra were refer-
the propagation, and thereby the monomer added to
both of the faces of the TadC bond, leading to the atactic
polymer.
1
enced to the residual solvent protons at δ 7.20 in the H NMR
Con clu sion
spectra and to the residual solvent carbons at δ 128.0 (triplet
for C6D6) in the 13C NMR spectra. Assignments for 1H and
13C NMR peaks for some of the complexes were aided by 2D
1H-1H NOESY and 2D 1H-13C COSY spectra, respectively.
Elemental analyses were performed at Elemental Analysis
Center, Faculty of Science, Osaka University. All melting
points of the complexes were measured in sealed tubes under
argon atmosphere and were not corrected.
Ta (CH2P h )2(η5-C5Me5)(η4-bu ta -1,3-d ien e) (2a ). To a so-
lution of 1a (0.592 g, 1.34 mmol) in THF (60 mL) cooled at
-78 °C was added an ethereal solution of PhCH2MgCl (2.11
equiv, 2.84 mmol) via a syringe. The reaction mixture was
stirred for 2.5 h at 25 °C. The color of the solution changed
from purple to dark red. All volatiles were removed under
reduced pressure to give a residue, from which the product
was extracted with hexane (120 mL). Recrystallization from
toluene (5 mL) at -20 °C afforded 2a as dark red crystals in
83% yield; mp 121-122 °C (dec). Complex 2a is thermally
unstable, and the content of 2a was reduced to ca. 10% after
4 days at 25 °C, it also decomposed to about 60% on heating
at 45 °C for 6 h. 1H NMR (270 MHz, C6D6, 303 K): δ -0.79,
(2H, m, dCH2 anti), 0.02 (2H, m, dCH2 syn), 0.40, 1.23 (4H,
In this contribution, we demonstrated that cis-dialkyl
complexes of tantalum bearing an η5-C5R5 and an
auxiliary ligand such as 1,3-butadiene and o-xylylene
are efficient catalyst precursors for the ROMP of nor-
bornene. The factors governing the unexpectedly high
stereoselectivity have been investigated by examining
the structures of well-defined alkylidene and metalla-
cyclobutane derivatives.
When the Cp*-butadiene complexes 2a , 6, and 7 are
employed, the main feature of the polymerization sys-
tem is that high cis, atactic poly(norbornene) is formed.
This is a consequence of (i) the alkylidene-propagating
species preferring to exist as the anti-rotamer assumed
on the basis of the isolation and crystallographic char-
acterization of 3a and (ii) subsequent exo-addition of
norbornene to the alkylidene without face selectivity to
produce another anti-alkylidene and a cis-vinylene bond.
The stereoselectivity of the reaction depends sensitively
on the ligand environment at the catalyst site. For
example the Cp-butadiene complex 2b did not show
any stereoselectivity in the ROMP of norbornene, and
this is attributed to the similarity of the steric bulk of
the Cp and butadiene ligands; therefore, both rotamers
are available for propagation. We also found that the
o-xylylene complexes 9 and 12 catalyzed the ROMP of
norbornene to yield high trans, atactic polymer and that
the benzylidene complex 12 was characterized crystal-
lographically to be a syn-rotamer, addition of monomer
to which affords a trans-vinylene bond and an anti-
rotamer. Fast conversion of the anti-rotamer 27 to a
syn-rotamer 28 is assumed to be the key step in the
mechanism.
2
AB quartet, J HH ) 10.9 Hz, Ta-CH2-Ph), 1.77 (15H, s,
C5Me5), 6.39 (2H, m, dCH-), 6.85 (4H, d, J HH ) 6.9 Hz,
3
3
3
o-C6H5), 6.94 (2H, t, J HH ) 7.6 Hz, p-C6H5), 7.32 (4H, t, J HH
) 7.6 Hz, m-C6H5). 13C NMR (68 MHz, C6D6, 303 K): δ 11.4
(q, 1J CH ) 127 Hz, C5Me5), 60.1 (t, 1J CH ) 150 Hz, dCH2), 70.6
1
(t, J CH ) 117 Hz, Ta-CH2-Ph), 119.2 (s, C5Me5), 120.9 (d,
1J CH ) 169 Hz, dCH-), 122.7 (d, 1J CH ) 157 Hz, p-C6H5), 126.5
1
1
(d, J CH ) 156 Hz, o-C6H5), 127.7 (d, J CH ) 154 Hz, m-C6H5),
153.9 (s, ipso-C6H5). Anal. Calcd for C28H35Ta: C, 60.87; H,
6.38. Found: C, 60.86; H, 6.37.
P r ep a r a tion of Ta (CH2P h )2(η5-C5H5)(η4-bu ta -1,3-d ien e)
(2b). To a solution of 1b (0.198 g, 0.534 mmol) in THF (45
mL) cooled at -78 °C was added a solution of PhCH2MgCl
(2.16 equiv, 1.16 mmol) in ether via a syringe. The color of
the reaction solution changed from violet to brown. All
volatiles were removed under reduced pressure to give a
residue, from which the product was extracted with toluene
and hexane (1:10) (100 mL). The solution was concentrated
to 3 mL and kept at -20 °C to afford 2b as brown crystals in
51% yield; mp 108-109.5 °C (dec). Complex 2b is air and
moisture sensitive. 1H NMR (270 MHz, C6D6, 303 K): δ -0.82,
(2H, m, dCH2 anti), 0.07 (2H, m, dCH2 syn), 1.27 and 1.86
(4H, AB quartet, J HH ) 10.9 Hz, Ta-CH2-Ph), 5.79 (5H, s,
C5H5), 6.69 (2H, m, dCH-), 6.75 (4H, d, J HH ) 6.9 Hz,
o-C6H5), 6.89 (2H, t, J HH ) 7.4 Hz, p-C6H5), 7.27 (4H, t, J HH
) 7.7 Hz, m-C6H5). Anal. Calcd for C23H25Ta: C, 57.27; H,
5.22. Found: C, 57.27; H, 5.22.
P r ep a r a tion of Ta (dCHP h )(η5-C5Me5)(η4-bu ta -1,3-d i-
en e)(P Me3) (3a ). To a solution of 2 (0.258 g, 0.47 mmol) in
toluene (6.0 mL) was added a solution of PMe3 (1.07 equiv,
0.50 mmol) in toluene (1.0 M, 0.50 mL) via a syringe at room
temperature. The reaction mixture was stirred for 16 h at 48
°C, during which time the color of the solution turned to
reddish brown. The solution was concentrated to ca. 3 mL and
then was kept at -20 °C to afford 3a as yellow-brown crystals
in 55% yield; mp 113-118 °C (dec). Complex 3a is air and
moisture sensitive both in solid form and in solution and is
thermally unstable in hydrocarbon solvents, being decomposed
to the extent of about 60% by heating at 60 °C for 24 h. The
1H and 13C NMR were referenced to THF-d8 at δ 3.60 and δ
67.4, respectively. The 2D 1H-1H NOESY spectrum displayed
among the protons H4-PMe3 (w), Pho-H2 (w), Pho-Cp* (w),
Pho-PMe3 (w), HR-Cp* (m), H1-H2 (s), H4-H3 (s), and so on.
1H NMR (270 MHz, THF-d8, 303 K): δ -1.01 (1H, m, H4′),
We could not detect any metallacyclobutanes during
the polymerization; however, we isolated and crystal-
lographically characterized an acenaphthylene adduct
15 that suggests exo-addition to the TadC bond with
the retention of the syn-geometry.
Exp er im en ta l Section
2
3
Gen er a l P r oced u r es. All manipulations involving air-
and moisture-sensitive organometallic compounds were carried
out by use of standard Schlenk techniques under an argon
atmosphere. Hexane, THF, toluene, and pentane were dried
and deoxygenated by distillation over sodium benzophenone
ketyl under argon. Methanol was refluxed over magnesium
and distilled under argon. Benzene-d6 was distilled from Na/K
alloy and thoroughly degassed by trap-to-trap distillation
before use. PMe3 purchased as toluene solution (1.0 M) from
Aldrich Chemical Co., was used as received. Ethylene was
purchased from Seitetsu Kagaku Co. and was used as received.
Norbornene (bicyclo[2.2.1]hept-2-ene) from the Aldrich was
refluxed over sodium and distilled prior to use. Acenaphth-
ylene was sublimed before use. Complexes TaCl2(η5-C5R5)(η4-
buta-1,3-diene) (R ) CH3 (1a ); R ) H (1b)) were prepared
according to the literature,40,48 and the complex Cp*(η4-
3
3
butadiene)Nb[CH2CH(C10H6)CH] (14) has been reported previ-
ously.44
(48) Okamoto, T.; Yasuda, H.; Nakamura, A.; Kai, Y.; Kanehisa, N.;
Kasai, N. J . Am. Chem. Soc. 1988, 110, 5008.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4193
2
-0.73 (1H, m, H1′), 0.47 (1H, m, H4), 1.18 (9H, d, J HP ) 7.3
m, dCH-), 7.87 (1H, m, dCH-). One proton resonance (d
CHH) of the butadiene ligand was not assigned because of
overlap by impurity peaks.
Hz, PMe3), 1.32 (1H, m, H1), 2.11 (15H, s, C5Me5), 5.57 (1H,
m, H3), 6.23 (1H, m, H2), 6.59 (1H, t, J HH ) 7.1 Hz, p-C6H5),
3
3
3
P r ep a r a tion of Ta (η5-C5Me5)[o-(CH2)2C6H4]Cl2 (8). To
a solution of Cp*TaCl4 (0.561 g, 1.23 mmol) in THF (45 mL)
cooled at -78 °C was added a suspension of o-C6H4(CH2MgCl)2
(1.2 equiv, 1.47 mmol) in THF (0.21 M, 7.00 mL) via a syringe.
The reaction mixture was allowed to warm to room temper-
ature, stirred overnight, and evaporated to dryness. The
product was extracted with hot hexane (180 mL) at 60 °C.
Recrystallization from toluene (2.5 mL) at -20 °C afforded 8
as red crystals in 38% yield; mp 241.0-243.5 °C (dec). 1H
NMR (270 MHz, C6D6, 303 K): δ 0.47 and 1.57 (4H, AB
quartet, 2J HH ) 7.9 Hz, -CH2), 1.94 (15H, s, C5Me5), 7.52 (4H,
6.91 (2H, d, J HH ) 7.6 Hz, o-C6H5), 7.01 (2H, t, J HH ) 7.8
Hz, m-C6H5),9.67 (1H, d, 3J HP ) 5.9 Hz, TadCHPh). 13C NMR
1
(68 MHz, THF-d8, 303 K): δ 12.5 (q, J CH ) 126 Hz, C5Me5),
1
1
1
18.4 (qd, J CH ) 129 Hz, J CP ) 22 Hz, PMe3), 43.7 (t, J CH
)
1
1
148 Hz, C4), 52.8 (t, J CH ) 149 Hz, C1), 104.2 (d, J CH ) 159
Hz, C3), 110.3 (d, J CH ) 166 Hz, C2), 110.8 (s, C5Me5), 122.5
1
1
1
(d, J CH ) 158 Hz, p-C6H5), 127.7 (d, J CH ) 155 Hz, m-C6H5),
1
128.7 (d, J CH ) 155 Hz, o-C6H5), 155.2 (s, ipso-C6H5), 237.3
(dd, J CH ) 112 Hz, J CP ) 19 Hz, TadCHPh). 31P{H} NMR
(109 MHz, THF-d8, 303 K): δ -17.1 (PMe3). Anal. Calcd for
C24H36PTa: C, 53.73; H, 6.76. Found: C, 53.32; H, 6.69.
1
2
1
m, C6H4). 13C NMR (100 MHz, C6D6, 303 K): δ 12.0 (q, J CH
1
) 128 Hz, C5Me5), 69.94 (t, J CH ) 142 Hz, -CH2), 123.9 (s,
1
C5Me5), 131.3 and 137.3 (d, J CH ) 160 and 161 Hz, respec-
tively, C6H4), 127.3 (s, ipso-C6H4). Anal. Calcd for C18H23Cl2-
Ta: C, 44.01; H, 4.72. Found: C, 44.68; H, 4.78.
P r ep a r a t ion of Ta (CH2P h )2(η5-C5Me5)[o-(CH2)2C6H 4]
(9). To a solution of 8 (0.199 g, 0.41 mmol) in THF (30 mL)
cooled to -78 °C was added an ethereal solution of PhCH2-
MgCl (2.3 equiv, 0.92 mmol) via a syringe. The reaction
mixture was stirred for 0.5 h at 25 °C, and the color of the
solution changed from red to dark red. All volatiles were
removed under reduced pressure to give a residue, from which
the product was extracted with hexane (50 mL). Recrystalli-
zation from toluene (1.5 mL) and hexane (1.0 mL) at -20 °C
afforded 9 as dark red crystals in 62% yield; mp 107-108 °C
(dec). Complex 9 is thermally unstable and almost completely
decomposed after 20 h at 20 °C to give 12. 1H NMR (400 MHz,
C6D6, 303 K): δ 0.26, (2H, br, -CH2 anti), 0.99 (2H, br, -CH2
3a
P r epar ation of Ta(dCHP h )(η5-C5H5)(η4-bu ta-1,3-dien e)-
(P Me3) (3b). Complex 2b (ca. 6 mg, ca. 0.01 mmol) was
dissolved in 0.6 mL of C6D6 in a 5 mm NMR tube. To the dark
brown solution was added an excess of PMe3 (ca. 4 µL, ca. 0.04
mmol) at 25 °C. After the NMR tube was sealed and placed
1
for 25 h in an oil bath heated at 40 °C, the H NMR spectrum
was measured. Peaks due to 3b were observed along with
signals due to free toluene (δ 2.15, s, PhCH3) and excess PMe3
(δ 0.83, d, J HP ) 2.3 Hz). 1H NMR (270 MHz, C6D6, 303 K):
2
2
syn), 0.81 and 1.78 (4H, AB quartet, J HH ) 11.5 Hz, -CH2-
2
3
δ -0.33, 0.27, 0.55 and 1.82 (4H, m, dCH2), 0.72 (9H, d, J HP
) 7.6 Hz, PMe3), 5.54 (5H, s, C5H5), 10.52 (1H, d, J HP ) 5.9
Hz, TadCHPh). The resonances of the C6H5 and the methine
(dCH-) of the butadiene ligand were not assigned because
they were overlapped by C6D6 and toluene signals. All
attempts to isolate 3b failed.
Ph), 1.72 (15H, s, C5Me5), 6.69 (4H, d, J HH ) 7.0 Hz, o-C6H5),
3
3
6.90 (2H, t, J HH ) 7.3 Hz, p-C6H5), 6.95 and 7.37 (4H, m,
3
C6H4), 7.24 (4H, t, J HH ) 7.8 Hz, m-C6H5). 13C NMR (100
MHz, C6D6, 303 K): δ 11.5 (q, 1J CH ) 128 Hz, C5Me5), 68.8 (br
1
1
t, J CH ) 137 Hz, -CH2), 79.9 (br t, J CH ) 119 Hz, -CH2Ph),
1
119.5 (s, C5Me5), 122.8 (d, J CH ) 158 Hz, p-C6H5), 127.4 (d,
P r ep a r a tion of Ta Me(CH2SiMe3)(η5-C5Me5)(η4-bu ta -1,3-
d ien e) (6). To a solution of 1a (0.432 g, 0.98 mmol) in THF
(40 mL) cooled at -78 °C was added a solution of Me3SiCH2-
MgCl (1.12 equiv, 1.10 mmol) in ether (0.38 M, 2.90 mL) via
a syringe. The reaction mixture was stirred for 5 h at 20 °C,
and then a solution of MeMgI (1.02 equiv, 1.00 mmol) in ether
(1.11 M, 0.90 mL) was added at -78 °C followed by stirring
for 2 h at 20 °C. All volatiles were removed under reduced
pressure, and the residue was extracted with pentane (15 mL).
The product 6 was very highly soluble in pentane and therefore
was not isolated but gave a red-purple oil on solvent removal.
1H NMR (270 MHz, C6D6, 303 K): δ -1.04, -0.77, and 0.06
(3H, m, dCH2), -1.66 and -0.30 (2H, AB quartet, 2J HH ) 12.2
Hz, Ta-CH2-SiMe3), -0.45 (3H, s, Ta-CH3), 0.25 (9H, s,
-SiMe3), 1.78 (15H, s, C5Me5), 7.03 (1H, m, dCH-), 7.26 (1H,
m, dCH-). One proton resonance (dCHH) of the butadiene
ligand was not assigned because of being overlapped by
impurity peaks.
1J CH ) 160 Hz, m-C6H5), 130.0 and 133.3 (d, J CH ) 163 and
1
156 Hz, respectively, C6H4), 129.3 (s, ipso-C6H4), 152.0 (s, ipso-
C6H5). The resonances of the o-C6H5 was overlapped by C6D6.
Anal. Calcd for C32H37Ta: C, 63.78; H, 6.19. Found: C, 63.63;
H, 6.10.
P r ep a r a t ion of Ta (dCH P h )(η5-C5Me5)[o-(CH2)2C6H 4]
(12). To a solution of 8 (0.294 g, 0.698 mmol) in THF (50 mL)
cooled at -78 °C was added PhCH2MgCl (2.2 equiv, 1.52 mmol)
in ether (0.66 M, 2.30 mL) via a syringe. The reaction mixture
was warmed to 20 °C, and the color of the solution changed
from red to dark red, indicating the formation of 9. All
volatiles were removed under reduced pressure to give a
residue, from which the product was extracted with hexane
(50 mL). The hexane solution of the product was heated at
40 °C for 9 h. Recrystallization from toluene (2.0 mL) and
hexane (2.0 mL) at -20 °C afforded 12 as dark red crystals in
17% yield; mp 257.0-258.5 °C (dec). 1H NMR (400 MHz, C6D6,
303 K): δ -0.07, (2H, d, 2J HH ) 12.8 Hz, -CH2 anti), 1.92 (15H,
s, C5Me5), 3.49 (2H, d, 2J HH ) 12.8 Hz, -CH2 syn), 4.99 (1H, s,
dCHPh), 6.72 (2H, d, 3J HH ) 8.2 Hz, o-C6H5), 6.79 (1H, t, 3J HH
) 7.3 Hz, p-C6H5), 7.09 (2H, m, m-C6H5), 7.32 and 7.45 (4H,
P r ep a r a tion of Ta Me(CH2CMe3)(η5-C5Me5)(η4-bu ta -1,3-
d ien e) (7). To a solution of 1a (0.108 g, 0.24 mmol) in THF
(40 mL) cooled at -78 °C was added a solution of Me3CCH2-
MgCl (1.17 equiv, 0.29 mmol) in ether (0.13 M, 2.20 mL) via
a syringe. The reaction mixture was stirred for 3 h at 20 °C,
and then a solution of MeMgI (1.12 equiv, 0.27 mmol) in ether
(0.42 M, 0.65 mL) was added at -78 °C followed by stirring
for 2 h at 20 °C. All volatiles were removed under reduced
pressure, and the residue was extracted with hexane (25 mL).
The extract was concentrated to afford 7 as a dark red-purple
oily compound (80% yield). 1H NMR (270 MHz, C6D6, 303 K):
δ -1.13, 0.86, and 0.11 (3H, m, dCH2), -1.27 and 0.17 (2H,
1
m, C6H4). 13C NMR (100 MHz, C6D6, 303 K): δ 11.6 (q, J CH
1
) 128 Hz, C5Me5), 56.1 (t, J CH ) 145 Hz, -CH2), 113.8 (s,
C5Me5), 118.7 (s, ipso-C6H4), 122.9 (d, 1J CH ) 158 Hz, p-C6H5),
1
127.3 (m-C6H5), 127.4 (o-C6H5), 131.1 (d, J CH ) 162, C6H4),
1
230.8 (d, J CH ) 85 Hz, dCHPh). The resonances of the ipso-
C6H4 and ipso-C6H5 were overlapped by C6D6 and impurity,
respectively.
P r ep a r a tion of Ta [CH(P h )CH(C10H6)CH](η5-C5Me5)[o-
(CH2)2C6H4] (15). To a solution of 12 (0.229 g, 0.47 mmol) in
THF (40 mL) cooled at -78 °C was added acenaphthylene (83
2
AB quartet, J HH ) 13.4 Hz, Ta-CH2-CMe3), -0.45 (3H, s,
Ta-CH3), 1.22 (9H, s, -CMe3), 1.78 (15H, s, C5Me5), 6.90 (1H,

4194 Organometallics, Vol. 17, No. 19, 1998
Mashima et al.
Ta ble 6. Cr ysta l Da ta a n d Da ta Collection
P a r a m eter s of 8
mg, 0.54 mmol) in 7 mL of THF and PhCH2MgCl (2.2 equiv,
1.06 mmol) in ether (0.66 M, 1.60 mL) via a syringe. The
reaction mixture was allowed to warm to 20 °C, the color of
the solution changed from red to dark red, and stirring was
continued for 30 h. All volatiles were removed under reduced
pressure to give a residue, from which the product was
extracted with hexane (170 mL). Recrystallization from
toluene (7.0 mL) at -20 °C afforded 15 as dark-red purple
crystals in 18% yield; mp 162.0-163.0 °C (dec). 1H NMR (500
formula
fw
C18H23Cl2Ta
491.23
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21/a
15.029(3)
8.280(3)
15.527(3)
115.63(1)
4
â, deg
MHz, C6D6, 303 K): δ 0.24, 0.54, 1.25, and 1.48 (4H, d, 2J HH
)
Z
9.5, 7.7, 7.7, and 10.3 Hz, respectively, -CH2 (xylylene)), 2.14
V, Å3
1742.1(7)
1.873
2
calcd, g/cm-3
(1H, m, â-H), 3.17, 4.12, (2H, d, J HH ) 7.9 and 7.7 Hz, R-H),
D
1.64 (15H, s, C5Me5), 5.8-7.6 (15H, aromatic protons). 13C
F(000)
952
1
NMR (125 MHz, C6D6, 303 K): δ 11.1 (q, J CH ) 128 Hz,
C5Me5), 20.6 (d, J CH ) 139 Hz, â-C), 63.6 (t J CH ) 139 Hz,
-CH2), 64.0 (t J CH ) 140 Hz, -CH2), 89.7 (d, J CH ) 134 Hz,
R-C), 92.1 (d, J CH ) 132 Hz, R-C), 112.3 (d, J CH ) 158 Hz),
radiation
Mo KR
0.2 × 0.2 × 0.2
65.98
ω-2θ
23
1
1
cryst size, mm
abs coeff, cm-1
scan mode
temp, °C
1
1
1
1
1
117.2 (s, C5Me5), 119.0 (s), 120.1 (d, J CH ) 159 Hz), 120.2 (d,
scan speed, deg/min
scan width, deg
2θmax, deg
unique data
unique data (I > 3σ(I))
no. of variables
R
16
1J CH ) 159 Hz), 122.4 (d, 1J CH ) 159 Hz), 122.5 (d, 1J CH ) 158
0.89 + 0.30 tan θ
55.0
4286 (Rint ) 0.026)
3111
190
1
Hz), 130.5 (s), 131.9 (s), 132.1 (d, J CH ) 159 Hz), 136.5 (d,
1J CH ) 161 Hz), 140.4 (s), 149.1 (s), 151.0 (s), 159.2 (s). The
other peaks of aromatic carbons were overlapped by C6D6.
Anal. Calcd for C37H37Ta: C, 67.07; H, 5.63. Found: C, 66.37;
H, 5.69.
0.033
Rw
0.035
P olym er iza tion of Nor bor n en e. In a typical reaction, a
solution of 2a (15 mg, 0.027 mmol) in toluene (1.0 mL) was
mixed with a solution of norbornene (100 equiv, 2.7 mmol) in
toluene (1.36 M, 2.0 mL) at 25 °C. After the solution was
stirred at 45 or 65 °C for a prescribed period (Table 3),
methanol (20 mL) was added to the resulting reddish brown
solution to precipitate a pale yellow polymer. The polymer
was washed with methanol and then dried in vacuo. Bulk
polymerizations were run by the reaction of 2a (15 mg, 0.027
mmol) and norbornene (1000 equiv, 27 mmol) at 65 °C. 1H
NMR (270 MHz, C6D6, 303 K): δ 1.21 (q (br)), 1.49 (br), 1.92
(br), 2.09 (br), 2.95 (br), 5.39 (br), 6.46 (d (br), 3J HH ) 10.9 Hz,
dCHPh), 7.04-7.33 (m (br), dCHC6H5). 13C{1H} NMR (68
MHz, C6D6, 303 K): δ 33.6 (C1), 39.1 (C2), 43.2 (C3), 134.2 (C4).
GOF
2.48
0.95, -1.17
∆, e Å-3
Ta ble 7. Cr ysta l Da ta a n d Da ta Collection
P a r a m eter s of 12
formula
fw
cryst syst
space group
a, Å
C25H29Ta
510.45
monoclinic
Cc
13.005(3)
17.733(5)
10.187(3)
116.89(2)
4
2095(1)
1.618
1008
Mo KR
0.3 × 0.2 × 0.1
52.43
ω-2θ
23
16
1.57 + 0.35 tan θ
55.0
2487 (Rint ) 0.040)
2058
251
b, Å
c, Å
b, deg
Z
V, Å3
Comparison of 1H NMR spectral data with that of the
literature7,8 determined the stereochemistry of the double bond
to be cis. Unfortunately, the 13C NMR spectrum of poly-
(norbornene) displayed no m/r splittings.
D
calcd, g/cm-3
F(000)
radiation
cryst size, mm
abs coeff, cm-1
scan mode
temp, °C
scan speed, deg/min
scan width, deg
2θmax, deg
unique data
unique data (I > 3σ(I))
no. of variables
R
GP C An a lysis of P oly(n or bor n en e). Gel permeation
chromatographic (GPC) analyses were carried out for polymers
obtained by using 3 and 6 as catalysts at 40 °C using a
Shimadzu LC-10A liquid chromatograph system and a RID-
10A differential refractometer, equipped with a Shodex KF-
806L column. THF was used as the eluent at a flow rate of
0.8 mL/min, with sample concentrations of 0.5 mg/mL. The
GPC column was calibrated with commercially available
polystyrene standards (Aldrich).
0.024
0.025
Rw
For polymers obtained by using other complexes as cata-
lysts, GPC analyses were carried out using Tosoh TSKgel
HXL-H and L columns connected to a Tosoh UV-8010 absor-
bance detector. Samples were prepared in THF (0.1-0.3% (w/
v)) and were filtered through an Advantec DISMIC-25J P filter
in order to remove particulates before injection. GPC columns
were calibrated versus commercially available polystyrene
standards (Polymer Laboratories Ltd.) whose molecular weight
ranged from 500 to 1.11 × 106.
GOF
1.31
0.59, -0.68
∆, e Å-3
Cr ysta llogr a p h ic Da ta Collection s a n d Str u ctu r e De-
ter m in a tion of 8, 12, 14, a n d 15. Da ta Collection . The
crystals of 8, 12, 14, and 15 suitable for X-ray diffraction
studies were sealed in glass capillaries under an argon
atmosphere, and then each crystal of the complexes was
mounted on a Rigaku AFC-5R four-circle diffractometer for
data collection using Mo KR radiation. Relevant crystal and
data statistics are summarized in Tables 6-9. The unit cell
parameters at 23 °C were determined by a least-squares fit to
2q values of 20 strong higher reflections for all complexes.
Three standard reflections were chosen and monitored every
150 reflections. For each complex, an empirical absorption
correction was carried out on the basis of azimuthal scans.
No sample showed any significant intensity decay during the
data collection. The data for all complexes were corrected for
Lorentz and polarization effects.
Hyd r ogen a tion of P oly(n or bor n en e). Polymers were
hydrogenated with diimine generated in situ from p-toluene-
sulfonylhydrazide.26 Typically, 50 mg samples of polymer were
dissolved, with heating, in 10 mL of xylene followed by the
addition of 1.5 g of p-toluenesulfonylhydrazide. The mixture
was stirred at 120 °C for 2.5 h and then poured into methanol.
The precipitated polymer was recovered by centrifugation,
washed several times with methanol, and dried under vaccuum
to yield the hydrogenated product in almost quantitative yield.
Results of the hydrogenated polymer are shown in Table 4.

ROMP of Norbornene
Organometallics, Vol. 17, No. 19, 1998 4195
Ta ble 8. Cr ysta l Da ta a n d Da ta Collection
P a r a m eter s of 14
with k ) odd, the space group of 8 was determined to be P21/
a. The systematic absence of (hkl) with h + k ) odd and (h0l)
with l ) odd in the data collected for complex 12 indicated
the space group to be Cc. Similarly, the systematic absences
of 14 and 15 led the space group of 14 and 15 to be P21/n and
P21/c, respectively. The structures of all complexes were solved
by a direct method (SHELXS 86)49 and refined by the full-
matrix least-squares method. Measured nonequivalent reflec-
tions with I > 3.0σ(I) were used for the structure determina-
tion. In the subsequent refinement the function ∑ω(|Fo| -
|Fc|)2 was minimized, where |Fo| and |Fc| are the observed and
calculated structure factor amplitudes, respectively. The
agreement indices are defined as R ) ∑||Fo| - |Fc||/∑|Fo| and
Rw ) [∑ω(|Fo| - |Fc|)2/∑ω(|Fo|)2 ]1/2, where ω-1 ) σ2(Fo) ) σ2-
(Fo2)/(4Fo2). The positions of all non-hydrogen atoms for all
complexes were found from a difference Fourier electron
density map and refined anisotropically. For 8 and 14 all
hydrogen atoms were placed in calculated positions (C-H )
0.95 Å) and kept fixed. For 12, four hydrogen atoms including
H(11) bound to the benzylidene carbon were found from a
difference Fourier map and refined isotropically, and the other
hydrogen atoms were placed in calculated positions (C-H )
0.95 Å) and constrained to ride on their respective carbon
atoms. Hydrogen atoms of the Cp* ligand of 15 were placed
in calculated positions (C-H ) 0.95 Å), and the other hydrogen
atoms of 15 were found from a difference Fourier syntheses
and were refined isotropically. All calculations were performed
using the TEXSAN crystallographic software package, and
illustrations were drawn with ORTEP.
formula
fw
C27H31Nb
448.45
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21/n
8.490(5)
20.075(5)
12.954(4)
93.25(3)
4
â, deg
Z
V, Å3
2204(1)
1.351
D
calcd, g/cm-3
F(000)
936.00
radiation
Mo KR
0.3 × 0.3 × 0.2
5.55
ω-2θ
23
cryst size, mm
abs coeff, cm-1
scan mode
temp, °C
scan speed, deg/min
scan width, deg
2θmax, deg
unique data
unique data (I > 3σ(I))
no. of variables
R
16
1.10 + 0.30 tan θ
55.0
4295 (Rint ) 0.048)
1439
254
0.079
Rw
0.084
GOF
3.09
1.08, -0.58
∆, e Å-3
Ta ble 9. Cr ysta l Da ta a n d Da ta Collection
P a r a m eter s of 15
formula
fw
C37H37Ta
662.65
cryst system
space group
a, Å
b, Å
c, Å
monoclinic
P21/c
13.989(4)
10.871(3)
18.257(3)
94.83(1)
4
Ack n ow led gm en t. K.M. acknowledges the support
by the Grant-in-Aid for Scientific Research on Priority
Areas (No. 283, “Innovative Synthetic Reactions”) from
the Ministry of Education, Science, Sports and Culture,
J apan. K.M. also appreciates financial support from the
Asahi Glass Foundation.
â, deg
Z
V, Å3
2766.6(10)
1.591
D
calcd, g/cm-3
F(000)
1328
radiation
Mo KR
0.3 × 0.2 × 0.1
39.92
ω-2θ
23
cryst size, mm
abs coeff, cm-1
scan mode
temp, °C
Su p p or tin g In for m a tion Ava ila ble: Tables of final
positional parameters, final thermal parameters, bond dis-
tances and angles, and best planes for 8, 12, 14, and 15
together with their drawings with an all atom-numbering
scheme (58 pages). Ordering information is given on any
current masthead page.
scan speed, deg/min
scan width, deg
2θmax, deg
unique data
unique data (I > 3σ(I))
no. of variables
R
16
1.05 + 0.35 tan θ
55.0
6684 (Rint ) 0.041)
4397
431
OM9802966
0.036
Rw
0.038
GOF
1.55
1.18, -2.16
∆, e Å-3
(49) Sheldrick, G. M. In Crystallographic Computing 3; Sheldrick,
G. M., Kru¨ger, C., Goddard, R., Eds.; Oxford University Press: Oxford,
1985; p 179.
(50) Hamilton, J . G.; Ivin, K. J .; Rooney, J . J . J . Mol. Catal. 1985,
28, 255.
Str u ctu r a l Deter m in a tion s a n d Refin em en ts. Based
on the sytematic absence of 8, (h0l) with h ) odd and (0k0)